JP2010282761A - Lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium secondary battery having an excellent life even when using a positive electrode active material containing a less expensive and more safe phosphate-based positive electrode material. <P>SOLUTION: The lithium secondary battery includes a positive electrode containing the positive electrode active material having a composition expressed by the following composition formula (1), and a nonaqueous electrolyte containing at least one compound selected from a group consisting monofluorophosphate and difluorophosphate. Composition formula (1) LiMPO<SB>4</SB>, [in the composition formula (1), M indicates at least one transition metal element]. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a lithium secondary battery. More specifically, the present invention relates to a lithium secondary battery mainly containing a non-aqueous electrolyte solution for a lithium secondary battery containing a specific component and a lithium transition metal phosphate compound-based positive electrode active material. The present invention relates to a long-life lithium secondary battery using a positive electrode active material for a secondary battery.

  Various active materials containing lithium and a transition metal are known as positive electrode active materials for lithium secondary batteries. Among them, a positive electrode active material mainly composed of cobalt or nickel as a transition metal is to be put into practical use as a battery. There were many. This is mainly because a battery using a positive electrode active material whose main component is cobalt or nickel as a transition metal has a longer life than a battery using another positive electrode active material. Various methods for improving the battery using such a positive electrode active material and further improving the battery life have been studied.

  However, since an oxide-based positive electrode containing a large amount of nickel has a low thermal stability, it has a problem in safety. Therefore, a positive electrode active material mainly composed of a lithium transition metal phosphate compound that is safe and inexpensive. In particular, various improvements of lithium iron phosphate have been studied. There is an increasing demand for life in lithium iron phosphate. For example, Japanese Patent Laid-Open No. 2002-117743 discloses that the positive electrode active material layer contains lithium carbonate to improve the charge / discharge cycle characteristics at high temperature use, and Japanese Patent Laid-Open No. 2005-135775 discloses that Although it has been proposed to improve the charge / discharge cycle characteristics by controlling the end-of-charge potential of the positive electrode and the negative electrode active material, further improvement is required. (See Patent Documents 1 and 2)

Japanese Patent Application Laid-Open No. 2002-117843 JP 2005-135775 A

  The present invention has been made in view of the background art, and the problem is that a positive electrode active material containing a lithium transition metal phosphate compound-based positive electrode material that is cheaper and higher in safety is used. Another object of the present invention is to provide a lithium secondary battery having a good lifetime.

  As a result of intensive studies to solve the above problems, the inventors use a positive electrode active material mainly composed of a lithium transition metal phosphate compound-based positive electrode material and an electrolytic solution containing a specific fluorophosphate compound. As a result, it was found that a high-power lithium secondary battery having a good lifetime was obtained, and the present invention was completed.

  That is, the present invention is a lithium secondary battery comprising a positive electrode and a negative electrode capable of inserting and extracting lithium, and a non-aqueous electrolyte obtained by mixing a lithium salt with a non-aqueous solvent, wherein the non-aqueous electrolyte is a non-aqueous electrolyte. A positive electrode active material containing at least one compound selected from the group consisting of monofluorophosphate and difluorophosphate, wherein the positive electrode has a composition represented by the following composition formula (1) The present invention provides a lithium secondary battery characterized by comprising:

LiMPO ₄ (1)
[In composition formula (1), M represents at least one transition metal element. ]

  According to the present invention, it is possible to provide a lithium secondary battery that is inexpensive, highly safe, and has a long life, and further, for example, a lithium secondary battery that is particularly suitable as a large battery for use in automobiles or the like can be provided.

DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail. However, the description of the constituent elements described below is an example (representative example) of an embodiment of the present invention, and the present invention is not limited to the gist of the present invention. The specific contents of are not specified.
<Non-aqueous electrolyte>
[Specific fluorophosphate compounds]
The non-aqueous electrolyte in the secondary battery of the present invention is at least one compound selected from the group consisting of monofluorophosphate and difluorophosphate (hereinafter referred to as “specific fluorophosphate compound”). It may be abbreviated) or added.

The counter cation of monofluorophosphate and difluorophosphate is not particularly limited. In addition to metal elements such as lithium, sodium, potassium, magnesium, calcium, iron, copper, NR ⁹ R ¹⁰ R ¹¹ R ¹² ( In the formula, R ^{9 to} R ¹² each independently represents a hydrogen atom or an organic group having 1 to 12 carbon atoms). Here, the organic group having 1 to 12 carbon atoms of R ^{9 to} R ¹² is an alkyl group which may be substituted with a halogen atom, a cycloalkyl group which may be substituted with a halogen atom, or a halogen atom. An aryl group which may be present, a nitrogen atom-containing heterocyclic group, and the like. R ^{9 to} R ¹² are each preferably a hydrogen atom, an alkyl group, a cycloalkyl group, or a nitrogen atom-containing heterocyclic group. Among these counter cations, from the viewpoint of battery characteristics when used for lithium secondary batteries, alkali metals such as lithium, sodium and potassium, alkaline earth metals such as magnesium and calcium, or NR ⁹ R ¹⁰ R ¹¹ R ¹² are used. Lithium is preferred and lithium is particularly preferred. Of these, difluorophosphate is preferable in terms of the output characteristic improvement rate and cycle characteristics. In addition, those compounds synthesized in a non-aqueous solvent may be used substantially as they are, or those synthesized separately and substantially isolated may be added to the non-aqueous solvent.

  Specific fluorophosphate compounds, that is, monofluorophosphate and difluorophosphate may be used alone, or two or more compounds may be used in any combination and ratio.

The ratio of these specific compounds in the non-aqueous electrolyte is preferably 10 ppm or more (0.001% by mass or more) in total with respect to the total non-aqueous electrolyte, more preferably 0.01% by mass or more, and still more preferably. It is 0.05 mass% or more, Most preferably, it is 0.1 mass% or more. Moreover, an upper limit becomes like this. Preferably it is 5 mass%, More preferably, it is 4 mass%, More preferably, it is 3 mass%. If the concentration of the specific compound is too low, the effect of improving battery output or the effect of improving battery life may be difficult to obtain, while if the concentration is too high, the charge / discharge efficiency may be reduced.
Although the action of the specific fluorophosphate compound is not clear, as described above, the action is not limited by the counter cation, so that the site other than the cation of the specific fluorophosphate compound acts on the electrode active material appropriately. Thus, the effect of the present invention is considered to be obtained.

  In addition, when these specific fluorophosphate compounds are actually used in the production of secondary batteries as non-aqueous electrolytes, the content in the non-aqueous electrolyte is remarkably increased even if the battery is disassembled and the non-aqueous electrolyte is taken out again. In many cases, it is decreasing. Therefore, what can detect the said specific compound at least from the non-aqueous electrolyte solution extracted from the battery is considered to be included in this invention.

[Electrolytes]
The non-aqueous electrolyte solution for secondary batteries containing a specific fluorophosphate used in the lithium secondary battery of the present invention includes an electrolyte and a non-aqueous solvent that dissolves the electrolyte, as in the case of ordinary non-aqueous electrolyte solutions. contains. The electrolyte is not particularly limited as long as it is known to be a lithium salt that can be used as an electrolyte for a non-aqueous electrolyte solution for a lithium secondary battery, and examples thereof include the following.

[[Lithium salt]]
Inorganic lithium salts: inorganic fluoride salts such as LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiSbF ₆ ; perhalogenates such as LiClO ₄ , LiBrO ₄ , LiIO ₄ ; inorganic chloride salts such as LiAlCl ₄ (however, monofluoro Excluding phosphate and difluorophosphate).
Fluorine-containing organic lithium salt: perfluoroalkane sulfonate such as LiCF ₃ SO ₃ ; LiN (CF ₃ SO ₂ ) ₂ , LiN (CF ₃ CF ₂ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ Perfluoroalkanesulfonylimide salts such as SO ₂ ); Perfluoroalkanesulfonylmethide salts such as LiC (CF ₃ SO ₂ ) ₃ ; Li [PF ₅ (CF ₂ CF ₂ CF ₃ )], Li [PF ₄ ( CF ₂ CF ₂ CF ₃ ) ₂ ], Li [PF ₃ (CF ₂ CF ₂ CF ₃ ) ₃ ], Li [PF ₅ (CF ₂ CF ₂ CF ₂ CF ₃ )], Li [PF ₄ (CF ₂ CF ₂ CF ₂ CF ₃ ) ₂ ], Li [PF ₃ (CF ₂ CF ₂ CF ₂ CF ₃ ) ₃ ] and other fluoroalkyl fluorophosphates.
Oxalatoborate salts such as lithium bis (oxalato) borate and lithium difluorooxalatoborate.

These may be used individually by 1 type, or may use 2 or more types together by arbitrary combinations and ratios. Among these, LiPF ₆ is preferable when comprehensively judging the solubility in a non-aqueous solvent, the charge / discharge characteristics in the case of a secondary battery, the output characteristics, the cycle characteristics, and the like. A preferred example in the case of using two or more types together is a combination use of LiPF ₆ and LiBF _{4. In} this case, the proportion of LiBF _{4 in} the total of both is 0.01% by mass or more and 20% by mass or less. It is preferable that it is 0.1 mass% or more and 5 mass% or less.

  Another example is the combined use of an inorganic fluoride salt and a perfluoroalkanesulfonylimide salt. In this case, the proportion of the inorganic fluoride salt in the total of both is 70% by mass or more and 99% by mass. Or less, more preferably 80% by mass or more and 98% by mass or less. The combined use of both has the effect of suppressing deterioration due to high temperature storage.

  The concentration of the lithium salt in the nonaqueous electrolytic solution is not particularly limited, but is usually 0.5 mol / L or more, preferably 0.6 mol / L or more, more preferably 0.7 mol / L or more. Moreover, the upper limit is 2 mol / L normally, Preferably it is 1.8 mol / L, More preferably, it is 1.7 mol / L. If the concentration is too low, the electrical conductivity of the electrolyte solution may be insufficient. On the other hand, if the concentration is too high, the electrical conductivity may decrease due to an increase in viscosity, and the performance of the lithium secondary battery decreases. There is a case.

[Nonaqueous solvent]
As the non-aqueous solvent, it can be appropriately selected from those conventionally proposed as solvents for non-aqueous electrolyte solutions. For example, the following are mentioned.
1) Cyclic carbonate:
As for carbon number of the alkylene group which comprises a cyclic carbonate, 2-6 are preferable, Most preferably, it is 2-4. Specific examples include ethylene carbonate, propylene carbonate, butylene carbonate, and the like. Of these, ethylene carbonate and propylene carbonate are preferable.
2) Chain carbonate:
As the chain carbonate, dialkyl carbonate is preferable, and the number of carbon atoms of the alkyl group is preferably 1 to 5, and particularly preferably 1 to 4, respectively. Specific examples include dialkyl carbonates such as dimethyl carbonate, diethyl carbonate, di-n-propyl carbonate, ethyl methyl carbonate, methyl-n-propyl carbonate, and ethyl-n-propyl carbonate. Of these, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate are preferable.
3) Cyclic ester:
Specific examples include γ-butyrolactone and γ-valerolactone.
4) Chain ester:
Specific examples include methyl acetate, ethyl acetate, propyl acetate, and methyl propionate.
5) Cyclic ether:
Specific examples include tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydropyran and the like.
6) Chain ether:
Specific examples include dimethoxyethane and dimethoxymethane.
7) Sulfur-containing organic solvent:
Specific examples include sulfolane and diethylsulfone.

  These may be used alone or in combination of two or more, but it is preferable to use in combination of two or more. For example, it is preferable to use a high dielectric constant solvent such as cyclic carbonates and cyclic esters in combination with a low viscosity solvent such as chain carbonates and chain esters.

  One preferred combination of non-aqueous solvents is a combination mainly composed of cyclic carbonates and chain carbonates. Especially, it is preferable that the sum total of the cyclic carbonate and chain carbonate which occupy in a non-aqueous solvent is 85 volume% or more, More preferably, it is 90 volume% or more, More preferably, it is 95 volume% or more. The capacity of the cyclic carbonates relative to the total of the cyclic carbonates and the chain carbonates is usually 5% or more, preferably 10% or more, more preferably 15% or more, and usually 50% or less, preferably 35%. Below, more preferably 30% or less. It is particularly preferred that the above preferred volume range of the total amount of carbonates occupying the entire non-aqueous solvent is combined with the preferred above volume range of cyclic carbonates relative to cyclic and chain carbonates.

  A non-aqueous electrolyte containing at least one compound selected from the group consisting of a lithium salt, a monofluorophosphate, and a difluorophosphate in this mixed solvent is a cycle of a battery produced using the nonaqueous electrolyte. This is particularly preferable because the balance between the characteristics and the high temperature storage characteristics (particularly, the remaining capacity and high load discharge capacity after high temperature storage) and the suppression of gas generation are improved.

  Specific examples of preferred combinations of cyclic carbonates and chain carbonates include ethylene carbonate and dimethyl carbonate, ethylene carbonate and diethyl carbonate, ethylene carbonate and ethyl methyl carbonate, ethylene carbonate and dimethyl carbonate and diethyl carbonate, ethylene carbonate and dimethyl carbonate And ethyl methyl carbonate, ethylene carbonate, diethyl carbonate, and ethyl methyl carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate. A combination in which propylene carbonate is further added to the combination of these ethylene carbonates and chain carbonates is also a preferable combination. When propylene carbonate is contained, the volume ratio of ethylene carbonate to propylene carbonate is usually 99: 1 to 40:60, preferably 95: 5 to 50:50.

  Among these, those containing asymmetric chain carbonates are more preferable, especially ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate, ethylene carbonate, diethyl carbonate and ethyl methyl carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate and ethyl. Those containing ethylene carbonate such as methyl carbonate, symmetric chain carbonates, and asymmetric chain carbonates are preferred because of a good balance between cycle characteristics and large current discharge characteristics. Among these, those in which the asymmetric chain carbonate is ethyl methyl carbonate are preferable, and the number of carbon atoms of the alkyl group constituting the dialkyl carbonate is preferably 1 or 2.

  Other examples of preferred non-aqueous solvents are those containing chain esters. As the chain ester, methyl acetate, ethyl acetate and the like are particularly preferable. The capacity of the chain ester in the non-aqueous solvent is usually 5% or more, preferably 8% or more, more preferably 15% or more, and the upper limit is usually 50% or less, preferably 35% or less, more preferably 30 % Or less, more preferably 25% or less. In particular, those containing a chain ester in the mixed solvent of cyclic carbonates and chain carbonates are preferable from the viewpoint of improving the low-temperature characteristics of the battery.

  Examples of other preferable non-aqueous solvents include one or more organic solvents selected from the group consisting of ethylene carbonate, propylene carbonate, γ-butyrolactone and γ-valerolactone occupying 60% by volume or more of the total. Is. Such a mixed solvent preferably has a flash point of 50 ° C. or higher, and particularly preferably has a flash point of 70 ° C. or higher. A non-aqueous electrolyte using this solvent reduces evaporation of the solvent and leakage even when used at high temperatures. Among them, the amount of γ-butyrolactone in the nonaqueous solvent is 60% by volume or more, and the total of ethylene carbonate and γ-butyrolactone in the nonaqueous solvent is 80% by volume or more, preferably 90% by volume or more. And the volume ratio of ethylene carbonate to γ-butyrolactone is 5:95 to 45:55, or the total of ethylene carbonate and propylene carbonate in the non-aqueous solvent is 80% by volume or more, preferably 90% It is preferable to use an ethylene carbonate / propylene carbonate having a volume ratio of 30:70 to 60:40 because the balance between cycle characteristics and large current discharge characteristics is improved.

  The non-aqueous electrolyte used in the lithium secondary battery of the present invention contains a non-aqueous solvent, a lithium salt, and the specific fluorophosphate compound, but may further contain other compounds as necessary. . In preparing the non-aqueous electrolyte, each raw material is preferably dehydrated in advance, and the water content is usually 50 ppm or less, preferably 30 ppm or less, particularly preferably 10 ppm or less.

[Other compounds]
The non-aqueous electrolyte solution used for the lithium secondary battery of the present invention can contain other compounds in an arbitrary amount as long as it does not impair the effects of the present invention. Examples of such other compounds include:
(1) aromatic compounds such as biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, dibenzofuran;
(2) Partially fluorinated products of the aromatic compounds such as 2-fluorobiphenyl, o-cyclohexylfluorobenzene, p-cyclohexylfluorobenzene; 2,4-difluoroanisole, 2,5-difluoroanisole, 2,6-difluoroanisole And overcharge inhibitors such as fluorine-containing anisole compounds such as 3,5-difluoroanisole;
(3) vinylene carbonate, vinyl ethylene carbonate, fluoroethylene carbonate, trifluoropropylene carbonate, succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, cyclohexanedicarboxylic anhydride, etc. Negative electrode film-forming agent: ethylene sulfite, propylene sulfite, dimethyl sulfite, propane sultone, butane sultone, methyl methanesulfonate, busulfan, methyl toluenesulfonate, dimethyl sulfate, ethylene sulfate, sulfolane, dimethyl sulfone, diethyl sulfone, dimethyl sulfoxide, diethyl sulfoxide , Tetramethylene sulfoxide, diphenyl sulfide, thioanisole, diphenyl disulfide, dipyridinium disulfide, etc. Agent;
Etc.

  As the overcharge inhibitor, aromatic compounds such as biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, and dibenzofuran are preferable. Two or more of these may be used in combination. When using 2 or more types together, it is particularly preferable to use cyclohexylbenzene or terphenyl (or a partially hydrogenated product thereof) together with t-butylbenzene or t-amylbenzene.

  As the negative electrode film forming agent, vinylene carbonate, vinyl ethylene carbonate, fluoroethylene carbonate, succinic anhydride, and maleic anhydride are preferable. Two or more of these may be used in combination. When using 2 or more types together, vinylene carbonate and vinyl ethylene carbonate, fluoroethylene carbonate, succinic anhydride, or maleic anhydride are preferable.

  As the positive electrode protective agent, ethylene sulfite, propylene sulfite, propane sultone, butane sultone, methyl methanesulfonate, and busulfan are preferable. Two or more of these may be used in combination.

  Moreover, the combined use of a negative electrode film forming agent and a positive electrode protective agent, or the combined use of an overcharge inhibitor, a negative electrode film forming agent, and a positive electrode protective agent is particularly preferable. The content ratio of these other compounds in the non-aqueous electrolyte solution is not particularly limited, but is preferably 0.01% by mass or more, particularly preferably 0.1% by mass or more, respectively, based on the whole non-aqueous electrolyte solution. The upper limit is preferably 5% by mass or less, particularly preferably 3% by mass or less, and further preferably 2% by mass or less. By containing these compounds, it is possible to suppress rupture / ignition of the battery at the time of abnormality due to overcharge, or to improve capacity maintenance characteristics and cycle characteristics after high temperature storage.

  There are no particular limitations on the method for preparing the non-aqueous electrolyte solution for secondary batteries of the present invention, and a lithium salt, a specific fluorophosphate compound, and other compounds as required are dissolved in a non-aqueous solvent according to a conventional method. Can be prepared.

<Positive electrode>
The positive electrode used for the lithium secondary battery of the present invention will be described below.
[Positive electrode active material]
The positive electrode active material used for the positive electrode is described below.
[[composition]]
The positive electrode active material is a substance containing a transition metal that can electrochemically occlude and release lithium ions and has a composition represented by the following composition formula (1) (hereinafter referred to as “positive electrode of composition formula (1)”. Abbreviated as “active material”).

LiMPO ₄ (1)
[In composition formula (1), M represents at least one transition metal element. ]

In the composition formula (1), M preferably contains at least one selected from V, Ti, Cr, Mn, Fe, Co, Ni and Cu, and particularly contains M, Fe, Mn, Ni and Co. preferable. In addition to the main transition metals, a small amount of elements such as Mo, Ta, W, Si, B, Al, and S can be added. When a small amount is added, it is preferably 0.1 mol% or more and 5 mol% or less, more preferably 0.2 mol% or more and 2.5 mol% or less.
Although there are still many unclear points regarding the mechanism of action when the positive electrode active material in the present invention is used, in addition to the action of a specific fluorophosphate on the central metal, the interaction with the anion moiety is also inferred. It is considered that the function is exhibited by the combined action and the effect is obtained.

[[Surface coating]]
Moreover, you may use what the substance of the composition different from this adhered to the surface of the positive electrode active material of compositional formula (1). Surface adhering substances include aluminum oxide, silicon oxide, titanium oxide, zirconium oxide, magnesium oxide, calcium oxide, boron oxide, antimony oxide, bismuth oxide, lithium sulfate, sodium sulfate, potassium sulfate, magnesium sulfate, calcium sulfate And sulfates such as aluminum sulfate, carbonates such as lithium carbonate, calcium carbonate, and magnesium carbonate, and carbon.

  For example, these surface adhering substances are dissolved or suspended in a solvent, impregnated and added to the positive electrode active material, and dried. After the surface adhering substance precursor is dissolved or suspended in a solvent and impregnated and added to the positive electrode active material, It can be made to adhere to the surface of the positive electrode active material by a method of reacting by heating or the like, a method of adding to the positive electrode active material precursor and firing simultaneously. In addition, when making carbon adhere, the method of making carbonaceous adhere mechanically later in the form of activated carbon etc. can also be used, for example.

  The amount of the surface adhering substance is, by mass, with respect to the positive electrode active material, preferably 0.1 ppm or more, more preferably 1 ppm or more, still more preferably 10 ppm or more, and the upper limit, preferably 20% or less, more preferably as the lower limit. Is used at 10% or less, more preferably 5% or less. The surface adhering substance can suppress the oxidation reaction of the electrolyte solution on the surface of the positive electrode active material and can improve the battery life. However, when the amount of the adhering quantity is too small, the effect is not sufficiently manifested. If it is too high, the resistance may increase in order to inhibit the entry and exit of lithium ions.

  In the present invention, a material in which a material having a composition different from that of the positive electrode active material of the composition formula (1) is attached is also referred to as a “positive electrode active material”.

[[shape]]
As the shape of the particles of the positive electrode active material in the present invention, a lump shape, a polyhedron shape, a spherical shape, an elliptical spherical shape, a plate shape, a needle shape, a columnar shape, and the like, which are conventionally used, are used. It is preferably formed by forming secondary particles, and the shape of the secondary particles is spherical or elliptical. In general, an electrochemical element expands and contracts as the active material in the electrode expands and contracts as it is charged and discharged. Therefore, the active material is easily damaged due to the stress or the conductive path is broken. Therefore, it is preferable that the primary particles are aggregated to form secondary particles, rather than a single particle active material having only primary particles, in order to relieve expansion and contraction stress and prevent deterioration. In addition, spherical or oval spherical particles are less oriented during molding of the electrode than plate-like equiaxed particles, so that the expansion and contraction of the electrode during charging and discharging is small, and the electrode is produced. The mixing with the conductive material is also preferable because it is easy to mix uniformly.

[[Tap density]]
The tap density of the positive electrode active material is preferably 0.5 g / cm ³ or more, more preferably 1.0 g / cm ³ or more, still more preferably 1.5 g / cm ³ or more, and most preferably 1.7 g / cm ^3. That's it. When the tap density of the positive electrode active material is lower than the lower limit, the amount of the required dispersion medium increases when the positive electrode active material layer is formed, and the required amount of the conductive material and the binder increases. The filling rate of the active material is restricted, and the battery capacity may be restricted. By using a complex oxide powder having a high tap density, a high-density positive electrode active material layer can be formed. In general, the tap density is preferably as large as possible, and there is no particular upper limit, but if it is too large, diffusion of lithium ions using the electrolytic solution in the positive electrode active material layer as a medium is rate-limiting, and load characteristics may be easily reduced. The upper limit is preferably 2.7 g / cm ³ or less, more preferably 2.5 g / cm ³ or less.

  In the present invention, the tap density is defined as the powder packing density (tap density) g / cc when 5 to 10 g of the positive electrode active material powder is put in a 10 ml glass measuring cylinder and tapped 200 times with a stroke of about 20 mm. Ask.

[[Median diameter d ₅₀ ]]
The median diameter d ₅₀ of the positive electrode active material particles (secondary particle diameter when primary particles are aggregated to form secondary particles) is preferably 0.5 μm or more, more preferably 1.2 μm or more. More preferably, it is 1.5 μm or more, most preferably 2 μm or more, and the upper limit is preferably 20 μm or less, more preferably 18 μm or less, still more preferably 16 μm or less, and most preferably 15 μm or less. If the lower limit is not reached, a high tap density product may not be obtained. If the upper limit is exceeded, it takes time for the diffusion of lithium in the particles. When a conductive material, a binder, or the like is slurried with a solvent and applied as a thin film, problems such as streaking may occur. Here, by mixing two or more kinds of the positive electrode active materials having different median diameters d ₅₀ , the filling property at the time of forming the positive electrode can be further improved.

The median diameter d ₅₀ in the present invention is measured by a known laser diffraction / scattering particle size distribution measuring apparatus. When LA-920 manufactured by HORIBA is used as a particle size distribution meter, a 0.1% by mass sodium hexametaphosphate aqueous solution is used as a dispersion medium for measurement, and a measurement refractive index of 1.24 is set after ultrasonic dispersion for 5 minutes. Measured.

[[Average primary particle size]]
When primary particles are aggregated to form secondary particles, the average primary particle diameter of the positive electrode active material is preferably 0.05 μm or more, more preferably 0.1 μm or more, and still more preferably 0.8. The upper limit is preferably 2 μm or less, more preferably 1.6 μm or less, still more preferably 1.3 μm or less, and most preferably 1 μm or less. If the above upper limit is exceeded, it is difficult to form spherical secondary particles, which adversely affects the powder filling property, or the specific surface area is greatly reduced, so that there is a high possibility that battery performance such as output characteristics will deteriorate. is there. On the other hand, when the value falls below the lower limit, there is a case where problems such as inferior reversibility of charge / discharge are usually caused because crystals are not developed.

  The primary particle diameter is measured by observation using a scanning electron microscope (SEM). Specifically, in a photograph at a magnification of 10000 times, the longest value of the intercept by the left and right boundary lines of the primary particles with respect to the horizontal straight line is obtained for any 50 primary particles and obtained by taking the average value. It is done.

[[BET specific surface area]]
The BET specific surface area of the positive electrode active material used for the secondary battery of the present invention is preferably 0.4 m ² /
g or more, more preferably 0.5 m ² / g or more, further preferably 0.6 m ² / g or more, and the upper limit is 50 m ² / g or less, preferably 40 m ² / g or less, more preferably 30 m ² / g. It is as follows. If the BET specific surface area is smaller than this range, the battery performance tends to be lowered. If the BET specific surface area is larger, the tap density is difficult to increase, and a problem may occur in applicability when forming the positive electrode active material layer.

  The BET specific surface area is determined by using a surface area meter (for example, a fully automated surface area measuring device manufactured by Okura Riken), preliminarily drying the sample for 30 minutes at 150 ° C. under nitrogen flow, and then measuring the relative pressure of nitrogen relative to atmospheric pressure. It is defined by a value measured by a nitrogen adsorption BET one-point method using a gas flow method using a nitrogen-helium mixed gas that is accurately adjusted to have a value of 0.3.

[[Production method]]
As a method for producing the positive electrode active material, a general method is used as a method for producing an inorganic compound. In particular, various methods are conceivable for producing a spherical or elliptical active material. For example, a phosphorus raw material such as phosphoric acid and a raw material M of the composition formula (1) are dissolved in a solvent such as water. Or, pulverize and disperse, adjust the pH while stirring, create and recover a spherical precursor, dry it as necessary, and then add a Li source such as LiOH, Li ₂ CO ₃ , LiNO _3, etc. Examples thereof include a method of obtaining an active material by baking at a high temperature.

For the production of the positive electrode in the present invention, the positive electrode active material of the composition formula (1) and / or the positive electrode active material of the composition formula (1) coated with the surface adhering material may be used alone or in different compositions. These may be used in any combination or ratio. As a preferable combination in this case, LiMn ₂ O ₄ or a combination with a part of this Mn substituted with another transition metal or the like, or LiCoO ₂ or a part of this Co with another transition metal or the like is substituted. The combination with what was done is mentioned. Here, the positive electrode active material of the composition formula (1) and / or the positive electrode active material of the composition formula (1) coated with the surface adhesion material is preferably 30% by mass or more of the whole positive electrode active material, More preferably, it is at least mass%. When the usage rate of the positive electrode active material of the composition formula (1) and / or the positive electrode active material of the composition formula (1) coated with the surface adhering material decreases, the battery capacity may be reduced. The “positive electrode active material of composition formula (1) and / or the positive electrode active material of composition formula (1) coated with the surface adhering material” and “the positive electrode active material of composition formula (1) and / or the surface adhering”. The “positive electrode active material other than the positive electrode active material of the composition formula (1) coated with the material” is collectively referred to as “positive electrode active material”.

[Configuration of positive electrode]
Below, the structure of the positive electrode used for this invention is described.
[[Electrode structure and fabrication method]]
The positive electrode for a lithium secondary battery of the present invention is produced by forming a positive electrode active material layer containing a positive electrode active material and a binder on a current collector. That is, the positive electrode for a lithium secondary battery of the present invention is produced by forming a positive electrode active material layer containing the positive electrode active material and a binder on a current collector. Manufacture of the positive electrode using a positive electrode active material can be performed by a conventional method. That is, a positive electrode active material and a binder, and if necessary, a conductive material and a thickener mixed in a dry form into a sheet form are pressure-bonded to the positive electrode current collector, or these materials are liquid media A positive electrode can be obtained by forming a positive electrode active material layer on the current collector by applying it to a positive electrode current collector and drying it as a slurry by dissolving or dispersing in a slurry.

  The content of the positive electrode active material used in the positive electrode of the lithium secondary battery of the present invention in the positive electrode active material layer is preferably 80% by mass or more, more preferably 82% by mass or more, and particularly preferably 84% by mass or more. is there. Moreover, an upper limit becomes like this. Preferably it is 95 mass% or less, More preferably, it is 93 mass% or less. If the content of the positive electrode active material in the positive electrode active material layer is low, the electric capacity may be insufficient. Conversely, if the content is too high, the strength of the positive electrode may be insufficient.

The positive electrode active material layer obtained by coating and drying is preferably consolidated by a hand press, a roller press or the like in order to increase the packing density of the positive electrode active material. Density of the positive electrode active material layer is preferably 1.5 g / cm ³ or more as a lower limit, more preferably 2 g / cm ^3, and even more preferably at 2.2 g / cm ³ or more, the upper limit, preferably 3.5g / Cm ³ or less, more preferably 3 g / cm ³ or less, and even more preferably 2.8 g / cm ³ or less. If it exceeds this range, the permeability of the electrolyte solution to the vicinity of the current collector / active material interface decreases, and the charge / discharge characteristics particularly at a high current density decrease, and a high output may not be obtained. On the other hand, if it is lower, the conductivity between the active materials is lowered, the battery resistance is increased, and a high output may not be obtained.

[[[Conductive material]]]
A known conductive material can be arbitrarily used as the conductive material. Specific examples include metal materials such as copper and nickel; graphite such as natural graphite and artificial graphite (graphite); carbon black such as acetylene black; and carbon materials such as amorphous carbon such as needle coke. In addition, these may be used individually by 1 type and may use 2 or more types together by arbitrary combinations and a ratio. The conductive material is usually 0.01% by mass or more, preferably 0.1% by mass or more, more preferably 1% by mass or more in the positive electrode active material layer, and the upper limit is usually 50% by mass or less, preferably It is used so as to contain 30% by mass or less, more preferably 15% by mass or less. If the content is lower than this range, the conductivity may be insufficient. Conversely, if the content is higher than this range, the battery capacity may decrease.

[[[Binder]]]
The binder used in the production of the positive electrode active material layer is not particularly limited, and in the case of a coating method, any material that can be dissolved or dispersed in the liquid medium used in electrode production may be used. , Polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, nitrocellulose, and other resin polymers; SBR (styrene-butadiene rubber), NBR (acrylonitrile-butadiene rubber), fluororubber, isoprene rubber , Rubber polymers such as butadiene rubber and ethylene-propylene rubber; styrene / butadiene / styrene block copolymer or hydrogenated product thereof, EPDM (ethylene / propylene / diene terpolymer), styrene / ethylene / butadiene / Ethylene copolymer, styrene Thermoplastic elastomeric polymer such as isoprene / styrene block copolymer or hydrogenated product thereof; syndiotactic-1,2-polybutadiene, polyvinyl acetate, ethylene / vinyl acetate copolymer, propylene / α-olefin copolymer Soft resinous polymers such as polymers; Fluoropolymers such as polyvinylidene fluoride (PVdF), polytetrafluoroethylene, fluorinated polyvinylidene fluoride, polytetrafluoroethylene / ethylene copolymers; alkali metal ions (especially lithium ions) And a polymer composition having ion conductivity. In addition, these substances may be used individually by 1 type, and may use 2 or more types together by arbitrary combinations and ratios.

  The ratio of the binder in the positive electrode active material layer is usually 0.1% by mass or more, preferably 1% by mass or more, more preferably 3% by mass or more, and the upper limit is usually 80% by mass or less, preferably 60%. It is not more than mass%, more preferably not more than 40 mass%, most preferably not more than 10 mass%. When the ratio of the binder is too low, the positive electrode active material cannot be sufficiently retained and the positive electrode has insufficient mechanical strength, which may deteriorate battery performance such as cycle characteristics. On the other hand, if it is too high, battery capacity and conductivity may be reduced.

[[[Liquid medium]]]
The liquid medium for forming the slurry may be any type of solvent that can dissolve or disperse the positive electrode active material, the conductive material, the binder, and the thickener used as necessary. There is no particular limitation, and either an aqueous solvent or an organic solvent may be used. Examples of the aqueous medium include water, a mixed medium of alcohol and water, and the like. Examples of the organic medium include aliphatic hydrocarbons such as hexane; aromatic hydrocarbons such as benzene, toluene, xylene, and methylnaphthalene; heterocyclic compounds such as quinoline and pyridine; ketones such as acetone, methyl ethyl ketone, and cyclohexanone. Esters such as methyl acetate and methyl acrylate; amines such as diethylenetriamine and N, N-dimethylaminopropylamine; ethers such as diethyl ether, propylene oxide and tetrahydrofuran (THF); N-methylpyrrolidone (NMP) Amides such as dimethylformamide and dimethylacetamide; aprotic polar solvents such as hexamethylphosphalamide and dimethylsulfoxide.

  In particular, when an aqueous medium is used, it is preferable to make a slurry using a thickener and a latex such as styrene-butadiene rubber (SBR). A thickener is usually used to adjust the viscosity of the slurry. The thickener is not particularly limited, and specific examples include carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, casein, and salts thereof. These may be used individually by 1 type, or may use 2 or more types together by arbitrary combinations and a ratio. When a thickener is further added, the ratio of the thickener to the active material is 0.1% by mass or more, preferably 0.5% by mass or more, more preferably 0.6% by mass or more. The upper limit is 5% by mass or less, preferably 3% by mass or less, more preferably 2% by mass or less. Below this range, applicability may be significantly reduced. If it exceeds, the ratio of the active material in the positive electrode active material layer may decrease, and there may be a problem that the capacity of the battery decreases and a problem that the resistance between the positive electrode active materials increases.

[[[Current collector]]]
There is no restriction | limiting in particular as a material of a positive electrode electrical power collector, A well-known thing can be used arbitrarily. Specific examples include metal materials such as aluminum, stainless steel, nickel plating, titanium, and tantalum; and carbon materials such as carbon cloth and carbon paper. Of these, metal materials, particularly aluminum, are preferred.

  Examples of the shape of the current collector include metal foil, metal cylinder, metal coil, metal plate, metal thin film, expanded metal, punch metal, and foam metal in the case of a metal material. A thin film, a carbon cylinder, etc. are mentioned. Of these, metal thin films are preferred. In addition, you may form a thin film suitably in mesh shape. Although the thickness of the thin film is arbitrary, it is usually 1 μm or more, preferably 3 μm or more, more preferably 5 μm or more, and the upper limit is usually 1 mm or less, preferably 100 μm or less, more preferably 50 μm or less. If the thin film is thinner than this range, the strength required for the current collector may be insufficient. Conversely, if the thin film is thicker than this range, the handleability may be impaired.

  The ratio of the thickness of the current collector to the positive electrode active material layer is not particularly limited, but the value of (thickness of the positive electrode active material layer on one side immediately before electrolyte injection) / (thickness of the current collector) is 20 The lower limit is preferably 15 or less, most preferably 10 or less, and the lower limit is preferably 0.5 or more, more preferably 0.8 or more, and most preferably 1 or more. Above this range, the current collector may generate heat due to Joule heat during high current density charge / discharge. Below this range, the volume ratio of the current collector to the positive electrode active material increases and the battery capacity may decrease.

[[[Electrode area]]]
When using the non-aqueous electrolyte of the present invention, it is preferable that the area of the positive electrode active material layer is larger than the outer surface area of the battery outer case from the viewpoint of increasing the stability at high output and high temperature. Specifically, the sum of the electrode areas of the positive electrode with respect to the surface area of the exterior of the secondary battery is preferably 15 times or more, and more preferably 40 times or more. The outer surface area of the outer case is the total area obtained by calculation from the vertical, horizontal, and thickness dimensions of the case part filled with the power generation element excluding the protruding part of the terminal in the case of a bottomed square shape. . In the case of a bottomed cylindrical shape, the geometric surface area approximates the case portion filled with the power generation element excluding the protruding portion of the terminal as a cylinder. The total electrode area of the positive electrode is the geometric surface area of the positive electrode mixture layer facing the mixture layer containing the negative electrode active material, and in the structure in which the positive electrode mixture layer is formed on both sides via the current collector foil. , The sum of the areas where each surface is calculated separately.

[[[Discharge capacity]]]
When the non-aqueous electrolyte for secondary battery of the present invention is used, the electric capacity of the battery element housed in one battery case of the secondary battery (electric capacity when the battery is discharged from the fully charged state to the discharged state) ) Is 3 ampere hours (Ah) or more, since the effect of improving output characteristics is increased. Therefore, the positive electrode plate is preferably designed so that the discharge capacity is fully charged and is 3 ampere hours (Ah) or more and 20 Ah or less, and more preferably 4 Ah or more and 10 Ah or less. If it is less than 3 Ah, the voltage drop due to the electrode reaction resistance becomes large when taking out a large current, and the power efficiency may deteriorate. If it exceeds 20 Ah, the electrode reaction resistance is reduced and the power efficiency is improved, but the temperature distribution due to the internal heat generation of the battery during pulse charge / discharge is large, the durability of repeated charge / discharge is inferior, and overcharge, internal short circuit, etc. The heat dissipation efficiency deteriorates due to sudden heat generation at the time of abnormality, and the probability that the internal pressure rises and the gas release valve operates (valve operation), and the battery contents erupt violently (explosion) increases. There is a case.

[[[Positive electrode plate thickness]]]
The thickness of the positive electrode plate is not particularly limited, but from the viewpoint of high capacity and high output, the thickness of the composite layer obtained by subtracting the metal foil thickness of the core material is relative to one side of the current collector. The lower limit is preferably 10 μm or more, more preferably 20 μm or more, and the upper limit is preferably 200 μm or less, more preferably 100 μm or less.

<Lithium secondary battery>
The lithium secondary battery of the present invention will be described in detail below.

[Battery shape]
The battery shape is not particularly limited, and examples thereof include a bottomed cylindrical shape, a bottomed square shape, a thin shape, a sheet shape, and a paper shape. When incorporating into a system or device, in order to increase the volumetric efficiency and improve the storage capacity, it may be of a different shape such as a horseshoe shape or a comb shape considering the fit in the peripheral system arranged around the battery. . From the viewpoint of efficiently releasing the heat inside the battery to the outside, a rectangular shape having at least one surface that is relatively flat and has a large area is preferable. In a battery having a bottomed cylindrical shape, since the outer surface area with respect to the power generating element to be filled becomes small, it is preferable to design so that Joule heat generated by the internal resistance at the time of charging and discharging efficiently escapes to the outside. Moreover, it is preferable to design so that the filling ratio of the substance having high thermal conductivity is increased and the temperature distribution inside is reduced. In the bottomed square shape, the ratio between the area S of the largest surface (the product of the width and height of the outer dimensions excluding the terminal portion, unit cm ² ) and the thickness T (unit cm) of the battery outer shape The 2S / T value is preferably 100 or more, and more preferably 200 or more. By increasing the maximum surface, it is possible to improve characteristics such as cycleability and high-temperature storage even for high-power and large-capacity batteries, and increase the heat dissipation efficiency during abnormal heat generation. It is possible to suppress a dangerous state of “rupture”.

[Battery configuration]
The rechargeable lithium secondary battery of the present invention includes the above-described positive electrode and negative electrode capable of occluding and releasing lithium ions, the above-described non-aqueous electrolyte, a separator disposed between the positive electrode and the negative electrode, a current collecting terminal, and It is comprised at least by an exterior case etc. If necessary, a protective element may be mounted inside the battery and / or outside the battery.

[Negative electrode]
Below, the negative electrode used for the lithium secondary battery of this invention is demonstrated.
[[Negative electrode active material]]
The negative electrode active material used for the negative electrode is described below.
[[[composition]]]
The negative electrode active material is not particularly limited as long as it can electrochemically occlude and release lithium ions. Carbonaceous materials, metal oxides such as tin oxide and silicon oxide, metal composite oxides, lithium simple substance And lithium alloys such as lithium aluminum alloy and metals that can form alloys with lithium such as Sn and Si. These may be used individually by 1 type, or may use 2 or more types together by arbitrary combinations and a ratio. Of these, carbonaceous materials or lithium composite oxides are preferably used from the viewpoint of safety. The metal composite oxide is not particularly limited as long as it can occlude and release lithium, but preferably contains titanium and / or lithium as a constituent component from the viewpoint of high current density charge / discharge characteristics.

As a carbonaceous material,
(1) natural graphite,
(2) artificial carbonaceous material and artificial graphite material; carbonaceous material (for example, natural graphite, coal-based coke, petroleum-based coke, coal-based pitch, petroleum-based pitch, or these pitches) oxidized, needle coke, Pitch coke and carbon materials partially graphitized from these, pyrolysis products of organic materials such as furnace black, acetylene black, pitch-based carbon fiber, carbonizable organic materials (for example, coal tar pitch from soft pitch to hard pitch, or Coal heavy oil such as dry distillation liquefied oil, normal pressure residual oil, direct current heavy oil of reduced pressure residual oil, crude oil, cracked petroleum heavy oil such as ethylene tar produced during thermal decomposition of naphtha, etc., and acenaphthylene , Aromatic hydrocarbons such as decacyclene, anthracene, phenanthrene, N-ring compounds such as phenazine and acridine, thiophene, bithioff S-ring compounds such as polyphenyl, polyphenylene such as biphenyl and terphenyl, polyvinyl chloride, polyvinyl alcohol, polyvinyl butyral, insolubilized products of these, organic polymers such as nitrogen-containing polyacrylonitrile and polypyrrole, sulfur-containing Organic polymers such as polythiophene and polystyrene, natural polymers such as polysaccharides such as cellulose, lignin, mannan, polygalacturonic acid, chitosan and saccharose, thermoplastic resins such as polyphenylene sulfide and polyphenylene oxide, and furfuryl alcohol resins And thermosetting resins such as phenol-formaldehyde resins and imide resins) and their carbides or carbonizable organic substances in a low molecular organic solvent such as benzene, toluene, xylene, quinoline, n-hexane, and This Carbon material is heat treated one or more times et carbide] is in the range of 400 to 3200 ° C.,
(3) a carbon material in which the negative electrode active material layer is composed of at least two types of carbonaceous materials having different crystallinity and / or has an interface in contact with the different crystalline carbonaceous materials,
(4) A carbon material in which the negative electrode active material layer is composed of at least two kinds of carbonaceous materials having different orientations and / or has an interface in contact with the carbonaceous materials having different orientations,
Is preferably a good balance between initial irreversible capacity and high current density charge / discharge characteristics.

[[Negative electrode configuration, physical properties, preparation method]]
Regarding the properties of the carbonaceous material, the negative electrode containing the carbonaceous material, the electrodeification method, the current collector, and the lithium ion secondary battery, any one or more of (1) to (19) shown below It is desirable to satisfy

(1) X-ray parameters For carbonaceous materials, the d value (interlayer distance) of the lattice plane (002 plane) determined by X-ray diffraction by the Gakushin method is preferably 0.335 nm or more, and the upper limit is usually It is desired that the thickness is 0.36 nm or less, preferably 0.35 nm or less, and more preferably 0.345 nm or less. The crystallite size (Lc) of the carbonaceous material determined by X-ray diffraction by the Gakushin method is preferably 1 nm or more, and more preferably 1.5 nm or more.

(2) Ash content The ash content in the carbonaceous material is 1% by mass or less, particularly 0.5% by mass or less, especially 0.1% by mass or less, and the lower limit is 1 ppm or more with respect to the total mass of the carbonaceous material. It is preferable that When the above range is exceeded, deterioration of the battery performance due to the reaction with the electrolyte during charging / discharging may not be negligible. Below this range, the manufacturing process requires a lot of time, energy and equipment for preventing contamination, which may increase costs.

(3) Volume-based average particle diameter The volume-based average particle diameter of the carbonaceous material is usually 1 μm or more, preferably 3 μm or more, more preferably a volume-based average particle diameter (median diameter) determined by a laser diffraction / scattering method. Is 5 μm or more, more preferably 7 μm or more. Moreover, an upper limit is 100 micrometers or less normally, Preferably it is 50 micrometers or less, More preferably, it is 40 micrometers or less, More preferably, it is 30 micrometers or less, Most preferably, it is 25 micrometers or less. If it falls below the above range, the irreversible capacity may increase, leading to loss of the initial battery capacity. On the other hand, when the above range is exceeded, when an electrode is produced by coating, a non-uniform coating surface tends to be formed, which may be undesirable in the battery production process.

  In the present invention, the volume-based average particle size is determined by dispersing carbon powder in a 0.2% by mass aqueous solution (about 10 mL) of polyoxyethylene (20) sorbitan monolaurate, which is a surfactant, and laser diffraction / scattering type particle size. It is defined by the median diameter measured using a distribution meter (for example, LA-700 manufactured by Horiba, Ltd.).

(4) Raman R value, Raman half-value width The Raman R value of the carbonaceous material measured using the argon ion laser Raman spectrum method is usually 0.01 or more, preferably 0.03 or more, more preferably 0.1 or more. The upper limit is 1.5 or less, preferably 1.2 or less, more preferably 1.0 or less, and still more preferably 0.5 or less. When the Raman R value is below this range, the crystallinity of the particle surface becomes too high, and there are cases where the number of sites where Li enters between layers decreases with charge / discharge. That is, charge acceptance may be reduced. In addition, when the negative electrode is densified by applying it to the current collector and then pressing it, the crystals are likely to be oriented in a direction parallel to the electrode plate, which may lead to a decrease in load characteristics. On the other hand, if it exceeds this range, the crystallinity of the particle surface will decrease, the reactivity with the electrolyte will increase, and the efficiency may decrease and the generation of gas may increase.

Further, the Raman half-width in the vicinity of 1580 cm ^{−1 of the} carbonaceous material of the present invention is not particularly limited, but is usually 10 cm ⁻¹ or more, preferably 15 cm ⁻¹ or more, and the upper limit is usually 100 cm ⁻¹ or less, preferably The range is 80 cm ⁻¹ or less, more preferably 60 cm ⁻¹ or less, and still more preferably 40 cm ⁻¹ or less. When the Raman half width is less than this range, the crystallinity of the particle surface becomes too high, and there are cases where the number of sites where Li enters between layers decreases with charge and discharge. That is, charge acceptance may be reduced. In addition, when the negative electrode is densified by applying it to the current collector and then pressing it, the crystals are likely to be oriented in a direction parallel to the electrode plate, which may lead to a decrease in load characteristics. On the other hand, if it exceeds this range, the crystallinity of the particle surface will decrease, the reactivity with the electrolyte will increase, and the efficiency may decrease and the generation of gas may increase.

The Raman spectrum is measured using a Raman spectrometer (for example, a Raman spectrometer manufactured by JASCO Corporation), the sample is naturally dropped into the measurement cell and filled, and the sample surface in the cell is irradiated with an argon ion laser beam. The cell is rotated in a plane perpendicular to the laser beam. The obtained Raman spectrum, the intensity I _A of the peak P _A in the vicinity of 1580 cm ^-1, and measuring the intensity I _B of a peak P _B in the vicinity of 1360 cm ^-1, the intensity ratio R (R = I _B / I _A ) Is calculated and defined as the Raman R value of the carbonaceous material. Further, the half width of the peak P _A near 1580 cm ^{−1 of} the obtained Raman spectrum is measured, and this is defined as the Raman half width of the carbonaceous material.

The Raman measurement conditions here are as follows.
Argon ion laser wavelength: 514.5nm
・ Laser power on the sample: 15-25mW
・ Resolution: 10-20cm ^-1
Measurement range: 1100 cm ^{−1 to} 1730 cm ⁻¹
・ Raman R value, half width analysis: Background processing ・ Smoothing processing: Simple average, 5 points of convolution

(5) BET specific surface area The specific surface area of the carbonaceous material of the present invention measured using the BET method is usually 0.1 m ² / g or more, preferably 0.7 m ² / g or more, more preferably 1.0 m ^2. / G or more, more preferably 1.5 m ² / g or more. The upper limit is usually 100 m ² / g or less, preferably 25 m ² / g or less, more preferably 15 m ² / g or less, and still more preferably 10 m ² / g or less. When the value of the specific surface area is less than this range, the acceptability of lithium is likely to deteriorate during charging when used as a negative electrode material, and lithium may easily precipitate on the electrode surface. On the other hand, when it exceeds this range, when used as a negative electrode material, the reactivity with the electrolyte increases, gas generation tends to increase, and a preferable battery may be difficult to obtain.

  The specific surface area according to the BET method is determined by using a surface area meter (for example, a fully automatic surface area measuring device manufactured by Riken Okura), preliminarily drying the sample at 350 ° C. for 15 minutes under a nitrogen flow, and then comparing the nitrogen relative to the atmospheric pressure. A value measured by a nitrogen adsorption BET one-point method using a gas flow method is used, using a nitrogen helium mixed gas accurately adjusted so that the pressure value becomes 0.3.

(6) Pore size distribution The pore size distribution of the carbonaceous material used in the present invention is determined by Hg porosimetry (mercury intrusion method), and the pore diameter is 0.01 μm to 1 μm. The amount of voids, irregularities due to particle surface steps, contact surfaces between particles, etc. is 0.01 mL / g or more, preferably 0.05 mL / g or more, more preferably 0.1 mL / g or more, and the upper limit is 0.6 mL. / G or less, preferably 0.4 mL / g or less, more preferably 0.3 mL / g or less. If it exceeds this range, a large amount of binder may be required when forming an electrode plate. If it is less than the range, the high current density charge / discharge characteristics may be deteriorated, and the effect of mitigating the expansion / contraction of the electrode during charge / discharge may not be obtained.

  Further, the total pore volume corresponding to the pore diameter in the range of 0.01 μm to 100 μm is preferably 0.1 mL / g or more, more preferably 0.25 mL / g or more, still more preferably 0.4 mL / g or more, The upper limit is 10 mL / g or less, preferably 5 mL / g or less, more preferably 2 mL / g or less. If this range is exceeded, a large amount of binder may be required during electrode plate formation. If it is less than that, it may not be possible to obtain the effect of dispersing the thickener or the binder during the electrode plate formation. The average pore diameter is preferably 0.05 μm or more, more preferably 0.1 μm or more, further preferably 0.5 μm or more, and the upper limit is 50 μm or less, preferably 20 μm or less, more preferably 10 μm or less. Beyond this range, a large amount of binder may be required. If it is less, the high current density charge / discharge characteristics may deteriorate.

  As an apparatus for Hg porosimetry, a mercury porosimeter (Autopore 9520: manufactured by Micromeritex Corporation) was used. About 0.2 g of a sample was sealed in a powder cell and pretreated by degassing for 10 minutes at room temperature under vacuum (50 μmHg or less). Subsequently, the pressure was reduced to 4 psia (about 28 kPa), mercury was introduced, the pressure was increased stepwise from 4 psia (about 28 kPa) to 40000 psia (about 280 MPa), and then the pressure was reduced to 25 psia (about 170 kPa). The number of steps at the time of pressure increase was 80 points or more, and the mercury intrusion amount was measured after an equilibration time of 10 seconds in each step. The pore size distribution was calculated from the mercury intrusion curve thus obtained using the Washburn equation. The surface tension (γ) of mercury was 485 dyne / cm, and the contact angle (ψ) was 140 °. As the average pore diameter, the pore diameter when the cumulative pore volume was 50% was used.

(7) Circularity Using circularity as the degree of sphericity of the carbonaceous material, the circularity of particles having a particle size in the range of 3 to 40 μm is preferably 0.1 or more, particularly preferably 0.5 or more. Preferably it is 0.8 or more, more preferably 0.85 or more, and most preferably 0.9 or more. High circularity is preferable because high current density charge / discharge characteristics are improved.
The circularity is defined by the following formula. When the circularity is 1, a theoretical sphere is obtained.
Circularity = (perimeter of equivalent circle having the same area as the particle projection shape) / (actual circumference of particle projection shape)

  As the value of the circularity, for example, a flow type particle image analyzer (for example, FPIA manufactured by Sysmex Industrial Co., Ltd.) is used, and about 0.2 g of a sample is mixed with polyoxyethylene (20) sorbitan monolaurate as a surfactant. Particles having a detection range of 0.6 to 400 μm and a particle size in the range of 3 to 40 μm after being dispersed in a 0.2 mass% aqueous solution (about 50 mL) and irradiated with an ultrasonic wave of 28 kHz for 1 minute at an output of 60 W Use the value measured for.

  The method for improving the degree of circularity is not particularly limited, but a spheroidized sphere is preferable because the shape of the interparticle void when the electrode body is formed is preferable. Examples of spheroidizing treatment include a method of mechanically approximating a sphere by applying a shearing force and a compressive force, a mechanical / physical processing method of granulating a plurality of fine particles by an adhesive force possessed by a binder or particles, etc. Is mentioned.

(8) True density The true density of the carbonaceous material is usually 1.4 g / cm ³ or more, preferably 1.6 g / cm ³ or more, more preferably 1.8 g / cm ³ or more, and still more preferably 2.0 g / cm. cm ³ and the upper limit is 2.26 g / cm ³ or less. The upper limit is the theoretical value of graphite. Below this range, the crystallinity of the carbon is too low and the initial irreversible capacity may increase. In the present invention, the true density is defined by a value measured by a liquid phase substitution method (pycnometer method) using butanol.

(9) Tap density The tap density of the carbonaceous material is usually 0.1 g / cm ³ or more, preferably 0.5 g / cm ³ or more, more preferably 0.7 g / cm ³ or more, particularly preferably 1.0 g / cm. It is desired to be cm ³ or more. The upper limit is preferably 2 g / cm ³ or less, more preferably 1.8 g / cm ³ or less, and particularly preferably 1.6 g / cm ³ or less. When the tap density is below this range, the packing density is difficult to increase when used as a negative electrode, and a high-capacity battery may not be obtained. On the other hand, if it exceeds this range, there are too few voids between the particles in the electrode, it becomes difficult to ensure conductivity between the particles, and it may be difficult to obtain preferable battery characteristics. The tap density is measured by a method similar to the method described in the section of the positive electrode and is defined thereby.

(10) Orientation ratio The orientation ratio of the carbonaceous material is usually 0.005 or more, preferably 0.01 or more, more preferably 0.015 or more, and the upper limit is theoretically 0.67 or less. Below this range, the high-density charge / discharge characteristics may deteriorate.

The orientation ratio is measured by X-ray diffraction after pressure-molding the sample. X-ray diffraction is performed by setting a molded body obtained by filling 0.47 g of a sample into a molding machine having a diameter of 17 mm and compressing it at 600 kgf / cm ^{2 so} that it is flush with the surface of the sample holder for measurement. Measure. A ratio represented by (110) diffraction peak intensity / (004) diffraction peak intensity is calculated from the (110) diffraction and (004) diffraction peak intensities of the obtained carbon, and defined as the orientation ratio of the active material.

The X-ray diffraction measurement conditions here are as follows. “2θ” indicates a diffraction angle.
・ Target: Cu (Kα ray) graphite monochromator ・ Slit:
Divergence slit = 0.5 degree, light receiving slit = 0.15 mm, scattering slit = 0.5 degree. Measurement range and step angle / measurement time:
(110) plane: 75 degrees ≦ 2θ ≦ 80 degrees 1 degree / 60 seconds (004) plane: 52 degrees ≦ 2θ ≦ 57 degrees 1 degree / 60 seconds

(11) Aspect ratio (powder)
The aspect ratio is theoretically 1 or more, and the upper limit is 10 or less, preferably 8 or less, more preferably 5 or less. If the upper limit is exceeded, streaking or a uniform coated surface may not be obtained during electrode plate formation, and the high current density charge / discharge characteristics may deteriorate.

  The aspect ratio is expressed as A / B when the longest diameter A of the carbonaceous material particles when observed three-dimensionally and the shortest diameter B orthogonal to the carbonaceous material particles. The carbon particles are observed with a scanning electron microscope capable of magnifying observation. Arbitrary 50 graphite particles fixed on the end face of a metal plate having a thickness of 50 μm or less are selected, and the stage on which the sample is fixed is rotated and tilted, and A and B are measured. Find the average value.

(12) Sub-material mixing “Sub-material mixing” means that two or more carbonaceous materials having different properties are contained in the negative electrode and / or the negative electrode active material. The properties described here include one or more of X-ray diffraction parameters, median diameter, aspect ratio, BET specific surface area, orientation ratio, Raman R value, tap density, true density, pore distribution, circularity, and ash content. Show the characteristics.

  Particularly preferred embodiments include that the volume-based particle size distribution is not symmetrical when the median diameter is centered, that two or more carbonaceous materials having different Raman R values are contained, and the X-ray parameters are It is different. As an example of the effect, carbonaceous material such as natural graphite, graphite such as artificial graphite, carbon black such as acetylene black, amorphous carbon such as needle coke, etc. is contained as a conductive material, so that electric resistance is increased. It can be reduced. These may be used individually by 1 type, or may use 2 or more types together by arbitrary combinations and a ratio. When added as a conductive material, it is 0.1% by mass or more, preferably 0.5% by mass or more, more preferably 0.6% by mass or more, and the upper limit is 45% by mass or less, preferably 40% by mass. It is a range. Below this range, the effect of improving conductivity may be difficult to obtain. If it exceeds, the initial irreversible capacity may increase.

(13) Electrode production The electrode may be produced by a conventional method. For example, it is formed by adding a binder, a solvent, and, if necessary, a thickener, a conductive material, a filler, etc. to a negative electrode active material to form a slurry, which is applied to a current collector, dried and then pressed. Can do. The thickness of the negative electrode active material layer per side in the stage immediately before the electrolytic solution pouring step of the battery is usually 15 μm or more, preferably 20 μm or more, more preferably 30 μm or more, and the upper limit is 150 μm or less, preferably 120 μm or less, More preferably, it is 100 μm or less. If it exceeds this range, the electrolyte solution hardly penetrates to the vicinity of the current collector interface, and thus the high current density charge / discharge characteristics may be deteriorated. On the other hand, below this range, the volume ratio of the current collector to the negative electrode active material increases, and the battery capacity may decrease. Further, the negative electrode active material may be roll-formed to form a sheet electrode, or may be formed into a pellet electrode by compression molding.

(14) Current collector Any known current collector can be used. Examples of the current collector for the negative electrode include metal materials such as copper, nickel, stainless steel, and nickel-plated steel. Of these, copper is particularly preferable from the viewpoint of ease of processing and cost. When the current collector is a metal material, examples of the shape of the current collector include metal foil, metal cylinder, metal coil, metal plate, metal thin film, expanded metal, punch metal, and foam metal. Among them, a metal thin film is preferable, a copper foil is more preferable, and a rolled copper foil by a rolling method and an electrolytic copper foil by an electrolytic method are more preferable, and both can be used as a current collector. When the thickness of the copper foil is less than 25 μm, a copper alloy (phosphor bronze, titanium copper, Corson alloy, Cu—Cr—Zr alloy, etc.) having higher strength than pure copper can be used.

  A current collector made of a copper foil produced by a rolling method is suitable for use in a small cylindrical battery because the copper crystals are arranged in the rolling direction so that the negative electrode is hard to crack even if it is rounded sharply or rounded at an acute angle. be able to. For example, an electrolytic copper foil is prepared by immersing a metal drum in an electrolytic solution in which copper ions are dissolved, and flowing current while rotating the copper drum, thereby depositing copper on the surface of the drum and peeling it off. It is obtained. Copper may be deposited on the surface of the rolled copper foil by an electrolytic method. One side or both sides of the copper foil may be subjected to a roughening treatment or a surface treatment (for example, a chromate treatment having a thickness of about several nm to 1 μm, a base treatment such as Ti).

The following physical properties are desired for the current collector substrate.
(a) Average surface roughness (Ra)
The average surface roughness (Ra) of the active material thin film forming surface of the current collector substrate defined by the method described in JIS B0601-1994 is not particularly limited, but is usually 0.05 μm or more, preferably 0.1 μm or more, particularly The upper limit is usually 1.5 μm or less, preferably 1.3 μm or less, particularly preferably 1.0 μm or less. By setting the average surface roughness (Ra) of the current collector substrate within the range between the lower limit and the upper limit described above, good charge / discharge cycle characteristics can be expected. By setting it to the above lower limit or more, the area of the interface with the active material thin film is increased, and the adhesion with the active material thin film is improved. The upper limit of the average surface roughness (Ra) is not particularly limited, but those having an average surface roughness (Ra) exceeding 1.5 μm are generally difficult to obtain as foils having a practical thickness as a battery. 1.5 μm or less is preferable.

(b) Tensile strength The tensile strength of the current collector substrate is not particularly limited, but is usually 100 N / mm ² or more, preferably 250 N / mm ² or more, more preferably 400 N / mm ² or more, and particularly preferably 500 N / mm ^2. That's it. The tensile strength is obtained by dividing the maximum tensile force required until the test piece breaks by the cross-sectional area of the test piece. The tensile strength in the present invention is measured by the same apparatus and method as the elongation rate. If the current collector substrate has a high tensile strength, cracking of the current collector substrate due to expansion / contraction of the active material thin film accompanying charging / discharging can be suppressed, and good cycle characteristics can be obtained.

(c) 0.2% proof stress of 0.2% proof stress current collector substrate is not particularly limited, normally 30 N / mm ² or more, preferably 150 N / mm ² or more, particularly preferably 300N / mm ² or more . The 0.2% proof stress is the magnitude of the load necessary to give a plastic (permanent) strain of 0.2%. It means that The 0.2% proof stress in the present invention is measured by the same apparatus and method as the elongation rate. If the current collector substrate has a high 0.2% proof stress, plastic deformation of the current collector substrate due to expansion / contraction of the active material thin film accompanying charging / discharging can be suppressed, and good cycle characteristics can be obtained. it can. Although the thickness of a metal thin film is arbitrary, it is 1 micrometer or more normally, Preferably it is 3 micrometers or more, More preferably, it is 5 micrometers or more. Moreover, an upper limit is 1 mm or less normally, Preferably it is 100 micrometers or less, More preferably, it is 30 micrometers or less. If the thickness is less than 1 μm, the strength may be reduced, and application may be difficult. On the other hand, if the thickness exceeds 100 μm, the shape of the desired electrode may be deformed by winding or the like. The metal thin film may be mesh.

(15) Ratio of current collector and active material layer thickness The ratio of current collector and active material layer thickness is not particularly limited, but (active material layer thickness on one side immediately before electrolyte injection) / The (current collector thickness) is preferably 150 or less, particularly preferably 20 or less, more preferably 10 or less, and the lower limit is preferably 0.1 or more, particularly preferably 0.4 or more, more preferably. Is a range of 1 or more. Above this range, the current collector may generate heat due to Joule heat during high current density charge / discharge. Below this range, the volume ratio of the current collector to the negative electrode active material increases and the battery capacity may decrease.

(16) Electrode density The electrode structure when the negative electrode active material is made into an electrode is not particularly limited, but the density of the active material present on the current collector is preferably 1.0 g / cm ³ or more, more preferably. Is 1.2 g / cm ³ , more preferably 1.3 g / cm ³ or more, and the upper limit is 2.0 g / cm ³ or less, preferably 1.9 g / cm ³ or less, more preferably 1.8 g / cm ^3. Hereinafter, the range is more preferably 1.7 g / cm ³ or less. If it exceeds this range, the active material particles are destroyed, which may lead to an increase in initial irreversible capacity and deterioration of high current density charge / discharge characteristics due to a decrease in the permeability of the electrolyte solution near the current collector / active material interface. On the other hand, if it is lower, the conductivity between the active materials is lowered, the battery resistance is increased, and the capacity per unit volume may be lowered.

(17) Binder The binder for binding the active material is not particularly limited as long as it is a material that is stable with respect to the electrolyte and the solvent used during electrode production. Specifically, resin-based polymers such as polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, aromatic polyamide, cellulose, nitrocellulose; SBR (styrene-butadiene rubber), isoprene rubber, butadiene rubber, fluorine rubber, NBR ( Acrylonitrile-butadiene rubber), rubber-like polymers such as ethylene-propylene rubber; styrene / butadiene / styrene block copolymer or hydrogenated product thereof; EPDM (ethylene / propylene / diene terpolymer), styrene / ethylene / Thermoplastic elastomeric polymer such as butadiene / styrene copolymer, styrene / isoprene / styrene block copolymer or hydrogenated product thereof; syndiotactic-1,2-polybutadiene, polyvinyl acetate, ethylene / vinegar Soft resinous polymers such as vinyl acid copolymers and propylene / α-olefin copolymers; Fluorine-based polymers such as polyvinylidene fluoride, polytetrafluoroethylene, fluorinated polyvinylidene fluoride, and polytetrafluoroethylene / ethylene copolymers Polymers: Polymer compositions having ionic conductivity of alkali metal ions (particularly lithium ions), and the like. These may be used individually by 1 type, or may use 2 or more types together by arbitrary combinations and a ratio.

  The solvent for forming the slurry is not particularly limited as long as it is a solvent that can dissolve or disperse the active material, the binder, and the thickener and conductive material used as necessary. Either an aqueous solvent or an organic solvent may be used. Examples of the aqueous solvent include water, a mixed solvent of alcohol and water, and examples of the organic solvent include N-methylpyrrolidone (NMP), dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, acrylic acid. Methyl, diethyltriamine, N, N-dimethylaminopropylamine, propylene oxide, tetrahydrofuran (THF), toluene, acetone, diethyl ether, dimethylacetamide, hexamethylphosphalamide, dimethyl sulfoxide, benzene, xylene, quinoline, pyridine , Methyl naphthalene, hexane and the like. In particular, when an aqueous solvent is used, a dispersant or the like is added in addition to the above-described thickener, and a slurry is formed using a latex such as SBR. In addition, these may be used individually by 1 type, or may use 2 or more types together by arbitrary combinations and a ratio.

  The ratio of the binder to the active material is preferably 0.1% by mass or more, particularly preferably 0.5% by mass or more, more preferably 0.6% by mass or more, and the upper limit is 20% by mass or less, preferably 15%. It is in the range of not more than mass%, more preferably not more than 10 mass%, still more preferably not more than 8 mass%. If it exceeds this range, the binder ratio in which the binder amount does not contribute to the battery capacity increases, and the battery capacity may be reduced. On the other hand, if it is lower, the strength of the negative electrode may be reduced. In particular, when the main component contains a rubbery polymer typified by SBR, the ratio of the binder to the active material is 0.1% by mass or more, preferably 0.5% by mass or more, and more preferably 0.8%. The upper limit is 5% by mass or less, preferably 3% by mass or less, and more preferably 2% by mass or less. Further, when the main component contains a fluorine-based polymer typified by polyvinylidene fluoride, the ratio to the active material is 1% by mass or more, preferably 2% by mass or more, more preferably 3% by mass or more. As an upper limit, it is 15 mass% or less, Preferably it is 10 mass% or less, More preferably, it is the range of 8 mass% or less.

  A thickener is usually used to adjust the viscosity of the slurry. The thickener is not particularly limited, and specific examples include carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, casein, and salts thereof. These may be used individually by 1 type, or may use 2 or more types together by arbitrary combinations and a ratio. When a thickener is further added, the ratio of the thickener to the active material is 0.1% by mass or more, preferably 0.5% by mass or more, more preferably 0.6% by mass or more. Is 5% by mass or less, preferably 3% by mass or less, more preferably 2% by mass or less. Below this range, applicability may be significantly reduced. If it exceeds the upper limit, the ratio of the active material in the negative electrode active material layer may be reduced, resulting in a problem that the capacity of the battery is reduced and a problem that the resistance between the negative electrode active materials is increased.

(18) Electrode orientation ratio The electrode orientation ratio is preferably 0.001 or more, particularly preferably 0.005 or more, more preferably 0.01 or more, and the upper limit is 0.67 or less, which is a theoretical value. Below this range, the high-density charge / discharge characteristics may deteriorate.

  The measurement of the electrode plate orientation ratio is as follows. About the negative electrode after pressing to the target density, the active material orientation ratio of the electrode is measured by X-ray diffraction. The specific method is not particularly limited. As a standard method, peak separation is performed by fitting peaks of carbon (110) diffraction and (004) diffraction by X-ray diffraction using asymmetric Pearson VII as a profile function. The integrated intensities of the peaks of (110) diffraction and (004) diffraction are calculated. From the obtained integrated intensity, a ratio represented by (110) diffraction integrated intensity / (004) diffraction integrated intensity is calculated. The electrode active material orientation ratio calculated by this measurement is defined as the electrode plate orientation ratio.

The X-ray diffraction measurement conditions here are as follows. “2θ” indicates a diffraction angle.
-Target: Cu (Kα ray) graphite monochromator-Slit: Divergence slit = 1 degree, Receiving slit = 0.1 mm, Scattering slit = 1 degree-Measurement range and step angle / measurement time:
(110) plane: 76.5 degrees ≦ 2θ ≦ 78.5 degrees 0.01 degrees / 3 seconds (004) plane: 53.5 degrees ≦ 2θ ≦ 56.0 degrees 0.01 degrees / 3 seconds Sample preparation: Fix the electrode to the glass plate with double-sided tape with a thickness of 0.1 mm

(19) Impedance The resistance of the negative electrode when charged to 60% of the nominal capacity from the discharged state is preferably 100Ω or less, particularly preferably 50Ω or less, more preferably 20Ω or less, and / or the double layer capacity is 1 ×. 10 ⁻⁶ F or more is preferable, particularly preferably 1 × 10 ⁻⁵ F, and more preferably 1 × 10 ⁻⁴ F. Within this range, the output characteristics are good and preferable.

The resistance and double layer capacity of the negative electrode are measured by the following procedure. The lithium-ion secondary battery to be measured is charged at a current value that can be charged for 5 hours in a nominal capacity, then maintained in a state where it is not charged / discharged for 20 minutes, and then discharged at a current value that can be discharged in 1 hour for a nominal capacity. The capacity when the capacity is 80% or more of the nominal capacity is used. About the lithium ion secondary battery of the above-mentioned discharge state, it charges to 60% of a nominal capacity with the electric current value which can charge a nominal capacity in 5 hours, Immediately transfers a lithium ion secondary battery in the glove box under argon gas atmosphere. Here, the lithium ion secondary battery is quickly disassembled and taken out in a state where the negative electrode is not discharged or short-circuited, and if it is a double-sided coated electrode, the electrode active material on one side is peeled off without damaging the electrode active material on the other side. Two electrodes are punched to 12.5 mmφ, and are opposed to each other so that the active material surface does not shift through a separator. 60 μL of the electrolytic solution used in the battery is dropped and adhered between the separator and both negative electrodes, and kept in a state where it is not in contact with the outside air, the current collectors of both negative electrodes are made conductive, and the AC impedance method is performed. The measurement was performed at a temperature of 25 ° C. and a complex impedance measurement was performed in a frequency band of 10 ⁻² to 10 ⁵ Hz, and the surface resistance (R) was obtained by approximating the arc of the negative resistance component of the obtained Cole-Cole plot with a semicircle. ) And double layer capacity (Cdl).

  The area of the negative electrode plate is not particularly limited, but is designed to be slightly larger than the opposing positive electrode plate so that the positive electrode plate does not protrude from the negative electrode plate. From the viewpoint of suppressing the life of a cycle in which charge and discharge are repeated and deterioration due to high temperature storage, it is preferable that the area be as close to the positive electrode as possible, since the ratio of electrodes that work more uniformly and effectively is increased and the characteristics are improved. In particular, when the electrode is used at a large current, the design of the electrode area is important. The thickness of the negative electrode plate is designed according to the positive electrode plate to be used and is not particularly limited, but the thickness of the composite layer obtained by subtracting the thickness of the metal foil of the core is usually 15 μm or more, Preferably it is 20 micrometers or more, More preferably, it is 30 micrometers or more, and an upper limit is 150 micrometers or less, Preferably it is 120 micrometers or less, More preferably, it is 100 micrometers or less.

[Separator]
The separator used in the present invention has a predetermined mechanical strength that electrically insulates both electrodes, has a high ion permeability, and has resistance to oxidation on the side in contact with the positive electrode and reduction on the negative electrode side. There is no particular limitation as long as it has both. As a material for the separator having such required characteristics, a resin, an inorganic material, glass fiber, or the like is used. As the resin, olefin polymer, fluorine polymer, cellulose polymer, polyimide, nylon and the like are used. Specifically, it is preferable to select from materials that are stable with respect to the electrolytic solution and have excellent liquid retention properties, and it is preferable to use a porous sheet or nonwoven fabric made of a polyolefin such as polyethylene or polypropylene.

  As the inorganic substance, oxides such as alumina and silicon dioxide, nitrides such as aluminum nitride and silicon nitride, and sulfates such as barium sulfate and calcium sulfate are used, and those having a particle shape or fiber shape are used. As the form, a thin film shape such as a non-woven fabric, a woven fabric, or a microporous film is used. In the thin film shape, those having a pore diameter of 0.01 to 1 μm and a thickness of 5 to 50 μm are preferably used. In addition to the independent thin film shape, a separator formed by forming a composite porous layer containing the inorganic particles on the surface layer of the positive electrode and / or the negative electrode using a resin binder can be used. For example, a porous layer may be formed on both surfaces of the positive electrode using alumina particles having a 90% particle size of 1 μm or less as a binder.

[Electrode group]
The electrode group has a laminated structure in which the positive electrode plate and the negative electrode plate are interposed via the separator, and a structure in which the positive electrode plate and the negative electrode plate are wound in a spiral shape via the separator. Either may be used. The ratio of the volume of the electrode group to the battery internal volume (hereinafter referred to as electrode group occupancy) is preferably 40% to 90%, and more preferably 50% to 80%. When the electrode group occupancy is less than 40%, the battery capacity becomes small, and when it exceeds 90%, the void space is small, the battery becomes hot, the member expands, or the vapor pressure of the liquid component of the electrolyte In some cases, the internal pressure rises due to an increase in the pressure, which deteriorates various characteristics such as charge / discharge repetition performance as a battery and high-temperature storage, and further, a gas release valve that releases the internal pressure to the outside may operate.

[Current collection structure]
The current collecting structure is not particularly limited, but in order to more effectively realize the output characteristics improvement by the non-aqueous electrolyte solution of the present invention, it is necessary to make the structure to reduce the resistance of the wiring part and the joint part. is there. When such an internal resistance is small, the effect of using the nonaqueous electrolytic solution of the present invention is exhibited particularly well.

  In the case where the electrode group has the laminated structure described above, a structure formed by bundling the metal core portions of the electrode layers and welding them to the terminals is preferably used. When the area of one electrode increases, the internal resistance increases. Therefore, it is also preferable to provide a plurality of terminals in the electrode to reduce the resistance. When the electrode group has the winding structure described above, the internal resistance can be lowered by providing a plurality of lead structures for the positive electrode and the negative electrode, respectively, and bundling the terminals.

  By optimizing the above structure, the internal resistance can be made as small as possible. In a battery used at a large current, the impedance measured by the 10 kHz AC method (hereinafter abbreviated as “DC resistance component”) is preferably 10 milliohms (mΩ) or less, and the DC resistance component is 5 milliohms (mΩ). It is more preferable to make it below. When the direct current resistance component is 0.1 milliohm or less, the output characteristics are improved, but the ratio of the current collecting structure used increases, and the battery capacity may decrease.

  The non-aqueous electrolyte used in the secondary battery of the present invention is effective in reducing reaction resistance related to lithium insertion / extraction with respect to the electrode active material, which is considered to be a factor that can realize good output characteristics. It is done. However, it was found that a battery having a large direct current resistance is inhibited by the direct current resistance, and the effect of reducing the reaction resistance cannot be reflected 100% on the output characteristics. This can be improved by using a battery having a small DC resistance component, and the effect of the non-aqueous electrolyte solution of the present invention can be sufficiently exhibited. From the viewpoint of drawing out the effect of the non-aqueous electrolyte and producing a battery having high output characteristics, this requirement and the electric capacity of the battery element housed in one battery exterior of the secondary battery described above ( It is particularly preferable to satisfy the requirement that the electric capacity when the battery is discharged from the fully charged state to the discharged state is 3 ampere hours (Ah) or more at the same time.

[Exterior case]
The material of the outer case is not particularly limited as long as it is a substance that is stable with respect to the non-aqueous electrolyte used. Specifically, a nickel-plated steel plate, stainless steel, aluminum, an aluminum alloy, a metal such as a magnesium alloy, or a laminated film (laminate film) of a resin and an aluminum foil is used. From the viewpoint of weight reduction, an aluminum or aluminum alloy metal or a laminate film is preferably used. In the exterior case using the above metals, a laser-sealed, resistance-welded, ultrasonic welding is used to weld the metals together to form a sealed sealed structure, or a caulking structure using the above-mentioned metals via a resin gasket To do. Examples of the outer case using the laminate film include those having a sealed and sealed structure by heat-sealing resin layers. In order to improve the sealing performance, a resin different from the resin used for the laminate film may be interposed between the resin layers. In particular, when a resin layer is heat-sealed through a current collecting terminal to form a sealed structure, a metal and a resin are joined, so that a resin having a polar group or a modified group having a polar group introduced as an intervening resin is used. Resins are preferably used.

[Protective element]
PTC (Positive Temperature Coefficient), thermal fuse, thermistor, which increases resistance when abnormal heat is generated or excessive current flows, the current flowing through the circuit due to a sudden rise in battery internal pressure or internal temperature during abnormal heat generation For example, a valve (current cutoff valve) that shuts off the current can be used. It is preferable to select a protective element that does not operate under normal use at a high current. From the viewpoint of high output, it is more preferable to design the protective element so as not to cause abnormal heat generation or thermal runaway even without the protective element.

  Although the mechanism for improving the life and output by using the non-aqueous electrolyte solution and the positive electrode described above is not clear, the adsorbent of the additive in the electrolyte solution on the surface of the positive electrode active material causes the positive electrode active material and the electrolyte solution to adhere to each other. Since undesirable side reactions, particularly side reactions between M in the compositional formula (1) and the electrolyte solution can be suppressed, it is considered that the resistance of the positive electrode surface is reduced, the output is improved, and the life is improved. Further, in the battery described above, since the ratio of the resistance on the positive electrode surface is large with respect to the resistance of the battery, it is preferable that reducing the resistance on the positive electrode surface in this way has a great effect on improving the output of the battery. Conceivable.

  EXAMPLES Hereinafter, although an Example and a comparative example are given and this invention is demonstrated further more concretely, this invention is not limited to these Examples, unless the summary is exceeded.

Example 1
[Production of positive electrode]
As a positive electrode active material, 75% by mass of LiFePO ₄ , 20% by mass of acetylene black, and 5% by mass of polytetrafluoroethylene powder were thoroughly mixed in a mortar and thinned into 9 mmφ and 12 mmφ. Punched with a punch. At this time, the total weight was adjusted to about 7.5 mg (9 mmφ) and about 17.5 mg (12 mmφ), respectively. This was pressure-bonded to an aluminum mesh to obtain 9 mmφ and 12 mmφ positive electrodes.

Using a 9 mmφ positive electrode as a test electrode and a lithium metal plate as a counter electrode, 1 mol of LiPF ₆ in a solvent of EC (ethylene carbonate): DMC (dimethyl carbonate): EMC (ethyl methyl carbonate) = 3: 3: 4 (volume ratio) A coin-type cell was assembled using a 25 μm-thick porous polyethylene film as a separator using an electrolytic solution dissolved at / L.

The obtained coin-type cell was subjected to a constant current / constant voltage charge of 0.2 mA / cm ² , that is, a reaction for releasing lithium ions from the positive electrode at an upper limit of 4.2 V. Next, a constant current discharge of 0.2 mA / cm ² , that is, a reaction for occluding lithium ions in the positive electrode was performed at a lower limit of 3.0V. In this case, the initial charge capacity per unit weight of the positive electrode active material was Qs (C) [mAh / g], and the initial discharge capacity was Qs (D) [mAh / g].

[Production of negative electrode]
Graphite powder (d ₀₀₂ = 3.35 Å) having an average particle diameter of 8 to 10 μm was used as the negative electrode active material, and polyvinylidene fluoride was used as the binder, and these were weighed at a weight ratio of 92.5: 7.5. Were mixed in an N-methylpyrrolidone solution to obtain a negative electrode mixture slurry. The slurry was applied to one side of a 20 μm thick copper foil, dried to evaporate the solvent, punched to 12 mmφ, and pressed at 0.5 ton / cm ² to form a negative electrode. At this time, the amount of the negative electrode active material on the electrode was adjusted to be about 6 to 8 mg.
A test in which a negative electrode is used as a test electrode, a battery cell is assembled using lithium metal as a counter electrode, and lithium ions are occluded in the negative electrode by a constant current-constant voltage method (cut current 0.05 mA) of 0.2 mA / cm ² -3 mV. The initial occlusion capacity per unit weight of the negative electrode active material at a lower limit of 0 V was defined as Qf [mAh / g].

[Preparation of non-aqueous electrolyte]
Under a dry argon atmosphere, it was sufficiently dried at a concentration of 1 mol / L in a mixed solvent of purified ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) in a volume ratio of 3: 3: 4. Lithium hexafluorophosphate (LiPF ₆ ) was dissolved. Furthermore, lithium difluorophosphate was contained so that it might become 1.0 mass%.

[Production of battery for performance test]
The positive electrode and the negative electrode were combined, a test battery was assembled using a coin cell, and the battery performance was evaluated. That is, the above-mentioned positive electrode prepared is placed on the positive electrode can of the coin cell, a porous polyethylene film having a thickness of 25 μm is placed thereon as a separator, pressed with a polypropylene gasket, and then the non-aqueous electrolyte prepared above. Was added to the inside of the can and sufficiently infiltrated into the separator, and then the above-described negative electrode was placed, the negative electrode can was placed and sealed, and a coin-type lithium secondary battery was produced. At this time, the balance between the weight of the positive electrode active material and the weight of the negative electrode active material was set so as to satisfy the following expression.
(Negative electrode active material weight [g] × Qf [mAh / g]) / (Positive electrode active material weight [g] × Qs (C) [mAh / g]) = 1.2

[Battery characteristics test]
First, 2 cycles of charge / discharge at a constant current of 0.2 C and 1 cycle of charge / discharge at a constant current of 1 C were performed at room temperature. Next, after constant charge / discharge of 0.2 C for the first time at 60 ° C., constant current charge / discharge is performed for 100 cycles at 1 C, and the capacity retention ratio before and after the cycle = (discharge capacity at the 100th cycle) / (1 cycle) (Eye discharge capacity) was obtained and the cycle characteristics were evaluated. In both cases, the upper limit of charging was 4.1 V, and the lower limit voltage was 3.0 V.

(Comparative Example 1)
The same operation as in Example 1 was carried out except that the electrolyte did not contain lithium difluorophosphate. Table 1 shows the results of the battery evaluation of Example 1 and Comparative Example 1.

  From the results of Table 1, it was found that the capacity retention rate was improved by the inclusion of the specific compound in the electrolytic solution, and the battery capacity was sufficiently maintained even after the cycle test.

  The use of the lithium secondary battery of the present invention is not particularly limited, and can be used for various known uses. Specific examples include notebook computers, pen input computers, mobile computers, electronic book players, mobile phones, mobile faxes, mobile copy, mobile printers, headphone stereos, video movies, LCD TVs, handy cleaners, portable CDs, minidiscs, and transceivers. Widely used in electronic notebooks, calculators, memory cards, portable tape recorders, radios, backup power supplies, motors, automobiles, motorbikes, motorbikes, bicycles, lighting equipment, toys, game equipment, watches, power tools, strobes, cameras, etc. It is what is done.

Claims (4)

A lithium secondary battery comprising a positive electrode and a negative electrode capable of inserting and extracting lithium, and a non-aqueous electrolyte obtained by mixing a lithium salt with a non-aqueous solvent, wherein the non-aqueous electrolyte is a monofluorophosphate And at least one compound selected from the group consisting of difluorophosphates, wherein the positive electrode includes a positive electrode active material having a composition represented by the following composition formula (1). Rechargeable lithium battery.
LiMPO ₄ (1)
[In composition formula (1), M represents at least one transition metal element. ]   The lithium secondary battery according to claim 1, wherein M in the composition formula (1) is at least one element selected from the group consisting of Mn, Fe, and Co.   The lithium according to claim 1 or 2, wherein the non-aqueous electrolyte contains 0.01% by mass or more of at least one compound selected from the group consisting of monofluorophosphate and difluorophosphate. Secondary battery.   The lithium secondary battery according to any one of claims 1 to 3, wherein the positive electrode contains a positive electrode active material having a composition different from that of the composition formula (1).