JP2008210529A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a combination of lithium salts and an electrolyte, capable of reducing the internal resistance of a battery in charging at high temperatures in a nonaqueous electrolyte secondary battery equipped with a negative electrode including silicon, tin or these compounds. <P>SOLUTION: In the nonaqueous electrolyte secondary battery equipped with a positive electrode, the negative electrode, a separator, and a nonaqueous electrolyte, the negative electrode includes silicon, tin, or these compounds, and the nonaqueous electrolyte includes vinylene carbonate, LiPF<SB>6</SB>, and 0.02-0.5 mol/L imide salt. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a non-aqueous electrolyte secondary battery using silicon, tin, or a compound thereof as a negative electrode active material.

  In recent years, small and lightweight non-aqueous electrolyte secondary batteries have been widely used as power sources for electronic devices such as mobile phones and digital cameras. This non-aqueous electrolyte secondary battery generally uses a lithium transition metal composite oxide for the positive electrode, a carbon material for the negative electrode, and a carbonate containing a lithium salt for the electrolyte, has a high operating voltage, and an energy density. It is characterized by high. In recent years, in order to further increase the energy density, it has been studied to change the negative electrode active material from a carbon material to silicon, tin, or an alloy containing these (hereinafter simply referred to as an alloy-based material).

As reported in Non-Patent Document 1, since materials such as silicon and tin have a higher Li ⁺ ion diffusion coefficient than carbon materials, it is considered that high rate discharge performance and output characteristics are improved. Therefore, it is expected that a nonaqueous electrolyte secondary battery using silicon, tin, or the like as a negative electrode active material will be enlarged and applied to uses such as an electric vehicle (EV) and a hybrid electric vehicle (HEV).

  When the nonaqueous electrolyte secondary battery is applied to HEV, it is desired to install the battery in the vicinity of an engine room having a space. However, since the vicinity of the HEV engine room is likely to be exposed to a high temperature of 60 ° C. or higher, the reliability of the battery at a high temperature of 60 ° C. or higher is required.

In Patent Document 1, a negative electrode in which Si is provided in a thin film shape is used. In a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3: 7, 0.9 mol / liter of LiN (as a solute) is used. A lithium secondary battery using an electrolytic solution in which C ₂ F ₅ SO ₂ ) ₂ ] and 0.1 mol / liter LiPF ₆ are dissolved is disclosed, and charge / discharge cycle characteristics at 25 ° C. are improved. Has been reported.

Further, in Patent Documents 2 to 4, in a non-aqueous electrolyte secondary battery using silicon, tin, or a compound thereof as a negative electrode active material, as an electrolyte lithium salt, Is reported to be able to be used by mixing LiPF ₆ with an imide salt such as LiN (CF ₃ SO ₂ ) ₂ .

Further, Patent Document 5 discloses a technique in which LiN (CF ₃ SO ₂ ) ₂ and LiPF ₆ are mixed and used as a lithium salt of an electrolyte in a nonaqueous electrolyte secondary battery using silicon or a silicon compound as a negative electrode active material. Is disclosed, and the concentration of LiPF ₆ is preferably less than 0.1 mol / dm ³ .

Patent Document 6 discloses a technique in which LiPF ₆ and an imide salt are mixed and used as a lithium salt in order to increase the capacity when low crystalline carbon is used as the negative electrode active material of a lithium secondary battery. Patent Document 7 discloses LiPF as a lithium salt in order to obtain a battery having excellent high rate characteristics, low temperature characteristics, and high temperature storage characteristics when artificial graphite is used as the negative electrode active material of a lithium secondary battery. _A technique using a mixture of ₆ and an imide salt is disclosed.
WO2002 / 058182 JP 2006-019274 A Japanese Patent Laid-Open No. 10-208742 JP 2001-185214 A JP 2001-176545 A JP 2000-195554 A Japanese Patent Laid-Open No. 2001-2223025 J. et al. O. Besenhard et. al. Electrochimica Acta 45, 31 (1999)

In a non-aqueous electrolyte secondary battery using silicon, tin, or a compound thereof as a negative electrode active material, a technique using a mixture of an imide salt and LiPF ₆ as a lithium salt of an electrolyte is disclosed in Patent Literature 1-5.

  However, the test conditions of Patent Document 1 are those under a normal temperature environment of 25 ° C., and it is impossible to estimate the output performance when the battery is used under a high temperature environment such as 60 ° C. from this data. In Patent Document 5, the charge / discharge cycle characteristics are measured at room temperature, and the relationship between the lithium salt and the high temperature characteristics of the battery is unknown.

Also, either in Patent Documents 2 to 4, in the case of using and the LiPF ₆ imide salt, and specific mixing ratio, there is no description of test temperature, is improved battery characteristics at a high temperature of at least 60 ° C. It was unknown.

Further, in Patent Document 6 and Patent Document 7, since there is no data on batteries using silicon or tin as a negative electrode active material or these compounds, these negative electrode active materials, imide salts and LiPF ₆ were combined. The effect of the case was unknown.

  An object of the present invention is to reduce the internal resistance of a non-aqueous electrolyte secondary battery having a negative electrode containing silicon, tin, or a compound thereof when charged at a high temperature. The object is to provide a combination of salts and an electrolyte.

The invention of claim 1 is a nonaqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, a separator, and a nonaqueous electrolyte, wherein the negative electrode contains silicon, tin or a compound thereof, and the nonaqueous electrolyte is vinylene carbonate and LiPF _6. And 0.02-0.5 mol / L of an imide salt.

As in the present invention, non-aqueous silicon, tin or their compounds used in the negative electrode active material of electrolyte secondary battery, in the electrolyte, and imide salts of vinylene carbonate and LiPF ₆ and 0.02~0.5mol / L By simultaneously containing, a flexible SEI film is formed on the surface of the negative electrode active material. As a result, even when the negative electrode active material undergoes a large volume change due to charge / discharge, the SEI film does not fall off from the negative electrode surface, so the thickness of the SEI film once formed does not increase greatly, and the battery during high temperature storage It is presumed that the increase in internal resistance is suppressed.

  The method of the present invention can provide a nonaqueous electrolyte secondary battery in which the internal resistance of the battery is reduced when charged in a high temperature environment of 60 ° C. or higher.

The present invention provides a nonaqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, a separator, and a nonaqueous electrolyte, wherein the negative electrode contains silicon, tin or a compound thereof as an active material, and the nonaqueous electrolyte is vinylene carbonate. It contains LiPF ₆ and 0.02-0.5 mol / L imide salt.

  The negative electrode active material used in the present invention includes silicon, tin, or a compound thereof. Here, the silicon compound or tin compound means a molar ratio of elements contained in the compound and silicon or tin exceeding 40%. This is because when these elements are not contained in an amount of 40% or more, the influence of the electrochemical characteristics of these materials on the performance of the battery is small, and the effects of the invention do not appear.

Examples of the negative electrode active material used in the present invention include Si, SiO _x (0.8 ≦ x ≦ 1.2), SiO / Si / SiO ₂ composite material, silicon compounds such as Si ₂ N, Sn, Sn ₄ Ni _3. , (Sn ₄ Ni ₃ ) _0.52 Sn _0.48 , Sn ₆ Cu ₅ and other tin compounds. Further, the negative electrode active material may have any shape, for example, a powder shape, a film shape formed by a plating method, a vapor deposition method, or the like.

In the nonaqueous electrolyte secondary battery of the present invention, the electrolytic solution contains vinylene carbonate, LiPF ₆ and 0.02 to 0.5 mol / L imide salt. In addition, the preferable density | concentration range of an imide salt is 0.03-0.08 mol / L.

As in the present invention, silicon, tin or a compound thereof is used as the negative electrode active material, and at the same time, the nonaqueous electrolyte contains vinylene carbonate, LiPF ₆ and 0.01 to 0.5 mol / L imide salt. An increase in the internal resistance of the battery during high temperature storage can be suppressed. The following can be considered as the reason.

  In the nonaqueous electrolyte secondary battery, an SEI film is formed on the negative electrode surface. Here, SEI (Solid Electrolyte Interface) coating is formed on the surface of metallic lithium or carbon material when the metal lithium or carbon material is charged for the first time in a non-aqueous electrolyte and the solvent in the electrolyte is reduced. Refers to the passivation film (edited by Masayuki Yoshio, edited by Akiya Ozawa, “Lithium ion secondary batteries-materials and applications”, Nikkan Kogyo Shimbun (1996)). The SEI film formed on the surface of metallic lithium or carbon material functions as a lithium ion conductive protective film, and the subsequent reaction between metallic lithium or carbon material and the solvent is suppressed.

The most suitable electrolyte salt for forming the SEI film is lithium hexafluorophosphate (LiPF ₆ ). An imide salt such as LiN (CF ₃ SO ₂ ) ₂ is also effective for SEI film formation, but the imide salt alone slightly reduces the conductivity of the electrolyte.

  When the negative electrode active material is a carbon material such as graphite, the volume change of the negative electrode active material due to charging / discharging is small, so the thickness of the SEI film once formed does not increase greatly by float charging, and at the time of high temperature float charging There was no problem that the internal resistance increased significantly.

  However, when silicon, tin, or a compound thereof is used as the negative electrode active material as in the present invention, the internal resistance of the battery is increased when float charging is performed at a high temperature of 60 ° C. or higher. The cause is mainly that the thickness of the SEI film on the negative electrode surface increases.

  That is, the formation of the SEI film at the time of float charging is caused by a reaction in which the negative electrode active material in the state where Li is occluded reduces the electrolyte solution solvent and the electrolyte salt. At this time, Li is released from the negative electrode active material.

  Since silicon, tin, or these compounds have a large volume change (expansion when Li is occluded and shrink when they are released) due to insertion and extraction (charge / discharge) of Li, SEI on the surface of the negative electrode active material is accompanied by charge and discharge of the battery. The coating cannot withstand the volume change and is easily peeled off. When the SEI film is peeled off, the negative electrode active material and the electrolytic solution are in direct contact with each other to cause a new SEI film forming reaction. Thus, it is inferred that the resistance of the battery is increased by repeating the formation and peeling of the SEI film.

When the lithium salt in the electrolytic solution is only LiPF ₆ , LiPF ₆ is decomposed together with the electrolytic solution solvent, and an SEI film mainly containing an inorganic component such as LiF is easily formed. Since the SEI film mainly composed of this inorganic component is poor in flexibility and easily peels off due to the volume change of the negative electrode active material, silicon, tin or a compound thereof is used as the negative electrode active material, and LiPF ₆ is used as the electrolyte salt. The battery used had a problem that the internal resistance during high-temperature float charging increased.

  The SEI film produced by the reductive decomposition of vinylene carbonate (VC) contains a lot of organic components, and is considered to be relatively excellent in flexibility. Even when a negative electrode active material such as Si or Sn having a large expansion and contraction is used for the negative electrode, as in the case of using a carbon material, if VC is added to the electrolyte, a good SEI film can be obtained at the beginning of charge and discharge. Is considered to be formed.

  However, when a negative electrode active material such as Si or Sn is used, even if a SEI film having excellent flexibility is formed at the initial stage of charge and discharge, the desorption and re-formation may occur when float charging or the like is performed thereafter. Prone to occur. At this time, when sufficient VC does not exist in the electrolytic solution, an SEI film mainly containing an inorganic component such as LiF is formed.

  Accordingly, it is considered that the increase in resistance when float charging or the like is performed cannot be suppressed unless a sufficient amount of VC is consumed in the liquid electrolyte than is consumed at the beginning of charge / discharge.

  However, not only is the VC oxidized and decomposed by the positive electrode with increased oxidizing power, but gas is generated, and the high temperature promotes the growth of the SEI film on the negative electrode, and conversely increases the resistance of the battery. There was a problem.

  As described above, simply adding VC to the electrolytic solution cannot suppress the increase in resistance when float charging or the like is performed at a high temperature.

In addition, when an appropriate amount of an imide salt such as LiN (CF ₃ SO ₂ ) ₂ is used instead of LiPF ₆ , the flexibility of the formed SEI film is increased, and the formation becomes stable, and further SEI film formation is suppressed. Therefore, although the effect which suppresses the increase in the internal resistance of a battery is seen, the effect was inadequate.

Therefore, the present invention provides LiPF ₆ alone on the surface of the negative electrode active material due to the synergistic effect of simultaneously containing vinylene carbonate (VC), LiPF ₆ and 0.01 to 0.5 mol / L imide salt in the electrolyte. In this case, it is possible to form a flexible SEI film that was not obtained by the combination of VC and LiPF ₆ , and as a result, it is possible to suppress an increase in the internal resistance of the battery during high-temperature float charging. Presumed.

  In addition, the density | concentration of the imide salt in electrolyte solution needs to be the range of 0.01-0.5 mol / L. If the concentration of the imide salt is less than 0.01 mol / L, there is no effect of suppressing the increase in the internal resistance of the battery during high-temperature storage, and if it is greater than 0.5 mol / L, the aluminum current collector used for the positive electrode is corroded. This tends to cause a battery failure.

  Further, the concentration of VC in the electrolytic solution is preferably 5% by mass or less. When the concentration of VC is larger than 5% by mass, a large amount of unreacted VC remains in the electrolyte. When such a battery is stored at a high temperature, VC is decomposed at the positive electrode or the negative electrode to generate gas. This is because resistance increases due to SEI film growth.

Furthermore, in the present invention, the imide salt is Li [N (CF ₃ SO ₂ ) ₂ ], Li [N (C ₂ F ₅ SO ₂ ) ₂ ], Li [N (CF ₃ SO ₂ ) (C ₄ F ₉ And at least one selected from the group consisting of SO ₂ )]. This is because these imide salts are easily available.

  There is no restriction | limiting in particular as a electrically conductive agent used for a negative electrode, A various material can be used suitably. For example, Ni, Ti, Al, Fe, or an alloy or carbon material of two or more kinds thereof can be given. Among these, it is preferable to use a carbon material. Examples of the carbon material include amorphous carbon such as natural graphite, artificial graphite, vapor-grown carbon fiber, acetylene black, ketjen black, and needle coke. Note that when the negative electrode active material is a conductive material, the conductive agent is not necessarily used.

  Examples of the separator used in the nonaqueous electrolyte battery of the present invention include a microporous membrane, a woven fabric, and a nonwoven fabric. Among these, a microporous membrane is preferable because it has a shutdown performance and is excellent in safety. Examples of the polyolefin include polyethylene, polypropylene, and polybutene. Among these, polyethylene, polypropylene, and copolymers thereof are preferable in terms of film strength. In addition, materials, laminates of multiple microporous membranes with different weight average molecular weights and porosity, and appropriate additives such as various plasticizers, antioxidants, and flame retardants are contained in these microporous membranes You can use what you are doing. For example, polyvinylidene fluoride or polyacrylonitrile can be supported.

There is no restriction | limiting in particular as a positive electrode active material used for this invention, A various material can be used suitably. For example, transition metal compounds such as manganese dioxide and vanadium pentoxide, transition metal chalcogen compounds such as iron sulfide and titanium sulfide, and complex oxides Li _x MO _2−δ of these transition metals and lithium (however, , M represents at least one element selected from Co, Ni, or Mn, and is a composite oxide in which 0.4 ≦ x ≦ 1.2 and 0 ≦ δ ≦ 0.5), or these composite oxides It can be used by containing at least one element selected from the group consisting of Al, Mn, Fe, Ni, Co, Cr, Ti, and Zn, or a nonmetallic element such as P or B.

Furthermore, a composite oxide of lithium and nickel, that is, a positive electrode active material represented by Li _x Ni _p M1 _q M2 _r O _{2 -δ} (where M1 and M2 are Al, Mn, Fe, Ni, Co, Cr, Ti And at least one element selected from the group consisting of Zn, or a non-metallic element such as P or B, 0.4 ≦ x ≦ 1.2, 0.8 ≦ p + q + r ≦ 1.2, 0 ≦ a complex oxide in which δ ≦ 0.5 can be used. Among them, high voltage, high energy density, and excellent cycle performance make it possible to use lithium / nickel / cobalt / manganese composite oxides, lithium / cobalt composite oxides, and lithium / cobalt / nickel composite oxides. preferable.

  As the conductive agent used for the positive electrode, the same material as that used for the negative electrode can be used.

  There is no restriction | limiting in particular as a binder used for a positive electrode, A various material can be used suitably. For example, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polytetrafluoroethylene, fluorinated polyvinylidene fluoride, ethylene-propylene-diene terpolymer, styrene-butadiene rubber, acrylonitrile-butadiene rubber, fluorine At least one selected from the group consisting of rubber, polyvinyl acetate, polymethyl methacrylate, polyethylene, nitrocellulose, or derivatives thereof can be used.

  There is no restriction | limiting in particular as a binder used for a negative electrode, A various material can be used suitably. For example, styrene-butadiene rubber (SBR) or carboxymethyl cellulose (CMC), polyvinylidene fluoride, carboxy-modified polyvinylidene fluoride, polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-chlorotrifluoroethylene Copolymer, polyvinylidene fluoride-hexafluoropropylene copolymer, ethylene-propylene-diene copolymer, acrylonitrile-butadiene rubber, fluororubber, polyvinyl acetate, polymethyl methacrylate, nitrocellulose, polyethylene, polypropylene or derivatives thereof At least one selected from the group consisting of and the like can be used. When the negative electrode active material is formed into a film formed by a plating method or a vapor deposition method, a binder need not be used.

  When preparing an electrode mixture, a non-aqueous solvent or an aqueous solution can be used as a solvent used when mixing an active material, a binder, a conductive agent, and the like. Non-aqueous solvents include N-methyl-2-pyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, NN-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran, etc. Can do. Moreover, you may add a dispersing agent, a thickener, etc. to these.

  As the current collector substrate of the electrode used in the present invention, copper, iron, nickel, SUS, or the like can be used. Of these, copper is preferred because of its high thermal conductivity and electronic conductivity. In addition, since it exposes to the electric potential range lower than 0.5V in a film formation process, an aluminum base material cannot be used. Examples of the shape include a sheet, a foam, a sintered porous body, and an expanded lattice. Further, a current collector having a hole in an arbitrary shape can be used.

  There is no restriction | limiting in particular as an organic solvent of the electrolyte solution used for the nonaqueous electrolyte battery of this invention, A various material can be used suitably. For example, ethers, ketones, lactones, nitriles, amines, amides, sulfur compounds, halogenated hydrocarbons, esters, carbonates, nitro compounds, phosphate ester compounds, sulfolane hydrocarbons, etc. Among these, ethers, ketones, esters, lactones, halogenated hydrocarbons, carbonates, and sulfolane hydrocarbons are preferable, and carbonates are particularly preferable.

  Further examples of these include tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydropyran, 1,4-dioxane, anisole, monoglyme, 4-methyl-2-pentanone, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, 1,2-dichloroethane, γ-butyrolactone, γ-valerolactone, dimethoxyethane, diethoxyethane, methyl formate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethylene carbonate, propylene carbonate, Vinylene carbonate, butylene carbonate, dimethylformamide, dimethyl sulfoxide, dimethylthioformamide, sulfolane, 3-methyl-sulfo Examples thereof include run, trimethyl phosphate, triethyl phosphate, and phosphazene derivatives, and mixed solvents thereof.

  Especially, it is preferable to use ethylene carbonate, propylene carbonate, (gamma) -butyrolactone, dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate individually or in mixture of 2 or more types.

In the electrolyte, carbonates such as butylene carbonate, benzenes such as biphenyl and cyclohexylbenzene, sulfurs such as propane sultone, ethylene sulfide, hydrogen fluoride, triazole-based cyclic compounds, fluorine-containing esters, and tetraethylammonium fluoride It can also be used by containing at least one selected from the group consisting of hydrogen fluoride complexes or derivatives thereof, phosphazenes and derivatives thereof, amide group-containing compounds, imino group-containing compounds, or nitrogen-containing compounds. Further, the electrolyte may contain at least one selected from CO ₂ , NO ₂ , CO, SO ₂ and the like.

  Further, a solid or gel ion conductive electrolyte can be used in combination with the above electrolyte. When combined, the non-aqueous electrolyte battery is composed of a combination of a positive electrode, a negative electrode and a separator, an organic or inorganic solid electrolyte and the non-aqueous electrolyte, or an organic or inorganic solid electrolyte membrane as a positive electrode, a negative electrode and a separator. A combination with the non-aqueous electrolyte is mentioned. A porous polymer solid electrolyte membrane can also be used for the ion conductive electrolyte.

Examples of the ion conductive electrolyte include polyethylene oxide, polypropylene oxide, polyacrylonitrile, polyethylene glycol and derivatives thereof, LiI, Li ₃ N, Li _{1 + x} M _x Ti _2-x (PO ₄ ) ₃ (M = Al, Sc, Y, La), Li _0.5-3x R _{0.5 + x} TiO ₃ (R = La, Pr, Nd, Sm), or using thiolysicon typified by Li _4-x Ge _1-x P _x S ₄ it can. Furthermore, oxide glass such as LiI-Li ₂ O—B ₂ O ₅ system, Li ₂ O—SiO ₂ system, or LiI—Li ₂ S—B ₂ S ₃ system, LiI—Li ₂ S—SiS ₂ system, Sulfide glass such as Li ₂ S—SiS ₂ —Li ₃ PO ₄ can be used.

  In addition, the shape of the battery is not particularly limited, and the present invention can be applied to non-aqueous electrolyte secondary batteries having various shapes such as a square, oval, cylindrical, coin, button, and sheet batteries. It is.

  Next, a preferred embodiment of the present invention will be described. However, the present invention is not limited to the following examples, and can be appropriately changed within a range in which the gist thereof does not change.

[Examples 1-6 and Comparative Examples 1-3]
[Example 1]
Silicon (purity 99.999%) as a negative electrode active material was attached to the surface of a copper foil having a thickness of 20 μm (surface roughness Ra = 0.4 μm) by RF sputtering. The amount of Si deposited was weighed with a precision balance and confirmed to be 0.7 mg / cm ² . A Ni wire with resin was ultrasonically welded to the lead portion of the copper foil to produce a negative electrode N1 having a silicon layer on one side of the copper foil.

Next, 90% by mass of LiNi _1/3 Mn _1/3 Co _1/3 O ₂ (positive electrode active material), 5% by mass of acetylene black (conductive agent), and 5% by mass of PVdF (binder) Is dispersed in N-methylpyrrolidone (NMP), and the paste is applied onto an aluminum foil having a thickness of 20 μm and dried at 150 ° C. to evaporate NMP. And compression molded. Further, an Al wire with resin was ultrasonically welded to the lead portion of the Al foil to produce a positive electrode having a positive electrode mixture layer on one side. The coating weight of the positive electrode mixture layer was 16 mg / cm ² .

  A positive electrode, a negative electrode, and a polyethylene separator, which is a continuous porous body having a thickness of 25 μm, were stacked and inserted into a battery container having a height of 48 mm and a width of 30 mm. As the battery container, a metal laminated resin film case having a thickness of 200 μm and made of polyethylene terephthalate (PET) film / aluminum foil / polyethylene film from the outside was used.

  Further, after injecting 0.4 g of the nonaqueous electrolyte into the container, the container was sealed and the nonaqueous electrolyte secondary battery A1 of Example 1 having a rated capacity of 20 mAh (hereinafter abbreviated as “battery A1”). The same applies to other non-aqueous electrolyte secondary batteries).

As the electrolytic solution, a mixed solvent X of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 1: 1: 1 (hereinafter abbreviated as “mixed solvent X”) 98 mol / L LiPF ₆ and 0.02 mol / L LiN (C ₂ F ₅ SO ₂ ) ₂ (hereinafter abbreviated as “2,2-imide”) were dissolved, and vinylene carbonate (VC ) Was added at 3% by weight.

Note that the rated capacity of the battery is the smaller value of the theoretical capacity determined from the following reaction formula from the weight of the positive electrode active material and the negative electrode active material contained in the battery. Even when the active materials are different, the following reaction formula is applied.
Positive electrode: LoCoO ₂ → 0.6Li ⁺ + Li _0.4 CoO ₂ + 0.6e ⁻
Negative electrode: Si + 4.4Li ⁺ + 4.4e ⁻ → Li _4.4 Si
[Example 2]
As the electrolytic solution, 0.95 mol / L LiPF ₆ and 0.05 mol / L 2,2-imide were dissolved in the mixed solvent X, and 3% by weight of VC was added to the electrolytic solution. A battery A2 of Example 2 was made in the same manner as Example 1 except for the above.

[Example 3]
As the electrolytic solution, 0.90 mol / L LiPF ₆ and 0.10 mol / L 2,2-imide were dissolved in the mixed solvent X, and 3% by weight of VC was added to the electrolytic solution. A battery A3 of Example 3 was made in the same manner as Example 1 except for the above.

[Example 4]
As the electrolytic solution, 0.85 mol / L LiPF ₆ and 0.15 mol / L 2,2-imide were dissolved in the mixed solvent X, and 3% by weight of VC was added to the electrolytic solution. A battery A4 of Example 4 was made in the same manner as Example 1 except for the above.

[Example 5]
As the electrolytic solution, 0.75 mol / L LiPF ₆ and 0.25 mol / L 2,2-imide were dissolved in the mixed solvent X, and 3% by weight of VC was added to the electrolytic solution. A battery A5 of Example 5 was made in the same manner as Example 1 except for the above.

[Example 6]
As the electrolytic solution, 0.50 mol / L LiPF ₆ and 0.50 mol / L 2,2-imide were dissolved in the mixed solvent X, and 3% by weight of VC was added to the electrolytic solution. A battery A6 of Example 6 was made in the same manner as Example 1 except for the above.

[Comparative Example 1]
A battery R1 of Comparative Example 1 was manufactured in the same manner as in Example 1 except that a solution obtained by dissolving 1.00 mol / L LiPF ₆ in mixed solvent X was used as the electrolytic solution, and VC was not added. did.

[Comparative Example 2]
Comparative Example as in Example 1 except that 1.00 mol / L LiPF ₆ was dissolved in mixed solvent X as electrolyte and 3% by mass of VC was added to this electrolyte. 2 batteries (R2) were produced.

[Comparative Example 3]
Except that 1.00 mol / L 2,2-imide was dissolved in the mixed solvent X as the electrolytic solution and 3% by mass of VC was added to the electrolytic solution, the same procedure as in Example 1 was performed. A battery (R3) of Comparative Example 3 was produced.

[Comparative Example 4]
As the electrolytic solution, 0.90 mol / L LiBF ₄ and 0.10 mol / L 2,2-imide were dissolved in the mixed solvent X, and 3% by weight of VC was added to the electrolytic solution. A battery A4 of Comparative Example 4 was produced in the same manner as Example 1 except for the above.

[AC impedance measurement after high temperature storage]
Using the batteries A1 to A6 produced in Examples 1 to 6 and the batteries R1 to R4 produced in Comparative Examples 1 to 4, AC impedance measurement after high-temperature storage was performed.

  These batteries were first initialized in a 25 ° C. environment. Initialization is a constant current charge of 4 mA up to 4.1 V, followed by a constant voltage charge until the total energization time reaches 8 hours, left open for 10 minutes, a constant current discharge of up to 2.5 V at 4 mA, 10 The circuit was left open for a minute.

  Next, a charge / discharge test was conducted. The test conditions were: charging at a constant current of 20 mA up to 4.1 V, followed by charging at a constant voltage of 4.1 V until the total energization time was 3 hours, leaving it in an open circuit state for 10 minutes, followed by 20 mA. Then, the battery was discharged at a constant current of up to 2.5 V and then left for 10 minutes in an open circuit state.

  Next, after putting this battery in a thermostat kept at 60 ° C. for 3 hours, 20 mA constant current charge up to 4.1 V and subsequent constant voltage charge until the total energization time reaches 72 hours. (Float charge) was performed and left in an open circuit state for 60 minutes. Further, the battery was transferred to a constant temperature bath at 25 ° C., and after 24 hours had elapsed, AC impedance measurement was performed. The conditions were an AC applied voltage of 10 mV and a measurement frequency range of 10 kHz to 10 mHz.

  Table 1 shows the compositions of the electrolytic solutions of batteries A1 to A6 and batteries R1 to R3 and the results of AC impedance measurement. In Table 1, the column “Impedance” indicates the value of AC impedance at 100 mHz. The relationship between the imide salt concentration and the impedance is shown in FIG.

From Table 1 and FIG. 1, the following became clear. When the battery R1 and the battery R2 are compared, the impedance of the battery R2 is very small. Therefore, it was found that when LiPF ₆ and VC coexist in the non-aqueous electrolyte, the impedance of the battery is lowered.

Next, when comparing the battery A3, the battery R2, and the battery R3, the battery R2 in which the electrolyte salt is LiPF ₆ alone or the battery R3 in which 2,2-imide is alone has an impedance of 4 when charged at a high temperature. Although large and .4Ω and 5.2Omu, in the case of LiPF ₆ 0.90 mol / L and 2,2 imide 0.10 mol / L were mixed battery A3 as the electrolyte salt is lowered to 2.4Omu, the LiPF ₆ It was found that the battery can be significantly reduced by 45% compared to the battery R2 used alone.

Further, from the results of the batteries A1 to A6, it is considered that the effect of reducing the impedance was caused by simultaneously using LiPF ₆ and 2,2-imide as the electrolyte salt and further allowing VC to exist in the electrolyte. .

Further, in the case of the battery R4 of Comparative Example 4 in which LiBF ₄ is used instead of LiPF _{6 as the} electrolyte salt, a stable SEI film is not formed and the resistance becomes very high. When LiPF ₆ is used, P (phosphorus) element is also contained in the SEI film. Therefore, in order to obtain a stable SEI coating, it is considered essential that the P (phosphorus) element is present in the electrolyte salt.

[Examples 7 to 12 and Comparative Example 5]
[Example 7]
A battery A7 of Example 7 was obtained in the same manner as in Example 1 except that LiN (CF ₃ SO ₂ ) ₂ (hereinafter abbreviated as “1,1-imide”) was used instead of 2,2-imide. Produced.

[Example 8]
A battery A8 of Example 8 was made in the same manner as Example 2 except that 1,1-imide was used instead of 2,2-imide.

[Example 9]
A battery A9 of Example 9 was made in the same manner as Example 3 except that 1,1-imide was used instead of 2,2-imide.

[Example 10]
A battery A10 of Example 10 was made in the same manner as Example 4 except that 1,1-imide was used instead of 2,2-imide.

[Example 11]
A battery A11 of Example 11 was made in the same manner as Example 5 except that 1,1-imide was used instead of 2,2-imide.

[Example 12]
A battery A12 of Example 12 was made in the same manner as Example 6 except that 1,1-imide was used instead of 2,2-imide.

[Comparative Example 5]
A battery R5 of Comparative Example 5 was produced in the same manner as in Comparative Example 3, except that 1,1-imide was used instead of 2,2-imide.

  Using batteries A7 to A12 produced in Examples 7 to 12 and battery R5 produced in Comparative Example 5, initialization and charge / discharge tests were performed under the same conditions as in Example 1, and after high temperature storage under the same conditions. AC impedance was measured. Table 2 shows the compositions of the electrolytic solutions of batteries A7 to A12 and battery R5 and the results of AC impedance measurement. In Table 2, the column “Impedance” indicates the value of AC impedance at 100 mHz. The relationship between the imide salt concentration and the impedance is shown in FIG.

From the results of Table 2 and FIG. 2, the same result was obtained when 1,1-imide was used instead of 2,2-imide, and LiPF ₆ and 1,1-imide were used simultaneously as the electrolyte salt. It was confirmed that the presence of VC in the electrolyte produced the effect of reducing impedance.

[Examples 13 to 18 and Comparative Example 6]
[Example 13]
In the same manner as in Example 1 except that LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ) (hereinafter abbreviated as “1,4-imide”) was used instead of 2,2-imide, A battery A13 of Example 13 was produced.

[Example 14]
A battery A14 of Example 14 was made in the same manner as Example 2 except that 1,4-imide was used instead of 2,2-imide.

[Example 15]
A battery A15 of Example 15 was made in the same manner as Example 3 except that 1,4-imide was used instead of 2,2-imide.

[Example 16]
A battery A16 of Example 16 was made in the same manner as Example 4 except that 1,4-imide was used instead of 2,2-imide.

[Example 17]
A battery A17 of Example 17 was made in the same manner as Example 5 except that 1,4-imide was used instead of 2,2-imide.

[Example 18]
A battery A18 of Example 18 was made in the same manner as Example 6 except that 1,4-imide was used instead of 2,2-imide.

[Comparative Example 6]
A battery R6 of Comparative Example 6 was produced in the same manner as in Comparative Example 3, except that 1,4-imide was used instead of 2,2-imide.

  Using batteries A13 to A18 produced in Examples 13 to 18 and battery R6 produced in Comparative Example 6, initialization and charge / discharge tests were performed under the same conditions as in Example 1, and after high-temperature storage under the same conditions. AC impedance was measured. Table 3 shows the composition of the electrolytic solutions of batteries A13 to A18 and battery R6 and the results of AC impedance measurement. In Table 3, the column “Impedance” indicates the value of AC impedance at 100 mHz. The relationship between the imide salt concentration and the impedance is shown in FIG.

From the results of Table 3 and FIG. 3, the same result was obtained when 1,4-imide was used instead of 2,2-imide, and LiPF ₆ and 1,4-imide were used simultaneously as the electrolyte salt. It was confirmed that the presence of VC in the electrolyte produced the effect of reducing impedance.

[Examples 19 to 24 and Comparative Example 7]
[Example 19]
Sn—Ni alloy was used instead of silicon as the negative electrode active material. The negative electrode was produced as follows. The copper foil was immersed in a commercially available Sn—Ni plating solution (manufactured by High Purity Chemical Laboratory, SNS-200E). Thereafter, using a Sn—Ni alloy containing 28% by mass of Ni as a counter electrode, current was applied so that the cathode current density was 2 A / dm ^3, and a Sn—Ni alloy was synthesized on the copper foil. This was washed with ion-exchanged water and then dried at 150 ° C. to make a negative electrode N2.

As a result of quantitative elemental analysis, the composition of the Sn—Ni alloy was Sn 85 mass% and Ni 15 mass%. Further, in the X-ray diffraction pattern of this Sn—Ni alloy, peaks due to the CuKα ₁ line were present at 2θ = 30.3 ° and 30.6 °. From this, it was found that this alloy contains Sn ₄ Ni ₃ and Sn. Assuming that this alloy is composed only of these phases, the composition of this alloy is determined to be (Sn ₄ Ni ₃ ) _0.52 Ni _0.48 . The mass of the plated product was 5 mg / cm ² .

  A battery A19 of Example 19 was made in the same manner as in Example 1 except that the negative electrode N2 was used instead of the negative electrode N1.

[Example 20]
A battery A20 of Example 20 was made in the same manner as Example 2 except that the negative electrode N2 was used instead of the negative electrode N1.

[Example 21]
A battery A21 of Example 21 was made in the same manner as Example 3 except that the negative electrode N2 was used instead of the negative electrode N1.

[Example 22]
A battery A22 of Example 22 was made in the same manner as Example 4 except that the negative electrode N2 was used instead of the negative electrode N1.

[Example 23]
A battery A23 of Example 23 was made in the same manner as Example 5 except that the negative electrode N2 was used instead of the negative electrode N1.

[Example 24]
A battery A24 of Example 24 was made in the same manner as Example 6 except that the negative electrode N2 was used instead of the negative electrode N1.

[Comparative Example 7]
A battery R7 of Comparative Example 7 was produced in the same manner as in Comparative Example 3, except that the negative electrode N2 was used instead of the negative electrode N1.

  Using batteries A19 to A24 produced in Examples 19 to 24 and battery R7 produced in Comparative Example 7, initialization and charge / discharge tests were performed under the same conditions as in Example 1, and after high temperature storage under the same conditions. AC impedance was measured. Table 4 shows the composition of the electrolytic solutions of batteries A19 to A24 and battery R7 and the results of AC impedance measurement. In Table 4, the column “Impedance” indicates the value of AC impedance at 100 mHz. The relationship between the imide salt concentration and the impedance is shown in FIG.

From the results of Table 4 and FIG. 4, when the negative electrode N2 is used instead of the negative electrode N1, in other words, when the negative electrode active material is changed from Si to a Sn—Ni alloy, the same result is obtained, and the electrolyte salt is LiPF. It was confirmed that the use of ₆ and 2,2-imide at the same time and the presence of VC in the electrolyte produced the effect of reducing impedance.

[Examples 25 to 29 and Comparative Example 8]
[Example 25]
A battery A25 of Example 25 was made in the same manner as Example 3 except that 0.3% by weight of VC was added to the electrolytic solution.

[Example 26]
A battery A26 of Example 26 was made in the same manner as Example 3 except that 0.5% by weight of VC was added to the electrolytic solution.

[Example 27]
A battery A27 of Example 27 was made in the same manner as Example 3 except that 2.0% by weight of VC was added to the electrolytic solution.

[Example 28]
A battery A28 of Example 28 was made in the same manner as Example 3 except that 5.0% by weight of VC was added to the electrolytic solution.

[Example 29]
A battery A29 of Example 29 was made in the same manner as Example 3 except that 7.5% by weight of VC was added to the electrolytic solution.

[Comparative Example 8]
A battery R8 of Comparative Example 8 was produced in the same manner as in Example 3 except that VC was not added to the electrolytic solution.

  Using batteries A25 to A29 produced in Examples 25 to 29 and battery R8 produced in Comparative Example 8, initialization and charge / discharge tests were performed under the same conditions as in Example 1, and after high temperature storage under the same conditions. AC impedance was measured. Table 5 shows the composition of the electrolytic solutions of batteries A25 to A29 and battery R8 and the results of AC impedance measurement. In Table 5, the column “Impedance” indicates the value of AC impedance at 100 mHz. Table 5 also shows the results of Example 3 for comparison. FIG. 5 shows the relationship between the amount of VC added to the electrolytic solution (wt%) and the AC impedance (Ω) at 100 mHz.

From the results of Table 5 and FIG. 5, the impedances of the batteries A25 to A29 in which LiPF ₆ and 2,2-imide were simultaneously used as the electrolyte salt and VC was added to the electrolyte solution were not added to the electrolyte solution. It was found that the impedance was reduced compared to R8. It was also found that the impedance was the smallest when the amount of VC added was 3 wt%.

In general, from the viewpoint of high ionic conductivity, the concentration of LiPF ₆ is preferably 0.5 mol / L or more and 1.5 mol / l.

  In the above examples and comparative examples, the internal resistance of the battery after high temperature storage was measured using 100 mHz alternating current. Since alternating current with a frequency of 100 mHz has a period of 10 seconds, the alternating current impedance at this frequency is considered to have a deep correlation with the direct current resistance value of about several seconds to 60 seconds required for HEV, and the value of the alternating current impedance is small. It is estimated that a battery with a large output of about several seconds to 60 seconds can be obtained.

The relationship between the AC impedance and the electrolyte composition at a battery frequency of 100 mHz will be examined. In the case where VC was not contained in the electrolytic solution, the AC impedance of the battery R1 of Comparative Example 1 using only LiPF _{6 as the} electrolyte salt was 15.0Ω, but the electrolyte salt contained LiPF ₆ and 2, 2- In the battery R8 of Comparative Example 8 in which imide was mixed and used, the AC impedance was reduced to 4.9Ω.

Further, in the battery R2 of Comparative Example 2 in which only the LiPF ₆ is used as the electrolyte salt when the electrolytic solution contains VC, the AC impedance is reduced to 4.4Ω, and the comparative example using only the lithium imide salt as the electrolyte salt 3. In batteries R3 and R5 to R7 of Comparative Examples 5 to 7, the AC impedance was 5.2 to 6.8Ω. Thus, in the battery R2, the batteries R3, and R5 to R7, the AC impedance did not become smaller than 4.0Ω.

On the other hand, as in the batteries R1 to R29 of Examples 1 to 29, when the electrolytic solution contains VC and LiPF ₆ and lithium imide salt are mixed and used as the electrolyte salt, the AC impedance is 3.5Ω or less. Declined. The AC impedance of 3.5Ω is 20% lower than the minimum value of 4.4Ω (battery R2 of Comparative Example 2) of the battery in which the electrolyte does not contain any one of VC, LiPF ₆ and lithium imide salt, This shows that the effect of reducing the internal resistance of the battery is extremely large.

That is, in order to improve the output of a non-aqueous electrolyte secondary battery for HEV using silicon or tin or a compound thereof as a negative electrode active material, a mixture of LiPF ₆ and lithium imide salt is used as an electrolyte salt. It is considered that adding vinylene carbonate (VC) to this is a very effective formulation.

  The mechanism that can reduce the 100 mHz AC impedance is not yet elucidated at this stage, but when the lithium imide salt was decomposed at the negative electrode because the properties of the film formed on the negative electrode were different by XPS analysis. This may be due to the formation of a relatively low resistance film.

  When the imide salt concentration is 0.02 mol / l or less, the impedance reduction effect is small, and when 0.09 mol / l or more is used, the impedance tends to increase. It can be said that it is preferable that it is 0.03-0.08 mol / l.

  A possible cause of the increase in impedance when the imide salt is added excessively is that Al as the positive electrode current collector may be corroded by an excessive imide salt.

The figure which shows the relationship between the imide salt density | concentration at the time of using 2, 2- imide and an impedance. The figure which shows the relationship between an imide salt density | concentration at the time of using 1, 1-imide and an impedance. The figure which shows the relationship between the imide salt density | concentration at the time of using 1, 4- imide and an impedance. The figure which shows the relationship between an imide salt density | concentration at the time of using a Sn-Ni alloy for a negative electrode and using 2, 2- imide. The figure which shows the relationship between the amount of VC addition to electrolyte solution, and impedance.

Claims (1)

In a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, the negative electrode contains silicon, tin, or a compound thereof, and the non-aqueous electrolyte is vinylene carbonate, LiPF ₆ , 0.02 to 0.0. A nonaqueous electrolyte secondary battery comprising 5 mol / L of an imide salt.

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