JP2021099969A - Power storage element - Google Patents

Abstract

To develop a power storage element that has an excellent effect of suppressing an increase in resistance after a charge/discharge cycle even when non-graphitizable carbon is used as a negative electrode active material of a battery with a deep negative electrode charging depth.SOLUTION: A power storage element includes a negative electrode having a negative electrode mixture layer containing a negative electrode active material, and a positive electrode having a positive electrode mixture layer containing a positive electrode active material. The negative electrode mixture layer contains a cellulose derivative in which the counter cation is a metal ion, the negative electrode active material contains non-graphitizable carbon as a main component, and when the true density of the non-graphitizable carbon is A [g/cm3], a charging electricity amount B [mAh/g] of the negative electrode in the fully charged state satisfies the following equation. -580×A+1258≤B≤-830×A+1800.SELECTED DRAWING: None

Description

The present invention relates to a power storage element.

Non-aqueous electrolyte secondary batteries typified by lithium-ion non-aqueous electrolyte secondary batteries are widely used in personal computers, electronic devices such as communication terminals, automobiles, etc. due to their high energy density. The non-aqueous electrolyte secondary battery generally includes an electrode body having a pair of electrodes electrically separated by a separator, and a non-aqueous electrolyte interposed between the electrodes, and transfers ions between both electrodes. It is configured to charge and discharge by doing so. In addition, capacitors such as lithium ion capacitors and electric double layer capacitors are also widely used as power storage elements other than non-aqueous electrolyte secondary batteries.

Carbon materials such as graphite, non-graphitic carbon, and amorphous carbon are widely used as the active material for the negative electrode of such a power storage element. For example, for the purpose of increasing the capacity, a lithium ion secondary battery using a graphitizable carbonaceous material having a deep chargeable / discharging capacity per unit mass as a negative electrode active material has been proposed (see Patent Document 1).

Japanese Unexamined Patent Publication No. 7-335262

However, when the charge depth of the negative electrode is deepened by using non-graphitizable carbon as the negative electrode active material for the purpose of increasing the capacity of the battery, the negative electrode potential becomes low and after the charge / discharge cycle due to precipitation of metallic lithium during charging. Resistance may increase. Therefore, even when non-graphitizable carbon is used as the negative electrode active material of a battery having a deep negative electrode charging depth for the purpose of increasing the capacity, a power storage element having an excellent effect of suppressing an increase in resistance after a charge / discharge cycle is required. ing.

The present invention has been made based on the above circumstances, and when non-graphitizable carbon is used as the active material of the negative electrode having a deep charging depth, it has an effect of suppressing an increase in resistance after a charge / discharge cycle. An object of the present invention is to provide an excellent power storage element.

One aspect of the present invention made to solve the above problems is a negative electrode having a negative electrode mixture layer containing a negative electrode active material and a positive electrode having a positive electrode mixture layer containing a positive electrode active material. The mixture layer contains a cellulose derivative in which the counter cation is a metal ion, the negative electrode active material contains non-graphitizable carbon as a main component, and the true density of the non-graphitizable carbon is A [g / cm ³ ]. When this is done, the charging electricity amount B [mAh / g] of the negative electrode in the fully charged state is a power storage element that satisfies the following formula 1.
−580 × A + 1258 ≦ B ≦ -830 × A + 1800 ・ ・ ・ 1

According to the present invention, when non-graphitizable carbon is used as the active material of the negative electrode having a deep charging depth, it is possible to provide a power storage element having an excellent effect of suppressing an increase in resistance after a charge / discharge cycle.

It is a schematic perspective view which shows the structure of the power storage element which concerns on one Embodiment of this invention. It is a schematic diagram which shows the power storage device which was configured by gathering a plurality of power storage elements which concerns on one Embodiment of this invention. It is a graph which shows the relationship between the true density of graphitized carbon in a negative electrode active material in a test example, and the charge electric energy of a negative electrode in a fully charged state.

As a result of various experiments, the present inventor has found that the true density A of graphitizable carbon contained in the negative electrode mixture layer and the charge carrier (lithium ion in the case of a lithium ion secondary battery) are added to the graphitizable carbon. ), It was realized that there is a certain correlation with the amount of charge carriers (charged electricity amount B) that can be occluded while suppressing precipitation, and by appropriately selecting the binder for the negative electrode mixture layer. , The present invention has been completed by finding that the precipitation of the charge carrier can be suppressed more effectively.
That is, the power storage element according to the embodiment of the present invention includes a negative electrode having a negative electrode mixture layer containing a negative electrode active material and a positive electrode having a positive electrode mixture layer containing a positive electrode active material. When the layer contains a cellulose derivative in which the counter cation is a metal ion, the negative electrode active material contains non-graphitizable carbon as a main component, and the true density of the non-graphitizable carbon is A [g / cm ³ ]. The charging electricity amount B [mAh / g] of the negative electrode in the fully charged state satisfies the following formula 1.
−580 × A + 1258 ≦ B ≦ -830 × A + 1800 ・ ・ ・ 1

The power storage element suppresses the precipitation of charge carriers when non-graphitizable carbon is used as the active material of the negative electrode having a deep charging depth for the purpose of increasing the capacity, and has an effect of suppressing an increase in resistance after a charge / discharge cycle. Excellent. The reason for this is not clear, but the following reasons can be inferred. When non-graphitizable carbon is used as the active material of the negative electrode, if the charging depth of the negative electrode is deepened, the negative electrode potential shifts to a low level, so that the charge carrier may easily precipitate. When the true density of non-graphitizable carbon is A [g / cm ³ ], the electricity storage element has a charge electricity amount B [mAh / g] of the negative electrode in a fully charged state of −830 × A + 1800 or less. Therefore, the amount of charging electricity B becomes an appropriate size, and it is possible to suppress excessive precipitation of charge carriers. On the other hand, in the range where the charging electricity amount B [mAh / g] of the negative electrode is −580 × A + 1258 or more, the negative electrode mixture layer has excellent reduction resistance and is reduced and decomposed even when the negative electrode potential is low. By using a cellulose derivative in which the counter cation, which is considered to be difficult, is a metal ion, an increase in resistance after the charge / discharge cycle can be suppressed. Therefore, the power storage element is excellent in suppressing the increase in resistance after the charge / discharge cycle when graphitizable carbon is used as the active material of the negative electrode having a deep charging depth.
Here, the "fully charged state" means a state in which the battery is charged until the rated upper limit voltage for securing the rated capacity determined by the battery design is reached. If there is no description about the rated capacity, when charging is performed using the charge control device adopted by the power storage element, the battery is charged until the charge end voltage at which the charging operation is stopped and controlled is reached. The state of being. For example, the power storage element is constantly charged with a current of (1/3) CA until it reaches the rated upper limit voltage or the charging end voltage, and then becomes 0.01 CA at the rated upper limit voltage or the charging end voltage. The state in which constant current and constant voltage (CCCV) charging is performed up to is a typical example of the "fully charged state" here.

It is preferable that the metal ion is a sodium ion. When the metal ion is a sodium ion, the effect of suppressing the increase in resistance after the charge / discharge cycle can be further improved.

The positive electrode active material contains a lithium transition metal oxide containing nickel, cobalt and manganese as a main component, and the molar ratio of nickel to the total of nickel, cobalt and manganese in the lithium transition metal oxide is 0.5 or more. preferable. By setting the molar ratio of nickel to the total sum of nickel, cobalt and manganese in the lithium transition metal oxide in the above range, the capacity of the power storage element can be increased and the above-mentioned effect can be more exerted.

Hereinafter, the power storage element according to the embodiment of the present invention will be described in detail.

<Power storage element>
The power storage element according to an embodiment of the present invention includes a negative electrode, a positive electrode, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. Hereinafter, a non-aqueous electrolyte secondary battery will be described as a preferable example of the power storage element. The positive electrode and the negative electrode usually form electrode bodies that are alternately superposed by stacking or winding through a separator. The electrode body is housed in a battery container, and the battery container is filled with a non-aqueous electrolyte.

[Negative electrode]
The negative electrode includes a negative electrode base material and a negative electrode mixture layer that is directly or indirectly laminated on at least one surface of the negative electrode base material. The negative electrode mixture layer contains a negative electrode active material. The negative electrode may include an intermediate layer arranged between the negative electrode base material and the negative electrode mixture layer.

(Negative electrode base material)
The negative electrode base material is a base material having conductivity. As the material of the negative electrode base material, metals such as copper, nickel, stainless steel and nickel-plated steel or alloys thereof are used, and copper or a copper alloy is preferable. Further, examples of the form of the negative electrode base material include foil, a vapor-deposited film, and the like, and foil is preferable from the viewpoint of cost. That is, copper foil is preferable as the negative electrode base material. Examples of the copper foil include rolled copper foil and electrolytic copper foil. Note that has a "conductive" means that the volume resistivity is measured according to JIS-H-0505 (1975 years) is not more than 1 × 10 ⁷ Ω · cm, "non-conductive "means that the volume resistivity is 1 × 10 ⁷ Ω · cm greater.

The average thickness of the negative electrode base material is preferably 2 μm or more and 35 μm or less, more preferably 3 μm or more and 25 μm or less, further preferably 4 μm or more and 20 μm or less, and particularly preferably 5 μm or more and 15 μm or less. By setting the average thickness of the negative electrode base material in the above range, it is possible to increase the strength of the negative electrode base material and the energy density per volume of the power storage element. The "average thickness of the base material" means a value obtained by dividing the punching mass when punching a base material having a predetermined area by the true density of the base material and the punched area.

(Negative electrode mixture layer)
The negative electrode mixture layer is formed from a so-called negative electrode mixture containing a negative electrode active material.

The negative electrode active material contains non-graphitizable carbon as a main component. Since the negative electrode active material contains non-graphitizable carbon as a main component, the capacity of the power storage element can be increased. Further, the negative electrode mixture may contain other negative electrode active materials other than the graphitizable carbon. The "main component in the negative electrode active material" means a component having the highest content, and means a component contained in an amount of 90% by mass or more with respect to the total mass of the negative electrode active material.

(Difficult graphitizing carbon)
The graphitizable carbon is a carbon substance having an average lattice spacing d (002) of the (002) plane measured by the X-ray diffractometry in a discharged state larger than 0.36 nm and smaller than 0.42 nm. Non-graphitizable carbon usually refers to a material in which fine graphite crystals are arranged in random directions and have nano-order voids between the crystal layers. Examples of the non-graphitizable carbon include a phenol resin fired body, a furan resin fired body, and a furfuryl alcohol resin fired body.

Here, the "discharged state" refers to a state in which the open circuit voltage is 0.7 V or more in a unipolar battery using a negative electrode containing a carbon substance as a negative electrode active material as a working electrode and a metal Li as a counter electrode. Since the potential of the metal Li counter electrode in the open circuit state is substantially equal to the oxidation-reduction potential of Li, the open circuit voltage in the single-pole battery is substantially equal to the potential of the negative electrode containing the carbon material with respect to the oxidation-reduction potential of Li. .. That is, the fact that the open circuit voltage in the single-pole battery is 0.7 V or more means that lithium ions that can be occluded and discharged are sufficiently released from the carbon material that is the negative electrode active material during charging and discharging. ..

The true density A of non-graphitizable carbon is not particularly limited as long as the relationship between the true density A and the amount of electricity charged B satisfies the above equation, but the lower limit thereof is 1.4 [g / cm ³ ]. Preferably, 1.45 [g / cm ³ ] is more preferable. In some embodiments, the true density A may be 1.5 [g / cm ³ ] or higher, 1.55 [g / cm ³ ] or higher, and 1.6 [g / cm] or higher. ³ ] or more. As the upper limit of the true density, 1.8 [g / cm ³ ] is preferable, and 1.7 [g / cm ³ ] is more preferable. In some embodiments, the true density A may be 1.65 [g / cm ³ ] or less, 1.58 [g / cm ³ ] or less, and 1.52 [g / cm] or less. ³ ] It may be less than or equal to. If the true density of graphitizable carbon is too small, impurities derived from raw materials and reaction active surfaces increase, resulting in a large irreversible capacity. If the true density is too high, the amount of lithium ions that can be occluded between crystal structures is small. turn into. That is, within the above range, the amount of lithium ions that can be occluded can be increased while suppressing the irreversible capacity. The true density is measured by the pycnometer method using butanol.

The lower limit of the content of the non-graphitizable carbon with respect to the total mass of the negative electrode active material is preferably 90% by mass. By setting the content of non-graphitizable carbon to the above lower limit or more, the capacity of the power storage element can be further increased. On the other hand, the upper limit of the content of the non-graphitizable carbon with respect to the total mass of the negative electrode active material may be, for example, 100% by mass.

(Other negative electrode active materials)
Other negative electrode active materials that may be contained in addition to non-graphitizable carbon include easily graphitizable carbon, metals such as graphite, Si, and Sn, oxides of these metals, or these metals and carbon materials. Complex and the like.

The content of the negative electrode active material in the negative electrode mixture layer is not particularly limited, but the lower limit thereof is preferably 50% by mass, more preferably 80% by mass, and even more preferably 90% by mass. On the other hand, as the upper limit of this content, 99% by mass is preferable, and 98% by mass is more preferable.

The negative electrode mixture layer contains a cellulose derivative whose counter cation is a metal ion. The cellulose derivative is a component that functions as a thickener when forming a negative electrode mixture layer by coating or the like. A cellulose derivative is a compound having a structure in which a hydrogen atom of a hydroxy group of cellulose is substituted with another group. Examples of the cellulose derivative having a counter cation include carboxyalkyl cellulose (carboxymethyl cellulose (CMC), carboxyethyl cellulose, carboxypropyl cellulose, etc.), alkyl cellulose (methyl cellulose, ethyl cellulose, etc.), hydroxyalkyl cellulose (hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxyethyl, etc.). Methyl cellulose, hydroxypropyl methyl cellulose, etc.), cellulose phthalate acetate, hydroxypropyl methyl cellulose phthalate, acetyl cellulose, etc. can be mentioned. Among these, carboxyalkyl cellulose is preferable, and CMC is more preferable. The above-mentioned cellulose derivative may be used alone or in combination of two or more. Examples of the metal ion serving as the counter cation include sodium ion, magnesium ion, lithium ion and the like.

The content of the cellulose derivative in the negative electrode mixture layer is not particularly limited, but the lower limit thereof is 0.1% by mass. The lower limit of the content of the cellulose derivative is preferably 0.3% by mass, more preferably 0.5% by mass. On the other hand, the upper limit of the content of the cellulose derivative is, for example, 10% by mass. The upper limit of the content of the cellulose derivative is preferably 5% by mass, more preferably 3% by mass. In some embodiments, the upper limit of the content of the cellulose derivative may be 2% by weight or 1.5% by weight (eg 1.2% by weight). By setting the content of the cellulose derivative to the above lower limit or more, sufficient viscosity can be given to the negative electrode mixture paste when forming the negative electrode mixture layer, and the negative electrode mixture layer can be efficiently formed. .. On the other hand, by setting the content of the cellulose derivative to the above upper limit or less, the above-mentioned performance improving effect (for example, the effect of suppressing the increase in resistance after the charge / discharge cycle) can be more effectively exhibited.

(Other optional ingredients)
The negative electrode mixture contains optional components such as a conductive agent, a binder, and a filler, if necessary.

The graphitizable carbon also has conductivity, but the conductive agent is not particularly limited as long as it is a conductive material. Examples of such a conductive agent include graphite, carbonaceous materials, metals, conductive ceramics and the like. Examples of the carbonaceous material include non-graphitized carbon and graphene-based carbon. Examples of non-graphitized carbon include carbon nanofibers, pitch-based carbon fibers, and carbon black. Examples of carbon black include furnace black, acetylene black, and ketjen black. Examples of graphene-based carbon include graphene, carbon nanotubes (CNT), and fullerenes. Examples of the shape of the conductive agent include powder and fibrous. As the conductive agent, one of these materials may be used alone, or two or more of these materials may be mixed and used. Further, these materials may be used in combination. For example, a material in which carbon black and CNT are composited may be used. Among these, carbon black is preferable from the viewpoint of electron conductivity and coatability, and acetylene black is particularly preferable.

As the binder, either an aqueous binder or a non-aqueous binder can be used, but an aqueous binder is preferable. A water-based binder and a non-water-based binder may be used in combination. The aqueous binder means a binder that can be dissolved or dispersed in an aqueous solvent when preparing a mixture. The aqueous solvent means water or a mixed solvent mainly composed of water. Examples of the solvent other than water constituting the mixed solvent include organic solvents (lower alcohols, lower ketones, etc.) that can be uniformly mixed with water. The non-aqueous binder means a binder that can be dissolved or dispersed in a non-aqueous solvent when preparing a mixture. Examples of the non-aqueous solvent include N-methyl-2-pyrrolidone (NMP) and the like. As the inder, known ones can be used, for example, fluororesin (polytetrafluoroethylene (PTFE), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), tetrafluoroethylene-hexafluoropropylene). Polymer (FEP), ethylene-tetrafluoroethylene copolymer (ETFE), etc.), vinyl acetate copolymer, styrene butadiene rubber (SBR), acrylic acid-modified SBR, ethylene-propylene-diene rubber (EPDM), sulfonated EPDM , Fluororubber, Arabic rubber, Polyfluorovinylidene (PVDF), Polychloride vinylidene (PVDC), Polypropylene, Polypropylene, Polyethylene oxide (PEO), Polypropylene oxide (PPO), Polyethyleneoxide-propylene oxide copolymer (PEO-PPO) Etc. can be used. Among these, rubber-based binders such as SBR, acrylic acid-modified SBR, EPDM, sulfonated EPDM, fluororubber, and Arabic rubber are preferable, and SBR is more preferable, from the viewpoint of binding property and resistance increase inhibitory property. When the binder has a functional group that reacts with lithium or the like, this functional group may be deactivated in advance by methylation or the like. The lower limit of the binder content in the negative electrode mixture layer is preferably 1% by mass, more preferably 2% by mass. On the other hand, as the upper limit of the content of the binder, 10% by mass is preferable, and 5% by mass is more preferable. By setting the content of the binder in the above range, it is possible to further improve the input performance and the like of the non-aqueous electrolyte power storage element at a low temperature.

The filler is not particularly limited. The main components of the filler are polyolefins such as polypropylene and polyethylene, inorganic oxides such as silicon dioxide, aluminum oxide, titanium dioxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide and aluminosilicate, magnesium hydroxide and hydroxide. Hydroxides such as calcium and aluminum hydroxide, carbonates such as calcium carbonate, sparingly soluble ionic crystals such as calcium fluoride, barium fluoride and barium sulfate, nitrides such as aluminum nitride and silicon nitride, talc and montmorillonite, Examples thereof include mineral resource-derived substances such as boehmite, zeolite, apatite, kaolin, mulite, spinel, olivine, cericite, bentonite, and mica, or man-made products thereof. When a filler is used in the negative electrode mixture layer, the proportion of the filler in the entire negative electrode mixture layer can be about 8.0% by mass or less, and usually about 5.0% by mass or less (for example, 1.0). It is preferably mass% or less).

(Middle layer)
The intermediate layer is a coating layer on the surface of the negative electrode base material, and contains conductive particles such as carbon particles to reduce the contact resistance between the negative electrode base material and the negative electrode mixture layer. The composition of the intermediate layer is not particularly limited, and can be formed by, for example, a composition containing a resin binder and conductive particles.

In the power storage element, when the true density of the graphitizable carbon is A [g / cm ³ ], the charging electricity amount B [mAh / g] of the negative electrode in the fully charged state satisfies the following formula 1.
−580 × A + 1258 ≦ B ≦ -830 × A + 1800 ・ ・ ・ 1
When the true density of graphitizable carbon is A [g / cm ³ ], the electricity storage element has a charge electricity amount B [mAh / g] of the negative electrode in a fully charged state of −830 × A + 1800 or less. Therefore, the amount of charging electricity B becomes an appropriate size, and it is possible to suppress excessive precipitation of metallic lithium. Further, in the range where the charging electricity amount B of the negative electrode is −580 × A + 1258 or more, the counter is considered to have excellent reduction resistance as a binder of the negative electrode mixture layer and is unlikely to be reduced and decomposed even when the negative electrode potential is low. By using a cellulose derivative in which the cation is a metal ion, an increase in resistance after the charge / discharge cycle can be suppressed. Therefore, the power storage element is excellent in suppressing the increase in resistance after the charge / discharge cycle when graphitizable carbon is used as the active material of the negative electrode having a deep charging depth.

The charging electricity amount B of the negative electrode 12 shall be measured by the following procedure.
(1) The target battery is discharged and disassembled in the glove box until the end of discharge (low SOC region).
(2) In the glove box controlled to an atmosphere having an oxygen concentration of 5 ppm or less, the positive electrode plate and the negative electrode plate are taken out to assemble a small laminate cell.
(3) After charging the small laminate cell to the fully charged state, constant current constant voltage (CCCV) discharge is performed up to 0.01 CA at the lower limit voltage when the rated capacity is obtained by the power storage element.
(4) In the glove box controlled to have an oxygen concentration of 5 ppm or less, the small laminate cell is disassembled, the negative electrode is taken out, and the small laminate cell in which lithium metal is arranged as the counter electrode is reassembled.
(5) Additional discharge is performed at a current density of 0.01 CA until the negative electrode potential reaches 2.0 V (vs. Li ^{/ Li +) to adjust the negative electrode to a completely discharged state.}
(6) The total amount of electricity in (3) and (5) above is divided by the mass of the negative electrode opposite the positive and negative electrodes in the small laminate cell to obtain the amount of electricity to be charged.

[Positive electrode]
The positive electrode includes a positive electrode base material and a positive electrode mixture layer that is directly or indirectly laminated on at least one surface of the positive electrode base material. The positive electrode mixture layer contains a positive electrode active material. The positive electrode may include an intermediate layer arranged between the positive electrode base material and the positive electrode mixture layer.

(Positive electrode base material)
The positive electrode base material 21 is a base material having conductivity. As the material of the positive electrode base material, metals such as aluminum, titanium, tantalum, and stainless steel or alloys thereof are used. Among these, aluminum and aluminum alloys are preferable from the viewpoint of balance of potential resistance, high conductivity and cost. Further, examples of the form of the positive electrode base material 21 include a foil, a vapor-deposited film, and the like, and the foil is preferable from the viewpoint of cost. That is, aluminum foil is preferable as the positive electrode base material 21. Examples of aluminum or aluminum alloy include A1085 and A3003 specified in JIS-H4000 (2014).

(Positive electrode mixture layer)
The positive electrode mixture layer is formed from a so-called positive electrode mixture containing a positive electrode active material. As the positive electrode active material, for example, a known positive electrode active material can be appropriately selected. As the positive electrode active material for a lithium ion non-aqueous electrolyte secondary battery, a material capable of occluding and releasing lithium ions is usually used. Examples of the positive electrode active material include _{a lithium transition metal composite oxide having an α-NaFeO type 2} crystal structure, a lithium transition metal oxide having a spinel type crystal structure, a polyanion compound, a chalcogen compound, sulfur and the like. Examples of the lithium transition metal composite oxide having an _{α-NaFeO type 2 crystal structure include Li [Li x} Ni _1-x ] O ₂ (0 ≦ x <0.5) and Li [Li _x Ni _γ Co _{(1-). x-γ)} ] O ₂ (0 ≦ x <0.5, 0 <γ <1), Li [Li _x Co _(1-x) ] O ₂ (0 ≦ x <0.5), Li [Li _x Ni _γ Mn _(1-x-γ) ] O ₂ (0 ≦ x <0.5, 0 <γ <1), Li [Li _x Ni _γ Mn _β Co _(1-x-γ-β) ] O ₂ (0 ≦ x <0.5, 0 <γ, 0 <β, 0.5 <γ + β <1), Li [Li _x Ni _γ Co _β Al _(1-x-γ-β) ] O ₂ (0 ≦ Examples thereof include x <0.5, 0 <γ, 0 <β, 0.5 <γ + β <1). Examples of the lithium transition metal oxide having a spinel-type crystal structure include Li _x Mn ₂ O ₄ and Li _x Ni _γ Mn _(2-γ) O ₄ . Examples of the polyanion compound include LiFePO ₄ , LiMnPO ₄ , LiNiPO ₄ , LiCoPO ₄ , Li ₃ V ₂ (PO ₄ ) ₃ , Li ₂ MnSiO ₄ , Li ₂ CoPO ₄ F and the like. Examples of the chalcogen compound include titanium disulfide, molybdenum disulfide, molybdenum dioxide and the like. The atoms or polyanions in these materials may be partially substituted with atoms or anion species consisting of other elements. The surface of these materials may be coated with other materials.

The positive electrode active material is preferably a nickel-containing lithium transition metal oxide containing nickel. The molar ratio of nickel to the total amount of metal elements other than lithium in the nickel-containing lithium transition metal oxide is preferably 0.5 or more (for example, 0.5 or more and 1 or less), and 0.55 or more (for example, 0.6). More than 0.9 or less) is more preferable. As a particularly preferable example of the positive electrode active material, a lithium transition metal oxide containing nickel, cobalt and manganese is used as a main component, and the molar ratio of nickel to the total of nickel, cobalt and manganese in the lithium transition metal oxide is 0.5 or more. (For example, 0.5 or more and 0.9
Hereinafter, typically 0.6 or more and 0.8 or less) can be mentioned. By setting the molar ratio of nickel to the sum of nickel, cobalt and manganese in the lithium transition metal oxide within the above range, the capacity of the power storage element can be increased.

In the positive electrode mixture layer, one of these materials may be used alone, or two or more of these materials may be mixed and used. In the positive electrode mixture layer, one of these compounds may be used alone, or two or more of these compounds may be mixed and used.

The content of the positive electrode active material in the positive electrode mixture layer is not particularly limited, but the lower limit thereof is preferably 50% by mass, more preferably 80% by mass, and even more preferably 90% by mass. On the other hand, as the upper limit of this content, 99% by mass is preferable, and 98% by mass is more preferable.

The charging electricity amount B of the negative electrode in the fully charged state is, for example, the mass N of the negative electrode active material per unit area of the negative electrode mixture layer with respect to the mass P of the positive electrode active material per unit area of the positive electrode mixture layer. It can be adjusted by changing the ratio N / P of. In one embodiment, when the true density of the non-graphitizable carbon is A [g / cm ³ ], the negative electrode mixture layer has a mass P of the positive electrode active material per unit area of the positive electrode mixture layer. It is preferable that the ratio N / P of the mass N of the negative electrode active material per unit area satisfies the following formula 2.
0.57 × A-0.53 ≦ N / P ≦ 0.83 × A-0.77 ・ ・ ・ 2
When N / P satisfying the above formula 2 is applied to a conventional battery using non-graphitizable carbon, the negative electrode potential becomes low as the charging depth becomes deeper than the normal charging depth, and metallic lithium precipitates during charging. This may cause an increase in resistance after the charge / discharge cycle. However, the power storage element is a cellulose derivative in which the counter cation is a metal ion, which is considered to be excellent in reduction resistance and is unlikely to be reduced and decomposed even when the negative electrode potential is low, as a binder for the negative electrode mixture layer. By using it in the range of N / P to be satisfied, it is possible to exert an effect of suppressing an increase in resistance after a charge / discharge cycle.

(Other optional ingredients)
The positive electrode mixture contains optional components such as a conductive agent, a binder, and a filler, if necessary. Optional components such as a conductive agent, a binder, and a filler can be selected from the materials exemplified by the negative electrode.

The conductive agent is not particularly limited as long as it is a conductive material. Such a conductive agent can be selected from the materials exemplified in the negative electrode. When a conductive agent is used, the proportion of the conductive agent in the entire positive electrode mixture layer can be about 1.0% by mass to 20% by mass, and usually about 2.0% by mass to 15% by mass (for example). It is preferably 3.0% by mass to 6.0% by mass).

The binder can be selected from the materials exemplified in the negative electrode. When a binder is used, the proportion of the binder in the entire positive mixture layer can be approximately 0.50% by mass to 15% by mass, and is usually approximately 1.0% by mass to 10% by mass (for example, 1. 5% by mass to 3.0% by mass) is preferable.

The filler can be selected from the materials exemplified in the negative electrode. When a filler is used, the proportion of the filler in the entire positive electrode mixture layer can be about 8.0% by mass or less, and usually about 5.0% by mass or less (for example, 1.0% by mass or less). It is preferable to do so.

(Middle layer)
The intermediate layer is a coating layer on the surface of the positive electrode base material 21, and includes conductive particles such as carbon particles to reduce the contact resistance between the positive electrode base material 21 and the positive electrode mixture layer. Similar to the negative electrode, the structure of the intermediate layer is not particularly limited, and can be formed by, for example, a composition containing a resin binder and conductive particles.

[Non-aqueous electrolyte]
As the non-aqueous electrolyte, a known non-aqueous electrolyte usually used for a general non-aqueous electrolyte secondary battery (storage element) can be used. The non-aqueous electrolyte contains a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. The non-aqueous electrolyte may be a solid electrolyte or the like.

As the non-aqueous solvent, a known non-aqueous solvent usually used as a non-aqueous solvent for a general non-aqueous electrolyte for a power storage element can be used. Examples of the non-aqueous solvent include cyclic carbonates, chain carbonates, esters, ethers, amides, sulfones, lactones, nitriles and the like. Among these, it is preferable to use at least cyclic carbonate or chain carbonate, and it is more preferable to use cyclic carbonate and chain carbonate in combination. When the cyclic carbonate and the chain carbonate are used in combination, the volume ratio of the cyclic carbonate to the chain carbonate (cyclic carbonate: chain carbonate) is not particularly limited, but may be, for example, 5:95 to 50:50. preferable.

Examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), vinylethylene carbonate (VEC), chloroethylene carbonate, fluoroethylene carbonate (FEC), and difluoroethylene. Examples thereof include carbonate (DFEC), styrene carbonate, catechol carbonate, 1-phenylvinylene carbonate, 1,2-diphenylvinylene carbonate, and among these, EC is preferable.

Examples of the chain carbonate include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diphenyl carbonate and the like, and among these, EMC is preferable.

As the electrolyte salt, a known electrolyte salt usually used as an electrolyte salt of a general non-aqueous electrolyte for a power storage element can be used. Examples of the electrolyte salt include lithium salt, sodium salt, potassium salt, magnesium salt, onium salt and the like, but lithium salt is preferable.

Examples of the lithium salt include inorganic lithium salts such _{as LiPF 6} , LiPO ₂ F ₂ , LiBF ₄ , LiClO ₄ , LiN (SO ₂ F) _{2 , LiSO 3} CF ₃ , LiN (SO ₂ CF _{3 ) 2} , and LiN (SO). ₂ C ₂ F ₅ ) ₂ , LiN (SO ₂ CF ₃ ) (SO ₂ C ₄ F ₉ ), LiC (SO ₂ CF ₃ ) ₃ , LiC (SO ₂ C ₂ F ₅ ) ₃ etc. Hydrogen is replaced with fluorine Examples thereof include a lithium salt having a sulfur group. Among these, an inorganic lithium salt is preferable, and LiPF ₆ is more preferable.

The lower limit of the concentration of the electrolyte salt in the nonaqueous electrolyte is preferably 0.1 mol / dm ^3, more preferably 0.3 mol / dm ^3, more preferably ^{0.5mol / dm 3, 0.7mol / dm} 3 Is particularly preferable. On the other hand, the upper limit is not particularly limited, but is preferably 2.5 mol / dm ^3, more preferably 2.0 mol / dm ^3, more preferably 1.5 mol / dm ^3.

Other additives may be added to the non-aqueous electrolyte. Further, as the non-aqueous electrolyte, a molten salt at room temperature, an ionic liquid, or the like can also be used.

[Separator]
The separator is interposed between the negative electrode and the positive electrode. As the separator, for example, a woven fabric, a non-woven fabric, a porous resin film, or the like is used. Among these, a porous resin film is preferable from the viewpoint of strength, and a non-woven fabric is preferable from the viewpoint of liquid retention of a non-aqueous electrolyte. As the main component of the separator, polyolefins such as polyethylene and polypropylene are preferable from the viewpoint of strength, and polyimide and aramid are preferable from the viewpoint of oxidative decomposition resistance. Moreover, you may combine these resins.

An inorganic layer may be laminated between the separator and the electrode (usually a positive electrode). This inorganic layer is a porous layer also called a heat-resistant layer or the like. Further, a separator having an inorganic layer formed on one surface or both surfaces of the porous resin film can also be used. The inorganic layer is usually composed of inorganic particles and a binder, and may contain other components.

[Specific configuration of power storage element]
The shape of the power storage element of the present embodiment is not particularly limited, and examples thereof include a cylindrical battery, a pouch film type battery, a square type battery, a flat type battery, a coin type battery, and a button type battery.

FIG. 1 shows a square non-aqueous electrolyte secondary battery 1 as an example of a power storage element. The figure is a perspective view of the inside of the battery container. The electrode body 2 having the positive electrode and the negative electrode wound around the separator is housed in the square battery container 3. The positive electrode is electrically connected to the positive electrode terminal 4 via the positive electrode current collector 41. The negative electrode is electrically connected to the negative electrode terminal 5 via the negative electrode current collector 51.

[Manufacturing method of power storage element]
The method for manufacturing the power storage element is as follows: producing a negative electrode, producing a positive electrode, preparing a non-aqueous electrolyte, and laminating or winding the positive electrode and the negative electrode via a separator so that the electrodes are alternately superimposed. The electrode body is housed in a container, and the non-aqueous electrolyte is injected into the container. The positive electrode can be obtained by laminating the positive electrode mixture layer directly on the positive electrode base material or via an intermediate layer. The positive electrode mixture layer is laminated by applying the positive electrode mixture paste to the positive electrode base material. Further, the negative electrode can be obtained by laminating the negative electrode mixture layer directly on the negative electrode base material or via an intermediate layer, similarly to the positive electrode. The negative electrode mixture layer is laminated by applying a negative electrode mixture paste containing non-graphitized carbon to the negative electrode base material. The positive electrode mixture paste and the negative electrode mixture paste may contain a dispersion medium. As the dispersion medium, for example, an aqueous solvent such as water or a mixed solvent mainly composed of water; or an organic solvent such as N-methylpyrrolidone or toluene can be used.

The method of accommodating the negative electrode, the positive electrode, the non-aqueous electrolyte, etc. in the case can be performed by a known method. After accommodating, a power storage element can be obtained by sealing the accommodating port. Details of each element constituting the power storage element obtained by the above manufacturing method are as described above.

According to the power storage element, when non-graphitizable carbon is used as the active material of the negative electrode having a deep charging depth, precipitation of metallic lithium is suppressed, and the effect of suppressing an increase in resistance after a charge / discharge cycle is excellent.

[Other Embodiments]
The power storage element of the present invention is not limited to the above embodiment, and various modifications may be made without departing from the gist of the present invention. For example, the configuration of one embodiment can be added to the configuration of another embodiment, and a part of the configuration of one embodiment can be replaced with the configuration of another embodiment or a well-known technique. In addition, some of the configurations of certain embodiments can be deleted. Further, a well-known technique can be added to the configuration of a certain embodiment.

In the above embodiment, the mode in which the power storage element is a non-aqueous electrolyte secondary battery has been mainly described, but other power storage elements may be used. Examples of other power storage elements include capacitors (electric double layer capacitors, lithium ion capacitors) and the like. Examples of the non-aqueous electrolyte secondary battery include a lithium ion non-aqueous electrolyte secondary battery.

The present invention can also be realized as a power storage device including the plurality of the power storage elements. In this case, the technique of the present invention may be applied to at least one power storage element included in the power storage device. Further, an assembled battery can be constructed by using a single or a plurality of power storage elements (cells) of the present invention, and a power storage device can be further configured by using the assembled battery. The power storage device can be used as a power source for automobiles such as electric vehicles (EV), hybrid electric vehicles (HEV), and plug-in hybrid vehicles (PHEV). Further, the power storage device can be used for various power supply devices such as an engine starting power supply device, an auxiliary power supply device, and an uninterruptible power supply (UPS).

FIG. 2 shows an example of a power storage device 30 in which a power storage unit 20 in which two or more electrically connected power storage elements 1 are assembled is further assembled. The power storage device 30 may include a bus bar (not shown) that electrically connects two or more power storage elements 1 and a bus bar (not shown) that electrically connects two or more power storage units 20. The power storage unit 20 or the power storage device 30 may include a condition monitoring device (not shown) that monitors the state of one or more power storage elements.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to the following Examples.

[Examples 1 to 21]
(Preparation of negative electrode)
A negative electrode mixture paste was prepared by mixing non-graphitizable carbon as a negative electrode active material, styrene-butadiene rubber (SBR) as a binder, carboxymethyl cellulose (CMC) as a thickener, and water as a dispersion medium. .. The mass ratio of graphitizable carbon, styrene-butadiene rubber, and (carboxymethyl cellulose (CMC)) was 97.4: 2.0: 0.6 in terms of solid content.

The negative electrode mixture paste was prepared by adjusting the viscosity with the amount of water and kneading with a multi-blender mill. This negative electrode mixture paste was applied to both sides of the copper foil. Next, a negative electrode mixture layer was prepared by drying. After the above drying, the negative electrode mixture layer was roll-pressed so as to have a predetermined packing density to obtain a negative electrode.

(Preparation of positive electrode)
Positive electrode using lithium nickel cobalt manganese composite oxide as a positive electrode active material, acetylene black (AB) as a conductive agent, vinylidene fluoride (PVDF) as a binder, and N-methylpyrrolidone (NMP) as a non-aqueous dispersion medium. A mixture paste (material for forming a positive electrode mixture layer) was prepared. The mass ratio of the positive electrode active material, the binder and the conductive agent was 94.5: 4.0: 1.5 in terms of solid content. This positive electrode mixture paste was applied to both sides of the aluminum foil so that a non-laminated portion was formed on one end edge of the aluminum foil. Next, a positive electrode mixture layer was prepared by drying. After the above drying, the positive electrode mixture layer was roll-pressed so as to have a predetermined packing density to obtain a positive electrode. The N / Ps of Examples 1 to 28 are shown in Table 1. Here, the molar ratio (Ni: Co: Mn ratio) of nickel, cobalt and manganese of the lithium nickel cobalt manganese composite oxide as the positive electrode active material was set to 6.0: 2.0: 2.0.

(Non-aqueous electrolyte)
For the non-aqueous electrolyte, LiPF ₆ was dissolved in a solvent in which propylene carbonate (PC) and diethyl carbonate (DEC) were mixed so that the volume ratio was 30:70 so that the salt concentration was 1.2 mol / dm ^3. Prepared.

(Separator)
A polyethylene microporous membrane having a thickness of 14 μm was used as the separator.

(Power storage element)
The positive electrode, the negative electrode, and the separator were laminated to prepare an electrode body. Then, the electrode body was sealed in a container. Next, after welding the container and the lid plate, the non-aqueous electrolyte was injected and sealed. In this way, the batteries (storage elements) of Examples 1 to 21 were obtained.

[Evaluation]
(True density of graphitized carbon)
The true density of non-graphitizable carbon was measured by the following procedure.
The discharged carbon-degraphitized carbon was immersed in water to remove the binder and the thickener, and then vacuum-dried at 25 ° C. for 12 hours, and then the non-graphitized carbon was taken out. Next, the non-graphitized carbon was dried at 120 ° C. for 2 hours and cooled to room temperature in a desiccator. The mass (m1) of the specific gravity bottle was accurately weighed, and about 3 g of non-graphitized carbon was added to accurately weigh the mass (m2). Next, 1-butanol was gently added to the specific gravity bottle to a depth of about 20 mm from the bottom, placed in a vacuum desiccator, and gradually exhausted to maintain the pressure from 2.0 kPa to 2.6 kPa. This pressure was maintained for 20 minutes, and after the generation of bubbles stopped, the densitometer was removed from the vacuum desiccator and 1-butanol was further added. The specific gravity bottle was immersed in a constant temperature water bath at 30 ± 0.5 ° C. for 30 minutes, and the 1-butanol liquid level was aligned with the marked line. The specific density bottle was taken out, the outside was wiped well, and the mass was accurately weighed. It was immersed in a constant temperature water tank again for 15 minutes, the 1-butanol liquid level was aligned with the marked line, the specific density bottle was taken out, the outside was wiped well, and the mass was weighed. This step is repeated 3 times, and the average value of each mass when repeated 3 times is defined as (m4). Next, the same specific density bottle is filled with 1-butanol, soaked in a constant temperature water tank in the same manner as above, and the process of measuring the mass after aligning with the marked line is repeated 4 times, and the average value of each mass when repeated 4 times is calculated. Let it be (m3). Further, the degassed water immediately before use is placed in a specific gravity bottle, immersed in a constant temperature water tank in the same manner as described above, aligned with the marked line, and then weighed four times, and the average value is taken as (m5). The true density A was calculated by the following formula 2. In the following formula 2, d is the specific gravity at 30 ° C., and d = 0.9946.
A = (m2-m1) / (m2-m1- (m4-m3)) x ((m3-m1) / (m5-m1)) x d ... 2

(Amount of electricity charged in the negative electrode)
The amount of electricity charged in the negative electrode was measured by the method described above.

(DCR (direct current resistance) increase rate after charge / discharge cycle)
(1) Initial discharge capacity confirmation test Constant current constant voltage (CCCV) charging is performed in a constant temperature bath at 25 ° C. under the conditions of charging current 13.6A and charging termination voltage 4.32V until the charging current becomes 0.4A or less. After that, a 20-minute rest period was provided. Then, a constant current (CC) discharge was performed with a discharge current of 40.9 A and a discharge end voltage of 2.4 V. The discharge capacity at this time was defined as the "initial discharge capacity".
(2) Charge / discharge cycle test For each power storage element after measuring the "initial discharge capacity", the charging current is 0.4A or less under the conditions of a charging current of 13.6A and a final charging voltage of 4.32V in a constant temperature bath at 45 ° C. It was charged with constant current and constant voltage (CCCV) until it became, and then a rest period of 10 minutes was provided. Then, a constant current (CC) discharge was performed with a discharge current of 40.9 A and a discharge end voltage of 2.4 V, and then a rest period of 10 minutes was provided. This charge / discharge cycle was carried out for 1000 cycles. After 1000 cycles, the discharge capacity was measured under the same conditions as in the "initial capacity" measurement test, and the discharge capacity at this time was defined as "capacity after 1000 cycles".
(3) DCR increase rate after charge / discharge cycle The DCR of the power storage element after the charge / discharge cycle test was evaluated. For each power storage element after the charge / discharge cycle test, 50 of the discharge capacity measured after the initial discharge capacity measurement and after the 1000 cycle test in a constant temperature bath at 25 ° C. under the same conditions as the above discharge capacity measurement method. The amount of electricity charged for% SOC was constantly charged with a current value of 13.6 A. After setting the SOC of the battery to 50% under the above conditions, discharge the battery at current values of 40.9A, 81.8A, 122.7A, and 300.0A for 10 seconds, respectively, and set the voltage 10 seconds after the start of discharge on the vertical axis. From the graph of current-voltage performance obtained by plotting the discharge current value on the horizontal axis, the DCR value, which is a value corresponding to the gradient, was obtained. Then, for each test example, the ratio of "DCR after charge / discharge cycle test" to "DCR before start of charge / discharge cycle test" at 25 ° C. ("DCR after charge / discharge cycle test" / "start of charge / discharge cycle test". The previous DCR ”) was calculated, and the“ DCR increase rate [%] ”was calculated. Regarding this DCR increase rate, Table 1 shows the ratio [%] of the DCR increase rate of each test example to the DCR increase rate of Example 1.

Table 1 below shows the true density of graphitized carbon of Examples 1 to 21, the counter cation of the cellulose derivative, the amount of charge electricity of the negative electrode, the N / P ratio, and the DCR increase rate of each Test Example with respect to the DCR increase rate of Example 1. Indicates the ratio of. Further, FIG. 3 shows the relationship between the true density of non-graphitized carbon in the negative electrode active material in Examples 1 to 21 and the charging electricity amount of the negative electrode in a fully charged state.

As shown in Table 1 and FIG. 3, when the true density of the graphitizable carbon is A [g / cm ³ ], the charging electricity amount B [mAh / g] of the negative electrode in the fully charged state is In the range of −580 × A + 1258 ≦ B ≦ −830 × A + 1800, Examples 1 to 5, Example 12, Example 14 and Example 18 in which the negative electrode mixture layer contains a cellulose derivative in which the counter cation is a metal ion are charged and discharged. The inhibitory effect on the DCR increase rate after the cycle was good.

On the other hand, the charge electricity amount B of the negative electrode is in the range of −580 × A + 1258 ≦ B ≦ -830 × A + 1800, but the negative electrode mixture layer contains a cellulose derivative in which the counter cation is not a metal ion. In 15 and Example 19, the effect of suppressing the increase in resistance after the charge / discharge cycle was reduced.
Further, in Example 8, Example 9, Example 16, Example 17, Example 20, and Example 21 in which the charge electricity amount B of the negative electrode is less than −580 × A + 1258, the DCR increase rate is good regardless of the counter cation of the cellulose derivative. It was.
Further, in Examples 6, 10 and 11 in which the charging electricity amount B of the negative electrode exceeds −830 × A + 1800, the negative electrode mixture layer is charged and discharged even though the negative electrode mixture layer contains a cellulose derivative in which the counter cation is a metal ion. The inhibitory effect on the increase in resistance after the cycle decreased.
The power storage element contains a cellulose derivative in which the counter cation is a metal ion when the charge electricity amount B of the negative electrode in the fully charged state is in a specific range satisfying −580 × A + 1258 ≦ B ≦ −830 × A + 1800. Therefore, it can be seen that the increase in resistance after the charge / discharge cycle can be suppressed even when the charge depth of the negative electrode is relatively deep.

As a result of the above, it was shown that the power storage element is excellent in suppressing the increase in resistance after the charge / discharge cycle when non-graphitizable carbon is used as the active material of the negative electrode having a deep charging depth.

INDUSTRIAL APPLICABILITY The present invention is suitably used as a power storage element such as a non-aqueous electrolyte secondary battery used as a power source for personal computers, electronic devices such as communication terminals, automobiles, and the like.

1 Power storage element 2 Electrode body 3 Battery container 4 Positive terminal 41 Positive current collector 5 Negative terminal 51 Negative negative current collector 20 Power storage unit 30 Power storage device

Claims (3)

A negative electrode having a negative electrode mixture layer containing a negative electrode active material, and a negative electrode
A positive electrode having a positive electrode mixture layer containing a positive electrode active material, and a positive electrode
Is equipped with
The negative electrode mixture layer contains a cellulose derivative whose counter cation is a metal ion.
The above negative electrode active material contains non-graphitizable carbon as a main component and contains
When the true density of the graphitizable carbon is A [g / cm ³ ], the charging electricity amount B [mAh / g] of the negative electrode in the fully charged state satisfies the following formula 1.
−580 × A + 1258 ≦ B ≦ -830 × A + 1800 ・ ・ ・ 1 The power storage element according to claim 1, wherein the metal ion is a sodium ion. The positive electrode active material is mainly composed of a lithium transition metal oxide containing nickel, cobalt and manganese.
The power storage element according to claim 1 or 2, wherein the molar ratio of nickel to the total sum of nickel, cobalt and manganese in the lithium transition metal oxide is 0.5 or more.

Images