Electrode mixture and power storage device

Abstract

An electrode mixture 38 used for a negative electrode of a power storage device includes: an electrode active material 40 including an alloying material that forms an alloy with an alkali metal element; and cellulose-based nanofibers 42 having an anionic functional group R that interacts with the alkali metal element.

Description

Electrode mixture and energy storage device

[0001] This disclosure relates to electrode mixtures and energy storage devices.

[0002] Conventionally, a technique has been known in which silicon-containing materials are included in the negative electrode active material used in energy storage devices (see, for example, Patent Document 1). Silicon-containing materials are alloying materials that alloy with lithium elements and are known to be able to absorb more lithium ions than carbon-based active materials such as graphite. Therefore, by including silicon-containing materials in the negative electrode active material, the capacity per unit area of ​​the negative electrode can be increased.

[0003] International Publication No. 2019 / 065704, International Publication No. 2022 / 190863, International Publication No. 2018 / 123324, International Publication No. 2015 / 098632

[0004] The alloying material in the negative electrode active material expands and contracts during the charging and discharging of the energy storage device. Silicon-containing materials, in particular, expand and contract significantly. The volume change of the alloying material causes voids to form between the particles of the negative electrode active material. When voids form between the particles of the negative electrode active material, the number of particles of the negative electrode active material that become isolated from the conductive paths in the negative electrode composite layer increases. This can degrade the charge-discharge cycle characteristics of the energy storage device. Furthermore, in recent years, there has been a strong demand for shortening the charging time of energy storage devices. Therefore, a reduction in resistance during charging of the energy storage device is desirable.

[0005] This disclosure is made in light of these circumstances, and one of its purposes is to provide technology for improving the performance of energy storage devices.

[0006] One aspect of this disclosure is an electrode mixture used in the negative electrode of an energy storage device. This electrode mixture comprises an electrode active material containing an alloying material that alloys with an alkali metal element, and a cellulose nanofiber having anionic functional groups that interact with the alkali metal element.

[0007] Another aspect of the present disclosure is an energy storage device. This energy storage device comprises an electrode group having a positive electrode and a negative electrode, and a non-aqueous electrolyte. The negative electrode includes the electrode mixture of the above aspect.

[0008] Any combination of the above components, as well as any conversion of the expressions of this disclosure between methods, apparatus, systems, etc., are also valid forms of this disclosure.

[0009] According to this disclosure, it is possible to improve the performance of energy storage devices.

[0010] This is a cross-sectional view of an energy storage device according to an embodiment. This is a cross-sectional view of a part of the negative electrode. This is a schematic diagram of the electrode mixture used in the negative electrode. This figure shows the type of additive, the expansion ratio at full charge, and the resistance in each example and comparative example.

[0011] The present disclosure will be described below with reference to the drawings, based on preferred embodiments. The embodiments are illustrative and not limiting, and not all features or combinations thereof described in the embodiments are necessarily essential to the present disclosure. The same or equivalent components, members, and processes shown in each drawing are denoted by the same reference numerals, and redundant descriptions are omitted where appropriate. The scale and shape of each part shown in each drawing are set for convenience to facilitate explanation and are not to be interpreted restrictively unless otherwise specified. Furthermore, where terms such as "first," "second," etc. are used in this specification or claims, unless otherwise specified, these terms do not indicate any order or importance, but are used to distinguish one configuration from another. In addition, some components that are not important for explaining the embodiments are omitted in each drawing.

[0012] Figure 1 is a cross-sectional view of an energy storage device 1 according to an embodiment. Note that in Figure 1, the distinction between the current collector 34 and the electrode mixture layer 36 (both of which are shown in Figure 2) is omitted. The energy storage device 1 is, for example, a rechargeable secondary battery such as a lithium-ion battery, a nickel-metal hydride battery, or a nickel-cadmium battery, or a capacitor such as an electric double-layer capacitor. An example of an energy storage device 1 includes an electrode group 2, a first insulating plate 4, a second insulating plate 6, an outer casing 8, and a non-aqueous electrolyte EL.

[0013] The electrode group 2 is, for example, cylindrical and has a wound structure in which a strip-shaped positive electrode 10 and a strip-shaped negative electrode 12 are stacked with a strip-shaped separator 14 in between and wound in a spiral shape. The separator 14 is, for example, made of polypropylene, polyethylene, cellulose, etc., and is composed of a microporous film that has ion permeability and insulating properties. A positive electrode lead 16 is attached to the positive electrode 10. A negative electrode lead 18 is attached to the negative electrode 12. Each lead is attached to the current collector 34 of each electrode by welding or the like. The positive electrode lead 16 protrudes from one end of the electrode group 2 in the axial direction. The negative electrode lead 18 protrudes from the other end of the electrode group 2 in the axial direction. In this embodiment, one lead is attached to each electrode, but multiple leads may be attached to each electrode.

[0014] The first insulating plate 4 and the second insulating plate 6 are arranged to sandwich the electrode group 2 in the axial direction of the electrode group 2. The electrode group 2, the first insulating plate 4, and the second insulating plate 6 are housed together with the non-aqueous electrolyte EL in an outer container 8. The outer container 8 is a bottomed cylindrical metal container. The first insulating plate 4 is positioned on the opening side of the outer container 8. The second insulating plate 6 is positioned on the bottom side of the outer container 8. A sealing body 20 is fitted into the opening of the outer container 8. A gasket 22 is provided between the outer container 8 and the sealing body 20. This seals the electrode group 2, the first insulating plate 4, the second insulating plate 6, and the non-aqueous electrolyte EL inside the outer container 8.

[0015] As an example, the sealing body 20 includes a filter 24, a lower valve body 26, an upper valve body 28, an insulating member 30, and a cap 32. Each component constituting the sealing body 20 has, for example, a disc shape or a ring shape. In addition, each component except the insulating member 30 is electrically connected to one another. The filter 24 has an opening 24a and covers the opening of the outer can 8. The lower valve body 26 and the upper valve body 28 cover the opening of the outer can 8 and close the opening 24a. The lower valve body 26 and the upper valve body 28 are connected at their respective central portions, with the insulating member 30 interposed between their respective peripheral portions. When the internal pressure of the outer can 8 rises due to heat generation caused by an internal short circuit or the like, for example, the lower valve body 26 may rupture. As a result, the upper valve body 28 bulges towards the cap 32 and separates from the lower valve body 26. Consequently, the electrical connection between the lower valve body 26 and the upper valve body 28 is interrupted. A cap 32 is placed over the outside of the upper valve body 28.

[0016] The positive electrode lead 16 extends through a through-hole in the first insulating plate 4 towards the sealing body 20. The negative electrode lead 18 extends outside the second insulating plate 6 towards the bottom of the outer can 8. The positive electrode lead 16 is connected to the filter 24 by welding or the like. The cap 32 is electrically connected to the filter 24 to form the positive electrode terminal. The negative electrode lead 18 is connected to the bottom of the outer can 8 by welding or the like. Therefore, the outer can 8 forms the negative electrode terminal. The structure of the energy storage device 1 can be changed as appropriate. For example, the outer can 8 may be rectangular or coin-shaped. The outer can 8 may also be an outer casing made of laminated sheet. The electrode group 2 may be laminated instead of wound.

[0017] A power storage device equipped with an outer casing made of laminate sheets, i.e., a laminate-type power storage device, has, for example, the following configuration. Note that the structure of a laminate-type power storage device is well known, so it is not illustrated. Specifically, the power storage device comprises a group of sheet-like electrodes, a non-aqueous electrolyte, a film outer casing that houses the group of electrodes and the non-aqueous electrolyte, and a reinforcing body disposed on the outer surface of the film outer casing. An adhesive layer is provided between the film outer casing and the reinforcing body. Positive electrode leads and negative electrode leads are connected to the group of electrodes. Parts of each of the positive electrode leads and negative electrode leads are exposed to the outside from the film outer casing and the reinforcing body, and the exposed parts function as positive electrode external terminals and negative electrode external terminals. As an example, a group of sheet-like electrodes has a pair of negative electrodes, a positive electrode placed between them, and a separator interposed between the negative electrode and the positive electrode. The separator is bonded to the negative electrode and the positive electrode. Furthermore, the sheet-like electrode group is not limited to the configuration in which one positive electrode is sandwiched between two negative electrodes as described above; it may also be configured in which one negative electrode is sandwiched between two positive electrodes. In addition, multiple layers of the configuration in which one positive electrode is sandwiched between two negative electrodes may be stacked, or multiple layers of the configuration in which one negative electrode is sandwiched between two positive electrodes may be stacked. Moreover, a separator may be sandwiched between one positive electrode and one negative electrode.

[0018] Next, the negative electrode 12, positive electrode 10, and non-aqueous electrolyte EL will be described in detail. Figure 2 is a cross-sectional view of a part of the negative electrode 12. Figure 3 is a schematic diagram of the electrode mixture 38 used in the negative electrode 12. The negative electrode 12 has a current collector 34 and an electrode mixture layer 36 (negative electrode mixture layer). The current collector 34 is made of a strip of metal foil. In the case of a typical lithium-ion secondary battery, the current collector 34 is made of copper foil or the like. The electrode mixture layer 36 is provided on the current collector 34. As an example, the electrode mixture layer 36 is laminated on both main surfaces of the current collector 34. Alternatively, the electrode mixture layer 36 may be laminated on only one main surface of the current collector 34.

[0019] The electrode mixture layer 36 is formed by laminating the electrode mixture 38 (negative electrode mixture) onto the current collector 34. The electrode mixture layer 36 may be provided on the current collector 34 by pressing an electrode mixture sheet, which is formed from a dry electrode mixture into a sheet, onto the surface of the current collector 34, or by coating, drying, and rolling a wet electrode mixture onto the current collector 34. For example, the dry electrode mixture has a solvent content of less than 5% by mass of the total mass of the electrode mixture. For example, the wet electrode mixture has a solvent content of 5% by mass or more of the total mass of the electrode mixture.

[0020] The electrode mixture 38 used in the negative electrode 12 includes an electrode active material 40 (negative electrode active material) and cellulose nanofibers 42. The electrode active material 40 includes an alloying material that alloys with at least one alkali metal element. The alkali metal element is one or more elements selected from the group consisting of lithium (Li), sodium (Na), and potassium (K), and acts as a carrier of electrical conductivity. The alloying material includes an element that alloys with the alkali metal element. Examples of such elements include aluminum (Al), gallium (Ga), indium (In), silicon (Si), germanium (Ge), tin (Sn), lead (Pb), arsenic (As), antimony (Sb), bismuth (Bi), phosphorus (P), etc. The alloying material may be an alloy, compound, composite material, etc. containing the element. Furthermore, the alloying material may contain only one of the above elements, or it may contain two or more of the above elements.

[0021] From the viewpoint of increasing capacity, the alloying material may contain Si. In this case, the alloying material may also contain silicon-containing material. The silicon-containing material can be any material that contains Si, and examples include Si alloys, Si compounds, and Si-containing composite materials. One type of silicon-containing material may be used alone, or two or more types may be used in combination.

[0022] The silicon-containing material may be a composite material including an ion conductive phase and a silicon phase dispersed in the ion conductive phase. The ion conductive phase includes, for example, at least one selected from the group consisting of a silicate phase, a carbon phase, a silicide phase, and a silicon oxide phase. The silicide phase is a phase of a compound composed of Si and an element more electropositive than Si, and as an example, NiSi, Mg ２ Si, TiSi ２ etc. may be mentioned. The silicon phase is formed by Si being dispersed in fine particle form. The ion conductive phase is a continuous phase composed of an aggregate of finer particles than the silicon phase. The ratio of the mass of the silicon phase to the total mass of the silicon-containing material may be 30% by mass or more and 60% by mass or less.

[0023] The above composite material may have a conductive layer covering the surface of the ion conductive phase. The conductive layer is composed of a material having higher conductivity than the ion conductive layer and forms a good conductive path in the electrode binder layer 36. The conductive layer is, for example, a carbon film composed of a conductive carbon material. Examples of the conductive carbon material include carbon black, acetylene black, ketjen black, graphite, amorphous carbon with low crystallinity or non-crystalline carbon. The thickness of the conductive layer may be 1 nm or more and 200 nm or less, or 5 nm or more and 100 nm or less in consideration of ensuring conductivity and the diffusibility of Li ions into the particles. The thickness of the conductive layer can be measured by observing the cross section of the composite material using a scanning electron microscope (SEM) or a transmission electron microscope (TEM).

[0024] The ion conductive phase may include at least one element selected from the group consisting of Group 1 elements and Group 2 elements of the periodic table. The ion conductive layer may be a silicon oxide phase doped with Li. Also, the ion conductive phase may include at least one element selected from the group consisting of boron (B), Al, zirconium (Zr), niobium (Nb), tantalum (Ta), vanadium (V), yttrium (Y), titanium (Ti), P, Bi, zinc (Zn), Sn, Pb, Sb, cobalt (Co), erbium (Er), fluorine (F), tungsten (W), and lanthanoids.

[0025] An example of the composite material containing Si has a sea-island structure in which fine Si is dispersed substantially uniformly in an amorphous silicon oxide phase, and as a whole, has a general formula SiO ｘ (0 < x ≤ 2), which is a composite particle. The main component of the silicon oxide may be silicon dioxide. Further, the silicon oxide phase may be doped with Li. The content ratio (x) of oxygen to Si may be, for example, 0.5 ≤ x < 2.0, or 0.8 ≤ x ≤ 1.5.

[0026] Another example of the composite material containing Si is a composite particle having a sea-island structure in which fine Si is dispersed substantially uniformly in an amorphous silicate phase. The silicate phase may be a lithium silicate phase containing Li. The lithium silicate phase is, for example, a phase of a composite oxide represented by the general formula Li ２ｚ SiO （２＋ｚ） (0 < z < 2). The lithium silicate phase may not contain Li ４ SiO ４ (Z = 2). Li ４ SiO ４ is an unstable compound and reacts with water to show alkalinity. Therefore, it can cause deterioration of Si and lead to a decrease in charge-discharge capacity. From the viewpoints of stability, productivity, Li ion conductivity, etc., the lithium silicate phase may mainly contain Li ２ SiO ３ (Z = 1) or Li ２ Si ２ O ５ (Z = 1 / 2). Note that the lithium silicate phase may contain Li ４ SiO ４ .

[0027] Another example of the composite material containing Si is a composite particle having a sea-island structure in which fine Si is dispersed substantially uniformly in a carbon phase. The carbon phase may be an amorphous carbon phase. The carbon phase may contain a crystalline phase component. In this case, the carbon phase may contain more amorphous phase components than crystalline phase components. The amorphous carbon phase is composed of, for example, a carbon material having an average interplanar spacing of the (002) plane measured by X-ray diffraction method exceeding 0.34 nm. Note that the composite material containing a carbon phase may or may not have a conductive layer different from the carbon phase.

[0028] The ratio of the mass of silicon element in the silicon-containing material to the total mass of the electrode mixture layer 36 may be 2% by mass or more, may be 5% by mass or more, may be 10% by mass or more, or may be 20% by mass or more. According to these, the capacity of the power storage device 1 can be effectively increased. Further, the ratio may be 50% by mass or less, may be 45% by mass or less, or may be 40% by mass or less. According to these, it is possible to more effectively suppress the volume change of the electrode mixture layer 36 during charge and discharge of the power storage device 1 from becoming excessively large, and it is possible to more effectively suppress the diffusibility of the non-aqueous electrolyte EL in the electrode group 2 from decreasing.

[0029] Examples of the alloying material other than the silicon-containing material include simple substances of elements selected from the group consisting of Al, Ga, In, Ge, Sn, Pb, As, Sb, Bi, and P described above, alloys containing at least one of these elements, and the like. Further, the alloying material may be a simple substance of an element selected from the group consisting of Ga, Ge, Sn, Pb, Sb, Bi, and P, or an alloy containing at least one of these elements. Note that the electrode active material 40 may contain a carbon material such as graphite. Examples of the carbon material include natural graphite such as flaky graphite, massive graphite, and earthy graphite; artificial graphite such as massive artificial graphite and graphitized mesophase carbon microbeads; hard carbon; mixtures thereof; and the like.

[0030] Cellulosic nanofibers 42 are chemically modified cellulose nanofibers (CNF) having an anionic functional group R that interacts with alkali metal elements. In this disclosure, "interaction" means electrical attraction with alkali metal elements. In cellulose nanofibers 42, one or more hydroxyl groups in multiple glucose molecules of the main chain cellulose are substituted with anionic functional groups R. As an example, in cellulose nanofibers 42, one hydroxyl group in each of the total glucose molecules is substituted with an anionic functional group R. The substitution ratio of anionic functional groups R in cellulose nanofibers 42 can be appropriately selected. CNF can be obtained, for example, by defibrating naturally derived cellulose from wood, plants, etc., to a size on the nanometer order. As defibration methods, for example, known mechanical grinding methods or known chemical fragmentation methods can be used.

[0031] Cellulose nanofibers 42 can be obtained by introducing an anionic functional group R to CNF by subjecting it to a known chemical modification treatment at the cellulose fiber raw material stage, during the defibration process, or after the defibration process. As an example, the anionic functional group R includes at least one selected from the group consisting of a carboxyl group, a phosphate group, a phosphite group, and a sulfonic acid group. In this disclosure, "carboxyl group" also includes carboxyl groups introduced into the main chain via an alkyl group such as a methyl group, i.e., a carboxymethyl group. Known compounds that react with the hydroxyl groups of cellulose can be used as the CNF modifier.

[0032] The cellulose nanofibers 42 are regularly arranged on the surface of the electrode active material 40 due to the crystallinity of the cellulose skeleton. This suppresses the expansion of the alloying material that occurs when the alloying material in the electrode active material 40 alloys with alkali metal elements. Therefore, the charge-discharge cycle characteristics of the energy storage device 1 can be improved. In particular, alloying materials containing Si tend to expand significantly, so a more pronounced improvement in charge-discharge cycle characteristics can be obtained when the alloying material contains Si.

[0033] Furthermore, the regular arrangement of the cellulose skeleton leads to a regular arrangement of the anionic functional groups R of the cellulose nanofibers 42. This alignment of the anionic functional groups R enables the conduction of alkali metal elements (alkali metal ions) via the Grotus mechanism. In other words, a high-speed conduction path for alkali metal elements is formed. Therefore, the resistance of the energy storage device 1 can be reduced. In Figure 3, Li is shown as an example of an alkali metal element. Also, the illustration of the anionic functional groups R for some of the cellulose nanofibers 42 is omitted.

[0034] The diameter of the cellulose nanofiber 42 is, for example, 1 nm to 1,000 nm, and may be 10 nm to 300 nm. By setting the diameter to 10 nm to 300 nm, the specific surface area and cross-sectional area of ​​the cellulose nanofiber 42 can be brought within a more suitable range, thereby more effectively improving the conductivity of alkali metal ions and suppressing the swelling of the electrode active material 40. The length of the cellulose nanofiber 42 is, for example, 100 nm to 100,000 nm. The ratio of the mass of the cellulose nanofiber 42 to the total mass of the electrode mixture 38 is, for example, 0.01% by mass to 10% by mass.

[0035] The anionic functional group R may be alkali metal chlorinated. That is, an alkali metal ion may be bonded to the anionic functional group R as a counterion. Examples of alkali metal ions used for alkali metal chlorination of the anionic functional group R include one or more selected from the group consisting of Li ions, Na ions, and K ions. When a proton is used as the counterion of the anionic functional group R, there is a risk of hydrogen generation in the energy storage device 1. In contrast, by using an alkali metal ion as the counterion of the anionic functional group R, the generation of such hydrogen can be suppressed. Alkali metal chlorination of the anionic functional group R can be carried out by known methods. Furthermore, alkali metal chlorination may be carried out before mixing the electrode active material 40 and the cellulose nanofiber 42, or after mixing.

[0036] The electrode mixture 38 may contain a binder. Examples of binders include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), styrene-butadiene rubber (SBR), nitrile-butadiene rubber (NBR), carboxymethylcellulose (CMC) or its salts, polyacrylic acid (PAA) or its salts (PAA-Na, PAA-K, partially neutralized salts, etc.), and polyvinyl alcohol (PVA). One type of binder may be used alone, or two or more types may be used in combination.

[0037] The electrode mixture 38 has a volumetric energy density of 600 mAh / cm³. ３ It may be even better. Also, the electrode mixture 38 has a volume energy density of 2500 mAh / cm³. ３ It is acceptable to be less than [a certain value].

[0038] The positive electrode 10, like the negative electrode 12, has a structure in which an electrode mixture layer (positive electrode mixture layer) is laminated on a current collector. In the case of a typical lithium-ion secondary battery, the current collector is made of aluminum foil or the like. The electrode mixture layer of the positive electrode 10 is formed by laminating an electrode mixture (positive electrode mixture) containing, for example, an electrode active material (positive electrode active material), a conductive material, and a binder onto a current collector.

[0039] The electrode active material mainly consists of a lithium-containing metal composite oxide. Examples of metal elements contained in the lithium-containing metal composite oxide include nickel (Ni), Co, manganese (Mn), Al, B, magnesium (Mg), Ti, V, chromium (Cr), iron (Fe), copper (Cu), Zn, Ga, strontium (Sr), Zr, Nb, In, Sn, Ta, W, calcium (Ca), Sb, Pb, Bi, Ge, etc. The lithium-containing metal composite oxide may also be a composite oxide containing at least one of the elements Ni, Co, Mn, and Al. In the case of a typical lithium-ion secondary battery, examples of electrode active materials for the positive electrode 10 include lithium nickel cobalt manganese composite oxide (NCM), lithium nickel cobalt aluminum composite oxide (NCA), lithium iron phosphate (LFP), etc.

[0040] Examples of conductive materials include carbon materials such as carbon black, acetylene black, Ketjenblack, carbon nanotubes, and graphite. Examples of binders include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF); polyacrylonitrile (PAN); polyimide; acrylic resin; and polyolefin. These resins may also be used in combination with carboxymethylcellulose (CMC) or its salts, polyethylene oxide (PEO), etc.

[0041] Non-aqueous electrolytes (EL) are ionic conductive and, as an example, include a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent.

[0042] Examples of non-aqueous solvents include esters, ethers, nitriles, amides, and mixed solvents containing two or more of these. For example, ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and methyl propionate (MP) can be used as non-aqueous solvents. Non-aqueous solvents may also contain halogen-substituted compounds in which at least some of the hydrogen atoms in these solvents are replaced with halogen atoms such as fluorine. Examples of halogen-substituted compounds include fluorinated cyclic carbonate esters such as fluoroethylene carbonate (FEC); fluorinated linear carbonate esters; and fluorinated linear carboxylic acid esters such as methyl fluoropropionate (FMP). The above-mentioned non-aqueous solvents may be used individually or in combination of two or more.

[0043] The electrolyte salt contains an alkali metal element that is alloyed with the alloying material of the electrode active material 40. As described above, the alkali metal element is one or more selected from the group consisting of Li, Na, and K, for example. When the alkali metal element is Li, the electrolyte salt is LiBF ４ LiPF ６ Examples of lithium salts include the following. When the alkali metal element is Na, the electrolyte salt is NaBF. ４ NaPF ６Examples of sodium salts include bis(trifluoromethanesulfonyl)imide sodium and bis(fluorosulfonyl)imide sodium. When the alkali metal element is K, the electrolyte salt is KBF. ４ KPF ６ Examples of potassium salts include bis(trifluoromethanesulfonyl)imide potassium and bis(fluorosulfonyl)imide potassium. The concentration of the electrolyte salt in the non-aqueous electrolyte EL is, for example, 1 mol / L or higher.

[0044] The non-aqueous electrolyte (EL) is not limited to a liquid electrolyte, i.e., a non-aqueous electrolyte solution, but may also be a solid electrolyte composed of a gel-like polymer or the like. Examples of solid electrolytes include solid or gel-like polymer electrolytes and inorganic solid electrolytes. A polymer electrolyte includes, for example, an electrolyte salt and a matrix polymer, or a non-aqueous solvent, an electrolyte salt and a matrix polymer. An example of a matrix polymer is a polymer material that absorbs a non-aqueous solvent and gels. Examples of such polymer materials include fluororesins, acrylic resins, and polyether resins. Examples of inorganic solid electrolytes include materials known for use in all-solid-state lithium-ion secondary batteries, such as oxide-based solid electrolytes, sulfide-based solid electrolytes, and halide-based solid electrolytes.

[0045] The embodiments of this disclosure have been described in detail above. The embodiments described above are merely examples of how to implement this disclosure. The content of the embodiments does not limit the technical scope of this disclosure, and many design changes, such as changes, additions, and deletions of components, are possible as long as they do not depart from the spirit of the invention as defined in the claims. A new embodiment with design changes will have the combined effects of both the embodiment and the variation. In the embodiments described above, the content in which such design changes are possible is emphasized with notations such as "of this embodiment" or "in this embodiment," but design changes are also permitted even if there are no such notations. Furthermore, any combination of components included in each embodiment is also valid as an embodiment of this disclosure. The hatching applied to the cross-section in the drawings does not limit the material of the object to which the hatching is applied.

[0046] The embodiments may be specified by the following items: [Item 1] An electrode mixture (38) used in the negative electrode (12) of an energy storage device (1), comprising: an electrode active material (40) containing an alloying material that alloys with an alkali metal element; and a cellulose nanofiber (42) having an anionic functional group (R) that interacts with an alkali metal element. [Item 2] The electrode mixture (38) of Item 1, wherein the alloying material contains silicon. [Item 3] The electrode mixture (38) of Item 1 or Item 2, wherein the anionic functional group (R) contains at least one selected from the group consisting of a carboxyl group, a phosphate group, a phosphite group, and a sulfonic acid group. [Item 4] The electrode mixture (38) of Item 1 or Item 2, wherein the anionic functional group (R) is alkali metal chlorided. [Item 5] The electrode mixture (38) of Item 2, wherein the alloying material comprises a silicon-containing material comprising an ionic conductive phase and a silicon phase dispersed in the ionic conductive phase. [Item 6] The electrode mixture (38) of Item 5, wherein the ionic conductive phase comprises at least one selected from the group consisting of a silicate phase, a carbon phase, a silicide phase, and a silicon oxide phase. [Item 7] The electrode mixture (38) of Item 5 or Item 6, wherein the ratio of the mass of the silicon phase to the total mass of the silicon-containing material is 30% by mass or more and 60% by mass or less. [Item 8] An energy storage device (1) comprising an electrode group (2) having a positive electrode (10) and a negative electrode (12), and a non-aqueous electrolyte (EL), wherein the negative electrode (12) comprises any electrode mixture (38) of Items 1 to 7.

[0047] The following describes embodiments of the present invention, but these embodiments are merely illustrative examples for suitably illustrating the present invention and do not limit the present invention in any way.

[0048] (Example 1) (Positive electrode) LiNi as the positive electrode active material ０．８ Co ０．１ Mn ０．１ O ２A lithium transition metal oxide represented by (NCM) was used. A positive electrode mixture layer slurry was prepared by mixing 100 parts by mass of positive electrode active material, 0.4 parts by mass of carbon nanotubes (CNT), and 0.6 parts by mass of polyvinylidene fluoride (PVdF). The positive electrode mixture layer slurry was applied to a positive electrode current collector made of aluminum foil, and the coating was dried. After rolling the coating using a roller, the electrode was cut so that the positive electrode mixture layer was 20 mm square, and a positive electrode was fabricated in which the positive electrode mixture layer was formed on one side of the positive electrode current collector.

[0049] (Negative Electrode) As negative electrode active materials, composite particles (Si-C) as an alloying material or silicon-containing material containing a carbon phase and a Si phase dispersed in the carbon phase, and graphite (Gr) were used. Si-C, Gr, carboxymethylated CNF (TFo10002: manufactured by Sugino Machine Co., Ltd.) as a cellulose nanofiber, polyacrylic acid (PAA), and a dispersion of styrene-butadiene copolymer (SBR) were mixed in a solid content mass ratio of 70:30:1:1:5, and an appropriate amount of water was added to prepare a negative electrode mixture layer slurry. Carboxymethylated CNF is an example of CNF into which a carboxyl group has been introduced.

[0050] A negative electrode mixture slurry was applied to a negative electrode current collector made of copper foil, and the coating was dried. Then, the coating was rolled using a roller, and the electrode was cut so that the negative electrode mixture layer was 20 mm square, thereby creating a negative electrode with the negative electrode mixture layer formed on one side of the negative electrode current collector.

[0051] (Non-aqueous electrolyte) A non-aqueous electrolyte was prepared by dissolving LiPF6 as a lithium salt at a concentration of 1.4 mol / L in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 7:3.

[0052] (Test Cell) Tabs were attached to both the negative and positive electrodes, and an electrode group was fabricated by stacking the negative and positive electrodes with a separator in between. A three-layer polypropylene separator was used. The fabricated electrode group was inserted into an outer casing made of aluminum laminate sheet, and the opening of the outer casing was sealed to create a test cell (laminate cell).

[0053] (Example 2) A test cell was prepared in the same manner as in Example 1, except that sulfonated CNF (sulfonic acid group content 4 mmol / g) was used as the cellulose nanofiber.

[0054] (Example 3) A test cell was prepared in the same manner as in Example 1, except that phosphorylated CNF (phosphorous group content 4 mmol / g) was used as the cellulose nanofiber.

[0055] (Example 4) A test cell was prepared in the same manner as in Example 1, except that phosphorylated CNF (phosphate group content 8 mmol / g) was used as the cellulose nanofiber.

[0056] (Comparative Example 1) A test cell was prepared in the same manner as in Example 1, except that carboxymethylcellulose was used instead of cellulose nanofibers.

[0057] (Comparative Example 2) A test cell was prepared in the same manner as in Example 1, except that phosphorylated cellulose was used instead of cellulose nanofibers.

[0058] (Comparative Example 3) A test cell was prepared in the same manner as in Example 1, except that unmodified CNF was used instead of cellulose nanofibers.

[0059] (Comparative Example 4) A test cell was prepared in the same manner as in Example 1, except that chitosan nanofibers were used instead of cellulose nanofibers.

[0060] (Comparative Example 5) A test cell was prepared in the same manner as in Example 1, except that chitin nanofibers were used instead of cellulose nanofibers.

[0061] (Evaluation of expansion suppression) For each example and each comparative example test cell, the expansion rate of the negative electrode when fully charged was calculated by the following method. That is, first, before assembling the test cell, the thickness T(0) of the negative electrode mixture layer was measured at nine predetermined locations on each negative electrode. １ ~T(0) ９ The following was measured: Thickness T(0) ｎ (where n is an integer from 1 to 9) is the initial thickness of the negative electrode mixture layer.

[0062] Furthermore, each test cell was charged with a constant current of 0.1C in a constant temperature bath at 25°C until the cell voltage reached 0.005V. 1C is the current value that discharges the design capacity in one hour. After that, it was left to rest for 20 minutes. Next, it was charged with a constant current of 0.01C until the cell voltage reached 0.005V. After that, it was left to rest for 20 minutes. Furthermore, it was charged with a constant current of 0.001C until the cell voltage reached 0.005V. After the above charging procedures, each test cell was disassembled and the negative electrode was removed. For each negative electrode, the thickness T(0) １ ~T(0) ９ The thickness T(C) of the negative electrode mixture layer in a fully charged state was measured at the same nine locations as the measurement points. １ ~T(C) ９ The following was measured. In addition, the expansion rate X at full charge at each measurement location was calculated based on the following formula. １ ~X ９ We calculated X １ ~X ９ The expansion rate of the negative electrode at full charge was obtained by taking the arithmetic mean of the following: Expansion rate at full charge Xn [%] = Thickness T (C) ｎ / Thickness T (0) ｎ (n is an integer from 1 to 9)

[0063] For each test cell in Examples 1-4 and Comparative Examples 2-5, the ratio of the full-charge expansion rate to the full-charge expansion rate of the test cell in Comparative Example 1 (hereinafter referred to as the full-charge expansion rate ratio) was calculated. The full-charge expansion rate ratio can be used as an indicator of the effect of additives such as cellulose nanofibers on suppressing the expansion of the anode mixture layer. A smaller full-charge expansion rate ratio means that the expansion of the anode mixture layer can be suppressed. The energy density of the anode in each test cell was calculated to be 1000 mAh / L from the charging capacity of the anode and the volume of the anode mixture before charging.

[0064] (Resistance Evaluation) The resistance of the test cells in each example and comparative example was measured. Specifically, each test cell was charged with a constant current of 0.1C in a constant temperature bath at 25°C until the cell voltage reached 0.005V. After that, it was left to rest for 20 minutes. Next, a constant current charge was performed with a current of 0.01C until the cell voltage reached 0.005V. After that, it was left to rest for 20 minutes. Furthermore, a constant current charge was performed with a current of 0.001C until the cell voltage reached 0.005V. After that, it was left to rest for 2 hours. Subsequently, a constant current discharge was performed with a current of 0.1C until the cell voltage reached 1V. After that, it was left to rest for 20 minutes. Next, a constant current discharge was performed with a current of 0.01C until the cell voltage reached 1V. After that, it was left to rest for 20 minutes. Furthermore, a constant current discharge was performed with a current of 0.001C until the cell voltage reached 1V.

[0065] Next, constant current charging was performed at a current of 0.1C until the cell voltage reached 0.005V. Afterward, it was allowed to rest for 20 minutes. Then, constant current charging was performed at a current of 0.01C until the cell voltage reached 0.005V. Afterward, it was allowed to rest for 20 minutes. Furthermore, constant current charging was performed at a current of 0.001C until the cell voltage reached 0.005V. Afterward, it was allowed to rest for 2 hours. Next, constant current discharge was performed at a current of 0.2C so that the open-circuit voltage 2 hours after the end of discharge was 0.1V. Afterward, constant current discharge was performed at a current of 0.5C for 30 seconds. The resistance value R at an open-circuit voltage of 0.1V was calculated from the difference V between the open-circuit voltage and the voltage after 30 seconds of discharge, and the current value I of the constant current discharge, based on the following formula. The resistance value R was also calculated similarly for cases where the open-circuit voltage was 0.3V and 0.5V. Resistance value R [Ω] = V / I

[0066] Figure 4 shows the type of additive, the expansion ratio at full charge, and the resistance for each example and comparative example. From the expansion ratio at full charge for Examples 1-4 and the expansion ratio at full charge for Comparative Examples 1-5, it was confirmed that adding cellulose nanofibers to the negative electrode mixture suppresses the expansion of the negative electrode mixture layer compared to adding additives other than cellulose nanofibers, such as non-nanofiber cellulose, unmodified CNF, or non-cellulose nanofibers. Furthermore, from the resistance at each voltage for Examples 1-4 and the resistance at each voltage for Comparative Examples 1-5, it was confirmed that adding cellulose nanofibers to the negative electrode mixture reduces the resistance of the energy storage device compared to adding additives other than cellulose nanofibers.

[0067] This disclosure can be used in electrode mixtures and energy storage devices.

[0068] 1. Energy storage device, 2. Electrode group, 10. Positive electrode, 12. Negative electrode, 38. Electrode mixture, 40. Electrode active material, 42. Cellulose nanofiber, EL: Non-aqueous electrolyte, R: Anionic functional group.

Claims

1. An electrode mixture used in the negative electrode of an energy storage device, comprising: an electrode active material containing an alloying material that alloys with an alkali metal element; and a cellulose nanofiber having an anionic functional group that interacts with the alkali metal element.

2. The electrode mixture according to claim 1, wherein the alloying material contains silicon.

3. The electrode mixture according to claim 1 or 2, wherein the anionic functional group comprises at least one selected from the group consisting of a carboxyl group, a phosphate group, a phosphite group, and a sulfonic acid group.

4. The electrode mixture according to claim 1 or 2, wherein the anionic functional group is alkali metal chloride.

5. The electrode mixture according to claim 2, wherein the alloying material comprises a silicon-containing material comprising an ionic conductive phase and a silicon phase dispersed in the ionic conductive phase.

6. The electrode mixture according to claim 5, wherein the ion conducting phase comprises at least one selected from the group consisting of a silicate phase, a carbon phase, a silicide phase, and a silicon oxide phase.

7. The electrode mixture according to claim 5 or 6, wherein the ratio of the mass of the silicon phase to the total mass of the silicon-containing material is 30% by mass or more and 60% by mass or less.

8. An energy storage device comprising an electrode group having a positive electrode and a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode contains the electrode mixture described in claim 1 or 2.

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