Secondary battery

Abstract

The present invention improves cycle characteristics. A secondary battery according to the present invention comprises a positive electrode, a negative electrode, a separator that is between a principal surface of the positive electrode and a principal surface of the negative electrode, and an electrolyte solution. The positive electrode includes a positive electrode active material layer. The positive electrode active material layer includes a positive electrode active material and a first polymer compound. The negative electrode includes a negative electrode active material layer. The negative electrode active material layer includes a negative electrode active material and a second polymer compound. The negative electrode active material includes a first negative electrode active material that includes silicon. The second polymer compound includes an NVA polymer that includes N-vinylacetamide as a monomer. The separator includes a base material, a positive electrode–side coating layer that is on the positive electrode side of the base material, and a negative electrode–side coating layer that is on the negative electrode side of the base material. The positive electrode–side coating layer includes the first polymer compound. The negative electrode–side coating layer includes the NVA polymer.

Description

secondary battery

[0001] The present invention relates to a secondary battery.

[0002] Patent Document 1 discloses a lithium ion battery containing, as a binder, a modified polymer containing, as a monomer unit, a structure derived from an N-alkyl unsaturated carboxylic acid amide.

[0003] International Publication No. 2021 / 006198

[0004] However, in the secondary battery disclosed in Patent Document 1, there is a possibility that the cycle characteristics may be degraded due to a decrease in adhesion between the separator and the negative electrode.

[0005] The present invention has been made in view of the above problems, and an object of the present invention is to improve cycle characteristics.

[0006] A secondary battery according to one aspect of the present invention includes a positive electrode, a negative electrode, a separator between a main surface of the positive electrode and a main surface of the negative electrode, and an electrolyte; the positive electrode has a positive electrode active material layer, and the positive electrode active material layer includes a positive electrode active material and a first polymer compound; the negative electrode has a negative electrode active material layer, and the negative electrode active material layer includes a negative electrode active material and a second polymer compound; the negative electrode active material includes a first negative electrode active material containing silicon; and the second polymer compound includes an NVA polymer containing N-vinylacetamide as a monomer; and the separator has a substrate, a positive electrode-side coating layer on the positive electrode side of the substrate, and an anode-side coating layer on the anode side of the substrate; the positive electrode-side coating layer includes the first polymer compound, and the anode-side coating layer includes the NVA polymer.

[0007] According to the present invention, cycle characteristics can be improved.

[0008] Fig. 1 is a cross-sectional view showing an example of a secondary battery according to a first embodiment. Fig. 2 is an enlarged cross-sectional view showing a part of a cross section of the electrode body according to Fig. 1. Fig. 3 is a cutaway view showing another example of the secondary battery according to the first embodiment. Fig. 4 is a schematic view of a cross section taken along line IV-IV in Fig. 3.

[0009] The following describes embodiments of the present invention, but the present invention is not limited to these embodiments.

[0010] (Secondary Battery) Fig. 1 is a cross-sectional view showing an example of a secondary battery according to the first embodiment. The secondary battery 1 shown in Fig. 1 is a laminated lithium-ion secondary battery. As shown in Fig. 1, the secondary battery 1 includes a battery element 20, an exterior member 30, and an adhesive 32.

[0011] The battery element 20 is provided inside an exterior member 30. As shown in FIG. 1 , the battery element 20 includes an electrode body 200, a positive electrode lead 21, and a negative electrode lead 22. The positive electrode lead 21 is a terminal drawn from a positive electrode 210 (described later) to the outside of the exterior member 30. That is, the positive electrode lead 21 is a terminal that serves as a positive electrode of the secondary battery 1. In FIG. 1 , the positive electrode lead 21 is provided on an end surface of the electrode body 200. The negative electrode lead 22 is a terminal drawn from the inside of a negative electrode 220 (described later) to the outside of the exterior member 30. That is, the negative electrode lead 22 is a terminal that serves as a negative electrode of the secondary battery 1. In FIG. 1 , the negative electrode lead 22 is provided on an end surface of the electrode body 200. Details of the electrode body 200 will be described later.

[0012] The exterior member 30 is a case in which the battery element 20 is housed. The exterior member 30 includes two exterior sheets 30a and 30b. The exterior sheets 30a and 30b each include an insulating layer, a metal layer, and an outermost layer. In the example of FIG. 1 , the exterior sheet 30a has a recess 31. As a result, the battery element 20 is housed in the exterior member 30 by housing the battery element 20 in the recess 31 and bonding the peripheral edges of the exterior sheets 30a and 30b.

[0013] The exterior sheets 30a, 30b are constructed by laminating an insulating layer, a metal layer, and an outermost layer in this order from the inside, i.e., the side where the battery element 20 is provided, and then bonding them together by lamination or the like. The insulating layers of the exterior sheets 30a, 30b are made of a resin such as polyethylene, polypropylene, modified polyethylene, modified polypropylene, or a polyolefin resin containing ethylene or propylene as a monomer. This allows the exterior sheets 30a, 30b to reduce the moisture permeability of the secondary battery 1 and improve its airtightness. The metal layers of the exterior sheets 30a, 30b are metal plate or foil materials such as aluminum, stainless steel, nickel, and iron. The outermost layer may be made of any material, but is preferably made of the same resin as the insulating layer or a material with high resistance to tearing, punctures, etc., such as nylon.

[0014] The adhesive 32 is a member for making the exterior member 30 airtight. The adhesive 32 is provided between the exterior member 30 and the positive electrode lead 21 and the negative electrode lead 22. The material of the adhesive 32 preferably has adhesion to the positive electrode lead 21 and the negative electrode lead 22. For example, when the positive electrode lead 21 and the negative electrode lead 22 are made of a metal material, the adhesive 32 is made of a polyolefin resin such as polyethylene, polypropylene, modified polyethylene, or modified polypropylene. This allows the adhesive 32 to seal the gap between the exterior member 30 and the positive electrode lead 21 and the negative electrode lead 22, thereby making the interior of the exterior member 30 airtight.

[0015] Fig. 2 is an enlarged cross-sectional view showing a portion of the cross section of the electrode assembly in Fig. 1. More specifically, Fig. 2 is a cross-sectional view showing a portion of one layer of a positive electrode 210 and one layer of a negative electrode 220 of the electrode assembly 200. As shown in Fig. 2, the electrode assembly 200 includes a positive electrode 210, a negative electrode 220, and a separator 230. In the secondary battery 1, the electrode assembly 200 has a structure in which the positive electrode 210 and the negative electrode 220 are stacked in the thickness direction with the separator 230 interposed therebetween. The positive electrode 210 and the negative electrode 220 included in the electrode assembly 200 are layered members for the charge / discharge reaction of the secondary battery according to the first embodiment.

[0016] The positive electrode 210 includes a positive electrode current collector 211 and a positive electrode active material layer 212. In the positive electrode 210, the positive electrode current collector 211 is laminated between the positive electrode active material layers 212.

[0017] The positive electrode current collector 211 is a conductive layer, and may be made of, for example, aluminum foil, stainless steel foil, etc. In the example of Fig. 1 , the positive electrode current collector 211 has a rectangular shape in plan view in the thickness direction, with protrusions on the positive electrode lead 21 side. The protrusions of the positive electrode current collector 211 are connected to the positive electrode lead 21.

[0018] The positive electrode active material layer 212 is a layer containing a positive electrode active material. The positive electrode active material layer 212 includes a positive electrode active material, a first polymer compound, and a conductive agent. The positive electrode active material layer 212 is not limited to the materials listed above, and may further include, for example, a dispersant.

[0019] The positive electrode active material is preferably a lithium-containing compound such as a lithium-containing composite oxide or a lithium-containing phosphate compound. The lithium-containing composite oxide is an oxide containing lithium and one or more elements other than lithium as constituent elements. The lithium-containing composite oxide has, for example, a layered rock salt type or a spinel type crystal structure. The lithium-containing phosphate compound is a phosphate compound containing lithium and one or more elements other than lithium as constituent elements. The lithium-containing phosphate compound has, for example, an olivine type crystal structure. A specific example of the lithium-containing composite oxide is LiNiO ２ , LiCoO ２ , LiCo ０．９８ Al ０．０１ Mg ０．０１ O ２ , LiNi ０．５ Co ０．２ Mn ０．３ O ２ , LiNi ０．８ Co ０．１５ Al ０．０５ O ２ , LiNi ０．３３ Co ０．３３ Mn ０．３３ O ２ , Li １．２ Mn ０．５２ Co ０．１７５ Ni ０．１ O２ , Li １．１５ (Mn ０．６５ Ni ０．２２ Co ０．１３ ) O ２ , LiMn ２ O ４ Specific examples of lithium-containing phosphate compounds include LiFePO ４ , LiMnPO ４ , LiFe ０．５ Mn ０．５ P.O. ４ , LiFe ０．３ Mn ０．７ P.O. ４ And so on.

[0020] The first polymer compound contained in the positive electrode active material layer 212 is a binder. The first polymer compound may be any material, and may include, for example, one or more of synthetic rubber and polymer compounds. Examples of synthetic rubber include styrene butadiene rubber, fluorine-based rubber, and ethylene propylene diene. Examples of polymer compounds include polyvinylidene fluoride (PVdF) and polyimide.

[0021] The conductive agent contained in the positive electrode active material layer 212 may be any material, and may include, for example, carbon. Examples of carbon include graphite, carbon black, acetylene black, and ketjen black. However, the conductive agent contained in the positive electrode active material layer 212 is not limited to these materials as long as it is a conductive material, and may also be a metal material, a conductive polymer, or the like.

[0022] The negative electrode 220 includes a negative electrode current collector 221 and a negative electrode active material layer 222. In the negative electrode 220, the negative electrode current collector 221 is laminated between the negative electrode active material layers 222.

[0023] The negative electrode current collector 221 is a conductor, and for example, copper foil or the like can be used. In the example of Fig. 1 , the shape of the negative electrode current collector 221 is a rectangular sheet having protrusions on the negative electrode lead 22 side when viewed in a plan view in the thickness direction. The protrusions of the negative electrode current collector 221 are connected to the negative electrode lead 22.

[0024] The negative electrode active material layer 222 is a layer containing a negative electrode active material. The negative electrode active material layer 222 contains a negative electrode active material and a second polymer compound.

[0025] The negative electrode active material preferably includes a first negative electrode active material and further includes a second negative electrode active material. Here, the negative electrode active material refers to a reducing agent that undergoes a reduction reaction upon charging of the secondary battery 1 and an oxidation reaction upon discharging of the secondary battery 1, and that can cause these oxidation-reduction reactions reversibly, and is capable of reacting to absorb and desorb charge carriers in the secondary battery 1. Therefore, for example, fibrous carbon such as carbon nanotubes and carbon fine particles such as carbon black do not substantially absorb or desorb lithium ions, which are charge carriers in lithium-ion secondary batteries, and therefore are not included in the negative electrode active material of the present disclosure.

[0026] The first negative electrode active material is a negative electrode active material containing silicon. The negative electrode active material containing silicon includes, for example, elemental silicon, a silicon alloy, and a silicon compound. Examples of silicon alloys that can be used as the first negative electrode active material include those containing, as a second constituent element other than silicon, at least one element selected from the group consisting of tin (Sn), nickel (Ni), copper (Cu), iron (Fe), cobalt (Co), manganese (Mn), zinc (Zn), indium (In), silver (Ag), titanium (Ti), germanium (Ge), bismuth (Bi), antimony (Sb), and chromium (Cr). Examples of silicon compounds that can be used as the first negative electrode active material include silicon oxide (SiO ｘ Examples of the first negative electrode active material include those containing oxygen (O) or carbon (C), such as silicon carbide (SiC), and may contain the second constituent element described above in addition to silicon. The first negative electrode active material may be doped with Li. ｘ In this case, it is preferable that Li is pre-doped by doping with Li in the negative electrode production process. ｘThe irreversible capacity of the first negative electrode active material can be reduced. The first negative electrode active material may be a composite of Si and another material such as carbon, or a composite of a Si alloy and another material such as carbon. In this case, the irreversible capacity can be reduced. The particle surfaces of the first negative electrode active material are preferably partially or entirely coated with carbon. This can improve the electronic conductivity of the particle surfaces of the first negative electrode active material.

[0027] The second negative electrode active material is a negative electrode active material containing carbon as a constituent element. Examples of materials that can be used as the second negative electrode active material include MCMB (MesoCarbon MicroBeads), artificial graphite, natural graphite, non-graphitizable carbon, and graphitizable carbon. More specifically, examples of materials that can be used as the second negative electrode active material include pyrolytic carbons, cokes, glassy carbon fiber, organic polymer compound sintered bodies, activated carbon, and carbon blacks. Examples of cokes include pitch coke, needle coke, and petroleum coke. The organic polymer compound sintered body is a material obtained by sintering a polymer compound such as a phenolic resin or a furan resin at an appropriate temperature and carbonizing it.

[0028] The negative electrode active material is not limited to the first negative electrode active material and the second negative electrode active material, and may include other negative electrode active materials, such as metals, semimetal alloys or compounds, and tin (Sn) alloys or compounds, capable of absorbing and releasing lithium. Examples of metals and semimetals that can be used as the negative electrode active material include tin (Sn), lead (Pb), aluminum (Al), indium (In), zinc (Zn), antimony (Sb), bismuth (Bi), cadmium (Cd), magnesium (Mg), boron (B), gallium (Ga), germanium (Ge), arsenic (As), silver (Ag), zirconium (Zr), yttrium (Y), and hafnium (Hf). Among these, germanium, tin, and lead are preferred. Tin is more preferred because it has a high ability to absorb and release lithium and can achieve a high energy density.

[0029] Here, the first negative electrode active material is preferably contained in the negative electrode active material at 30% by mass or more. That is, the mass fraction of the first negative electrode active material relative to the negative electrode active material is preferably 30% or more. This improves the negative electrode capacity. The mass fraction of the first negative electrode active material relative to the negative electrode active material can be calculated by distinguishing between particles containing silicon (Si) and particles containing only carbon (C) in an EDX (Energy Dispersive X-ray Spectroscopy) mapping image of a cross section of the negative electrode active material layer 222 cut along the thickness direction. More specifically, particle size distributions are obtained for particles containing Si (particles of the first negative electrode active material) and particles containing only C (particles of the second negative electrode active material) using image analysis software such as Image J. Volume distributions are calculated from the obtained particle size distributions, and the volumes of each particle are summed to calculate the sum of the volumes of particles containing Si and particles containing only C, respectively. Since the density of particles containing Si and the density of particles containing only C can be approximately assumed to be the same, the mass fraction of the first negative electrode active material can be calculated using the following calculation formula (1): (Mass fraction of first negative electrode active material (%)) = (Total volume of particles containing Si) / [(Total volume of particles containing Si) + (Total volume of particles containing only C)] (1) Here, if the region for obtaining the particle size distribution is too narrow, the error will be large, so the particle size distribution is obtained from a region with a maximum width of 100 μm or more. In addition, the particle size distribution is obtained by determining the particle size distribution using image analysis software such as Image J based on the particle sizes of multiple particles measured using the image analysis software.

[0030] Examples of tin alloys that can be used as the negative electrode active material include those containing at least one element selected from the group consisting of nickel, copper, iron, cobalt, manganese, zinc, indium, silver, titanium, germanium, bismuth, antimony, and chromium as a second constituent element other than tin. Examples of tin compounds that can be used as the negative electrode active material include those containing oxygen or carbon, and may contain the above-mentioned second constituent element in addition to tin.

[0031] The second polymer compound contained in the negative electrode active material layer 222 is a binder containing a polymer (NVA polymer) containing N-vinylacetamide (NVA) as a monomer. Examples of NVA polymers that can be used as the second polymer compound include poly-N-vinylacetamide (PNVA) and copolymers containing NVA as monomers. The copolymers containing NVA as a monomer that can be used as the second polymer compound further contain at least one of an alkali metal acrylate, an alkali metal methacrylate, and derivatives of an alkali metal acrylate and an alkali metal methacrylate as a monomer. Here, the alkali metal salt refers to a salt containing an alkali metal such as lithium (Li), sodium (Na), or potassium (K). This can suppress a decrease in electronic conductivity between negative electrode active material particles due to expansion and contraction of the negative electrode active material layer 222 during charge / discharge cycles, thereby improving cycle retention.

[0032] The negative electrode active material layer 222 on the separator 230 side has a higher mass fraction of NVA polymer than the negative electrode active material layer 222 on the negative electrode current collector 221 side. Specifically, for example, the portion of the negative electrode active material layer 222 in contact with the separator 230 has a higher mass fraction of NVA polymer than the portion of the negative electrode active material layer 222 in contact with the negative electrode current collector 221. In the first embodiment, the negative electrode active material layer 222 has a first layer 222a on the negative electrode current collector 221 side and a second layer 222b on the separator 230 side. The second layer 222b has a higher mass fraction of NVA polymer than the first layer 222a. This further improves adhesion between the separator 230 and the negative electrode 220, thereby further improving cycle characteristics. Note that the method for varying the mass fraction of the NVA polymer is not limited to the above. For example, by changing the manufacturing conditions of the anode active material layer 222, such as the drying rate of the anode mixture slurry, the mass fraction of the NVA polymer in the anode active material layer 222 on the separator 230 side may be made larger than that in the anode active material layer 222 on the anode current collector 221 side. The thickness direction distribution of the mass fraction of the NVA polymer in the anode active material layer 222 can be measured by quantifying the amount of N (nitrogen) contained in the NVA polymer in an EDX mapping image of a cross section of the anode active material layer 222 cut along the thickness direction. The anode active material layer 222 is divided into two halves in the thickness direction, and the mass concentration of N contained in the first layer 222a on the anode current collector 221 side and the mass concentration of N contained in the second layer 222b on the separator 230 side are measured. Based on the measured mass concentration of N, the difference in mass concentration of N between the first layer 222a and the second layer 222b is calculated using the following formula (2): If the difference in mass concentration of N is 10% or more, it can be said that the mass fraction of the NVA polymer is greater in the negative electrode active material layer 222 on the separator 230 side than in the negative electrode active material layer 222 on the negative electrode current collector 221 side. (Difference in Mass Concentration of N Between the First Layer 222a and the Second Layer 222b) (%) = [(% by mass of N contained in the second layer 222b) - (% by mass of N contained in the first layer 222a)] / [(% by mass of N contained in the second layer 222b) + (% by mass of N contained in the first layer 222a)] × 100 (2)

[0033] The anode active material layer 222 is not limited to containing only the anode active material and the second polymer compound, and may further contain, for example, an anode conductive agent. The anode conductive agent includes at least one of a carbon material, a metal material, and a conductive polymer compound. Specific examples of carbon materials used as the anode conductive agent include particulate carbon materials such as carbon black, acetylene black, and ketjen black, and fibrous carbon materials such as carbon nanotubes. Examples of carbon nanotubes include single-wall carbon nanotubes (SWCNTs). This improves the electronic conductivity of the particle surfaces of the first anode active material. The mass fraction of the anode conductive agent relative to the anode active material layer 222 is preferably 5% or less, more preferably 2% or less. This improves the paintability of the anode slurry.

[0034] The separator 230 is a film that insulates the positive electrode 210 from the negative electrode 220. The separator 230 is provided between the main surface of the positive electrode 210 and the main surface of the negative electrode 220 so that the positive electrode 210 and the negative electrode 220 do not come into direct contact with each other. In the example of FIG. 1 , the separator 230 has a rectangular sheet shape when viewed in a plan view in the thickness direction. The separator 230 includes a substrate 231, a positive electrode-side coating layer 232, and a negative electrode-side coating layer 233.

[0035] The material of the substrate 231 is preferably electrically stable, chemically stable with respect to the positive electrode active material, the negative electrode active material, and the electrolyte, and has insulating properties. The substrate 231 may be, for example, a polymer nonwoven fabric, a porous film, or a layer made of glass or ceramic fibers. The material of the substrate 231 more preferably includes a porous polyolefin film. This improves the safety of the battery by preventing short circuits and providing a shutdown effect.

[0036] The positive electrode side coating layer 232 is a layer that coats the positive electrode 210 side of the substrate 231. The positive electrode side coating layer 232 contains a first polymer compound, which improves the adhesion between the separator 230 and the positive electrode 210, thereby improving the cycle characteristics.

[0037] The negative electrode coating layer 233 is a layer that coats the negative electrode 220 side of the substrate 231. The negative electrode coating layer 233 contains an NVA polymer, which improves the adhesion between the separator 230 and the negative electrode 220, thereby improving the cycle characteristics.

[0038] Preferably, the positive electrode side coating layer 232 and the negative electrode side coating layer 233 further contain particles made of an inorganic substance. Examples of the inorganic substance include metals, semiconductors, oxides of metals and semiconductors, and nitrides of metals and semiconductors. Examples of metals used as the inorganic substance include aluminum (Al) and titanium (Ti). Examples of semiconductors used as the inorganic substance include silicon (Si) and boron (B). Examples of oxides and nitrides used as the inorganic substance include alumina (Al). ２ O ３ ), boron nitride (BN), aluminum nitride (AlN), titanium dioxide (TiO ２ ), silicon dioxide (SiO ２ ) and the like. In addition, it is preferable that the inorganic material has insulating properties, is easily available, and has a large heat capacity. The particle size of the inorganic material particles is not particularly limited, and for example, when alumina is used as the inorganic material, an inorganic material having an average particle size of 0.5 μm can be used.

[0039] The electrolyte solution is impregnated into the separator 230. In the example of Fig. 1, the electrolyte solution fills the space inside the exterior member 30. The electrolyte solution is a non-aqueous electrolyte solution containing an electrolyte salt and a solvent that dissolves the electrolyte salt.

[0040] The electrolyte salt is, for example, lithium perchlorate (LiClO ４ ), lithium hexafluorophosphate (LiPF ６ ), lithium tetrafluoroborate (LiBF ４ ), lithium bis(trifluoromethanesulfonyl)imide (LiN(SO ２ CF ３ ) ２ ), lithium bis(pentafluoroethanesulfonyl)imide (LiN(SO ２ C ２ F ５ )２ ), lithium hexafluoroarsenate (LiAsF ６ ) and other lithium salts.

[0041] Examples of the solvent include lactone-based solvents such as γ-butyrolactone, γ-valerolactone, δ-valerolactone, and ε-caprolactone; carbonate-based solvents such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate; ether-based solvents such as 1,2-dimethoxyethane, 1-ethoxy-2-methoxyethane, 1,2-diethoxyethane, tetrahydrofuran, and 2-methyltetrahydrofuran; nitrile-based solvents such as acetonitrile; sulfolane-based solvents; phosphoric acids; phosphate ester solvents; and pyrrolidones.

[0042] The electrolyte preferably contains at least one additive selected from the group consisting of fluorinated carboxylic acid esters, sulfonic acid esters, sulfonic acid anhydrides, and carboxylic acid anhydrides. This promotes the formation of a low-resistance solid electrolyte interphase (SEI), thereby improving the charge load characteristics. Examples of fluorinated carboxylic acid esters include fluoroethylene carbonate (FEC). Examples of sulfonic acid anhydrides include propanedisulfonic acid anhydride (PSAH). Examples of sulfonic acid esters include 1,3-propane sultone. Examples of carboxylic acid anhydrides include 1,4-dioxane-2,6-dione.

[0043] The battery according to the first embodiment has been described above, but the secondary battery according to the first embodiment is not limited to that shown in Fig. 1. Other examples will be described below with reference to the drawings, but the same components as those in Figs. 1 and 2 will be denoted by reference numerals and will not be described again.

[0044] Fig. 3 is a cutaway view showing a different example of the secondary battery according to the first embodiment. Fig. 4 is a schematic view of a cross section taken along line IV-IV in Fig. 3. The secondary battery 1A shown in Figs. 3 and 4 differs from the example shown in Fig. 1 in that the electrode body 200 has a structure in which the positive electrode lead 21A and the negative electrode lead 22A are wound around the center.

[0045] The battery element 20A is provided inside the exterior member 30. As shown in FIG. 4 , the battery element 20A includes an electrode body 200A, a positive electrode lead 21A, a negative electrode lead 22A, and a protective material 23. The positive electrode lead 21A is a terminal drawn from inside the battery element 20A to the outside of the exterior member 30, and the positive electrode lead 21A is provided near the center of the battery element 20A. The negative electrode lead 22A is a terminal drawn from inside the battery element 20A to the outside of the exterior member 30, and the negative electrode lead 22A is provided near the center of the battery element 20A. The protective material 23 is a member that protects the outside of the battery element 20A. The protective material 23 is provided so as to be wrapped around the electrode body 200A. The protective material 23 is, for example, an insulating tape.

[0046] 4, the electrode assembly 200A is a laminate for charge / discharge reactions of the secondary battery according to the first embodiment. The electrode assembly 200A includes a positive electrode 210A including a positive electrode current collector 211A and a positive electrode active material layer 212A, a negative electrode 220A including a negative electrode current collector 221A and a negative electrode active material layer 222A, and a separator 230A. The electrode assembly 200A has a structure in which the positive electrode lead 21A and the negative electrode lead 22A are wound around the center, and is laminated in the following order from the outside, i.e., from the protective material 23 side: the negative electrode current collector 221A, the negative electrode active material layer 222A, the separator 230A, the positive electrode active material layer 212A, the positive electrode current collector 211A, the positive electrode active material layer 212A, the separator 230A, and the negative electrode active material layer 222A. In the electrode body 200A, no layers other than the negative electrode current collector 221A, separator 230A, and positive electrode current collector 211A are provided near the positive electrode lead 21A and the negative electrode lead 22A. With this structure, the positive electrode current collector 211A is connected to the positive electrode lead 21A, and the negative electrode current collector 221A is connected to the negative electrode lead 22A.

[0047] As described above, the secondary battery 1 according to the first embodiment includes a positive electrode 210, a negative electrode 220, a separator 230 between the positive electrode 210 and the negative electrode 220, and an electrolyte. The positive electrode 210 includes a positive electrode active material layer 212. The positive electrode active material layer 212 includes a positive electrode active material and a first polymer compound. The negative electrode 220 includes a negative electrode active material layer 222. The negative electrode active material layer 222 includes a negative electrode active material and a second polymer compound. The negative electrode active material includes a first negative electrode active material containing silicon. The second polymer compound includes an NVA polymer containing N-vinylacetamide as a monomer. The separator 230 includes a substrate 231, a positive electrode coating layer 232 on the positive electrode 210 side of the substrate 231, and a negative electrode coating layer 233 on the negative electrode 220 side of the substrate 231. The positive electrode coating layer 232 includes a first polymer compound, and the negative electrode coating layer 233 includes an NVA polymer.

[0048] This makes it possible to maintain the bond between the separator 230 and the electrode during charge-discharge cycles, and also to suppress a decrease in electronic conductivity between the negative electrode active material particles due to expansion and then contraction of the negative electrode active material layer 222 during charge-discharge cycles, thereby improving cycle characteristics.

[0049] In a desirable embodiment, the negative electrode active material layer 222 on the separator 230 side has a larger mass fraction of NVA polymer than the negative electrode active material layer 222 on the opposite side from the separator 230. This makes it possible to maintain the bond between the separator 230 and the negative electrode 220, thereby further improving the cycle characteristics.

[0050] In a preferred embodiment, the negative electrode active material further includes a second negative electrode active material containing carbon, and the mass fraction of the first negative electrode active material relative to the negative electrode active material is 30% or more, thereby further improving cycle characteristics.

[0051] Examples will be described below. Table 1 shows examples and comparative examples. Note that the present invention is not limited to these examples.

[0052]

[0053] Example 1 A positive electrode according to Example 1 was fabricated by the following method. Lithium cobalt oxide (LiCoO２ 95% by mass of PVDF (Polyvinyl Alcohol), 2% by mass of amorphous carbon powder (Ketjen Black) as a conductive agent, and 3% by mass of PVdF as a first polymer compound were mixed and dispersed in N-methyl-2-pyrrolidone (NMP) to obtain a positive electrode mixture slurry. The prepared positive electrode mixture slurry was uniformly applied to both sides of a strip-shaped aluminum foil having a thickness of 10 μm as a positive electrode current collector to prepare a positive electrode coated body. The obtained positive electrode coated body was dried with hot air and then compression-molded using a roll press to form a positive electrode sheet. The positive electrode sheet formed through the above process was cut into a strip of 70 mm x 800 mm to prepare the positive electrode according to Example 1. A positive electrode lead was attached to the exposed portion of the positive electrode current collector of the prepared positive electrode.

[0054] The negative electrode according to Example 1 was fabricated by the following method. ｘ 47.5 mass% of the first negative electrode active material, 47.5 mass% of MCMB as the second negative electrode active material, 4 mass% of PNVA as the second polymer compound, 0.7 mass% of carbon black and 0.3 mass% of SWCNT as the negative electrode conductive agent were added to an appropriate amount of ion-exchanged water, and the mixture was kneaded and stirred using a planetary mixer to obtain a negative electrode mixture slurry for the first layer. ｘ 46.5% by mass of the first negative electrode active material, 46.5% by mass of MCMB as the second negative electrode active material, 6% by mass of PNVA as the second polymer compound, and 0.7% by mass of carbon black and 0.3% by mass of SWCNT as the negative electrode conductive agent were added to an appropriate amount of ion-exchanged water and kneaded and stirred using a planetary mixer to obtain a second layer of negative electrode mixture slurry. The first layer of the negative electrode mixture slurry and the second layer of the negative electrode mixture slurry were uniformly and continuously applied to both sides of an 8 μm thick copper foil as a negative electrode current collector, in that order, to form a negative electrode coated body. The obtained negative electrode coated body was dried with hot air and then compression-molded using a roll press to form a negative electrode sheet. The negative electrode sheet was cut into a 72 mm x 810 mm strip to produce the negative electrode according to Example 1. A negative electrode lead was attached to the exposed portion of the negative electrode current collector of the produced negative electrode.

[0055] In the following description, it is assumed that the first negative electrode active material is A1 mass % and the second negative electrode active material is B1 mass % in the first layer. In this case, the mass fraction w1 (%) of the first negative electrode active material relative to the negative electrode active material in the first layer satisfies the relationship of the following formula (3). Furthermore, it is assumed that the first negative electrode active material is A2 mass % and the second negative electrode active material is B2 mass % in the second layer. In this case, the mass fraction w2 (%) of the first negative electrode active material relative to the negative electrode active material in the second layer satisfies the relationship of the following formula (4).

[0056] w1=A1 / (A1+B1)×100...(3) w2=A2 / (A2+B2)×100...(4)

[0057] In Example 1, the first negative electrode active material in the first layer and the second layer is a silicon compound (SiO ｘ ), and the second negative electrode active material is MCMB. As shown in Table 1, the mass fraction w1 of Example 1 is 50% (= 47.5% by mass / (47.5% by mass + 47.5% by mass)). The mass fraction w2 of Example 1 is 50% (= 46.5% by mass / (46.5% by mass + 46.5% by mass)).

[0058] The separator according to Example 1 was produced by the following method. PVdF having an average molecular weight of 150,000 as a binder and NMP as a solvent were mixed and dissolved sufficiently in a mass ratio of 10:90, and alumina (Al 2 O 3 ) having an average particle size of 0.5 μm as inorganic particles was added to the solution. ２ O ３ ) fine powder was added in an amount twice the mass of PVdF, and the mixture was thoroughly stirred to prepare a coating slurry for the positive electrode side sheet layer. PNVA having an average molecular weight of 1 million as a binder and ion-exchanged water as a solvent were mixed in a mass ratio of 3:97 to prepare a solution, to which alumina (Al ) having an average particle size of 0.5 μm as an inorganic substance was added. ２ O ３) fine powder was added in an amount twice the mass of PNVA and thoroughly stirred to prepare a coating slurry for the negative electrode side sheet layer. The prepared coating slurries for the positive electrode side sheet layer and the negative electrode side sheet layer were applied to each side of a 15 μm thick microporous polyethylene film substrate, forming a positive electrode side sheet layer and a negative electrode side sheet layer to a thickness of 3 μm, thereby preparing a separator sheet. The separator sheet was cut into a 74 mm × 860 mm strip to prepare the separator according to Example 1.

[0059] The electrolyte solution according to Example 1 was prepared by dissolving lithium hexafluorophosphate (LiPF ) as an electrolyte salt in a solvent obtained by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a mass ratio of 5:5. ６ ) was dissolved to a molar concentration of 1 mol / L.

[0060] The secondary battery according to Example 1 was fabricated by the following method. The positive and negative electrodes were stacked with the separator interposed therebetween, tightly adhered, and wound longitudinally. A protective tape was then applied to the outermost periphery of the wound laminate to fabricate an electrode assembly. The fabricated electrode assembly was then loaded into a housing member, and three sides of the housing member were heat-sealed, with one side remaining unsealed to form an opening. The housing member was a laminate film consisting of a 25 μm-thick nylon film as the outermost layer, a 40 μm-thick aluminum foil as the metal layer, and a 30 μm-thick polypropylene film as the insulating layer. The electrolyte solution prepared above was then injected through the opening in the housing member, and the remaining side of the housing member was heat-sealed in a reduced-pressure environment to seal the housing member, thereby fabricating the secondary battery according to Example 1.

[0061] In Example 1, the negative electrode capacity was calculated by the following method. Here, the negative electrode capacity refers to the capacity of the negative electrode active material.

[0062] A counter electrode Li coin cell was separately prepared using a positive electrode in which the positive electrode active material layer according to Example 1 was formed on only one side of the positive electrode current collector, and charging was carried out under the following conditions to measure the electrical capacity and determine the charge capacity per thickness of the positive electrode active material layer: Charge rate: 0.1 C, Charging method: CCCV, Charge control voltage: 4.45 V, Charge cut-off current: 0.01 C

[0063] Similarly, a counter electrode Li coin cell was prepared using a negative electrode in which a negative electrode active material layer according to Example 1 was formed on only one side of a negative electrode current collector using the components according to Example 1, and charging was carried out under the following conditions to measure the electrical capacity and determine the charge capacity per thickness of the negative electrode active material: Charge rate: 0.1 C Charging method: CCCV Charge control voltage: 0 V Charge cut-off current: 0.01 C

[0064] Using the charge capacity per thickness of the positive electrode active material and the charge capacity per thickness of the negative electrode active material measured above, a test battery was fabricated by adjusting the thickness of the positive electrode active material layer and the negative electrode active material layer by changing the solid content ratio of the positive electrode mixture slurry and the negative electrode mixture slurry, the application speed, etc., so that the ratio of the charge capacity of the positive electrode to the charge capacity of the negative electrode was 0.9. The test battery fabricated was discharged under the following conditions to measure the capacity and calculate the negative electrode capacity. Here, the negative electrode capacity was calculated as a relative value to the value in Example 6 described below. Discharge rate: 0.1 C Discharge method: CC Discharge end voltage: 1.0 V

[0065] In Example 1, a cycle characteristic test was carried out in the following manner: The cycle characteristic test was carried out at 23°C.

[0066] The battery prepared above was subjected to a first charge / discharge cycle under the following conditions: charge rate: 0.2 C, charge method: CCCV, charge control voltage: 4.40 V, charge cut-off current: 0.025 C, discharge rate: 0.1 C, discharge method: CC, discharge cut-off voltage: 3.0 V.

[0067] Thereafter, the second to 100th cycles were performed under the following conditions, and the discharge capacities at the second and 100th cycles were measured. The ratio of the discharge capacity at the 100th cycle to the discharge capacity at the second cycle was calculated as the cycle retention rate: Charge rate: 1.0 C, Charging method: CCCV, Charge control voltage: 4.40 V, Charge cut-off current: 0.025 C, Discharge rate: 1.0 C, Discharge method: CC, Discharge cut-off voltage: 3.0 V.

[0068] Example 2 In Example 2, as shown in Table 1, a battery was fabricated and measured in the same manner as in Example 1, except that the second polymer compound and the binder of the negative electrode-side coating layer were changed to a copolymer of NVA and sodium acrylate (NVA-AANa).

[0069] Example 3 In Example 3, as shown in Table 1, a battery was produced in the same manner as in Example 1, except that the binder of the second polymer compound and the negative electrode-side coating layer was changed to a copolymer of NVA and lithium acrylate (NVA-AALi), and measurements were performed.

[0070] Example 4 In Example 4, as shown in Table 1, a battery was fabricated and measured in the same manner as in Example 1, except that the second polymer compound and the binder of the negative electrode-side coating layer were changed to a copolymer of NVA and potassium acrylate (NVA-AAK).

[0071] (Example 5) In Example 5, as shown in Table 1, a battery was fabricated and measured in the same manner as in Example 1, except that the mass fraction of the second polymer compound in the first layer and the second layer was the same, 5%.

[0072] Example 6 In Example 6, as shown in Table 1, a battery was fabricated and measured in the same manner as in Example 1, except that the mass fraction w1 of the first negative electrode active material relative to the negative electrode active material in the first layer was changed to 10% (= 9.5 mass % / (9.5 mass % + 85.5 mass %)) and the mass fraction w2 of the second negative electrode active material relative to the negative electrode active material in the second layer was changed to 10% (= 9.3 mass % / (9.3 mass % + 83.7 mass %)).

[0073] Example 7 In Example 7, as shown in Table 1, a battery was fabricated and measured in the same manner as in Example 1, except that the mass fraction w1 of the first negative electrode active material relative to the negative electrode active material in the first layer was changed to 30% (= 28.5 mass % / (28.5 mass % + 66.5 mass %), and the mass fraction w2 of the second negative electrode active material relative to the negative electrode active material in the second layer was changed to 30% (= 27.9 mass % / (27.9 mass % + 65.1 mass %)).

[0074] Example 8 In Example 8, as shown in Table 1, a battery was fabricated and measured in the same manner as in Example 1, except that the mass fraction w1 of the first negative electrode active material relative to the negative electrode active material in the first layer was changed to 70% (= 66.5 mass % / (66.5 mass % + 28.5 mass %)) and the mass fraction w2 of the second negative electrode active material relative to the negative electrode active material in the second layer was changed to 70% (= 65.1 mass % / (65.1 mass % + 27.9 mass %)).

[0075] Example 9 In Example 9, as shown in Table 1, a battery was fabricated and measured in the same manner as in Example 1, except that the mass fraction w1 of the first negative electrode active material relative to the negative electrode active material in the first layer was changed to 100% (= 91.8 mass % / (91.8 mass % + 0 mass %)) and the mass fraction w2 of the second negative electrode active material relative to the negative electrode active material in the second layer was changed to 100% (= 88.2 mass % / (88.2 mass % + 0 mass %)).

[0076] Example 10 In Example 10, as shown in Table 1, a battery was fabricated and measured in the same manner as in Example 9, except that the first negative electrode active material in the first layer and the second layer was changed to elemental silicon (Si), the mass fraction w1 of the first negative electrode active material relative to the negative electrode active material in the first layer was changed to 100% (= 91.8 mass % / (91.8 mass % + 0 mass %)), and the mass fraction w2 of the second negative electrode active material relative to the negative electrode active material in the second layer was 100% (= 88.2 mass % / (88.2 mass % + 0 mass %)).

[0077] Example 11 In Example 11, as shown in Table 1, the first negative electrode active material of the first layer and the second layer was a silicon-containing alloy (SiTi ０．０１ ), the mass fraction w1 of the first anode active material relative to the anode active material in the first layer was changed to 100% (= 91.8 mass % / (91.8 mass %+0 mass %)), and the mass fraction w2 of the second anode active material relative to the anode active material in the second layer was changed to 100% (= 88.2 mass % / (88.2 mass %+0 mass %)). A battery was fabricated and measured in the same manner as in Example 9.

[0078] Comparative Example 1 In Comparative Example 1, as shown in Table 1, a battery was fabricated in the same manner as in Example 1, except that the binder in the positive electrode side coating layer of the separator was changed to PNVA, and measurements were carried out.

[0079] Comparative Example 2 In Comparative Example 2, as shown in Table 1, a battery was fabricated in the same manner as in Example 1, except that the binder in the negative electrode side coating layer of the separator was changed to PVdF, and measurements were carried out.

[0080] Comparative Example 3 In Comparative Example 3, as shown in Table 1, a battery was fabricated in the same manner as in Example 1, except that the second polymer compound was changed to polyacrylic acid (PAA), and measurements were carried out.

[0081] Comparative Example 4 In Comparative Example 4, as shown in Table 1, a battery was fabricated in the same manner as in Example 1, except that the binder of the second polymer compound and the negative electrode-side coating layer was changed to PAA, and measurements were performed.

[0082] As shown in Table 1, in Examples 1 to 11, by using the same binder as the first polymer compound in the positive electrode side coating layer of the separator, the cycle retention rate could be improved compared to Comparative Example 1 in which the binder included in the positive electrode side coating layer of the separator was different from the first polymer compound.

[0083] As shown in Table 1, in Examples 1 to 11, by using the same binder as the second polymer compound in the negative electrode side coating layer of the separator, the cycle retention rate could be improved compared to Comparative Example 2, in which the binder in the negative electrode side coating layer of the separator was different from the second polymer compound.

[0084] As shown in Table 1, in Examples 1 to 11, by using an NVA polymer as the second polymer compound, the cycle retention rate was improved compared to Comparative Examples 3 and 4, in which the second polymer compound was not an NVA polymer.

[0085] As shown in Table 1, in Examples 1 to 4, by making the mass fraction of the second polymer compound in the second layer larger than that in the first layer, the cycle retention rate was improved more than in Example 5, in which the mass fraction of the second polymer compound in the second layer was equal to that of the first layer.

[0086] As shown in Table 1, in Examples 1 to 5 and 7 to 9, by setting the mass fractions w1 and w2 of the first negative electrode active material relative to the negative electrode active material in the first layer and the second layer to 30% or more, the negative electrode capacity could be improved compared to Example 6 in which the mass fractions w1 and w2 of the first negative electrode active material relative to the negative electrode active material in the first layer and the second layer were less than 30%.

[0087] The above-described embodiments are intended to facilitate understanding of the present invention and are not intended to limit the present invention. The present invention may be modified or improved without departing from the spirit thereof, and equivalents thereof are also included in the present invention.

[0088] REFERENCE SIGNS LIST 1, 1A Secondary battery 20, 20A Battery element 21, 21A Positive electrode lead 22, 22A Negative electrode lead 23 Protective material 30 Exterior member 30a, 30b Exterior sheet 31 Recess 32 Adhesive material 200, 200A Electrode body 210, 210A Positive electrode 211, 211A Positive electrode current collector 212, 212A Positive electrode active material layer 220, 220A Negative electrode 221, 221A Negative electrode current collector 222, 222A Negative electrode active material layer 222a First layer 222b Second layer 230, 230A Separator 231 Substrate 232 Positive electrode side coating layer 233 Negative electrode side coating layer

Claims

1. A secondary battery comprising: a positive electrode, a negative electrode, a separator between a main surface of the positive electrode and a main surface of the negative electrode, and an electrolyte; the positive electrode has a positive electrode active material layer, the positive electrode active material layer including a positive electrode active material and a first polymer compound; the negative electrode has a negative electrode active material layer, the negative electrode active material layer including a negative electrode active material and a second polymer compound, the negative electrode active material including a first negative electrode active material including silicon, and the second polymer compound including an NVA polymer including N-vinylacetamide as a monomer; the separator has a substrate, a positive electrode-side coating layer on the positive electrode side of the substrate, and an anode-side coating layer on the negative electrode side of the substrate, the positive electrode-side coating layer including the first polymer compound, and the anode-side coating layer including the NVA polymer.

2. The secondary battery according to claim 1, wherein the negative electrode active material layer on the separator side has a larger mass fraction of the NVA polymer than the negative electrode active material layer on the opposite side to the separator.

3. The secondary battery according to claim 1 or 2, wherein the negative electrode active material further comprises a second negative electrode active material containing carbon, and the mass fraction of the first negative electrode active material relative to the negative electrode active material is 30% or more.

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