Cured product for lithium ion secondary battery, negative electrode for lithium ion secondary battery, and lithium ion secondary battery

Abstract

This invention provides a cured material for lithium-ion secondary batteries, comprising a water-soluble polymer, a divalent cation, and an anion. The water-soluble polymer has carboxyl groups. The divalent cation crosslinks the water-soluble polymer. The anion contains sulfur.

Description

Technical Field

[0001] This invention relates to a solidified material for lithium-ion secondary batteries, a negative electrode for lithium-ion secondary batteries, and a lithium-ion secondary battery.

[0002] This invention claims priority based on Japanese Patent Application No. 2024-037719 filed in Japan on March 12, 2024, the contents of which are incorporated herein by reference. Background Technology

[0003] Lithium-ion secondary batteries are also widely used as power sources for mobile devices such as mobile phones and laptops, as well as hybrid vehicles.

[0004] The capacity of lithium-ion secondary batteries depends primarily on the active materials of the electrodes. Graphite is typically used as the negative electrode active material, but higher capacity negative electrode active materials are needed. Therefore, silicon (Si), with a theoretical capacity significantly larger than that of graphite (372 mAh / g), has attracted considerable attention.

[0005] Silicon-containing negative electrode active materials undergo significant volume expansion during charging. This volume expansion contributes to the reduced cycle performance of the battery. When the negative electrode active material expands, it can lead to various problems, such as cracking, delamination at the interface between the negative electrode active material layer and the current collector, cracking at the SEI (Solid Electrolyte Interphase) coating, and electrolyte decomposition. All of these factors negatively impact the battery's cycle performance.

[0006] For example, Patent Document 1 discloses an adhesive formed by crosslinking a water-soluble polymer containing a carboxyl group with a water-soluble crosslinking agent.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent Application Publication No. 2010-129363 Summary of the Invention

[0010] The technical problem that the invention aims to solve

[0011] Further improvements in cyclic performance are required.

[0012] The present invention was made in view of the above-mentioned problems, and its object is to provide a solidified material for lithium-ion secondary batteries that can improve the cycle characteristics of lithium-ion secondary batteries.

[0013] Technical solutions for solving technical problems

[0014] To address the aforementioned technical problems, the following solution is provided.

[0015] (1) The solidified material for lithium-ion secondary batteries involved in the first method includes a water-soluble polymer, a divalent cation, and an anion. The water-soluble polymer has a carboxyl group. The divalent cation crosslinks the water-soluble polymer. The anion contains sulfur.

[0016] (2) The negative electrode for lithium-ion secondary batteries involved in the second method includes the negative electrode active material and the solidified material for secondary batteries involved in the first method.

[0017] (3) The lithium-ion secondary battery involved in the third method has a negative electrode for secondary battery, a positive electrode and a separator located between the negative electrode for secondary battery involved in the second method and the positive electrode.

[0018] Invention Effects

[0019] The lithium-ion solidified material involved in the above method can improve the cycle characteristics of lithium-ion secondary batteries. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the lithium-ion secondary battery according to the first embodiment.

[0021] Figure 2 This is a schematic diagram of the adhesive involved in the first embodiment. Detailed Implementation

[0022] Hereinafter, embodiments will be described in detail with appropriate reference to the accompanying drawings. For ease of understanding and convenience, the drawings used in the following description may sometimes be enlarged representations of certain features, and the dimensional ratios of structural elements may differ from the actual dimensions. The materials, dimensions, etc., illustrated in the following description are examples only; the present invention is not limited to these examples, and appropriate modifications and implementations can be made without altering its spirit (technical conditions).

[0023] (1) The solidified material for lithium-ion secondary batteries involved in the first method includes a water-soluble polymer, a divalent cation, and an anion. The water-soluble polymer has a carboxyl group. The divalent cation crosslinks the water-soluble polymer. The anion contains sulfur.

[0024] (2) In the lithium-ion secondary battery cured product involved in the above method (1), the divalent cation can be selected from Mg 2+ Ca 2+ 、Sr 2+ Ba 2+ Co 2+ Ni 2+ Cu 2+Zn 2+ At least one of the groups.

[0025] (3) In the lithium-ion secondary battery cured product involved in the above method (1) or (2), the above water-soluble polymer may have a carboxyl group replaced by an alkali metal.

[0026] (4) The negative electrode for lithium-ion secondary batteries involved in the second method includes the negative electrode active material and the solidified material for secondary batteries involved in any of the above methods (1) to (3).

[0027] (5) In the negative electrode for lithium-ion secondary batteries involved in the above method (4), when the amount (parts by mass) of all water-soluble polymers contained in the negative electrode is set to 100 parts by mass, the amount (parts by mass) of sulfur element can be more than 0.002 parts by mass and less than 0.22 parts by mass.

[0028] (6) In the negative electrode for lithium-ion secondary batteries involved in the above method (4), when the amount (parts by mass) of all water-soluble polymers contained in the negative electrode is set to 100 parts by mass, the amount (parts by mass) of sulfur element can be more than 0.005 parts by mass and less than 0.2 parts by mass.

[0029] (7) In the negative electrode for lithium-ion secondary batteries involved in any of the above methods (4) to (6), when the amount (parts by mass) of all water-soluble polymers contained in the negative electrode is set to 100 parts by mass, the amount (parts by mass) of the above divalent cations can be more than 0.005 parts by mass and less than 11.00 parts by mass.

[0030] (8) In the negative electrode for lithium-ion secondary batteries involved in any of the above methods (4) to (6), when the amount (parts by mass) of all water-soluble polymers contained in the negative electrode is set to 100 parts by mass, the amount (parts by mass) of the above divalent cations can be more than 0.01 parts by mass and less than 10 parts by mass.

[0031] (9) The lithium-ion secondary battery involved in the third method has a negative electrode, a positive electrode and a separator located between the negative electrode and the positive electrode of the secondary battery involved in any one of the above methods (4) to (8).

[0032] [Lithium-ion secondary battery]

[0033] Figure 1 This is a schematic diagram of the lithium-ion secondary battery according to the first embodiment. Figure 1The lithium-ion secondary battery 100 shown has a power generation element 40, an outer casing 50, and a non-aqueous electrolyte (not shown). The outer casing 50 covers the area around the power generation element 40. The power generation element 40 is connected to the outside via a pair of terminals 60, 62 connected to it. The non-aqueous electrolyte is contained within the outer casing 50. Figure 1 The example shown illustrates a case where one power generation element 40 is housed within the outer packaging 50, but multiple power generation elements 40 may also be stacked. Furthermore, the lithium-ion secondary battery 100 can be any type, such as cylindrical, square, laminated, or button-shaped.

[0034] (Power generation components)

[0035] The power generation element 40 has a diaphragm 10, a positive electrode 20, and a negative electrode 30.

[0036] <Positive electrode>

[0037] The positive electrode 20, for example, has a positive current collector 22 and a positive active material layer 24. The positive active material layer 24 is in contact with at least one side of the positive current collector 22.

[0038] [Positive current collector]

[0039] The positive current collector 22 is, for example, a conductive plate. The positive current collector 22 is, for example, a thin plate of metal such as aluminum, copper, nickel, titanium, or stainless steel. Lightweight aluminum is well-suited for the positive current collector 22. The average thickness of the positive current collector 22 is, for example, 10 μm or more and 30 μm or less.

[0040] [Positive electrode active material layer]

[0041] The positive electrode active material layer 24 may contain, for example, a positive electrode active material. The positive electrode active material layer 24 may also contain conductive additives or binders as needed.

[0042] The positive electrode active material includes an electrode active material that can reversibly perform lithium ion adsorption and release, lithium ion desorption and insertion (intercalation), or lithium ion and counter anion doping and dedoping.

[0043] Positive electrode active materials are, for example, composite metal oxides. Examples of composite metal oxides include lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2), lithium manganese spinel (LiMn2O4), and general-form LiNi... x Co y Mn z M aO2 compounds (in the general formula, x + y + z + a = 1, 0 ≤ x < 1, 0 ≤ y < 1, 0 ≤ z < 1, 0 ≤ a < 1, and M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, and Cr), lithium vanadium compounds (LiV₂O₅), olivine-type LiMPO₄ (where M represents one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, and Zr, or VO), and lithium titanate (Li₄Ti₅O₄). 12 LiNi x Co y Al z O2 (0.9<x+y+z<1.1).

[0044] The positive electrode active material can also be an organic compound. For example, the positive electrode active material can be a conjugated polymer system containing organic conductive materials. Examples of conjugated polymer systems containing organic conductive materials include polyacetylene, polyaniline, polypyrrole, polythiophene, and poly(benzo[n]benzene).

[0045] The positive electrode active material can also be a lithium-free material. Examples of lithium-free materials include FeF3, conjugated polymer systems containing organic conductive materials, Chevrel phase compounds, transition metal chalcogenides, vanadium oxides, and niobium oxides. Only one lithium-free material can be used, or multiple materials can be combined. When the positive electrode active material is lithium-free, for example, discharge is performed first. Lithium is inserted into the positive electrode active material through discharge. Furthermore, for lithium-free positive electrode active materials, lithium can also be pre-doped chemically or electrochemically.

[0046] Conductive additives improve the electronic conductivity between positive electrode active materials. Examples of conductive additives include carbon powder, carbon nanotubes, carbon materials, metal powders, mixtures of carbon materials and metal powders, and conductive oxides. Examples of carbon powders include carbon black, acetylene black, and Ketjen black. Examples of metal powders include powders of copper, nickel, stainless steel, and iron.

[0047] There is no particular limitation on the content of the conductive additive in the positive electrode active material layer 24. For example, relative to the total mass of the positive electrode active material, conductive additive, and binder, the content of the conductive additive is 0.5% by mass or more and 20% by mass or less, preferably 1% by mass or more and 5% by mass or less.

[0048] The binder in the positive electrode active material layer 24 binds the positive electrode active materials together. Known binders can be used. The binder can also be the same binder used in the negative electrode active material layer 34 described later. The binder is preferably an adhesive that is insoluble in the electrolyte, has antioxidant properties, and possesses adhesiveness. Examples of binders include fluoropolymers. Examples of binders include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyamide (PA), polyimide (PI), polyamide-imide (PAI), polybenzimidazole (PBI), polyethersulfone (PES), polyacrylic acid and its copolymers, metal ion crosslinkers of polyacrylic acid and its copolymers, maleic anhydride-grafted polypropylene (PP) or polyethylene (PE), and mixtures thereof. PVDF is particularly preferred as the binder used in the positive electrode active material layer.

[0049] There is no particular limitation on the content of the binder in the positive electrode active material layer 24. For example, relative to the total mass of the positive electrode active material, conductive additive, and binder, the content of the binder is 1% by mass or more and 15% by mass or less, preferably 1.5% by mass or more and 5% by mass or less. When the content of the binder is low, the bonding strength of the positive electrode 20 is weakened. When the content of the binder is high, since the binder is electrochemically inert and detrimental to the discharge capacity, the energy density of the lithium-ion secondary battery 100 decreases.

[0050] <Negative electrode>

[0051] The negative electrode for a lithium-ion secondary battery according to this embodiment includes a negative electrode active material and a solidified material for a secondary battery according to this embodiment.

[0052] The negative electrode 30, for example, has a negative current collector 32 and a negative active material layer 34. The negative active material layer 34 is formed on at least one side of the negative current collector 32. The negative electrode 30 is an example of a negative electrode for a lithium-ion secondary battery.

[0053] [Negative current collector]

[0054] The negative current collector 32 is, for example, a conductive plate. The negative current collector 32 can be made of the same material as the positive current collector 22.

[0055] [Negative electrode active material layer]

[0056] The negative electrode active material layer 34 contains a negative electrode active material and a binder. The negative electrode active material layer 34 may also contain conductive additives if needed. An example of the binder is a cured product used in lithium-ion secondary batteries.

[0057] The negative electrode active material contains silicon or silicon compounds. It may also contain both silicon and silicon compounds. Examples of silicon compounds include silicon alloys and silicon oxide. For instance, silicon or silicon compounds can be crystalline, amorphous, or amorphous with crystalline components dispersed within them. Amorphous silicon or silicon compounds can be produced using methods such as melt quenching or gas atomization. The negative electrode active material can also be any known material other than silicon or silicon compounds.

[0058] Silicon alloy X n Si represents silicon. X is a cation. Examples of X include Ba, Mg, Al, Zn, Sn, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ge, Y, Zr, Nb, Mo, W, Au, Ti, Na, and K. n satisfies 0 ≤ n ≤ 0.5. Silicon oxide is made from SiO₂. x This indicates that x satisfies, for example, 0.8 ≤ x ≤ 2. Silicon oxide can be composed solely of SiO2, solely of SiO, or a mixture of SiO and SiO2. Additionally, silicon oxide can also lack some oxygen.

[0059] The negative electrode active material can also be a composite of silicon or silicon compounds. The composite is obtained by coating at least a portion of the surface of the silicon or silicon compound particles with a conductive material.

[0060] Conductive materials include, for example, carbon materials, Al, Ti, Fe, Ni, Cu, Zn, Ag, Sn, etc. For example, silicon-carbon composite materials (Si-C) are an example of a composite. The amount of conductive material coating the silicon or silicon compound particles relative to the total mass of the composite is, for example, 0.01% by mass or more and 30% by mass or less, preferably 0.1% by mass or more and 20% by mass or less. The composite can be manufactured, for example, by mechanical alloying, chemical vapor deposition, wet deposition, or by coating a polymer followed by thermal decomposition and carbonization of the polymer.

[0061] The specific surface area of ​​the negative electrode active material, calculated using the BET method, is, for example, 0.5 m². 2 / g or more and 100m 2 / g or less, preferably 1.0m 2 / g or more and 20m 2 Below / g. When the specific surface area is small, Li ions are not easily inserted and detached between the negative electrode active materials. When the specific surface area is large, a large amount of binder is required during polarization, and the capacity per unit volume becomes smaller.

[0062] The adhesive binds the negative electrode active material together with each other, as well as the negative electrode active material and the negative electrode current collector. Figure 2 This is a schematic diagram of the adhesive 1 according to the first embodiment. The adhesive 1 includes a water-soluble polymer 2, a divalent cation 3, and an anion.

[0063] Water-soluble polymer 2, for example, has a carboxyl group 2A. Water-soluble polymer 2 may also have a carboxyl group 2B substituted with an alkali metal. The alkali metal-substituted carboxyl group 2B is a carboxyl group in which the hydrogen atom of the carboxyl group is replaced by an alkali metal. Examples of alkali metals that substitute the hydrogen atom of the carboxyl group include Li, Na, and K.

[0064] When the water-soluble polymer 2 has alkali metal-substituted carboxyl groups 2B, the presence rate of alkali metal-substituted carboxyl groups 2B is, for example, 10% or more and 80% or less, preferably 20% or more and 60% or less. The presence rate of alkali metal-substituted carboxyl groups 2B is the ratio of alkali metal-substituted carboxyl groups 2B to the sum of carboxyl groups 2A and alkali metal-substituted carboxyl groups 2B. This ratio corresponds to the ion exchange rate of the carboxyl groups in the water-soluble polymer 2.

[0065] When a portion of the carboxyl groups is ion-exchanged, excessive intramolecular crosslinking and gelation of the water-soluble polymer 2 can be inhibited. As a result, the adhesive 1 forms a large network structure, and its strength increases. Furthermore, carboxyl group 2A is responsible for binding with the negative electrode active material and crosslinking with divalent cations. The presence of carboxyl group 2A enhances the adhesion between the adhesive 1 and the negative electrode active material. The proportion of alkali metal-substituted carboxyl groups 2B in the water-soluble polymer 2 can be determined by the amount of alkali consumed during the hydrolysis of the copolymer unit or by compositional analysis using nuclear magnetic resonance (NMR).

[0066] Water-soluble polymer 2 may be, for example, polyacrylic acid, carboxymethyl cellulose, copolymers of acrylic acid and vinyl alcohol, copolymers of sodium acrylate and vinyl alcohol, sodium carboxymethyl cellulose, polynorbornene dicarboxylic acid, or polymethacrylic acid. Water-soluble polymer 2 may be used alone or in combination. Additionally, adhesive 1 may also contain water-soluble polymers other than water-soluble polymer 2.

[0067] The weight-average molecular weight of water-soluble polymer 2 is, for example, 9000 or more and 1,000,000 or less. When the molecular weight of water-soluble polymer 2 is small, there are fewer cross-linking points in adhesive 1, and the elasticity of adhesive 1 decreases. When the molecular weight of water-soluble polymer 2 is large, adhesive 1 gels and is difficult to disperse uniformly. The weight-average molecular weight of water-soluble polymer 2 can be determined by analyzing the uncross-linked water-soluble polymer 2 if it is available. If the uncross-linked water-soluble polymer 2 is not available, it can be estimated based on the weight-average molecular weight of adhesive 1 and the ratio of the presence of water-soluble polymer 2, divalent cations 3, and anions in adhesive 1.

[0068] Whether a polymer is "water-soluble" can be determined by the following steps: First, add 1 part by weight of the polymer (equivalent to the solid component) to every 100 parts by weight of ion-exchanged water and stir to prepare a mixture. Adjust the mixture to a temperature range of 20°C to 95°C and a pH range of 3 to 12 (using NaOH aqueous solution and / or HCl aqueous solution for pH adjustment). Next, pass the mixture through a 250-mesh sieve. When the weight of the solid component remaining on the sieve that does not pass through the sieve does not exceed 50% by weight relative to the solid component of the added polymer, the polymer can be considered water-soluble. Furthermore, even if the above polymer-water mixture separates into two phases into an emulsion state upon standing, as long as the above definition is met, the polymer is also water-soluble.

[0069] Divalent cation 3 crosslinks the water-soluble polymers 2. Divalent cation 3 binds to the carboxyl groups of the water-soluble polymers 2 before crosslinking.

[0070] The divalent cation 3, preferably selected from Mg, is preferred. 2+ Ca 2+ 、Sr 2+ Ba 2+ Co 2+ Ni 2+ Cu 2+ Zn 2+ At least one of the groups.

[0071] When the total amount (parts by mass) of all water-soluble polymers contained in the negative electrode active material layer 34 is set to 100 parts by mass, the amount of divalent cations 3 is, for example, 0.005 parts by mass or more and 11.00 parts by mass or less, preferably 0.01 parts by mass or more and 10 parts by mass or less, more preferably 0.1 parts by mass or more and 10 parts by mass or less, and even more preferably 1 part by mass or more and 7 parts by mass or less. When the mass ratio of divalent cations 3 is within this range, a binder 1 with sufficient elasticity will be formed, and the strength of the binder 1 will be improved. The term "all water-soluble polymers contained in the negative electrode active material layer 34" is not limited to the water-soluble polymer 2 having a carboxyl group 2A, but also includes other types of water-soluble polymers. "All water-soluble polymers contained in the negative electrode active material layer 34" refers to polymers in the negative electrode active material layer 34 that satisfy the above definition of "water-soluble".

[0072] The mass ratio of divalent cations 3 can be obtained using high-frequency inductively coupled plasma (ICP) luminescence spectrophotometry. Specifically, this can be achieved by: using standard solutions corresponding to each divalent cation to prepare a standard curve for ICP luminescence spectrophotometry, and then using the calculated standard curve to determine the amount of divalent cations in the adhesive aqueous solution.

[0073] Anions (not shown in the diagram) are present within binder 1. Although the exact location of the anions is difficult to determine, it is thought that they may be present near the divalent cation 3 or near the carboxyl group 2B substituted with an alkali metal.

[0074] The anions contain sulfur. The sulfur in the anions helps form a high-quality SEI (Solid Electrolyte Interphase) coating on the surface of the negative electrode active material in lithium-ion secondary batteries. The SEI coating inhibits irreversible reactions between the negative electrode active material and the electrolyte, preventing electrolyte decomposition. Electrolyte decomposition is one of the reasons for the decline in the cycle characteristics of lithium-ion secondary batteries.

[0075] Anions such as SO4 2- CF3SO3 - (CF3SO2)2N - (C4F9SO2)2N - , C3H2F6NO4S2, (CF3CF2SO2)2N - (CF3SO2)(CF3(CF2)3SO2)N - (FSO2)2N - (FSO2)(CF3SO2)N - .

[0076] When the total amount (parts by mass) of all water-soluble polymers contained in the negative electrode active material layer 34 is set to 100 parts by mass, the amount of sulfur is, for example, 0.002 parts by mass or more and 0.22 parts by mass or less, preferably 0.005 parts by mass or more and 0.2 parts by mass or less, more preferably 0.01 parts by mass or more and 0.1 parts by mass or less. When the mass ratio of sulfur is within this range, the formation of the SEI coating can be promoted. When there is too much sulfur, the pH in the adhesive changes, which may hinder the crosslinking reaction between the water-soluble polymer 2 and the divalent cation 3.

[0077] The mass ratio of sulfur can be determined using high-frequency inductively coupled plasma (ICP) luminescence spectrophotometry. Specifically, this can be achieved by using a sulfate ion standard solution to prepare a standard curve for ICP luminescence spectrophotometry, and then using the calculated standard curve to determine the amount of sulfur in the adhesive aqueous solution.

[0078] The weight-average molecular weight of adhesive 1 is, for example, 9,000 or more and 1,000,000 or less. When the molecular weight of adhesive 1 is small, there are fewer cross-linking points within adhesive 1, resulting in lower adhesion to the active material. When the molecular weight of adhesive 1 is large, gelation occurs in the adhesive.

[0079] There is no particular limitation on the content of binder 1 in the negative electrode active material layer 34. For example, relative to the total mass of the negative electrode active material, conductive additive, and binder 1, the content of binder 1 is 0.5% by mass or more and 20% by mass or less, preferably 5% by mass or more and 15% by mass or less. When the content of binder 1 is low, the bonding strength of the negative electrode 30 becomes weak. When the content of binder 1 is high, since binder 1 is electrochemically inert and detrimental to discharge capacity, the energy density of the lithium-ion secondary battery 100 becomes low.

[0080] The conductive additive in the negative electrode active material layer 34 can improve the electronic conductivity between the negative electrode active materials. The conductive additive can be the same material as the conductive additive in the positive electrode active material layer 24.

[0081] There is no particular limitation on the content of the conductive additive in the negative electrode active material layer 34. For example, relative to the total mass of the negative electrode active material, conductive additive, and binder, the content of the conductive additive is 5% by mass or more and 20% by mass or less, preferably 1% by mass or more and 12% by mass or less.

[0082] <Septum>

[0083] The separator 10 is sandwiched between the positive electrode 20 and the negative electrode 30. The separator 10 isolates the positive electrode 20 and the negative electrode 30, preventing short circuits between them. The separator 10 extends in-plane along the positive electrode 20 and the negative electrode 30. Lithium ions can pass through the separator 10.

[0084] The separator 10 may have, for example, a porous structure with electrical insulation. The separator 10 may be, for example, a monolayer or laminate of a polyolefin membrane. The separator 10 may also be a stretched membrane of a mixture of polyethylene or polypropylene. The separator 10 may also be a nonwoven fabric made of fibers selected from at least one constituent material chosen from cellulose, polyester, polyacrylonitrile, polyamide, polyethylene, and polypropylene. The separator 10 may also be a solid electrolyte. Solid electrolytes may include, for example, polymeric solid electrolytes, oxide solid electrolytes, and sulfide solid electrolytes. The separator 10 may also be an inorganic coated separator. An inorganic coated separator is a separator formed by coating the surface of the above-mentioned membrane with a mixture of resins such as PVDF or CMC and inorganic materials such as alumina or silica. Inorganic coated separators exhibit excellent heat resistance and inhibit the deposition of transition metals dissolved from the positive electrode onto the negative electrode surface.

[0085] Electrolyte

[0086] The electrolyte is sealed within the outer casing 50 and is contained within and permeates the power generation element 40. The non-aqueous electrolyte may contain, for example, a non-aqueous solvent and an electrolyte salt (electrolyte, electrolyte salt). The electrolyte salt is dissolved in the non-aqueous solvent. Known electrolytes may be used.

[0087] The electrolytic salt is, for example, a lithium salt. Examples of electrolytic salts include LiPF6, LiClO4, LiBF4, LiCF3SO3, LiCF3CF2SO3, LiC(CF3SO2)3, LiN(CF3SO2)2, LiN(CF3CF2SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(CF3CF2CO)2, LiBOB, and LiN(FSO2)2. Regarding lithium salts, one type can be used alone, or two or more can be used in combination. From the viewpoint of degree of ionization, the electrolytic salt preferably contains LiPF6. The concentration of the electrolytic salt is, for example, 0.8 mol / L or more and 5.0 mol / L or less.

[0088] Non-aqueous solvents include, for example, aprotic organic solvents. Organic solvents include, for example, cyclic carbonates, chain carbonates, esters, ethers, and mixtures thereof. Solvents can also be ionic liquids.

[0089] Cyclic carbonates solubilize the electrolyte salts. Examples of cyclic carbonates include ethylene carbonate, propylene carbonate, butyl carbonate, and fluoroethylene carbonate. Preferably, the cyclic carbonate contains at least fluoroethylene carbonate. Fluoroethylene carbonate (FEC) has a high redox potential and is easily reduced and decomposed. Through the partial reduction and decomposition of FEC, the electrolyte salts or residual solvents in the electrolyte are less likely to decompose. Furthermore, during the initial use of the lithium-ion secondary battery, FEC forms a thin and stable coating (SEI coating) on ​​the entire surface of the negative electrode active material. The SEI coating prevents direct contact between the negative electrode active material and the electrolyte, and also prevents the decomposition of the electrolyte.

[0090] Chain carbonates can reduce the viscosity of cyclic carbonates. Examples of chain carbonates include diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate.

[0091] In addition, non-aqueous solvents may also include methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, etc.

[0092] The electrolyte may also contain SEI coating forming materials, surfactants, etc. Additives include, for example, vinylene carbonate, vinyl ethylene carbonate, phenyl ethylene carbonate, succinic anhydride, lithium dioxatate, lithium tetrafluoroborate, dinitro compounds, propane sulpholactone, butane sulpholactone, propene sulpholactone, 3-cyclobutene sulfone, fluoroallyl ethers, fluoroacrylates, etc.

[0093] <Outer Packaging>

[0094] The outer packaging 50 seals the power generation element 40 and the non-aqueous electrolyte inside. The outer packaging 50 prevents the non-aqueous electrolyte from leaking out to the outside and prevents moisture from entering the interior of the lithium-ion secondary battery 100.

[0095] like Figure 1 As shown, the outer packaging 50 has a metal foil 52 and resin layers 54 laminated on each side of the metal foil 52. The outer packaging 50 is a metal laminate film formed by coating the metal foil 52 with a polymer film (resin layer 54) from both sides.

[0096] For example, aluminum foil can be used as the metal foil 52. The resin layer 54 can be a polymer film such as polypropylene. The materials constituting the resin layer 54 can be different on the inner and outer sides. For example, as the material on the outer side, a polymer with a high melting point can be used, such as polyethylene terephthalate (PET), polyamide (PA), etc.; as the material of the polymer film on the inner side, polyethylene (PE), polypropylene (PP), etc. can be used.

[0097] <Terminal>

[0098] Terminals 60 and 62 are connected to the negative terminal 30 and the positive terminal 20, respectively. Terminal 62, connected to the positive terminal 20, is the positive terminal, and terminal 60, connected to the negative terminal 30, is the negative terminal. Terminals 60 and 62 are responsible for making electrical connections to external sources. Terminals 60 and 62 are made of conductive materials such as aluminum, nickel, and copper. The connection method can be welding or screw connection. To prevent short circuits, it is preferable to protect terminals 60 and 62 with insulating tape.

[0099] [Manufacturing method of lithium-ion secondary batteries]

[0100] Prepare a negative electrode 30, a positive electrode 20, a separator 10, an electrolyte, and an outer packaging 50, and assemble them to manufacture a lithium-ion secondary battery 100. The following is an example of a method for manufacturing a lithium-ion secondary battery 100.

[0101] First, the negative electrode 30 is fabricated. For example, the negative electrode 30 is fabricated by sequentially performing the precursor solution preparation process, slurry preparation process, electrode coating process, curing process, and calendering process.

[0102] First, the precursor solution preparation process is carried out. First, a water-soluble polymer solution and a compound containing a divalent cation are prepared. Examples of compounds containing divalent cations include hydroxides and chlorides of divalent cations. Examples of compounds containing divalent cations include Mg(OH)₂, MgCl₂, Ca(OH)₂, CaCl₂, Sr(OH)₂, SrCl₂, Ba(OH)₂, BaCl₂, Co(OH)₂, CoCl₂, Ni(OH)₂, NiCl₂, Cu(OH)₂, CuCl₂, Zn(OH)₂, and ZnCl₂.

[0103] For water-soluble polymer solutions, a portion of the carboxyl groups can undergo ion exchange. This can be achieved by adding aqueous solutions of NaOH, Na₂CO₃, LiOH, Li₂CO₃, K₂CO₃, etc., dropwise to the polymer solution while stirring. The cations in these solutions neutralize the carboxyl groups, allowing the protons of the carboxyl groups to exchange with the cations.

[0104] Next, the water-soluble polymer solution and the compound containing divalent cations are mixed. Although the method varies depending on the type of water-soluble polymer, the polymer is dissolved in water by stirring at a temperature between room temperature and below 100°C for at least one hour. The stirring speed is set between 550 rpm and 1500 rpm to ensure complete dissolution of the water-soluble polymer solution. Then, the compound containing divalent cations is added to the water-soluble polymer solution and stirred at room temperature for at least 5 minutes. The stirring speed is, for example, between 180 rpm and 600 rpm. By ensuring the compound containing divalent cations is fully dispersed in the water-soluble polymer solution, the cross-linking reaction can proceed without deviation.

[0105] Alternatively, the mixture can be purified. Specifically, the mixture is added to acetone to precipitate the water-soluble polymers. The precipitate is then removed, thoroughly dried using a vacuum dryer, and subsequently dissolved in water to obtain the purified solution.

[0106] Then, a compound containing sulfur as an anion is added to the mixed solution. Examples of compounds containing sulfur as an anion include Li₂SO₄ and salts of imide anions. The mixing ratio of the water-soluble polymer solution, divalent cations, and anions is adjusted according to the composition of the target adhesive. Following the above steps, a precursor solution for the adhesive is obtained.

[0107] Next, the slurry preparation process is carried out. In the slurry preparation process, a negative electrode active material (silicon or silicon compound) and conductive additives are added to the precursor solution of the binder.

[0108] The solvent is, for example, water. The composition ratio of the precursor solution of the negative electrode active material, conductive additive, and binder is, for example, 70wt%–99wt% : 0wt%–10wt% : 1wt%–20wt% by mass. Their mass ratio is adjusted to make the total 100wt%.

[0109] The negative electrode active material is obtained by mixing and compounding active material particles and conductive materials while applying shear force. When shear force is applied and mixed to a degree that does not deteriorate the active material particles, the surface of the active material particles is coated with conductive material. Furthermore, the particle size of the negative electrode active material can be adjusted according to the degree of mixing. Additionally, the prepared negative electrode active material can be sieved to ensure uniform particle size.

[0110] Next, the electrode coating process is performed. The electrode coating process involves applying a slurry to the surface of the negative electrode current collector 32. There are no particular restrictions on the slurry coating method. For example, slot die coating or blade coating can be used as the slurry coating method.

[0111] Next, a curing process is performed. In this process, the slurry is annealed. Additionally, the solvent is removed during the curing process. The curing process is carried out, for example, in a nitrogen atmosphere. The curing temperature is, for example, 120°C or higher and 150°C or lower. Furthermore, the heating rate to reach the curing temperature is set, for example, 2°C / min or higher and 5°C / min or lower. Furthermore, the cooling rate after curing is set, for example, 2°C / min or higher and 5°C / min or lower. By curing the adhesive under the above conditions, no bubbles are generated in the slurry, and the solvent is removed uniformly.

[0112] The calendering process is a process performed as needed. The calendering process involves applying pressure to the negative electrode active material layer 34 to adjust its density. The calendering process is performed, for example, using a rolling mill.

[0113] The positive electrode 20 can be manufactured using the same steps as the negative electrode 30. The separator 10 and the outer packaging 50 can be commercially available products.

[0114] Next, the diaphragm 10 is stacked between the positive electrode 20 and the negative electrode 30 to form a power generation element 40. In the case that the power generation element 40 is a wound body, the positive electrode 20, the negative electrode 30 and the diaphragm 10 are wound around one end of the diaphragm 10 as an axis.

[0115] Finally, the power generation element 40 is placed inside the outer packaging body 50. A non-aqueous electrolyte is injected into the outer packaging body 50. By applying pressure reduction or heating after the non-aqueous electrolyte is injected, the non-aqueous electrolyte is contained and permeates into the power generation element 40. The outer packaging body 50 is sealed by heating or the like, thereby obtaining a lithium-ion secondary battery 100. Alternatively, instead of injecting electrolyte into the outer packaging body 50, the power generation element 40 can be immersed in electrolyte.

[0116] The lithium-ion secondary battery 100 according to the first embodiment exhibits excellent cycle characteristics. This is because the binder 1 contained in the negative electrode active material layer 34 has a large network structure formed by cross-linking of water-soluble polymer 2 and divalent cation 3, thereby exhibiting high strength and high elasticity. In addition, the sulfur element contained in the binder 1 helps to form an SEI coating on the surface of the negative electrode active material and inhibits the irreversible reaction between the negative electrode active material and the electrolyte, which is also one of the reasons for the improved cycle characteristics of the lithium-ion secondary battery 100. Furthermore, the unreacted carboxyl groups 2A in the binder 1 interact with the negative electrode active material and bond firmly, which is also one of the reasons for the improved cycle characteristics of the lithium-ion secondary battery 100.

[0117] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the structures and combinations thereof in each embodiment are merely examples, and additions, omissions, substitutions and other modifications to the structures can be made without departing from the spirit (technical requirements) of the present invention.

[0118] Example

[0119] [Example 1]

[0120] First, polyacrylic acid with a weight-average molecular weight of 25,000 was prepared as the base polymer. Then, 60% of the carboxyl groups in the polyacrylic acid were replaced with Li by ion exchange. This ion exchange was carried out by slowly adding an aqueous solution of Li₂CO₃ while stirring the polyacrylic acid aqueous solution at 500 rpm.

[0121] Next, add 100 parts by weight of water-soluble polymer and water, and stir at room temperature for at least 2 hours, setting the stirring speed to between 550 rpm and 1500 rpm, until completely dissolved. Then, add 3.90 parts by weight of Zn(OH)₂ to the aqueous solution of the water-soluble polymer. The amount of Zn(OH)₂ added at this point is almost identical to the amount of divalent cations in the adhesive. Therefore, the amount of divalent cations added can also be replaced by the amount of divalent cations in the adhesive.

[0122] Next, 0.04 parts by mass of Li₂SO₄ are added to the mixed solution. The amount of sulfur in the added Li₂SO₄ (parts by mass) is almost identical to the amount of sulfur in the binder (parts by mass). Therefore, the amount of sulfur added can be replaced by the amount of sulfur in the binder (parts by mass).

[0123] A negative electrode active material and a conductive additive are added to a solution containing a binder. The negative electrode active material is SiO₂ obtained by a disproportionation reaction through heat treatment at 1000℃ under reduced pressure. xThe conductive additive is Super-P (registered trademark). Then, a coating solution (slurry) is prepared by mixing 25g of the negative electrode active material, 1.4g of the conductive additive, and 13.5g of the adhesive aqueous solution (10% solids concentration). The total solids concentration in the coating solution is 40wt%. Next, the slurry is applied to the copper foil serving as the negative electrode current collector using a doctor blade.

[0124] Next, the negative electrode current collector coated with the slurry was annealed in a nitrogen atmosphere. Annealing was performed under the following conditions: heating at 5°C / min, holding at 150°C for 2 hours, and cooling at 5°C / min. Then, the solidified negative electrode current collector was calendered to form the negative electrode active material layer 34. The negative electrode was then fabricated by stamping it into an electrode size of 22×32mm using a die.

[0125] Li is used as the positive electrode active material x CoO2. Ketjen black was used as the conductive additive. Polyvinylidene fluoride (PVDF) was used as the binder. N-methyl-2-pyrrolidone was used as the solvent. A positive electrode slurry was prepared by mixing 96 parts by weight of the positive electrode active material, 2 parts by weight of the conductive additive, 2 parts by weight of the binder, and 70 parts by weight of the solvent. The positive electrode slurry was then coated onto one side of an aluminum foil with a thickness of 15 μm, vacuum dried at 100°C for 2 hours, and calendered to form the positive electrode active material layer 24. The electrode was then die-cut to an electrode size of 22 × 32 mm to fabricate the positive electrode.

[0126] Next, the electrolyte is prepared. For the solvent, a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a mass ratio of 3:7 is used. LiPF6 is used as the electrolytic salt. The concentration of LiPF6 is 1 mol / L.

[0127] (Evaluation of the fabrication of lithium-ion secondary batteries)

[0128] The prepared negative and positive electrodes are stacked with positive and negative active material layers facing each other, separated by a separator (porous polyethylene sheet) to obtain a laminate. A nickel negative electrode lead is attached to the negative electrode of the laminate. An aluminum positive electrode lead is attached to the positive electrode of the laminate. The positive and negative electrode leads are welded using an ultrasonic welding machine. The laminate is inserted into an aluminum laminated film outer packaging, and the outer packaging is heat-sealed except for one surrounding area, thus forming a closed section. Finally, the electrolyte is injected into the outer packaging, and then the remaining section is sealed using a vacuum sealing machine while under reduced pressure through heat sealing, thereby producing a lithium-ion secondary battery.

[0129] (Determination of volume retention after 100 cycles)

[0130] The cycle characteristics of the lithium-ion secondary battery were measured. The cycle characteristics were measured using a secondary battery charge-discharge test apparatus (manufactured by Hokuto Electric Co., Ltd.).

[0131] Charge the battery at a constant current rate of 0.5C (the current value required to complete charging in 2 hours when charged at a constant current rate of 25°C) until the battery voltage reaches 4.2V. Discharge the battery at a constant current rate of 1.0C until the battery voltage reaches 2.5V. Measure the discharge capacity after the charge and discharge cycles to determine the battery capacity Q1 before the cycle test.

[0132] For the battery whose capacity Q1 was determined under the above conditions, a secondary battery charge-discharge test device was used again. The battery was charged at a constant current rate of 0.5C until the battery voltage reached 4.2V, and then discharged at a constant current rate of 1.0C until the battery voltage reached 2.5V. This charge-discharge cycle was defined as one cycle, and 100 cycles were performed. Afterward, the discharge capacity was measured after 100 cycles, and the battery capacity Q2 after 100 cycles was determined.

[0133] Based on the capacities Q1 and Q2 obtained under the above conditions, the capacity retention rate E after 100 cycles is calculated. The capacity retention rate E is obtained by E = Q2 / Q1 × 100. The capacity retention rate in Example 1 is 78%.

[0134] [Example 2, Example 3]

[0135] Examples 2 and 3 differ from Example 1 in that the molecular weight of the base polymer of the water-soluble polymer is changed. Other conditions are essentially the same as in Example 1, and the capacity retention rate is determined.

[0136] [Examples 4-10]

[0137] Examples 4-10 differ from Example 1 in that the mass fraction of sulfur was changed. The mass fraction of sulfur was altered by varying the amount of Li₂SO₄ added. Other conditions were essentially the same as in Example 1, and the capacity retention rate was determined.

[0138] [Examples 11-19]

[0139] Examples 11-19 differ from Example 1 in that the mass fraction of the divalent cation was changed. The mass fraction of the divalent cation was altered by changing the amount of Zn(OH)₂ added. Other conditions were essentially the same as in Example 1, and the capacity retention rate was determined.

[0140] [Examples 20-23]

[0141] The difference between Examples 20-23 and Example 1 is that the alkali metal substitution rate of the carboxyl groups in the water-soluble polymers was changed. No alkali metal substitution was performed in Example 20. Other conditions were essentially the same as in Example 1, and the capacity retention rate was determined.

[0142] [Examples 24-30]

[0143] Examples 24-30 differ from Example 1 in that the type of anion contained in the adhesive was changed. The type of anion in the adhesive was changed by using other compounds containing sulfur as an anion instead of Li₂SO₄. Other conditions were essentially the same as in Example 1, and the capacity retention rate was determined.

[0144] [Examples 31-38]

[0145] Examples 31-38 differ from Example 1 in that the type of divalent cation was changed. This was achieved by replacing Zn(OH)₂ with other hydroxides containing divalent cations. Other conditions remained essentially the same as in Example 1, and the capacity retention rate was determined.

[0146] [Example 39, Example 40]

[0147] Examples 39 and 40 differ from Example 1 in that the type of alkali metal replacing the carboxyl group of the water-soluble polymer is changed. Other conditions are essentially the same as in Example 1, and the capacity retention rate is determined.

[0148] [Example 41]

[0149] Example 41 differs from Example 1 in that the molecular weight of the base polymer of the water-soluble polymer is changed. Other conditions are essentially the same as in Example 1, and the capacity retention rate is determined.

[0150] [Example 42]

[0151] Example 42 differs from Example 1 in that the type and molecular weight of the base polymer of the water-soluble polymer are changed. Other conditions are essentially the same as in Example 1, and the capacity retention rate is determined.

[0152] [Comparative Example 1]

[0153] The difference between Comparative Example 1 and Example 1 is that Li₂SO₄ was not added to the precursor solution. That is, the binder in Comparative Example 1 differs from that in Example 1 in that it does not contain sulfur-containing anions. Other conditions were essentially the same as in Example 1, and the capacity retention rate was determined.

[0154] [Comparative Example 2]

[0155] The difference between Comparative Example 2 and Example 1 is that Zn(OH)₂ containing divalent cations was not added to the precursor solution. That is, the binder in Comparative Example 2 differs from that in Example 1 in that it does not contain divalent cations and is not fully cross-linked. Other conditions were essentially the same as in Example 1, and the capacity retention rate was determined.

[0156] [Comparative Example 3]

[0157] The difference between Comparative Example 3 and Example 1 is that polyvinyl alcohol (PVA) was used instead of polyacrylic acid as the water-soluble polymer. Other conditions were basically the same as in Example 1, and the capacity retention rate was determined.

[0158] The conditions and measurement results of Examples 1-42 and Comparative Examples 1-3 are summarized in Tables 1 and 2.

[0159] [Table 1]

[0160]

[0161] [Table 2]

[0162]

[0163] [Table 3]

[0164]

[0165] [Table 4]

[0166]

[0167] [Table 5]

[0168]

[0169] The cycling characteristics of Examples 1 through 42 are all excellent. Comparative Example 1 has low cycling characteristics. This is because sulfur was not sufficiently utilized to promote the formation of the SEI coating on the surface of the negative electrode active material. In Comparative Example 2, the cycling characteristics are low because cross-linking with divalent cations was not utilized. In Comparative Example 3, a water-soluble polymer without carboxyl groups, namely PVA (polyvinyl alcohol), was used; however, in the case of hydroxyl groups, cross-linking with divalent cations was insufficient, resulting in low cycling characteristics.

[0170] [Industry availability]

[0171] The cured material for lithium-ion secondary batteries of this embodiment can be suitably used as an adhesive for the negative electrode of lithium-ion secondary batteries.

[0172] [Symbol Explanation]

[0173] 1: Adhesive; 2: Water-soluble polymer; 2A: Carboxyl group; 2B: Carboxyl group substituted with alkali metal; 3: Divalent cation; 10: Separator; 20: Positive electrode; 22: Positive electrode current collector; 24: Positive electrode active material layer; 30: Negative electrode; 32: Negative electrode current collector; 34: Negative electrode active material layer; 40: Power generation element; 50: Outer packaging; 52: Metal foil; 54: Resin layer; 60, 62: Terminals; 100: Lithium-ion secondary battery.

Claims

1. A cured material for lithium-ion secondary batteries, comprising a water-soluble polymer, a divalent cation, and an anion. The water-soluble polymer has a carboxyl group. The divalent cations crosslink the water-soluble polymers. The anion contains sulfur.

2. The cured material for lithium-ion secondary batteries as described in claim 1, wherein, The divalent cation is selected from Mg. 2+ Ca 2+ 、Sr 2+ Ba 2+ Co 2+ Ni 2+ Cu 2+ Zn 2+ At least one of the groups.

3. The cured material for lithium-ion secondary batteries as described in claim 1, wherein, The water-soluble polymer has carboxyl groups that are replaced by alkali metals.

4. A negative electrode for a lithium-ion secondary battery, comprising a negative electrode active material and a solidified material for a lithium-ion secondary battery as described in claim 1.

5. The negative electrode for a lithium-ion secondary battery as described in claim 4, wherein, When the total mass fraction of all water-soluble polymers contained in the negative electrode is set to 100 parts by mass, the mass fraction of sulfur element is more than 0.002 parts by mass and less than 0.22 parts by mass.

6. The negative electrode for a lithium-ion secondary battery as described in claim 4, wherein, When the total mass fraction of all water-soluble polymers contained in the negative electrode is set to 100 parts by mass, the mass fraction of sulfur element is more than 0.005 parts by mass and less than 0.2 parts by mass.

7. The negative electrode for a lithium-ion secondary battery as described in claim 4, wherein, When the mass fraction of all water-soluble polymers contained in the negative electrode is set to 100 parts by mass, the mass fraction of the divalent cation is 0.005 parts by mass or more and 11.00 parts by mass or less.

8. The negative electrode for a lithium-ion secondary battery as described in claim 4, wherein, When the mass fraction of all water-soluble polymers contained in the negative electrode is set to 100 parts by mass, the mass fraction of the divalent cation is 0.01 parts by mass or more and 10 parts by mass or less.

9. A lithium-ion secondary battery, wherein, It comprises: a negative electrode for a lithium-ion secondary battery, a positive electrode, and a separator located between the negative electrode and the positive electrode, as described in any one of claims 4 to 8.

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