JP2023121481A - Cured product for lithium-ion secondary battery, negative electrode for lithium-ion secondary battery, and lithium-ion secondary battery - Google Patents

Abstract

To provide a cured product for lithium-ion secondary battery capable of improving cycle characteristics of a lithium-ion secondary battery.SOLUTION: The cured product for lithium-ion secondary battery has a polymer in which a water-soluble polymer is crosslinked with a titanium compound. When measuring a wide-angle X-ray scattering (WAXS) using CuKα rays, the diffraction angle 2θ has a peak in a range of 16° to 21° and the half-width of the peak is 7.1° or less.SELECTED DRAWING: Figure 2

Description

TECHNICAL FIELD The present invention relates to a cured product for lithium ion secondary batteries, a negative electrode for lithium ion secondary batteries, and a lithium ion secondary battery.

Lithium ion secondary batteries are also widely used as power sources for mobile devices such as mobile phones and laptop computers, and hybrid cars.

The capacity of a lithium ion secondary battery mainly depends on the active material of the electrodes. Graphite is generally used as a negative electrode active material, but there is a demand for a negative electrode active material with a higher capacity. Therefore, silicon (Si), which has a much larger theoretical capacity than that of graphite (372 mAh/g), has attracted attention.

A negative electrode active material containing Si undergoes a large volume expansion during charging. The volume expansion of the negative electrode active material causes deterioration in the cycle characteristics of the battery. When the negative electrode active material expands in volume, for example, cracks occur in the negative electrode active material, separation occurs at the interface between the negative electrode active material layer and the current collector, or cracks occur in the SEI (Solid Electrolyte Interphase) coating, resulting in decomposition of the electrolyte. etc. may occur. These degrade the cycle characteristics of the battery.

A binder is used in the negative electrode active material layer to improve the cycle characteristics of the battery. For example, Patent Literature 1 discloses a binder composition containing a crosslinked compound of polyvinyl acrylic acid and polyvinyl alcohol substituted with alkali cations.

JP 2012-64574 A

Further improvement in cycle characteristics is required.

The present disclosure has been made in view of the above problems, and an object thereof is to provide a cured product for lithium ion secondary batteries that can improve the cycle characteristics of lithium ion secondary batteries.

In order to solve the above problems, the following means are provided.

(1) The cured product for a lithium ion secondary battery according to the first aspect has a polymer in which a water-soluble polymer is crosslinked with a titanium compound, and is measured by wide-angle X-ray scattering (WAXS) using CuKα rays. Furthermore, it has a peak in the range of the diffraction angle 2θ of 16° or more and 21° or less, and the half width of the peak is 7.1° or less.

(2) In the cured product for a lithium ion secondary battery according to the above aspect, the water-soluble polymer is any selected from the group consisting of polyvinyl alcohol, carboxymethyl cellulose, methyl cellulose, and a copolymer of acrylic acid and vinyl alcohol. or one or more.

(3) In the cured product for a lithium ion secondary battery according to the above aspect, the water-soluble polymer may have a weight average molecular weight of 9000 or more and 200000 or less.

(4) A negative electrode for a lithium ion secondary battery according to the second aspect has a negative electrode active material and the cured product for a secondary battery according to the above aspect.

(5) A lithium ion secondary battery according to a third aspect includes the lithium ion secondary battery negative electrode according to the above aspect, a positive electrode, and a separator between the lithium ion secondary battery negative electrode and the positive electrode. , provided.

The lithium ion cured product according to the above aspect improves the cycle characteristics of the lithium ion secondary battery.

1 is a schematic diagram of a lithium ion secondary battery according to a first embodiment; FIG. 1 is a schematic diagram of a polymer that constitutes a binder according to the first embodiment; FIG. FIG. 2 is a schematic diagram of a polymer constituting a binder according to the first embodiment before cross-linking; It is an example of wide-angle X-ray scattering (WAXS) measurement results of the binder according to the first embodiment.

Hereinafter, embodiments will be described in detail with appropriate reference to the drawings. In the drawings used in the following description, characteristic portions may be enlarged for convenience in order to make the characteristics easier to understand, and the dimensional ratios and the like of each component may differ from the actual. The materials, dimensions, and the like exemplified in the following description are examples, and the present invention is not limited to them, and can be implemented with appropriate modifications without changing the gist of the invention.

"Lithium-ion secondary battery"
FIG. 1 is a schematic diagram of a lithium ion secondary battery according to a first embodiment. A lithium-ion secondary battery 100 shown in FIG. 1 includes a power generation element 40, an exterior body 50, and a non-aqueous electrolyte (not shown). The exterior body 50 covers the periphery of the power generation element 40 . The power generation element 40 is connected to the outside by a pair of connected terminals 60 and 62 . A non-aqueous electrolyte is contained in the exterior body 50 . Although FIG. 1 illustrates the case where there is one power generation element 40 in the exterior body 50, a plurality of power generation elements 40 may be stacked. The lithium-ion secondary battery 100 may be cylindrical, square, laminate, button, or the like.

(power generation element)
The power generation element 40 includes a separator 10 , a positive electrode 20 and a negative electrode 30 .

<Positive electrode>
The cathode 20 has, for example, a cathode current collector 22 and a cathode active material layer 24 . The cathode active material layer 24 is in contact with at least one surface of the cathode current collector 22 .

[Positive collector]
The positive electrode current collector 22 is, for example, a conductive plate. The positive electrode current collector 22 is, for example, a metal thin plate made of aluminum, copper, nickel, titanium, stainless steel, or the like. Aluminum, which is light in weight, is preferably used for the positive electrode current collector 22 . The average thickness of the positive electrode current collector 22 is, for example, 10 μm or more and 30 μm or less.

[Positive electrode active material layer]
The positive electrode active material layer 24 contains, for example, a positive electrode active material. The positive electrode active material layer 24 may contain a conductive aid and a binder as needed.

The positive electrode active material is an electrode active material that can reversibly absorb and release lithium ions, desorb and insert (intercalate) lithium ions, or dope and dedope lithium ions and counter anions. including.

The positive electrode active material is, for example, a composite metal oxide. Composite metal oxides include, for example, lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganate (LiMnO ₂ ), lithium manganese spinel (LiMn ₂ O ₄ ), and general formula: LiNi _x Co _{yMn z} M _a O ₂ compound (in the general formula, x + y + z + a = 1, 0 ≤ x < 1, 0 ≤ y < 1, 0 ≤ z < 1, 0 ≤ a < 1, M is Al, Mg, Nb, one or more elements selected from Ti, Cu, Zn, and Cr), lithium vanadium compound (LiV ₂ O ₅ ), olivine-type LiMPO ₄ (where M is Co, Ni, Mn, Fe, Mg, Nb, Ti , Al, and one or more elements selected from Zr or VO), lithium titanate (Li ₄ Ti ₅ O ₁₂ ), LiNi _x Co _y Al _z O ₂ (0.9<x+y+z<1.1) be. The positive electrode active material may be organic. For example, the positive electrode active material may be polyacetylene, polyaniline, polypyrrole, polythiophene, polyacene.

The positive electrode active material may be a non-lithium containing material. Lithium-free materials are, for example, FeF ₃ , conjugated polymers containing organic conductive materials, Chevrell phase compounds, transition metal chalcogenides, vanadium oxides, niobium oxides, and the like. Only one of the lithium-free materials may be used, or a plurality of materials may be used in combination. When the positive electrode active material is a lithium-free material, for example, it is first discharged. Lithium is inserted into the positive electrode active material by discharging. In addition, lithium may be chemically or electrochemically pre-doped into a positive electrode active material that does not contain lithium.

A conductive aid enhances the electronic conductivity between the positive electrode active materials. Examples of conductive aids include carbon powder, carbon nanotubes, carbon materials, metal fine powders, mixtures of carbon materials and metal fine powders, and conductive oxides. Examples of carbon powder include carbon black, acetylene black, and ketjen black. Metal fine powder is, for example, powder of copper, nickel, stainless steel, iron, or the like.

The content of the conductive aid in the positive electrode active material layer 24 is not particularly limited. For example, the content of the conductive aid with respect to the total mass of the positive electrode active material, the conductive aid, and the binder 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. .

The binder in the positive electrode active material layer 24 binds the positive electrode active materials together. A known binder can be used. Also, the binder may be the same as that used for the negative electrode active material layer 34, which will be described later. The binder is preferably insoluble in the electrolytic solution, has oxidation resistance, and has adhesiveness. The binder is, for example, fluororesin. Binders include, for example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyamide (PA), polyimide (PI), polyamideimide (PAI), polybenzimidazole (PBI), polyethersulfone (PES), Polyacrylic acid and its copolymers, metal ion crosslinked polyacrylic acid and its copolymers, maleic anhydride-grafted polypropylene (PP) or polyethylene (PE), and mixtures thereof. PVDF is particularly preferable as the binder used for the positive electrode active material layer.

The binder content in the positive electrode active material layer 24 is not particularly limited. For example, the binder content 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 with respect to the total mass of the positive electrode active material, the conductive aid, and the binder. When the binder content is low, the adhesive strength of the positive electrode 20 is weakened. If the binder content is high, the binder is electrochemically inactive and does not contribute to the discharge capacity, so the energy density of the lithium ion secondary battery 100 is low.

<Negative Electrode>
The negative electrode 30 has, for example, a negative electrode current collector 32 and a negative electrode active material layer 34 . The negative electrode active material layer 34 is formed on at least one surface of the negative electrode current collector 32 . The negative electrode 30 is an example of a negative electrode for a lithium ion secondary battery.

[Negative electrode current collector]
The negative electrode current collector 32 is, for example, a conductive plate. The negative electrode current collector 32 can be the same as the positive electrode current collector 22 .

[Negative electrode active material layer]
The negative electrode active material layer 34 contains a negative electrode active material and a binder. The negative electrode active material layer 34 may contain a conductive aid as needed. A binder is an example of a cured product for a lithium ion secondary battery.

The negative electrode active material contains silicon or a silicon compound. The silicon compound is, for example, a silicon alloy, silicon oxide, or the like. For example, the silicon or silicon compound may be crystalline, amorphous, or crystalline dispersed in amorphous. Amorphous silicon or a silicon compound can be produced by a melt spun method, a gas atomization method, or the like.

A silicon alloy is represented by XnSi. X is a cation. X is, for example, Ba, Mg, Al, Zn, Sn, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ge, Y, Zr, Nb, Mo, W, Au, Ti, Na , K, etc. n satisfies 0≦n≦0.5. Silicon oxide is denoted by SiO _x . x satisfies 0.8≦x≦2, for example. The silicon oxide may consist of only _SiO2 , may consist of only SiO, or may be a mixture of SiO and _SiO2 . In addition, silicon oxide may be partially deficient in oxygen.

The negative electrode active material may be a composite of silicon or a silicon compound. The composite is obtained by coating at least part of the surface of silicon or silicon compound particles with a conductive material. The conductive material is, for example, a carbon material, Al, Ti, Fe, Ni, Cu, Zn, Ag, Sn, or the like. For example, a silicon carbon composite (Si—C) is one example of a composite. The coating amount of the conductive material on the silicon or silicon compound particles 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 with respect to the total mass of the composite. is. The composite can be produced by, for example, a mechanical alloying method, a chemical vapor deposition method, a wet method, a method of coating a polymer and then thermally decomposing the polymer to carbonize it, or the like.

The specific surface area of the negative electrode active material determined by the BET method is, for example, 0.5 m ² /g or more and 100 m 2 /g or less, preferably 1.0 ^{m 2 /} g or more and 20 m ² /g or less. If the specific surface area is small, it becomes difficult for Li ions to intercalate and deintercalate between the negative electrode active materials. If the specific surface area is large, a large amount of binder is required for electrode formation, and the capacity per unit volume becomes small.

The binder in the negative electrode active material layer 34 binds the negative electrode active materials together. FIG. 2 is a schematic diagram of the polymer 1 that constitutes the binder according to the first embodiment. Polymer 1 has water-soluble polymer 2 and titanium compound 3 .

Here, whether or not the polymer is "water-soluble" can be determined by the following procedure. First, 1 part by weight of polymer (equivalent to solid content) is added to 100 parts by weight of ion-exchanged water and stirred to prepare a mixture. The mixture is adjusted to one condition at a temperature of 20-95° C. and a pH within the range of 3-12 (using aqueous NaOH and/or aqueous HCl for pH adjustment). The mixture is then passed through a 250 mesh screen. A polymer is said to be water-soluble if the weight of the residual solids remaining on the screen that does not pass through the screen does not exceed 50% by weight relative to the solids content of the added polymer. Even if the mixture of the polymer and water is in an emulsion state in which it separates into two phases when allowed to stand still, the polymer is water-soluble as long as it satisfies the above definition.

FIG. 3 is a schematic diagram of the polymer 1 before cross-linking that constitutes the binder according to the first embodiment. For example, when the hydroxyl groups of the water-soluble polymer 2 react with the hydroxyl groups of the precursor 4 of the titanium compound 3, the water-soluble polymer 2 and the titanium compound 3 are crosslinked. Part of the hydroxyl groups of the water-soluble polymer 2 remain unreacted without reacting with the precursor 4 of the titanium compound 3 . The remaining hydroxyl groups interact with the negative electrode active material to bind the negative electrode active material and the polymer 1 . Although an example of reaction between hydroxyl groups is shown here, the present invention is not limited to this example. For example, the alkoxy group of the titanium compound 3 is eliminated and reacts with the hydroxyl group of the water-soluble polymer 2, or the isocyanate group, carboxy group, or the like of the titanium compound 3 reacts with the hydroxyl group of the water-soluble polymer 2 to crosslink. good. Polymer 1 is obtained by cross-linking water-soluble polymer 2 with titanium compound 3 .

The titanium compound per 100 parts by weight of the water-soluble polymer is, for example, 5 parts by weight or more and less than 53 parts by weight, preferably 13 parts by weight or more and less than 26 parts by weight. If the abundance ratio of the titanium compound 3 is small, the number of cross-linking points in the polymer 1 is small and the elasticity of the binder is lowered. When the abundance ratio of the titanium compound 3 is high, the proportion of hydroxyl groups that can be freely present in the polymer 1 decreases, and the binding properties between the negative electrode active material and the negative electrode current collector 32 decrease.

The water-soluble polymer 2 is, for example, a polymer having hydroxyl groups. The water-soluble polymer 2 is, for example, polyvinyl alcohol, carboxymethylcellulose, methylcellulose, a copolymer of acrylic acid and vinyl alcohol, a copolymer of sodium acrylate and vinyl alcohol, sodium carboxymethylcellulose, polynorbornene dicarboxylic acid. . The water-soluble polymer 2 is preferably polyvinyl alcohol, carboxymethylcellulose, methylcellulose, or a copolymer of acrylic acid and vinyl alcohol. A copolymer of acrylic acid and vinyl alcohol can be obtained, for example, by copolymerizing a vinyl ester and an ethylenically unsaturated carboxylic acid ester and hydrolyzing the copolymer by acidification. One type of the water-soluble polymer 2 may be used alone, or a plurality of types may be used in combination.

The weight average molecular weight of the water-soluble polymer 2 is, for example, 9000-200000. In particular, when polyvinyl alcohol is used as the water-soluble polymer 2, the weight average molecular weight is preferably 9,000 to 200,000. When the molecular weight of the water-soluble polymer 2 is small, the number of cross-linking points in the polymer 1 is small and the elasticity of the binder is lowered. If the molecular weight of the water-soluble polymer 2 is large, the polymer 1 gels, making uniform dispersion difficult. The weight average molecular weight of the water-soluble polymer 2 can be determined by analyzing the water-soluble polymer before cross-linking, if the water-soluble polymer 2 before cross-linking is available. If the water-soluble polymer 2 before cross-linking is not available, it can be roughly calculated from the weight average molecular weight of the polymer 1 and the abundance ratio of the water-soluble polymer 2 and the titanium compound 3 in the polymer 1 .

When polyvinyl alcohol is used as the water-soluble polymer 2, partially saponified polyvinyl alcohol and fully saponified polyvinyl alcohol produced using saponified polyvinyl acetate are preferable. Polyvinyl alcohol is, for example, vinyl alcohol-vinyl acetate copolymer, vinyl alcohol-vinyl butyral copolymer, ethylene-vinyl alcohol copolymer, preferably vinyl alcohol-vinyl acetate copolymer.

The copolymerization ratio of polyvinyl alcohol is represented by the degree of saponification. The degree of saponification of polyvinyl alcohol is, for example, 60 mol % or more and 99 mol % or less. When the degree of saponification of polyvinyl alcohol is 60 mol % or more, it becomes easy to crosslink with the precursor 4 of the titanium compound 3 . The degree of saponification of polyvinyl alcohol can be determined by the amount of alkali consumption required for hydrolysis of copolymer units such as vinyl acetate or composition analysis by NMR.

Titanium compound 3 contains the element titanium in its structure. Titanium compound 3 is an organic titanium compound, such as a titanium chelate. Titanium compound 3 is, for example, titanium lactate, titanium triethanolamine, titanium lactate ammonium salt, titanium diethanolamine, titanium aminoethylaminoethrate, or the like. The titanium compound 3 is a cross-linking agent that connects the water-soluble polymer 2 . The titanium compound 3 is a compact cross-linking agent and does not easily destroy the crystallinity of the water-soluble polymer 2 .

FIG. 4 is an example of wide-angle X-ray scattering (WAXS) measurement results of the binder according to the first embodiment. Wide-angle X-ray scattering is performed using CuKα radiation. As shown in FIG. 4, the binder according to the first embodiment has a peak p in the diffraction angle 2θ range of 16° or more and 21° or less. The half width (full width at half maximum: FWHM) of the peak p is 7.1° or less. When the peak p generated by wide-angle X-ray scattering satisfies the above conditions, the binder has high strength and high elasticity. In general, the narrower the half width of X-ray scattering, the higher the crystallinity of the structure. The fact that the half-value width of the binder is within the above range is considered to indicate that the binder has a certain degree of crystallinity, and the strength of the binder is high. The position and half width of the peak p of the binder can be controlled by controlling the ratio of the water-soluble polymer in the binder, the effect conditions of the binder, and the like.

The binder content in the negative electrode active material layer 34 is not particularly limited. For example, the content of the binder 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 with respect to the total mass of the negative electrode active material, conductive aid, and binder. When the binder content is low, the adhesive strength of the negative electrode 30 is weakened. If the binder content is high, the binder is electrochemically inactive and does not contribute to the discharge capacity, so the energy density of the lithium ion secondary battery 100 is low.

The conductive aid in the negative electrode active material layer 34 enhances electronic conductivity between the negative electrode active materials. The conductive aid used in the positive electrode active material layer 24 can be used.

The content of the conductive aid in the negative electrode active material layer 34 is not particularly limited. For example, the content of the conductive aid is 5% by mass or more and 20% by mass or less, preferably 1% by mass or more and 12% by mass or less with respect to the total mass of the negative electrode active material, the conductive aid, and the binder.

<Separator>
Separator 10 is sandwiched between positive electrode 20 and negative electrode 30 . The separator 10 separates the positive electrode 20 and the negative electrode 30 and prevents short circuit between the positive electrode 20 and the negative electrode 30 . The separator 10 extends in-plane along the positive electrode 20 and the negative electrode 30 . Lithium ions can pass through the separator 10 .

The separator 10 has, for example, an electrically insulating porous structure. The separator 10 is, for example, a monolayer or laminate of polyolefin films. Separator 10 may be a stretched film of a mixture such as polyethylene or polypropylene. The separator 10 may be a fibrous nonwoven fabric made of at least one constituent material selected from the group consisting of cellulose, polyester, polyacrylonitrile, polyamide, polyethylene and polypropylene. Separator 10 may be, for example, a solid electrolyte. Solid electrolytes are polymer solid electrolytes, oxide-based solid electrolytes, and sulfide-based solid electrolytes, for example. Separator 10 may be an inorganic coated separator. The inorganic coated separator is obtained by coating the surface of the above film with a mixture of a resin such as PVDF or CMC and an inorganic material such as alumina or silica. The inorganic coated separator has excellent heat resistance and suppresses deposition of transition metals eluted from the positive electrode onto the surface of the negative electrode.

<Electrolyte>
The electrolytic solution is enclosed in the exterior body 50 and impregnates the power generating element 40 . The non-aqueous electrolyte has, for example, a non-aqueous solvent and an electrolytic salt. The electrolytic salt is dissolved in a non-aqueous solvent.

A known electrolytic solution can be used as the electrolytic solution. The electrolytic solution contains, for example, a non-aqueous solvent and an electrolytic salt.

An electrolytic salt is, for example, a lithium salt. The electrolyte is, for _example , _{LiPF6 , LiClO4} , _LiBF4 , _LiCF3SO3 , _{LiCF3CF2SO3 ,} LiC( _{CF3SO2 ) 3 ,} LiN( _CF3SO2 ) ₂ , _LiN ( _CF3CF2 SO ₂ ) ₂ , LiN(CF ₃ SO ₂ )(C ₄ F ₉ SO ₂ ), LiN(CF ₃ CF ₂ CO) ₂ , LiBOB, LiN(FSO ₂ ) ₂ and the like. Lithium salt may be used individually by 1 type, and may use 2 or more types together. From the point of view of the degree of ionization, the electrolyte 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.

Non-aqueous solvents are, for example, aprotic organic solvents. Examples of organic solvents include cyclic carbonates, chain carbonates, ethers, mixtures thereof, and the like. Alternatively, the solvent may be an ionic liquid.

Cyclic carbonates solvate electrolytes. Cyclic carbonates are, for example, ethylene carbonate, propylene carbonate, butylene carbonate, fluoroethylene carbonate. The cyclic carbonate preferably contains at least fluoroethylene carbonate. Fluoroethylene carbonate (FEC) has a high redox potential and is easily reduced and decomposed. Part of the fluoroethylene carbonate (FEC) is reductively decomposed, making it difficult for the electrolyte and the remaining solvent in the electrolytic solution to be decomposed. In addition, fluoroethylene carbonate (FEC) forms a thin and stable film (SEI film) on the entire surface of the negative electrode active material at the initial stage of use of the lithium ion secondary battery. The SEI coating prevents direct contact between the negative electrode active material and the electrolytic solution, and prevents decomposition of the electrolytic solution.

Chain carbonates reduce the viscosity of cyclic carbonates. Chain carbonates are, for example, diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate. Non-aqueous solvents may also include methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, and the like. .

The electrolytic solution may also contain an SEI (Solid Electrolyte Interface) forming material, a surfactant, and the like. Additives include, for example, vinylene carbonate, vinylethylene carbonate, phenylethylene carbonate, succinic anhydride, lithium bisoxalate, lithium tetrafluoroborate, dinitrile compounds, propanesultone, butanesultone, propenesultone, 3-sulfolene, fluorinated Allyl ether, fluorinated acrylate, and the like.

<Exterior body>
The exterior body 50 seals the power generation element 40 and the non-aqueous electrolyte therein. The exterior body 50 prevents the leakage of the non-aqueous electrolyte to the outside and the intrusion of moisture into the inside of the lithium ion secondary battery 100 from the outside.

The exterior body 50 has a metal foil 52 and a resin layer 54 laminated on each surface of the metal foil 52, as shown in FIG. 1, for example. The exterior body 50 is a metal laminate film in which a metal foil 52 is coated from both sides with polymer films (resin layers 54).

For example, aluminum foil can be used as the metal foil 52 . A polymer film such as polypropylene can be used for the resin layer 54 . The material forming the resin layer 54 may be different between the inner side and the outer side. For example, a polymer with a high melting point such as polyethylene terephthalate (PET) or polyamide (PA) is used as the outer material, and polyethylene (PE) or polypropylene (PP) is used as the inner polymer film material. be able to.

<Terminal>
Terminals 60 and 62 are connected to positive electrode 20 and negative electrode 30, respectively. A terminal 60 connected to the positive electrode 20 is a positive terminal, and a terminal 62 connected to the negative electrode 30 is a negative terminal. Terminals 60 and 62 are responsible for electrical connection with the outside. Terminals 60, 62 are made of a conductive material such as aluminum, nickel, or copper. The connection method may be welding or screwing. Terminals 60, 62 are preferably protected with insulating tape to prevent short circuits.

"Manufacturing method of lithium ion secondary battery"
The lithium ion secondary battery 100 is manufactured by preparing a negative electrode 30, a positive electrode 20, a separator 10, an electrolytic solution, and an outer package 50, and assembling them. An example of a method for manufacturing the lithium ion secondary battery 100 will be described below.

First, the negative electrode 30 is produced. The negative electrode 30 is manufactured, for example, by sequentially performing a precursor solution preparation process, a slurry preparation process, an electrode coating process, a curing process, and a rolling process.

First, a precursor solution preparation step is performed. First, a water-soluble polymer solution and a precursor 4 of a titanium compound 3 are prepared. The precursor 4 of the titanium compound 3 is a substance before cross-linking with the water-soluble polymer, and the precursor 4 itself is also a titanium compound. Next, the water-soluble polymer solution and the precursor 4 are mixed to prepare a binder precursor solution. The mixing ratio of the water-soluble polymer solution and the precursor 4 of the titanium compound 3 is adjusted according to the composition of the intended binder.

Although it depends on the type of water-soluble polymer, when dissolving the water-soluble polymer in water, it is stirred at room temperature to 100° C. or less for 2 hours or more. The number of revolutions during stirring is set to 550 rpm or more and 1500 rpm or less to completely dissolve. After that, the precursor 4 of the titanium compound 3 is added to the water-soluble polymer solution and stirred at room temperature for 5 minutes or longer. The rotation speed during stirring is, for example, 180 rpm or more and 400 rpm or less. By sufficiently dispersing the precursor 4 of the titanium compound 3 in the water-soluble polymer solution, the crystallinity of the cured binder can be enhanced.

Next, a slurry preparation step is performed. In the slurry preparation step, a negative electrode active material (silicon or silicon compound) and a conductive aid are added to a binder precursor solution.

Solvents are, for example, water, N-methyl-2-pyrrolidone, and the like. The composition ratio of the negative electrode active material, the conductive aid, and the binder precursor solution is, for example, 70 wt % to 100 wt %:0 wt % to 10 wt %:0 wt % to 20 wt % in mass ratio. These mass ratios are adjusted so that the total is 100 wt %.

The negative electrode active material may be a composite obtained by mixing active material particles and a conductive material while applying a shearing force. When the active material particles are mixed by applying a shearing force to such an extent that the active material particles are not degraded, the surfaces of the active material particles are coated with the conductive material. In addition, the particle size of the negative electrode active material can be adjusted by the degree of mixing. Further, the negative electrode active material after production may be sieved to make the particle size uniform.

Next, an electrode coating process is performed. The electrode application step is a step of applying slurry to the surface of the negative electrode current collector 32 . The slurry application method is not particularly limited. For example, a slit die coating method and a doctor blade method can be used as a slurry coating method.

A curing step is then performed. In the curing step, the slurry is annealed. By annealing the slurry, the water-soluble polymer 2 and the titanium compound 3 are crosslinked. The curing step also removes the solvent. A hardening process is performed in nitrogen atmosphere, for example. The curing temperature is, for example, 120° C. or higher and 150° C. or lower. Also, the rate of temperature increase up to the curing temperature is, for example, 2° C./min or more and 5° C./min or less. Also, the temperature drop rate after curing is, for example, 2° C./min or more and 5° C./min or less. As described above, in the precursor solution preparation step, the precursor 4 of the titanium compound 3 is sufficiently dispersed in the water-soluble polymer solution, and the mixing ratio of the water-soluble polymer 2 and the precursor 4 of the titanium compound 3 in the binder is adjusted. By curing the binder under the above conditions, the position and half width of the peak p in the wide-angle X-ray scattering (WAXS) measurement results of the cured binder can be controlled.

A rolling process is performed as needed. The rolling step is a step of applying pressure to the negative electrode active material layer 34 to adjust the density of the negative electrode active material layer 34 . The rolling process is performed by, for example, a roll press device.

The positive electrode 20 can be produced in the same procedure as the negative electrode 30 . A commercially available product can be used for the separator 10 and the outer package 50 .

Next, the positive electrode 20 and the negative electrode 30 are laminated so that the separator 10 is positioned between them to produce the power generation element 40 . When the power generating element 40 is a wound body, the positive electrode 20, the negative electrode 30, and the separator 10 are wound around one end side of the separator.

Finally, the power generation element 40 is enclosed in the exterior body 50 . A non-aqueous electrolyte is injected into the exterior body 50 . After injecting the non-aqueous electrolyte, the power generation element 40 is impregnated with the non-aqueous electrolyte by depressurizing, heating, or the like. The lithium ion secondary battery 100 is obtained by applying heat or the like to seal the exterior body 50 . Instead of injecting the electrolytic solution into the exterior body 50, the power generation element 40 may be impregnated with the electrolytic solution.

The lithium ion secondary battery 100 according to the first embodiment has excellent cycle characteristics. This is probably because the binder contained in the negative electrode active material layer 34 has high strength and high elasticity. The binder according to the present embodiment maintains the crystal structure of the water-soluble polymer by cross-linking the water-soluble polymer with a compact titanium compound. This can also be confirmed from the fact that the wide-angle X-ray scattering (WAXS) measurement result has a predetermined peak p. A high-strength binder can be obtained by maintaining the crystallinity of the water-soluble polymer. A highly elastic binder can be obtained by cross-linking water-soluble polymers with a titanium compound.

As described above, the embodiments of the present invention have been described in detail with reference to the drawings. , substitutions, and other modifications are possible.

"Example 1"
First, a binder precursor solution was prepared. First, 100 parts by mass of a water-soluble polymer and water were added, and the mixture was stirred at room temperature for 2 hours or more. After that, 3 parts by mass of a titanium compound was added to the water-soluble polymer aqueous solution. This solution was stirred at room temperature at 300 rpm for 5 minutes or longer. Carboxymethyl cellulose was used as the water-soluble polymer. Titanium lactate (Orgatics TC-315 manufactured by Matsumoto Fine Chemicals Co., Ltd.) was used as the titanium compound.

Next, a negative electrode active material and a conductive aid were added to the binder precursor solution. The negative electrode active material was SiO _x that had undergone a disproportionation reaction by heat treatment at 1000° C. under reduced pressure. Super-P (registered trademark) was used as the conductive aid. Then, 25 g of the negative electrode active material, 1.4 g of the conductive aid, and 4.5 g of the binder precursor solution (solid concentration: 30%) were mixed to prepare a coating liquid (slurry). The total solid concentration in the coating liquid was 35 wt%. Then, the slurry was applied on a copper foil as a negative electrode current collector with a doctor blade.

Next, the slurry-coated negative electrode current collector was annealed in a nitrogen atmosphere. Annealing was performed under the conditions of increasing the temperature at 5° C./min, keeping the temperature at 150° C. for 2 hours, and decreasing the temperature at 5° C./min. Annealing causes the water-soluble polymer and the titanium compound to crosslink, and the slurry hardens. Then, the negative electrode active material layer 34 was formed by rolling the negative electrode current collector after curing the slurry. A metal mold was used to punch an electrode size of 22×32 mm to prepare a negative electrode.

Here, a binder precursor solution was prepared under the same conditions as above, and this binder precursor solution was annealed to prepare a binder (excluding the negative electrode active material and the conductive aid). This binder was subjected to wide-angle X-ray scattering measurement using CuKα rays. The diffraction angle 2θ of the binder of Example 1 was 16.2° and the half width was 3.4°.

Li _x CoO ₂ was used as the positive electrode active material. Ketjenblack was used as the conductive aid. 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 mass of a positive electrode active material, 2 parts by mass of a conductive aid, 2 parts by mass of a binder, and 70 parts by mass of a solvent. Then, the positive electrode slurry was applied to one surface of an aluminum foil having a thickness of 15 μm, vacuum-dried at 100° C. for 2 hours, and rolled to form a positive electrode active material layer 24 . Then, an electrode size of 22×32 mm was punched out using a mold to prepare a positive electrode.

Next, an electrolytic solution was prepared. The solvent used was a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) at a mass ratio of 3:7. LiPF ₆ was used as the electrolytic salt. The concentration of LiPF ₆ was 1 mol/L.

(Production of lithium ion secondary battery for evaluation)
The produced negative electrode and positive electrode were laminated via a separator (porous polyethylene sheet) so that the positive electrode active material layer and the negative electrode active material layer faced each other to obtain a laminate. A negative electrode lead made of nickel was attached to the negative electrode of the laminate. A positive electrode lead made of aluminum was attached to the positive electrode of the laminate. The positive lead and negative lead were welded by an ultrasonic welder. This laminate was inserted into an outer package made of an aluminum laminate film, and heat-sealed except for one peripheral portion to form a closed portion. Finally, after the electrolytic solution was injected into the exterior body, the remaining one portion was heat-sealed while the pressure was reduced by a vacuum sealer to fabricate a lithium ion secondary battery.

(Measurement of capacity retention rate after 200 cycles)
Cycle characteristics of lithium ion secondary batteries were measured. Cycle characteristics were measured using a secondary battery charge/discharge test device (manufactured by Hokuto Denko Co., Ltd.).

Charge until the battery voltage reaches 4.2 V at a constant current charge of 0.5C (current value at which charging ends in 1 hour when constant current charge is performed at 25°C), discharge rate 1.0C. The battery was discharged at a constant current until the battery voltage reached 2.5V. The discharge capacity after charging/discharging was detected to obtain the battery capacity _Q1 before the cycle test.

The battery for which the battery capacity _Q1 was obtained as described above was again charged with a constant current charge at a charge rate of 0.5C using the secondary battery charge/discharge test equipment until the battery voltage reached 4.2V. The battery was discharged at a constant current of 5C until the battery voltage reached 2.5V. The above charge/discharge was counted as one cycle, and 200 cycles of charge/discharge were performed. After that, the discharge capacity after 200 cycles of charging and discharging was detected, and the battery capacity _Q2 after 200 cycles was obtained.

From the capacities Q ₁ and Q ₂ obtained above, the capacity retention rate E after 200 cycles was obtained. The capacity retention rate E is obtained by E=Q ₂ /Q ₁ ×100. The capacity retention rate of Example 1 was 66%.

"Examples 2-28"
Examples 2 to 28 differ from Example 1 in that either the type of water-soluble polymer, the type of titanium compound, or the weight part of the titanium compound relative to the water-soluble polymer was changed. Moreover, when polyvinyl alcohol is used as the water-soluble polymer, it is different from Example 1 in that it is dissolved at 100° C. to obtain an aqueous solution of polyvinyl alcohol. Other conditions were the same as in Example 1, and the peak position and half width of the binder and the capacity retention rate were determined.

"Comparative Example 1"
Comparative Example 1 is different from Example 1 in that no titanium compound was added during the preparation of the binder. Other conditions were the same as in Example 1, and the peak position and half width of the binder and the capacity retention rate were determined.

"Comparative Example 2"
Comparative Example 2 differs from Example 1 in that the type of water-soluble polymer, the weight part of the titanium compound with respect to the water-soluble polymer, and the curing conditions of the binder were changed. Other conditions were the same as in Example 1, and the peak position and half width of the binder and the capacity retention rate were determined.

The results of Examples 1 to 28 and Comparative Examples 1 and 2 are summarized in Table 1. TC-300 in Table 1 means Orgatics TC-300 manufactured by Matsumoto Fine Chemical Co., Ltd. TC-300 is a titanium lactate ammonium salt. Similarly, TC-315 in Table 1 means Orgatics TC-315 manufactured by Matsumoto Fine Chemicals. TC-315 is titanium lactate. The weight average molecular weight of the partially saponified polyvinyl alcohol is also shown.

The lithium ion secondary batteries using the binders according to Examples 1 to 28 exhibited high capacity retention rates even after 200 cycles, and exhibited high cycle characteristics. On the other hand, in the lithium ion secondary battery using the binder according to Comparative Example 1, the binder did not exhibit sufficient elasticity and could not be charged and discharged multiple times. The lithium ion secondary battery using the binder according to Comparative Example 2 had insufficient cycle characteristics as compared with Examples 1-28.

1 polymer 2 water-soluble polymer 3 titanium compound 4 precursor 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 exterior body 52 metal foil 54 resin Layers 60, 62 Terminal 100 Lithium ion secondary battery

Claims (5)

The water-soluble polymer has a polymer crosslinked with a titanium compound,
When measured by wide-angle X-ray scattering (WAXS) using CuKα rays, the diffraction angle 2θ has a peak in the range of 16 ° or more and 21 ° or less,
A cured product for a lithium ion secondary battery, wherein the half width of the peak is 7.1° or less. The lithium ion secondary according to claim 1, wherein the water-soluble polymer is at least one selected from the group consisting of polyvinyl alcohol, carboxymethylcellulose, methylcellulose, and a copolymer of acrylic acid and vinyl alcohol. Cured material for batteries. The cured product for a lithium ion secondary battery according to claim 1 or 2, wherein the water-soluble polymer has a weight average molecular weight of 9000 or more and 200000 or less. A negative electrode for a lithium ion secondary battery, comprising a negative electrode active material and the cured product for a secondary battery according to any one of claims 1 to 3. A lithium ion secondary battery comprising: the negative electrode for lithium ion secondary batteries according to claim 4; a positive electrode; and a separator interposed between the negative electrode for lithium ion secondary batteries and the positive electrode.

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