JP5725075B2 - Secondary battery negative electrode binder, secondary battery negative electrode, and lithium ion secondary battery - Google Patents

Description

The present invention relates to a negative electrode binder used in a lithium ion secondary battery, a lithium ion secondary battery negative electrode, and a lithium ion secondary battery.

  A lithium ion secondary battery is a secondary battery having a high charge / discharge capacity and capable of high output. Currently, it is mainly used as a power source for portable electronic devices, and further expected as a power source for electric vehicles that are expected to be widely used in the future. Lithium ion secondary batteries have active materials capable of inserting and extracting lithium (Li) in the positive electrode and the negative electrode, respectively. And it operates by moving lithium ions in the electrolyte provided between the two electrodes.

  In lithium ion secondary batteries, lithium-containing metal composite oxides such as lithium cobalt composite oxide are mainly used as the active material for the positive electrode, and carbon materials having a multilayer structure are mainly used as the active material for the negative electrode. Yes. The performance of the lithium ion secondary battery depends on the materials of the positive electrode, the negative electrode, and the electrolyte constituting the secondary battery. In particular, research and development of active material that forms an active material is being actively conducted. For example, silicon or silicon oxide having a higher capacity than carbon has been studied as a negative electrode active material.

  By using silicon as the negative electrode active material, a battery having a higher capacity than that using a carbon material can be obtained. However, silicon has a large volume change due to insertion and extraction of Li during charge and discharge. Moreover, it is known that silicon itself is pulverized by repeated occlusion / release. Therefore, there is a problem that silicon is pulverized and falls off or peels from the current collector, and the charge / discharge cycle life of the battery is short. In order to solve this problem, studies have been made to suppress volume change associated with insertion and extraction of Li during charge and discharge rather than silicon by using silicon oxide as a negative electrode active material.

For example, the use of silicon oxide (SiO _x : x is about 0.5 ≦ x ≦ 1.5) as a negative electrode active material has been studied. It is known that SiO _x segregates in SiO ₂ within particles when heat-treated. This is called disproportionation reaction, and it is separated into two phases of Si phase and SiO ₂ phase by solid internal reaction. The Si phase obtained by separation is very fine. Further, the SiO ₂ phase covering the Si phase has a function of suppressing the decomposition of the electrolytic solution. Therefore, the secondary battery using the negative electrode active material composed of SiO _x decomposed into Si and SiO ₂ has excellent cycle characteristics.

As the silicon particles constituting the Si phase of SiO _x are finer, the secondary battery using the silicon particles as the negative electrode active material has improved cycle characteristics. Therefore, Japanese Patent No. 3806333 (Patent Document 1) describes a method of heating and sublimating metallic silicon and SiO ₂ to form silicon oxide gas and cooling it to produce SiO _x . According to this method, the particle size of the silicon particles constituting the Si phase can be set to a nanosize of 1-5 nm.

JP 2009-102219 (Patent Document 2) discloses that a silicon raw material is decomposed to an elemental state in a high-temperature plasma, and rapidly cooled to a liquid nitrogen temperature to obtain silicon nanoparticles. A manufacturing method for fixing in a SiO ₂ —TiO ₂ matrix by a sol-gel method or the like is described.

However, in the manufacturing method described in Patent Document 1, the matrix is limited to a sublimable material. For this reason, since the matrix is limited to SiO ₂ , the matrix and lithium react irreversibly during lithium occlusion. Such a reaction is a main factor of cell capacity reduction. Further, in the manufacturing method described in Patent Document 2, high energy is required for plasma discharge. Furthermore, the silicon composites obtained by these production methods have a disadvantage that the dispersibility of Si-phase silicon particles is low and they are likely to aggregate. When Si particles are aggregated to increase the particle size, a secondary battery using the same as a negative electrode active material has a low initial capacity and cycle characteristics.

By the way, in recent years, nanosilicon materials that are expected to be used in various fields such as semiconductors, electrical / electronics, and the like have been developed. For example, Physical Review B (1993), vol 48, 8172-8189 (Non-Patent Document 1) describes a method of synthesizing layered polysilane by reacting hydrogen chloride (HCl) with calcium disilicide (CaSi ₂ ). In addition, it is described that the layered polysilane thus obtained can be used for a light emitting device or the like. Since this material can be manufactured by a method other than the sublimation method or plasma discharge, problems such as irreversible capacity and initial efficiency reduction due to aggregation can be solved.

  JP 2011-090806 A (Patent Document 3) describes a lithium ion secondary battery using layered polysilane as a negative electrode active material.

Japanese Patent No. 3806333 JP 2009-102219 A JP 2011-090806 JP

Physical Review B (1993), vol48,8172-8189

  However, the negative electrode active material made of layered polysilane described in Patent Document 3 has a disadvantage that it is not preferable as a negative electrode active material for a secondary battery because of its large BET specific surface area. For example, in the negative electrode of a lithium ion secondary battery, if the BET specific surface area is large, the irreversible capacity consumed in the negative electrode in order to promote the decomposition of the electrolytic solution increases, and it is difficult to increase the capacity. There is also a problem that SEI is likely to occur and the cycle characteristics are low.

  Thus, it has been recalled that the layered polysilane described in Patent Document 3 is fired in a non-oxidizing atmosphere. According to this method, nanosilicon having a crystallite size of several nanometers can be obtained, which is preferable as a negative electrode active material. However, a lithium ion secondary battery using nanosilicon produced by this method as a negative electrode active material has a problem that initial efficiency is low and capacity retention after a cycle test is low.

  This invention is made | formed in view of such a situation, and it aims at solving this problem by improving the binder of a negative electrode.

The feature of the binder for a lithium ion secondary battery negative electrode of the present invention that solves the above problems is a reaction between polyacrylic acid and at least one metal compound selected from basic copper acetate, basic nickel acetate, or copper hydroxide It consists of things.

  And the characteristic of the secondary battery negative electrode of this invention consists of a collector and the negative electrode active material layer formed in the surface of a collector, and a negative electrode active material layer is a negative electrode active material and the binder of this invention. And to include.

The binder for a lithium ion secondary battery negative electrode of the present invention comprises a reaction product of polyacrylic acid and at least one metal compound selected from basic copper acetate, basic nickel acetate, or copper hydroxide . By including this metal element, the conductivity is improved as compared with a binder made of only a normal polymer, so that the initial efficiency of the lithium ion secondary battery is improved. Furthermore, since this metal element can change the valence, electrons are transferred according to the potential change during charging and discharging, and the lithium ion secondary battery using the negative electrode of the present invention further improves initial efficiency and cycle. Later efficiency is also improved.

2 is an SEM image of a gray powder prepared in Example 1.

Lithium-ion secondary battery negative electrode binder of the present invention, polyacrylic acid, basic copper acetate, and at least one metal compound selected from basic nickel acetate or copper hydroxide, ing from the reaction of.

The metal compound is preferably contained in an amount of 0.01 to 10 parts by mass with respect to 100 parts by mass of polyacrylic acid . If the amount of the metal compound is less than 0.01 parts by mass, it will be difficult to achieve the effect of containing the metal element. If the amount of the metal compound exceeds 10 parts by mass, the viscosity of the reaction product of the polymer and the metal compound becomes too high, and the negative electrode active material layer is formed. Preparation of the slurry for forming becomes difficult.

Examples of the reaction product of polyacrylic acid and a metal compound include salt compounds and complex compounds.

In order to obtain a reactant by reacting polyacrylic acid and a metal compound , a method of mixing and heating both, a method of heating by dissolving both polyacrylic acid and a metal compound in a soluble solvent, etc. are used. be able to. In order to obtain a uniform reaction product at the molecular level, a method of heating in a state where both are mixed in a liquid state is preferable, as in the latter method.

The binder for a secondary battery negative electrode of the present invention can be used together with a known negative electrode active material such as graphite, hard carbon, silicon, carbon fiber, tin (Sn), and silicon oxide. Among the negative electrode active materials, silicon oxide represented by SiO _x (0.3 ≦ x ≦ 1.6), or nanosilicon obtained by firing layered polysilane described in Non-Patent Document 1 and Patent Document 3, etc. preferable.

The inventors of the present application conducted intensive studies on the layered polysilanes described in Non-Patent Document 1 and Patent Document 3. By reacting calcium disilicide (CaSi ₂ ) with an acid such as an aqueous solution of hydrogen chloride (HCl), a layered polysilane can be obtained. Calcium disilicide (CaSi ₂ ) forms a layered crystal in which a Ca atomic layer is inserted between the (111) faces of diamond-type Si, and the layered polysilane is formed by extracting calcium (Ca) by reaction with acid. Is obtained.

Alternatively, a mixture of an aqueous hydrogen fluoride (HF) solution and an aqueous hydrogen chloride (HCl) solution can be used as the acid. The composition ratio of hydrogen fluoride (HF) and hydrogen chloride (HCl) is preferably such that the molar amount of hydrogen fluoride (HF) is 100 or less, where the molar amount of hydrogen chloride (HCl) is 100. If the amount of hydrogen fluoride (HF) exceeds this ratio, impurities such as CaF ₂ and CaSiO are generated, and it is difficult to separate this impurity from the layered polysilane, which is not preferable.

The reaction ratio between the acid and calcium disilicide (CaSi ₂ ) is preferably such that the acid is in excess of the equivalent. The reaction atmosphere is preferably performed in an inert gas atmosphere. Although reaction time and reaction temperature are not specifically limited, Usually, reaction temperature is 0 degreeC-100 degreeC, and reaction time is 0.25-24 hours.

  Nanosilicon obtained by the above reaction is aggregated into aggregated particles. For this reason, there is a problem that the aggregated particles are pulverized due to repeated expansion and contraction during charging and discharging as the power storage device, and the specific surface area is increased and the cycle characteristics are degraded due to the generation of SEI.

  Therefore, it is particularly preferable to use a negative electrode active material comprising aggregated particles made of nanosilicon, a carbon layer made of amorphous carbon and composited covering at least a part of the aggregated particles. The carbon layer made of amorphous carbon covers at least a part of the aggregated particles. The effect that aggregated particles are reinforced by this carbon layer is expressed. A conductive aid such as graphite, acetylene black, or ketjen black may be used for the negative electrode, but these carbons are crystalline and not amorphous. The thickness of the carbon layer composited over the nanosilicon aggregate is preferably in the range of 1 to 100 nm, and more preferably in the range of 5 to 50 nm. If the thickness of the carbon layer is too thin, it will be difficult to achieve the effect, and if the thickness of the carbon layer is too thick, the battery resistance may increase and charging / discharging may become difficult.

In the case of forming a carbon layer, mixing amorphous carbon separately produced by some method with nanosilicon agglomerated particles becomes inhomogeneous and carbon may cover at least a part of the agglomerated particles. Have difficulty. Therefore, a method has been developed in which amorphous carbon reliably covers at least a part of the aggregated particles to produce a homogeneous negative electrode active material. The manufacturing method includes an aggregated particle forming step of obtaining a nanosilicon aggregated particle by heat-treating a layered polysilane represented by the composition formula (SiH) _n having a structure in which a plurality of six-membered rings composed of silicon atoms are connected, and an aggregation It is desirable to perform in this order a polymerization step for polymerizing the aromatic heterocyclic compound in a state where the particles and the aromatic heterocyclic compound are mixed, and a carbonization step for carbonizing the polymer of the aromatic heterocyclic compound. .

  The formation process of the nanosilicon agglomerated particles is as described above.

  In the polymerization step, the aromatic heterocyclic compound is polymerized in a state where the nanosilicon aggregated particles and the aromatic heterocyclic compound are mixed. As a result, a polymer of the aromatic heterocyclic compound in a state of being attached to the nanosilicon aggregated particles is obtained. Here, the aromatic heterocyclic compound includes five-membered aromatic heterocyclic compounds such as furan, pyrrole, thiophene, imidazole, pyrazole, oxazole, isoxazole, thiazole and isothiazole, and polycyclic rings such as indole, benzimidazole and benzofuran. A polymerizable substance such as an aromatic heterocyclic compound can be used.

  Various polymerization methods can be employed to polymerize these compounds. In the case of pyrrole or the like, a method of heating in the presence of a polymerization catalyst such as concentrated hydrochloric acid or iron trichloride is simple. In particular, when iron trichloride is used, it can be polymerized in a non-aqueous atmosphere and the oxidation of Si can be suppressed, so that there is an effect that the initial capacity is increased when a power storage device is obtained.

  In the carbonization step, the polymer of the aromatic heterocyclic compound is carbonized in a state of being mixed with the nanosilicon aggregate particles. This step may be heat-treated in an inert atmosphere at a temperature of 100 ° C. or higher, and preferably 400 ° C. or higher as in the case of nanosilicon production. Since aromatic heterocyclic compounds are polymers, carbonization proceeds without evaporation even when heated, and a composite in which a carbon layer composed of amorphous carbon is bonded to the surface of nanosilicon agglomerated particles Is obtained. If heat treatment is performed in a state where nanosilicon agglomerated particles and an aromatic heterocyclic compound are mixed without performing a polymerization step, the aromatic heterocyclic compound evaporates and carbonization is difficult.

  In the negative electrode active material in which amorphous carbon covers at least a part of the aggregated particles, the composition ratio of silicon to carbon is desirably Si / C = 3/1 to 20/1 in terms of weight ratio. If this ratio exceeds 20/1, the effect of forming a carbon layer is not exhibited, and if it is less than 3/1, the capacity of the secondary battery decreases.

<Secondary battery negative electrode>
The secondary battery negative electrode of the present invention comprises a current collector and a negative electrode active material layer formed on the surface of the current collector. The negative electrode active material layer comprises a negative electrode active material and the binder of the present invention. Including.

  A current collector is a chemically inert electronic high conductor that keeps current flowing through an electrode during discharging or charging. The current collector can adopt a shape such as a foil or a plate, but is not particularly limited as long as it has a shape according to the purpose. As the current collector, for example, a copper foil or an aluminum foil can be suitably used.

  The negative electrode active material is as described above. In order to produce a negative electrode for a non-aqueous secondary battery using a negative electrode active material, a negative electrode active material powder, a conductive additive such as carbon powder, the binder of the present invention, and an appropriate amount of an organic solvent are added and mixed. The slurry can be produced by applying it onto a current collector by a roll coating method, a dip coating method, a doctor blade method, a spray coating method, a curtain coating method, or the like, and drying or curing the binder. .

  The binder is required to bind the active material and the like in as small an amount as possible, but the addition amount is preferably 0.5 wt% to 50 wt% of the total of the active material, the conductive auxiliary agent, and the binder. When the binder is less than 0.5 wt%, the moldability of the electrode is lowered, and when it exceeds 50 wt%, the energy density of the electrode is lowered.

  The binder includes, in addition to the binder of the present invention, polyvinylidene fluoride (PolyVinylidene DiFluoride: PVdF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyimide (PI), polyamideimide (PAI), Carboxymethyl cellulose (CMC), polyvinyl chloride (PVC), methacrylic resin (PMA), polyacrylonitrile (PAN), modified polyphenylene oxide (PPO), polyethylene oxide (PEO), polyethylene (PE), polypropylene (PP), polyacryl An acid (PAA) or the like can also be mixed.

  The conductive assistant is added to increase the conductivity of the electrode. Carbon black, graphite, acetylene black (AB), ketjen black (KB), vapor grown carbon fiber (VGCF), etc., which are carbonaceous fine particles, are used alone or in combination of two or more as conductive aids. Can be added. The amount of the conductive auxiliary agent used is not particularly limited, but can be, for example, about 20 to 100 parts by mass with respect to 100 parts by mass of the negative electrode active material. If the amount of the conductive auxiliary is less than 20 parts by mass, an efficient conductive path cannot be formed, and if it exceeds 100 parts by mass, the moldability of the electrode deteriorates and the energy density decreases.

  There is no restriction | limiting in particular in an organic solvent, The mixture of a some solvent may be sufficient. N-methyl-2-pyrrolidone and N-methyl-2-pyrrolidone and ester solvents (ethyl acetate, n-butyl acetate, butyl cellosolve acetate, butyl carbitol acetate, etc.) or glyme solvents (diglyme, triglyme, tetraglyme, etc.) The mixed solvent is particularly preferred.

  When the secondary battery is a lithium ion secondary battery, the negative electrode can be pre-doped with lithium. In order to dope lithium into the negative electrode, for example, an electrode formation method in which a half battery is assembled using metallic lithium as the counter electrode and electrochemically doped with lithium can be used. The amount of lithium doped is not particularly limited.

  When the secondary battery is a lithium ion secondary battery, known positive electrodes, electrolytes, and separators that are not particularly limited can be used. The positive electrode may be anything that can be used in a lithium ion secondary battery. The positive electrode has a current collector and a positive electrode active material layer bound on the current collector. The positive electrode active material layer includes a positive electrode active material and a binder, and may further include a conductive additive. The positive electrode active material, the conductive auxiliary agent, and the binder are not particularly limited as long as they can be used in the lithium ion secondary battery.

Examples of the positive electrode active material include lithium metal, LiCoO ₂ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , Li ₂ MnO ₃ , and sulfur. The current collector is not particularly limited as long as it is generally used for the positive electrode of a lithium ion secondary battery, such as aluminum, nickel, and stainless steel. As the conductive auxiliary agent, the same ones as described in the above negative electrode can be used.

The electrolytic solution is obtained by dissolving a lithium metal salt as an electrolyte in an organic solvent. The electrolytic solution is not particularly limited. As the organic solvent, an aprotic organic solvent such as propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC) or the like should be used. Can do. As the electrolyte to be dissolved, a lithium metal salt soluble in an organic solvent such as LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiI, LiClO ₄ , LiCF ₃ SO ₃ can be used.

For example, lithium metal salts such as LiClO ₄ , LiPF ₆ , LiBF ₄ , and LiCF ₃ SO ₃ in organic solvents such as ethylene carbonate, dimethyl carbonate, propylene carbonate, and dimethyl carbonate at a concentration of about 0.5 mol / L to 1.7 mol / L. A dissolved solution can be used.

  A separator will not be specifically limited if it can be used for a lithium ion secondary battery. The separator separates the positive electrode and the negative electrode and holds the electrolytic solution, and a thin microporous film such as polyethylene or polypropylene can be used.

  When the secondary battery is a lithium ion secondary battery, the shape is not particularly limited, and various shapes such as a cylindrical shape, a stacked shape, and a coin shape can be employed. Regardless of the shape, a separator is sandwiched between the positive electrode and the negative electrode to form an electrode body, and the space between the positive electrode current collector and the negative electrode current collector to the positive electrode terminal and the negative electrode terminal is used for current collection. After connecting using a lead or the like, the electrode body is sealed in a battery case together with an electrolytic solution to form a battery.

  Hereinafter, embodiments of the present invention will be specifically described with reference to Examples and Comparative Examples.

<Preparation of binder>
2 g of copper acetate was dissolved in 20 ml of pure water, and 10 ml of 0.5N NaOH aqueous solution was added dropwise with stirring. Stir for 2 hours after dropping the entire amount, precipitate deposited, filter and wash twice with 10 ml of pure water, rinse with 10 ml of acetone, and then dry under vacuum for 3 hours to obtain blue basic copper acetate (see Formula 1) 1 g of powder was obtained.

  Next, 0.8 g of polyacrylic acid having a number average molecular weight of 500,000 (see Formula 2) was dissolved in 9.2 g of N-methyl-2-pyrrolidone (NMP). 1 mg of the above basic copper acetate powder is further dissolved in this solution, and the mixture is reacted by stirring at 60 ° C. for 3 hours to prepare an NMP solution of the polyacrylic acid-basic copper acetate reactant shown in Chemical Formula 3. did. In the polyacrylic acid-basic copper acetate reactant, it is considered that basic copper acetate is bonded to the carboxyl group of a part of the side chain of polyacrylic acid and has an acetyl group at the end.

<Preparation of negative electrode active material>
20 ml of a mixed solution of 3 ml of HF aqueous solution with a concentration of 46% by mass and 300 ml of HCl aqueous solution with a concentration of 36% by mass was brought to 0 ° C. in an ice bath, and 5 g of calcium disilicide (CaSi ₂ ) was passed there in an argon gas stream Was added and stirred. After confirming the completion of foaming, the temperature was raised to room temperature, and the mixture was further stirred at room temperature for 2 hours, and then 20 ml of distilled water was added and further stirred for 5 minutes. At this time, yellow powder floated.

The obtained mixed solution was filtered, and the residue was washed with 10 ml of distilled water, then washed with 10 ml of ethanol, and vacuum dried to obtain 5.5 g of layered polysilane. The layered polysilane was heat-treated in argon gas containing O ₂ in an amount of 1% by volume or less at 500 ° C. for 1 hour to obtain a powder composed of nanosilicon aggregated particles. This powder was subjected to X-ray diffraction measurement (XRD measurement) using CuKα rays. According to the XRD measurement, halos that are considered to be derived from Si fine particles were observed. The Si fine particles had a crystal grain size of about 7 nm calculated from Scherrer's equation from the half-value width of the diffraction peak of the (111) plane of the X-ray diffraction measurement result.

  1 g of this nanosilicon powder was vacuum impregnated with 0.5 ml of furan for 3 hours, and concentrated hydrochloric acid was added. After adding concentrated hydrochloric acid, it was treated at 60 ° C. for 3 hours to polymerize furan, filtered and washed to remove concentrated hydrochloric acid. The obtained powder was vacuum-dried for 3 hours, and then calcined at 500 ° C. in an argon gas to carbonize the furan polymer to obtain a gray powder. The yield of gray powder was 1.22 g with respect to 1 g of nanosilicon powder. In this example, the composition ratio between silicon and carbon was Si / C = 82/18 by weight.

  An SEM photograph of the obtained gray powder is shown in FIG. From FIG. 1, a composite structure in which nano-silicon aggregated particles on the order of μm are wrapped in a carbon layer having a maximum thickness of about 200 nm is confirmed. Table 1 shows the results of measuring the specific surface areas of the gray powder and the nanosilicon powder used in the production of the gray powder by the BET method.

  It can be seen that the specific surface area is reduced by coating the nanosilicon agglomerated particles with the carbon layer.

  X-ray diffraction measurement (XRD measurement) using CuKα rays was performed on the gray powder. As a result, the gray powder had no 2θ = 26 ° peak (crystalline carbon peak) present in acetylene black, indicating that the carbon contained in the gray powder was amorphous. From the half-width, it was also found that the particle size of Si in the gray powder was 10 nm or less.

<Preparation of lithium ion secondary battery>
45 parts by mass of the obtained gray powder, 40 parts by mass of natural graphite powder, 5 parts by mass of acetylene black, and 10 parts by mass of the NMP solution of the polyacrylic acid-basic copper acetate compound obtained above were mixed. A slurry was prepared. This slurry was applied to the surface of an electrolytic copper foil (current collector) having a thickness of about 20 μm using a doctor blade to form a negative electrode active material layer on the copper foil. Thereafter, the current collector and the negative electrode active material layer were firmly and closely joined by a roll press. This was vacuum-dried at 100 ° C. for 2 hours to form a negative electrode having a negative electrode active material layer thickness of 16 μm.

  A lithium ion secondary battery (half cell) was produced using the negative electrode produced by the above procedure as an evaluation electrode. The counter electrode was a metal lithium foil (thickness 500 μm).

  The counter electrode was cut to φ13 mm and the evaluation electrode was cut to φ11 mm, and a separator (Hoechst Celanese glass filter and Celgard “Celgard2400”) was interposed between them to form an electrode body battery. This electrode body battery was accommodated in a battery case (CR2032 type coin battery member, manufactured by Hosen Co., Ltd.). In the battery case, a nonaqueous electrolyte solution in which LiPF6 is dissolved at a concentration of 1M is injected into a mixed solvent in which ethylene carbonate and diethyl carbonate are mixed at a ratio of 1: 1 (volume ratio). The next battery was obtained.

<Preparation of binder>
2 g of copper acetate was dissolved in 20 ml of pure water, and 20 ml of 0.5N NaOH aqueous solution was added dropwise with stirring. After the entire amount is dropped, the mixture is stirred for 2 hours. The deposited precipitate is filtered off, washed twice with 10 ml of pure water, rinsed with 10 ml of acetone, and dried under vacuum for 3 hours to obtain a powder of copper hydroxide (see Formula 4). 1.5g was obtained. The color tone of this powder was greener blue than the basic copper acetate powder of Example 1.

  Next, 0.8 g of the same polyacrylic acid as in Example 1 was dissolved in 9.2 g of N-methyl-2-pyrrolidone (NMP). 1 mg of the above-mentioned copper hydroxide powder was further dissolved in this solution and reacted at 60 ° C. with stirring to prepare an NMP solution of a polyacrylic acid-copper reactant represented by Chemical Formula 5. The polyacrylic acid-copper reaction product is considered to have a structure in which two molecules of polyacrylic acid are crosslinked by copper (Cu).

<Preparation of lithium ion secondary battery>
A lithium ion secondary battery was obtained in the same manner as Example 1 except that the same amount of the NMP solution of the polyacrylic acid-copper reactant was used instead of the NMP solution of the polyacrylic acid-basic copper acetate reactant. .

<Preparation of binder>
Nickel acetate (2.1 g) was dissolved in pure water (20 ml), and 0.5 N NaOH aqueous solution (10 ml) was added dropwise with stirring. After the total amount was dropped, the mixture was stirred for 2 hours, and the deposited precipitate was filtered off, washed twice with 10 ml of pure water, rinsed with 10 ml of acetone, dried under vacuum for 12 hours, and basic nickel acetate [Ni (OH) (OOCCH3 )] Powder 1.2 g was obtained.

  Next, 0.8 g of the same polyacrylic acid as in Example 1 was dissolved in 9.2 g of N-methyl-2-pyrrolidone (NMP). 1 mg of the above basic nickel acetate powder was further dissolved in this solution, and the mixture was allowed to react at 60 ° C. for 3 hours while stirring to prepare an NMP solution of the polyacrylic acid-basic nickel acetate reactant shown in Chemical Formula 6. did.

<Preparation of lithium ion secondary battery>
A lithium ion secondary battery was carried out in the same manner as in Example 1 except that the same amount of the NMP solution of the polyacrylic acid-basic nickel acetate reactant was used instead of the NMP solution of the polyacrylic acid-basic copper acetate reactant. Got.

[Comparative Example 1]
Instead of NMP solution of polyacrylic acid-basic copper acetate compound, NMP of polyacrylic acid prepared by dissolving 0.8 g of polyacrylic acid similar to Example 1 in 9.2 g of N-methyl-2-pyrrolidone (NMP) A lithium ion secondary battery was obtained in the same manner as in Example 1 except that the same amount of the solution was used.

<Battery characteristics test 1>
The lithium ion secondary batteries of Examples 1 to 3 and Comparative Example 1 were measured for the initial charge capacity when charged under conditions of a temperature of 25 ° C. and a current of 0.2 mA, and the results are shown in Table 2. In addition, the initial discharge capacity when discharged under the condition of current 0.2 mA was measured, and the results are shown in Table 2. Further, the initial efficiency (initial charge capacity / initial discharge capacity) was calculated, and the results are shown in Table 2.

  Also, using the lithium ion secondary batteries of Examples 1 to 3 and Comparative Example 1, charging to 1V under the conditions of a temperature of 25 ° C. and a current of 0.2 mA, and resting for 10 minutes, then 0.01V under the conditions of a current of 0.2 mA A cycle test was performed in which the cycle of discharging to 10 minutes and stopping for 10 minutes was repeated. Then, the capacity maintenance ratio, which is the ratio of the charge capacity of the 10th cycle to the charge capacity of the first cycle, and the coulomb efficiency, which is the ratio of the discharge capacity after 10 cycles to the charge capacity after 10 cycles, were measured, and the results are shown in Table 2. Shown in

  The lithium ion secondary batteries of Examples 1 to 3 all have improved initial efficiency as compared with Comparative Example 1. This is thought to be due to the efficient transfer of electrons due to the change in the valence of copper or nickel. Further, the lithium ion secondary battery of Example 2 has the highest initial efficiency, but other battery characteristics are low. This is probably because the polyacrylic acid main chains are cross-linked by copper as shown in the chemical formula (5), so that aggregation occurs and the binder is unevenly distributed. Therefore, when the cycle characteristics are regarded as important, the compound of the metal element used for the production of the binder of the present invention is preferably a basic acetate rather than a hydroxide, and preferably has an acyl group at the end of the side chain. .

Lithium-ion secondary battery negative electrode of the present invention can be used lithium-ion secondary batteries, such as lithium ion capacitor. Further, the lithium ion secondary battery of the present invention is useful as a non-aqueous secondary battery used for motor driving of electric vehicles and hybrid vehicles, personal computers, portable communication devices, home appliances, office equipment, industrial equipment, etc. In particular, it can be suitably used for driving a motor of an electric vehicle or a hybrid vehicle that requires a large capacity and a large output.

Claims (8)

  A binder for a negative electrode of a lithium ion secondary battery, comprising a reaction product of polyacrylic acid and at least one metal compound selected from basic copper acetate, basic nickel acetate, or copper hydroxide.   2. The binder for a lithium ion secondary battery negative electrode according to claim 1, wherein the reactant has an acyl group at the end of the side chain. 3. The binder for a lithium ion secondary battery negative electrode according to claim 1, wherein the metal element of the metal compound is contained in an amount of 0.01 to 10 parts by mass with respect to 100 parts by mass of the polyacrylic acid .   A current collector and a negative electrode active material layer formed on the surface of the current collector, the negative electrode active material layer comprising a negative electrode active material and the binder according to any one of claims 1 to 3, A lithium ion secondary battery negative electrode comprising: The negative electrode active material is an agglomerated particle composed of nanosilicon produced by heat-treating a layered polysilane represented by a composition formula (SiH) _n having a structure in which a plurality of six-membered rings composed of silicon atoms are connected. Item 5. The negative electrode of the lithium ion secondary battery according to Item 4.   6. The negative electrode of a lithium ion secondary battery according to claim 5, comprising a carbon layer made of amorphous carbon and composited so as to cover at least a part of the aggregated particles.   7. The lithium ion secondary battery negative electrode according to claim 6, wherein the composition ratio of silicon and carbon is Si / C = 3/1 to 20/1 by weight.   A lithium ion secondary battery comprising the lithium ion secondary battery negative electrode according to any one of claims 4 to 7.