JP5129066B2 - Negative electrode for lithium ion secondary battery and method for producing the same - Google Patents

Description

  The present invention relates to a negative electrode for a lithium ion secondary battery and a method for producing the same.

  As electronic devices become smaller and lighter, secondary batteries with high energy density are desired as power sources. A secondary battery is one that extracts chemical energy of a positive electrode active material and a negative electrode active material as electric energy to the outside by a chemical reaction via an electrolyte. Among such secondary batteries, a secondary battery having a high energy density among the practically used secondary batteries is a lithium ion secondary battery. Among them, organic electrolyte-based lithium ion secondary batteries (hereinafter simply referred to as “lithium ion secondary batteries”) are in widespread use.

  In lithium ion secondary batteries, lithium-containing metal composite oxides such as lithium cobalt composite oxide are mainly used as the positive electrode active material, and lithium ion insertion (interlayer of lithium intercalation compounds) is used as the negative electrode active material. And carbon materials having a multilayer structure capable of releasing lithium ions from the interlayer are mainly used. The positive and negative electrode plates are prepared by dispersing these active materials and binder resin in a solvent to form a slurry on both sides of the current collector metal foil, drying the solvent to remove the mixture layer. After formation, it is produced by compression molding with a roll press.

  Other secondary batteries also have different types of active materials, current collectors, etc., but there are also those in which the active material is similarly fixed to the current collector by a binder resin.

  As the binder resin in this case, polyvinylidene fluoride (hereinafter abbreviated as “PVdF”) is frequently used for both electrodes. Since this binder resin is a fluorine-based resin, its adhesiveness with the current collector is inferior, and the active material may fall off.

  In recent years, a next-generation negative electrode active material having a charge / discharge capacity that greatly exceeds the theoretical capacity of a carbon material has been developed as a negative electrode active material of a lithium ion secondary battery. For example, a material containing a metal that can be alloyed with lithium such as Si or Sn is expected. When Si, Sn, etc. are used for the active material, the volume of the active material changes greatly due to the insertion / desorption of Li during charging / discharging. Therefore, even if the fluororesin is used as a binder, the adhesive state with the current collector Is difficult to maintain well. These materials have a very large volume change rate due to the insertion and desorption of lithium, and are repeatedly expanded and contracted by the charge / discharge cycle, so that the active material particles are pulverized or desorbed, so the cycle deterioration is very large. There is a drawback.

  Patent Document 1 includes a negative electrode active material composed of a first phase containing Si and a second phase containing a transition metal silicide, a binder composed of polyimide and polyacrylic acid, and a conductive material that is a carbon material. A negative electrode for a nonaqueous electrolyte secondary battery has been proposed.

Patent Document 2 discloses a negative electrode for a lithium secondary battery in which a negative electrode active material particle containing silicon and / or a silicon alloy and a negative electrode mixture layer containing a binder are heat-treated on the surface of the negative electrode current collector. A negative electrode for a lithium secondary battery is disclosed that includes an imide compound in which a binder precursor made of polyimide or polyamic acid is decomposed by heat treatment as a binder.
JP 2007-95670 A JP 2007-242405 A

  As described in Patent Document 1 and Patent Document 2, various combinations of an active material and a binder resin that binds the active material have been studied. There is a need for a binder resin with improved performance. In addition, since the processing temperature of polyvinylidene fluoride conventionally used as a binder resin is about 140 ° C., it is required to improve the performance without setting the processing temperature as high as possible because of the equipment used conventionally. In Patent Document 1, the processing temperature is 400 ° C., and in Patent Document 2, the processing temperature is 200 ° C. to 300 ° C.

  The present invention has been made in view of such circumstances, and lithium ion secondary that suppresses peeling and dropping of the active material from the current collector, has excellent cycle performance, and can suppress the processing temperature. An object is to provide a negative electrode for a battery.

As a result of intensive studies by the present inventors, a specific binder that has not been used as a binder for secondary battery electrodes until now, that is, formula (I): R ¹ _m SiO _{(4-m) / 2} (m = 0 to 0) 2 integer, R ¹ is has an alkoxysilyl group represented by an alkyl group or an aryl group having 8 or less carbon atoms), and an imide group and an amic acid group 99: 1 to 70: alkoxy of 30 a rate of Lithium ion secondary battery that suppresses peeling and dropping of active material from current collector by using group-containing silane-modified polyimide resin cured product as electrode binder, has excellent cycle performance, and can suppress processing temperature It has been found that a negative electrode can be provided.

That is, the negative electrode for a lithium ion secondary battery of the present invention is a negative electrode for a lithium ion secondary battery manufactured through a coating process in which a binder resin and an active material are applied to the surface of a current collector.
The binder is an alkoxysilyl group represented by the formula (I): R ¹ _m SiO _{(4-m) / 2} (m = 0 to 2 integer, R ¹ represents an alkyl group or aryl group having 8 or less carbon atoms). And an alkoxy group-containing silane-modified polyimide resin cured product having an imide group and an amic acid group in a ratio of 99: 1 to 70:30.

  The binder having an alkoxysilyl group structure represented by the formula (I) is a hybrid of resin and silica. By forming a hybrid of resin and silica, the thermal stability becomes higher than that of the resin alone.

  The binder has imide groups and amic acid groups in a ratio of 99: 1 to 70:30. The amic acid group is imidized by heat treatment. By adjusting the heating temperature and heating time of the heat treatment, the imidation ratio of the polyamic acid group can be controlled. By having an imide group within the above range, the strength is strong and the heat resistance and durability are excellent. Moreover, heating temperature can be lowered | hung by making it have an amic acid group in the said range.

  The active material may contain Si and / or Sn. In that case, the volume change rate accompanying the insertion and desorption of lithium is very large, and the expansion and contraction are repeated by the charge and discharge cycle. Therefore, the active material particles are pulverized or desorbed by using the binder. Can be prevented.

  Further, the active material may include a lithium inert metal that does not form an intermetallic compound with lithium, or a silicide of the lithium inert metal and Si alone. In this case, even when the volume expands due to occlusion of lithium into Si during charging, the stress during expansion is relieved by the lithium inert metal or the silicide of the lithium inert metal, cracking of the active material and peeling from the current collector Is suppressed. This is because the lithium inactive metal or the silicide of the lithium inactive metal does not form an intermetallic compound with lithium, so that the portion occupied in the active material does not fluctuate during charge and discharge. Therefore, it is considered that the stress at the time of expansion of the simple substance of Si is relaxed with respect to the entire active material, and the active material is prevented from peeling and dropping from the current collector due to the volume change of the simple substance of Si accompanying insertion and extraction of lithium.

The lithium inert metal may be at least one selected from the group consisting of Ti, Zr, Ni, Cu, Fe, and Mo. The lithium inert metal or the silicide of the lithium inert metal has a high electronic conductivity and has a higher strength than Si alone. Therefore, the stress at the time of expansion is easily relieved, and it is possible to suppress a decrease in conductivity due to the peeling of the active material. In particular, Mo or MoSi ₂ is preferable as the lithium inert metal or the silicide of the lithium inert metal because of its high hardness.

  The method for producing a negative electrode for a lithium ion secondary battery according to the present invention includes an application step of applying a binder resin and an active material to the surface of the current collector, and curing the binder resin to fix the active material to the surface of the current collector. And a curing step for producing a negative electrode for a lithium ion secondary battery, wherein the binder resin is a resin having a structure represented by the formula (II) and containing an alkoxysilyl group and an amic acid group. In addition, the curing step includes a heating step of heating the binder resin at 150 ° C. or higher and 450 ° C. or lower.

  The binder resin has a structure represented by the formula (II). The structure represented by the formula (II) is a sol-gel reaction site structure, which indicates that an unreacted site for sol-gel reaction remains. For this reason, a sol-gel reaction also takes place when the binder resin is cured, and the sol-gel reaction sites react with the OH groups of the resin. It is also possible to react with the current collector surface. As a result, the current collector and the active material can be firmly held together.

  The binder resin further has an amic acid group, and the curing step includes a heating step of heating the binder resin at 150 ° C. or higher and 450 ° C. or lower, whereby the amic acid group is imidized by heating. At this time, the binder resin can be cured even when the curing temperature is in a range lower than the curing temperature of 400 ° C. usually recommended for amic acid groups. And the cycling performance of the obtained electrode is also favorable.

  The above temperature range is preferably 150 ° C. or more and 250 ° C. or less, more preferably 150 ° C. or more and 200 ° C. or less, from the viewpoint of improving cycle characteristics while lowering the processing temperature. By setting it within this temperature range, the negative electrode for a lithium secondary battery to be manufactured can improve the cycle characteristics significantly more than that using PVdF as the binder resin, while the processing temperature is the same as that of PVdF.

  Moreover, it is preferable that the said temperature range shall be 400 degreeC or more and 450 degrees C or less at the point which further improves cycling characteristics. This temperature range is a range in which a cured product of alkoxy group-containing silane-modified polyimide resin having a ratio of imide group: amidic acid group of approximately 99: 1 can be obtained. Cycle characteristics can be improved.

  By setting it as such a manufacturing method, the negative electrode for lithium ion secondary batteries which an active material cannot peel easily from the collector surface can be manufactured.

  By using the negative electrode for a lithium ion secondary battery and the method for producing the same of the present invention, excellent cycle performance can be obtained.

  The negative electrode for a lithium ion secondary battery of the present invention is manufactured through a coating process in which a binder resin and an active material are applied to the surface of a current collector. Applying means placing a binder resin and an active material on a current collector. As a coating method, a coating method generally used when producing an electrode for a secondary battery, such as a roll coating method, a dip coating method, a doctor blade method, a spray coating method, or a curtain coating method, can be used.

  A current collector is a chemically inert electronic high conductor that keeps current flowing through an electrode during discharging or charging. The current collector is in the form of a foil, a plate or the like formed of the electronic high conductor. If it is the shape according to the objective, it will not specifically limit. For example, examples of the current collector include copper foil and aluminum foil.

  An active material is a substance that directly contributes to electrode reactions such as charge reaction and discharge reaction.

  In the case of a lithium ion secondary battery, the active material of the negative electrode is a carbon-based material capable of inserting and extracting lithium, a metal capable of alloying lithium, or an oxide thereof. These active materials can be used alone or in combination of two or more. The active material used in the present invention is not particularly limited. However, such an active material is in a powder form and is applied and fixed to the surface of the current collector via a binder resin. The powder particle diameter is preferably 100 μm or less.

Examples of the metal capable of alloying lithium include Al, Si, Zn, Ge, Cd, Sn, and Pb. In particular, Si and Sn are effective. The theoretical capacity of carbon is 372 mAhg ^−1, while the theoretical capacity of Si, which is a metal that can be alloyed with lithium, is 4200 mAhg ⁻¹ , Ge is 1620 mAhg ⁻¹ , and Sn is 994 mAhg ⁻¹ . However, metals that can be alloyed or their oxides have a very large volume change rate due to insertion and extraction of lithium as compared with carbon-based materials.

  Further, the active material may include a lithium inert metal that does not form an intermetallic compound with lithium or a silicide of the lithium inert metal other than Si alone. Lithium inert metals or silicides of lithium inert metals are not involved in charge / discharge. Therefore, the stress at the time of expansion | swelling of Si simple substance which occludes lithium is relieve | moderated as the whole active material, and the crack of an active material and peeling from a collector are suppressed.

  As the lithium inert metal, at least one selected from the group consisting of Ti, Zr, Ni, Cu, Fe, and Mo is preferable, and Mo is particularly preferable. By including the lithium inert metal or its silicide in addition to Si having low electron conductivity in the active material, the electron conductivity can be further improved in accordance with the above-described effects. In the charge / discharge reaction of the active material, it is essential to exchange electrons between the active material and the current collector at the same time as the exchange of lithium ions. Therefore, deterioration of cycle characteristics can be suppressed by improving the electronic conductivity of the active material.

  A composite powder of a lithium inert metal or a lithium inert metal silicide that does not form an intermetallic compound with lithium and Si alone can be produced, for example, by a mechanical alloying method. In this method, it is possible to easily form fine primary particles having a particle size of about 10 to 200 nm. As a specific method, a composite powder which is a target active material is obtained by mixing raw materials composed of a plurality of components and performing mechanical alloying treatment so that the primary particle diameter is about 10 to 200 nm. Can do. It is also possible to obtain a mixture of Si simple substance and a silicide of lithium inert metal using only Si simple substance and lithium inert metal as raw materials. That is, a silicide of lithium inert metal can be made from Si and lithium inert metal as raw materials by mechanical alloying treatment. The centrifugal acceleration (input energy) in the mechanical alloying process is preferably about 5 to 20G, and more preferably about 7 to 15G.

  The mechanical alloying process itself may be applied as it is. For example, a composite powder that is a target active material can be obtained by compounding (partial alloying) the raw material mixture while repeating mixing and adhesion by mechanical bonding force. As an apparatus used for the mechanical alloying treatment, a mixer, a disperser, a pulverizer and the like generally used in the powder field can be used as they are. Specific examples include a reiki machine, a ball mill, a vibration mill, an agitator mill, and the like. In particular, in order to reduce the stacking of powders mainly composed of battery active materials existing between networks, it is necessary to efficiently disperse the powders that are overlapped or aggregated during the compositing operation one by one. Therefore, it is desirable to use a mixer that can give a shearing force. The operating conditions of these devices are not particularly limited. Moreover, it can also be set as composite powder by mixing the Si simple substance separately manufactured with said method, and the lithium inert metal or the silicide of a lithium inert metal.

  The mixing ratio of Si simple substance and lithium inert metal or lithium inert metal silicide is such that the molar ratio of Si simple substance and lithium inert metal or lithium inert metal silicide is 1: 1-3: 1 is preferable. Moreover, it is preferable that the amount of silicic acid of lithium inert metal or lithium inert metal is 40 wt% or more per 100 wt% of the negative electrode active material. “Wt%” means “mass%”.

  A conductive additive can be fixed to the surface of the current collector together with the active material. The conductive auxiliary agent is added to increase the conductivity when the active material is fixed to the current collector via the binder resin. As the conductive aid, carbon black, graphite, acetylene black, kettin black, carbon fiber, etc., which are carbonaceous fine particles, may be added alone or in combination of two or more.

  The binder is used as a binder when these active materials and conductive assistants are applied to the current collector. The binder is required to bind the active material and the conductive assistant in as small an amount as possible, and the amount is preferably 0.5 wt% to 50 wt% of the total of the active material, the conductive auxiliary and the binder.

The binder of the present invention is represented by the formula (I): R ¹ _m SiO _{(4-m) / 2} (m is an integer of 0 to 2, R ¹ is an alkyl group or aryl group having 8 or less carbon atoms). It is an alkoxy group-containing silane-modified polyimide resin cured product having an alkoxysilyl group and having an imide group and an amic acid group in a ratio of 99: 1 to 70:30.

The binder has a structure represented by the formula (I): R ¹ _m SiO _{(4-m) / 2} (m = 0 to 2 and R ¹ represents an alkyl group or aryl group having 8 or less carbon atoms). It is an alkoxysilyl group-containing resin cured product. The structure represented by the formula (I) is a gelled fine silica site structure (high-order network structure of siloxane bonds). This structure is a structure of an organosilicon polymer composed of a siloxane bond, and is a structure obtained by polycondensation of silanol of the following formula (A).

_{nR m Si (OH) 4-} m → (R m SiO (4-m) / 2) n ···· formula (A)
(R: organic group, m = 1 to 3, n> 1)
Further, the binder has imide groups and amic acid groups in a ratio of 99: 1 to 70:30. The amic acid group is imidized (dehydration polymerization) by heat treatment to form an imide group. This imidization reaction starts from about 150 ° C. and easily proceeds at 200 ° C. or higher. The imidation ratio of the amic acid is desirably 70% or more. Specifically, it is preferable to imidize until having an imide group and an amic acid group in a ratio of 99: 1 to 70:30. If it exists in this range, it fully functions as a binder and can maintain the cycling characteristics of a negative electrode.

  Such an imidation rate can be controlled, for example, by adjusting a heating temperature or a heating time, and the imidization rate can also be obtained using infrared spectroscopy (IR).

  Moreover, the manufacturing method of the negative electrode for lithium ion secondary batteries of this invention has an application | coating process and a hardening process.

  The coating process is a process of coating the binder resin and the active material on the surface of the current collector. Moreover, you may apply | coat a conductive support agent together in an application | coating process. As described above, as the active material, a carbon-based material capable of inserting and extracting lithium, a metal capable of alloying lithium, or an oxide thereof is used. There is no particular limitation. Si and Sn are particularly effective as the active material. Further, a lithium inactive metal that does not form an intermetallic compound with lithium or a silicide of the lithium inactive metal and Si alone may be included.

  The curing step is a step of curing the binder resin and fixing the active material to the current collector surface. The binder resin is an alkoxysilyl group-containing resin having a structure represented by the formula (II) and is a resin containing an amic acid group.

  In the coating step, the binder resin and the active material are mixed in advance, and a solvent or the like is added to form a slurry, which can be applied to the current collector. The conductive auxiliary agent may also be applied as a slurry. The coating thickness is preferably 10 μm to 300 μm. Further, the mixing ratio of the binder resin and the active material is preferably in mass parts, and active material: binder resin = 99: 1 to 70:30. The mixing ratio in the case of containing a conductive auxiliary agent is preferably active material: conductive auxiliary agent: binder resin = 98: 1: 1 to 60:20:20.

  The curing step is a step of curing the binder resin that is an alkoxysilyl group-containing resin. The curing step includes a heating step of heating the binder resin at 150 ° C. or higher and 450 ° C. or lower. The active material is fixed to the current collector surface by curing the binder resin. When the conductive assistant is included, the conductive assistant is fixed in the same manner. When the binder resin is cured, a sol-gel curing reaction is also caused by the structure represented by the formula (II) of the binder resin. The structure represented by the formula (II) includes a sol-gel reaction site structure.

The sol-gel reaction site structure is a structure that contributes to a reaction when performing the sol-gel method. In the sol-gel method, a solution of an inorganic or organometallic salt is used as a starting solution, this solution is converted into a colloidal solution (Sol) by hydrolysis and condensation polymerization reaction, and a solid (Gel) that loses fluidity by further promoting the reaction. It is a method of forming. In general, the sol-gel method uses a metal alkoxide (a compound represented by M (OR) _x , M is a metal, and R is an alkyl group) as a raw material.

The compound represented by M (OR) _x reacts as shown in the following formula (B) by hydrolysis.

nM (OR) _x + nH ₂ O → nM (OH) (OR) _x−1 + nROH (B)
When the reaction shown here is further promoted, it finally becomes M (OH) _x , and when a polycondensation reaction occurs between the two molecules of hydroxide generated here, it reacts as shown in the following formula (C).

M (OH) _x + M (OH) _x → (OH) _x-1 M-OM (OH) _x-1 + H ₂ O (C)
At this time, all the OH groups can be polycondensed, and a dehydration condensation polymerization reaction with an organic polymer having an OH group at the terminal is also possible.

  Since the binder resin has a sol-gel reaction site structure represented by the formula (II), the sol-gel reaction sites can react with each other or with the OH groups of the resin when the binder resin is cured. Further, since it is a hybrid of resin and silica, it has good adhesion to the current collector, the active material, and the conductive additive, which are inorganic components, and the current collector and the active material can be firmly held in the current collector. Since the alkoxysilyl group-containing resin in which the sol-gel curing reaction has occurred has a fine gelled silica site structure (a high-order network structure of siloxane bonds), it has good adhesion to the active material, the conductive assistant and the current collector.

  At this time, as a resin having an amic acid group that becomes a hybrid with silica, a polyamic acid resin can be mentioned.

  Resin and silica can be made into a hybrid having a structure represented by the formula (II) by a sol-gel method, and becomes an alkoxy group-containing silane-modified polyamic acid resin. At this time, the binder resin has a structure represented by the formula (II), which indicates that the sol-gel reaction site still remains. Therefore, when the binder resin is an alkoxysilyl group-containing resin having a structure represented by the formula (II), the sol-gel reaction sites can react with the OH groups of the resin when the binder resin is cured.

  The binder resin can be synthesized by a known technique. When the binder resin is an alkoxy group-containing silane-modified polyamic acid resin, it can be formed by reacting a precursor of a polyamic acid composed of a carboxylic acid anhydride component and a diamine component with an alkoxysilane partial condensate. The alkoxysilane partial condensate is obtained by partially condensing a hydrolyzable alkoxysilane monomer in the presence of an acid or base catalyst and water. At this time, the alkoxysilane partial condensate may be reacted with an epoxy compound in advance to form an epoxy group-containing alkoxysilane partial condensate and then reacted with a polyamic acid to form an alkoxy group-containing silane-modified polyamic acid resin.

  Moreover, a commercial item can be used suitably for said binder resin. For example, there are various commercially available products such as trade name “COMPOCERAN H800” (manufactured by Arakawa Chemical Industries, Ltd.) which is an alkoxy group-containing silane-modified polyamic acid resin.

  The chemical formula of the basic skeleton of the above-mentioned trade name “COMPOCERAN H800” (manufactured by Arakawa Chemical Industries) is shown below.

  The curing step includes a heating step of heating the binder resin at 150 ° C. or higher and 450 ° C. or lower. In the curing step, the amic acid group is imidized (dehydration polymerization) by heat treatment to form an imide group. Moreover, this imidation reaction is started from about 150 ° C. and is likely to proceed at 200 ° C. or higher. Therefore, even when heated at a temperature of 150 ° C. or higher and 250 ° C. or lower, it functions sufficiently as a binder and can maintain the cycle characteristics of the negative electrode. Thereby, even if it does not raise a heating temperature to the general recommended curing temperature of 400 degreeC polyimide, the negative electrode for lithium ion secondary batteries excellent in cycling characteristics can be manufactured.

  Hereinafter, the present invention will be described in more detail with reference to examples.

  A negative electrode for a lithium ion secondary battery of the present invention was produced as follows, and a discharge cycle test was performed using a model battery for evaluation. The test used a coin-type lithium ion secondary battery with the negative electrode as the evaluation electrode.

<Resin characteristics evaluation by difference in curing temperature>
First, the resin characteristic by the difference in curing temperature was measured. Alkoxy-containing silane-modified polyamic acid resin (made by Arakawa Chemical Industry Co., Ltd., trade name Composeran, product number H850D, solvent composition: N, N-dimethylacetamide (DMAc)) is cured at 150 ° C., 200 ° C., and 430 ° C. curing temperatures. The imidation ratio of the resin cured at each curing temperature was measured. The imidization rate is determined by infrared spectroscopy (IR) measurement of each cured product, assuming that the imidization rate of the heat-treated product at 430 ° C. is 100%, benzene ring skeleton stretching vibration band (near 1500 cm ⁻¹ ), imide carbonyl The imidation ratio of the cured product at another curing temperature was calculated from the absorbance ratio of the group (near 1780 cm ⁻¹ ).

  The results are shown in Table 1.

  As shown in Table 1, when the curing temperature was 150 ° C. or higher, the imidization rate was found to be about 70% or higher.

  Next, the tensile strength was measured using a 200 ° C. cured product and a 430 ° C. cured product having the above curing temperatures. At that time, PVdF (manufactured by Kureha) was similarly evaluated for comparison. A film having a thickness of 20 to 30 μm of each resin was prepared as a test piece, and a test piece having a width of 5 mm and a length of 20 mm was prepared. The test conditions were such that the distance between chucks was 20 mm and the tensile speed was 5 mm / min, and the breaking strength and the like were obtained. The results are shown in Table 2.

  As shown in Table 2, even when the alkoxy-containing silane-modified polyamic acid resin (Arakawa Chemical Industries, Ltd., product number H850D) is a 200 ° C. cured product, the elastic modulus is 4.4 GPa and the elastic modulus of PVdF is 1.5 GPa. It was about 3 times larger than. From this, it was found that even a 200 ° C. cured product can suppress the expansion and contraction of the active material and has no problem as a binder resin.

  As a precaution, an immersion experiment of the film in an electrolytic solution was performed. A 430 ° C. cured film of the alkoxy-containing silane-modified polyamic acid resin used in the tensile test was cut into 5 cm × 5 cm, and placed in a 50 ° C. thermostat for 24 hours to adjust the humidity. Here, the weight was measured and immersed in EC (ethylene carbonate): DEC (diethyl carbonate) (1: 1 v / v%) and stored at 23 ° C. for 24 hours. After the film was taken out and the liquid on the surface was wiped off, the weight was measured. When the weight increase rate was calculated from the weight before and after the immersion, the weight change rate was -0.3%. From this, the weight change rate was almost 0%, and it was judged that there was no particularly problematic influence by the electrolytic solution.

<Production of electrode for evaluation>
An electrode was prepared using Si powder as the active material and the above-described alkoxy-containing silane-modified polyamic acid resin (manufactured by Arakawa Chemical Industries, Ltd., product number H850D) and PVdF as the binder resin. Si particles having a particle diameter of 4 μm or less (manufactured by High-Purity Chemical) were used as they were as the Si powder. Further, KB (Ketchin Black) manufactured by Ketjen Black International Co., Ltd. was used as a conductive aid.

(Test Example 1)
In Test Example 1, alkoxy-containing silane-modified polyamic acid resin (manufactured by Arakawa Chemical Industry Co., Ltd., trade name Composeran, product number H850D, solvent composition: N, N-dimethylacetamide (DMAc), curing residue 15%, viscosity 4100 mPa · s / 25 ° C., silica in the cured residue, 2 wt%) was used. The alkoxy-containing silane-modified polyamic acid resin is one of the above-mentioned trade name Composeran H800 series and has the structure shown in [Chemical Formula 2].

  In Test Example 1, Si powder was put into a paste in which an alkoxy-containing silane-modified polyamic acid resin was dissolved in N-methylpyrrolidone (NMP), and kettin black (KB) was added and mixed to prepare a slurry. The mixing ratio was Si: resin: KB = 80: 15: 5 (wt%).

  After slurry adjustment, the slurry was placed on an electrolytic copper foil having a thickness of 18 μm, and a film was formed on the copper foil using a doctor blade.

After the obtained sheet was dried at 80 ° C. for 20 minutes to volatilize and remove NMP, the current collector made of electrolytic copper foil and the negative electrode layer made of the above composite powder were firmly adhered and bonded by a roll press machine. I let you. This was extracted with a 1 cm ² circular punch and vacuum dried at 200 ° C. for 3 hours to obtain an electrode having a thickness of 100 μm or less. This electrode is used as the electrode of Test Example 1.

(Test Example 2)
The electrode of Test Example 2 was prepared in the same manner as Test Example 1 except that the heating temperature at the time of electrode preparation was 120 ° C. for 10 minutes, then 200 ° C. for 10 minutes, and subsequently 430 ° C. for 10 minutes.

(Test Example 3)
The electrode of Test Example 3 was prepared in the same manner as in Test Example 1 except that PVdF (manufactured by Kureha) was used as the binder resin and the heating temperature during electrode preparation was 140 ° C. for 3 hours.
<Production of coin-type battery>
1 mol of LiPF ₆ / ethylene carbonate (EC) + diethyl carbonate (DEC) (EC: DEC = 1: 1 (volume ratio)) solution using the above electrode as the negative electrode and metal lithium as the positive electrode Was used as an electrolyte, and a coin-type model battery (CR2032 type) was produced in a glove box in an Ar atmosphere. The coin-type model battery was manufactured by sequentially stacking a spacer, a Li foil having a thickness of 500 μm as a counter electrode, a separator (trade name Celgard # 2400, manufactured by Celgard), and an evaluation electrode, followed by caulking.

<Coin-type battery evaluation>
Evaluation of the evaluation electrode in this model battery was performed by the following method.

  First, the model battery was discharged at a constant current of 0.2 mA until reaching 0 V, and after resting for 5 minutes, it was charged until it reached 2.0 V at a constant current of 0.2 mA. This was made into 1 cycle, charging / discharging was performed repeatedly and the charge capacity was investigated.

  A graph showing the number of cycles and the charge capacity for the model battery of each test example is shown in FIG.

  As is clear from FIG. 1, in the battery using the electrodes of Test Examples 1 and 2 as the evaluation electrode, the amount of decrease in the initial charge capacity was smaller than that of the battery using the electrode of Test Example 3 as the evaluation electrode.

  In other words, in the electrode of Test Example 3 using PVdF, which is a conventional binder resin, the charge capacity has dropped sharply to about 5% in one cycle test, whereas the alkoxy resin-containing silane-modified polyamic acid is added to the binder resin. In the electrodes of Test Examples 1 and 2 using a resin, the charge capacity was maintained about 90%. Moreover, in the electrode of Test Example 3, the charge capacity after 2 cycles was 0, whereas in the electrodes of Test Examples 1 and 2, the charge capacity after 13 cycles was also maintained at 60% or more.

  Here, in Test Example 1 and Test Example 2, the curing temperature of the binder resin is different, and thus the imidization ratio of the resin is different. The electrode of Test Example 2 having a higher curing temperature than FIG. 1 was slightly better in cycle characteristics than the electrode of Test Example 1, but the electrode of Test Example 1 having a curing temperature of 200 ° C. was charged after 13 cycles. Capacity was maintained over 60%. From this, even if the curing temperature was low, an excellent effect as a binder resin could be confirmed.

  From the above, it is considered that cracking and peeling due to volume expansion of Si accompanying occlusion and release of lithium can be suppressed by using the binder resin, and cycle characteristics are improved.

The graph which compares cycling characteristics about the battery using the negative electrode of Test Examples 1-3 is shown.

Claims (3)

In the negative electrode for a lithium ion secondary battery in which an active material is fixed to the surface of the current collector via a binder,
The binder is an alkoxysilyl group represented by the formula (I): R ¹ _m SiO _{(4-m) / 2} (m = 0 to 2 integer, R ¹ represents an alkyl group or aryl group having 8 or less carbon atoms). And an alkoxy group-containing silane-modified polyimide resin cured product having an imide group and an amic acid group in a ratio of 99: 1 to 70:30.   The negative electrode for a lithium ion secondary battery according to claim 1, wherein the active material contains Si and / or Sn. An application step of applying a binder resin and an active material to the surface of the current collector;
A curing step of curing the binder resin and fixing the active material to the surface of the current collector;
A method for producing a negative electrode for a lithium ion secondary battery comprising:
The binder resin is a resin having a structure represented by the formula (II) and containing an alkoxysilyl group and an amic acid group, and the curing step is heating by heating the binder resin at 150 ° C. or higher and 450 ° C. or lower. The manufacturing method of the negative electrode for lithium ion secondary batteries characterized by including a process.