JP2008066272A - Anode for nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an anode for a nonaqueous electrolyte secondary battery, the cycle characteristics of which are improved. <P>SOLUTION: The anode 10 for a nonaqueous electrolyte secondary battery has an active substance layer 12, containing particles 12a of an active substance. A metal material deposited electrolessly exists in between the particles 12a. At least part of the surface of the particles 12a is covered by a metal material 13, having a low capability of forming a lithium compound. Voids are formed between the particles 12a covered with the metal material 13. The metal material 13 exists on the surface of the particles, across the entire region in the thickness direction of the active substance layer. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a negative electrode for a non-aqueous electrolyte secondary battery such as a lithium secondary battery.

  In Patent Document 1, a paste obtained by adding graphite, polyvinyl alcohol, carboxymethyl cellulose, and water to a silicon powder as an active material and kneading as a negative electrode for a lithium secondary battery was applied onto a current collector. Things have been proposed. However, since silicon has a large volume change caused by insertion and extraction of lithium, when charging and discharging are repeated, it is pulverized by stress generated by expansion and contraction and easily falls off the current collector. As a result, cycle characteristics are degraded.

  In order to eliminate such inconvenience, the present applicant has first made a non-aqueous electrolysis comprising an active material layer including a pair of front and back surfaces that are in contact with an electrolytic solution and have conductivity, and active material particles between the surfaces. A negative electrode for a liquid secondary battery was proposed (see Patent Document 2). A metal material having a low lithium compound forming ability is infiltrated into the active material layer of the negative electrode, and active material particles are present in the infiltrated metal material. Since the active material layer has such a structure, even if the particles are pulverized due to expansion and contraction of the particles due to charge and discharge, the active material layer is unlikely to fall off. As a result, when this negative electrode is used, there is an advantage that the cycle life of the battery becomes long.

  However, as a result of further studies by the present inventors, the negative electrode previously proposed by the present applicant can prevent the active material particles from falling off due to repeated charge and discharge, but the degree of penetration of the metal material described above It has been found that there are cases where sufficient voids are not formed between the particles of the active material and the electrolytic solution cannot be smoothly circulated.

JP 11-2114010 A International Publication No. 2005/055345 Pamphlet

  Accordingly, an object of the present invention is to provide a negative electrode for a non-aqueous electrolyte secondary battery whose performance is further improved as compared with the negative electrode of the prior art described above.

  The present invention provides a negative electrode for a non-aqueous electrolyte secondary battery comprising an active material layer containing particles of an active material, wherein a metal material deposited by electroless plating is present between the particles. To do.

  According to the present invention, since the metal material is deposited on the surface of the active material particles by electroless plating, sufficient electron conductivity is imparted to the particles. In addition, since the metal material is deposited between the particles by electroless plating, the adhesion between the particles and between the particles and the current collector is improved. Furthermore, according to electroless plating, a metal material can be uniformly deposited over the thickness direction of the active material layer. For these reasons, the cycle characteristics are improved.

  The present invention will be described below based on preferred embodiments with reference to the drawings. FIG. 1 shows a schematic diagram of a cross-sectional structure of an embodiment of a negative electrode for a non-aqueous electrolyte secondary battery of the present invention. The negative electrode 10 of this embodiment includes a current collector 11 and an active material layer 12 formed on at least one surface thereof. 1 shows a state in which the active material layer 12 is formed only on one side of the current collector 11 for convenience, the active material layer may be formed on both sides of the current collector. .

  The active material layer 12 includes active material particles 12a. As the active material, a material capable of occluding and releasing lithium ions is used. Examples of such materials include silicon-based materials, tin-based materials, aluminum-based materials, and germanium-based materials. In order to improve the capacity density per negative electrode weight, silicon-based materials and tin-based materials are particularly preferable, and silicon-based materials are particularly preferable.

  As the active material particles 12a made of a silicon-based material or a tin-based material, a material capable of occluding lithium and containing silicon or tin, for example, a) silicon or tin particles, and b) silicon or tin and metal. And c) particles obtained by coating a metal on the surface of particles of silicon alone or tin alone. Moreover, a silicon oxide is also mentioned. These materials can be used alone or in combination.

  In the case where the silicon-based particles or tin-based particles are compound particles of silicon or tin and metal of the above b), the compound contains silicon or an alloy of tin and metal, and 1) silicon or tin and metal 2) Intermetallic compound of silicon or tin and metal, or 3) Si single phase or single phase of tin, single phase of metal, solid solution of silicon or tin and metal, intermetallic compound of silicon or tin and metal Any one of the composites composed of two or more phases. As the metal, Cu, Ag, Ni, Co, and Ce are preferable. In particular, Cu, Ag, and Ni are desirably used from the viewpoint of excellent electronic conductivity and low ability to form a lithium compound. The composition of silicon or tin and metal in the compound particles of (b) is preferably such that the amount of silicon or tin is 30 to 99.9% by weight and the amount of metal is 0.1 to 70% by weight. A more preferable composition is selected in an appropriate range depending on the method for producing compound particles. For example, when the compound particles are a binary alloy of silicon or tin and metal, and the alloy is manufactured using a rapid cooling method, the amount of silicon or tin is preferably 40 to 90% by weight. On the other hand, the amount of metal is preferably 10 to 60% by weight. In the case where the compound particles are a ternary or higher alloy of silicon or tin and metal, B, Al, Sn, Fe, Cr, Zn, In, V, Y are further added to the binary alloy described above. , Zr, Nb, Ta, W, La, Pr, Pd and Nd may be contained in a small amount. Thereby, the additional effect that pulverization is suppressed is produced. In order to further enhance this effect, these elements are preferably contained in the compound particles in an amount of 0.01 to 10% by weight, particularly 0.05 to 1.0% by weight. In particular, an alloy containing tin, cobalt, carbon, and at least one of nickel and chromium is preferably used as the tin-based alloy particles. Further, lithium may be occluded in the active material particles made of a silicon-based material or a tin-based material before or after the negative electrode is incorporated in the battery.

  When the compound particles of silicon or tin and metal of b) above are alloy particles, the alloy particles are produced by, for example, a rapid cooling method, so that the crystallites of the alloy have a fine size and are uniformly dispersed. By this, it is preferable from the point by which pulverization is suppressed and electronic conductivity is maintained. In this rapid cooling method, first, a raw material melt containing silicon or tin and a metal is prepared. The raw material is made into molten metal by high frequency melting. The ratio of silicon or tin to metal in the molten metal is in the range described above. The temperature of the molten metal is preferably 1200 to 1500 ° C., particularly 1300 to 1450 ° C., in relation to the rapid cooling conditions. An alloy is obtained from this molten metal using a mold casting method. That is, the molten metal is poured into a copper or iron mold to obtain a rapidly cooled silicon alloy or tin alloy ingot. The ingot is pulverized and sieved, and for example, those having a particle size of 40 μm or less are provided for the present invention.

  In the case where the silicon-based particles or tin-based particles are particles in which a metal is coated on the surface of the silicon simple substance or tin simple particles in the above (c) (this particle is referred to as a metal-coated particle), The same metals as those contained in the particles b) mentioned above, such as copper, are used. The amount of silicon or tin in the metal-coated particles is preferably 70 to 99.9% by weight, particularly 80 to 99% by weight, especially 85 to 95. On the other hand, the amount of the coating metal including copper is preferably 0.1 to 30% by weight, particularly 1 to 20% by weight, particularly 5 to 15% by weight. The metal-coated particles are produced using, for example, an electroless plating method. In this electroless plating method, first, a plating bath in which silicon particles or tin particles are suspended and containing a coating metal such as copper is prepared. In this plating bath, silicon particles or tin particles are electrolessly plated to coat the surface of the silicon particles or tin particles with the coating metal. The concentration of silicon particles or tin particles in the plating bath is preferably about 400 to 600 g / l. When electrolessly plating copper as the coating metal, it is preferable to contain copper sulfate, Rochelle salt or the like in the plating bath. In this case, the concentration of copper sulfate is preferably 6 to 9 g / l, and the concentration of Rochelle salt is preferably 70 to 90 g / l from the viewpoint of controlling the plating rate. For the same reason, the pH of the plating bath is preferably 12 to 13, and the bath temperature is preferably 20 to 30 ° C. As the reducing agent contained in the plating bath, for example, formaldehyde or the like is used, and the concentration thereof can be about 15 to 30 cc / l. Moreover, a commercial item can also be used as an electroless-plating bath.

  Of the various active material particles 12a described above, silicon particles or silicon oxide particles are particularly preferable from the viewpoint of high lithium storage capacity.

  In the active material layer 12, the metal material 13 exists between the particles 12a. The metal material 13 is a material different from the constituent material of the particles 12a, and is preferably a metal material having a low lithium compound forming ability. This metal material 13 is deposited by electroless plating. The metal material 13 covers at least a part of the surface of the particle 12a. Gaps are formed between the particles 12 a covered with the metal material 13. That is, the metal material 13 is deposited between the particles 12a in a state in which a gap is secured so that the non-aqueous electrolyte containing lithium ions can reach the particles 12a. In FIG. 1, the metal material 13 is conveniently represented as a thick line surrounding the periphery of the particle 12a. This figure is a schematic view of the active material layer 12 viewed two-dimensionally. In reality, each particle is in direct contact with other particles or through a metal material 13. “Low lithium compound forming ability” means that lithium does not form an intermetallic compound or solid solution, or even if formed, lithium is in a very small amount or very unstable.

  The metal material 13 has conductivity, and examples thereof include copper, nickel, iron, cobalt, or alloys of these metals, which are materials having a low lithium compound forming ability. Silver can also be used as a metal other than these. In particular, the metal material 13 is preferably a highly ductile material in which even if the active material particles 12a expand and contract, the coating on the surface of the particles 12a is not easily broken. It is preferable to use copper as such a material.

  The metal material 13 deposited by electroless plating is preferably present on the surface of the active material particles 12 a over the entire thickness direction of the active material layer 12. The active material particles 12 a are preferably present in the matrix of the metal material 13. Thus, even if the particles 12a are pulverized due to expansion and contraction due to charge / discharge, the particles are less likely to fall off. In addition, since the electronic conductivity of the entire active material layer 12 is ensured through the metal material 13, the electrically isolated active material particles 12 a are generated, and in particular, the electrically isolated active material is deep in the active material layer 12. Generation of the particles 12a of the substance is effectively prevented. This is particularly advantageous when a material that is a semiconductor and has poor electron conductivity, such as a silicon-based material, is used as the active material. The fact that the metal material 13 is deposited on the surface of the active material particles 12a over the entire thickness direction of the active material layer 12 can be confirmed by electron microscope mapping using the material 13 as a measurement target.

  The metal material 13 deposited by electroless plating covers the surfaces of the particles 12a continuously or discontinuously. When the metal material 13 continuously covers the surfaces of the particles 12a, it is preferable to form fine voids in the coating of the metal material 13 that allow the non-aqueous electrolyte to flow. When the metal material 13 discontinuously coats the surface of the particle 12a, the non-aqueous electrolyte is supplied to the particle 12a through a portion of the surface of the particle 12a that is not covered with the metal material 13.

  The average thickness of the metal material 13 covering the surface of the active material particles 12a is preferably 0.05 to 2 μm, more preferably 0.1 to 0.25 μm. That is, the metal material 13 covers the surface of the active material particles 12a with a minimum thickness. As a result, while the energy density is increased, the particles 12a are prevented from falling off due to expansion / contraction and pulverization due to charge / discharge. Here, the “average thickness” is a value calculated based on a portion of the surface of the active material particle 12 a that is actually covered with the metal material 13. Accordingly, the portion of the surface of the active material particle 12a that is not covered with the metal material 13 is not used as a basis for calculating the average value.

  The voids formed between the particles 12a coated with the metal material 13 deposited by electroless plating have a function as a flow path for the non-aqueous electrolyte containing lithium ions. Since the non-aqueous electrolyte smoothly flows in the thickness direction of the active material layer 12 due to the presence of the voids, cycle characteristics can be improved. Further, the voids formed between the particles 12a also have a function as a space for relieving stress caused by the volume change of the active material particles 12a due to charge and discharge. The increase in the volume of the active material particles 12a whose volume has been increased by charging is absorbed by the voids. As a result, pulverization of the particles 12a is difficult to occur, and significant deformation of the negative electrode 10 is effectively prevented.

  As will be described later, the active material layer 12 is preferably formed by applying a slurry containing a particle 12a and a binder onto a current collector and drying the coating film using a predetermined plating bath. It is formed by performing electrolytic plating and depositing a metal material 13 between the particles 12a.

  The ratio of voids formed in the active material layer, that is, the void ratio, is preferably about 15 to 45% by volume, particularly about 20 to 40% by volume. By setting the porosity within this range, it is possible to form necessary and sufficient voids in the active material layer 12 that allow the nonaqueous electrolyte to flow. The void amount of the active material layer 12 is measured by a mercury intrusion method (JIS R 1655). The mercury intrusion method is a method for obtaining information on the physical shape of a solid by measuring the size and volume of pores in the solid. The principle of the mercury intrusion method is to apply a pressure to mercury to inject it into the pores of the object to be measured, and to measure the relationship between the pressure applied at that time and the volume of mercury that has been pushed in (intruded). In this case, mercury enters sequentially from the large voids present in the active material layer 12. In the present invention, the void amount measured at a pressure of 90 MPa is regarded as the entire void amount. The porosity (%) of the active material layer 12 is obtained by dividing the void amount per unit area measured by the above method by the apparent volume of the active material layer 12 per unit area and multiplying it by 100. .

The porosity can also be controlled by appropriately selecting the particle size of the active material particles 12a. From this viewpoint, the particle 12a has a maximum particle size of preferably 30 μm or less, more preferably 10 μm or less. Moreover, when the particle diameter of the particle is expressed by a D ₅₀ value, it is preferably 0.1 to 8 μm, particularly preferably 0.3 to 4 μm. The particle size of the particles is measured by laser diffraction / scattering particle size distribution measurement and electron microscope observation (SEM observation).

  In the negative electrode of the present embodiment, the surface of the active material particles 12a is coated with a metal material having a low lithium compound forming ability by electrolytic plating, and the same or different type of lithium as the metal material is formed on the metal material. A metal material having a low compound forming ability may be deposited by electroless plating. Further, as the active material particles 12a, particles whose surfaces are previously coated with metal may be used. By doing in this way, since the plating nucleus exists in advance on the surface of the particle 12a prior to the electroless plating, the electroless plating can be performed successfully.

  If the amount of the active material relative to the whole negative electrode is too small, it is difficult to sufficiently improve the energy density of the battery. Conversely, if the amount is too large, the strength decreases and the active material tends to fall off. Considering these, the thickness of the active material layer is 10 to 40 μm, preferably 15 to 30 μm, and more preferably 18 to 25 μm.

  In the negative electrode 10 of the present embodiment, a thin surface layer (not shown) may be formed on the surface of the active material layer 12. Further, the negative electrode 10 may not have such a surface layer. The thickness of the surface layer is 0.25 μm or less, preferably 0.1 μm or less. There is no restriction | limiting in the lower limit of the thickness of a surface layer. By forming the surface layer, the pulverized active material particles 12a can be further prevented from falling off. However, in the present embodiment, by setting the porosity of the active material layer 12 within the above-described range, it is possible to sufficiently prevent the pulverized active material particles 12a from dropping without using a surface layer. Is possible.

  When the negative electrode 10 has the thin surface layer or does not have the surface layer, a secondary battery is assembled using the negative electrode 10 to reduce the overvoltage when the battery is initially charged. Can do. This means that lithium can be prevented from being reduced on the surface of the negative electrode 10 when the secondary battery is charged. The reduction of lithium leads to the generation of dendrites that cause a short circuit between the two electrodes.

  When the negative electrode 10 has a surface layer, the surface layer covers the surface of the active material layer 12 continuously or discontinuously. When the surface layer continuously covers the surface of the active material layer 12, the surface layer has a large number of microscopic voids (not shown) that are open at the surface and communicate with the active material layer 12. It is preferable. The fine voids are preferably present in the surface layer so as to extend in the thickness direction of the surface layer. The fine voids allow the non-aqueous electrolyte to flow. The role of the fine voids is to supply a non-aqueous electrolyte into the active material layer 12. When the surface of the negative electrode 10 is viewed in plan by electron microscope observation, the fine voids are such that the ratio of the area covered with the metal material 13, that is, the coverage is 95% or less, particularly 80% or less, especially 60% or less. It is preferable that the size is large. When the coverage exceeds 95%, it is difficult for the non-aqueous electrolyte having high viscosity to enter, and the range of selection of the non-aqueous electrolyte may be narrowed.

  The surface layer is made of a metal material having a low lithium compound forming ability. This metal material may be the same as or different from the metal material 13 deposited in the active material layer 12. The surface layer may have a structure of two or more layers made of two or more different metal materials. Considering the ease of manufacture of the negative electrode 10, the metal material 13 present in the active material layer 12 and the metal material constituting the surface layer are preferably the same type.

  In the negative electrode 10 of this embodiment, when the porosity in the active material layer 12 has the above-described value, the resistance of the negative electrode 10 to bending increases. Specifically, the MIT folding resistance measured according to JIS C 6471 is preferably 30 times or more, more preferably 50 times or more. High folding resistance is extremely advantageous since the negative electrode 10 is less likely to be folded when the negative electrode 10 is folded or wound and accommodated in a battery container. As the MIT folding endurance device, for example, a film folding endurance fatigue tester (product number 549) manufactured by Toyo Seiki Seisakusho is used, and measurement can be performed with a bending radius of 0.8 mm, a load of 0.5 kgf and a sample size of 15 × 150 mm. it can.

  As the current collector 11 in the negative electrode 10, the same one as conventionally used as the current collector of the negative electrode for the non-aqueous electrolyte secondary battery can be used. The current collector 11 is preferably made of a metal material having a low lithium compound forming ability as described above. Examples of such metallic materials are as already described. In particular, it is preferably made of copper, nickel, stainless steel or the like. Also, it is possible to use a copper alloy foil represented by a Corson alloy foil. Further, as the current collector, a metal foil having a normal tensile strength (JIS C 2318) of preferably 500 MPa or more, for example, a copper film layer formed on at least one surface of the aforementioned Corson alloy foil can be used. Furthermore, it is preferable to use a current collector having a normal elongation (JIS C 2318) of 4% or more. This is because when the tensile strength is low, wrinkles are generated due to stress when the active material expands, and when the elongation is low, the current collector may crack. By using these current collectors, it is possible to further improve the folding resistance of the negative electrode 10 described above. The thickness of the current collector 11 is preferably 9 to 35 μm considering the balance between maintaining the strength of the negative electrode 10 and improving the energy density. In addition, when using copper foil as the electrical power collector 11, it is preferable to give the rust prevention process using organic compounds, such as a chromate process and a triazole type compound and an imidazole type compound.

  Next, the preferable manufacturing method of the negative electrode 10 of this embodiment is demonstrated. In this production method, a coating film is formed on the current collector 11 using a slurry containing active material particles and a binder, and then the electroless plating is performed on the coating film.

  First, a slurry containing active material particles 12 a is applied onto the current collector 11 to form a coating film. The surface roughness of the coating film forming surface of the current collector 11 is preferably 0.5 to 4 μm at the maximum height of the contour curve. If the maximum height exceeds 4 μm, the formation accuracy of the coating film 15 is lowered. When the maximum height is less than 0.5 μm, the adhesion of the active material layer 12 tends to be lowered.

  The slurry contains a binder and a diluting solvent in addition to the active material particles. The slurry may contain a small amount of conductive carbon material particles such as acetylene black and graphite. In particular, when the active material particles 12a are made of a silicon-based material, the conductive carbon material is preferably contained in an amount of 1 to 3% by weight based on the weight of the active material particles 12a. When the content of the conductive carbon material is less than 1% by weight, the viscosity of the slurry is lowered and the sedimentation of the active material particles 12a is promoted, so that it is difficult to form a good coating film and uniform voids. On the other hand, if the content of the conductive carbon material exceeds 3% by weight, plating nuclei concentrate on the surface of the conductive carbon material, and it becomes difficult to form a good coating.

  As the binder, styrene butadiene rubber (SBR), polyvinylidene fluoride (PVDF), polyethylene (PE), ethylene propylene diene monomer (EPDM), or the like is used. As a diluting solvent, N-methylpyrrolidone, cyclohexane or the like is used. The amount of the active material particles 12a in the slurry is preferably about 30 to 70% by weight. The amount of the binder is preferably about 0.4 to 4% by weight. A dilution solvent is added to these to form a slurry.

  The formed coating film has a large number of minute spaces between the particles 12a. The current collector 11 on which the coating film is formed is immersed in a plating bath for electroless plating. By dipping in the plating bath, the plating solution enters the minute space in the coating film and reaches the interface between the coating film and the current collector 11. Under the state, electroless plating is performed to deposit the plating metal species on the surfaces of the particles 12a. The deposition rate of the plating metal species is preferably 0.05 to 1.5 μm / 20 minutes, particularly preferably 0.2 to 0.5 μm / 20 minutes from the viewpoint of forming appropriately sized crystal grains.

  As a plating solution for electroless plating, various commercially available products can be used depending on the type of metal to be deposited, and the type is not critical in the present invention. As the plating solution, it is preferable to use a plating solution that is excellent in uniform precipitation, covering property, and followability to unevenness from the viewpoint of depositing a metal uniformly and densely on the surface of the active material particles 12a. From this point of view, it is preferable to use a plating solution that is used for performing electroless plating on a through hole or a via hole such as a printed wiring board (PWB).

  When the particles of the active material contain Si, the pH of the plating solution is more than 7 to 13, particularly 7.1 to 12.5, from the viewpoint of successfully progressing electroless plating by a substitution reaction accompanying the elution of Si. It is preferable to use what is. This pH is the pH at the temperature during plating.

  The electroless plating is terminated when the metal material 13 is deposited over the entire thickness direction of the coating film. By adjusting the end point of plating, a surface layer (not shown) can be formed on the upper surface of the active material layer 12. Thus, the target negative electrode shown in FIG. 1 is obtained.

  Prior to electroless plating, a current collector having a coating film containing active material particles is immersed in an electrolytic plating bath to perform electrolytic plating, and a metal material is deposited between the particles in the coating film. Also good. By doing so, plating nuclei for the next electroless plating are formed, so that the electroless plating can be performed more successfully. This is particularly effective when using particles 12a of an active material containing Si, which is a material with poor electron conductivity. The amount of metal deposition in the electroplating prior to the electroless plating is sufficient to be small enough to form plating nuclei. The metal deposited by electrolytic plating and the metal deposited by subsequent electroless plating may be the same or different.

  As a plating solution used for electrolytic plating performed prior to electroless plating, when copper is used as a metal to be deposited, for example, a copper pyrophosphate bath can be used. When nickel is used as the metal to be deposited, for example, an alkaline nickel bath can be used. By using these plating solutions, it is possible to form uniform fine plating nuclei on the surfaces of the particles 12a.

When using a copper pyrophosphate bath, the bath composition, electrolysis conditions and pH are preferably as follows.
Copper pyrophosphate trihydrate: 85-120 g / l
-Potassium pyrophosphate: 300-600 g / l
-Potassium nitrate: 15-65 g / l
-Bath temperature: 45-60 ° C
・ Current density: 1 to 7 A / dm ²
PH: Ammonia water and polyphosphoric acid are added to adjust the pH to 7.1 to 9.5.

When an alkaline nickel bath is used, the bath composition, electrolysis conditions and pH are preferably as follows.
Nickel sulfate: 100 to 250 g / l
Ammonium chloride: 15-30 g / l
・ Boric acid: 15-45 g / l
-Bath temperature: 45-60 ° C
・ Current density: 1 to 7 A / dm ²
-PH: 25% by weight ammonia water: Adjust to pH 8-11 within the range of 100-300 g / l.

  When particles containing Si, which is a material with poor electron conductivity, are used as the active material particles 12, the surface of the active material particles 12 is covered with metal instead of performing electroplating prior to electroless plating. Electroless plating may be performed using the prepared one. By using such particles 12a, the same effects as in the case of performing electroplating prior to electroless plating are exhibited.

  It is also preferable to subject the negative electrode 10 to rust prevention after electroless plating. As the rust prevention treatment, for example, organic rust prevention using triazole compounds such as benzotriazole, carboxybenzotriazole, tolyltriazole, and imidazole, and inorganic rust prevention using cobalt, nickel, chromate and the like can be employed.

The negative electrode 10 thus obtained is suitably used as a negative electrode for a nonaqueous electrolyte secondary battery such as a lithium secondary battery. In this case, the positive electrode of the battery is prepared by suspending a positive electrode active material and, if necessary, a conductive agent and a binder in a suitable solvent to prepare a positive electrode mixture, applying this to a current collector, drying, roll rolling, It is obtained by pressing, further cutting and punching. As the positive electrode active material, conventionally known positive electrode active materials such as lithium transition metal composite oxides such as lithium nickel composite oxide, lithium manganese composite oxide, and lithium cobalt composite oxide are used. Further, as the positive electrode active material, a lithium transition metal composite oxide in which LiCoO ₂ contains at least both Zr and Mg, a lithium transition metal composite oxide having a layered structure and containing at least both Mn and Ni, A mixture thereof can also be preferably used. The use of such a positive electrode active material can be expected to increase the end-of-charge voltage without deteriorating charge / discharge cycle characteristics and thermal stability. The average value of the primary particle diameter of the positive electrode active material is preferably 5 μm or more and 10 μm or less from the viewpoint of the packing density and the reaction area. The weight average molecular weight of the binder used for the positive electrode is 350,000 or more and 2,000. It is preferable that the polyvinylidene fluoride is 1,000 or less. This is because it can be expected to improve discharge characteristics in a low temperature environment.

  As the battery separator, a synthetic resin nonwoven fabric, a polyolefin such as polyethylene or polypropylene, or a polytetrafluoroethylene porous film is preferably used. From the viewpoint of suppressing the heat generation of the electrode that occurs when the battery is overcharged, it is preferable to use a separator in which a thin film of a ferrocene derivative is formed on one side or both sides of a polyolefin microporous membrane. The separator preferably has a puncture strength of 0.2 N / μm thickness or more and 0.49 N / μm thickness or less, and a tensile strength in the winding axis direction of 40 MPa or more and 150 MPa or less. This is because even when a negative electrode active material that greatly expands and contracts with charge and discharge is used, damage to the separator can be suppressed, and occurrence of internal short circuits can be suppressed.

The nonaqueous electrolytic solution is a solution in which a lithium salt as a supporting electrolyte is dissolved in an organic solvent. The lithium _{salt, CF 3 SO 3 Li, (} CF 3 SO 2) NLi, (C 2 F 5 SO 2) 2 NLi, LiClO 4, LiA1Cl 4, LiPF 6, LiAsF 6, LiSbF 6, LiCl, LiBr, LiI And LiC ₄ F ₉ SO ₃ . These can be used alone or in combination of two or more. Of these lithium salts, CF ₃ SO ₃ Li, (CF ₃ SO ₂ ) NLi, and (C ₂ F ₅ SO ₂ ) ₂ NLi are preferably used because of their excellent water decomposition resistance. Examples of the organic solvent include ethylene carbonate, diethyl carbonate, dimethyl carbonate, propylene carbonate, butylene carbonate, and the like. In particular, 0.5 to 5% by weight of vinylene carbonate and 0.1 to 1% by weight of divinyl sulfone and 0.1 to 1.5% by weight of 1,4-butanediol dimethanesulfonate with respect to the whole non-aqueous electrolyte It is preferable from the viewpoint of further improving the charge / discharge cycle characteristics. The details are not clear, but 1,4-butanediol dimethanesulfonate and divinylsulfone are decomposed stepwise to form a film on the positive electrode, so that the film containing sulfur becomes denser. It is thought that it is to become.

  In particular, as the non-aqueous electrolyte, 4-fluoro-1,3-dioxolan-2-one, 4-chloro-1,3-dioxolan-2-one or 4-trifluoromethyl-1,3-dioxolane-2- It is also preferable to use a high dielectric constant solvent having a relative dielectric constant of 30 or more, such as a cyclic carbonate derivative having a halogen atom such as ON. This is because it has high resistance to reduction and is not easily decomposed. Further, an electrolytic solution obtained by mixing the high dielectric constant solvent and a low viscosity solvent having a viscosity of 1 mPa · s or less such as dimethyl carbonate, diethyl carbonate, or methyl ethyl carbonate is also preferable. This is because higher ionic conductivity can be obtained. Furthermore, it is also preferable that the content of fluorine ions in the electrolytic solution is in the range of 14 mass ppm to 1290 mass ppm. This is because, when an appropriate amount of fluorine ions is contained in the electrolytic solution, a coating film such as lithium fluoride derived from fluorine ions is formed on the negative electrode, and it is considered that the decomposition reaction of the electrolytic solution in the negative electrode can be suppressed. . Furthermore, it is preferable that 0.001 mass%-10 mass% of at least 1 sort (s) of additives from the group which consists of an acid anhydride and its derivative (s) are contained. This is because a film is formed on the surface of the negative electrode, and the decomposition reaction of the electrolytic solution can be suppressed. As this additive, a cyclic compound containing a —C (═O) —O—C (═O) — group in the ring is preferable. For example, succinic anhydride, glutaric anhydride, maleic anhydride, phthalic anhydride, anhydrous Phthalic anhydride derivatives such as 2-sulfobenzoic acid, citraconic anhydride, itaconic anhydride, diglycolic anhydride, hexafluoroglutaric anhydride, 3-fluorophthalic anhydride, 4-fluorophthalic anhydride, or anhydrous 3,6 -Epoxy-1,2,3,6-tetrahydrophthalic acid, 1,8-naphthalic anhydride, 2,3-naphthalene carboxylic acid anhydride, 1,2-cyclopentane dicarboxylic acid anhydride, 1,2-cyclohexane dicarboxylic acid, etc. 1,2-cycloalkanedicarboxylic anhydride, or cis-1,2,3,6-tetrahydrophthalic anhydride or 3,4,5,6-tetrahydrophthal Tetrahydrophthalic anhydride such as acid anhydride, or hexahydrophthalic anhydride (cis isomer, trans isomer), 3,4,5,6-tetrachlorophthalic anhydride, 1,2,4-benzene Examples thereof include tricarboxylic acid anhydride, dianhydropyromellitic acid, and derivatives thereof.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the scope of the present invention is not limited to such examples. Unless otherwise specified, “%” means “% by weight”.

[Example 1]
A current collector made of an electrolytic copper foil having a thickness of 18 μm was acid washed at room temperature for 30 seconds. After the treatment, it was washed with pure water for 15 seconds. A slurry containing Si particles was applied on the current collector to a thickness of 15 μm to form a coating film. The composition of the slurry was particles: styrene butadiene rubber (binder): acetylene black = 100: 1.7: 2 (weight ratio). The average particle diameter D ₅₀ of the Si particles was 2.5 μm. The average particle diameter D ₅₀ was measured using a Microtrac particle size distribution measuring device (No. 9320-X100) manufactured by Nikkiso Co., Ltd.

  The current collector on which the coating film was formed was immersed in Melplate Cu-390, which is an electroless copper plating solution manufactured by Meltex. Electroless plating was performed at 25 ° C. for 60 minutes to form an active material layer. The electroless plating was terminated when copper was deposited over the entire thickness direction of the coating film, washed with water, and subjected to rust prevention treatment with benzotriazole (BTA) to obtain a target negative electrode. A scanning electron micrograph of a longitudinal section of the active material layer in the obtained negative electrode is shown in FIG.

[Example 2]
In Example 1, the current collector on which the coating film was formed was immersed in a copper pyrophosphate bath having the following bath composition, and electrolytic plating was performed. The electrolysis conditions were as follows. DSE was used for the anode. A DC power source was used as the power source. The electrolysis time was 1.5 minutes. Thereafter, copper electroless plating was performed in the same manner as in Example 1 to obtain a negative electrode. A scanning electron micrograph of a longitudinal section of the active material layer in the obtained negative electrode is shown in FIG.
Copper pyrophosphate trihydrate: 105 g / l
-Potassium pyrophosphate: 450 g / l
・ Potassium nitrate: 30 g / l
・ Bath temperature: 50 ° C
・ Current density: 3 A / dm ²
-PH: Ammonia water and polyphosphoric acid were added to adjust to pH 8.2.

Example 3
In a plating bath in which silicon particles are suspended and containing copper sulfate and Rochelle salt, the silicon particles are electrolessly plated, and the surfaces of the silicon particles are coated with copper to obtain copper-coated silicon particles. The concentration of silicon particles in the plating bath was 500 g / l, the concentration of copper sulfate was 7.5 g / l, and the concentration of Rochelle salt was 85 g / l. The pH of the plating bath was 12.5, and the bath temperature was 25 ° C. Formaldehyde was used as a reducing agent, and its concentration was 22 cc / l. The obtained copper-coated silicon particles had a D ₅₀ value of 2.5 μm. The composition of the particles was Si80 / Cu20. A negative electrode was obtained in the same manner as in Example 1 except that the copper-coated silicon particles were used. A scanning electron micrograph of the longitudinal section of the active material layer in the obtained negative electrode is shown in FIG.

Example 4
80% silicon. A 1400 ° C. molten metal containing 20% nickel was poured into a copper mold to obtain a rapidly cooled silicon-nickel alloy ingot. The ingot was pulverized with a jet mill and sieved to obtain active material particles. The obtained active material particles were put into 20% KOH and etched for 20 minutes. The active material particles thus obtained had a D ₅₀ value of 2.5 μm. A negative electrode was obtained in the same manner as in Example 1 except that this active material particle was used.

[Comparative Example 1]
A negative electrode was obtained in the same manner as in Example 1 except that electroless plating was not performed.

[Comparative Example 2]
In Example 1, the current collector on which the coating film was formed was immersed in a copper sulfate bath having the following bath composition with reference to Patent Document 2, and electrolytic plating was performed. The electrolysis conditions were as follows.
CuSO ₄ : 250 g / l
・ H ₂ SO ₄ : 70 g / l
・ Bath temperature: 40 ℃
・ Current density: 5 A / dm ²
The electrolytic plating was terminated when copper was deposited over the entire thickness direction of the coating film. Other than that was carried out similarly to Example 1, and obtained the negative electrode.

[Evaluation]
About the negative electrode obtained by the Example and the comparative example, the tape peeling test was done based on JISC5012, and the adhesiveness of a collector and an active material layer was evaluated. Of 50 samples, the ratio (%) of ◯ was calculated when the sample peeled at the interface between the current collector and the active material layer was indicated as x, and the sample that did not peel as ◯.

Further, the porosity of the active material layer was measured by the method described above. Furthermore, comprehensive evaluation was performed according to the following criteria. These results are shown in Table 1.
A: Adhesiveness exceeds 90% and porosity is more than 15%.
○: Adhesiveness is more than 50% and 90% or less, and porosity is more than 15%.
X: Adhesiveness is 50% or less or porosity is 15% or less.

  As is clear from the results shown in Table 1, in the negative electrode of the example, the adhesion between the current collector and the active material layer is high, and a sufficient amount of voids are formed in the active material layer. I understand. On the other hand, in the negative electrode of Comparative Example 1, it can be seen that a sufficient amount of voids are formed in the active material layer, but the adhesion between the current collector and the active material layer is poor. On the contrary, in the negative electrode of Comparative Example 2, it can be seen that the adhesiveness between the current collector and the active material layer is high, but the amount of voids formed in the active material layer is not sufficient.

It is a schematic diagram which shows the cross-sectional structure of one Embodiment of the negative electrode for nonaqueous electrolyte secondary batteries of this invention. 2 is a scanning electron micrograph image of a longitudinal section of an active material layer in a negative electrode obtained in Examples 1 and 2. FIG. 4 is a scanning electron micrograph image of a longitudinal section of an active material layer in a negative electrode obtained in Example 3. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Negative electrode for non-aqueous electrolyte secondary batteries 11 Current collector 12 Active material layer 12a Active material particles 13 Metal material with low lithium compound forming ability

Claims (6)

  A negative electrode for a non-aqueous electrolyte secondary battery, comprising an active material layer containing active material particles, wherein a metal material deposited by electroless plating is present between the particles.   The nonaqueous electrolyte secondary battery according to claim 1, wherein at least a part of the surface of the particle is coated with the metal material, and a gap is formed between the particles coated with the metal material. Negative electrode.   The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the metal material is present on the surface of the particles over the entire thickness direction of the active material layer.   The surface of the particle is coated with a metal material by electrolytic plating, and a metal material of the same type or different from the metal material is deposited on the metal material by electroless plating. A negative electrode for a non-aqueous electrolyte secondary battery according to 1. A method for producing a negative electrode for a non-aqueous electrolyte secondary battery according to claim 1,
Applying a slurry containing active material particles to at least one surface of the current collector to form a coating film,
For a non-aqueous electrolyte secondary battery, wherein the current collector formed with the coating film is immersed in a bath to perform electroless plating, and a metal material is deposited between the particles in the coating film. Manufacturing method of negative electrode. The current collector on which the coating film is formed is immersed in an electrolytic plating bath to perform electrolytic plating, and a metal material is deposited between the particles in the coating film.
The manufacturing method according to claim 5, wherein the current collector is immersed in an electroless plating bath to perform the electroless plating.