JP2008016196A - Negative electrode for polymer electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a negative electrode for a polymer electrolyte secondary battery having enhanced cycle characteristics. <P>SOLUTION: The negative electrode 10 for the polymer electrolyte secondary battery is equipped with an active material layer 12 containing particles 12a of an active material. At least a part of the surface of the particle 12a is covered with a metallic material 13 having low lithium compound forming property. Pores are formed between the particles 12a covered with the metallic material 13. The porosity of the active material layer is 15-45%. Preferably, the metallic materials 13 are present on the surfaces of the particles over the whole area in the thickness direction of the active material layer. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

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

  The present applicant previously includes a pair of current collecting surface layers whose surfaces are in contact with the electrolytic solution, and an active material layer including active material particles having a high ability to form a lithium compound interposed between the surface layers. In addition, a negative electrode for a nonaqueous electrolyte secondary battery was proposed (see Patent Document 1). 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.

  By the way, as the secondary battery using the occlusion and release of lithium ions, a polymer electrolyte secondary battery using a polymer gel electrolyte is known in addition to the non-aqueous electrolyte secondary battery. The polymer gel electrolyte used for the polymer electrolyte secondary battery has a low fluidity. Therefore, even if a negative electrode for a secondary battery using a non-aqueous electrolyte, which is a liquid with high fluidity, is directly converted to a negative electrode for a polymer electrolyte secondary battery, a battery having desired characteristics can be obtained. Can not.

Japanese Patent No. 3612669

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

  The present invention includes an active material layer containing particles of an active material, and at least a part of the surface of the particles is coated with a metal material having a low ability to form a lithium compound, and the particles coated with the metal material A polymer electrolyte secondary characterized in that voids are formed between the active material layers and the porosity of the active material layer is 15 to 45% measured in accordance with a mercury intrusion method (JIS R 1655) A negative electrode for a battery is provided.

  According to the present invention, even if it is a polymer gel electrolyte having low fluidity, a path through which it can flow is formed in the active material layer, and the polymer gel electrolyte easily reaches the active material layer. The entire thickness direction of the active material layer is used for the electrode reaction. As a result, cycle characteristics are improved. In addition, even if the particles are pulverized due to expansion and contraction due to charging / discharging, the particles do not easily fall off.

  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 polymer 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. As the tin-based material, for example, an alloy containing tin, cobalt, carbon, and at least one of nickel and chromium is preferably used. In order to improve the capacity density per weight of the negative electrode, a silicon-based material is particularly preferable.

  As the silicon-based material, a material that can occlude lithium and contains silicon, for example, silicon alone, an alloy of silicon and metal, silicon oxide, or the like can be used. These materials can be used alone or in combination. Examples of the metal include one or more elements selected from the group consisting of Cu, Ni, Co, Cr, Fe, Ti, Pt, W, Mo, and Au. Among these metals, Cu, Ni, and Co are preferable, and Cu and Ni are preferably used from the viewpoint of excellent electronic conductivity and a low ability to form a lithium compound. Further, lithium may be occluded in an active material made of a silicon-based material before or after the negative electrode is incorporated in the battery. A particularly preferable silicon-based material is silicon or silicon oxide in view of the high occlusion amount of lithium.

  In the active material layer 12, at least a part of the surface of the particle 12a is covered with a metal material having a low lithium compound forming ability. The metal material 13 is a material different from the constituent material of the particles 12a. Gaps are formed between the particles 12a coated with the metal material. That is, the metal material covers the surface of the particle 12a in a state in which a gap is provided such that the polymer gel electrolyte can reach the particle 12a. In FIG. 1, the metal material 13 is conveniently represented as a thick line surrounding the periphery of the particle 12a. In the figure, among the particles 12a included in the active material layer 12, there are particles that are drawn so that there is no contact with other particles. In fact, each particle is in direct contact with other particles or through the 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, and alloys of these metals. 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 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 presence of the metal material 13 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 coats the surface of the particle 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 in which the polymer gel electrolyte can flow. When the metal material 13 discontinuously coats the surface of the particle 12a, the polymer gel 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. In order to form the coating of the metal material 13 having such a structure, the metal material 13 may be deposited on the surfaces of the particles 12a by, for example, electrolytic plating in accordance with conditions described later.

  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 serve as a flow path for the polymer gel electrolyte. Since the polymer gel electrolyte smoothly circulates in the thickness direction of the active material layer 12 due to the presence of the voids, the 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. The presence of the polymer gel electrolyte in the voids also has the advantage that the polymer gel electrolyte acts as a cushion material that relieves stress generated by the expansion and contraction of the particles 12a.

  When the present inventors examined the voids formed in the active material layer 12, the porosity of the active material layer 12 is set to 15 to 45%, preferably 20 to 40%, and more preferably 25 to 35%. It has been found that the distribution of the polymer gel electrolyte in the active material layer 12 becomes extremely good, and is extremely effective for stress relaxation accompanying expansion and contraction of the particles 12a of the active material. Furthermore, it has been found that the liquid retention of the polymer gel electrolyte in the voids is improved. In particular, setting the upper limit to 35% is extremely effective in improving the conductivity and maintaining the strength in the active material layer, and setting the lower limit to 25% can widen the range of selection of the electrolyte. The porosity in this range is higher than the porosity in the conventional negative electrode active material layer, for example, the porosity in the negative electrode described in Patent Document 1 described above. By using the negative electrode 10 having such a high porosity active material layer, it is possible to use a polymer gel electrolyte which is a material having low fluidity.

  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. In the present invention, the porosity (%) of the active material layer 12 is obtained by dividing the void amount per unit area measured by the above-mentioned method by the apparent volume of the active material layer 12 per unit area, and dividing it by 100. Find by multiplying.

The active material layer 12 is preferably formed by subjecting a coating obtained by applying a slurry containing particles 12a and a binder onto a current collector and drying it, and performing electrolytic plating using a predetermined plating bath. It is formed by depositing a metal material 13 between 12a. The degree of precipitation of the metal material 13 affects the value of the porosity of the active material layer 12. In order to achieve a desired porosity, it is necessary that a space in which the plating solution can permeate is formed in the coating film. As a result of the examination by the present inventors, it was found that the particle size distribution of the active material particles 12a is a large factor in order to form a space in which the plating solution can penetrate into the coating film as necessary and sufficient. . Specifically, by adopting particles having a particle size distribution represented by D ₁₀ / D ₉₀ of preferably 0.05 to 0.5, more preferably 0.1 to 0.3, as the active material particles 12a. It was found that a desired space was formed in the coating film, and the plating solution was sufficiently permeated. It was also found that the coating film can be effectively prevented from peeling off during electrolytic plating. It can be seen that the closer the D ₁₀ / D ₉₀ is to 1, the closer the particle size of the particles 12a is to monodisperse, so the particle size distribution in the above range is sharper. That is, in this embodiment, it is preferable to use the particles 12a having a sharp particle size distribution. By using the particles 12a having a sharp particle size distribution, the voids between the particles can be increased when the particles 12a are packed at a high density. On the other hand, when particles having a broad particle size distribution are used, small particles easily enter between large particles, and it is not easy to increase the voids between the particles. In addition, the use of the particles 12a having a sharp particle size distribution has an advantage that variations in reaction are less likely to occur.

In order to obtain a negative electrode having excellent cycle characteristics, in addition to the particle size distribution of the active material particles 12a being in the above range, the particle size of the particles 12a itself is also important. When the particle size of the active material particles 12a is excessively large, the particles 12a are easily pulverized by repeating expansion and contraction, thereby frequently generating electrically isolated particles 12g. If the particle size of the active material particles 12a is too small, the gaps between the particles 12a may be too small, and the gaps may be filled by penetration plating described later. This has a negative effect on the improvement of cycle characteristics. Therefore, in this embodiment, it is preferable that the average particle diameter of the active material particles 12a is 0.1 to 5 μm, particularly 0.2 to 3 μm, expressed as D ₅₀ .

The values of the particle size distribution D ₁₀ / D ₉₀ and the average particle size D ₅₀ of the active material particles 12a are measured by laser diffraction scattering type particle size distribution measurement or electron microscope observation (SEM observation).

  In order to set the porosity of the active material layer 12 within the above range, it is preferable that the plating solution is sufficiently permeated into the coating film. In addition to this, it is preferable to make conditions suitable for depositing the metal material 13 by electrolytic plating using the plating solution. The plating conditions include the composition of the plating bath, the pH of the plating bath, and the current density of electrolysis. Regarding the pH of the plating bath, it is preferable to adjust it to more than 7 and 11 or less, particularly 7.1 or more and 11 or less. By controlling the pH within this range, the dissolution of the active material particles 12a is suppressed, the surface of the particles 12a is cleaned, and plating on the particle surface is promoted. Is formed. The value of pH is measured at the temperature at the time of plating.

When using copper as the metal material 13 for plating, it is preferable to use a copper pyrophosphate bath. When nickel is used as the metal material, for example, an alkaline nickel bath is preferably used. In particular, it is preferable to use a copper pyrophosphate bath, even if the active material layer 12 is thick, because the voids can be easily formed over the entire thickness direction of the layer. Further, since the metal material 13 is deposited on the surface of the active material particles 12a and the metal material 13 is less likely to be deposited between the particles 12a, the voids between the particles 12a are also successfully formed. preferable. 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.

In particular, when a copper pyrophosphate bath is used, it is preferable to use one having a P ratio defined by a ratio of P ₂ O ₇ weight to Cu weight (P ₂ O ₇ / Cu) of 5 to 12. . When the P ratio is less than 5, the metal material 13 covering the active material particles 12a tends to be thick, and it may be difficult to form desired voids between the particles 12a. Further, when a P ratio exceeding 12 is used, current efficiency is deteriorated, and gas generation is likely to occur, so that production stability may be lowered. When a copper pyrophosphate bath having a P ratio of 6.5 to 10.5 is used as a more preferable copper pyrophosphate bath, the size and number of voids formed between the active material particles 12a may be reduced. This is very advantageous for the distribution of the gel electrolyte.

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 this alkaline nickel bath and the above-described copper pyrophosphate bath are compared, there is a tendency that an appropriate void is formed in the active material layer 12 when the copper pyrophosphate bath is used, thereby extending the life of the negative electrode. It is preferable because it is easy to plan.

  The characteristics of the metal material 13 can be appropriately adjusted by adding various additives used in the electrolytic solution for producing copper foil such as protein, active sulfur compound, and cellulose to the above various plating baths.

  In the negative electrode 10 of the present embodiment, the porosity calculated from the void amount of the active material layer 12 measured by the mercury intrusion method was within the above range, and was measured by the mercury intrusion method at 10 MPa. The porosity calculated from the void amount of the active material layer 12 is preferably 10 to 40%. Moreover, it is preferable that the porosity calculated from the void amount of the active material layer 12 measured by the mercury intrusion method at 1 MPa is 0.5 to 15%. Furthermore, it is preferable that the porosity calculated from the void amount of the active material layer 12 measured by the mercury intrusion method at 5 MPa is 1 to 35%. As described above, in the mercury intrusion measurement, mercury intrusion conditions are gradually increased. Under low pressure conditions, mercury is pressed into large gaps, and under high pressure conditions, mercury is pressed into small gaps. Therefore, the porosity measured at a pressure of 1 MPa is mainly derived from large voids. On the other hand, the porosity measured at a pressure of 10 MPa reflects the presence of small voids.

  As described above, the active material layer 12 is preferably subjected to electrolytic plating using a predetermined plating bath on the coating film obtained by applying and drying the slurry containing the particles 12a and the binder, It is formed by depositing a metal material 13 between the particles 12a. Therefore, as shown in FIG. 2, the large void S1 described above is mainly derived from the space between the particles 12a, while the small void S2 described above is mainly formed of the metal material 13 deposited on the surface of the particles 12a. It is thought that it originates in the space between the crystal grains 14. The large void S1 mainly functions as a space for relieving stress caused by the expansion and contraction of the particles 12a. On the other hand, the small void S2 mainly serves as a path for supplying the polymer gel electrolyte to the particles 12a. By balancing the abundance of these large voids S1 and small voids S2, the cycle characteristics are further improved.

  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 12 is preferably 10 to 40 μm, more preferably 15 to 30 μm, and still 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 polymer gel electrolyte to flow. The role of the fine voids is to supply the polymer gel 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.

  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 present 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.

  Since the porosity in the active material layer 12 has a high value, the negative electrode 10 of this embodiment has high resistance to bending. Specifically, the MIT folding resistance measured according to JIS C 6471 is preferably 30 times or more, and 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 a current collector of a negative electrode for a polymer 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, referring FIG. In this manufacturing method, a coating film is formed on the current collector 11 using a slurry containing particles of an active material and a binder, and then electrolytic plating is performed on the coating film.

  First, a current collector 11 is prepared as shown in FIG. Then, a slurry containing active material particles 12 a is applied onto the current collector 11 to form a coating film 15. 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 and current concentration of the permeation plating tends to occur on the convex portions. When the maximum height is less than 0.5 μm, the adhesion of the active material layer 12 tends to be lowered. As the active material particles 12a, those having the above-described particle size distribution and average particle size are preferably used.

  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 15 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 15 has a large number of minute spaces between the particles 12a. The current collector 11 on which the coating film 15 is formed is immersed in a plating bath containing a metal material having a low lithium compound forming ability. By immersion in the plating bath, the plating solution enters the minute space in the coating film 15 and reaches the interface between the coating film 15 and the current collector 11. Under this condition, electrolytic plating is performed to deposit plating metal species on the surfaces of the particles 12a (hereinafter, this plating is also referred to as permeation plating). The osmotic plating is performed by using the current collector 11 as a cathode, immersing a counter electrode as an anode in a plating bath, and connecting both electrodes to a power source.

  The deposition of the metal material by the osmotic plating is preferably progressed from one side of the coating film 15 to the other side. Specifically, as shown in FIGS. 3B to 3D, electrolytic plating is performed so that deposition of the metal material 13 proceeds from the interface between the coating film 15 and the current collector 11 toward the surface of the coating film. I do. By precipitating the metal material 13 in this way, the surface of the active material particles 12a can be successfully coated with the metal material 13, and a void is successfully formed between the particles 12a coated with the metal material 13. be able to. In addition, it becomes easy to set the void ratio of the voids to the above-described preferable range.

  As described above, the conditions of the infiltration plating for depositing the metal material 13 include the composition of the plating bath, the pH of the plating bath, the current density of electrolysis, and the like. Such conditions are as already described.

  As shown in FIGS. 3B to 3D, when plating is performed so that the deposition of the metal material 13 proceeds from the interface between the coating film 15 and the current collector 11 toward the surface of the coating film. In the forefront portion of the precipitation reaction, fine particles 13a made of plating nuclei of the metal material 13 are present in layers with a substantially constant thickness. As the deposition of the metal material 13 proceeds, adjacent microparticles 13a combine to form larger particles, and when the deposition proceeds further, the particles combine to continuously cover the surface of the active material particles 12a. It becomes like this.

  The permeation plating is terminated when the metal material 13 is deposited on the entire thickness direction of the coating film 15. 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. In this way, the target negative electrode is obtained as shown in FIG.

  It is also preferable to subject the negative electrode 10 to rust prevention after the osmotic 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 polymer electrolyte secondary battery such as a lithium ion polymer 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. The active material layer of the positive electrode is impregnated with a polymer gel electrolyte in advance to be combined. As the positive electrode active material, conventionally known positive electrode active materials such as lithium-containing 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 in view of the packing density and the reaction area, and 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.

The polymer gel electrolyte includes a matrix polymer, an organic solvent, and a lithium salt. As the matrix polymer, polyethylene oxide, polypropylene oxide, polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, polyacrylic acid, polymethacrylic acid, polyacrylonitrile, polycarbonate, polyethylene glycol and the like can be used. The lithium _{salt, LiC1O 4, LiA1Cl 4, LiPF} 6, LiAsF 6, LiSbF 6, LiBF 4, LiSCN, LiC1, LiBr, LiI, etc. _{LiCF 3 SO 3, LiC 4 F} 9 SO 3 are exemplified. Examples of the organic solvent include ethylene carbonate, diethyl carbonate, dimethyl carbonate, propylene carbonate, butylene carbonate, and γ-butyrolactone.

  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.

[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 particle size distribution D ₁₀ / D ₉₀ was 0.07. The average particle size D ₅₀ and the particle size distribution D ₁₀ / D ₉₀ were 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 a copper pyrophosphate bath having the following bath composition, and copper was permeated to the coating film by electrolysis to form an active material layer. The electrolysis conditions were as follows. DSE was used for the anode. A DC power source was used as the power source.
Copper pyrophosphate trihydrate: 105 g / l
-Potassium pyrophosphate: 450 g / l
・ Potassium nitrate: 30 g / l
-P ratio: 7.7
・ Bath temperature: 50 ° C
・ Current density: 3 A / dm ²
-PH: Ammonia water and polyphosphoric acid were added to adjust to pH 8.2.

  The permeation plating was terminated when copper was deposited over the entire thickness direction of the coating film, and was washed with water and subjected to rust prevention treatment with benzotriazole (BTA) to obtain a target negative electrode.

[Examples 2 to 5]
A negative electrode was obtained in the same manner as in Example 1 except that Si particles having the average particle size D ₅₀ and the particle size distribution D ₁₀ / D ₉₀ shown in Table 1 were used.

[Comparative Examples 1 and 2]
As the Si particles, those having the average particle size D ₅₀ and the particle size distribution D ₁₀ / D ₉₀ shown in Table 1 were used. Further, instead of the copper pyrophosphate bath used in Example 1, a copper sulfate bath having the following composition was used. The current density was 5 A / dm ² and the bath temperature was 40 ° C. A DSE electrode was used as the anode. A DC power source was used as the power source. A negative electrode was obtained in the same manner as Example 1 except for these.
・ CuSO ₄・ 5H ₂ O 250g / l
・ H ₂ SO ₄ 70 g / l

[Comparative Examples 3 and 4]
A negative electrode was obtained in the same manner as in Example 1 except that Si particles having the average particle size D ₅₀ and the particle size distribution D ₁₀ / D ₉₀ shown in Table 1 were used.

[Evaluation]
About the negative electrode obtained by the Example and the comparative example, the measurement by a mercury porosimeter was performed. The results are shown in Table 1. Separately, lithium ion polymer secondary batteries were manufactured using the negative electrodes obtained in Examples and Comparative Examples. LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ was used as the positive electrode. The positive electrode active material layer was impregnated with the following polymer gel electrolyte. As the polymer gel electrolyte, one containing 6% by weight of polyacrylonitrile, 40% by weight of ethylene carbonate, 44% by weight of diethyl carbonate, and 10% by weight of LiPF ₆ was used. The capacity retention rate up to 100 cycles was measured for the obtained secondary battery. The capacity retention rate was calculated by measuring the discharge capacity at each cycle, dividing those values by the initial discharge capacity, and multiplying by 100. The charging conditions were 0.5 C, 4.2 V, and a constant current / constant voltage (CCCV). The discharge conditions were 0.5 C, 2.7 V, and a constant current (CC). However, the first cycle was 0.05 C, the second to fourth cycles were 0.1 C, the fifth to seventh cycles were 0.5 C, and the eighth to tenth cycles were 1 C. The results are shown in Table 1.

  As apparent from the results shown in Table 1, it can be seen that the secondary battery including the negative electrode of the example has better cycle characteristics than the secondary battery including the negative electrode of the comparative example. Although not shown in the table, it was confirmed that electrical conductivity was obtained between the front and back surfaces of the negative electrode of each example.

It is a schematic diagram which shows the cross-section of one Embodiment of the negative electrode for polymer electrolyte secondary batteries of this invention. FIG. 2 is a schematic diagram illustrating an enlarged main part of a cross section of an active material layer in the negative electrode illustrated in FIG. 1. It is process drawing which shows the manufacturing method of the negative electrode shown in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Negative electrode for polymer electrolyte secondary batteries 11 Current collector 12 Active material layer 12a Active material particle 13 Metal material with low lithium compound forming ability 15 Coating film

Claims (9)

  An active material layer including particles of the active material, wherein at least a part of the surface of the particles is coated with a metal material having a low ability to form a lithium compound, and between the particles coated with the metal material A negative electrode for a polymer electrolyte secondary battery, wherein voids are formed, and the porosity of the active material layer is 15 to 45%.   The negative electrode for a polymer 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. 3. The negative electrode for a polymer electrolyte secondary battery according to claim 1, wherein the particle size distribution of the particles is 0.05 to 0.5 in terms of D ₁₀ / D ₉₀ . 4. The negative electrode for a polymer electrolyte secondary battery according to claim 1, wherein the average particle diameter of the particles is 0.1 to ₅ μm in terms of D ₅₀ .   The active material particles are made of a silicon-based material, and the active material layer contains a conductive carbon material in an amount of 1 to 3% by weight based on the weight of the active material particles. A negative electrode for a polymer electrolyte secondary battery as described in 1.   The negative electrode for a polymer electrolyte secondary battery according to any one of claims 1 to 5, wherein the coating of the metal material is formed by electrolytic plating using a plating bath having a pH of more than 7 and 11 or less.   The porosity of the active material layer measured by mercury intrusion method (JIS R 1655) at 10 MPa is 10 to 40%, and the porosity of the active material layer measured by mercury intrusion method (JIS R 1655) at 1 MPa. The negative electrode for a polymer electrolyte secondary battery according to claim 1, wherein the content is 0.5 to 15%.   The polymer electrolyte secondary according to any one of claims 1 to 7, wherein a maximum peak value of pore size distribution of voids in the active material layer measured by a mercury intrusion method (JIS R 1655) is between 100 and 2000 nm. Battery negative electrode. A polymer electrolyte secondary battery comprising the negative electrode for a polymer electrolyte secondary battery according to claim 1.