JP2008016192A - Anode for nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an anode for a nonaqueous electrolyte secondary battery with a low overvoltage at initial charging and excellent in cycle characteristics. <P>SOLUTION: The anode for the nonaqueous electrolyte secondary battery 10 is provided with an active material layer 12 containing particles 12a of an active material. At least a part of the surface of the particles 12a is coated with a metal material 13 and a solid electrolyte film (SEI) 14. In addition, gaps are formed between the particles 12a coated with the metal material 13 layer. The SEI 14 is preferred to coat at least a part the surface of the particles 12a with a part of the metal material 13 in a taken-in state. <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.

  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.

  In order for the particles in the active material layer to successfully occlude and release lithium ions, it is necessary that the non-aqueous electrolyte containing lithium ions can smoothly flow through the active material layer. For this purpose, it is advantageous to provide a path through which the non-aqueous electrolyte can flow in the active material layer. When the amount of penetration of the metal material in the active material layer is excessively large, the path is not sufficiently formed, lithium ions do not easily reach the active material particles, and the initial charge overvoltage is high. Tend to be. An increase in overvoltage causes generation of lithium dendrites on the surface of the negative electrode and decomposition of the non-aqueous electrolyte. Moreover, the electrode reaction in the thickness direction of the active material layer becomes non-uniform. On the other hand, when the amount of penetration of the metal material in the active material layer is too small, the active material is pulverized and dropped due to expansion and contraction of the active material due to charge and discharge, and the cycle characteristics are deteriorated. .

Japanese Patent No. 3612669

  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 includes an active material layer containing particles of an active material, and at least a part of the surface of the particles is covered with a metal material having a low lithium compound forming ability and a solid electrolyte membrane, The present invention provides a negative electrode for a nonaqueous electrolyte secondary battery in which voids are formed.

  The present invention also includes an active material layer containing active material particles, 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 there are voids between the particles. A non-aqueous electrolyte secondary that is formed and has an average thickness of the metal material of 0.05 to 1 μm, and the metal material is present on the surface of the particles over the entire thickness direction of the active material layer A negative electrode for a battery is provided.

The present invention also applies a slurry containing active material particles on a current collector to form a coating film,
The coating film is subjected to a press treatment of 50 to 500 MPa,
The negative electrode for a non-aqueous electrolyte secondary battery in which the coated film after pressing is immersed in a plating bath containing a metal material having a low lithium forming capacity to perform electrolytic plating, and the metal material is deposited on the surface of the particles. A manufacturing method is provided.

  When the active material particles expand and contract due to charge and discharge, the solid electrolyte membrane deforms following the expansion and contraction, and the metal material having a low lithium compound forming capacity also deforms following the deformation. It becomes difficult to peel off the coating from the surface of the particles. As a result, cycle characteristics are improved. In particular, when the thickness of the metal material coating is set to a small value, the non-aqueous electrolyte containing lithium ions easily reaches the particle surface of the active material, and the initial charging overvoltage is reduced.

  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. The active material layer 12 is formed, for example, by applying a slurry containing active material particles 12a. Examples of the active material include materials that can occlude lithium, such as 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, 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. In a particularly preferable silicon-based material, silicon is 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 12 a is covered with a layer of the metal material 13. The metal material 13 is a material different from the constituent material of the particles 12a. Voids are formed between the particles 12 a covered with the layer of the metal material 13. That is, the layer of the metal material 13 covers the surface of the particle 12a in a state in which a gap is provided so that the non-aqueous electrolyte containing lithium ions can reach the particle 12a. In FIG. 1, the layer of the metal material 13 is conveniently represented as a thick line surrounding the periphery of the particle 12a. In the figure, among the particles 12 a included in the active material layer 12, there are particles that are drawn so that there is no contact with other particles. This is because the active material layer 12 is two-dimensionally illustrated. In fact, each particle is in contact with other particles directly or through the metal material 13.

  The metal material 13 is preferably present 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 layer of the metal material 13 covers the surface of the particle 12a continuously or discontinuously. When the layer of the metal material 13 covers the surface of the particle 12a continuously, it is preferable to form a fine void in the layer of the metal material 13 that allows the non-aqueous electrolyte to flow. When the layer of the metal material 13 covers the surface of the particle 12a discontinuously, the non-aqueous electrolyte is applied to the particle 12a through a portion of the surface of the particle 12a that is not covered with the layer of the metal material 13. Supplied. In order to form the layer 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.

  Gaps are formed between the particles 12 a covered with the layer of the metal material 13. This void serves as a flow path for the non-aqueous electrolyte containing lithium ions. Since the non-aqueous electrolyte easily reaches the active material particles 12a due to the presence of the voids, it is possible to reduce the overvoltage of the initial charge. As a result, lithium dendrite is prevented from being generated on the surface of the negative electrode. The generation of dendrite causes a short circuit between the two poles. The ability to reduce the overvoltage is also advantageous from the viewpoint of preventing decomposition of the non-aqueous electrolyte. This is because the irreversible capacity increases when the non-aqueous electrolyte is decomposed. Furthermore, the ability to reduce the overvoltage is advantageous in that the positive electrode is less susceptible to damage.

  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.

  FIG. 2A is a schematic diagram showing an enlarged main part of the active material particles 12 a included in the active material layer 12. A part of the surface of the particle 12 a is covered with a metal material 13 in a thin layer shape. Further, a part of the surface of the particle 12 a is covered with a solid electrolyte interface (hereinafter also referred to as “SEI”) 14. SEI is generally considered to be a reductive decomposition product of a non-aqueous electrolyte. SEI is said to have lithium ion permeability but not electronic conductivity. It has been found that various types of SEI are formed depending on the type of non-aqueous electrolyte, the type of active material, the type of additive, and the like. By forming SEI on the surface of the particle 12a, the surface of the particle 12a is inactivated and stabilized, and an increase in irreversible capacity due to reduction of the non-aqueous electrolyte is prevented.

  The state of coating with the metal material 13 and the SEI 14 has various modes depending on the forming method. For example, the aspect which the metal material 13 and SEI14 coat | cover the surface of the particle | grains 12a separately, and the aspect by which the metal material 13 and SEI14 are laminated | stacked up and down in layers are mentioned. In the latter case, there is a case where the metal material is located on the lower side and the SEI 14 is located on the upper side, and vice versa. A particularly preferable embodiment is an embodiment in which at least a part of the surface of the particle 12 a is covered with the SEI 14 taking in a part of the metal material 13 for the reason described later. The state of the coating of the particles 12a with the SEI 14 and the metal material 13 can be confirmed, for example, by observing the cross section of the active material layer 12 with an SEM.

  FIG. 2B shows a state in which the particles 12a shown in FIG. 2A are contracted by the release (discharge) of lithium ions. When the particle 12a contracts, the SEI 14 covering the surface thereof deforms following the contraction of the particle 12a. That is, the SEI 14 is deformed without peeling from the surface of the particle 12a. Following the deformation of the SEI 14, the coating of the metal material 13 is also deformed. That is, the SEI functions as a binder between the particles 12 a and the metal material 13. As a result, the surface of the particle 12a is covered with the SEI 14 and the metal material 13 even in the state after the contraction.

  As described above, in the negative electrode 10 according to the present embodiment, even when the active material particles 12 a are repeatedly expanded and contracted by charging and discharging, the state in which the surfaces of the particles 12 a are covered with the SEI 14 and the metal material 13 is maintained. As a result, the particles 12a are effectively prevented from falling off. Further, electrical contact between the particles 12a is maintained. For these reasons, the cycle characteristics are improved.

  From the viewpoint of reliably deforming the metal material 13 following the contraction of the active material particle 12a, the SEI 14 covers at least a part of the surface of the particle 12a in a state in which a part of the metal material 13 is taken in. Is preferred.

  Further, from the viewpoint of reliably deforming the metal material 13 following the contraction of the active material particles 12a, it is preferable that the coating with the metal material 13 is not excessively thick. If the coating with the metal material 13 is too thick, when the coating of the metal material 13 is deformed following the contraction of the particles 12a, the coating of the metal material 13 is not deformed due to a small space for absorbing the deformation. This is because the contraction of the active material particles 12a cannot be followed, and as a result, the particles are separated from the surface of the particles 12a. From this viewpoint, the average thickness of the coating of the metal material 13 is preferably 0.05 to 1 μm, particularly 0.05 to 0.25 μm. Moreover, by setting it as the thickness of this range, the non-aqueous electrolyte containing lithium ion can permeate | transmit the coating | cover of the metal material 13 smoothly, and can reach | attain the surface of the particle | grains 12a. Thereby, the overvoltage of the initial charge can be lowered. Further, the coating having a thickness in this range can effectively prevent the particles 12a from falling off when the particles 12a are pulverized due to expansion and contraction. The average thickness of the coating of the metal material 13 can be measured, for example, by observing the cross section of the active material layer 12 with an SEM.

Even when the thickness of the coating of the metal material 13 is set within the above range, when the particle size of the active material particles 12a is excessively small, the filling rate of the particles 12a in the active material layer 12 is increased. Therefore, the deformation of the coating of the metal material 13 cannot follow the contraction of the active material particles 12a, and as a result, the coating of the metal material 13 may be peeled off from the surface of the particles 12a. From the viewpoint of preventing this, the ratio of the average thickness of the coating of the metal material 13 to the average particle diameter of the particles 12a (the former / the latter) is set to 1/50 to 1/2, particularly 1/50 to 1/5. It is preferable. The average particle diameter of the particles 12a itself is preferably 0.1 to 8 μm, particularly preferably 1 to 5 μm. The average particle diameter here is a value of D ₅₀ , and is measured by, for example, laser diffraction scattering type particle size distribution measurement or electron microscope observation (SEM observation). The thickness of the coating of the metal material 13 can be controlled by adjusting the plating conditions described later.

  As described later, the active material layer 12 is preferably electrolyzed using a predetermined plating bath on a coating film obtained by applying a slurry containing particles 12a and a binder onto a current collector and drying the slurry. It forms by plating and depositing the metal material 13 between the particle | grains 12a. The SEI 14 is formed by immersing the active material layer 12 thus formed in a non-aqueous electrolyte and performing charging. There are no particular restrictions on the charging conditions. If normal charging conditions are used, the SEI 14 can be easily formed.

  In order to form necessary and sufficient voids in the active material layer through which the non-aqueous electrolyte can be circulated, it is preferable to sufficiently infiltrate the plating solution 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. The pH of the plating bath is preferably adjusted to 7.1-11. 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. In the acidic bath as disclosed in Patent Document 1, it is not easy to coat the surface of the active material particles 12a thinly and uniformly with the metal material 13 over the entire active material layer.

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: 2-7 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 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 flow of the water 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.

The void ratio in the active material layer 12 formed by the various methods described above, that is, the void ratio, is preferably about 15 to 45% by volume, and particularly preferably 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 porosity is measured by the following procedures (1) to (7).
(1) The weight per unit area of the coating film formed by applying the slurry is measured, and the weight of the particles 12a and the weight of the binder are calculated from the blending ratio of the slurry.
(2) From the weight change per unit area after electrolytic plating, the weight of the plated metal species is calculated.
(3) After electrolytic plating, the thickness of the active material layer 12 is determined by SEM observation of the cross section of the negative electrode.
(4) The volume of the active material layer 12 per unit area is calculated from the thickness of the active material layer 12.
(5) The respective volumes are calculated from the weight of the particles 12a, the weight of the binder, the weight of the plating metal species, and the respective compounding ratios.
(6) From the volume of the active material layer 12 per unit area, the volume of the voids is calculated by subtracting the volume of the particles 12a, the volume of the binder, and the volume of the plating metal species.
(7) The void volume calculated in this way is divided by the volume of the active material layer 12 per unit area, and a value obtained by multiplying by 100 is defined as a void ratio (%).

  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.

  The metal material 13 having a low ability to form a lithium compound deposited in the active material layer 12 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.

  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.

  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.

  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.

  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. The thickness of the current collector 11 is not critical in this embodiment. Considering the balance between maintaining the strength of the negative electrode 10 and improving the energy density, it is preferably 9 to 35 μm. 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 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 coating is subjected to electrolytic plating.

  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 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. This coating film 15 is subjected to press treatment. The pressing process is performed under a pressure that reduces the microspace between the particles 12a but does not cause the microspace to disappear. Next, the current collector 11 having the coating film 15 subjected to the press treatment 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.

  By subjecting the coating film 15 to press treatment prior to osmotic plating, the progress of plating can be accelerated, and a thin and uniform coating of the metal material 13 can be formed on the surface of the active material particles 12a. It became clear as a result of examination of inventors. The reason for this is that the degree of contact between the particles 12a is increased by the press treatment, whereby the electron conductivity in the coating film 15 is improved, and the region where plating nuclei are generated increases in the thickness direction of the coating film 15. This is thought to be caused by From this viewpoint, the pressure of the press treatment is preferably 50 to 500 MPa, particularly 100 to 500 MPa.

  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 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. 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 manner, as shown in FIG. 3D, a negative electrode having a structure in which the coating of the metal material 13 is formed on the surfaces of the active material particles 12a is obtained.

  Next, this negative electrode is charged by immersing it in a non-aqueous electrolyte containing lithium ions. By this charging, SEI 14 is formed on the surface of the active material particles 12a. In this case, since the metal material 13 deposited on the surface of the particle 12a by performing the above-described press treatment is thin and uniform, the SEI 14 is easily formed in a state in which a part of the metal material 13 is taken in.

As described above, various types of SEI are formed according to the type of non-aqueous electrolyte, the type of active material, the type of additive, and the like. From the viewpoint of forming SEI 14 having high lithium ion permeability, it is preferable to use a solution in which a lithium salt is dissolved in an organic solvent mainly composed of ethylene carbonate as the non-aqueous electrolyte. Examples of other organic solvents used in combination with ethylene carbonate include diethyl carbonate, dimethyl carbonate, propylene carbonate, butylene carbonate, and the like. It is preferable to add an additive such as vinylene carbonate to the nonaqueous electrolytic solution containing these organic solvents because it is easy to form SEI having higher lithium ion permeability. Examples of the lithium salt include LiClO ₄ , LiA1Cl ₄ , LiPF ₆ , LiAsF ₆ , LiSbF ₆ , LiBF ₄ , LiSCN, LiCl, LiBr, LiI, LiCF ₃ SO ₃ , LiC ₄ F ₉ SO _{3 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, non-aqueous electrolytes include 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.

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-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.

  As the battery separator, a synthetic resin nonwoven fabric, a polyolefin such as polyethylene or polypropylene, or a polytetrafluoroethylene porous film is preferably used. In particular, for example, a porous polyethylene film (manufactured by Asahi Kasei Chemicals; N9420G) can be preferably used as the separator. 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.

  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 20 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 passed through a roll press machine, and the coating film was pressed. The press pressure was 300 MPa. A current collector having a coating film subjected to press working was immersed in a copper pyrophosphate bath having the following bath composition, and electrolytic coating was performed on the coating film 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. The permeation plating was terminated when copper was deposited over the entire thickness direction of the coating film. Thus, a negative electrode having a structure including an active material layer in which a copper coating was formed on the surface of Si particles was obtained. The SEM image of the longitudinal cross-section of the active material layer before the first charge in the obtained negative electrode is shown in FIG. As is clear from FIG. 4A, the surface of the Si particles was covered with copper, and appropriate voids were formed between the Si particles.
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.

A lithium secondary battery was manufactured using the obtained negative electrode. LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ was used as the positive electrode. As the electrolytic solution, a solution obtained by adding 2% by volume of vinylene carbonate to a solution of 1 mol / l LiPF ₆ dissolved in a 1: 1 volume% mixed solvent of ethylene carbonate and diethyl carbonate was used. As the separator, a 20 μm thick polypropylene porous film was used. The obtained secondary battery was charged for the first time to form SEI on the surface of the Si particles. The SEM image of the longitudinal section of the active material layer after the initial charge in the negative electrode thus obtained is shown in FIG. As a result of SEM observation, it was confirmed that SEI covered the surface of Si particles in a state where a part of copper was taken in. The average thickness of the copper coating was 0.2 μm, and the ratio of the average thickness of the copper coating to the average particle size of the Si particles (the former / the latter) was 1/5.

[Comparative Example 1]
Instead of using a copper pyrophosphate bath as a bath for permeation plating, a copper sulfate bath having the following composition disclosed in Patent Document 1 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. Moreover, the press process was not performed with respect to the coating film. A negative electrode was obtained in the same manner as Example 1 except for these. The SEM image of the longitudinal section of the active material layer before the first charge in the negative electrode obtained in this way is shown in FIG. As is clear from FIG. 5A, the space between the Si particles was almost filled with copper. When this negative electrode was charged and discharged in the same manner as in the Examples and the cross section was observed with an SEM, the generation of SEI was not observed.
・ CuSO ₄・ 5H ₂ O 250g / l
・ H ₂ SO ₄ 70 g / l

[Comparative Example 2]
Instead of using a copper pyrophosphate bath as the bath for the permeation plating, a Watt bath having the following composition disclosed in Patent Document 1 was used. The electrolysis conditions were as follows. A nickel electrode was used as the anode. A DC power source was used as the power source. Moreover, the press process was not performed with respect to the coating film. A negative electrode was obtained in the same manner as Example 1 except for these. FIG. 5B shows a SEM image of the longitudinal section of the active material layer before the first charge in the negative electrode thus obtained. As apparent from FIG. 5B, the space between the Si particles was almost filled with copper. When this negative electrode was charged and discharged in the same manner as in the Examples and the cross section was observed with an SEM, the generation of SEI was not observed.
・ NiSO ₄・ 6H ₂ O 250g / l
・ NiCl ₂・ 6H ₂ O 45g / l
・ H ₃ BO ₄ 30g / l
・ Bath temperature 50 ℃
・ Current density 5A / dm ²
・ PH: 5

[Evaluation]
The capacity retention rate at the 100th cycle was measured for the above-described secondary batteries obtained using the negative electrodes obtained in Examples and Comparative Examples. The capacity retention rate was calculated by measuring the discharge capacity at the 100th cycle, dividing those values by the initial discharge capacity, and multiplying by 100. The charging conditions were 0.5 C, 4.2 V, and constant current / constant voltage. The discharge condition was 0.5 C, 2.7 V, and a constant current. 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. As a result, the capacity retention rate in Example 1 was 88%, compared with 60% in Comparative Example 1 and 70% in Comparative Example 2, and the capacity retention rate was significantly lower than that in Example 1. Note that the first cycle here also serves as the SEI formation step.

  Along with the measurement of the capacity maintenance rate, after performing one cycle of charge and discharge corresponding to 50% of the maximum capacity of the negative electrode for the above-mentioned battery, the negative electrode is taken out from the battery, and the Raman of Si is separated at an interval sufficient for the thickness direction of the active material layer. The spectrum was measured. The end-of-charge voltage was 4.2V, and the end-of-discharge voltage was 2.7V. The charge / discharge rate was 0.05C. As a measuring apparatus, a laser Raman spectrophotometer “NRS-2100” (trade name) manufactured by JASCO Corporation was used. The excitation wavelength was 514.5 nm. The measurement results are shown in FIG.

From the measurement results shown in FIG. 6, it can be determined whether or not the active material uniformly contributes to the electrode reaction over the entire thickness direction of the active material layer. Details are as follows. The structure of Si changes from crystalline to amorphous by an electrode reaction. In the analysis using the Raman spectra, since the spectra due to the crystallinity of the difference in Si is different, with the spectrum derived from the amorphous spectrum derived from the crystalline (521 cm around ^-1) (480 cm around ^-1) By determining the area ratio, it is possible to know how much the active material has contributed to the electrode reaction.

  In FIG. 6, in Example 1, the ratio of the spectrum derived from the crystalline substance and the spectrum derived from the amorphous substance is substantially constant regardless of the thickness direction of the active material layer. This means that the active material contributes uniformly to the electrode reaction over the entire thickness direction of the active material layer. The reason for this is considered to be that the non-aqueous electrolyte is smoothly distributed in the active material layer due to the thin coating of the Si particle surfaces. On the other hand, in Comparative Examples 1 and 2, a large amount of amorphous Si is present on the surface side of the active material layer, while a large amount of Si remains crystalline on the current collector side. This means that the electrode reaction occurs only on the surface of the active material layer and in the vicinity thereof, and the active material existing in the deep part of the active material layer does not contribute to the electrode reaction. This is due to the fact that the gap between the Si particles is almost filled with copper or nickel, and that there are not enough voids in the active material layer to allow the nonaqueous electrolyte to flow therethrough. it is conceivable that.

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. FIG. 2 is a schematic diagram illustrating an enlarged main part of active material particles included in 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. It is a SEM image of the longitudinal section of the active material layer in the negative electrode obtained in the example. It is a SEM image of the longitudinal section of the active material layer in the negative electrode obtained by the comparative example. It is a graph which shows the Raman spectrum in the thickness direction in the active material layer of the negative electrode obtained by the Example and the comparative example.

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 14 SEI
15 Coating film

Claims (8)

  An active material layer containing active material particles is provided, and at least a part of the surface of the particles is covered with a metal material having a low lithium compound forming ability and a solid electrolyte membrane, and voids are formed between the particles. A negative electrode for a non-aqueous electrolyte secondary battery.   The negative electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein the solid electrolyte membrane covers at least a part of the surface of the particles in a state in which a part of the metal material is taken in.   3. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the average thickness of the metal material is 0.05 to 1 μm.   4. The non-aqueous electrolyte secondary battery according to claim 1, wherein a ratio of the average thickness of the metal material to the average particle diameter of the particles (the former / the latter) is 1/50 to 1/2. Negative electrode.   5. 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. A nonaqueous electrolyte secondary battery comprising the negative electrode for a nonaqueous electrolyte secondary battery according to any one of claims 1 to 5.   An active material layer including particles of the 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 voids are formed between the particles; A negative electrode for a non-aqueous electrolyte secondary battery, wherein the average thickness of the metal material is 0.05 to 1 μm, and the metal material is present on the surface of the particles over the entire thickness direction of the active material layer. It is a manufacturing method of the negative electrode for nonaqueous electrolyte secondary batteries according to claim 7,
A slurry containing active material particles is applied onto a current collector to form a coating film,
The coating film is subjected to a press treatment of 50 to 500 MPa,
The negative electrode for a non-aqueous electrolyte secondary battery in which the coated film after pressing is immersed in a plating bath containing a metal material having a low lithium forming capacity to perform electrolytic plating, and the metal material is deposited on the surface of the particles. Production method.