JP2008034347A - Non-aqueous electrolyte secondary battery - Google Patents

Abstract

Provided is a negative electrode for a non-aqueous electrolyte secondary battery in which a path through which a non-aqueous electrolyte can flow is necessary and sufficiently formed in an active material layer and an overvoltage during initial charging can be reduced. .
A non-aqueous electrolyte secondary battery includes a positive electrode, a negative electrode, and a separator interposed therebetween. The negative electrode includes an active material layer including active material particles 12 a and a conductivity-imparting component 13. When the active material layer is charged and discharged at least 5% of the battery capacity to the battery at least five times and then repeatedly charged and discharged, the conductivity-imparting component 13 contained in the active material layer is charged and discharged. It has a structure that gradually becomes fine powder by repeated discharge. The active material particles 12a may be pulverized by repeated charge and discharge.
[Selection] Figure 1

Description

  The present invention relates to 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. However, if the metal material penetrates into the active material layer excessively, the non-aqueous electrolyte solution may not easily flow through the active material layer, and only the surface of the active material layer and the vicinity thereof may contribute to the electrode reaction. . In addition, as the active material particles are pulverized, electrically isolated pulverized particles may be generated.

Japanese Patent No. 3612669

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

The present invention provides a non-aqueous electrolyte secondary battery having a positive electrode, a negative electrode, and a separator interposed between them, the negative electrode including an active material layer containing particles of an active material and a conductivity-imparting component.
The active material layer includes the conductivity-imparting component included in the active material layer when the battery is repeatedly charged and discharged after at least 5 times of charging and discharging at 50% or more of the battery capacity. However, the present invention provides a non-aqueous electrolyte secondary battery characterized by having a structure in which the powder is gradually pulverized by repeated charging and discharging.

  In the battery of the present invention, the conductivity-imparting component contained in the active material layer of the negative electrode is gradually pulverized by repetition of charge and discharge, whereby the active material layer is in a mixed state of the active material and the pulverized conductivity-imparting component. Become. In this mixed state, the electrical contact between the particles of the active material is ensured by the finely divided particles of the conductivity-imparting component. As a result, the active material is not easily isolated electrically, and the cycle characteristics of the battery are improved.

  The present invention will be described below based on preferred embodiments with reference to the drawings. The schematic diagram which expanded the principal part of the negative electrode active material layer in the battery of this invention is shown by Fig.1 (a)-(c). Among these figures, FIG. 1A shows a state before the first charge. The active material layer in a state before charge / discharge includes active material particles 12 a and a conductivity-imparting component 13. As the active material, a material capable of occluding and releasing lithium 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 before the first charge, the coating of the conductivity imparting component 13 is located on the surface of the particle 12a. That is, at least a part of the surface of the particle 12 a is covered with the conductivity imparting component 13. The conductivity imparting component 13 is a material different from the constituent material of the particles 12a. The conductivity-imparting component 13 is interposed between the particles 12a and is mainly used for the purpose of ensuring electronic conductivity between the particles 12a. As the conductivity imparting component 13, a metal material having a low ability to form a lithium compound is preferably used. Examples thereof include copper, nickel, iron, cobalt, and alloys of these metals. In particular, the metal material is preferably a highly ductile material in which even when 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. “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.

  In the active material layer before the first charge, voids S are formed between the particles 12 a covered with the conductivity imparting component 13. That is, the conductivity-imparting component 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 the active material layer before the first charge, the conductivity imparting component 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, as will be described later, when the conductivity imparting component 13 is pulverized due to expansion and contraction of the particles 12a due to charge and discharge, the mixed state of the particles 12a and the pulverized conductivity imparting component 13 is Become good. As a result, the electronic conductivity of the entire active material layer 12 is ensured through the conductivity-imparting component 13, so that electrically isolated active material particles 12 a are generated, particularly in the deep part of the active material layer 12. Generation of isolated active material particles 12a 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 conductivity-imparting component 13 on the surface of the active material particles 12a throughout the thickness direction of the active material layer 12 means that the conductivity-imparting component 13 is an electron to be measured for the active material layer before the first charge. It can be confirmed by microscopic mapping.

  The conductivity imparting component 13 coats the surface of the particle 12a continuously or discontinuously. When the conductivity-imparting component 13 covers the surfaces of the particles 12a continuously, it is preferable to form fine voids in the coating of the conductivity-imparting component 13 that allow the non-aqueous electrolyte to flow. When the conductivity-imparting component 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 coated with the conductivity-imparting component 13. Supplied. In order to form the coating of the conductivity-imparting component 13 having such a structure, the conductivity-imparting component 13 may be deposited on the surface of the particle 12a by, for example, electrolytic plating according to the conditions described later.

  FIG. 1B shows the state of the active material layer after the battery in the state shown in FIG. 1 is charged and discharged at least 50% at least five times. Lithium is occluded and released from the active material particles 12a by charging and discharging. Corresponding to this occlusion release, the particles 12a expand and contract. The coating of the conductivity-imparting component 13 present on the surface of the particle 12a is divided without being able to withstand the stress generated due to the expansion and contraction. Further, when the particle 12a is made of a material having a large degree of expansion / contraction, the particle 12a itself may be cracked by applying stress to the particle 12a itself due to expansion / contraction to cause cracks. is there. When cracking occurs in the particles 12a, the division of the coating of the conductivity imparting component 13 becomes more prominent.

  The charging / discharging conditions are not particularly limited, but charging / discharging at 50% of the battery capacity at a charging end voltage of 4.2 and a discharging end voltage of 2.7 at a charge / discharge rate of 0.2 C is measured. This is preferable because the reproducibility is the best.

  When charging / discharging is further repeated from the state shown in FIG. 1B, the coating of the conductivity-imparting component 13 existing on the surface of the particle 12a is further divided and pulverized following the expansion and contraction of the particle 12a. Further, when the material is made of a material having a large degree of expansion and contraction, the particles 12a themselves are easily pulverized. When the pulverization of the conductivity imparting component 13 proceeds, the active material layer becomes a mixed state of the particles 12a and the pulverized conductivity imparting component 13. When the pulverization of the particles 12a has progressed in addition to the pulverization of the conductivity-imparting component 13, as shown in FIG. 1 (c), the active material layer has the pulverized particles 12a and the pulverized conductivity. It becomes a mixed state with the component 13. These mixed states preferably occur in the entire thickness direction of the active material layer. There are no particular restrictions on the charge / discharge conditions at this time. As an example, there may be mentioned conditions for charging and discharging an amount of electricity corresponding to 50% of the maximum capacity of the battery at a current density of 5 hours (0.2 C rate).

  The pulverization referred to in the present invention refers to a phenomenon in which the average major axis of the active material particles and the conductivity-imparting component particles grasped when the negative electrode is observed in cross section is reduced. In the conductivity imparting component, the crystal particle group having electrical contact is defined as one particle. As the degree of pulverization, the average major axis of the conductivity-imparting component particles is 1 μm or more, the average major axis of the particles of the active material is 2 μm or more, and at least after 100 cycles of charge / discharge after 5 cycles after charge / discharge The average major axis of the conductivity imparting component particles is preferably less than 1 μm, and the average major axis of the active material particles is preferably less than 2 μm.

  As described above, the negative electrode active material layer in the battery according to the present embodiment is included in the active material layer when 50% or more of the battery is charged and discharged at least five times and then repeatedly charged and discharged. The conductivity-imparting component 13 has a structure that is gradually pulverized by repeated charge and discharge. In the active material layer having such a structure, electrical contact between the particles 12a of the active material is ensured by the finely divided particles of the conductivity imparting component. As a result, the active material particles 12a are not easily electrically isolated. Thereby, the cycle characteristics of the battery are improved. In addition, when the pulverization of the active material particles 12a proceeds in addition to the pulverization of the conductivity imparting component, the electrical contact between the pulverized active material particles 12a is similarly pulverized conductivity imparting. Secured by component particles. Thus, regardless of the presence or absence of pulverization of the active material particles 12a, the pulverized particles of the conductivity-imparting component ensure electrical contact between the active material particles 12a and improve the cycle characteristics of the battery. . That is, the conductivity imparting component 13 has a structure in which it is pulverized by repeated charging / discharging. In particular, the active material particles 12a are also gradually pulverized by repeated charging / discharging. In this case, it is extremely advantageous in that the electrical contact between the active material particles 12a is ensured.

  On the other hand, in the negative electrode active material layer in the conventional battery, for example, in the active material layer having a structure in which the active material layer is filled with a metal material having a low lithium compound forming ability, the charge / discharge is less than 5 times. Therefore, the pulverization of the active material particles almost reaches the limit, and the subsequent pulverization does not substantially proceed in charge / discharge. As a result, the pulverization of the metal material having a low ability to form a lithium compound does not substantially proceed. The reason for this is that only the particles present on the surface of the active material layer and in the vicinity thereof are concentrated because the voids that allow smooth flow of the non-aqueous electrolyte are not sufficiently formed between the particles of the active material. This is because it is used for the electrode reaction, whereby the pulverization of the particles proceeds at an accelerated rate. Therefore, in the active material layer having such a structure, the particles of the active material located at a position far from the surface are not used for the electrode reaction and remain in the state before the initial charge. As a result, in a battery having a negative electrode including an active material layer having such a structure, capacity deterioration occurs at an early stage of the charge / discharge cycle.

By the way, some of the negative electrodes of the prior art are obtained by applying a slurry in which negative electrode active material particles and metal powder are mixed from the beginning to form a coating film, and sintering the coating film (for example, special features). No. 2002-260637). It can be said that the active material layer of the negative electrode is similar to the state of the active material layer after charging / discharging of the negative electrode according to the present invention in that the active material particles and the metal powder are in a mixed state. However, in the negative electrode as disclosed in the prior art, when ultrafine active material particles or metal powder is used, there are the following disadvantages.
(1) The surface of the active material particles and metal powder of these ultrafine powders are significantly oxidized, and a large amount of oxygen is brought into the battery.
(2) Since a large amount of binder, such as polyimide or polyvinylidene fluoride, is required, the rate characteristics during charge / discharge are inferior and the irreversible capacity becomes significant.
(3) The use of ultrafine powder, the use of a large amount of binder, and the refinement / high dispersion of active material particles are economically disadvantageous.
(4) The amount of voids formed between the active material particles and the metal powder is small.

  In the present invention, the reason why the conductivity-imparting component 13 is gradually pulverized by repeated charge / discharge is as follows. In general, a commercially available secondary battery is usually activated by one or more charging / discharging operations, and the battery is usually ready for use and put on the market. Therefore, if the above criteria are set after at least 5 times of charging / discharging, it can be objectively evaluated that the cycle characteristics are good based on the ready-to-use state. For these reasons, the reference is after charging and discharging at least five times. The upper limit value is not particularly limited as long as the number of times of charging / discharging is at least 5, but if it is set intentionally, 10 times is a realistic value. The reason why the charge / discharge level is 50% or more is that, in the case of a commercially available secondary battery, the charge / discharge level is 50% when the battery is put into a ready-to-use state after being charged / discharged. By being normal. Here, 50% means that 50% charge / discharge was performed with respect to the maximum capacity of the battery. The maximum capacity of the battery depends on the capacity of the smaller capacity of the positive and negative electrodes. The upper limit is not particularly limited as long as the degree of charge / discharge is 50% or more, and may be 100%. In addition, although the degree of charging / discharging of each time may be the same or may differ, it is preferable that it is the same from a viewpoint of obtaining the result of favorable reproducibility.

  The pulverization of the conductivity-imparting component 13 and the pulverization of the particles 12a of the active material due to repeated charge / discharge are not permanent, and do not proceed further when a certain amount of charge / discharge is repeated. . This is because the minimum unit of pulverization is determined by the crystallite size, and pulverization does not proceed beyond the crystallite size. In the present embodiment, from the viewpoint of improving cycle characteristics, it is preferable that pulverization of the conductivity imparting component 13 proceeds until at least 200 cycles, particularly at least 100 cycles of charge / discharge. When the active material particles 12a are also pulverized, the pulverization preferably proceeds to the same cycle as described above. In order to lengthen the charge / discharge cycle in which pulverization proceeds, it is advantageous to reduce the crystallite size of the conductivity-imparting component 13. The same applies when the active material particles 12a are pulverized. Means for reducing the size of the crystallite will be described later.

  In order for the conductivity-imparting component 13 to have a structure in which the powder is gradually pulverized by repeated charge and discharge, it is advantageous that the active material particles 12a are made of a material that causes a volume change due to insertion and extraction of lithium ions. In this case, the degree of pulverization of the conductivity-imparting component 13 is larger as the material that causes a larger volume change. From this viewpoint, the particles 12a are preferably made of a silicon-based material that is a material that causes a large volume change. The active material particles 12a made of a material that causes such a large volume change are apt to be gradually pulverized by repeated charge and discharge.

  In order to have a structure in which the conductivity imparting component 13 is gradually pulverized by repetition of charge and discharge, it is also preferable that the conductivity imparting component 13 is easily pulverized. Specifically, the conductivity-imparting component 13 that covers the particles 12a is preferably composed of an aggregate of crystallites of the metal material. This is because the coating of the conductivity-imparting component 13 is easily divided in crystallite units. As described above, since the metal material is a material having a low ability to form a lithium compound, the conductivity-imparting component 13 does not occlude / release lithium ions, and therefore the component 13 does not change in volume. .

  Even if the coating of the conductivity-imparting component 13 is composed of an aggregate of crystallites of a metal material, the crystallites can be filled between the active material particles 12a depending on the size of the crystallites. In some cases, it is difficult to ensure electrical contact between the particles 12a. Further, depending on the size of the crystallite, the mixed state of the active material particles 12a and the finely divided conductivity imparting component 13 may be difficult to be improved. As a result, electrically isolated particles 12a are likely to be generated, and it may be difficult to improve cycle characteristics. The same can be said when the active material particles 12a are also pulverized. From these viewpoints, the average particle size of the crystallites is preferably 0.01 to 1 μm, particularly 0.05 to 0.25 μm. A crystallite having such an average particle diameter can be obtained by, for example, performing electroplating under the conditions described later to form a coating of the conductivity imparting component 13. The average particle diameter of the crystallite is measured by observing the cross section of the active material layer with SEM observation or SIM observation. Note that, as described above, the conductivity imparting component 13 does not change in volume due to charge / discharge due to the properties of its constituent materials, so the size of the crystallite is substantially constant throughout the charge / discharge cycle. Does not change.

  In order to further improve the cycle characteristics, it is advantageous to realize a mixed state of the particles 12a and the pulverized conductivity imparting component 13 over the entire region in the thickness direction of the active material layer. When the particles 12a are also pulverized, it is advantageous to realize a mixed state of the pulverized particles 12a and the pulverized conductivity imparting component 13 over the entire thickness direction of the active material layer. For this purpose, it is necessary to uniformly occlude and release lithium ions throughout the thickness direction of the active material layer. From this viewpoint, it is preferable that the active material layer has voids through which the nonaqueous electrolytic solution containing lithium ions can smoothly flow over the entire region in the thickness direction. The fact that the non-aqueous electrolyte easily reaches the active material particles 12a is also advantageous in that the overcharge voltage during initial charging can be reduced. This is because 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.

  In order to successfully form voids in the active material layer, it is preferable that the conductivity-imparting component 13 covering the surfaces of the active material particles 12a is thin. Specifically, the conductivity imparting component 13 covering the surface of the active material particles 12a has an average thickness of preferably 0.05 to 2 μm, more preferably 0.1 to 0.25 μm. It is. That is, the conductivity imparting component 13 covers the surface of the active material particles 12a with a minimum thickness. Thus, a necessary and sufficient gap is formed in the active material layer while ensuring the electron conductivity between the particles 12a. 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.

  As described later, the active material layer 12 in the state shown in FIG. 1A described above is preferably obtained by applying a slurry containing particles 12a and a binder onto a current collector and drying it. The coating film is formed by performing electrolytic plating using a predetermined plating bath and depositing a metal material 13 between the particles 12a.

  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 order to reduce the crystallite size of the conductivity imparting component 13 described above, it is preferable to add a predetermined amount of brightener to the plating solution to generate a large amount of plating nuclei. As the brightening agent, those similar to those conventionally used in the technical field of electrolytic plating can be used. For example, thiol compounds such as mercaptothiazole and sodium mercaptopropane sulfonate, and ammonia can be used. What is necessary is just to adjust the addition amount of a brightener suitably according to the kind of plating solution to be used. In particular, when a copper pyrophosphate bath described below is used as the plating solution, it is preferable to use ammonia as a brightening agent because ammonia acts as a pH adjusting agent in addition to acting as a brightening agent.

When copper is used as the conductivity-imparting component 13, it is preferable to use a copper pyrophosphate bath. 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: 7.1-9.5

  When ammonia is added to the copper pyrophosphate bath as the brightener described above, the amount added should be about 5 to 40 g / l, especially about 10 to 20 g / l in the case of a 25 wt% aqueous solution. This is preferable in that a crystallite having a desired particle diameter can be obtained. When ammonia in this range is added, the pH of the copper pyrophosphate bath may be outside the above range. In that case, an appropriate amount of acid such as polyphosphoric acid may be added to adjust the pH.

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.

  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 plating bath.

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 (%).

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

  In the battery of this embodiment, if the amount of the active material with respect 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.

  The negative electrode in the battery of this embodiment includes a current collector and an active material layer formed on at least one surface thereof. As the current collector in the negative electrode, the same current collector as that conventionally used as the negative electrode current collector for non-aqueous electrolyte secondary batteries can be used. The current collector is preferably composed 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 is not critical in this embodiment. Considering the balance between maintaining the strength of the negative electrode 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.

The positive electrode in the battery of this embodiment is prepared by suspending a positive electrode active material and, if necessary, a conductive agent and a binder in an appropriate solvent to prepare a positive electrode mixture, and applying this to at least one surface of a current collector and drying. Then, it is obtained by roll rolling, 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.

  A separator is interposed between the positive electrode and the negative electrode. As the 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.

The space between the positive electrode and the negative electrode is filled with a non-aqueous electrolyte. The nonaqueous electrolytic solution is a solution in which a lithium salt as a supporting electrolyte is dissolved in an organic solvent. 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. Examples of the organic solvent include ethylene carbonate, diethyl carbonate, dimethyl carbonate, propylene carbonate, butylene carbonate, and the like. In particular, 0.5 to 5% by weight of vinylene carbonate and 0.1 to 1% by weight of divinyl sulfone and 0.1 to 1.5% by weight of 1,4-butanediol dimethanesulfonate with respect to the whole non-aqueous electrolyte. It is preferable from the viewpoint of further improving the charge / discharge cycle characteristics. The details are not clear, but 1,4-butanediol dimethanesulfonate and divinylsulfone are decomposed stepwise to form a film on the positive electrode, so that the film containing sulfur becomes denser. It is thought that it is to become.

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

  Next, the preferable manufacturing method of the negative electrode in the battery of this embodiment is demonstrated, referring FIG. In this production method, a coating film is formed on a current collector using a slurry containing particles of an active material and a binder, and then the electroplating 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 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. 2B to 2D, the deposition of the conductivity imparting component 13 proceeds from the interface between the coating film 15 and the current collector 11 toward the surface of the coating film. Perform electrolytic plating. In FIGS. 2B to 2D, the conductivity imparting component 13 is conveniently represented as a thick line surrounding the periphery of the particle 12a. By precipitating the conductivity imparting component 13 in this way, the surface of the active material particles 12a can be successfully coated with the conductivity imparting component 13, and between the particles 12a coated with the conductivity imparting component 13. The void can be formed successfully. 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 osmotic plating for depositing the conductivity imparting component 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. In particular, it is preferable to add the various brighteners described above to the plating bath in order to make the size of the crystallites of the conductivity imparting component within the preferable range described above.

  As shown in FIGS. 2B to 2D, when plating is performed such 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. In this way, the target negative electrode is obtained as shown in FIG.

  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 μm. The average particle diameter D ₅₀ was measured using a Microtrac particle size distribution measuring device (No. 9320-X100) manufactured by Nikkiso Co., Ltd.

The current collector on which the coating film was formed was immersed in 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: 25 wt% aqueous ammonia 15 g / l and 10 wt% polyphosphoric acid were added to adjust the pH to 8.2.

  The permeation plating was terminated when copper was deposited over the entire thickness direction of the coating film. In this way, a target negative electrode was obtained. SEM observation of the longitudinal section of the active material layer confirmed that the active material particles were covered with a copper film having an average thickness of 300 nm in the active material layer. Moreover, it confirmed that the magnitude | size of the crystallite of copper was 150 nm on average. Furthermore, the porosity of the active material layer measured by the above method was 28%.

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.

[Comparative Example 1]
Instead of using a copper pyrophosphate bath as the osmotic plating bath, 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 secondary battery was obtained in the same manner as in Example 1 except for the above.
・ CuSO ₄・ 5H ₂ O 250g / l
・ H ₂ SO ₄ 70 g / l

[Evaluation]
About the battery obtained by the Example and the comparative example, the capacity maintenance rate to 100 cycles was measured. 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 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 of the battery of the example was 90%, whereas the capacity retention rate of the battery of the comparative example was 68%.

  Separately from this, the state of the negative electrode active material layer when the battery obtained in the example was repeatedly charged and discharged by 50% was observed by SEM. The charge / discharge conditions were a charge end voltage of 4.2 V, a discharge end voltage of 2.7 V, and a charge / discharge rate of 0.2 C. Observations were made at the 5th, 50th and 150th cycles. The result is shown in FIG. Moreover, about the battery obtained by the comparative example, the state of the negative electrode active material layer before the first charge and the 30th cycle was observed by SEM. The result is shown in FIG.

  As is clear from the SEM image shown in FIG. 3, it can be seen that the battery of the example progresses in the pulverization of the active material layer by repeating the charge / discharge cycle. Specifically, in the fifth cycle shown in FIG. 3A, a large number of copper crystallites are formed around the Si particles, and the crystallites are connected in close contact with adjacent crystallites. Is configured. In the 50th cycle shown in FIG.3 (b), it turns out that pulverization of Si particle | grains is advancing. On the other hand, in FIG. 3A, it can be seen that the connected copper crystallites are partially cut, and the pulverization is still progressing. In the 150th cycle shown in FIG.3 (c), it turns out that the pulverization of Si particle | grains is further progressing. On the other hand, it can be seen that the copper crystallites are almost cut into individual crystallites and mixed with finely divided Si particles.

  On the other hand, in the battery of the comparative example, as shown in FIG. 4A, it can be seen that the space between the Si particles before the first charge is substantially filled with copper. Moreover, it turns out that micronization has not already advanced in the 30th cycle shown in FIG.4 (b). That is, after the 30th cycle, the conductivity of the entire electrode is lost due to the pulverization of the active material particles, and it is assumed that the capacity is reduced. It should be noted that the state of the active material layer near the current collector is hardly changed from the state before the first charge. This means that in the comparative example, the active material present in a portion far from the surface of the negative electrode active material layer is not used for the electrode reaction.

It is a schematic diagram which expands and shows the principal part of the negative electrode active material layer in the non-aqueous-electrolyte secondary battery of this invention. It is process drawing which shows the manufacturing method of the negative electrode active material layer in the non-aqueous-electrolyte secondary battery of this invention. It is a SEM image of the negative electrode active material layer in the battery of an Example. It is a SEM image of the negative electrode active material layer in the battery of a 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 Conductivity imparting component 15 Coating film

Claims (6)

In a non-aqueous electrolyte secondary battery having a positive electrode, a negative electrode, and a separator interposed between them, the negative electrode including an active material layer containing active material particles and a conductivity-imparting component,
The active material layer includes the conductivity-imparting component included in the active material layer when the battery is repeatedly charged and discharged after at least 5 times of charging and discharging at 50% or more of the battery capacity. However, the non-aqueous electrolyte secondary battery is characterized in that it is gradually pulverized by repeated charge and discharge.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the active material particles contained in the active material layer have a structure in which the particles are gradually pulverized by repeated charge and discharge. The active material layer is composed of the particles made of a material that causes a volume change by occlusion and release of lithium ions, and the conductivity-imparting component coating made of a metal material that is located on the surface of the particles and has a low ability to form a lithium compound; Including
The non-aqueous electrolyte secondary battery according to claim 1, wherein the coating has a structure made of an aggregate of crystallites of the metal material having an average particle diameter of 0.01 to 1 μm.   4. The non-aqueous electrolyte secondary battery according to claim 1, wherein the metal material is present on the surface of the particles of the active material over the entire thickness direction of the active material layer. 5. The particles are made of a material containing silicon and capable of occluding and releasing lithium ions,
The non-aqueous electrolyte secondary battery according to claim 3 or 4, wherein the coating is formed by electrolytic plating using a copper plating bath containing a brightener.   The non-aqueous electrolyte secondary battery according to claim 5, wherein the brightener is ammonia, and the copper plating bath is a copper pyrophosphate bath.