JP2007299801A - Energy storing element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-density, high-output, and long-life energy storing element. <P>SOLUTION: The energy storing element has an electrolyte made of a nonaqueous solvent containing positive and negative electrodes and lithium salts. Active carbon is used as the active material of the positive electrode, and silicon or silicide to which lithium of not less than 35% of theoretical capacity is prestored is used as the active material of the negative electrode. The negative electrode has an active material layer including particles of silicon or silicide to which lithium is prestored. Preferably, a metal material having low formation capability of a lithium compound penetrates between the particles in the active material layer. Preferably, the metal material is penetrated by electrolytic plating. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an energy storage element such as a hybrid capacitor.

  An energy storage element having a high energy density is required for a power source of a portable electronic device or an electric vehicle. As such elements, secondary batteries such as lithium secondary batteries and electric double layer capacitors have been developed so far. An electric double layer capacitor using activated carbon for both positive and negative electrodes is advantageous to a lithium secondary battery in terms of output and life. However, it is disadvantageous in that the energy density is low.

  For the purpose of increasing the energy density of the capacitor, a hybrid capacitor using activated carbon used for an electric double layer capacitor as a positive electrode and graphite as a negative electrode has been proposed (for example, see Patent Document 1). In addition, for the purpose of further increasing the energy density of such a hybrid capacitor, it has also been proposed to previously store lithium in graphite used as a negative electrode (see, for example, Patent Documents 2 and 3).

  In addition to the above-described high energy density, energy storage devices such as secondary batteries, electric double layer capacitors, and hybrid capacitors are also required to have a long life. However, as long as graphite is used for the negative electrode as described above, it is expected that it will be difficult to meet the needs of high energy density and long life in the near future.

JP 60-182670 A Japanese Patent Laid-Open No. 8-1007048 JP-A-9-55342

  Accordingly, an object of the present invention is to provide an energy storage element that can eliminate the drawbacks of the prior art described above.

  The present invention has an electrolyte composed of a positive electrode, a negative electrode, and a non-aqueous solvent containing a lithium salt, and uses activated carbon as an active material of the positive electrode, and lithium of 35% or more of theoretical capacity is used as the active material of the negative electrode. The object is achieved by providing an energy storage element characterized by using silicon or a silicon compound stored in advance.

  ADVANTAGE OF THE INVENTION According to this invention, in addition to high energy density and high output, a long life energy storage element is provided.

  The present invention will be described below based on preferred embodiments with reference to the drawings. The energy storage element of the present invention has a positive electrode, a negative electrode, and an electrolytic solution as its basic configuration. In general, the positive electrode and the negative electrode are disposed to face each other via a separator that can transmit electrolyte, and are hermetically accommodated in a container having a predetermined shape. The container is filled with an electrolytic solution composed of a non-aqueous solvent containing a lithium salt.

  The energy storage device of the present invention takes the form of a power storage device using different active materials for the positive electrode and the negative electrode. In particular, a hybrid capacitor using an active material that accumulates charges by a non-Faraday process (polarization) in the positive electrode and an active material that accumulates charges by a Faraday process is used in the negative electrode.

  Activated carbon is used as the active material for the positive electrode in the energy storage device of the present invention. Unlike activated carbon, which is the same carbon material, activated carbon does not have a layered structure. Therefore, when the energy storage device of the present invention is charged, lithium ions are not occluded and released. Instead, an electric double layer is formed at the interface with the electrolyte by polarization.

As the activated carbon, various activated carbons such as coconut shell activated carbon and petroleum coke activated carbon can be used. The activated carbon may be in the form of particles or fibers. The specific surface area of activated carbon 500~3000m ² / g, especially 1000~3000m ² / g, it is preferable that especially 1500~3000m ² / g.

  Activated carbon increases its activity upon activation. As the activation method, for example, a steam activation method or a molten KOH activation method can be used. From the viewpoint of further increasing the capacity of the energy storage element of the present invention, it is preferable to use a molten KOH activation method.

  The positive electrode having activated carbon can be obtained, for example, by mixing activated carbon, a conductive agent, and a binder, press-molding them into a predetermined shape, firing the molded body in an inert gas, and activating it in a steam atmosphere. Alternatively, the activated carbon, the conductive agent and the binder can be kneaded together with a solvent to form a slurry, which can be formed into a predetermined shape and dried. You may use the positive electrode obtained by these methods, joining to the electrical power collector which consists of various metals, such as aluminum and stainless steel, with a conductive adhesive. For example, carbon black or acetylene black can be used as the conductive agent. As the binder, for example, phenol resin or polytetrafluoroethylene can be used. As the solvent, a volatile alcohol solvent can be used.

  On the other hand, the negative electrode uses silicon or a silicon compound (hereinafter collectively referred to as a silicon-based material) as an active material. The silicon-based material occludes lithium ions and accumulates charges when the energy storage device of the present invention is charged. Accordingly, silicon-based materials that do not occlude lithium ions do not belong to the category of silicon-based materials referred to in the present invention.

  Silicon-based materials are used, for example, in the form of particles or thin layers. When using a silicon-based material in the form of particles, a slurry containing the particles is applied onto a current collector to form a coating film, and the coating film is dried to form a layer containing the silicon-based material. can do. In this case, the coating film after drying may be baked to increase the bonding state of the particles. On the other hand, when the silicon-based material is used in the form of a thin film, the thin film can be formed on the current collector by various thin film forming means such as sputtering, CVD, and PVD. In particular, when the negative electrode has a structure described later, the silicon-based material is preferably used in the form of particles.

  Examples of the silicon compound used as the negative electrode active material include an alloy of silicon and a metal and a silicon oxide. When the silicon compound is an alloy of silicon and metal, examples of the metal include Ni, Cu, Co, Cr, Fe, Ag, Ti, Pt, W, Mo, and Au. The amount of the metal in the alloy is preferably 0.1 to 70% by weight, in particular 5 to 50% by weight, in particular 5 to 30% by weight. If the composition is within this range, it is possible to increase the capacity and life of the energy storage device of the present invention.

  The reaction in which the silicon-based material occludes and releases lithium occurs within 0.5 V with respect to lithium. By the way, the activated carbon negative electrode in the electric double layer capacitor operates in a range of about 1.5 to 3.0 V with respect to lithium. Therefore, by using a silicon-based material instead of the activated carbon negative electrode in the electric double layer capacitor, an increase in energy density due to an increase in operating voltage can be expected.

  In particular, it is possible to further increase the operating voltage by preliminarily storing lithium in the silicon-based material, thereby further increasing the energy density. The reason is as follows. When a silicon-based material is used as the active material, for example, when graphite is used as the active material, the discharge voltage rapidly decreases at the end of discharge. This is because the potential of the negative electrode changes remarkably in a region where there is little lithium present in the negative electrode using a silicon-based material as an active material. The amount of lithium occluded in the silicon-based material and the potential of the negative electrode are not in a linear relationship, and the potential of the negative electrode greatly changes as the amount of lithium decreases. At the end of discharge, if the potential of the negative electrode using a silicon-based material as an active material increases, the voltage of the energy storage element decreases, and the energy density cannot be improved. The present invention intends to achieve a high energy density by designing an energy storage element capable of avoiding a significant portion of the potential change and charging / discharging in a stable lithium amount region.

  Further, the cycle life can be improved by preliminarily inserting lithium into the silicon-based material. Specifically, a silicon-based material is a material that has a large volume change due to insertion and extraction of lithium due to charge / discharge, and the amount of lithium previously stored in the silicon-based material is determined according to the degree of volume change. By setting in a small range, it is possible to effectively prevent the silicon-based material from being pulverized due to the volume change. As a result, the cycle life is improved.

  From the viewpoint of achieving the high energy density and improving the cycle life, the amount of lithium previously occluded in the silicon-based material is set to 35% or more of the theoretical capacity. This lithium occlusion amount is used to achieve a high energy density when the energy storage device of the present invention is operated at a relatively low cycle number (for example, 5,000 to 10,000 cycles) at room temperature and in a temperature range around. The minimum amount required. The energy storage device of the present invention is operated under severe conditions, for example, at a high temperature (for example, around 60 ° C.) and a high number of cycles (for example, 10,000 to 100,000 cycles), thereby achieving high energy density and improving cycle life. In this case, it is advantageous to further increase the amount of occlusion of lithium. From this viewpoint, the lithium storage amount is preferably 50% or more. On the other hand, there is no restriction | limiting in particular in the upper limit of lithium occlusion amount, The higher is preferable. However, when the amount of occlusion of lithium is increased, lithium is less likely to be occluded in the silicon-based material, and lithium may be deposited on the surface of the silicon-based material. From this point of view, the occlusion amount of lithium is preferably 90% or less, and more preferably 80% or less.

Silicon can occlude lithium to the state represented by the composition formula SiLi _4.4 . In this occlusion state, it is defined that the occlusion amount of lithium is 100% of the theoretical capacity. Therefore, “lithium is occluded to 35% of the theoretical capacity” means that 1.54 mol (= 4.4 × 0.35) of Li is occluded with respect to 1 mol of Si. The theoretical capacity when the silicon-based material is a silicon compound is calculated based on the amount of silicon contained in the silicon compound. For example, when the silicon compound is a silicon alloy, the amount of elements other than silicon is subtracted from the alloy, and the amount of occlusion of lithium is calculated based on the amount of silicon contained in the alloy. The same applies when the silicon compound is an oxide of silicon.

  In the present invention, pre-occlusion of lithium in the silicon-based material of the negative electrode means that lithium is occluded in the negative electrode in a state before the energy storage element is assembled, and when it is incorporated in the energy storage element. However, lithium is not occluded, but includes both cases in which lithium is occluded in the negative electrode by a predetermined means before charging the energy storage element.

  In order to occlude lithium in the silicon-based material, for example, the following method may be used. First, the negative electrode and the metal lithium foil are brought into contact with each other, and further, the positive electrode is stacked on the metal lithium foil via a separator, and these three components are enclosed in a container together with a non-aqueous electrolyte. Then, lithium is occluded in the silicon-based material by utilizing a local battery reaction formed between lithium and the silicon-based material in the presence of the non-aqueous electrolyte. The entire system may be warmed to promote lithium storage.

Instead of the above method, the following method can also be used. First, a layer containing a silicon-based material is formed on a conductive foil. This is used as a working electrode. Apart from the working electrode, an electrode having metallic lithium is used as the counter electrode. Both electrodes are immersed in an electrolytic solution made of a non-aqueous solvent obtained by dissolving a lithium salt in an organic solvent. Under this condition, both poles are short-circuited. As a result, current flows due to the potential difference between the two electrodes. That is, lithium ions are eluted from the metal lithium at the counter electrode, and the eluted lithium ions are occluded by the silicon-based material at the working electrode. By controlling the short circuit time, the amount of lithium occluded in the silicon-based material can be adjusted. Lithium can also be occluded by energizing using an external power supply instead of short-circuiting both electrodes, that is, occlusion of lithium without using an external power supply. In this occlusion method, as an electrolytic solution composed of a non-aqueous solvent obtained by dissolving a lithium salt in an organic solvent, for example, a solution obtained by dissolving LiPF ₆ in propylene carbonate (for example, concentration 1 mol / l) can be used. .

  Compared to graphite, which is a material capable of occluding and releasing lithium ions as in the case of silicon-based materials, silicon-based materials have a large volume change due to insertion and extraction of lithium ions. Therefore, when charging / discharging of the energy storage element of the present invention is repeated, the silicon material may be pulverized due to the volume change, and sufficient cycle characteristics may not be obtained. One embodiment of a negative electrode aimed at avoiding such inconvenience is shown in FIG.

  A negative electrode 10 of the embodiment shown in FIG. 1 includes a current collector 11 and an active material layer 12 formed on at least one surface thereof. Although FIG. 1 shows a state in which the active material layer 12 is formed only on one side of the current collector 11, 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. As the active material, the silicon-based material described above is used.

  In the active material layer 12, as shown in FIG. 1, a metal material 13 having a low lithium compound forming ability permeates between the particles 12a contained in the layer. The metal material 13 is deposited between the particles by electrolytic plating. In the figure, the metal material 13 is conveniently represented as a thick line surrounding the periphery of the particle 12a. It is preferable that the metal material 13 exists over the entire thickness direction of the active material layer 12. The active material particles 12 a are preferably present in the deposited 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 silicon-based material that is a semiconductor and has a poor electron conductivity is used as the active material. The permeation of the metal material 13 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.

  Inside the active material layer 12, the surfaces of the active material particles 12 a are covered with the deposited metal material 13. However, the metal material 13 does not cover the surface of the particles 12a in such a manner that the non-aqueous electrolyte does not reach the particles 12a. That is, the metal material 13 covers the surface of the particle 12a while ensuring a gap that allows the non-aqueous electrolyte to reach the particle 12a.

  The average thickness of the metal material 13 covering the surface of the active material particles 12a is preferably 0.05 to 2 μm, more preferably 0.1 to 0.25 μm. That is, the metal material 13 covers the surface of the active material particles 12a almost continuously with a minimum thickness. As a result, while the energy density is increased, the particles 12a are prevented from falling off due to expansion / contraction and pulverization due to charge / discharge. In some cases, the thickness of the metal material 13 covering the surfaces of the active material particles 12a may exceed the thickness of the surface layer 14 described later. This is because, when the negative electrode 10 is manufactured by the method described later, the particles 12 a located in the deep part of the active material layer 12 (that is, near the current collector 11), the particles 12 a located on the surface of the negative electrode 10, This is because the degree of electrolytic plating is different, and the particles 12a located in the deep part of the active material layer 12 tend to be covered with the metal material 13 thickly.

  Gaps are formed between the particles 12 a covered with the metal material 13. This void has a function as a space for relieving the stress caused by the volume change of the active material particles 12a caused by charging and discharging. 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. Furthermore, the voids formed between the particles 12a also have a function as a flow path for the non-aqueous electrolyte. From these viewpoints, the ratio of voids in the active material layer 12, that is, the void ratio, is preferably 20 to 40% by volume, and particularly preferably 20 to 35% by volume. Below this porosity, hole processing by laser processing or punching processing or hole processing by etching is necessary to secure the entire negative electrode, which may increase production costs and decrease production stability. There is. The porosity can be determined by electron microscope mapping. In order to make the porosity within the above range, for example, a condition for depositing the metal material 13 between the particles 12a by electrolytic plating may be appropriately selected. Examples of such conditions include the composition and pH of the plating bath and the current density during electrolysis.

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).

  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. “Lithium compound forming ability is low” 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 negative electrode 10 of the present embodiment, a thin surface layer 14 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 14 is as thin as 0.25 μm or less, preferably 0.1 μm or less. The lower limit value of the thickness of the surface layer 14 is not limited and may be zero. That the thickness of the surface layer is 0 means that the negative electrode 10 has no surface layer.

  When the negative electrode 10 has the thin surface layer 14 or does not have the surface layer, an energy storage device is assembled using the negative electrode 10 to reduce overvoltage when the device is initially charged. be able to. This means that lithium can be prevented from being reduced on the surface of the negative electrode 10 during charging of the energy storage element. The reduction of lithium leads to the generation of dendrites that cause a short circuit between the two electrodes. In addition, the ability to reduce the overvoltage is advantageous from the viewpoint of preventing decomposition of the nonaqueous electrolytic solution. 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.

  When the negative electrode 10 has the surface layer 14, the surface layer 14 covers the surface of the active material layer 12 continuously or discontinuously. As the thickness of the surface layer 14 decreases, the covering state of the surface layer changes from continuous to discontinuous. When the thickness of the surface layer 14 is further reduced, fine particles (not shown) of the metal material 13 are attached to the surface of the active material layer 12 and the particles 12a located in the vicinity thereof. The fine particles are derived from plating nuclei generated when the metal material 13 is deposited by electrolytic plating.

  When the surface layer 14 continuously covers the surface of the active material layer 12, the surface layer 14 has a large number of fine voids (not shown) that are open on the surface and communicate with the active material layer 12. It is preferable. The fine voids are preferably present in the surface layer 14 so as to extend in the thickness direction of the surface layer 14. 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 14 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. Considering the ease of manufacturing the negative electrode 10, the metal material 13 present in the active material layer 12 and the metal material constituting the surface layer 14 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. 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.

  Next, a preferred method for manufacturing the negative electrode 10 of the embodiment shown in FIG. 1 will be described with reference to FIG. In the present manufacturing method, a step of forming a coating film on the current collector 11 using a slurry containing particles of an active material and then performing electrolytic plating on the coating film is performed.

  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 particles of a conductive material such as acetylene black or graphite. 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 in the slurry is preferably about 14 to 40% 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 (hereinafter, this plating is also referred to as penetration 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.

  In this production method, it is important to control the precipitation of the metal material by the permeation plating. Specifically, it is important to deposit the metal material from one side of the coating film 15 toward the other side. Specifically, as shown in FIGS. 2B and 2C, 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 gap can be successfully formed between the particles 12a coated with the metal material 13. Can be formed. In addition, it becomes easy to set the void ratio of the voids to the above-described preferable range.

  As shown in FIGS. 2 (b) and 2 (c), the fine particles 13a of the metal material 13 are attached to the front surface and the particles 12a located in the vicinity thereof on the front surface where the electrolytic plating is performed. It will be in the state. The fine particles 13a are derived from plating nuclei generated when the metal material 13 is deposited by electrolytic plating.

As described above, the conditions of the osmotic plating for depositing the metal material 13 include the composition of the plating bath, the pH of the plating bath, and the current density of electrolysis. For example, when the active material is silicon alone, it is preferable to adjust the pH of the plating bath to 7.1 to 11. By setting the pH to 7.1 or more, the surface of the active material particles 12a is cleaned and becomes a homogeneous surface state, enabling uniform precipitation on the surfaces of the particles 12a, and at the same time, moderate voids between the particles 12a. Is formed. On the other hand, by setting the pH to 11 or less, significant dissolution of the particles 12a can be prevented. When copper is used as the metal material for the osmotic plating, it is preferable to use a copper pyrophosphate bath. It is preferable to use a copper pyrophosphate bath even when the active material layer 12 is thick because the voids can be easily formed over the entire thickness direction of the layer. Further, 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, which is preferable in that the space between the particles 12a is maintained. When using a copper pyrophosphate bath, the bath composition 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: 1-7A / dm2
PH: Ammonia water and polyphosphoric acid are added to adjust the pH to 7.1 to 11, preferably 7.1 to 9.5.

  In the case where conductive film particles are contained in the coating film, the conductive material particles are preferentially given to the deposition of the metal material 13 from the interface between the coating film 15 and the current collector 11. It becomes easy for the metal material 13 to precipitate as a precipitation site. As a result, the metal material 13 may not be successfully deposited so that a desired void is formed between the active material particles 12a. For the purpose of preventing this, it is preferable that the coating film does not contain particles of the conductive material, or a small amount even if contained.

  The electrolytic 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 the electroplating, the thickness of the surface layer 14 can be appropriately controlled. In this way, the target negative electrode 10 is obtained as shown in FIG.

  The electrolytic solution in the energy storage element of the present invention is composed of a non-aqueous solvent containing a lithium salt. Examples of the non-aqueous solvent include ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, acetonitrile, dimethoxyethane, tetrahydrofuran, dioxolane, methylene chloride, sulfolane and the like. Two or more of these may be used in combination.

The lithium salt to be dissolved in the nonaqueous solvent described above, for example _{LiC1O 4, LiA1Cl 4, LiPF 6} , LiAsF 6, LiSbF 6, LiSCN, LiC1, LiBr, LiI, etc. _{LiCF 3 SO 3, LiC 4 F} 9 SO 3 is Illustrated. These lithium salts can be used alone or in combination of two or more. The concentration of the lithium salt in the electrolytic solution is preferably 0.1 to 1.5 mol / l, particularly 0.5 to 1.5 mol / l from the viewpoint of reducing internal resistance.

  In the energy storage element of the present invention, for example, a separator is sandwiched between a positive electrode and a negative electrode, and these are accommodated in a container together with an electrolytic solution, and the metal case and the sealing plate of the container are insulated by a gasket by the sealing plate and the gasket. Sealed and configured. Alternatively, these three members are wound by sandwiching a separator between a positive electrode and a negative electrode in the form of a sheet, and the wound body is impregnated with an electrolytic solution and accommodated in a metal case of the container, and the opening of the metal case is It is configured by sealing with a sealing member so that the electrolyte does not evaporate. In addition, as a separator, the thing similar to what has been used in the said technical field, for example, a porous film, a nonwoven fabric, etc. can be used.

  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 average particle size of the particles was D ₅₀ = 2 μm. The composition of the slurry was particle: styrene butadiene rubber (binder) = 100: 1.7 (weight ratio). The slurry did not contain conductive material particles.

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: 15 g / l
・ Bath temperature: 54 ℃
・ Current density: 3 A / dm ²
-PH: Ammonia water and polyphosphoric acid were added to adjust to pH 8.2.

  The permeation plating was terminated when copper was deposited over the entire thickness direction of the coating film. In this way, a target negative electrode was obtained. When the surface of the obtained negative electrode was observed with an electron microscope, the surface of the active material layer was discontinuously coated with copper particles. The average particle size of the particles was 0.2 μm. Further, as a result of electron microscope mapping using copper as a measurement target, the porosity in the active material layer was 35% by volume. In the active material layer, the active material particles were coated with a copper film having an average thickness of 0.05 μm.

The negative electrode (8.1 mAh / cm ² ) thus obtained was punched into a disk shape having a diameter of 14 mm. Separately, a metal lithium foil having a thickness of 30 μm was punched into a disk shape having a diameter of 14 mm (9.5 mAh / cm ² ) and overlaid on the surface of the active material layer in the negative electrode.

Apart from these operations, a commercially available activated carbon sheet (0.33 mAh / cm ² ) was punched into a disk shape having a diameter of 13 mm and used as a positive electrode. The positive electrode was overlaid on the metal lithium foil, and the whole was housed in a container to produce a 2032 type coin power storage device. The container was filled with an electrolytic solution in which LiPF ₆ was dissolved to a concentration of 1 mol / l in a mixed solvent in which ethylene carbonate and dimethyl carbonate were mixed at a volume ratio of 1: 1.

  A separator made of a polyethylene porous film was sandwiched between a positive electrode and a negative electrode, and these were enclosed in a container to produce a 2032 type coin power storage device.

  In the electric storage element, metallic lithium was completely occluded in the negative electrode Si. The storage amount of lithium was 73% with respect to the theoretical capacity of silicon. Charging / discharging was repeatedly performed with respect to the electrical storage element after metal lithium was occluded completely. The charge / discharge conditions were a voltage range of 2.9 to 3.9 V, a 10 C rate, and a temperature of 30 ° C. Charging / discharging was repeated 1000 times, the discharge capacity at the 1000th cycle relative to the discharge capacity at the 100th cycle was calculated, and the value was taken as the capacity retention rate. The energy density (Wh / L) at 1 kW / L was determined from the discharge power capacity when the constant current charge / discharge was performed at a current value corresponding to 1 kW / L. The results are shown in Table 1.

[Examples 2 and 3]
A 2032 type coin power storage device was produced in the same manner as in Example 1 except that the amount of occlusion of lithium was 37% (Example 2) and 42% (Example 3) with respect to the theoretical capacity of silicon. The capacity retention rate and energy density of the obtained electricity storage device were determined in the same manner as in Example 1. The results are shown in Table 1.

[Comparative Example 1]
A 2032 type coin power storage device was produced in the same manner as in Example 1 except that the amount of occlusion of lithium was 24% of the theoretical capacity of silicon. The capacity retention rate and energy density of the obtained electricity storage device were determined in the same manner as in Example 1. The results are shown in Table 1.

Example 4
A negative electrode punched out in a disk shape obtained in Example 1 was used as a working electrode, and a cell using metallic lithium as a counter electrode was constructed. As an electrolytic solution, a solution obtained by dissolving LiPF ₆ to a concentration of 1 mol / l in a mixed solvent in which ethylene carbonate and dimethyl carbonate were mixed at a volume ratio of 1: 1 was used. Charging was performed until the amount of lithium occluded in the negative electrode silicon reached 75% of the theoretical capacity. Next, the metal lithium of the counter electrode was replaced with the positive electrode punched out into a disk shape obtained in Example 1, and a final test cell was obtained. This test cell was repeatedly charged and discharged. The charge / discharge conditions were a voltage range of 2.3 to 4.3 V, a 30 C rate, and a temperature of 60 ° C. Charging / discharging was repeated 5000 times, the discharge capacity at the 5000th cycle relative to the discharge capacity at the 1000th cycle was calculated, and the value was taken as the capacity retention rate. The energy density was determined in the same manner as in Example 1. The results are shown in Table 2.

[Comparative Example 2]
A 2032 type coin battery was fabricated in the same manner as in Example 4 except that the amount of occlusion of lithium was 25% of the theoretical capacity of silicon. The capacity retention rate and energy density of the obtained battery were determined in the same manner as in Example 4. The results are shown in Table 2.

Example 5
A negative electrode (7.5 mAh / cm ² ) produced by the same method as in Example 1 was punched into a disk shape having a diameter of 14 mm. Separately, a metal lithium foil having a thickness of 30 μm was punched into a disk shape having a diameter of 13 mm (8.2 mAh / cm ² ), and was superimposed on the surface of the active material layer in the negative electrode. Except for this, a 2032 type coin power storage device was fabricated in the same manner as in Example 1. In the electric storage element, metallic lithium was completely occluded in the negative electrode Si. The storage amount of lithium was 71% with respect to the theoretical capacity of silicon. Charging / discharging was repeatedly performed with respect to the electrical storage element after metal lithium was occluded completely. The charge / discharge conditions were a voltage range of 2.3 to 4.3 V, a 50 C rate, and a temperature of 60 ° C. Charging / discharging was repeated 10,000 times, the discharge capacity at the 10,000th cycle relative to the discharge capacity at the 1000th cycle was calculated, and the value was taken as the capacity retention rate. The energy density was determined in the same manner as in Example 1. The results are shown in Table 3.

Example 6
A 2032 type coin power storage device was fabricated in the same manner as in Example 5 except that the amount of occlusion of lithium was 60% of the theoretical capacity of silicon. The capacity retention rate and energy density of the obtained electricity storage device were determined in the same manner as in Example 5. The results are shown in Table 3.

Example 7
A 2032 type coin power storage device was produced in the same manner as in Example 5 except that the amount of occlusion of lithium was 51% with respect to the theoretical capacity of silicon. The capacity retention rate and energy density of the obtained electricity storage device were determined in the same manner as in Example 5. The results are shown in Table 3.

Example 8
A 2032 type coin power storage device was produced in the same manner as in Example 5 except that the amount of occluded lithium was 42% of the theoretical capacity of silicon. The capacity retention rate and energy density of the obtained electricity storage device were determined in the same manner as in Example 5. The results are shown in Table 3.

  As is apparent from the results shown in Tables 1 to 3, according to Examples 1 to 8 (products of the present invention), it can be seen that the capacity retention rate can be maintained at a high value even when charging and discharging are repeated. On the other hand, in each comparative example with a small amount of occlusion of lithium, it can be seen that the capacity retention rate decreases when charging and discharging are repeated. In particular, when operating at a high temperature and a high number of cycles, it is clear from the results shown in Table 3 that it is advantageous to increase the lithium storage amount.

FIG. 1 is a schematic diagram showing a cross-sectional structure of an embodiment of a negative electrode suitably used in the energy storage element of the present invention. 2A to 2D are process diagrams showing a method for manufacturing the negative electrode shown in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Negative electrode 11 Current collector 12 Active material layer 12a Active material particle 13 Metal material with low lithium compound forming ability 13a Metal material fine particle 14 Surface layer 15 Coating film

Claims (6)

  It has an electrolyte composed of a positive electrode, a negative electrode, and a non-aqueous solvent containing a lithium salt. Activated carbon is used as an active material of the positive electrode, and lithium of 35% or more of the theoretical capacity is occluded in advance as the active material of the negative electrode. An energy storage element using silicon or a silicon compound.   The negative electrode includes an active material layer containing silicon or silicon compound particles in which lithium is previously occluded, and a metal material having a low ability to form a lithium compound penetrates between the particles in the active material layer. Item 1. An energy storage element according to Item 1.   The energy storage element according to claim 2, wherein the metal material permeates throughout the thickness direction of the active material layer.   The energy storage element according to claim 2 or 3, wherein the metal material is copper, nickel, iron, cobalt, or an alloy of these metals.   The energy storage element according to claim 2, wherein the metal material is infiltrated by electrolytic plating. 2. A surface layer made of a metal material having a low ability to form a lithium compound formed on the active material layer, wherein the surface layer has a number of fine voids extending in the thickness direction. The energy storage element according to any one of 5.

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