Capacitor and method for manufacturing same

Abstract

The present invention provides: a capacitor (10) in which cracking of a silicon compound and separation of an electrode can be reduced; and a method for manufacturing the same. This capacitor comprises a positive electrode mixture layer (17) that contains an active material, a negative electrode mixture layer (13) that contains a silicon-containing substance, and a separator (14) that separates the positive electrode mixture layer and the negative electrode mixture layer, and the value obtained by dividing the capacity (mAh / cm2) of the positive electrode mixture layer by the capacity (mAh / cm2) of the negative electrode mixture layer is smaller than the value obtained by multiplying the mass fraction (%) of silicon in the negative electrode mixture layer by 0.87. In this method for manufacturing a capacitor, the positive electrode mixture layer and the negative electrode mixture layer are produced so that the value obtained by dividing the capacity of the positive electrode mixture layer by the capacity of the negative electrode mixture layer is smaller than the value obtained by multiplying the mass fraction of silicon in the negative electrode mixture layer by 0.87, and the positive electrode mixture layer and the negative electrode mixture layer are separated by a separator.

Description

Capacitor and manufacturing method thereof

[0001] The present invention relates to a capacitor and a method for manufacturing the same.

[0002] A capacitor in which a silicon-containing negative electrode and a silicon-containing positive electrode are separated by a separator can have a high theoretical electric capacity. However, because the volume of the silicon compound changes significantly with charging and discharging, repeated charging and discharging, as described in Non-Patent Documents 1 and 2, causes cracks and peeling of the electrodes due to expansion and contraction of the silicon compound, resulting in a rapid decrease in capacity.

[0003] Electrochimica Acta, 2021,394, 139115Batteries 2022, 8(6), 49

[0004] To improve the durability of capacitors, there is a demand for technology that reduces cracking of silicon compounds and peeling of electrodes.

[0005] The present invention has been made to meet this demand, and an object of the present invention is to provide a capacitor that can reduce cracking of the silicon compound and peeling of the electrodes, and a method for manufacturing the same.

[0006] To achieve this object, a first aspect of the present invention is a capacitor, comprising: a positive electrode mixture layer containing an active material; a negative electrode mixture layer containing a substance containing silicon; and a separator separating the positive electrode mixture layer from the negative electrode mixture layer, wherein the capacity (mAh / cm) of the positive electrode mixture layer is ２ ) is the capacity (mAh / cm ２ ) is smaller than the value obtained by multiplying the mass fraction (%) of silicon in the negative electrode mixture layer by 0.87.

[0007] In a second aspect, in the first aspect, the mass fraction of silicon in the negative electrode mixture layer is 5 wt % or more and 95 wt % or less.

[0008] In a third aspect, in the first or second aspect, the thickness of the negative electrode mixture layer is thinner than the thickness of the positive electrode mixture layer.

[0009] In a fourth aspect, in any one of the first to third aspects, the negative electrode mixture layer contains carbon fibers.

[0010] A fifth aspect is a method for producing a capacitor including a positive electrode mixture layer containing an active material and a negative electrode mixture layer containing a substance containing silicon, wherein the capacity (mAh / cm ２ ) is the capacity (mAh / cm ２ The positive electrode mixture layer and the negative electrode mixture layer are prepared so that the value obtained by dividing the mass fraction (%) of silicon in the negative electrode mixture layer by 0.87 is smaller than the value obtained by multiplying the mass fraction (%) of silicon in the negative electrode mixture layer by 0.87, and the positive electrode mixture layer and the negative electrode mixture layer are separated by a separator.

[0011] According to the present invention, the capacity (mAh / cm ２ ) is the capacity (mAh / cm ２ ) is smaller than the value obtained by multiplying the mass fraction (%) of silicon in the negative electrode mixture layer by 0.87, so the capacitor can be charged and discharged within a range where the volume change of the silicon compound is small, thereby reducing cracking of the silicon compound and peeling of the electrode.

[0012] 1 is a cross-sectional view of an electricity storage device according to an embodiment; 2 is a graph in which the horizontal axis represents the mass fraction of silicon in a negative electrode mixture layer and the vertical axis represents the value obtained by dividing the capacity of a positive electrode mixture layer by the capacity of a negative electrode mixture layer; 3 is a graph showing the relationship between the mass fraction of silicon in the total of carbon and silicon in a negative electrode mixture layer and the thickness of the negative electrode mixture layer;

[0013] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Fig. 1 is a schematic cross-sectional view of a capacitor 10 according to one embodiment. The capacitor 10 includes, in order, a negative electrode 11, a separator 14, and a positive electrode 15.

[0014] 1 shows a capacitor 10 having one set of anode 11, separator 14, and cathode 15, but the present invention is not limited to this and may include a plurality of sets of these. Capacitor 10 is not limited to a laminated structure, but may also have a wound structure in which anode 11 and cathode 15 are laminated with separator 14 interposed therebetween and wound.

[0015] The capacitor 10 is exemplified by a lithium ion capacitor. Examples of the lithium ion capacitor include a redox capacitor that utilizes a redox reaction of an electrode or a redox reaction of ions in a non-aqueous electrolyte, and a hybrid capacitor that combines an electric double layer and a redox reaction, or that combines them with a secondary battery material.

[0016] The negative electrode 11 has a current collecting layer 12 and a negative electrode composite layer 13 superimposed on each other. Examples of materials for the current collecting layer 12 include a metal selected from Ni, Ti, Fe, Cu, and Al, an alloy containing two or more of these elements, and stainless steel. There are no particular restrictions on the shape of the current collecting layer 12. The current collecting layer 12 may be a porous foil having a plurality of holes penetrating the current collecting layer 12.

[0017] The negative electrode composite layer 13 contains an active material and a conductive additive. Examples of the active material include a material containing silicon. Examples of the material containing silicon include Si, a Si-Mn alloy, a Si-Fe alloy, a Si-Cu alloy, a Si-Co alloy, a composite material of Si powder and carbon powder, Si with a carbon film, and a compound containing Si and O as constituent elements (hereinafter referred to as "SiO ｘ ", where 0.5≦X≦1.5) is exemplified.

[0018] SiO ｘ (Silicon oxide) is an oxide of Si, amorphous SiO ２ Examples include those with a structure in which microcrystalline or amorphous Si is dispersed in a matrix. ｘ Since SiO has poor conductivity, ｘ A conductive layer is provided to cover the surface of SiO ｘ It is preferable to combine SiO with a conductive material. ｘ Granulation is an example of a means for combining the conductive layer with the conductive material. Examples of materials for the conductive layer include metals such as Pt and Os, and carbon. Examples of conductive materials include graphite, low-crystalline carbon, easily graphitizable carbon (soft carbon), hardly graphitizable carbon (hard carbon), carbon black, and carbon fiber. Carbon fiber includes carbon nanotubes.

[0019] The negative electrode mixture layer 13 may contain other active materials in addition to the silicon-containing material. Examples of other active materials include carbon-based materials such as graphite, low-crystalline carbon, graphitizable carbon, non-graphitizable carbon, and carbon fiber. In order to ensure the capacity of the negative electrode mixture layer 13 while reducing peeling of the current collecting layer 12 due to expansion and contraction of silicon, the mass fraction of silicon in the negative electrode mixture layer 13 is preferably 5% to 95%, more preferably 5% to 70%, and particularly preferably 5% to 40%.

[0020] When the negative electrode composite layer 13 contains carbon fibers, the carbon fibers are bound together to reduce the occurrence of destruction of the negative electrode composite layer 13 due to volume changes of the silicon-containing substance, and furthermore, the conductivity of the negative electrode composite layer 13 can be ensured even when the silicon-containing substance expands, which is preferable.

[0021] The positive electrode 15 includes a current collecting layer 16 and a positive electrode composite layer 17 attached to the current collecting layer 16. There are no particular limitations on the material of the current collecting layer 16, and examples include Cu, Al, a Cu alloy, an Al alloy, and stainless steel. There are also no particular limitations on the shape of the current collecting layer 16. The current collecting layer 16 may be a porous foil having a plurality of holes penetrating the current collecting layer 16.

[0022] Positive electrode mixture layer 17 contains an active material. There are no limitations on the active material, but examples of the active material include activated carbon, graphite, low-crystalline carbon, graphitizable carbon, non-graphitizable carbon, and carbon fiber.

[0023] Negative electrode mixture layer 13 and positive electrode mixture layer 17 may contain a conductive additive. The conductive additive electrically connects the active materials so that the active materials in negative electrode mixture layer 13 and positive electrode mixture layer 17 do not become isolated. Examples of the conductive additive include graphite, low-crystalline carbon, graphitizable carbon, non-graphitizable carbon, and carbon fiber.

[0024] The negative electrode mixture layer 13 and the positive electrode mixture layer 17 may contain a binder and a thickener. The binder binds the active material and the conductive additive. Examples of the binder include fluororesin, acrylic resin, polyolefin, and rubber. Examples of the fluororesin include polytetrafluoroethylene, polyvinylidene fluoride, and vinylidene fluoride-hexafluoropropylene copolymer. Examples of the acrylic resin include polyacrylic acid, polyacrylonitrile, polyacrylamide, and polyacrylimide. Examples of the polyacrylic acid include polymethacrylic acid ester, polyacrylic acid ester, and sodium polyacrylate. Examples of the polyolefin include polyethylene and polypropylene. Examples of the rubber include styrene-butadiene rubber. Examples of the thickener include carboxymethyl cellulose.

[0025] The separator 14 is made of a porous material that is durable against the active materials and electrolyte solution contained in the negative electrode 11 and the positive electrode 15, and that is non-electron-conductive while allowing lithium ions to pass through. Examples of the separator 14 include nonwoven fabrics and porous films made of cellulose, polypropylene, polyethylene, polyimide, alumina, etc.

[0026] The electrolytic solution is a solution of an electrolyte dissolved in a solvent. The electrolyte is a compound used to transfer charge carriers between the positive electrode 15 and the negative electrode 11, and an example of the electrolyte is a lithium salt. The anion of the lithium salt is a halide ion (I － , Cl － ,Br － etc.), SCN － , B.F. ４ － , B.F. ３ (CF ３ ) － , B.F. ３ (C ２ F ５ ) － , P.F. ６ － , ClO ４ － , SbF ６ － , N(SO ２ F) ２ － , N(SO ２ CF ３ ) ２－ , N(SO ２ C ２ F ５ ) ２ － , B(C ６ H ５ ) ４ － , B(O ２ C ２ H ４ ) ２ － , C(SO ２ F) ３ － , C(SO ２ CF ３ ) ３ － , C.F. ３ COO － , C.F. ３ SO ２ O － , C ６ F ５ SO ２ O － , B(O ２ C ２ O ２ ) ２ － is exemplified.

[0027] The solvent for the electrolyte is not particularly limited as long as it is liquid in the temperature range in which the capacitor 10 is used. Examples of the solvent include carbonate esters, aliphatic carboxylic acid esters, phosphate esters, γ-lactones, ethers, nitriles, sulfolane, dimethyl sulfoxide, fluorous solvents, and ionic liquids. Mixtures of these solvents are also acceptable.

[0028] Capacitor 10 is manufactured, for example, as follows. A slurry is prepared by dispersing an active material and a conductive additive in a solution in which a binder is dissolved. The prepared slurry is applied to current collecting layer 16, and then the slurry is dried to obtain positive electrode 15 in which positive electrode composite layer 17 is formed on current collecting layer 16. After drying, positive electrode composite layer 17 may be rolled with a roller or the like.

[0029] A slurry is prepared by dispersing a silicon-containing active material and a conductive additive in a solution containing a binder. The slurry is applied onto current collecting layer 12 and then dried to obtain negative electrode 11 in which negative electrode composite layer 13 is formed on current collecting layer 12.

[0030] Next, lithium is supplied to the negative electrode 11, and the negative electrode 11 is pre-doped with an amount of lithium ions corresponding to at least a portion of the irreversible capacity. Pre-doping can be exemplified by short-circuiting the current collecting layer 12 and the metallic lithium with a separator disposed between the negative electrode composite layer 13 and the metallic lithium, and then immersing the negative electrode 11, the separator, and the metallic lithium in an electrolyte solution. The potential difference between the current collecting layer 12 and the metallic lithium causes electrons to flow from the metallic lithium to the current collecting layer 12, and at the same time, the metallic lithium is ionized and released into the electrolyte solution, and the lithium ions in the electrolyte solution are supported on the negative electrode composite layer 13. Pre-doping results in a negative electrode 11 in which some of the atoms constituting the negative electrode composite layer 13 have been replaced with lithium atoms. During pre-doping, a current may be passed between the current collecting layer 12 and the metallic lithium.

[0031] By pre-doping the negative electrode mixture layer 13 with lithium ions, the potential of the negative electrode 11 drops to around 0 V. By pre-doping, a silicon compound in which lithium is combined with silicon is formed in the negative electrode mixture layer 13. When the potential of the negative electrode 11 reaches 0.06 V, the silicon compound Li ３．３ When the potential of the negative electrode 11 becomes 0.00 V, the silicon compound Li ４．４ When lithium combines with silicon, the silicon compound Li ３．３ The volume change to Si is large, but the silicon compound Li ３．３ Silicon compound Li from Si ４．４ The change in volume up to Si is small.

[0032] After cutting the negative electrode 11, separator 14, and positive electrode 15 into predetermined shapes, the positive electrode 15, separator 14, and negative electrode 11 are stacked in this order, terminals (not shown) are connected to the current collecting layers 12 and 16, respectively, and the resultant is sealed in a case (not shown) together with an electrolyte to obtain a capacitor 10.

[0033] When negative electrode mixture layer 13 and positive electrode mixture layer 17 are produced, the capacity (mAh / cm ２ ) is the capacity (mAh / cm ) of negative electrode mixture layer 13 ２ The types and amounts of the active materials in negative electrode mixture layer 13 and positive electrode mixture layer 17 are set so that the value obtained by dividing the mass fraction (%) of silicon in negative electrode mixture layer 13 by Li is smaller than the value obtained by multiplying the mass fraction (%) of silicon in negative electrode mixture layer 13 by 0.87. ３．３ Si to Li ４．４ Since the range of Si can be made to be in the range of 0.1 to 1.0, the volume change of the silicon compound can be reduced. This can reduce cracking of the silicon compound in negative electrode mixture layer 13 and peeling of current collecting layer 12 due to charging and discharging, thereby improving the durability of capacitor 10.

[0034] Because negative electrode composite layer 13 contains a substance containing silicon, even if the thickness of negative electrode composite layer 13 is thinner than the thickness of positive electrode composite layer 17, it is possible to ensure a capacity of negative electrode 11 that is comparable to the capacity of positive electrode 15. Because silicon has a relatively low density, by making the thickness of negative electrode composite layer 13 thinner than the thickness of positive electrode composite layer 17, it is possible to improve the output density and energy density per unit volume and unit mass compared to a capacitor having a negative electrode composite layer 13 with the same thickness as the thickness of positive electrode composite layer 17.

[0035] When analyzing capacitor 10 to examine the capacity of negative electrode composite layer 13, first, negative electrode composite layer 13 is cut along current collecting layer 12 to prepare a cross section. Next, qualitative and quantitative analyses of the cross section are performed to identify the material that appears in the cross section and the proportion of the cross section that is occupied by that material. Then, the capacity per unit area (mAh / cm) of the cross section of negative electrode composite layer 13 is calculated by taking a weighted average of the capacities of the materials. ２ In addition, the area of ​​each material that appears in the cross section of negative electrode composite layer 13 is multiplied by the density of each material to determine a value related to the mass of the material, and the mass fraction (%) of silicon is calculated from the proportion of the value related to the mass of silicon to the sum of the values ​​related to the mass of the materials.

[0036] Similar to the analysis of negative electrode mixture layer 13, a cross section prepared by cutting positive electrode mixture layer 17 along current collecting layer 16 was analyzed, and the capacity per unit area of ​​the cross section of positive electrode mixture layer 17 (mAh / cm ２The capacity (mAh / cm ) of the positive electrode mixture layer 17 thus obtained is calculated. ２ ) is the capacity (mAh / cm ) of negative electrode mixture layer 13 ２ ) and the value obtained by multiplying the mass fraction (%) of silicon in negative electrode mixture layer 13 by 0.87 are compared in magnitude.

[0037] The present invention will be explained in more detail by way of examples, but the present invention is not limited to these examples.

[0038] (Preparation of negative electrode) Graphite particles (100-N) parts by mass, N parts by mass of silicon particles whose surfaces are coated with carbon, 0.2 parts by mass of carbon fiber, 8.2 parts by mass of polyacrylamide (binder), and 1.1 parts by mass of carboxymethyl cellulose (thickener) were weighed. N was selected as 11 numbers ranging from 5 to 100. The weighed materials were added to 110 parts by mass of pure water and mixed in a mixer to prepare 11 types of slurries.

[0039] The slurry was applied to a current collecting layer made of 8 μm thick copper foil, and vacuum dried at 70° C. for 1 hour to form a negative electrode composite layer. The layers were then punched out into squares with sides of 25 mm, to obtain 11 types of negative electrodes. The amount of slurry applied was determined based on the theoretical capacity of graphite and silicon, so that the negative electrode capacity would be 4.12 mAh / cm. ２ The capacity of the negative electrode was adjusted to 4.12 mAh / cm ２ This is because the capacity is such that the potential of the negative electrode can be set to 0 V by pre-doping (described later) in which a current flows between the current collecting layer and the counter electrode.

[0040] (Pre-doping of negative electrode) A separator and a metal lithium foil (counter electrode) pressed onto copper foil were attached in order on the negative electrode composite layer to prepare a half cell. After immersing the half cell in the electrolyte, it was vacuum degassed and sealed. The electrolyte was a non-aqueous solvent mixed with lithium battery grade ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate in a ratio of 3:3:4 (volume ratio), further mixed with 1 wt% vinylene carbonate, and the electrolyte lithium bis(fluorosulfonyl)imide (LiFSI) was dissolved to 1 mol / L.

[0041] A current was passed between the copper foil of the current collecting layer of the half cell and the copper foil of the counter electrode, and pre-doping was performed by charging at a constant current of 0.1 C to 0 V at 25°C, followed by charging at a constant voltage of 0.01 C, while measuring the voltage of the half cell and the time for which the current was passed.

[0042] The relationship between the capacity of the negative electrode and the voltage of the half cell (the relationship between time and the voltage of the half cell) was plotted, and the capacity of the negative electrode was 4.12 mAh / cm ２ Eleven types of graphs of curves passing through the point where the voltage is 0.00 V were created, and the capacity C of the negative electrode when the voltage is 0.06 V was calculated on each graph. This means that when the voltage is 0.00 V, ４．４ Si is formed, and when the voltage is 0.06 V, Li ３．３ This is based on the assumption that Si is formed.

[0043] The silicon compound of the negative electrode is Li ３．３ Si and Li ４．４ To operate the capacitor so that it changes between Si and (4.12-C) mAh / cm ２ Assuming that a capacity of (4.12-C) mAh / cm is required, the positive electrode ２ Therefore, the upper limit of the capacity of the positive electrode is (4.12-C) mAh / cm ２ The capacity of the negative electrode was 4.12 mAh / cm ２ Therefore, the capacity of the positive electrode mixture layer (mAh / cm ２ ) is the capacity (mAh / cm ２ ) was (4.12-C) / 4.12.

[0044] FIG. 2 shows the mass fraction (%) of silicon in the negative electrode composite layer for 11 types of negative electrodes on the horizontal axis (X axis) and the capacity (mAh / cm) of the positive electrode composite layer. ２ ) is the capacity (mAh / cm ２ ) on the vertical axis (Y axis). The equation of the line passing through the 11 points was Y = 0.87X. Therefore, the capacity (mAh / cm ２ ) is the capacity (mAh / cm ２) is made smaller than the value obtained by multiplying the mass fraction (%) of silicon in the negative electrode mixture layer by 0.87, thereby obtaining a silicon compound having a small volume change. ３．３ Si and Li ４．４ It was found that the ability to operate a capacitor between silicon and silicon reduces cracking of the silicon and peeling of the current collecting layer. The results of a cycle characteristic test conducted to confirm this are described below.

[0045] Example 1 Graphite particles (100-M) parts by mass, M parts by mass of silicon particles whose surfaces are coated with carbon, 0.2 parts by mass of carbon fiber, 8.2 parts by mass of polyacrylamide (binder), and 1.1 parts by mass of carboxymethyl cellulose (thickener) were weighed out. M was adjusted so that the mass fraction of silicon in the negative electrode composite layer was 27%. The weighed materials were added to 110 parts by mass of pure water and mixed in a mixer to prepare a slurry.

[0046] The slurry was applied to a current collecting layer made of copper foil with a thickness of 8 μm, and vacuum dried at 70° C. for 1 hour to form a negative electrode composite layer. The layer was then punched into a square shape with a side length of 25 mm to obtain a negative electrode. The amount of the slurry applied was determined so that the capacity of the negative electrode would be 4.12 mAh / cm based on the theoretical capacity of graphite and silicon. ２ It was adjusted so that

[0047] A separator and a metal lithium foil (counter electrode) pressed onto the copper foil were attached in order on the negative electrode mixture layer to prepare a half cell. After immersing the half cell in the electrolyte, it was vacuum degassed and sealed. A current was passed between the copper foil of the current collecting layer of the half cell and the copper foil of the counter electrode, and pre-doping was performed by charging at a constant current of 0.1 C to 0 V at 25 ° C, followed by constant voltage charging to 0.01 C. The electrolyte was a non-aqueous solvent mixture of lithium battery grade ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate in a volume ratio of 3:3:4, further containing 1 wt% vinylene carbonate, in which the electrolyte lithium bis(fluorosulfonyl)imide (LiFSI) was dissolved to a concentration of 1 mol / L.

[0048] 30 parts by mass of activated carbon particles, 5 parts by mass of carbon black (conductive additive), and 5 parts by mass of polyacrylic acid (binder) were added to 150 parts by mass of pure water and mixed in a mixer to prepare a slurry. The slurry was applied to a current collecting layer made of 15 μm thick aluminum foil and vacuum dried at 70° C. for 1 hour to form a positive electrode composite layer, which was then punched out into a square shape with a side length of 20 mm to obtain a positive electrode. The amount of slurry applied was determined based on the capacity (mAh / cm) of the positive electrode. ２ ) to a negative electrode capacity of 4.12 mAh / cm ２ The value was adjusted based on the theoretical capacity of activated carbon so that the value obtained by dividing by 0.03 was 0.03.

[0049] The half-cell was disassembled, and a separator and a positive electrode were attached in that order onto the removed negative electrode mixture layer to prepare a cell. After the electrolyte was poured into the cell, the cell was degassed under vacuum, and excess electrolyte was squeezed out. The cell was then sealed under vacuum to obtain the capacitor of Example 1.

[0050] Example 2 M was set so that the mass fraction of silicon in the negative electrode mixture layer was 37%, and the capacity of the positive electrode (mAh / cm ２ ) to a negative electrode capacity of 4.12 mAh / cm ２ The capacitor of Example 2 was obtained in the same manner as in Example 1, except that the positive electrode mixture layer was formed by applying the slurry so that the value obtained by dividing by 0.04 was 0.04.

[0051] (Example 3) M was set so that the mass fraction of silicon in the negative electrode mixture layer was 55%, and the capacity of the positive electrode (mAh / cm ２ ) to a negative electrode capacity of 4.12 mAh / cm ２ The capacitor of Example 3 was obtained in the same manner as in Example 1, except that the positive electrode mixture layer was formed by applying the slurry so that the value obtained by dividing by 0.04 was 0.04.

[0052] Example 4 M was set so that the mass fraction of silicon in the negative electrode mixture layer was 73%, and the capacity of the positive electrode (mAh / cm ２ ) to a negative electrode capacity of 4.12 mAh / cm ２ The capacitor of Example 3 was obtained in the same manner as in Example 1, except that the positive electrode mixture layer was formed by applying the slurry so that the value obtained by dividing by 0.40 was 0.40.

[0053] (Comparative Example) M was set so that the mass fraction of silicon in the negative electrode mixture layer was 37%, and the capacity of the positive electrode (mAh / cm ２ ) to a negative electrode capacity of 4.12 mAh / cm ２ A capacitor for the comparative example was obtained in the same manner as in Example 1, except that the positive electrode mixture layer was formed by applying the slurry so that the value obtained by dividing by was 0.40.

[0054] (Cycle Characteristics Test) For each of the capacitors in Examples 1-4 and the Comparative Example, 10,000 charge-discharge cycles were repeated at 25°C, with one cycle consisting of a 100C constant current charge to 3.8 V and a constant current discharge to 2.2 V. The capacitance was calculated by dividing the amount of electricity passed by a value excluding the voltage consumed for the IR drop at the start of discharge. The change in capacitance per cycle was evaluated according to the root rule, and the number of cycles at which the capacitance reached 80% of the capacitance at the first cycle was defined as the capacitor's life. Capacitors with a life exceeding 150,000 cycles were rated as A, those with a life of 100,000 or more but less than 150,000 cycles as B, those with a life of 50,000 or more but less than 100,000 cycles as C, and those with a life of less than 50,000 cycles as D.

[0055] The mass fraction (%) of silicon in the negative electrode mixture layer and the capacity (mAh / cm ２ ) is the capacity (mAh / cm ２ The capacitors in Examples 1 to 4 and the comparative example were plotted in FIG. 2 based on the values ​​(ratios) obtained by dividing the average capacitance by the capacitance (%) by the capacitance (%). The results of the cycle characteristic test were shown in Table 1.

[0056]

[0057] 2 and Table 1, in Example 1-4 in the range of Y<0.87X, the cycle characteristic test was judged as A, B, or C, whereas in the comparative example in the range of Y>0.87X, the test result was judged as D. It is presumed that Example 1-4, which satisfied Y<0.87X, was able to reduce cracking of the negative electrode mixture layer and peeling of the current collecting layer, and therefore, the charge-discharge cycle characteristic was improved.

[0058] In particular, Example 1-3, in which Y<0.5X, was in the range of Y<0.5X, was evaluated as A or B in the cycle characteristic test. It is presumed that Example 1-3, which satisfied Y<0.5X, was able to further reduce cracking of the negative electrode mixture layer and peeling of the current collecting layer, and therefore, improved charge-discharge cycle characteristics.

[0059] Example 5 Fig. 3 is a graph showing the relationship between the mass fraction of silicon in the active material (carbon and silicon) of the negative electrode composite layer and the thickness of the negative electrode composite layer. The graph is obtained by increasing the proportion of silicon while keeping the active material capacity constant, using the capacity, mass, and volume (thickness of the negative electrode composite layer) of the active material when the carbon content is 100% (0% silicon) as a reference, calculating the mass of the active material at that time by a weighted average, determining the density of the active material, and plotting the volume of the active material (thickness of the negative electrode composite layer).

[0060] For the calculation, the capacity per unit mass of carbon (graphite) is 372 mAh / g, the capacity per unit mass of silicon is 4212 mAh / g, and the capacity per unit mass of LiC is 4212 mAh / g. ６ The density is 2.16 g / cm ３ Lithium is compounded with silicon １５ Si ４ The density is 1.22 g / cm ３ It was decided.

[0061] As shown in Figure 3, it was revealed that the negative electrode composite layer became thinner as the silicon percentage increased. Compared to the thickness of the negative electrode composite layer when the silicon percentage was 0%, the thickness of the negative electrode composite layer was approximately 70% when the silicon percentage was 5%, approximately 50% when the silicon percentage was 10%, approximately 30% when the silicon percentage was 30%, and approximately 20% when the silicon percentage was 50%. It was confirmed that according to this embodiment, the thickness of the negative electrode composite layer can be reduced, and therefore the power density and energy density per unit volume and unit mass can be improved.

[0062] The present invention has been described above based on the embodiments, but the present invention is not limited to the above embodiments, and it can be easily inferred that various improvements and modifications are possible within the scope of the present invention.

[0063] In the embodiment, the capacitor 10 has been described as including the negative electrode 11 in which the negative electrode composite layer 13 is provided on one side of the current collecting layer 12, and the positive electrode 15 in which the positive electrode composite layer 17 is provided on one side of the current collecting layer 16, but this is not necessarily limited to this. For example, it is of course possible to apply each element in the embodiment to a capacitor including electrode layers (so-called bipolar electrodes) in which the positive electrode composite layer 17 and the negative electrode composite layer 13 are provided on both sides of the current collecting layer 12. A so-called bipolar structure capacitor can be obtained by alternately stacking the bipolar electrodes and the separators 14 and housing the resultant in a case (not shown).

[0064] In the embodiment, the negative electrode 11 in which the negative electrode composite material layer 13 is provided on one side of the current collecting layer 12 has been described, but this is not necessarily limited to this. It is of course possible to provide the negative electrode composite material layer 13 on both sides of the current collecting layer 12.

[0065] 10 Capacitor 13 Negative electrode mixture layer 14 Separator 17 Positive electrode mixture layer

Claims

1. A capacitor comprising a positive electrode mixture layer containing an active material, a negative electrode mixture layer containing a material containing silicon, and a separator separating the positive electrode mixture layer and the negative electrode mixture layer, wherein the capacity (mAh / cm) of the positive electrode mixture layer is ２ ) to the capacity (mAh / cm ２ ) is smaller than the value obtained by multiplying the mass fraction (%) of silicon in the negative electrode mixture layer by 0.

87.

2. The capacitor according to claim 1, wherein the mass fraction of silicon in the negative electrode mixture layer is 5 wt % or more and 95 wt % or less.

3. The capacitor according to claim 1 or 2, wherein the thickness of the negative electrode mixture layer is thinner than the thickness of the positive electrode mixture layer.

4. The capacitor according to claim 1 or 2, wherein the negative electrode mixture layer contains carbon fiber.

5. A method for manufacturing a capacitor including a positive electrode mixture layer containing an active material and a negative electrode mixture layer containing a material containing silicon, wherein the capacity (mAh / cm) of the positive electrode mixture layer is ２ ) to the capacity (mAh / cm ２ a mass fraction (%) of silicon in the negative electrode mixture layer is multiplied by 0.87, and the positive electrode mixture layer and the negative electrode mixture layer are separated by a separator.