Electric power storage device

Abstract

This electric power storage device comprises: a positive electrode containing lithium vanadium phosphate expressed by Li3−xV2(PO4)3 (−2 ≤ x ≤ 3); and a negative electrode containing a substance (other than metallic lithium) that is capable of reversibly occluding lithium ions. At least one of the positive electrode and the negative electrode is pre-doped with lithium ions. A first electric current capacity C1, which is defined below, is 5-700% of a second electric current capacity C2, which is defined below. First electric current capacity C1: the electric current capacity when the negative electrode that is removed from the electric power storage device is completely discharged in a state in which x is 0. Second electric current capacity C2: the electric current capacity when the electric power storage device is charged from a state in which x is 0 to a state in which x is 2.

Description

Energy storage devices

[0001] This disclosure relates to an energy storage device.

[0002] Energy storage devices are disclosed in Patent Documents 1-2 and Non-Patent Documents 1-2. These energy storage devices contain lithium vanadium phosphate as the positive electrode active material.

[0003] Patent No. 6020580 Patent No. 6104245

[0004] Z. Ahsan et. Materials Today Communications 29(2021)102955Zihan Song et. Al. Nano Energy, Volume 66, December 2019, 104175

[0005] There is a need to increase the energy density of energy storage devices. In one aspect of this disclosure, it is preferable to provide an energy storage device with high energy density.

[0006] One aspect of this disclosure is Li ３－ｘ V ２ (PO ４ ) ３ An energy storage device comprising a positive electrode containing lithium vanadium phosphate represented by (-2 ≤ x ≤ 3) and a negative electrode containing a material capable of reversibly intercepting lithium ions (excluding metallic lithium), wherein at least one of the positive electrode and the negative electrode is pre-doped with lithium ions, and the first current capacity C1, as defined below, is 5% or more and 700% or less of the second current capacity C2, as defined below.

[0007] First current capacity C1: The current capacity when the negative electrode taken from the energy storage device is completely discharged when x is 0.

[0008] Second current capacity C2: The current capacity when the energy storage device is charged from a state where x is 0 to a state where x is 2.

[0009] One aspect of this disclosure is that energy storage devices have high energy density.

[0010] This is a side view showing the configuration of the energy storage device. This is an explanatory diagram showing the first current capacity C1 and the second current capacity C2. This is a graph showing the relationship between the negative electrode discharge capacity and the potentials of the positive and negative electrodes. This is a plan view showing the configuration of the positive electrode fabricated in the embodiment. This is a plan view showing the configuration of the negative electrode fabricated in the embodiment. This is a plan view showing the configuration of the control electrode fabricated in the embodiment.

[0011] Exemplary embodiments of the present disclosure will be described with reference to the drawings. <First Embodiment> 1. Configuration of the Energy Storage Device 1 The configuration of the energy storage device 1 will be described based on Figure 1. The energy storage device 1 includes a positive electrode 3. The positive electrode 3 has a shape such as a strip or rectangle. The positive electrode 3 includes a positive electrode current collector 5 and a positive electrode active material layer 7. Examples of the positive electrode current collector 5 include metal foil such as aluminum, copper, nickel, and stainless steel. However, copper and nickel should not be used for the positive electrode 3 whose oxidation-reduction potential exceeds 3V. The positive electrode current collector 5 may also be a metal foil on which a conductive layer mainly composed of carbon material is formed. The thickness of the positive electrode current collector 5 is, for example, 5 μm or more and 50 μm or less.

[0012] The positive electrode active material layer 7 is formed on the surface of the positive electrode current collector 5. For example, the positive electrode active material layer 7 is formed on both sides of the positive electrode current collector 5. The positive electrode active material layer 7 can be manufactured by applying a slurry containing the positive electrode active material and binder to the surface of the positive electrode current collector 5 and drying it.

[0013] Examples of binders include rubber-based binders such as styrene-butadiene rubber (SBR) and NBR; fluororesins such as polytetrafluoroethylene and polyvinylidene fluoride; polypropylene, polyethylene, and fluorine-modified (meth)acrylic binders as disclosed in Japanese Patent Application Publication No. 2009-246137.

[0014] In addition to the positive electrode active material and the binder, the slurry may contain other components. Examples of the other components include a conductive agent, a thickener, and the like. Examples of the conductive agent include carbon black, graphite, vapor-grown carbon fiber, metal powder, and the like. Examples of the thickener include carboxymethyl cellulose, its Na salt or ammonium salt, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, hydroxypropyl cellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, casein, and the like.

[0015] The thickness of the positive electrode active material layer 7 is not particularly limited. The thickness of the positive electrode active material layer 7 is, for example, 5 μm or more and 500 μm or less, preferably 10 μm or more and 200 μm or less, and particularly preferably 10 μm or more and 100 μm or less.

[0016] The positive electrode active material layer 7 contains a positive electrode active material. The positive electrode active material contains lithium vanadium phosphate represented by Li ３－ｘ V ２ (PO ４ ) ３ . x is -2 or more and 3 or less. The positive electrode active material may further contain something other than lithium vanadium phosphate.

[0017] The positive electrode active material other than lithium vanadium phosphate is not particularly limited as long as it is an electrode active material applicable to a battery or a capacitor that utilizes insertion and desorption of alkali metal ions. Examples of the positive electrode active material other than lithium vanadium phosphate include transition metal oxides such as lithium iron phosphate, cobalt oxide, nickel oxide, manganese oxide, vanadium oxide; sulfur-based active materials such as elemental sulfur, metal sulfide, and the like.

[0018] The power storage device 1 includes a negative electrode 9. The negative electrode 9 has a shape such as a strip shape or a rectangular shape, for example. The negative electrode 9 includes a negative electrode current collector 11 and a negative electrode active material layer 13. Examples of the negative electrode current collector 11 include the same ones as the positive electrode current collector 5. However, it is better not to use aluminum for the negative electrode 9 whose redox potential is 1 V or less.

[0019] The negative electrode active material layer 13 is formed on the surface of the negative electrode current collector 11. The negative electrode active material layer 13 is formed, for example, on both surfaces of the negative electrode current collector 11. The negative electrode active material layer 13 can be produced by applying a slurry containing a negative electrode active material and a binder to the surface of the negative electrode current collector 11 and drying it.

[0020] As the binder, for example, the same binder as that contained in the slurry for producing the positive electrode active material layer 7 can be used. The slurry may contain other components in addition to the negative electrode active material and the binder. As the other components, for example, the same other components as those contained in the slurry for producing the positive electrode active material layer 7 can be used.

[0021] The thickness of the negative electrode active material layer 13 is not particularly limited. The thickness of the negative electrode active material layer 13 is, for example, 5 μm or more and 500 μm or less, preferably 10 μm or more and 200 μm or less, and particularly preferably 10 μm or more and 100 μm or less.

[0022] The negative electrode active material layer 13 contains a negative electrode active material. The negative electrode active material contains a substance that can reversibly occlude lithium ions (excluding metallic lithium). Examples of the substance that can reversibly occlude lithium ions include graphite, metal oxides, etc.

[0023] Examples of the negative electrode active material include carbon materials such as graphite, graphitizable carbon, non-graphitizable carbon, or composite carbon materials in which graphite particles are coated with carbonized pitch or resin; materials containing metals or semi-metals such as Si and Sn that can be alloyed with lithium or their oxides.

[0024] The power storage device 1 includes a separator 15. The separator 15 is provided between the positive electrode 3 and the negative electrode 9. The separator 15 suppresses physical contact between the positive electrode 3 and the negative electrode 9. Examples of the separator 15 include non-woven fabrics or porous films made of cellulose rayon, polyethylene, polypropylene, polyamide, polyester, polyimide, etc. as raw materials.

[0025] The energy storage device 1 includes an electrolytic solution 17. The electrolytic solution 17 is impregnated in the positive electrode 3 and the negative electrode 9. The electrolytic solution 17 contains an alkali metal salt and a solvent. The alkali metal salt is, for example, a lithium salt or a sodium salt.

[0026] Examples of the anion part constituting the alkali metal salt include PF ６ － , PF ３ (C ２ F<UNK>0000013) ３ － , PF ３ (CF ３ ) ３ － , and phosphorus anions having a fluoro group such as these; BF ４ － , BF ２ (CF) ２ － , BF ３ (CF ３ ) － , B(CN) ４ － , and boron anions having a fluoro group or a cyano group such as these; N(FSO ２ ) ２ <000003'2, N(CF ３ SO ２ ) ２ － , N(C ２ F ５ SO ２ ) ２ － , and sulfonylimide anions having a fluoro group such as these; CF ３ SO ３ － , and organic sulfonic acid anions having a fluoro group such as these.

[0027] The alkali metal salt is preferably a fluorine-containing compound. The concentration of the alkali metal salt in the electrolytic solution 17 is preferably 0.1 mol / L or more, and more preferably within the range of 0.5 to 1.5 mol / L.

[0028] Examples of solvents included in the electrolyte 17 include ethylene carbonate, propylene carbonate, butylene carbonate, 1-fluoroethylene carbonate, dipropyl carbonate, γ-butyrolactone, sulfolane, diethylene glycol dimethyl ether (diglyme), diethylene glycol methyl ethyl ether, triethylene glycol dimethyl ether (triglime), triethylene glycol butyl methyl ether, and tetraethylene glycol dimethyl ether (tetraglime).

[0029] Furthermore, as the solvent contained in the electrolyte 17, ionic liquids such as quaternary imidazolium salts, quaternary pyridinium salts, quaternary pyrrolidinium salts, and quaternary piperidinium salts can also be used. The solvent may consist of a single component or a mixed solvent of two or more components.

[0030] The energy storage device 1 includes, for example, a case 19. The positive electrode 3, the negative electrode 9, the separator 15, and the electrolyte 17 are housed inside the case 19.

[0031] At least one of the positive electrode 3 and the negative electrode 9 is pre-doped with lithium ions. For example, only the positive electrode 3 is pre-doped with lithium ions. For example, only the negative electrode 9 is pre-doped with lithium ions. For example, both the positive electrode 3 and the negative electrode 9 are pre-doped with lithium ions.

[0032] In the energy storage device 1, the first current capacity C1 and the second current capacity C2 are defined as follows, respectively.

[0033] First current capacity C1: The current capacity when the negative electrode 9 taken from the energy storage device 1 is completely discharged when x is 0.

[0034] Second current capacity C2: The current capacity when charging the energy storage device 1 from a state where x is 0 to a state where x is 2.

[0035] The first current capacity C1 and the second current capacity C2 are shown in Figure 2. The horizontal axis in Figure 2 represents the negative electrode discharge capacity. The negative electrode discharge capacity shown on the horizontal axis in Figure 2 is normalized so that the second current capacity C2 is 100%. The negative electrode discharge capacity shown on the horizontal axis in Figure 2 is 0% when x is 0. The vertical axis in Figure 2 represents the potential of the positive electrode 3. When the negative electrode discharge capacity is -100%, x is 2. When the negative electrode discharge capacity is 0%, x is 0. When the negative electrode discharge capacity is C1, the negative electrode 9 is completely discharged.

[0036] The first current capacity C1 is 5% to 700% of the second current capacity C2. For example, the first current capacity C1 is 75% to 200% of the second current capacity C2. For example, the first current capacity C1 is 5% to 100% of the second current capacity C2. The greater the amount of lithium ions pre-doped into at least one of the positive electrode 3 and the negative electrode 9, the larger the first current capacity C1 becomes.

[0037] Examples of energy storage devices 1 include batteries and capacitors. Examples of batteries include lithium-ion secondary batteries. Examples of capacitors include lithium-ion capacitors.

[0038] 2. Effects of the Energy Storage Device 1 (1A) The discharge capacity of the energy storage device 1 is the sum of the first current capacity C1 and the second current capacity C2. The first current capacity C1 is between 5% and 700% of the second current capacity C2. Therefore, the energy density of the energy storage device 1 is high.

[0039] (1B) For example, the first current capacity C1 is 75% or more and 200% or less of the second current capacity C2. In this case, the energy density of the energy storage device 1 becomes particularly high.

[0040] (1C) For example, the first current capacity C1 is between 5% and 100% of the second current capacity C2. In this case, the energy storage device 1 can discharge down to 0V. In other words, even if discharged down to 0V, the positive electrode 3 and negative electrode 9 are less likely to be damaged. The reason for this is explained below. Figure 3 is a graph showing the relationship between the negative electrode discharge capacity and the potentials of the positive electrode 3 and negative electrode 9. The horizontal axis of Figure 3 represents the negative electrode discharge capacity. The negative electrode discharge capacity shown on the horizontal axis of Figure 3 is normalized so that the second current capacity C2 is 100%. The negative electrode discharge capacity shown on the horizontal axis of Figure 3 is 0% when x is 0. The vertical axis of Figure 3 represents the potentials of the positive electrode 3 and negative electrode 9.

[0041] The curve Vp shown in Figure 3 represents the potential of the positive electrode 3 during discharge (hereinafter referred to as the discharge curve). Vn_1, Vn_2, Vn_3, and Vn_4 shown in Figure 3 are the discharge curves of the negative electrode 9, respectively. The energy storage device 1 can discharge down to 0V, that is, until the potential of the positive electrode 3 and the potential of the negative electrode 9 become equal.

[0042] When the discharge curve of the negative electrode 9 is represented by Vn_1, the negative electrode discharge capacity when the potential of the positive electrode 3 and the potential of the negative electrode 9 are equal is a negative value. In this case, the first current capacity C1 does not exist. Also, when the discharge curve of the negative electrode 9 is represented by Vn_1, the potential of the negative electrode 9 will be 3V or higher before the potential of the positive electrode 3 and the potential of the negative electrode 9 become equal. In this case, the negative electrode current collector 11 dissolves (i.e., ionizes), and the negative electrode 9 becomes brittle. Furthermore, if the cell is charged with the negative electrode current collector 11 ionized and released into the electrolyte, ionized metal ions may deposit on the surface of the negative electrode 9, potentially causing a short circuit, which is dangerous.

[0043] When the discharge curve of the negative electrode 9 is represented by Vn_2, the negative electrode discharge capacity when the potential of the positive electrode 3 and the potential of the negative electrode 9 are equal is 5%. In this case, the first current capacity C1 is 5% of the second current capacity C2.

[0044] When the discharge curve of the negative electrode 9 is represented by Vn_3, the negative electrode discharge capacity is 100% when the potential of the positive electrode 3 and the potential of the negative electrode 9 are equal. In this case, the first current capacity C1 is 100% of the second current capacity C2.

[0045] When the discharge curve of the negative electrode 9 is between Vn_2 and Vn_3 (i.e., when the first current capacity C1 is between 5% and 100% of the second current capacity C2), even if discharge is performed until the potential of the positive electrode 3 and the potential of the negative electrode 9 become equal, the potential of the positive electrode 3 will not become excessively low, and the potential of the negative electrode 9 will not become excessively high. As a result, the positive electrode 3 and the negative electrode 9 are less likely to be damaged.

[0046] When the discharge curve of negative electrode 9 is represented by Vn_4, the negative electrode discharge capacity when the potential of positive electrode 3 and the potential of negative electrode 9 are equal is greater than 100%. In this case, the first current capacity C1 is greater than 100% of the second current capacity C2.

[0047] When the discharge curve of the negative electrode 9 is represented by Vn_4 (i.e., when the first current capacity C1 is greater than 100% of the second current capacity C2), the potential of the positive electrode 3 becomes significantly lower before the potential of the positive electrode 3 and the potential of the negative electrode 9 become equal. In this case, the positive electrode current collector 5 is prone to alloying reactions with lithium ions and is easily damaged.

[0048] As described above, when the first current capacity C1 is between 5% and 100% of the second current capacity C2, the positive electrode 3 and the negative electrode 9 are less likely to be damaged.

[0049] 3. Examples (3-1) Manufacturing of Energy Storage Devices Three units each of energy storage devices 1_1 to 1_10 were manufactured. The manufacturing methods for energy storage devices 1_1 to 1_10 were basically the same. However, as will be described later, the thickness of the negative electrode and the negative electrode pre-doping capacity differed.

[0050] (i) Preparation of positive electrode A slurry for the positive electrode was prepared by mixing the following components.

[0051] Li ３－ｘ V ２ (PO ４ ) ３(-2≦x≦3): 80 parts by mass Acetylene black: 10 parts by mass PVDF binder: 10 parts by mass N-methyl-2-pyrrolidone: the amount that results in a solid content mass ratio of 30% by mass in the positive electrode slurry A strip of aluminum foil with a width of 280 mm and a thickness of 12 μm was prepared. This aluminum foil is the positive electrode current collector. Using a vertical die type double-sided coating machine, the positive electrode slurry was simultaneously coated on both sides of the aluminum foil. The coating of the positive electrode slurry was performed along the longitudinal direction of the aluminum foil. The coating speed was 2 m / min.

[0052] The positive electrode slurry was applied to the central part of the aluminum foil in the width direction. The ends in the width direction were not coated with the positive electrode slurry. The width of the portion coated with the positive electrode slurry was 200 mm. The coated portion 23, described later, is a part of the portion coated with the positive electrode slurry. The uncoated portion 25, described later, is a part of the portion that was not coated with the positive electrode slurry.

[0053] Next, the thickness of the positive electrode was adjusted to 60 μm by pressing it with a roll press. Then, it was vacuum dried at 100°C for 12 hours. Through these steps, the positive electrode was obtained.

[0054] (ii) Preparation of the negative electrode A slurry for the negative electrode was prepared by mixing the following components.

[0055] Graphite: 87 parts by mass Acetylene black: 4 parts by mass SBR-based binder: 6 parts by mass Carboxymethylcellulose: 3 parts by mass Ion-exchanged water: Amount that results in a solid content mass ratio of 35% by mass in the negative electrode slurry A strip of copper foil with a width of 300 mm and a thickness of 8 μm was prepared. This copper foil is the negative electrode current collector. Using a vertical die type double-sided coating machine, the negative electrode slurry was simultaneously coated on both sides of the copper foil. The coating of the negative electrode slurry was performed along the longitudinal direction of the copper foil. The coating speed was 2 m / min.

[0056] The negative electrode slurry was applied to the central part of the copper foil in the width direction. The ends in the width direction were not coated with the negative electrode slurry. The width of the portion coated with the negative electrode slurry was 200 mm. The coated portion 33, described later, was a part of the portion coated with the negative electrode slurry. The uncoated portion 35, described later, was a part of the portion that was not coated with the negative electrode slurry. Next, vacuum drying was performed at 135°C for 12 hours. The coating film of the negative electrode slurry became the negative electrode active material layer. Through the above process, a negative electrode was obtained.

[0057] When fabricating the negative electrode, the thickness of the coating film of the negative electrode slurry and the thickness of the negative electrode active material layer were adjusted by controlling the discharge amount of the negative electrode slurry from the die head of the double-sided coating machine. The negative electrode thicknesses for each of the energy storage devices 1_1 to 1_10 are shown in Table 1. The negative electrode thickness is the sum of the thickness of the copper foil and the thickness of the negative electrode active material layer.

[0058] Next, the negative electrode was pre-doped with lithium. The lithium pre-doping method was a continuous method in which the negative electrode was pre-doped while being transported in the doping solution. The lithium pre-doping method is disclosed in Japanese Patent Application Publication No. 2024-067440. The doping solution contained lithium ions. During lithium pre-doping, the negative electrode was electrically connected to the counter electrode unit via the doping solution, and a current flowed between the negative electrode and the counter electrode unit. The counter electrode unit was made of lithium metal.

[0059] The amount of lithium ions pre-doped into the negative electrode (hereinafter referred to as the negative electrode pre-doping capacity) was adjusted by controlling the current value of the current flowing between the negative electrode and the counter electrode unit. For each of the energy storage devices 1_1 to 1_10, the negative electrode pre-doping capacity was as shown in Table 1.

[0060] (iii) Assembly of the energy storage device Seventeen positive electrodes 3, as shown in Figure 4, were fabricated by cutting the positive electrode. Each positive electrode 3 was rectangular in shape. The length of the long side of the rectangle was 145 mm, and the length of the short side was 100 mm.

[0061] Each positive electrode 3 had one coated portion 23 and one uncoated portion 25. The coated portion 23 was the part coated with the positive electrode slurry. The uncoated portion 25 was the part that was not coated with the positive electrode slurry. The uncoated portion 25 was located at the end of the positive electrode 3 in the direction of the longer side of the rectangle.

[0062] The coated portion 23 and the uncoated portion 25 were rectangular in shape. The coated portion 23 measured 100 mm x 130 mm. The uncoated portion 25 measured 100 mm x 15 mm. A portion of the uncoated portion 25 was the terminal weld portion 26.

[0063] Furthermore, 18 negative electrodes 9, as shown in Figure 5, were fabricated by cutting the negative electrode. Each negative electrode 9 was rectangular in shape. The length of the long side of the rectangle was 145 mm, and the length of the short side was 100 mm.

[0064] Each negative electrode 9 had one coated portion 33 and one uncoated portion 35. The coated portion 33 was the part coated with the negative electrode slurry. The uncoated portion 35 was the part that was not coated with the negative electrode slurry. The uncoated portion 35 was located at the end of the negative electrode 9 in the direction of the longer side of the rectangle.

[0065] The coated portion 33 and the uncoated portion 35 were rectangular in shape. The coated portion 33 measured 100 mm x 130 mm. The uncoated portion 35 measured 100 mm x 15 mm. A portion of the uncoated portion 35 was the terminal weld portion 36.

[0066] In addition, 36 separators were prepared. Each separator had the same shape and size as the positive electrode 3 and the negative electrode 9 when viewed from above. The thickness of each separator was 15 μm. The material of the separators was polypropylene.

[0067] Next, a laminate was formed by stacking 17 positive electrodes 3, 18 negative electrodes 9, and 36 separators. The stacking method involved repeatedly stacking separators, negative electrodes 9, separators, and positive electrodes 3 in that order. Both ends of the laminate in the thickness direction were separators. Negative electrodes 9 were adjacent to each separator at both ends.

[0068] When the laminate was viewed from the stacking direction, the side with the terminal weld 26 was on the opposite side from the side with the terminal weld 36. Next, the four sides of the laminate were taped together. Through these steps, an electrode unit was obtained.

[0069] Next, an aluminum positive electrode terminal was placed on top of the positive electrode terminal weld in the electrode unit, and the two were ultrasonically welded together. The positive electrode terminal weld was a collection of terminal welds 26 on 17 positive electrodes 3. A sealant film had been pre-heat-fused to the sealing portion of the aluminum positive electrode terminal. The shape of the aluminum positive electrode terminal was strip-shaped. The dimensions of the aluminum positive electrode terminal were 50 mm in width, 50 mm in length, and 0.2 mm in thickness.

[0070] Next, a copper negative electrode terminal was placed on top of the negative electrode terminal weld in the electrode unit, and the two were resistance welded together. The negative electrode terminal weld was a collection of terminal welds 36 on 18 negative electrodes 9. A sealant film had been pre-heat-fused to the sealing portion of the copper negative electrode terminal. The copper negative electrode terminal was strip-shaped. The dimensions of the copper negative electrode terminal were 50 mm in width, 50 mm in length, and 0.2 mm in thickness.

[0071] Next, the entire electrode unit was sandwiched between two outer aluminum laminate films. Each of the two outer aluminum laminate films was rectangular in shape. The two outer aluminum laminate films sandwiched the entire electrode unit in the stacking direction of the laminate.

[0072] Next, the two outer aluminum laminate films were heat-sealed together along three of their edges. As a result, the two outer aluminum laminate films formed a bag with three edges closed and one edge open. One of the three edges that was heat-sealed was the edge facing the positive electrode terminal weld. The other edge that was heat-sealed was the edge facing the negative electrode terminal weld.

[0073] Next, add LiPF to the solvent. ６ The electrolyte was prepared by dissolving [the substance]. The solvent was a mixture of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a volume ratio of 3:4:3. LiPF in the electrolyte ６ The concentration was 1.2 mol / L.

[0074] Next, the electrolyte solution was vacuum-impregnated into a bag consisting of two outer aluminum laminate films. Then, one of the four sides of the two outer aluminum laminate films that had been open was heat-sealed and vacuum-sealed. Through these steps, the energy storage device was completed. The energy storage device was a laminate-type cell.

[0075] (3-2) Measurement of the first current capacity C1 The first current capacity C1 was measured for each of the energy storage devices 1_1 to 1_10. The measurement method was as follows: One of the three manufactured energy storage devices was disassembled and the positive electrode 3 and negative electrode 9 were recovered.

[0076] Next, two control electrodes 41, as shown in Figure 6, were fabricated. Each control electrode 41 consisted of a copper foil 42 with a lithium foil 43 attached to one side. In plan view, the copper foil 42 had the same shape and size as the positive electrode 3 and the negative electrode 9. The thickness of the copper foil 42 was 8 μm. The lithium foil 43 was rectangular in shape. The length of the long side of the rectangle was 128 mm, and the length of the short side was 98 mm. The thickness of the lithium foil 43 was 100 μm.

[0077] The lithium foil 43 was provided on one end 42A of the copper foil 42 in the long-side direction. On the side of the copper foil 42 opposite to the end 42A, there was an exposed portion 42B to which the lithium foil 43 was not attached. Part of the exposed portion 42B was a terminal welding portion 46.

[0078] Four separators were also prepared. These separators were the same as those used in the fabrication of the energy storage device. Next, the separators, control electrode 41, separator, negative electrode 9, separator, control electrode 41, and separator were stacked in this order to form a laminate. When the laminate was viewed from the stacking direction, the side with the terminal weld 36 was on the opposite side from the side with the terminal weld 46. Next, the four sides of the laminate were taped together. Through the above steps, an electrode unit for measuring the first current capacity C1 (hereinafter referred to as the C1 measuring electrode unit) was obtained.

[0079] Next, copper terminals were placed on the terminal welds 46 of the two reference electrodes 41 in the C1 measuring electrode unit, and the two were ultrasonically welded together. A sealant film had been pre-heat-fused to the sealing portion of the copper terminals. The copper terminals were strip-shaped. The dimensions of the copper terminals were 50 mm in width, 50 mm in length, and 0.2 mm in thickness.

[0080] Next, a copper negative electrode terminal was placed on the terminal weld 36 of one negative electrode 9 in the C1 measuring electrode unit, and the two were resistance welded together. A sealant film had been pre-heat-fused to the sealing portion of the copper negative electrode terminal. The copper negative electrode terminal was strip-shaped. The dimensions of the copper negative electrode terminal were 50 mm in width, 50 mm in length, and 0.2 mm in thickness.

[0081] Next, the entire C1 measurement electrode unit was sandwiched between two outer aluminum laminate films. The shape of each of the two outer aluminum laminate films was rectangular. The two outer aluminum laminate films sandwiched the entire C1 measurement electrode unit in the stacking direction of the laminate.

[0082] Next, the two outer aluminum laminate films were heat-sealed together along three of their edges. As a result, the two outer aluminum laminate films formed a bag with three edges closed and one edge open. One of the three edges that was heat-sealed was the edge facing the terminal weld 46. The other edge that was heat-sealed was the edge facing the terminal weld 36.

[0083] Next, add LiPF to the solvent. ６ The electrolyte was prepared by dissolving [the substance]. The solvent was a mixture of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a volume ratio of 3:4:3. LiPF in the electrolyte ６ The concentration was 1.2 mol / L.

[0084] Next, the electrolyte solution was vacuum-impregnated into a bag consisting of two outer aluminum laminate films. Then, one of the four sides of the two outer aluminum laminate films that had been open was heat-sealed and vacuum-sealed. Through these steps, a half-cell for measuring the first current capacity C1 was completed.

[0085] A half-cell used for measuring the first current capacity C1 was charged using a charge / discharge device under the conditions of 3.0V and 7.6mA using CC charging. The electrical capacity at this time was measured. The measured electrical capacity is the first current capacity C1. The measured first current capacity C1 is shown in Table 1.

[0086] (3-3) Measurement of the second current capacity C2 The second current capacity C2 was measured for each of the energy storage devices 1_1 to 1_10. The measurement method was as follows.

[0087] A laminate was formed by stacking a separator, a control electrode 41, a separator, a positive electrode 3, a separator, a control electrode 41, and a separator in this order. The positive electrode 3 was the one recovered in (3-2) above. The separator and the control electrode 41 were the same as those used to make the half cell for measuring the first current capacity C1. When the laminate was viewed from the stacking direction, the side with the terminal weld 26 was on the opposite side from the side with the terminal weld 46. Next, the four sides of the laminate were taped together. Through the above steps, an electrode unit for measuring the second current capacity C2 (hereinafter referred to as the C2 measuring electrode unit) was obtained.

[0088] Next, an aluminum positive electrode terminal was placed on the terminal weld 26 of the C2 measuring electrode unit, and the two were ultrasonically welded together. A sealant film had been pre-heat-fused to the sealing portion of the aluminum positive electrode terminal. The aluminum positive electrode terminal was strip-shaped. The dimensions of the aluminum positive electrode terminal were 50 mm in width, 50 mm in length, and 0.2 mm in thickness.

[0089] Next, a copper negative electrode terminal was placed on the terminal weld 46 of the C2 measuring electrode unit, and the two were resistance welded together. A sealant film had been pre-heat-fused to the sealing portion of the copper negative electrode terminal. The copper negative electrode terminal was strip-shaped. The dimensions of the copper negative electrode terminal were 50 mm in width, 50 mm in length, and 0.2 mm in thickness.

[0090] Next, the entire C2 measurement electrode unit was sandwiched between two outer aluminum laminate films. Each of the two outer aluminum laminate films was rectangular in shape. The two outer aluminum laminate films sandwiched the entire electrode unit in the direction of the lamination of the laminate.

[0091] Next, the two outer aluminum laminate films were heat-sealed together along three of their edges. As a result, the two outer aluminum laminate films formed a bag with three edges closed and one edge open. One of the three edges that were heat-sealed was the edge facing the terminal weld 26. The other edge that was heat-sealed was the edge facing the terminal weld 46.

[0092] Next, add LiPF to the solvent. ６The electrolyte was prepared by dissolving [the substance]. The solvent was a mixture of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a volume ratio of 3:4:3. LiPF in the electrolyte ６ The concentration was 1.2 mol / L.

[0093] Next, the electrolyte solution was vacuum-impregnated into a bag consisting of two outer aluminum laminate films. Then, one of the four sides of the two outer aluminum laminate films that had been open was heat-sealed and vacuum-sealed. Through these steps, a half-cell for measuring the second current capacity C2 was completed.

[0094] A half-cell used for measuring the second current capacity C2 was charged using a charge / discharge device under the conditions of 4.3V and 7.6mA using CC charging. The electrical capacity at this time was measured. The measured electrical capacity is the second current capacity C2. The measured second current capacity C2 is shown in Table 1.

[0095] (3-4) Calculation of C1 / C2 For each of the energy storage devices 1_1 to 1_10, C1 / C2 was calculated by dividing the first current capacity C1 by the second current capacity C2. The calculated C1 / C2 is shown in Table 1. The larger the negative electrode pre-doping capacity, the larger the C1 / C2. Also, the thicker the negative electrode, the larger the C1 / C2.

[0096] (3-5) Calculation of Energy Density The energy density was measured for each of the energy storage devices 1_1 to 1_10. The method for measuring the energy density was as follows.

[0097] One of the three manufactured energy storage devices was charged to 4.2V at a constant current of 1.3A using a charge / discharge tester. Then, it was discharged to 1.0V at a constant current of 1.3A, and the discharge energy was measured. Next, the energy density was calculated by dividing the measured discharge energy by the cell weight of the energy storage device. The calculated energy densities are shown in Table 1. High energy densities were observed when C1 / C2 was between 5% and 700%. Particularly high energy densities were observed when C1 / C2 was between 75% and 200%.

[0098] (3-6) Evaluation of Electrode State During Over-V Discharge The state of the electrodes during over-V discharge was evaluated for each of the energy storage devices 1_1 to 1_10. The evaluation method was as follows: One of the three manufactured energy storage devices was used for 0V discharge. 0V discharge meant that the energy storage device was discharged to 0V with a constant current using a charge / discharge tester, followed by a constant voltage discharge for one hour. Next, after charging to 3V, the energy storage device was disassembled and the appearance of the electrodes was observed. The results of the observation are shown in Table 1. There were no problems with the electrodes in energy storage devices 1_1 to 1_4. In energy storage devices 1_5 to 1_8 and 1_10, the positive electrode became brittle due to an alloying reaction between the aluminum of the positive electrode current collector and lithium ions during 0V discharge. In energy storage device 1_9, copper from the negative electrode current collector dissolved during 0V discharge and deposited on the negative electrode surface during 3V charging, causing a short circuit. <Other Embodiments> Although embodiments of the present disclosure have been described above, the present disclosure is not limited to the embodiments described above and can be implemented with various modifications.

[0099] (1) The function of one component in each of the above embodiments may be divided among multiple components, or the function of multiple components may be performed by one component. Also, some of the configurations of each of the above embodiments may be omitted. Also, at least some of the configurations of each of the above embodiments may be added to, replaced with, etc., the configurations of other embodiments.

[0100] (2) In addition to the energy storage device 1 described above, this disclosure can also be realized in various forms, such as a system that uses the energy storage device 1 as a component, a method for manufacturing the energy storage device 1, etc.

[0101] 1...Energy storage device, 3...Positive electrode, 5...Positive electrode current collector, 7...Positive electrode active material layer, 9...Negative electrode, 11...Negative electrode current collector, 13...Negative electrode active material layer, 15...Separator, 17...Electrolyte, 19...Case, 23, 33...Coated parts, 25, 35...Uncoated parts, 26, 36, 46...Terminal connection parts, 41...Control electrode, 42...Copper foil, 43...Lithium foil

Claims

1. Li ３－ｘ V ２ (PO ４ ) ３ A power storage device comprising: a positive electrode containing lithium vanadium phosphate represented by (-2 ≤ x ≤ 3); and a negative electrode containing a material capable of reversibly intercepting lithium ions (excluding metallic lithium), wherein at least one of the positive electrode and the negative electrode is pre-doped with lithium ions, and the first current capacity C1, as defined below, is 5% or more and 700% or less of the second current capacity C2, as defined below. First current capacity C1: The current capacity when the negative electrode is completely discharged after being removed from the power storage device when x is 0. Second current capacity C2: The current capacity when the power storage device is charged from the state where x is 0 to the state where x is 2.

2. An energy storage device according to claim 1, wherein the first current capacity C1 is 75% or more and 200% or less of the second current capacity C2.

3. An energy storage device according to claim 1, wherein the first current capacity C1 is 5% or more and 100% or less of the second current capacity C2.

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