Secondary batteries

Abstract

[Problem] To provide a secondary battery which is made of materials with abundant resources and has a large energy density, and of which the capacity is less likely to decrease even when charging and discharging are repeated. [Solution] The problem is solved by a secondary battery that comprises at least a positive electrode, a negative electrode, and an electrolyte. The positive electrode is an olivine type phosphate. The negative electrode contains silicon pre-doped with lithium, or a silicon compound. The ratio (AC ratio) of the capacity per unit area of the negative electrode to the capacity per unit area of the positive electrode is 1.5 or more.

Description

[Technical Field]

[0001] The present invention relates to a secondary battery. More specifically, the present invention relates to a secondary battery comprising at least a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode is an olivine-type phosphate, the negative electrode contains silicon or a silicon compound pre-doped with lithium, and the ratio of the capacity per unit area of ​​the negative electrode to the capacity per unit area of ​​the positive electrode (AC ratio) is 1.5 or more, the secondary battery being made of resource-free materials, having high energy density, excellent stability, and being resistant to capacity loss even with repeated charge and discharge. [Background technology]

[0002] As the markets for portable electronic devices and electric vehicles expand, batteries used in these devices are required to have high energy densities. Secondary batteries have been developed that utilize the electrochemical reactions associated with the transfer of electric charge using alkali metal ions such as lithium as charge carriers. Lithium-ion secondary batteries, in particular, are widely used due to their high energy density.

[0003] Lithium-ion secondary batteries use lithium-containing transition metal oxides as the positive electrode active material and graphite as the negative electrode active material. Charging and discharging are achieved through lithium ion insertion and desorption reactions between these positive and negative electrode active materials. For such lithium-ion secondary batteries, the development of new technologies to achieve even higher energy densities is required. To meet these demands, efforts are being made to increase the capacity density of the negative electrode and reduce the irreversible capacity. For example, Patent Document 1 discloses a secondary battery having a negative electrode with a lithium occlusion layer made of silicon or a silicon compound, which is capable of reacting with a large number of electrons and has a theoretical capacity more than twice that of graphite. Patent Document 2 discloses a secondary battery in which the irreversible capacity is reduced by electrochemically pre-doping lithium into a silicon-based electrode layer under pressure.

[0004] However, lithium-containing transition metal oxides such as cobalt, nickel, and manganese, which constitute the positive electrode of lithium-ion batteries, have limited reserves as resources, and increasing their use raises the cost of these materials. For this reason, lithium iron phosphate has attracted attention as a positive electrode active material that does not require these rare metals (Patent Document 2). This compound is made from iron, a basic metal, and is therefore abundant in resources. Its capacity density as an electrode active material is 140 to 170 Ah / kg, equivalent to that of lithium cobalt oxide. However, its discharge voltage is only 2.8 V, 70% of that of lithium cobalt oxide, and its energy density is also only about 70%. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-87251 [Patent Document 2] Patent No. 7170330 [Patent Document 3] Japanese Patent Application Laid-Open No. 2002-110162 Summary of the Invention [Problem to be solved by the invention]

[0006] In Patent Document 1, high-capacity silicon is used as the active material, but its irreversible capacity is greater than that of graphite, and its volume change during charging and discharging is large, resulting in problems such as insufficient capacity and charge-discharge cycle life. In Patent Document 2, the silicon negative electrode is electrochemically pre-doped to compensate for the capacity lost as irreversible capacity and form a stable passive layer to improve charge-discharge cycle life, but this requires the continued use of rare metals such as cobalt and nickel, which have limited reserves as resources.

[0007] Thus, there has been a problem that a secondary battery made of abundantly available materials, having a high energy density, and an improved charge / discharge cycle life has not yet been found.

[0008] The present invention has been made to solve the above-mentioned problems, and its object is to provide a secondary battery made of a material that is abundant in resources, has a high energy density, and is resistant to capacity reduction even with repeated charge and discharge. [Means for solving the problem]

[0009] The secondary battery according to the present invention is characterized in that, in a secondary battery having at least a positive electrode, a negative electrode, and an electrolyte as constituent elements, the positive electrode active material constituting the positive electrode is an olivine-type phosphate, the negative electrode contains silicon pre-doped with lithium or a silicon compound, and the ratio of the capacity per unit area of ​​the negative electrode to the capacity per unit area of ​​the positive electrode (AC ratio) is 1.5 or more. [Effects of the Invention]

[0010] According to the present invention, in a secondary battery comprising at least a positive electrode, a negative electrode, and an electrolyte, the positive electrode is an olivine-type phosphate, which allows for an active material that does not use materials with limited reserves, such as cobalt or nickel. Furthermore, the negative electrode contains lithium-predoped silicon or a silicon compound, which eliminates irreversible capacity and results in an increased overall battery energy density. Furthermore, an AC ratio of 1.5 or higher allows for a battery with excellent stability, with little capacity loss even after repeated charge and discharge.

[0011] By implementing the present invention, it is possible to realize a battery that has an energy density comparable to that of batteries that use materials such as cobalt and nickel, which have limited reserves, and that has a long charge-discharge cycle life and excellent stability. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a cross-sectional view showing an example of a secondary battery according to the present invention. [Figure 2]1 is a graph showing the change in voltage during repeated charge and discharge (1 to 10 times) of the secondary battery produced in Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the technical scope of the present invention should be determined based on the claims and is not limited to the following embodiments.

[0014] [Secondary battery] The secondary battery according to the present invention has at least a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode is an olivine-type phosphate, the negative electrode contains silicon pre-doped with lithium, or a silicon compound, and the ratio of the capacity per unit area of ​​the negative electrode to the capacity per unit area of ​​the positive electrode (referred to as the "AC ratio") is 1.5 or more. The secondary battery is not particularly limited in form, but the example shown in FIG. 1 is a coin-type secondary battery. In this example, the battery has a positive electrode 4, a negative electrode 6, and an electrolyte 10, and is also composed of a positive electrode case 2, a negative electrode case 3, a gasket 9, a separator 5, and a metal spring 8.

[0015] (Structural Elements) The positive electrode case 2 serves as an exterior member for the positive electrode 4 and functions as a positive electrode current collector. An electrode layer composed of an electrochemically oxidizable and reducible positive electrode active material and a conductive material is disposed in the center of the bottom of the positive electrode case 2. A separator 5 made of a porous sheet or film, such as a microporous membrane, nonwoven fabric, or woven fabric, is laminated on the positive electrode 4. A negative electrode 6 is disposed on the separator. A metal negative electrode current collector 7 is laminated on the negative electrode 6. A metal spring 8 is placed on the negative electrode current collector 7. While a coin-type secondary battery is described here, the battery shape is not particularly limited, and cylindrical, prismatic, sheet, and other types can also be used. The method of packaging is also not particularly limited, and a metal case, molded resin, aluminum laminate film, and the like may be used.

[0016] In Fig. 1, the negative electrode case 3 is fixed to the positive electrode case 2 against the biasing force of a metal spring 8. A positive electrode 4, a separator 5, a negative electrode 6, and a negative electrode current collector 7 are stacked and placed in the internal space formed by fixing the positive electrode case 2 and the negative electrode case 3, and the internal space is filled with an electrolyte 10. The gap between the positive electrode case 2 and the negative electrode case 3 that forms the internal space is sealed via a gasket 9.

[0017] In the present invention, the AC ratio is 1.5 or more. Typically, a secondary battery uses a positive electrode and a negative electrode facing each other. Therefore, the capacity ratio per unit area of ​​the facing electrodes affects the battery characteristics. In the present invention, the overall capacity of the positive and negative electrodes is not particularly limited. Furthermore, the thickness, density, and basis weight of the positive and negative electrodes are not particularly limited. In the present invention, the capacity per unit area for determining the AC ratio is the design capacity, which is calculated from the amount of electrode active material in each electrode layer and the theoretical capacity. Specific examples of theoretical capacity are 169.9 mAh / g for lithium iron phosphate and 4199 mAh / g for silicon. Specifically, the calculation is performed using an active material consisting of lithium iron phosphate with a concentration of 90 wt.% and a basis weight of 20 mg / cm. 2 The capacity density of the positive electrode is 3.06 mA / cm 2 The capacity density of the negative electrode, which has an active material concentration of 75 wt.% and a basis weight of 4 mg / cm, is 12.6 mA / cm. 2 The AC ratio in this case is 12.6 / 3.06=4.12. In the present invention, a mixture of silicon and graphite can also be used as the negative electrode. In this case, the capacity can be calculated using the theoretical capacity of each and the mixing ratio. For example, if the graphite and silicon mixing ratio is 9:1, the active material concentration is 94 wt.%, and the basis weight is 8 mg / cm. 2 In this case, the negative electrode has a capacity density of 8.54 and an AC ratio of 2.79.

[0018] (positive and negative electrodes) In the present invention, the main electrode active material constituting the positive electrode is an olivine-type phosphate, which is used alone or in combination with other electrode active materials. Olivine-type phosphate is represented by the general formula LiMPO4 (M = Fe, Co, Mn, etc.). In the present invention, olivine-type lithium iron phosphate with M = Fe is particularly preferred in terms of resource issues and the effectiveness of the invention.

[0019] In the present invention, the positive electrode 4 can be produced by a conventionally known method, that is, by mixing a positive electrode active material such as lithium iron phosphate with a conductive material and a binder, adding a solvent to prepare a slurry, applying the slurry to an electrode foil serving as a current collector by a conventionally known method, and drying the slurry.

[0020] In the present invention, the negative electrode 6 can be prepared by a conventional method. The electrode active material constituting the negative electrode is silicon or a silicon compound pre-doped with lithium. Examples of silicon or silicon compounds include Si, SiO, and SiOC. In the present invention, the negative electrode can be formed in the same manner as in the conventional method, for example, by mixing the electrode active material with a conductive material (such as graphite) and a binder, adding a solvent to prepare a slurry, and then coating the slurry on an electrode foil serving as a current collector by a conventional method and drying it. In the present invention, the electrode foil serving as the negative electrode current collector is not particularly limited, but copper foil and stainless steel foil are preferred in terms of the effects of the invention. In the present invention, the electrode made of pre-doped silicon or a silicon compound may be one coated with pre-doped silicon or a silicon compound in advance, or one coated with silicon or a silicon compound and then pre-doped.

[0021] In the present invention, the lithium-predoped silicon or silicon compound is not particularly limited as long as it is lithium-predoped silicon or silicon compound, but from the viewpoint of the effects of the invention, it is preferable that 0.02 mol or more of lithium is predoped per silicon or silicon compound. When predoping silicon by electrolytic predoping, 0.02 mol per active material corresponds to 19.1 Ah / kg, so it is preferable to do it with more than that. In the present invention, predoping reduces irreversible capacity and compensates for lithium loss during repeated charge and discharge, resulting in a high-energy, long-life secondary battery.

[0022] In the present invention, the conductive material used to prepare the positive electrode and negative electrode is not particularly limited, and examples thereof include carbonaceous particles such as carbon black, ketjen black, and acetylene black; carbonaceous fibers such as vapor-grown carbon fibers, carbon nanotubes, and carbon nanohorns; and carbonaceous sheets such as graphene. Two or more of these conductive materials can be used in combination as needed. The solvent is also not particularly limited, and examples thereof include aprotic solvents such as N-methylpyrrolidone, dimethyl sulfoxide, dimethylformamide, propylene carbonate, diethyl carbonate, dimethyl carbonate, γ-butyrolactone, acetonitrile, tetrahydrofuran, nitrobenzene, and acetone, as well as methanol, ethanol, and water. The binder is not particularly limited as long as it binds the electrode active material and the conductive material, and examples thereof include various resins such as polyethylene, polyvinylidene fluoride, polyhexafluoropropylene, polytetrafluoroethylene, polyethylene oxide, carboxymethyl cellulose, styrene butadiene rubber, polyacrylic acid, and polyimide resin.

[0023] According to the research of the present inventors, when a negative electrode active material such as graphite or silicon is used, if the AC ratio exceeds 1 during charging, the proportion of irreversible capacity increases, and the capacity of the battery decreases. Furthermore, if the AC ratio is 1 or less, lithium precipitates on the electrode during charging. Therefore, the AC ratio is usually used in the range of about 1 to 1.05. In the present invention, the inventors discovered that pre-doping with silicon or a silicon compound reduces the irreversible capacity, and that if the AC ratio is 1.5 or more, the capacity decrease due to repeated charging and discharging is small, leading to the present invention.

[0024] (electrolyte) In the present invention, the electrolyte 10 is interposed between the positive electrode 4 and the negative electrode 6 to transport charge carriers between the two electrodes. There are no particular limitations on the electrolyte 10 as long as it has ion conductivity. -6 Liquid, gel, and solid electrolytes having an ionic conductivity of 0.1 S / cm or more can be used. In the present invention, liquid and gel electrolytes are preferably used as the electrolyte due to their ease of reaction. In the case of a liquid, the electrolyte is an organic solvent containing an electrolyte salt. Examples of the electrolyte salt include LiPF6, LiClO4, LiBF4, LiCF3SO3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiC(CF3SO3)3, and LiC(C2F5SO2)3. Examples of the organic solvent include ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, γ-butyrolactone, tetrahydrofuran, dioxane, methyl methyl acrylate, γ-butylolactone, methyl methyl acrylate ... Examples of suitable solvents include solan, dimethyl sulfone, ethyl methyl sulfone, diethyl sulfone, propyl methyl sulfone, isopropyl methyl sulfone, propyl ethyl sulfone, isopropyl ethyl sulfone, dipropyl sulfone, diisopropyl sulfone, sulfolane, pentamethylene sulfone, hexamethylene sulfone, 3-methyl sulfolane, 2,4-dimethyl sulfolane, N,N-dimethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone, and the like, as well as mixed solvents thereof.

[0025] In the present invention, the concentration of the electrolyte salt is not particularly limited, but can be selected arbitrarily within the range of 0.1 mol / L to 2.5 mol / L, and a common concentration such as 1 mol / L may be used. Furthermore, an electrolyte additive such as fluoroethylene carbonate, vinylene carbonate, hydrofluoroether, or biphenyl may be added to the electrolyte.

[0026] The electrolyte 10 may be a gel electrolyte obtained by impregnating a polymer compound with a solvent to form a gel, an ionic liquid, a symmetric glycol diether such as glyme, or a chain sulfone. Examples of the polymer compound include vinylidene fluoride-based polymers such as polyvinylidene fluoride, vinylidene fluoride-tetrafluoroethylene copolymer, and vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, acrylonitrile-based polymers such as acrylonitrile-methyl methacrylate copolymer, and further, polyethylene oxide, ethylene oxide-propylene oxide copolymer, and polymers of these acrylates or methacrylates impregnated with an electrolytic solution.

[0027] [Secondary battery manufacturing method] The method for manufacturing a secondary battery according to the present invention is a method for manufacturing the aforementioned secondary battery 1, and is a method for manufacturing a secondary battery having at least a positive electrode 4, a negative electrode 6, and an electrolyte 10. The secondary battery 1 manufactured by this method has an AC ratio of 1.5 or more, and is therefore characterized by little capacity loss due to repeated charge and discharge.

[0028] A typical manufacturing method for a lithium ion secondary battery includes an electrode forming step, an electrode and separator lamination step, an electrolyte injection / impregnation step, an electrode extraction step, an exterior packaging step, etc. In the present invention, a typical manufacturing method for a lithium ion secondary battery can be used except that silicon or a silicon compound is used for the negative electrode, pre-doping is performed, and the above AC ratio is set under certain conditions.

[0029] The technical scope of the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention. [Example]

[0030] The present invention will be further described in more detail below with reference to examples and comparative examples.

[0031] [Example 1] (Secondary battery production) 7.5g of silicon powder, 1.5g of acetylene black, and 7.7g of a 13wt% aqueous solution of polyacrylonitrile copolymer with an average molecular weight of 2,300,000 were weighed out, and purified water was added to adjust the viscosity to prepare a negative electrode ink for coating. This ink was applied to a 30μm thick, 130mm wide, 12μm thick copper foil using a knife roll coater and dried at 80°C. A 20μm thick negative electrode active material layer was then formed on the copper foil current collector by roll pressing, resulting in a negative electrode with silicon as the active material. The capacity density of this negative electrode was 8.8mAh / cm. 2 It was.

[0032] The resulting negative electrode was cut into a length of 20 mm and a width of 50 mm, and a tab for electrode extraction was welded to the uncoated portion. Next, in a dry room with a dew point of -45 °C or less, a 100 μm thick nonwoven fabric was laminated, and an electrolyte consisting of a mixed solution of 10 vol% fluoroethylene carbonate, 27 vol% ethylene carbonate, and 63 vol% diethyl carbonate containing 1 M LiPF5 was dripped onto it. A Li foil-laminated copper foil (Li foil thickness 30 μm) cut into a length of 25 mm and a width of 55 mm was laminated. An insulating stainless steel plate was placed on the outside of the electrode and the Li foil-laminated copper foil, and a pressure of 0.01 MPa was applied to both stainless steel plates. Under pressure, a constant current electrolysis reaction was performed on the negative electrode at a current of 0.37 mA up to 0.01 V. After completion of the reaction, the negative electrode, which had turned black, was removed, washed with dimethyl carbonate, and air-dried in a dry room to obtain a negative electrode made of silicon pre-doped with lithium. The amount of pre-doping calculated from the amount of current flowing during pre-doping was 0.1 mol per silicon.

[0033] Next, an N-methylpyrrolidone (NMP) solution containing 9.0 g of lithium iron phosphate, 0.5 g of acetylene black, and 0.5 g of polyvinylidene fluoride (PVDF) was weighed out and kneaded with NMP to form a mixture. NMP was further added to this mixture to adjust the viscosity, producing a positive electrode slurry for coating. This positive electrode slurry was applied to a 60 μm thick, 130 mm wide, 12 μm thick aluminum foil using a knife roll coater and dried at 120 °C to obtain a single-sided coated electrode. The same positive electrode slurry was then applied to the back side to a 60 μm thick coating, obtaining a double-sided coated foil. The resulting double-sided coated electrode was then roll-pressed to form a 50 μm thick positive electrode active material layer on each side of the aluminum foil current collector, yielding a positive electrode using lithium iron phosphate as the positive electrode active material. The capacity density of this positive electrode was 3.4 mAh / cm. 2 It was.

[0034] The positive electrode prepared as described above was punched out to a diameter of 13 mm and placed on the positive electrode case of a coin-type battery. A polyolefin separator film was then laminated on top of it. Furthermore, a negative electrode made of lithium-predoped silicon punched out to a diameter of 14 mm was laminated on top of it, and an electrolyte consisting of a 1M LiPF6-containing solution containing 10% by volume of fluoroethylene carbonate, 27% by volume of ethylene carbonate, and 63% by volume of diethyl carbonate was impregnated. Next, as shown in Figure 1, a negative electrode 6 and a metal spring 8 were placed on top of it, and a negative electrode member with a gasket 9 covering the periphery was placed on top of it, and the exterior was sealed using a crimping machine. In this way, a sealed coin-type secondary battery 1 was fabricated using abundantly available materials: lithium iron phosphate as the positive electrode active material and lithium-predoped silicon as the negative electrode active material. The AC ratio of this secondary battery was 2.6.

[0035] (Secondary battery operation check) The fabricated secondary battery was charged at a constant current of 0.5 mA until the voltage reached 4.2 V, and then discharged at a constant current of 0.5 mA until the voltage reached 2.0 V. As a result, it was confirmed that this cell was a secondary battery with a discharge capacity of 3.8 mAh. After that, it was repeatedly charged and discharged in the range of 2.0 to 4.2 V, and the capacity density remained at 90% or more of the initial value even after 100 cycles, confirming that this was a secondary battery with a long cycle life and little capacity loss even after repeated charge and discharge.

[0036] [Comparative Example 1] (Secondary battery production) The negative electrode ink for coating in Example 1 was applied to copper foil 130 mm wide and 12 μm thick using a knife roll coater in the same manner as in Example 1, except that the coating thickness was changed from 30 μm to 15 μm and 12 μm, and then dried at 80°C. Subsequently, by roll pressing, negative electrode active material layers of 10 μm and 8 μm thick were formed on the copper foil current collector, yielding negative electrodes using silicon as the active material. The capacity densities of these negative electrodes were 4.4 mAh / cm. 2 , and 3.5mAh / cm 2 It was.

[0037] The obtained negative electrode was cut into a length of 20 mm and a width of 50 mm as in Example 1, and a tab for electrode extraction was welded to the uncoated portion. Next, in a dry room with a dew point of -45 ° C or less, a nonwoven fabric was laminated in the same manner as in Example 1, an electrolyte solution was dripped, and a lithium foil-laminated copper foil (Li foil thickness 30 μm) was further laminated. An insulating stainless steel plate was placed on the outside of the electrode and the Li foil-laminated copper foil thus prepared, and a pressure of 0.01 MPa was applied to both sides of the stainless steel plate. Under pressure, a constant current electrolysis reaction was carried out on the negative electrode at a current of 0.37 mA up to 0.01 V. After the reaction was completed, the negative electrode, which had turned black, was removed, washed with dimethyl carbonate, and air-dried in a dry room to obtain a negative electrode made of silicon pre-doped with lithium. The pre-doping amount calculated from the amount of current flowing during pre-doping was 0.1 mol per silicon.

[0038] As described above, except for using a pre-doped negative electrode prepared with different coating thicknesses, sealed coin-type secondary batteries were fabricated using abundantly available materials, including lithium iron phosphate as the positive electrode active material and lithium-pre-doped silicon as the negative electrode active material, in the same manner as in Example 1. The AC ratios of these secondary batteries were 1.29 and 1.03, respectively.

[0039] (Secondary battery operation check) The fabricated secondary batteries were charged and discharged in the same manner as in Example 1. As a result, it was confirmed that both of these cells were 3.8 mAh secondary batteries (FIG. 2). After that, when they were repeatedly charged and discharged in the range of 2.0 to 4.2 V, the capacity density was 80% or less of the initial value after 100 cycles, confirming that the secondary batteries' capacity decreased with repeated charging and discharging.

[0040] Comparative Example 2 (Secondary battery production) A sealed coin-type secondary battery was fabricated by laminating a positive electrode and a separator film in the same manner as in Example 1, except that a negative electrode using silicon as the active material was used without pre-doping instead of the negative electrode made of silicon pre-doped with lithium in Example 1. The AC ratio of this battery was 2.98.

[0041] (Secondary battery operation check) The fabricated secondary batteries were charged and discharged in the same manner as in Example 1. As a result, it was confirmed that both cells were 3.0 mAh secondary batteries, which was 20% lower than the capacity predicted from the basis weight of the positive electrode. After that, when they were repeatedly charged and discharged in the range of 2.0 to 4.2 V, the capacity density was 80% or less of the initial value after 100 cycles, confirming that the secondary batteries' capacity decreased with repeated charging and discharging.

[0042] [Example 2] (Secondary battery production) 1.8g of silicon powder, 7.2g of artificial graphite, 1.0g of acetylene black, and KF Polymer L#1100 manufactured by Kureha Corporation were weighed out, and NMP was added to adjust the viscosity to prepare a negative electrode ink for coating. This ink was applied to a 60μm thick, 130mm wide, 12μm thick copper foil using a knife roll coater, and dried at 80°C. A 40μm thick negative electrode active material layer was then formed on the copper foil current collector by roll pressing, yielding a negative electrode with silicon and artificial graphite as active materials. The capacity density of this negative electrode was 7.2mAh / cm 2 It was.

[0043] The obtained negative electrode was cut out by the method of Example 1, and a tab for electrode extraction was welded in the same manner as in Example 1. An electrolyte solution was dropped and a Li foil-laminated copper foil (Li foil thickness 30 μm) was laminated. Next, a constant current electrolysis reaction was carried out in the same manner as in Example 1. After the reaction was completed, the negative electrode, which had turned golden, was removed, washed with dimethyl carbonate, and air-dried in a dry room to obtain a negative electrode made of silicon and artificial graphite pre-doped with lithium. The pre-doping amount calculated from the amount of current flowing during pre-doping was 0.2 mol per silicon.

[0044] A sealed coin-type secondary battery was fabricated using lithium iron phosphate as the positive electrode active material and lithium pre-doped silicon and artificial graphite as the negative electrode active material, which are abundant resources, in the same manner as in Example 1, except that a negative electrode made of lithium pre-doped silicon and artificial graphite was used instead of the negative electrode made of lithium pre-doped silicon as described above. The AC ratio of this secondary battery was 2.12.

[0045] (Secondary battery operation check) The fabricated secondary battery was charged at a constant current of 0.6 mA until the voltage reached 4.2 V, and then discharged at a constant current of 0.6 mA until the voltage reached 2.0 V. As a result, it was confirmed that this cell was a secondary battery with a discharge capacity of 3.7 mAh. After that, it was repeatedly charged and discharged in the range of 2.0 to 4.2 V, and the capacity density remained at 90% or more of the initial value even after 100 cycles, confirming that this was a secondary battery with a long cycle life and little capacity loss even after repeated charge and discharge. [Explanation of symbols]

[0046] 1 Secondary battery 2 Positive electrode case 3 Negative electrode case 4 Positive electrode 5 Separator 6 negative electrode 7 Negative electrode current collector 8 Metal Springs 9 Gasket 10 Electrolytes

Claims

1. A secondary battery comprising at least a positive electrode, a negative electrode, and an electrolyte as constituent elements, wherein the positive electrode contains olivine-type lithium iron phosphate as a positive electrode active material, the negative electrode contains silicon pre-doped with lithium or a silicon compound as a negative electrode active material, and the silicon pre-doped with lithium or the silicon compound is pre-doped with 0.02 mol or more of lithium per silicon or silicon compound, and the ratio (AC ratio) of the capacity per unit area of ​​the negative electrode to the capacity per unit area of ​​the positive electrode is 1.5 or more.

2. A secondary battery as described in claim 1, wherein the AC ratio is 1.5 or more and 4.12 or less.

3. 3. The secondary battery according to claim 1, wherein the silicon or silicon compound pre-doped with lithium is pre-doped with 0.02 mol or more and 0.2 mol or less of lithium per silicon or silicon compound.

4. 3. The secondary battery according to claim 1, wherein the silicon or silicon compound pre-doped with lithium is silicon.

Images