Secondary battery

Abstract

[Problem] To provide a secondary battery which is composed of a material having a rich amount of resources, has a high energy density, and does not readily decrease in capacity even when repeatedly charged and discharged. [Solution] The problem is solved by a secondary battery having at least a positive electrode, a negative electrode, and an electrolyte, the positive electrode being an olivine-type phosphate, the negative electrode containing silicon or a silicon compound pre-doped with lithium, and the electrolyte being a lithium-containing electrolyte. 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.

Description

TECHNICAL FIELD

[0001] The present application relates to a secondary battery. More specifically, the present application relates to a secondary battery characterized by comprising at least a positive electrode, a negative electrode, and an electrolyte, the positive electrode being an olivine-type phosphate, the negative electrode being a negative electrode comprising silicon or a silicon compound pre-doped with lithium, and a ratio (AC ratio) of a capacity per unit area of the negative electrode to a capacity per unit area of the positive electrode being 1.5 or more, the secondary battery being formed of a material that does not have a resource problem, having a large energy density and excellent stability, and not easily reducing the capacity even if charging and discharging are repeated. BACKGROUND

[0002] With the expansion of the market for portable electronic devices, electric vehicles, and the like, high energy density is required for batteries used for them. Heretofore, a secondary battery that uses alkali metal ions such as lithium as charge carriers and utilizes electrochemical reactions accompanying the charge and discharge thereof has been developed. In particular, a lithium ion secondary battery is widely popular because of its large energy density.

[0003] In a lithium ion secondary battery, a lithium-containing transition metal oxide is used as a positive electrode active material, and graphite is used as a negative electrode active material. Charging and discharging are performed by lithium ion intercalation and deintercalation reactions with respect to these positive and negative electrode active materials. In such a lithium ion secondary battery, development of new technologies that achieve further high energy density is required. In response to such a requirement, high capacity density of the negative electrode, reduction of irreversible capacity, and the like have been studied. For example, a secondary battery composed of a negative electrode having a lithium storage layer of silicon or a silicon compound capable of reacting with a large number of electrons and having a theoretical capacity of 2 times or more that of graphite is disclosed in Patent Literature 1. A secondary battery having reduced irreversible capacity by electrochemically pre-doping lithium to an electrode layer mainly composed of silicon under pressure is disclosed in Patent Literature 2.

[0004] However, cobalt, nickel, manganese, and the like, which are lithium-containing transition metal oxides constituting the positive electrode of a lithium ion battery, have limited reserves as resources, and there is a problem in that the material price becomes high if the use amount increases. Therefore, as a positive electrode active material that does not use these rare metals, lithium iron phosphate has attracted attention (Patent Literature 2). This compound uses iron as a base metal as a raw material, and thus has a large resource amount, and the capacity density as an electrode active material is 140 to 170 Ah / kg, which is equivalent to that of lithium cobaltate. However, the discharge voltage is 2.8 V, which is only 70% of that of lithium cobaltate, and thus the energy density is also only about 70%.

[0005] PRIOR ART DOCUMENTS

[0006] PATENT LITERATURE

[0007] Patent Literature 1: Japanese Patent Application Laid-Open No. 2004-87251

[0008] Patent Document 2: Japanese Patent No. 7170330

[0009] Patent Document 3: Japanese Patent Application Publication No. 2002-110162 Summary of the Invention

[0010] The problem the invention aims to solve

[0011] In Patent Document 1, high-capacity silicon is used as the active material, but its irreversible capacity is larger than that of graphite, resulting in significant volume changes during charge and discharge, thus leading to insufficient capacity and charge-discharge cycle life. In Patent Document 2, silicon anodes are pre-doped electrochemically to compensate for the capacity loss due to irreversible capacity, and a stable passivation layer is formed to improve charge-discharge cycle life. However, this requires the continued use of rare metals such as cobalt and nickel, which have limited reserves.

[0012] Thus, there remains a challenge of discovering a high-energy-density secondary battery made from abundant materials with improved charge-discharge cycle life.

[0013] This invention was made to solve the above-mentioned problems, and its purpose is to provide a secondary battery with high energy density made of abundant resources, whose capacity is not easily reduced even after repeated charging and discharging.

[0014] Solution for solving the problem

[0015] The secondary battery of the present invention is characterized in that it comprises at least a positive electrode, a negative electrode and an electrolyte, wherein the positive electrode active material constituting the aforementioned positive electrode is an olivine-type phosphate, the aforementioned negative electrode is a negative electrode containing silicon or a silicon compound pre-doped with lithium, and the ratio of the capacity per unit area of ​​the aforementioned negative electrode to the capacity per unit area of ​​the aforementioned positive electrode (AC ratio) is 1.5 or more.

[0016] The effects of the invention

[0017] According to the present invention, in a secondary battery comprising at least a positive electrode, a negative electrode, and an electrolyte, by using an olivine-type phosphate as the positive electrode, it is possible to obtain an active material that does not require materials with limited reserves, such as cobalt and nickel. Furthermore, by including lithium-doped silicon or silicon compounds in the negative electrode, there is no irreversible capacity, resulting in an increase in the overall energy density of the battery. Moreover, by setting the AC ratio to 1.5 or higher, it is possible to produce a battery with excellent stability, exhibiting minimal capacity reduction even after repeated charge-discharge cycles.

[0018] By implementing this invention, it is possible to achieve batteries with the same energy density, long charge-discharge cycle life, and excellent stability as batteries that use materials such as cobalt and nickel, which have limited reserves, without using such materials. Attached Figure Description

[0019] Figure 1 This is a cross-sectional view illustrating an example of the secondary battery of the present invention.

[0020] Figure 2 This is a graph showing the voltage changes during repeated charge-discharge cycles (1 to 10 times) of the secondary battery manufactured in Example 1. Detailed Implementation

[0021] Hereinafter, embodiments for carrying out the present invention will be described with reference to the accompanying drawings. However, the scope of the present invention should be determined based on the scope of the claims and is not limited to the following embodiments.

[0022] [Rechargeable Battery]

[0023] The secondary battery of the present invention is characterized in that it has at least a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode is an olivine-type phosphate, the negative electrode is a silicon or 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 (referred to as the "AC ratio") is 1.5 or more. The form of the secondary battery is not particularly limited. Figure 1 An example is a coin-shaped secondary battery. This example has a positive electrode 4, a negative electrode 6, and an electrolyte 10, and is composed of a positive electrode housing 2, a negative electrode housing 3, a gasket 9, a separator 5, and a metal spring 8.

[0024] (Construction elements)

[0025] The positive electrode casing 2 serves as the outer casing of the positive electrode 4 and functions as a positive electrode current collector. An electrode layer composed of a positive electrode active material capable of electrochemical oxidation-reduction and a conductive material is disposed at the center of the bottom of the positive electrode casing 2. A separator 5, composed of a porous sheet or film such as a microporous membrane, non-woven fabric, or woven cloth, is stacked on the positive electrode 4. Furthermore, a negative electrode 6 is disposed on the separator. A negative electrode current collector 7 made of metal is stacked on the negative electrode 6. Furthermore, a metal spring 8 is mounted on the negative electrode current collector 7. Here, a coin-shaped secondary battery is described, but the battery shape is not particularly limited; cylindrical, square, and flat shapes are also applicable. Additionally, the outer casing method is not particularly limited; metal casings, molding resins, aluminum-plastic laminates, etc., can be used.

[0026] exist Figure 1In this configuration, the negative electrode housing 3 is fixed to the positive electrode housing 2 against the force of the metal spring 8. Within the internal space formed by the fixing of the positive electrode housing 2 and the negative electrode housing 3, the positive electrode 4, the separator 5, the negative electrode 6, and the negative electrode current collector 7 are stacked and filled with electrolyte 10. The gap between the positive electrode housing 2 and the negative electrode housing 3 forming the internal space is sealed by a gasket 9.

[0027] In this invention, the AC ratio is 1.5 or higher. Typically, in secondary batteries, the positive and negative electrodes are used with opposite sides. Therefore, the capacity ratio per unit area of ​​the opposite electrodes affects the battery characteristics. In this invention, the overall capacity of the positive and negative electrodes is not particularly limited. Furthermore, the thickness, density, and weight per unit area of ​​the positive and negative electrodes are not particularly limited. In this invention, the capacity per unit area with the AC ratio is the design capacity, calculated from the amount of electrode active material in each electrode layer and the theoretical capacity. Specific examples of theoretical capacity include 169.9 mAh / g for lithium iron phosphate and 4199 mAh / g for silicon. For specific calculations, the active material concentration composed of lithium iron phosphate is 90 wt.%, and the weight per unit area is 20 mg / cm³. 2 In the positive electrode, the capacity density is 3.06 mA / cm³. 2 In a negative electrode composed of silicon with an active material concentration of 75 wt.% and a unit area weight of 4 mg / cm², the capacity density is 12.6 mA / cm². 2 The AC ratio at this point is 12.6 / 3.06 = 4.12. In this invention, a mixture of silicon and graphite can also be used as the negative electrode. The capacity in this case can be calculated using their respective theoretical capacities and mixing ratios. For example, a graphite to silicon mixing ratio of 9:1 and an active material concentration of 94 wt.% and a unit area weight of 8 mg / cm² are used. 2 In the negative electrode, the capacity density is 8.54, and the AC ratio is 2.79.

[0028] (Positive and negative electrodes)

[0029] In this invention, the main electrode active material constituting the positive electrode is 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 this invention, from the viewpoint of resource considerations and the effectiveness of the invention, olivine-type lithium iron phosphate with M = Fe is particularly preferred.

[0030] In this invention, the positive electrode 4 can be manufactured using conventionally known methods. That is, it can be manufactured by mixing a positive electrode active material such as lithium iron phosphate with a conductive material and a binder, adding a solvent to make a slurry, coating the slurry onto an electrode foil that serves as a current collector using conventionally known methods, and then drying it.

[0031] In this invention, the negative electrode 6 can be fabricated using conventionally known methods. The electrode active material constituting the negative electrode is lithium-doped silicon or a silicon compound. Examples of silicon or silicon compounds include Si, SiO, and SiOC. In this invention, the negative electrode can be formed using the same steps as conventional methods: for example, mixing the electrode active material with a conductive material (graphite, etc.) and a binder, adding a solvent to prepare a slurry, coating the slurry onto an electrode foil that serves as a current collector using conventionally known methods, and then drying it. In this invention, there are no particular limitations on the electrode foil that serves as the negative electrode current collector; however, copper foil and stainless steel foil are preferred from the perspective of the invention's effectiveness. In this invention, the electrode composed of pre-doped silicon or a silicon compound can be an electrode pre-coated with pre-doped silicon or a silicon compound, or an electrode pre-doped after coating with silicon or a silicon compound.

[0032] In this invention, the silicon or silicon compound pre-doped with lithium is not particularly limited, but from the viewpoint of achieving the desired effect, it is preferable to pre-dope with 0.02 mol or more of lithium per unit of silicon or silicon compound. In the case of electrolytic doping of pre-doped silicon, 0.02 mol per unit of active material corresponds to 19.1 Ah / kg, so it is preferred to achieve this or higher. In this invention, by performing pre-doping, irreversible capacity can be reduced, compensating for the loss of lithium during repeated charge-discharge cycles, thus resulting in a high-energy, long-life secondary battery.

[0033] In this invention, the conductive materials used in the fabrication of the positive and negative electrodes are not particularly limited. Examples include carbon microparticles such as carbon black, Ketjen black, and acetylene black; carbon fibers such as vapor-grown carbon fibers, carbon nanotubes, and carbon nanofibers; and carbon sheets such as graphene. Two or more of these conductive materials can be used in combination as needed. Furthermore, the solvents are not particularly limited. Examples include non-protic solvents such as N-methylpyrrolidone, dimethyl sulfoxide, dimethylformamide, propylene carbonate, diethyl carbonate, dimethyl carbonate, γ-butyrolactone, acetonitrile, tetrahydrofuran, nitrobenzene, and acetone; methanol; ethanol; and water. The binder is not particularly limited; it can be any resin that binds the electrode active material and the conductive material. Examples include various resins such as polyethylene, polyvinylidene fluoride, polyhexafluoropropylene, polytetrafluoroethylene, polyethylene oxide, carboxymethyl cellulose, styrene-butadiene rubber, polyacrylic acid, and polyimide resin.

[0034] According to the research of the inventors, when using negative electrode active materials such as graphite and silicon, if the AC ratio is greater than 1 during charging, the proportion of irreversible capacity increases, resulting in a decrease in battery capacity. Furthermore, when the AC ratio is below 1, lithium is deposited on the electrode during charging. Therefore, the AC ratio is typically used in the range of approximately 1 to 1.05. The present invention has discovered that by pre-doping silicon or silicon compounds, the irreversible capacity is reduced. In this case, if the AC ratio is 1.5 or higher, the capacity reduction accompanying repeated charge and discharge cycles becomes smaller, thus completing the present invention.

[0035] (electrolytes)

[0036] In this invention, the electrolyte 10 is located between the positive electrode 4 and the negative electrode 6 and facilitates charge carrier transport between the two electrodes. It is not particularly limited as long as it has ionic conductivity, and can use electrolytes with a conductivity of 10 at room temperature. -6 Electrolytes in liquid, gel, and solid forms with an ionic conductivity of S / cm or higher are preferred. In this invention, from the perspective of ease of reaction, electrolytes in liquid and gel forms are preferred as electrolytes. In the liquid state, it is an organic solvent containing an electrolyte salt. Examples of electrolyte salts include LiPF6, LiClO4, LiBF4, LiCF3SO3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiC(CF3SO3)3, and LiC(C2F5SO2)3. Examples of organic solvents include ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, γ-butyrolactone, tetrahydrofuran, dioxolane, dimethyl sulfone, ethyl sulfone, diethyl sulfone, propyl methyl sulfone, isopropyl methyl sulfone, propyl ethyl sulfone, isopropyl ethyl sulfone, dipropyl sulfone, diisopropyl sulfone, sulfolane, pentamethyl sulfone, hexamethylene sulfone, 3-methylcyclobutane sulfone, 2,4-dimethylcyclobutane sulfone, N,N-dimethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone, and / or mixtures thereof.

[0037] In this invention, the concentration of the electrolyte salt is not particularly limited and can be arbitrarily selected within the range of 0.1 mol / L to 2.5 mol / L, or a common concentration such as 1 mol / L can be used. Additionally, electrolyte additives such as fluoroethylene carbonate, and / or vinylene carbonate, hydrofluoroether, and biphenyl can be added to the electrolyte.

[0038] Electrolyte 10 can also be a gel electrolyte, an ionic liquid, a symmetrical glycol diether such as ethylene glycol dimethyl ether, or a chain sulfone, in which a polymer compound is contained in a solvent to form a gel. Examples of polymer compounds include polyvinylidene fluoride, polyvinylidene fluoride-tetrafluoroethylene copolymer, polyvinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, acrylonitrile-methyl methacrylate copolymer, and other acrylonitrile-based polymers, as well as polyethylene oxide, ethylene oxide-propylene oxide copolymer, and other substances formed by containing an electrolyte in these acrylate or methacrylate polymers.

[0039] [Manufacturing method of secondary batteries]

[0040] The manufacturing method of the secondary battery of the present invention is a method for manufacturing the described secondary battery 1, which 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 higher, and therefore has the characteristic of minimal capacity reduction with repeated charging and discharging.

[0041] The conventional manufacturing method for lithium-ion secondary batteries consists of an electrode formation process, an electrode and separator stacking process, an electrolyte injection / impregnation process, an electrode lead-out process, and an assembly process. This invention, except that it uses silicon or a silicon compound as the negative electrode and sets the AC ratio to a certain condition through pre-doping, can utilize the conventional manufacturing method for lithium-ion secondary batteries.

[0042] It should be noted that the technical scope of the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention.

[0043] Example

[0044] The present invention will be further described below with examples and comparative examples.

[0045] [Example 1]

[0046] (Making a secondary battery)

[0047] 7.5 g of silicon powder, 1.5 g of acetylene black, and 7.7 g of a 13 wt.% aqueous solution of polyacrylonitrile copolymer with an average molecular weight of 2,300,000 were weighed out. Purified water was added to adjust the viscosity to prepare the negative electrode ink for coating. This ink was coated onto a 12 µm thick copper foil using a roller coater to a thickness of 30 µm and a width of 130 mm, and dried at 80 °C. Then, by rolling, a 20 µm thick layer of negative electrode active material was formed on the copper foil, which serves as the current collector, resulting in a silicon-based negative electrode. The capacity density of this negative electrode was 8.8 mAh / cm³. 2 .

[0048] The obtained negative electrode was cut into pieces 20 mm long and 50 mm wide, and electrode tabs for lead-out were welded to the uncoated portion. Next, a 100 µm thick nonwoven fabric was laminated in a drying chamber with a dew point below -45°C. An electrolyte solution consisting of a mixture of 10 vol% fluoroethylene carbonate, 27 vol% ethylene carbonate, and 63 vol% diethyl carbonate containing 1 M LiPF5 was added. Li foil cut into pieces 25 mm long and 55 mm wide and bonded to copper foil (Li foil thickness 30 µm) was then further laminated. An insulated stainless steel plate was overlapped on the outside of the electrode and the Li foil bonded to copper foil, 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 performed on the negative electrode at 0.37 mA until 0.01 V was reached. After the reaction, the negative electrode, which had turned black, was removed, washed with dimethyl carbonate, and air-dried in a drying chamber to obtain a negative electrode composed of lithium-doped silicon. The predoping amount, calculated based on the current flowing through it, is 0.1 mol per unit of silicon.

[0049] 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 mixed with NMP to form a mixture. Further NMP was added to this mixture to adjust the viscosity, thus preparing a positive electrode slurry for coating. This positive electrode slurry was coated onto a 12 µm thick aluminum foil using a roller coater to a thickness of 60 µm and a width of 130 mm, and dried at 120 °C to obtain a single-sided coated electrode. Then, the same positive electrode slurry was used to coat the back side to a thickness of 60 µm, resulting in a double-sided coated foil. The double-sided coated electrode was then rolled to form a 50 µm thick positive electrode active material layer on each side of the aluminum foil, which serves as the current collector, resulting in a positive electrode with lithium iron phosphate as the positive electrode active material. The capacity density of this positive electrode is 3.4 mAh / cm³. 2 .

[0050] The positive electrode, prepared as described above, is punched to a diameter of 13 mm and placed on the positive electrode casing of a coin-shaped battery. A polyolefin-based separation membrane is then laminated on top of this membrane. Next, a negative electrode, punched to a diameter of 14 mm and made of lithium-predoped silicon, is laminated and impregnated in an electrolyte solution consisting of a mixture of 10 vol% fluoroethylene carbonate, 27 vol% ethylene carbonate, and 63 vol% diethyl carbonate containing 1 M LiPF6. Then, as... Figure 1 As shown, a negative electrode 6 and a metal spring 8 are mounted, and the negative electrode side component, with its periphery covered by a gasket 9, is overlapped and externally sealed using a riveting machine. Thus, a sealed coin-shaped secondary battery 1, using abundant materials, is manufactured, comprising lithium iron phosphate as the positive electrode active material and pre-doped lithium silicon as the negative electrode active material. The AC ratio of this secondary battery is 2.6.

[0051] (Confirmation of secondary battery operation)

[0052] The fabricated secondary battery was charged at a constant current of 0.5mA to a voltage of 4.2V, and then discharged at a constant current of 0.5mA to 2.0V. This confirmed that the battery cell was a secondary battery with a discharge capacity of 3.8mAh. Further charge-discharge cycles within the range of 2.0 to 4.2V were performed, and the capacity density remained above 90% of the initial value after 100 cycles, confirming it as a long-cycle-life secondary battery with minimal capacity reduction even after repeated charge-discharge cycles.

[0053] [Comparative Example 1]

[0054] (Making a secondary battery)

[0055] Except that the negative electrode ink used for coating in Example 1 was coated with 15µm and 12µm instead of the 30µm coating thickness using a razor roller coater, it was coated on a 12µm thick copper foil with a width of 130mm, just like in Example 1, and dried at 80°C. Then, by rolling, negative electrode active material layers with thicknesses of 10µm and 8µm were formed on the copper foil, which serves as the current collector, to obtain a negative electrode with silicon as the active material. The capacity density of this negative electrode was 4.4mAh / cm³. 2 and 3.5mAh / cm 2 .

[0056] The obtained negative electrode was cut into 20 mm long and 50 mm wide sections, similar to that in Example 1, and electrode tabs for lead-out were welded to the uncoated portion. Next, in a drying chamber with a dew point below -45°C, nonwoven fabric was layered using the same method as in Example 1, electrolyte was added, and lithium foil was further layered onto copper foil (Li foil thickness 30 µm). An insulating stainless steel plate was overlapped on the outside of the electrode and the Li foil-coated 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 until 0.01 V was reached. After the reaction, the negative electrode, which had turned black, was removed, washed with dimethyl carbonate, and air-dried in a drying chamber to obtain a negative electrode composed of pre-doped lithium silicon. The pre-doping amount, calculated based on the current flowing through the pre-doping, was 0.1 mol per unit of silicon.

[0057] As described above, except for the use of a pre-doped negative electrode fabricated with varying coating thickness, a closed-cell coin-type secondary battery using abundant materials was fabricated using the same method as in Example 1. This secondary battery consisted of lithium iron phosphate as the positive electrode active material and lithium-doped silicon as the negative electrode active material. The AC ratios of these secondary batteries were 1.29 and 1.03, respectively.

[0058] (Confirmation of secondary battery operation)

[0059] The fabricated secondary batteries were charged and discharged using the method described in Example 1. The results confirmed that these battery cells were all 3.8mAh secondary batteries. Figure 2 Then, the battery was repeatedly charged and discharged within the range of 2.0 to 4.2V. As a result, the capacity density became less than 80% of the initial value after 100 cycles, confirming that it was a secondary battery whose capacity decreased with repeated charging and discharging.

[0060] [Comparative Example 2]

[0061] (Making a secondary battery)

[0062] Except for replacing the pre-doped lithium-silicon anode of Example 1 with an undoped silicon anode as the active material, a closed-cell coin-shaped secondary battery was fabricated by stacking a cathode, separating the membrane, and immersing it in an electrolyte, using the same method as in Example 1. The battery consisted of lithium iron phosphate as the positive electrode active material and undoped silicon as the negative electrode active material. The AC ratio of this battery was 2.98.

[0063] (Confirmation of secondary battery operation)

[0064] The fabricated secondary batteries were charged and discharged using the method described in Example 1. The results confirmed that these battery cells were all 3.0 mAh secondary batteries, which is 20% lower than the capacity expected based on the unit area weight of the positive electrode. Then, repeated charge-discharge cycles were performed within the range of 2.0 to 4.2 V, and the capacity density became less than 80% of the initial value after 100 cycles, confirming that these are secondary batteries whose capacity decreases with repeated charge-discharge cycles.

[0065] [Example 2]

[0066] (Making a secondary battery)

[0067] 1.8 g of silicon powder, 7.2 g of artificial graphite, 1.0 g of acetylene black, and KF polymer L#1100 (KUREHA Co., Ltd.) were weighed out, and NMP was added to adjust the viscosity to prepare the negative electrode ink for coating. This ink was coated onto a 12 µm thick copper foil using a roller coater to a thickness of 60 µm and a width of 130 mm, and dried at 80 °C. Then, by rolling, a 40 µm thick layer of negative electrode active material was formed on the copper foil, which serves as the current collector, resulting in a negative electrode with silicon and artificial graphite as active materials. The capacity density of this negative electrode was 7.2 mAh / cm³. 2 .

[0068] The negative electrode obtained in Example 1 was cut, and electrode tabs for lead-out were welded using the same method as in Example 1. Electrolyte was added, and Li foil was laminated with copper foil (Li foil thickness 30µm). Next, a constant current electrolysis reaction was performed using the same method as in Example 1. After the reaction, the negative electrode, which had turned golden yellow, was removed, washed with dimethyl carbonate, and air-dried in a drying chamber to obtain a negative electrode composed of lithium-doped silicon and artificial graphite. The pre-doping amount, calculated based on the current flowing through the pre-doping, was 0.2 mol per unit of silicon.

[0069] Except as described above, a closed-cell coin-type secondary battery using abundant materials was fabricated, consisting of lithium iron phosphate as the positive electrode active material, lithium iron phosphate as the positive electrode active material, and lithium-doped silicon and artificial graphite as the negative electrode active material. The AC ratio of this secondary battery was 2.12.

[0070] (Confirmation of secondary battery operation)

[0071] The fabricated secondary battery was charged at a constant current of 0.6mA to a voltage of 4.2V, and then discharged at a constant current of 0.6mA to 2.0V. This confirmed that the battery cell was a secondary battery with a discharge capacity of 3.7mAh. Further charge-discharge cycles within the range of 2.0 to 4.2V were performed, and the capacity density remained above 90% of the initial value after 100 cycles, confirming it as a long-cycle-life secondary battery with minimal capacity reduction even after repeated charge-discharge cycles.

[0072] Explanation of reference numerals in the attached figures

[0073] 1. Secondary battery

[0074] 2 Positive electrode casing

[0075] 3. Negative electrode casing

[0076] 4 Positive electrode

[0077] 5. Dividers

[0078] 6 Negative electrode

[0079] 7. Negative current collector

[0080] 8. Metal springs

[0081] 9 gaskets

[0082] 10 Electrolytes

Claims

1. A secondary battery, characterized in that, It comprises at least a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode is an olivine-type phosphate, the negative electrode comprises 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, i.e., the AC ratio, is 1.5 or more.

2. The secondary battery according to claim 1, wherein, The olivine-type phosphate is olivine-type lithium iron phosphate.

3. The secondary battery according to claim 1 or 2, wherein, The lithium-doped silicon or silicon compound is formed by predoping more than 0.02 mol of lithium per unit of silicon or silicon compound.

4. The secondary battery according to claim 1 or 2, wherein, The lithium-doped silicon or silicon compound is silicon.

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