Lithium secondary battery

Abstract

This invention provides a lithium secondary battery with excellent cycle characteristics. The invention relates to a lithium secondary battery comprising a Si-containing negative electrode active material and an electrolyte, wherein the average particle size of the negative electrode active material is 5.0 μm or more and 30 μm or less, and the electrolyte contains LiFSI, LiPF6, and LiPO2F2 as lithium salts, satisfying the following conditions when the molar fraction of LiFSI is set as x, the molar fraction of LiPF6 as y, and the molar fraction of LiPO2F2 as z, and x + y + z = 1: 0.05 ≤ x ≤ 0.95, 0.05 ≤ y ≤ 0.90, and 0.005 ≤ z ≤ 0.35, and the lithium ion concentration in the electrolyte is 0.7 M or more and 3.0 M or less.

Description

Technical Field

[0001] This invention relates to lithium secondary batteries. Background Technology

[0002] Previously, technologies that convert natural energy sources such as solar or wind power into electrical energy have received attention. Consequently, various secondary batteries have been developed as energy storage devices that are highly safe and capable of storing large amounts of electrical energy.

[0003] Among them, lithium-ion batteries, which are known to charge and discharge by the movement of lithium ions between the positive and negative electrodes, exhibit high voltage and high energy density. As a typical lithium-ion battery, there is a lithium-ion battery (LIB) that has active materials in both the positive and negative electrodes that can retain lithium elements, and charges and discharges by accepting and accepting lithium ions between these active materials.

[0004] In addition, in order to achieve high energy density and improve production, lithium secondary batteries (LMB: Lithium-metal battery) have been developed that use lithium metal as the negative electrode active material to replace materials that can intercalate lithium ions, such as carbon materials, and lithium secondary batteries (AFB: Anode-free battery) that use a negative electrode composed of a negative electrode current collector that does not have negative electrode active materials such as carbon materials or lithium metal.

[0005] On the other hand, in lithium-ion secondary batteries capable of achieving high voltage, the high reactivity of battery components and electrolyte leads to a decrease in cycle performance. Specifically, repeated charging and discharging cause side reactions between the negative electrode material, positive electrode material, and electrolyte, resulting in a reduction in battery capacity. Therefore, methods to prevent this capacity reduction are needed.

[0006] For example, in Patent Document 1, in order to improve the high-temperature storage performance, high-temperature cycle performance and power performance of lithium-ion batteries, an electrolyte containing an electrolyte salt and an organic solvent is disclosed. The electrolyte salt contains a lithium salt and the organic solvent contains a cyclic ether. When the mass fraction of the lithium salt relative to the electrolyte is set as W1 and the mass fraction of the cyclic ether relative to the electrolyte is set as W2, the condition 0.2≤W1 / W2≤1.06 is satisfied.

[0007] In addition, in Patent Document 2, in order to combine the low internal resistance, high temperature storage performance, and high temperature cycling performance of the battery, an electrolyte comprising an electrolyte salt and an organic solvent is disclosed. The electrolyte salt comprises lithium bis(fluorosulfonyl)imide, and the mass percentage of lithium bis(fluorosulfonyl)imide in the electrolyte is 4.5% to 11%. The organic solvent comprises ethylene carbonate, and the electrolyte satisfies 0.9 ≤ WLiFSI / (16.77%-WEC) ≤ 2.9 (where WLiFSI is the mass percentage of lithium bis(fluorosulfonyl)imide in the electrolyte, and WEC is the mass percentage of ethylene carbonate in the electrolyte).

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: Japanese Patent Publication No. 2023-537443

[0011] Patent Document 2: Japanese Patent Publication No. 2023-520319 Summary of the Invention

[0012] The problem the invention aims to solve

[0013] However, in lithium secondary batteries containing Si-containing active materials, Si tends to expand and contract easily when repeatedly charged and discharged, thus the decrease in capacity and cycle life becomes more significant.

[0014] The present invention was made in view of the above circumstances, and its object is to provide a lithium secondary battery with improved cycle characteristics.

[0015] Solution for solving the problem

[0016] One embodiment of the present invention relates to a lithium secondary battery comprising a Si-containing negative electrode active material and an electrolyte, wherein the average particle size of the negative electrode active material is 5.0 μm or more and 30 μm or less, and the electrolyte comprises LiFSI, LiPF6, and LiPO2F2 as lithium salts, wherein the molar fraction of LiFSI is set as x, the molar fraction of LiPF6 is set as y, the molar fraction of LiPO2F2 is set as z, and x+y+z=1, satisfying the ranges of 0.05≤x≤0.95, 0.05≤y≤0.90, and 0.005≤z≤0.35, and the concentration of lithium ions in the electrolyte is 0.7M or more and 3.0M or less.

[0017] Invention Effects

[0018] According to the present invention, a lithium secondary battery with excellent cycle characteristics can be provided. Attached Figure Description

[0019] Figure 1 This is a schematic diagram illustrating an example of the lithium secondary battery of the present invention. Detailed Implementation

[0020] The following describes in detail the method for implementing the present invention (hereinafter referred to as "this embodiment"), but the present invention is not limited to the following embodiment. Various modifications can be made to the present invention without departing from its spirit. Furthermore, in the accompanying drawings, the same reference numerals are used to label the same elements, and repeated descriptions are omitted. In addition, unless otherwise specified, positional relationships such as up, down, left, and right are based on the positional relationships shown in the accompanying drawings. Moreover, the scale of the accompanying drawings is not limited to the scale shown in the drawings.

[0021] 1. Lithium secondary battery

[0022] Reference Figure 1 The basic structure of a lithium secondary battery according to one embodiment of the present invention will be described. For example, the lithium secondary battery according to one embodiment of the present invention is as follows: Figure 1 As shown, it includes multiple negative electrodes 10, multiple separators 20, and multiple positive electrodes 30, with the negative electrodes 10 and positive electrodes 30 separated by the separators 20. The structure will be described in detail below.

[0023] 1.1 Negative electrode

[0024] The negative electrode 10 typically includes a negative current collector and a negative active material layer formed on the negative current collector. In this embodiment, the negative electrode 10 includes a negative active material layer and a negative current collector, wherein the negative active material layer contains a Si-containing negative active material (hereinafter also simply referred to as "Si active material").

[0025] The negative electrode active material refers to the material that undergoes electrode reactions, namely oxidation and reduction reactions, in the negative electrode 10. The negative electrode active material in this embodiment includes Si active material; however, it may also include other negative electrode active materials that do not contain Si, depending on the need. Si has a high theoretical lithium intercalation capacity per unit weight, and by including Si active material, a high-capacity lithium secondary battery can be achieved.

[0026] Si-based active materials are active materials containing the element Si, such as Si and Li. y -SiO x SiC, SiO x As a Si active material, it is preferred to contain Li. y -SiO x Or SiC, more preferably containing Li y -SiO x By incorporating such Si active materials, the battery exhibits excellent cycle characteristics and tends to achieve high capacity. Furthermore, it is compatible with SiO₂. xIn comparison, Li y -SiO x The irreversible capacity is small during the initial charge and discharge, which can improve the energy density, making it the preferred choice.

[0027] In this embodiment, the average particle size of the Si active material is 5.0 μm or more and 30 μm or less, preferably 7.0 μm or more and 20 μm or less, or 8.0 μm or more and 15 μm or less. By making the average particle size of the Si active material 5.0 μm or more, side reactions with electrolyte components are tended to be suppressed, thereby suppressing the increase in resistance; by making it 30 μm or less, the volume change of the charge-discharge battery during charging and discharging is tended to be suppressed. Furthermore, in this embodiment, the particle size distribution is measured using a laser diffraction particle size analyzer, namely the MT-3000 manufactured by MicrotracBEL.

[0028] Relative to the total amount of negative electrode active material, the content (amount) of Si active material is preferably 1% by mass or more and 90% by mass or less, 3% by mass or more and 80% by mass or less, or 5% by mass or more and 70% by mass or less. Setting the amount of Si active material used within the above range tends to achieve a balance between energy density and cycle life. From the same viewpoint, relative to 100 parts by mass of the negative electrode active material composition excluding solvent components, the content (amount) of Si active material is preferably 2 parts by mass or more and 90 parts by mass or less, 5 parts by mass or more and 80 parts by mass or less, or 7 parts by mass or more and 70 parts by mass or less.

[0029] Other anode active materials that do not contain Si include, for example: carbon-based active materials such as graphite, graphene, hard carbon, and carbon nanotubes; metal oxide-based active materials such as titanium oxide compounds and cobalt oxide compounds; and metal-alloy-based active materials consisting of germanium, tin, lead, aluminum, and gallium, as well as those pre-doped with lithium. From the viewpoint of making the present invention more effective and reliable, it is preferable to use carbon-based active materials together with Si active materials, and among carbon-based active materials, graphite is preferred.

[0030] Relative to the total amount of negative electrode active material, the content of the aforementioned other negative electrode active materials is preferably 10% by mass or more and 99% by mass or less, 30% by mass or more and 97% by mass or less, or 80% by mass or more and 95% by mass or less. Setting the content of other negative electrode active materials within the above range tends to achieve a balance between energy density and cycle life. From the same viewpoint, relative to 100 parts by mass of the negative electrode active material composition excluding the solvent component, the content (amount) of other negative electrode active materials is preferably 10 parts by mass or more and 95 parts by mass or less, 30 parts by mass or more and 90 parts by mass or less, or 50 parts by mass or more and 85 parts by mass or less.

[0031] The negative electrode active material layer can be formed by mixing the negative electrode active material with a solvent or additives, binders, conductive agents, etc., and then coating it onto one or both sides of the negative electrode current collector and pressing it in place. There are no particular limitations on the solvent; water can be used. There are also no particular limitations on the additives; for example, carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), etc., can be used as binders, and conductive agents such as carbon black, carbon nanofibers (VGCF), single-walled carbon nanotubes (SWCNTs), and multi-walled carbon nanotubes (MWCNTs) can also be used.

[0032] When forming the negative electrode active material layer, the negative electrode active material composition is coated onto one or both sides of the negative electrode current collector. The coating amount of the negative electrode active material composition is preferably 1 mg / cm² or more and 30 mg / cm² or less, 2 mg / cm² or more and 25 mg / cm² or less, or 3 mg / cm² or more and 20 mg / cm² or less, based on the unit area weight of one side. By keeping the coating amount of the negative electrode active material composition within the above range, the cycle characteristics of the battery tend to be further improved.

[0033] The thickness of the negative electrode active material layer is preferably 20 μm or more and 200 μm or less, 30 μm or more and 175 μm or less, or 40 μm or more and 150 μm or less. By keeping the thickness of the negative electrode active material layer within the above range, the cycle characteristics of the battery tend to be further improved.

[0034] Examples of current collectors for the negative electrode include current collector films having metal layers on both sides of a metal foil and a resin layer. For example, copper foil can be used as a metal foil. In this embodiment, from the viewpoints of ease of processing and high energy density, a current collector film having metal layers on both sides of the resin layer is preferred. As a current collector film for the negative electrode, a thin film having copper layers on both sides of the resin layer is preferred.

[0035] In the current collector film for the negative electrode, the resin layer preferably comprises at least one selected from polyethylene terephthalate (PET), polypropylene, polyamide, acrylic resin, polycarbonate, polyethylene, polyvinyl chloride, and polystyrene, and more preferably is composed of polyethylene terephthalate or polypropylene. By forming such a resin layer, the cycle characteristics of the battery tend to be further improved.

[0036] In the current collector film for the negative electrode, the resin layer is preferably in the form of a thin film (sheet), and its thickness is preferably 2.0 μm or more and 20 μm or less, 3.0 μm or more and 10 μm or less, or 4.0 μm or more and 7.0 μm or less. By keeping the thickness of the resin layer within the above range, the cycle performance of the battery tends to be further improved.

[0037] In the current collector film for the negative electrode, the metal layer is preferably a copper (Cu) layer, wherein the Cu content in the copper layer is 99% by mass or more, 99.9% by mass or more, 99.99% by mass or more, or 99.9999% by mass or more. The thickness of each of the aforementioned metal or copper layers is preferably 0.2 μm or more and 10 μm or less, 0.3 μm or more and 5.0 μm or less, 0.5 μm or more and 3.0 μm or less, or 0.8 μm or more and 2.0 μm or less. By keeping the thickness of the aforementioned metal or copper layers within the above ranges, the cycle characteristics of the battery tend to be further improved.

[0038] 1.2 Electrolyte

[0039] The electrolyte of this embodiment contains LiFSI, LiPF6, and LiPO2F2 as lithium salts. When the molar fraction of LiFSI is set as x, the molar fraction of LiPF6 as y, and the molar fraction of LiPO2F2 as z, and x + y + z = 1, the electrolyte satisfies the ranges of 0.05 ≤ x ≤ 0.95, 0.05 ≤ y ≤ 0.90, and 0.005 ≤ z ≤ 0.35, and the lithium ion concentration in the electrolyte is 0.7 M or more and 3.0 M or less. Furthermore, LiFSI refers to LiN(SO2F)2.

[0040] Previously, it was known that using Si-containing negative electrode active materials resulted in high lithium intercalation capacity per unit weight, making it easy to manufacture high-capacity lithium secondary batteries. On the other hand, Si is a material that expands significantly in volume due to repeated charging and discharging, leading to increased resistance and reduced battery life. The inventors conducted in-depth research on this issue and discovered that by using the type and concentration of salts in the electrolyte as described above, the effects of Si volume expansion can be mitigated, enabling the manufacture of long-life lithium secondary batteries.

[0041] The reasons are not necessarily clear, but are speculated as follows. In lithium-ion secondary batteries, the commonly used lithium salt LiPF6, while exhibiting excellent performance as an electrolyte, tends to exhibit active side reactions with Si under conditions of temperature rise due to repeated charge-discharge cycles, thus hindering its function as an electrolyte. Here, by simultaneously including LiFSI and LiPO2F2 as lithium salts, it is believed that even under temperature rise, it helps suppress electrolyte side reactions while maintaining ion dissociation. Furthermore, by ensuring that the ratio of LiFSI, LiPF6, and LiPO2F2 and the lithium ion concentration in the electrolyte meet the aforementioned ranges, not only can side reactions with Si be suppressed, but also side reactions with the cathode material 30 can be suppressed, thereby further improving the battery's cycle characteristics and capacity retention. Thus, the synergistic effect of the above-described structure of the lithium secondary battery in this embodiment results in excellent cycle characteristics. However, the reasons are not limited to the above.

[0042] 1.2.1 Lithium Salts

[0043] The electrolyte in this embodiment contains LiFSI, LiPF6, and LiPO4. ２ F ２ As a lithium salt, other salts may be included as needed. Examples of other salts include: LiI, LiCl, LiBr, LiF, and LiBF. ４ LiAsF6, LiSO ３ CF ３ LiN(SO) ２ CF ３ ) ２ LiN(SO) ２ CF ３ CF ３ ) ２ LiBF ２ (C ２ O ４ LiB(O) ２ C ２ H ４ ) ２ LiB(O) ２ C ２ H ４ )F ２ LiB(OCOCF) ３ ) ４ LiNO ３ Li ２ SO ４ Lithium salts, or salts of metals such as Na, K, Ca, and Mg.

[0044] In the electrolyte of this embodiment, when the mole fraction of LiFSI is set as x, the mole fraction of LiPF6 as y, and the mole fraction of LiPO2F2 as z, and x + y + z = 1, the ranges of 0.05 ≤ x ≤ 0.95, 0.05 ≤ y ≤ 0.90, and 0.005 ≤ z ≤ 0.35 are satisfied. The mole fraction x of LiFSI is preferably 0.06 ≤ x ≤ 0.90, 0.10 ≤ x ≤ 0.80, or 0.30 ≤ x ≤ 0.70. The mole fraction y of LiPF6 is preferably 0.06 ≤ y ≤ 0.70, 0.10 ≤ y ≤ 0.50, or 0.20 ≤ y ≤ 0.40. The mole fraction z of LiPO2F2 is preferably 0.008 ≤ z ≤ 0.30, 0.01 ≤ z ≤ 0.25, or 0.03 ≤ z ≤ 0.18. By keeping the molar fractions of LiFSI, LiPF6, and LiPO2F2 within the aforementioned ranges, side reactions with the negative and positive electrode materials are suppressed, and the battery's cycle characteristics tend to be superior.

[0045] The lithium-ion concentration in the electrolyte is 0.7M or higher and 3.0M or lower, preferably 0.8M or higher and 2.5M or lower, 1.0M or higher and 2.0M or lower, or 1.3M or higher and 1.7M or lower. By keeping the lithium-ion concentration within the above range, the battery's cycle characteristics tend to be better.

[0046] The concentration of LiFSI in the electrolyte is preferably 0.1 M or higher and 2.0 M or lower, 0.3 M or higher and 1.5 M or lower, or 0.5 M or higher and 1.2 M or lower. By keeping the concentration of LiFSI within the above range, the cycle characteristics of the battery tend to be better.

[0047] The concentration of LiPF6 in the electrolyte is preferably 0.1 M or more and 2.0 M or less, 0.2 M or more and 1.0 M or less, or 0.3 M or more and 0.8 M or less. By keeping the concentration of LiPF6 within the above range, the cycle characteristics of the battery tend to be better.

[0048] The concentration of LiPO2F2 in the electrolyte is preferably 0.01M or higher and 1.0M or lower, 0.03M or higher and 0.8M or lower, or 0.05M or higher and 0.4M or lower. By keeping the concentration of LiPO2F2 within the above range, the cycle characteristics of the battery tend to be better.

[0049] In the electrolyte, the concentrations of salts other than LiFSI, LiPF6, and LiPO2F2 are preferably 1.0 M or less, 0.8 M or less, or 0.6 M or less. By keeping the concentrations of other salts within the above ranges, the cycle characteristics of the battery tend to be further improved. The concentrations of other salts in the electrolyte can be, for example, 0 M or more, or 0 M or more, 0.0001 M or more, 0.001 M or more, or 0.005 M or more.

[0050] 1.2.2 Solvent

[0051] The solvent for the electrolyte is not particularly limited as long as it can dissolve the lithium salt composition of this embodiment. Examples of such solvents include: chain or cyclic carbonate compounds, ether compounds, and chain or cyclic fluorinated compounds in which one or more hydrogen atoms in the carbon skeleton are replaced by fluorine atoms. Furthermore, a single solvent or two or more solvents may be used in combination for the electrolyte.

[0052] As a solvent for the electrolyte, it is preferable to include at least one selected from chain carbonate compounds, cyclic carbonate compounds, chain fluorinated compounds, and cyclic fluorinated compounds; more preferably, it includes at least one selected from chain carbonate compounds, cyclic carbonate compounds, and cyclic fluorinated compounds; even more preferably, it includes chain carbonate compounds, cyclic carbonate compounds, chain fluorinated compounds, and cyclic fluorinated compounds; and still more preferably, it includes chain carbonate compounds, cyclic carbonate compounds, chain fluorinated compounds, and cyclic fluorinated compounds. By giving the solvent the above-described composition, the improved cycle performance of the present invention tends to become more significant.

[0053] Chain carbonate compounds refer to carbonate compounds in which one or more hydrogen atoms in the carbon skeleton are not replaced by fluorine atoms and do not have cyclic structures such as aromatic rings, alicyclic rings, monocyclic rings, or heterocyclic rings. Examples of chain carbonate compounds include dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, propylene carbonate, and chloroethylene carbonate. It is preferable to include dimethyl carbonate or ethylene carbonate, and more preferably, dimethyl carbonate and ethylene carbonate. By including chain carbonate compounds, the improved cycling performance of the present invention tends to become more significant.

[0054] Cyclic carbonate compounds refer to carbonate compounds in which one or more hydrogen atoms in the carbon skeleton are not replaced by fluorine atoms and have cyclic structures such as aromatic rings, alicyclic rings, monocyclic rings, and heterocyclic rings. Examples of cyclic carbonate compounds include ethylene carbonate and propylene carbonate. Ethyl carbonate is preferred. By including cyclic carbonate compounds, the cycling performance improvement effect of the present invention tends to become more significant.

[0055] Chain fluorine compounds are compounds that do not have cyclic structures such as aromatic rings, alicyclic rings, monocyclic rings, or heterocyclic rings, and in which one or more hydrogen atoms in the carbon skeleton are replaced by fluorine atoms. Examples of chain-like fluorinated compounds include: 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, 1,2-bis(1,1,2,2-tetrafluoroethoxy)ethane, methyl perfluoroisobutyl ether, ethyl nonafluoroisobutyl ether, 1,1,1,2,2,3,4,5,5,5-decafluoro-3-methoxy-4-(trifluoromethyl)pentane, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 1,1,2,2-tetrafluoroethoxy-2,2,3,3-tetrafluoropropoxymethane, methyl-1,1,2,2-tetrafluoroethyl ether, ethyl-1,1,2,2-tetrafluoroethyl ether, propyl-1,1,2,2-tetrafluoroethyl ether, etc. Preferably, the compound comprises a chain-like fluorinated compound having an ether group; more preferably, it comprises at least one selected from 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, 1,2-bis(1,1,2,2-tetrafluoroethoxy)ethane, methyl perfluoroisobutyl ether, ethyl nonafluoroisobutyl ether, and 1,1,1,2,2,3,4,5,5,5-decafluoro-3-methoxy-4-(trifluoromethyl)pentane; and even more preferably, it comprises 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, 1,2-bis(1,1,2,2-tetrafluoroethoxy)ethane, or ethyl nonafluoroisobutyl ether. By including the above-mentioned chain-like fluorinated compounds, the improvement in cycling characteristics of the present invention tends to become more significant.

[0056] Cyclic fluorinated compounds are compounds having cyclic structures such as aromatic rings, alicyclic rings, monocyclic rings, and heterocyclic rings, and in which one or more hydrogen atoms in the carbon skeleton are replaced by fluorine atoms. Examples of cyclic fluorinated compounds include: fluoroethylene carbonate, 1,1,2,2,3,3,4-heptafluorocyclopentane, hexadecylfluoro(1,3-dimethylcyclohexane), tetradecylfluoromethylcyclohexane, fluorocyclohexane, fluorocyclopentane, and octadecylfluorodecahydronaphthalene. Among these, fluoroethylene carbonate is preferred. By including the above-mentioned cyclic fluorinated compounds, the cycling performance improvement effect of the present invention tends to become more significant.

[0057] The electrolyte of this embodiment contains a chain-like fluorine compound having at least one of the monovalent groups represented by formulas (A) to (D) below. The content of the chain-like fluorine compound is preferably 0.5% by mass or more and 30% by mass or less, more preferably 1% by mass or more and 25% by mass or less, and even more preferably 3% by mass or more and 20% by mass or less, relative to the total amount of components other than the chain-like fluorine compound in the electrolyte. By including such a chain-like fluorine compound, the cycle characteristics of the lithium secondary battery tend to be further improved. Furthermore, the wavy lines in the formulas represent bonding sites in the monovalent groups.

[0058] [Chemical Formula 1]

[0059]

[0060] [Chemical Formula 2]

[0061]

[0062] [Chemical Formula 3]

[0063]

[0064] [Chemical Formula 4]

[0065]

[0066] The electrolyte preferably contains a chain-like fluorine compound having formula (A), formula (B), or formula (C) above, and more preferably contains a chain-like fluorine compound having formula (A) or formula (B) above. By containing a chain-like fluorine compound having this structure, the cycle characteristics of the lithium secondary battery tend to be further improved.

[0067] The electrolyte in this embodiment preferably contains a chain-like or cyclic fluorinated carbonate. By containing a chain-like or cyclic fluorinated carbonate, the improvement in cycle characteristics of the present invention tends to become more significant. From the same viewpoint, it is preferable to contain fluoroethylene carbonate as a chain-like or cyclic fluorinated carbonate.

[0068] Ether compounds are compounds that have an ether group and are not fluorinated. Examples of ether compounds include: 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE), 1,2-dimethoxypropane (DMP), diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 1,3-dimethoxypropane, 1,4-dimethoxybutane, 1,1-dimethoxyethane, 2,2-dimethoxypropane, 1,2-diethoxybutane, and 2,3-diethoxybutane.

[0069] Relative to the total amount of electrolyte, the content of carbonate compound is preferably 30% by mass or more and 100% by mass or less, 35% by mass or more and 95% by mass or less, 45% by mass or more and 90% by mass or less, or 55% by mass or more and 85% by mass or less. By keeping the content of carbonate compound within the above range, the improvement in cycle characteristics of the present invention tends to become more significant. Furthermore, the carbonate compound includes both fluorinated carbonate compounds and unfluorinated carbonate compounds.

[0070] Relative to the total amount of chain carbonate compounds and cyclic carbonate compounds, the content of chain carbonate compounds is preferably 10% by volume or more and 60% by volume or less, 20% by volume or more and 50% by volume or less, and 25% by volume or more and 45% by volume or less. By keeping the content of chain carbonate compounds within the above range, the improvement in the cycling characteristics of the present invention tends to become more significant. Furthermore, the chain carbonate compounds include both fluorinated chain carbonate compounds and unfluorinated chain carbonate compounds.

[0071] Relative to the total amount of chain carbonate compounds and cyclic carbonate compounds, the content of cyclic carbonate compounds is preferably 5% by volume or more and 60% by volume or less, 10% by volume or more and 50% by volume or less, and 20% by volume or more and 40% by volume or less. By keeping the content of cyclic carbonate compounds within the above range, the improvement in the cycling characteristics of the present invention tends to become more significant. Furthermore, the cyclic carbonate compounds include both fluorinated cyclic carbonate compounds and unfluorinated cyclic carbonate compounds.

[0072] Relative to the total amount of chain carbonate compounds and cyclic carbonate compounds, the content of chain fluorinated compounds is preferably 0% by mass or more and 50% by mass or less, 0% by mass or more and 40% by mass or less, 1% by mass or more and 30% by mass or less, or 3% by mass or more and 25% by mass or less. By keeping the content of chain fluorinated compounds within the above ranges, the improvement in the cycling characteristics of the present invention tends to become more significant.

[0073] Relative to the total amount of chain carbonate compounds and cyclic carbonate compounds, the content of cyclic fluorinated compounds is preferably 0% by mass or more and 30% by mass or less, 1% by mass or more and 20% by mass or less, or 3% by mass or more and 10% by mass or less. By keeping the content of cyclic fluorinated compounds within the above range, the improvement in the cycling characteristics of the present invention tends to become more significant.

[0074] The content of fluorinated carbonate is preferably 0% by mass or more and 30% by mass or less, 1% by mass or more and 20% by mass or less, or 3% by mass or more and 10% by mass or less, relative to the total amount of chain carbonate compounds and cyclic carbonate compounds. By keeping the content of fluorinated carbonate within the above range, the improvement in the cycling characteristics of the present invention tends to become more significant.

[0075] 1.3 Diaphragm

[0076] As for the separator 20 in this embodiment, there are no particular limitations as long as it has the functions of physically isolating and / or electrically isolating the positive electrode 30 and the negative electrode 10, and ensuring the ionic conductivity of lithium ions. Examples of such separators include insulating porous components, polymer electrolytes, gel electrolytes, and inorganic solid electrolytes. Typically, at least one selected from insulating porous components, polymer electrolytes, and gel electrolytes can be used. In addition, the separator 20 can be used alone or in combination with two or more components.

[0077] As the separator 20, it is preferable to use one or more of the following: an insulating porous component, a polymer electrolyte, or a gel electrolyte, or a combination of two or more. Furthermore, when an insulating porous component is used alone as the separator 20, the lithium secondary battery needs to further include an electrolyte.

[0078] There are no particular limitations on the polymer electrolytes mentioned above. Examples include solid polymer electrolytes that mainly contain polymers and electrolytes, and semi-solid polymer electrolytes that mainly contain polymers, electrolytes, and plasticizers.

[0079] There are no particular limitations on the above-mentioned gel electrolytes. For example, gel electrolytes that mainly consist of polymers and liquid electrolytes (i.e., solvents and electrolytes) can be listed.

[0080] There are no particular limitations on the polymers that can be included in polymer electrolytes and gel electrolytes. Examples include polymers containing oxygen-containing functional groups, halogen groups, and polar groups such as cyano groups, such as ethers and esters. Specifically, examples include resins such as polyethylene oxide (PEO) with ethylene oxide units in the main chain and / or side chains, resins such as polypropylene oxide (PPO) with propylene oxide units in the main chain and / or side chains, acrylic resins, vinyl resins, ester resins, nylon resins, polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polysiloxanes, polyphosphononitrile, polymethyl methacrylate, polyamides, polyimides, aramids, and polytetrafluoroethylene. The above resins can be used alone or in combination of two or more.

[0081] Examples of electrolytes contained in polymer electrolytes and gel electrolytes include salts of Li, Na, K, Ca, and Mg. Typically, in this embodiment, the polymer electrolyte and gel electrolyte contain a lithium salt. There are no particular limitations on the lithium salt; any lithium salt that can be contained in the aforementioned electrolyte is acceptable. This salt or lithium salt can be used alone or in combination of two or more.

[0082] The ratio of polymer to lithium salt in polymer electrolytes and gel electrolytes can be determined based on the ratio of polar groups in the polymer to lithium atoms in the lithium salt. For example, if the polymer has oxygen atoms, it can be determined based on the ratio of the number of oxygen atoms in the polymer to the number of lithium atoms in the lithium salt ([Li] / [O]). In polymer electrolytes and gel electrolytes, the ratio of polymer to lithium salt can be adjusted to the above ratio ([Li] / [O]) for example, 0.02 or more and 0.20 or less, 0.03 or more and 0.15 or less, or 0.04 or more and 0.12 or less.

[0083] There are no particular limitations on the solvent contained in the gel electrolyte; for example, one of the solvents that can be contained in the electrolyte described above can be used alone or in combination of two or more. Preferred examples of solvents are also the same as those in the electrolyte described above.

[0084] There are no particular limitations on the plasticizers contained in semi-solid polymer electrolytes; for example, the same components as the solvents that can be contained in gel electrolytes and various oligomers can be listed.

[0085] When the diaphragm 20 includes an insulating porous component, the component exhibits ion conductivity by filling the pores of the component with a substance that has ion conductivity. Therefore, in this embodiment, for example, the electrolyte of this embodiment and a gel electrolyte containing the electrolyte of this embodiment are filled.

[0086] There are no particular limitations on the materials used to form the porous insulating components. Examples of suitable materials include insulating polymers, specifically polyethylene (PE) and polypropylene (PP). That is, the diaphragm 20 can be a porous polyethylene (PE) membrane, a porous polypropylene (PP) membrane, or a laminate of these membranes.

[0087] 1.4 Positive electrode

[0088] The positive electrode 30 in this embodiment has a positive current collector and a positive active material layer. The average thickness of the positive electrode 30 is not particularly limited, and for example, it can be 20 μm or more and 100 μm or less, 30 μm or more and 80 μm or less, or 40 μm or more and 70 μm or less. However, the average thickness of the positive electrode 30 can be appropriately adjusted according to the desired battery capacity.

[0089] The positive current collector in this embodiment may have a positive current collector film, which includes a resin layer containing polyethylene terephthalate and metal layers disposed on both sides of the resin layer, or it may have a metal layer but no resin layer. When the positive current collector film is provided, the metal layers are formed by vapor deposition, sputtering, electroplating, or by attaching them to the surfaces of both sides of the resin layer using an adhesive.

[0090] The resin layer of the positive current collector is an insulator, preventing the metal layers on both sides of the resin layer from conducting to each other. There is no particular limitation on the resin constituting the resin layer; for example, it can be composed of sheet (film) or fibrous resin. The resin may contain at least one selected from polyethylene terephthalate (PET), polypropylene, polyamide, acrylic resin, polycarbonate, polyethylene, polyvinyl chloride, and polystyrene. The above-mentioned resins may be used alone or in combination of two or more.

[0091] In addition to the resins mentioned above, the resin layer of the positive current collector may also contain other additives depending on the desired physical properties. There are no particular limitations on the additives; examples include colorants, flame retardants, and surfactants.

[0092] The thickness of the resin layer of the positive current collector is, for example, 2 μm or more and 15 μm or less, 3 μm or more and 12 μm or less, or 4 μm or more and 10 μm or less.

[0093] The metal layer of the positive electrode current collector is in physical and / or electrical contact with the positive electrode active material layer, functioning by accepting and donating electrons to the positive electrode active material layer. The metal layer of the positive electrode current collector is composed of a conductive material such as a metal that does not react with lithium in the battery. There is no particular limitation on the metal constituting the metal layer of the positive electrode current collector; it is selected from at least one of aluminum, titanium, stainless steel, nickel, and alloys thereof. From the viewpoint of more effectively and reliably achieving the effects of the invention, aluminum or aluminum alloys are preferred, and aluminum is particularly preferred. One type of metal or a combination of two or more can be used as the metal. Furthermore, in this specification, "metal that does not react with lithium" refers to a metal that does not react with lithium ions or lithium metal to alloy under the operating conditions of a lithium secondary battery.

[0094] When the positive current collector has a positive current collector film, there is no particular limitation on the thickness of the metal layer, for example, it can be 0.1μm or more and 4.0μm or less, 0.2μm or more and 3.0μm or less, 0.3μm or more and 2.5μm or less, or 0.4μm or more and 2.0μm or less.

[0095] When the positive current collector has a metal layer but no resin layer, there is no particular limitation on the thickness of the metal layer, for example, it can be 4.0 μm or more and 20.0 μm or less, 6.0 μm or more and 17.5 μm or less, or 8.0 μm or more and 15.0 μm or less.

[0096] The positive electrode active material refers to the substance that undergoes electrode reactions, namely oxidation and reduction reactions, in the positive electrode 30. There is no particular limitation on the positive electrode active material in this embodiment; for example, it may be included in a positive electrode active material composition containing a binder, conductive additive, sacrificial positive electrode agent, and other additives. The positive electrode active material composition is coated onto at least one or both sides of the positive electrode current collector and pressed into shape, thereby disposing of the positive electrode active material layer on at least one or both sides of the positive electrode current collector.

[0097] Methods for configuring a positive electrode active material layer on a positive electrode current collector are not limited to compression molding. Examples include: a method of including a thermosetting compound in a positive electrode active material composition and heating it to cure it; a method of including a photocurable compound in a positive electrode active material composition and irradiating it with light to cure it; and a method of curing a positive electrode active material composition as a two-component curable composition by mixing the two components.

[0098] The positive electrode active material layer in this embodiment may contain one or more substances of the general formula: Li z Ni x Co y M 1-x-y O 2+α (where 0.5≤x≤1.0, 0≤y≤0.35, 0.9≤z≤1.3, -0.2≤α≤0.15, and M is a compound selected from one or more elements selected from Mn, Al, V, Mg, Mo, Nb, Ti, Zr, Fe, Cu, Cr, Zn, F and B)

[0099] Furthermore, preferably, the positive electrode active material layer comprises one or more substances of the general formula: Li z Ni x Co y M 1-x-y O 2+α (where 0.7≤x≤1.0, 0≤y≤0.35, 0.9≤z≤1.3, -0.2≤α≤0.15, and M is an element selected from Mn, Al, V, Mg, Mo, Nb, Ti, Zr, Fe, Cu, Cr, Zn, F, and B) represents the compound. By using the above-mentioned compound with a high nickel content as the positive electrode active material, the energy density of lithium secondary batteries tends to be further improved. In addition, if the nickel content is increased, redox shuttle reaction is more likely to occur, but by including the additives detailed below in the electrolyte, this reaction tends to be suppressed, resulting in excellent performance stability at high temperatures.

[0100] As the positive electrode active material, other positive electrode active materials besides the compounds mentioned above can be included. Specifically, other positive electrode active materials in this embodiment include: host materials for lithium (typically lithium ions). There are no particular limitations on such other positive electrode active materials; for example, metal oxides and metal phosphates can be included. There are no particular limitations on the aforementioned metal oxides; for example, cobalt oxide compounds, manganese oxide compounds, and nickel oxide compounds can be included. There are no particular limitations on the aforementioned metal phosphates; for example, iron phosphate compounds and cobalt phosphate compounds can be included. Typical other positive electrode active materials include: LiNi. 0.8 Co 0.15 Al 0.05 O2, LiCoO2, LiNi x Mn y O(x+y=1), LiNiO2, LiMn2O4, LiFePO4, LiCoPO4, LiFeOF, LiNiOF, and TiS2. Other positive electrode active materials can be used alone or in combination of two or more.

[0101] The positive electrode active material composition may include a binder. By including a binder, the positive electrode active material layer is more easily bonded to the positive electrode current collector, and the flexibility is improved after the positive electrode active material layer is disposed on the positive electrode current collector.

[0102] The positive electrode adhesive used in this embodiment is not particularly limited, and examples include: polyvinylidene fluoride (PVDF); modified PVDF obtained by introducing functional groups such as hydroxyl, amino, carbonyl, carboxyl, phenyl, and methyl groups into PVDF; polytetrafluoroethylene (PTFE); modified PTFE obtained by introducing functional groups such as hydroxyl, amino, carbonyl, carboxyl, phenyl, and methyl groups into PTFE; block copolymers, random copolymers, or graft copolymers having PTFE as a structural unit; styrene-butadiene rubber; carboxymethyl cellulose; acrylic resin; polyimide resin, etc. One adhesive may be used alone or in combination of two or more.

[0103] The positive electrode active material composition of this embodiment may include a sacrificial positive electrode agent. The sacrificial positive electrode agent in this embodiment refers to a lithium-containing compound that undergoes oxidation within the charge / discharge potential range of the positive electrode active material and substantially does not undergo reduction. There is no particular limitation on the sacrificial positive electrode agent; examples include: lithium oxides such as Li₂O₂; lithium nitrides such as Li₃N; lithium sulfide solid solutions such as Li₂S-P₂S₅, Li₂S-LiCl, Li₂S-LiBr, and Li₂S-LiI; and Li… 1+x (Ti 1-y Fe y ) 1-xO2(0<x≤0.25, 0.4<y≤0.9), Li 2-x Ti 1-z Fe z O 3-y Iron-based lithium oxides such as Li5FeO4 (0≤x<2、0≤y≤1、0.05≤z≤0.95) can be used. Sacrificial cathode agents can be used alone or in combination of two or more.

[0104] 2. Manufacturing method of lithium secondary batteries

[0105] As for the manufacturing method of lithium secondary batteries, any method that can manufacture lithium secondary batteries with the above-described battery structure is acceptable, and there are no particular limitations. For example, the following methods can be listed.

[0106] Prepare the aforementioned negative electrode 10, separator 20, and positive electrode 30. Each structural component or reagent used in the components can be manufactured using conventionally known methods, or commercially available products can be used. Stack the prepared positive electrode 30, separator 20, and negative electrode 10 in this order, with the positive electrode 30 facing the separator 20, to obtain a laminate. Seal the obtained laminate together with the electrolyte of this embodiment into a sealed container to obtain a lithium secondary battery. The sealed container is not particularly limited; for example, a laminated film can be used.

[0107] In addition, such as Figure 1 As shown, a separator 20 can be sandwiched between the positive electrode 30 and the negative electrode 10, and the positive electrode 30 and the negative electrode 10 can be alternately stacked in multiple layers, thereby tending to further improve the performance of the battery, such as energy density. As a stacking method, for example, the negative electrode 10 and the positive electrode 30 can be coated so that they do not contact each other, facing the opposite side of the separator 20, without cutting the separator 20. From the viewpoint of preventing short circuits and improving productivity, such stacking is preferred.

[0108] Although the negative electrode 10 of this embodiment is not shown, it has a negative electrode active material layer containing Si active material. Therefore, the negative electrode active material layer is obtained by coating a negative electrode active material composition, which is prepared by mixing the aforementioned negative electrode active material with a binder, additives, conductive additives, solvents, etc., onto both sides or one side of the negative electrode current collector and then pressing it into shape. The resulting molded body is then punched to a specified size by a punching process to obtain the negative electrode 10 of this embodiment.

[0109] The shape of the lithium secondary battery in this embodiment is not particularly limited; for example, it can be a sheet type, a stacked sheet type, a thin shape, a bottomed cylindrical shape, a bottomed square shape, etc. From the viewpoint of more effectively and reliably achieving the effects of this embodiment, a sheet type, a stacked sheet type, or a thin shape is preferred.

[0110] Example

[0111] The present invention will be further described in detail below through embodiments, but the present invention is not limited to these embodiments.

[0112] 1. Manufacturing of lithium secondary batteries

[0113] The following procedures were followed to produce lithium-ion secondary batteries of Examples 1-15 and Comparative Examples 1-11.

[0114] 1.1 Preparation of the negative electrode

[0115] As the negative electrode current collector, a material with a 1.0 μm copper layer deposited on both sides of a 6.0 μm thick polyethylene terephthalate (PET) film was prepared. Then, a negative electrode active material composition was prepared by mixing 18 parts by mass of a Si-containing active material with the particle size listed in Table 1 as the negative electrode active material, 75 parts by mass of graphite as another negative electrode active material, 2 parts by mass of carbon black as a conductive additive, 1 part by mass of carboxymethyl cellulose (CMC) as a binder, and 4 parts by mass of styrene-butadiene rubber (SBR) in 100 parts by mass of water as a solvent. This composition was coated onto both sides of the negative electrode current collector to a weight per unit area of ​​6.2 mg / cm², pressed, and punched into a size of 4.4 cm × 4.4 cm. A negative electrode metal sheet (a 4.0 μm thick copper foil) was then ultrasonically welded to the ends of the resulting negative electrode 10.

[0116] 1.2 Preparation of the diaphragm

[0117] A polyethylene microporous membrane (thickness: 15 μm, 4.8 cm × 4.8 cm) coated with a mixture of polyvinylidene fluoride (PVDF) and Al2O3 was prepared as a diaphragm 20.

[0118] 1.3 Preparation of the positive electrode

[0119] A current collector film consisting of a 6.0 μm thick PET film as a resin layer and 1.0 μm thick aluminum metal layers deposited on both sides is used as the positive electrode current collector. Then, 96 parts by mass of LiNi as the positive electrode active material are mixed in N-methylpyrrolidone (NMP) as a solvent. 0.8 Co 0.15 Al 0.05 A positive electrode active material composition was prepared by using O2, 2 parts by mass of carbon black as a conductive additive, and 2 parts by mass of polyvinylidene fluoride (PVDF) as a binder. This positive electrode active material composition was coated onto both sides of a positive electrode current collector to a unit area weight of 15 mg / cm², and then pressed to form positive electrode active material layers on both sides of the current collector, resulting in a molded body. The molded body was then punched to a specified size (4cm × 4cm) to obtain positive electrode 30.

[0120] 1.4 Electrolyte Preparation

[0121] A solvent solution was prepared by mixing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 30:35:35. 5 wt% of fluoroethylene carbonate (FEC) and the amount (wt%) of hydrofluoroether (HFE) listed in Table 1 were added as further solvents relative to the total volume of this solvent solution. In the hydrofluoroether (HFE) compounds, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether was used as "HFE1", 1,2-bis(1,1,2,2-tetrafluoroethoxy)ethane as "HFE2", and ethyl nonafluoroisobutyl ether as "HFE3".

[0122] Lithium salts of lithium bis(fluorosulfonyl)imide (LiFSI), lithium hexafluorophosphate (LiPF6), and lithium difluorophosphate (LiPO2F2) were dissolved in the above solution to achieve the mole fractions and lithium ion concentrations listed in Table 1. In Comparative Example 8, however, no lithium secondary battery was fabricated because the lithium salts were not completely dissolved.

[0123] 1.5 Battery Assembly

[0124] The negative electrode 10, separator 20, and positive electrode 30 obtained by the above operation are as follows: Figure 1 The layers were stacked multiple times in the sequence shown to obtain a laminate. Then, terminals were joined at the leads of the stacked positive and negative electrodes using ultrasonic welding; specifically, a 100μm Al terminal was joined to the positive electrode, and a 100μm Ni terminal was joined to the negative electrode. The laminated casing was then inserted. Finally, the electrolyte obtained above was injected into the casing and sealed, thus obtaining the lithium-ion secondary batteries of the various embodiments and comparative examples.

[0125] 2. Evaluation of lithium secondary batteries

[0126] [Capacity Maintenance Rate]

[0127] The lithium secondary battery obtained above was initially charged at 23mA in an environment of 45°C until the voltage reached 4.2V, and then initially discharged at 23mA until the voltage reached 2.7V. Then, in an environment of 45°C, the following cycle was performed: CC charging at 460mA until the voltage reached 4.2V, and then CC discharging at 460mA until the voltage reached 2.7V (first cycle).

[0128] Next, this cycle was performed 299 times, for a total of 300 cycles. In the above cycles of the embodiments and comparative examples, the capacity retention rate was obtained by dividing the capacity obtained from the CC discharge of the 300th cycle by the capacity obtained from the CC discharge of the 1st cycle. The results are shown in Table 1.

[0129] [Table 1]

[0130]

[0131] As shown in Table 1, Examples 1 to 15 are lithium secondary batteries containing a Si-containing negative electrode active material and an electrolyte. The average particle size of the negative electrode active material is 5.0 μm or more and 30 μm or less. The electrolyte contains LiFSI, LiPF6, and LiPO2F2 as lithium salts. When the molar fraction of LiFSI is set as x, the molar fraction of LiPF6 is set as y, and the molar fraction of LiPO2F2 is set as z, and x+y+z=1, the ranges of 0.05≤x≤0.95, 0.05≤y≤0.90, and 0.005≤z≤0.35 are satisfied. The concentration of lithium ions in the electrolyte is 0.7M or more and 3.0M or less. Since the capacity retention rate is higher than that of Comparative Examples 1 to 11, which do not have this structure, it is confirmed that this is a lithium secondary battery with excellent cycle characteristics.

[0132] <Note>

[0133] The embodiments disclosed herein include the following schemes.

[0134] [1] A lithium secondary battery comprising a Si-containing negative electrode active material and an electrolyte, wherein,

[0135] The average particle size of the negative electrode active material is greater than 5.0 μm and less than 30 μm.

[0136] The electrolyte contains LiFSI, LiPF6, and LiPO2F2 as lithium salts.

[0137] When the mole fraction of LiFSI is set as x, the mole fraction of LiPF6 as y, and the mole fraction of LiPO2F2 as z, and x+y+z=1, the ranges satisfying 0.05≤x≤0.95, 0.05≤y≤0.90, and 0.005≤z≤0.35 are met.

[0138] In the electrolyte, the concentration of lithium ions is above 0.7M and below 3.0M.

[0139] [2] According to the lithium secondary battery of [1], wherein the electrolyte comprises a chain fluorine compound having at least one of the monovalent groups represented by formulas (A) to (D) above.

[0140] The content of the chain fluorine compound is 0.5% by mass or more and 30% by mass or less, relative to the total amount of components other than the chain fluorine compound in the electrolyte.

[0141] [3] The lithium secondary battery according to [1] or [2], wherein the negative electrode active material comprises Li y -SiO x Or SiC.

[0142] [4] The lithium secondary battery according to any one of [1] to [3], wherein the concentration of lithium ions is 1.0 M or more and 2.5 M or less.

[0143] [5] The lithium secondary battery according to any one of [1] to [4], wherein the average particle size of the negative electrode active material is 7.0 μm or more and 20 μm or less.

[0144] [6] The lithium secondary battery according to any one of [1] to [5], wherein the electrolyte comprises fluorinated carbonate.

[0145] [7] A lithium secondary battery according to any one of [1] to [6], wherein the electrolyte comprises a chain fluorine compound A having the formula (A) or the formula (B).

[0146] Industrial availability

[0147] The lithium secondary battery involved in this invention has excellent cycle characteristics, and therefore has industrial applicability as an energy storage device for various applications.

[0148] Explanation of reference numerals in the attached figures

[0149] 10: Negative electrode; 20: Separator; 30: Positive electrode.

Claims

1. A lithium secondary battery comprising a Si-containing negative electrode active material and an electrolyte, wherein, The average particle size of the negative electrode active material is greater than 5.0 μm and less than 30 μm. The electrolyte contains LiFSI, LiPF6, and LiPO2F2 as lithium salts. When the mole fraction of LiFSI is set as x, the mole fraction of LiPF6 as y, and the mole fraction of LiPO2F2 as z, and x + y + z = 1, the ranges satisfying 0.05 ≤ x ≤ 0.95, 0.05 ≤ y ≤ 0.90, and 0.005 ≤ z ≤ 0.35 are met. In the electrolyte, the concentration of lithium ions is above 0.7M and below 3.0M.

2. The lithium secondary battery according to claim 1, wherein, The electrolyte contains a chain-like fluorine compound having at least one monovalent group represented by formulas (A) to (D). The content of the chain fluorine compound is 0.5% by mass or more and 30% by mass or less, relative to the total amount of components other than the chain fluorine compound in the electrolyte. [Chemical Formula 1] [Chemical Formula 2] [Chemical Formula 3] [Chemical Formula 4] In the formula, the wavy line represents the bonding site in the monovalent group.

3. The lithium secondary battery according to claim 1, wherein, The negative electrode active material contains Li y -SiO x Or SiC.

4. The lithium secondary battery according to claim 1, wherein, The concentration of lithium ions is above 1.0 M and below 2.0 M.

5. The lithium secondary battery according to claim 1, wherein, The average particle size of the negative electrode active material is greater than 7.0 μm and less than 20 μm.

6. The lithium secondary battery according to claim 1, wherein, The electrolyte contains fluorinated carbonate.

7. The lithium secondary battery according to claim 1, wherein, The electrolyte contains a chain-like fluorine compound A having formula (A) or formula (B).

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