Lithium secondary battery and non-aqueous electrolyte therefor

Abstract

A lithium secondary battery comprises a positive electrode, a negative electrode, a separator disposed between the positive and negative electrodes, and a non-aqueous electrolyte. Lithium metal precipitates from the negative electrode during charging and dissolves during discharge. The non-aqueous electrolyte contains lithium ions, cations of a metal M1 that forms an alloy with lithium, and halide ions.

Description

Technical Field

[0001] The present disclosure relates to a lithium secondary battery and a nonaqueous electrolyte therefor. Background Art

[0002] Lithium-ion secondary batteries are known as high-capacity secondary batteries. Lithium-ion secondary batteries use, for example, carbon materials as negative electrode active materials. Carbon materials reversibly intercalate and deintercalate lithium ions, allowing for charge and discharge.

[0003] On the other hand, lithium secondary batteries (also known as lithium metal secondary batteries) that use lithium metal as the negative electrode active material have a higher theoretical capacity density. In lithium secondary batteries, lithium metal is deposited on the negative electrode current collector during charging, and the deposited lithium metal dissolves in the non-aqueous electrolyte during discharge.

[0004] However, it is difficult to control the precipitation morphology of lithium metal in lithium secondary batteries. When lithium metal precipitates in a dendritic crystal form, the specific surface area of ​​the negative electrode increases, and side reactions with the non-aqueous electrolyte increase. In addition, inactive lithium is generated that is not conducive to charge and discharge, resulting in a decrease in discharge capacity.

[0005] Patent Document 1 proposes a non-aqueous electrolyte battery comprising: a positive electrode having an active material containing lithium, a negative electrode having an active material capable of doping / dedoping a metal in an ionic state and / or precipitating / dissolving the metal, and a non-aqueous electrolyte containing an electrolyte salt, characterized in that an additive having a redox potential higher than the potential of the metal when precipitated at the negative electrode and a redox potential lower than the potential of the positive electrode active material in a charged state is added to the non-aqueous electrolyte. In addition, lithium iodide can be cited as an example of the additive. When the lithium metal precipitated at the negative electrode becomes insulated from the negative electrode and becomes unfavorable for charge and discharge, the additive oxidizes and ionizes the lithium metal that is unfavorable for charge and discharge, thereby preventing deterioration of the charge and discharge cycle characteristics.

[0006] Patent Document 2 proposes a non-aqueous electrolyte secondary battery comprising a positive electrode having a positive electrode current collector and a positive electrode composite material layer formed on the current collector, a negative electrode having a negative electrode current collector, and a non-aqueous electrolyte. During charge, lithium metal is deposited on the negative electrode current collector, and during discharge, the lithium metal dissolves in the non-aqueous electrolyte, which contains a lithium salt having an oxalate complex as an anion. By adding the lithium salt having an oxalate complex as an anion to the non-aqueous electrolyte, the lithium metal is uniformly deposited on the negative electrode, specifically suppressing expansion of the negative electrode.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: Japanese Patent Application Laid-Open No. 2003-243030

[0010] Patent Document 2: International Publication No. 2018 / 179782 Summary of the Invention

[0011] The methods proposed in Patent Documents 1 and 2 are not sufficient for improving the charge-discharge cycle characteristics of lithium secondary batteries.

[0012] One aspect of the present disclosure relates to a lithium secondary battery, comprising: a positive electrode, a negative electrode, a separator arranged between the positive electrode and the negative electrode, and a non-aqueous electrolyte, wherein lithium metal is precipitated in the negative electrode during charging and dissolved during discharging, and the non-aqueous electrolyte comprises: lithium ions, cations of a metal M1 that forms an alloy with lithium, and halide ions.

[0013] Another aspect of the present disclosure relates to a non-aqueous electrolyte for a lithium secondary battery, comprising lithium ions, cations of a metal M1 that forms an alloy with lithium, and halide ions.

[0014] According to the present disclosure, the charge and discharge cycle characteristics of a lithium secondary battery can be improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is a partial cross-sectional view schematically showing an example of the lithium secondary battery of the present disclosure. DETAILED DESCRIPTION

[0016] The lithium secondary battery disclosed herein comprises a positive electrode, a negative electrode, a separator disposed between the positive and negative electrodes, and a non-aqueous electrolyte. Lithium metal is deposited in the negative electrode during charging and dissolves during discharge. Specifically, the negative electrode comprises at least a negative electrode current collector, on which the lithium metal is deposited. The lithium secondary battery disclosed herein is also referred to as a lithium metal secondary battery.

[0017] In a lithium (metal) secondary battery, more than 70% of the rated capacity is reflected by the precipitation and dissolution of lithium metal. The electron migration in the negative electrode during charging and discharging is mainly based on the precipitation and dissolution of lithium metal in the negative electrode. Specifically, 70 to 100% (for example, 80 to 100%, 90 to 100%) of the electron migration (current from other perspectives) in the negative electrode during charging and discharging is based on the precipitation and dissolution of lithium metal. That is, the negative electrode of the present embodiment is different from the negative electrode in which the electron migration in the negative electrode during charging and discharging is mainly based on the storage and release of lithium ions by the negative electrode active material (graphite, etc.).

[0018] Here, the nonaqueous electrolyte contains lithium ions, cations of the metal M1 that forms an alloy with lithium, and halide ions.

[0019] It is believed that when the cations of the metal M1 that forms an alloy with lithium are contained in a non-aqueous electrolyte, a region doped with the metal M1 is formed in the lithium metal initially precipitated at the negative electrode during charging (hereinafter also referred to as the doping region). When charging is then performed, the lithium metal is mainly precipitated between the thin layer formed by the doping region (hereinafter also referred to as the doping layer M1) and the negative electrode collector. As a result, the lithium metal is pressed by the doping layer M1. It is believed that, through the effect of this pressing, the elongation of the dendritic precipitates is suppressed, and the side reactions and the reduction of the discharge capacity are suppressed. As a result, the charge and discharge cycle characteristics of the lithium secondary battery are improved.

[0020] However, during charging, protruding precipitates may form at any time on the negative electrode. Using these protruding precipitates as nuclei, dendritic precipitates of lithium metal grow. If these protruding precipitates (hereinafter also referred to as dendrite precursors) are left standing, it becomes difficult to suppress the growth of dendritic precipitates.

[0021] When halide ions are contained in a non-aqueous electrolyte, the dendrite precursors can be dissolved (oxidized) by a redox reaction. The dendrite precursors are dissolved by the halide ions, thereby making the lithium metal surface flatter. Furthermore, the halide ions also dissolve the dendritic precipitates, so even if dendritic precipitates are generated, their elongation can be suppressed.

[0022] It should be noted that if the halide ions dissolve (oxidize) the dendrite precursors, they themselves are reduced. When the reduced halide ions move to the positive electrode side and react with the positive electrode active material, the positive electrode active material is reduced (discharged), and the halide ions are oxidized to their original state. If the halide ions repeat this reaction excessively, the self-discharge of the positive electrode proceeds, and the storage characteristics of the lithium secondary battery will deteriorate. In this regard, the halide ions preferentially react with dendritic lithium with a large specific surface area. In addition, in the presence of the doping layer M1, the reaction of the halide ions with the lithium metal in a good precipitation state is suppressed by the doping layer M1, and therefore, the self-discharge of the positive electrode becomes difficult to proceed. Thus, the cycle characteristics are improved, and the degradation of the storage characteristics can also be suppressed.

[0023] As the initial halogen to become the halide ion, fluorine (F), chlorine (Cl), bromine (Br), iodine (I) and the like are preferred. These can be used alone or in combination of two or more. Among them, in terms of the greatest effect of dissolving dendrite precursors or dendritic crystal-like precipitates, at least one of bromine and iodine is preferred, and iodine is most preferred.

[0024] Initially during charging, an alloy of metal M1 and lithium may form as a doped region in the negative electrode. In this case, lithium metal is believed to be primarily deposited between the alloyed doped layer M1 (hereinafter also referred to as alloy layer M1) and the negative electrode current collector. The alloy layer M1 is a more stable layer within the doped layer M1 and is believed to more stably press the lithium metal.

[0025] The metal M1 may be at least one selected from the group consisting of In, Sn, Au, Ag, Pt, Zn, Sb, Bi, Si, and Mg. These metals M1 react with lithium during the initial stage of charging to form an alloy layer M1. The metal M1 may be at least one of Sn and Au. Sn and Au are effective in improving charge-discharge cycle characteristics.

[0026] The concentration of the cations of the metal M1 in the non-aqueous electrolyte is not limited, for example, it can be greater than 0.5 mmol / L and less than 100 mmol / L, or it can be greater than 5 mmol / L and less than 50 mmol / L. When the metal M1 in the above range is used, the required sufficient doping region is formed in the lithium metal. However, in the battery, the cations of the metal M1 are used to form the doping region (such as the alloy layer M1) due to the reaction with lithium. Therefore, when analyzing the non-aqueous electrolyte collected from the battery, the content of the cations of the metal M1 can be less than 0.5 mmol. On the other hand, it is rare for the cations of the metal M1 to be completely consumed. From the viewpoint of obtaining the effect of the present disclosure, the non-aqueous electrolyte collected from the battery only needs to contain metal M1 above the detection limit.

[0027] It should be noted that the lithium metal deposited on the negative electrode forms a doped region of the metal M1, so when the negative electrode is analyzed, the metal M1 is detected. The metal M1 can be detected as an alloy with lithium.

[0028] The concentration of halide ions in the non-aqueous electrolyte is not limited, for example, it can be 0.5 mmol / L or more and 100 mmol / L or less, or it can be 5 mmol / L or more and 50 mmol / L or less. When the halide ions in the above range are used, a sufficiently significant effect is obtained on dissolving dendrite precursors or dendrite-like precipitates. However, halide ions are used for redox reactions in the battery. Therefore, when analyzing the non-aqueous electrolyte collected from the battery, the content of halide ions can be less than 0.5 mmol.

[0029] The cations and halide ions of the metal M1 can be derived from a salt represented by the general formula: MXn (hereinafter referred to as MXn salt). Here, M represents an atom of the metal M1 (or M2), and X represents a halogen atom. n is, for example, an integer of 1 to 4. Specific examples of MXn salts include InCl3, InBr3, InBr, InI3, InI, SnCl4, SnBr4, SnBr2, SnI4, SnI2, AuCl, AuBr, AuI, AgCl, AgBr, AgI, PtI2, PtBr2, etc. Among them, in terms of easy ion dissociation in the non-aqueous electrolyte, SnI4, AuI, etc. are preferred, and at least one of SnI4 and AuI is more preferred.

[0030] The non-aqueous electrolyte may further contain oxalate complex anions. The oxalate complex anions may be derived, for example, from an oxalate complex salt. Compared to other components contained in the non-aqueous electrolyte, the oxalate complex anions decompose at a higher potential, forming a thin and uniform film on the surface of the lithium metal. When halide ions are contained in the non-aqueous electrolyte, the film derived from the oxalate complex anions significantly improves the storage characteristics of the lithium secondary battery.

[0031] The coating derived from the oxalate complex anion has flexibility. In addition to the doping layer (or alloy layer) M1, a soft coating derived from the oxalate complex anion is also formed, so that the lithium metal is appropriately and firmly pressed by the doping layer M1 and the coating. In addition, the coating derived from the oxalate complex anion can easily follow the changes in its surface shape when the lithium metal dissolves. That is, the coating is in a state of constant contact with the lithium metal, and it is easy to exert a pressing effect. As a result, the formation of dendrite precursors is significantly suppressed, and the number of dendrite precursors is reduced, so that the amount of reaction between the dendrite precursors and the halide ions is also reduced. It is believed that by exerting these effects simultaneously, the self-discharge of the positive electrode is significantly suppressed, and the storage characteristics of the lithium secondary battery are significantly improved.

[0032] As the oxalate complex anion, bisoxalatoborate anion (BOB anion), difluorooxalatoborate anion (FOB anion) and the like are preferred. Among them, lithium difluorooxalatoborate (LiFOB) is ideal in terms of forming a stable film on the negative electrode surface even at high temperatures.

[0033] The concentration of the oxalate complex anion in the nonaqueous electrolyte is not limited, and may be, for example, 50 mmol / L to 500 mmol / L, 50 mmol / L to 300 mmol / L, or 80 mmol / L to 150 mmol / L.

[0034] The content of each component of the non-aqueous electrolyte can be determined using, for example, high performance liquid chromatography.

[0035] Hereinafter, the lithium secondary battery of the present disclosure will be described more specifically according to each component.

[0036] (Positive electrode)

[0037] The positive electrode contains a positive electrode active material. The positive electrode generally includes a positive electrode current collector and a positive electrode mixture held on the positive electrode current collector. The positive electrode mixture may contain a positive electrode active material as an essential component, and may contain a binder, a thickener, a conductive agent, etc. as optional components. The positive electrode generally has a layered positive electrode mixture (hereinafter referred to as a positive electrode mixture layer) held by the positive electrode current collector. The positive electrode mixture layer can be formed as follows: a positive electrode paste in which the constituent components of the positive electrode mixture are dispersed in a dispersion medium is coated on the surface of the positive electrode current collector and dried, whereby it can be formed. The dried coating film can be calendered as needed.

[0038] As the positive electrode active material, for example, a lithium transition metal composite oxide having a layered rock salt structure is used. Among them, a lithium transition metal composite oxide containing Ni, Co, and at least one of Al and Mn (hereinafter also referred to as composite oxide NC) is promising because it exhibits high capacity and high voltage.

[0039] The composition of the composite oxide NC can be represented, for example, by Li α Ni (1-x1-x2-x3-y) Co x1 Mn x2 Al x3 M y O 2+β (0.95 ≤ α ≤ 1.05, 0.5 ≤ 1 - x1 - x2 - x3 - y ≤ 0.95, 0 < x1 ≤ 0.04, 0 ≤ x2 ≤ 0.1, 0 ≤ x3 ≤ 0.1, 0 < x2 + x3 ≤ 0.2, 0 ≤ y ≤ 0.1, -0.05 ≤ β ≤ 0.05). Among them, M is at least one selected from the group consisting of Ti, Zr, Nb, Mo, W, Fe, Zn, B, Si, Mg, Ca, Sr, and Y. From the viewpoint of increasing the capacity, (1 - x1 - x2 - x3 - y) representing the ratio (atomic ratio) of Ni is expected to satisfy 0.8 ≤ 1 - x1 - x2 - x3 - y ≤ 0.95, and more preferably satisfy 0.9 ≤ 1 - x1 - x2 - x3 - y ≤ 0.95.

[0040] Examples of the binder include fluororesins (e.g., polytetrafluoroethylene, polyvinylidene fluoride), polyolefin resins (e.g., polyethylene, polypropylene), polyamide resins (e.g., aromatic polyamide resins), polyimide resins (e.g., polyimide, polyamideimide), acrylic resins (e.g., polyacrylic acid, polymethacrylic acid, acrylic acid-methacrylic acid copolymers, ethylene-acrylic acid copolymers, or salts thereof), vinyl resins (e.g., polyvinyl acetate), and rubber-like materials (e.g., styrene-butadiene copolymer rubber (SBR)).

[0041] Examples of thickeners include cellulose derivatives such as cellulose ethers. Examples of cellulose derivatives include CMC and its modified products, methylcellulose, and the like. Modified CMC products also include CMC salts. Examples of salts include alkali metal salts (e.g., sodium salts) and ammonium salts.

[0042] Examples of the conductive agent include conductive fibers and conductive particles. Examples of the conductive fibers include carbon fibers, carbon nanotubes, and metal fibers. Examples of the conductive particles include conductive carbon (carbon black, graphite, etc.) and metal powders.

[0043] The positive electrode current collector can be selected according to the type of nonaqueous electrolyte secondary battery. Metal foil can be used as the positive electrode current collector. Examples of the material of the metal foil include stainless steel, aluminum, aluminum alloys, and titanium. The thickness of the positive electrode current collector is not particularly limited, but can be, for example, 1 to 50 μm, or 5 to 30 μm.

[0044] (negative electrode)

[0045] The negative electrode has a negative electrode collector. During charging, lithium metal is deposited on the negative electrode collector, and during discharging, the lithium metal is dissolved. The lithium ions that form lithium metal are supplied by the non-aqueous electrolyte, and the lithium ions are supplied to the non-aqueous electrolyte by the positive electrode. The negative electrode may include a lithium ion storage layer (a layer that reflects the capacity by the storage and release of lithium ions generated by the negative electrode active material (graphite, etc.)) supported on the negative electrode collector. In this case, the open circuit potential of the negative electrode when fully charged can be 70 mV or less relative to lithium metal (lithium dissolution potential). When the open circuit potential of the negative electrode when fully charged is 70 mV or less relative to lithium metal, lithium metal exists on the surface of the lithium ion storage layer when fully charged. That is, the negative electrode reflects the capacity generated by the precipitation and dissolution of lithium metal.

[0046] Here, a fully charged state refers to a state in which the battery is charged to a charge level of 0.98×C or higher, for example, with the rated capacity of the battery being C. The open circuit potential of the negative electrode at a fully charged state can be measured by disassembling the fully charged battery under an argon atmosphere, removing the negative electrode, and assembling a battery cell using lithium metal as the counter electrode. The non-aqueous electrolyte in the battery cell can have the same composition as the non-aqueous electrolyte in the disassembled battery.

[0047] The lithium ion storage layer is formed by forming a layer of a negative electrode mixture containing a negative electrode active material. The negative electrode mixture contains a binder, a thickener, a conductive agent, etc. in addition to the negative electrode active material.

[0048] Examples of negative electrode active materials include carbonaceous materials, Si-containing materials, and Sn-containing materials. The negative electrode may contain a single negative electrode active material or a combination of two or more. Examples of carbonaceous materials include graphite, easily graphitizable carbon (soft carbon), and non-graphitizable carbon (hard carbon).

[0049] Binders and conductive agents, for example, those described for the positive electrode can be used. The shape and thickness of the negative electrode current collector can be selected from the shapes and ranges described for the positive electrode current collector. Examples of materials for the negative electrode current collector (metal foil) include stainless steel, nickel, nickel alloys, copper, and copper alloys.

[0050] (non-aqueous electrolyte)

[0051] The term "non-aqueous electrolyte" refers to a concept encompassing liquid non-aqueous electrolytes (i.e., non-aqueous electrolytes), gel electrolytes, and solid electrolytes, and is a concept excluding aqueous electrolytes. A gel electrolyte may be a non-fluid electrolyte formed by a composite of a non-aqueous electrolyte and a gelling agent or a matrix material. The non-aqueous electrolyte comprises a non-aqueous solvent, a lithium salt dissolved in the non-aqueous solvent, and an additive. The cation of the metal M1 is contained in the additive. In the case where it is derived from a lithium salt, the halide ion is contained in the additive. In the case where it is derived from a lithium salt, the oxalate complex anion is contained in the additive.

[0052] Examples of the non-aqueous solvent constituting the non-aqueous electrolyte include cyclic carbonates, chain carbonates, cyclic carboxylates, chain carboxylates, and chain ethers. Examples of cyclic carbonates include propylene carbonate (PC) and ethylene carbonate (EC). Examples of chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylates include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of chain carboxylates include methyl formate, ethyl formate, propyl formate, methyl acetate (MA), ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. Examples of chain ethers include dialkyl ethers and difluoroalkyl ethers having 1 to 4 carbon atoms. The non-aqueous solvent may contain one type alone or a combination of two or more types.

[0053] Examples of lithium salts constituting the non-aqueous electrolyte include LiClO4, LiBF4, LiPF6, LiAlCl4, LiSbF6, LiSCN, LiCF3SO3, LiCF3CO2, LiAsF6, LiB 10 Cl10, lithium lower aliphatic carboxylates, borates, and imide salts. Examples of borates include lithium bis(1,2-benzenediolate (2-)-O,O')borate, lithium bis(2,3-naphthalenediolate (2-)-O,O')borate, lithium bis(2,2'-biphenylediolate (2-)-O,O')borate, and lithium bis(5-fluoro-2-olato-1-benzenesulfonic acid-O,O')borate. Examples of imide salts include lithium bis(fluorosulfonyl)imide (LiN(FSO2)2) (hereinafter also referred to as LiFSI), lithium bis(trifluoromethylsulfonyl)imide (LiN(CF3SO2)2), lithium trifluoromethanesulfonate fluorosulfonylimide (LiN(CF3SO2)(FSO2)), lithium trifluoromethanesulfonate nonafluorobutanesulfonylimide (LiN(C3SO2)(C4F9SO2)), and lithium bis(pentafluoroethanesulfonyl)imide (LiN(C2F5SO2)2). The nonaqueous electrolyte may contain one lithium salt or a combination of two or more.

[0054] The concentration of the lithium salt in the non-aqueous electrolyte is, for example, 0.5 mol / L or more and 2 mol / L or less.

[0055] (Separator)

[0056] It is generally desirable to sandwich a separator between the positive electrode and the negative electrode. The separator has high ion permeability and possesses appropriate mechanical strength and insulation properties. As the separator, for example, a microporous film, a woven fabric, or a non-woven fabric, or a laminate of at least two selected from these can be used. As the material of the separator, polyolefins (e.g., polypropylene, polyethylene) are preferred.

[0057] (other)

[0058] As an example of the structure of a lithium secondary battery, an electrode group in which a positive electrode and a negative electrode are wound across a separator, and a structure in which a non-aqueous electrolyte is housed in an outer shell can be cited. Alternatively, an electrode group in other forms such as a stacked electrode group in which a positive electrode and a negative electrode are stacked across a separator can be used instead of a wound electrode group. The lithium secondary battery can be, for example, in any form such as a cylindrical, square, coin-shaped, button-shaped, or laminated type.

[0059] In partial cross-sectional view Figure 1 Schematically shows the structure of an example of the lithium secondary battery of the present disclosure. Figure 1 The illustrated lithium secondary battery 100 is a cylindrical secondary battery.

[0060] The lithium secondary battery 100 is a wound battery comprising a wound electrode assembly 40 and a non-aqueous electrolyte. The electrode assembly 40 comprises a strip-shaped positive electrode 10, a strip-shaped negative electrode 20, and a separator 30. The separator 30 is disposed between the positive electrode 10 and the negative electrode 20. A positive electrode lead 13 is connected to the positive electrode 10. A negative electrode lead 23 is connected to the negative electrode 20.

[0061] One end of the positive electrode lead 13 is connected to the positive electrode 10, and the other end is connected to the sealing member 50. The sealing member 50 includes a positive electrode terminal 50a. The sealing member 50 generally includes a mechanism that operates as a safety valve when the internal pressure of the battery rises.

[0062] One end of the negative electrode lead 23 is connected to the negative electrode 20, and the other end is connected to the bottom of the shell (shell body) 60. The shell 60 functions as a negative electrode terminal. The shell 60 is a cylindrical can with a bottom. The shell 60 is made of metal, for example, iron. The inner surface of the iron shell 60 is usually nickel-plated. An upper insulating ring 81 and a lower insulating ring 82 made of resin are respectively arranged at the upper and lower parts of the electrode group 40. The electrode group 40 and the non-aqueous electrolyte are arranged inside the shell 60. The shell 60 is sealed by a sealing body 50 and a gasket 70.

[0063] [Example]

[0064] Hereinafter, the present invention will be specifically described based on Examples and Comparative Examples, but the present invention is not limited to the following Examples.

[0065] Examples 1 to 4 and Comparative Examples 1 to 4

[0066] A lithium secondary battery was produced and evaluated according to the following procedures.

[0067] (1) Preparation of positive electrode

[0068] The positive electrode active material particles (composition: LiNi 0.9 Co 0.05 Al0.05 100 parts by mass of 1,000 parts by mass of 1,000 parts by mass of acetylene black, 1 part by mass of polyvinylidene fluoride, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) were mixed to obtain a positive electrode slurry. Next, the positive electrode slurry was applied to one side of an aluminum foil, and the coating was dried and then rolled to form a positive electrode mixture layer (thickness 95 μm, density 3.6 g / cm 3 ), and obtain the positive electrode.

[0069] (2) Preparation of negative electrode

[0070] The negative electrode was produced by cutting an electrolytic copper foil (thickness 10 μm) into a predetermined electrode size.

[0071] (3) Preparation of non-aqueous electrolyte

[0072] LiN(FSO2)2 (i.e., LiFSI) was dissolved at a concentration of 1 mol / L in a mixed nonaqueous solvent containing DME (dimethoxyethane) and 1,1,1-trifluoroethyl-2,2,3,3-tetrafluoroethyl ether in a volume ratio of 1:3. The additives listed in Tables 1 and 2 (MXn salts (AuI, SnI4, LiI), LiFOB) were dissolved at the concentrations shown in Tables 1 and 2 to prepare a nonaqueous electrolyte. LiFOB is lithium difluorooxalatoborate.

[0073] (4) Production of lithium secondary batteries

[0074] An aluminum tab is installed on the positive electrode, and a nickel tab is installed on the negative electrode. Next, the positive electrode, negative electrode, and separator are arranged in a manner such that a separator is arranged between the positive and negative electrodes, and they are combined and wound into a spiral shape. The wound electrode group is housed in a bag-shaped outer shell formed by a laminate sheet containing an aluminum layer. After injecting a non-aqueous electrolyte into the outer shell, the outer shell is sealed to obtain a lithium secondary battery (evaluation battery cell).

[0075] (5) Battery evaluation

[0076] (Charge and discharge cycle characteristics)

[0077] The evaluation battery cell was charged at a constant current of 0.3 It until the voltage reached 4.1 V at a temperature of 25°C. Thereafter, the battery cell was charged at a constant voltage of 4.1 V until the current reached 0.05 It. Next, the battery cell was discharged at a constant current of 0.3 It until the voltage reached 2.5 V. This charge and discharge cycle was repeated 100 times, and the ratio of the discharge capacity at the 100th cycle to the discharge capacity at the 1st cycle was calculated as the capacity retention rate (R 100 The results are shown in Table 1. Battery cells A1 to A4 are examples, and battery cells B1 to B3 are comparative examples.

[0078] [Table 1]

[0079]

[0080] As can be understood from Table 1, when the non-aqueous electrolyte solution containing the MXn salt is used, the capacity retention rate is significantly improved compared to the comparative example.

[0081] (Save Properties)

[0082] For a portion of the evaluation battery cells, constant current charging was performed at a current of 0.3 It until the voltage reached 4.1 V at a temperature of 25°C, and then constant voltage charging was performed at a constant voltage of 4.1 V until the current reached 0.05 It. Next, the charged battery cells were stored at 25°C for 10 days, and then constant current discharge was performed at a current of 0.3 It until the voltage reached 2.5 V. The ratio of the discharge capacity after storage for 10 days to the discharge capacity of the first cycle was calculated as the capacity retention rate after storage (R St25 ). The results are shown in Table 2. Battery cell B4 is a comparative example.

[0083] [Table 2]

[0084]

[0085] As can be understood from Table 2, particularly good storage characteristics are obtained when MXn salt and LiFOB are used in combination, and the storage characteristics exhibit completely different behaviors depending on the presence or absence of cations of metal M1 other than lithium.

[0086] Industrial applicability

[0087] The present disclosure can be used in lithium secondary batteries.

[0088] Description of Reference Numerals

[0089] 10 positive electrode

[0090] 13 Positive lead

[0091] 20 negative electrode

[0092] 23 Negative lead

[0093] 30 dividers

[0094] 40 electrode set

[0095] 50 Sealing body

[0096] 50a positive terminal

[0097] 60 Shell (housing body)

[0098] 70 gasket

[0099] 81 Upper insulation ring

[0100] 82 Lower insulation ring

[0101] 100 lithium secondary batteries

Claims

1. A lithium secondary battery comprising: a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte, In the negative electrode, lithium metal is deposited during charging and dissolved during discharging. The non-aqueous electrolyte comprises lithium ions, cations of a metal M1 that forms an alloy with lithium, oxalate complex anions, and halide ions. in, The metal M1 is at least one selected from the group consisting of In, Sn, Au, Ag, Pt, Zn, Sb, Bi, Si, and Mg.

2. The lithium secondary battery according to claim 1, wherein The metal M1 is at least one selected from the group consisting of Au, Ag, Pt, Sb, Bi, and Si.

3. The lithium secondary battery according to claim 1, wherein The concentration of the metal M1 cation in the non-aqueous electrolyte is 100 mmol / L or less.

4. The lithium secondary battery according to any one of claims 1 to 3, wherein The concentration of the halide ions in the non-aqueous electrolyte is 0.5 mmol / L or more and 100 mmol / L or less.

5. The lithium secondary battery according to any one of claims 1 to 3, wherein The lithium metal and the metal M1 form an alloy on the negative electrode.

6. A non-aqueous electrolyte for a lithium secondary battery, comprising: lithium ions, cations of a metal M1 that forms an alloy with lithium, oxalate complex anions, and halide ions, in, The metal M1 is at least one selected from the group consisting of In, Sn, Au, Ag, Pt, Zn, Sb, Bi, Si, and Mg.

7. The non-aqueous electrolyte for lithium secondary batteries according to claim 6, wherein The concentration of the metal M1 cation is greater than or equal to 0.5 mmol / L and less than or equal to 100 mmol / L.

8. The non-aqueous electrolyte for lithium secondary batteries according to claim 6 or 7, wherein The concentration of the halide ions is 0.5 mmol / L or more and 100 mmol / L or less.

9. The non-aqueous electrolyte for lithium secondary batteries according to claim 6 or 7, wherein The metal M1 is at least one selected from the group consisting of Au, Ag, Pt, Sb, Bi, and Si.

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