Lithium secondary battery

Abstract

This lithium secondary battery comprises a positive electrode, a negative electrode, a separator that is arranged between the positive electrode and the negative electrode, and a nonaqueous electrolyte. The positive electrode contains a lithium transition metal composite oxide. The lithium transition metal composite oxide contains at least Ni, and the proportion of Ni in all metal elements other than Li is 90% by mole or more. At the negative electrode, lithium metal is deposited during charging, while lithium metal is dissolved during discharging. The nonaqueous electrolyte contains a nonaqueous solvent, lithium ions, oxalate complex anions containing fluorine, and nitrate anions.

Description

[Technical Field]

[0001] The present disclosure relates to lithium secondary batteries. [Background technology]

[0002] Lithium-ion secondary batteries are known as high-capacity secondary batteries. In lithium-ion secondary batteries, for example, carbon materials are used as the negative electrode active material. The carbon materials are charged and discharged by reversibly inserting and extracting lithium ions.

[0003] On the other hand, lithium secondary batteries (also called lithium metal secondary batteries) that use lithium metal as the negative electrode active material have an even 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 discharging.

[0004] However, it is difficult to control the morphology of lithium metal deposition in lithium secondary batteries. When lithium metal deposits in a dendrite-like form, the specific surface area of ​​the negative electrode increases, increasing the risk of side reactions with the non-aqueous electrolyte. Furthermore, inactive lithium that cannot contribute to charge and discharge is generated, resulting in a decrease in discharge capacity.

[0005] Patent Document 1 proposes a nonaqueous electrolyte secondary battery that includes a positive electrode having a positive electrode current collector and a positive electrode composite layer formed on the current collector, a negative electrode having a negative electrode current collector, and a nonaqueous electrolyte, in which lithium metal is deposited on the negative electrode current collector during charging and the lithium metal dissolves in the nonaqueous electrolyte during discharging, the nonaqueous electrolyte containing a lithium salt having an oxalate complex as the anion. By adding a lithium salt having an oxalate complex as the anion to the nonaqueous electrolyte, lithium metal is uniformly deposited on the negative electrode, and swelling of the negative electrode is specifically suppressed.

[0006] Patent Document 2 proposes a lithium-ion electrochemical battery comprising a metal oxide cathode, an anode that is lithium metal or a lithium metal alloy, a separator between the anode and the cathode, a non-aqueous electrolyte containing one or more non-aqueous solvents and one or more lithium salts, and a nitrogen-containing material, the nitrogen-containing material comprising an inorganic nitrate and being soluble in the electrolyte. Patent Document 2 states, "During operation or cycling of the electrochemical cell, the nitrogen-containing compound can form a uniform ion-conducting surface layer on the lithium anode. The formation of the ion-conducting surface layer facilitates uniform deposition of lithium on the anode during charging of the battery by suppressing dendrite formation and the growth of high-surface-area lithium on the anode." [Prior art documents] [Patent documents]

[0007] [Patent Document 1] International Publication No. 2018 / 179782 [Patent Document 2] Japanese Patent Application Publication No. 2019-24009 Summary of the Invention

[0008] To achieve a higher capacity lithium secondary battery, it is desirable to use a lithium transition metal composite oxide containing a large amount of Ni as the positive electrode active material. Therefore, the use of a lithium transition metal composite oxide containing at least Ni, in which the proportion of Ni to all metal elements other than Li is 90 mol % or more, has been investigated. However, in this case, the methods proposed in Patent Documents 1 and 2 have difficulty in sufficiently improving the decrease in discharge capacity when the lithium secondary battery is repeatedly charged and discharged.

[0009] One aspect of the present disclosure relates to a lithium secondary battery including a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte, wherein the positive electrode includes a lithium transition metal composite oxide that includes at least Ni, where the proportion of Ni relative to all metal elements other than Li is 90 mol % or more, lithium metal precipitates in the negative electrode during charging and dissolves during discharging, and the non-aqueous electrolyte includes a non-aqueous solvent, lithium ions, fluorine-containing oxalate complex anions, and nitrate anions.

[0010] According to the present disclosure, it is possible to suppress a decrease in discharge capacity when a lithium secondary battery is repeatedly charged and discharged. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a partial cross-sectional view schematically illustrating an example of a lithium secondary battery according to the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0012] The lithium secondary battery according to the present disclosure includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. Lithium metal precipitates in the negative electrode during charging and dissolves during discharging. Specifically, the negative electrode includes at least a negative electrode current collector, and lithium metal precipitates on the negative electrode current collector. The lithium secondary battery according to the present disclosure is also referred to as a lithium metal secondary battery.

[0013] In a lithium (metal) secondary battery, for example, 70% or more of the rated capacity is achieved by the deposition and dissolution of lithium metal. The movement of electrons at the negative electrode during charging and discharging is mainly due to the deposition and dissolution of lithium metal at the negative electrode. Specifically, 70 to 100% (e.g., 80 to 100% or 90 to 100%) of the movement of electrons (current from another perspective) at the negative electrode during charging and discharging is due to the deposition and dissolution of lithium metal. In other words, the negative electrode according to this embodiment differs from a negative electrode in which the movement of electrons at the negative electrode during charging and discharging is mainly due to the absorption and release of lithium ions by the negative electrode active material (such as graphite).

[0014] Here, the positive electrode contains a lithium transition metal composite oxide as a positive electrode active material. The lithium transition metal composite oxide contains at least Ni, and the proportion of Ni in all metal elements other than Li is 90 mol % or more. Hereinafter, a lithium transition metal composite oxide containing at least Ni, and having a proportion of Ni in all metal elements other than Li of 90 mol % or more, will also be referred to as composite oxide A. Composite oxide A has a particularly high capacity among lithium transition metal composite oxides.

[0015] It is desirable that composite oxide A further contains Al. Lithium transition metal composite oxides with a high Ni content tend to have an unstable crystal structure. In contrast, Al improves the stability of the crystal structure of composite oxide A, thereby contributing to improved thermal stability and durability. In composite oxide A, the proportion of Al to all metal elements other than Li may be 10 mol % or less. From the viewpoint of further increasing capacity, the Al proportion may be 7 mol % or less, or may be 5 mol % or less. On the other hand, from the viewpoint of the stability of the crystal structure of composite oxide A and increasing durability, the Al proportion may be 0 mol % or more, preferably 1 mol % or more, and more preferably 3 mol % or more.

[0016] It is desirable that the lithium transition metal composite oxide further contains Co. Co contributes to the stability of the crystal structure of composite oxide A, thereby improving output characteristics and durability, as well as capacity development. In composite oxide A, the proportion of Co in all metal elements other than Li may be 10 mol % or less. From the viewpoint of further increasing capacity, the Co proportion may be 7 mol % or less, or may be 5 mol % or less. On the other hand, from the viewpoint of improving the stability of the crystal structure and output characteristics of composite oxide A, the Co proportion may be 0 mol % or more, preferably 1 mol % or more, and more preferably 3 mol % or more.

[0017] Next, the non-aqueous electrolyte contains a non-aqueous solvent, lithium ions, oxalate complex anions having fluorine (or a fluoro group), and nitrate anions.

[0018] The fluorine-containing oxalate complex anion (hereinafter also referred to as F-containing oxalate complex anion) has the effect of suppressing lithium metal from depositing in a dendritic state. In this embodiment, even if a fluorine-free oxalate complex anion is used instead of the F-containing oxalate complex anion, it is difficult to suppress lithium metal from depositing in a dendritic state. As a result, a sudden short circuit may occur during the charge / discharge cycle, and the cycle life may be significantly shorter than expected.

[0019] During charging, protruding deposits may be formed on the negative electrode at any time. Dendritic deposits of lithium metal grow from the protruding deposits as nuclei. If the protruding deposits (hereinafter also referred to as dendrite precursors) are left as they are, it becomes difficult to suppress the formation of dendritic deposits.

[0020] In contrast, the F-containing oxalate complex anion decomposes at a higher potential than the other components in the non-aqueous electrolyte, forming a thin, uniform coating on the surface of lithium metal, which is thought to deposit mainly between the coating and the negative electrode current collector.

[0021] However, when the non-aqueous electrolyte contains an F-containing oxalate complex anion, the composite oxide A contained in the positive electrode is likely to deteriorate. In particular, composite oxide A with a low Co content and a high Ni content is significantly deteriorated by the F-containing oxalate complex anion. Furthermore, it is believed that the improved stability of the negative electrode makes it easier to notice the impact of positive electrode deterioration on battery performance.

[0022] On the other hand, when nitrate anions are contained in the non-aqueous electrolyte, the deterioration of composite oxide A, which has a high Ni content, is suppressed. Nitrate anions significantly suppress the decrease in discharge capacity during repeated charge-discharge cycles. This is thought to be because nitrate anions are adsorbed onto composite oxide A and suppress the side reaction between oxalate complex anions and composite oxide A.

[0023] Furthermore, the uniformity of the coating derived from the F-containing oxalate complex anion is significantly improved, particularly when nitrate anions are contained in the nonaqueous electrolyte. The highly uniform coating derived from nitrate anions and F-containing oxalate complex anions covers a larger area of ​​the lithium metal and has high flexibility. It is believed that the formation of a flexible coating on most of the lithium metal surface firmly presses the lithium metal against the coating. This pressing effect suppresses the elongation of dendritic precipitates. Furthermore, the flexible coating can easily follow the changes in the surface shape of the lithium metal as it dissolves. In other words, the coating is always in contact with the lithium metal, making the pressing effect more likely to be exerted. As a result, the generation of dendrite precursors is significantly suppressed, and the reduction in dendrite precursors is believed to significantly improve the effect of suppressing dendritic precipitation of lithium metal.

[0024] The F-containing oxalate complex anion may be, for example, one derived from an F-containing oxalate complex salt. As the F-containing oxalate complex salt, for example, an F-containing oxalate complex lithium salt can be used, but is not limited thereto.

[0025] The F-containing oxalate complex anion preferably contains boron (B) or phosphorus (P). Examples of the F-containing oxalate complex anion containing boron include difluorooxalate borate anion (BF(CO)). - ) (hereinafter also referred to as FOB anion). Examples of F-containing oxalate complex anions containing phosphorus include LiPF4(C2O4) and LiPF2(C2O2)2. Among them, F-containing oxalate complex anions containing boron are more suitable than F-containing oxalate complex anions containing phosphorus in that they form a stable coating on the surface of lithium metal even at high temperatures, and FOB anions are the most preferred.

[0026] The concentration C1 of the F-containing oxalate complex anion in the nonaqueous electrolyte may be, for example, 0.1 mmol / L or more and 500 mmol / L or less, 10 mmol / L or more and 300 mmol / L or less, or 80 mmol / L or more and 150 mmol / L or less.

[0027] The concentration C2 of nitrate anions in the nonaqueous electrolyte may be, for example, 0.1 mmol / L or more and 50 mmol / L or less, 0.5 mmol / L or more and 20 mmol / L or less, or 1 mmol / L or more and 10 mmol / L or less.

[0028] Even a small amount of nitrate anion has the effect of reducing the influence of the F-containing oxalate complex anion on the positive electrode. Therefore, the ratio of concentration C2 to concentration C1 (C2 / C1) may be, for example, 0.01 or more and less than 1, 0.01 or more and 0.6 or less, or 0.01 or more and 0.5 or less.

[0029] The nonaqueous solvent of the nonaqueous electrolyte preferably contains an ether compound. Ether compounds have high resistance to reduction at the negative electrode and are less likely to cause side reactions with lithium metal. The ether compound preferably accounts for, for example, 50% by volume or more of the total nonaqueous solvent, but may also account for 70% by volume or more, or 90% by volume or more, and the entire nonaqueous solvent may be an ether compound. Here, the content of the ether compound is calculated assuming that all substances other than ionic substances that dissociate in the nonaqueous electrolyte are considered to be nonaqueous solvent.

[0030] The content of each component of the non-aqueous electrolyte can be determined using, for example, high performance liquid chromatography, gas chromatography-mass spectrometry (GC-MS), NMR, inductively coupled plasma mass spectrometry (ICP-MS), elemental analysis, or the like.

[0031] The ether compound includes, for example, a dialkyl ether having 1 to 5 carbon atoms. The dialkyl ether having 1 to 5 carbon atoms is, for example, a compound represented by the general formula: R1-(OCH2CH2)n-OR2 The non-aqueous solvent may be a chain ether compound (hereinafter also referred to as ether compound A) represented by the following formula: R1 and R2 are each independently an alkyl group having 1 to 5 carbon atoms, and preferably an alkyl group having 1 to 2 carbon atoms. Furthermore, n is 1 to 4, and preferably 1 or 2. By using ether compound A as the main component of the non-aqueous solvent, the solubility of the lithium salt in the non-aqueous electrolyte is increased, and high fluidity and high lithium ion conductivity of the non-aqueous electrolyte are ensured.

[0032] Here, the main component of the non-aqueous solvent refers to a component that occupies, for example, 20% by volume or more of the non-aqueous solvent. It is desirable that the ether compound A occupies, for example, 20% by volume or more and 80% by volume or less of the non-aqueous solvent.

[0033] Examples of ether compounds include tetrahydrofuran, 1,2-dimethoxyethane (DME), 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol ethyl methyl ether, diethylene glycol dibutyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, etc. One type of ether compound may be used alone, or two or more types may be used in combination.

[0034] The ether compound may be a fluorinated ether compound. Examples of the fluorinated ether compound include difluoroalkyl ethers having 1 to 4 carbon atoms. The fluorinated ether compound may account for, for example, 20% by volume or more and 80% by volume or less of the non-aqueous solvent.

[0035] The fluorination rate of the fluorinated ether compound may be 60% or more, or may be 100%. Here, the fluorination rate of the fluorinated ether compound is the ratio of the number of fluorine atoms to the total number of fluorine atoms and hydrogen atoms contained in the fluorinated ether compound, expressed as a percentage (%).

[0036] The use of fluorinated ether compounds reduces the interaction of oxygen atoms in the ether backbone with lithium ions. Due to their strong electronegativity, the fluorine atoms in fluorinated ether compounds attract electrons from the entire molecule toward the inner core. The introduction of fluorine into ether compounds lowers the orbital level of the lone electron pair of oxygen atoms in the ether backbone, which would otherwise interact with lithium ions. The overlap between orbitals is reduced, weakening the interaction between lithium ions and the ether. Because fluorinated ether compounds are less likely to capture lithium ions, they are more likely to be reduced to lithium metal on the anode surface. This allows for the formation of a uniform SEI coating, suppressing the formation of dendrite-like lithium metal. This suppresses side reactions between lithium metal and the nonaqueous electrolyte, allowing for more uniform charge / discharge reactions.

[0037] Specific examples of the fluorinated ether compound include, for example, 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether, 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, and the like. The fluorinated ether compound may be used alone or in combination of two or more.

[0038] When the non-aqueous solvent contains an ether compound, the charge-discharge reaction at the negative electrode proceeds more uniformly, but the influence of the deterioration of the positive electrode becomes more apparent. Therefore, the effect of suppressing the deterioration of the composite oxide A with a high Ni content by nitrate anions also becomes relatively large. In other words, when the positive electrode contains the composite oxide A and the non-aqueous electrolyte contains an F-containing oxalate complex anion and an ether compound, nitrate anions have a great influence on improving the charge-discharge cycle characteristics of the lithium secondary battery.

[0039] Hereinafter, the lithium secondary battery according to the present disclosure will be described more specifically for each component.

[0040] (Positive electrode) The positive electrode contains a positive electrode active material. The positive electrode usually includes a positive electrode current collector and a positive electrode mixture held by the positive electrode current collector. The positive electrode mixture contains, as an essential component, a positive electrode active material, and may contain, as optional components, a binder, a thickener, a conductive agent, and the like. The positive electrode usually includes 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 by applying a positive electrode slurry in which the constituent components of the positive electrode mixture are dispersed in a dispersion medium to the surface of the positive electrode current collector and drying it. The dried coating film may be rolled if necessary.

[0041] As the positive electrode active material, at least the composite oxide A already described is used. The composite oxide A is a lithium transition metal composite oxide having a layered rock salt structure. The composite oxide A only needs to be the main component of the positive electrode active material.

[0042] Here, the main component of the positive electrode active material refers to, for example, a component that occupies 50% by mass or more of the positive electrode active material. It is desirable that the composite oxide A occupies 50% by mass or more, further 70% by mass or more, and further 90% by mass or more of the positive electrode active material.

[0043] The composition of the composite oxide A is, for example, Li α Ni (1-x1-x2-x3-y) Co x1 Mn x2 Al x3 M y O 2+β (0.95 ≤ α ≤ 1.05, 0.9 ≤ 1 - x1 - x2 - x3 - y ≤ 0.99, 0 ≤ x1 ≤ 0.1, 0 ≤ x2 ≤ 0.1, 0 ≤ x3 ≤ 0.1, 0 ≤ y ≤ 0.1, -0.05 ≤ β ≤ 0.05). However, 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.

[0044] The x3 indicating the ratio (atomic ratio) of Al preferably satisfies 0 < x3 ≤ 0.1, and more preferably satisfies 0.01 ≤ x3 ≤ 0.1 from the viewpoints of thermal stability and durability.

[0045] The x1 indicating the ratio (atomic ratio) of Co preferably satisfies 0 < x1 ≤ 0.1, and more preferably satisfies 0.01 ≤ x1 ≤ 0.1 from the viewpoints of output characteristics and durability.

[0046] The (1 - x1 - x2 - x3 - y) indicating the ratio (atomic ratio) of Ni preferably satisfies 0.9 ≤ 1 - x1 - x2 - x3 - y ≤ 0.95 from the viewpoints of high capacity and stability.

[0047] Examples of binders include fluororesins (e.g., polytetrafluoroethylene, polyvinylidene fluoride), polyolefin resins (e.g., polyethylene, polypropylene), polyamide resins (e.g., aramid resin), polyimide resins (e.g., polyimide, polyamideimide), acrylic resins (e.g., polyacrylic acid, polymethacrylic acid, acrylic acid-methacrylic acid copolymer, ethylene-acrylic acid copolymer, or salts thereof), vinyl resins (e.g., polyvinyl acetate), and rubber-like materials (e.g., styrene-butadiene copolymer rubber (SBR)).

[0048] Examples of thickeners include cellulose derivatives such as cellulose ether. Examples of cellulose derivatives include carboxymethyl cellulose (CMC) and its modified products, methyl cellulose, etc. Modified CMC products also include CMC salts.

[0049] Examples of conductive agents include conductive fibers and conductive particles. Examples of conductive fibers include carbon fibers, carbon nanotubes, and metal fibers. Examples of conductive particles include conductive carbon (carbon black, graphite, etc.) and metal powder.

[0050] The positive electrode current collector is selected depending on the type of nonaqueous electrolyte secondary battery. A metal foil may be used as the positive electrode current collector. Examples of materials for the metal foil include stainless steel, aluminum, aluminum alloy, and titanium. The thickness of the positive electrode current collector is not particularly limited, but may be, for example, 1 to 50 μm.

[0051] (Negative electrode) The negative electrode includes a negative electrode current collector. During charging, lithium metal is deposited on the negative electrode current collector, and during discharging, the lithium metal dissolves. The lithium ions that form the lithium metal are supplied from a non-aqueous electrolyte, and the lithium ions are supplied from the positive electrode to the non-aqueous electrolyte. The negative electrode may include a lithium ion absorption layer (a layer that develops capacity by the absorption and desorption of lithium ions by a negative electrode active material (such as graphite)) supported on the negative electrode current collector. In this case, the open circuit potential of the negative electrode at full charge may be 70 mV or less relative to lithium metal (lithium dissolution and deposition potential). When the open circuit potential of the negative electrode at full charge is 70 mV or less relative to lithium metal, lithium metal is present on the surface of the lithium ion absorption layer at full charge. In other words, the negative electrode develops capacity by the deposition and dissolution of lithium metal.

[0052] Here, "fully charged" refers to a state in which the battery is charged to a state of charge of, for example, 0.98 × C or more, where C is the rated capacity of the battery. The open circuit potential of the negative electrode at full charge can be measured by disassembling a fully charged battery under an argon atmosphere, removing the negative electrode, and assembling a cell with lithium metal as the counter electrode. The nonaqueous electrolyte in the cell may have the same composition as the nonaqueous electrolyte in the disassembled battery.

[0053] The lithium ion occlusion layer is a layer of a negative electrode mixture containing a negative electrode active material. The negative electrode mixture may contain a binder, a thickener, a conductive agent, etc. in addition to the negative electrode active material.

[0054] Examples of the negative electrode active material include a carbonaceous material, a Si-containing material, and a Sn-containing material. The negative electrode may contain one type of negative electrode active material or a combination of two or more types. Examples of the carbonaceous material include graphite, easily graphitizable carbon (soft carbon), and hardly graphitizable carbon (hard carbon).

[0055] The binder, conductive agent, etc. may be, for example, those exemplified for the positive electrode. The shape and thickness of the negative electrode current collector can be selected from the shape and range of the positive electrode current collector. Examples of materials for the negative electrode current collector (metal foil) include stainless steel, nickel, nickel alloy, copper, and copper alloy.

[0056] (non-aqueous electrolyte) The term "non-aqueous electrolyte" encompasses liquid non-aqueous electrolytes (i.e., non-aqueous electrolyte solutions), gel electrolytes, and solid electrolytes, but excludes aqueous electrolytes. The gel electrolyte and solid electrolyte may be non-fluidic electrolytes formed by combining a non-aqueous electrolyte solution with a gelling agent or a matrix material.

[0057] The non-aqueous electrolyte solution includes a non-aqueous solvent, lithium ions, fluorine-containing oxalate complex anions, and nitrate anions. For example, the non-aqueous electrolyte solution includes a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent, and the lithium salt may include an F-containing oxalate complex lithium and lithium nitrate. However, the F-containing oxalate complex anions and the nitrate anions do not necessarily have to be derived from the lithium salt; for example, the nitrate anions may be derived from a nitrate salt other than the lithium salt.

[0058] Lithium salts include, for example, LiClO4, LiBF4, LiPF6, LiAlCl4, LiSbF6, LiSCN, LiCF3SO3, LiCF3CO2, LiAsF6, LiB 10 Cl 10 , lower aliphatic lithium carboxylate, lithium borate, lithium imide, etc. These may be used alone or in combination of two or more.

[0059] Examples of lithium imide include lithium bisfluorosulfonylimide (LiN(FSO2)2) (hereinafter also referred to as LiFSI), lithium bistrifluoromethanesulfonyl imide (LiN(CF3SO2)2), lithium trifluoromethanesulfonate fluorosulfonylimide (LiN(CF3SO2)(FSO2)), lithium trifluoromethanesulfonate nonafluorobutanesulfonate imide (LiN(CF3SO2)(C4F9SO2)), and lithium bispentafluoroethanesulfonate imide (LiN(C2F5SO2)2).

[0060] The total concentration of lithium salts in the non-aqueous electrolyte may be, for example, 0.5 mol / L or more and 2 mol / L or less.

[0061] Although the ether compounds already mentioned can be used as the non-aqueous solvent, other compounds may also be used. For example, cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, chain carboxylic acid esters, chain ethers, etc. may be used as at least a portion of the non-aqueous solvent. 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 carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of chain carboxylic acid esters include methyl formate, ethyl formate, propyl formate, methyl acetate (MA), ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate.

[0062] (separator) It is desirable to interpose a separator between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulating properties. For example, a microporous thin film, a woven fabric, a nonwoven fabric, etc. can be used as the separator. The separator is preferably made of polyolefin (e.g., polypropylene, polyethylene).

[0063] (others) An example of the structure of a lithium secondary battery is a structure in which an electrode group in which a positive electrode and a negative electrode are wound with a separator interposed therebetween and a non-aqueous electrolyte are housed in an outer casing. Alternatively, a stacked electrode group in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween, or an electrode group of another form may be used. The lithium secondary battery may be in any form, such as a cylindrical type, a prismatic type, a coin type, a button type, or a laminate type.

[0064] The configuration of an example of a lithium secondary battery according to the present disclosure is shown in a partial cross-sectional view schematically in Figure 1. The lithium secondary battery 100 shown in Figure 1 is a cylindrical secondary battery.

[0065] The lithium secondary battery 100 is a wound type battery and includes a wound electrode group 40 and a non-aqueous electrolyte. The electrode group 40 includes 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.

[0066] One end of the positive electrode lead 13 is connected to the positive electrode 10, and the other end is connected to the sealing body 50. The sealing body 50 includes a positive electrode terminal 50a. Typically, the sealing body 50 includes a mechanism that operates as a safety valve when the internal pressure of the battery increases.

[0067] 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 case (case body) 60. The case 60 functions as a negative electrode terminal. The case 60 is a cylindrical can with a bottom. The case 60 is made of metal, for example, iron. The inner surface of the iron case 60 is usually nickel-plated. An upper insulating ring 81 and a lower insulating ring 82 made of resin are arranged on the top and bottom of the electrode group 40, respectively. The electrode group 40 and a non-aqueous electrolyte are arranged inside the case 60. The case 60 is sealed with a sealing body 50 and a gasket 70.

[0068] [Example] The present disclosure will be specifically described below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0069] Example 1 (1) Preparation of the positive electrode Positive electrode active material particles (composition: LiNi 0.9 Co 0.05 Al 0.05 100 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. The positive electrode slurry was then applied to both sides of an aluminum foil (positive electrode current collector). The coating was dried and then rolled to form a positive electrode mixture layer (thickness 95 μm, density 3.6 g / cm) on both sides of the aluminum foil. 3 ) was formed to obtain a positive electrode. (2) Preparation of the negative electrode The negative electrode (negative electrode current collector) was prepared by cutting electrolytic copper foil (thickness: 10 μm) to a predetermined size. (3) Preparation of non-aqueous electrolyte A non-aqueous electrolyte solution was prepared by dissolving LiN(FSO2)2 (i.e., LiFSI) at a concentration of 1 mol / L in a non-aqueous solvent mixture containing DME (1,2-dimethoxyethane) and PC (propylene carbonate) in a volume ratio of 2:1. Furthermore, lithium difluorooxalatoborate (LiFOB) as an F-containing oxalate complex lithium salt at a concentration of 100 mmol / L and lithium nitrate at a concentration of 5 mmol / L were dissolved in the mixture. (4) Fabrication of lithium secondary batteries An aluminum tab was attached to the positive electrode, and a nickel tab was attached to the negative electrode. Next, the positive electrode, negative electrode, and separator were arranged so that the separator was disposed between the positive electrode and negative electrode, and then they were spirally wound together. The wound electrode group was housed in a bag-shaped exterior body formed of a laminate sheet including an aluminum layer. After a nonaqueous electrolyte solution was poured into the exterior body, the exterior body was sealed to obtain a cell A1 for evaluation of a lithium secondary battery.

[0070] Example 2 A cell A2 was produced in the same manner as in Example 1, except that the concentration of lithium nitrate in the non-aqueous electrolyte was changed to 50 mmol / L.

[0071] Example 3 A cell A3 was prepared in the same manner as in Example 1, except that dimethyl carbonate (DMC) was used in place of PC in the non-aqueous solvent.

[0072] Example 4 A cell A4 was produced in the same manner as in Example 1, except that the concentration of LiFOB in the non-aqueous electrolyte was changed to 500 mmol / L.

[0073] Example 5 A cell A5 was prepared in the same manner as in Example 1, except that DMC was used in place of DME in the non-aqueous solvent.

[0074] Comparative Example 1 A cell B1 was fabricated in the same manner as in Example 1, except that the non-aqueous electrolyte did not contain either LiFOB or lithium nitrate.

[0075] Comparative Example 2 A cell B2 was prepared in the same manner as in Example 1, except that the non-aqueous electrolyte did not contain lithium nitrate.

[0076] Comparative Example 3 A cell B3 was prepared in the same manner as in Example 1, except that the non-aqueous electrolyte did not contain LiFOB.

[0077] Comparative Example 4 The composition of the positive electrode active material is LiNi 0.5 Co 0.2 Al 0.3 The gas pressure was changed to O2, and a cell B4 was fabricated in the same manner as in Example 1.

[0078] Comparative Example 5 A cell B5 was prepared in the same manner as in Example 5, except that the non-aqueous electrolyte did not contain lithium nitrate.

[0079] Comparative Example 6 A cell B6 was prepared in the same manner as in Example 5, except that the non-aqueous electrolyte did not contain LiFOB.

[0080] Comparative Example 7 A cell B7 was prepared in the same manner as in Comparative Example 4, except that the non-aqueous electrolyte did not contain lithium nitrate.

[0081] Comparative Example 8 A cell B8 was fabricated in the same manner as in Example 1, except that lithium dioxalatoborate (LiBOB) was added to the non-aqueous electrolyte solution instead of LiFOB.

[0082] Table 1 shows the compositions of the non-aqueous electrolyte and the positive electrode active material.

[0083] [Table 1]

[0084] <Battery evaluation> (Charge / discharge cycle characteristics) The evaluation cell was charged at a constant current of 0.3 It at a temperature of 25°C until the voltage reached 4.1 V, and then charged at a constant voltage of 4.1 V until the current reached 0.05 It. The cell was then discharged at a constant current of 0.3 It until the voltage reached 2.5 V. This charge-discharge cycle was repeated 100 times, and the ratio of the discharge capacity at the 100th cycle to the discharge capacity at the first cycle (C1) was calculated as the capacity retention rate (R 100 The results are shown in Table 2. The discharge capacity (C1) is shown as a relative value (Index) with the value obtained for cell A1 in Example 1 set at 100.

[0085] (Short circuit life (dendrite generation)) The above charge-discharge cycle was repeated after 100 cycles, and the number of cycles (N) until a short circuit due to metallic lithium dendrites occurred and the voltage suddenly dropped was determined. The results are shown in Table 2.

[0086] [Table 2]

[0087] Comparing Example 1 and Comparative Example 2, by adding a fluorine-containing oxalate complex anion to the non-aqueous electrolyte, the generation of lithium metal dendrites was significantly suppressed, while the charge-discharge cycle characteristics (R 100 In contrast, in the example in which the non-aqueous electrolyte solution contained fluorine-containing oxalate complex anions and nitrate anions, the charge-discharge cycle characteristics (R 100 ) has improved significantly.

[0088] In addition, when the results of Comparative Example 4 and Comparative Example 7, both of which used lithium transition metal composite oxides with low Ni content, are compared, it can be seen that the presence or absence of nitrate anions has only a small effect on the charge-discharge cycle characteristics. Furthermore, when the results of Comparative Example 2 and Comparative Example 7, both of which did not use nitrate anions, are compared, it can be seen that the results of Comparative Example 7 (R 100 =61.2), the comparative example 2 (R 100 = 58.6), the charge-discharge cycle characteristics are considerably reduced. The effect of the nitrate anion in improving such a reduction in charge-discharge cycle characteristics is particularly remarkable when composite oxide A with a high Ni content is used.

[0089] Furthermore, when comparing the improvement effect of Example 5 in Comparative Example 5, both of which contain no ether compounds in their non-aqueous electrolyte solutions, with the improvement effect of Example 1 in Comparative Example 2, both of which contain ether compounds in their non-aqueous electrolyte solutions, the improvement effect of Example 1 in Comparative Example 2 is significant. This indicates that the action and effect of nitrate anions is particularly pronounced when the non-aqueous electrolyte solution contains an ether compound.

[0090] The results of Comparative Example 8 show that even if LiBOB is contained in the non-aqueous electrolyte, neither the effect of suppressing dendrites nor the effect of improving charge-discharge cycle characteristics cannot be expected. [Industrial Applicability]

[0091] The present disclosure can be used in lithium secondary batteries in which lithium metal precipitates during charging and dissolves during discharging. [Explanation of symbols]

[0092] 10 positive electrode 13 Positive lead 20 negative electrode 23 Negative lead 30 Separator 40 electrode groups 50 Sealing body 50a positive terminal 60 Case (Case Body) 70 Gasket 81 Upper insulating ring 82 Lower insulating ring 100 Lithium secondary battery

Claims

1. a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte; the positive electrode contains a lithium transition metal composite oxide, the lithium transition metal composite oxide contains at least Ni, and the proportion of Ni in all metal elements other than Li is 90 mol % or more; In the negative electrode, lithium metal is precipitated during charging, and the lithium metal is dissolved during discharging, the non-aqueous electrolyte includes a non-aqueous solvent, lithium ions, fluorine-containing oxalate complex anions, and nitrate anions; a ratio C2 / C1 of a concentration C2 (mmol / L) of the nitrate anion to a concentration C1 (mmol / L) of the fluorine-containing oxalate complex anion in the nonaqueous electrolyte is 0.01 or more and less than 1; a concentration C2 of the nitrate anions in the non-aqueous electrolyte of 0.1 mmol / L or more and 50 mmol / L or less.

2. The lithium secondary battery according to claim 1 , wherein the lithium transition metal composite oxide further contains Al.

3. 3. The lithium secondary battery according to claim 1, wherein the lithium transition metal composite oxide further contains Co.

4. 4. The lithium secondary battery according to claim 1, wherein the fluorine-containing oxalate complex anion contains boron.

5. 5. The lithium secondary battery according to claim 1, wherein the concentration C1 of the fluorine-containing oxalate complex anion in the non-aqueous electrolyte is 0.1 mmol / L or more and 500 mmol / L or less.

6. 6. The lithium secondary battery according to claim 1, wherein 50% by volume or more of the non-aqueous solvent is an ether compound.

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