Non-aqueous electrolyte secondary battery

Abstract

A non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure comprises: an electrode group that includes a positive electrode, a negative electrode, and a separator that is between the positive electrode and the negative electrode; and a non-aqueous electrolyte. A protective layer that includes a polymer is provided to the surface of the negative electrode, and the non-aqueous electrolyte includes a cyclic imide compound, the cyclic imide compound content of the non-aqueous electrolyte being no more than 100 ppm.

Description

Nonaqueous electrolyte secondary battery

[0001] This invention relates to a non-aqueous electrolyte secondary battery.

[0002] A non-aqueous electrolyte secondary battery comprises an electrode group and a non-aqueous electrolyte. The electrode group includes, for example, a positive electrode, a negative electrode, and a separator placed between the positive and negative electrodes. In non-aqueous electrolyte secondary batteries, it is known that the surface of the negative electrode is covered with a protective layer containing a polymer.

[0003] Patent Document 1 discloses a negative electrode comprising a laminate in which a fluoropolymer is laminated on a layer containing a metal, wherein the metal is at least one selected from lithium, sodium, magnesium, and zinc, and the fluoropolymer is a fluorine-containing polymer containing vinylidene fluoride units and monomer units (A) having a specific structure. Patent Document 1 discloses that in a negative electrode comprising a laminate in which the above-mentioned fluorine-containing polymer is laminated on a layer containing a metal, uniform metal deposition is possible due to the interaction between the fluorine-containing polymer and lithium ions, thereby suppressing the generation of dendrites. Furthermore, Patent Document 1 discloses that by suppressing the generation of dendrites, the battery life of a secondary battery can be improved.

[0004] Patent Document 2 discloses a lithium secondary battery comprising a positive electrode that absorbs lithium ions during discharge and releases them during charging, a negative electrode that deposits lithium metal during charging and dissolves the lithium metal during discharge, and a non-aqueous electrolyte having lithium ion conductivity, wherein the surface of the negative electrode is covered with a protective layer, and the protective layer includes a block polymer in which a first polymer portion having a repeating structure of monomer unit A and a second polymer portion having a repeating structure of monomer unit B are bonded together. Patent Document 2 discloses, for example, that by forming the protective layer with a block polymer in which a first polymer portion having high mechanical strength is terminated to a second polymer portion having high lithium conductivity, it is possible to obtain high lithium ion conductivity in the second polymer portion while increasing the mechanical strength of the protective layer with the first polymer portion. Furthermore, Patent Document 2 discloses that by including the first and second polymer portions as described above in the protective layer, it is possible to achieve both high film stability and high lithium ion conductivity in the protective layer, thereby realizing a lithium-ion battery with excellent cycle characteristics.

[0005] International Publication No. 2024 / 010057, International Publication No. 2022 / 163538

[0006] In non-aqueous electrolyte secondary batteries, achieving excellent cycle characteristics is accomplished by suppressing the decrease in capacity retention rate. Therefore, in non-aqueous electrolyte secondary batteries in which the surface of the negative electrode is covered with a polymer-containing protective layer, there is room for further investigation into how to suppress the decrease in capacity retention rate.

[0007] Therefore, the objective of this disclosure is to provide a non-aqueous electrolyte secondary battery that can suppress the decrease in capacity retention rate.

[0008] One aspect of the present invention relates to a non-aqueous electrolyte secondary battery. The non-aqueous electrolyte secondary battery comprises an electrode group including a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. A protective layer containing a polymer is disposed on the surface of the negative electrode. The non-aqueous electrolyte contains a cyclic imide compound, and the content of the cyclic imide compound in the non-aqueous electrolyte is 100 ppm or less.

[0009] According to this disclosure, it is possible to provide a non-aqueous electrolyte secondary battery that can suppress the decrease in capacity retention rate.

[0010] This is a schematic cross-sectional view showing a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure.

[0011] The embodiments of this disclosure will be described below with examples, but this disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be given as examples, but other numerical values, materials, etc. may be applied as long as the effects of this disclosure are obtained. Notwithstanding, known components may be applied to components of parts that are characteristic of this disclosure. In this specification, when "the range of numerical values ​​A to numerical values ​​B" is used, that range includes numerical values ​​A and B.

[0012] In the following explanation, when examples are given for the lower and upper limits of numerical values ​​related to specific physical properties or conditions, any combination of either of the given lower limits and any of the given upper limits is permitted, as long as the lower limit does not exceed the upper limit. When multiple materials are given as examples, unless otherwise specified, one type may be selected and used alone, or two or more types may be used in combination.

[0013] This disclosure includes any combination of two or more claims that can be arbitrarily selected from the claims set forth in the attached claims. In other words, any combination of two or more claims that can be arbitrarily selected from the claims set forth in the attached claims is possible, as long as it does not result in a technical inconsistency.

[0014] [Non-aqueous electrolyte secondary battery] A non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure comprises an electrode group including a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte.

[0015] In the non-aqueous electrolyte secondary battery according to the embodiments of this disclosure, a protective layer containing a polymer is disposed on the surface of the negative electrode. In the non-aqueous electrolyte secondary battery according to the embodiments of this disclosure, the non-aqueous electrolyte contains a cyclic imide compound. In the non-aqueous electrolyte secondary battery according to the embodiments of this disclosure, the content of the cyclic imide compound in the non-aqueous electrolyte is 100 ppm or less.

[0016] In the non-aqueous electrolyte secondary battery according to the embodiments of this disclosure, it is important that (i) the non-aqueous electrolyte contains a cyclic imide compound, and (ii) the content of the cyclic imide compound in the non-aqueous electrolyte is 100 ppm or less. In short, it is important that the non-aqueous electrolyte contains a cyclic imide compound at a predetermined concentration or less. The reasons for this are explained below.

[0017] In non-aqueous electrolyte secondary batteries, a protective layer is sometimes placed on the surface of the negative electrode. In this case, the deposition of lithium metal in a dendrite-like manner on the surface of the negative electrode during charging can be suppressed. This suppresses the degree of expansion caused by the dendrite deposition of lithium metal in the electrode group, which includes the negative electrode, the positive electrode, and the separator placed between the negative and positive electrodes. Furthermore, by placing a protective layer on the surface of the negative electrode, contact between the surface of the negative electrode and the non-aqueous electrolyte is suppressed, thereby suppressing the occurrence of side reactions between the surface of the negative electrode and the non-aqueous electrolyte. As a result, a decrease in the capacity retention rate of the non-aqueous electrolyte secondary battery can be suppressed.

[0018] However, it is difficult to arrange the protective layer on the surface of the negative electrode with uniform thickness, and inconsistencies in the thickness of the protective layer are likely to occur. Therefore, when charging and discharging are repeated in a non-aqueous electrolyte secondary battery, this inconsistency in thickness can cause lithium metal deposition to concentrate on a part of the negative electrode surface (the part where the protective layer is thin). Then, the lithium metal deposited in a concentrated part of the negative electrode surface grows in a dendrite-like manner, and after penetrating the protective layer, it may even penetrate the separator. In this case, a short circuit occurs between the positive electrode and the negative electrode in the non-aqueous electrolyte secondary battery, and this short circuit causes a decrease in the capacity retention rate.

[0019] Furthermore, if a protective layer is formed on the surface of the negative electrode, repeated charging and discharging may cause the protective layer to peel off from the negative electrode surface. In this case, contact occurs between the negative electrode surface and the non-aqueous electrolyte in the area where the protective layer has peeled off, causing a side reaction to occur between the negative electrode surface and the non-aqueous electrolyte. This also reduces the capacity retention rate of the non-aqueous electrolyte secondary battery.

[0020] In the non-aqueous electrolyte secondary battery of this disclosure, as described above, (i) the non-aqueous electrolyte contains a cyclic imide compound, and (ii) the content of the cyclic imide compound in the non-aqueous electrolyte is 100 ppm or less. In short, the non-aqueous electrolyte contains a cyclic imide compound at a predetermined concentration or less. Therefore, it is thought that the cyclic imide compound reaches the surface of the negative electrode from the non-aqueous electrolyte via the protective layer, thereby forming an SEI (Solid Electrolyte Interphase) containing the cyclic imide compound between the protective layer and the surface of the negative electrode. Furthermore, it is thought that the formation of an SEI containing the cyclic imide compound between the protective layer and the surface of the negative electrode can suppress the concentration of lithium metal deposition on a part of the negative electrode surface due to uneven thickness of the protective layer. In other words, it is thought that the growth of lithium metal in a dendrite-like manner on a part of the surface of the negative electrode can be suppressed. In addition, it is thought that the formation of an SEI containing the cyclic imide compound between the protective layer and the surface of the negative electrode can change the surface polarity of the negative electrode so that delamination between the protective layer and the surface of the negative electrode is less likely to occur. As a result of the above, it is possible to suppress the deposition of lithium metal in a dendrite-like manner and to suppress side reactions between the surface of the negative electrode and the non-aqueous electrolyte, thereby suppressing a decrease in the capacity retention rate.

[0021] Furthermore, if the content of cyclic imide compounds in the non-aqueous electrolyte exceeds 100 ppm, that is, if the content of cyclic imide compounds in the non-aqueous electrolyte is too high, a thick SEI containing cyclic imide compounds will form. While a thickly formed SEI can sufficiently suppress the progression of side reactions between the negative electrode surface and the non-aqueous electrolyte, it will result in inferior lithium-ion conductivity. Consequently, the internal resistance of the non-aqueous electrolyte secondary battery will increase. And when the internal resistance of the non-aqueous electrolyte secondary battery increases, this causes a decrease in the capacity retention rate of the non-aqueous electrolyte secondary battery. Therefore, from the viewpoint of sufficiently suppressing the decrease in the capacity retention rate of the non-aqueous electrolyte secondary battery, it is important that the content of cyclic imide compounds in the non-aqueous electrolyte be 100 ppm or less.

[0022] The configuration of the non-aqueous electrolyte secondary battery according to the embodiments of this disclosure will be described in detail below.

[0023] (Non-aqueous electrolytes) Non-aqueous electrolytes are lithium ion conductive. Non-aqueous electrolytes may be liquid or gel-like. Liquid non-aqueous electrolytes can be prepared by dissolving a lithium salt in a non-aqueous solvent. When a lithium salt dissolves in a non-aqueous solvent, lithium ions and anions are generated.

[0024] The gel-like non-aqueous electrolyte may contain a lithium salt and a matrix polymer, or a lithium salt, a non-aqueous solvent, and a matrix polymer. Examples of the matrix polymer include polymer materials that absorb the non-aqueous solvent and gel. Examples of polymer materials include fluororesins, acrylic resins, and polyether resins.

[0025] Various known solvents can be used as the non-aqueous solvent. Examples of the non-aqueous solvent include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, chain carboxylic acid esters, chain ethers, fluorinated chain ethers, cyclic ethers, and fluorinated cyclic ethers. Examples of the cyclic carbonate include propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC). Examples of the chain carbonate include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of the cyclic carboxylic acid ester include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of the chain carboxylic acid ester include ethyl acetate, methyl propionate, and methyl fluoropropionate. Examples of the cyclic ether include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, and 2-methyltetrahydrofuran. Examples of the chain ether include 1,2-dimethoxyethane, diethyl ether, ethyl vinyl ether, methyl phenyl ether, benzyl ethyl ether, diphenyl ether, dibenzyl ether, 1,2-diethoxyethane, and diethylene glycol dimethyl ether. The non-aqueous solvent may be used singly or in combination of two or more kinds.

[0026] Examples of the lithium salt include a lithium salt of a chlorine-containing acid, a lithium salt of a fluorine-containing acid, a lithium salt of a fluorine-containing acid imide, a lithium halide, and a lithium salt containing an oxalate complex. Examples of the lithium salt of a chlorine-containing acid include LiClO ４ , LiAlCl ４ , and LiB １０ Cl １０ and the like. Examples of the lithium salt of a fluorine-containing acid include LiPF ６ , LiPF ２ O ２ , LiBF ４ , LiSbF ６LiAsF ６ LiCF ３ SO ３ , and LiCF ３ CO ２ Examples include LiN(FSO). ２ ) ２ ,LiN(CF ３ SO ２ ) ２ ,LiN(CF ３ SO ２ ) (FSO ２ ), LiN (CF ３ SO ２ ) (C ４ F ９ SO ２ ), LiN(C ２ F ５ SO ２ ) ２ Examples include LiCl, LiBr, and LiI as lithium halides. Examples of oxalate complex-containing lithium salts include LiB(C) ２ O ４ ) ２ LiBr ２ (C ２ O ４ ), LiPF ４ (C ２ O ４ ), and LIPF ２ (C ２ O ４ ) ２ These are some examples. Lithium salts may be used individually or in combination of two or more types.

[0027] The concentration of lithium salt in the non-aqueous electrolyte may be 0.5 mol / L or higher, 1.0 mol / L or higher, or 1.5 mol / L or higher. The concentration of lithium salt in the non-aqueous electrolyte may be 3.5 mol / L or lower, 2.0 mol / L or lower, or 1.5 mol / L or lower. By setting the concentration of lithium salt within the above range, a non-aqueous electrolyte with excellent ionic conductivity and appropriate viscosity can be obtained.

[0028] The non-aqueous electrolyte may contain various known additives. Examples of additives include 1,3-propanesalton, methylbenzenesulfonate, cyclohexylbenzene, biphenyl, and fluorobenzene. These additives function, for example, to form a good quality SEI (Solid Electrolyte Interphase) in a non-aqueous electrolyte secondary battery.

[0029] In the non-aqueous electrolyte secondary battery according to the embodiments of this disclosure, the non-aqueous electrolyte includes a cyclic imide compound as described above. Cyclic imide compounds are preferred because they have higher stability in the non-aqueous electrolyte compared to imide compounds that do not have a ring structure, and they do not readily react with other components contained in the non-aqueous electrolyte.

[0030] The content of the cyclic imide compound in the non-aqueous electrolyte is 100 ppm or less. This suppresses the decrease in capacity retention rate in the non-aqueous electrolyte secondary battery. More preferably, the content of the cyclic imide compound in the non-aqueous electrolyte secondary battery is 50 ppm or less, and more preferably 15 ppm or less. By having a cyclic imide compound content of 50 ppm or less, the decrease in capacity retention rate in the non-aqueous electrolyte secondary battery can be further suppressed. It is particularly preferable that the content of the cyclic imide compound in the non-aqueous electrolyte secondary battery is 10 ppm or less. By having a cyclic imide compound content of 10 ppm or less, the decrease in capacity retention rate in the non-aqueous electrolyte secondary battery can be particularly suppressed. The lower limit of the content of the cyclic imide compound in the non-aqueous electrolyte secondary battery is not particularly limited, but may be 1 ppm or more, or 3 ppm or more. This allows for the formation of a protective layer with a thickness that can suppress the progress of side reactions between the surface of the negative electrode and the non-aqueous electrolyte. The content of the cyclic imide compound in the non-aqueous electrolyte secondary battery may be 1 ppm or more and 100 ppm or less, 1 ppm or more and 50 ppm or less, 3 ppm or more and 15 ppm or less, or 3 ppm or more and 10 ppm or less.

[0031] The content of cyclic imide compounds in non-aqueous electrolytes can be measured by gas chromatography-mass spectrometry (GC-MS). GC-MS can be performed, for example, under the following analytical conditions: <Analytical Conditions> ・Instrument: TurboMatrix 650ATD (Perkin Elmer) / JSM-Q1600GC (JEOL) ・Column: Equity ＴＭ -5 (60 m × 0.25 mm × 0.25 μm) Column heating conditions (1) Hold at 35°C for 5 minutes. (2) Heat to 100°C at a heating rate of 10°C / min. (3) Heat to 290°C at a heating rate of 20°C / min, then hold for 39 minutes. Sample heating conditions 250°C × 10 min Carrier gas helium (flow rate 1 mL / min) Injection volume 14.3%

[0032] The cyclic imide compound preferably contains a succinimide compound. By including a succinimide compound, a good SEI can be formed between the surface of the negative electrode and the protective layer. Examples of succinimide compounds include N-alkylsuccinimide, N-alkenylsuccinimide, and N-arylsuccinimide. Examples of N-alkylsuccinimide include N-methylsuccinimide, N-ethylsuccinimide, N-isopropylsuccinimide, N-cyclohexylsuccinimide, N-isobutylsuccinimide, and N-tert-amylsuccinimide. Examples of N-alkenylsuccinimide include N-vinylsuccinimide and N-allylsuccinimide. Examples of N-arylsuccinimide include N-phenylsuccinimide.

[0033] The cyclic imide compound preferably contains 50% by mass or more of the succinimide compound, more preferably 70% by mass or more, and even more preferably 90% by mass or more. Furthermore, the entirety of the cyclic imide compound may consist of the succinimide compound. That is, the cyclic imide compound may contain 100% by mass of the succinimide compound. The cyclic imide compound preferably contains N-methylsuccinimide as the succinimide compound. The inclusion of N-methylsuccinimide in the cyclic imide compound allows for the formation of an even higher quality SEI between the negative electrode surface and the protective layer.

[0034] The succinimide compound may also be a borated succinimide compound that has been treated with a boron compound.

[0035] The non-aqueous electrolyte preferably contains a hydrofluoroether. The inclusion of a hydrofluoroether in the non-aqueous electrolyte further enhances the capacity retention rate of the non-aqueous electrolyte secondary battery. Furthermore, when the negative electrode contains a magnesium-containing lithium alloy, the inclusion of a hydrofluoroether in the non-aqueous electrolyte secondary battery particularly enhances the capacity retention rate. This is thought to be because the interaction between the magnesium-containing lithium alloy and the hydrofluoroether-containing non-aqueous electrolyte forms a SEI with excellent film quality, synergistically improving the charge-discharge cycle characteristics. As will be described later, the negative electrode includes a negative electrode current collector. Therefore, by using a magnesium-containing lithium alloy as the material for forming the negative electrode current collector, the negative electrode can be made to contain a magnesium-containing lithium alloy.

[0036] Hydrofluoroethers are ether compounds having hydrogen and fluorine atoms. The non-aqueous solvent contained in the non-aqueous electrolyte contains at least a hydrofluoroether. The non-aqueous solvent may also contain non-aqueous solvents other than hydrofluoroethers. The non-aqueous solvents other than hydrofluoroethers can be the non-aqueous solvents mentioned above. Hydrofluoroethers may be in the form of chains or rings. Hydrofluoroethers may be compounds having an ether bond (-O-) and an organic group containing a fluorine atom, and also containing a hydrogen atom.

[0037] The hydrofluoroether preferably contains at least one selected from the group consisting of 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

[0038] (Positive electrode) The positive electrode includes, for example, a positive electrode mixture layer containing a positive electrode active material. The positive electrode may also include a positive electrode current collector and a positive electrode mixture layer disposed on at least one surface of the positive electrode current collector. The positive electrode mixture layer contains a positive electrode active material as an essential component. It may also contain various additives as optional components. Examples of additives include conductive agents, binders, and conductive agents.

[0039] The positive electrode can be obtained by known methods. For example, first, a positive electrode slurry containing a positive electrode active material and various additives is prepared. The positive electrode slurry can be prepared, for example, by dispersing the positive electrode active material and various additives in a dispersion medium (e.g., N-methyl-2-pyrrolidone). Next, the positive electrode slurry is applied to at least one surface of a positive electrode current collector to form a coating, and then this coating is dried. Next, the coating is rolled as needed. This gives the positive electrode. The obtained positive electrode may be cut to a predetermined size as needed.

[0040] The positive electrode active material can be a substance capable of reversibly intercalating and releasing lithium ions. Examples of positive electrode active materials include lithium-containing transition metal oxides, transition metal fluorides, polyanions, fluorinated polyanions, and transition metal sulfides. Among these, lithium-containing transition metal oxides are preferred because they have low manufacturing costs and a high average discharge voltage.

[0041] Examples of transition metal elements included in lithium-containing transition metal oxides include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, and W. Lithium-containing transition metal oxides may contain one transition metal element alone or two or more transition metal elements. The transition metal element may be at least one selected from the group consisting of Co, Ni, and Mn. Lithium-containing transition metal oxides may also contain one or more main group elements. Examples of main group elements include Mg, Al, Ca, Zn, Ga, Ge, Sn, Sb, Pb, Bi, and B.

[0042] For example, carbon materials can be used as conductive agents. Examples of carbon materials include carbon black, carbon nanotubes, and graphite. Examples of carbon black include acetylene black and Ketjenblack.

[0043] Examples of binders include fluororesins, polyacrylonitrile, polyimide resins, acrylic resins, polyolefin resins, and rubbery polymers. Examples of fluororesins include polytetrafluoroethylene and polyvinylidene fluoride.

[0044] For example, cellulose derivatives can be used as thickeners. Examples of cellulose derivatives include carboxymethylcellulose (CMC) and its modified forms, as well as methylcellulose. Modified forms of CMC also include salts of CMC. Examples of salts include alkali metal salts (e.g., sodium salts) and ammonium salts.

[0045] For example, a conductive sheet can be used as the positive electrode current collector. For example, a metal foil can be used as the conductive sheet.

[0046] When the positive electrode current collector is a conductive sheet, the material used to form the positive electrode current collector may include, for example, a metallic material containing at least one selected from the group consisting of Al, Ti, and Fe. Examples of metallic materials include Al, Ti, Al alloys, Ti alloys, and Fe alloys (e.g., stainless steel). The thickness of the positive electrode current collector is not particularly limited. The positive electrode current collector may have a thickness in the range of 5 μm to 300 μm.

[0047] (Negative electrode) The negative electrode includes a negative electrode current collector. A protective layer containing a polymer is placed on the surface of the negative electrode. In the case of a lithium secondary battery, lithium metal is deposited on the surface of the negative electrode during charging. More specifically, lithium ions contained in the non-aqueous electrolyte accept electrons at the surface of the negative electrode during charging, becoming lithium metal, which is deposited on the surface of the negative electrode. The deposited lithium metal dissolves as lithium ions in the non-aqueous electrolyte during discharge. Therefore, the negative electrode may be an electrode in which lithium metal is deposited during charging and the lithium metal dissolves in the non-aqueous electrolyte during discharge.

[0048] Furthermore, the negative electrode of a lithium secondary battery differs from that of a lithium-ion secondary battery, where electron movement during charging and discharging is primarily due to the intercalation and release of lithium ions in the negative electrode active material (e.g., graphite). Also, if the negative electrode consists only of a negative electrode current collector, the surface of the negative electrode refers to the surface of the negative electrode current collector. The lithium ions contained in the non-aqueous electrolyte may originate from lithium salts added to the non-aqueous electrolyte or may be supplied from the positive electrode active material. If lithium metal is not deposited on the surface of the negative electrode current collector, the protective layer covers the surface of the negative electrode current collector. If lithium metal is deposited on the surface of the negative electrode current collector, the protective layer covers the surface of the deposited lithium metal.

[0049] The protective layer may be formed on the surface of the negative electrode and then placed on the surface of the negative electrode (first arrangement). Alternatively, the protective layer may be formed on the surface of a separator facing the negative electrode, and then placed on the surface of the negative electrode by bringing the protective layer into contact with the surface of the negative electrode (second arrangement). In the second arrangement, the protective layer may be placed on the surface of the negative electrode without being bonded to it, or it may be placed on the surface of the negative electrode while being bonded to it. Furthermore, as will be described later, if a spacer is placed between the negative electrode and the separator to secure space for the deposition of lithium metal, the protective layer may be formed on the surface of the spacer facing the negative electrode, and then placed on the surface of the negative electrode by bringing the protective layer into contact with the surface of the negative electrode (third arrangement). In the third arrangement, the protective layer may also be placed on the surface of the negative electrode without being bonded to it, or it may be placed on the surface of the negative electrode while being bonded to it.

[0050] For example, a conductive sheet can be used as the negative electrode current collector. For example, a metal foil can be used as the conductive sheet.

[0051] When the negative electrode current collector is a conductive sheet, the material used to form the negative electrode current collector may be a conductive material other than lithium metal and lithium alloy. The conductive material may be a metallic material. The conductive material may be a material that does not react with lithium. Examples of conductive materials include copper (Cu), nickel (Ni), iron (Fe), alloys containing these metallic elements, and graphite with preferentially exposed basal surfaces. Examples of alloys include copper alloys and stainless steel (SUS). From the viewpoint of high conductivity, it is preferable to use copper or a copper alloy as the conductive material. The negative electrode current collector may be a copper foil, a copper alloy foil, or a stainless steel foil. The negative electrode current collector may have a thickness in the range of 5 μm to 300 μm.

[0052] The negative electrode may include a negative electrode current collector and a lithium layer disposed on at least one surface of the negative electrode current collector. In this case, the protective layer is also disposed to cover the lithium layer. Therefore, when the negative electrode includes a negative electrode current collector and a lithium layer, the surface of the negative electrode means the surface of the negative electrode current collector and the surface of the lithium layer.

[0053] The lithium layer is a layer containing at least one of lithium metal and / or a lithium alloy. When the lithium layer contains a lithium alloy, the content of elements other than lithium in the lithium layer is small (10 atomic percent or less). Examples of elements other than lithium contained in lithium alloys include aluminum, magnesium, indium, and zinc. By forming a lithium layer, even if lithium ions become deactivated during charging and discharging, these deactivated lithium ions can be replenished. This suppresses the decrease in discharge capacity that occurs with repeated charging and discharging. Furthermore, by forming a lithium layer, the deposition of lithium metal in a dendrite-like manner can be suppressed.

[0054] The method for forming the lithium layer is not particularly limited. The lithium layer can be formed by various known methods. For example, the lithium layer can be formed by pressing a lithium metal foil or lithium alloy foil onto at least one surface of the negative electrode current collector. The structure of the lithium layer is dense, while the lithium metal deposited during charging is usually porous. Therefore, the lithium layer and the lithium metal deposited during charging can be distinguished on the negative electrode current collector.

[0055] The polymer is preferably a fluorinated polymer. A fluorinated polymer is a polymer containing fluorine. A polymer other than a block copolymer can be used as the fluorinated polymer. Examples of polymers other than block copolymers include homopolymers and random copolymers.

[0056] Examples of fluorinated polymers include polyvinylidene fluoride homopolymers, tetrafluoroethylene homopolymers, copolymers containing polyvinylidene fluoride as a monomer, or combinations of the above homopolymers and copolymers. Preferably, the fluorinated polymer is a polyvinylidene fluoride homopolymer, a copolymer containing polyvinylidene fluoride as a monomer, or a combination of the above homopolymers and copolymers. In other words, it is preferable that the fluorinated polymer contains polyvinylidene fluoride in its structure. Fluorinated polymers containing polyvinylidene fluoride in their structure are preferable because they do not dissolve in non-aqueous electrolytes and have relatively high flexibility and tensile strength. A polyvinylidene fluoride homopolymer refers to polyvinylidene fluoride (PVDF), and a tetrafluoroethylene homopolymer refers to polytetrafluoroethylene (PTFE). Examples of copolymers containing vinylidene fluoride as a monomer include copolymers of vinylidene fluoride and hexafluoropropylene, and copolymers of vinylidene fluoride and trifluoroethylene.

[0057] The number-average molecular weight of the fluorinated polymer may be 500,000 or more, or 800,000 or more. Alternatively, it may be 1,500,000 or less, or 1,000,000 or less. The number-average molecular weight of the fluorinated polymer is a polystyrene equivalent value measured by gel permeation chromatography (GPC). GPC measurement is typically performed using a polystyrene gel column and water / methanol (volume ratio 8 / 2) as the mobile phase. Fluorinated polymers with a number-average molecular weight within the above range are preferred because they are less soluble in non-aqueous electrolytes and have higher tensile strength. Furthermore, fluorinated polymers with a number-average molecular weight within the above range are easily processed by dissolving them in a predetermined organic solvent. Commercially available fluorinated polymers may be used, or fluorinated polymers synthesized by various known synthesis methods may be used.

[0058] The protective layer may contain a cyclic imide compound. This facilitates the formation of a SEI containing the cyclic imide compound between the protective layer and the surface of the negative electrode. The cyclic imide compound contained in the protective layer may be an impurity contained in the coating solution used to form the protective layer, as described later. For example, it may be an impurity derived from a liquid component (e.g., N-methyl-2-pyrrolidone) contained in the coating solution. Alternatively, the cyclic imide compound contained in the protective layer may be a cyclic imide compound contained in a non-aqueous electrolyte that has permeated into the protective layer.

[0059] From the viewpoint of suppressing contact between the surface of the negative electrode and the non-aqueous electrolyte, and from the viewpoint of suppressing dendrites from penetrating the protective layer, the average thickness of the protective layer may be 0.1 μm or more and 5 μm or less, or 0.5 μm or more and 2 μm or less. The average thickness of the protective layer can be measured by the following procedure. First, a cross-sectional image of the protective layer is obtained using a scanning electron microscope (SEM). Next, the thickness of 10 arbitrary points on the protective layer is measured using this cross-sectional image. Then, the average thickness of the protective layer can be obtained by taking the arithmetic mean of the 10 measured thicknesses.

[0060] The protective layer may contain a lithium salt. The inclusion of a lithium salt in the protective layer facilitates the charge-discharge reaction at the start of battery use. It is preferable that the lithium salt be included in the protective layer during its formation. That is, it is preferable to form the protective layer using a material containing the above-mentioned polymer and lithium salt.

[0061] Lithium salts are LiBF ４ LiPF ６ LiClO ４ LiCF ３ SO ３ LiC (CF ３ SO ２ ) ３ , LiN (SO ２ C ２ F ５ ) ２ , LiN (SO ２ CF ３ ) ２ , Li(S.O. ２ F) ２ LiPF ３(CF ２ CF ３ ) ３ , and LiPF ３ (CF ３ ) ３ It is preferable to include at least one selected from the group consisting of the above. The lithium salt may be any one selected from the above group.

[0062] The protective layer may contain substances other than the polymer and lithium salt described above. Examples of other substances include resins other than the polymer described above, and inorganic particles. Examples of resins other than the polymer described above include polyolefin resins, silicone resins, and epoxy resins.

[0063] Inorganic particles are particles made of inorganic materials (e.g., metal oxides, metal hydroxides, metal composite oxides, metal nitrides, metal carbides, and metal fluorides). Within the protective layer, the mixing of the polymer and inorganic particles easily increases the lithium ion conductivity of the protective layer. As a result, during charging and discharging, the movement of lithium ions between the negative electrode and the non-aqueous electrolyte proceeds smoothly through the protective layer. To enhance the strength of the protective layer, the density of the inorganic particles is 6 g / cm³. ３ That's fine too.

[0064] The density of inorganic particles may be 3.5 times or more the density of the areas within the protective layer occupied by materials other than inorganic particles. In this case, the inorganic particles tend to deposit thinly and densely when forming the protective layer. As a result, uneven distribution due to aggregation of inorganic particles is suppressed, and the thickness and strength of the protective layer tend to become uniform.

[0065] Examples of inorganic materials constituting inorganic particles include copper oxide, bismuth oxide, tungsten oxide, indium oxide, and silver oxide. The inorganic particles may also contain at least one selected from the group consisting of copper oxide particles and bismuth oxide particles. In this case, the density of the inorganic particles is 6 g / cm³. ３ This makes it easier to adjust the density to the above. Furthermore, it becomes easier to adjust the density of inorganic particles to 3.5 times or more the density of the region within the protective layer occupied by materials other than inorganic particles.

[0066] The proportion of inorganic particles in the protective layer may be 50% by mass or less, 35% by mass or less, or 20% by mass or less. The proportion of inorganic particles in the protective layer may be 5% by mass or more and 20% by mass or less. By having the proportion of inorganic particles in the protective layer within the above range, flexibility, strength, and lithium ion conductivity are sufficiently ensured in the protective layer.

[0067] The method for forming the protective layer is not particularly limited. The protective layer may be formed by the following method. First, a coating solution for forming the protective layer is prepared. The coating solution can be prepared by mixing a fluorinated polymer, a liquid component, and, if necessary, other materials (such as lithium salts, resins, and inorganic particles). The liquid component is not particularly limited. Examples of liquid components include N-methyl-2-pyrrolidone, dimethyl ether, and tetrahydrofuran. Next, in the case of the first configuration described above, the coating solution is applied to at least one surface of the negative electrode current collector to form a coating film, and then the coating film is dried. If a lithium layer is arranged on at least one surface of the negative electrode current collector, the coating solution is applied to the surface of the lithium layer to form a coating film, and then the coating film is dried. Furthermore, in the case of the second configuration described above, the coating solution is applied to the surface of the separator facing the negative electrode to form a coating film, and then the coating film is dried. Furthermore, in the case of the third configuration described above, the coating solution is applied to the surface of the spacer facing the negative electrode to form a coating film, and then the coating film is dried. The coating and drying methods are not particularly limited. Coating may be carried out using, for example, a bar coater, an applicator, and a gravure coater. In this way, a protective layer is formed on the surface of the negative electrode, the surface of the separator, or the surface of the spacer.

[0068] The fluorinated polymer may be present in the protective layer in particulate form. However, if the fluorinated polymer is present in the protective layer in particulate form, its properties may not be fully exhibited. Therefore, it is preferable that the fluorinated polymer is not present in the protective layer in particulate form. For this reason, it is preferable to form the protective layer using a coating solution containing the fluorinated polymer in a non-particulate form.

[0069] It is preferable that a space for the deposition of lithium metal is formed between the negative electrode and the positive electrode. This space can be formed by placing a spacer between the negative electrode and the positive electrode. From the viewpoint of improving productivity, the same material as the protective layer may be used for the spacer. When forming the protective layer, the thickness of the protective layer may be partially increased, and the thicker portion may be used as a spacer. In other words, the protective layer and the spacer may be integrated. Alternatively, a coating liquid for spacer formation may be applied linearly to the protective layer to form linear protrusions (spacers) on the protective layer. The coating liquid for forming the protective layer may be used as the coating liquid for spacer formation.

[0070] The spacer may be placed between at least one of the positive and negative electrodes and the separator. If the spacer is placed between the negative electrode and the separator, it may be placed between the protective layer on the surface of the negative electrode and the separator, or between the negative electrode and the protective layer. The linear protrusions may be arranged in a stripe pattern or a mesh pattern (e.g., honeycomb pattern). The height of the spacer may be between 10 μm and 100 μm. The width of the spacer may be between 200 μm and 2000 μm. The spacer may be formed on the positive electrode, on the negative electrode, on the protective layer, or on the separator.

[0071] Aside from forming a protective layer, the method for manufacturing a non-aqueous electrolyte secondary battery is not particularly limited. A non-aqueous electrolyte secondary battery may be manufactured by combining the method described herein with known methods.

[0072] (Separator) A porous sheet having ion permeability and insulating properties is used as the separator. Examples of porous sheets include microporous membranes, woven fabrics, and nonwoven fabrics. The material forming the separator is not particularly limited. The material forming the separator may be, for example, a polymer material. Examples of polymer materials include olefin resins, polyamide resins, and cellulose. Examples of olefin resins include polyethylene, polypropylene, and copolymers of ethylene and propylene. The separator may contain additives such as inorganic fillers as needed.

[0073] The thickness of the separator is not particularly limited. The separator may have a thickness of, for example, 5 μm to 20 μm, or 10 μm to 20 μm.

[0074] (Outer casing) The outer casing houses the non-aqueous electrolyte and the electrode group. Various known outer casings can be used. The shape of the outer casing is appropriately selected according to the shape of the non-aqueous electrolyte secondary battery. The shape of the non-aqueous electrolyte secondary battery is not particularly limited and may be cylindrical, rectangular, or any other shape. The outer casing may include a bottomed cylindrical battery case and a sealing body and gasket that seal the opening of the battery case.

[0075] Hereinafter, an example of a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure will be described with reference to the drawings. The components of the non-aqueous electrolyte secondary battery according to this example can be the components described above. Furthermore, the example of components described below can be modified as appropriate based on the above. In addition, the matters described below may be applied to the above embodiment. Furthermore, in the following description of the non-aqueous electrolyte secondary battery, components that are not essential to the non-aqueous electrolyte secondary battery according to the embodiment of the present disclosure may be omitted.

[0076] Figure 1 is a schematic longitudinal cross-sectional view showing a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure. As shown in Figure 1, the non-aqueous electrolyte secondary battery 10 is a cylindrical battery comprising a cylindrical case, a wound electrode group 14, and a non-aqueous electrolyte (not shown). The electrode group 14 is housed in the battery case and is in contact with the non-aqueous electrolyte.

[0077] The battery case includes a case body 15, which is a bottomed cylindrical metal container, and a sealing body 16 that seals the opening of the case body 15. A gasket 27 is placed between the case body 15 and the sealing body 16. The placement of the gasket 27 ensures that the battery case is airtight. Inside the case body 15, insulating plates 17 and 18 are placed at both ends of the electrode group 14 in the direction of the winding axis, respectively.

[0078] The case body 15 has, for example, a stepped portion 21. The stepped portion 21 is formed, for example, by partially pressing the side wall of the case body 15 from the outside. The stepped portion 21 may be formed in an annular shape on the side wall of the case body 15 along the circumferential direction of a virtual circle defined by the case body 15. In this case, the sealing body 16 is supported, for example, by the opening side surface of the stepped portion 21.

[0079] The sealing body 16 includes a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26. These members are stacked in this order in the sealing body 16. The sealing body 16 is installed in the opening of the case body 15 such that the cap 26 is located on the outside of the case body 15 and the filter 22 is located on the inside of the case body 15.

[0080] The lower valve body 23 and the upper valve body 25 are connected at their respective centers. An insulating member 24 is positioned between the peripheral edge of the lower valve body 23 and the peripheral edge of the upper valve body 25. The filter 22 and the lower valve body 23 are connected at their respective peripheral edges. The upper valve body 25 and the cap 26 are connected at their respective peripheral edges. Each of the above-mentioned components constituting the sealing body 16 has, for example, a disc shape or a ring shape. Each of the above-mentioned components is electrically connected to one another, except for the insulating member 24.

[0081] The lower valve body 23 has a ventilation hole. Therefore, if the internal pressure of the battery case rises due to abnormal heat generation or other reasons, the upper valve body 25 bulges towards the cap 26 and separates from the lower valve body 23. This disconnects the electrical connection between the lower valve body 23 and the upper valve body 25. If the internal pressure rises further, the upper valve body 25 ruptures, and gas is released through the opening formed in the cap 26.

[0082] The electrode group 14 includes a positive electrode 11, a negative electrode 12, and a separator 13. The positive electrode 11, the negative electrode 12, and the separator 13 are all strip-shaped. The width direction of the positive electrode 11 and the negative electrode 12 is, for example, parallel to the winding axis direction of the electrode group 14. The separator 13 is positioned between the positive electrode 11 and the negative electrode 12. The positive electrode 11 and the negative electrode 12 are wound in a spiral shape with the separator 13 interposed between them.

[0083] In the non-aqueous electrolyte secondary battery 10, when observing a cross-section of the electrode group 14 in a direction perpendicular to the winding axis, the positive electrode 11 and the negative electrode 12 are alternately stacked in the radial direction of a virtual circle defined by the case body 15, with a separator 13 interposed between them.

[0084] The positive electrode 11 is electrically connected to the cap 26, which functions as a positive electrode terminal, via a positive electrode lead 19. The positive electrode lead 19 extends from the positive electrode 11 to the filter 22 through a through hole formed in the insulating plate 17. One end of the positive electrode lead 19 is connected, for example, near the center of the positive electrode 11 in the longitudinal direction, and the other end of the positive electrode lead 19 is welded, for example, to the electrode group 14 side of the filter 22.

[0085] The negative electrode 12 is electrically connected to the case body 15, which functions as a negative electrode terminal, via a negative electrode lead 20. One end of the negative electrode lead 20 is connected, for example, to the end of the negative electrode 12 in the longitudinal direction, and the other end of the negative electrode lead 20 is welded, for example, to the inner bottom surface of the case body 15.

[0086] In the above, as an example of a non-aqueous electrolyte secondary battery, the configuration shown in Figure 1, specifically, a configuration in which an electrode group formed by winding a positive electrode and a negative electrode with a separator in between, and an electrolyte are housed in an outer casing (battery case), is described. However, the configuration of a non-aqueous electrolyte secondary battery is not limited to the configuration shown in Figure 1. That is, a non-aqueous electrolyte secondary battery is not limited to an example of a cylindrical configuration, but may also be configured as a rectangular, coin-shaped, button-shaped, or laminate-shaped configuration. Furthermore, the electrode group of a non-aqueous electrolyte secondary battery is not limited to an example of a wound type configuration. For example, the electrode group of a non-aqueous electrolyte secondary battery may be formed as a laminated type by stacking the positive electrode and negative electrode with a separator in between.

[0087] (Note) The following technologies are disclosed by the above description. (Technology 1) A non-aqueous electrolyte secondary battery comprising: an electrode group including a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode; and a non-aqueous electrolyte, wherein a protective layer containing a polymer is disposed on the surface of the negative electrode; the non-aqueous electrolyte contains a cyclic imide compound; and the content of the cyclic imide compound in the non-aqueous electrolyte is 100 ppm or less. (Technology 2) The non-aqueous electrolyte secondary battery according to Technology 1, wherein the protective layer contains the cyclic imide compound. (Technology 3) The non-aqueous electrolyte secondary battery according to Technology 1 or 2, wherein the content of the cyclic imide compound in the non-aqueous electrolyte is 15 ppm or less. (Technology 4) The non-aqueous electrolyte secondary battery according to any one of Technologies 1 to 3, wherein the negative electrode comprises a negative electrode current collector and a lithium layer containing at least one of lithium metal and lithium alloy, and disposed on at least one surface of the negative electrode current collector. (Technology 5) The aqueductive secondary battery according to any one of Technologies 1 to 4, wherein the negative electrode is an electrode in which lithium metal is deposited during charging and the lithium metal dissolves in the non-aqueous electrolyte during discharge. (Technology 6) The non-aqueous secondary battery according to any one of Technologies 1 to 5, wherein the cyclic imide compound comprises a succinimide compound. (Technology 7) The non-aqueous secondary battery according to Technology 6, wherein the cyclic imide compound comprises N-methylsuccinimide as the succinimide compound. (Technology 8) The non-aqueous secondary battery according to any one of Technologies 1 to 7, wherein the polymer is a fluorinated polymer. (Technology 9) The non-aqueous secondary battery according to Technology 8, wherein the fluorinated polymer is a homopolymer of vinylidene fluoride, a copolymer containing vinylidene fluoride as a monomer, or a combination of the homopolymer or the copolymer. (Technology 10) The non-aqueous secondary battery according to any one of Technologies 1 to 9, wherein the protective layer comprises a lithium salt. (Technical 11) The lithium salt is LiBF ４ LiPF ６ LiClO ４ LiCF ３ SO ３ LiC (CF ３ SO ２ )３ , LiN(SO ２ C ２ F ５ ) ２ , LiN(SO ２ CF ３ ) ２ , Li(SO ２ F) ２ , LiPF ３ (CF ２ CF ３ ) ３ , and LiPF ３ (CF ３ ) ３ The non-aqueous electrolyte secondary battery according to Technique 10, comprising at least one selected from the group consisting of. (Technique 12) The non-aqueous electrolyte secondary battery according to any one of Techniques 1 to 11, wherein the non-aqueous electrolyte comprises hydrofluoroether. (Technique 13) The non-aqueous electrolyte secondary battery according to Technique 12, wherein the hydrofluoroether comprises at least one selected from the group consisting of 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

[0088] Hereinafter, the present disclosure will be specifically described based on Examples and Comparative Examples, but the present disclosure is not limited to the following Examples.

[0089] (Example 1) (1) Production of Positive Electrode A positive electrode active material, acetylene black (AB, conductive agent), and polyvinylidene fluoride (PVDF, binder) were mixed at a mass ratio of positive electrode active material: AB: PVDF = 95:2.5:2.5 to obtain a mixture, then N-methyl-2-pyrrolidone (dispersion medium) was added to this mixture and stirred to prepare a positive electrode mixture slurry. As the positive electrode active material, a layered rock salt type lithium-containing metal oxide containing Li, Ni, Co, and Al (the molar ratio of Li to the total of Ni, Co, and Al is 1.0) was used.

[0090] A positive electrode mixture slurry was applied to both surfaces of an Al foil (positive electrode current collector) to form a coating film, and then the coating film was dried. Next, the dried coating film (positive electrode mixture layer) was rolled using a roller. Next, the laminate of the positive electrode mixture layer and the positive electrode current collector was cut into a predetermined size. Thereby, a positive electrode including the positive electrode current collector and the positive electrode mixture layers arranged on both surfaces of the positive electrode current collector was obtained. The positive electrode current collector had a strip shape.

[0091] (2) Production of Negative Electrode A Li foil (thickness: 25 μm) was attached to both surfaces of a negative electrode current collector. A strip-shaped electrolytic copper foil (thickness: 15 μm) was used as the negative electrode current collector. Next, a fluoropolymer, a lithium salt, and N-methyl-2-pyrrolidone (dispersion medium) were mixed to prepare a coating liquid for forming a protective layer. Polyvinylidene fluoride (PVDF) was used as the fluoropolymer. The addition amount of the lithium salt was set to 1 / 10 of the mass of the fluoropolymer. Lithium bis(fluorosulfonyl)imide was used as the lithium salt. The coating liquid for forming a protective layer was applied to both surfaces of the negative electrode to form a coating film, and then the coating film was dried to form a protective layer (thickness: 2 μm).

[0092] (3) Preparation of Nonaqueous Electrolyte Ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed at a volume ratio of EC:DMC=30:70 to prepare a nonaqueous solvent. Next, LiPF ６ was adjusted to have a concentration of 1 mol / L, and LiBF ２ (C ２ O ４ ) was adjusted to have a concentration of 0.1 mol / L, LiPF ６ and LiBF ２ (C ２ O ４ ) were dissolved in the nonaqueous solvent. Next, N-methylsuccinimide was dissolved in the nonaqueous solvent such that the content of N-methylsuccinimide was 4 ppm. Thereby, a nonaqueous electrolyte (nonaqueous electrolytic solution) was prepared.

[0093] (4) Battery Assembly One end of an aluminum positive electrode lead was welded to the positive electrode current collector of the positive electrode. Next, one end of a nickel negative electrode lead was welded to the negative electrode current collector of the negative electrode. The electrode group was fabricated by winding the positive electrode, negative electrode, and separator in the longitudinal direction in an inert gas atmosphere so that a separator was placed between the positive electrode and the negative electrode. A microporous polyethylene membrane was used as the separator. The separator had a strip shape.

[0094] The electrode group was housed in a bag-shaped outer casing. At this time, the other end of the positive electrode lead and the other end of the negative electrode lead were exposed to the outside of the outer casing. After injecting a non-aqueous electrolyte into the outer casing, the opening of the outer casing was sealed. In this way, a non-aqueous electrolyte secondary battery according to Example 1 was fabricated.

[0095] (Example 2) A non-aqueous electrolyte secondary battery according to Example 2 was prepared in the same manner as in Example 1, except that in the preparation of the non-aqueous electrolyte, N-methylsuccinimide was dissolved in a non-aqueous solvent so that the N-methylsuccinimide content was 8 ppm.

[0096] (Example 3) A non-aqueous electrolyte secondary battery according to Example 3 was prepared in the same manner as in Example 1, except that in the preparation of the non-aqueous electrolyte, N-methylsuccinimide was dissolved in a non-aqueous solvent so that the N-methylsuccinimide content was 10 ppm.

[0097] (Example 4) A non-aqueous electrolyte secondary battery according to Example 4 was prepared in the same manner as in Example 1, except that in the preparation of the non-aqueous electrolyte, N-methylsuccinimide was dissolved in a non-aqueous solvent so that the N-methylsuccinimide content was 12 ppm.

[0098] (Example 5) A non-aqueous electrolyte secondary battery according to Example 5 was prepared in the same manner as in Example 1, except that in the preparation of the non-aqueous electrolyte, N-methylsuccinimide was dissolved in a non-aqueous solvent so that the N-methylsuccinimide content was 50 ppm.

[0099] (Example 6) A non-aqueous electrolyte secondary battery according to Example 6 was prepared in the same manner as in Example 1, except that in the preparation of the non-aqueous electrolyte, N-methylsuccinimide was dissolved in a non-aqueous solvent so that the N-methylsuccinimide content was 98 ppm.

[0100] (Comparative Example 1) A non-aqueous electrolyte secondary battery according to Comparative Example 1 was prepared in the same manner as in Example 1, except that N-methylsuccinimide was not dissolved in the non-aqueous electrolyte during the preparation of the non-aqueous electrolyte. Therefore, in the non-aqueous electrolyte secondary battery according to Comparative Example 1, the N-methylsuccinimide content in the non-aqueous electrolyte was 0 ppm.

[0101] (Comparative Example 2) A non-aqueous electrolyte secondary battery according to Comparative Example 2 was manufactured in the same manner as in Comparative Example 1, except that a protective layer was not formed on both sides of the negative electrode during the preparation of the negative electrode. Therefore, in the non-aqueous electrolyte secondary battery according to Comparative Example 2, no protective layer was formed on both sides of the negative electrode, and the N-methylsuccinimide content in the non-aqueous electrolyte was 0 ppm.

[0102] (Comparative Example 3) A non-aqueous electrolyte secondary battery according to Comparative Example 3 was prepared in the same manner as in Example 3, except that a protective layer was not formed on both sides of the negative electrode during the preparation of the negative electrode. Therefore, in the non-aqueous electrolyte secondary battery according to Comparative Example 3, no protective layer was formed on both sides of the negative electrode, and the N-methylsuccinimide content in the non-aqueous electrolyte was 10 ppm.

[0103] (Comparative Example 4) A non-aqueous electrolyte secondary battery according to Comparative Example 4 was prepared in the same manner as in Example 1, except that in the preparation of the non-aqueous electrolyte, N-methylsuccinimide was dissolved in a non-aqueous solvent so that the content of N-methylsuccinimide was 110 ppm.

[0104] Table 1 below summarizes the presence or absence of protective layers on both sides of the negative electrode and the N-succinimide content in the non-aqueous electrolyte for each example (Examples 1-6 and Comparative Examples 1-4) of the non-aqueous electrolyte secondary battery. In Table 1 below, the presence or absence of protective layers on both sides of the negative electrode is simply referred to as "presence or absence of protective layer," and the N-succinimide content in the non-aqueous electrolyte is simply referred to as "succinimide content."

[0105]

[0106] [Evaluation] <Capacity Retention Rate> Charge-discharge cycle tests were conducted on the non-aqueous electrolyte secondary batteries for each example (Examples 1-6 and Comparative Examples 1-4). In the charge-discharge cycle tests, the following cycle was repeated 500 times in a constant temperature bath at 25°C: charging was performed under the following conditions, followed by a 20-minute pause, and then discharging under the following conditions.

[0107] 《Charging》 Constant current charging was performed at a current value of 10 mA per unit area (square centimeter) of the electrodes until the battery voltage reached 4.3 V. Then, constant voltage charging was performed at a voltage of 4.3 V until the current value per unit area of ​​the electrodes reached 1 mA.

[0108] 《Discharge》 Constant current discharge was performed at a current value of 10 mA per unit area (square centimeter) of the electrode until the battery voltage reached 3 V.

[0109] The capacity retention rate at 500 cycles was calculated using the discharge capacity C1 value at 1 cycle and the discharge capacity C2 value at 500 cycles, based on the following formula. The results are shown in Table 2 below. Capacity retention rate (%) = (C2 / C1) × 100

[0110]

[0111] Table 2 shows that in each of the examples (Examples 1 to 6), the capacity retention rate at 500 cycles was 70% or higher. In contrast, in the non-aqueous electrolyte secondary batteries of Comparative Examples 1 to 3, the capacity retention rate at 500 cycles was 0%, and in the non-aqueous electrolyte secondary battery of Comparative Example 4, the capacity retention rate at 500 cycles had decreased to 31.1%. From this, it can be seen that by covering the surface of the negative electrode with a protective layer containing a polymer and keeping the N-methylsuccinimide content in the non-aqueous electrolyte to 100 ppm or less, the decrease in the capacity retention rate of the non-aqueous electrolyte secondary battery can be suppressed.

[0112] Furthermore, comparing the capacity retention rates of the non-aqueous electrolytes in each example (Examples 1 to 6), it can be seen that the capacity retention rates of the non-aqueous electrolyte secondary batteries in Examples 1 to 4 are all 80% or higher, while the capacity retention rates of the non-aqueous electrolyte secondary batteries in Examples 5 and 6 are all less than 80%. From this, it can be seen that by limiting the N-methylsuccinimide content in the non-aqueous electrolyte secondary battery to 15 ppm or less, the decrease in the capacity retention rate of the non-aqueous electrolyte secondary battery can be further suppressed.

[0113] Although the present invention has been described in relation to preferred embodiments at present, such disclosure should not be interpreted restrictively. Various modifications and alterations will undoubtedly become apparent to those skilled in the art in the field to which the invention pertains by reading the above disclosure. Accordingly, the appended claims should be interpreted as encompassing all modifications and alterations without departing from the true spirit and scope of the invention.

[0114] The non-aqueous electrolyte secondary battery described herein can be used in applications where suppressing a decrease in capacity retention rate is required.

[0115] 10: Non-aqueous electrolyte secondary battery, 11: Positive electrode, 12: Negative electrode, 13: Separator, 14: Electrode group

Claims

1. A non-aqueous electrolyte secondary battery comprising: an electrode group including a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode; and a non-aqueous electrolyte, wherein a protective layer containing a polymer is disposed on the surface of the negative electrode; the non-aqueous electrolyte contains a cyclic imide compound; and the content of the cyclic imide compound in the non-aqueous electrolyte is 100 ppm or less.

2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the protective layer comprises the cyclic imide compound.

3. The non-aqueous electrolyte secondary battery according to claim 1, wherein the content of the cyclic imide compound in the non-aqueous electrolyte is 15 ppm or less.

4. The non-aqueous electrolyte secondary battery according to claim 1, wherein the negative electrode comprises a negative electrode current collector and a lithium layer comprising at least one of lithium metal and lithium alloy, and disposed on at least one surface of the negative electrode current collector.

5. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein the negative electrode is an electrode in which lithium metal is deposited during charging and the lithium metal dissolves in the non-aqueous electrolyte during discharge.

6. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein the cyclic imide compound comprises a succinimide compound.

7. The non-aqueous electrolyte secondary battery according to claim 6, wherein the cyclic imide compound comprises N-methylsuccinimide as the succinimide compound.

8. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein the polymer is a fluorinated polymer.

9. The non-aqueous electrolyte secondary battery according to claim 8, wherein the fluorinated polymer is a homopolymer of vinylidene fluoride, a copolymer containing vinylidene fluoride as a monomer, or a combination of the homopolymer and the copolymer.

10. The protective layer comprises a lithium salt, the non-aqueous electrolyte secondary battery according to any one of claims 1 to 4.

11. The lithium salt is LiBF ４ , LiPF ６ , LiClO ４ , LiCF ３ SO ３ , LiC(CF ３ SO ２ ) ３ , LiN(SO ２ C ２ F ５ ) ２ , LiN(SO ２ CF ３ ) ２ , Li(SO ２ F) ２ , LiPF ３ (CF ２ CF ３ ) ３ , and LiPF ３ (CF ３ ) ３ The non-aqueous electrolyte secondary battery according to claim 10, comprising at least one selected from the group consisting of.

12. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein the non-aqueous electrolyte comprises a hydrofluoroether.

13. The non-aqueous electrolyte secondary battery according to claim 12, wherein the hydrofluoroether comprises at least one selected from the group consisting of 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

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