Secondary battery and electronic device

Abstract

The application relates to a secondary battery and an electronic device. The secondary battery comprises a positive electrode, a negative electrode and an electrolyte, the positive electrode comprises lithium cobalt oxide and a polymer P, the negative electrode comprises graphite and silicon carbon, and the electrolyte comprises lithium difluorophosphate, 1,3,6-hexanetrimethylene cyanide and a nitrogen-containing lithium salt. The design can not only improve the cycle performance and low-temperature rate performance of the secondary battery, but also significantly improve the electrode expansion resistance after the cycle.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of energy storage, in particular to a secondary battery and an electronic device. BACKGROUND

[0002] At present, with the rapid development and continuous expansion of the new energy vehicle industry, the development of high-power, high-capacity, and high-safety battery systems is imminent. Silicon-based negative electrode materials have attracted widespread attention and extensive research due to their high theoretical specific capacity, low lithium intercalation potential, abundant raw materials, non-toxicity, and environmental friendliness, and are expected to replace carbon negative electrode materials to become the next generation of high-performance lithium battery negative electrode materials.

[0003] However, silicon-based negative electrode materials have a huge volume expansion during repeated lithium intercalation and deintercalation, with a volume change of up to 400%, which leads to active particle breakage and pulverization, unstable and continuous growth of the surface SEI film structure, and serious electrode structure collapse, which causes rapid decay of the electrochemical performance of the silicon negative electrode, and reduces the battery capacity, cycle life, and coulombic efficiency. In addition, due to the poor conductivity of silicon, the internal resistance increases, and the battery cycle performance deteriorates significantly, especially in low temperature environments. SUMMARY

[0004] Embodiments of the present application adjust the composition of the electrode applied in the secondary battery and the components in the electrolyte to solve the problems existing in the prior art to some extent.

[0005] The present inventors have found that a positive electrode comprising lithium cobalt oxide and a polymer P containing a nitrile group-containing monomer unit, a negative electrode comprising graphite and a silicon-carbon compound, and an electrolyte comprising lithium difluorophosphate, 1,3,6-hexanetrinitrile, and a nitrogen-containing lithium salt can not only improve the cycle performance and low-temperature rate performance of the secondary battery, but also significantly improve the electrode swelling resistance after cycling.

[0006] In a secondary battery in which the positive electrode comprises lithium cobalt oxide and a polymer P, the negative electrode uses a silicon-carbon material, and the electrolyte contains (I) lithium difluorophosphate, (II) 1,3,6-hexanetrinitrile, and (III) a nitrogen-containing lithium salt, and more importantly, the total content of (I) to (III) is set to a specific range, although the reason for improving the cycle performance and low-temperature rate performance of the secondary battery is not clear, it is believed that the components of (I) to (III) in the electrolyte not only form a low-impedance coating film on the surface of the positive electrode, but also form a stable coating film on the surface of the negative electrode during the first charge-discharge cycle. As a result, not only can the cycle performance and low-temperature rate performance of the secondary battery be improved, but also the electrode swelling resistance after cycling can be significantly improved.

[0007] In another aspect of the present application, the present application provides an electronic device comprising the secondary battery described in the present application.

[0008] The present application can improve the cycle performance and low-temperature rate performance of the secondary battery and significantly improve the electrode expansion resistance after cycling by using a specific combination of electrode composition and electrolyte.

[0009] Additional aspects and advantages of the embodiments of the present application will be described, shown, or illustrated in part in the following description. DETAILED DESCRIPTION

[0010] The embodiments of the present application will be described in detail below. The embodiments of the present application should not be interpreted as limiting the present application.

[0011] The following terms used in the present application have the meanings indicated below, unless explicitly stated otherwise.

[0012] The present application can improve the cycle performance and low-temperature rate performance of the secondary battery and significantly improve the electrode expansion resistance after cycling by using a specific combination of electrode composition and electrolyte.

[0013] In one embodiment, the present application provides a secondary battery comprising a positive electrode, a negative electrode and an electrolyte as described below.

[0014] I. Electrolyte

[0015] The electrolyte used in the secondary battery of the present application comprises an electrolyte and a solvent dissolving the electrolyte. In some embodiments, the electrolyte of the present application further comprises a combination of lithium difluorophosphate, 1,3,6-hexanetrione and a nitrogen-containing lithium salt.

[0016] Since the coating film formed by lithium difluorophosphate on the surface of silicon-carbon material is unstable and prone to decomposition at low temperature, the inventors found that when the electrolyte further contains 1,3,6-hexanetrione and a nitrogen-containing lithium salt, the decomposition of the coating film can be significantly reduced; and the combination of lithium difluorophosphate, 1,3,6-hexanetrione and a nitrogen-containing lithium salt can also inhibit the swelling of the polymer P contained in the positive electrode in the electrolyte, which can not only improve the cycle performance and low-temperature rate performance of the secondary battery, but also significantly improve the electrode expansion resistance after cycling.

[0017] Specifically, from the perspective of improving the cycle performance of lithium ion batteries, the content of lithium difluorophosphate is 0.01 wt% or more based on the mass of the electrolyte, preferably the content of lithium difluorophosphate is 0.02 wt% or more, preferably 0.03 wt% or more, and more preferably 0.04 wt% or more.

[0018] Further, as an upper limit of the content of lithium difluorophosphate, from the viewpoint of suppressing electrode expansion, the content of lithium difluorophosphate is 0.8 wt% or less, preferably 0.78 wt% or less, more preferably 0.59 wt% or less, further preferably 0.37 wt% or less, and particularly preferably 0.25 wt% or less.

[0019] In some embodiments, the content of lithium difluorophosphate is set to a1 wt%, a1 being 0.01, 0.02, 0.03, 0.04, 0.05, 0.08, 0.11, 0.25, 0.37, 0.59, 0.78, 0.8, or within a range consisting of any two of the above values. For example, 0.01 to 0.05, 0.02 to 0.08, 0.03 to 0.11, 0.05 to 0.37, 0.08 to 0.25, 0.01 to 0.25, 0.05 to 0.59, 0.04 to 0.78, 0.03 to 0.8, when within the above ranges, contribute to further suppressing electrode expansion.

[0020] Specifically, from the viewpoint of improving the cycle performance of the lithium ion battery, the content of 1,3,6-hexanetrione is 0.3 wt% or more, preferably the content of 1,3,6-hexanetrione is 0.6 wt% or more, preferably 0.8 wt% or more, and more preferably 1.2 wt% or more, based on the mass of the electrolyte.

[0021] Further, as an upper limit of the content of 1,3,6-hexanetrione, from the viewpoint of suppressing electrode expansion, the content of 1,3,6-hexanetrione is 5 wt% or less, preferably 4.9 wt% or less, more preferably 4.2 wt% or less, further preferably 3.5 wt% or less, and particularly preferably 2.1 wt% or less.

[0022] In some embodiments, the content of 1,3,6-hexanetrione is set to a2 wt%, a2 being 0.3, 0.6, 0.8, 1.2, 1.7, 1.9, 2.1, 3.5, 4.2, 4.9, 5, or within a range consisting of any two of the above values. For example, 0.3 to 1.9, 0.6 to 2.1, 0.8 to 3.5, 1.2 to 4.2, 1.7 to 4.9, 1.2 to 5, when within the above ranges, contribute to further suppressing electrode expansion.

[0023] In some embodiments, the nitrogen-containing lithium salt includes at least one of LiN(FCO)2, LiN(FCO)(FSO2), LiN(FSO2)2, LiN(FSO2)(CF3SO2), LiN(CF3SO2)2, LiN(C2F5SO2)2, lithium cyclic 1,2-perfluoroethane disulfonimide, lithium cyclic 1,3-perfluoropropane disulfonimide, LiN(CF3SO2)(C4F9SO2), lithium 4,5-dicyano-2-trifluoromethylimidazole salt, lithium 4,5-dicyano-2-pentafluoroethylimidazole salt, lithium 2,4,5-tricyanoimidazole salt, lithium 5,6-dicyano-2-trifluoromethylbenzimidazole salt, lithium 5,6-dicyano-2-pentafluoroethylbenzimidazole salt, lithium 2,5,6-tricyanobenzimidazole salt, lithium 4,7-dicyano-2-trifluoromethylbenzimidazole salt, lithium 4,7-dicyano-2-pentafluoroethylbenzimidazole salt, lithium 2,4,7-tricyanobenzimidazole salt, lithium 4,5,6,7-tetracyano-2-trifluoromethylbenzimidazole salt, lithium 4,5,6,7-tetracyano-2-pentafluoroethylbenzimidazole salt, or lithium 2,4,5,6,7-pentacyanobenzimidazole salt. Since the stability of the formed coating film is excellent, the battery performance is further improved. The nitrogen-containing lithium salt (III) can be only one, or two or more.

[0024] In some embodiments, the nitrogen-containing lithium salt is preferably at least one of LiN(FCO)2, LiN(FCO)(FSO2), LiN(FSO2)2, LiN(FSO2)(CF3SO2), LiN(CF3SO2)2, LiN(C2F5SO2)2.

[0025] Specifically, from the viewpoint of improving the cycle performance of the lithium ion battery, the content of the nitrogen-containing lithium salt is 0.2 wt% or more, preferably the content of the nitrogen-containing lithium salt is 0.6 wt% or more, preferably 0.8 wt% or more, more preferably 1.1 wt% or more, based on the mass of the electrolyte.

[0026] In addition, as the upper limit of the content of the nitrogen-containing lithium salt, from the viewpoint of suppressing electrode expansion, the content of the nitrogen-containing lithium salt is 3 wt% or less, preferably 2.8 wt% or less, more preferably 2.4 wt% or less, further preferably 2.1 wt% or less, particularly preferably 1.7 wt% or less.

[0027] In some embodiments, the content of the nitrogen-containing lithium salt is set to a3wt%, a3 is 0.2, 0.6, 0.7, 0.8, 1.1, 1.9, 2.1, 2.4, 2.8, 3, or within a range consisting of any two of the aforementioned values. For example, 0.2 to 1.9, 0.6 to 3, 0.7 to 2.8, 0.8 to 2.1, 1.1 to 2.8, 0.2 to 1.1, 0.2 to 0.8, when within the aforementioned range, helps to further improve the cycle performance.

[0028] Further, from the viewpoint of suppressing electrode expansion, the total content of (I), (II), and (III) is 1.35wt% or more, preferably 2.35wt% or more, based on the mass of the electrolyte solution.

[0029] In addition, as the upper limit of the total content of (I), (II), and (III), from the viewpoint of improving the electrochemical characteristics at low temperature rates, the total content of (I), (II), and (III) is 5.65wt% or less, preferably 4.85wt% or less.

[0030] In some embodiments, the total content of (I), (II), and (III) is awt%, a is 1.35, 1.45, 1.55, 1.85, 2.35, 2.55, 2.75, 4.15, 4.85, 5.55, 5.65, or within a range consisting of any two of the aforementioned values. For example, 1.35 to 4.15, 1.55 to 4.85, 1.85 to 5.55, 2.35 to 5.65, when within the aforementioned range, helps to further suppress electrode expansion.

[0031] In addition, the electrolyte solution can further include other nitrile-based compounds, and the inventors have also unexpectedly found that the other nitrile-based compounds can reduce the impedance of the aforementioned coating, improve lithium ion charge transport, improve low-temperature performance, and improve electrode expansion resistance.

[0032] The other nitrile-based compounds include at least one of butanedinitrile, hexanedinitrile, ethylene glycol bis(propionitrile) ether, 1,3,5-pentanetricarbonitrile, 1,2,3-propanetricarbonitrile, 1,2,6-hexanetricarbonitrile, 1,2,4-tris(2-cyanoethoxy)butane, 1,1,1-tris(cyanoethoxymethylene)ethane, 1,1,1-tris(cyanoethoxymethylene)propane, 3-methyl-1,3,5-tris(cyanoethoxy)pentane, 1,2,7-tris(cyanoethoxy)heptane, 1,2,6-tris(cyanoethoxy)hexane, or 1,2,5-tris(cyanoethoxy)pentane.

[0033] The aforementioned nitrile-based compounds can be only one, or two or more. For example, butanedinitrile and hexanedinitrile; or butanedinitrile and ethylene glycol bis(propionitrile) ether; or hexanedinitrile and ethylene glycol bis(propionitrile) ether.

[0034] Specifically, from the viewpoint of improving the low-temperature rate performance and suppressing electrode expansion, the content of the other nitrile-based compound is 0.3% by mass or more, preferably 0.6% by mass or more, and more preferably 0.9% by mass or more, based on the mass of the electrolyte solution.

[0035] Further, from the viewpoint of improving the low-temperature rate performance and suppressing electrode expansion, the content of the other nitrile-based compound is 8% by mass or less, preferably 7.9% by mass or less, more preferably 7.1% by mass or less, further preferably 6.2% by mass or less, and particularly preferably 5.3% by mass or less, as the upper limit of the content of the other nitrile-based compound.

[0036] In some embodiments, the total content of the other nitrile-based compound is b% by mass, b being 0.3, 0.35, 0.4, 0.45, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.4, 1.5, 2, 2.5, 3, 3.5, 3.9, 4, 4.6, 5.3, 6.2, 7.1, 7.9, 8, or within a range composed of any two of the above values. For example, 0.3 to 5.3, 0.6 to 4.6, 0.9 to 3.9, 0.4 to 2, 0.45 to 4.6, 0.8 to 2.5, 1.4 to 3.9, 2.5 to 8, 0.9 to 6.2, 0.45 to 5.3, 0.6 to 1.4, when within the above ranges, further improve the low-temperature rate performance and suppress electrode expansion.

[0037] Further, the electrolyte solution can further include other additives, and the inventors have also unexpectedly found that the other additives can suppress the decomposition and regeneration of the aforementioned coating film during charge and discharge, thereby further improving the low-temperature performance and cycle performance.

[0038] The other additives include at least one of lithium monofluorophosphate, 1,2-bis(difluorophosphoroxo)ethane, lithium fluorosulfonate, vinylene carbonate, fluoroethylene carbonate, 1,3-propane sultone, 1,3-propene sultone, 1,3-propanediol cyclic sulfate, fluorobenzene, cyclohexylbenzene, biphenyl, tris(trimethylsilyl)phosphate, or tris(trimethylsilyl)borate.

[0039] The above other additives can be only one or two or more.

[0040] Specifically, from the viewpoint of improving the low-temperature rate performance and cycle performance, the content of the other additives is 0.3% by mass or more, preferably 0.9% by mass or more, and more preferably 1.6% by mass or more, based on the mass of the electrolyte solution.

[0041] Further, the content of the other additive is preferably 10% by weight or less, more preferably 9.7% by weight or less, even more preferably 8.2% by weight or less, further more preferably 7.1% by weight or less, particularly preferably 6.7% by weight or less, from the viewpoint of improving the low-temperature rate performance and cycle performance.

[0042] In some embodiments, the total content of the other additive is c% by weight, where c is 0.3, 0.35, 0.4, 0.45, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.5, 1.6, 2, 2.5, 2.8, 3, 3.5, 3.9, 4, 4.5, 5, 5.5, 6, 6.5, 7.1, 7.5, 8.2, 8.6, 9, 9.3, 9.7, 10, or within a range defined by any two of the above values. For example, 0.3 to 18.2, 0.45 to 7.5, 0.6 to 9.7, 5.5 to 9.7, 6 to 8.6, 2.8 to 6.5, 1 to 6.5, 0.7 to 7.1, 1.5 to 9.3, 0.45 to 3.9, 0.7 to 4.5, when within the above ranges, contribute to further improvement of the low-temperature rate performance and cycle performance.

[0043] In some embodiments, the electrolyte is not particularly limited and can be used arbitrarily using a substance known as an electrolyte. The quality of the electrolyte is not particularly limited as long as the effects of the present application are not impaired.

[0044] For example, the lithium salt used in the electrolyte of the present application includes lithium hexafluorophosphate, and the content of lithium hexafluorophosphate is 9 to 15% by weight, preferably 9 to 13% by weight, more preferably 9 to 12% by weight, based on the mass of the electrolyte. By setting the content within the above range, the cycle and the improvement of the low-temperature discharge characteristics can be more balanced.

[0045] In some embodiments, the electrolyte further includes any non-aqueous solvent known in the art as a solvent for an electrolyte.

[0046] In some embodiments, the non-aqueous solvent includes, but is not limited to, one or more of the following: cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, chain carboxylic acid esters, cyclic ethers, chain ethers, phosphorus-containing organic solvents, and sulfur-containing organic solvents.

[0047] II. Negative electrode

[0048] The negative electrode includes a negative electrode current collector and a negative electrode mixture layer disposed on the surface of the negative electrode current collector, the negative electrode mixture layer containing a negative electrode active material. In some embodiments, the chargeable capacity of the negative electrode active material is greater than the discharge capacity of the positive electrode active material, in order to minimize the unintended deposition of lithium metal on the negative electrode during charging.

[0049] Further, as the negative electrode active material, there are no particular limitations, and examples include carbon-based negative electrode active materials, metal-based negative electrode active materials, and negative electrode active materials combining these.

[0050] Carbon-based negative electrode active materials

[0051] Here, the carbon-based negative electrode active material refers to an active material having a carbon-based skeleton into which lithium can be inserted, and examples include carbonaceous materials and graphitic materials.

[0052] As the carbonaceous material, examples include, for example, easy-graphitizable carbon, difficult-graphitizable carbon having an amorphous structure, and the like. Here, as the easy-graphitizable carbon, examples include carbon materials obtained from tar pitch as a raw material, which is derived from petroleum or coal. When specific examples are given, examples include coke, mesocarbon microbeads (MCMB), mesophase pitch-based carbon fibers, and pyrolytic vapor grown carbon fibers. Further, as the difficult-graphitizable carbon, examples include, for example, phenol resin sintered bodies, polyacrylonitrile-based carbon fibers, quasi-isotropic carbon, furfuryl alcohol resin sintered bodies (PFA), hard carbon, and the like.

[0053] Further, as the graphitic material, examples include, for example, natural graphite, artificial graphite, and the like. Among these, as the artificial graphite, examples include, for example, artificial graphite obtained by heat-treating carbon containing easy-graphitizable carbon at 2800°C or higher, graphitic MCMB obtained by heat-treating MCMB at 2000°C or higher, graphitic mesophase pitch-based carbon fibers obtained by heat-treating mesophase pitch-based carbon fibers at 2000°C or higher, and the like. In addition, in the present application, as the carbon-based negative electrode active material, natural graphite in which at least a portion of the surface is covered with amorphous carbon (amorphous-coated natural graphite) can be used.

[0054] Metal-based negative electrode active materials

[0055] Further, the metal-based negative electrode active material is an active material containing metal, and generally refers to an active material containing an element capable of intercalating lithium or alloying with lithium in the structure, and having a theoretical current capacity of 500 mAh / g or more per unit mass in the case of intercalating lithium or alloying with lithium. As the metal-based negative electrode active material, examples include, for example, lithium metal, elemental metals that can form lithium alloys (for example, Ag, Al, Ba, Bi, Cu, Ga, Ge, In, Ni, P, Pb, Sb, Si, Sn, Sr, Zn, Ti, and the like), alloys thereof, and oxides, sulfides, nitrides, silicides, carbides, phosphides, and the like thereof. Among these, as the metal-based negative electrode active material, an active material containing silicon (silicon-based negative electrode active material) is preferable. This is because the lithium ion secondary battery can be made high-capacity by using the silicon-based negative electrode active material.

[0056] As the silicon-based negative electrode active material, for example, silicon (Si), an alloy containing silicon, SiO, SiO x , a silicon-containing material coated with or compounded with electrically conductive carbon.

[0057] From the viewpoint of improving the capacity of the battery, a silicon-carbon material, such as a composite material of porous carbon supporting silicon, is preferred.

[0058] In addition, the negative electrode active material can be used alone or in combination of two or more at an arbitrary ratio.

[0059] Volume average particle diameter of active material

[0060] Here, the volume average particle diameter of the negative electrode active material is preferably 1 μm or more, more preferably 5 μm or more, and preferably 30 μm or less, more preferably 20 μm or less. If the volume average particle diameter of the negative electrode active material is above the lower limit value described above, the electrode expansion can be effectively suppressed. In addition, if the volume average particle diameter of the negative electrode active material is below the upper limit value described above, the cycle of the obtained battery can be effectively suppressed.

[0061] Mass per unit area of negative electrode mixture layer

[0062] Specifically, from the viewpoint of improving the cycle performance, the mass per unit area of the negative electrode mixture layer is 4.5 mg / cm 2 or more, preferably 5.1 mg / cm 2 or more, and more preferably 5.5 mg / cm 2 or more, and more preferably 6.2 mg / cm 2 or more.

[0063] In addition, as the upper limit of the mass per unit area of the negative electrode mixture layer, from the viewpoint of improving the low-temperature rate performance, the mass per unit area of the negative electrode mixture layer is 12.5 mg / cm 2 or less, preferably 11.2 mg / cm 2 or less, more preferably 10.5 mg / cm 2 or less, further preferably 9.3 mg / cm 2 or less, particularly preferably 8.1 mg / cm 2 or less.

[0064] In some embodiments, the mass per unit area of the negative electrode mixture layer is w mg / cm 2w is 4.5, 4.8, 5.1, 5.3, 5.5, 6.2, 7.8, 8.1, 9.3, 10.5, 11.2, 12.3, 12.5, or within a range consisting of any two of the above values. For example, 4.5 to 9.3, 4.8 to 12.3, 5.1 to 8.1, 5.3 to 10.5, 5.5 to 12.3, 6.2 to 11.2, 7.8 to 12.5, 8.1 to 10.5, 9.3 to 12.5, 5.3 to 9.3, 6.2 to 8.1, when within the above ranges, help to further suppress electrode expansion.

[0065] Further, from the viewpoint of suppressing electrode expansion, the aforementioned a / w is 0.124 or greater and 1.009 or less. In some embodiments, a / w is 0.124, 0.241, 0.259, 0.33, 0.42, 0.455, 0.491, 0.555, 0.608, 0.647, 0.741, 0.866, 0.991, 1.009, or within a range consisting of any two of the above values. For example, 0.124 to 0.741, 0.241 to 0.491, 0.33 to 1.009, 0.555 to 0.866, when within the above ranges, help to further suppress electrode expansion.

[0066] The mass per unit area of the negative electrode mixture layer is the ratio of the mass of the negative electrode mixture layer (mg) to the area of the mixture layer (cm 2 ). The mass and area of the negative electrode mixture layer are obtained by cutting a test piece of appropriate size from the negative electrode, measuring the area S1 and mass W0, then peeling the negative electrode current collector from the negative electrode, measuring the mass of the negative electrode current collector W1, and calculating the mass of the negative electrode mixture layer from (W0-W1). The mass per unit area = (W0-W1) / S1. If the selected negative electrode has a double-sided mixture layer, the mass per unit area = (W0-W1) / S1 / 2.

[0067] As a method of peeling the negative electrode mixture layer, for example, methods such as immersing the negative electrode mixture layer in a solvent that dissolves or swells the negative electrode mixture layer, and wiping the mixture layer with a cloth or the like can be cited.

[0068] The mass per unit area of the negative electrode mixture layer can be adjusted using known methods. For example, in the case of forming the negative electrode mixture layer by coating, it can be adjusted by changing the solid content concentration of the coating liquid used to form the negative electrode mixture layer, the number of times of coating, the gap of the coating liquid inlet of the coating machine, and the like. The mass per unit area of the negative electrode mixture layer can be increased by increasing the solid content concentration, increasing the number of times of coating, or increasing the gap, and the like. The mass per unit area of the negative electrode mixture layer can be decreased by decreasing the solid content concentration, decreasing the number of times of coating, or decreasing the gap, and the like.

[0069] The negative electrode mixture layer can further include a negative electrode binder. The negative electrode binder can improve the binding of the negative electrode active material particles to each other and the binding of the negative electrode active material to the current collector. The type of the negative electrode binder is not particularly limited, as long as it is a material stable to an electrolyte or a solvent used in the electrode manufacturing. In some embodiments, the negative electrode binder includes a resin binder. Examples of the resin binder include, but are not limited to, a fluorine resin, a polyacrylonitrile (PAN), a polyimide resin, an acrylic resin, a polyolefin resin, and the like. When a water-based solvent is used to prepare a negative electrode mixture slurry, the negative electrode binder includes, but is not limited to, carboxymethyl cellulose (CMC) or a salt thereof, styrene-butadiene rubber (SBR), polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol, and the like.

[0070] As the current collector that holds the negative electrode active material, a publicly known current collector can be arbitrarily used. Examples of the negative electrode current collector include, but are not limited to, a metal material such as copper, nickel, stainless steel, nickel-plated steel, and the like. In some embodiments, the negative electrode current collector is copper.

[0071] The negative electrode can be prepared by coating a negative electrode mixture slurry including a negative electrode active material, a resin binder, and the like on a negative electrode current collector, drying, and then calendering to form a negative electrode mixture layer on both surfaces of the negative electrode current collector, whereby the negative electrode can be obtained.

[0072] III. Positive Electrode

[0073] The positive electrode includes a positive electrode current collector and a positive electrode mixture layer disposed on a surface of the positive electrode current collector.

[0074] The positive electrode mixture layer includes a positive electrode active material, and the positive electrode mixture layer can be one layer or multiple layers. The positive electrode active material is any material capable of reversibly intercalating and deintercalating lithium ions.

[0075] For example, as the positive electrode active material for a lithium ion battery, a complex metal oxide containing one or two or more selected from the group consisting of cobalt, manganese, and nickel, or a lithium-containing olivine-type phosphate containing one or two or more selected from iron, cobalt, nickel, and manganese is used. These positive electrode active materials can be used alone or in combination of two or more.

[0076] As such a lithium complex metal oxide, for example, a solid solution of LiCoO2, LiMn2O4, LiNiO2, LiCoO2(0.01 < x < 1), LiNiO2(x + y + z = 1), Li2MnO3, and LiMO2(M is a transition metal such as Co, Ni, Mn, Fe, and the like) selected from LiCoO2, LiMn2O4, LiNiO2, LiCoO2(0.01 < x < 1), LiNiO2(x + y + z = 1), Li2MnO3, and LiMO2(M is a transition metal such as Co, Ni, Mn, Fe, and the like) can be appropriately cited. 1-x Ni x O2(0.01 < x < 1), LiNi x Mn y Co z O2(x + y + z = 1), Li2MnO3, and LiMO2(M is a transition metal such as Co, Ni, Mn, Fe, and the like) can be appropriately cited. 1 / 2 Mn3 / 2 O4, LiFePO4, LiMnPO4, and LiMn 1-x One or more of FexPO4(0.01 < x < 1), more preferably two or more. A part of these complex metal oxides with lithium or olivine-type lithium-containing phosphate can be substituted with other elements, or a part of cobalt, nickel, manganese, iron can be substituted with one or two or more elements selected from Co, Mn, Ni, Mg, Al, B, Ti, V, Nb, Cu, Zn, Mo, Ca, Sr, W, and Zr, or coated with a compound containing these other elements or carbon material.

[0077] For example, the positive electrode contains a lithium cobalt oxide having at least three kinds of impurity elements of aluminum, magnesium, titanium, zirconium, lanthanum, iridium, cerium, and tungsten. From the viewpoint of improving the cycle characteristics of the lithium ion battery, the content of any one of the impurity elements is preferably 0.01 wt% or more, preferably 0.03 wt% or more, and more preferably 0.05 wt% or more, based on the mass of the lithium cobalt oxide. In addition, as an upper limit of the content of the impurity elements, the content of any one of the impurity elements is 1 wt% or less, preferably 0.5 wt% or less, more preferably 0.3 wt% or less, further preferably 0.15 wt% or less, and particularly preferably 0.1 wt% or less.

[0078] If a lithium complex metal oxide that operates at a high charging voltage is used, the electrochemical characteristics are easily degraded at a high temperature environment by a reaction with a nonaqueous electrolyte solution at the time of charging, but in the lithium ion battery described in the present application, the degradation of these electrochemical characteristics can be suppressed.

[0079] As the voltage at the time of charging, from the viewpoint of high voltage, the positive electrode potential is preferably 4.4 V (vs. Li / Li+) or more, more preferably 4.5 V (vs. Li / Li+) or more, and particularly preferably 4.6 V (vs. Li / Li+) or more.

[0080] From the viewpoint of suppressing electrode expansion, the positive electrode binder includes a polymer P that can exert a binding action between electrode active materials and between an electrode active material and a current collector.

[0081] As described above, the polymer P contains at least: a monomer unit containing a nitrile group; and preferably polyacrylonitrile.

[0082] As the nitrile group-containing monomer capable of forming a nitrile group-containing monomer unit, an α,β-ethylenically unsaturated nitrile monomer can be given. Specifically, as the α,β-ethylenically unsaturated nitrile monomer, any α,β-ethylenically unsaturated compound having a nitrile group can be given, and examples thereof include acrylonitrile; α-halogenated acrylonitriles such as α-chloroacrylonitrile and α-bromoacrylonitrile; α-alkyl acrylonitriles such as methacrylonitrile and α-ethylacrylonitrile; and the like. In addition, the nitrile group-containing monomer can be used alone or in combination of two or more kinds at an arbitrary ratio.

[0083] The conductive agent of the positive electrode is not particularly limited as long as it is an electron-conducting material that does not cause chemical changes. Examples thereof include natural graphite (flaky graphite and the like), artificial graphite, acetylene black, ketjen black, slot carbon black, furnace black, lamp black, or thermal cracking carbon black, and the like. In addition, the graphite and the carbon black can be appropriately mixed and used. The amount of the conductive agent added to the positive electrode mixture is preferably 1 to 10% by weight, and particularly preferably 1.5 to 5% by weight.

[0084] The positive electrode can be produced by mixing the above-described positive electrode active material with a conductive agent such as acetylene black or carbon black, and a binder such as polytetrafluoroethylene, polyvinylidene fluoride, polyacrylonitrile, a copolymer of styrene and butadiene, or carboxymethyl cellulose, mixing a high-boiling solvent such as 1-methyl-2-pyrrolidone into the mixture, and kneading the mixture to produce a positive electrode mixture slurry, coating the slurry on an aluminum foil or the like serving as a current collector, drying the coated product, and pressing the dried product to form a positive electrode mixture layer, thereby producing the positive electrode.

[0085] The density of the portion of the positive electrode other than the current collector is generally 3.5 g / cm 3 The density of the portion of the positive electrode other than the current collector is generally 3.5 g / cm 3 The density of the portion of the positive electrode other than the current collector is generally 3.5 g / cm 3 The density of the portion of the positive electrode other than the current collector is generally 3.5 g / cm 3 The density of the portion of the positive electrode other than the current collector is generally 3.5 g / cm 3 The density of the portion of the positive electrode other than the current collector is generally 3.5 g / cm

[0086] The type of the positive electrode current collector is not particularly limited, and it can be any known material suitable for use as a positive electrode current collector. Examples of the positive electrode current collector can include, but are not limited to, metal materials such as aluminum, stainless steel, nickel plating, titanium, tantalum, and the like; and carbon materials such as carbon cloth and carbon paper. In some embodiments, the positive electrode current collector is a metal material. In some embodiments, the positive electrode current collector is aluminum.

[0087] In order to reduce the electronic contact resistance of the positive electrode current collector and the positive electrode mixture layer, the surface of the positive electrode current collector can include a conductive aid or a conductive coating. Examples of the conductive aid can include, but are not limited to, carbon and noble metals such as gold, platinum, and silver. Examples of the conductive coating can include a mixture layer containing an inorganic oxide, a conductive agent, and a binder.

[0088] The positive electrode can be produced by forming a positive electrode mixture layer containing a positive electrode active material and a binder on a current collector. The production of the positive electrode using the positive electrode active material can be performed by a conventional method, i.e., dry mixing of the positive electrode active material and the binder, and, as necessary, a conductive material and a thickening agent, etc., making a sheet, and pressure bonding the obtained sheet to a positive electrode current collector; or dissolving or dispersing these materials in a liquid medium to make a slurry, coating the slurry on a positive electrode current collector and drying, thereby forming a positive electrode mixture layer on the current collector, whereby the positive electrode can be obtained.

[0089] IV. Separation film

[0090] In order to prevent short circuit, a separation film is usually provided between the positive electrode and the negative electrode. In this case, the electrolyte of the present application is usually used by permeating into the separation film.

[0091] The material and shape of the separation film are not particularly limited as long as the effects of the present application are not significantly impaired. The separation film can be a resin, a glass fiber, an inorganic substance, or the like formed of a material stable to the electrolyte of the present application. In some embodiments, the separation film includes a porous sheet or a nonwoven fabric-like substance having excellent liquid retention, or the like. Examples of the material of the resin or glass fiber separation film can include, but are not limited to, polyolefin, aramid, polytetrafluoroethylene, polyethersulfone, or the like. In some embodiments, the polyolefin is polyethylene or polypropylene. In some embodiments, the polyolefin is polypropylene. The above-mentioned materials of the separation film can be used alone or in any combination.

[0092] The separation film can also be a material in which the above-mentioned materials are layered, examples of which include, but are not limited to, a three-layer separation film in which polypropylene, polyethylene, and polypropylene are layered in this order, or the like.

[0093] Examples of the material of the inorganic substance can include, but are not limited to, oxides such as alumina and silica, nitrides such as aluminum nitride and silicon nitride, sulfates (e.g., barium sulfate, calcium sulfate, etc.). The form of the inorganic substance can include, but is not limited to, a granular or fibrous form.

[0094] The shape of the separation film can be a film shape, examples of which include, but are not limited to, a nonwoven fabric, a woven fabric, a microporous film, or the like. In the film shape, the separation film has a pore diameter of 0.01 μm to 1 μm and a thickness of 5 μm to 50 μm. In addition to the above-mentioned independent film-shaped separation film, a separation film formed by forming a composite porous layer containing the above-mentioned inorganic substance particles on the surface of the positive electrode and / or the negative electrode using a resin-based adhesive, for example, a separation film in which 90% of alumina particles having a particle size of less than 1 μm are formed into a porous layer on both surfaces of the positive electrode using a fluororesin as an adhesive, can be used.

[0095] The thickness of the separator film is arbitrary. In some embodiments, the thickness of the separator film is greater than 1 μm, greater than 5 μm, or greater than 8 μm. In some embodiments, the thickness of the separator film is less than 50 μm, less than 40 μm, or less than 30 μm. When the thickness of the separator film is within the above range, insulation and mechanical strength can be ensured, and the rate characteristics and energy density of the secondary battery can be ensured.

[0096] The present application also provides an electronic device including the secondary battery according to the present application.

[0097] The use of the secondary battery of the present application is not particularly limited, and it can be used in any electronic device known in the art. In some embodiments, the secondary battery of the present application can be used in, but not limited to, notebook computers, pen input computers, mobile computers, electronic book players, portable telephones, portable facsimile machines, portable copying machines, portable printers, head-mounted stereo headphones, video recorders, liquid crystal televisions, portable cleaners, portable CD players, mini-disc players, transceivers, electronic organizers, calculators, memory cards, portable audio players, radios, backup power supplies, motors, automobiles, motorcycles, power-assisted bicycles, bicycles, lighting appliances, toys, game machines, timepieces, power tools, flashlights, cameras, home-use large storage batteries, and lithium ion capacitors, etc.

[0098] The preparation of the secondary battery will be described below with reference to specific examples, and those skilled in the art will understand that the preparation methods described in the present application are only examples, and any other suitable preparation method is within the scope of the present application.

[0099] Examples

[0100] Hereinafter, examples of the non-aqueous electrolyte solution of the present application will be shown, but the present application is not limited to these examples.

[0101] Manufacture of lithium ion battery

[0102] The positive electrode active material of Table 1-1: 97 wt%, acetylene black: 1.5 wt% was mixed, added to a solution obtained by previously dissolving polyacrylonitrile: 1.5 wt% in 1-methyl-2-pyrrolidinone, and mixed to prepare a positive electrode mixture paste. The positive electrode mixture paste was coated on an aluminum foil, dried, and pressure treated, and then cut into a prescribed size to manufacture a positive electrode.

[0103] Further, a negative electrode active material of Table 1-1 : 96 wt%, a butadiene-styrene rubber: 2 wt% was mixed, added to a solution in which lithium carboxymethyl cellulose: 2 wt% was previously dissolved in deionized water and mixed to prepare a negative electrode mixture paste. The negative electrode mixture paste was coated on a copper foil, dried, pressure treated and cut into a predetermined size to produce a negative electrode. The mass per unit area of the negative electrode mixture layer was obtained by adjusting the coating thickness.

[0104] The positive and negative electrodes produced as above were each connected to a lead wire. Lamination was performed via a polypropylene porous film having a thickness of 10 μm to obtain a laminate. Further, LiPF6 as a supporting electrolyte was additionally dissolved in a solution containing (I) lithium difluorophosphate, (II) 1,3,6-hexanetrinitrile, (III) a lithium salt containing nitrogen, and propyl propionate, ethyl propionate, vinyl carbonate and propylene carbonate (mass ratio 2.3:2:1.2:0.9). The contents and components of (I) to (III), other nitrile-based compounds, other additives were as shown in Tables 1 and 2 based on 100 parts by mass of the total mass of the nonaqueous electrolyte, and the content of LiPF6 was 14%.

[0105] Then, the laminate was housed in an aluminum laminate case together with 3.2 g of the electrolyte. The opening portion of the case was heat-sealed, and a lithium ion battery was produced through steps of formation, capacity, etc. The lithium ion battery was a pouch having a width of 35 mm, a height of 48 mm and a thickness of 5 mm.

[0106] Table 1

[0107] The positive and negative electrode materials and the electrolyte partial component codes of the lithium ion battery produced are shown in Table 1, and the detailed compositions are shown in Tables 1-1, 1-2 and 1-3, respectively.

[0108] Table 1 Positive and negative electrode materials and electrolyte partial component codes of the lithium ion battery

[0109]

[0110]

[0111]

[0112] The values in parentheses in the above tables are wt%

[0113] Table 1-1 Positive electrode active material component codes

[0114] No. Component Positive electrode 1 Lithium cobaltate containing 0.3 mass% aluminum element + 0.2 mass% magnesium element Positive electrode 2 Lithium cobaltate containing 0.2 mass% magnesium element + 0.1 mass% zirconium element Positive electrode 3 Lithium cobaltate containing 0.3 mass% aluminum element + 0.1 mass% magnesium element + 0.2 mass% lanthanum element Positive electrode 4 Lithium cobaltate containing 0.2 mass% aluminum element + 0.2 mass% magnesium element + 0.2 mass% iridium element Positive electrode 5 Lithium cobaltate containing 0.3 mass% aluminum element + 0.2 mass% tungsten element Positive electrode 6 Lithium cobaltate containing 0.3 mass% magnesium element + 0.2 mass% cerium element

[0115] Table 1-2 Negative electrode active material component codes

[0116] No. Component Negative electrode 1 Artificial graphite + silicon carbon (mass ratio 97:3) Negative electrode 2 Artificial graphite + silicon carbon (mass ratio 95:5) Negative electrode 3 Artificial graphite + silicon carbon (mass ratio 92:8) Negative electrode 4 Artificial graphite + silicon carbon (mass ratio 90:10) Negative electrode 5 Artificial graphite + silicon carbon + hard carbon (mass ratio 94:3:3) Negative electrode 6 Artificial graphite + silicon carbon + hard carbon (mass ratio 90:3:7)

[0117] Table 1-3 Electrolyte component code

[0118] Code Substance name Code Substance name D1 LiN(FSO2)2 C1 Lithium monofluorophosphate D2 LiN(FSo2)(CF3SO2) C2 Vinylene carbonate D3 LiN(CF3SO2)2 C3 Fluoroethylene carbonate D4 LiN(FCO)(FSO2) C4 Lithium fluorosulfonate D5 LiN(C2F5So2)2 C5 1,3-propane sulfone propyl ester D6 LiN(FCO)2 C6 1,2-bis(difluorophosphoryl)ethane D7 Cyclic 1,2-perfluoroethane disulfonimide lithium C7 Fluorobenzene D8 Cyclic 13-perfluoropropane disulfonimide lithium C8 Biphenyl D9 LiN(CF3SO2)(C4F9SO2) C9 Tris(trimethylsilyl)phosphate D10 4,5-dicyano-2-trifluoromethylimidazole lithium salt C10 Tris(trimethylsilyl)borate D11 4,5-dicyano-2-pentafluoroethylimidazole lithium salt C11 1,3-propanediol cyclic sulfate D12 2,4,5-tricyanoimidazole lithium salt C12 Cyclohexylbenzene N1 Butanedinitrile N7 1,3,5-pentanetricarbonitrile N2 Hexanedinitrile N8 1,2,3-propanetricarbonitrile N3 Ethylene glycol bis(propionitrile) ether N9 1,2,6-tris(cyanoethoxy)hexane N4 1,2,6-hexanetricarbonitrile N10 1,2,4-tris(2-cyanoethoxy)butane N5 1,1,1-tris(cyanoethoxymethylene)ethane N11 1,1,1-tris(cyanoethoxymethylene)propane N6 1,2,5-tris(cyanoethoxy)pentane N12 3-methyl-1,3,5-tris(cyanoethoxy)pentane

[0119] Test method

[0120] Cycle characteristics

[0121] The lithium ion batteries produced in the examples and comparative examples were left to stand for 5 hours at a temperature of 25°C after electrolyte injection. First, the batteries were charged at a constant current of 0.2 C to a battery voltage of 4.6 V at a temperature of 25°C, and then subjected to aging treatment at a temperature of 60°C for 12 hours. Then, the batteries were discharged at a constant current of 0.2 C to a battery voltage of 3.0 V at a temperature of 25°C. Then, CC-CV (constant current-constant voltage) charging (upper limit battery voltage 4.6 V) was performed at a constant current of 0.2 C, and CC discharging to 3.0 V was performed at a constant current of 0.2 C, and the initial discharge capacity Xl was measured. Then, charging and discharging operations were performed at a battery voltage of 4.6-3.0 V and a charge-discharge rate of 1.0 C for 50 cycles at a temperature of 45°C. Subsequently, charging and discharging operations were performed at a battery voltage of 4.6-3.0 V and a charge-discharge rate of 0.5 C for 50 cycles at a temperature of 0°C. Further, CC-CV charging (battery voltage 4.6 V) was performed at a constant current of 0.2 C, and discharging to a battery voltage of 3.00 V was performed at a constant current of 0.2 C at a temperature of 25°C, and the discharge capacity X2 was measured. Using the initial discharge capacity Xl and the discharge capacity X2, the capacity retention rate indicated by ΔC' = (X2 / Xl) x 100 (%) was calculated, and evaluated according to the following criteria. A indicates the most excellent cycle characteristics, and D indicates the lowest cycle characteristics.

[0122] A: The capacity retention rate ΔC' is 85% or more

[0123] B: The capacity retention rate ΔC' is 80% or more and less than 85%

[0124] C: The capacity retention rate ΔC' is 75% or more and less than 80%

[0125] D: The capacity retention rate ΔC' is less than 75%

[0126] Low temperature rate

[0127] The lithium ion battery produced in the Examples and Comparative Examples was left to stand for 5 hours at a temperature of 25°C after the electrolyte was injected. Subsequently, the battery was charged to a battery voltage of 4.6 V at a constant current of 0.2 C at a temperature of 25°C, and then subjected to aging treatment at a temperature of 60°C for 12 hours. Subsequently, the battery was discharged to a battery voltage of 3.0 V at a constant current of 0.2 C at a temperature of 25°C. Then, CC-CV (constant current-constant voltage) charging (upper limit battery voltage 4.6 V) was performed at a constant current of 0.2 C, and CC discharging to a battery voltage of 3.0 V was performed at a constant current of 0.2 C. This 0.2 C charging and discharging was repeated 3 times. Subsequently, 0.5 C constant current charging and discharging between a battery voltage of 4.6 V and 3.0 V was performed at a temperature of 25°C, and the discharge capacity at this time was defined as Co. Then, CC-CV charging was performed at a constant current of 0.2 C, and discharging to 2.5 V was performed at a constant current of 0.5 C at a temperature of -20°C, and the discharge capacity at this time was defined as Ci. Then, as the rate characteristic, the capacity retention rate indicated by ΔC = (Ci / Co) x 100 (%) was calculated, and evaluation was performed in accordance with the following criteria. A indicates the most excellent low temperature characteristic (high discharge capacity under high current conditions in a low temperature environment, and low internal resistance), and D indicates the lowest low temperature characteristic.

[0128] A: The capacity retention rate ΔC is 80% or more

[0129] B: The capacity retention rate ΔC is more than 75% and less than 80%

[0130] C: The capacity retention rate ΔC is more than 70% and less than 75%

[0131] D: The capacity retention rate ΔC is less than 70%

[0132] Expansion resistance of electrode after cycling

[0133] The battery after the first 50 cycles in the above <rate characteristic> was charged at 1 C in a 25°C environment, and the positive electrode was removed from the battery in the charged state. The thickness (d2) of the positive electrode (thickness excluding the current collector) was measured. Then, the change rate of the thickness of the positive electrode after cycling relative to the thickness (dO) of the positive electrode before cycling (before the lithium ion battery was produced) [{(d2-dO) / dO} x 100 (%)] was calculated. Then, evaluation was performed in accordance with the following criteria. A indicates the most excellent expansion resistance of electrode after cycling, and D indicates the lowest expansion resistance of electrode after cycling.

[0134] A: The change rate of the thickness is less than 10%

[0135] B: The change rate of the thickness is more than 10% and less than 15%

[0136] C: The change rate of the thickness is more than 15% and less than 20%

[0137] D: the rate of change of thickness is 25% or more

[0138] Table 2

[0139] In Table 2, a1 represents the content of (I) lithium difluorophosphate (unit: wt%), a2 represents the content of (II) 1,3,6-hexanetrinitrile (unit: wt%), a3 represents the content of (III) nitrogen-containing lithium salt (unit: wt%), a represents the total content of (I), (II), (III), w represents the mass per unit area of the negative electrode mixture layer (unit: mg / cm 2 ), b represents the total content of other nitrile-based compounds (unit: wt%), and c represents the total content of other additives (unit: wt%).

[0140] Table 2: Content of some components in electrolyte and electrochemical performance of lithium ion battery

[0141]

[0142]

[0143]

[0144] In a secondary battery in which the positive electrode comprises lithium cobalt oxide and a polymer P and the negative electrode uses silicon-carbon material, the electrolyte contains (I) lithium difluorophosphate, (II) 1,3,6-hexanetrinitrile and (III) nitrogen-containing lithium salt, and the components of (I)-(III) form a stable coating on the surface of the electrode during the initial cycle, inhibit the destruction of the electrolyte to the negative electrode during the charge-discharge cycle, reduce the consumption of active lithium by the decomposition and regeneration of the coating, thereby not only improving the cycle performance and low-temperature rate performance of the secondary battery, but also significantly improving the electrode swelling resistance after cycling.

[0145] In particular, when the electrolyte further contains other nitrile-based compounds, the impedance of the aforementioned coating can be reduced, the lithium ion charge transport can be improved, and the low-temperature rate performance and electrode swelling resistance of the lithium ion battery can be significantly improved.

[0146] In particular, when the electrolyte further contains other additives, the inventors have also unexpectedly found that the other additives can inhibit the decomposition of the aforementioned coating during the charge-discharge cycle, thereby improving the cycle performance and low-temperature rate performance.

[0147] References throughout this specification to "an embodiment", "particular embodiments", "one embodiment", "another embodiment", "certain embodiments", "some embodiments", "one example" or "an example" mean that a particular feature, structure, material or characteristic is included in at least one embodiment or example of the present application. Thus, the appearances of the phrases such as "in some embodiments", "in an embodiment", "in one embodiment", "in another embodiment", "in one example", "in particular embodiments" or "in certain embodiments" in various places throughout this specification are not necessarily referring to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics can be combined in any suitable manner in one or more embodiments or examples.

[0148] While the illustrative embodiments have been described and illustrated, it will be understood by those skilled in the art that the above-described embodiments are not the only ways in which the present application can be practiced. Changes, substitutions and modifications can be made to the embodiments without departing from the spirit, principles and scope of the application.

Claims

1. A secondary battery comprising: a positive electrode, a negative electrode, and an electrolyte solution, the positive electrode includes lithium cobalt oxide and a polymer P including a monomer unit containing a nitrile group, the negative electrode includes graphite and silicon carbon, the electrolyte solution contains (I) lithium difluorophosphate, (II) 1,3,6-hexanetrinitrile, and (III) a lithium salt containing nitrogen, the total content of the (I), (II), and (III) being 1.35 wt% or more and 5.65 wt% or less based on the mass of the electrolyte solution, the mass content of the lithium salt containing nitrogen is 0.2 wt% or more and 3 wt% or less, the mass content of the lithium difluorophosphate is 0.01 wt% or more and 0.8 wt% or less, and the mass content of the 1,3,6-hexanetrinitrile is 0.3 wt% or more and 5 wt% or less; wherein the lithium salt containing nitrogen includes at least one of LiN(FCO)2, LiN(FCO)(FSO2), LiN(FSO2)2, LiN(FSO2)(CF3SO2), LiN(CF3SO2)2, LiN(C2F5SO2)2, cyclic 1,2-perfluoroethane disulfonimide lithium, cyclic 1,3-perfluoropropane disulfonimide lithium, LiN(CF3SO2)(C4F9SO2), 4,5-dicyano-2-trifluoromethylimidazolium lithium salt, 4,5-dicyano-2-pentafluoroethylimidazolium lithium salt, 2,4,5-tricyanoimidazolium lithium salt, 5,6-dicyano-2-trifluoromethylbenzimidazolium lithium salt, 5,6-dicyano-2-pentafluoroethylbenzimidazolium lithium salt, 2,5,6-tricyanobenzimidazolium lithium salt, 4,7-dicyano-2-trifluoromethylbenzimidazolium lithium salt, 4,7-dicyano-2-pentafluoroethylbenzimidazolium lithium salt, 2,4,7-tricyanobenzimidazolium lithium salt, 4,5,6,7-tetracyano-2-trifluoromethylbenzimidazolium lithium salt, 4,5,6,7-tetracyano-2-pentafluoroethylbenzimidazolium lithium salt, or 2,4,5,6,7-pentacyanobenzimidazolium lithium salt.

2. The secondary battery according to claim 1, wherein the lithium cobalt oxide has at least three kinds of impurity elements among aluminum, magnesium, titanium, zirconium, lanthanum, iridium, cerium, and tungsten; the content of any one of the impurity elements is 0.01 wt% or more and 1 wt% or less based on the mass of the lithium cobalt oxide.

3. The secondary battery according to claim 1, wherein the polymer P includes polyacrylonitrile.

4. The secondary battery according to any one of claims 1 to 3, wherein the total content of the lithium difluorophosphate, 1,3,6-hexanetrinitrile, and lithium salt containing nitrogen is 1.85 wt% or more and 5.55 wt% or less based on the mass of the electrolyte solution.

5. The secondary battery according to any one of claims 1 to 3, wherein The negative electrode includes a negative electrode current collector and a negative electrode mixture layer disposed on the negative electrode current collector, wherein the mass per unit area of ​​the negative electrode mixture layer is 4.5 mg / cm³. 2 Above and 12.5 mg / cm 2 Based on the mass of the electrolyte, the total content of lithium difluorophosphate, 1,3,6-hexanetricarbonyl nitrile and nitrogen-containing lithium salt is a, and the mass per unit area of ​​the negative electrode mixture layer is w, where a and w satisfy: a / w is 0.124 or more and 1.009 or less.

6. The secondary battery according to any one of claims 1 to 3, wherein the electrolyte solution further includes other nitrile group compounds including at least one of succinonitrile, adiponitrile, ethyleneglycol bis(propionitrile) ether, 1,3,5-pentanetrinitrile, 1,2,3-propanetrinitrile, 1,2,6-hexanetrinitrile, 1,2,4-tris(2-cyanoethoxy)butane, 1,1,1-tris(cyanoethoxymethylene)ethane, 1,1,1-tris(cyanoethoxymethylene)propane, 3-methyl-1,3,5-tris(cyanoethoxy)pentane, 1,2,7-tris(cyanoethoxy)heptane, 1,2,6-tris(cyanoethoxy)hexane, or 1,2,5-tris(cyanoethoxy)pentane; The content of the other nitrile-based compound is 0.3 wt% or more and 8 wt% or less based on the mass of the electrolyte solution.

7. The secondary battery according to claim 6, wherein The content of the other nitrile-based compound is 0.6 wt% or more and 7.1 wt% or less based on the mass of the electrolyte solution.

8. The secondary battery according to claim 7, wherein The content of the other nitrile-based compound is 1.4 wt% or more and 6.2 wt% or less based on the mass of the electrolyte solution.

9. The secondary battery according to any one of claims 1 to 3, wherein The electrolyte solution further includes other additives including at least one of lithium monofluorophosphate, 1,2-bis(difluorophosphorooxy)ethane, vinylene carbonate, fluoroethylene carbonate, lithium fluorosulfonate, 1,3-propane sultone, 1,3-propene sultone, 1,3-propanediol cyclic sulfate, fluorobenzene, cyclohexylbenzene, biphenyl, tris(trimethylsilyl)phosphate, or tris(trimethylsilyl)borate; The content of the other additives is 0.3 wt% or more and 10 wt% or less based on the mass of the electrolyte solution.

10. The secondary battery according to claim 9, wherein The content of the other additives is 0.6 wt% or more and 7.1 wt% or less based on the mass of the electrolyte solution.

11. The secondary battery according to claim 10, wherein The content of the other additives is 1.4 wt% or more and 6.2 wt% or less based on the mass of the electrolyte solution.

12. An electronic device including the secondary battery according to any one of claims 1 to 11.

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