Secondary battery and electronic device

Abstract

A secondary battery and an electronic device, belonging to the technical field of batteries. The secondary battery comprises a positive electrode sheet and an electrolyte, the electrolyte comprising a carboxylate compound and lithium difluorophosphate. The carboxylate compound comprises at least one of compounds of which the molecular formula is R1COOR2, where R1 and R2 are each independently selected from an alkyl group or halogenated alkyl group of C1 to C6. Using the electrolyte as a reference, the mass content of the carboxylate compound is a%, and the mass content of lithium difluorophosphate is b%, 2≤b≤8.8 and 6≤a / b≤28. The coordination between lithium difluorophosphate and the carboxylate compound improves the cycle performance and high-temperature storage performance of secondary batteries.

Description

Secondary battery and electronic device Technical Field

[0001] The present application belongs to the field of battery technology, and specifically relates to a secondary battery and an electronic device. Background Art

[0002] Secondary batteries, as rechargeable energy storage devices, are widely used in electronic devices such as mobile phones, laptops, and cameras. There is a growing demand for improved performance in secondary batteries, including power, lifespan, and safety. As a key component of secondary batteries, the electrolyte directly impacts their performance.

[0003] In existing technologies, electrolytes typically utilize a mixture of organic solvents and lithium salts. However, deficiencies in ionic conductivity, stability, and fluidity can lead to issues such as poor cycling performance. While some existing technologies propose adjusting the electrolyte formulation to improve it, this process can also lead to poor compatibility between components, which can reduce the safety and performance of secondary batteries. Therefore, it is necessary to comprehensively consider the advantages and disadvantages of electrolyte improvements to further enhance the cycling performance of secondary batteries.

[0004] Summary of the Invention

[0005] In view of this, the present application provides a secondary battery that can exhibit better cycle performance while also taking into account higher storage performance. In another aspect, the present application also provides an electronic device including the secondary battery.

[0006] In a first aspect, the present application provides a secondary battery comprising a positive electrode and an electrolyte, the electrolyte comprising a carboxylate compound and lithium difluorophosphate; the carboxylate compound comprises at least one of the molecular formula R1COOR2, wherein R1 and R2 are each independently selected from an alkyl group or a halogenated alkyl group from C1 to C6; the mass content of the carboxylate compound is a%, and the mass content of the lithium difluorophosphate is b%, based on the electrolyte; wherein the value of b ranges from 2≤b≤8.8, and a and b satisfy the relationship: 6≤a / b≤28. The present application utilizes a close combination of lithium difluorophosphate and the carboxylate compound. By achieving an appropriate carboxylate compound content, the stability of the lithium difluorophosphate in the electrolyte can be improved, enabling high concentrations of lithium difluorophosphate. Furthermore, a high concentration of lithium difluorophosphate can promote the formation of a stable CEI and SEI, inhibiting the decomposition of the carboxylate compound, thereby preventing the performance of the secondary battery from deteriorating due to the inherent instability of the carboxylate compound at an excessively high concentration. Furthermore, the carboxylate compound can reduce or avoid the impact of low conductivity of high concentrations of lithium difluorophosphate on electrolyte dynamics. The present application controls the content relationship so that the two interact with each other, which can not only ensure the complete dissolution of lithium difluorophosphate, but also improve the defect of insufficient stability of the carboxylate compound, while balancing the dynamics of the entire battery system, thereby improving the cycle performance and high-temperature storage performance of the secondary battery.

[0007] In some embodiments, the carboxylate compound includes at least one of methyl formate, methyl acetate, methyl propionate, methyl butyrate, ethyl formate, ethyl acetate, ethyl propionate, ethyl butyrate, propyl formate, propyl acetate, propyl propionate, propyl butyrate, or ethyl fluoroacetate.

[0008] Furthermore, the carboxylate compound is selected from at least one of methyl formate, ethyl formate, methyl acetate, or ethyl acetate. These carboxylate compounds can ensure that high-concentration lithium difluorophosphate is fully dissolved. When combined with lithium difluorophosphate, they can further improve the stability of lithium difluorophosphate, resulting in a secondary battery with better cycle performance and high-temperature storage performance.

[0009] In some embodiments, the value range of a is 12≤a≤60.

[0010] Furthermore, the value range of b is 2≤b≤7.5.

[0011] In some preferred embodiments, a and b satisfy the relationship: 6≤a / b≤15, for example, a and b satisfy the relationship: 7≤a / b≤15; further, a and b satisfy the relationship: 6≤a / b≤13.64.

[0012] In some embodiments, the positive electrode sheet includes a positive electrode material having a specific surface area of ​​cm 2 / g; the value range of c is 0.2≤c≤1.0.

[0013] In some embodiments, c and b satisfy the relationship: 3≤c / b≤45.

[0014] In some embodiments, the positive electrode material includes a ternary material, and the ternary material includes manganese and / or aluminum.

[0015] In some embodiments, the molar content of the manganese and / or aluminum element is d based on the molar amount of the metal elements other than Li in the positive electrode material, and the value of d is in the range of 5%≤d≤35%. For example, the molar content of the manganese and / or aluminum element is 1 / 20 to 1 / 3 based on the molar amount of the metal elements other than Li in the positive electrode material.

[0016] In some embodiments, the electrolyte further includes a first substance; the first substance includes at least one of 1,3-propane sultone, 1,3-propylene sultone, vinyl sulfate, 1,3-propylene glycol sulfate, 2,4-butane sultone, 1,4-butane sultone, vinylene carbonate, and fluoroethylene carbonate. In addition to the combination of lithium difluorophosphate and a carboxylate compound, the present application introduces the first substance to synergize with lithium difluorophosphate to enhance the stability of the SEI / CEI, further improving the battery's cycling performance.

[0017] In some embodiments, based on the electrolyte, the mass content of the first substance is x%, where x is in the range of 0.05 ≤ x ≤ 12; and 2.05 ≤ (x + b) ≤ 20.8. This application controls the content relationship between the first substance and lithium difluorophosphate to ensure a reasonable compatibility between the lithium difluorophosphate and the first substance, thereby reducing or avoiding the increase in the overall impedance of the battery cell caused by the addition of excessively high-impedance additives, thereby improving the performance of the secondary battery.

[0018] In some embodiments, the electrolyte further comprises a second substance; the second substance comprises at least one of lithium dioxalatoborate, lithium tetrafluoroborate, lithium difluorooxalatoborate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium 4,5-dicyano-2-(trifluoromethyl)isopyrazole, and lithium tetraborate.

[0019] In some embodiments, the mass content of the second substance is y%, based on the electrolyte, where y is in the range of 0.05 ≤ y ≤ 8, and 2.05 ≤ (y + b) ≤ 16.6. By rationally adjusting the content ratio of lithium difluorophosphate and the second substance, a synergistic relationship between the two during film formation is achieved, which further improves the formation of a strong and low-impedance SEI / CEI at the electrode interface, further improving battery performance.

[0020] In a second aspect, the present application further provides an electronic device comprising any of the aforementioned secondary batteries. The secondary battery provided herein has low impedance, good low-temperature cycling performance, and high-temperature storage performance, thereby providing the electronic device with a long service life and good performance.

[0021] Based on the secondary battery provided by the present application, a carboxylate compound is used to cooperate with lithium difluorophosphate, and an excellent interfacial film is formed by lithium difluorophosphate to promote the formation of a good interface film, reduce or avoid the carboxylate compound due to its poor stability and the occurrence of side reactions with the positive and negative electrode materials, while the carboxylate compound can improve the stability of lithium difluorophosphate in the electrolyte, play a role in promoting the dissolution of lithium difluorophosphate and preventing precipitation. The present application controls the content relationship of the carboxylate compound and lithium difluorophosphate so that the two complement each other, and closely cooperate to improve the circulation and high temperature storage performance of the secondary battery. On the other hand, the present application is based on the coordination of the carboxylate compound with lithium difluorophosphate, further combined with the specific surface area of ​​the positive electrode material, can reduce the side reaction between the positive electrode material and the electrolyte, and thus make the circulation and high temperature storage of the secondary battery more significantly improved. DETAILED DESCRIPTION

[0022] In order to make the purpose, technical solutions and advantages of this application more clearly understood, the present application is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain this application and are not intended to limit this application.

[0023] For simplicity, the present invention only explicitly discloses certain numerical ranges. However, any lower limit can be combined with any upper limit to form an unspecified range; and any lower limit can be combined with other lower limits to form an unspecified range, and similarly, any upper limit can be combined with any other upper limit to form an unspecified range. In addition, although not explicitly stated, unless otherwise specified, each point or individual value between the endpoints of a range is included in the range. Thus, each point or individual value can serve as its own lower limit or upper limit and be combined with any other point or individual value, or with other lower limits or upper limits, to form an unspecified range.

[0024] Unless otherwise specified, the terms "include" and "comprising" used in this application may be open-ended or closed-ended. For example, "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0025] The inventors of this application have discovered that in conventional electrolytes, such as carbonate electrolytes, the mass content of lithium difluorophosphate is difficult to exceed 1%, otherwise the electrolyte performance may deteriorate. In view of this, the present application provides, in a first aspect, a secondary battery comprising a positive electrode and an electrolyte, wherein the electrolyte comprises a carboxylate compound and lithium difluorophosphate; the carboxylate compound comprises at least one of the molecular formulas R1COOR2, wherein R1 and R2 are each independently selected from an alkyl group or a halogenated alkyl group from C1 to C6; based on the electrolyte, the mass content of the carboxylate compound is a%, and the mass content of the lithium difluorophosphate is b%, wherein the value of b ranges from 2≤b≤8.8, and a and b satisfy the relationship: 6≤a / b≤28. The electrolyte of the present application utilizes a carboxylate compound in combination with lithium difluorophosphate (LiPO2F2) to improve the solubility and stability of lithium difluorophosphate in the electrolyte solvent system, breaking through the 1% mass content limit of lithium difluorophosphate in the electrolyte solvent system, making it possible to use high-concentration lithium difluorophosphate in the electrolyte. On the other hand, because carboxylate compounds are less stable than carbonate compounds, the high-concentration lithium difluorophosphate in the present application can promote the formation of an excellent interfacial film, which can reduce the side reactions between the carboxylate compound and the positive and negative electrodes. The combination of the two can further improve the cycling and storage performance of the secondary battery.

[0026] Optionally, in the molecular formula of the carboxylate compound, the alkyl group can be selected from a C1 to C6 chain alkyl group or a cyclic alkyl group; the halogenated alkyl group can be selected from a C1 to C6 chain alkyl group or a cyclic alkyl group substituted by any one of a F atom, a Cl atom, a Br atom, and an I atom.

[0027] In some exemplary embodiments, based on the electrolyte, the mass content of lithium difluorophosphate is b%, the value range of b is 2≤b≤8.8, and a and b satisfy the relationship: 6≤a / b≤28. For example, b can be selected from 2, 2.5, 3, 3.5, 4, 4.4, 5, 5.8, 6, 6.6, 7, 7.5, 8, 8.6, 8.8, or a range consisting of any two of the above values, and a / b can be selected from 6, 6.67, 6.98, 7, 8, 9.09, 9.5, 10, 10.34, 12, 13.5, 13.64, 14, 15, 16, 17.5, 18, 20, 21, 22, 24, 26, 28, or a range consisting of any two of the above values.

[0028] In some exemplary embodiments, the carboxylate compound includes at least one of methyl formate (MF), methyl acetate (MA), methyl propionate (MP), methyl butyrate (MB), ethyl formate (EF), ethyl acetate (EA), ethyl propionate (EP), ethyl butyrate (EB), propyl formate, propyl acetate, propyl propionate, propyl butyrate, or ethyl fluoroacetate. These carboxylate compounds have relatively high DN numbers and, when combined with lithium difluorophosphate, can significantly enhance the solubility and stability of lithium difluorophosphate in the electrolyte, thereby increasing the applied mass concentration of lithium difluorophosphate in the electrolyte and thereby enhancing the performance of the secondary battery. Among them, ethyl acetate has good wettability and excellent power, but ethyl acetate is not as good as carbonate in side reactions at the positive and negative electrodes, which leads to impaired high temperature and cycle performance of secondary batteries. Lithium difluorophosphate can form an excellent interfacial film, which can improve these defects of ethyl acetate. Lithium difluorophosphate has low solubility in carbonate solvents, but has a higher solubility in ethyl acetate due to its higher DN number. It can cooperate with high-concentration lithium difluorophosphate to overcome the problems between them, solving the problem of secondary battery wettability while improving the cycle and high-temperature storage performance.

[0029] In some exemplary embodiments, based on the electrolyte, the mass content of the carboxylate compound is a%, and the value range of a is 12≤a≤60. For example, a can be selected from 12, 19, 27, 35, 42, 56, 60 or a range consisting of any two of the above values.

[0030] In some embodiments, the electrolyte further includes a first substance; the first substance includes at least one of 1,3-propane sultone (1,3-PS), 1,3-propylene sultone, vinyl sulfate, 1,3-propylene glycol sulfate, 2,4-butane sultone, 1,4-butane sultone (1,4-BS), vinylene carbonate (VC), and fluoroethylene carbonate (FEC).

[0031] In some embodiments, based on the electrolyte, the mass content of the first substance is x%, where x is in the range of 0.05 ≤ x ≤ 12, and 2.05 ≤ (x + b) ≤ 20.8. For example, x can be selected from 0.05, 2, 3, 5, 8, 10, 12, or a range consisting of any two of the above values; and x + b can be selected from 2.05, 4.43, 6.4, 7.4, 8.85, 14.4, 16.4, 17.8, 18.7, 19.5, 19.9, 20.8, or a range consisting of any two of the above values.

[0032] In some embodiments, the electrolyte further comprises a second substance; the second substance comprises at least one of lithium dioxalatoborate (LiBOB), lithium tetrafluoroborate (LiBF4), lithium difluorooxalatoborate (LiDFOB), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiCF3SO3), lithium 4,5-dicyano-2-(trifluoromethyl)isopyrazole (LiTDI), and lithium tetraborate (Li2B4O7).

[0033] In some embodiments, the mass content of the second substance is y%, based on the electrolyte, and the value of y is in the range of 0.05≤y≤8, and 2.05≤(y+b)≤16.6. For example, y can be selected from 0.05, 0.10, 0.20, 0.60, 1.20, 1.80, 2, 4, 6, 8, or a range consisting of any two of the above values; y+b can be selected from 2.05, 4.42, 4.45, 4.5, 4.6, 5, 5.6, 6.2, 9.3, 10.3, 10.5, 11.4, 11.5, 12.4, 15.6, 16.6, or a range consisting of any two of the above values.

[0034] In the present application, the electrolyte also includes a non-aqueous solvent. The present application has no particular restrictions on the type of non-aqueous solvent, as long as the purpose of the present application can be achieved. For example, it may include but is not limited to at least one of a carbonate compound and an ether compound. The above-mentioned carbonate compound may include but is not limited to at least one of a chain carbonate compound, a cyclic carbonate compound, and a fluorinated carbonate compound. The above-mentioned chain carbonate compound may include but is not limited to at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC) or methylethyl carbonate (MEC). Cyclic carbonate may include but is not limited to at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC) or vinylethylene carbonate (VEC). The fluorinated carbonate compound may include, but is not limited to, at least one of fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, or trifluoromethylethylene carbonate. The above-mentioned ether compound may include, but is not limited to, at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran.

[0035] The preparation process of the electrolyte in this application is well known to those skilled in the art, and there is no particular limitation in this application. For example, it may include but is not limited to the following steps: in an argon atmosphere glove box with a water content of <10ppm, lithium difluorophosphate, a carboxylate compound and a sufficiently dried lithium salt LiPF6 are added to a non-aqueous solvent in a mass ratio for dissolution treatment to prepare the electrolyte in this application. Optionally, the first substance and / or the second substance may also be added to the non-aqueous solvent to prepare the electrolyte in some embodiments of this application. This application does not particularly limit the order of adding lithium difluorophosphate, a carboxylate compound, a lithium salt LiPF6, the first substance and the second substance, and can be selected according to actual needs, as long as the purpose of this application can be achieved.

[0036] In some exemplary embodiments, the positive electrode sheet in the present application includes a positive electrode material, and the specific surface area of ​​the positive electrode material is cm 2 / g; the value range of c is 0.2≤c≤1.0, for example, c can be selected from 0.2, 0.4, 0.5, 0.6, 0.8, 1.0 or the range consisting of any two of the above values. The inventors of the present application found that as the specific surface area of ​​the positive electrode material increases, the capacity, cycle and storage performance of the secondary battery will decline. The reason may be that there are residual alkali impurities on the surface of the positive electrode material, and the decomposition of the residual alkali during the service of the battery will cause deterioration of the performance. On the basis of the coordination of the carboxylate compound and lithium difluorophosphate, the present application further controls the specific surface area of ​​the positive electrode material to coordinate and control the content of residual alkali, and promotes the generation of a suitable and more stable CEI film through lithium difluorophosphate to inhibit the decomposition of the residual alkali, thereby improving the capacity, cycle and storage performance of the secondary battery.

[0037] In this application, the positive electrode material refers to the substance disposed on the surface of the positive electrode current collector, which may include but is not limited to the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder. In this application, the positive electrode material can be disposed on one surface of the positive electrode current collector along its own thickness direction, or on two surfaces of the positive electrode current collector along its thickness direction. It should be noted that the "surface" here can be the entire area of ​​the positive electrode current collector or a partial area of ​​the positive electrode current collector. This application is not particularly limited, as long as the purpose of this application can be achieved.

[0038] In some exemplary embodiments, the positive electrode material includes a ternary material, and the ternary material includes manganese and / or aluminum. When the ternary material with a high nickel content comes into contact with air, the residual alkali content on its surface will increase. The high residual alkali content will not only cause the processing performance of the ternary material to drop sharply, but also significantly deteriorate the cycle performance of the secondary battery. After the ternary material of the present application is combined with a carboxylate compound and lithium difluorophosphate, it can reduce or avoid the impact of residual alkali on the performance of the secondary battery, give full play to the advantages of the ternary material with high specific capacity, low cost and good safety performance, and ensure the excellent capacity, cycle and storage performance of the secondary battery.

[0039] This application does not particularly limit the method for regulating the specific surface area of ​​the cathode material, as long as it can achieve the objectives of this application. For example, cathode materials with different specific surface areas can be obtained by mechanical crushing, grinding, screening, etc. For example, cathode materials with different specific surface areas can be obtained by ball milling during mechanical crushing. Generally, extending the ball milling time increases the specific surface area, while shortening the ball milling time decreases the specific surface area.

[0040] In some exemplary embodiments, the ternary material includes lithium nickel cobalt manganese oxide (NCM) and / or lithium nickel cobalt aluminum oxide (NCA). Optionally, lithium nickel cobalt manganese oxide is selected from LiNi 0.8 Mn 0.1 Co 0.1 O2(NCM811), LiNi 0.6 Mn 0.1 Co 0.3 O2(NCM613), LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 O2、LiNi 0.35 Mn 0.28 Co 0.37 O2、LiNi 0.6 Mn 0.2 Co 0.2 O2、LiNi 0.5 Mn 0.3 Co 0.2 O2、LiNi 0.7 Mn 0.15 Co 0.15 At least one of O2. Lithium nickel cobalt aluminum oxide includes but is not limited to LiNi 0.8 Al 0.05 Co 0.15 O2. Mn.

[0041] In some exemplary embodiments, the positive electrode sheet of the present application may further include a positive electrode current collector. The present application has no particular limitation on the positive electrode current collector, as long as it can achieve the purpose of the present application. For example, it may include but is not limited to aluminum foil, aluminum alloy foil, or a composite current collector (e.g., an aluminum-carbon composite current collector).

[0042] The positive electrode material may also include a positive electrode conductor and a positive electrode binder. The present application does not particularly limit the types of the positive electrode conductor and the positive electrode binder, as long as the purpose of the present application can be achieved. For example, the positive electrode binder may include but is not limited to at least one of polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon; the positive electrode conductor may include but is not limited to at least one of a carbon-based material, a metal-based material, or a conductive polymer. For example, the carbon-based material may include at least one of natural graphite, artificial graphite, conductive carbon black (Super P), or carbon fiber; the metal-based material may include but is not limited to at least one of metal powder, metal fiber, copper, nickel, aluminum, or silver; and the conductive polymer may include but is not limited to polyphenylene derivatives. The present application does not particularly limit the mass ratio of the ternary material, positive electrode conductor, and positive electrode binder in the positive electrode material, and can be selected according to actual needs, as long as the purpose of the present application can be achieved.

[0043] This application does not impose any particular restrictions on the negative electrode sheet, as long as the purpose of this application can be achieved. For example, the negative electrode sheet includes a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector. In this application, the negative electrode material layer can be disposed on one surface in the thickness direction of the negative electrode current collector, or on two surfaces in the thickness direction of the negative electrode current collector. It should be noted that the "surface" here can be the entire area of ​​the negative electrode current collector or a partial area of ​​the negative electrode current collector. This application does not impose any particular restrictions, as long as the purpose of this application can be achieved.

[0044] The present application has no particular restrictions on the negative electrode current collector, as long as the purpose of the present application can be achieved. For example, it may include but is not limited to copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam or composite current collector (such as carbon copper composite current collector, nickel copper composite current collector, titanium copper composite current collector, etc.). In the present application, there is no particular restriction on the thickness of the negative electrode current collector and the negative electrode material layer, as long as the purpose of the present application can be achieved, for example, the thickness of the negative electrode current collector is 6μm to 12μm, and the thickness of the negative electrode material layer is 30μm to 130μm. In the present application, there is no particular restriction on the thickness of the negative electrode pole piece, as long as the purpose of the present application can be achieved, for example, the thickness of the negative electrode pole piece is 50μm to 280μm.

[0045] The negative electrode material layer of the present application includes a negative electrode active material, which may include but is not limited to graphite, mesophase microcarbon beads (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, SiO x (0.5<x<1.6), LiSn alloy, LiSnO alloy, Sn, SnO, SnO2, spinel structure lithium titanate TiO2Li4Ti5O 12 , at least one of Li Al alloy and metallic lithium.

[0046] The negative electrode material layer in the present application may further include a negative electrode binder and a negative electrode conductor, or the negative electrode material layer may further include a negative electrode binder, a negative electrode conductor and a thickener. The present application has no particular restrictions on the types of negative electrode binders and negative electrode conductors, as long as the purpose of the present application can be achieved. For example, the negative electrode binder may include but is not limited to at least one of the above-mentioned positive electrode binders, and the negative electrode conductor may include but is not limited to at least one of the above-mentioned positive electrode conductors. The present application has no particular restrictions on the type of thickener, as long as the purpose of the present application can be achieved. For example, the thickener may include but is not limited to at least one of sodium carboxymethyl cellulose or carboxymethyl cellulose.

[0047] The present application has no particular restrictions on the isolation membrane, as long as the purpose of the present application can be achieved. For example, the material of the isolation membrane may include, but is not limited to, polyethylene (PE), polypropylene (PP)-based polyolefins (PO), polyesters (such as polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex or aramid; the type of isolation membrane may include at least one of woven membrane, non-woven membrane, microporous membrane, composite membrane, rolled membrane or spun membrane. For example, the isolation membrane may include a substrate layer and a surface treatment layer. The substrate layer may be a non-woven fabric, a membrane or a composite membrane with a porous structure, and the material of the substrate layer may include at least one of polyethylene, polypropylene, polyethylene terephthalate or polyimide. Optionally, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene non-woven fabric, a polyethylene non-woven fabric or a polypropylene polyethylene polypropylene porous composite membrane may be used. Optionally, a surface treatment layer is provided on at least one surface of the substrate layer, and the surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by a mixed polymer and an inorganic substance. For example, the inorganic layer includes inorganic particles and a binder. The present application has no particular restrictions on the inorganic particles. For example, it may include at least one of aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide or barium sulfate. The present application has no particular restrictions on the binder. For example, it may be at least one of the above-mentioned positive electrode binders. The polymer layer contains a polymer. The present application has no particular restrictions on the polymer. For example, the polymer includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinyl pyrrolidone, polyvinyl ether or polyvinylidene fluoride or poly (vinylidene fluoride hexafluoropropylene). In the present application, the thickness of the isolation membrane is not particularly limited, as long as the purpose of the present application can be achieved. For example, the thickness of the isolation membrane can be 5μm to 500μm.

[0048] The secondary battery of this application also includes a packaging bag for containing the positive electrode sheet, separator, negative electrode sheet, and electrolyte, as well as other components of secondary batteries known in the art. This application does not particularly limit these other components. This application does not particularly limit the packaging bag and can be any packaging bag known in the art, as long as it can achieve the purpose of this application. For example, an aluminum-plastic film packaging bag can be used.

[0049] The preparation process of a secondary battery is well known to those skilled in the art and is not particularly limited in this application. For example, it may include but is not limited to the following steps: stacking the positive electrode sheet, the separator, and the negative electrode sheet in order, and winding, folding, and other operations as needed to obtain an electrode assembly with a wound structure, placing the electrode assembly in a packaging bag, injecting an electrolyte into the packaging bag and sealing it to obtain a secondary battery; or stacking the positive electrode sheet, the separator, and the negative electrode sheet in order, and then fixing the four corners of the entire stacked structure with tape to obtain an electrode assembly with a stacked structure, placing the electrode assembly in a packaging bag, injecting an electrolyte into the packaging bag and sealing it to obtain a secondary battery. In addition, as needed, overcurrent protection elements, guide plates, etc. may be placed in the packaging bag to prevent pressure rise and overcharging and discharging inside the secondary battery.

[0050] The present application provides an electronic device in a second aspect, which includes the secondary battery provided in the first aspect of the present application.

[0051] The electronic device of the present application is not particularly limited and can be any electronic device known in the art. For example, the electronic device can include, but is not limited to, a laptop computer, a pen-type computer, a mobile computer, an electronic book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a headset, a video recorder, an LCD television, a portable cleaner, a portable CD player, a mini-disc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, a power tool, a flashlight, a camera, a large household battery, and a lithium-ion capacitor.

[0052] The scheme of the present application is described below with reference to the following specific examples. Unless otherwise specified, the raw materials used in the following examples are all from common commercial products, and the devices or equipment used are all purchased from conventional market sales channels.

[0053] Test methods and equipment:

[0054] Surface Area: The surface area of ​​the cathode material was measured using a Tristar II 3020M surface area analyzer using nitrogen adsorption. The specific test was conducted in accordance with the national standard GB / T19587-2017, "Determination of the Specific Surface Area of ​​Solids by the BET Method for Gas Adsorption."

[0055] Cycling performance test method: At 25°C, charge the battery to 4.28V at 1C, charge it to 0.05C at a constant voltage at 4.28V, and then discharge it to 2.5V at 1C. Cycle this way for 800 cycles, and record the cycle capacity retention rate after 800 cycles.

[0056] 60°C high-temperature storage performance test method: The battery was charged at 0.5C constant current to 4.28V at 25°C, then charged at constant voltage to a current of 0.05C. The battery thickness was measured and recorded as d0. After storage in a 60°C oven for 180 days, the battery thickness was measured and recorded as d. The thickness expansion rate (%) of the battery after 60°C high-temperature storage = (d - d0) / d0 × 100%.

[0057] 50% SOC impedance test: At 25°C, discharge the battery to 2.5V at a current of 0.5C, let it stand for 5 minutes, then charge it to 4.28V at a current of 0.5C, and keep the voltage constant at 0.025C at 4.28V. Let it stand for 5 minutes, and discharge it to 2.5V at a current of 0.1C. The capacity released at this time is marked as C1. Charge it to 4.28V with a capacity of 0.5C1, keep the voltage constant at 0.025C1 at 4.28V, let it stand for 5 minutes, and discharge it for 5 hours with a current of 0.1C1. The battery voltage at this time is recorded as V1. Then, discharge it with a current of 1C for 1 second, and the voltage at the end of discharge is recorded as V2. The formula for calculating 50% SOC impedance is: (V1-V2) / (1C-0.1C1).

[0058] Conductivity test at 25°C: The electrolyte was placed in a constant temperature water bath at 25°C for 30 min, and the conductivity of the electrolyte was tested using a platinum black electrode (DJS-1C).

[0059] Example I-1

[0060] The secondary battery of this embodiment includes a positive electrode plate and an electrolyte, wherein the electrolyte includes a carboxylate compound and lithium difluorophosphate; the carboxylate compound includes ethyl acetate; based on the electrolyte, the mass content a% of ethyl acetate is 12%, the mass content b% of lithium difluorophosphate is 2%, and a and b satisfy the relationship: a / b=6.

[0061] The method for preparing the secondary battery of this embodiment includes the following steps:

[0062] Preparation of the electrolyte: In an argon atmosphere glove box with a water content of <10 ppm, ethyl methyl carbonate and ethylene carbonate were mixed in a mass ratio of 48.5:25 to prepare a non-aqueous solvent, and then lithium difluorophosphate, ethyl acetate and fully dried lithium salt LiPF6 (the molar content of LiPF6 is 1 M) were added to the non-aqueous solvent in a mass ratio to dissolve and prepare the electrolyte in this embodiment.

[0063] Preparation of positive electrode sheet: NCM613 (active material), conductive agent Super-P, and binder polyvinylidene fluoride (PVDF) are fully stirred and mixed in N-methylpyrrolidone (NMP) solvent in a weight ratio of 96:2:2 to form a uniform positive electrode slurry; this slurry is coated on the positive electrode current collector Al foil, dried, and cold pressed to obtain a positive electrode sheet. In the obtained positive electrode sheet, the molar content d% of manganese element is 10% based on the molar content of metal elements other than Li in the positive electrode material. In this positive electrode sheet, the positive electrode material includes NCM613, positive electrode conductive agent and positive electrode binder, and the specific surface area c of the positive electrode material is 0.8m 2 / g; c and b satisfy the relationship: c / b=40.

[0064] Preparation of negative electrode sheet: The negative electrode active material graphite, the binder styrene-butadiene rubber, and the thickener sodium carboxymethyl cellulose are thoroughly stirred and mixed in a deionized water solvent in a weight ratio of 97.4:1.4:1.2 to form a uniform negative electrode slurry; this slurry is coated on the negative electrode current collector Cu foil, dried, and cold pressed to obtain the negative electrode sheet.

[0065] Isolation membrane: A single-layer PE porous polymer film is used as the isolation membrane, with a thickness of 16 microns and a porosity of 39%. The inorganic coating is Al2O3, and the organic particles are polyvinylidene fluoride.

[0066] Preparation of the secondary battery (i.e., lithium-ion battery) of the present application: stack the positive electrode sheet, separator, and negative electrode sheet in order, so that the separator is between the positive electrode sheet and the negative electrode sheet to play an isolating role, and then wind to obtain a bare cell; place the bare cell in an outer packaging foil, inject the above-prepared electrolyte into the dried battery, and complete the preparation of the lithium-ion battery through vacuum packaging, standing, formation, shaping and other processes. Among them, the formation process is as follows: at 45±5℃, perform the first cycle of charge and discharge, and the process is as follows: first, charge at a constant current rate of 0.1C for 10 minutes, then charge at a constant current rate of 0.5C to a specified voltage of 4.3V, then charge at a constant voltage until the current is less than or equal to 0.05C, and then discharge at a constant current rate of 0.5C to 2.5V.

[0067] The secondary batteries of Examples I-2 to I-14 and Comparative Examples I-1 to I-8 differ from Example I-1 only in the mass content of lithium difluorophosphate and the type and / or mass content of the carboxylate compound. The specific differences and performance test results are shown in Table 1 below.

[0068] Table 1

[0069] As can be seen from Table 1, Examples I-1 to I-6 of the present application, on the basis of adding 2 wt % of lithium difluorophosphate, control the mass content a% of the carboxylate compound and the mass content b% of lithium difluorophosphate to satisfy the mathematical relationship: 6≤a / b≤28. Compared with Comparative Examples I-1 to I-5, they can show a higher 25°C cycle retention rate and a lower 60°C storage thickness expansion rate while maintaining a good 50% SOCDCR performance, and have more excellent cycle performance and high temperature storage performance. However, in Comparative Examples I-1 to I-5, when a and b satisfy a / b=4, Comparative Example I-2 has a problem of incomplete dissolution of lithium difluorophosphate during electrolyte preparation, resulting in serious deterioration of electrolyte performance. Comparative Examples I-1 and Comparative Examples I-3 to Comparative Examples I-5 use more ethyl acetate to make the a / b value exceed 28, and the electrolyte viscosity is lower, so the DCR performance is relatively small, but the cycle retention rate after a long cycle of 800 cycles is low.

[0070] In Examples I-7 to I-12, based on Comparative Example I-3, the a / b value is reduced by increasing the content of lithium difluorophosphate. Test results show that the increase in lithium difluorophosphate will increase the impedance of the secondary battery to a certain extent, resulting in an increase in the DCR performance at 50% SOC. However, by controlling 6≤a / b≤28, the present application allows ethyl acetate and lithium difluorophosphate to interact and cooperate, still achieving higher cycle performance and high-temperature storage performance. At the same time, compared with Comparative Example I-6, the present application controls the mass content of lithium difluorophosphate to 2-8.8%, and after combining with ethyl acetate, the secondary battery can have even better cycle performance and high-temperature storage performance.

[0071] In addition, compared with Example I-7, Examples I-1 to I-3 and I-8 to I-12 have higher 25°C cycle retention rates when a / b is adjusted to meet 6≤a / b≤13.64, which proves that when the mass content b% of lithium difluorophosphate is 2-7.5%, the present application controls 6≤a / b≤13.64 to make ethyl acetate and lithium difluorophosphate closely matched, which can further improve the cycle performance of the secondary battery. After 800 cycles at 25°C, the capacity retention rate can reach 76.5%-79.9%, which has extremely excellent cycle performance.

[0072] From Examples I-1 to I-14, it can be seen that the present application uses lithium difluorophosphate and carboxylate compounds in close combination to promote the formation of stable CEI and SEI, which can improve the cycle performance and high-temperature storage performance of the secondary battery.

[0073] The secondary batteries of Examples II-1 to II-9 differ from Example I-8 only in the mass content of lithium difluorophosphate and / or the specific surface area of ​​the positive electrode material. The specific differences and performance test results are shown in Table 2 below.

[0074] Table 2

[0075] As can be seen from Table 2, Examples II-1 to II-9 of the present application, on the basis of the coordination of the carboxylate compound and lithium difluorophosphate, further control the specific surface area c of the positive electrode material to ensure that 3≤c / b≤45. Through the joint coordination of the three, the 50% SOC DCR performance of the secondary battery can be reduced, and the 25°C, 800-cycle cycle retention rate and 60°C high-temperature storage performance of the secondary battery can be improved.

[0076] Among them, Examples II-3 to II-8 further control 9.09≤c / b≤15.52, and after close cooperation with lithium difluorophosphate and carboxylate compounds, the cycle performance and high-temperature storage performance of the secondary battery can be further improved.

[0077] The secondary batteries of Examples III-1 to III-12 differ from those of Example I-8 only in that the electrolyte further includes a first substance, and the mass content of lithium difluorophosphate and / or the type and content of the first substance are different. The specific differences and performance test results are shown in Table 3 below.

[0078] Table 3

[0079] As shown in Table 3, in Examples III-1 to III-12, based on the coordination of the carboxylate compound with lithium difluorophosphate, controlling the content of the added first substance to satisfy the relationship 2.05≤(x+b)≤20.8 with the lithium difluorophosphate is beneficial for improving the cycling performance and high-temperature storage performance of the secondary battery. Further preferably, in the examples of the present application, controlling the content of the added first substance to satisfy the relationship 7.4≤(x+b)≤19.5 with the lithium difluorophosphate can promote the synergistic coordination of the lithium difluorophosphate and the first substance, thereby further improving the cycling performance and high-temperature storage performance of the secondary battery.

[0080] In addition, a comparison of Example III-3 and Example III-12 shows that the type of the first substance also affects the impedance, cycling performance, and storage performance of the secondary battery. By optimizing the type and content of the first substance, the present application effectively improves the cycling performance and high-temperature storage performance of the secondary battery by combining the first substance with lithium difluorophosphate.

[0081] The secondary batteries of Examples IV-1 to IV-11 differ from those of Example I-8 only in that the electrolyte further includes a second substance, and the mass content of lithium difluorophosphate and / or the type and content of the second substance are different. The specific differences and performance test results are shown in Table 4 below.

[0082] Table 4

[0083] As shown in Table 4, the type and content of the second substance will affect the cycle performance and high-temperature storage performance of the secondary battery. In Examples IV-1 to IV-11, based on the coordination of the carboxylate compound with lithium difluorophosphate, the content of the added second substance is controlled to satisfy the relationship 2.05≤(y+b)≤16.6 with the lithium difluorophosphate, which can improve the cycle performance and high-temperature storage performance of the secondary battery. Further preferably, in the examples of the present application, the content of the added second substance is controlled to satisfy the relationship 4.5≤(x+b)≤11.5 with the lithium difluorophosphate, which can promote the synergistic coordination between the lithium difluorophosphate and the second substance, so that the secondary battery has better cycle performance and high-temperature storage performance.

[0084] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent replacements and improvements made within the principles of the present application should be included in the scope of protection of the present application.

Claims

1. A secondary battery comprising a positive electrode and an electrolyte, characterized in that: The electrolyte comprises a carboxylate compound and lithium difluorophosphate; the carboxylate compound comprises at least one compound of the molecular formula R1COOR2, wherein R1 and R2 are each independently selected from a C1 to C6 alkyl group or a halogenated alkyl group; Based on the electrolyte, the mass content of the carboxylate compound is a%, the mass content of the lithium difluorophosphate is b%, 2≤b≤8.8, 6≤a / b≤28.

2. The secondary battery according to claim 1, wherein The secondary battery satisfies at least one of the following conditions: (1) The carboxylate compound includes at least one of methyl formate, methyl acetate, methyl propionate, methyl butyrate, ethyl formate, ethyl acetate, ethyl propionate, ethyl butyrate, propyl formate, propyl acetate, propyl propionate, propyl butyrate, or ethyl fluoroacetate; (2)12≤a≤60； (3)2≤b≤7.5； (4)6≤a / b≤15.

3. The secondary battery according to claim 1, wherein The positive electrode sheet includes a positive electrode material, and the specific surface area of ​​the positive electrode material is cm 2 / g; 0.2≤c≤1.0; and / or, 3≤c / b≤45.

4. The secondary battery according to any one of claims 1 to 3, characterized in that The positive electrode plate includes a positive electrode material, the positive electrode material includes a ternary material, and the ternary material includes manganese and / or aluminum.

5. The secondary battery according to claim 4, wherein Based on the molar amount of the metal elements other than Li in the positive electrode material, the molar content of the manganese element and / or the aluminum element is d, and the value range of d is 5%≤d≤35%.

6. The secondary battery according to any one of claims 1 to 3, characterized in that The electrolyte further includes a first substance; The first substance includes at least one of 1,3-propane sultone, 1,3-propylene sultone, vinyl sulfate, 1,3-propylene glycol sulfate, 2,4-butane sultone, 1,4-butane sultone, vinylene carbonate, and fluoroethylene carbonate.

7. The secondary battery according to claim 6, characterized in that Based on the electrolyte, the mass content of the first substance is x%, 0.05≤x≤12, and 2.05≤(x+b)≤20.

8.

8. The secondary battery according to any one of claims 6, wherein The electrolyte further comprises a second substance; The second substance includes at least one of lithium bis(oxalatoborate), lithium tetrafluoroborate, lithium difluorooxalatoborate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium 4,5-dicyano-2-(trifluoromethyl)isopyrazole, and lithium tetraborate.

9. The secondary battery according to claim 8, characterized in that Based on the electrolyte, the mass content of the second substance is y%, 0.05≤y≤8, and 2.05≤(y+b)≤16.

6.

10. An electronic device, characterized in that: The electronic device includes the secondary battery according to any one of claims 1 to 9.

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