Secondary battery and electric device

Abstract

The invention provides a secondary battery and an electric device. The secondary battery comprises a positive pole piece, a diaphragm, a negative pole piece and an electrolyte, the positive pole piece comprises a positive current collector and a positive active material layer arranged on at least one surface of the positive current collector, and the positive active material layer contains lithium iron phosphate and a solid electrolyte additive; the solid electrolyte additive comprises oxysalt formed by at least one element of P, Ti, Al, La, Zr and V and Li, and the mass ratio A of the lithium iron phosphate to the solid electrolyte additive is greater than or equal to 30 and less than or equal to 200; the electrolyte comprises lithium fluorosulfonate, a salt organic additive, a chalcogenide organic additive and a carbonic ester additive, the mass percentage content of the lithium fluorosulfonate in the electrolyte is B%, and B is larger than or equal to 0.2 and smaller than or equal to 2.

Description

Technical Field

[0001] The present application relates to the field of battery technology, in particular to a secondary battery and an electrical device. Background Art

[0002] Lithium-ion secondary batteries are widely used in new energy vehicles due to their high operating voltage, long cycle life, high energy density, low self-discharge, and lack of memory effect. However, the discharge capacity of existing lithium-ion secondary batteries is significantly affected in low-temperature conditions due to the slowed migration rate of lithium ions in such environments, severely restricting the development of new energy vehicles. Furthermore, existing lithium-ion secondary batteries also have shortcomings in their high-temperature cycling performance. Summary of the Invention

[0003] The purpose of this application is to provide a secondary battery and an electrical device to improve the low-temperature discharge performance and high-temperature cycle performance of the battery.

[0004] To achieve the above objectives, the first aspect of the present application provides a secondary battery, the secondary battery comprising a positive electrode sheet, a separator, a negative electrode sheet, and an electrolyte, the positive electrode sheet comprising a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, the positive electrode active material layer comprising lithium iron phosphate and a solid electrolyte additive,

[0005] The solid electrolyte additive includes an oxyacid salt formed by at least one element of P, Ti, Al, La, Zr, and V and Li, and the mass ratio of the lithium iron phosphate to the solid electrolyte additive is A, 30≤A≤200;

[0006] The electrolyte contains lithium fluorosulfonate, salt organic additives, sulfur organic additives and carbonate additives. The mass percentage of lithium fluorosulfonate in the electrolyte is B%, and 0.2≤B≤2.

[0007] As an embodiment of the present application, the solid electrolyte additive includes at least one of lithium aluminum titanium phosphate, lithium lanthanum titanate, and lithium lanthanum zirconate.

[0008] As an embodiment of the present application, the separator includes a substrate and a coating provided on at least one surface of the substrate, the thickness of the coating is α μm, and the compaction density of the positive electrode sheet is PD g / cm 3 , the conductivity of the electrolyte is σmS / cm, satisfying:

[0009] As an embodiment of the present application, the compaction density of the positive electrode sheet is PD g / cm 3 Meets: 2.2≤PD≤2.8.

[0010] As an embodiment of the present application, the thickness α μm of the coating layer on the separator satisfies: 1.5≤α≤3.

[0011] As an embodiment of the present application, the conductivity σmS / cm of the electrolyte satisfies: 7≤σ≤14.

[0012] As an embodiment of the present application, the mass percentage of the salt organic additive in the electrolyte is C%, the mass percentage of the sulfur organic additive in the electrolyte is D%, the mass percentage of the carbonate additive in the electrolyte is E%, 0.1≤C≤2, 0.1≤D≤3, 0.5≤E≤5.

[0013] As an embodiment of the present application, the salt organic additive includes at least one of lithium dioxalatoborate, lithium difluorooxalatoborate, lithium difluorobisoxalatophosphate, and lithium difluorophosphate.

[0014] As an embodiment of the present application, the sulfur-based organic additive includes at least one of methylene methanedisulfonate, vinyl sulfate, vinyl sulfite, 1,3-propyl sulfonate, and 1,3-propylene sultone.

[0015] As an embodiment of the present application, the carbonate additive includes at least one of vinylene carbonate and vinyl ethylene carbonate.

[0016] As an embodiment of the present application, the salt organic additive includes lithium difluorooxalatoborate, the sulfur-based organic additive includes vinyl sulfate, and the mass percentage of vinyl sulfate in the electrolyte is greater than the mass percentage of lithium difluorooxalatoborate.

[0017] As an embodiment of the present application, the salt organic additive includes lithium difluorooxalatoborate, the carbonate additive includes vinylene carbonate, and the mass percentage of vinylene carbonate in the electrolyte is greater than the mass percentage of lithium difluorooxalatoborate.

[0018] As an embodiment of the present application, the salt organic additive includes lithium difluorooxalatoborate, the sulfur-based organic additive includes vinyl sulfate, the carbonate additive includes vinylene carbonate, and the mass percentages of lithium difluorooxalatoborate, vinyl sulfate and vinylene carbonate in the electrolyte satisfy: vinylene carbonate>vinyl sulfate>lithium difluorooxalatoborate.

[0019] In a second aspect of the present application, an electrical device is provided, wherein the electrical device includes the secondary battery described in the first aspect of the present application.

[0020] Compared with the prior art, the present invention has the following advantages:

[0021] The present application improves the low-temperature discharge performance and high-temperature cycle performance of the secondary battery by controlling the mass ratio of lithium iron phosphate and solid electrolyte additives in the positive electrode sheet, the type of additives in the electrolyte, and the content of lithium fluorosulfonate in the electrolyte within a certain range. DETAILED DESCRIPTION

[0022] To make the purpose, technical solutions, and advantages of the embodiments of this application more clear, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are part of the embodiments of this application, not all of them. Based on the embodiments of this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0023] In this application, the technical features described in an open manner include closed technical solutions composed of the listed features, and also include open technical solutions containing the listed features. In this application, when it comes to numerical ranges, unless otherwise specified, the above numerical ranges are deemed to be continuous and include the minimum and maximum values ​​of the range, as well as every value between such minimum and maximum values. Further, when a range refers to an integer, it includes every integer between the minimum and maximum values ​​of the range. In addition, when multiple ranges are provided to describe features or characteristics, the ranges can be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges included therein.

[0024] The reagents and instruments used in this application without manufacturer indication are all conventional products that can be purchased commercially.

[0025] An embodiment of the present application provides a secondary battery, the secondary battery comprising a positive electrode plate, a separator, a negative electrode plate, and an electrolyte. The positive electrode plate comprises a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer comprises lithium iron phosphate and a solid electrolyte additive.

[0026] The solid electrolyte additive includes an oxyacid salt formed by at least one element of P, Ti, Al, La, Zr, and V and Li, and the mass ratio of the lithium iron phosphate to the solid electrolyte additive is A, 30≤A≤200;

[0027] The electrolyte contains lithium fluorosulfonate, salt organic additives, sulfur organic additives and carbonate additives. The mass percentage of lithium fluorosulfonate in the electrolyte is B%, and 0.2≤B≤2.

[0028] The inventors of this application have discovered that by adding a solid electrolyte additive to the positive electrode of the battery, a continuous ion transmission channel can be constructed in the positive electrode. By further controlling the mass ratio of lithium iron phosphate to the solid electrolyte additive in the positive electrode, the type of additive in the electrolyte, and the content of lithium fluorosulfonate in the electrolyte within a certain range, the low-temperature discharge performance and high-temperature cycle performance of the secondary battery can be improved at the same time.

[0029] In some embodiments of the present application, the mass ratio A of the lithium iron phosphate to the solid electrolyte additive in the positive electrode active material layer satisfies the following: 30 ≤ A ≤ 200, where A can be any value between 30, 48, 95, 120, 140, 160, 190, and 200, or a range formed between any two values. The solid electrolyte additive and the positive electrode active material work together to construct a continuous ion transmission path in the positive electrode sheet, increase the ion transmission channels, and improve the low-temperature discharge performance of the secondary battery.

[0030] In some embodiments of the present application, the solid electrolyte additive includes an oxyacid salt formed by at least one element of P, Ti, Al, La, Zr, and V and the element Li. For example, it can be at least one of lithium aluminum titanium phosphate, lithium lanthanum titanate, and lithium lanthanum zirconate. The unique structure of the oxyacid salt can provide three-dimensional continuous transmission channels and abundant shared corner oxygen sites for ions, reducing the energy barrier for ion transmission and achieving higher ionic conductivity. At the same time, it also has good compatibility with the positive electrode active material, which is conducive to the uniform deposition and stripping of ions, forming a more continuous and uniform ion transmission channel, reducing the resistance to ion transmission, and improving the low-temperature discharge performance of the secondary battery.

[0031] In some embodiments of the present application, the compaction density of the positive electrode sheet is PD g / cm 3 Satisfies: 2.2≤PD≤2.8. Appropriate compaction density can increase the content of active material per unit volume, thereby improving the energy density of the battery; at the same time, it can also ensure a good pore structure, ensure sufficient electrolyte infiltration and rapid ion transport, reduce concentration polarization, and optimize the kinetic performance of the electrochemical reaction. The compaction density of the positive electrode active material layer is PD g / cm 3 Specifically, it can be selected from 2.2 g / cm 3 , 2.5g / cm 3 , 2.65g / cm 3 , 2.8g / cm 3The test method for the compacted density PD of the positive electrode sheet is as follows: measure the thickness L1 of the positive electrode current collector and the thickness L2 of the positive electrode sheet with the positive electrode active material evenly coated on both sides, and calculate PD according to the formula PD = 2*CW / (L2-L1), where CW is in g / 1540.25mm 2 , L1 and L2 are in mm, CW is the single surface density of the positive electrode active material layer, and the test method is: first weigh an area of ​​1540.25mm 2 The mass of the positive electrode current collector is m1, and the area is 1540.25mm 2 The mass m2 of the positive electrode sheet uniformly coated with positive electrode active material on both sides is calculated according to the formula CW=(m2-m1) / 2 to obtain CW, where m1 and m2 are in g.

[0032] It should be noted that the positive electrode active material layer in this application can be one layer or multiple layers, and each layer of the multilayer positive electrode active material can contain the same or different positive electrode active materials. The positive electrode active material can also include any substance that can reversibly intercalate and deintercalate metal ions such as lithium ions.

[0033] In some embodiments of the present application, the mass percentage B% of lithium fluorosulfonate in the electrolyte satisfies: 0.1≤B≤2. The mass percentage B% of lithium fluorosulfonate in the electrolyte can specifically be any value between 0.1%, 0.5%, 1%, 1.5%, 2%, or an interval formed between any two values. Lithium fluorosulfonate is a new type of lithium salt that has high ion conductivity and can maintain its high ion conductivity at lower temperatures. When the amount of lithium fluorosulfonate added is within the above-mentioned appropriate range, the battery can achieve the best state in terms of cycle stability, rate performance, safety performance, etc. on the basis of ensuring the low-temperature performance of the battery.

[0034] In some embodiments of the present application, the thickness αμm of the coating on the diaphragm satisfies: 1.5≤α≤3. In order to prevent battery short circuit, a diaphragm is usually provided between the positive electrode plate and the negative electrode plate. In the present application, the type of diaphragm is not particularly limited and can be selected according to actual needs. The diaphragm can be a polypropylene film, polyethylene film or non-woven fabric film containing an organic coating such as polyvinylidene fluoride, spandex film, aramid film, etc.; or a polypropylene film, polyethylene film, or non-woven fabric film containing an inorganic coating layer of boehmite, aluminum oxide or other ceramics. When the thickness of the coating on the surface of the diaphragm is within an appropriate range, it can improve the thermal stability and mechanical strength of the diaphragm, prevent the positive and negative extreme area contact caused by the shrinkage of the diaphragm, and improve the puncture resistance of the diaphragm to prevent short circuit caused by lithium dendrites piercing the diaphragm under long-term battery cycle conditions, thereby improving the safety of the battery. The thickness α μm of the coating on the diaphragm can be any value among 1.5 μm, 2.0 μm, 2.5 μm, 3.0 μm, or an interval formed between any two values.

[0035] In some embodiments of the present application, the conductivity σmS / cm of the electrolyte satisfies: 7≤σ≤14. When the conductivity is within an appropriate range, it can reduce the internal resistance of the battery, improve the rate performance of the battery, improve the low-temperature performance of the battery, extend the cycle life of the battery, and improve safety. The conductivity σmS / cm of the electrolyte can be any value between 7mS / cm, 8mS / cm, 9mS / cm, 10mS / cm, 14mS / cm, or an interval formed between any two values. The conductivity of the electrolyte described in the present application is directly measured using a conductivity meter.

[0036] In some embodiments of the present application, the mass percentage of the salt organic additive in the electrolyte is C%, the mass percentage of the sulfur-based organic additive in the electrolyte is D%, and the mass percentage of the carbonate additive in the electrolyte is E%, with 0.1≤C≤2, 0.1≤D≤3, and 0.5≤E≤5. The contents of the salt organic additive, sulfur organic additive, and carbonate additive in the electrolyte within the above ranges are beneficial for simultaneously improving the low-temperature discharge performance and high-temperature cycle performance of the secondary battery.

[0037] In some embodiments of the present application, the mass percentage of the salt organic additive in the electrolyte is C%, the mass percentage of the sulfur organic additive in the electrolyte is D%, and the mass percentage of the carbonate additive in the electrolyte is E%, 0.5≤C≤1, 0.5≤D≤1.5, 2≤E≤5.

[0038] In some embodiments of the present application, a lithium salt is further added to the electrolyte, wherein the lithium salt includes at least one of lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide. The mass percentage of the lithium salt in the electrolyte is 8-16%.

[0039] In some embodiments of the present application, the salt organic additive includes at least one of lithium dioxalatoborate (LiBOB), lithium difluorooxalatoborate (LiODFB), lithium difluorobis(oxalatophosphate) (LiODFP), and lithium difluorophosphate (LiPO2F2).

[0040] In some embodiments of the present application, the sulfur-based organic additive includes at least one of methylene methanedisulfonate (MMDS), diethylene sulfate (DTD), vinyl sulfite (ES), 1,3-propyl sulfonate (PS), and 1,3-propylene sultone (PST).

[0041] In some embodiments of the present application, the carbonate additive includes at least one of vinylene carbonate (VC) and vinyl ethylene carbonate (VEC).

[0042] In some embodiments of the present application, a phosphorus-based organic additive may be added to the electrolyte as needed. The phosphorus-based organic additive includes a fluorophosphate. The weight percentage of the phosphorus-based organic additive is 0.1-3%. The addition of the phosphorus-based organic additive can improve the thermal stability of the electrolyte to a certain extent, thereby enhancing the cycling stability of the battery at high temperatures.

[0043] In some embodiments of the present application, the salt organic additive includes lithium difluorooxalatoborate (LiODFB), the sulfur-based organic additive includes vinyl sulfate (DTD), and the mass percentage of vinyl sulfate (DTD) in the electrolyte is greater than that of lithium difluorooxalatoborate (LiODFB).

[0044] In some embodiments of the present application, the salt organic additive includes lithium difluorooxalatoborate (LiODFB), the carbonate additive includes vinylene carbonate (VC), and the mass percentage of vinylene carbonate (VC) in the electrolyte is greater than the mass percentage of lithium difluorooxalatoborate (LiODFB).

[0045] In some preferred embodiments of the present application, the salt organic additive includes lithium difluorooxalatoborate (LiODFB), the sulfur-based organic additive includes vinyl sulfate (DTD), and the carbonate additive includes vinylene carbonate (VC). The mass percentages of lithium difluorooxalatoborate (LiODFB), vinyl sulfate (DTD), and vinylene carbonate (VC) in the electrolyte satisfy the following relationship: VC > DTD > LiODFB. When the amounts of the additives in the electrolyte satisfy this relationship, the battery exhibits superior low-temperature discharge performance and high-temperature cycling performance.

[0046] In the secondary battery described herein, the type of organic solvent forming the electrolyte is not particularly limited and can be selected according to actual needs. The organic solvent includes, but is not limited to, at least one of ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methylpropyl carbonate, diphenyl carbonate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, ethyl fluoroacetate, fluoroether, γ-butyrolactone, acetonitrile, and sulfolane.

[0047] In some preferred embodiments of the present application, the solvent comprises ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate.

[0048] Positive electrode

[0049] In some embodiments of the present application, the positive electrode active material layer further includes a positive electrode conductor and a positive electrode binder.

[0050] In this application, the type of positive electrode conductive agent is not limited; any known conductive agent may be used. Positive electrode conductive agents include, but are not limited to, at least one of natural graphite, artificial graphite, acetylene black, amorphous carbon such as needle coke, carbon nanotubes, and graphene. These positive electrode conductive agents may be used alone or in any combination.

[0051] In the present application, the type of positive electrode binder used in the production of the positive electrode active material layer is not particularly limited. In the case of a coating method, any material may be used as long as it is soluble or dispersible in the liquid medium used in electrode production. The positive electrode binder includes, but is not limited to, one or more of the following: resin polymers such as polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, and nitrocellulose; rubber polymers such as styrene-butadiene rubber (SBR), nitrile rubber (NBR), fluororubber, isoprene rubber, polybutadiene rubber, and ethylene-propylene rubber; thermoplastic elastomer polymers such as styrene-butadiene-styrene block copolymers or their hydrogenates, ethylene-propylene-diene terpolymers (EPDM), styrene-ethylene-butadiene-ethylene copolymers, styrene-isoprene-styrene block copolymers or their hydrogenates; soft resin polymers such as polyvinyl acetate, ethylene-vinyl acetate copolymers, and propylene-α-olefin copolymers; fluorinated polymers such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene, fluorinated polyvinylidene fluoride, and polytetrafluoroethylene-ethylene copolymers; and polymer compositions having ion conductivity for alkali metal ions (particularly lithium ions). The above-mentioned positive electrode binders can be used alone or in any combination.

[0052] In this application, lithium iron phosphate, a solid electrolyte additive, a conductive agent, and a binder are prepared into a positive electrode slurry, which is then coated onto a positive electrode current collector to prepare a positive electrode sheet. The type of solvent used to form the positive electrode slurry is not limited, as long as it can dissolve or disperse the lithium iron phosphate, solid electrolyte additive, conductive agent, and binder. Solvents used to form the positive electrode slurry include, but are not limited to, aqueous solvents and organic solvents. Aqueous solvents include, but are not limited to, water and a mixture of alcohol and water. Organic solvents include, but are not limited to, aliphatic hydrocarbons such as hexane; aromatic hydrocarbons such as benzene, toluene, xylene, and methylnaphthalene; heterocyclic compounds such as quinoline and pyridine; ketones such as acetone, methyl ethyl ketone, and cyclohexanone; esters such as methyl acetate and methyl acrylate; amines such as diethylenetriamine and N,N-dimethylaminopropylamine; ethers such as diethyl ether, propylene oxide, and tetrahydrofuran (THF); amides such as N-methylpyrrolidone (NMP), dimethylformamide, and dimethylacetamide; and aprotic polar solvents such as hexamethylphosphoramide and dimethyl sulfoxide.

[0053] In the present application, there is no particular restriction on the type of positive electrode current collector, which can be any material known to be suitable for use as a positive electrode current collector. Positive electrode current collectors include but are not limited to metal materials such as aluminum, stainless steel, nickel plating, titanium, tantalum; carbon materials such as carbon cloth and carbon paper; composite materials formed by polymers and metal layers. In some embodiments, the positive electrode current collector is a metal material. In some embodiments, the positive electrode current collector is aluminum. There is no particular restriction on the form of the positive electrode current collector. When the positive electrode current collector is a metal material, the form of the positive electrode current collector may include but is not limited to metal foil, metal cylinder, metal strip roll, metal plate, metal foil, metal mesh, stamped metal, foamed metal, etc. When the positive electrode current collector is a carbon material, the form of the positive electrode current collector may include but is not limited to carbon plate, carbon film, carbon cylinder, etc.

[0054] Negative electrode

[0055] In some embodiments of the present application, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector. The negative electrode active material layer contains a negative electrode active material. This application does not limit the type of negative electrode active material; the negative electrode active material may specifically be a graphite material, a silicon material, or a hard carbon material. In some embodiments of the present application, a conductive agent, a binder, or a dispersant may also be added to the negative electrode active material layer as needed.

[0056] In some embodiments of the present application, the type of negative electrode current collector is not limited, and any current collector material commonly used in the art can be used to prepare the secondary battery in the present application. The negative electrode current collector is preferably made of copper foil or carbon-coated copper foil.

[0057] In the present application, the type of solvent used to form the negative electrode slurry is not limited, as long as it can dissolve or disperse the negative electrode active material, the conductive agent, the binder, and the dispersant.

[0058] In some embodiments of the present application, the preparation of a secondary battery includes: stacking the positive electrode sheet, the diaphragm, and the negative electrode sheet in order, so that the diaphragm is located between the positive and negative electrode sheets to play an isolating role, and then winding them into a bare battery cell, placing them in a battery shell, and then baking them at 55 to 110°C to remove water, injecting electrolyte, sealing, and obtaining a secondary battery after standing, hot and cold pressing, formation, clamping, and capacity separation.

[0059] In some embodiments of the present application, a secondary battery may include an outer packaging, which may be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery may also be a soft shell, such as a bag-type soft shell, which may be made of a plastic such as one or more of polypropylene, polybutylene terephthalate, and polybutylene succinate. The shape of the secondary battery is not particularly limited, and it may be cylindrical, square, or any other shape.

[0060] The second aspect of the present application provides an electrical device, which includes the secondary battery described in the first aspect of the present application. The electrical device can be an application device such as a vehicle, a mobile phone, a portable device, a laptop computer, a ship, a spacecraft, an electric toy, and an electric tool. The vehicle can be a new energy vehicle, which can be a pure electric vehicle, a hybrid vehicle, or an extended-range vehicle; the spacecraft includes an airplane, a rocket, a space shuttle, and a spacecraft; the electric toy includes a fixed or mobile electric toy, such as a game console, an electric car toy, an electric ship toy, and an electric airplane toy; the electric tool includes a metal cutting electric tool, a grinding electric tool, an assembly electric tool, and a railway electric tool, such as an electric drill, an electric grinder, an electric wrench, an electric screwdriver, an electric hammer, an impact drill, a concrete vibrator, and an electric planer. The embodiments of the present application do not impose any special restrictions on the above-mentioned devices.

[0061] The following are specific examples of the present application, and the technical solutions of the present application are further described in conjunction with the examples, but the present application is not limited to these examples. The reagents, methods and equipment used in this application, unless otherwise specified, are conventional reagents, methods and equipment in the art.

[0062] Example 1

[0063] This embodiment provides a secondary battery, which is prepared according to a method comprising the following steps:

[0064] Preparation of positive electrode

[0065] The positive electrode active material LiFePO4 (LFP), solid electrolyte additive lithium lanthanum zirconium oxide (chemical formula Li7La3Zr2O 12 , abbreviated as LLZO), conductive agent acetylene black (Super P) and binder polyvinylidene fluoride (PVDF) were mixed uniformly in a mass ratio of 95:0.5:2.5:2, and evenly dispersed in 1-methyl-2-pyrrolidone (NMP) to prepare a uniform black slurry. The mixed slurry was coated on both sides of the positive electrode current collector aluminum foil, and then baked, roll-pressed, and cut into pieces to obtain the positive electrode sheets. The compaction density of the positive electrode sheets is shown in Table 1.

[0066] Preparation of negative electrode sheet

[0067] The negative electrode active material artificial graphite (AG), conductive agent acetylene black (Super P) and binder SBR were mixed in a mass ratio of 84.6:9.4:3:3 and evenly dispersed in deionized water to form a uniform black slurry. The mixed slurry was coated on both sides of the copper foil, baked, rolled and cut into pieces with a surface density of 7 mg / cm 2 The negative electrode.

[0068] Preparation of electrolyte

[0069] EC, EMC and DMC are mixed in a volume ratio of 2.5:3:4.5 to obtain a mixed solution for standby use; at room temperature (25°C), lithium salt LiPF6 is added successively to the obtained mixed solvent in a glove box filled with argon, and the mixture is stirred continuously and cooled, and then additives such as lithium fluorosulfonate, VC, DTD, LiODFB and LiPO2F2 are added, wherein, based on the total mass of the electrolyte: the mass percentage of lithium fluorosulfonate is 0.2%, the mass percentage of VC is 0.5%, the mass percentage of DTD is 1%, the mass percentage of LiODFB is 1%, the mass percentage of LiPO2F2 is 0.5%, and the mass percentage of LiPF6 is 12.5%, thereby obtaining the electrolyte of Example 1.

[0070] Production of secondary batteries

[0071] The prepared positive electrode sheet, separator, and negative electrode sheet are stacked in order so that the separator is located between the positive and negative electrode sheets. The coating parameters of the separator are shown in Table 1 (the coating material of the separator is aluminum oxide ceramic). After winding, hot pressing and shaping, and tab welding, a bare cell is obtained. The bare cell is placed in an outer packaging aluminum-plastic film and baked in an oven at 85±10°C for 24 hours. The above-prepared electrolyte is injected into the dried battery, allowed to stand, formed, and divided to complete the preparation of the lithium-ion soft-pack battery.

[0072] The remaining examples and comparative examples refer to Example 1, with the differences shown in Table 1.

[0073] Table 1

[0074]

[0075] Example 19

[0076] This embodiment provides a secondary battery, which is prepared by referring to the method of Example 1. The difference from Example 1 is that the amount of additives added to the electrolyte is different from that in Example 1, specifically:

[0077] In the electrolyte, the mass percentage of lithium fluorosulfonate is 0.2%, the mass percentage of VC is 1%, the mass percentage of DTD is 0.5%, and the mass percentage of LiODFB is 0.2%. That is, the amount of additives satisfies the relationship of VC>DTD>LiODFB, and the mass percentage of LiPO2F2 is 0.5%.

[0078] Example 20

[0079] This embodiment provides a secondary battery, which is prepared by referring to the method of Example 1. The difference from Example 1 is that the solid electrolyte additive lithium lanthanum zirconium oxide is replaced with an equal mass of lithium titanium aluminum phosphate in the positive electrode active material.

[0080] Performance Testing

[0081] (1) Low-temperature DCR test: In an environment of 25±1°C, the soft-pack batteries obtained in the above examples and comparative examples were charged to 3.65V at a current of 1C, then discharged at 1C for 30 minutes. After adjusting to 50% SOC, the temperature was adjusted to -20°C, left to rest for 120 minutes, and then pulse-discharged at 0.5C for 10 seconds and then charged for 10 seconds. DCR was calculated as (voltage before pulse discharge - voltage after pulse discharge) / discharge current. The recorded results are shown in Table 2.

[0082] (2) -10°C low temperature cycle performance test: At -10±2°C, the soft-pack batteries obtained in the above examples and comparative examples were charged at a constant current of 0.2C to 3.65V, and then charged at a constant voltage until the current reached 0.05C, at which point the charging was terminated; then, they were discharged at a constant current of 0.3C until the voltage reached 2.5V, at which point the discharge was terminated. During the charging and discharging process, the discharge capacity and other parameters of the battery were recorded to calculate the discharge specific capacity, and the number of cycles at which the battery capacity retention rate was 80% was recorded. The recorded data of the number of cycles are shown in Table 2.

[0083] (3) High-temperature storage gas generation test: The soft-pack batteries obtained in the examples and comparative examples were charged at a constant current rate of 1C to 3.65V at 25±2°C, and then charged at a constant voltage of 3.65V until the current was less than 0.05C, so that they were in a fully charged state of 3.65V;

[0084] The volume of the fully charged battery before storage was measured and recorded as V0. The fully charged battery was then placed in a 70±2°C oven. After 120 days, the battery was removed and its volume after storage was immediately measured and recorded as V1. The volume expansion rate = (V1-V0) / V0×100%. The results are shown in Table 2.

[0085] (4) High-temperature cyclic gas generation test: At 45±2°C, the soft-pack batteries obtained in Examples 1 to 18 and Comparative Examples 1 to 6 were subjected to a charge and discharge cycle test in the range of 2.5 to 3.65 V at a charge and discharge rate of 1C / 1C (i.e., constant current charging at 1C to 3.65 V, then constant voltage charging until the current reaches 0.05C, and then charging is terminated; then constant current discharge is performed at 1C until the voltage reaches 2.5 V, and the discharge is terminated, which is considered one cycle). The number of cycles at which the battery capacity retention rate is 80% is recorded. The results are shown in Table 2.

[0086] Table 2

[0087]

[0088] It can be seen from the above results that the present application improves the low-temperature discharge performance and high-temperature cycle performance of the secondary battery by controlling the mass ratio of lithium iron phosphate to solid electrolyte additive in the positive electrode sheet, the type of additive in the electrolyte, and the content of lithium fluorosulfonate in the electrolyte within a certain range.

[0089] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application and are not intended to limit the scope of protection of the present application. Although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present application may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present application.

Claims

1. A secondary battery comprising a positive electrode sheet, a separator, a negative electrode sheet, and an electrolyte, characterized in that: The positive electrode plate includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, the positive electrode active material layer comprising lithium iron phosphate and a solid electrolyte additive, the solid electrolyte additive comprising an oxyacid salt formed by at least one element selected from P, Ti, Al, La, Zr, and V and Li, and a mass ratio of the lithium iron phosphate to the solid electrolyte additive being A, 30≤A≤200; The electrolyte contains lithium fluorosulfonate, salt organic additives, sulfur organic additives and carbonate additives. The mass percentage of lithium fluorosulfonate in the electrolyte is B%, and 0.2≤B≤2.

2. The secondary battery according to claim 1, wherein The separator includes a substrate and a coating provided on at least one surface of the substrate, wherein the thickness of the coating is α μm, and the compaction density of the positive electrode sheet is PD g / cm 3 , the conductivity of the electrolyte is σmS / cm, satisfying:

3. The secondary battery according to claim 2, wherein The secondary battery satisfies at least one of the following conditions: (1)2.2≤PD≤2.8; (2)1.5≤α≤3； (3)7≤σ≤14.

4. The secondary battery according to claim 1, wherein The solid electrolyte additive includes at least one of lithium aluminum titanium phosphate, lithium lanthanum titanate, and lithium lanthanum zirconate.

5. The secondary battery according to claim 1, wherein The mass percentage of the salt organic additive in the electrolyte is C%, the mass percentage of the sulfur organic additive in the electrolyte is D%, and the mass percentage of the carbonate additive in the electrolyte is E%, 0.1≤C≤2, 0.1≤D≤3, 0.5≤E≤5.

6. The secondary battery according to claim 5, characterized in that Meet at least one of the following characteristics: (4) The salt organic additive includes at least one of lithium dioxalatoborate, lithium difluorooxalatoborate, lithium difluorobisoxalatophosphate, and lithium difluorophosphate; (5) The sulfur-based organic additive includes at least one of methylene disulfonate, vinyl sulfate, vinyl sulfite, 1,3-propyl sulfonate, and 1,3-propylene sultone; (6) The carbonate additive includes at least one of vinylene carbonate and vinyl ethylene carbonate.

7. The secondary battery according to claim 5, characterized in that The salt organic additive includes lithium difluorooxalatoborate, the sulfur organic additive includes vinyl sulfate, and the mass percentage of vinyl sulfate in the electrolyte is greater than the mass percentage of lithium difluorooxalatoborate.

8. The secondary battery according to claim 5, characterized in that The salt organic additive includes lithium difluorooxalatoborate, the carbonate additive includes vinylene carbonate, and the mass percentage of vinylene carbonate in the electrolyte is greater than the mass percentage of lithium difluorooxalatoborate.

9. The secondary battery according to claim 5, characterized in that The salt organic additive includes lithium difluorooxalatoborate, the sulfur organic additive includes vinyl sulfate, and the carbonate additive includes vinylene carbonate. The mass percentages of lithium difluorooxalatoborate, vinyl sulfate, and vinylene carbonate in the electrolyte satisfy the following: vinylene carbonate>vinyl sulfate>lithium difluorooxalatoborate.

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

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