Secondary battery and electric device

Abstract

This application discloses a secondary battery and an electrical device, belonging to the field of battery technology. The secondary battery includes a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. The positive active material layer contains a binder polymer containing C-F bonds. The electrolyte includes a lithium salt, specifically lithium bis(fluorosulfonyl)imide. The secondary battery satisfies the following condition: 0.2 ≤ b × c ≤ 9; where b% is the mass percentage of lithium bis(fluorosulfonyl)imide in the electrolyte; and cg / Ah is the ratio of the mass of the binder polymer containing C-F bonds in the secondary battery to the rated capacity of the secondary battery. The secondary battery described in this application has excellent safety performance and can effectively reduce the generation of reducing gas H2.

Description

Technical Field

[0001] This application relates to the field of battery technology, specifically to a secondary battery and an electrical device. Background Technology

[0002] Since the 1990s, secondary batteries have been widely used in commercial applications. With the commercial application of new energy technologies such as photovoltaics, lithium-ion secondary batteries, mainly lithium iron phosphate, have begun to be widely used in energy storage applications such as power generation, grid, and user sides. Industrial production capacity has been increasing year by year, and the demand for energy storage is broad, entering the TWh era. At the same time, the market requirements for the safety and reliability of secondary batteries have also been further improved.

[0003] Therefore, it is necessary to provide a secondary battery that can solve the above problems. Summary of the Invention

[0004] The purpose of this application is to overcome the shortcomings of the existing technology and provide a secondary battery and power device. The secondary battery can effectively reduce the generation of reducing gas H2 and has excellent safety performance.

[0005] To achieve the above objectives, a first aspect of this application provides a secondary battery, the secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte; the positive electrode comprises a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector, the positive active material layer comprising a binder polymer containing CF bonds; the electrolyte comprises a lithium salt, the lithium salt comprising lithium bis(fluorosulfonyl)imide.

[0006] The secondary battery satisfies: 0.2≤b×c≤9;

[0007] Wherein, b wt% is the mass percentage of the lithium bis(fluorosulfonyl)imide in the electrolyte;

[0008] cg / Ah is the ratio of the mass of the binder polymer containing CF bonds in the secondary battery to the rated capacity of the secondary battery.

[0009] As an implementation scheme of this application, b satisfies: 4≤b≤18.

[0010] As an implementation of this application, c satisfies: 0.05≤c≤0.5.

[0011] As an implementation scheme of this application, b satisfies: 6≤b≤16.

[0012] As an embodiment of this application, the lithium salt further includes lithium hexafluorophosphate, wherein the lithium hexafluorophosphate accounts for 6 to 12 wt% of the mass percentage of the electrolyte.

[0013] As an embodiment of this application, the mass percentage of lithium difluorosulfonylimide in the electrolyte is greater than the mass percentage of lithium hexafluorophosphate in the electrolyte.

[0014] As an embodiment of this application, the binder polymer containing CF bonds includes at least one of polyvinylidene fluoride, polytetrafluoroethylene, and fluorinated polyvinylidene fluoride.

[0015] As an embodiment of this application, the electrolyte further includes additives, which include a first additive, a second additive, and a third additive;

[0016] The first additive includes at least one of tris(trimethylsilane)borate, tris(trimethylsilane)phosphate, ethyl(2-fluoro)acetate, ethyl fluoroacetate, and ethylene fluorocarbonate.

[0017] The second additive includes at least one of vinyl sulfate, 1,3-propane sulpholactone, methylene disulfonate, propenyl-1,3-sulfonate lactone, N,N'-thiodiimidazole, triphenyl phosphonite, n-butyric anhydride, hexanetrionitrile, succinate, and tetrafluoroterephthalonitrile.

[0018] The third additive includes at least one of lithium bis(oxalato)borate, lithium difluorophosphate, lithium 4,5-dicyano-2-trifluoromethyl-imidazolium, and lithium trifluoromethanesulfonate.

[0019] As an embodiment of this application, the first additive accounts for 0.1 to 3 wt% of the electrolyte by mass.

[0020] As an embodiment of this application, the second additive accounts for 0.1 to 1.5 wt% of the electrolyte by mass.

[0021] As an embodiment of this application, the third additive accounts for 0.1 to 5 wt% of the electrolyte by mass.

[0022] As an embodiment of this application, the additive further includes a fourth additive, which comprises at least one of lithium difluorooxalatoborate and lithium difluorodioxalatophosphate.

[0023] A second aspect of this application provides an electrical device including the aforementioned secondary battery, wherein the secondary battery serves as the power supply for the electrical device.

[0024] The beneficial effects of this application are as follows: By controlling the content of lithium bis(fluorosulfonyl)imide in the electrolyte and the ratio of the mass of the binder polymer containing CF bonds in the secondary battery to the rated capacity of the secondary battery to satisfy the following relationship: 0.2≤b×c≤9, the amount of reducing gas H2 generated is reduced, thereby improving the safety performance of the secondary battery. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0026] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0027] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0028] This application provides a secondary battery, which includes a positive electrode, a negative electrode, a separator, and an electrolyte; the positive electrode includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector, the positive active material layer comprising a binder polymer containing CF bonds; the electrolyte includes a lithium salt, the lithium salt including lithium bis(fluorosulfonyl)imide (LiFSI);

[0029] The secondary battery satisfies: 0.2≤b×c≤9, for example, it can be 0.2, 0.4, 0.5, 0.8, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 6.5, 7, 7.5, 8, 8.5, 9 or any two of these values.

[0030] Wherein, b wt% is the mass percentage of the lithium bis(fluorosulfonyl)imide in the electrolyte;

[0031] cg / Ah is the ratio of the mass of the binder polymer containing CF bonds in the secondary battery to the rated capacity of the secondary battery.

[0032] The inventors of this application have discovered that during the thermal runaway process of a secondary battery, the positive electrode and negative electrode come into contact. A large-area short circuit triggers a significant reaction. The CF-bonded binder polymer of the positive electrode comes into contact with the highly reducing active surface of the negative electrode, generating a large amount of reducing gas H2, thus reducing the safety performance of the secondary battery. Adding lithium bis(fluorosulfonyl)imide to the electrolyte can generate a less active SEI interface layer on the negative electrode surface, improving interfacial electronic insulation and reducing the reaction contact between the active surface sites of the negative electrode and the CF-bonded binder polymer of the positive electrode after thermal runaway. This significantly reduces the reaction of the binder under high-temperature conditions, reduces the amount of reducing gas H2 generated, and thus improves the safety performance of the secondary battery.

[0033] In one embodiment, b satisfies: 4 ≤ b ≤ 18, for example, it can be a range of 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18 or any two of these values; especially when the mass percentage of lithium bis(fluorosulfonyl)imide in the electrolyte is within this range, bis(fluorosulfonyl)imide has good high-temperature stability, which can reduce the risk of thermal runaway, effectively reduce the gas generated by side reactions, maintain the chemical stability of the electrolyte at high temperatures, promote the formation of a less active SEI and CEI interface layer, improve interface stability, and further improve the safety performance of the secondary battery.

[0034] In one embodiment, c satisfies: 0.05 ≤ c ≤ 0.5, for example, it can be a range of 0.05, 0.06, 0.08, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, or any two of these values. In particular, when the ratio of the mass of the binder polymer containing CF bonds in the secondary battery to the rated capacity of the secondary battery is within this range, the structural stability of the positive electrode sheet can be further improved, the positive electrode active material can be firmly bonded to the surface of the positive electrode current collector, and safety problems caused by structural damage can be reduced. In the formation of a stable SEI and CEI interface layer, the positive electrode sheet can be protected from further erosion by the electrolyte, internal side reactions can be reduced, the amount of reducing gas H2 generated can be reduced, and the safety performance of the secondary battery can be improved.

[0035] In one implementation, b satisfies: 6≤b≤16.

[0036] In one embodiment, the mass of the binder polymer containing CF bonds in the secondary battery is 40 to 150 g, for example, it can be 40 g, 45 g, 50 g, 60 g, 70 g, 80 g, 90 g, 100 g, 110 g, 120 g, 130 g, 140 g, 150 g, or any two of these values.

[0037] It should be noted that the mass of the binder polymer containing CF bonds in the secondary battery can be obtained through thermogravimetric analysis (TGA). For example, the following steps can be taken: after weighing the positive electrode, perform a thermogravimetric analysis at a high temperature of 600°C under a helium protective atmosphere, weigh it again, and calculate the weight loss ratio of the electrode. This weight is the mass of the binder polymer containing CF bonds in the secondary battery.

[0038] In one embodiment, the rated capacity of the secondary battery is 314–750 Ah, for example, it can be a range of 314 Ah, 315 Ah, 320 Ah, 330 Ah, 340 Ah, 350 Ah, 360 Ah, 380 Ah, 400 Ah, 450 Ah, 500 Ah, 550 Ah, 600 Ah, 650 Ah, 700 Ah, 750 Ah, or any two of these values. By designing the rated capacity of the secondary battery within this range, the secondary battery has a high rated capacity, releases more chemical energy per unit volume, and has a relatively low overall current density under constant discharge current, reducing electrode polarization and heat generation. Together with the binder polymer containing CF bonds, it improves the gas generation effect, reduces the amount of reducing gas H2 generated, and improves the safety performance of the secondary battery.

[0039] It should be noted that the rated capacity of a secondary battery refers to the average capacity of a fully charged battery at room temperature after three cycles of charging and discharging at 0.5C under a voltage of 2.5 to 3.65V.

[0040] In one embodiment, the lithium salt further includes lithium hexafluorophosphate (LiPF6), wherein the lithium hexafluorophosphate accounts for 6 to 12 wt% of the electrolyte by mass, for example, 6 wt%, 7 wt%, 8 wt%, 9 wt%, 10 wt%, 11 wt%, 12 wt%, or any two of these values. This application uses a combination of lithium hexafluorophosphate and lithium difluorosulfonylimide as the lithium salt, which can improve ionic conductivity and thermal stability, enhance the stability of the secondary battery during cycling, effectively suppress battery gas expansion, further improve electrolyte stability, inhibit electrolyte decomposition, effectively mitigate the problem of positive electrode thickness during use, improve the stability of the positive electrode interface, reduce the generation of reducing gas H2, and improve the safety performance of the secondary battery.

[0041] In one embodiment, the electrolyte comprises a lithium salt, which includes lithium bis(fluorosulfonyl)imide and lithium hexafluorophosphate, wherein the mass percentage of lithium bis(fluorosulfonyl)imide in the electrolyte is greater than the mass percentage of lithium hexafluorophosphate in the electrolyte. By controlling the content of lithium bis(fluorosulfonyl)imide in the electrolyte to be greater than that of lithium hexafluorophosphate, the generation of reducing gas H2 can be reduced to a certain extent, thereby improving the safety performance of the secondary battery and maintaining better cycle performance.

[0042] In one embodiment, the electrolyte further includes additives, including a first additive, a second additive, and a third additive.

[0043] In one embodiment, the first additive includes at least one of tris(trimethylsilane)borate, tris(trimethylsilane)phosphate, ethyl(2-fluoro)acetate, ethyl fluoroacetate, and ethylene fluorocarbonate.

[0044] In one embodiment, the first additive comprises tris(trimethylsilane) phosphate and fluoroethylene carbonate.

[0045] In one embodiment, the second additive comprises at least one of vinyl sulfate, 1,3-propanesulfonyl lactone, methylene disulfonate, propenyl-1,3-sulfonyl lactone, N,N'-thiodiimidazole, triphenyl phosphonite, n-butyric anhydride, hexanetrionitrile, succinate, and tetrafluoroterephthalonitrile.

[0046] In one embodiment, the second additive comprises vinyl sulfate and / or methylene disulfonate.

[0047] In one embodiment, the third additive includes at least one of lithium bis(oxalato)borate, lithium difluorophosphate, lithium 4,5-dicyano-2-trifluoromethyl-imidazolium, and lithium trifluoromethanesulfonate.

[0048] In one embodiment, the third additive comprises lithium difluorophosphate.

[0049] This application employs a combination of the three types of additives mentioned above. The first additive effectively improves the stability of the initial interface film formation of the negative electrode and reduces internal resistance. The second additive improves the stability of the high-temperature interface film formation of the initial interface between the positive and negative electrodes. The third additive effectively ensures the interface state of the initial SEI inorganic layer of the secondary battery, forming a structure with low impedance, high temperature resistance, dense film formation, and high electronic insulation. After thermal runaway, the reactivity of polymers such as binders containing CF bonds on the active surface of the negative electrode and the positive electrode sheet is reduced, thereby reducing H2 generation and improving the safety performance of the secondary battery.

[0050] In one embodiment, the first additive constitutes 0.1 to 2 wt% of the electrolyte by mass, for example, 0.1 wt%, 0.2 wt%, 0.5 wt%, 0.6 wt%, 0.8 wt%, 1 wt%, 1.2 wt%, 1.5 wt%, 1.8 wt%, 2.0 wt%, or any combination of two such values. In particular, controlling the mass percentage of the first additive within this range helps improve the thermal stability of the electrolyte, maintaining good stability under high-temperature or overheating conditions in the secondary battery. This promotes improved initial interface film formation stability at the negative electrode, facilitates the formation of a uniform interface film on the positive electrode surface, suppresses electrolyte decomposition and side reactions under high voltage, and thus improves the safety and cycle performance of the secondary battery.

[0051] In one embodiment, the first additive accounts for 0.5 to 1 wt% of the electrolyte by mass. Having the first additive within this range further improves the cycle performance of the secondary battery.

[0052] In one embodiment, the second additive accounts for 0.1 to 1.5 wt% of the electrolyte by mass, for example, it can be 0.1 wt%, 0.2 wt%, 0.5 wt%, 0.6 wt%, 0.8 wt%, 1.0 wt%, 1.2 wt%, 1.5 wt%, or any two of these values. In particular, when the mass percentage of the second additive in the electrolyte is controlled within this range, a high-temperature resistant and chemically stable interfacial film is formed at the positive and negative electrode interface, effectively reducing side reactions at high temperatures and mitigating corrosion of the positive and negative electrodes.

[0053] In one embodiment, the second additive accounts for 0.5 to 1.0 wt% of the electrolyte by mass. When the second additive accounts for a mass percentage of the electrolyte within the above range, the cycle performance of the secondary battery can be further improved.

[0054] In one embodiment, the third additive accounts for 0.1 to 5 wt% of the electrolyte by mass, for example, 0.1 wt%, 0.2 wt%, 0.5 wt%, 0.6 wt%, 0.8 wt%, 1.0 wt%, 1.2 wt%, 1.5 wt%, 1.8 wt%, 2.0 wt%, 2.2 wt%, 2.5 wt%, 2.8 wt%, 3.0 wt%, 3.2 wt%, 3.5 wt%, 3.8 wt%, 4.0 wt%, 4.2 wt%, 4 The range of 0.5 wt%, 4.8 wt%, 5.0 wt%, or any two of these values, especially when the mass percentage of the third additive in the electrolyte is controlled within this range, can more effectively ensure the film-forming interface state of the initial SEI inorganic layer of the secondary battery, forming a structure with low impedance, high temperature resistance, dense film formation, and high electronic insulation. After thermal runaway, the reactivity of polymers such as binders containing CF bonds on the active surface of the negative electrode and the positive electrode sheet is reduced, thereby reducing H2 generation and improving the safety performance of the secondary battery.

[0055] In one embodiment, the third additive accounts for 0.1 to 1.5 wt% of the electrolyte by mass. When the third additive accounts for a percentage of the electrolyte within the above range, it ensures that the secondary battery possesses both excellent safety performance and superior cycle performance.

[0056] In one embodiment, the additive further includes a fourth additive, which comprises at least one of lithium difluorooxalatoborate and lithium difluorodioxalatophosphate.

[0057] In one embodiment, the fourth additive constitutes 0.05 to 0.5 wt% of the electrolyte by mass. For example, it can be 0.05 wt%, 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, or any combination of two such values. Further inclusion of lithium difluorooxalatoborate or lithium difluorodioxalatophosphate in the electrolyte additive can improve the energy efficiency of the secondary battery.

[0058] In one embodiment, the organic solvent includes cyclic carbonates and chain carbonates.

[0059] In one embodiment, the cyclic carbonate includes at least one of ethylene carbonate, vinylene carbonate, and propylene carbonate.

[0060] In one embodiment, the chain carbonate includes at least one of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, and dipropyl carbonate.

[0061] In one embodiment, the cyclic carbonate accounts for 30 to 40 wt% of the organic solvent, for example, 30 wt%, 31 wt%, 32 wt%, 33 wt%, 34 wt%, 35 wt%, 36 wt%, 37 wt%, 38 wt%, 39 wt%, 40 wt%, or any combination of two of these values.

[0062] In one embodiment, the chain carbonate accounts for 60 to 70 wt% of the organic solvent by mass, for example, 60 wt%, 61 wt%, 62 wt%, 63 wt%, 64 wt%, 65 wt%, 66 wt%, 67 wt%, 68 wt%, 69 wt%, 70 wt%, or any combination of two of these values.

[0063] In one embodiment, the binder polymer containing CF bonds includes at least one of polyvinylidene fluoride, polytetrafluoroethylene, and fluorinated polyvinylidene fluoride.

[0064] In one embodiment, the positive electrode active material layer comprises a positive electrode active material, the positive electrode active material comprising the chemical formula Li. a Fe 1-b M b Compounds of PO4, wherein 0.8 ≤ a ≤ 1.2, 0 ≤ b ≤ 0.9, and M is selected from at least one of Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Ti, V, Mg, and Al.

[0065] In one embodiment, the type of positive electrode current collector is not particularly limited, and it can be any known material suitable for use as a positive electrode current collector. In one embodiment, the positive electrode current collector includes metallic materials such as aluminum, stainless steel, nickel plating, titanium, and tantalum, as well as carbon materials such as carbon cloth and carbon paper.

[0066] There are no particular restrictions on the form of the positive electrode current collector. When the positive electrode current collector is a metallic material, it can be in the form of metal foil, metal cylinder, metal strip, metal plate, metal foil, metal mesh, stamped metal, foamed metal, etc. When the positive electrode current collector is a carbon material, it can be in the form of carbon plate, carbon film, carbon cylinder, etc.

[0067] In one embodiment, the positive electrode active material layer further includes a conductive agent.

[0068] In one embodiment, the secondary battery further includes a negative electrode sheet, the negative electrode sheet including a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector, the negative active material layer including a negative active material.

[0069] In this application, there are no particular restrictions on the negative electrode current collector, as long as it can achieve the purpose of this application. For example, it can be copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, foamed nickel, foamed copper, or composite current collector, etc.

[0070] In one embodiment, the negative electrode active material can be natural graphite, artificial graphite, mesophase microcarbon spheres (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, SiO, Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, or spinel-structured lithium titanate Li4Ti5O. 12 At least one of Li-Al alloys and metallic lithium.

[0071] In one embodiment, the negative electrode active material layer further includes a conductive agent and a negative electrode binder.

[0072] In one embodiment, there is no limitation on the type of conductive agent mentioned in this application, and any known conductive agent may be used.

[0073] In one embodiment, the conductive agent includes at least one of carbon materials such as acetylene black, needle coke, carbon nanotubes, and graphene.

[0074] In one embodiment, there is no limitation on the type of negative electrode binder mentioned in this application, and any known negative electrode binder may be used.

[0075] In one embodiment, the negative electrode binder comprises at least one of polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, nitrocellulose, styrene-butadiene rubber, nitrile rubber, fluororubber, isoprene rubber, polybutadiene rubber, ethylene-propylene rubber, polyvinyl acetate, polyvinylidene fluoride, polytetrafluoroethylene, and fluorinated polyvinylidene fluoride.

[0076] In the secondary battery mentioned in this application, a separator is usually provided between the positive and negative electrodes to prevent short circuits. There are no particular restrictions on the material and shape of the separator, as long as it does not significantly impair the effectiveness of this application.

[0077] In the secondary battery mentioned in this application, a separator is usually provided between the positive and negative electrodes to prevent short circuits. There are no particular restrictions on the material and shape of the separator, as long as it does not significantly impair the effectiveness of this application.

[0078] In one embodiment, the diaphragm comprises a porous sheet-like or nonwoven material with excellent liquid retention properties. Materials for resin or glass fiber diaphragms include, but are not limited to, polyolefins, aromatic polyamides, polytetrafluoroethylene, and polyethersulfone.

[0079] In one embodiment, the polyolefin is polyethylene or polypropylene. In some embodiments, the polyolefin is polypropylene. The materials of the diaphragm described above can be used alone or in any combination.

[0080] In one embodiment, the secondary battery may include an outer packaging that can be used to encapsulate the aforementioned electrode assembly and electrolyte.

[0081] In one embodiment, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0082] This application does not impose any particular restrictions on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape.

[0083] One embodiment of this application provides an electrical device including the secondary battery described above.

[0084] For example, the aforementioned electrical devices may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but are not limited thereto.

[0085] The present application is further illustrated below with specific embodiments:

[0086] Example 1

[0087] Example 1 provides a secondary battery with a rated capacity of 600 Ah. The preparation method of the secondary battery includes the following steps:

[0088] (1) Preparation of positive electrode sheet

[0089] Lithium iron phosphate (LiFePO4), acetylene black (Super P), and a binder polymer containing CF bonds (PVDF, polyvinylidene fluoride) and polyether phosphate were mixed evenly in a mass ratio of 97.55:1.8:0.5:0.15 and then uniformly dispersed in N-methyl-2-pyrrolidone (NMP) to form a uniform black slurry. The mixed slurry was coated on both sides of aluminum foil, and then baked, rolled, and cut to obtain the positive electrode sheet.

[0090] (2) Preparation of negative electrode sheet

[0091] Artificial graphite, acetylene black (Super P), CMC (sodium carboxymethyl cellulose), SBR (styrene-butadiene rubber), and PAA (polyacrylic acid) are mixed evenly in a mass ratio of 96.7:0.7:0.6:1.5:0.5 and then evenly dispersed in deionized water to form a uniform black slurry. The mixed slurry is then coated on both sides of a copper foil, baked, rolled, and cut to obtain the negative electrode sheet.

[0092] (3) Preparation of electrolyte

[0093] At room temperature, in a glove box filled with argon (H2O < 1 ppm, O2 < 1 ppm), DMC, EMC, and EC were mixed thoroughly at a volume ratio of 30:30:40. Molecular sieve dehydration is performed. 16 wt% LiFSI and 0 wt% LiPF6 are added sequentially to the resulting mixed solvent, with continuous stirring and cooling. Lithium salt can be added further when the electrolyte temperature rises by no more than 2°C, ultimately yielding a colorless and transparent liquid. Then, 0.50 wt% tris(trimethylsilane)phosphate (TMSP), 0.50 wt% ethylene sulfate (DTD), 0.50 wt% fluoroethylene carbonate (FEC), and 0.50 wt% lithium difluorophosphate (LiPO2F2) are added and stirred until homogeneous to obtain the electrolyte described in this application.

[0094] (4) Assembly of secondary batteries:

[0095] The positive electrode, separator (made of Xingyuan material PE + ceramic + adhesive layer, with a total thickness of 14um) and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrode. After stacking, a core is obtained. The core is then encapsulated in a square aluminum shell, dried, and injected with 2400g of electrolyte. After wetting, formation, sealing, and capacity testing, a secondary battery is obtained.

[0096] Examples 2-5

[0097] The difference between Examples 2-5 and Example 1 is that the amounts of LiFSI and LiPF6 are changed in Examples 2-5, thereby changing b, as shown in Table 1.

[0098] Examples 6-9

[0099] The difference between Examples 6-9 and Example 1 is that the PVDF loading in the positive electrode is changed in Examples 6-9, thereby changing c, as shown in Table 1.

[0100] Examples 10-15

[0101] The difference between Examples 10-15 and Example 1 is that Examples 10-15 change the PVDF loading in the positive electrode and adjust the amount of LiFSI and LiPF6 in the electrolyte, thereby changing b and c, as shown in Table 1.

[0102] Examples 16-29

[0103] The difference between Examples 16-29 and Example 1 is that Examples 16-29 change the amount of additives and the system of the electrolyte, as shown in Table 1.

[0104] Comparative Example 1

[0105] The difference between Comparative Example 1 and Example 1 is that Comparative Example 1 uses an equal amount of LiPF6 to replace LiFSI, as shown in Table 1.

[0106] Comparative Example 2

[0107] The difference between Comparative Example 2 and Example 1 is that Comparative Example 2 changes the PVDF loading in the positive electrode, thereby changing c, as shown in Table 1.

[0108] Table 1

[0109]

[0110]

[0111] Table 1 (continued)

[0112]

[0113]

[0114] Examples 30-32

[0115] The difference between Examples 30-32 and Example 1 is that the electrolytes of the secondary batteries in Examples 30-32 also contain lithium difluorooxalate borate at amounts equivalent to 0.05 wt%, 0.2 wt%, and 0.5 wt% of the total mass of the electrolyte, respectively, as shown in Table 3.

[0116] Performance testing

[0117] 1. Safety test: The results of H2 content test after thermal runaway test are shown in Table 2.

[0118] 1) Initial charging: Let the battery stand at 25±1℃ for 300 minutes, discharge at 0.5CP (1126.4W) to 2.5V and stop charging; let it stand for 10 minutes, charge at 0.5CP (1126.4W) to 3.65V and stop charging and let it stand for 10 minutes.

[0119] 2) Use one 1800W heating element (heating element dimensions: 352mm long * 204mm wide). The heating element should be attached to one side of the large surface of the battery sample and secured with a steel clamp (4Nm). The heating element is connected in parallel to a regulated power supply, using 220V AC power.

[0120] 3) Heating-Gas Collection: The heating elements are connected in parallel to a regulated power supply, using AC 220V power. The sample is placed in an explosion-proof container, and the air inside the chamber is replaced with nitrogen at least 4 times, with the oxygen volume concentration below 1%. After the replacement is completed, one bag of gas is collected before testing, and the gas composition is analyzed.

[0121] 4) Heat the battery with heating elements: Adjust the controller to ensure that the battery is heated at a rate of 4-7°C / min; stop heating immediately when the battery triggers thermal runaway; thermal runaway is defined as: simultaneously satisfying a) and c) or simultaneously satisfying b) and c);

[0122] a) Voltage drop ≥ 25%; b) T2 temperature reaches 150℃; c) T2 temperature rise rate dT / dt ≥ 1℃ / s, and lasts for more than 3s;

[0123] 5) After the temperature of the internal environment (T8) reaches 25±2℃, collect 3 bags of gas from the tank and perform gas composition analysis.

[0124] 2. Cyclic performance test method: At 25±2℃, the secondary batteries obtained in Examples 1 to 29 and Comparative Examples 1 to 2 were charged to 3.65V with a charging power of 0.5P, left to stand for 10 minutes, and then discharged to 2.5V with a discharging power of 0.5P. The cycle test was continued until 60% SOH, and the number of cycles was recorded as shown in Table 2.

[0125] 3. Energy efficiency test method: At 25±2℃, the secondary batteries of Examples 1 and 30-32 were charged to 3.65V with a charging power of 0.5P, and then discharged to 2.5V with a discharging power of 0.5P. The energy efficiency = discharge energy / charging energy * 100%. The test results are shown in Table 3.

[0126] Table 2

[0127]

[0128]

[0129] Table 3

[0130]

[0131]

[0132] As shown in Table 2, by controlling the content of lithium bis(fluorosulfonyl)imide in the electrolyte and the ratio of the mass of the binder polymer containing CF bonds in the secondary battery to the rated capacity of the secondary battery to satisfy a certain relationship, the amount of reducing gas H2 generated is reduced, thereby improving the safety performance of the secondary battery.

[0133] As shown in Table 3, the energy efficiency of secondary batteries can be improved by further adding an appropriate amount of lithium difluorooxalate borate to the electrolyte.

[0134] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit the scope of protection of this application. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the substance and scope of the technical solutions of this application.

Claims

1. A secondary battery, characterized in that, The secondary battery includes a positive electrode, a negative electrode, a separator, and an electrolyte; the positive electrode includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector, the positive active material layer comprising a binder polymer containing CF bonds; the electrolyte includes a lithium salt, the lithium salt including lithium bis(fluorosulfonyl)imide. The secondary battery satisfies: 0.2≤b×c≤9; Wherein, bwt% is the mass percentage of the lithium bis(fluorosulfonyl)imide in the electrolyte; and b satisfies: 4≤b≤18; cg / Ah is the ratio of the mass of the binder polymer containing CF bonds in the secondary battery to the rated capacity of the secondary battery; where c satisfies: 0.05 ≤ c ≤ 0.5; The electrolyte also includes additives, which include a first additive, a second additive, and a third additive. The first additive includes at least one of tris(trimethylsilane)borate, tris(trimethylsilane)phosphate, ethyl(2-fluoro)acetate, ethyl fluoroacetate, and ethylene fluorocarbonate. The second additive includes at least one of vinyl sulfate, 1,3-propane sulpholactone, methylene disulfonate, propenyl-1,3-sulfonate lactone, N,N'-thiodiimidazole, triphenyl phosphonite, n-butyric anhydride, hexanetrionitrile, succinate, and tetrafluoroterephthalonitrile. The third additive includes at least one of lithium bis(oxalato)borate, lithium difluorophosphate, lithium 4,5-dicyano-2-trifluoromethyl-imidazolium, and lithium trifluoromethanesulfonate.

2. The secondary battery according to claim 1, characterized in that, The condition b satisfies: 6 ≤ b ≤ 16.

3. The secondary battery according to claim 1, characterized in that, The lithium salt also includes lithium hexafluorophosphate, which accounts for 6 to 12 wt% of the electrolyte.

4. The secondary battery according to claim 3, characterized in that, The mass percentage of lithium difluorosulfonylimide in the electrolyte is greater than the mass percentage of lithium hexafluorophosphate in the electrolyte.

5. The secondary battery according to claim 1, characterized in that, The binder polymer containing CF bonds includes at least one of polyvinylidene fluoride, polytetrafluoroethylene, and fluorinated polyvinylidene fluoride.

6. The secondary battery according to claim 1, characterized in that, Satisfy at least one of the following (1) to (3): (1) The first additive accounts for 0.1~2 wt% of the electrolyte by mass; (2) The second additive accounts for 0.1~1.5 wt% of the electrolyte by mass; (3) The third additive accounts for 0.1 to 5 wt% of the electrolyte.

7. The secondary battery according to claim 1, characterized in that, The additive also includes a fourth additive, which comprises at least one of lithium difluorooxalatoborate and lithium difluorodioxalatophosphate.

8. The secondary battery according to claim 7, characterized in that, The fourth additive accounts for 0.05~0.5wt% of the electrolyte by mass.

9. An electrical device, characterized in that, The device includes the secondary battery as described in any one of claims 1 to 8, wherein the secondary battery serves as the power supply for the electrical device.

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