Electrolyte and secondary battery

Abstract

The application provides an electrolyte and a secondary battery. The electrolyte comprises an additive, an electrolyte and a solvent. The additive comprises a first additive. The electrolyte comprises lithium bisfluorosulfonylimide and lithium hexafluorophosphate. The first additive is selected from at least one of the compounds shown in formula I. The electrolyte comprises lithium bisfluorosulfonylimide, lithium hexafluorophosphate and the compound shown in formula I. The three components synergistically act, which is beneficial to improving the rate performance and high-temperature performance of the secondary battery, and is also beneficial to slowing down thermal runaway, effectively improving the critical temperature of thermal runaway and improving the thermal safety performance of the secondary battery.

Description

Technical Field

[0001] This application relates to the field of electrochemical technology, and in particular to an electrolyte and a secondary battery. Background Technology

[0002] Secondary batteries, such as lithium-ion batteries, are widely used in consumer electronics, power batteries, and energy storage batteries due to their advantages such as high energy density, long cycle life, and no memory effect. However, the safety issues caused by secondary batteries, such as fires and explosions, have gradually attracted social attention. In particular, the development trend of secondary batteries with large capacity and large size, such as 280Ah batteries, has exacerbated the occurrence of safety accidents.

[0003] Thermal runaway is a common and critical cause of safety issues in secondary batteries. It occurs when the rate of heat generation exceeds the rate of heat dissipation within the battery, triggering side reactions that spontaneously increase the battery's temperature. At relatively high temperatures, this can lead to severe exothermic redox reactions, generating substantial amounts of heat and causing a dangerous situation. Therefore, reducing the occurrence of exothermic reactions during thermal runaway and improving the thermal safety performance of secondary batteries is a pressing issue that needs to be addressed. Summary of the Invention

[0004] The purpose of this application is to provide an electrolyte and a secondary battery that improve the rate performance and high-temperature performance of the secondary battery, while also reducing the occurrence of exothermic reactions during thermal runaway and improving the thermal safety performance of the secondary battery. The specific technical solution is as follows:

[0005] A first aspect of this application provides an electrolyte comprising an additive, an electrolyte, and a solvent, wherein the additive comprises a first additive, and the electrolyte comprises lithium difluorosulfonylimide and lithium hexafluorophosphate, wherein the first additive is selected from at least one compound of formula I.

[0006]

[0007] In this context, A1 and A2 are each independently selected from a single bond, an oxygen atom, a C1-C3 alkylene group, or a C1-C3 alkoxide group, and n and m are each independently 0 or 1, and n and m are not simultaneously 0.

[0008] In some embodiments of this application, based on the mass of the electrolyte, the mass percentage of lithium bis(fluorosulfonyl)imide is A, 1% ≤ A ≤ 10%, preferably 3% ≤ A ≤ 7%, and the mass percentage of the first additive is B, 0.1% ≤ B ≤ 3%, preferably 0.5% ≤ B ≤ 1%.

[0009] In some embodiments of this application, A:B = (3-9):1.

[0010] In some embodiments of this application, the compound represented by Formula I is selected from at least one of the following compounds:

[0011]

[0012] In some embodiments of this application, the mass percentage of lithium hexafluorophosphate is D, based on the mass of the electrolyte, where 8% ≤ D ≤ 15%.

[0013] In some embodiments of this application, the solvent includes cyclic organic solvents and / or chain organic solvents, wherein the cyclic organic solvent is selected from at least one of ethylene carbonate and propylene carbonate, and the chain organic solvent is selected from at least one of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, and methyl propyl carbonate.

[0014] The second aspect of this application provides a secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte provided in the first aspect of this application.

[0015] In some embodiments of this application, the negative electrode sheet includes a negative current collector and a negative electrode material layer located on at least one surface of the negative current collector, the negative electrode material layer including a negative electrode active material, the negative electrode active material including graphite.

[0016] In some embodiments of this application, the positive electrode sheet includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector. The positive electrode material layer includes a positive electrode active material selected from at least one of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide, lithium manganese oxide, and lithium manganese iron phosphate.

[0017] The beneficial effects of this application are:

[0018] This application provides an electrolyte and a secondary battery. The electrolyte includes an additive, an electrolyte, and a solvent. The additive includes a first additive, and the electrolyte includes lithium bis(fluorosulfonyl)imide and lithium hexafluorophosphate. The first additive is selected from at least one compound shown in Formula I. The electrolyte, comprising lithium bis(fluorosulfonyl)imide, lithium hexafluorophosphate, and the compound shown in Formula I, works synergistically to improve the rate performance and high-temperature performance of the secondary battery, while also mitigating thermal runaway. This effectively raises the critical temperature for thermal runaway, thereby improving the thermal safety performance of the secondary battery.

[0019] Of course, implementing any product or method of this application does not necessarily require achieving all of the advantages described above at the same time. Attached Figure Description

[0020] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other embodiments can be obtained based on these accompanying drawings.

[0021] Figure 1 These are the exothermic test curves of differential scanning calorimetry (DSC) for Examples 1-4 of this application;

[0022] Figure 2 The differential scanning calorimetry (DSC) exothermic test curves are for comparative examples 1-4 of this application. Detailed Implementation

[0023] The technical solutions of this application will be clearly and completely described below with reference to the embodiments and accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0024] It should be noted that, in the specific embodiments of this application, lithium-ion batteries are used as an example of secondary batteries to explain this application, but the secondary batteries in this application are not limited to lithium-ion batteries.

[0025] A first aspect of this application provides an electrolyte comprising an additive, an electrolyte, and a solvent. The additive includes a first additive, and the electrolyte includes lithium bis(fluorosulfonyl)imide (LiFSI) and lithium hexafluorophosphate. The first additive is selected from at least one compound shown in Formula I.

[0026]

[0027] In this configuration, A1 and A2 are each independently selected from a single bond, an oxygen atom, a C1-C3 alkylene group, or a C1-C3 alkoxyene group, and n and m are each independently 0 or 1, and n and m are not simultaneously 0. For example, A1 and A2 can each be independently selected from a single bond, an oxygen atom, a methylene group, an ethylene group, a propylene group, a methoxy group, an ethoxy group, or a propoxy group.

[0028] The inventors discovered that LiFSI possesses excellent thermal stability and high ion conductivity, which is beneficial for improving the rate performance and high-temperature performance of secondary batteries. However, although LiFSI exhibits good thermal stability, during the operation of a secondary battery, if extreme failure occurs, at around 200°C, LiFSI will react with charged negative electrode materials (such as lithium graphite), releasing a large amount of heat, triggering thermal runaway, and posing a violent explosion hazard. The electrolyte in this application also includes LiPF6. When the battery experiences extreme failure behavior, LiPF6 decomposes at around 200°C, undergoing an endothermic reaction, thereby reducing the heat release during thermal runaway evolution and raising the critical temperature for thermal runaway. Furthermore, by introducing the compound shown in Formula I into the electrolyte, the large π bond on the benzene ring of the compound shown in Formula I affects the activity of the entire compound, lowering its energy barrier and facilitating the ring-opening reaction of sulfonyl lactones. This results in the formation of a solid electrolyte interphase (SEI) film rich in inorganic sulfates, inorganic sulfites, alkyl sulfonates, or alkyl lithium sulfates on the negative electrode surface during the battery formation stage, enhancing the interaction between the negative electrode and the electrolyte. The chemical stability at the electrolyte interface is enhanced. Furthermore, the opening of the SF and S=O bonds in LiFSI can decompose into inorganic salts such as LiF, Li2S, and Li2O. These substances can synergistically form a denser SEI layer with the products of the ring-opening reaction of sulfonyl lactone, further improving the thermal stability of the SEI film and enhancing the high-temperature performance of the secondary battery. Moreover, the thermally stable SEI film constructed from the products of the ring-opening reaction of LiFSI and sulfonyl lactone can effectively isolate the exothermic reaction between lithiated graphite and undecomposed LiFSI at around 200℃, mitigating thermal runaway and effectively raising the critical temperature for thermal runaway, thus improving the thermal safety performance of the secondary battery.

[0029] Based on this, when the additive is the compound shown in Formula I-3, the chain length of the molecules in the SEI film is increased when the six-membered ring is opened, thereby improving the thermal stability of the SEI film. The presence of O atoms in the six-membered ring also increases the Li2O concentration of the SEI film components. The Li2O-rich structure makes the SEI layer more compact, thereby improving thermal stability, effectively suppressing the exothermic reaction between lithiated graphite and undecomposed LiFSI, and slowing down thermal runaway.

[0030] The electrolyte of this application includes LiFSI, LiPF6, and a compound shown in Formula I. The synergistic effect of these three compounds improves the rate performance and high-temperature performance of the secondary battery, while also mitigating thermal runaway and enhancing the thermal safety of the secondary battery. "High temperature" as used in this application refers to a temperature greater than or equal to 45°C.

[0031] In some embodiments of this application, based on the mass of the electrolyte, the mass percentage of lithium difluorosulfonylimide is A, where 1% ≤ A ≤ 10%, preferably 3% ≤ A ≤ 7%, and the mass percentage of the first additive is B, where 0.1% ≤ B ≤ 3%, preferably 0.5% ≤ B ≤ 1%. For example, the mass percentage of lithium difluorosulfonylimide can be 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10%, or a range consisting of any two of these values; the mass percentage of the first additive can be 0.1%, 0.3%, 0.5%, 0.8%, 1%, 1.3%, 1.5%, 1.8%, 2%, 2.3%, 2.5%, 2.8%, 3%, or a range consisting of any two of these values. The electrolyte comprises LiFSI, LiPF6, and a compound of Formula I, and the mass percentages of LiFSI and the compound of Formula I are controlled within the scope of this application. The synergistic effect of these three compounds is beneficial for further improving the rate performance and high-temperature performance of the secondary battery, while also helping to mitigate thermal runaway and improve the thermal safety performance of the secondary battery.

[0032] In some embodiments of this application, A:B = (3-9):1. For example, the value of A:B can be 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, or 9:1. The electrolyte of this application includes LiFSI, LiPF6, and the compound shown in Formula I, and the mass percentage ratio of LiFSI to the compound shown in Formula I is controlled within the scope of this application. This is beneficial for further improving the rate performance and high-temperature performance of the secondary battery, while also helping to mitigate thermal runaway and improve the thermal safety performance of the secondary battery.

[0033] In some embodiments of this application, the compound represented by Formula I is selected from at least one of the following compounds:

[0034]

[0035] In some embodiments of this application, the mass percentage of lithium hexafluorophosphate (LiPF6) is D, based on the mass of the electrolyte, where 8% ≤ D ≤ 15%. For example, the mass percentage of LiPF6 can be 8%, 8.5%, 9%, 9.5%, 10%, 10.5%, 11%, 11.5%, 12%, 12.5%, 13%, 13.5%, 14%, 14.5%, 15%, or a range of any two of these values. By including LiPF6 in the electrolyte and controlling the mass percentage of LiPF6 within the scope of this application, the secondary battery can possess high ionic conductivity and good electrochemical stability. Simultaneously, when the battery experiences extreme failure behavior, LiPF6 decomposes at approximately 200°C, undergoing an endothermic reaction, thereby reducing the heat release during thermal runaway evolution, delaying thermal runaway, and raising the critical temperature for thermal runaway. This improves both the cycle performance and rate performance of the secondary battery, and enhances its thermal safety performance.

[0036] In some embodiments of this application, the solvent includes cyclic organic solvents and / or chain organic solvents. The cyclic organic solvents are selected from at least one of ethylene carbonate and propylene carbonate, and the chain organic solvents are selected from at least one of dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, and methyl propyl carbonate. The electrolyte, including solvents within the above range, can give the electrolyte suitable viscosity, high ionic conductivity, and good electrochemical stability, further improving the rate performance, high-temperature performance, and thermal stability of the secondary battery.

[0037] In some embodiments of this application, the electrolyte comprises lithium bis(fluorosulfonyl)imide, lithium hexafluorophosphate, a first additive, and the solvent described above. The mass percentages of lithium bis(fluorosulfonyl)imide, lithium hexafluorophosphate, and the first additive, based on the mass of the electrolyte, are as described above, and the mass percentage of the solvent is 72% to 90.9%.

[0038] In some embodiments of this application, the additive further includes vinylene carbonate (VC), wherein the mass percentage of vinylene carbonate is 0.5% to 5% based on the mass of the electrolyte, preferably 1% to 2%. For example, the mass percentage of vinylene carbonate can be 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, 4.2%, 4.5%, 4.8%, 5%, or a range of any two of these values. Based on the electrolyte including LiFSI and the compound shown in Formula 1, VC is further introduced and the mass percentage of VC is controlled within the scope of this application. Compared with other additives, VC can be reduced to form a stable SEI film on the negative electrode surface first, which can slow down the occurrence of thermal failure in the early stage of thermal runaway, further suppress the exothermic reaction between LiFSI and lithiated graphite, and further improve the high temperature performance and thermal safety performance of secondary batteries.

[0039] The second aspect of this application provides a secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte provided in the first aspect of this application.

[0040] In some embodiments of this application, the negative electrode sheet includes a negative current collector and a negative electrode material layer located on at least one surface of the negative current collector, the negative electrode material layer including a negative electrode active material, the negative electrode active material including graphite.

[0041] In this application, the negative electrode active material includes graphite, which is selected from at least one of natural graphite and artificial graphite. In some embodiments of this application, the negative electrode active material is entirely selected from graphite. In other embodiments of this application, the negative electrode active material may also include, but is not limited to, silicon-based materials, soft carbon, hard carbon, or mesophase microcarbon spheres, wherein the silicon-based material may include, but is not limited to, at least one of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, or silicon alloys.

[0042] In this application, 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. The phrase "the negative electrode material layer is disposed on at least one surface of the negative electrode current collector" means that the negative electrode material layer can be disposed on one surface of the negative electrode current collector along its thickness direction, or on two surfaces of the negative electrode current collector along its thickness direction. It should be noted that the "surface" here can be the entire surface area of ​​the negative electrode current collector, or only a portion of the surface area; this application has no particular limitation, as long as the purpose of this application is achieved. This application has no particular limitation on the negative electrode current collector, as long as the purpose of this application is achieved; for example, the negative electrode current collector can be copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, foamed nickel or foamed copper, aluminum foil, or a composite negative electrode current collector. The aforementioned composite negative electrode current collector can be a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The material of the polymer material base layer can be, but is not limited to, at least one of polypropylene (PP), polyethylene terephthalate (PET), or polybutylene terephthalate (PBT). The material of the metal layer can be, but is not limited to, at least one of copper, copper alloy, nickel, or nickel alloy. This application does not impose any particular limitation on the thickness of the negative electrode material layer and the negative electrode current collector, as long as the purpose of this application can be achieved. For example, the thickness of the single-sided negative electrode material layer can be 30 μm to 70 μm, and the thickness of the negative electrode current collector can be 3 μm to 10 μm.

[0043] The negative electrode material layer also includes a negative electrode conductive agent and a negative electrode binder. This application does not impose any particular limitation on the types of negative electrode conductive agents and negative electrode binders, as long as they achieve the purpose of this application. For example, the negative electrode conductive agent may include, but is not limited to, at least one of superconducting carbon black (Super P), acetylene black, Ketjen black, carbon nanotubes, graphene, or carbon fibers. The aforementioned carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The aforementioned carbon fibers may include, but are not limited to, vapor-grown carbon fibers (VGCF) and / or carbon nanofibers. For example, the negative electrode binder may include, but is not limited to, at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethyl methacrylate (PMAA), or carboxymethyl chitosan (CMCS). This application does not impose any particular limitation on the mass ratio of the negative electrode active material, negative electrode conductive agent, and negative electrode binder in the negative electrode material layer. Those skilled in the art can select according to actual needs, as long as the purpose of this application is achieved. In some embodiments of this application, the negative electrode material layer may further include a thickener, which may include, but is not limited to, sodium carboxymethyl cellulose (CMC). This application does not impose any particular limitation on the mass ratio of the negative electrode active material, negative electrode conductive agent, negative electrode binder, and thickener in the negative electrode material layer; those skilled in the art can select these components according to actual needs, as long as the purpose of this application is achieved.

[0044] In this application, there are no particular limitations on the preparation method of the negative electrode sheet, as long as it achieves the purpose of this application. For example, it can be prepared by the following method: adding negative electrode active material, negative electrode conductive agent, and negative electrode binder to deionized water and stirring evenly to obtain a negative electrode slurry with a solid content of 45wt% to 70wt%. The negative electrode slurry is uniformly coated on both surfaces of the negative electrode current collector, and after drying, a negative electrode sheet with a double-sided negative electrode material layer is obtained. Then, it is cold-pressed and cut to obtain the negative electrode sheet.

[0045] In some embodiments of this application, the positive electrode sheet includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector. The positive electrode material layer includes a positive electrode active material selected from at least one of lithium nickel cobalt manganese oxide (e.g., NCM811, NCM712, NCM622, NCM523, NCM111), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide, lithium manganese oxide, and lithium manganese iron phosphate. Preferably, the positive electrode active material is selected from lithium iron phosphate.

[0046] In this application, a lithium-ion battery includes a positive electrode sheet, which includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector. The phrase "positive electrode material layer disposed on at least one surface of the positive current collector" means that the positive electrode material layer can be disposed on one surface of the positive current collector along its thickness direction, or on two surfaces of the positive current collector along its thickness direction. It should be noted that the "surface" here can be the entire surface area of ​​the positive current collector, or only a portion thereof; this application does not have any particular limitation, as long as the purpose of this application is achieved. This application does not have any particular limitation on the positive current collector, as long as the purpose of this application is achieved; for example, the positive current collector can be an aluminum foil, an aluminum alloy foil, or a composite positive current collector. The aforementioned composite positive electrode current collector can be a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The material of the polymer material base layer can be, but is not limited to, at least one of polypropylene (PP), polyethylene terephthalate (PET), or polybutylene terephthalate (PBT). The material of the metal layer can be, but is not limited to, at least one of aluminum, aluminum alloy, nickel, or nickel alloy. This application does not impose any particular limitation on the thickness of the positive electrode material layer and the positive electrode current collector, as long as the purpose of this application can be achieved. For example, the thickness of the single-sided positive electrode material layer can be from 50 μm to 250 μm, and the thickness of the positive electrode current collector can be from 7 μm to 15 μm.

[0047] The positive electrode material layer may further include a positive electrode conductive agent and a positive electrode binder. This application does not impose any particular limitation on the types of positive electrode conductive agents and positive electrode binders, as long as they achieve the purpose of this application. For example, the positive electrode conductive agent may include, but is not limited to, at least one of superconducting carbon black (Super P), acetylene black, Ketjen black, carbon nanotubes, graphene, or carbon fiber. For example, the positive electrode binder may include, but is not limited to, at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, or fluorinated acrylate resin. This application does not impose any particular limitation on the mass ratio of the positive electrode active material, positive electrode conductive agent, and positive electrode binder in the positive electrode material layer. Those skilled in the art can select according to actual needs, as long as the purpose of this application is achieved.

[0048] In this application, there are no particular limitations on the preparation method of the positive electrode sheet, as long as it achieves the purpose of this application. For example, it can be prepared by the following method: mixing positive electrode active material, positive electrode conductive agent, and positive electrode binder, adding N-methylpyrrolidone (NMP) and stirring evenly to obtain a positive electrode slurry with a solid content of 50wt% to 85wt%. The positive electrode slurry is uniformly coated on both surfaces of the positive electrode current collector, and after drying, a positive electrode sheet with a double-sided coating of positive electrode material layer is obtained. The positive electrode sheet is then obtained by cold pressing and cutting.

[0049] The secondary battery of this application also includes a separator to separate the positive and negative electrode plates, prevent internal short circuits, allow electrolyte ions to pass freely, and not affect the electrochemical charging and discharging process. This application does not impose any particular limitation on the type of separator; any porous structure separator with good chemical and mechanical stability can be selected. For example, the separator material can include at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator type can include, but is not limited to, at least one of woven membrane, nonwoven membrane (nonwoven fabric), microporous membrane, composite membrane, rolled membrane, or spun membrane. The separator can be a single-layer thin film or a multi-layer composite thin film. In this application, the thickness of the separator is not particularly limited, as long as it achieves the purpose of this application; for example, the thickness can be from 5 μm to 20 μm.

[0050] In this application, the secondary battery also includes a casing for housing the positive electrode, separator, negative electrode, and electrolyte, as well as other components known in the field of lithium-ion batteries. This application does not limit the scope of these other components. This application does not impose any particular limitation on the casing; it can be a casing known in the art, as long as it achieves the purpose of this application. For example, the casing can be a rigid casing or a flexible casing. The material of the rigid casing can be metal; this application does not limit the type of metal and can use known metal rigid casings, as long as they achieve the purpose of this application. The flexible casing can be a metal plastic film, such as aluminum-plastic film, steel-plastic film, etc.

[0051] The secondary battery described in this application is not particularly limited and may include any device in which an electrochemical reaction occurs. In one embodiment of this application, the secondary battery may include, but is not limited to, lithium-ion secondary batteries (lithium-ion batteries), sodium-ion secondary batteries (sodium-ion batteries), etc.

[0052] The fabrication process of the secondary battery described in this application is well known to those skilled in the art, and this application does not impose any particular limitations. For example, the fabrication process of a lithium-ion battery may include, but is not limited to, the following steps: stacking the positive electrode, separator, and negative electrode in sequence, and performing operations such as winding and folding as needed to obtain a wound electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain a secondary battery. Alternatively, stacking the positive electrode, separator, and negative electrode in sequence, and then fixing the four corners of the entire stacked structure with tape to obtain a stacked electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain a secondary battery. In addition, overcurrent protection elements, conductive plates, etc., may be placed in the housing as needed to prevent pressure rise and overcharging / discharging inside the secondary battery.

[0053] The secondary battery of this application may be in the form of a single battery cell, a battery module, or a battery pack. A single battery cell can be assembled into a battery module, and a battery module may contain one or more battery cells; the specific number can be selected by those skilled in the art based on the application and capacity of the battery module. The battery modules of this application can also be assembled into a battery pack, and a battery pack may contain one or more battery modules; the specific number can be selected by those skilled in the art based on the application and capacity of the battery pack.

[0054] Example

[0055] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" are quality standards.

[0056] Test methods and equipment:

[0057] Ratio Performance Test

[0058] The lithium-ion battery was placed in a 25°C constant temperature test chamber and left to stand for 30 minutes to allow it to reach a constant temperature. It was then charged at a constant current of 3C to 3.65V, followed by constant voltage charging at 3.65V until the cutoff current was 0.05C. Finally, it was discharged at a constant current of 3C to 2V, and the initial discharge capacity was recorded as C0. This constitutes one charge-discharge cycle. The above charge-discharge cycle was repeated, and the number of cycles when the capacity decayed to 80% of the initial capacity was recorded. The charge-discharge cycle test instrument was a Newway BTS. A higher number of cycles indicates better rate performance of the lithium-ion battery.

[0059] When conducting rate performance tests on the lithium-ion batteries of Examples 1-21, Comparative Examples 1-2, and Comparative Examples 1-5, the upper limit voltage of 3.65V in the above steps was adjusted to 4.4V, and the lower limit voltage of 2V was adjusted to 3V. When conducting rate performance tests on the lithium-ion batteries of Examples 1-22, Comparative Examples 1-3, and Comparative Examples 1-6, the upper limit voltage of 3.65V in the above steps was adjusted to 4.45V, and the lower limit voltage of 2V was adjusted to 3V. The remaining examples and comparative examples were tested with an upper limit voltage of 3.65V and a lower limit voltage of 2V.

[0060] High-temperature cycling performance test

[0061] The lithium-ion battery was placed in a 45℃ constant temperature chamber and left to stand for 4 hours. Then, it was charged at a constant current of 1C to a voltage of 3.65V, followed by constant voltage charging at 3.65V to a current of 0.05C. Finally, it was discharged at a constant current of 1C to a voltage of 2.00V. The initial discharge capacity was recorded as C1. This constituted one charge-discharge cycle. This process was repeated 500 times, and the discharge capacity after the 500th cycle was recorded as C2. The charge-discharge cycle test instrument was a Xinwei BTS.

[0062] 45℃ capacity retention rate (%) = C2 / C1 × 100%; The 45℃ capacity retention rate is used to evaluate the high-temperature cycle performance of lithium-ion batteries. The higher the capacity retention rate, the better the high-temperature cycle performance of lithium-ion batteries.

[0063] When conducting high-temperature cycle performance tests on the lithium-ion batteries of Examples 1-21, Comparative Examples 1-2, and Comparative Examples 1-5, the upper limit voltage of 3.65V in the above steps was adjusted to 4.4V, and the lower limit voltage of 2V was adjusted to 3V. When conducting high-temperature cycle performance tests on the lithium-ion batteries of Examples 1-22, Comparative Examples 1-3, and Comparative Examples 1-6, the upper limit voltage of 3.65V in the above steps was adjusted to 4.45V, and the lower limit voltage of 2V was adjusted to 3V. The remaining examples and comparative examples were tested with an upper limit voltage of 3.65V and a lower limit voltage of 2V.

[0064] High-temperature storage performance test

[0065] A lithium-ion battery was placed at 25°C and charged at a constant current of 1C to 3.65V. Then, it was charged at a constant voltage of 3.65V to a current of 0.05C. The thickness of the lithium-ion battery was measured and recorded as D1. The battery was then placed in a 60°C explosion-proof oven and left to stand for 30 days. After the battery was removed and completely cooled, its thickness was measured again and recorded as D2. The thickness change rate is calculated as (D2 - D1) / D1 × 100%. A smaller thickness change rate indicates better high-temperature storage performance of the lithium-ion battery.

[0066] When conducting high-temperature storage performance tests on the lithium-ion batteries of Examples 1-21, Comparative Examples 1-2, and Comparative Examples 1-5, the upper limit voltage of 3.65V in the above steps was adjusted to 4.4V. When conducting high-temperature storage performance tests on the lithium-ion batteries of Examples 1-22, Comparative Examples 1-3, and Comparative Examples 1-6, the upper limit voltage of 3.65V in the above steps was adjusted to 4.45V. The remaining examples and comparative examples were tested with an upper limit voltage of 3.65V.

[0067] Heat release test

[0068] The lithium-ion battery was placed at 25°C and charged at a constant current of 1C to 3.65V, then charged at a constant voltage of 3.65V to a current of 0.05C to achieve a fully charged state. The battery was then disassembled in an argon-filled glove box (moisture <10ppm, oxygen <1ppm), the negative electrode was removed, cut, and 3mg of the negative electrode and 5μl of the corresponding electrolyte were weighed and placed in a 27μl DSC high-pressure crucible (Netschler). The crucible was then sealed using a press to obtain the sample. Differential scanning calorimetry (DSC) was then performed on the sample using a Netschler DSC214 differential scanning calorimeter. The heating range was 25°C to 400°C at a heating rate of 10°C / min. The main exothermic peak temperatures were recorded, and the heat of exothermic activity from 150°C to 350°C was calculated from the peak area. The higher the value of the main exothermic peak temperature, the higher the temperature at which thermal runaway occurs, and the more delayed the point at which thermal runaway occurs, meaning the better the thermal stability.

[0069] In Examples 1-21, Comparative Examples 1-2, and 1-5, the upper limit voltage of 3.65V in the above steps was adjusted to 4.4V before charging the lithium-ion batteries before the heat dissipation test. In Examples 1-22, Comparative Examples 1-3, and 1-6, the upper limit voltage of 3.65V in the above steps was adjusted to 4.45V before charging the lithium-ion batteries. The remaining examples and comparative examples were charged with an upper limit voltage of 3.65V.

[0070] Example 1-1

[0071] <Preparation of Electrolyte>

[0072] In an argon-filled glove box (moisture <10ppm, oxygen <1ppm), ethylene carbonate (EC), propylene carbonate (PC), and ethyl methyl carbonate (EMC) were mixed in a mass ratio of 32:4:45.7 to obtain a base solvent. Then, lithium bis(fluorosulfonyl)imide electrolyte (LiFSI), lithium hexafluorophosphate (LiPF6), the compound shown in Formula I-3, and ethylene carbonate were added to the base solvent and mixed thoroughly to obtain the electrolyte. Based on the mass of the electrolyte, the mass percentages of LiFSI, LiPF6, and EMC were 1%, 14%, 0.8%, and 2.5%, respectively. The remainder was the base solvent, of which EC accounted for 32%, PC for 4%, and EMC for 45.7%.

[0073] <Preparation of Negative Electrode Sheets>

[0074] Artificial graphite (anode active material), Super P (conductive agent), sodium carboxymethyl cellulose (CMC) (binder), and styrene-butadiene rubber (SBR) were mixed in a mass ratio of 95:1.5:1.5:2. Deionized water was added as a solvent to prepare a slurry with a solid content of 49 wt%. The slurry was stirred evenly in a vacuum mixer to obtain the anode slurry. The anode slurry was uniformly coated on both surfaces of a 9 μm thick copper foil anode current collector. After drying at 85°C, it was cold-pressed to obtain a double-sided coated anode electrode sheet. The areal density of the single-sided anode material layer was 8 mg / cm³. 2 The thickness of the single-sided negative electrode material layer is 50μm. Then, it is trimmed, cut into sheets, and slit. After slit, it is dried at 85℃ for 4 hours under vacuum. Then, the electrode tabs are welded to obtain a negative electrode sheet with a specification of 660mm×59mm for later use.

[0075] <Preparation of the positive electrode>

[0076] Lithium iron phosphate (LiFePO4), a positive electrode active material, Super P, a positive electrode conductive agent, and polyvinylidene fluoride (PVDF), a positive electrode binder, were mixed in a mass ratio of 96:2:2. N-methylpyrrolidone (NMP) was added as a solvent to prepare a slurry with a solid content of 55 wt%. After vacuum stirring, the slurry was obtained. The positive electrode slurry was uniformly coated onto both surfaces of a 12 μm thick aluminum foil used as a positive electrode current collector. After drying at 85°C, it was cold-pressed to obtain a positive electrode sheet with a double-sided coated positive electrode material layer. The areal density of the single-sided positive electrode material layer was 17.5 mg / cm³. 2 The thickness of the single-sided positive electrode material layer is 76.5 μm. The material is then trimmed, cut, and slit. After slitting, it is dried at 85°C under vacuum for 4 hours, and then tabs are welded to obtain positive electrode sheets with dimensions of 540 mm × 55 mm for later use.

[0077] <Preparation of the diaphragm>

[0078] A 16μm thick polypropylene (PP) porous membrane (provided by Shenzhen Xingyuan Material Technology Co., Ltd.) was used as the separator.

[0079] <Preparation of Lithium-ion Batteries>

[0080] The prepared positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to act as a separator. The electrode assembly is then wound to obtain the electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag and vacuum-baked at 85°C for 48 hours. The electrolyte prepared above is then injected, and the lithium-ion battery is obtained through vacuum sealing, settling, formation, and shaping processes.

[0081] Examples 1-2 to Examples 1-8

[0082] Except for the preparation of the electrolyte, in which the mass percentages of LiFSI and LiPF6 are adjusted according to Table 1, wherein the mass percentage of solvent EMC changes accordingly when the total mass percentage of LiFSI and LiPF6 changes, while the mass percentages of other components remain unchanged, the rest is the same as in Example 1-1.

[0083] Examples 1-9 to Examples 1-17

[0084] Except for the preparation of the electrolyte, in which the types and mass percentages of the compounds shown in Formula I are adjusted according to Table 1, and the mass percentage of the solvent EMC changes accordingly when the mass percentage of the compounds shown in Formula I changes, while the mass percentages of the other components remain unchanged, the rest is the same as in Examples 1-4.

[0085] Examples 1-18 to Examples 1-20

[0086] Except for the preparation of the electrolyte, in which the mass percentages of LiPF6 and LiFSI are adjusted according to Table 1, and the mass percentage of solvent EMC changes accordingly when the total mass percentage of LiPF6 and LiFSI changes, while the mass percentages of other components remain unchanged, the rest is the same as in Examples 1-4.

[0087] Examples 1-21

[0088] Except in the section on "Preparation of Positive Electrode Sheet", where the positive electrode active material is replaced with LiNi. 0.8 Co 0.1 Mn 0.1In the <Preparation of Electrolyte>, O2 (NCM811) was used without the addition of vinylene carbonate. The mass percentage of solvent EMC was changed accordingly. Except for the mass percentage of other components, the rest was the same as in Examples 1-4.

[0089] Examples 1-22

[0090] Except for replacing the positive electrode active material with LiCoO2 in the <Preparation of Positive Electrode Sheet>, not adding vinylene carbonate in the <Preparation of Electrolyte>, changing the mass percentage of solvent EMC accordingly, and keeping the mass percentage of other components unchanged, the rest are the same as in Examples 1-4.

[0091] Examples 1-23 to Examples 1-24

[0092] Except for the preparation of the electrolyte, in which the mass percentage of the compound shown in Formula I is adjusted according to Table 1, and the mass percentage of the solvent EMC changes accordingly when the mass percentage of the compound shown in Formula I changes, while the mass percentage of the other components remains unchanged, the rest is the same as in Examples 1-4.

[0093] Examples 1-25 to Examples 1-26

[0094] Except for the preparation of the electrolyte, in which the mass percentages of LiFSI, the compound shown in Formula I, and LiPF6 are adjusted according to Table 1, and the mass percentage of solvent EMC is changed accordingly, while the mass percentages of other components remain unchanged, the rest is the same as in Example 1-1.

[0095] Comparative Example 1-1

[0096] Except for <Preparation of Electrolyte>, where LiFSI was not added according to Table 1 and the mass percentage of LiPF6 was adjusted, the rest is the same as in Example 1-1.

[0097] Comparative Examples 1-2

[0098] Except for <Preparation of Electrolyte>, where LiFSI was not added according to Table 1 and the mass percentage of LiPF6 was adjusted, the rest is the same as in Examples 1-21.

[0099] Comparative Examples 1-3

[0100] Except for <Preparation of Electrolyte>, where LiFSI was not added according to Table 1 and the mass percentage of LiPF6 was adjusted, the rest is the same as in Examples 1-22.

[0101] Comparative Examples 1-4

[0102] Except for <Preparation of Electrolyte>, where the compound shown in Formula I was not added according to Table 1, the mass percentage of solvent EMC changed accordingly, and the mass percentages of other components remained unchanged, the rest was the same as in Examples 1-4.

[0103] Comparative Examples 1-5

[0104] Except for the preparation of the electrolyte, in which the compound shown in Formula I was not added according to Table 1, the mass percentage of solvent EMC changed accordingly, and the mass percentage of other components remained unchanged, the rest was the same as in Examples 1-21.

[0105] Comparative Examples 1-6

[0106] Except for <Preparation of Electrolyte>, where the compound shown in Formula I was not added according to Table 1, the mass percentage of solvent EMC changed accordingly, and the mass percentages of other components remained unchanged, the rest were the same as in Examples 1-22.

[0107] Comparative Examples 1-7

[0108] Except for the preparation of the electrolyte, in which LiPF6 was not added according to Table 1, the mass percentage of solvent EMC changed accordingly, and the mass percentages of other components remained unchanged, the rest was the same as in Examples 1-4.

[0109] The relevant parameters and performance parameters of each embodiment and comparative example are shown in Table 1.

[0110] Table 1

[0111]

[0112]

[0113] Note: In Table 1, " / " indicates that the corresponding substance or parameter does not exist.

[0114] As can be seen from Examples 1-1 to 1-26 and Comparative Examples 1-1 to 1-7, the electrolytes comprising lithium difluorosulfonylimide, lithium hexafluorophosphate, and the compound shown in Formula I within the scope of this application can enable lithium-ion batteries to exhibit higher cycle counts at 25°C, higher capacity retention at 45°C, lower thickness change rate, higher main exothermic peak temperature, and lower heat release, indicating that the lithium-ion batteries have good rate performance and high-temperature performance, as well as good thermal safety performance. The electrolytes of Comparative Examples 1-1 to 1-3 did not include lithium difluorosulfonylimide, and their lithium-ion batteries exhibited lower cycle counts at 25°C, lower capacity retention at 45°C, and higher thickness change rate, indicating poorer rate performance and high-temperature performance. Comparative Examples 1-4, 1-5, and 1-6 did not include the compound shown in Formula I, and their lithium-ion batteries exhibited lower main exothermic peak temperatures and higher heat release, indicating poorer thermal safety. Figure 1 and Figure 2 It can be seen that the main exothermic peak temperature of Examples 1-4 is 257.8℃, while the main exothermic peak temperature of Comparative Examples 1-4 is 220.9℃. Comparative Examples 1-7 do not include lithium hexafluorophosphate, and their lithium-ion batteries have a lower number of cycles at 25℃ and a higher heat release, indicating that the rate performance and thermal safety of the lithium-ion batteries are poor. This shows that the electrolyte does not meet the scope of this application, and the rate performance, high-temperature cycle performance, and thermal safety performance of the lithium-ion batteries cannot be simultaneously achieved.

[0115] The mass percentage of lithium difluorosulfonylimide affects the high-temperature performance, rate performance, and thermal safety performance of lithium-ion batteries. As can be seen from Examples 1-1 to 1-8, by controlling the mass percentage of lithium difluorosulfonylimide within the range specified in this application, lithium-ion batteries can exhibit higher cycle counts at 25°C, higher capacity retention at 45°C, lower thickness change rate, higher main exothermic peak temperature, and lower heat release. This indicates that the lithium-ion battery possesses good rate performance and high-temperature performance, while also exhibiting good thermal safety performance.

[0116] The mass percentage content and type of the compound shown in Formula I affect the high-temperature performance, rate performance, and thermal safety performance of lithium-ion batteries. As can be seen from Examples 1-4, 1-9 to 1-17, selecting the compound shown in Formula I within the scope of this application and adjusting the mass percentage content of the compound shown in Formula I within the scope of this application can result in lithium-ion batteries exhibiting higher cycle counts at 25°C, higher capacity retention at 45°C, lower thickness change rate, higher main exothermic peak temperature, and lower heat release. This indicates that the lithium-ion battery possesses good rate performance and high-temperature performance, as well as good thermal safety performance.

[0117] The ratio of the mass percentage A of lithium bis(fluorosulfonyl)imide to the mass percentage B of the compound shown in Formula I affects the high-temperature performance, rate performance, and thermal safety performance of lithium-ion batteries. As can be seen from Examples 1-1 to 1-26, by adjusting the ratio of A to B within the range of this application, lithium-ion batteries can exhibit higher cycle counts at 25°C, higher capacity retention at 45°C, lower thickness change rate, higher main exothermic peak temperature, and lower heat release. This indicates that the lithium-ion battery possesses good rate performance and high-temperature performance, while also exhibiting good thermal safety performance, thus improving the overall performance of the lithium-ion battery.

[0118] The mass percentage of lithium hexafluorophosphate affects the high-temperature performance, rate performance, and thermal safety performance of lithium-ion batteries. As can be seen from Examples 1-1 to 1-8 and Examples 1-18 to 1-20, by adjusting the mass percentage of lithium hexafluorophosphate within the range specified in this application, lithium-ion batteries can exhibit higher cycle counts at 25°C, higher capacity retention at 45°C, lower thickness change rate, higher main exothermic peak temperature, and lower heat release. This indicates that the lithium-ion battery possesses good rate performance and high-temperature performance, as well as good thermal safety performance.

[0119] The type of positive electrode active material affects the high-temperature performance, rate performance, and thermal safety performance of lithium-ion batteries. As can be seen from Examples 1-4, 1-21 to 1-22, and Comparative Examples 1-1 to 1-6, by selecting positive electrode active materials within the scope of this application and adjusting the mass percentage of the compound shown in Formula I and LiFSI within the scope of this application, lithium-ion batteries can exhibit higher cycle counts at 25°C, higher capacity retention at 45°C, lower thickness change rate, higher main exothermic peak temperature, and lower heat release. This indicates that lithium-ion batteries possess good rate performance and high-temperature performance, as well as good thermal safety performance.

[0120] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. An electrolyte comprising an additive, an electrolyte, and a solvent, said additive comprising a first additive, said electrolyte comprising lithium difluorosulfonylimide and lithium hexafluorophosphate, wherein the first additive is selected from at least one compound of formula I: , in, A1 and A2 are each independently selected from single bonds, oxygen atoms, C1-C3 alkylene groups, or C1-C3 alkoxide groups, and n and m are each independently 0 or 1, and n and m are not simultaneously 0; Based on the mass of the electrolyte, the mass percentage of lithium difluorosulfonyl imide is A, 1% ≤ A ≤ 10%; the mass percentage of the first additive is B, 0.3% ≤ B ≤ 3%; and the mass percentage of lithium hexafluorophosphate is D, 8% ≤ D ≤ 12%. Where A:B = (3~9):

1.

2. The electrolyte according to claim 1, wherein it satisfies at least one of the following conditions: （1）3%≤A≤7%； （2）0.5%≤B≤1%。 3. The electrolyte according to claim 1, wherein, The compound represented by Formula I is selected from at least one of the following compounds: 。 4. The electrolyte according to any one of claims 1 to 3, wherein, The solvent includes cyclic organic solvents and / or chain organic solvents, wherein the cyclic organic solvent is selected from at least one of ethylene carbonate and propylene carbonate, and the chain organic solvent is selected from at least one of dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, and methyl propyl carbonate.

5. A secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte according to any one of claims 1 to 4.

6. The secondary battery according to claim 5, wherein, The negative electrode sheet includes a negative current collector and a negative electrode material layer located on at least one surface of the negative current collector. The negative electrode material layer includes a negative electrode active material, which includes graphite.

7. The secondary battery according to claim 5, wherein, The positive electrode sheet includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector. The positive electrode material layer includes a positive electrode active material selected from at least one of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide, lithium manganese oxide, and lithium manganese iron phosphate.

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