Electrolyte, lithium ion battery and electric device

Abstract

The application provides an electrolyte, a lithium ion battery and a power device, the electrolyte comprising a solvent, a lithium salt and an additive; wherein the solvent comprises a fluoroether, a carbonate compound and a carboxylic acid ester compound, and the mass ratio of the fluoroether, the carbonate compound and the carboxylic acid ester compound is (10-20):(40-65):(20-45); and the additive comprises a first additive, and the first additive is a tetranitrile compound as shown in formula I. Compared with the prior art, the electrolyte of the application adds the fluoroether in the solvent, so that a SEI film with better toughness can be formed on the metal lithium negative electrode side, thereby improving the stability of the metal lithium negative electrode interface; meanwhile, the tetranitrile compound is also added as the additive, which can effectively improve the stability of the positive electrode, and cooperates with the fluoroether to effectively balance the transmission rate of lithium ions, effectively improves the lithium dendrite problem prone to occur in the metal lithium negative electrode system, and thereby improves the cycle life and safety performance of the battery.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion batteries, specifically to an electrolyte, a lithium-ion battery, and an electrical device. Background Technology

[0002] Lithium-ion batteries are widely used in many fields, such as electric vehicles, power tools, digital products, and energy storage power stations, due to their high energy density, excellent fast-charging performance, and good safety. Currently, graphite is the most commonly used anode material in lithium-ion batteries, characterized by high electronic conductivity and lithium-ion diffusion coefficient, low lithium intercalation potential, and readily available and inexpensive raw materials. However, graphite's highest energy density is only 372 mAh / g, which greatly limits the development of lithium-ion batteries. Therefore, there is an urgent need to develop next-generation lithium-ion battery anode materials.

[0003] Lithium metal has a theoretical specific capacity of 3860 mAh / g and excellent electronic conductivity, making it a viable alternative to graphite as a lithium-ion battery anode material. However, due to localized polarization, lithium metal deposition is accompanied by dendrite growth and other problems, leading to dead lithium and low coulombic efficiency. In severe cases, it can even puncture the separator, causing safety issues. Furthermore, lithium metal anodes undergo significant volume changes during charging and discharging, severely damaging the SEI film and causing electrolyte consumption, which greatly restricts the application of lithium metal anodes.

[0004] In view of this, it is indeed necessary to provide a technical solution to the above problems. Summary of the Invention

[0005] One of the objectives of this invention is to provide an electrolyte that addresses the shortcomings of existing technologies, thereby improving the cycle life and safety performance of lithium-ion batteries with lithium metal anodes, which are prone to lithium dendrite formation and SEI film damage due to changes in the volume of lithium metal.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] An electrolyte comprising a lithium salt, a solvent, and an additive; wherein,

[0008] The solvent includes fluorinated ethers, carbonate compounds, and carboxylic acid esters, wherein the mass ratio of the fluorinated ethers, carbonate compounds, and carboxylic acid esters is (10-20):(40-65):(20-45);

[0009] The additive includes a first additive, which is a tetranitrile compound as shown in Formula I.

[0010]

[0011] R1, R2, R3, and R4 are each independently selected from substituted or unsubstituted C1-C10 alkyl or alkoxy, C2-C10 alkenyl or alkenoxy alkynyl or alkynoxy, or one or more heteroatomic groups among N, S, and O. When substituted, the substituent is selected from F atoms.

[0012] Let the mass percentage of the fluorinated ether in the electrolyte be a%, the mass percentage of the carbonate compound in the electrolyte be b%, the mass percentage of the carboxylic acid ester compound in the electrolyte be c%, and the mass percentage of the first additive in the electrolyte be d. Then the following relationship is satisfied: (a+b+c)*3%≥d≥(a+b+c)*0.5%.

[0013] Preferably, the mass percentage of the fluoroether in the electrolyte is less than the mass percentage of the carboxylic acid ester compound in the electrolyte; and the mass percentage of the carboxylic acid ester compound in the electrolyte is less than the mass percentage of the carbonate compound in the solvent.

[0014] Preferably, the total mass percentage of fluoroethers, carbonate compounds, and carboxylic acid ester compounds in the electrolyte is greater than or equal to 50%.

[0015] Preferably, the carbonate compound is a fluorinated or non-fluorinated carbonate compound; the carboxylic acid ester compound is a fluorinated or non-carboxylic acid ester compound.

[0016] Preferably, the fluoroether has the following structural formula: The structural formula of carboxylic acid ester compounds is The structural formula of carbonate compounds is Among them, R5 and R6 are each independently selected from fluorinated C1-C10 alkyl or alkoxy groups, fluorinated C2-C10 alkenyl or alkenyloxy groups, and C2-C10 fluorinated alkynyl or alkynyloxy groups; R7-R 12 Each is independently selected from fluorinated or non-fluorinated C1-C10 alkyl or alkoxy groups, fluorinated or non-fluorinated C2-C10 alkenyl or alkenyloxy groups, and fluorinated or non-fluorinated C2-C10 alkynyl or alkynyloxy groups.

[0017] Preferably, the first additive is any one of the following structural formulas:

[0018]

[0019]

[0020] Preferably, the additive further includes a second additive, which is at least one selected from 1,3-propanesulfonate lactone, vinyl sulfate, fluorovinyl carbonate, tris(trimethylsilane)borate, and tris(trimethylsilane)phosphate, and the second additive accounts for 0.1% to 10% of the electrolyte by mass.

[0021] Preferably, the lithium salt is at least one selected from lithium hexafluorophosphate, lithium difluorophosphate, lithium difluorobis(oxalate)phosphate, lithium tetrafluoro(oxalate)phosphate, lithium bis(oxalate)borate, lithium difluoro(oxalate)borate, lithium tetrafluoroborate, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium perchlorate, and lithium bis(fluorosulfonyl)imide, and the lithium salt accounts for 0.1% to 25% of the mass of the electrolyte.

[0022] The second objective of this invention is to provide a secondary battery, comprising a positive electrode, a negative electrode, a separator spaced between the positive electrode and the negative electrode, and an electrolyte, wherein the electrolyte is the electrolyte described above, and the negative electrode is a lithium metal negative electrode.

[0023] A third objective of this invention is to provide an electrical device comprising the aforementioned secondary battery.

[0024] Compared to existing technologies, the beneficial effects of this invention are as follows: The electrolyte provided by this invention contains fluorinated ethers in the solvent, which allows for the formation of a more resilient SEI film on the side of the lithium metal anode, thereby improving the stability of the lithium metal anode interface; at the same time, tetranitrile compounds are added as additives, which, compared to conventional dinitrile and trinitrile additives, have a stronger complexing ability with transition metal ions, effectively improving the stability of the positive electrode. In addition, the stability of the anode interface is also enhanced. The synergistic effect of both can effectively balance the lithium ion transport rate, thereby effectively improving the problem of lithium dendrite formation in lithium metal anode materials, thus improving the cycle life and safety performance of lithium-ion batteries with lithium metal anodes. Detailed Implementation

[0025] To make the technical solution and advantages of the present invention clearer, the present invention and its beneficial effects will be described in further detail below in conjunction with specific embodiments, but the embodiments of the present invention are not limited thereto.

[0026] A first aspect of this invention aims to provide an electrolyte comprising a lithium salt, a solvent, and an additive; wherein,

[0027] The solvent includes fluorinated ethers, carbonate compounds, and carboxylic acid esters, wherein the mass ratio of the fluorinated ethers, carbonate compounds, and carboxylic acid esters is (10-20):(40-65):(20-45);

[0028] The additive includes a first additive, which is a tetranitrile compound as shown in Formula I.

[0029]

[0030] R1, R2, R3, and R4 are each independently selected from substituted or unsubstituted C1-C10 alkyl or alkoxy, C2-C10 alkenyl or alkenyloxy, C2-C10 alkynyl or alkynoxy, or one or more heteroatomic groups among N, S, and O. When substituted, the substituent is selected from F atoms.

[0031] Let the mass percentage of the fluorinated ether in the electrolyte be a%, the mass percentage of the carbonate compound in the electrolyte be b%, the mass percentage of the carboxylic acid ester compound in the electrolyte be c%, and the mass percentage of the first additive in the electrolyte be d. Then the following relationship is satisfied: (a+b+c)*3%≥d≥(a+b+c)*0.5%.

[0032] The electrolyte solvent includes fluorinated ethers. Compared to conventional non-fluorinated ether solvents (such as dimethyl ether (DME), fluorinated ethers can penetrate the solvation structure, altering its stability and allowing Li to enter the electrolyte. + It is easier to combine with anionic groups, and lithium salt additives in the electrolyte can precipitate on the positive and negative electrode surfaces, decomposing to produce interfacial film components such as Li3PO4, Li2CO3, and LiF, which makes the SEI film more tough, improves the interfacial stability of the lithium metal anode, and greatly reduces the occurrence of side reactions. The carbonate compounds contained have high dielectric constant, dissociation constant, and low viscosity, with a balanced performance; the carboxylic acid ester compounds have both high conductivity and oxidation resistance, which can improve the oxidation resistance of the electrolyte while maintaining a low viscosity.

[0033] Simultaneously, the mass ratio of fluorinated ethers to carbonate compounds and carboxylic acid esters is controlled within the range of (10-20):(40-65):(20-45). This ensures that the fluorinated ether content is neither too high, affecting other electrolyte properties, nor too low, failing to effectively improve SEI film stability, thus guaranteeing long-term cycling performance of the lithium metal anode. Specifically, the mass ratio of fluorinated ethers, carbonate compounds, and carboxylic acid esters can be 10:40:50, 10:45:45, 10:50:40, 10:55:35, 10:60:30, 10:65:25, 15:40:45, 15:50:35, 15:60:25, 20:40:40, 20:45:35, 20:50:30, 20:60:20, but is not limited to these ratios.

[0034] Furthermore, if the mass percentage of the first additive in the electrolyte is d%, then the following relationship is satisfied: (a+b+c)*3% ≥ d ≥ (a+b+c)*0.5%. The content of the first additive varies with the content of fluorinated ethers, carbonate compounds, and carboxylic acid esters. Satisfying the above relationship ensures that there are sufficient tetranitrile compounds to complex with transition metal ions, effectively ensuring the stability of the positive electrode. It also prevents excessive impedance due to excessive content, which would affect the lithium ion transport rate. Combined with the fluorinated ether solvent within the specified content, it can effectively improve the problem of lithium dendrite formation in the lithium metal anode system.

[0035] Preferably, the total mass percentage of fluorinated ethers, carbonate compounds, and carboxylic acid esters in the electrolyte is greater than or equal to 50%. This ensures that a certain amount of tetranitrile compounds can complex with transition metal ions, guaranteeing the stability of the positive electrode. Combined with the fluorinated ether solvent in the specified content, it improves the problem of lithium dendrite formation in the lithium metal anode system, effectively balancing the lithium ion transport rate in the lithium metal anode system lithium-ion battery, thereby improving battery performance.

[0036] In some embodiments, the mass percentage of the fluorinated ether in the solvent is less than the mass percentage of the carboxylic acid ester compound in the electrolyte; and the mass percentage of the carboxylic acid ester compound in the electrolyte is less than the mass percentage of the carbonate compound in the solvent. That is, with the volume percentage of the fluorinated ether in the electrolyte being a%, the volume percentage of the carbonate compound in the electrolyte being b%, and the volume percentage of the carboxylic acid ester compound in the electrolyte being c%, a < c, c < b. Specifically, the preferred mass ratio of the fluorinated ether, carbonate compound, and carboxylic acid ester compound is 10:50:40, 10:55:35, 10:60:30, 10:65:25, 15:50:35, 15:60:25, 20:45:35, or 20:50:30, but it is not limited to these.

[0037] In some embodiments, the fluoroether has the following structural formula: R5 and R6 are each independently selected from fluorinated C1-C10 alkyl or alkoxy groups, fluorinated C2-C10 alkenyl or alkenyloxy groups, and C2-C10 fluorinated alkynyl or alkynyloxy groups. Preferably, they contain at least difluorinated substitution. More preferably, they contain at least tetrafluorinated substitution.

[0038] Specifically, fluoroethers can be at least one of the following structural formulas:

[0039]

[0040] In some embodiments, the carbonate compound is a fluorinated or non-fluorinated carbonate compound; it may be a fluorinated or non-fluorinated linear carbonate compound with the structural formula [insert structural formula here]. It can also be a fluorinated or non-fluorinated cyclic carbonate compound. R9~R 12 Each is independently selected from fluorinated or non-fluorinated C1-C10 alkyl or alkoxy groups, fluorinated or non-fluorinated C2-C10 alkenyl or alkenyloxy groups, and fluorinated or non-fluorinated C2-C10 alkynyl or alkynyloxy groups.

[0041] Specifically, non-fluorinated linear carbonate compounds may be at least one of ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and methyl propyl carbonate (MPC).

[0042] Fluorinated linear carbonate compounds may be at least one of the following structural formulas:

[0043]

[0044] Non-fluorinated cyclic carbonate compounds may be at least one of ethylene carbonate (EC), propylene carbonate (PC), and butene carbonate (BC).

[0045] Fluorinated cyclic carbonate compounds may be at least one of the following structural formulas:

[0046] In some embodiments, the carboxylic acid ester compound is a fluorinated or non-carboxylic acid ester compound with the structural formula shown below. R7 to R8 are each independently selected from fluorinated or non-fluorinated C1 to C10 alkyl or alkoxy groups, fluorinated or non-fluorinated C2 to C10 alkenyl or alkenyloxy groups, and fluorinated or non-fluorinated C2 to C10 alkynyl or alkynyloxy groups.

[0047] Specifically, the non-fluorinated carboxylic acid ester compound may be at least one of methyl formate (MF), propyl propionate (PP), ethyl acetate (EA), ethyl formate (EF), propyl formate (PF), butyl formate (BF), methyl acetate (MA), propyl formate (PA), methyl propionate (MP), ethyl propionate (EP), ethyl butyrate (EB), propyl butyrate (PB), and γ-butyrolactone (GBL).

[0048] Fluorocarboxylic acid esters may be at least one of the following structural formulas:

[0049]

[0050] In addition, the electrolyte of this invention also incorporates a tetranitrile compound with the structure of Formula I as an electrolyte additive. Compared with conventional dinitrile additives (such as adiponitrile ADN, butadionitrile SN) and trinitrile additives (such as 1,3,6-hexanetrionitrile HTCN), the tetranitrile compound with this structure has a stronger complexing ability with transition metal ions and works synergistically with fluorinated ether solvents. It is suitable for lithium-ion batteries with lithium metal anode systems, can effectively balance the lithium ion transport rate, and effectively improve the problem of lithium dendrite formation that is prone to occur in lithium metal anode systems, thereby improving the cycle life and safety performance of lithium-ion batteries with lithium metal anode systems.

[0051] In some embodiments, the first additive is any one of the following structural formulas:

[0052]

[0053] In some embodiments, the additive further includes a second additive, which is at least one selected from 1,3-propanesulfonate lactone (PS), vinyl sulfate (DTD), fluoroethylene carbonate (FEC), tris(trimethylsilane)borate (TMSB), and tris(trimethylsilane)phosphate (TMSP). The second additive accounts for 0.1% to 10% of the electrolyte by mass; preferably, the second additive accounts for 3% to 8% of the electrolyte by mass.

[0054] Among them, 1,3-propanesulfonate lactone (PS) can be used as a second additive for both positive and negative electrodes to improve battery cycle performance and suppress gas generation during high-temperature storage; vinyl sulfate (DTD), as a second additive for both positive and negative electrodes, can also improve battery cycle performance; fluoroethylene carbonate (FEC), as a second additive for the negative electrode, can repair damaged SEI films; tris(trimethylsilane)borate ester (TMSB), as a second additive for the negative electrode, has the effect of reducing impedance, and when used in combination with phosphite additives, it can effectively improve SEI stability and cycle performance; tris(trimethylsilane)phosphate ester (TMSP), as a second additive for the negative electrode, also has the effect of reducing impedance, improving SEI stability, and improving cycle performance. Preferably, the inventors have found that the combination of PS, DTD, and tetranitrile compounds is more suitable for lithium-ion batteries with a lithium metal negative electrode system, and the electrochemical performance of the battery is superior compared to the combination with other additives.

[0055] In some embodiments, the lithium salt is at least one selected from lithium hexafluorophosphate, lithium difluorophosphate, lithium difluorobis(oxalato)phosphate, lithium tetrafluoro(oxalato)phosphate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium tetrafluoroborate, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium perchlorate, and lithium bis(fluorosulfonyl)imide, and the lithium salt accounts for 0.1% to 25% of the electrolyte by mass. Preferably, the lithium salt accounts for 5% to 20% of the electrolyte by mass. More preferably, the lithium salt accounts for 10% to 18% of the electrolyte by mass.

[0056] A second aspect of the present invention aims to provide a secondary battery, comprising a positive electrode, a negative electrode, a separator spaced between the positive electrode and the negative electrode, and an electrolyte, wherein the electrolyte is the electrolyte described above, and the negative electrode is a lithium metal negative electrode.

[0057] In some embodiments, the positive electrode includes a positive current collector and a positive active layer coated on at least one surface of the positive current collector. The positive active layer further includes a positive active material, which may be, but is not limited to, a chemical formula such as Li. x Ni h Co y M z O 2-d N d (where 0.95≤x≤1.2, h>0, y≥0, z≥0, and h+y+z=1, 0≤d≤1, M is selected from one or more combinations of Mn and Al, and N is selected from one or more combinations of F, P, and S) The positive electrode active material may also be, but is not limited to, LiCoO2, LiNiO2, LiVO2, LiCrO2, LiMn2O4, LiCoMnO4, Li2NiMn3O8, LiNi 0.5 Mn 1.5 The positive electrode active material can be one or more combinations of O4, LiCoPO4, LiMnPO4, LiFePO4, LiNiPO4, LiCoFSO4, CuS2, FeS2, MoS2, NiS, and TiS2. The positive electrode active material can also be modified. Methods for modifying the positive electrode active material are known to those skilled in the art. For example, coating, doping, and other methods can be used to modify the positive electrode active material. The materials used for modification can be one or more combinations of Al, B, P, Zr, Si, Ti, Ge, Sn, Mg, Ce, and W, including but not limited to. The positive electrode current collector can be any material suitable for use as a positive electrode current collector in lithium-ion batteries. For example, the positive electrode current collector can be, but is not limited to, metal foil, and more specifically, aluminum foil, among others.

[0058] The separator can be any material suitable for lithium-ion battery separators in the art, such as, but not limited to, one or more of polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester, and natural fibers.

[0059] A third aspect of the present invention aims to provide an electrical device comprising the aforementioned secondary battery.

[0060] The electrical devices can be vehicles, mobile phones, portable devices, laptops, ships, spacecraft, electric toys, and power tools, etc. Vehicles can be gasoline-powered cars, natural gas-powered cars, or new energy vehicles. New energy vehicles can be pure electric vehicles, hybrid electric vehicles, or range-extended electric vehicles, etc. Spacecraft include airplanes, rockets, space shuttles, and spacecraft, etc. Electric toys include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc. Power tools include metal cutting power tools, grinding power tools, assembly power tools, and railway power tools, such as electric drills, electric grinders, electric wrenches, electric screwdrivers, electric hammers, impact drills, concrete vibrators, and electric planers, etc.

[0061] To better verify the excellent performance of the electrolyte provided by the present invention, the present invention and its beneficial effects will be further described in detail below with reference to specific embodiments, but the embodiments of the present invention are not limited thereto.

[0062] Example 1

[0063] An electrolyte comprising lithium salt, solvent, and additives, specifically composed and prepared as follows: In an argon-filled glove box, ethylene carbonate (EC): ethyl methyl carbonate (DEC): propyl propionate (PP): 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (HFE, i.e., the above structural formula M3) are mixed in a mass ratio of 20:30:40:10, with the total volume of solvent being 100%. Then, lithium hexafluorophosphate (LiPF6) is slowly added to the mixed solution at 14.5 wt% based on the total weight of the electrolyte. Finally, a tetranitrile compound with structural formula A1, 0.4 wt% 1,3-propanesulfonic acid lactone (PS), and 1 wt% vinyl sulfate (DTD) are added based on the total weight of the electrolyte. After stirring evenly, the electrolyte of this embodiment is obtained.

[0064] By adjusting the composition and proportion of solvent, lithium salt and additives according to the above composition, Examples 2-15 and Comparative Examples 1-9 were obtained. The specific composition is shown in Table 1 below.

[0065] Table 1

[0066]

[0067]

[0068]

[0069] The electrolytes obtained in Examples 1-15 and Comparative Examples 1-9 were applied to lithium-ion batteries.

[0070] Preparation of lithium-ion batteries:

[0071] 1) Positive electrode sheet: The active material LiCoO2 (manufacturer: Xiamen Tungsten New Energy): lithium supplementer Li5FeO4: conductive agent Super P: polyvinylidene fluoride PVDF = 96:1.7:1.3:1 by weight, is mixed with N-methylpyrrolidone (NMP) and mixed evenly to prepare a lithium-ion battery positive electrode slurry; the positive electrode slurry is coated on both sides of the current collector aluminum foil, dried at 85°C and then cold-pressed, then trimmed, cut into sheets and slits, and dried under vacuum at 85°C for 4 hours, and then the tabs are welded to prepare the lithium-ion battery positive electrode sheet.

[0072] 2) The negative electrode is a lithium metal negative electrode.

[0073] 3) Preparation of soft-pack battery: The prepared positive electrode sheet, separator and negative electrode sheet are stacked in sequence, with the separator in the middle of the positive and negative electrode sheets, and wound to obtain a bare cell; the bare cell is placed in an aluminum-plastic film outer packaging, the electrolyte prepared above is injected into the dried battery, and then it is packaged, left to stand, formed, shaped and capacity tested to complete the preparation of lithium-ion battery.

[0074] The electrochemical performance of the lithium-ion batteries obtained in Examples 1-15 and Comparative Examples 1-9 was tested.

[0075] Performance testing:

[0076] 1) Room temperature cycle performance test: In a 25℃ environment, the battery after capacity gradation is charged to 4.45V at a constant current and constant voltage of 0.7C, with a cutoff current of 0.05C, and then discharged to 3.0V at a constant current of 0.5C. This cycle is repeated for 500 charge-discharge cycles. The capacity retention rate at the 500th cycle is calculated using the following formula:

[0077] 500-week cycle capacity retention (%) = (500-week cycle discharge capacity / initial cycle discharge capacity) × 100%.

[0078] 2) 60℃ 14d High-Temperature Storage Test: The battery was placed at room temperature and charged and discharged once at 0.5C (4.45V-3.0V). The discharge capacity C0 before storage was recorded. Then, the battery was charged to a full state of 4.45V using constant current and constant voltage. The thickness d1 of the battery before high-temperature storage was measured using a PPG battery thickness gauge (500g). The battery was then stored in a 60℃ constant temperature chamber for 14 days. After storage, the battery was removed and the thermal thickness d2 after storage was measured. The battery thickness expansion rate after 14 days of storage at 60℃ was calculated. After the battery cooled at room temperature for 24 hours, it was discharged again at 0.5C to 3.0V using constant current, and then charged to 4.45V at 0.5C using constant current and constant voltage. The discharge capacity C1 and charging capacity C2 after storage were recorded. The remaining capacity and recovery rate of the battery after 14 days of storage at 60℃ were calculated using the following formulas:

[0079] Thickness expansion rate after storage at 60℃ for 14 days = (d2-d1) / d1*100%;

[0080] The remaining capacity after storage at 60℃ for 14 days = C1 / C0*100%;

[0081] Capacity recovery rate after 14 days of storage at 60℃ = C2 / C0*100%.

[0082] 3) DCR (Direct Current Resistance) Test: At room temperature (23℃±3℃), maintain a constant current and voltage of 0.5C to 4.45V, with a cutoff current of 0.02C. Then discharge at 0.1C for 9 hours (adjusted to 10% SOC), followed by a 0.1C discharge for 10 seconds. Record the ending voltage V1. Discharge at 1C for 1 second and record the ending voltage V2. The calculation formula is as follows:

[0083] DCR = (V1 - V2) / (1C - 0.1C).

[0084] 4) Thermal shock performance: Under 25℃ ambient conditions, discharge to 3.0V with a given current of 0.2C; rest for 5 minutes; charge to 4.45V with a charging current of 0.2C. When the cell voltage reaches 4.45V, switch to constant voltage charging at 4.45V until the charging current is less than or equal to the given cutoff current of 0.05C; after resting for 1 hour, put the cell into an oven. The oven temperature is increased to 135±2℃ at a rate of 5±2℃ / min and maintained for 30 minutes before stopping. The judgment criterion is that the cell does not catch fire or explode.

[0085] The experimental results are shown in Table 2 below.

[0086] Table 2

[0087]

[0088]

[0089] The results of Examples 1-15 and Comparative Examples 1-9 show that the electrolyte of the present invention simultaneously adds fluorinated ether solvents and tetranitrile additives, and controls the fluorinated ethers, carbonate compounds and carboxylic acid ester compounds within the above range. This electrolyte system is compatible with lithium-ion batteries with lithium metal anodes, effectively improves the lithium dendrite problem of lithium metal anodes, stabilizes the positive and negative electrodes, and effectively improves the cycle performance, high-temperature performance and safety performance of lithium-ion batteries with lithium metal anodes.

[0090] The comparisons of Examples 1, 5-6, and Comparative Examples 5-9 show that, under the premise of controlling the content of fluorinated ethers, carbonate compounds, and carboxylic acid esters, adjusting the content of tetranitrile additives results in lithium-ion batteries with metal lithium anodes exhibiting better cycle performance, high-temperature performance, and safety performance. In particular, when the mass ratio of fluorinated ethers, carbonate compounds, and carboxylic acid esters is 10:50:40, the battery with 0.5 wt% tetranitrile additive shows even better performance. Under this ratio, selecting tetranitrile additives with different structures also results in lithium-ion batteries with metal lithium anodes exhibiting relatively good cycle performance, high-temperature performance, and safety performance, as shown in the test results comparison of Examples 1-4.

[0091] Furthermore, a comparison between Examples 6 and Examples 9-12 also verifies that when tetranitrile additives are matched with a mass ratio of fluorinated ethers, carbonate compounds, and carboxylic acid esters of 10:50:40, lithium-ion batteries with metal lithium anodes exhibit better cycle performance, high-temperature performance, and safety performance. Through numerous experiments, the inventors have verified that maintaining a lower mass percentage of fluorinated ethers in the electrolyte than that of carboxylic acid esters in the solvent, and a lower mass percentage of carboxylic acid esters in the electrolyte than that of carbonate compounds in the electrolyte, is more conducive to improving various battery performance characteristics.

[0092] In summary, the electrolyte provided by this invention has good compatibility with lithium-ion batteries with a lithium metal anode system, and can effectively improve the cycle performance, high-temperature performance and safety performance of the battery system, and extend the battery's service life.

[0093] Based on the disclosure and teachings of the foregoing specification, those skilled in the art can make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the specific embodiments described above, and any obvious improvements, substitutions, or modifications made by those skilled in the art based on the present invention are within the scope of protection of the present invention. Furthermore, although some specific terms are used in this specification, these terms are only for convenience of explanation and do not constitute any limitation on the present invention.

Claims

1. An electrolyte, characterized by, The electrolyte comprises a solvent, a lithium salt and an additive; wherein The solvent comprises a fluorinated ether, a carbonate compound and a carboxylic ester compound, and the mass ratio of the fluorinated ether, the carbonate compound and the carboxylic ester compound is 10:50:40; The mass proportion of the fluorinated ether in the electrolyte is less than that of the carboxylic ester compound, and the mass proportion of the carboxylic ester compound in the electrolyte is less than that of the carbonate compound; The additive comprises a first additive, and the first additive is any one of the following structural formulae: The carboxylic ester compound is at least one of the following structural formulae: The mass proportion of the fluorinated ether in the electrolyte is a%, the mass proportion of the carbonate compound in the electrolyte is b%, the mass proportion of the carboxylic ester compound in the electrolyte is c%, and the mass proportion of the first additive in the electrolyte is d%, and the following relationship is met: (a+b+c)*3% ≥ d ≥ (a+b+c)*0.5%.

2. The electrolyte according to claim 1, characterized in that, The sum of the mass proportions of the fluorinated ether, the carbonate compound and the carboxylic ester compound in the electrolyte is greater than or equal to 50%.

3. The electrolyte according to any one of claims 1 to 2, characterized in that The carbonate compound is a fluorinated or non-fluorinated carbonate compound.

4. The electrolyte according to claim 3, characterized in that, The fluorinated ether has a structural formula of The carbonate compound has a structural formula of wherein R5and R6are each independently selected from a fluorine-substituted C1-C10alkyl or alkoxy group, a fluorine-substituted C2-C10alkenyl or alkenyloxy group, a C2-C10fluorine-substituted alkynyl or alkynyloxy group; R9-R 12 each independently selected from a fluorine-substituted or non-fluorine-substituted C1-C10alkyl or alkoxy group, a fluorine-substituted or non-fluorine-substituted C2-C10alkenyl or alkenyloxy group, a fluorine-substituted or non-fluorine-substituted C2-C10alkynyl or alkynyloxy group.

5. The electrolyte of claim 1, wherein The additive further comprises a second additive, and the second additive is at least one of 1,3-propane sultone, ethylene sulfate, fluorinated ethylene carbonate, tris(trimethylsilyl)borate, tris(trimethylsilyl)phosphate, and the mass proportion of the second additive in the electrolyte is 0.1% to 10%.

6. The electrolyte of claim 1, wherein The lithium salt is at least one of lithium hexafluorophosphate, lithium difluorophosphate, lithium difluorobisoxalate phosphate, lithium tetrafluorooxalate phosphate, lithium bisoxalate borate, lithium difluorooxalate borate, lithium tetrafluoroborate, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium perchlorate and lithium bisfluorosulfonylimide, and the mass proportion of the lithium salt in the electrolyte is 0.1% to 25%.

7. A secondary battery comprising a positive electrode sheet, a negative electrode sheet, a separator interposed between the positive electrode sheet and the negative electrode sheet, and an electrolyte, characterized by, The electrolyte is the electrolyte of any one of claims 1 to 6, and the negative electrode piece is a metal lithium negative electrode piece.

8. An electrical device, characterized by The secondary battery comprises the secondary battery of claim 7.

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