Electrolyte, preparation method thereof, battery and electric device

Abstract

This invention belongs to the field of battery technology, and particularly relates to an electrolyte, its preparation method, a battery, and an electrical device. The electrolyte comprises a flame retardant and a mixed solvent. The flame retardant is PFPN, and the mixed solvent is a carbonate-based solvent. The concentration of the flame retardant in the electrolyte is 5 wt% to 16 wt%. The mixed solvent is composed of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC), with the volume fraction of EC in the mixed solvent being 20% ​​to 50%. The electrolyte of this invention can effectively improve the needle penetration safety of lithium-ion batteries while maintaining their high performance.

Description

Technical Field

[0001] This invention belongs to the field of battery technology, and particularly relates to an electrolyte and its preparation method, a battery, and an electrical device. Background Technology

[0002] The 21700 cylindrical lithium-ion battery (21 mm in diameter, 70 mm in height) is characterized by its high energy density (single cell capacity 4.5–5.5 Ah, energy density >300 Wh kg). -1 With its advantages of high consistency and mature manufacturing process, it has already achieved large-scale commercial application in electric vehicles (such as Tesla Model 3 / Y), energy storage systems, and high-power tools, becoming one of the largest cylindrical battery specifications in terms of industrial scale. This provides a solid foundation for its application in a wider range of fields.

[0003] The nail penetration test is one of the core tests recognized internationally for evaluating the safety performance of lithium-ion batteries. The nail penetration test simulates the internal short circuit behavior of a lithium-ion battery when it is punctured by a foreign object by piercing it with a steel needle, thereby assessing the degree of thermal runaway risk and safety protection capability of the battery under current extreme conditions.

[0004] At the moment a steel needle punctures a battery, a large amount of joules are generated inside the battery due to a short circuit. This heat is released in a very short time, triggering an oxidation decomposition reaction at the interface between the battery's positive electrode and the electrolyte. The reaction process produces O2. Most of the electrolytes used in existing lithium batteries are carbonate electrolytes, including EC (ethylene carbonate), DMC (dimethyl carbonate), and EMC (ethyl methyl carbonate). Carbonate electrolytes have a flash point of 20-25°C. When they come into contact with O2, they will burn rapidly and may even cause an explosion, resulting in a high risk of thermal runaway.

[0005] Compared to traditional 18650 lithium-ion batteries, 21700 cylindrical lithium-ion batteries have more winding layers, larger effective electrode area, and more concentrated energy density. This also means that after a short circuit caused by a needle puncture, the thermal runaway propagation rate of 21700 cylindrical lithium-ion batteries is much higher than that of 18650 batteries. The highest temperature after a needle puncture often exceeds 600°C, and the flame duration can reach more than 20 seconds, resulting in a relatively higher safety risk.

[0006] With the widespread application of high-nickel ternary cathode materials (NCM811, NCA) in 21700 batteries, the single-cell energy density of 21700 batteries has been further improved. However, this has also led to an increasingly prominent contradiction between the highly active oxidation state of the cathode, the highly flammable electrolyte, and the thermal runaway chain reaction of 21700 batteries, which seriously restricts its expanded application in safety-sensitive fields such as aerospace, special equipment, and medical emergency.

[0007] Regarding the safety issues of 21700 batteries, the industry has developed a variety of safety countermeasures, mainly including: (1) Modification of cathode material: By coating the surface of cathode material with inert oxide layers such as Al2O3 and ZrO2, the oxygen production temperature of cathode can be effectively delayed. However, this method can only act on the surface of cathode material and cannot fundamentally solve the core problem of the flammability of electrolyte itself.

[0008] (2) Membrane coating modification: The battery separator is coated with ceramic coating (Al2O3 / SiO2) to improve the thermal shut-off temperature of the separator. When the battery is slightly overheated or has a local short circuit, the separator can block ion transport and suppress the further expansion of the internal short circuit. However, in the case of severe internal short circuit induced by puncture, the ceramic coating separator still cannot prevent the electrolyte from contacting the oxygen generated by the positive electrode and causing fire.

[0009] (3) Solid / quasi-solid electrolyte: All-solid electrolytes (such as oxides, polymers, sulfides, etc.) are used to replace traditional liquid electrolytes. Although the risk of combustion of liquid electrolytes can be fundamentally eliminated, all-solid electrolytes have disadvantages such as low ionic conductivity, high interfacial impedance and immature manufacturing process compared with liquid electrolytes. There is still a long way to go before the large-scale application of 21700 cylindrical batteries.

[0010] (4) Electrolyte flame retardant additives: Introducing flame retardant additives into carbonate-based electrolytes without changing the existing manufacturing process is currently the most economical and easily achievable way to improve safety performance. Among them, phosphate esters (triethyl phosphate TEP, dimethyl methyl phosphate DMMP) are the most widely studied type of flame retardant additives. Their flame retardant mechanism is as follows: During the combustion process of the electrolyte, the flame retardant additive decomposes in the gas phase to generate PO• free radicals, which interrupt the continuous combustion reaction by capturing the active intermediates in the thermal runaway chain reaction; however, this type of flame retardant additive has high viscosity and a narrow electrochemical window (<4.4V vs. Li / Li). + It has poor compatibility with high-nickel cathode materials and graphite anodes, which will lead to the deterioration of the battery SEI film (solid electrolyte interface film) and rapid capacity decay. Typically, after 400 cycles, the battery capacity retention rate will be less than 80%, which is difficult to meet the actual application requirements of 21700 cylindrical batteries.

[0011] In summary, existing methods cannot effectively improve the needle penetration safety of 21700 cylindrical lithium-ion batteries while maintaining their high performance. Summary of the Invention

[0012] To address the shortcomings of existing technologies, this invention provides an electrolyte, its preparation method, a battery, and an electrical device. This electrolyte can effectively improve the needle penetration safety of lithium-ion batteries while maintaining their high performance.

[0013] In a first aspect, the present invention provides an electrolyte comprising a flame retardant and a mixed solvent, wherein the flame retardant is PFPN and the mixed solvent is a carbonate-based solvent.

[0014] According to an embodiment of the present invention, the electrolyte does not contain phosphate esters.

[0015] According to an embodiment of the present invention, the mass percentage of the flame retardant in the electrolyte is 5% to 16%, preferably 6% to 14%, and more preferably 8% to 10%.

[0016] According to an embodiment of the present invention, the carbonate-based solvent includes cyclic carbonates and linear carbonates, wherein the cyclic carbonate is selected from at least one of propylene carbonate EC, propylene carbonate PC, and propylene carbonate BC, and the linear carbonate is selected from at least one of dimethyl carbonate DMC, ethyl methyl carbonate EMC, diethyl carbonate DEC, methyl propyl carbonate MPC, and ethyl propyl carbonate EPC.

[0017] According to an embodiment of the present invention, the mixed solvent includes one or more of ethylene carbonate EC, dimethyl carbonate DMC, ethyl methyl carbonate EMC, and diethyl carbonate DEC.

[0018] According to an embodiment of the present invention, the mixed solvent is composed of ethylene carbonate EC, dimethyl carbonate DMC and ethyl methyl carbonate EMC, and the volume fraction of ethylene carbonate EC in the mixed solvent is 20% to 50%, preferably 40% to 50%, for example 25%, 33% and 43%.

[0019] According to an embodiment of the present invention, the electrolyte further includes a synergistic additive selected from one or more of vinylene carbonate (VC), fluoroethylene carbonate (FEC), lithium difluorooxalate borate (LiDFOB), lithium oxalate borate (LiBOB), and vinyl sulfite (DTD), preferably a mixture of vinylene carbonate (VC) and lithium oxalate borate (LiBOB). The synergistic additive works synergistically with PFPN to form a stable composite SEI film rich in LiF and organoborate at the graphite anode, and an interfacial protective layer rich in phosphorus and fluorine components at the cathode.

[0020] According to an embodiment of the present invention, the mass percentage of the synergistic additive in the electrolyte is 0.5-2%, preferably 0.8-1.5%, for example, the mass percentage of vinylene carbonate (VC) in the electrolyte is 0.5-1.5%, or the mass percentage of fluoroethylene carbonate (FEC) in the electrolyte is 0.8-1.5%, or the mass percentage of VC+LiDFOB in the electrolyte is 1.0-1.5%.

[0021] According to an embodiment of the present invention, the electrolyte further includes a lithium salt, which is selected from one or a combination of LiPF6, LiFSI, and LiBF4, preferably LiPF6 or a mixture of LiPF6 and LiFSI.

[0022] According to an embodiment of the present invention, the lithium salt and the mixed solvent form a basic solvent system, wherein the concentration of the lithium salt in the basic solvent system is 0.8 to 1.3 M, preferably 1.0 to 1.2 M.

[0023] According to an embodiment of the present invention, when the lithium salt is a mixture of LiPF6 and LiFSI, the concentration of LiPF6 in the base solvent system is greater than 0.8 M, for example, the concentration of LiPF6 is 1.0 M and the concentration of LiFSI is 0.2 M.

[0024] According to an embodiment of the present invention, the electrolyte is composed of a flame retardant, a mixed solvent, a synergistic additive, and a lithium salt. The flame retardant PFPN has a mass percentage of 5% to 16%. The mixed solvent is composed of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC), with the volume fraction of EC in the mixed solvent being 20% ​​to 50%. The synergistic additive is vinylene carbonate (VC) and / or fluoroethylene carbonate (FEC), with a mass percentage of 0.8% to 1.5%. The lithium salt is LiPF6 or a mixture of LiPF6 and LiFSI, with a concentration of 0.8% to 1.3 M in the base solvent system.

[0025] According to an embodiment of the present invention, the basic solvent system has a viscosity ≤ 5 mPa·s and an ionic conductivity ≥ 2.5 mS / cm at 25°C. -1 .

[0026] Secondly, the present invention provides a method for preparing the above-mentioned electrolyte, comprising the following steps: Lithium salts are mixed with a mixed solvent, and then synergistic additives and flame retardants are added.

[0027] According to an embodiment of the invention, the mixing is carried out in a dry environment with a dew point ≤ -40°C.

[0028] According to an embodiment of the present invention, after adding the synergistic additive and flame retardant, the mixture is mixed until completely dissolved and then aged at room temperature for 10-15 hours.

[0029] According to an embodiment of the present invention, the water content of the electrolyte is ≤20ppm.

[0030] Thirdly, the present invention provides a battery comprising the electrolyte as described above.

[0031] According to an embodiment of the present invention, the battery is a liquid lithium-ion battery, a semi-solid lithium-ion battery, or an all-solid lithium-ion battery.

[0032] According to an embodiment of the present invention, the battery is a lithium-ion battery, preferably a cylindrical lithium-ion battery, such as a 21700 cylindrical lithium-ion battery.

[0033] According to an embodiment of the present invention, the battery includes a positive electrode sheet, wherein the positive electrode active material of the positive electrode sheet is selected from at least one of NCM and NCA, and the NCM is selected from at least one of NCM622, NCM811, and NCM9.5.5.

[0034] According to an embodiment of the present invention, the compaction density of the positive electrode sheet is 3.4–3.7 g / cm³. -3 .

[0035] According to an embodiment of the present invention, the positive electrode is a nickel-rich positive electrode, wherein the nickel molar fraction in the positive electrode is greater than 80%.

[0036] According to an embodiment of the present invention, the battery includes a negative electrode sheet, wherein the negative electrode active material of the negative electrode sheet is selected from one or a combination of graphite, silicon-carbon composite material, carbon nanotubes, and conductive carbon black, and the silicon content in the silicon-carbon composite material is ≤10 wt%.

[0037] According to an embodiment of the present invention, the compaction density of the negative electrode sheet is 1.5–1.7 g / cm³. -3 .

[0038] According to an embodiment of the present invention, the battery includes a separator, the separator being selected from single-layer PE, single-layer PP or ceramic-coated composite separator, and the thickness of the separator being 10 to 18 μm.

[0039] According to an embodiment of the present invention, the coating in the ceramic-coated composite diaphragm is an Al2O3 or boehmite coating.

[0040] According to an embodiment of the present invention, the self-extinguishing time of the battery is <8s g -1 The maximum temperature of the needle puncture test is ≤120℃, and it will not catch fire or explode.

[0041] According to an embodiment of the present invention, the battery discharge capacity at 25°C and 3C rate is not less than 95% of the design capacity; after 500 charge-discharge cycles at 45°C and 1C rate, the capacity retention rate is ≥80%; and the DC internal resistance increases by ≤50% after 500 cycles.

[0042] Fourthly, the present invention provides an electrical device comprising the battery described above.

[0043] Beneficial effects 1) The electrolyte in this invention uses ethoxypentafluorocyclotriphosphazene (PFPN) as a flame retardant, and its content in the electrolyte is controlled at 5-12 wt%. When the battery is punctured, PFPN effectively improves the puncture safety of the lithium-ion battery while maintaining its high performance through the following mechanisms: ① Gas-phase flame retardant mechanism: At high temperature, PFPN decomposes to generate F• and PO• free radicals. F• combines with H• in the combustion chain (binding energy −0.1539 eV, stronger than the combination of H• and O2), and PO• captures highly active HO• (binding energy −1.6510 eV, much stronger than the combination of HO• and EC / DMC), completely interrupting the combustion chain reaction and forming a flame-retardant atmosphere of HF, NH3, and N2; ② Interface modification mechanism: The fluorinated segments of PFPN participate in the formation of the SEI film on the negative electrode surface, constructing a phosphorus-rich F-rich protective layer, while inhibiting the dissolution of transition metals on the high-nickel positive electrode surface; ③ Solvation regulation mechanism: PFPN enters the Li-ion battery as a weakly coordinated macromolecular solvent. + The first solvation layer lowers the Li-solvent binding energy, reduces the desolvation energy barrier, and enhances the Li... + The battery features well-designed embedding dynamics and high-rate discharge capability, meeting the electrolyte SET < 8 s / g requirement. It also achieves a capacity retention rate of 85.6% after 600 cycles and a needle penetration temperature of less than or equal to 100℃, thus maintaining high battery performance while achieving flame retardancy.

[0044] 2) The inventors unexpectedly discovered that by using only ethylene carbonate EC, dimethyl carbonate DMC, and ethyl methyl carbonate EMC, without requiring too many other components, good results can be achieved. This not only reduces the difficulty of electrolyte preparation but also lowers costs. Furthermore, by controlling the volume percentage of ethylene carbonate EC at 20%–50%, especially at a high content of 40%–50%, this invention can synergistically work with PFPN to reduce the overall flammability of the electrolyte, improve the stability of the negative electrode interface, improve SEI stability, and increase ionic conductivity.

[0045] 3) This invention modulates the interface of the PFPN electrolyte system by adding synergistic additives, such as VC, FEC, VC+ LiDFOB, etc., to the electrolyte, thereby improving the flexibility and uniformity of the SEI film and effectively buffering the volume expansion of the silicon-carbon anode. Detailed Implementation

[0046] To enable those skilled in the art to better understand the present application, the technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0047] In the description of this application, "same chemical composition" should be interpreted broadly, that is, the main components of the two have the same chemical composition, or the two have substantially the same chemical composition, but may have errors or impurities within the acceptable range that can be understood by those skilled in the art.

[0048] In the description of this application, "A and / or B" can include any of the cases of A alone, B alone, or A and B, where A and B are merely examples and can be any technical feature connected by "and / or" in this application.

[0049] Unless otherwise stated, all technical terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. All patents and publications referenced in this application are incorporated herein by reference in their entirety. The terms "comprising" or "including" are open-ended expressions, meaning they include the content specified in this application but do not exclude other aspects. Example 1 Synthesis and purification of PFPN flame retardant additives: PFPN (molecular formula: N3P3F5OC2H5) was synthesized via the following route: Starting with hexachlorocyclotriphosphazene (N3P3Cl6), it was reacted with NaOC2H5 (1.0 equivalent) in anhydrous tetrahydrofuran at 0°C for 4 h to undergo monosubstitution, yielding a monoethoxypentachlorocyclotriphosphazene intermediate; this intermediate was then reacted with anhydrous KF (5.5 equivalent) in acetonitrile at 70°C for 24 h, replacing the remaining 5 P-Cl bonds with PF bonds to obtain the crude product; the crude product was purified by vacuum distillation (boiling point 125°C / 760 mmHg), and the purity was confirmed by GC-MS to be ≥99%. 1 H NMR, 31 P NMR, 19 The structure was characterized by F NMR, and the moisture content was confirmed to be <20 ppm by Karl Fischer method. The total amount of metal ion impurities (Fe, Cr, Cu, Ni) was confirmed to be <1 ppm by ICP-MS. Alternatively, commercial products from Sigma-Aldrich that meet battery-grade purity requirements (≥99%, moisture <20 ppm) can be purchased directly.

[0050] Electrolyte preparation: Prepare in a dry room with a dew point ≤ -40℃: Dissolve LiPF6 (battery grade, purity ≥ 99.9%) in a mixed solvent of EC ethylene carbonate / EMC methyl ethyl carbonate / DMC dimethyl carbonate (volume ratio 1:1.2:0.8) to prepare a 1.2 mol / L LiPF6 basic electrolyte (90 wt%); then add 1.5 wt% VC (ethylene carbonate, purity ≥ 99.5%) and 8.5 wt% ethoxypentafluorocyclotriphosphazene (PFPN), stir until completely dissolved, and let stand at 25℃ for 12 h to mature. The moisture content (Karl Fischer method) is ≤ 20 ppm. After passing the test, seal and store for later use.

[0051] Method for manufacturing positive electrode sheets: Take lithium nickel cobalt manganese oxide (NCM811, D) 50 =10μm, BET=0.35 m 2 / g), conductive carbon black (Super P, BET=62m) 2 Multi-walled carbon nanotubes (MWCNTs, diameter 10–20 nm, length 5–7 μm) and polyvinylidene fluoride (PVDF, Mw≈300000) were weighed at a mass ratio of 96:1:1:2. An appropriate amount of NMP solvent was added, and the mixture was stirred in a dual planetary mixer at 30 rpm revolution and 1800 rpm rotation for 4 h until the slurry was homogeneous. The solid content was controlled at 60 wt%, and the viscosity (25℃, 10 rpm) was 4000–6000 mPa·s, yielding the positive electrode slurry. The positive electrode slurry was then double-sided coated onto the aluminum foil with the previously coated thermally responsive protective layer using a slot-coating process, with a target double-sided areal density of 35.0 mg / cm³. 2 (17.5 mg / cm² on one side) 2 After drying in four drying tunnels (100 / 110 / 120 / 130℃, each section for approximately 80 seconds), the product is rolled to a compacted density of 3.50 g / cm³. 3 Cut it to the required size to obtain the positive electrode sheet.

[0052] Method for manufacturing negative electrode plates: The negative electrode sheet comprises an 8μm thick copper current collector and a negative electrode coating material coated on both sides of the current collector. A negative electrode coating material with a solid content of 50% is formed by adding 20.0wt% deposited silicon carbon, 76.0wt% graphite, 0.5wt% single-walled carbon nanotubes (SWCNTs), 0.9wt% conductive carbon black (Super P), 1.0wt% sodium carboxymethyl cellulose (CMC), 0.8wt% polyacrylic acid (PAA), and 0.8wt% styrene-butadiene rubber (SBR) to deionized water and stirring. This negative electrode coating material is then coated onto both sides of the current collector, dried at 100℃ for 20 min, and cold-pressed at 8 MPa. The negative electrode sheet has a thickness of 80μm and a compaction density of 1.6 g / cm³. 3 .

[0053] Lithium-ion battery assembly: After the positive and negative electrode sheets are rolled and slit, they are wound together with a separator (a wet-process separator from Xingyuan Material) according to a set process to form a 21700 cylindrical battery core. Subsequently, the battery core is fixed to a pre-made connecting piece by welding and then installed into a metal battery casing. Using methods known in the art, after completing key processes such as electrolyte injection, sealing, and formation, a lithium-ion battery (Example 1) is obtained. This lithium-ion battery uses a cylindrical casing with an external dimension of 21.0 mm in diameter and 70.0 mm in length, conforming to the 21700 standard specification.

[0054] The preparation methods of PFPN, electrolyte, positive electrode, negative electrode, and lithium battery used in the following examples and comparative examples are the same, except that the composition of the electrolyte is different. See Table 1 for details: Table 1 Electrolyte composition in Examples 1-16 and Comparative Examples 1-10 Factor gradient one: PFPN dosage (Comparative Example 1, Examples 2-4, Comparative Examples 2-3) Fixed variables: 1.2 mol / L LiPF6, solvent is EC / EMC / DMC (volume ratio 1:1.2:0.8), 1.5 wt% VC; Variable: PFPN mass fraction.

[0055] Comparative Example 1 (blank reference, PFPN = 0 wt%) electrolyte composition: 1.2 mol / L LiPF6 (battery grade, purity ≥99.9%) dissolved in a mixed solvent of EC / EMC / DMC (volume ratio 1:1.2:0.8), with 1.5 wt% VC (purity ≥99.5%) added. No flame retardant additives were added, and the mixture was stirred thoroughly to prepare a commercial reference electrolyte. This comparative example represents the baseline performance of current mainstream carbonate-based electrolytes and is used to evaluate the needle penetration safety risk and electrochemical baseline without flame retardant treatment.

[0056] Comparative Example 2 (low PFPN dosage, PFPN = 3 wt%) electrolyte composition: 1.2 mol / L LiPF6, EC / EMC / DMC (volume ratio 1:1.2:0.8), 1.5 wt% VC, 3 wt% PFPN, with the balance (94.5 wt%) being the base solvent electrolyte system (the base solvent electrolyte system refers to the blank base electrolyte formed by lithium salt and solvent EC / EMC / DMC). During preparation, LiPF6 was first dissolved in the mixed solvent, then VC and PFPN were added sequentially, and the mixture was stirred until completely clear and dissolved.

[0057] Example 2 (PFPN = 5 wt%) Electrolyte composition: 1.2 mol / L LiPF6, EC / EMC / DMC (volume ratio 1:1.2:0.8), 1.5 wt% VC, 5 wt% PFPN, the balance (93.5 wt%) is the basic solvent electrolyte system, prepared in the same way as in Example 1.

[0058] Example 3 (PFPN = 10 wt%) Electrolyte composition: 1.2 mol / L LiPF6, EC / EMC / DMC (volume ratio 1:1.2:0.8), 1.5 wt% VC, 10 wt% PFPN, the balance (88.5 wt%) is the basic solvent electrolyte system, prepared in the same way as in Example 1.

[0059] Example 4 (PFPN = 12 wt%) Electrolyte composition: 1.2 mol / L LiPF6, EC / EMC / DMC (volume ratio 1:1.2:0.8), 1.5 wt% VC, 12 wt% PFPN, the balance (86.5 wt%) is the basic solvent electrolyte system, prepared in the same way as in Example 1.

[0060] Comparative Example 3 (high PFPN content, PFPN = 16 wt%) electrolyte composition: 1.2 mol / L LiPF6, EC / EMC / DMC (volume ratio 1:1.2:0.8), 1.5 wt% VC, 16 wt% PFPN, with the balance (82.5 wt%) being the base solvent electrolyte system. The preparation method was the same as in Example 1. This comparative example aims to investigate the negative impact of excess PFPN on the physicochemical properties and electrochemical performance of the electrolyte.

[0061] Factor gradient two: Solvent composition (Examples 5-7, Comparative Examples 4-5) Fixed variables: 1.2 mol / L LiPF6, 8.5 wt% PFPN, 1.5 wt% VC; Variable: EC / EMC / DMC volume ratio (mainly changing the volume fraction of EC in the mixed solvent).

[0062] Comparative Example 4 (low EC content, EC / EMC / DMC = 1:0.5:2.5, EC accounts for 25 vol%) Electrolyte composition: LiPF6 was dissolved in a mixed solvent of EC / EMC / DMC (volume ratio 1:0.5:2.5) (EC volume percentage 25%) to form a LiPF6 solution with a concentration of 1.2 mol / L. 8.5 wt% PFPN and 1.5 wt% VC were added. The preparation method was the same as in Example 1. The low EC content led to a weakened solvation ability, and the SEI membrane quality was expected to decrease, and the cycle stability was reduced.

[0063] Example 5 (EC / EMC / DMC = 1:1:1, EC accounts for 33 vol%) Electrolyte composition: LiPF6 is dissolved in a mixed solvent of EC / EMC / DMC (volume ratio 1:1:1) (EC volume percentage 33 vol%) to form a LiPF6 solution with a concentration of 1.2 mol / L. 8.5 wt% PFPN and 1.5 wt% VC are added. The preparation method is the same as in Example 1.

[0064] Example 6 (EC / EMC / DMC = 1.5:1.2:0.8, EC accounts for 43 vol%) Electrolyte composition: LiPF6 was dissolved in a mixed solvent of EC / EMC / DMC (volume ratio 1.5:1.2:0.8) (EC volume percentage 43%) to form a LiPF6 solution with a concentration of 1.2 mol / L. 8.5 wt% PFPN and 1.5 wt% VC were added, and the preparation method was the same as in Example 1.

[0065] Example 7 (EC / EMC / DMC = 2:1.2:0.8, EC accounts for 50 vol%) Electrolyte composition: LiPF6 is dissolved in a mixed solvent of EC / EMC / DMC (volume ratio 2:1.2:0.8) (EC volume percentage 50%) to form a LiPF6 solution with a concentration of 1.2 mol / L. 8.5 wt% PFPN and 1.5 wt% VC are added. The preparation method is the same as in Example 1.

[0066] Comparative Example 5 (excessively high EC content, EC / EMC / DMC = 3:1:1, EC volume percentage 60 vol%) electrolyte composition: LiPF6 was dissolved in a mixed solvent of EC / EMC / DMC (volume ratio 3:1:1) (EC volume percentage 60%) to form a 1.2 mol / L LiPF6 solution. 8.5 wt% PFPN and 1.5 wt% VC were added, prepared using the same method as in Example 1. EC is a room-temperature solid (melting point 36℃). An excessively high proportion led to a sharp increase in electrolyte viscosity, a significant decrease in ionic conductivity, and deterioration in low-temperature performance and rate performance.

[0067] Factor gradient three: Lithium salt type and concentration (Examples 8-10, Comparative Examples 6-7) Fixed variables: EC / EMC / DMC (volume ratio 1:1.2:0.8), 8.5 wt% PFPN, 1.5 wt% VC; Variables: lithium salt type and concentration.

[0068] Example 8 (Low concentration LiPF6, 0.8 mol / L) Electrolyte composition: LiPF6 was dissolved in EC / EMC / DMC (volume ratio 1:1.2:0.8) to form a 0.8 mol / L LiPF6 solution. 8.5 wt% PFPN and 1.5 wt% VC were added, prepared using the same method as in Example 1. Reducing the LiPF6 concentration to 0.8 mol / L is expected to decrease ionic conductivity, but the electrolyte viscosity is reduced, which is beneficial for low-temperature and high-rate performance.

[0069] Example 9 (LiFSI system, 1.2 mol / L) Electrolyte composition: LiFSI (lithium bisfluorosulfonylimide, battery grade, purity ≥99.9%, water <20 ppm) was dissolved in EC / EMC / DMC (volume ratio 1:1.2:0.8) to form a 1.2 mol / L LiFSI solution. 8.5 wt% PFPN and 1.5 wt% VC were added, prepared using the same method as in Example 1. LiFSI exhibits higher ionic conductivity, better thermal stability, and a wider electrochemical window (vs. LiPF6), and is expected to further improve high-temperature cycling performance.

[0070] Example 10 (LiPF6 / LiFSI composite lithium salt, 1.0 + 0.2 mol / L) Electrolyte composition: LiPF6 and LiFSI were dissolved in EC / EMC / DMC (volume ratio 1:1.2:0.8) to form a 1.0 mol / L LiPF6 and 0.2 mol / L LiFSI lithium salt solution, with 8.5 wt% PFPN and 1.5 wt% VC added. During preparation, LiPF6 was first dissolved in the mixed solvent, then LiFSI was added, and after stirring until homogeneous, PFPN and VC were added sequentially. The composite lithium salt combines the passivation effect of LiPF6 on Al current collectors with the high conductivity advantage of LiFSI.

[0071] Comparative Example 6 (Very Low Concentration LiPF6, 0.5 mol / L) Electrolyte Composition: LiPF6 was dissolved in EC / EMC / DMC (volume ratio 1:1.2:0.8) to form a 0.5 mol / L LiPF6 solution. 8.5 wt% PFPN and 1.5 wt% VC were added, prepared using the same method as in Example 1. The excessively low lithium salt concentration resulted in insufficient ionic conductivity (expected <1.5 mS·cm). -1 ), Li + Due to limited transmission dynamics, both rate performance and capacity are significantly reduced.

[0072] Comparative Example 7 (LiTFSI system, 1.2 mol / L) electrolyte composition: LiTFSI (lithium bis(trifluoromethanesulfonyl)imide) was dissolved in EC / EMC / DMC (volume ratio 1:1.2:0.8) to form a 1.2 mol / L LiTFSI solution. 8.5 wt% PFPN and 1.5 wt% VC were added, prepared using the same method as in Example 1. LiTFSI exhibits severe corrosiveness to aluminum current collectors above 4.0 V. The oxidation and dissolution of the positive electrode aluminum foil leads to numerous side reactions, resulting in a sharp deterioration in cycle performance. This comparative example was used to verify the importance of lithium salt anion compatibility with the current collector.

[0073] Factor gradient four: Types of synergistic additives (Examples 11-13, Comparative Examples 8-9) Fixed variables: 1.2 mol / L LiPF6, EC / EMC / DMC (volume ratio 1:1.2:0.8), 8.5 wt% PFPN, total amount of synergistic additives approximately 1.5 wt%; Variable: type of synergistic additives.

[0074] Comparative Example 8 (without synergistic additives) electrolyte composition: 1.2 mol / L LiPF6, EC / EMC / DMC (volume ratio 1:1.2:0.8), 8.5 wt% PFPN, without any synergistic additives, with the balance being the basic electrolyte, prepared in the same manner as in Example 1. This comparative example was used to evaluate the SEI membrane quality and cycle stability of PFPN alone without synergistic additives, revealing the contribution of synergistic additives.

[0075] Example 11 (FEC, 1.5 wt%) Electrolyte composition: 1.2 mol / L LiPF6, EC / EMC / DMC (volume ratio 1:1.2:0.8), 8.5 wt% PFPN, 1.5 wt% FEC (fluoroethylene carbonate, purity ≥99.5%), balance being the basic electrolyte. FEC is a fluorinated film-forming additive that preferentially reduces to form a LiF-rich SEI film at the negative electrode, improving the expansion performance and cycle stability of the silicon-carbon negative electrode. The preparation method is the same as in Example 1.

[0076] Example 12 (VC + LiDFOB composite, 0.5 wt% + 1.0 wt%) Electrolyte composition: 1.2 mol / L LiPF6, EC / EMC / DMC (volume ratio 1:1.2:0.8), 8.5 wt% PFPN, 0.5 wt% VC + 1.0 wt% LiDFOB (lithium difluorooxalate borate, purity ≥99%), with the balance being the basic electrolyte. LiDFOB has both negative electrode film formation and positive electrode surface protection functions. Combined with a small amount of VC, it can construct a high-quality bifunctional interface on both electrodes. During preparation, it is added in the order of VC→LiDFOB and fully dissolved.

[0077] Example 13 (DTD, 1.5 wt%) Electrolyte composition: 1.2 mol / L LiPF6, EC / EMC / DMC (volume ratio 1:1.2:0.8), 8.5 wt% PFPN, 1.5 wt% DTD (ethylene sulfite, purity ≥99%), with the balance being the basic electrolyte. DTD contains sulfonyl groups, which can form a lithium sulfate-containing interfacial protective layer at the positive electrode, effectively inhibiting the dissolution of transition metals from the high-nickel positive electrode, and participating in the formation of a stable SEI film at the negative electrode. The preparation method is the same as in Example 1.

[0078] Comparative Example 9 (using TEP instead of PFPN, TEP = 8.5 wt%) Electrolyte composition: 1.2 mol / L LiPF6, EC / EMC / DMC (volume ratio 1:1.2:0.8), 8.5 wt% TEP (triethyl phosphate, purity ≥99%, traditional phosphate flame retardant additive), 1.5 wt% VC, with the remainder being the basic electrolyte. The preparation method was the same as in Example 1. This comparative example completely replaced PFPN with an equal amount of TEP to systematically compare the comprehensive trade-off between traditional phosphate esters and the PFPN of this invention in terms of flame retardant effect and electrochemical performance, and to verify the technical superiority of PFPN.

[0079] Factor gradient five: VC dosage (Examples 14-16, Comparative Example 10) Fixed variables: 1.2 mol / L LiPF6, EC / EMC / DMC (volume ratio 1:1.2:0.8), 8.5 wt% PFPN; Variable: VC mass fraction.

[0080] Comparative Example 10 (VC = 0 wt%) electrolyte composition: 1.2 mol / L LiPF6, EC / EMC / DMC (volume ratio 1:1.2:0.8), 8.5 wt% PFPN, without added VC, prepared in the same way as Example 1. This comparative example examines the interfacial stability of PFPN in negative electrode film formation without VC synergy, revealing the necessity of VC for SEI optimization of PFPN electrolyte.

[0081] Example 14 (VC = 0.5 wt%) Electrolyte composition: 1.2 mol / L LiPF6, EC / EMC / DMC (volume ratio 1:1.2:0.8), 8.5 wt% PFPN, 0.5 wt% VC, balance is basic electrolyte, prepared by the same method as in Example 1.

[0082] Example 15 (VC = 1.0 wt%) Electrolyte composition: 1.2 mol / L LiPF6, EC / EMC / DMC (volume ratio 1:1.2:0.8), 8.5 wt% PFPN, 1.0 wt% VC, balance being the basic electrolyte. Preparation method is the same as in Example 1.

[0083] Example 16 (VC = 2.0 wt%) Electrolyte composition: 1.2 mol / L LiPF6, EC / EMC / DMC (volume ratio 1:1.2:0.8), 8.5 wt% PFPN, 2.0 wt% VC, with the balance being the basic electrolyte. The preparation method was the same as in Example 1. The effect of excess VC on initial capacity, gas evolution, and high-temperature cycling performance was investigated.

[0084] Test case Self-extinguishing time (SET) test In a fume hood (ambient temperature 20±2℃), take 1.0 mL of electrolyte and completely saturate a glass fiber cotton ball with a diameter of about 30 mm and a mass of about 0.10 g. Ignite the cotton ball with an alcohol lamp and then remove the flame. Use a stopwatch to record the time (s) required for the electrolyte to self-extinguish. The result is expressed as time divided by the total mass (g) of the cotton ball and electrolyte, in units of s / g. Report the average of three parallel measurements.

[0085] DC internal resistance (DCIR) test The battery was discharged at a constant current of 0.5C to 50% SOC and then placed in a 25℃ constant temperature chamber for 30 min. A 2C current pulse discharge was applied for 10 s, and the voltage values ​​V0 and V2 before and after the pulse application were collected. 10s Calculate DCIR = (V0− V 10s ) / I (mΩ); Record the DCIR after the first activation cycle and after 500 cycles respectively, and calculate the growth rate = (DCIR) / I (mΩ). 500 − DCIR0) / DCIR0× 100%.

[0086] Ratio Performance Test The battery was charged to 4.2V (cutoff current 0.05C) at 0.2C CC-CV in a constant temperature environment of 25℃ and then left to stand for 10 minutes. It was then discharged to 2.5V at constant current of 0.2C and 3C respectively. The average value was taken for 3 cycles at each rate. The capacity retention rate (%) at each rate was calculated based on the 0.2C capacity.

[0087] Ion conductivity test A platinum black electrode conductivity cell was used (electrode constant K = 1.0 cm). -1 The electrolyte was placed in a 25°C constant temperature water bath, and an impedance scan was performed using an LCR meter in the frequency range of 1 kHz to 100 kHz. The real part of the impedance R at the high-frequency intercept of the Nyquist plot was taken, and σ = K / R (mS·cm) was calculated. -1 ).

[0088] Viscosity test The dynamic viscosity of the prepared electrolyte was measured using a rotational viscometer (cone-plate type, cone angle 1°) at 25°C and a shear rate of 100 rpm, with the unit being mPa·s.

[0089] Methods for measuring diaphragm thickness: A smooth diaphragm cross-section sample free from mechanical stress damage was prepared using an argon ion cross-section polisher (CP). The sample was then magnified and observed under a scanning electron microscope (SEM). The thickness of the polyethylene base film in the middle and the ceramic coatings on both sides was read and the average value was calculated at multiple different positions in the cross-section image using image software to verify the specifications of the base film and coatings.

[0090] Test method for compaction density of positive / negative electrode sheets: First, the positive (or negative) electrode sheet, after being washed with dimethyl carbonate and vacuum dried, was cut into nine standard-sized square samples (2.0cm × 2.0cm). Next, the active material on both sides of three of these square samples was removed, rinsed with NMP, dried, weighed, and the average mass M1 was calculated. Simultaneously, the average thickness L1 of the sample was measured using a micrometer. Then, the mass of the remaining three square samples was weighed, and the average mass M2 was calculated. The average thickness L2 of the sample was also measured. The compaction density of the positive (or negative) electrode sheet was calculated. Unit: g / cm 3 .

[0091] Method for determining the rated capacity of a cylinder: The determination of the rated capacity of cylindrical batteries is usually carried out under standard room temperature conditions (e.g., 25±2℃). The specific steps are as follows: First, the formed battery is charged with a standard current (usually 0.2C) at a constant current until the rated upper limit voltage (e.g., 4.2V). Then, it is switched to constant voltage charging until the current drops to the specified cutoff value (e.g., 0.05C) and left to stand for 30 minutes to achieve internal electrochemical equilibrium. Next, the fully charged battery is continuously discharged with a standard discharge current (usually 0.2C constant current) until the voltage drops to the discharge cutoff voltage specified in the product specification (e.g., 2.5V). During this discharge stage, the total amount of electricity released (i.e., ampere-hour integral) is recorded and calculated using a high-precision battery test cabinet.

[0092] Needle prick test experiment The battery to be tested was fully charged to 100% SOC (State of Charge) and placed in a 25°C environment for 2 hours to stabilize. Then, the battery was fixed to an insulating clamp, ensuring its positive and negative terminals were horizontal. Using a 3mm diameter stainless steel needle, perpendicular to the battery surface and along the direction between the positive and negative terminals, the needle was inserted into the battery at a constant speed of 25±1mm / s until it was completely penetrated (the penetration depth should exceed 90% of the battery thickness). After insertion, the needle was kept still, and the battery status was continuously observed for 5 minutes. Any abnormal phenomena such as fire, explosion, or smoke were recorded, and temperature changes were monitored using a thermocouple attached to the battery surface.

[0093] Table 2. Effects of different PFPN contents on the flame retardant properties and discharge capacity of the electrolyte. As shown in Table 2, with the increase of PFPN content in the electrolyte, the flame retardant performance of the electrolyte showed a significant monotonic improvement trend, while the electrochemical performance showed an inverted U-shaped pattern of first increasing and then decreasing, with a clear optimal dosage range. Comparative Example 1 (0 wt% PFPN) had a self-extinguishing time (SET) exceeding 120 s / g, and a peak temperature exceeding 650°C after needle puncture, resulting in violent ignition and explosion, clearly demonstrating the extremely high safety risk of pure carbonate electrolyte under puncture-induced internal short-circuit conditions. The SET of Comparative Example 2 (3 wt%) was 72.4 s / g, which was lower than that of the blank but still far exceeded the safety threshold. The needle puncture result showed that the flame ignition and continued burning, indicating that when the PFPN dosage was below the critical concentration, the density of F• and PO• free radicals generated in the gas phase was insufficient to cut off the chain reaction of carbonate combustion. The SET of Example 2 (5 wt%) dropped to 18.6 s / g, and only smoke was emitted without open flame after needle puncture, indicating that it had entered the lower end of the effective flame retardant range. The SET of Example 1 (8.5 wt%) was 5.2 s / g, which met the requirement of SET < 8 s / g in the claims. The peak needle puncture temperature dropped to 98°C, which was far below the thermal runaway trigger temperature of NCM811 (about 180°C), and it completely passed the needle puncture safety test. As PFPN continued to increase, the SET further decreased in Examples 3 and 4, and the flame retardant effect was better. Comparative Example 3 (16 wt%) was close to the self-sustaining combustion limit and had extremely strong flame retardancy.

[0094] The initial discharge capacity decreased monotonically with increasing PFPN dosage. This is related to the dilution effect of PFPN as a macromolecular weak solvent on carbonate-based solvents and its slight impediment to electrolyte ion conduction. The key indicator was the capacity retention rate after 600 cycles: Comparative Example 1, lacking the interfacial protection effect of PFPN, had a retention rate of only 82.3%, while Example 1 (8.5 wt%) reached a peak of 85.6%, indicating that PFPN significantly improved interfacial stability by participating in SEI film formation (a phosphorus-rich F-containing protective layer). Example 3 (10 wt%) decreased to 84.3%, and Example 4 (12 wt%) further decreased to 82.1%, while Comparative Example 3 (16 wt%) only reached 73.8%, indicating that excessive PFPN resulted in high viscosity, leading to Li... + The transport kinetics deteriorate, and the large amount of HF decomposition products of PFPN may corrode the positive electrode interface. The degradation trend of 3C rate performance is consistent with this. Considering the two key indicators of flame retardancy and cycling, 8.5-10wt% is the optimal range for PFPN dosage. Within this range, the electrolyte SET < 8 s / g, the needle penetration temperature ≤ 100°C, and the retention rate ≥ 84% after 600 cycles are all in optimal balance.

[0095] Table 3. Effects of different solvent compositions on flame retardant and electrochemical properties. Solvent composition has a dual impact on flame retardant and electrochemical performance by influencing the physicochemical properties (viscosity, ionic conductivity) and interfacial film-forming behavior (the contribution of EC reduction products to SEI). EC is a high-boiling-point (248°C) and high-flash-point (160°C) solvent, while chain carbonates DMC (boiling point 90°C, flash point 17°C) and EMC (boiling point 107°C, flash point 23°C) are more flammable. Therefore, increasing the EC ratio (Comparative Example 4 → Comparative Example 5) helps reduce the overall flammability of the electrolyte, which is reflected in the SET value showing a trend of first decreasing and then increasing with the increase of EC ratio. However, it should be noted that a high EC ratio (Comparative Example 5, 60 vol%), although helping to suppress open flame due to its high boiling point, leads to a sharp increase in viscosity (6.2 mPa·s), which seriously degrades the overall battery performance. Its needle penetration temperature rises to 115°C (due to heat accumulation caused by decreased thermal conductivity). This paradox reveals the multidimensional complexity of the impact of solvent ratio on safety. Comparative Example 4 (EC only 25 vol%) suffered from poor SEI film quality due to insufficient EC (ethylene carbonate, the EC reduction product, is the main component of high-quality SEI), resulting in low anode interface stability and a capacity retention rate of only 78.4% after 600 cycles, the lowest among all example groups. With increasing EC ratio, SEI stability improved, and capacity retention rate increased to its peak (Example 1, 85.6%; Example 6, 85.2%). However, Comparative Example 5 (60 vol% EC) exhibited an excessively high viscosity (6.2 mPa·s), resulting in an ionic conductivity of only 1.8 mS·cm. -1 (Below the ≥2.5 mS·cm specified in the claims) -1 Threshold), Li + Transport obstruction caused the capacity retention rate to drop to 79.2%, and the initial discharge capacity was only 4.61 Ah. Ionic conductivity is a key indicator for evaluating solvent ratio: the ionic conductivity of Examples 1 and 5 both reached 2.8–2.9 mS·cm. -1 The optimal range is defined as follows. Based on comprehensive evaluation, when the EC volume fraction is between 33% and 43% (i.e., the range of Examples 1, 5, and 6), the electrolyte viscosity is ≤4 mPa·s and the ionic conductivity is ≥2.6 mS·cm. -1 The cycle retention rate is ≥84%, which is the optimal solvent ratio window.

[0096] Table 4. Effects of different lithium salts on flame retardant and electrochemical properties Lithium salts are the only ion source in the electrolyte, and their type and concentration directly determine ionic conductivity, electrochemical window, aluminum current collector compatibility, and interfacial film-forming chemistry, thus profoundly affecting the battery's power performance and cycle life. Flame retardant properties (SET and needle penetration temperature) show minimal differences among different lithium salts, all falling within the range of 5.0–5.3 S / g and 96–99°C, indicating that the flame retardant mechanism of PFPN mainly stems from its own thermal decomposition behavior, with limited influence from the type of lithium salt. This is an important manifestation of the wide compatibility of PFPN as a flame retardant additive. LiPF6 concentration effect (Comparative Example 6, Example 8, Example 1): The ionic conductivity of Comparative Example 6 (0.5 mol / L LiPF6) is only 1.6 mS·cm. -1 The severe deficiency directly leads to limited Li+ transport, resulting in an initial discharge capacity of only 4.52 Ah, the lowest cycle retention rate after 600 cycles (75.3%), and a DCIR growth rate as high as 68.2%, indicating that the Li+ concentration at the interface is extremely low. + Unstable supply induces increased concentration polarization and side reactions. As the LiPF6 concentration increases from 0.8 mol / L to 1.2 mol / L, all performance characteristics steadily improve, reaching the optimal performance equilibrium point at 1.2 mol / L. Lithium salt type effect (Examples 9, 10, and Comparative Example 7): LiFSI (Example 9) exhibits higher thermal stability (decomposition temperature >200°C, while LiPF6 is approximately 80°C), lower viscosity contribution, and higher ionic conductivity (3.6 mS·cm) compared to LiPF6. -1 Therefore, the retention rate (87.2%) and DCIR growth rate (33.1%) after 600 cycles were both superior to Example 1. Example 10 (LiPF6 + LiFSI composite salt) combines the advantages of both salts: the AlF3 passivation layer formed by LiPF6 on the aluminum current collector surface inhibits aluminum corrosion, and LiFSI provides additional high-temperature stability. The composite salt system achieved the best performance with a retention rate of 88.5% after 600 cycles and the lowest DCIR growth rate (31.4%). Comparative Example 7 (LiTFSI, 1.2 mol / L) showed better performance, although its ionic conductivity (3.1 mS·cm) was lower. -1 Similar to Example 9, but TFSI - Anions cannot form an effective passivation layer on the aluminum current collector (positive electrode current collector), initiating aluminum corrosion at high potentials (≥3.8 V), generating a large amount of AlTFSI3 dissolution products. This leads to a sharp deterioration in cycle retention (71.2%) and a surge in DCIR (85.6% increase). This result clearly verifies the critical limiting role of lithium salt compatibility with aluminum current collectors on the lifespan of 21700 full cells. Considering ionic conductivity, cycle stability, and cost, 1.2 mol / L LiPF6 (Example 1) is the preferred engineering solution, while LiPF6 / LiFSI composite salt (Example 10) represents an optimized upgrade direction for pursuing higher cycle life.

[0097] Table 5. Effects of different synergistic additive compositions on flame retardant and electrochemical properties. This group of experiments revealed the interfacial regulation effect of synergistic additives on the PFPN electrolyte system, and the technical superiority of PFPN over traditional phosphate ester flame retardants. The role of synergistic additives (Comparative Example 8 vs. all examples): Comparative Example 8, without synergistic additives, had an SEI film mainly composed of carbonate and PFPN reduction products, with a cycle retention rate of only 79.2% after 600 cycles. This indicates that although PFPN alone can participate in film formation at the anode interface, the resulting SEI film lacks flexibility and uniformity, and cannot effectively buffer the volume expansion of the silicon-carbon anode. After adding VC (Example 1), the cycle retention rate increased to 85.6%; after adding FEC (Example 11), the cycle retention rate increased to 86.8%, because FEC preferentially reduces to form a LiF-rich elastic SEI film on the silicon-carbon anode surface, which is more effective than VC in suppressing the volume expansion of silicon-carbon. Composite Additive Strategy (Example 12): The combination of VC (0.5 wt%) and LiDFOB (1.0 wt%) demonstrated optimal overall performance: 88.3% retention rate after 600 cycles and the highest 3C rate retention rate (93.6%). The oxalate ligand of LiDFOB (lithium difluorooxalate borate) can form a boron-containing interfacial layer on the cathode surface, inhibiting Ni dissolution and phase transition in the NCM811 high-nickel cathode. At the same time, its lithium borate reduction product forms a composite SEI film on the anode, synergistically improving the interfacial quality with the lithium propylene carbonate product of a small amount of VC, demonstrating the synergistic effect of multifunctional additives. PFPN vs. TEP Comparative Example 9: Comparative Example 9, which completely replaced PFPN with an equal amount of TEP (triethyl phosphate), is the most important control experiment of this invention. The SET of TEP (8.5 wt%) was 8.3 s / g (slightly exceeding the 8 s / g threshold of the claim), and the needle penetration temperature was 135°C, exceeding the first-level trigger temperature of NCM811 thermal runaway, and a brief smoke phenomenon was observed, indicating that the flame retardancy of traditional phosphate esters is significantly inferior to that of PFPN (SET difference of up to 60%). More importantly, TEP severely damages electrochemical performance: the retention rate after 600 cycles is only 72.4%, and the 3C rate retention rate is only 85.3%. The high viscosity of TEP (approximately 3.5 times that of the corresponding carbonate solvent) increases the viscosity of the electrolyte, and its strong coordination ability (with Li) + (High binding energy), occupying a large amount of Li +The solvation shell hinders desolvation kinetics; simultaneously, the reduction decomposition products (lithium phosphate compounds) of TEP on the graphite anode surface continuously consume active lithium, leading to continuous thickening of the SEI film and the formation of an unstable "breathing" interface, ultimately causing rapid cycle decay. This result experimentally proves the core argument of this invention: fluorinated triphosphazene (PFPN) is superior to traditional phosphate ester additives in both flame retardancy and electrochemical performance compatibility, and the two are not interchangeable.

[0098] Table 6. Effects of different VC dosages on flame retardant and electrochemical properties. This experimental group systematically investigated the fine-tuning effect of VC dosage on the interfacial stability of the PFPN electrolyte system. Flame retardant properties (SET and needle penetration temperature) were almost unaffected by VC dosage within the range of 0–2 wt% VC (both within the range of 5.0–5.3 s / g and 96–99°C), indicating that VC makes no substantial contribution to the flame retardant mechanism; its effect is entirely concentrated on film-forming modification of the electrode interface. This is highly consistent with VC's role as a film-forming additive (rather than a flame retardant additive). As the VC dosage increased from 0 to 1.5 wt% (Comparative Example 10 → Example 1): all interfacial related indicators showed monotonic improvement; the 600-cycle retention rate gradually increased from 79.2% to 85.6%, an increase of 6.4 percentage points; the DCIR (direct current internal resistance) growth rate decreased from 52.3% to 38.5%; and the reduction potential of VC (approximately 1.0 V vs. Li / Li) decreased. + The VC content is higher than that of carbonate solvents. During the first charge, it preferentially undergoes ring-opening polymerization on the surfaces of graphite and silicon-carbon anodes to form a flexible SEI film with a polyvinyl carbonate (PVC-Li) framework. This effectively buffers the volume expansion of silicon-carbon (volume change during cycling ~300%), thereby significantly reducing impedance growth. Notably, the initial discharge capacity decreases slightly with increasing VC content (because VC reduction consumes some active lithium for SEI formation), but the decrease is extremely small (only 0.02 Ah), indicating that the VC content in the range of 0–2 wt% does not pose a practical limitation on capacity.

[0099] Increasing the VC dosage from 1.5 wt% to 2.0 wt% (Example 1 → Example 16): the cycle retention rate slightly decreased from 85.6% to 84.8%, while the DCIR growth rate also rebounded slightly (38.5% → 40.2%). The mechanism of this "excess VC effect" is that excess VC continues to undergo reduction polymerization after film formation, producing a thicker organic SEI layer and increasing interfacial impedance. Simultaneously, under cycling conditions, the thick SEI film is more prone to cracking, and the fresh negative electrode surface exposed at the cracks further consumes active lithium, thus accelerating capacity decay. Considering all data, the optimal VC dosage is 1.5 wt% (Example 1), at which the cycle retention rate and interfacial stability are optimal; 1.0–1.5 wt% are all effective dosage ranges, with 1.5 wt% representing the best performance.

[0100] The specific embodiments of the present invention have been described above by way of example. However, the scope of protection of the present invention is not limited to the above exemplary embodiments. Any modifications, equivalent substitutions, improvements, etc., made by those skilled in the art within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. An electrolyte, characterized in that, The electrolyte contains a flame retardant and a mixed solvent. The flame retardant is PFPN, and the mixed solvent is a carbonate-based solvent. The concentration of the flame retardant in the electrolyte is 5 wt% to 16 wt%. The mixed solvent is composed of ethylene carbonate EC, dimethyl carbonate DMC, and ethyl methyl carbonate EMC, and the volume fraction of ethylene carbonate EC in the mixed solvent is 20% to 50%.

2. The electrolyte according to claim 1, characterized in that, The concentration of the flame retardant in the electrolyte is 6wt% to 14wt%, and the volume fraction of ethylene carbonate EC in the mixed solvent is 40% to 50%.

3. The electrolyte according to claim 1, characterized in that, The electrolyte also includes a synergistic additive, which is selected from one or more of vinylene carbonate (VC), fluoroethylene carbonate (FEC), lithium difluorooxalate borate (LiDFOB), lithium oxalate borate (LiBOB), and vinyl sulfite (DTD); the mass percentage of the synergistic additive in the electrolyte is 0.5-2%.

4. The electrolyte according to claim 1, characterized in that, The electrolyte also includes a lithium salt, which is selected from one or more of LiPF6, LiFSI, and LiBF4. The lithium salt and the mixed solvent form a basic solvent system, in which the concentration of the lithium salt is 0.8 to 1.3 M.

5. The electrolyte according to any one of claims 1-4, characterized in that, The electrolyte is composed of a flame retardant, a mixed solvent, a synergistic additive, and a lithium salt. The flame retardant PFPN has a mass percentage of 5% to 16%. The mixed solvent is composed of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC), with EC having a volume fraction of 20% to 50%. The synergistic additive is vinylene carbonate (VC) and / or fluoroethylene carbonate (FEC), with a mass percentage of 0.8% to 1.5%. The lithium salt is LiPF6 or a mixture of LiPF6 and LiFSI, with a concentration of 0.8% to 1.3 M in the base solvent system.

6. The electrolyte according to claim 5, characterized in that, The base solvent system has a system viscosity of < 5 mPa-s and an ionic conductivity of > 2.5 mS cm at 25°C -1 .

7. A method for preparing the electrolyte according to any one of claims 1-6, characterized in that, Includes the following steps: The lithium salt is mixed with the mixed solvent, and then the synergistic additives and flame retardants are added and mixed evenly.

8. A battery comprising the electrolyte according to any one of claims 1-6 or the electrolyte prepared according to claim 7.

9. The battery according to claim 8, characterized in that, The battery's self-extinguishing time is <8s g -1 The maximum temperature of the needle penetration test is ≤120℃; the discharge capacity of the battery at 25℃ and 3C rate is not less than 95% of the design capacity; after 500 charge-discharge cycles at 45℃ and 1C rate, the capacity retention rate is ≥80%; and the DC internal resistance increases by ≤50% after 500 cycles.

10. An electrical device, characterized in that, The electrical device includes the battery as described in claim 8 or 9.

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