A ternary lithium ion battery electrolyte and a preparation method thereof

Abstract

The application discloses a ternary lithium ion battery electrolyte and a preparation method thereof. The electrolyte comprises the following components in mass fraction: 0.5-1% of polycyclic additive, 10-12% of electrolyte lithium salt, 0.6-1.5% of auxiliary lithium salt, 0.3-3.5% of film forming agent, and the rest is organic solvent. The polycyclic additive has a structure shown in formula (I), wherein R and R' are independently selected from O or S, R1, R2, R3, R4...R 2n , R 2n+1 are independently selected from H, vinyl carbonate or vinyl sulfite, and n>=0. The application effectively improves the high-voltage stability and high-temperature stability of the electrolyte by adding a new polycyclic additive and compounding the polycyclic additive with auxiliary lithium salt and film forming agent through synergistic effect, significantly reduces the CEI film impedance and charge transfer impedance of the electrolyte at the electrode interface, and improves the capacity retention rate and cycle capacity of the ternary lithium ion battery.

Description

Technical Field

[0001] This invention relates to an electrolyte and its preparation method, and more particularly to a ternary lithium-ion battery electrolyte and its preparation method. Background Technology

[0002] Since their commercialization in the 1990s, lithium-ion batteries have rapidly become the preferred power source for portable electronic devices, electric vehicles, and large-scale energy storage systems due to their core advantages such as high energy density, long cycle life, and low self-discharge rate. Currently, the mainstream lithium-ion batteries on the market use either lithium iron phosphate (LFP) or ternary lithium battery systems. LFP batteries dominate the low-to-mid-range market due to their excellent safety performance and lower cost; while ternary lithium batteries, relying on their high energy density resulting from high voltage and better low-temperature performance, have secured a place in the high-end market, but their high-voltage resistance, safety, and cycle life still need improvement.

[0003] In lithium-ion batteries, the electrolyte is considered the "blood" of the battery, undertaking three key functions: ion transport, electrode isolation, and interface stability. Its performance directly determines the battery's energy density, cycle life, fast-charging capability, and safety. However, lithium-ion batteries use flammable organic solvents as electrolytes, which can easily lead to thermal runaway under abnormal conditions such as overheating, impact, or overcharging, posing a risk of fire or even explosion.

[0004] CN119340486A discloses a non-aqueous electrolyte and a lithium-ion battery, which suppresses the problem of rapid impedance growth under high-temperature fast charging cycles by adding specific additives. However, it does not solve the safety problem of the battery under thermal runaway conditions, nor does it solve the problem of improving the electrolyte's high-voltage resistance performance at high temperatures. CN117525586A discloses an electrolyte and a lithium-ion battery containing the electrolyte, which forms an interface film on the positive and negative electrode surfaces of the battery through the synergistic effect of specific additives, reducing impedance and improving the electrolyte's room temperature, high-temperature cycling and high-temperature resistance performance. However, it does not solve the problem of high-voltage stability. Summary of the Invention

[0005] Purpose of the invention: The purpose of this invention is to provide a ternary lithium-ion battery electrolyte with good high-pressure stability and high-temperature stability. Another purpose of this invention is to provide a method for preparing the ternary lithium-ion battery electrolyte.

[0006] Technical solution: The ternary lithium-ion battery electrolyte of the present invention comprises the following components by mass fraction: 0.5-1% polycyclic additive, 10-12% electrolyte lithium salt, 0.6-1.5% auxiliary lithium salt, 0.3-3.5% film-forming agent, and the balance being an organic solvent. The polycyclic additive has the structure shown in formula (I): , Where R and R' are independently selected from C or S, R1, R2, R3, R4…R 2n R 2n+1 Each of the following is independently selected from H, ethylene carbonate, or ethylene sulfite, and n≥0.

[0007] Preferably, in the polycyclic additive structure, n=0, and R1 is selected from H, ethylene carbonate, or ethylene sulfite.

[0008] Preferably, the polycyclic additive is at least one of compounds A, B, C, and D with the following structures:

[0009] ,

[0010] Compounds A to D are cyclic carbonate / sulfite compounds with high dielectric constants, which can improve the ion conduction of the electrolyte and increase the electrolyte conductivity. Under high voltage conditions, compounds A to D can effectively reduce ohmic polarization and electrochemical polarization, reduce internal self-discharge of the battery, reduce the capacity loss rate of the battery under high voltage conditions, and improve the cycle life of the battery under high voltage conditions.

[0011] Further preferably, the polycyclic additive is at least one of compound B and compound D, and its structure contains sulfite groups, which can undergo polymerization reaction at higher temperatures to form non-flammable polyester, which can block the direct contact between the positive electrode material and the electrolyte and reduce the risk of combustion, thereby improving the safety of the battery during high-voltage operation. In addition, the electron-deficient sulfite can also capture the lattice oxygen released by the ternary material due to structural collapse, reduce the risk of combustion, and further improve the safety of the battery.

[0012] The electrolyte lithium salt is lithium hexafluorophosphate (LiPF6), with a concentration of 1.0~1.5 mol / L in the electrolyte, used to provide lithium ions.

[0013] The auxiliary lithium salt is a combination of lithium difluorooxalate borate (LiODFB) and lithium difluorophosphate (LiPO2F2). LiODFB has good ionic conductivity, which can increase the migration rate of lithium ions in the electrolyte, thereby improving the charge and discharge efficiency and rate performance of the battery. Moreover, the highest energy orbital (HOMO) of both LiODFB and LiPO2F2 is higher than that of the solvent used, which can preferentially decompose to generate lithium fluoride on the electrode surface, passivate the positive electrode-electrolyte interface, block the direct contact between the highly active positive electrode material and the electrolyte, suppress the oxidation side reaction of the electrolyte and the dissolution of transition metal ions, thereby effectively slowing down the capacity decay and impedance growth of the battery during high-voltage cycling and improving long-term cycle stability.

[0014] Furthermore, the lithium difluorooxalate borate has a mass fraction of 0.5-1.0% in the electrolyte, and the lithium difluorophosphate has a mass fraction of 0.1-0.5% in the electrolyte.

[0015] The film-forming agent is a compound of three components: tris(trimethylsilyl)phosphate (TMSP), fluoroethylene carbonate (FEC), and ethylene sulfate (DTD). TMSP can oxidize and decompose on the surface of the positive electrode of the battery at high potential to form a stable CEI film, effectively reducing the polarization voltage during charging and discharging. FEC can form a fluorine-containing SEI film on the surface of the negative electrode of the battery, improving high-voltage stability and reducing impedance. DTD can coordinate with lithium ions while forming a film on the electrode surface, improving the lithium ion coordination ability, reducing the decomposition of electrolyte and lithium salt under high voltage, improving the electrolyte's resistance to high-voltage oxidation, and reducing interfacial impedance.

[0016] Further, the mass fraction of the tris(trimethylsilyl)phosphate in the electrolyte is 0.1~0.5%, the mass fraction of the fluoroethylene carbonate in the electrolyte is 0.1~2.0%, and the mass fraction of the ethylene sulfate in the electrolyte is 0.1~1.0%.

[0017] The organic solvent includes at least one of ethylene carbonate, methyl ethyl carbonate, and diethyl carbonate.

[0018] Preferably, the organic solvent is a mixture of ethylene carbonate, methyl ethyl carbonate and diethyl carbonate in a volume ratio of (2~3):(3~4):(3~4).

[0019] The preparation method of the ternary lithium-ion battery electrolyte of the present invention includes the following steps: weighing and mixing organic solvents of corresponding mass fractions, adding electrolyte lithium salts of corresponding mass fractions to the mixed organic solvents, and mixing to form a homogeneous solution; adding polycyclic additives, auxiliary lithium salts and film-forming agents of corresponding mass fractions to the aforementioned homogeneous solution, and mixing evenly to obtain the electrolyte.

[0020] Preferably, the preparation of the ternary lithium-ion battery electrolyte is carried out in an anhydrous and oxygen-free environment.

[0021] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: By adding novel polycyclic additives and compounding them with auxiliary lithium salts and film-forming agents, the high-voltage stability and high-temperature stability of the electrolyte are effectively improved through synergistic effects, the CEI film impedance and charge transfer impedance of the electrolyte at the electrode interface are significantly reduced, and the capacity retention rate and cycle capacity of ternary lithium-ion batteries are improved. Detailed Implementation

[0022] The technical solution of the present invention will be further described below with reference to the embodiments. Unless otherwise specified, all reagents used are commercially available and used directly without purification. The polycyclic additives A, B, C, and D are prepared according to the following method.

[0023] Preparation of polycyclic additive A

[0024] Erythritol (butanetetroxide) and dimethyl carbonate in a molar ratio of 1:2.5 were added to the reactor and dissolved using an excess of toluene / acetonitrile mixture (molar ratio 9:1). The mixture was then reacted at 55°C and 500 mbar under the catalysis of 5% DBU (1,8-diazabicyclo[5.4.0]undecene) of the total raw material mass until the white solid in the reactor disappeared and regenerated. The reaction was continued for 2 hours, then the reaction was stopped, and the reaction solution was filtered. The solid obtained was polycyclic additive A.

[0025] Preparation of polycyclic additive B

[0026] Erythritol (butanetetroxide) and dimethyl sulfate were added to the reactor in a molar ratio of 1:2.5 and dissolved using an excess of toluene / acetonitrile mixture (molar ratio 9:1). The reaction was carried out under the catalysis of 5% DBU (1,8-diazabicyclo[5.4.0]undecene) of the total raw material mass at 55°C and a vacuum of 500 mbar until the white solid in the reactor disappeared and regenerated. The reaction was continued for 2 hours, then the reaction was stopped, and the reaction solution was filtered. The solid obtained was polycyclic additive B.

[0027] Preparation of polycyclic additive C

[0028] Sorbitol (hexanediol) and dimethyl carbonate in a molar ratio of 1:3.75 were added to the reactor and dissolved using an excess of toluene / acetonitrile mixture (molar ratio 9:1). The mixture was catalyzed by 5% DBU (1,8-diazabicyclo[5.4.0]undecene) of the total raw material mass and reacted at 55°C and 500 mbar vacuum until the white solid in the reactor disappeared and regenerated. The reaction was continued for 2 hours, then stopped. The reaction solution was filtered, and the solid obtained was polycyclic additive C.

[0029] Preparation of polycyclic additive D

[0030] Sorbitol (hexanediol) and tert-butyldimethylsilyl ether were added to a reactor in a molar ratio of 1:2.5 and dissolved in excess DMF (N,N-dimethylformamide). The reaction was carried out at 5°C for 6 hours under the catalysis of sodium tert-butoxide at 1% of the total mass of the raw materials. After the reaction was completed, the mixture was quenched with water and the organic phase was concentrated to obtain intermediate 1.

[0031] Intermediate 1 and dimethyl carbonate were added to the reactor in a molar ratio of 1:1.25 and dissolved in an excess toluene / acetonitrile mixture (molar ratio 9:1). The mixture was then reacted at 55°C and 500 mbar under the catalysis of 5% DBU (1,8-diazabicyclo[5.4.0]undecene) of the total raw material mass until the white solid in the reactor disappeared and regenerated. The reaction was continued for 2 hours, then the reaction was stopped, and the reaction solution was filtered. The solid obtained was intermediate 2.

[0032] At room temperature, an ethanol solution of 10% HCl was added to intermediate 2, and the mixture was stirred for 4 hours. The mixture was then quenched with water, and the organic phase was concentrated to obtain intermediate 3. Intermediate 3 and dimethyl sulfate were added to the reactor in a molar ratio of 1:2.5 and dissolved using an excess toluene / acetonitrile mixture (molar ratio 9:1). The mixture was then reacted at 55°C and a vacuum of 500 mbar under the catalysis of 5% DBU (1,8-diazabicyclo[5.4.0]undecene) from the total mass of the raw materials. The reaction continued for 2 hours until the white solid in the reactor disappeared and regenerated. The reaction was then stopped, and the reaction solution was filtered. The solid obtained was polycyclic additive D.

[0033] Examples 1-10

[0034] The ternary lithium-ion battery electrolyte provided by this invention contains the following components by mass percentage, with the balance being organic solvent. The specific proportions are shown in Table 1.

[0035] In an anhydrous and oxygen-free environment, weigh out the corresponding mass fractions of organic solvent and mix them. Add the corresponding mass fraction of LiPF6 to the mixed organic solvent and mix to form a homogeneous solution. Add the corresponding mass fractions of polycyclic additives, LiODFB, LiPO2F2, TMSP, FEC and DTD to the above homogeneous solution and mix them evenly to obtain the electrolyte.

[0036] Table 1 Component Allocation Ratio of Examples Serial Number Solvent (V / V / V) Lithium salts (wt%) TMSP (wt%) FEC (wt%) DTD (wt%) Polycyclic additives (wt%) 1 EC:EMC:DEC2:3:3 <![CDATA[LiPF6:12%LiODFB:1.0%LiPO2F2:0.5%]]> 0.5% 2.0% 1.0% A：1.0% 2 EC:EMC:DEC2:3:3 <![CDATA[LiPF6:12%LiODFB:1.0%LiPO2F2:0.5%]]> 0.5% 2.0% 1.0% B：1.0% 3 EC:EMC:DEC2:3:3 <![CDATA[LiPF6:12%LiODFB:1.0%LiPO2F2:0.5%]]> 0.5% 2.0% 1.0% C：1.0% 4 EC:EMC:DEC2:3:3 <![CDATA[LiPF6:12%LiODFB:1.0%LiPO2F2:0.5%]]> 0.5% 2.0% 1.0% D：1.0% 5 EC:EMC:DEC2:3:3 <![CDATA[LiPF6:12%LiODFB:1.0%LiPO2F2:0.5%]]> 0.5% 2.0% 1.0% A：0.5% 6 EC:EMC:DEC2:3:3 <![CDATA[LiPF6:12%LiODFB:1.0%LiPO2F2:0.5%]]> 0.5% 2.0% 1.0% B：0.5% 7 EC:EMC:DEC2:3:3 <![CDATA[LiPF6:12%LiODFB:1.0%LiPO2F2:0.5%]]> 0.5% 2.0% 1.0% C：0.5% 8 EC:EMC:DEC2:3:3 <![CDATA[LiPF6:12%LiODFB:1.0%LiPO2F2:0.5%]]> 0.5% 2.0% 1.0% D：0.5% 9 EC:EMC:DEC2:3:3 <![CDATA[LiPF6:10%LiODFB:0.5%LiPO2F2:0.1%]]> 0.1% 0.1% 0.1% B：1.0% 10 EC:EMC:DEC2:3:3 <![CDATA[LiPF6:11%LiODFB:0.7%LiPO2F2:0.3%]]> 0.3% 0.15% 0.5% B：1.0%

[0037] Note: In Table 1, “V / V / V” represents the volume ratio.

[0038] The electrolyte of the example was subjected to the following electrochemical performance tests, and the test results are shown in Table 2:

[0039] (1) High voltage stability: Pt sheet was used as working electrode, lithium metal as counter electrode and reference electrode, and assembled into a three-electrode system in a glove box under nitrogen protection. The assembled three electrodes were added to the electrolyte to be tested and linear voltammetry scan was performed to test its oxidation potential.

[0040] (2) Cyclic capacity: After activating the above three electrodes at a rate of 0.1C for two cycles, cycle them at a rate of 1C for 500 cycles and perform cyclic voltammetry test to test their cyclic capacity;

[0041] (3) Capacity retention: using LiNi 0.8 Mn 0.1 Co 0.1 O2 was used as the positive electrode and lithium metal was used as the negative electrode to assemble a button battery. Charge and discharge tests were conducted within a charge and discharge voltage range of 2.5~4.8V and a charge and discharge current rate of 1C=280mA / g to test its capacity retention rate.

[0042] (4) Impedance: Using the aforementioned button cell, the impedance was tested using a CHI760E electrochemical workstation under an AC excitation signal with an amplitude of ±5mV, a frequency range of 10mHz~105Hz, and a test potential of 4.3V.

[0043] (5) Thermal stability: The battery was subjected to a nail penetration test, and the subsequent condition of the battery was observed.

[0044] Table 2 Electrochemical performance test results of the examples Serial Number Oxidation potential (V) Capacity (mAh / g) after 500 cycles Capacity retention Impedance (mΩ) thermal stability 1 4.75 188.3 88.8% <![CDATA[R CEI =23.6R CT =80.5]]> No fire or explosion 2 4.76 189.2 89.8% <![CDATA[R CEI =22.1R CT =78.4]]> No fire or explosion 3 4.79 186.0 85.9% <![CDATA[R CEI =25.4R CT =92.5]]> No fire or explosion 4 4.79 187.1 88.7% <![CDATA[R CEI =24.2R CT =86.4]]> No fire or explosion 5 4.73 182.5 85.3% <![CDATA[R CEI =23.5R CT =79.7]]> No fire or explosion 6 4.72 181.3 84.7% <![CDATA[R CEI =21.9R CT =78.1]]> No fire or explosion 7 4.76 181.8 85.0% <![CDATA[R CEI =25.2R CT =89.6]]> No fire or explosion 8 4.77 180.8 84.5% <![CDATA[R CEI =23.9R CT =86.1]]> No fire or explosion 9 4.69 178.5 83.2% <![CDATA[R CEI =29.8R CT =100.1]]> No fire or explosion 10 4.72 176.2 82.3% <![CDATA[R CEI =27.9R CT =96.3]]> No fire or explosion

[0045] Comparative Examples 1-16

[0046] Electrolytes for each comparative example were prepared in a similar manner to those in the examples, with variations in the proportions of each component and the types of polycyclic additives. The specific proportions are shown in Table 3.

[0047] Table 3 Comparative group distribution ratio Serial Number Solvent (V / V / V) Lithium salts (wt%) TMSP (wt%) FEC (wt%) DTD (wt%) Polycyclic additives (wt%) 1 EC:EMC:DEC2:3:3 <![CDATA[LiPF6:12%LiODFB:1.0%LiPO2F2:0.5%]]> 0.5% 2.0% 1.0% / 2 EC:EMC:DEC2:3:3 <![CDATA[LiPF6:12%LiODFB:1.0%LiPO2F2:0.5%]]> 0.5% 2.0% 1.0% VC: 1.0% 3 EC:EMC:DEC2:3:3 <![CDATA[LiPF6:12%LiODFB:1.0%LiPO2F2:0.5%]]> 0.5% 2.0% 1.0% ES: 1.0% 4 EC:EMC:DEC2:3:3 <![CDATA[LiPF6:12%LiODFB:1.0%LiPO2F2:0.5%]]> 0.5% 2.0% 1.0% VC: 0.5% ES: 0.5% 5 EC:EMC:DEC2:3:3 <![CDATA[LiPF6:12%]]> / / / / 6 EC:EMC:DEC2:3:3 <![CDATA[LiPF6:12%LiODFB:1.0%LiPO2F2:0.5%]]> 0.5% 2.0% 1.0% A：0.2% 7 EC:EMC:DEC2:3:3 <![CDATA[LiPF6:12%LiODFB:1.0%LiPO2F2:0.5%]]> 0.5% 2.0% 1.0% B：0.2% 8 EC:EMC:DEC2:3:3 <![CDATA[LiPF6:12%LiODFB:1.0%LiPO2F2:0.5%]]> 0.5% 2.0% 1.0% C：0.2% 9 EC:EMC:DEC2:3:3 <![CDATA[LiPF6:12%LiODFB:1.0%LiPO2F2:0.5%]]> 0.5% 2.0% 1.0% D：0.2% 10 EC:EMC:DEC2:3:3 <![CDATA[LiPF6:12%LiODFB:1.0%LiPO2F2:0.5%]]> 0.5% 2.0% 1.0% A：1.1% 11 EC:EMC:DEC2:3:3 <![CDATA[LiPF6:12%LiODFB:1.0%LiPO2F2:0.5%]]> 0.5% 2.0% 1.0% B：1.1% 12 EC:EMC:DEC2:3:3 <![CDATA[LiPF6:12%LiODFB:1.0%LiPO2F2:0.5%]]> 0.5% 2.0% 1.0% C：1.1% 13 EC:EMC:DEC2:3:3 <![CDATA[LiPF6:12%LiODFB:1.0%LiPO2F2:0.5%]]> 0.5% 2.0% 1.0% D：1.1% 14 EC:EMC:DEC2:3:3 <![CDATA[LiPF6:12%LiODFB:1.0%LiPO2F2:0.5%]]> 0.5% 2.0% / B：1.0% 15 EC:EMC:DEC2:3:3 <![CDATA[LiPF6:12%LiODFB:1.0%LiPO2F2:0.5%]]> / / / B：1.0% 16 EC:EMC:DEC2:3:3 <![CDATA[LiPF6:12%]]> 0.5% 2.0% 1.0% B：1.0%

[0048] Note: In Table 3, "V / V / V" represents the volume ratio. VC represents vinylene carbonate. ES represents vinyl sulfite. " / " indicates that it does not exist.

[0049] The electrolytes of each comparative example were subjected to the same electrochemical performance tests as those in the examples, and the test results are shown in Table 4.

[0050] Table 4 Comparative electrochemical performance test results Serial Number Oxidation potential (V) Capacity (mAh / g) after 500 cycles Capacity retention Impedance (mΩ) thermal stability 1 4.51 150.6 74.2% <![CDATA[R CEI =31.2R CT =103.4]]> thermal runaway 2 4.62 155.7 75.6% <![CDATA[R CEI =37.8R CT =126.3]]> thermal runaway 3 4.57 150.8 70.5% <![CDATA[R CEI =32.1R CT =109.4]]> thermal runaway 4 4.59 152.4 71.2% <![CDATA[R CEI =33.5R CT =113.2]]> thermal runaway 5 4.32 90.5 46.2% <![CDATA[R CEI =52.8R CT =161.2]]> Spontaneous combustion 6 4.64 176.9 86.3% <![CDATA[R CEI =23.8R CT =81.2]]> No fire or explosion 7 4.71 181.8 85.0% <![CDATA[R CEI =22.3R CT =79.5]]> No fire or explosion 8 4.70 180.7 84.4% <![CDATA[R CEI =25.5R CT =92.6]]> No fire or explosion 9 4.75 181.1 84.6% <![CDATA[R CEI =24.4R CT =86.8]]> No fire or explosion 10 4.75 179.9 84.1% <![CDATA[R CEI =25.1R CT =88.6]]> No fire or explosion 11 4.76 178.2 83.3% <![CDATA[R CEI =22.5R CT =80.1]]> No fire or explosion 12 4.77 175.3 81.9% <![CDATA[R CEI =25.5R CT =92.8]]> No fire or explosion 13 4.81 177.6 83.0% <![CDATA[R CEI =26.1R CT =94.3]]> No fire or explosion 14 4.73 178.0 83.2% <![CDATA[R CEI =25.5R CT =92.1]]> No fire or explosion 15 4.58 161.2 75.3% <![CDATA[R CEI =31.1R CT =101.2]]> No fire or explosion 16 4.67 158.2 73.9% <![CDATA[R CEI =34.4R CT =116.1]]> No fire or explosion

[0051] To further investigate the effect of polycyclic additives on the flame retardancy of electrolytes under thermal runaway conditions, dimer polycyclic additive A (referred to as Example 17) and trimer polycyclic additive C (referred to as Example 18) were added to the electrolyte of Comparative Example 1, respectively. Their thermal stability was tested using the same method as in the examples. The corresponding polycyclic additives were added until the electrolyte was completely non-flammable, and the amount of polycyclic additive used was recorded. In Example 17, when the electrolyte was completely non-flammable, 8% by mass of dimer polycyclic additive A was added; in Example 18, when the electrolyte was completely non-flammable, 6.5% by mass of trimer polycyclic additive C was added. This shows that the flame retardancy of a unit content of trimer polycyclic additive is better than that of dimer polycyclic additive. This is because the longer the carbon chain, the more special functional groups it links, which can effectively capture the lattice oxygen released by the ternary material, thus improving its flame retardancy.

[0052] As shown in Tables 2 and 4, the ternary lithium-ion battery electrolyte of the present invention exhibits superior performance in terms of high-voltage stability, capacity retention, capacity after 500 cycles, charge transfer impedance, CEI film impedance, and thermal stability. It solves the problems of severe electrolyte decomposition and gas generation and poor safety under high-voltage conditions in ternary lithium-ion batteries, and has significant advantages in practical applications of ternary lithium-ion batteries.

[0053] Comparing Examples 1-8 with Comparative Examples 1-4, it can be seen that polycyclic additives play a dominant role in improving the overall electrochemical performance and heat resistance of the electrolyte. The addition of polycyclic additives optimizes the formed film, introducing organic components such as polycarbonate / sulfite into the SEI and CEI films. This mitigates the embrittlement caused by the formation of films with excessive inorganic components such as LiF, improves film elongation, enhances the mechanical strength of the film, and effectively prevents problems such as continuous electrolyte decomposition and battery short circuits caused by CEI film rupture. This, in turn, increases the oxidation potential of the electrolyte, ultimately improving the battery's high-voltage resistance. Simultaneously, the special polycarbonate / sulfite groups in the polycyclic additives can undergo polymerization at higher temperatures to form non-flammable polyesters, which can block direct contact between the cathode material and the electrolyte and reduce the risk of combustion, improving the safety of the battery during high-voltage operation. Furthermore, the special electron-deficient sulfite can also capture lattice oxygen released by the ternary material due to structural collapse, reducing the risk of combustion and further improving battery safety.

[0054] When all other components are the same, without the addition of polycyclic additives, the high-voltage stability, capacity retention, capacity after 500 cycles, charge transfer impedance, CEI film impedance, and thermal stability of the electrolyte are significantly reduced. Furthermore, even using equal amounts of polycyclic additive monomers VC or ES has limited improvement on the electrochemical performance and heat resistance of the electrolyte. This is because VC or ES have a strong ability to participate in film formation, resulting in a thicker SEI film with higher internal resistance. In addition, regarding the type of polycyclic additive, it can be seen that when the dosage is the same, trimer-based polycyclic additives have a higher impedance than dimer-based polycyclic additives. However, trimers, as additives, have stronger high-voltage resistance than dimers. This is because longer carbon chains have more linked special functional groups, which can effectively capture lattice oxygen released from the ternary material. Moreover, longer molecular carbon chains have higher oxidation potentials, thus effectively improving the electrolyte's high-voltage resistance. Meanwhile, when the number of polymerizations of polycyclic additives is the same, polycyclic additives with R=S in the structure have better cycle performance and lower impedance than polycyclic additives with R=C in the same amount. This is because sulfite groups are easy to open in the electrolyte and combine with lithium ions to form lithium sulfite / sulfonate, which is used to regulate the CEI film. This type of lithium salt can effectively adjust the thickness of the CEI film, improve conductivity and reduce impedance.

[0055] Further comparison of Examples 1-8 and Comparative Examples 6-13 shows that the amount of polycyclic additives also has a significant impact on the overall electrochemical performance and heat resistance of the electrolyte. When using the same type of polycyclic additive, within a certain range, as the content increases, the high-voltage stability, capacity retention, and capacity after 500 cycles of the electrolyte increase, but the charge transfer impedance and CEI film impedance decrease. However, when the amount of polycyclic additive is further increased, its electrochemical performance decreases. This is because the longer the carbon chain of the polycyclic additive, the higher its impedance, which leads to a decrease in cycle performance. Therefore, adding polycyclic additives to improve flame retardancy comes at the cost of reduced cycle life. Only by adding polycyclic additives within the range described in this patent can the safety performance of the electrolyte be significantly improved, and the high-voltage resistance of the electrolyte be enhanced.

[0056] Comparing Example 2 with Comparative Examples 5, 14, 15, and 16, it can also be seen that the addition of film-forming agents and auxiliary lithium salts in the electrolyte plays a certain role in improving the electrolyte performance. Specifically, DTD and polycyclic additives have a synergistic effect. Due to the large steric hindrance of polycyclic additives, coordination with lithium ions is difficult. However, their similar structure to DTD can promote the dissociation of lithium ions and enhance the coordination between lithium ions and DTD, thereby improving the interaction between lithium ions and hexafluoride anions. This keeps the hexafluoride anions away from the positive electrode surface, reducing the decomposition of the electrolyte and lithium salt under high voltage, improving the electrolyte's resistance to high-voltage oxidation, and reducing interfacial impedance.

Claims

1. A ternary lithium-ion battery electrolyte, characterized in that, The composition comprises the following components by mass fraction: 0.5-1% polycyclic additive, 10-12% electrolyte lithium salt, 0.6-1.5% auxiliary lithium salt, 0.3-3.5% film-forming agent, and the balance being an organic solvent. The polycyclic additive has the structure shown in formula (I). ， Where R and R' are independently selected from C or S, R1, R2, R3, R4…R 2n R 2n+1 Each of the following is independently selected from H, ethylene carbonate, or ethylene sulfite, and n≥0.

2. The ternary lithium-ion battery electrolyte according to claim 1, characterized in that, In the polycyclic additive structure, n=0, and R1 is selected from H, ethylene carbonate, or ethylene sulfite.

3. The ternary lithium-ion battery electrolyte according to claim 1 or 2, characterized in that, The polycyclic additive is at least one of compounds A, B, C, and D with the following structures: 。 4. The ternary lithium-ion battery electrolyte according to claim 1, characterized in that, The electrolyte lithium salt is lithium hexafluorophosphate.

5. The ternary lithium-ion battery electrolyte according to claim 1, characterized in that, The auxiliary lithium salt is a combination of lithium difluorooxalate borate and lithium difluorophosphate.

6. The ternary lithium-ion battery electrolyte according to claim 5, characterized in that, The lithium difluorooxalate borate has a mass fraction of 0.5-1.0% in the electrolyte, and the lithium difluorophosphate has a mass fraction of 0.1-0.5% in the electrolyte.

7. The ternary lithium-ion battery electrolyte according to claim 1, characterized in that, The film-forming agent is a compound of three components: tris(trimethylsilyl)phosphate, fluoroethylene carbonate, and ethylene sulfate.

8. The ternary lithium-ion battery electrolyte according to claim 7, characterized in that, The mass fraction of the tris(trimethylsilyl)phosphate in the electrolyte is 0.1-0.5%, the mass fraction of the fluoroethylene carbonate in the electrolyte is 0.1-2.0%, and the mass fraction of the ethylene sulfate in the electrolyte is 0.1-1.0%.

9. The ternary lithium-ion battery electrolyte according to claim 1, characterized in that, The organic solvent includes at least one of ethylene carbonate, methyl ethyl carbonate, and diethyl carbonate.

10. A method for preparing a ternary lithium-ion battery electrolyte as described in claim 1, characterized in that, The process includes the following steps: weighing and mixing organic solvents of corresponding mass fractions, adding electrolyte lithium salts of corresponding mass fractions to the mixed organic solvents, and mixing to form a homogeneous solution; adding polycyclic additives, auxiliary lithium salts and film-forming agents of corresponding mass fractions to the aforementioned homogeneous solution, and mixing evenly to obtain an electrolyte.

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