Electrolyte and lithium battery

Abstract

This application relates to the field of batteries, and discloses an electrolyte and a lithium battery. The electrolyte includes an organic solvent, a lithium salt, a diluent, and an additive. The diluent is selected from compounds with the structure shown in formula (1): Formula (1), where R1~R 10 Each is independently selected from H or F, and R1~R 10 The electrolyte contains at least four F atoms. The mass percentage of the diluent in the electrolyte is 'a', and the mass percentage of the organic solvent in the electrolyte is 'b', where the ratio of a to b is greater than 0 and less than or equal to 0.7. The electrolyte of this embodiment can be used in high-nickel / silicon-carbon batteries to improve battery stability and reduce the rate of battery capacity decay.

Description

Technical Field

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

[0002] Current high-energy-density battery systems, such as those using high-nickel ternary materials and lithium-rich manganese-based cathodes, require high voltages, such as >4.5 V, to leverage their advantages of high specific capacity (>210 mAh / g) and high energy density (>350 Wh / kg). However, traditional carbonate-based electrolytes are prone to oxidative decomposition at high voltages, leading to intensified interfacial side reactions such as transition metal dissolution, lattice oxygen release, and cathode phase transitions, which in turn cause capacity decay and voltage drop. To improve the electrolyte's oxidation resistance, a high-concentration electrolyte strategy can be adopted, increasing the lithium salt concentration to 3-5 mol / L. This increases the complexation degree between lithium ions and the solvent, forming a stable solvation structure, inhibiting electrolyte decomposition, and enhancing electrode-electrolyte interface passivation, thereby broadening the electrochemical window.

[0003] However, in existing high-concentration electrolytes, traditional diluents such as cyclic carbonates (e.g., ethylene carbonate EC) and chain carbonates (e.g., dimethyl carbonate DMC, diethyl carbonate DEC) can effectively dissociate lithium salts due to their high dielectric constant (ε≈89) and low viscosity (η≈0.59 mPa·s), optimizing lithium-ion migration rates and improving the ionic conductivity of the electrolyte (>10 mS / cm). Simultaneously, they form a rich electrolyte on the surface of the negative electrode material through reduction and decomposition. Solid electrolyte interface (SEI) films are formed with LiF. However, existing diluents have weak stability and are prone to oxidation and decomposition in the high-voltage operating range above 4.3V of high-nickel cathodes, generating byproducts such as CO2 and H2O, leading to the dissolution of transition metals and structural collapse of the cathode material. On the other hand, for batteries with silicon-carbon anode materials, the high specific surface area and volume expansion characteristics of silicon-carbon anodes require higher wettability of the electrolyte. The SEI film formed by traditional diluents has low mechanical strength and cannot withstand the 300% volume expansion of silicon-carbon anodes, resulting in repeated cracking and repair of the electrode-electrolyte interface, which in turn leads to the growth of lithium dendrites and rapid capacity decay of the battery. Summary of the Invention

[0004] This application discloses an electrolyte and a lithium battery to solve the problem of rapid battery capacity decay caused by the poor stability of existing electrolytes.

[0005] To achieve the above objectives, this application provides the following technical solution:

[0006] In a first aspect, this application provides an electrolyte comprising an organic solvent, a lithium salt, a diluent, and an additive, wherein the diluent is selected from compounds with the structure shown in formula (1):

[0007] ,

[0008] Equation (1)

[0009] In equation (1), R1~R 10 Each is independently selected from H or F, and R1~R 10 It contains at least four Fs;

[0010] The mass percentage of the diluent in the electrolyte is 'a', the mass percentage of the organic solvent in the electrolyte is 'b', and the ratio of 'a' to 'b' is greater than 0 and less than or equal to 0.7.

[0011] The electrolyte of this application contains a diluent with a cyclic fluorinated structure, which can limit the solvent itself and Li + The coordination ability of Li prevents the solvent from participating in the solvation sheath, thus enabling Li to... + It tends to combine with anions, ultimately forming a superior SEI film on the negative electrode surface, reducing the reduction and decomposition of the electrolyte on the negative electrode material surface, and maintaining the high ionic conductivity of the electrolyte. Simultaneously, the low surface tension of the diluent (e.g., less than or equal to 22 mN / m) can improve the porosity wettability of the silicon negative electrode, alleviate expansion stress during cycling, and improve the battery's high-temperature storage and cycle performance. By controlling the ratio of diluent to solvent, the electrolyte in high-nickel / silicon-carbon battery systems can enhance the interaction between the solvent and lithium salt, raising the battery's stability window to above 4.5V, thereby increasing the battery's energy density, while also giving the battery excellent high-temperature resistance and low gas production performance. Therefore, the electrolyte of this application, when applied to lithium-ion batteries, can form a more stable and uniform SEI film on the negative electrode surface, reducing the continuous reaction between the electrolyte and electrode materials and lowering the capacity decay rate.

[0012] Furthermore, in batteries containing silicon-carbon materials, the surface energy of the silicon-carbon materials can range from 0.5 to 1.2 J / m². 2 The higher the silicon content, the denser the carbon layer coating, and the closer the surface energy is to pure silicon (e.g., 1.3 J / m²). 2 By adjusting the ratio of diluent to silicon-carbon anode, the surface tension of the electrolyte and the surface energy of the anode can be matched, improving the interfacial compatibility between the electrode and electrolyte, forming a uniform and dense SEI film on the electrode surface, increasing the ionic conductivity of the electrolyte, and enhancing the rate performance of the battery.

[0013] Secondly, this application also provides a lithium battery, which includes a positive electrode, a negative electrode, a separator, and the electrolyte of this application.

[0014] The battery of this application contains the electrolyte of this application, and by adding a diluent with a cyclic fluorinated structure to the electrolyte, the solvent itself and Li can be limited. + The coordination ability of Li prevents the solvent from participating in the solvation sheath, thus enabling Li to... + It tends to combine with anions, ultimately forming a superior SEI film on the negative electrode surface, reducing the reduction and decomposition of the electrolyte on the negative electrode material surface, and maintaining the high ionic conductivity of the electrolyte. Simultaneously, the low surface tension of the diluent improves the porosity wettability of the silicon negative electrode, alleviating expansion stress during cycling. By controlling the ratio of diluent to solvent, the electrolyte in a high-nickel / silicon-carbon battery system can enhance the interaction between the solvent and lithium salt, raising the battery's stability window to above 4.5V, thereby increasing the battery's energy density, while also giving the battery excellent high-temperature resistance and low gas production performance. Therefore, the battery of this application, when containing the electrolyte of this application, can form a more stable and uniform SEI film on the surface of the negative electrode, reducing the continuous reaction between the electrolyte and electrode materials, and lowering the capacity decay rate.

[0015] Furthermore, in batteries containing silicon-carbon materials, the surface energy of the silicon-carbon materials can range from 0.5 to 1.2 J / m². 2 The higher the silicon content, the denser the carbon layer coating, and the closer the surface energy is to pure silicon (e.g., 1.3 J / m²). 2 By adjusting the ratio of diluent to silicon-carbon anode, the surface tension of the electrolyte and the surface energy of the anode can be matched, improving the interfacial compatibility between the electrode and electrolyte, forming a uniform and dense SEI film on the electrode surface, increasing the ionic conductivity of the electrolyte, and enhancing the rate capability and high-temperature performance of the battery. Detailed Implementation

[0016] The technical solutions in the embodiments of this application will be clearly and completely described below with reference to the embodiments of this application. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0017] In batteries with high-concentration electrolytes, to improve the stability of the SEI film due to the poor antioxidant performance of existing diluents, this application provides an electrolyte and a high-nickel / silicon-carbon battery with optimal content. The diluent of this application embodiment can be used in batteries with high-concentration electrolytes in a high-nickel / silicon-carbon system to improve the battery's rate capability and high-temperature stability, and reduce the battery capacity decay rate. The electrolyte of this application embodiment has the following properties: (1) High voltage compatibility: high oxidation stability, able to resist the strong oxidation environment of high-nickel cathode materials, avoiding electrolyte decomposition and gas generation and thickening of the cathode-electrolyte interface (CEI) film; (2) Silicon-carbon anode compatibility: low Li + Coordination ability and low surface tension, the diluent should avoid participating in the solvation sheath, reduce reduction and decomposition on the silicon surface, and improve the porosity wettability of the silicon anode to effectively alleviate volume expansion; (3) Kinetic optimization: low viscosity and wide temperature range, which can reduce the migration resistance of ions and improve the rate performance of the battery. At the same time, the surface energy of the electrode material is moderate, and the electrolyte can be uniformly covered on its surface to form a dense and uniform SEI film, which improves the high-temperature storage and cycle performance of the battery.

[0018] To achieve the above effects, the electrolyte in this application embodiment comprises the following components: an organic solvent, a lithium salt, a diluent, and an additive, wherein the diluent is selected from compounds with the structure shown in formula (1):

[0019] ,

[0020] Equation (1)

[0021] In equation (1), R1~R 10 Each is independently selected from H or F, and R1~R 10 It contains at least four Fs; the mass percentage of the diluent in the electrolyte is a, the mass percentage of the organic solvent in the electrolyte is b, and the ratio of a to b is greater than 0 and less than or equal to 0.7.

[0022] In the electrolyte of this application embodiment, the diluent shown in formula (1) contains at least 4 F elements, or for example, 4 to 10, or for example, 4 to 8. However, the specific location of F is not limited in this application embodiment. F can be present in R1 to R2. 10 Any position in the middle. For example, the number of F can be 4, 5, 6, 7, 8, 9 or 10.

[0023] In one embodiment, in equation (1), R3~R8 is F, R1~R2, R9~R 10 H is present. Therefore, the compound shown in formula (1) is 3,3,4,4,5,5-hexafluorotetrahydropyran, and its structure can be described as follows:

[0024] Formula (II).

[0025] The electrolyte of this application embodiment contains a diluent with a cyclic fluorinated structure. The fluorine element (F) has strong electron-withdrawing properties, which lowers the highest occupied molecular orbital energy level of the system, resulting in a low HOMO energy level for the diluent, below -10.8 eV. This diluent structure is difficult to oxidize, thus increasing the overall antioxidant capacity of the electrolyte. This improved antioxidant performance allows the electrolyte to withstand higher operating voltages of the cathode material, preventing electrolyte decomposition, extending battery cycle life, and ultimately increasing the battery's energy density by widening the battery's operating voltage window. Therefore, the electrolyte of this application embodiment, by adding a diluent with the structure shown in formula (1), can improve the stability of the electrolyte to the cathode material, facilitate the formation of a lithium fluoride-rich electrode-electrolyte interface (CEI) film, inhibit lithium dendrite growth, reduce interfacial side reactions, improve the stability of the electrode interface, enhance battery cycle performance, and solve the problem of rapid battery capacity decay caused by poor stability of existing electrolytes.

[0026] Furthermore, on the negative electrode side, the diluent with the cyclic fluorination structure of formula (1), after being added to the electrolyte, can limit the solvent itself and Li. + The coordination ability of Li prevents the solvent from participating in the solvation sheath, thus enabling Li to... + It tends to combine with anions, ultimately forming a superior SEI film on the negative electrode surface, reducing the reduction and decomposition of the electrolyte on the negative electrode material surface, and maintaining the high ionic conductivity of the electrolyte. When the silicon-carbon content in the negative electrode material is moderate, the electrode has the optimal surface energy (0.8-1.1 J / m). 2 The diluent with a cyclic fluorinated structure in the electrolyte of this application has a low surface tension that matches the high surface energy of the negative electrode material, thereby improving the interfacial compatibility between the electrode and the electrolyte, alleviating the expansion stress during battery cycling, and improving the battery's rate and high-temperature performance.

[0027] In this embodiment, the mass ratio of diluent to organic solvent in the electrolyte is controlled within a range greater than 0 and less than or equal to 0.7. The ratio of the diluent in the electrolyte to the silicon-carbon material in the negative electrode material is controlled within a range greater than 1 and less than or equal to 6. Only when both are in synergy can the battery's rate capability and high-temperature performance be improved, while reducing the battery degradation rate. The mass percentage of diluent in the electrolyte can be represented by 'a', the mass percentage of organic solvent in the electrolyte by 'b', and the mass percentage of silicon-carbon material in the negative electrode material by 'c'. In one embodiment, the mass ratio of diluent to organic solvent a / b can be 0.05~0.7, or for example, 0.1~0.7, 0.1~0.6, or 0.15~0.5. For example, the value of a / b can be any value between two numbers, such as 0.01, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.55, 0.6, 0.65, or 0.7 or higher. The value of a / c can be any value or a value between two numbers, such as 1, 2, 2.5, 3, 4.5, 5, 5.5, or 6 or higher.

[0028] In one embodiment of this application, the diluent accounts for 10% to 40% of the mass of the electrolyte. Exemplarily, the mass percentage of the diluent in the electrolyte can be any two values ​​between 10%, 12%, 15%, 18%, 20%, 22%, 25%, 28%, 30%, 32%, 35%, 38%, or 40% or more, and the mass percentage of the silicon-carbon material in the negative electrode material can be any two values ​​between 5%, 8%, 10%, 12%, 15%, 18%, 20%, 22%, 25%, 28%, or 30% or more.

[0029] If the ratio of diluent to organic solvent is not properly matched, the diluent and solvent cannot achieve effective synergy, which will not only reduce the overall conductivity and stability of the electrolyte, but also cause the lithium salt to not completely dissolve due to insufficient dilution, ultimately weakening the cycle performance of the battery and causing a significant increase in the direct current resistance (DCR) of the battery. If the surface tension of the electrolyte and the surface of the electrode cannot be matched, it will lead to (1) the surface tension being too low, causing the electrolyte to over-wet the electrode, resulting in binder swelling and electrode structure collapse; (2) the surface tension being too high, resulting in insufficient wetting, leaving more air in the negative electrode pores, leading to uneven growth of the SEI film, aggravating the side reactions of the battery, and accelerating capacity decay. Therefore, the electrolyte of this application, by adding a diluent with a cyclic fluorinated structure, can effectively reduce the viscosity of the electrolyte, improve the kinetics of the electrolyte, and make the active ions in the electrolyte, such as lithium ions, migrate more quickly in the electrolyte, reducing the DCR growth rate of high nickel / silicon-carbon batteries during use.

[0030] Tests have shown that the electrolyte of this application, when applied to corresponding high-nickel / silicon-carbon batteries, can increase the stable operating voltage window of the battery to above 4.5V, thereby improving the battery energy density, while also giving the battery excellent high-temperature resistance and low gas production performance.

[0031] In one embodiment of this application, the organic solvent in the electrolyte includes at least one of ethyl methyl carbonate, ethylene carbonate, propylene carbonate, or fluoroethylene carbonate.

[0032] The electrolyte in this embodiment can be a high-concentration electrolyte, wherein the lithium ion concentration can be greater than 3 mol / L. In the high-concentration electrolyte, a diluent with the structure shown in formula (1) of this application is miscible with ethylene carbonate EC, propylene carbonate PC, methyl ethyl carbonate EMC, and fluoroethylene carbonate FEC, which are organic solvents, but hardly dissolves lithium salts, maintaining the characteristics of the high-concentration electrolyte and well preserving the unique solvation structure of the high-concentration electrolyte, ensuring the stability of the battery during use. In this embodiment, the diluent with the structure shown in formula (1) is used in conjunction with the organic solvents ethylene carbonate, propylene carbonate, methyl ethyl carbonate, and fluoroethylene carbonate to stabilize the silicon-carbon material and form a stable SEI film on the negative electrode, improving the high-temperature stability of the battery.

[0033] In one embodiment of this application, the lithium salt includes lithium hexafluorophosphate (LiPF6) and lithium bisfluorosulfonyl imide (LiFSI), and the total concentration of LiPF6 and LiFSI in the electrolyte is greater than or equal to 3 mol / L.

[0034] In the electrolyte of this application embodiment, a mixed lithium salt of lithium hexafluorophosphate (LiPF6) and lithium bisfluorosulfonyl imide (LiFSI) is selected as the electrolyte. The total concentration, based on lithium ion concentration, is 3 mol / L, meaning the total molar concentration of LiPF6 and LiFSI in the electrolyte is 3 mol / L. This concentration of electrolyte is considered a high-efficiency electrolyte, which can reduce HF generation, improve battery thermal stability, and reduce the risk of corrosion of the aluminum current collector.

[0035] In one embodiment of this application, the molar ratio of LiPF6 to LiFSI is 0.7 to 2.3. Exemplarily, the molar ratio of LiPF6 to LiFSI can be any two values ​​between 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, or 2.3. Excessive LiPF6 content can react with FEC in the organic solvent to produce HF, further exacerbating side reactions within the battery. Excessive LiFSI can corrode the aluminum current collector, affecting battery performance. Therefore, controlling the molar ratio of LiPF6 to LiFSI within the range of 0.7 to 2.3 can reduce side reactions while minimizing electrolyte corrosion of the current collector.

[0036] In one embodiment of this application, the additive includes lithium difluorooxalate borate (LiODFB) and a compound additive. It is understood that the compound additive does not contain lithium difluorooxalate borate. As an example, the compound additive includes at least one of tris(trimethylsilyl)phosphate (TMSP), vinyl sulfate (DTD), 1,3-propenesulfonate lactone, and lithium difluorophosphate (LiPO2F2).

[0037] Lithium difluorooxalate borate was chosen as the additive. On one hand, it forms a boron-oxalate complex electrode-electrolyte interface film on the surface of the positive electrode, anchoring transition metal ions and reducing their dissolution, thus lowering battery gas production. On the other hand, it generates a low-resistance SEI film rich in LiF at the negative electrode, reducing the consumption of active lithium, improving battery cycle performance, and reducing DCR growth. Tris(trimethylsilyl) phosphate, vinyl sulfate, and lithium difluorophosphate were also selected as compounding additives. These can be compounded with lithium difluorooxalate borate to further improve the overall performance of the electrolyte.

[0038] In one embodiment, the lithium difluorooxalate borate comprises 1.5% to 2.5% of the electrolyte by mass. Exemplarily, the lithium difluorooxalate borate comprises 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, 2.3%, 2.4%, or 2.5% or more of any two values. In one embodiment, the compound additive comprises 1.5% to 2.5% of the electrolyte by mass.

[0039] The addition of lithium difluorooxalate borate (LiODFB) in a specific mass percentage to the electrolyte has the following effects: (1) LiODFB has high ionic conductivity, which helps to improve the charge and discharge efficiency of the battery and enhance its rate performance; (2) LiODFB can form a stable SEI film rich in LiF on the electrode surface, which can effectively inhibit the decomposition of the electrolyte and improve the cycle performance of the battery; (3) LiODFB has excellent high and low temperature performance, which can improve the adaptability of the battery to different ambient temperatures and reduce the influence of temperature on battery performance. In addition, at least one of tris(trimethylsilyl) phosphate, vinyl sulfate, 1,3-propenesulfonate lactone and lithium difluorophosphate is selected as a compound additive, and its total mass percentage in the electrolyte can be 1.8%~2.3%.

[0040] The electrolyte in this application has high oxidation stability and can be used in high-nickel / silicon-carbon battery systems, enabling the battery to maintain high stability under high voltages greater than 4.3V, thereby improving the rate capability and high-temperature cycle performance of high-energy-density batteries.

[0041] Based on the same technical objective, this application provides a lithium battery. The lithium battery includes a positive electrode, a negative electrode, a separator, and an electrolyte according to this application. The separator is disposed between the positive and negative electrode, and the electrolyte wets the separator.

[0042] The positive electrode sheet may include a positive current collector and a positive electrode material layer disposed on at least one side of the positive current collector. The positive electrode material layer may include components such as a positive electrode active material, a binder, and a conductive agent. The positive electrode active material may be, for example, a nickel-cobalt-manganese positive electrode material or a nickel-cobalt-aluminum positive electrode material. The conductive agent may be, for example, conductive carbon black, acetylene black, graphite, carbon nanofibers, etc. The binder may be, for example, polyvinylidene fluoride, styrene-butadiene rubber, sodium carboxymethyl cellulose, etc.

[0043] The negative electrode sheet may include a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector. The negative electrode material layer may include a negative electrode active material, a binder, and a conductive agent. The negative electrode active material may be graphite or silicon carbide. The conductive agent may be, for example, conductive carbon black, acetylene black, graphite, carbon nanofibers, etc. The binder may be, for example, a combination of styrene-butadiene rubber and sodium carboxymethyl cellulose.

[0044] The diaphragm can be a polyethylene film or a polypropylene film.

[0045] In one embodiment of this application, the ratio of a to c is greater than 1 and less than or equal to 6. Exemplarily, the ratio of a to c can be any two values ​​between 1.1, 1.5, 1.8, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, or 6.0. By adjusting the ratio of diluent to solvent, the ionic conductivity of the electrolyte is improved. By adjusting the ratio of diluent to silicon-carbon anode, side reactions of the anode and electrolyte are reduced, thereby improving the battery's rate capability and high-temperature performance.

[0046] The electrolyte of this application has been explained above. The performance of the electrolyte of this application will be explained below with reference to specific embodiments.

[0047] Example 1

[0048] This embodiment is an electrolyte comprising an organic solvent, a lithium salt, a diluent, and additives. The organic solvent includes ethyl methyl carbonate, ethylene carbonate, propylene carbonate, and fluoroethylene carbonate. The lithium salt includes LiPF6 and LiFSI, with the total concentration of LiPF6 and LiFSI in the electrolyte being greater than or equal to 3 mol / L. The diluent is 3,3,4,4,5,5-hexafluorotetrahydropyran, the structure of which is shown below: The additives include lithium difluorooxalate borate, tris(trimethylsilyl) phosphate, vinyl sulfate, 1,3-propenesulfonate lactone, and lithium difluorophosphate.

[0049] The specific values ​​for the mass fractions of each component in the organic solvent, the concentration of lithium salt, and the content of additives are listed in Table 1.

[0050] Examples 1-9 and Comparative Examples 1-2

[0051] Examples 1-9 and Comparative Examples 1-2 are electrolytes, the components of which are listed in Table 1.

[0052] Comparative Example 3

[0053] The comparative example is an electrolyte in which the diluent is tetrahydropyran, and its structural formula is as follows:

[0054] .

[0055] Comparative Example 4

[0056] The comparative example is an electrolyte in which the diluent is 3,3-difluorotetrahydropyran, the structural formula of which is as follows:

[0057] .

[0058] Lithium-ion batteries were constructed using the electrolytes from Examples 1-9 and Comparative Examples 1-4, respectively. The positive electrode material of the lithium-ion battery was lithium nickel cobalt manganese oxide, and the negative electrode material was a mixture of silicon carbon and graphite. The specific assembly method of the lithium-ion battery includes the following steps:

[0059] S1, Preparation of the electrode: The positive electrode active material LiNi 0.92 Mn 0.01 Co 0.07 O2, conductive carbon nanotubes, and binder polyvinylidene fluoride are dispersed in N-methylpyridinone in a certain proportion, stirred under vacuum to form a slurry, coated on both sides of an aluminum current collector, and then dried, rolled, and slit to obtain the positive electrode sheet.

[0060] The negative electrode active material, conductive agent acetylene black, carbon nanotubes, and binder styrene-butadiene rubber were dispersed in water in a certain proportion, vacuum stirred into a slurry, coated on both sides of a copper current collector, and then subjected to drying, rolling, and slitting processes to obtain the negative electrode sheet. The negative electrode active material is a composite material composed of graphite and silicon-carbon materials in a mass ratio of 9:1. The proportion of silicon-carbon materials in the negative electrode material layer is listed in Table 1.

[0061] S2. Preparation of electrolyte: Prepare raw materials according to the composition of electrolyte in each embodiment and comparative example. In a glove box protected by high-purity nitrogen, add organic solvent, lithium salt, diluent and additives to polytetrafluoroethylene bottle in a certain proportion. After the lithium salt is completely dissolved in a shaker, add the corresponding additives and shake again to make the electrolyte fully mixed. Store the prepared electrolyte in a low temperature environment of 5°C for later use.

[0062] S3. Preparation of lithium-ion battery: The positive electrode, separator and negative electrode are assembled into a cell in a Z-shaped stack in sequence. The cell is placed in an aluminum-plastic film outer packaging shell, dried until the moisture content is below a certain value, and then injected with electrolyte. After vacuum sealing, standing, formation and volume determination, the lithium-ion battery used in this application is obtained.

[0063] Table 1

[0064]

[0065] The lithium-ion batteries prepared with the electrolytes corresponding to each embodiment and comparative example were tested as follows:

[0066] 1. Electrolyte conductivity test: The conductivity of the electrolyte was tested using a Lei Magnetic Conductivity Meter. The test results are shown in Table 2.

[0067] 2. DCR test: (1) Use Dahua charging and discharging equipment to discharge the lithium-ion battery (1C discharge to cutoff voltage 2.6V); (2) Charge the battery with a constant current of 0.3C to 4.25V and then switch to constant voltage charging to 0.05C. Then discharge it with 1C to 2.6V. This is one cycle. Charge and discharge 3 times. The capacity of the third discharge is used as the capacity of the battery; (3) After the lithium-ion battery is calibrated, charge it to 4.25V with 0.3C and then switch to constant voltage charging. The cutoff current is 0.05C; (4) Discharge it with 1C to adjust the load to 50% SOC. After standing for 30 minutes, discharge it with 1C for 18s. Record the voltage V1 when the discharge starts and the voltage V2 when the discharge ends after 18s. The discharge current is I. The internal resistance of the battery is DCR=|V1-V2| / I.

[0068] 3. Rate charging test: (1) After the battery is capped, it is charged with constant current at 1C and 2C to 4.25V, then switched to constant voltage charging. Charging is stopped when the charging termination current drops to 0.05C; (2) After charging, it is left to stand for 60 minutes; (3) After standing, it is discharged to 2.5V at 1C. The rate charging performance of the battery is judged by the constant current ratio and the equal voltage ratio. The test results are shown in Table 2.

[0069] 4. Rate Discharge Test: (1) After the battery is capacitated, it is charged at a constant current of 0.3C to 4.25V, then switched to constant voltage charging, and charging is stopped when the charging termination current drops to 0.05C; (2) After charging, it is left to stand for 60 minutes; (3) After standing, it is discharged to 2.5V with currents of 1C, 2C and 3C respectively. The rate discharge performance of the battery is judged by the discharge capacity retention rate and the discharge equalization voltage. The test results are shown in Table 2.

[0070] 5. Gas production test at 55℃: (1) After the battery is capacitated, test the DCR of the battery and record it as DCR1; (2) Discharge the battery with a current of 1C to 2.6V, then charge it with a constant current of 0.3C to 4.25V and then charge it with a constant voltage to 0.05C. After the battery is fully charged, test the battery volume by the water displacement method and record it as V0; (3) Place the battery in a constant temperature chamber at 65℃ for 28 days. Take the battery out every 7 days. After the battery temperature drops to room temperature, test the battery volume again by the water displacement method and record it as V1. The difference between V1 and V0 is the amount of gas produced by the battery during the storage period. Divide this value by the capacitated capacity of the battery and the time to get the amount of gas produced per ampere-hour of battery. (4) After the test, discharge the battery at a current of 0.3C to adjust the charge to 50% SOC. After standing for 30 minutes, discharge at 1C for 18 seconds and test the internal resistance of the battery after gas production, which is recorded as DCR2; (5) The gas production DCR growth rate of the battery = DCR2 / DCR1-1. The test results are shown in Table 3.

[0071] 3. 55℃ storage test: (1) After the battery is capacitated, test the internal resistance DCR of the battery and record it as DCR1; (2) Charge the battery with a constant current of 0.3C to 4.25V and then switch to constant voltage charging to 0.05C. After the battery is fully charged, store it in a constant temperature chamber at 55℃ for 7 days; (3) After storage, discharge the battery with a current of 1C to 2.6V. The discharge capacity / capacitated capacity at this time is the capacity retention rate of the battery; (4) Charge the battery with a constant current of 0.3C to 4.25V and then switch to constant voltage charging to 0.05C. After the battery is fully charged, discharge the battery with a current of 1C to 2.6V. The discharge capacity and capacitated capacity at this time are the capacity recovery rate of the battery; (5) Test the internal resistance DCR of the battery at room temperature and record it as DCR2. The storage DCR growth rate of the battery = DCR2 / DCR1-1. The test results are shown in Table 3.

[0072] 4. 55℃ Cyclic Test: (1) After the battery is capped, test the DCR of the battery and record it as DCR1; (2) Charge the battery with a constant current of 0.3C to 4.25V and then switch to constant voltage charging to 0.05C. After the battery is fully charged, perform a 2.6~4.25V cyclic test in a constant temperature chamber at 55℃. The test steps are: charge the battery with a constant current of 0.5C to 4.25V, cut off the current of 0.05C, let it stand for 30 minutes, discharge the battery with a constant current of 0.5C to 2.0V, let it stand for 30 minutes, and repeat the charge and discharge cycle 300 times; (3) After the cycle is completed, test the DCR of the battery at room temperature and record it as DCR2. The cycle DCR growth rate of the battery = DCR2 / DCR1-1. The test results are shown in Table 3.

[0073] Table 2

[0074]

[0075] Table 3

[0076]

[0077] As can be seen from the test data in Tables 2 and 3, the electrolytes in the embodiments of this application are based on 3,3,4,4,5,5-hexafluorotetrahydropyran as a specific diluent, used in combination with other organic solvents within a certain proportion range. This not only limits its interaction with Li due to its own cyclic fluorination structure, but also... +The diluent's coordination ability prevents the diluent from decomposing and contaminating the SEI film, maintaining the electrolyte's high ionic conductivity. It also exhibits high-voltage stability >4.5V (vs. Li+ / Li), ensuring good compatibility with high-nickel cathodes at 4.25V high-temperature cycling and improving battery cycle stability. LiPF6 and LiFSI are chosen as a mixed lithium salt, which improves battery ionic conductivity and high-temperature cycling performance while reducing the risk of LiFSI corrosion of the aluminum foil. The diluent works synergistically with LiFSI / LiPF6 to form a negative electrode-electrolyte interface film structure rich in a mechanically strong LiF outer layer and a highly ionicly conductive Li2O inner layer, effectively suppressing the expansion of the negative electrode material. Furthermore, the surface tension of the diluent matches the surface energy of the electrode material, improving electrode-electrolyte interfacial compatibility and enhancing the battery's rate capability and high-temperature performance. Therefore, the lithium-ion batteries corresponding to the electrolytes in Examples 1-9 of this application exhibit excellent performance in rate charging and discharging, high capacity retention and low DCR growth rate during high-temperature storage and cycling, while maintaining low gas production at high temperatures, with the gas production rate remaining below 4 mL / Ah after 28 days of storage. In contrast, if the ratio of solvent to lithium salt is improper, as shown in the test results of Comparative Examples 1 and 2, the lithium-ion battery cannot simultaneously achieve high temperature performance and a high DCR growth rate.

[0078] When the diluent is the compound of Comparative Example 3 and Comparative Example 4, the capacity retention rate of the corresponding lithium-ion batteries is relatively low. The specification states that when the composition of the diluent is not within the scope defined in this application, the performance of the corresponding lithium-ion battery cannot achieve the effects of the embodiments of this application.

[0079] Furthermore, the test results from the examples and comparative examples show that the ratio of diluent to solvent (ethyl methyl carbonate, ethylene carbonate, propylene carbonate, fluoroethylene carbonate) in the electrolyte has a significant impact on battery performance. This is especially true when the mass percentage ratio of diluent to solvent is preferably between (0.15-0.4):1, and the molar ratio of LiPF6 to LiFSI is 2.3, resulting in a stronger synergistic effect between the diluent, solvent, and lithium salt. On the other hand, the mass percentage of diluent in the electrolyte also affects its impact on battery performance. The amount of diluent added affects the ionic conductivity of the electrolyte, thus influencing the battery's impedance during testing. The amount of lithium salt LiFSI added affects the corrosion degree of the aluminum current collector and the battery's high-temperature performance. Simultaneously, the mass percentage of silicon-carbon material in the negative electrode material is also crucial to battery performance. A moderate silicon-carbon content allows the surface energy of the negative electrode and the surface tension of the electrolyte to complement each other, improving the battery's rate capability and high-temperature performance. Therefore, within the above preferred range, if the mass percentage of diluent and solvent is further controlled within the range of (0.2-0.35):1, and the ratio of the proportion of diluent in the electrolyte to the proportion of silicon-carbon material in the negative electrode material layer is further controlled within the range of (3-5):1, the high-temperature cycling performance of lithium-ion batteries can be maintained at more than 90% of the battery capacity after 300 cycles.

[0080] Obviously, those skilled in the art can make various modifications and variations to the embodiments of this application without departing from the spirit and scope of this application. Therefore, if these modifications and variations of this application fall within the scope of the claims of this application and their equivalents, this application also intends to include these modifications and variations.

Claims

1. A lithium battery, characterized in that, It includes a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the electrolyte includes an organic solvent, a lithium salt, a diluent, and additives; The lithium salt includes LiPF6 and LiFSI, and the total concentration of LiPF6 and LiFSI in the electrolyte is greater than or equal to 3 mol / L; The diluent is selected from 3,3,4,4,5,5-hexafluorotetrahydropyran, the mass percentage of the diluent in the electrolyte is a, the mass percentage of the organic solvent in the electrolyte is b, and the ratio of a to b is 0.15~0.45:1; The negative electrode includes a negative electrode sheet and a negative electrode material layer disposed on the surface of the negative electrode sheet. The negative electrode material layer contains graphite and silicon carbide materials. The mass percentage c of the silicon carbide material in the negative electrode material layer is 5% to 30%. The ratio of a to c is 2 to 3:

1.

2. The lithium battery according to claim 1, characterized in that, The diluent accounts for 10% to 40% of the mass of the electrolyte.

3. The lithium battery according to claim 1, characterized in that, The molar ratio of LiPF6 to LiFSI is 0.7 to 2.

3.

4. The lithium battery according to any one of claims 1-3, characterized in that, The organic solvent includes at least one of methyl ethyl carbonate, ethylene carbonate, propylene carbonate, or fluoroethylene carbonate.

5. The lithium battery according to any one of claims 1-3, characterized in that, The additives include lithium difluorooxalate borate and compound additives, wherein the lithium difluorooxalate borate accounts for 1.5% to 2.5% of the mass in the electrolyte, and the compound additives account for 1.5% to 2.5% of the mass in the electrolyte.

6. The lithium battery according to claim 5, characterized in that, The compound additives include tris(trimethylsilyl) phosphate, vinyl sulfate, 1,3-propenesulfonate lactone, and lithium difluorophosphate.

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