Lithium ion battery electrolyte and preparation method and application thereof

Abstract

The invention discloses a lithium ion battery electrolyte used under the condition of-30 DEG C to-70 DEG C and a preparation method and application thereof, the lithium ion battery electrolyte comprises a lithium salt, an organic solvent and an additive, and the organic solvent is a sulfite solvent or a mixture of the sulfite solvent and a carbonic ester solvent. According to the lithium ion battery electrolyte provided by the invention, sulfite is selected as the solvent of the electrolyte, the electrochemical performance of the lithium ion battery in a low-temperature environment is comprehensively improved, and the lithium ion battery electrolyte has the characteristics of low freezing point, high low-temperature conductivity and good cycling stability.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, and more specifically, relates to a lithium-ion battery electrolyte that can be used in a temperature range of -30 to -70°C. Furthermore, this invention also relates to a method for preparing the said lithium-ion battery electrolyte and its applications. Background Technology

[0002] Lithium-ion batteries, with their advantages of high energy density, long cycle life, and no memory effect, have been widely used in portable electronic devices, electric vehicles (EVs), and large-scale energy storage systems. However, as application scenarios continue to expand, users are placing higher demands on batteries in terms of fast charging capabilities and adaptability to extreme environments, especially maintaining efficient and stable operation under low-temperature conditions, which has become a key technological bottleneck.

[0003] At low temperatures, the performance of lithium-ion batteries deteriorates significantly, primarily manifested as capacity decay, increased internal resistance, and charging difficulties. The root cause of this phenomenon lies in the sluggish ion transport kinetics and the impaired electrode / electrolyte interface processes. Specifically, currently widely used carbonate electrolytes face three core challenges in low-temperature environments: a significant increase in electrolyte viscosity and a sharp decrease in conductivity, severely limiting the potential of Li-ion batteries. + Migration rate in bulk phase; Li + The increased desolvation energy barrier makes it difficult for Li to effectively embed on the electrode surface; the increased impedance of the SEI (solid electrolyte interphase) and CEI (cathode electrolyte interphase) films further hinders Li's embedding. + The diffusion rate in the interfacial film is significantly reduced, further deteriorating the interfacial reaction kinetics.

[0004] To improve the low-temperature performance of batteries, researchers are currently focusing on three main areas: electrode material modification, electrolyte system design, and interface engineering. However, all three approaches have limitations: the low-temperature performance of electrode materials is constrained by their intrinsic ionic conductivity, making breakthroughs through structural or doping modifications difficult; while interface modification can improve interfacial stability, the process is complex and may introduce additional impedance, affecting overall electrochemical performance. In contrast, controlling the electrolyte composition is considered the most promising and efficient optimization approach due to its low cost, ease of operation, and significant effects.

[0005] Currently, mainstream low-temperature electrolyte systems typically consist of lithium salts (such as LiPF6), high dielectric constant solvents (such as ethylene carbonate EC), low-viscosity linear carbonates (such as DMC and DEC), and a small amount of film-forming additives (such as VC). EC helps to improve the dissociation degree of lithium salts and enhance ion concentration; linear carbonates are used to reduce the system viscosity; and by blending multiple components (usually a combination of 4-5 organic solvents), a balance between dielectric properties and flowability is achieved, thus broadening the liquid operating temperature range. However, this multi-component electrolyte system still has the following significant drawbacks:

[0006] (1) High cost and complex process: Multi-solvent system not only increases the cost of raw materials, but also requires repeated adjustment of the ratio to optimize performance, which greatly prolongs the research and development cycle and is not conducive to rapid industrialization.

[0007] (2) Solvation structure imbalance at low temperatures: As the temperature decreases, highly polar solvents (such as EC) are more likely to enter Li. + The first solvation shell forms a strong coordination structure, which leads to an increase in the desolvation energy barrier and a deterioration in the interfacial charge transfer kinetics.

[0008] In summary, electrolyte regulation is a key breakthrough for improving the low-temperature performance of lithium-ion batteries. However, existing mainstream systems based on multi-component carbonate blends still face fundamental bottlenecks in terms of low-temperature ion transport efficiency and interfacial stability. Therefore, developing novel electrolyte systems to achieve rational regulation of solvation structure and synergistic construction of high-performance interfacial films is a crucial direction for overcoming obstacles in low-temperature applications. Summary of the Invention

[0009] In view of this, the present invention provides a lithium-ion battery electrolyte for use under conditions of -30 to -70°C, its preparation method and application. The lithium-ion battery electrolyte uses sulfite esters as the solvent, which can comprehensively improve the electrochemical performance of lithium-ion batteries under low temperature conditions, and has the characteristics of low freezing point, high low temperature conductivity and good cycle stability.

[0010] To achieve the above objectives, a first aspect of the present invention provides a lithium-ion battery electrolyte for use at -30 to -70°C, comprising a lithium salt, an organic solvent, and additives, wherein the organic solvent is a sulfite solvent, or a mixture of a sulfite solvent and a carbonate solvent.

[0011] In this invention, the lithium-ion battery electrolyte can be a sulfite-based solvent. Sulfite-based solvents have a moderate dielectric constant and a relatively weak lithium-ion coordination ability, which can effectively promote lithium salt dissociation while avoiding reaction with Li. + This leads to excessively strong solvation interactions. This property is particularly important at low temperatures (-30 to -70°C)—excessively strong Li... +Solvent binding leads to an increase in the desolvation energy barrier, while sulfite solvents, due to their weak solvation ability, can significantly reduce the Li- ionization energy barrier. + The difficulty of solvent removal increases the kinetics of interfacial reactions.

[0012] Furthermore, due to the interaction between sulfite compounds and Li + With weaker binding ability, more anions can participate in Li + The inner solvation structure forms an anion-dominated solvation sheath. This unique solvation structure, upon decomposition at the electrode surface, facilitates the preferential reduction or oxidation of anions, promoting the formation of sulfide-rich solid electrolyte interphase (SEI) and positive electrode electrolyte interphase (CEI) films at the anode and cathode interfaces, respectively. These sulfide-dominated interfacial films offer the following advantages: high ionic conductivity, significantly reducing interfacial impedance; high mechanical modulus, providing good structural stability and crack propagation resistance; and excellent chemical stability, effectively inhibiting continuous electrolyte decomposition. Therefore, sulfite solvents not only optimize bulk ion transport and desolvation processes through low viscosity and weak solvation characteristics, but also induce the formation of high-performance interfacial films by regulating the solvation structure, achieving synergistic optimization of "bulk transport" and "interfacial kinetics," thereby comprehensively improving the electrochemical performance of lithium-ion batteries at low temperatures.

[0013] Furthermore, sulfite solvents are single-component, which can reduce raw material costs and eliminate the need for repeated adjustments to solvent ratios for formula optimization, significantly reducing R&D cycle and manpower investment.

[0014] In this invention, the organic solvent for the lithium-ion battery electrolyte can also be a mixture of sulfite solvents and carbonate solvents. Carbonate solvents have a high dielectric constant, which is beneficial for the complete dissociation of lithium salts, increasing the concentration of free lithium ions in the electrolyte and thus improving ionic conductivity. Sulfite solvents typically have low viscosity and melting point, effectively reducing the overall viscosity of the mixed solvent and improving the migration rate of lithium ions. The combined use of these two solvents achieves a good balance between high dielectric constant and low viscosity, significantly improving the ionic conductivity of the electrolyte, especially exhibiting excellent performance over a wide temperature range.

[0015] Furthermore, the low freezing point and low viscosity of sulfite solvents can effectively suppress the problem of a sharp increase in electrolyte viscosity at low temperatures. When mixed with carbonate solvents, the electrolyte can maintain good fluidity at temperatures of -30°C or even lower, ensuring efficient lithium-ion transport and thus improving the battery's discharge capacity and cycle performance in cold environments.

[0016] On the surface of the negative electrode (such as graphite), sulfite solvents tend to preferentially reduce and decompose, which helps to form a dense, uniform solid electrolyte interphase (SEI) film rich in inorganic components (such as Li2SO3 and Li2SO4). This film has low interfacial impedance and good lithium-ion conductivity, while effectively suppressing the continuous consumption of electrolyte and the growth of lithium dendrites.

[0017] On the positive electrode surface, sulfite solvents also help form a stable positive electrode electrolyte interphase (CEI) film, protecting the positive electrode material and reducing side reactions. Some carbonate solvents (such as PC) tend to co-intercalate on graphite negative electrodes, leading to graphite layer stripping. Introducing sulfate solvents preferentially reduces and forms a film on the graphite surface, effectively blocking direct contact between PC molecules and graphite, preventing graphite stripping, and improving battery cycle stability. Therefore, using a mixture of sulfate and carbonate solvents as the organic solvent system for lithium-ion battery electrolytes achieves complementary advantages—carbonates provide excellent lithium salt dissolution, while sulfite improves fluidity, broadens the electrochemical window, and optimizes interfacial film formation performance. This composite solvent system is particularly suitable for lithium-ion batteries requiring high energy density, wide temperature range operation, long cycle life, and high safety. Furthermore, since sulfite and carbonate solvents are a binary system, repeated adjustments using multiple components are unnecessary.

[0018] Preferably, the lithium-ion battery electrolyte is in the voltage range of 2.5 to 4.3V.

[0019] In this invention, because the LCO cathode can achieve a suitable capacity at voltages below 4.3V, while the capacity increases at higher voltages (>4.4V), the cycle life decreases significantly. Furthermore, sulfite has limited antioxidant properties, and the sulfite solvent may decompose at higher voltages (>4.4V). Therefore, it is more suitable to use lithium-ion battery electrolytes in the voltage range of 2.5 to 4.3V, which can both achieve the desired capacity and maintain the cycle life.

[0020] Preferably, the volume ratio of the carbonate solvent to the sulfite solvent is 1:(2-4).

[0021] In this invention, the volume ratio of carbonate solvent to sulfite solvent is controlled at 1:(2-4). Sulfite solvents typically have low melting points and viscosities. Controlling their proportion in the mixed solvent can significantly reduce the overall viscosity of the electrolyte, enhance the migration ability of lithium ions at low temperatures, and thus effectively improve the discharge performance and cycle life of the battery in low-temperature environments (such as -30°C or even lower).

[0022] Carbonate solvents (such as EC, DEC, EMC, etc.) have high dielectric constants, which are beneficial to the dissociation of lithium salts; sulfite solvents improve fluidity. When the two are combined in a ratio of 1:(2-4), a good balance can be achieved between lithium salt solubility, ionic conductivity, viscosity and electrochemical stability, thereby improving the overall electrochemical performance of the battery.

[0023] Preferably, the sulfite solvent is at least one of dimethyl sulfite and diethyl sulfite; more preferably, the sulfite solvent is dimethyl sulfite.

[0024] In this invention, compared with other sulfite solvents, dimethyl sulfite has a low melting point, a high dielectric constant, and a weak resistance to lithium-ion binding, making it the most suitable solvent for low-temperature electrolytes among sulfite solvents.

[0025] Preferably, the carbonate solvent is at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

[0026] Preferably, in the lithium-ion battery electrolyte of the present invention, the lithium salt is at least one selected from lithium bis(fluorosulfonyl)imide, lithium bis(trifluorosulfonyl)imide, lithium tetrafluoroborate, lithium bis(oxalate-borate), lithium difluorooxalate-borate, and lithium difluorophosphate.

[0027] In the electrolyte of this invention, lithium bis(fluorosulfonyl)imide and lithium bis(trifluorosulfonyl)imide have high ionic conductivity, while lithium tetrafluoroborate, lithium bis(oxalate-borate), lithium difluorooxalate-borate, and lithium difluorophosphate have the function of forming an interfacial film. Preferably, the lithium salt includes at least one of lithium bis(fluorosulfonyl)imide, lithium bis(trifluorosulfonyl)imide, and lithium tetrafluoroborate, and at least one of lithium bis(oxalate-borate), lithium difluorooxalate-borate, and lithium difluorophosphate. This combination can balance the ionic conductivity and film-forming properties of the electrolyte.

[0028] Preferably, in the lithium-ion battery electrolyte of the present invention, the concentration of the lithium salt is 0.5-2.0 mol / L, such as 0.5 mol / L, 0.8 mol / L, 1.0 mol / L, 1.3 mol / L, 1.6 mol / L, 1.8 mol / L, 2.0 mol / L, etc.

[0029] In this invention, the concentration of lithium salt is controlled to be 0.5-2.0 mol / L, and the electrolyte has high ionic conductivity within this concentration range.

[0030] Preferably, in the lithium-ion battery electrolyte of the present invention, the additive is at least one of fluoroethylene carbonate, lithium difluorooxalate phosphate, and lithium difluorophosphate.

[0031] In this invention, these additives can form an interface film with low impedance, thereby meeting the requirements of low-temperature electrolytes.

[0032] Preferably, in the lithium-ion battery electrolyte of the present invention, the amount of the additive accounts for 0.1-10 wt% of the total mass of the electrolyte.

[0033] In this invention, the amount of additive is set to 0.1-10 wt% of the total mass of the electrolyte, because too much additive will cause problems such as a decrease in the ionic conductivity of the electrolyte itself, excessive film formation, and increased cost.

[0034] In a second aspect of the present invention, a method for preparing the lithium-ion battery electrolyte described in the first aspect is provided, wherein lithium salt, organic solvent and additives are mixed to obtain the electrolyte.

[0035] In the preparation method of the present invention, the mixing is a conventional operation in the art. From a safety point of view, it is recommended to mix in argon gas at room temperature.

[0036] The preparation method of this invention is simple and easy to operate, and it is very easy to achieve large-scale production, which can also provide conditions for its large-scale use.

[0037] In a third aspect of the invention, the application of the lithium-ion battery electrolyte described in the first aspect in a lithium battery is provided.

[0038] The lithium-ion battery electrolyte of this invention, while maintaining good conductivity, also exhibits good compatibility with the electrodes, significantly improving the cycle stability of the battery.

[0039] In a fourth aspect of the present invention, a lithium metal battery is provided, comprising a positive electrode, a negative electrode, a separator, and the lithium-ion battery electrolyte described in the first aspect of the present invention. The lithium metal battery of the present invention exhibits excellent electrochemical performance at low temperatures, characterized by a low freezing point, high low-temperature conductivity, and good cycle stability.

[0040] In the lithium metal battery provided by the present invention, the negative electrode can be any type of lithium metal negative electrode in the art, and the positive electrode can also be any type of positive electrode in the art. There are no strict limitations on the positive electrode, negative electrode and separator.

[0041] The lithium metal battery provided in the fourth aspect of the present invention can also be used in electrical devices in the form of battery packs, including but not limited to automobiles, drones, mobile phones, cameras, etc.

[0042] Compared with the prior art, the present invention has the following beneficial effects:

[0043] This invention provides a lithium-ion battery electrolyte, wherein the organic solvent of the lithium-ion battery electrolyte is a sulfite solvent. Sulfite solvents have a moderate dielectric constant and weak lithium-ion coordination ability, which can promote the dissociation of lithium salts and avoid reaction with Li. + Overly strong bonding reduces Li at low temperatures + The difficulty of desolvation; its weak binding allows more anions to participate in the inner solvation structure, which is conducive to the formation of anion-dominated solvation sheath layer, induces the formation of high-performance sulfide interface film, reduces interface impedance, enhances structural and chemical stability, realizes the synergistic optimization of "bulk transport" and "interface dynamics", and improves the low-temperature electrochemical performance of lithium-ion batteries. Attached Figure Description

[0044] Figure 1 The graph shows the cycle performance test results of LCO||Gr soft-pack full cells assembled with the electrolytes of Example 6 and Comparative Example 1 at room temperature and within a voltage range of 2.5-4.3V.

[0045] Figure 2 Cyclic diagram of the LCO||Gr pouch cell assembled with the electrolyte of Example 6, at 30°C and 20C charge-discharge cycle.

[0046] Figure 3 The circuit diagram shows the 0.5C charge-discharge cycle of the LCO||Gr pouch cell assembled with the electrolyte of Example 6 at -30°C.

[0047] Figure 4 The discharge curves and capacity retention diagrams of the LCO||Gr soft-pack battery assembled with the electrolyte of Example 6 are shown in the figure from -70°C to -30°C. Detailed Implementation

[0048] The present disclosure will be further illustrated by the following examples, but the present disclosure is not limited thereto.

[0049] Unless otherwise specified, all reagents and materials used in the following examples, comparative examples, and test cases were commercially available. Lithium bis(difluorosulfonyl)imide (LiFSI) and lithium bis(oxalateborate)borate (LiBOB) were purchased from Suzhou Duoduo Chemical Technology Co., Ltd., and the graphite anode and lithium cobalt oxide cathode (LiCoO2) were purchased from Tianjin Zhongneng Lithium Industry Co., Ltd.

[0050] Examples 1-7 and Comparative Examples 1-3

[0051] The electrolyte was prepared at room temperature in a glove box with an oxygen content of <0.01 ppm and a moisture content of <0.01 ppm. The preparation process was as follows: Organic solvents were mixed evenly according to a specified ratio, then lithium salt was added. After the lithium salt was completely dissolved (in this example, the concentration of lithium salt was 1.3 mol / L), an additive was added (the amount of additive accounted for 10 wt% of the total mass of the electrolyte). The entire process was carried out with stirring to ensure uniform mixing, resulting in a clear and transparent electrolyte.

[0052] The amounts of each component used in Examples 1-7 and Comparative Examples 1-3 are shown in Table 1 below.

[0053] Table 1. Dosage of each component in Examples 1-7 and Comparative Examples 1-3

[0054]

[0055] Note: Lithium bis(fluorosulfonyl)imide (LiFSI); Lithium bis(oxalato)borate (LiBOB); Dimethyl sulfite (DMS); Diethyl sulfite (DES); Vinyl sulfite (ES); Vinyl carbonate (EC); Ethyl methyl carbonate (methyl ethyl carbonate); Fluorinated vinyl carbonate (FEC)

[0056] Application Examples

[0057] The lithium-ion battery electrolytes prepared in Examples 1-7 and the electrolytes obtained in Comparative Examples 1-3 were injected in appropriate amounts into a pouch cell with a designed capacity of 1.2 Ah and then vacuum-sealed. The positive electrode material of the pouch cell was lithium cobalt oxide, and the negative electrode was graphite. The battery was then subjected to resting, formation, and capacity testing processes to complete the preparation of an LCO||Gr pouch cell.

[0058] Test Example 1

[0059] Cyclic performance tests were conducted on LCO||Gr pouch cells composed of electrolytes from Comparative Examples 1-3 and Examples 1-7. The specific method was as follows: After five formation cycles at 30°C, the cells were transferred to a -30°C environment. The cells were then charged and discharged within a voltage range of 2.5–4.3V using a constant current-constant voltage charging method and a constant current discharging method. The charging rate was 0.1C, and the cycle was stopped when the time reached 10 hours or the current was less than 0.05C, at which point the discharge rate was 0.1C. The low-temperature capacity retention rate was calculated by dividing the initial discharge specific capacity at low temperature by the formation discharge specific capacity at 30°C. The results are shown in Table 2.

[0060] Table 2. Low-temperature electrical performance data of Examples 1-7 and Comparative Examples 1-3

[0061] Capacity retention rate at -30℃ and 0.1C Example 1 82.5％ Example 2 83.0％ Example 3 83.5％ Example 4 82.3％ Example 5 83.1％ Example 6 86.7％ Example 7 81.2％ Comparative Example 1 29.3％ Comparative Example 2 50.3％ Comparative Example 3 62.7％

[0062] As shown in Table 2, the LCO||Gr soft-pack full cell composed of the electrolyte in Comparative Example 1 has a capacity retention rate of only 29.3% at -30℃, while the LCO||Gr soft-pack full cell composed of the electrolyte in Comparative Example 2 has a capacity retention rate of 50.3% at -30℃. This indicates that when the content of carbonate solvents or sulfite solvents is low, the ionic conductivity decreases sharply, the viscosity increases, and lithium ion migration becomes difficult at low temperatures, resulting in severe battery polarization, ineffective utilization of active materials, and poor low-temperature performance. The LCO||Gr soft-pack full cell composed of the electrolyte in Comparative Example 3 had a capacity retention rate of 62.7% at -30°C. This indicates that when the solvent is entirely vinyl sulfite, the ionic conductivity decreases, the viscosity increases, and lithium-ion migration becomes difficult at low temperatures. Therefore, it can be concluded that the sulfite solvent of this invention is at least one of dimethyl sulfite and diethyl sulfite, and its electrolyte can basically maintain a battery capacity retention rate of over 80%. When the volume ratio of carbonate solvent to sulfite solvent is 1:2, the battery capacity retention rate at -30°C is 82.5%. When the volume ratio of solvent to dimethyl sulfite is 1:3, the battery capacity retention rate at -30℃ is 83.0%. When the ratio of carbonate solvent to dimethyl sulfite is 1:4, the battery capacity retention rate at -30℃ exceeds 83.5%. When the solvent is entirely dimethyl sulfite, the battery capacity retention rate at -30℃ can reach 86.7%. From 1:2 to 1:4, and then to all solvents being dimethyl sulfite, the low-temperature capacity retention rate further improves with the increase of the proportion of sulfite solvent, indicating that increasing the proportion of sulfite solvent is more beneficial to the performance at low temperatures. When the proportion of dimethyl sulfite is higher, its excellent low-temperature fluidity and interface regulation ability are more fully demonstrated. When the solvent is entirely diethyl sulfite, the capacity retention rate of the battery at -30℃ is 81.2%, which is 6.34% lower than that when the solvent is entirely diethyl sulfite. This indicates that dimethyl sulfite is the most suitable solvent for low-temperature electrolytes among sulfite solvents.

[0063] Test Example 2: Rate Performance Test

[0064] The rate performance of LCO||Gr pouch cells composed of the electrolytes of Comparative Example 1 and Example 6 was measured. Specifically, the cells were cycled at room temperature within a voltage range of 2.5–4.3 V to determine the rate performance. The experimental results are as follows: Figure 1 As shown, from Figure 1As can be seen from the above, compared with Comparative Example 1, the capacity retention rate of the LCO||Gr soft-pack full cell assembled with the electrolyte of Example 6 of the present invention is significantly improved under different current densities. This indicates that in the electrolyte, sulfite solvent is more conducive to improving the rate performance of the electrolyte than carbonate solvent. It can significantly reduce the polarization current of the battery when charging at high rates, thereby improving the charge and discharge capacity of the battery under high rate charging conditions.

[0065] Test Example 3: Fast Charging Performance Measurement

[0066] Fast-charging performance of the LCO||Gr soft-pack full battery composed of the electrolyte from Example 6 was tested. Specifically, the battery was charged using a constant current and constant voltage method within a voltage range of 2.5–4.3V, and discharged using a constant current method. Charging was performed at 20C, and the discharge was stopped when the time reached the corresponding cutoff time of this rate or when the current was less than 0.05C. The discharge rate was 20C. The experimental results are as follows: Figure 2 As shown, the results indicate that the electrolyte of Example 6 enables the LCO||Gr pouch cell to cycle stably for 600 cycles at 20C with a capacity retention rate as high as 96.2%. This demonstrates that the LCO||Gr pouch cell composed of the electrolyte of Example 6 can still complete 600 cycles with minimal capacity decay (only 3.8%) under extreme fast charging conditions of 20C (i.e., fully charged in 3 minutes), indicating its excellent resistance to fast charging aging and its ability to withstand high current surges over long periods. It is suitable for applications requiring frequent fast charging. 20C charge / discharge places extremely high demands on ion diffusion rate and electrode / electrolyte interface reaction rate. Such a high capacity retention rate indicates that this electrolyte significantly improves the transport efficiency and insertion / extraction rate of lithium ions within the electrolyte bulk, electrode materials, and at the interface, effectively reducing polarization during fast charging. Furthermore, in constant current / constant voltage charging, the control of the voltage plateau and cutoff current places extremely high demands on the stability of the SEI (solid electrolyte interface). The battery showed almost no performance degradation after 600 cycles, indicating that the electrolyte helps form SEI and CEI films with high ionic conductivity, low impedance, and strong mechanical / chemical stability, suppressing side reactions and interface degradation during cycling. Furthermore, the battery remained stable despite the significant Joule heat generated during 20C fast charging, demonstrating that the electrolyte system possesses low impedance and excellent thermal management capabilities, effectively suppressing the risk of localized overheating and thermal runaway, thus ensuring safety during fast charging.

[0067] It should be noted that under 20C fast charging at room temperature, the LCO||Gr soft-pack full battery composed of the electrolyte in Comparative Example 1 will be directly damaged and unable to function normally.

[0068] Test Example 4: Low-Temperature Cycling Stability Determination

[0069] Low-temperature cycle stability tests were conducted on the LCO||Gr pouch cells assembled with the electrolyte of Example 6. Specifically, the LCO||Gr pouch cells were charged at -30°C using a constant current and constant voltage method within a voltage range of 2.5–4.3V, and discharged using a constant current method. Charging was performed at 0.5C, and discharge was stopped when the time reached the cutoff point corresponding to this rate or when the current was less than 0.05C. The discharge rate was 0.5C. Experimental results are as follows: Figure 3 As shown, the electrolyte of Example 6 enables the LCO||Gr pouch cell to cycle 100 times at a high rate of 0.5C at -30°C with 87.1% capacity retention. The LCO||Gr pouch cell assembled with the electrolyte of Example 6 exhibits very little capacity decay (only about 12.9%) after 100 charge-discharge cycles in the harsh low-temperature environment of -30°C, indicating that the electrode / electrolyte interface (especially the SEI film) remains stable at low temperatures (-30°C), with few side reactions and slow structural degradation. 0.5C is a relatively high rate (i.e., completing charge-discharge in 2 hours), and the ability to achieve stable cycling at low temperatures demonstrates that the electrolyte obtained in Example 6 has high low-temperature ionic conductivity and fast lithium-ion transport kinetics, supporting charge-discharge at higher currents. Furthermore, the LCO||Gr pouch cell assembled with the electrolyte of Example 6 exhibits good fast-charging capability at -30°C within a voltage range of 2.5–4.3V. Furthermore, the positive and negative electrode materials and the electrolyte maintain good compatibility during low-temperature cycling, without significant interface deterioration. This also indicates that the battery can not only be discharged once at low temperatures, but also be used repeatedly for a long time, thus possessing practical application value and being suitable for electronic devices or power battery systems that need to operate continuously in cold environments.

[0070] Test Example 5: Determination of Battery Operating Temperature Limits

[0071] The operating temperature limit range of the soft-pack full cell with the electrolyte composition LCO||Gr in Example 6 was tested. Specifically, the full cell was charged and discharged within the range of -30°C to -70°C. The low-temperature capacity retention rate was calculated by dividing the initial discharge specific capacity at low temperature by the converted discharge specific capacity at 30°C. The experimental results are as follows: Figure 4 ,Depend on Figure 4The results show that, within the extreme operating temperature range of the battery, the electrolyte of Example 6 enables the LCO||Gr pouch cell to release 85% of its capacity at a minimum of -70°C. This indicates that the electrolyte maintains sufficient ionic conductivity at extremely low temperatures, allowing lithium ions to migrate effectively between the positive and negative electrodes, thereby maintaining a high capacity output. Even at -70°C, the solid electrolyte interface (SEI) film formed by the electrolyte and electrode materials (especially the graphite negative electrode) remains stable and exhibits good ion transport characteristics, reducing charge transfer resistance. During charging or discharging at low temperatures, lithium ions tend to deposit unevenly on the negative electrode surface. This electrolyte may help form a stable SEI film, inhibiting lithium dendrite growth and improving safety. The ability to release 85% of the capacity at -70°C fully demonstrates that the electrolyte of Example 6 significantly improves the battery's low-temperature usability, energy retention capability, and cycle stability.

[0072] This invention provides a lithium-ion battery electrolyte, which uses sulfate esters as the solvent to comprehensively improve the electrochemical performance of lithium-ion batteries at low temperatures. It features a low freezing point, high low-temperature conductivity, and good cycle stability.

[0073] Based on the disclosure and teachings of the foregoing specification, those skilled in the art can make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the specific embodiments disclosed and described above, and some modifications and changes to the invention should also fall within the protection scope of the claims of the present invention. Furthermore, although some specific terms are used in this specification, these terms are only for convenience of explanation and do not constitute any limitation on the present invention.

Claims

1. A lithium-ion battery electrolyte for use at -30 to -70°C, characterized in that, It includes lithium salts, organic solvents, and additives, wherein the organic solvent is a sulfite solvent, or a mixture of a sulfite solvent and a carbonate solvent.

2. The lithium-ion battery electrolyte according to claim 1, characterized in that, The volume ratio of the carbonate solvent to the sulfite solvent is 1:(2-4).

3. The lithium-ion battery electrolyte according to claim 1, characterized in that, The sulfite solvent is at least one of dimethyl sulfite and diethyl sulfite.

4. The lithium-ion battery electrolyte according to claim 1, characterized in that, The carbonate solvent is at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate.

5. The lithium-ion battery electrolyte according to claim 1, characterized in that, The lithium salt is at least one of lithium bis(fluorosulfonyl)imide, lithium bis(trifluorosulfonyl)imide, lithium tetrafluoroborate, lithium bis(oxalate)borate, lithium difluorooxalateborate, and lithium difluorophosphate.

6. The lithium-ion battery electrolyte according to claim 1, characterized in that, The concentration of the lithium salt is 0.5–2.0 mol / L.

7. The lithium-ion battery electrolyte according to claim 1, characterized in that, The additive is at least one of fluoroethylene carbonate, lithium difluorooxalate phosphate, and lithium difluorophosphate, and the amount of the additive is 0.1 to 10 wt% of the total mass of the electrolyte.

8. A method for preparing a lithium-ion battery electrolyte according to any one of claims 1-7, characterized in that, The lithium salt, organic solvent, and additives are mixed to obtain the product.

9. The application of any one of the lithium-ion battery electrolytes according to claims 1-7 in a lithium battery.

10. A lithium metal battery, characterized in that, It includes a positive electrode, a negative electrode, a separator, and a lithium-ion battery electrolyte as described in any one of claims 1-7.

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