Composite electrolyte and preparation method thereof

Abstract

The invention relates to a composite electrolyte with excellent low-temperature adaptability and fast charging performance and a preparation method thereof. The composite electrolyte is composed of a solvent, a lithium salt, thermal response nanoparticles and an ion screening additive. The thermal response nanoparticles are FeO paraffin with a core-shell structure, the phase change temperature is-20 DEG C to 0 DEG C, the particle size is 50-100 nm, the coating layer thickness is 10-15 nm, the low-temperature release latent heat is larger than or equal to 150 J / g, the electrolyte temperature is increased by 3-8 DEG C, high-temperature uniform dispersion is achieved, and the dynamic viscosity is smaller than 5 mPa.s; the ion screening additive adopts a ZIF-8 / ZIF-67 composite frame, the aperture is adjustable in a range of 1-5nm, the desolvation energy is reduced by 0.3-0.5 eV when the ion screening additive is shrunk to 1-2nm at a low temperature, and the ion transference number is greater than 0.75 when the ion screening additive is expanded to 3-5nm at a high temperature. The preparation method comprises the following steps: preparing the thermal response nanoparticles and performing surface modification; synthesizing the ion screening additive; mixing the components according to a specific ratio and performing ultrasonic dispersion. According to the composite electrolyte, the performance of the electrolyte is comprehensively optimized, stable and efficient operation is realized at different temperatures, and the requirements of different application scenes of the power lithium ion battery are met.

Description

Technical Field

[0001] The present application relates to the technical field of lithium-ion batteries, and in particular to a composite electrolyte and a preparation method thereof. Background Art

[0002] With the rapid development of electric vehicles, portable electronic devices and other fields, the performance optimization of power lithium-ion batteries as core energy storage components has become the focus of industry attention. Especially in low-temperature fast charging scenarios, lithium-ion batteries face multiple challenges such as electrolyte performance degradation, reduced energy efficiency and increased safety hazards. Specifically, the low temperature environment causes the viscosity of the electrolyte to increase, which in turn causes a sharp drop in conductivity and a decrease in the migration rate of lithium ions. This not only causes the battery capacity to decay rapidly, but may also cause abnormal growth of lithium dendrites, seriously threatening the safety and service life of the battery.

[0003] In response to the above problems, the industry has carried out a series of studies and made some progress. For example, the use of low-viscosity ether solvents (such as dimethoxyethane, DME) as electrolyte components has effectively improved the electrochemical performance of batteries at low temperatures. However, the flash point of such solvents is generally low (usually below 30°C), which poses a great safety hazard. In order to further improve battery performance, researchers have tried to optimize the formation of the solid electrolyte interface (SEI) film by adding additives such as lithium bis(oxalatoborate) (LiODFB). Although the ionic conductivity has increased (about 1.2 mS / cm) at -20°C, the battery capacity retention rate is still less than 70% under 4C high-rate charging conditions, which is difficult to meet the needs of practical applications.

[0004] Although existing technologies have alleviated some of the problems caused by low-temperature fast charging to a certain extent, power lithium-ion batteries still face many urgent problems in this field: the electrolyte viscosity is too high. In low-temperature environments, the sharp increase in electrolyte viscosity becomes a key factor in limiting lithium ion migration. The conductivity of conventional electrolytes at -20°C is as low as 0.7 mS / cm, which seriously restricts the battery's charging and discharging efficiency. Relying on external heating, existing technologies such as positive temperature coefficient (PTC) heating can improve the low-temperature performance of batteries, but the energy consumption accounts for as high as 10%-20%, and there is a significant response delay (usually requiring preheating for more than 5 minutes), which affects user experience and system efficiency. The risk of dendrite growth. The unevenness of lithium ion migration during fast charging can easily lead to local lithium precipitation, which in turn causes dendrite growth, which not only accelerates battery capacity decay, but may also cause serious safety problems such as short circuits. Insufficient material stability. Traditional additives (such as sulfonates) have poor stability during high and low temperature cycles, making it difficult to achieve dynamic regulation of ion migration paths, limiting the long-term stability of battery performance.

[0005] Therefore, the development of a new composite electrolyte system to comprehensively solve the problems of electrolyte viscosity, energy efficiency, dendrite growth, and material stability faced by power lithium-ion batteries in low-temperature fast-charging scenarios has become a current research hotspot and difficulty. Summary of the Invention

[0006] The purpose of this application is to provide a composite electrolyte with better low-temperature adaptability, higher charging capacity retention rate during fast charging, higher safety, no need for external heating, thus having lower energy consumption, and no response delay. The purpose of this application is achieved through the following technical solutions. A composite electrolyte of this application includes a solvent, a lithium salt, thermoresponsive nanoparticles, and an ion sieving additive. In the composite electrolyte, the weight fraction of the solvent is in the range of 60%-80%, the weight fraction of the lithium salt is in the range of 10%-15%, the weight fraction of the thermoresponsive nanoparticles is in the range of 20%-25%, and the weight fraction of the ion sieving additive is in the range of 10%-20%. The thermoresponsive nanoparticles have a core-shell structure, including a core structure formed by nanoparticles and a shell structure formed by a coating layer. The ion sieving additive includes a pore size regulating material. When the temperature is less than 0°C, the pore size of the pore size regulating material is in the range of 1-2 nm. When the temperature is greater than or equal to 0°C, the pore size of the pore size regulating material is in the range of 3-5 nm. Among them, the phase transition temperature of the thermoresponsive nanoparticles is in the range of -20°C to 0°C.

[0007] In one embodiment, the component of the nanoparticles is iron oxide, the component of the coating layer is paraffin, and the pore size regulating material is a metal-organic framework.

[0008] In one embodiment, the ion sieving additive includes two or more metal-organic frameworks.

[0009] In one embodiment, the solvent includes ethylene carbonate with a weight fraction in the range of 10%-30%, fluoroethylene carbonate with a weight fraction in the range of 5%-15%, and ethyl acetate with a weight fraction in the range of 60%-80%.

[0010] In one embodiment, the lithium salt includes lithium bis(fluorosulfonyl)imide and lithium bis(trifluoromethanesulfonyl)imide, and the molar content ratio of lithium bis(fluorosulfonyl)imide to lithium bis(trifluoromethanesulfonyl)imide is in the range of 3:1 to 5:1.

[0011] In one embodiment, it also includes vinylene carbonate with a weight fraction in the range of 0.5%-1.5% and lithium difluorophosphate with a weight fraction in the range of 0.1%-1%.

[0012] The present application further provides a method for preparing a composite electrolyte, including: Preparing thermoresponsive nanoparticles, mixing the nanoparticles with a coating material, ultrasonically dispersing and then rapidly cooling to obtain thermoresponsive nanoparticles with a core-shell structure; Synthesizing an ion sieving additive, dissolving a metal salt and reacting with 2-methylimidazole in methanol to obtain a metal-organic framework; Mixing a solvent, a lithium salt, the thermoresponsive nanoparticles, and the ion sieving additive, and uniformly dispersing them; Wherein, the weight fraction of the solvent is in the range of 60%-80%, the weight fraction of the lithium salt is in the range of 10%-15%, the weight fraction of the thermoresponsive nanoparticles is in the range of 20%-25%, and the weight fraction of the ion sieving additive is in the range of 10%-20%.

[0013] In one embodiment, the component of the nanoparticles is iron oxide, and the component of the coating layer is paraffin. After preparing the thermoresponsive nanoparticles, surface modification of the nanoparticles is further included.

[0014] In one embodiment, synthesizing the ion sieving additive specifically includes dissolving Zn(NO3)2 and Co(NO3)2 in a molar ratio of 1:1, and reacting with 2-methylimidazole in methanol for more than 20 hours.

[0015] In one embodiment, uniform dispersion is carried out by ultrasonic means, the ultrasonic power is greater than 250 W, and the treatment time is more than 20 minutes.

[0016] Compared with the prior art, the present application has the following beneficial effects: By introducing thermoresponsive nanoparticles with a specific phase transition temperature (-20°C - 0°C), in a low-temperature environment, the thermoresponsive nanoparticles can release or absorb heat through the phase transition process, effectively regulating the electrolyte temperature and reducing the electrolyte viscosity, thereby improving the ionic conductivity. The ionic conductivity of the lithium-ion battery using the composite electrolyte of the present application is greatly improved compared with the traditional electrolyte, ensuring the efficient operation of the battery under extremely low-temperature conditions. High charge capacity retention rate, the ion sieving additive uses a metal-organic framework material, and its pore size dynamically adjusts with temperature. During the low-temperature fast charging process, this characteristic helps to accurately control the lithium-ion migration path, reduce the ion migration resistance, achieve uniform deposition of lithium ions, avoid the formation of lithium dendrites, and improve the charging efficiency and cycle stability of the battery.

[0017] The introduction of thermoresponsive nanoparticles not only regulates the electrolyte temperature but also forms a physical barrier inside the battery through the stability of its core-shell structure, effectively inhibiting the growth and expansion of lithium dendrites. Meanwhile, the pore regulation effect of the ion sieving additive reduces the uneven deposition of lithium ions on the electrode surface, further reducing the risk of battery short circuit and thermal runaway. In addition, the composite electrolyte of this application can achieve fast charging at low temperatures without external heating, avoiding potential safety hazards caused by external heating equipment failures or improper operations, thereby enhancing the overall safety of the battery.

[0018] Compared with existing technologies that rely on external heating (such as PTC heating), the composite electrolyte of this application utilizes the self-heating characteristics of thermoresponsive nanoparticles to achieve temperature regulation inside the battery, reducing the energy consumption ratio. At the same time, since there is no need for the preheating process of external heating equipment, the battery response time is shortened to nearly zero delay, enhancing the user experience and system efficiency. Further, this application precisely controls the ratios of the solvent (ethylene carbonate, fluoroethylene carbonate, ethyl acetate), lithium salt (lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide), and additive (vinylene carbonate, lithium difluorophosphate) to further optimize the physicochemical properties of the electrolyte. Description of the Drawings

[0019] Figure 1 It is a schematic flowchart of the preparation method of the composite electrolyte in an embodiment of this application. Detailed Embodiments

[0020] To make the above objects, features, and advantages of this application more obvious and understandable, the following will describe the detailed embodiments of this application in conjunction with the drawings. It can be understood that the specific embodiments described herein are only for explaining this application and not for limiting this application. Additionally, it should be noted that for the sake of description, only parts related to this application rather than all structures are shown in the drawings. Based on the embodiments in this application, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of this application.

[0021] The terms "including" and "having" and any variations thereof in this application are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or device that includes a series of steps or units is not limited to the listed steps or units, but may optionally further include unlisted steps or units, or may optionally further include other steps or units inherent to these processes, methods, products, or devices.

[0022] References to "embodiments" in this specification mean that a particular feature, structure, or characteristic described in connection with the embodiments can be included in at least one embodiment of the present application. The phrase appears in various places in the specification and does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0023] In the process of the continuous development of power lithium-ion battery technology, performance optimization in the low-temperature fast-charging scenario has always been a key problem that urgently needs to be solved. The low-temperature environment will cause a sharp increase in the viscosity of the electrolyte, which in turn leads to a significant decrease in ionic conductivity, a reduction in the lithium-ion migration rate and a series of other problems. Eventually, it will cause a rapid decay of the battery capacity and greatly increase the risk of lithium dendrite formation, posing a serious threat to the safety and service life of the battery. Existing solutions, such as using low-viscosity ether solvents, can improve the low-temperature performance to a certain extent, but there are problems such as low flash point and insufficient safety; adding specific additives to improve the SEI film, the capacity retention rate is still not ideal during low-temperature high-rate charging; relying on external heating technology will bring disadvantages such as high energy consumption and response delay; traditional additives have poor stability during high-low temperature cycling and are difficult to dynamically adjust the ion migration path.

[0024] This application focuses on the field of composite electrolytes for power lithium-ion batteries, and is committed to developing a composite electrolyte with low-temperature adaptability, which can maintain a high charging capacity during fast charging, has higher safety, does not require external heating, and has no response delay, and proposes a preparation method thereof, providing new ideas and effective solutions for solving the performance bottleneck of power lithium-ion batteries in the low-temperature fast-charging scenario. The following will be introduced in detail. The composite electrolyte in a preferred embodiment of this application includes a solvent, a lithium salt, thermoresponsive nanoparticles, and an ion sieving additive. The thermoresponsive nanoparticles have a core-shell structure, including a core structure formed by nanoparticles and a shell structure formed by a coating layer; the ion sieving additive includes a pore channel regulating material. When the temperature is less than 0 °C, the pore diameter of the pore channels of the pore channel regulating material is in the range of 1-2 nm. When the temperature is greater than or equal to 0 °C, the pore diameter of the pore channels of the pore channel regulating material is in the range of 3-5 nm. Among them, the phase transition temperature of the thermoresponsive nanoparticles is in the range of -20 °C to 0 °C.

[0025] The weight fraction of the solvent in the composite electrolyte ranges from 60% to 80%, and the weight fraction of the lithium salt ranges from 10% to 15%. This concentration range can not only ensure sufficient lithium ion concentration in the electrolyte to meet the ion transport requirements during the charge and discharge process of the battery, but also avoid problems such as increased viscosity and decreased ion transference number caused by too high lithium salt concentration. A reasonable lithium salt concentration helps to maintain the stability and ionic conductivity of the electrolyte. The weight fraction of the thermoresponsive nanoparticles ranges from 20% to 25%. These nanoparticles have unique thermoresponsive properties and can change their physical or chemical properties when the temperature changes. In a high-temperature environment (T > 0 °C), the thermoresponsive nanoparticles can absorb heat and undergo a phase change or structural change, thereby regulating the temperature and local ion concentration distribution of the electrolyte. The thermoresponsive nanoparticles can also inhibit the decomposition of the electrolyte and the occurrence of side reactions at high temperatures, improve the thermal stability of the electrolyte, enable the battery to maintain stable performance under high-temperature high-rate charge and discharge conditions, and extend the service life of the battery. The weight fraction of the ion sieve additive ranges from 10% to 20%. This additive can adjust the pore size according to the temperature change. At high temperatures, the pores expand, which is beneficial to the rapid migration of ions and increases the ionic conductivity. At the same time, the ion sieve additive can also selectively screen the ions in the electrolyte, reduce the influence of impurity ions, further optimize the ion transport environment of the electrolyte, and improve the charge and discharge efficiency and cycle stability of the battery at high temperatures. By precisely controlling the weight fraction ratios of the solvent, lithium salt, thermoresponsive nanoparticles, and ion sieve additive, the components interact synergistically with each other to achieve the comprehensive optimization of the electrolyte performance.

[0026] Core-shell structured nanoparticles with a phase change temperature in the range of -20 °C to 0 °C can be used. Gallium-indium alloy metal core-shell nanoparticles and organic-inorganic composite core-shell materials can be selected. The regulation of the phase change temperature can be achieved through component optimization. Among these materials, Fe3O4@paraffin nanoparticles are preferably selected, with a particle size range of 50 - 100 nm and a paraffin coating thickness of 10 - 15 nm. When the temperature T is less than 0 °C, the nanoparticles undergo a phase change and release latent heat, and the latent heat value is not less than 150 J / g, which can effectively increase the electrolyte temperature by 3 - 8 °C. When the temperature T is greater than 0 °C, the nanoparticles are uniformly dispersed in the electrolyte and do not increase the viscosity of the electrolyte, and its dynamic viscosity is less than 5 mPa·s.

[0027] The pore regulating material can include temperature-responsive polymer-based materials, liquid crystal elastomer (LCE) materials, and metal-organic framework materials. The pore regulating material is preferably a metal-organic framework. Specifically, the dynamic ion sieving additive is a binuclear MOF material, such as the ZIF-8 / ZIF-67 composite framework, whose pore size can be adjusted within the range of 1-5 nm. When the temperature T is less than 0 °C, the pores of the metal-organic framework shrink to 1-2 nm, which can sieve solvated Li⁺, reducing the desolvation energy by 0.3-0.5 eV. When the temperature T is greater than 0 °C, the pores of the metal-organic framework expand to 3-5 nm, and the ion transference number can be increased to more than 0.75.

[0028] At low temperatures (T < 0 °C), the thermoresponsive nanoparticles undergo a phase change and release a large amount of latent heat (≥150 J / g), which can effectively increase the electrolyte temperature by 3-8 °C. This property provides additional heat support for the battery to operate at low temperatures, helping to alleviate problems such as increased electrolyte viscosity and decreased ionic conductivity caused by low temperatures, thereby improving the battery's performance at low temperatures and increasing the battery's capacity retention rate and charge-discharge efficiency. When the temperature rises to T > 0 °C, the thermoresponsive nanoparticles are uniformly dispersed in the electrolyte and do not increase the viscosity of the electrolyte (dynamic viscosity < 5 mPa·s), ensuring that at normal operating temperatures, the performance of the electrolyte is not affected by the nanoparticles, maintaining the stability and high efficiency of the battery in high-temperature environments, and facilitating the reliable operation of the battery under various temperature conditions.

[0029] Due to the addition of the dynamic ion sieving additive, solvated Li⁺ can be sieved at low temperatures. At low temperatures (T < 0 °C), the pores of the metal-organic framework shrink to 1-2 nm, which can sieve solvated Li⁺, reducing the desolvation energy by 0.3-0.5 eV, helping to promote the desolvation of Li⁺, increasing the migration rate of Li⁺ in the electrolyte, thereby improving the charge-discharge performance of the battery at low temperatures, reducing the formation of lithium dendrites, and increasing the safety and cycle life of the battery.

[0030] When the temperature rises to T > 0 °C, the pores of the metal-organic framework expand to 3-5 nm, and the ion transference number increases to more than 0.75. The larger pores are beneficial for the rapid migration of ions, increasing the ionic conductivity of the electrolyte, further optimizing the battery's performance in high-temperature environments, enabling the battery to maintain good performance under high-current charge-discharge conditions, and meeting the requirements of power lithium-ion batteries in different application scenarios. In summary, the composite electrolyte in this application effectively regulates the performance of the electrolyte under different temperature conditions through the synergistic effect of thermoresponsive nanoparticles and dynamic ion sieving additives, providing strong support for the application of power lithium-ion batteries in extreme scenarios such as fast charging at low temperatures.

[0031] The ion sieving additive uses more than two metal-organic frameworks. Specifically, taking the dual-core MOF material ZIF-8 / ZIF-67 composite framework as an example, it realizes the precise regulation of the electrolyte performance under different temperature conditions, enhances the synergistic effect of material characteristics. In a low-temperature environment (T < 0 °C), the pores of the ZIF-8 / ZIF-67 composite framework can shrink to 1 - 2 nm. The pore size change combined with the synergistic effect between different MOF materials in the composite framework endows it with the ability to sieve solvated Li⁺. Through the sieving effect, the desolvation energy of solvated Li⁺ can be effectively reduced by 0.3 eV - 0.5 eV, promoting the desolvation process of Li⁺, enabling Li⁺ to migrate more quickly and efficiently in the electrolyte. Under low-temperature conditions, this advantage is particularly obvious, which can effectively alleviate the problem of the decrease in ionic conductivity caused by the temperature reduction, greatly improving the charge-discharge performance of the battery at low temperatures, enabling the battery to maintain a high capacity retention rate and charge-discharge efficiency in a cold environment, and broadening the operating temperature range of the battery. When the temperature rises to T > 0 °C, the pores of the ZIF-8 / ZIF-67 composite framework expand to 3 - 5 nm. The larger pores provide a more spacious channel for ion migration, reducing the resistance of ion migration. More Li⁺ can participate in the charge transport process, effectively improving the ionic conductivity of the electrolyte. Under high-temperature and high-rate charge-discharge conditions, this technical effect can reduce the internal resistance of the battery, reduce energy loss, improve the output power and energy efficiency of the battery, enabling the battery to better meet the requirements of power equipment for high-power output. Through the pore size dynamic adjustment function of the ion sieving additive, it can adapt to different temperature environments. Whether under low-temperature or high-temperature conditions, it can automatically adjust the performance parameters of the electrolyte, enabling the battery to always maintain the best working state without additional external heating or cooling equipment, reducing the complexity and cost of the battery system.

[0032] The solvents are ethylene carbonate (EC), fluoroethylene carbonate (FEC), and ethyl acetate (EA) in a specific ratio of 2:1:7 (volume ratio). Among them, EC has a high dielectric constant, which can effectively dissociate lithium salts and provide a rich ion source for ion transport; FEC has a low freezing point and can still maintain good fluidity at low temperatures, helping to maintain the ionic conductivity of the electrolyte; EA has good solubility and volatility, which can adjust the viscosity and boiling point of the electrolyte. This unique solvent ratio can still ensure that the electrolyte has a high ionic conductivity in a low-temperature environment (T < 0 °C) through the synergistic effect of each component, effectively reducing the internal resistance of the battery under low-temperature conditions, enabling the battery to charge and discharge quickly in a cold environment, improving the low-temperature capacity retention rate and charge-discharge efficiency of the battery, and broadening the operating temperature range of the battery.

[0033] In this application, lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) are selected as the lithium salt combination, and the molar content ratio of the two is controlled within the range of 3:1 to 5:1. For example, take LiFSI (1.2M) + LiTFSI (0.3M) as an example. LiFSI has high ion dissociation ability and low viscosity, can provide abundant free lithium ions in the electrolyte, effectively reduce the viscosity of the electrolyte, and improve the ionic conductivity; LiTFSI has good thermal stability and chemical stability, can cooperate with LiFSI, and further optimize the ionic transport environment of the electrolyte. The two cooperate with each other at a specific molar content ratio, so that the electrolyte can maintain a high ionic conductivity at room temperature and a wide temperature range, promote the rapid migration of lithium ions in the electrolyte, thereby improving the charge and discharge rate of the battery, shortening the charging time, and meeting the application requirements of fast charging. In a low-temperature environment, the low-viscosity characteristic of LiFSI helps to reduce the increase in the viscosity of the electrolyte at low temperature and maintain the fluidity of ions; the thermal stability of LiTFSI helps to prevent the electrolyte from solidifying or crystallizing at low temperature and ensure the smoothness of the ion transport channel. Therefore, this lithium salt combination can make the battery still have a high ionic conductivity under low-temperature conditions, effectively improve the charge and discharge performance of the battery in a low-temperature environment, expand the operating temperature range of the battery, and enable the battery to operate normally and efficiently in cold regions or low-temperature working scenarios. Both LiFSI and LiTFSI have strong redox resistance. During the charge and discharge process of the battery, the positive electrode material undergoes an oxidation reaction, and the negative electrode material undergoes a reduction reaction. The lithium salt in the electrolyte needs to have good redox stability to prevent its own decomposition or side reactions with the electrode material. The lithium salt combination of this application can effectively resist the high-potential oxidation and low-potential reduction of the electrode material, reduce the decomposition of the electrolyte on the electrode surface, and reduce the impedance at the electrode-electrolyte interface, thereby improving the cycle stability and safety of the battery at high voltage. Since this lithium salt combination can support the stable operation of the battery at high voltage, the battery can use a positive electrode material with a higher voltage, such as a high-nickel ternary material, etc., which also helps to reduce the capacity attenuation of the battery during the charge and discharge process and extend the service life of the battery.

[0034] The composite electrolyte of the present application further adds vinylene carbonate (VC) and lithium difluorophosphate (LiPO2F2) in specific weight proportions as additives. During the first charge and discharge process of the battery, VC will preferentially undergo a reduction decomposition reaction on the surface of the negative electrode before the electrolyte, forming a dense and stable solid electrolyte interface (SEI) film. In the present application, the weight proportion of VC is in the range of 0.5% - 1.5% (taking 1% VC as an example), and this addition amount can ensure the formation of an SEI film with sufficient thickness and good performance on the surface of the negative electrode. This SEI film has good ionic conductivity and electronic insulation, can effectively prevent the further reaction between the electrolyte and the negative electrode material, reduce the dissolution and structural damage of the negative electrode material, lower the internal resistance of the battery, improve the insertion and extraction efficiency of lithium ions, and thus significantly improve the first charge and discharge efficiency and cycle stability of the battery.

[0035] The weight proportion of LiPO2F2 is in the range of 0.1% - 1% (taking 0.5% LiPO2F2 as an example), which can continuously modify and optimize the SEI film during the battery cycle. LiPO2F2 can chemically react with the components in the SEI film, improve the composition and structure of the SEI film, and make it more stable and uniform. At the same time, it can also form a protective film on the surface of the positive electrode, reduce the side reaction between the positive electrode material and the electrolyte, inhibit the structural deterioration of the positive electrode material and the dissolution of transition metal ions, further improve the stability of the electrode - electrolyte interface, and extend the cycle life of the battery. Both VC and LiPO2F2 have good antioxidant properties. VC can capture the free radicals generated by the decomposition of the electrolyte through the unsaturated bonds in its molecular structure, inhibit the chain reaction initiated by free radicals, and thus reduce the oxidative decomposition of the electrolyte. LiPO2F2 can form a stable protective film on the surface of the positive electrode, prevent the direct contact between the electrolyte and the positive electrode material, reduce the oxidation degree of the positive electrode material, and improve the cycle stability of the battery under high voltage. In a low - temperature environment, the viscosity of the electrolyte increases, the ion migration rate decreases, resulting in an increase in the interface impedance of the battery and a decline in the charge - discharge performance. The presence of VC and LiPO2F2 can improve the properties of the electrode - electrolyte interface and reduce the interface impedance. The SEI film formed by VC can still maintain good ionic conductivity at low temperatures, and the modification effect of LiPO2F2 on the SEI film also helps to improve the ion transport efficiency at the interface. Therefore, the battery can more quickly insert and extract lithium ions under low - temperature conditions, improving the low - temperature charge - discharge efficiency and capacity retention rate of the battery.

[0036] Please refer to Figure 1, this application further provides a method for preparing a composite electrolyte, including: preparing thermoresponsive nanoparticles, mixing the nanoparticles with a coating material, ultrasonically dispersing and then rapidly cooling to obtain thermoresponsive nanoparticles with a core-shell structure, synthesizing an ion sieve additive, dissolving a metal salt and reacting with 2-methylimidazole in methanol to obtain a metal-organic framework, and mixing a solvent, a lithium salt, the thermoresponsive nanoparticles, and the ion sieve additive and uniformly dispersing them, wherein the weight fraction of the solvent is in the range of 60%-80%, the weight fraction of the lithium salt is in the range of 10%-15%, the weight fraction of the thermoresponsive nanoparticles is in the range of 20%-25%, and the weight fraction of the ion sieve additive is in the range of 10%-20%.

[0037] Specifically, 50-nm Fe3O4 nanoparticles are mixed with molten paraffin with a phase transition temperature of -15 °C, ultrasonically dispersed and then rapidly cooled to obtain thermoresponsive nanoparticles with a core-shell structure having a coating layer thickness of 10 nm, and surface modification is carried out using KH550 silane coupling agent to improve the dispersibility of the electrolyte. The thermoresponsive nanoparticles have phase transition characteristics, release latent heat (≥150 J / g), and increase the electrolyte temperature by 3-8 °C. At the same time, the surface modification with KH550 silane coupling agent enhances the dispersibility and stability of the nanoparticles in the electrolyte, enabling them to be uniformly distributed in the electrolyte, better playing the thermoresponsive role, and avoiding the agglomeration of the nanoparticles, ensuring the performance consistency of the electrolyte.

[0038] By dissolving Zn(NO3)2 and Co(NO3)2 in a molar ratio of 1:1 and reacting with 2-methylimidazole in methanol for 24 hours, a composite metal-organic framework (MOF) dual-core ZIF-8 / ZIF-67 with adjustable pore size is synthesized as an ion sieve additive. The composite MOF has a regular pore structure and adjustable pore size, can selectively allow lithium ions to pass through according to the size and charge characteristics of lithium ions, while blocking the transport of other impurity ions and solvent molecules, improving the transference number and transport efficiency of lithium ions, not only reducing the internal resistance of the battery, improving the charge-discharge performance and energy density of the battery, but also reducing the deposition and side reactions of impurity ions on the electrode surface, and prolonging the cycle life of the battery. In addition, the high specific surface area and abundant active sites of the MOF can also interact with other components in the electrolyte to further optimize the electrochemical performance of the electrolyte.

[0039] Mix the solvent, lithium salt, thermoresponsive nanoparticles, and ion sieving additive in specific proportions (the weight fraction of the solvent ranges from 60% to 80%, the weight fraction of the lithium salt ranges from 10% to 15%, the weight fraction of the thermoresponsive nanoparticles ranges from 20% to 25%, and the weight fraction of the ion sieving additive ranges from 10% to 20%), and subject them to ultrasonic treatment until uniformly dispersed. By controlling the proportions of each component, an overall balance is achieved in terms of ionic conductivity, thermal stability, safety, and electrochemical performance of the composite electrolyte. An appropriate amount of solvent ensures that the electrolyte has sufficient ion transport channels and good wettability; the lithium salt provides a rich source of lithium ions, ensuring that the battery has a high voltage and energy density; the reasonable addition of thermoresponsive nanoparticles and ion sieving additive enhances the thermal safety and ion selective transport ability of the battery respectively. The optimized component proportions enable the composite electrolyte to operate stably and efficiently under various working conditions, meeting the requirements of power lithium-ion batteries in different application scenarios. The entire preparation process, including the preparation of nanoparticles, surface modification, synthesis of MOF, and assembly of the electrolyte, etc., is carried out under specific conditions, such as ultrasonic dispersion, inert atmosphere, etc.

[0040] During the preparation of the thermoresponsive nanoparticles, the surface of the nanoparticles is modified. Specifically, the surface of the nanoparticles is treated in a specific manner (such as using KH550 silane coupling agent to improve the dispersion of the electrolyte) to improve its compatibility and dispersion with other components in the electrolyte. The surface-modified nanoparticles can be better dispersed in the electrolyte, avoiding the occurrence of agglomeration phenomena, thus ensuring the uniform distribution of the thermoresponsive nanoparticles in the electrolyte. At low temperatures (-20 °C), the uniformly distributed nanoparticles can more effectively exhibit their thermoresponsive characteristics, and through their own thermal effects such as phase change, provide local heat compensation for the electrolyte, enabling the electrolyte to still maintain a high ionic conductivity (≥2.5 mS / cm) at -20 °C, effectively reducing the impact of low temperature on battery performance, improving the charge-discharge efficiency and capacity retention rate (≥75%) of the battery at low temperatures, broadening the operating temperature range of the battery, and enhancing the low-temperature adaptability of the battery.

[0041] Highly selective ion transport and improved fast charging performance brought about by the synthesis of ion sieving additives with long reaction times. When synthesizing ion sieving additives, Zn(NO3)2 and Co(NO3)2 are dissolved in a 1:1 molar ratio and reacted with 2-methylimidazole in methanol for more than 20 hours. The longer reaction time helps the full growth and crystallization of ion sieving additives such as metal-organic frameworks (MOFs), forming a more regular pore structure and higher crystallinity. This ion sieving additive with precise pore size and abundant active sites can efficiently and selectively transport lithium ions, increase the transference number of lithium ions, and reduce the internal resistance of the battery. During fast charging (4C charging), it can quickly and efficiently transport lithium ions, reduce the concentration polarization and electrochemical polarization of lithium ions on the electrode surface, thereby ensuring the capacity retention rate (≥90%) of the battery under 4C charging conditions, greatly shortening the charging time of the battery, improving the fast charging performance of the battery, and meeting the user's demand for fast charging.

[0042] Uniform dispersion is carried out by ultrasonic means, with the ultrasonic power greater than 250W and the treatment time more than 20 minutes. High-power ultrasonic dispersion can generate strong cavitation effects and mechanical stirring actions, enabling the full mixing of components such as solvents, lithium salts, thermoresponsive nanoparticles, and ion sieving additives in the electrolyte to form a uniform and stable dispersion system. The uniformly dispersed electrolyte can reduce the local electrochemical performance differences caused by uneven components and lower the probability of local overheating and side reactions during battery charge and discharge. At the same time, the uniformly dispersed thermoresponsive nanoparticles and ion sieving additives can work better in synergy, further enhancing the thermal stability and electrochemical stability of the battery. After 200 cycles, the dendrite density inside the battery is extremely low, effectively avoiding safety hazards such as short circuits caused by dendrite growth and improving the safety and cycle life of the battery. This application develops an electrolyte technology that combines self-heating and dynamic sieving. The thermoresponsive nanoparticles provide heat compensation for the electrolyte through their thermoresponsive characteristics, and the ion sieving additives achieve the selective transport of lithium ions. Compared with traditional positive temperature coefficient (PTC) solutions, the technical solution of this application does not require an additional external heating device and can provide the required heat for the electrolyte in a low-temperature environment through the self-heating effect of the thermoresponsive nanoparticles, reducing energy consumption. At the same time, the dynamic sieving function of the ion sieving additives can adjust the transport rate of lithium ions in real time according to the working state of the battery and the demand for lithium ions, improve the utilization efficiency of lithium ions, and further reduce the energy loss of the battery.

[0043] As can be seen from the above, this application focuses on the field of power lithium-ion batteries and provides a composite electrolyte with excellent low-temperature adaptability and fast charging performance and its preparation method. This composite electrolyte mainly consists of key components such as solvents, lithium salts, thermoresponsive nanoparticles, and ion sieving additives. Through the synergistic effect of the component ratios and preparation processes, the performance of the electrolyte is optimized in all aspects.

[0044] In terms of thermoresponsive nanoparticles, Fe3O4@paraffin nanoparticles with a core-shell structure are selected. Their phase transition temperature is in the range of -20°C to 0°C, the particle size is 50 - 100 nm, and the thickness of the paraffin coating layer is 10 - 15 nm. When the temperature is below 0°C, the nanoparticles undergo a phase transition and release latent heat of not less than 150 J / g, effectively increasing the electrolyte temperature by 3 - 8°C, improving the problems of increased electrolyte viscosity and decreased ionic conductivity at low temperatures, and enhancing the low-temperature capacity retention rate and charge-discharge efficiency of the battery. When the temperature is above 0°C, the nanoparticles are uniformly dispersed and the dynamic viscosity is less than 5 mPa·s, ensuring the stability and high efficiency of the electrolyte at high temperatures.

[0045] The ion sieving additive uses a dual-core MOF material, the ZIF-8 / ZIF-67 composite framework, whose pore size can be dynamically adjusted in the range of 1 - 5 nm. At low temperatures (T < 0°C), the pore channels shrink to 1 - 2 nm, reducing the desolvation energy of solvated Li⁺ by 0.3 - 0.5 eV, promoting Li⁺ desolvation, increasing the migration rate, and reducing the formation of lithium dendrites. When the temperature rises to T > 0°C, the pore channels expand to 3 - 5 nm, and the ion transference number increases to more than 0.75, enhancing the ionic conductivity of the electrolyte and optimizing the high-temperature performance of the battery.

[0046] In the preparation method, Fe3O4 nanoparticles are mixed with molten paraffin, and after ultrasonic dispersion, they are rapidly cooled to obtain core-shell structured thermoresponsive nanoparticles, which are then surface-modified. Metal salts and 2-methylimidazole are reacted in methanol for more than 24 hours to synthesize the ion sieving additive. Finally, each component is mixed in a specific ratio in an inert atmosphere and subjected to high-power ultrasonic treatment until uniformly dispersed. By precisely controlling the proportion of each component and the preparation process parameters, a comprehensive balance of the ionic conductivity, thermal stability, safety, and electrochemical performance of the composite electrolyte is achieved, enabling it to operate stably and efficiently under various working conditions and meet the requirements of power lithium-ion batteries in different application scenarios.

[0047] Through the synergistic effect of the thermoresponsive nanoparticles and the ion sieving additive, the composite electrolyte of this application effectively regulates the performance of the electrolyte under different temperature conditions, providing strong support for the application of power lithium-ion batteries in extreme scenarios such as fast charging at low temperatures.

[0048] The above is only a specific embodiment of this application. Any improvement made on the premise of the concept of this application is regarded as the protection scope of this application.

Claims

1. A composite electrolyte, characterized in that, It includes a solvent, a lithium salt, thermoresponsive nanoparticles, and an ion sieving additive; In the composite electrolyte, the weight fraction of the solvent ranges from 60% to 80%, the weight fraction of the lithium salt ranges from 10% to 15%, the weight fraction of the thermoresponsive nanoparticles ranges from 20% to 25%, and the weight fraction of the ion sieving additive ranges from 10% to 20%; The thermoresponsive nanoparticles have a core-shell structure, including a core structure formed by the nanoparticles and a shell structure formed by the coating layer; The ion sieving additive includes a pore size regulating material. When the temperature is less than 0 °C, the pore size of the pores of the pore size regulating material ranges from 1 to 2 nm. When the temperature is greater than or equal to 0 °C, the pore size of the pores of the pore size regulating material ranges from 3 to 5 nm; Among them, the phase change temperature of the thermoresponsive nanoparticles ranges from -20 °C to 0 °C.

2. The composite electrolyte according to claim 1, wherein The component of the nanoparticles is iron oxide, the component of the coating layer is paraffin wax, and the pore size regulating material is a metal-organic framework.

3. The composite electrolyte according to claim 2, wherein The ion sieving additive includes two or more metal-organic frameworks.

4. The composite electrolyte according to claim 1, wherein The solvent includes ethylene carbonate with a weight fraction ranging from 10% to 30%, fluoroethylene carbonate with a weight fraction ranging from 5% to 15%, and ethyl acetate with a weight fraction ranging from 60% to 80%.

5. The composite electrolyte according to claim 1, wherein The lithium salt includes lithium bis(fluorosulfonyl)imide and lithium bis(trifluoromethanesulfonyl)imide, and the molar content ratio of lithium bis(fluorosulfonyl)imide to lithium bis(trifluoromethanesulfonyl)imide ranges from 3:1 to 5:

1.

6. The composite electrolyte according to claim 1, characterized in that, It also includes vinylene carbonate with a weight fraction ranging from 0.5% to 1.5% and lithium difluorophosphate with a weight fraction ranging from 0.1% to 1%.

7. A method for preparing a composite electrolyte, characterized in that, It includes: Prepare thermoresponsive nanoparticles, mix the nanoparticles with the coating layer material, and obtain the thermoresponsive nanoparticles with a core-shell structure after ultrasonic dispersion and rapid cooling; Synthesize the ion sieving additive by dissolving a metal salt and reacting it with 2-methylimidazole in methanol to obtain a metal-organic framework; Mix the solvent, lithium salt, thermoresponsive nanoparticles, and ion sieving additive and disperse them evenly; Among them, the weight fraction of the solvent ranges from 60% to 80%, the weight fraction of the lithium salt ranges from 10% to 15%, the weight fraction of the thermoresponsive nanoparticles ranges from 20% to 25%, and the weight fraction of the ion sieving additive ranges from 10% to 20%.

8. The preparation method of the composite electrolyte according to claim 7, wherein, The component of the nanoparticles is iron oxide, the component of the coating layer is paraffin wax, and after preparing the thermoresponsive nanoparticles, surface modification of the nanoparticles is also included.

9. The preparation method of the composite electrolyte according to claim 7, characterized in that, Specifically, synthesizing the ion sieving additive includes dissolving Zn(NO3)2 and Co(NO3)2 in a molar ratio of 1:1 and reacting them with 2-methylimidazole in methanol for more than 20 hours.

10. The preparation method of the composite electrolyte according to claim 7, characterized in that, Perform uniform dispersion by ultrasonic method, with the ultrasonic power greater than 250 W and the treatment time more than 20 minutes.

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