Preparation And Properties Of Safety Electrolytes For Lithium Ion Batteries

With the rapid development of society and the increasing demand for energy from all walks of life,lithium battery plays an important role in energy storage and power cell as energy storage/conversion carrier.Lithium batteries are increasingly used in consumer electronics,electric vehicles,and energy storage devices.Lithium batteries have been widely concerned by academia and industry because of their high energy density and good cyclic stability.At the same time,the energy structure of our country is dominated by fossil energy,with high carbon emission pressure and very high environmental climate potential risks.In view of this,our country has put forward the goal of"carbon peaking and carbon neutrality",making an important deployment to the energy structure,launched new energy storage materials and low carbon zero carbon negative carbon,etc.The potential safety risks of liquid electrolytes in lithium batteries,such as high volatility,flammability,and leakage,limit their large-scale applications.In 2022,the Comprehensive Department of the National Energy Administration issued a notice on further strengthening the prevention of the power industry in accordance with the General Secretary Xi Jinping’s discussion on"two adherence"and"three transformations"for disaster prevention and reduction.Improving the chemical stability of electrolytes and ensuring their heat and flame resistance to a certain extent can greatly reduce the safety hidden danger,which is beneficial to the further popularization and application of lithium batteries.All solid electrolyte has great advantages in fire safety performance,but their low ionic conductivity and large interface resistance make their industrial applications difficult.In this paper,through the solid-liquid phase combination method,a variety of gel quasi-solid electrolytes based on ionic liquids were studied from the aspects of electrochemical performance and fire safety performance,and a new type of electrolyte-quasi-solid electrolyte was prepared,which combined the excellent electrochemical performance of traditional liquid electrolytes and the fire safety performance of solid electrolytes.The main contents and results of this thesis are as follows:（1）Firstly,To solve the problem that the assembly of liquid electrolytes is easy to leak and contains flammable organic components,a hydrotalcite ionic liquid gel quasi-solid electrolyte is designed in this paper,and its safety and electrochemical performance are studied.A gel quasi-solid electrolyte based on hydrotalcite was prepared by sol-gel method using hydrotalcite as a frame structure,lithium trifluoromethane sulfonimide as lithium salt（Li TFSI）and 1-ethyl-3-methylimidazoline bis（trifluoromethyl sulfonyl）imide as an ionic liquid.The electrolyte has a superior interface wetting effect,resulting in a tight interface contact between the electrolyte and the electrode,thereby building an effective Li⁺conduction network structure throughout the battery,with a lithium ion conductivity of 10^-3 S cm^-1.Excellent charge-discharge battery performance in half battery Li|Li Fe PO₄ test.The results of combustion experiments show that the electrolyte has excellent flame retardancy.（2）A gel quasi-solid electrolyte based on MCM-41 and solvated ionic liquid（Li（G4）₁TFSI）was optimized to reduce the battery performance caused by hydrostatic being easily exposed to moisture and the high price of ionic liquid.The quasi-solid electrolyte fixed the solvated ionic liquid in the inorganic mesoporous material molecular sieve,improved the conductivity of lithium ions at room temperature,ion migration number,corrosion resistance,and reduced the interface resistance between electrolyte and electrode,and inhibited the growth of lithium dendrites.Compared with carbonic acid-based electrolytes,solvated ionic liquid has excellent flame retardancy and stability.In the half battery test,the charge and discharge cycle reached more than 2700 times.（3）Gel polymer quasi-solid electrolytes（GPE）based on poly（ethylene oxide）（PEO）and solvent ionic liquids have been developed to improve the flexibility and mechanical properties of electrolytes.In this paper,impedance,chronoamperometry,cyclic voltammetry,and linear sweep voltammetry were used to analyze the electrochemical performance of the electrolyte,thermogravimetric analysis was used to analyze the thermal stability of the electrolyte,and the combustion method was used to measure the flame retardant.The results show that the electrolysis has not only flexibility,but also good charge-discharge ability and coulomb efficiency.（4）Aiming at the disadvantage of low ionic conductivity of polyoxyethylene reference solid electrolyte,combined with the advantages of inorganic electrolyte and organic electrolyte,the performance of gel quasi-solid electrolyte composed of polymer（PVDF-HFP）and inorganic oxide（MCM-41/LDH）combined with solvated ionic liquid（Li（G4）₁TFSI）was designed and compared.The new electrolytes prepared in this paper have been further improved in the aspects of lithium-ion conductivity,lithium-ion migration number,and electrochemical window stability.The half cell fabricated by gel quasi-solid electrolyte shows good cycling performance.The results of ignition experiments show that the electrolyte has good flame retardancy,which is of great significance for improving the safety of lithium-ion batteries and preventing fire and other safety hazards.