Applications Of Ionic Liquids In The Synthesis Of Electrode And Electrolyte Materials Of Lithium-Ion Batteries

Ionic liquids,as a kind of room temperature molten salts composed only of anions and cations,which have been widely used in chemistry,biology,materials and other fields due to their series of unique physical and chemical properties.With the increasing demand for clean and sustainable energy,one of the most important application fields of ionic liquids is undoubtedly the storage and conversion devices for new energy,especially the booming lithium-ion batteries.It is well known that electrode and electrolyte,as the main components,have a decisive impact on the performance of lithium-ion batteries.On the one hand,polyanion-type cathode material lithium vanadium phosphate(LVP)has attracted much attention due to its high theoretical specific capacity,average charging/discharging voltage,structural stability and safety,but its low intrinsic electronic conductivity limits its large-scale application.On the other hand,solid polymer electrolytes(SPE)have attracted wide attention due to their much higher safety and energy density than conventional organic electrolytes,but their low ionic conductivity at room temperature has hindered the commercialization process.This thesis aims at the rational applications of ionic liquids,and optimizing the electrochemical properties of lithium vanadium phosphate and solid polymer electrolytes by morphological regulation,coating and doping,so as to promote the rapid development of lithium-ion batteries towards the direction of light weight,thin film,high safety and high energy density.The main contents are as follows:Adhering to the "integration" of reactant,structure directing agent and carbon source,the ionic liquid was used as the precursor,which assisted electrospinning technology to prepare self-supported lithium vanadium phosphate nanocubes/carbon nanofibers composite in situ.The material showed excellent electrochemical properties for lithium-ion batteries,it could still deliver 143.6 mAh g-1 high discharge capacity after 1000 cycles at 5 C rate,and the capacity retention of 95.54%was obtained.This was attributed to the fact that the ionic liquid not only induced the formation of lithium vanadium phosphate nanocubic crystal with high orientation,but also formed nitrogen doped carbon coating layer with high conductivity in the process of high temperature heat treatment,which ensured excellent electrochemical reaction kinetics of lithium vanadium phosphate.Using coating flow method,two kinds of solid polymer electrolyte films doped with ionic liquids were prepared.By optimizing the preparation process,it was found that the thickness of the film and the type of ionic liquid would affect the ionic conductivity and other properties of the electrolytes.The room-temperature ionic conductivity of the as-prepared electrolytes could reach 1.5×10-4 S cm-1,and the electrochemical stability window highly reached 5 V.The self-supported lithium vanadium phosphate/carbon fibers composite material was selected as the cathode,and the solid-state lithium-ion battery was assembled by combining the solid-state polymer electrolyte.And then the electrochemical properties of the battery at room temperature and high temperature 60? were investigated.At room temperature,the LVP/SPE-PYR14TFSI/Li solid-state lithium-ion battery delivered a discharge capacity of 101.5 mAh g-1 after 500 cycles at 1 C rate,and the coulombic efficiency was stable at around 98%,indicating its good electrochemical performance.