Preparation And Performance Study Of PEO/ionic Liquid-based Lithium Salt All-solid Electrolyte

In general,solid polymer electrolytes?SPEs?are prepared by the dissolution of lithium salts in polymer matrix.Due to their high energy density,flexibility,and good safety performance,SPEs have attracted widespread attention.However,the current room temperature conductivity of SPEs is low,which cannot meet the conductivity requirements of SPEs in practical application.In order to improve the room temperature conductivity of SPEs and promote the commercialization of solid polymer electrolytes,in this paper,we have successfully synthesized two lithium salts composed of lithium cations and bis?ionic liquid?borate anions?LiBIB,LiBMB?through a two-step process,which have similar structure with bis?oxalate?borate?LiBOB?,but have better solubility than compared lithium salts;then,solution casting technology has been used to prepare different lithium salt based SPEs membranes doping with amount of polyethylene oxide?PEO?.Fourier transform infrared?FTIR?,¹H-NMR,and ¹³C-NMR techniques were used to characterize the molecular structure of both the intermediate and the final lithium salts.The polymer electrolyte films were characterized by DSC,TGA and XRD as well.The electrochemical properties,such as ionic conductivity,electrochemical stability window,and Li⁺ions transport numbers were studied by EIS and LSV technique.The main conclusions are obtained as follows.?1?At room temperature,both LiBIB and LiBMB have partially crystalline properties and good thermal stability.Compared with LiBIB?T_d=180°C?,LiBMB?T_d=251°C?has higher thermal stability,and the melting temperature of the two lithium salts are all around40°C.After 45°C,the semi-solid state lithium salt is completely transformed into liquid state;?2?The doping of lithium salt in PEO polymer matrix can improve the thermal stability of SPEs,and the thermal stability increases with increasing the salt concentration.The highest thermal decomposition temperature of LiBIB and LiBMB based SPEs are at around 248°C and 286°C,respectively.The lithium salt doping can reduce the melting point of SPEs,and the melting point decreases with increasing the lithium salt content.?3?The ion conductivity of SPEs increases with both temperature and lithium salt concentration.The maximum room temperature conductivity of LiBIB and LiBMB based SPEs reaches 3.85×10^-4 S cm^-1and4.44×10^-4 S cm^-1,respectively,which are much higher than traditional SPEs.?4?Both the electrochemical stability window and the lithium ion transport number of SPEs increase with increasing the lithium salt concentration.The PEO/LiBIB based SPEs have electrochemical stability window of up to 8V and maximum lithium ion transport number of up to 0.51.Meanwhile,the PEO/LiBMB based SPEs have the most stable electrochemical window of 8V and the maximum lithium ion transport number is 0.59.