Modification Of Polyimide-based Polymer Electrolyte And Electrochemical Performance Study

Solid-state lithium batteries(SSLBs)are considered as one of the most promising chemical energy sources for electronic devices and electric vehicles due to their high energy density and high safety.As an important component of SSLBs,solid electrolyte has a great influence on battery performance.In recent years,the composite polymer electrolytes(CPEs)synthesized by doping the polymer electrolytes with inorganic fillers have attracted wide attention due to their strong mechanical tensile and high interfacial compatibility with lithium electrodes.Polyimide(PI)has become one of the most interesting polymers to be used as the matrix of polymer electrolyte because of its excellent thermal stability,chemical corrosion resistance,insulation and thermal stability.In this study,functionalized boron nitride nanosheets(BNNSS-NH₂)were successfully prepared by liquid-phase stripping method,and used as the fillers to dope into the PI-based polymer electrolyte matrix forming a PI/h-BN composite polymer electrolyte.Meanwhile,polyamide acid(PAA)was synthesized by in-situ polycondensation of PDMA and ODA monomers through chemical imidization at a low temperature.In addition,PI/PEO blended polymer electrolytes reinforced by the PI fibers were synthesized by electrospinning method.The electrochemical properties of the resultant polymer electrolytes were studied by related characterizations and electrochemical tests.The study contents are listed as follows:1.Boron nitride was simultaneously peeled and functionalized by a liquid phase peeling method.The effects of the balling time and the mass ratio of urea to h-BN on the structure and morphology of the synthesized BNNSS-NH₂ nanosheets were studied.By comparing the effects of various experimental conditions on the related properties of BNNSS-NH₂,the optimum synthesis conditions were selected as:20h of ball-milling reaction time,1:50 of mass ratio of urea to h-BN.2.PMDA/ODA was selected as polymer monomers to prepare polyamic acid PAA,and the PAA/BNNSs-NH₂ composite gel film was prepared at low temperature in the solution containing different mass fraction of BNNSs-NH₂.After chemical imidization,the PI/h-BN composite polymer film was obtained.After adsorption of electrolyte solution,the PI/h-BN composite polymer electrolyte was formed.The optimal doping amount of BNNSS-NH₂ is 4%,at which the inorganic particles of BNNSs-NH₂ are evenly dispersed in PI matrix.The composite polymer electrolyte(BPI-4)with this doping amount has an ionic conductivity of 8.204×10^-5 S cm^-1 at room temperature.The initial decomposition potential of this polymer electrolyte reaches 5.0 V(vs.Li⁺/Li).The interface between lithium metal and BPI-4 has excellent electrochemical stability,which can meet the working requirements of lithium secondary batteries.The Li Fe PO₄/Li battery assembled by it can deliver a specific capacity of 151 m Ah g^-1at 0.1 C rate in the first cycle,and retains 65 m Ah g^-1after 200 cycles.The specific discharge capacities are 135 m Ah g^-1,120 m Ah g^-1 and90 m Ah g^-1at 0.2 C,0.5 C and 1 C rates,respectively.The results indicate that BPI-4polymer electrolyte has excellent electrochemical properties.3.PI/PEO blended electrolyte(PIE)was prepared by electrospinning method using polyamic acid(PAA)originated from PMDA/ODA monomers.The PAA fiber membrane was imidized to obtain the PI/PEO blend electrolyte(PIE),which was then combined with PEO based polymer electrolyte.When the mass ratio of PI to PEO is1:20,the as-prepared PIE-20 electrolyte has an ionic conductivity of 4.325×10^-4 S cm^-1 at room temperature,and the electrochemical stability window is stable above4.8 V(vs.Li⁺/Li),which can meet the demands of lithium batteries.At a current density of 0.1 C,the Li Fe PO₄/Li battery assembled with PIE-20 can deliver a specific capacity of 153 m Ah g^-1.Both at room temperature and 60°C,the battery presented good rate performance,indicating its relative stability with lithium metal under high temperature and high rate.