Fabrication And Modification Of Novel PVDF-HFP Based Gel Polymer Electrolyte Materials

Lithium-ion batteries?LIBs? as a major energy storage device have achieved great success in the portable electronics and electric vehicle markets.However,the volatile and flammable liquid electrolytes may cause safety issues such as fire and explosion.Gel polymer electrolyte?GPE?can function?as liquid electrolyte,which have attracted strong attention due to its advantages of high ionic conductivity and safety,strong shape plasticity.However,the safety,mechanical strength,ionic conductivity and interface stability of GPE need to be enhanced.Poly?vinylidenefluoride-hexafluoropropylene??PVDF-HFP?with high dielectric constant was selected as the matrix of the GPE for modification research.The polar solvent with a high boiling point and a high flash point is helpful to improve safety and electrochemical performance.The ionic conductivity and interface stability with lithium metal are enhanced by doping inorganic active fillers and using interface stabilizer.The above modification methods are very promising for high-performance polymer electrolytes,research contents are as follows:?1?N-methyl pyrrolidone?NMP?as plasticizer was introduced into PVDF-HFP by solvent evaporation method.The molecular structure of NMP has a strong polar carbonyl group that can efficiently transport lithium ions.The GPE film with excellent mechanical properties was obtained by reasonably controlling the electrolyte uptake ratio and adding high concentration lithium salt.The structural and electrochemical performances of GPE with different electrolyte uptake ratio were systematically investigated.The results showed that the thermal stability of electrolyte membrane was significantly enhanced by the introduction of NMP.The GPE with electrolyte uptake ratio of 130 wt.% shows the highest conductivity(3.16×10^-4S cm^-1).The Li//Li cells can be continuously operated for 600 hours.The result shows that electrolyte membrane exhibits good interfacial stability.The LiFePO₄//Li cells show excellent cycling performance and rate capacity,the capacity retention rate was 70 % after 500 cycles at 0.5 C,and the discharge capacity of 86 mAh g^-1at 10 C.?2?The composite gel polymer electrolyte?CGPE?was prepared by doping inorganic active filler Li₇La₃Zr₂O₁₂?LLZO?and vinylene carbonate?VC?as interface stabilizer.X-ray diffraction results show that the introduction of LLZO reduces the crystallinity of the polymer matrix,and explore the effect of mass fraction of inorganic filler LLZO on ionic conductivity.At ambient temperature,the CGPE with 15 wt.% LLZO reaches the highest ionic conductivity(4.16 × 10^-4S cm^-1),and electrochemical window more than 4.9 V.Combining the voltage polarization test and AC impedance test in Li//Li cells,the CGPE exhibits high stability to lithium metal and good dendrite suppression capability,which can be attributed to VC solution form a stable passivation layer on the lithium metal surface.The LiFePO₄//Li cells possess a high discharge capacity of 151 mAh g^-1after 100 cycles at 0.1 C,and the capacity retention rate is up to 97 %.