Preparation And Properties Of Wide Temperature Solid Polymer Electrolyte For Solid State Lithium Battery

Lithium-ion batteries（LIBs）have become the most widely used energy storage devices in commercial applications due to their high operating voltage,energy density,and long cycle life.However,highly flammable organic liquid electrolyte is utilized in traditional LIBs,and huge safety hazards hinder the further development of LIBs.Solid Polymer Electrolytes（SPEs）have become a new type of electrolyte to replace liquid electrolytes due to their strong processability,high safety performance,and excellent lithium metal interface compatibility,which can effectively inhibit the growth of lithium dendrites.In particular,SPEs can utilize lithium metal anode to improve the energy density of LIBs.The traditional SPEs are to incorporate a small amount of eutectic lithium salt into the polymer matrix,which can be called Salt-in-Polymer（SIP）.However,the crystallinity of these SPEs is relatively high at room temperature,and the electrochemical properties such as ionic conductivity are difficult to achieve practical application conditions.In contrast,Polymer-in-Salt（PIS）is a special SPE composed of a small amount of polymer incorporated into a large amount of eutectic lithium salt.This kind of new SPE can maintain certain mechanical properties while exhibiting excellent room temperature ionic conductivity.In view of the poor electrochemical performance of traditional SPEs at room temperature,the microstructure and electrochemical performance of PIS were investigated in this paper.The main research contents are as follows:（1）A Polymer-in-Salt solid electrolyte with a three-dimensional framework structure was rationally designed and prepared with the porous PVDF membrane as the frame and Polymer-in-Salt as the filling（named PPIS）.Through a series of structural characterization and electrochemical performance tests,the differences in chemical structure and electrochemical performance between the Polymer-in-Salt and the traditional Salt-in-Polymer system were explored,which provided a theoretical basis for the rational design of room temperature SPEs.（2）PPIS exhibits high ionic conductivity of 4.5×10^-5 S cm^-1 and lithium transference number of 0.54 at room temperature with a wide electrochemical window beyond 5.5 V.Meanwhile,it exhibits excellent mechanical property and interfacial stability,which effectively suppresses the growth of lithium dendrite even after cycling for 1000 h.The assembled Li Fe PO₄/PPIS/Li solid-state battery exhibits excellent rate performance and cycling stability at 30°C and 60°C.Li Fe PO₄/PPIS/Li exhibits discharge capacity of 119mAh g^-1 after 100 cycles at 30°C with a capacity retention rate of 91.3%and 152 mAh g^-1after 100 cycles at 60°C with a capacity retention rate of 98.8%.In addition,LiFePO₄/PPIS/Li pouch cell displays significant safety and functional flexibility which improves the practical application prospects of PPIS.（3）The effects of different PEO molecular weights on the microstructure and electrochemical properties of the Polymer-in-Salt system were explored.With the increase of molecular weight,the film forming property of PIS gradually increases and the surface becomes more compact.The solvation effect between PEO and Li TFSI of high molecular weight is stronger,and the PIS solid electrolyte is more stable.At the same time,the electrochemical performance of the solid electrolyte prepared with higher PEO molecular weight is also better.Among them,the PPIS-100W solid electrolyte prepared with 100W molecular weight PEO reaches 1.57×10^-4 S cm^-1 at 30°C,and the assembled LiFePO₄/PPIS-100W/Li can achieve a specific discharge capacity of 145 mAh g^-1 at 0.1 C rate during 30°C.