Synthesis And Performance Study Of PVDF-HFP Based Solid-state Electrolytes For Lithium-ion Batteries

Conventional lithium-ion batteries usually use organic liquid electrolytes,which have relatively low resistance,but suffer from disadvantages such as liquid leakage,flammability of organic solvents and low energy density.Lithium batteries with solid electrolyte instead of the liquid electrolyte will have a higher theoretical energy density and better safety performance.Composite electrolytes not only utilize polymeric materials to improve the interfacial compatibility between electrodes/electrolytes,but also inorganic fillers to regulate ion migration.In this paper,PVDF-HFP polymer was used as a research object and lithium scandium titanium phosphate（LSTP）as an inorganic nano-filler to enhance the ionic conductivity and mechanical properties of polymer films,and the physical-chemical and electrochemical properties of the synthesized composite gel electrolytes and composite solid-state electrolytes are investigated.A composite electrolyte microporous membrane was prepared by using LSTP with a mass ratio of 5%compounded with PVDF-HFP so that it can fully absorb organic solvents containing Li PF₆.It was found that the composite gel electrolyte with the addition of LSTP could achieve an electrochemical stability window of 4.8 V.The Li-Li cells assembled with the composite gel electrolyte underwent 400 h constant current cycling experiments without micro-short circuits and with small polarization voltages.The Coulomb efficiency of Li Fe PO₄-Li battery assembled with composite gel electrolyte was stable at 96.38%after 200 charge/discharge cycles at 1 C(145.2 m Ah·g^-1).The addition of LSTP can reduce the lithium ion transport impedance,broaden the electrochemical stability window and improve the cycling performance of the battery.The LSTP@PVLi-x（0.4,0.6,0.8）solid electrolytes with different mass ratios were designed and synthesized based on LSTP@PVDF-HFP with Li TFSI instead of organic solvent.The crystallinity of the composite electrolyte decreases with the increase of lithium salt ratio.LSTP@PVLi-0.6 at 0.01 m A/cm² can maintain a stable cycling performance for 840 h.LSTP@PVLi-0.6 exhibits excellent cycling performance,with the discharge specific capacity remaining at 118 m Ah·g^-1 after 400 cycles at 1C.Li TFSI,as a plasticiser,can increase the ionic conductivity of the electrolyte and increase the discharge specific capacity.However,as the proportion of lithium salts increases it deteriorates the mechanical properties of the composite solid electrolyte and the stability of the lithium-metal interface,resulting in poor cycling performance of the battery.