Fabrication And Properties Of Solid Polymer Electrolytes For Lithium Ion Batteries

In contemporary society,with the popularity of mobile phones and electric vehicles,the demand for lithium-ion batteries has led scientists to study energy sources with high energy density and stable operation.The conventional electrolyte is liquid.During the use of the battery,it is easy to cause electrolyte leakage problems,which is not conducive to battery safety.In order to solve the public’s concerns about safety hazards,solid electrolytes with high safety and good cycle life have emerged.Therefore,in this paper,polyvinylidene fluoride-hexafluoropropylene copolymer(PVDF-HFP)is the core research object.The properties of PVDF-HFP-based polymer electrolytes were improved by doping inorganic ceramic particles,blending and constructing three-dimensional network gel electrolytes.After the corresponding polymer electrolyte was successfully prepared,its conductivity,lithium ion mobility and electrochemical stability potential were studied.At the same time,the lithium ion battery was synthesized and its charge and discharge performance and cycle stability were systematically studied.(1)Firstly,based on the low conductivity of pure PVDF-HFP-based polymer at room temperature,the organic-inorganic composite electrolyte was prepared by doping garnet-type inorganic ceramic powder Li7La3Zr2O12 and the optimum doping was determined.Quantity(10%).At room temperature,the composite polymer electrolyte(CPE)has a good lithium ion conductivity of 3.71 × 10-4 S cm-1.The composite polymer electrolyte exhibited a good lithium ion transfer number(0.58)and a stable electrochemical window(up to 4.65 V vs Li/Li+).Compared with the pure PVDF-HFP electrolyte,the electrochemical stability of the lithium ion battery using the composite polymer electrolyte is improved to exhibit an excellent initial discharge capacity.In the Li/CPE/LiFePO4 battery system,the discharge capacity of the battery at a rate of 0.2 C can reach 163.1 mAh g-1.During the long cycle of evaluating the battery,the doped modified polymer electrolyte exhibits a more stable electrochemical charge and discharge capability.After 200 cycles of charge and discharge cycles,the Coulomb efficiency can still be maintained above 99%,and the capacity retention rate can be maintained 83.8%.(2)Polypropylene carbonate(PPC)containing a very polar carbonate group is introduced into a pure PVDF-HFP polymer system by blending,and the introduction of a high dielectric group constitutes a stable and low-crystal interior.The three-dimensional carrier improves the transport of lithium ions and improves the cycle stability of the electrolyte.The electrochemical stability window of the polymer electrolyte after blending modification can reach 4.8V vs Li/Li+.In the assembled Li/LiFe0.2Mn0.8PO4 battery system,the 0.2C reversible capacity ratio of the new blended polymer electrolyte system after 100 cycles is about 89.8%,and the cycle stability is better than the pure PVDF-HFP electrolyte.In addition,in the Li/LiFe0.2Mn0.8PO4 battery,the rate performance of the polymer electrolyte at 0.1C,0.2C and 0.5C is 162.3mAh g-1,155.5mAh g-1 and 130.1mAh g-1.(3)In order to alleviate the multi-sulfide shuttle effect of lithium-sulfur batteries,the service life of lithium-sulfur batteries is improved.In this work,a PVD-HFP-based 3D network gel polymer electrolyte was successfully constructed by introducing PETT and Ester monomers into the PVDF-HFP system in situ under UV light.Studies have shown that the electrolyte exhibits a stable electrochemical window(5.05V vs Li/Li+).A solid lithium-sulfur battery assembled with a 3D network gel polymer electrolyte membrane has excellent capacity retention as compared with a liquid electrolyte.At a discharge rate of 0.5 C,the capacity retention rate can still reach 69.1%after 300 cycles.At the same time,the ionic conductivity of GPE under different strain conditions and different temperatures was studied.