PVDF-HFP-based Ionic Gel Polymer Electrolytes For Lithium Metal Batteries

The most promising anode material for lithium batteries is lithium metal,which has a high theoretical capacity(3860 m Ah g^-1)and low electrode potential（-3.04 V）.However,the continuous parasitic reaction of conventional organic liquid electrolytes with the active lithium metal and side effects such as anode volume expansion can cause short circuits and even a series of safety problems in lithium metal batteries.Therefore,replacing conventional liquid organic electrolytes with solid electrolytes that are easy to mold and process,have high safety and high energy density,as an effective approach.Gel polymer electrolytes have the advantages of both solid electrolytes and liquid electrolytes.However,the electrolytes have low ionic conductivity at room temperature,poor electrochemical properties,and poor mechanical properties.In order to solve these problems,this thesis modified and designed a practical composite gel polymer solid electrolyte by introducing different additive materials,polyvinylidene fluoride-hexafluoropropylene（PVDF-HFP）as a polymer matrix and ionic liquid 1-ethyl-3-methylimidazolium bis（trifluoromethanesulfonyl）imide salt（EMIm TFSI）as a plasticizer to construct ionic gel polymers,and using this as a blank ionogel control,three composite ionic gel polymer electrolyte systems were designed by introducing different fillers,and the properties such as plating/exfoliation process,coulomb efficiency,battery capacity and cycling stability were explored in an in-depth study as follows:（1）MXene modified flame retardant ionic gel polymer electrolytes and their application in lithium metal batteries.A composite ionogel-in-MXene electrolyte（CIME）with PVDF-HFP polymer as a substrate introducing EMIm TFSI and monolayer MXene（Ti₃C₂T_x）were constructed by a simple solution casting method,where PVDF-HFP and monolayer MXene（Ti₃C₂T_x）were interconnected by hydrogen bonds.The composite ionogel has a 3D porous polymer network structure with excellent room temperature ionic conductivity(1.54×10^-3S cm^-1)and lithium-ion mobility（0.67）,thermogravimetric analysis and combustion experiments show that CIME has not only great thermal stability but also flame retardancy.The excellent lithium plating/exfoliation performance was determined by assembling lithium symmetric cells for long-term cycling,and the LiFePO₄/Li and NCM811/Li cells based on CIME assembly also exhibited great cycling performance and Coulomb efficiency.（2）Ionic gel polymer electrolytes anchored by metal-organic frameworks and their application in lithium metal batteries.A composite solid electrolyte for high-rate lithium metal batteries was prepared by anchoring heat-treated activated MOF（ZIF-67）into a PVDF-HFP-based ionic gel network.The MOF/ionogel has good size,thermal stability,intrinsically high porosity and good electrolyte adsorption capacity with good lithium-ion transport capability.Li/Li symmetric batteries assembled with MOF/ionic gels show improved Coulomb efficiency,suppressed lithium dendrite formation and excellent long-term cycling stability with a low voltage hysteresis of about 400 m V after more than 500 h of lithium plating/exfoliation process.At the same time,the LiFePO₄/Li half-cell can be discharged with a specific capacity close to 100 m Ah g^-1after 100 cycles of stable cycling at a high magnification of 1 C,showing excellent ion transport capability.（3）Heterogeneous bilayer structured ionic gel polymer electrolytes and their application in lithium metal batteries.Herein,a composite ionic gel polymer electrolyte with a heterogeneous structure is prepared,in which MOF（ZIF-67）is deposited on one side of the ionic gel polymer matrix to homogenize the lithium-ion transport and stabilize the lithium metal anode interface.It possesses good mechanical properties,and when the composite ionic gel with heterogeneous structure is used,the lithium-ion mobility number increases to 0.69and the lithium-metal electrode shows enhanced cycling stability over 1000 h.Furthermore,the solid-state NCM811/Li batteries provides an initial discharge ratio capacity of 180 m Ah g^-1with a retention of 89.8%at a multiplicity of 0.1 C for 100 cycles.This offers new prospects for the development of gel electrolytes in lithium-metal batteries.