Organic-inorganic Hybrid All-solid-state Polymer Electrolytes For Lithium Ion Batteries

In the era of global ecological problems,such as fossil energy crisis,environmental pollution,and climate warming,getting more severe,to develop the environmentally-friendly,high-energy-density lithium ion batteries has become significant for promoting energy structure adjustment and sustainable development.Given the limitations of commercial liquid electrolytes,it is urgent to study all-solid-state electrolytes to acquire lithium batteries with better safety performance and higher energy density.The solid polymer electrolyte is suitable for commercialization for its advantages such as high flexibility,good compatibility with electrodes,easy molding,yet confronts several issues,like low ionic conductivity.We report a series of organic-inorganic hybrid all-solid-state polymer electrolytes in this thesis.Structures and thermal properties of the electrolytes were characterized by infrared,scanning electron microscopy and differential scanning calorimetry,and methods such as electrochemical impedance spectroscopy and linear sweep voltammetry were used to study electrochemical properties of the electrolytes.And we investigated the application of the electrolytes in LiFePO₄/Li solid batteries by galvanostatic charge-discharge experiments.We prepared a nanoclay in-situ composited comb polymer solid electrolyte?HCPE?through UV polymerization of the composition of a natural hollow nanotubular halloysite clay?HNT?with methoxy poly?ethylene glycol?acrylate?mPEGA?.The introduction of flexible PEG branches and nanotubes inhibits crystallization of the polymer,resulting in amorphous HCPE with a glass transition temperature as low as-48.6?,which facilitates Li⁺transportation.With negatively charged outer surfaces of HNT interacting with Li⁺and longer nanotubes bridging different PEG branches,an ion-conducting network was formed in the system;meanwhile,positively charged inner surfaces of HNT can weaken the interactions between Li⁺and anion by its interaction with the latter,which improves the mobility of Li⁺.Therefore,ionic conductivity of HCPE with 6 wt%HNT and ethoxy-to-Li⁺molar ratio of18/1 reaches 5.62×10^-5 S cm^-1 at room temperature and 3.89×10^-4 S cm^-1 at 60?.And HCPE has a electrochemical window of 5.28 V,and a lithium ion transference number of 0.159.The method of photocuring on top of lithium metal electrode provides the electrolyte with good compatibility with lithium.Due to the high ionic conductivity,the prepared LiFePO₄/HCPE/Li solid battery can achieve specific capacity as high as 136.8,134.2 mAh g^-1 at current density of 0.1 C and 0.2 C,respectively,with a discharge platform of around 3.4 V,under the condition of 30?.When cycling at 0.2 C,initial coulombic efficiency reaches 98.1%,and a specific capacity of 117.4 mAh g^-1 is preserved with a capacity retention rate of 84.9%after150 cycles.It is indicated that the prepared HCPE is promising for achieving all-solid-state lithium ion batteries with high performances.Furthermore,we synthesized a metal organic framework ZIF-8,characterized by XRD,N2 isothermal adsorption and SEM,and found the prepared ZIF-8 owning a crystal form consistent with the theory,having a particle size of 100-200 nm with pore size of 5-20 nm,and its specific surface area reaches 1174 m² g^-1.We prepared a MOF in-situ composited comb polymer solid electrolyte?MCPE?through UV polymerization of the composition of the synthesized ZIF-8 with mPEGA.Both interaction sites between the polymer and Li⁺and interphase volumes between the polymer and ZIF-8 particles have greatly increased owing to numerous ordered nanopores and high specific surface area of ZIF-8.Thus,MCPE with 5wt%ZIF-8 and 30 wt%LiTFSI acquires ionic conductivities up to 9.96×10^-5 S cm^-1 at 30? and 1.03×10^-3 S cm^-1 at 80?,which are 3.5 times as high as those of polymer solid electrolyte without ZIF-8.In addition,MCPE has an electrochemical window of 5.42 V,a lithium ion transference number of 0.147,and good stability with lithium.At 30?,the prepared LiFePO₄/MCPE/Li solid battery has a specific capacity of 130.4 mAh g^-1 and 130.7mAh g^-1 at 0.1 C and 0.2 C,respectively.Capacity retention rate is as high as 88.8% after 150cycles at 0.5 C,and the average coulombic efficiency reaches 99.9%.It is suggested that the MCPE-based solid lithium battery can be stably cycled at 0.5 C under a condition close to room temperature.In the thesis,two kinds of inorganic nanofillers with different structures were in-situ composited with poly?mPEGA?to prepare organic-inorganic hybrid all-solid-state polymer electrolytes with high ionic conductivity and wide electrochemical window.And the electrolytes can be applied to fabricate solid lithium ion batteries with high capacity and good cycling stability,providing implications for the research of composited all-solid-state polymer electrolytes.