Fabrication Of Branching Solid State Polymer Electrolytes For Lithium Ion Battery

Owing to their high energy density and efficiency,lithium-ion batteries have been widely applied in our daily life as energy storage devices during the last two decades.To satisfy the demand of various shaped electronic devices,bendable and flexible lithium-ion batteries with unique functionality are growing rapidly as a promising power sources.Due to the leakage,flammability and poor chemical stability of the organic liquid electrolytes,the solid-state polymer electrolytes are ideal candidates for deformable lithium ion batteries because it has great advantages of electrochemical stability window,high mechanical stability,flexibility and safety.However,solid polymer electrolyte including lithium salt,generally offers low ionic conductivity,it is critical issue to improve the ion conductivity of solid polymer electrolyte.Modification can increase the conductivity of the polymers.Blending,grafting,branching and cross-linking are employed for polymer structure modification.To solve these problems,the polymer is modified by branching and grafting to obtain a polymer electrolyte with high conductivity and good safety performance.This dissertation includes two major parts.Part 1:In this part,a novel composite polymer electrolyte consist of 8-arm block liquid crystalline copolymer（8-PEG-MALC）,8-arm poly（ethylene glycol）（8-PEG）,polyethylene（glycol）diacrylate（PEGDA）and bistrifluoromethanesulfonimide lithium salt（LiTFSI）was prepared successfully.The crosslink agent PEGDA ensure the good mechanical property of the solid-state polymer,branching 8-PEG responsible for the high ionic conductivity of the solid polyelectrolytes,and 8-arm block liquid crystalline copolymer with a birefringent mesogens to tuning the electrolytes morphology.The composite polymer electrolytes can form a flexible and transparent film with nanoscale microphase separation structure,which creating well-defined ion conducting channels.The electrochemical properties of composite polymer electrolytes are analyzed and the highest ionic conductivity reaches 6.2×10^-5 S cm^-1 at room temperature after annealed from liquid crystal state.It also displays high temperature stability up to 150℃,which is higher than traditional electrolytes.More intriguingly,the assembled LiFePO₄/Li cells using the composite polymer electrolytes exhibit good charge/discharge cycles at 95℃.The good electrochemical properties,temperature stability and bendability of the composite polymer electrolytes indicate it potentially as a very promising material for all-solid-state flexible lithium ion batteries.Part 2:In this part,a SiO₂-based state solid polymer electrolyte,in which vinyl-functionalized SiO₂ nanoparticles connect with imidazole monomers are synthesized by RAFT.A Grafting polymer electrolyte composed of imidazole monomer and vinyl-functionalized SiO₂ to improve ionic conductivity and mechanical strength.The polymer electrolytes（SiO₂-MOBIm₆-BF₄）were characterized by differential scanning calorimetry（DSC）,thermogravimetric analysis（TGA）,scanning electron microscopy（SEM）,linear sweep voltammetry（LSV）,AC impedance and charge/discharge cycle test,etc.The SiO₂-MOBIm₆-BF₄ display high thermal stability,electrochemical stability and high ionic conductivity.The polymer electrolyte of SiO₂-MOBIm₆-BF₄ increases when temperature is increased.The SiO₂-MOBIm₆-BF₄reaches highest ionic conductivity reaches 5.96×10^-5 S cm^-1 at 25℃ and 9.54×10^-4 S cm^-1 at 95℃.The electrochemical stability window of SiO₂-MOBIm₆-BF₄ is about4.4 V versus Li⁺/Li at room temperature.Discharge performance of lithium battery using this SiO₂-MOBIm₆-BF₄ shows an initial discharge capacity of 151.2 mAh g^-1and a stable cycle performance.