Study Of Novel Polymer Electrolyte For Solid Lithium Batteries

The defects of liquid electrolyte,such as organic solvent leakage,flammability,and that of inorganic solid electrolyte with complicated preparation process,interfacial incompatibility,poor mechanical properties have hindered their applications in lithium metal batteries(LMBs).Solid polymer electrolyte has no liquid leakage,good mechanical properties and interfacial compatibility.These merits make it more suitable for high energy density LMBs.However,the comprehensive properties of polymer electrolytes reported were difficult to meet the demand of applications in LMBs.There were still many issues,such as low ionic conductivity,low ion transport number,narrow electrochemical window.Therefore,this thesis focused on exploiting novel solid polymer electrolytes with high ionic conductivity,wide electrochemical stability window and good interfacial compatibility with electrode materials,providing theory and experimental support for the practical application of solid polymer electrolytes in LMBs.Firstly,a series of novel block copolymer electrolytes(BC-SPE)with both soft segments(Poly ethylene glycol,PEG)and hard segments(hexamethylene diisocyanate trimer,HDI)are designed by tailoring the ratio of these functional units.The hexamethylene diisocyanate trimer(HDI)with hexamethylene unit possesses a good mechanical strength,the PEG has a plenty of ether bonds in the backbone chains for Li⁺migration,in which the ratio of different functional units can be tailored with n_PEG/n _HDIt defined as R value to change the overall performance.The tensile strength,Shao hardness,electrochemical stability window and interfacial impedance of the BC-SPE are decreased as the R value increased.The ionic conductivity is increased with the R value increased.The block copolymer electrolyte has a high ionic conductivity of5.7?10^-4 S cm^-1 and wide electrochemical window up to 4.65 V when R equalled to1.87.The solid polymer LMBs was assembled with Li Fe PO₄ cathode material and BC-SPE,showing a discharge specific capacity retention of 86%and a coulomb efficiency of about 99%at the current density of 1 C.The properties of BC-SPE can be regulated by changing the R value according to the actual requirements of LMBs.Then,a novel modified silyl-terminated polyether based solid polymer electrolyte(MS-SPE)was synthesized with the advantages both poly(propylene oxide)polymer electrolyte and polysiloxane polymer electrolyte.The main chain containing a large amount ether bonds of MS can be utilized as transport path of Li⁺.On the one hand,the capping group of methoxysilane can reduce the crystallinity to improve the amorphous region.On the other hand,the dehydration and condensation reactions occurred can form a three-dimensional network structure to improve the mechanical properties and thermal stability of the polymer electrolyte.As a result,this amorphous MS-SPE has a high ionic conductivity of 3.6?10^-4 S cm^-1,high lithium ion transference number of 0.65,stable electrochemical window up to 5.0 V at room temperature.Using the MS-SPE as electrolyte,there was no capacity decay for LMBs composed of Li Fe PO₄ cathode after cycing of 600 times at the current density of 3 C.The capacity was kept around104 m A h g^-1 and the coulomb efficiency was close to 99.7%.The preparation process of MS-SPE was simple and can be produced in batches,the mechanical properties were good,which was beneficial to eliminate the hidden trouble of short circuit in the battery and improve the safety of lithium battery.Thirdly,a wide-temperature Poly(vinyl ethylene carbonate)solid polymer electrolyte(PVEC-SPE)was constructed.The polar group[-O-(C=O)-O-]can promote Li salt ionization,and carbonyl group(C=O)has weaker complexation with Li⁺that can facilitate ions migration for high ionic conductivity.Furthermore,the composite structure of a small amount of low molecular polymer and high molecule polymer was designed,a small amount of low molecular polymer may fill in the space of high molecular polymer backbone,which was beneficial for lowering the crystallinity of SPE,reducing interfacial resistance and improving interfacial compatibility between PVEC-SPE and electrodes.The ionic conductivity of PVEC-SPE were2.1?10^-3 S cm^-1 and 2.28?10^-4 S cm^-1 after optimization at 25?and subzero 15?,respectively.The interfacial compatibility of the polymer electrolyte prepared by in-situ method with the electrode material is much higher than that by the ex-situ method.The solid polymer LMB assembled between LFP cathode material and PVEC-SPE by in-situ method can operate 100 cycles without capacity decay at the current density of1 C.Therefore,the PVEC-SPE designed by in-situ method in this work is one of the most promising candidates for the practical applications of solid polymer LMBs with high ionic conductivity.