Novel Solid-State Electrolytes For All-Solid-State Lithium Metal Batteries

Li metal batteries(LMBs)are expected to achieve higher energy density due to the high specific capacity and the low electrochemical potential of Li metal anodes.However,the commonly used liquid electrolytes in LMBs usually suffer from insufficient electrochemical and thermal stability,high flammability,and also lead to uncontrollable lithium dendrite growth,which limits the development and application of LMBs.In contrast,all-solid-state batteries(ASSBs)based on solid-state electrolytes(SSEs)are promising candidates to replace traditional LMBs due to their potential high energy density,good thermal stability and high safety.Although different kinds of SSEs have made great progress,they still face the key challenges such as the insufficient room-temperature ionic conductivity,the growth of lithium dendrites,electrode/electrolyte interface problems and the limited operation temperature range and voltage range.In view of these challenges,this dissertation conducts corresponding research from the perspective of designing materials and preparing new materials,provides the design and synthesis methods of novel solid polymer electrolytes(SPEs)and inorganic solid electrolytes(ISEs),and then realizes the wider application of SSEs in ASSBs in different scenarios.The low melting points of traditional SPEs limit the usable temperature range of ASSBs.In this dissertation,a technique is designed to provide a stable lithium electrode/electrolyte interface,high mechanical strength and excellent thermal stability by introducing an electrospun polyacrylonitrile(PAN)nanofiber membrane into the commonly used polyethylene oxide(PEO)/lithium bis(trifluoromethanesulfonyl)imide(LiTFSI)electrolyte.The stable electrode/electrolyte interface and high mechanical strength can effectively inhibit the growth of lithium dendrites and prevent short circuit.The excellent rate performance and cycling stability of the novel electrolyte PANPEO/LiTFSI-based symmetrical Li-Li cells and Li-LiFePO4(LFP)full cells are demonstrated at 60 ℃.Furthermore,the excellent thermal stability of PANPEO/LiTFSI enables full cells to exhibit high cycling efficiency and long lifetime at extreme temperatures of 120℃ and even 150℃.This work breaks through the usable temperature range of SPEs and promotes the development and application of hightemperature batteries with high safety.SPEs generally need to work at temperatures higher than room temperature due to the low room-temperature ionic conductivity,which may lead to short circuit,thermal failure in the batteries and even cause safety issues.In this dissertation,a novel polyoxymethylene(POM)/LiTFSI SPE is designed to replace commonly used PEO with POM with a shorter chain unit.Compared with PEO/LiTFSI,the lower diffusion activation energy of lithium ions in POM enables POM/LiTFSI to provide higher roomtemperature ionic conductivity.And the inherent high modulus of POM also enables POM/LiTFSI to exhibit higher mechanical strength.Therefore,POM/LiTFSI can work stably at room temperature and effectively inhibit lithium dendrites.The excellent rate performance and cycling stability of POM/LiTFSI-based symmetrical Li-Li cells and Li-LFP full cells are demonstrated at room temperature.This work overcomes the limitation that traditional SPEs can not work at room temperature due to the insufficient ionic conductivity and promotes more extensive study and application of SPEs.The narrow electrochemical stability windows of sulfide solid electrolytes limit the working voltage of full batteries.In this dissertation,a novel lithium thioborate SSELi10B10S20 is successfully synthesized by sintering method,and the electrochemical properties of lithium thioborate materials are reported for the first time.Li10B10S20 exhibits high ionic conductivity,negligible electronic conductivity,and a wide electrochemical stability window.Its high critical current density and good cycling stability in symmetrical Li-Li cells at room temperature are demonstrated.In addition,the excellent stability of Li10B10S20 ensures that it can be stored and used for a long time.This work provides guiding principles for synthesizing more kinds of lithium thioborates and promotes the further development and wider application of sulfide system solid electrolytes.