Preparation,Surface Coating And Enhancement On Gel Electrolyte Of Nano Garnet

Rechargeable lithium-ion batteries have been the most promising high-efficiency secondary batteries for portable electronics,owing to the outstanding advantages of high energy density,excellent cycle performance,and green environmental protection etc,and they are currently expected to play an important role in electric vehicles and stationary energy storage.However,the energy densities of the traditional lithium-ion batteries are hard to be improved further and the safety is also problematic due to they use flammable organic liquid electrolytes.To this end,the all-solid-state lithium batteries derived fromthe replacement of organic liquid electrolytes for intrinsically safe solid electrolytes attract great attention.The all-solid-state lithium batteries would deliver much high energy density if the high-capcity electrode active materials,such as lithium metal anode and nickel cobalt manganese(NCM)cathode,are used.The solid electrolyte is the core material of the solid-state lithium battery.Among them,the lithium garnet material Li₇La₃Zr₂O₁₂(LLZO)and its derivatives in inorganic crystalline oxide electrolytes have a series of advantages,such as high ionic conductivity,stability to metal lithium anodes,and excellent mechanical properties,thus are very promising lithium ion solid electrolyte materials.However,the LLZO suffers from high synthesis temperature,coarse particles,poor cubic phase stability,inferior interfacial characteristic with electrodes,etc.To address these above issues,in this thesis,we first developed the synthesis method of nano garnet-type electrolytes.Cubic garnet Li_6.5Mg_0.05La₃Zr_1.6Ta_0.4O₁₂is prepared via a coprecipitation reaction under 1%of graphene oxide template at temperature of 650^oC,which is one of the lowest systhesis temperature for the cubic garnets.Additionaly,the nano garnets show small and uniform particles based on the SEM images.These characteristics make this method a particular process for obtaining nano garnets.In view of the interface issues,we study the influence of nano garnet on PVDF-HFP gel polymer electrolyte.We prepare the composite membrane of LLZO and PVDF-HFP,and subsequent gelate the composite membrane with organic liquid electrolyte.The composite gel polymer electrolyte shows much wider electrochemical stability window of above 6 V,which is one of the widest electrochemical stability window among the reported gel polymer electrolytes.This wide electrochemical stability window makes the composite gel polymer electrolyte match all the high-voltage cathodes.Besides,the symmetric Li/composite gel polymer electrolyte/Li cell can be cycled at high current density of 0.5 m A cm^-2,demonstrating the robust capability of the composite gel polymer electrolyte on lithium dendrite growth.A solid-state lithium battery is built with lithium metal as anode,NCM523 as cathode,and the composite gelpolymer electrolyte.The solid Li/NCM battery shows an initial discharge capacity of 170.0 m Ah g^-1,and Coulombic efficiency above 98%.In this thesis,it is proposed to prepare garnet-type solid electrolyte powder material Li_6.75La₃Zr_1.75Ta_0.25O₁₂ with nano-scale?-Al₂O₃ coating by mixed microemulsion solid-phase method.The addition of nano?-Al₂O₃ coating stabilizes the cubic phase of the garnet-type solid electrolyte,improves the ionic conductivity,and improves the elastic modulus,hardness and density of the garnet-type solid electrolyte sheet due to the inward diffusion of Al element at high temperature to improve the sinterability.Thanks to the dense structure,the?-Al₂O₃-Li_6.75La₃Zr_1.75Ta_0.25O₁₂electrolyte sheet shows a good inhibitory effect on the growth of metallic lithium dendrites,and can be safely cycled for a long time at a large current density of 0.5 m A cm^-2.The initial discharge capacity of the full cell with the structure Li/?-Al₂O₃-Li_6.75La₃Zr_1.75Ta_0.25O₁₂/NCM523 is 164.2 m Ah g^-1,and the capacity retention rate is 90.16%after 30 cycles at room temperature,with excellent electrochemical performance.