Preparation And Properties Of Li₇La₃Zr₂O₁₂/PEO Composite Electrolyte Membrane For Solid-state Batteries

At present,commercial lithium-ion batteries use organic electrolytes,which are flammable and explosive.With the application of lithium-ion batteries in large-scale energy storage systems,the safety of lithium-ion batteries should be given higher attention.Using solid electrolyte instead of traditional organic electrolyte can greatly improve the safety of lithium ion batteries.Solid electrolyte can be divided into inorganic material and organic polymer material.Among the inorganic electrolytes,Li₇La₃Zr₂O₁₂?LLZO?has the advantages of high ionic conductivity,good stability to lithium,wide electrochemical window and excellent mechanical properties at room temperature.However,inorganic electrolytes are brittle and have large interface impedance with electrodes,which makes it difficult to be widely used in a short time.The polymer electrolyte has a small contact resistance with the electrode,but the ion conductivity is too low at room temperature which limits its use.In this paper,the inorganic electrolyte LLZO and the polymer electrolyte PEO were combined to prepare the composite electrolyte with excellent comprehensive performance.And then assemble the all-solid-state batteries to study the electrochemical performance.The main work includes:?1?Al-doped cubic LLZO electrolyte was prepared by solid-state reaction.The effects of Li content,Al₂O₃ addition amount,sintering temperature and time on the phase formation of LLZO were investigated.Al-LLZO cubic phase electrolyte was prepared by adding 20%excess Li source and 1 mass%Al₂O₃ for 15 hours at 1100 C.Al-LLZO cubic phase electrolyte was prepared by adding 20%excess Li source and 1 mass%Al₂O₃ to the raw material and calcining at 1100 C for 15h.At room temperature,the ionic conductivity is 4.84×10^-4S/cm,the density is 90.8%,and the activation energy of lithium conduction is 0.31eV.?2?On the basis of the previous work,the properties of electrolyte were further improved by co-doping Al and Ta.The LLZO electrolyte consisting of Li Li_6.35La₃Zr_1.8Ta_0.2Al_0.15O₁₂2 was prepared.The ionic conductivity was increased to7.31×10^-4S/cm at room temperature,the density was 94.9%,and the lithium conduction activation energy was reduced to 0.29eV.Li|Li symmetrical battery was assembled,and the limiting current density was increased to 0.1mA/cm² by co-doping.LiFePO₄|LLZO|Li all-solid-state battery was assembled.After 50 cycles at 0.5C current,the specific discharge capacity of the battery was reduced from 152.3mAh/g to 99.1mAh/g,and the capacity retention rate was 65.1%.After 50 cycles at 1C current,the discharge specific capacity decreased from 149.9mAh/g to 92.8mAh/g,and the capacity retention rate was61.2%.?3?PEO/LiTFSI polymer electrolyte was prepared,and the polymer electrolyte with the best comprehensive performance was obtained when the molar ratio of EO:Li is 16:1.And the ionic conductivity is 1.16×10^-5S/cm at room temperature.Then LLZO and PEO were compounded to prepare composite electrolyte.The performance was best when the amount of LLZO added was 30 mass%.By adding LLZO filler,the ionic conductivity was increased to 3.08×10^-5S/cm at room temperature.In addition,the composite electrolyte can increase the stability voltage from 4.3V to 4.8V,which improves the stability of Li interface.The charge-discharge cycle of LiFePO4|SPE|Li solid-state battery was tested at 50?.After 100 cycles at 0.5C,the specific discharge capacity decreased from 151.8mAh/g to 137.6mAh/g,and the capacity retention rate was 90.6%.After 50cycles at 1C current,the capacity decreased from 142.9mAh/g to 126.9mAh/g and the capacity retention rate was 88.8%.