Sintering Optimization Of Garnet Electrolyte By Spark Plasma Sintering And Construction Of All Solid-State State Batteries

Lithium-ion batteries are widely used in our lives,such as electric vehicles and various portable electronic devices.The organic electrolyte used in traditional lithiumion batteries is usually flammable,causing safety hazards.The application of solid electrolytes is considered as a wise choose to enhance safety of high energy batteries.Garnet solid-state electrolyte is known for its high ionic conductivity,wide electrochemical window and good stability with lithium metal.However,the low density and poor interfacial contact hinder the development of garnet all-solid-state batteries.Spark plasma sintering is a new technology for preparing garnet electrolyte,which has the advantages of fast heating rate and short sintering time,but the process conditions are severe and complicated.In addition,the high cathode/electrolyte interfacial resistance of all-solid-state lithium batteries can cause capacity decay and low coulombic efficiency.In this paper,spark plasma sintering process is optimized systematically,and the influence of different process parameters on the mechanical strength,phase structure and ionic conductivity of electrolyte is analyzed.Li6.5La3Zr1.5Ta0.5O12 garnet solid electrolyte with good performance is prepared.Heating rate and pressure are important for the mechanical strength of electrolyte.Improper sintering conditions lead to the formation of pores between crystal grains,which reduces the mechanical properties of electrolyte.The optimized heating conditions and sintering pressure are 50℃ min-1 and 40 MPa,respectively.Electrolyte precursor with the particle size of 1μm easily decomposes to La2Zr2O7 impurity after sintering at 450℃,while cubic garnet phase of 6 pm precursor sintered at 1100℃ remains stable.In addition,precursor calcining time,sintering temperature and holding time are important for the ionic conductivity of electrolyte.Through optimization,the garnet electrolyte prepared by SPS at 1125 ℃10 min-40 MPa shows high density of 99%and high ion conductivity of 1.03 ×10-3 S cm-1.The interface between cathode and electrolyte is a severe problem in garnet allsolid-state batteries.In order to improve rigid contact and reduce interface impedance,Li3BO3 is used as sintering aid to prepare LiCoO2|LLZTO|Li and NCM 811|LLZTO|Li all-solid-state batteries.Li3BO3 help lower sintering temperature and improve the interfacial contact of all-solid-state battery.The interface impedance of LiCoO2 and NCM 811 all-solid-state batteries are respectively 89 and 263 Ω cm2 at 100℃.LiCoO2 and NCM 811 all-solid-state batteries show stable cycle at 100℃ benefiting from the lower interface impedance.Volume change of cathode during the charge/discharge process causes the interface contact to deteriorate.Cracks appeared on the surface of NCM 811 composite cathode after 50 cycles,and the interface impedance of the battery was 2104 Ω cm2.The LiCoO2 composite cathode shows a complete surface and lower interface impedance due to the lower volume change.After 50 cycles,the battery’s capacity retention rate is 40%,and the coulombic efficiency is 94%.