Preparation And Properties Of Li₇La₃Zr₂O₁₂-based Solid Electrolyte For Lithium Ion Batteries

Garnet-type Li₇La₃Zr₂O₁₂（LLZO）material is one of solid electrolyte for lithium ion battery due to high lithium ion conductivity(10^-4 S/cm),wide electrochemical window（6 V）and good electrochemical compatibility with lithium metal.As one of the most commonly used preparations to prepare LLZO solid electrolye,solid state reaction method makes lithium content in the final reaction product hard to control for the lithium volatilization in the reaction process during calcination with high temperature and long time and easily produces impurity phase such as tetragonal phase,which is two orders of magnitude lower than the ionic conductivity than LLZO cubic phase.In addition,the application-oriented application of LLZO solid electrolyte needs to further improve the lithium-ion conductivity.The large interfacial impedance of lithium ion batteries based on LLZO solid electrolyte is one of the challenges to realize reversible charging and discharging of batteries.In this paper,we focus on the preparation of LLZO-based solid electrolyte with high density and lithium-ion conductivity and try to assemble lithium ion batteries based on Al-doped LLZO solid electrolyte.The main results of this paper are as follows:Firstly,Li_6.28Al_0.24La₃Zr₂O₁₂（Al-LLZO）samples were prepared by microwave sintering with different temperature and time.And the crystal structure,microstructure and ionic conductivity for the sintered Al-LLZO samples were measured.Experimental results show that microwave sintering process could prepare the Al-LLZO samples with cubic phase at 1000℃.The relative density of the Al-LLZO samples sintered at 1200℃for 1 h reaches 89.3%and the lithium ion total conductivity of Al-LLZO samples was 1.06×10^-4S cm^-1.Research results of this study indicate microwave sintering is a great mehod to synthesize Al-LLZO solid electrolyte with pure cubic phase,high density and high ionic conductivity.Secondly,solid-state reaction method was used to prepare Li_7-2xZn_xLa₃Zr₂O₁₂（Zn-LLZO;x=0,0.01,0.03,0.05,0.25,0.30,0.35）powder.The powders were pressed to be green pellets which were then sintered.The crystal structure,microstructure and ionic conductivity for the sintered Zn-LLZO samples were also measured.The results show ZnO can activate the sintering of Zn-LLZO solid electrolyte.Moreover,the doping amount of Zinc has different effects on lithium ion conductivity of Zn-LLZO solid electrolyte.When the doping amount of Zinc is less than 0.01,the vacancies brought by the introduction of Zn into LLZO will promote lithium ion conduction of the sintered Zn-LLZO samples.And the lithium ion total conductivity further increases as the doping amount of Zinc increase to 0.03 due to the reduction of grain boundary.However,as doping amount of Zinc more than 0.05and further increase,the blocking effect of Zn²⁺increases and dominates,which will result in the decrease of lithim-ion total conductivity.The lithium ion total conductivity of sintered Zn-LLZO samples reaches 1.14×10^-4 S cm^-1 when doping amount of Zinc x=0.03,and the conductance activation energy is 0.27 eV.Finally,Al-LLZO solid electrolyte and Li(Ni_0.5Co_0.2Mn_0.3)O₂（NCM）were mixed to construct compound cathode and the lithium metal was matched to assembled lithium ion batteries.And the microstructure and electrochemical performance for the lithium ion batteries were tested and analyzed.The results show that increasing the solid electrolyte content in the cathode material of the battery to form a local ionic conductor network can reduce the interface impedance of the battery.Compared with the polarization impedance of the lithium ion batteries with 75%:25%mass ratio of NCM to Al-LLZO in cathode marerial,the polarization impedance of the lithium ion batteries with 50%:50%mass ratio of NCM to Al-LLZO in cathode marerial decrease from 88909Ωto 10490Ω,which will contribute to the charge process for the lithium ion batteries.