Preparation Of Garnet Solid Electrolytes And Their Application In Lithium Ion Batteries

Solid-state lithium batteries are considered as promising candidates for the next generation of lithium batteries due to their high specific energy density and safety.Cubic garnet is considered as the promising solid electrolyte due to its high chemical stability,wide electrochemical window and high ionic conductivity.This work focuses on preparation and application of the garnet solid electrolyte for solid-state lithium batteries.Firstly,Li_6.75La₃Zr_1.75Ta_0.25O₁₂(LLZTO)electrolyte pellet with high density and high conductivity is prepared,and submicron LLZTO powder is used as a ceramic filler in polyethylene oxide(PEO)-based composite solid electrolyte membrane,and then,the composite solid electrolyte is applied to the Li Fe PO₄positive electrode,and finally,the lithium-rich manganese-based cathode material is surface-coated and modified with Li₇La₃Zr₂O₁₂to improve its electrochemical performance.The details are as follows:(1)Li_6.75La₃Zr_1.75Ta_0.25O₁₂(LLZTO)electrolyte pellet is prepared by solid statemethod.The effects of calcination temperature,calcination time and amount of lithium on the morphology structure,density,ion conductivity and activation energy of the LLZTO pellet are systematically studied.It is found that as the calcination temperature increase,the particle growth speeds up and the density increases,but too high temperature is not conducive to density and ionic conductivity.With a short calcination time,the electrolyte pellet shrinks incompletely;and as the calcination time increases,the density of pellet gradually increases.However,too long calcination time will cause more lithium to volatilize,resulting in the appearance of tetragonal phases.In addition,when the amount of excess lithium is low,it cannot compensate for the lithium volatilization during the calcination process,resulting in low density and low ion conductivity of the LLZTO electrolyte pellet.As the amount of excess lithium increases,the density of the electrolyte pellet is improved,but too much lithium excess content will cause more energy required for the reaction during the calcination process,resulting in the appearance of tetragonal phases.Finally,the LLZTO electrolyte pellet obtained by sintering at 1150?for 14 h with15%excess lithium shows the best performance.The ionic conductivity,relative density and activation energy of the LLZTO pellet are 4.7699×10^-4S cm^-1,94.31%and 0.30 e V,respectively.(2)Composite electrolyte membranes incorporating Li_6.75La₃Zr_1.75Ta_0.25O₁₂ (LLZTO)are prepared by solvent-free hot rolling method.It is found that the addition of LLZTO to the PEO-Li TFSI-based electrolyte can effectively reduce the crystallinity of PEO and improve the ionic conductivity,voltage window,mechanical properties and interfacial stability of the composite solid electrolyte membrane.The composite electrolyte membrane with the optimal LLZTO content of 10 wt.%exhibits a maximum ionic conductivity of 3.03×10^-4S cm^-1at 55?and a maximum elongation break of 74.8%.And also,the lithium symmetric cell assembled with 10 wt.%LLZTO membrane(PEO-Li TFSI-10LLZTO)exhibits the lowest polarization voltage and the best cycle performance,which can be attributed to the enhanced mechanical properties of the membrane and the lower and stable interfacial resistance between the membrane and lithium metal.The all-solid-state Li Fe PO₄/Li battery delivers a high specific capacity of 155.8 m Ah g^–1at 0.1C rate at55?,and exhibits excellent cycle performance and rate performance.In addition,the pouch cells assembled with PEO-Li TFSI-10LLZTO composite electrolyte membrane can still maintain the pristine voltage of 3.39 V and power light-emitting diodes(LEDs)under harsh conditions such as cutting,showing an excellent safety performance.(3)PEO-Li TFSI-10LLZTO is applied to Li Fe PO₄positive electrode,and the fixed single-sided coating surface capacity density is 1 m Ah cm^-2.It is found that the contact between the active material particles in the electrode is closer after rolling,and the porosity is greatly reduced.In addition,the internal impedance of the electrode and the interface impedance between the electrode and the composite solid electrolyte membrane are found to decrease significantly at 25°C.The rolled electrode exhibits excellent electrochemical performance at low current density,and its capacity retention is 85.5%after 110 cycles at 0.2C.In addition,as the content of the composite solid electrolyte in the electrode increases,the porosity of the electrode decreases and the density increases.At the same time,the interface impedance between the electrode and the composite solid electrolyte membrane also decreases significantly,but its internal impedance increases instead due to increased electrode thickness and low ion conductivity of the composite solid electrolyte.Among them,the electrode with a composite solid electrolyte ratio of 15 wt.%(85-LFP)has the best ionic and electronic conductive properties,and exhibits excellent electrochemical performance.The discharge capacity of the electrode is123.8 m Ah g^-1and 83.1 m Ah g^-1at 0.5C and 1C rates,respectively,and the capacity retention is 92.9%after 110 cycles at 0.2C.(4)Li₇La₃Zr₂O₁₂-Li_1.2Mn_0.54Ni_0.13Co_0.13O₂composite cathode material is synthesized by a simple one-pot sol-gel process.Microstructural characterization shows that Zr⁴⁺is doped into the lattice of lithium-rich manganese-based cathode material,and the particles surface of lithium-rich manganese-based cathode is coated and connected to each other by Li₇La₃Zr₂O₁₂and La Ni O₃.The resistance of charge transfer and Li ion diffusion in the region of the surface layer of the particle are reduced and the rate performance is improved due to the synergistic effect of the electronically conductive La Ni O₃and the ionically conductive Li₇La₃Zr₂O₁₂composite coating layer.Moreover,the coating layer is beneficial to inhibit the side reaction between the active particles and the electrolyte,and improve the initial coulombic efficiency and the cycle capacity retention.In addition,the Li⁺diffusion coefficient of the modified sample at SOC>50%is significantly higher than that of the pure phase lithium-rich manganese-based cathode material,which can be attributed to Zr⁴⁺doping into the lithium-rich manganese-based cathode material structure.The d Q/d V results reveal that the Zr⁴⁺doping and the electronic/ionic conductive composite coating could help to stabilize the structure of the lithium-rich manganese-based cathode material,inhibit the conversion of the layered structure to the spinel structure during the cycle,thereby slowing the capacity fading of lithium-rich manganese-based cathode materials during the cycle.