Studies On The Preparation And Modification Of High Lithium Ion Conductivity LLZO/LLTO Solid-State Electrolyte Materials

In the last three decades,lithium-ion batteries,based on liquid electrolytes,are widely applied in various fields which arrange from consumer electronics,electric vehicles,large-scale electrical power storage systems and so forth.However,traditional lithium-ion batteries use electrolyte technology,which has limited potential to improve energy density and power density.Solid-state lithium batteries have the high potential to break this limitation and increase the safety of the battery at the same time.Oxide solid-state electrolytes are expected to offer several advantages over the currently commercialized liquid electrolytes such as the higher thermal stability,the absence of leakage and pollution,and a large electrochemical stability window.However,compared with traditional liquid electrolytes,oxide solid-state electrolytes have low ionic conductivity and high fabrication costs.To address these challenges,we proposed a machine learning model for lithium-ion conductivity prediction,optimized the gallium ratio in Li₇La₃Zr₂O₁₂?LLZO?via high-throughput experimental methods for screening Ga,fabricated high lithium ion conductivity LLZO thin-films,and presented a one-pot hydrothermal synthesis method for Li_0.33La_0.56TiO₃?LLTO?powders.The detailed contents of this dissertation are as follows.?1?The data-driven prediction model for lithium-ion conductivity of gallium doped LLZO solid-state electrolytes was developed.20 types of parameters from different categories have been selected to compute the descriptors to develop a screening model.7descriptors that are responsible for high lithium ionic conductivity in oxide solid-state electrolytes were identified via the logistic regression model.For training machine learning models of this work,470 inorganic compounds,which consisted of 431 oxides and 39 other kinds of materials,are collected.The supporting vector regression?SVR?based model then predicted the lithium conductivity of 0 at.%to 20 at.%Ga dopant ratio in LLZO,which was validated in the subsequent high-throughput experiments.This work offers practical insights for researchers into the selection of appropriate concentration Ga dopants in LLZO to archive high lithium ionic conductivity.?2?The gallium doped ratio in the LLZO was optimized via high-throughput experimentation.Based on the estimated results of the data-driven prediction model,a gallium doped LLZO thin-film library?0 at.%to 20 at.%?is fabricated by high-throughput multilayered radio frequency magnetron sputtering.Using element diffusion kinetic mechanism and thin-film crystallization kinetic mechanism,the presented LLZO sample library was prepared via repeatedly depositing the sequentially stacked nanolayers of LLZO and Ga₂O₃,and the following annealing.The ultra-thin thicknesses of each layer facilitate the interdiffusion in the multilayered structure,in turn,enable Ga₂O₃ to help to stabilize the cubic phase of LLZO.To obtain the ideal LLZO with low electric conductivity and high lithium-ion conductivity,the high-throughput Hall effect test technique and electrochemical impedance spectroscopy test technique are used to characterize the electrochemical performance of LLZO thin-films.By carefully analyzing the electric conductivities and lithium-ion conductivities of different chemical compositions,the 16 at.%gallium doped LLZO thin film was identified as the best ratio.?3?Reduced energy barrier for Li⁺transport across grain boundaries with amorphous domains in LLZO thin-films.The high-resistive grain boundaries are the bottleneck for Li⁺transport in LLZO solid electrolyte.Herein,high-conductive LLZO thin-films with cubic phase and amorphous domains between crystalline grains are prepared,via annealing the repetitive LLZO/Li₂CO₃/Ga₂O₃ multi-nanolayers at 600?C for 2 h.The amorphous domains may provide additional vacant sites for Li⁺,thus relaxing the accumulation of Li⁺at grain boundaries.The significantly improved ionic conductivity across grain boundaries demonstrates that the high energy barrier for Li⁺migration caused by the space charge layer is effectively reduced.Benefiting from the Li⁺transport paths with low energy barriers,the LLZO thin-film proposed in this dissertation has a cutting-edge value of ionic conductivity as high as 6.36×10^-4 S/cm,which is expected to be applied to thin-film lithium batteries.?4?High quality perovskite-type lithium lanthanum titanate powders were synthesized by low cost one-pot hydrothermal synthesis.The effects of different solvents and reaction temperatures on the phase and crystalline size of Li_0.33La_0.56TiO₃?LLTO?were studied.At 260?,the pure phase of LLTO was obtained in KOH?4 mol/L?solution.The average particle size of the LLTO is 568 nm.Powder neutron diffraction was used to study the phase stability of LLTO at different temperatures?25?⁶00??.The results show that the phase of LLTO is stable at high temperatures.The crystal unit cell parameters of LLTO increased via the temperature increased.Finally,the lithium ionic conductivity of the LLTO solid electrolyte synthesized by this method is 3.1×10^-5 S/cm,indicating that the hydrothermal synthesis method designed in this paper is effective.In summary,this dissertation provides several valuable techniques and insights for the oxide solid-state electrolytes rational design,optimization fabrication,and low-cost synthesis,which can be applied in new-generation all-solid-state lithium batteries.