Study On Li-ion Conduction Of Perovskite Li_0.33La_0.56TiO₃ Based Solid-state Electrolytes And Its Applications

Rapidly growing demands for rechargeable Li-ion batteries used in electronic equipment,vehicles and large-scale energy storage systems require new generation lithium-ion batteries with high energy density and enhanced safety.Conventional organic liquid electrolytes in commercial Li-ion batteries with an intrinsic fire risk and also limit the further increase of their energy density.Solid-state electrolytes are gaining continuing attention due to its excellent safety performance and improvement of lithium-ion batteries energy density.Perovskite Li_0.33La_0.56Ti O₃（LLTO）solid-state electrolyte has long been considered as a promising candidate enabling all-solid-state Li batteries due to its high bulk Li-ion conductivity and wide electrochemical window.However,the Li-ion conductivity at grain boundary is much lower than that of the bulk phase,which limits the total Li-ions conductivity of LLTO solid-state electrolyte.Besides,the electrode/electrolytes interfacial contact is bad due to the hard texture of LLTO pellets.The Ti⁴⁺in LLTO pellets are easily reduced to Ti³⁺,inducing severe interfacial reaction.Li dendrite growth induced short-circuit of Li-ion batteries which worsen electrochemical cycle stability is still a critical problem that limits the application of LLTO pellets.Until now,the mechanism of low grain boundary Li-ion conductivity,the Li dendrite growth and Li-ion conduction at grain boundaries and interface in LLTO pellets are not totally clear.Aiming above questions,the main research of this thesis mainly focuses on the following three aspects:（1）The influence of structure and phase defects on Li-ions conductivity mechanism in LLTO pellets was studied.There are two types grain boundaries in LLTO pellets identified by electron microscopy,namely“macro”and“micro”GBs by their size,respectively.A phase defects of Li-poor orthogonal phase Li_0.18La_0.6Ti O₃grains which is distinct with the normal tetragonal phase Li_0.33La_0.56Ti O₃ grains were widely located at micro grain boundaries.Combined by electrochemical measurements,the influence of Li-poor grains contents on grain boundary Li-ion conductivity of LLTO pellets was studied.Besides,structural defects are also identified in LLTO pellets.The influence of structural defects on Li-ion diffusion in LLTO micro grains is investigated by DFT theoretical calculation.These results reveal the structural origin of low Li-ion conductivity in LLTO pellets and provide insights to improve electrochemical performance of LLTO pellets.（2）The Li dendrite growth mechanism in LLTO solid-state electrolytes was studied.We investigate the Li-dendrite growth induced by volids and grain boundaries in LLTO ceramic pellets through detailed electron microscopy and spectroscopy analysis.Electrochemical cycling and thermal chemical reaction between Li and LLTO show much similar Li dendrite growth mechanism in LLTO pellets.Li dendrites growth prefer to nucleate at the voids in the LLTO pellet,then diffusion along the grain boundary and nucleate again inside the voilds,repeated until the short-circuit.Besides,Li dendrite growth also induce the reduction of Ti⁴⁺to Ti³⁺,leading to the decomposition of LLTO pellets.These results reveal the mechanism of Li dendrite growth in LLTO pellets and provide insights for the suppression of Li dendrite growth.（3）The inorganic-organic interfacial Li-ions conduction in LLTO based composite solid-state electrolytes is studied.LLTO nanotubes were prepared by electrospinning method and added in inorganic-organic composite solid-state electrolytes as inorganic fillers.Compared with LLTO nanoparticles and LLTO nanowires,the LLTO nanotubes fillers increase the interfacial area between organic polymer and LLTO fillers,leading to a facile transportation of Li-ions through these interfacial pathways which enhance the Li-ion conductivity of composite solid-state electrolytes.Besides,the electrochemical performance of composite solid-state electrolytes was largely enhanced after LLTO nanotubes fillers adding.These results demonstrate an effective strategy to improve composite solid-state electrolytes performance by nanotubes fillers.