Atomic Scale In-Situ Electron Microscopy Study Of All-Solid-State Batteries

Due to the better safety properties and high theoretical energy density of all-solid-state lithium ion battery,it has become one of the promising candidates to replace liquid electrolyte lithium ion battery in the field of portable smart devices and electrical vehicles et al.However,the existence of solid electrolyte totally changed the interface structure within lithium ion battery.Fully contacted solid-liquid interface becomes solid-solid interface,resulting in difficulties for the full contact of electrode and electrolyte,and higher contact resistance.Besides,most of the solid electrolytes have lower ion conductivity,which makes it hard to fulfill the expectation in terms of power energy density and cycle ability.Properties of matters are correlated with its atomic structures and electronic structures.Under this circumstance,acquiring structural evolution of the bulk and interfaces of all-solid-state batteries is important.This dissertation based on the work of development and application of atomic resolution in-situ electron microscopy on migration of lithium ions and structural evolution in three dimensions which are as follows:1.By utilizing the combination of focused ion beam milling,chip-based in-situ holder and probe aberration corrected microscope,we built an all-solid-state battery on micro-scale.Through in-situ electrochemical dilithiation,we observed the migration of lithium ions and the corresponding bulk structural evolution under high voltage condition.The results indicate that uneven delithiation caused by uneven contact condition of solid-solid interface in all-solid-state lithium ion battery.Besides,formation of twin boundaries and antiphase boundaries induced by the relaxation of internal stress caused by the existence of solid electrolyte is observed,which is the new lithium ions pathways to allow whole LiCoO₂ cathode fully delithiated.2.Based on that,we continued to complete the atomic scale in-situ electron microscopy to acquire three-dimensional structure evolution information.We observed ordered spinel LiNi_0.5Mn_1.5O₄ cathode materials in all-solid-state batteries along different zone axes,hence established the atomic structure and electronic structure evolution in three dimensions which provided important structural information of this cathode materials.Through observations of different zone axes,we found the inhomogeneity of the structural evolution.Along with the delithiation process,we can found homogenous transition metal elements migration from ordered into disordered along [100],[110],[111] zone axes.Whereas,in [112] zone axis,we found inhomogeneous transition metal elements migration and the formation of new boundaries.We successfully found the phenomenon hidden under two-dimensional projection observation in perspective of three dimensions.In combination with theoretical calculations,we came into conclusions that the formation of new boundaries are energy favorable alongside with the delithiation process.Further atomic scale doping calculation suggested that doping with lower valence state cations can effectively hinder the formation of boundaries,and then improve the homogeneity of the structural evolution during electrochemical cycle which makes this electrode materials more stable in battery.