Limitation And Enhancement Of Anode Reaction Kinetics For Lithium-ion Battery

With the rapid development of electric tools such as portable electronic devices and electric vehicles,high energy density,fast charging and discharging,high safety,low cost,and wide temperature range are required for batteries to meet the application needs of various scenarios.Lithium-ion batteries(LIBs)operating under fast charging/low temperature conditions can cause serious performance degradation,resulting in poor environmental adaptability and short drive range of electric tools,which has become a bottleneck restricting their promotion and application.To address the above issues,this dissertation investigates the rate-determining steps for lithium ion(Li~+)diffusion and charge transfer under fast charging/low-temperature operating conditions.Graphite is the key anode material for commercial LIBs.Li~+intercalating in graphite can form graphite intercalation compounds with a series of staging structures.However,many open questions remained unsolved or unclear,especially those associated with the nature of their staging structure and the kinetics of the stage formation and transitions,which hinders the further application of graphite at different operating conditions.Combining cryogenic-transmission electron microscopy(cryo-TEM)and other characterization techniques with theoretical simulations,this work unraveled the nature of the staging structure and evolution of the lithium-intercalated graphite at nanoscale.The results show that the Li ions are intercalated unevenly,generating local stress and dislocations in the graphitic structure.A combination of Li~+diffusion and dislocation interaction facilitate the transition among different staging phases.It is found that each staging compound is macroscopically ordered but microscopically inhomogeneous.Accordingly,"Localized-domains"model was proposed to describe the structural evolution of graphite during lithium intercalation.The sluggish kinetics of graphite anode at high-rate charging/discharging hinder its application in fast-charging LIBs.This work applied cryo-TEM,potential relaxation technique and other characterization techniques to uncover the structural evolution at high current densities,and correlate it with the reaction kinetics and electrochemical performance.The results show that compared with bulk diffusion in a thin electrode,interfacial Li~+transportation is proved to be the main rate-determining step,including desolvation and Li~+diffusion through the solid electrolyte interphase(SEI)layer,which is highly dependent on the electrolyte chemistry and SEI layer property.However,when a thick electrode is used,the diffusion of Li~+through the graphite electrode become one of the bottlenecks that limiting the graphite rate performance.Combining electrode structure design with electrolyte regulation is an efficient way to promote both of the Li~+diffusion through graphite electrode and interfacial Li~+transportation,so as to achieve high-performance graphite anode for fast-charging LIBs.Graphite anodes face serious capacity degradation and uncontrollable dendritic Li plating at low temperatures.Thoroughly understanding the influence of temperature on the underlying microstructure and performance of the battery is essential to solving the kinetic bottlenecks and achieving high performance at a low temperature.The temperature-dependent Li~+behavior in Li metal batteries and its relationship with the electrochemical performance were revealed by various characterization tools,including cryo-TEM,electron energy loss spectroscopy,and X-ray photoelectron spectroscopy.The kinetics bottleneck is deciphered as Li~+diffusion through the SEI layer at low temperatures.Lowering the temperature not only slows down the kinetic of Li~+transportation but also changes the thermodynamic reaction of the electrolyte decomposition,forming an SEI layer consisting of intermediate products rich in organic species,thus increasing the resistance for Li~+transportation.Tuning the solvation structure of electrolytes with a low LUMO(low unoccupied molecular orbital)energy level and polar groups is beneficial to readily generate an inorganics-rich SEI layer,which helps to reduce the energy barrier of the above process and improve the low temperature performance.