Physical Insight Of Reaction Mechanism And Modification For Electrode-electrolyte Interface In High Energy Density Electrochemical Devices

With the increasing energy and environmental crisis,the development of safe,clean and high energy density electrochemical energy storage devices has attracted great attention.The interface between the electrode and the electrolyte serves as a place for electrochemical energy storage reaction,which determines the electrochemical performance such as capacity,voltage and safety.How to obtain high energy density electrochemical devices by improving effective interface reactivity under the premiss of ensuring safety has become an urgent problem to be solved.And the electrode-electrolyte interface reaction is determined by the electrolyte potential window,the charge storage activity and capacity of the electrode material,and the transmission of electrons and ions.In this paper,several tyoical and promising high energy density electrochemical energy storage interfaces are selected as research objects and their energy storage mechanisms are deeply investigated,thus improving the electrochemical performace.First,since the intrinsic charge storage capability of the electrode material within the electrolyte potential window is directly related to the energy density,a first-principles calculation model is proposed validate to predict the intrinsic charge storage capacity by calculating the electronic structure of the material.The effect of central transition atoms and functional groups of MXenes were studied using the model.It was found that the modification of the functional groups would greatly affect the electrochemical stability and open circuit potential of M₂CT₂-type MXenes,and basically satisfy the law:1)for the same transition metal,the open circuit potential of M₂CT₂ with different terminals satisfies the rule of M₂CO₂>M₂CF₂>M₂C?OH?₂;2)for the same fuctional group,the open circuit potential of the transition metals of the same main group/period decreases with the increase/decrease of the atomic number.Second,electrolyte ions play a crucial role in the charge storage capability of the electrode material.The influence of electrolye pH on the charge storage capacity of layered Ti₂CT₂ was analyzed by first-principles calculation.It was found that Ti₂CT₂ gained electrons with increasing acidity and alkalinity.The embedding of electrolyte cations can reduce the electrostatic energy of the system and increase the charge storage capacity and energy density of the system.Ti₂CO₂ has a sodium storage voltage range of 2.78 to 0.73 V?vs.Na/Na⁺?and a sodium-encapsulated capacity of 283 mAh/g.Moreover,the intercalation of sodium ion leads to changes in the layered structure which also causes changes in electrochemical perfomances.Next,improving the kinetics of interfacial reactive ion diffusion can activate the electrochemical energy of the material.Electrochamical energy release in high-valence ion batteries with high energy density potential is often limited or even ruined by kinetic factors.Frim the perspective of thermodynamics and kinetics,this study used a combination of therotical and experimental work to investigate the phase transition reaction process of VOPO₄ under electrolytes with different water activities.The results show that the electrolyte with water activity above 10^-2 can ensure the cointercalation of Mg²⁺ions and water mlecules into the electrode.Water molecules can simultaneously reduce the desolvation energy of the interface and the energy barrier of ion diffusion in the lattice,and improve the kinetics of the electrochemical reaction.The activated VOPO₄ can process the reversible cycle of VOPO₄·H₂O?Mg_0.5VOPO₄·2H₂O?MgVOPO₄·2H₂O,and simultaneously increases the capacity and potential,thus increasing the reverible capacity from 8.0 to 91.7 mAh/g.Finally,the parasitic reaction of the all-solid lithium metal battery and the suppression of the lithium dendrite ensure stable release of high energy density of lithium metal.All-solid electrolytes offer new possibilities for the use of lithium metal as anode,however,side reactions and dendrite limit its application.Based on the first-principles calculation,this study reveals the instability of the interface between lithium metal and Li₃PS₄solid-state electrolyte,and proposes that the parasitic reaction and the dendrite can be reduced by generating interphase with high elastic modulus,high interfacial energy with Li metal,and poor electronic conducitivity.The theoretical results and experimental characters suggest assembling an all-solid-state battery with a high LiF content at the interface between Li metal and electrolyte,achieving a significant increase in critical current dendity from 0.7 to 2 mA/cm² and the Coulombic efficiency also increased to⁹8%.The improved Li|LiCoO₂ solid-state full cell enables continuous specific capacity of 120 mAh/g.