Anode Stabilization Design And Electrochemical Performance Of "Anode-free" Lithium Metal Batteries

Due to the advantages of high energy density,low cost,and high safety,“anode-free”lithium（Li）metal batteries are becoming one of the most promising next-generation high-energy-density battery systems.However,the lithium metal deposited on the anode has extremely high reactivity and can cause huge volume changes,which leads to the growth of Li dendrites,the generation and accumulation of dead Li,and the side reactions between metallic Li and the electrolyte,making it difficult to form stable anode interphase,and eventually accelerating the consumption of active Li and causing battery failure.In this thesis,based on the“anode-free”Li metal battery where the Li source is provided only by the cathode,the effects of alloying on nucleation growth,the controllable construction of favorable solid electrolyte interphase（SEI）,the mechanism of reactivating dead Li by the shuttling of redox mediators,as well as the selective matching of redox mediators’potentials are systematically studied.Furthermore,the importance of the anode interphase to the stable cycling of the full battery is verified.The main research contents and conclusions are as follows:（1）Aiming at the problems of the anode Cu current collectors with lithophobicity,high nucleation overpotential,and unstable interface after lithium deposition,the strategy to modify Cu current collectors by trace amounts of Au is proposed.The alloying of Au and Li enhances the lithiophilicity of the current collector surface,reduces the nucleation overpotential,and improves the Li deposition morphology.The favorable Li deposition morphology reduces the damage to the SEI and helps to form stable Li anode interphase.In the subsequent Li deposition and dissolution process,the Au can be converted between the form of Li_xAu alloy and solid solution,so as to play a long-term role.When applied in an“anode-free”full cell with high areal capacity(about 4 m Ah cm^-2),the capacity retention rate of the cell can maintain 60%after 100 cycles of stable cycling.（2）In order to directly tune the SEI to improve the anode interphase stability,a method to modify the Cu current collector by simple heat treatment is proposed to simultaneously enhance the lithiophilicity and regulate the chemical composition of SEI.It is found that the heat treatment can reconstruct the surface morphology of the Cu current collector,make Cu₂O species generated and improve the surface lithophilicity.When Li is deposited on Cu foil,Cu₂O can be converted into Li₂O and participate in the formation of SEI,thereby enhancing the stability of the anode interphase.Thus,the“anode-free”Li Fe PO₄（LFP）|Cu full cell based on this current collector achieves a capacity retention rate of 62%after 100cycles.（3）The stability of the anode interphase can be improved by modifying the current collector,but it cannot solve the problems of the increased impedance caused by the accumulation of dead lithium,and excessive consumption of active lithium during cycling.In this chapter,a modification method with Li I as an electrolyte additive is proposed to solve the problem of dead lithium accumulation.It is found that I^-can be reversibly converted to I₃^-during cycling,which constitutes a redox mediator（I^-/I₃^-）and shuttles between the cathode and the anode through the electrolyte.Among them,I₃^-can chemically react with dead Li to realize the elimination and activation of dead Li,thus the interphase stability is dynamically maintained during the process.When the addition amount of Li I is 50 m M,the capacity retention rate of the“anode-free”LFP|Cu full cell can keep 72%after 100 cycles.Meanwhile,Li I additive is found to speed up the self-discharge rate of the battery.（4）In order to solve the problem of self-discharge caused by redox mediators in the electrolyte,the relationship between the potential of the redox mediator,the potential of the cathode material,the charge cut-off voltage,and the self-discharge behavior of the battery is systematically studied in this chapter.The study finds that the potential of a suitable redox mediator should be higher than that of the cathode material and lower than the charge cut-off voltage,which can avoid serious self-discharge while reactivating dead Li.Based on this,a universal selection principle of redox mediators for reactivating dead Li is proposed.This principle is further confirmed in“anode-free”LFP|Cu full cells by reactivation of dead Li and self-discharge tests of redox mediators with three different potentials.