Investigation On Lithium-Zinc Alloy Based Lithium Metal Anode Materials

With the rapid development of consumer electronics,electric vehicles,grid-scale energy storage,etc.,the demand for high-energy-density lithium-ion batteries becomes more urgent.The commercial lithium-ion battery is limited by the extremely low theoretical specific capacity of graphite anodes,and the practical energy density is approaching its theoretical value.Lithium（Li）metal anode,due to its ultrahigh theoretical specific capacity(3860 mAh·g^-1)and low electrochemical potential（-3.04 V vs.standard hydrogen electrode）is one of the most potential anode materials for the next generation of high-energy-density Li secondary batteries.Unfortunately,the Li dendrite growth,serious side reactions,and infinite volume change of Li metal anode have brought serious safety risks and hindered its commercial application.Great effort has been devoted to solving these problems,and the proposed methods include electrolyte optimization,constructing artificial solid electrolyte interphase（SEI）,introducing three-dimensional（3D）structures,and so on.Among them,the 3D framework can not only inhibit the formation of Li dendrites but also suppress the volume change of Li metal during Li plating/stripping.Li-rich alloy,as a novel type of3D structure anode material,contains an embedded network of micro-nano alloy structures,which can be used as a 3D skeleton of Li metal anode to induce Li plating/stripping uniformly.This dissertation is to tackle the serious issues concerning Li dendrites and volume expansion of the Li metal anode by fabricating Li-Zn alloy based Li composite anodes,where the effect of the 3D alloy framework on the nucleation and deposition behaviors of Li are systematically investigated.A series of high-performance and high-stability Li alloy composite anodes with the optimized scaffold are developed for the commercial application of Li metal anode.The main achievements are summarized as follows:（1）The deposition behavior of Li metal on Zn foil and other common current collector foils is studied,and the alloying reaction between Li and Zn is the key factor in achieving uniform Li nucleation and regulating electrochemical deposition.Li-rich Li-Zn alloy anodes with different ratios of Li/Zn are prepared by molten metallurgy,which has a 3D skeletal structure composed of LiZn intermetallic compounds and exhibits excellent cycling stability.The Li-rich Li-Zn alloy with a lower Zn content exhibits a micro-nano scale framework and higher theoretical specific capacity but leads to poor structural stability.When the Zn content is higher,the alloy framework becomes robust with improved structural stability.Correspondingly,the surface area decreases,and the theoretical specific capacity declines.Therefore,the Zn content should be optimized to achieve a balance between the cycling performance and the practical specific capacity in Li-rich Li-Zn alloy anodes.（2）Li-rich Li-Zn binary alloy is composited with Cu foam to improve the dimensional stability of the LiZn micro-nano skeleton and lithiophilic property of the porous Cu framework.A facile thermal infusion method is used to produce a composite of Li-Zn alloy and Cu foam skeleton,where the micro-nano network of the LiZn alloy phase with ion/electron mixed conducting capability is constructed within the microsized pores of the Cu foam.Remarkably,the LiZn skeleton is"welded"onto the Cu foam framework,resulting in the enhanced stability of the whole skeleton structure.Additionally,a Cu-Zn alloy layer is in situ formed on the surface of the Cu foam skeleton,which shows the promoted lithiophilicity.Consequently,the deposition of Li ions in the 3D skeleton is greatly induced by the secondary skeleton of the LiZn network,inhibiting the Li“top-deposition”behavior on the Cu foam skeleton and suppressing the volume change of the Li metal anode during cycling.（3）A Li-rich ternary Li-Cu-Zn alloy is developed while doping Cu element in Li-rich binary Li-Zn alloy.Two Cu-Zn alloy phases,namely CuZn₅ and CuZn,are formed as the main components of the framework,exhibiting higher lithiophilicity and structural stability with respect to the binary alloy skeleton.The CuZn₅ phase can react with Li to form the LiZn phase,thereby inducing Li uniform nucleation and deposition.Alternatively,the lithiophilic CuZn phase is electrochemically inert and acts as a buffer structure,ensuring the geometric stability of the anode.By virtue of the synergistic effect of the two Cu-Zn alloys,Li-rich ternary Li-Cu-Zn alloy exhibits extremely low Li nucleation overpotential and excellent structural stability,which can induce a uniform Li deposition process.As a result,suppress the volume change during the Li deposition/stripping process.As a result,the electrochemical performance far exceeds that of binary Li-Zn alloy.（4）The ultrathin Li metal anode is the key part to assemble the high energy density Li metal battery.Herein,a hot melt coating method is proposed to one-step produce an ultrathin Li metal or Li-Zn alloy layer on the Cu sheet.The effect of the heating temperature on the wettability of molten Li to the Cu substrate has been systematically investigated.The high temperature can accelerate the alloying reaction rate between the molten Li with the surface of Cu foil,leading to enhanced wettability via the formation of the Li-Cu alloy buffer layer.As a result,the molten Li can be directly coated on the surface of Cu foil to prepare the ultrathin Li layer.This idea can be extended to the preparation of an ultrathin Li-rich alloy layer,where an ultrathin Li/Li-Zn alloy layer coated Cu foil with a coating layer thickness of 5～50μm and a width of 10 cm is achieved.The ultrathin Li-Zn alloy anode has excellent electrochemical properties due to the introduction of the 3D alloy skeleton structure,and the Li Fe PO₄-based full cell shows ultralong stability even under low negative/positive capacity（N/P）ratio conditions.