Modification Of Lithium Metal Anode For Advancedelectrochemical Performance

With rapid development of electronic equipment and electric vehicle?EV?,higher energy density is urgently required for electrochemical energy storage devices.Lithium?Li?metal is considered as the“Holy Grail”of high energy density batteries due to its ultra-high specific capacity and low redox potential.The application of Li metal anode in Li batteries can greatly improve the energy density of Li batteries.Thus,Li metal as a promising anode material for Li batteries has become a research hotspot.However,the practical application of Li anode is still hindered by some problems,including the irreversible reaction between Li metal and electrolyte,the growth of Li dendrites and the huge volume change of Li metal during plating and stripping process.Therefore,it is significant to modify Li metal anode so as to improve its safety and electrochemical properties.In this dissertation,the surface modification and bulk phase modification of Li metal were systematically investigated.The highlights are summarized as follows:?1?Iodic acid?HIO₃?was used to build an artificial solid-electrolyte interphase?SEI?on the surface of Li anode and the components of this artificial SEI mainly includes lithium iodate?LiIO₃?and lithium iodide?LiI?.The artificial SEI on Li metal surface can mitigate the side reaction between electrolyte and Li metal to stabilize the interface and inhibit the growth of Li dendrites to uniform the deposition of Li metal.The symmetric cell assembled with the modified Li foil has a longer cycle lifetime than the pristine Li foil.The surface modification of the Li metal improves the discharge capacity and Coulombic efficiency of lithium-sulfur?Li-S?cell.?2?Alkali metal nitrates were used as electrolyte additive and their electrochemical properties in Li batteries were studied systematically.Potassium nitrate?KNO₃?shows excellent electrochemical performance as the cation K⁺can surround the top of Li dendrites by electrostatic attraction and then delay their further growth,while the anion NO₃^-can react with Li metal in situ to improve the quality of SEI on Li metal surface,and jointly suppress the growth of Li dendrites.KNO₃ as electrolyte additive can improve the Coulombic efficiency of Li|Cu cell and the electrochemical cycling performance of Li-S cell.?3?The carbon nanofibers?CNFs?were synthesized by electrostatic spinning and high temperature carbonization,which were then used as three-dimensional?3D?current collector for Li metal anode.Its large specific surface area can reduce the actual current density of collector surface.Besides,CNFs network as a porous equipotential body exhibits electric field shielding effect,thus the growth of Li dendrites is effectively restrained.The internal pores of CNFs can provide space for the deposition of Li metal,which can effectively alleviate the volume change of electrode during the process of charge and discharge.Furthermore,the uniform distributed copper nanoparticles?CuNPs?were anchored on CNFs as heterogeneous nucleation sites to induce more uniform deposition of Li metal in 3D current collector.The Coulombic efficiency and cycling lifespan of Li|Cu batteries can be effectively improved by using CNFs+CuNPs as 3D current collector.Li metal coated with CNFs+CuNPs as composite Li electrode can improve the discharge capacity and cycling performance of LiCoO₂?LCO?cell.?4?The surface modification and the bulk modification of Li metal are combined to improve the performance of Li metal anode.Molten Li metal was infiltrated into the 3D scaffold structure of metal foam to form composite Li electrode,and the surface of the composite Li electrode was coated with a layer of zinc fluoride?ZnF₂?thin film by magnetron sputtering technology.The 3D scaffold structure can suppress the growth of Li dendrites and alleviate the volume change of the electrode during the charge and discharge process.The surface modification can reduce the side reaction between Li metal and electrolyte.The cycling performances of symmetric cell and LCO cell were tested to understand the roles of Cu foam and Ni foam as 3D scaffold structures.Metallic Cu and Li can form alloy at high temperature.Nanowires structure appeared in a composite Li electrode after cooling,which increased the specific surface area of the3D scaffold structure and lithiophilicility,so the composite Li electrode prepared with Cu foam exhibited better electrochemical performance.?5?The Li-rich dual-phase Li-Cu alloy was synthesized with one-step high temperature metallurgical method and used as a composite Li electrode.The Li-rich dual-phase Li-Cu alloy includes Li metal and inherent 3D conductive scaffold of Cu_xLi solid solution.Li atoms act as replacement atoms to form solid solution with Cu crystal at high temperature.Cu_xLi solid solution presents the electrochemical inert,which means there is no reversible electrochemical reaction,but Cu_xLi solid solution is lithiophilic,so it can induce uniform deposition of Li metal in 3D scaffold.When the proportion of Cu in the Li-Cu alloy is high,the Cu_xLi solid solution appears as microparticles;when the Cu content decreases,the Cu_xLi solid solution appears as thick scaffold;when the Cu content is low,the Cu_xLi solid solution mainly appears as nanowires.The 3D scaffold of Cu_xLi solid solution inside the Li-Cu alloy was obtained after removing the Li metal phase from the Li-Cu alloy by chemical etching.Using the Cu_xLi solid solution nanowires as 3D current collector can effectively inhibit the growth of Li dendrites and improve the Coulombic efficiency and cycling lifespan of Li|Cu cell.The electrochemical cycling performance of symmetric cell and Li₄Ti₅O₁₂?LTO?cell can be improved by using Li-rich dual-phase CuLi60 alloy as composite Li electrode.?6?The Li-rich dual-phase non-inert alloy was synthesized by one-step high temperature metallurgical method and used as a composite Li electrode,including Li metal and inherent 3D conductive scaffold of intermetallic compound.Notably,the intermetallic compound is lithiophilic,and induce uniform deposition of Li metal in 3D scaffold,which presents reversible electrochemical alloying/de-alloying reaction.Besides,the intermetallic compound is an ion-electron mixed conductor enabling the3D scaffold structure to transport Li ions.Taking Li-Ca alloy and Li-Ba alloy as examples,the 3D scaffold structure in the Li-rich dual-phase CaLi10 alloy appears as a porous foam of CaLi₂ intermetallic compound;the 3D scaffold structure in the Li-rich dual-phase BaLi24 alloy appears as ordered microporous array of BaLi₄ intermetallic compound.The Li metal is preferentially stripped from the dual-phase alloy during delithiation process,and then intermetallic compound is gradually de-alloyed as metal foam.In the lithiation process,the metal foam is preferentially alloyed to form intermetallic compound,and then Li metal deposition on the intermetallic compound surface forms a composite Li electrode.Conclusively,the 3D scaffold structure of Li-rich dual-phase non-inert alloy can effectively suppress the growth of Li dendrites and alleviate the volume change of electrode during charge and discharge process.