Modification Of Metallic Lithium Anodes And Their Electrochemical Performances For Secondary Lithium Batteries

With the development of electric automobile and consumer electronic industry,there are growing demands for high-performance energy storage systems.Lithium metal anode is an ideal candidate for high energy density batteries based on its high theoretical specific capacity?3860 mAh g-1?and the low electrochemical potential?-3.04 V?.However,large-scale application of rechargeable Li metal anodes still faces two major barriers:?1?metallic Li is too active and it will easily react with electrolyte,resulting in low Coulombic efficiency and poor cyclic life due to the rapid loss of Li anode;?2?the formation of "dead Li" or "dendritic Li" will lead to virtually infinite volumetric change and cause short circuit of the batteries,as a result,safety hazards will be occurred.The growth of Li dendrites and the low Coulombic efficiency during the Li plating and stripping processes have limited its commercial applications for decades.Up to now,the progresses of lithium metal anode for rechargeable lithium batteries can be summarized in four aspects:electrolytes,separator,current collectors and lithium metal anode itself.In this article,several methods are applied to enhance the performance of lithium metal anode:?1?Polycrystalline Li₃N films are coated on Li foil by an in-situ nitridation method.In this work,the main response parameters including reaction time,reaction temperature and nitrogen gas flow.The control variable method is applied to research the influence of reaction parameters to the electrochemical performance of Li₃N/Li composite anodes,and the optimized nitridation conditions are determined.The Li₃N/Li composite anodes are fabricated under the optimized nitridation conditions.Compared to the pure Li,Li₃N/Li electrodes display much better electrochemical performance and the dendritic formation are inhibited efficiently.It indicates that the Li₃N films on Li surface can prevent the contact between Li and the electrolyte,and reduce the side reaction,thus suppressing the formation of dendritic lithium effectively.?2?Amorphous carbon?a-C?films are deposited onto the surface of metallic lithium.foil by magnetron sputtering technique.According to the work,the thickness of a-C film affects the electrochemical performances from two aspects;the thick film can prevent the formation of dendritic lithium very efficiently,but lead to a large resistance of Li+ transfer.The enhanced performance of a-C/Li electrodes need a moderate thickness of a-C films,otherwise the promoting effect of a-C/Li electrodes will be weakened.?3?Combine with the advantages of a-C and Li₃N films,the nitrogen-doped amorphous carbon?a-CN_x?films in nanosized thickness are deposited onto the surface of metallic lithium foil by magnetron sputtering technique.In this work,the nitrogen atmosphere is controlled at 0 sccm,5 sccm and 10 sccm,and the influence of nitrogen content is investigated.As a result,the a-CN_x/Li?10 sccm?electrodes possess the best electrochemical performance.The in-situ visualizing confocal microscopic observation is adopted to monitor the dendritic formation of pure Li,a-C/Li and a-CN_x/Li?10 sccm?electrodes.For the a-CN_x/Li electrode,the separator is transfixed when the current density increases to 10 C,while the separator of a-C/Li electrode is broken at 5 C.As to the pure Li electrode,the dendrite is observed clearly when the current density is only 1 C.The introduction of nitrogen in a-C film can improve the performance of a-C/Li electrodes.?4?An integrated reduced graphene oxide?rGO?multilayer/Li composite electrode is fabricated to enhance the performance of metallic Li anode.From the SEM images after cycling,the top surface of pure Li electrode displays obvious granule morphology and the cross-section topography is broken and collapse.When tested as an electrode for rechargeable Li metal batteries,the rGO/Li composite exhibits noticeable enhancement of electrochemical performances with better cycling stability than the unmodified Li.The interconnected rGO layers not only help to suppress the formation of dendritic Li,but also store the dead Li and restrain the uneven surface potential.At last,the morphologies of the rGO/Li electrode after different cycles are tested to explain the process of the formation of dendritic lithium.?5?An automatic spreading method is applied to coat graphene oxide?GO?layers on the metallic lithium foil to form a composite electrode.In order to choose the appropriate organic solvent,the ethanol,acetonitrile,ether and DMC are dropped on the surface of lithium foils.Among them,DMC is chemical inertness to metallic Li and is the ideal choice.The electrochemical performance of GO modified Li?GO/Li?electrodes have been thoroughly studied.The GO/Li electrode displays enhanced electrochemical performances in lithium-sulfur battery,and better cycling stability in symmetric lithium-metal coin cells than the unmodified pure Li electrode.?6?Vertical graphene?VG?arrays are used as the scaffold structure for high performance Li metal batteries.The melt infusion method is employed to encapsulate Li inside the VG scaffold structure,and the lithiophilic Si layer is coated onto the array surface by magnetron sputtering to assist this melt-infusion process.The porous scaffold structure can control the volume expansion and inhibit the formation of dendritic lithium significantly,leading to the excellent electrochemical performance of the Li composite anode.In addition,the Li-S full batteries with the composite anode display enhanced cycling reversibility.