Research On The Protection Of Metallic Lithium Anode Based On The Electrolyte Modification

The metallic lithium anode,named the“holy grail”electrode,possesses an ultrahigh specific capacity(3860 m A h/g)and the most negative potential(-3.04 V).However,the practical application of lithium anode is hindered by serious volume effect and lithium dendrite growth.It is a simple and feasible strategy to obtain a stable SEI by modifying the electrolyte to protect the lithium metal anode,thereby improving the cycling performance of the metallic lithium anode and promoting the commercialization of metallic lithium batteries.In this paper,on the basis of ether-based conventional electrolyte and local high concentration electrolyte with good compatibility to lithium metal,different additives have been introduced to optimize the composition of SEI film on metallic lithium anode,inhibiting the growth of lithium dendrite and the occurrence of parasitic reactions,and improving the cycling performance of metallic lithium anode.Through the characterization of physical and electrochemical properties of electr olyte and morphology components of circulating metallic lithium anode,the protective mechanism s of additives have also been clarified.This paper mainly includes the following six chapters.The first chapter introduces the problems and solutions of lithi um metal anode which have been reported,leading to the main research content of this paper.The second chapter introduces the related methods of representation test.The third chapter is about the interface modification of lithium metal anode with new electrolyte additive(Li DFOP).On the one hand,Li DFOP can induce in situ polymerization of DOL to form gelated electrolyte and inhibit the occurrence of parasitic reactions.On the other hand,Li DFOP can participate in the formation of SEI film of lithium me tal anode prior to the reduction and decomposition of the other components in the electrolyte.Morphology and composition characterization show that Li DFOP constructed a unique SEI film with organic matter as frame and inorganic material as the filler on the surface of lithium metal cathode by inducing DOL polymerization and its preferential reduction decomposition.Under the protection of this unique SEI film,lithium metal presents a relatively dense and flat deposition morphology.The electrochemical tes t results show that the SEI film induced by Li DFOP can significantly improve the coulombic efficiency and cyclic stability of metallic lithium anode.The fourth chapter is about the interface modification of lithium metal anode by silicon nitrogen additive(BSA).The electrochemical performance of the electrolyte,the morphology of lithium metal anode after circulation and the composition of SEI film have been analyzed and the conclusions as below:The characterization of electrochemical performance of elec trolyte shows that BSA can participate in the formation of SEI film of lithium metal anode prior to the reduction and decomposition of the other components in the electrolyte.Morphology characterization and mechanism analysis show that:BSA reacts with trace water in the electrolyte,and after two-step hydrolysis,Si-N and Si-O bonds are broken,yielding acetamide and trimethylsilanol(TMSi A).TMSi A reacts with lithium metal to produce lithium silanol,and acetamide is further electronically reduced and combined with free lithium ions in the electrolyte.SEI films containing Si-O,C-N and Li_xN were constructed on the surface of lithium metal.The electrochemical test results show that the SEI film induced by BSA has high ionic conductivity and stability,wh ich improves the cycling stability of lithium metal battery under high current density.In chapter five,the synergistic effect of fluoride additive(FEC)and silicon additive(BSA)on the interface modification of lithium metal anode was studied.The electrochemical properties of the electrolyte and the morphology components of the lithium metal anode after recycling have been characterized and the conclusions as below:The electrochemical properties of the electrolyte showed that FEC could participate in the formation of SEI film of lithium metal anode prior to the reduction and decomposition of the other components in the electrolyte.In the dual additives localized-high concentration electrolyte(DALE)electrolyte,the electrochemical properties of the two additives did not change.Morphology and mechanism analysis show that FEC can increase the content of Li F in the SEI film,improve the strength of SEI film and increase the diameter of lithium deposit ion,but the density of lithium deposition is not improved.When FEC and BSA are used together,BSA is hydrolyzed to produce acetamide and trimethylsilanol lithium.After that,FEC is defluorinated to produce Li F and VC,and acetamide is further reacted with VC to form polymer.Thus,a Li F enhanced organic/inorganic hybrid SEI film has been constructed on the surface of lithium metal anode.The electrochemical test results show that the single additive(FEC)has limited cycling performance improvement,whil e the combination of BSA and FEC has the best cycling performance improvement.The sixth chapter is the conclusion part of this paper,summarizing the work of the whole thesis,and looks forward to the shortcomings of the dissertation and the future resear ch direction.