Electrolyte Structure Optimization And Solid Electrolyte Interphase Study Of Lithium Metal Batteries

With the rapid development of society,the emission of carbon dioxide has gradually increased,and the problem of global warming is becoming increasingly serious,so there is an urgent need to develop new energy and secondary energy storage equipment.Secondary batteries,such as lithium-ion batteries,are the most successful energy storage devices so far.With the commercial development of secondary battery systems,our daily life has changed dramatically.The increasing demand for the range of electronic devices and electric vehicles make the existing lithium-ion battery system insufficient in the energy density.Therefore,lithium metal batteries are revisited.Lithium metal is regarded the "holy grail" due to its ultra-high capacity(3860 mAh g-1)and lowest electrochemical potential(-3.040 V compared to standard hydrogen electrode).When matched with high voltage positive electrode materials,a high energy density of 500 Wh kg-1 is achieved.However,the uncontrolled Li dendrite,the severe side reaction and the huge volumetric change of the Li metal greatly hindered the commercial applications of lithium metal batteries.Li dendrite can penetrate the separator,causing a short circuit in the battery.Side reactions can consume lithium metal and electrolyte,resulting in short lifespan.Meanwhile,the huge volumetric change can cause the safety concerns.Electrolyte is an important component of batteries and directly contacts with the lithium metal anode.It directly influences the composition and distribution of the solid electrolyte interphase(SEI)on the surface of the lithium metal,as well as the morphology of the lithium metal deposition.Therefore,designing the composition and structure of the electrolyte can effectively improve the cycling performance of the lithium metal negative electrode.This paper aims to improve physicochemical properties of the solid electrolyte interphase on the surface of the lithium metal anode via optimizing the components and structure of the electrolyte.Meanwhile,we also focus on the reduction mechanism of the electrolytes on Li metal surface.We hope this paper can shed light on the development of new electrolytes.The main works are as follows:1.Ether solvents have good compatibility with lithium metal anode with limited side reactions.Therefore,a new ether solvent,triethyl orthoformate,is used to prepare a localized high concentration electrolyte with lithium bis(fluorosulfonyl)imide.Triethyl orthoformate has a large steric hindrance,which will further weaken the Li+solvent connection and promote the coordination of lithium ion and FSI" anion.In this localized high-concentration electrolytes system,contact electron pairs(CIP)and aggregates(AGG)are remarkably formed.X-ray photoelectron spectroscopy characterizations demonstrate that the Li-F SEI is formed in this kind of electrolyte,thus stabilizing the electrochemical performance of Li metal anode.Furthermore,full cells with NMC811 and LCO deliver excellent electrochemical performance in this local concentrated electrolyte,shedding a new sight on the development of high energy density lithium metal batteries.2.Carbonate-based electrolyte has received lots of interest owing to the wide electrochemical window and safety.However,the drastic side reactions would lead to the battery degradations.LiNO3,a kind of film-formation additive,is effective to protect Li metal anode.While the low solubility limits its application in the carbonatebased electrolytes.In this section,we successfully induce the nitrate/nitrite groups into the carbonate-based electrolytes by regulating the polarity of the solvent and changing the state of the additives.The work mechanism is revealed by the constant current charge-discharge test,cyclic voltammetry and scan electron microscopy.Nitrate/nitrite groups are proved to be preferentially reduced on Li metal anode,which effectively improve the electrochemical performance of Li anode.Meanwhile,the morphology of Li deposits is also improved after the addition of Nitrate/nitrite groups.However,the addition of nitrate/nitrite groups cannot fully change the intrinsic properties of the carbonate solvents,therefore,next work will be focused on the exploration of new electrolytes systems and further improve the electrode/electrode interface.