Hierarchically Porous Lithium Fluoride Electrolyte Additives For Stablizing Lithium Metal Batteries

Low-carbon energy is very significant for sustainable development,in which the electrochemical energy storage as an important part of its market scale is increasingly expanding.At present,the capacity of lithium-ion battery with graphite anode is approaching its theoretical limit and unable to satisify the market demand for higher energy density storage.Replacing graphite with lithium metal with lower potential and higher energy density is an effective strategy to develop the next generation of highenergy lithium batteries.However,the application of lithium metal cathodes still faces many problems and challenges.For example,lithium metal anode causes repeated destruction and re-formation of the solid electrolyte interface(SEI)layer during cycling due to its huge volume change.This leads to the continuous depletion of the lithium metal with the electrolyte,resulting in a rapid decay of the cycling stability and coulombic efficiency of the lithium metal battery.In addition,during the deposition/extraction process of lithium metal,due to its uneven,lithium dendrites will grow,resulting in "dead lithium" and internal short circuits,which will cause the degradation of the electrochemical performance of the lithium metal battery and even cause safety problem.To solve these problems,researchers have studied the lithium alloy anode,electrode structure design,artificial SEI and electrolyte additives,etc.Compared with these strategies,the development of new electrolyte additives is a more effective method.It can directly control the composition of the solid electrolyte interface layer without changing the existing battery assembly process,and improve the stability of the solid electrolyte interface layer.It can inhibit the growth of lithium dendrites and improve the electrochemical stability and safety of lithium metal batteries.In order to improve the stability of lithium metal anode,our research focuses on the fluorinated electrolyte additives,which have excellent thermal and chemical stability to improve the oxidation resistance of electrolytes and form more stable solid electrolyte interfaces(SEI).It is also one of the effective means to enhance the safety of organic liquid electrolytes.In this thesis,I first introduce the preparation of hierarchically porous lithium fluoride(LiF)as the electrolyte additive by thermal injection method.Then,various advanced characterization techniques were used to explain the mechanism of hierarchically porous LiF as the electrolyte additive.Finally,in order to enhance its effect as a flame retardant,we modified phosphorus-containing organics on the surface of hierarchically porous LiF.The main studies include the followings.1.Firstly,I studied the synthesis conditions for preparing various fluoride nanoparticles.The physical phases and morphology of the synthesized lithium fluoride,calcium fluoride,magnesium fluoride and aluminum fluoride were investigated.In the process of adding these fluoride nanoparticles to the electrolyte,only LiF nanoparticles can be stably existed in the carbonate electrolyte.Subsequent studies demonstrated that the morphology of lithium fluoride is hierarchically porous with high porosity and specific surface area.Therefore,the addition of LiF nanoboxes into the electrolyte enables uniform dispersion and does not hinder lithium ions while they are on the electrode surface due to the low diffusivity barrier.2.To understand the mechanism of action of hierarchically porous LiF to enhance the performance of lithium metal batteries,we performed experiments and calculations by Cryo-TEM,XPS,DFT calculations and ab initio-MD simulations.We found that the addition of hierarchically porous LiF led to the formation of a solid electrolyte interface(SEI)layer with higher fluorine content(>30%)on the lithium metal surface.This is due to the high specific surface area of LiF exposing a large number of highly active LiF(111)crystalline,which are able to increase the amount of the solvated LiF in the electrolyte by strong interactions with solvent molecules in the electrolyte.This allows the lithium metal anode to cycle very stably even at a high current density(4 mA cm-2)and a high capacity(4 mAh cm-2).Through experiments combined with theoretical calculations,we unravled the mechanism of the new LiF additive’s high-efficiency effect on lithium metal anodes.3.To futher improve the safety of lithium metal batteries,we modified the hierarchically porous LiF with hexamethylphosphoric acid(HMPA).This modification retains the advantages of LiF while bringing new functions,such as enhancing the overall safety of the battery by making the electrolyte flame retardant.The abundance of P-N and P-O bonds on the surface of LiF-HMPA can effectively block the combustion reaction of H· and OH· radicals.This surface modification enhances the stability of the lithium metal anode without hindering the lithium ion conductivity in the electrolyte.This functionalized modification of the LiF additive surface provides a new solution for enhancing the safety of lithium metal batteries.