Study On Modification Of Lithium Anodes For Lithium Secondary Batteries

Lithium has long been regarded as the most promising anode for next-generation lithium battery,which has a very high theoretical specific capacity(3860 mAh/g)and the most negative electrode potential(-3.04 V vs.standard hydrogen electrode).However,the formation of lithium dendrites during the cycling and the reaction between lithium metal with the electrolyte,resulting in safety hazards and poor stability of the lithium anode,severely limiting the application of lithium anode.To settle above problems,this paper propose two modification methods,including engineering of the electrolyte and the structure of the lithium anode,to inhibit the formation of lithium dendrites and improve the safety and stability lithium metal secondary battery.For the engineering of the electrolyte solution,the use of the additive tris(2,2,2-trifluoroethyl)borate(TTFEB)in the carbonate-based electrolyte improves the properties of the electrolyte and the lithium/electrolyte interface properties.On the one hand,the coupling of the boron atoms of the TTFEB and the anions in the electrolyte to promotes the dissociation of the lithium salt,and the lithium ion transference number of the electrolyte is also increased,which promoting the migration of the Li+ in the electrolyte.As a consequence,the uniform distribution of the Li+ concentration on the surface of the anode suppresses the generation of lithium dendrites.On the other hand,the theoretical calculation and electrochemical test results show that TTFEB has a higher reduction potential than the solvents,which can be preferentially decompose on the surface of lithium anode to form LiF-rich SEI film,improve the stability of SEI film,and inhibit lithium dendrites.After adding 2% TTFEB,the cycling efficiency of Li|Cu cell is increased from 90% to 96%,and the cycle life of Li|Li symmetrical battery is increased to more than 1000 h.Moreover,the interfacial impedance is smaller and more stable.The SEM results show that TTFEB can inhibit the formation of lithium dendrites during cycling.In the study of components of SEI,it is confirmed that TTFEB can form LiFrich SEI film and inhibit the reduction and decomposition of electrolyte.When the 2% TTFEB was applied in the lithium secondary battery,the cycling stability and rate performance of Li|LiFePO4 cells were significantly improved,and the pulverization of the lithium after long cycling was inhibited.In the aspect of modification of the structure of lithium anode,a small amount of In is incorporated into the lithium by vacuum thermal deposition method to prepare Li-In alloy,and the lithium was replaced by Li-In alloy as the anode material for the lithium secondary battery.The electrochemical performance of the lithium-rich Li-In alloy is the best when the In content in the evaporation source is 20% and the output power is 95 W.The cycle stability is improved significantly compared with that of the Li electrode.The cycle life in Li|LiFePO4 cells can be increased from 140 to more than 300 times,the interface impedance stability and cycling stability of the symmetrical cell are significantly improved,and the reversibility of lithium dissolution/deposition is improved obviously.The Li-In electrode has a similar rate performance and theoretical specific capacity compare to that of the Li electrode.Furthermore,the prepared Li-In alloy anode is also compatible well with the lithium-sulfur battery and the all-solid-state lithium secondary battery.The mechanism of Li-In alloy to improve the cycling stability of lithium electrode was studied by characterization and analysis of electrode structure,morphology and surface film composition.The results show that In is involved in the formation of porous conductive skeleton,which can inhibit the formation of lithium dendrites during the cycling and inhibit the pulverization caused by the drastic changes of the volume of electrode.At the same time,the reactivity of the lithium electrode and the electrolyte is reduced and the stability of the SEI film improve,thereby reducing the consumption of active lithium,significantly enhance the lithium electrode cycle stability.