Construction Of In Situ SEI Membrane Induced By Amino Acids And Its Application In Lithium Anode

Under the background of the "dual carbon" target,it has become a global consensus to vigorously develop renewable energy to replace traditional fossil fuels.Lithium-ion batteries have also developed at an astonishing rate in the past 30 years.However,due to the limitation of theoretical capacity（372 mAh g-1）,it may be difficult to meet the needs of future social development with the increasing energy demand of human society.Therefore,it is urgent to study and explore the battery system with higher energy density.Lithium metal has an extremely high specific capacity（3861 mAh g-1,more than 10 times higher than graphite）and the lowest chemical potential（3.04 V vs.Standard hydrogen electrode）,is the ideal anode material.However,the growth of lithium dendrite and low coulombic efficiency seriously hinder its practical application in terms of circularity and safety.Both the growth of lithium dendrite and coulomb efficiency are affected by the unstable solid electrolyte interface film（SEI）formed on the surface of lithium metal.For lithium anode,SEI film is very important to avoid continuous side reactions and regulate the nucleation and growth of lithium metal.However,the structure of SEI film formed spontaneously is uneven and easily broken,which will lead to large volume changes and rapid consumption of lithium and electrolyte in the cycle process,seriously reducing the performance of lithium metal battery.Therefore,it is very important to design a uniform and stable SEI film during charging and discharging.Based on this,in this paper,an artificial solid electrolyte interface protective film was constructed by in-situ electropolymerization of biological small molecule amino acids on the surface of lithium metal to stabilize the lithium metal/electrolyte interface and achieve uniform lithium deposition,thus inhibiting the growth of lithium dendrites and improving the electrochemical performance of lithium metal batteries.（1）In order to improve the cyclic stability and coulomb efficiency of lithium metal battery,glutamate（Glu）is introduced into ether electrolyte as electrolyte additive,and a net protective layer of polyglutamate is formed to stabilize the lithium metal anode by in-situ electropolymerization of glutamate on the surface of lithium metal.The polar groups（-COOH and-OH）on glutamate surface can reduce the nucleation overpotential of lithium ions,uniformly deposit lithium ions,inhibit the disordered growth of lithium dendrites,and significantly improve the stability of lithium metal anode in the cycle process.Introducing 2 wt%Glu to 1 M lithium bis（trifluoromethanesulfosulfonyl）imide/1,3-dioxane/dimethoxyethane electrolyte（LiTFSI DOL/DME）at 0.5 mA cm-2 current density,the Li-Li symmetrical cell cycle is stable for 500 h.At the current density of 1 mA cm-2,Li-Li symmetric cells can cycle stably for 400 h.After Glu was introduced,Li-LTO cell cycles exceeded 400 times and capacity retention rate reached 96.2%.The results show that the introduction of Glu can effectively enhance the cycle stability of lithium metal battery and improve its coulomb efficiency.（2）In order to inhibit the disordered deposition of lithium dendrites and the decomposition of lithium salt LiTFSI,aspartic acid（Asp）was introduced as the electrolyte additive,which can form porous polyaspartic acid film on the surface of lithium metal by in-situ electropolymerization,enrich the polar groups（-OH,-COOH）on the surface of lithium metal,and increase the lithium philicity of the surface.The nucleation overpotential in the deposition process is reduced,which makes the continuous nucleation of lithium ions easier and inhibits the disordered growth of lithium dendrites.XPS confirmed that the introduction of Asp inhibited the decomposition of lithium salt（LiTFSI）,increased the contents of inorganic components Li3N and LiF,optimized the composition of SEI membrane,and improved the stability of SEI membrane.In 1 M LiTFSI-DOL/DME electrolyte containing 2 wt%Asp,the stability of the lithium anode was significantly improved.At a current density of 0.5 mA cm-2,Li-Li symmetric cells can cycle stably for nearly 700 h.At a high current density of 3 mA cm-2,Li-Li symmetric cells can cycle stably for 1000 h.Through the introduction of Asp additive,Li-LTO battery rate performance was significantly improved.The capacity retention rate of Li-LTO cell reaches 99.5%after 3000 cycles at 2 C rate.The results show that the introduction of Asp can effectively inhibit the decomposition of lithium salt（LiTFSI）,alleviate the disordered deposition of lithium dendrite,and greatly improve the electrochemical performance of lithium metal anode.（3）In order to further improve the coulomb efficiency of lithium metal anode and broaden the oxidation decomposition voltage of ether electrolyte,tyrosine（L-Tyr）was introduced as electrolyte additive.Introduction of 2 wt%L-Tyr into conventional DOL/DME electrolyte as an additive can effectively alleviate the disorder deposition of lithium dendrite and brittleness of SEI film.L-Tyr can be electrically polymerized in situ on the anode of lithium metal to form ordered lamellar.The polar groups（-OH,COOH）on the surface of L-Tyr can reduce the nuclear potential of lithium ions and inhibit the growth of lithium dendrites.The carboxyl group on L-Tyr can form hydrogen bond with DME,which reduces the content of free DME in the electrolyte and makes more TFSI-reach the electrode surface,promoting the formation of SEI film dominated by electrolyte anions and increasing the content of LiF in,SEI film.After the introduction of Tyr,the Li-Li symmetric battery cycle stably at 0.5 mA cm-2 for 450 h,and the overpotential is～42 mV during the whole cycle.When the electrolyte concentration was 2 wt%Tyr,li-Cu cells were cycled more than 200 times at a current density of 0.5 mA cm-2,with an average coulomb efficiency of 93.1%（77.6%for blank commercial electrolyte）.With the introduction of Tyr,Li-LTO battery showed excellent cycle performance,maintaining a specific capacity of 119 mAh g-1 after a steady cycle of 5000 times at 5 C rate.More importantly,the introduction of L-Tyr increases the oxidation decomposition voltage of the electrolyte to 4.7 V,which broades the application range of ether electrolyte and enables it to be applied to the high voltage lithium metal battery system.The results show that the introduction of Tyr can effectively enhance the oxidation resistance of ether electrolyte and greatly improve the coulomb efficiency of lithium metal anode.