Study On The Cycle Performance Of Lithium Metal Anode For Rechargeable Lithium Battery

The growing demand for high-energy-density lithium(Li)batteries motivates the extensive research for higher-capacity electrode materials.Among them,Li metal is considered as the ultimate anode due to its ultra-high specific capacity(3860 mA h g-1)and the lowest electrochemical potential(-3.04 V vs SHE).However,during the repeated plating/stripping process of lithium metal anodes(LMAs),there are many serious issues including the uncontrollable growth of Li dendrites,the continuous side reactions between Li metal and related electrolytes used and the huge volume change,seriously hindering their commercial development.Therefore,modification of LMAs has been focused on recently.Herein,based on the research ideas of regulating and optimizing the electrolyte compositions,a new fluorinated ester solvent ethyl-(2,2,2-trifluoromethyl)carbonate(abbreviated as ETFEC)was introduced as a co-solvent to improve the cycling performance of LMAs.First of all,theoretical calculation results show that the ETFEC solvent has a lower LUMO energy value than ethylene carbonate(EC)and diethyl carbonate(DEC),which means that the ETFEC molecules will preferentially get electrons from electrode to undergo reductive decomposition and its products are responsible for the film-formation on the surface of LMAs.the Li?Cu cells test results show that when ETFEC completely replaces DEC as co-solvent in the electrolyte(1 M LiFSI/EC-ETFEC,3:7 by volume,denoted as 1 M-70%ETFEC),there shows the best improvement on the cycling performance of LMAs.Combined with EIS,SEM and XPS characterizations,it shows that 1 M-70%ETFEC electrolyte can facilitate the formation of a high Li+conductive and stable SEI film enriched with LiF on the surface of LMAs.This SEI film is able to regulate a uniform and dense Li deposition and effectively suppress the side reactions between electrolyte and Li metal.Compared with the base electrolyte,the LMAs can achieve more stable electrochemical performance in 1 M-70%ETFEC electrolyte.Then,based on the above experimental results,high-concentration Lithium bis(fluorosulfonyl)imide(LiFSI)salt was added to the modified electrolyte discussed above to further optimize the electrolyte formulation.Based on Li?Cu cells results,there exists the best cyclic Coulombic efficiency(CE)when using high concentration up to 5 M(denoted as 5 M70%ETFEC).Combined with XPS,EIS,SS-NMR,EDS mapping,SEM and operando OM results,it can be demonstrated that a SEI film enriched with LiF and inorganic sulfur-containing species was formed on the LMAs by using 5 M-70%ETFEC electrolyte,which shows high Li+conductivity,uniform and stable properties.This SEI film can effectively suppress the serious side reactions and the formation of Li dendrites during cycling,and there almost no dead Li found.Thus,an ultra-high Li plating/stripping CE of 99.1%can be achieved in Li?Cu cells and a superior cycling performance of Li?LiFePO4 full cells can also be obtained even a limited Li metal(5 mA h cm-2)involved in 5 M-70%ETFEC electrolyte.Finally,we successfully prepared Li13In3·nLi(n=86.3,36.6)composite anode materials with a uniform composition by a simple hot-melting method.It was found that Li13In3·36.6Li composite anodes will expose a uniform framework composed of particles after Li stripping.When combined with garnet-type solid electrolyte Li6.75La3Zr1.75W0.25O12(LLZWO),Li 13In3·3 6.6Li composite anode obtains more stable Li plating/stripping performance than pure Li metal anode.It is speculated that the framework of the particles in the Li13In3·36.6Li composite anode can effectively control the volume change during the lithium plating/stripping process,thereby improving the electric contact between the solid electrolyte and the electrode.