Study On Artificial Solid Electrolyte Interphase Formed By Controlled Electrochemical Reduction Of LiTFSI/DME-LiNO₃ And Its Application In Lithium Metal Anode

With the increasing demand for batteries with high energy density and high power density,lithium?Li?metal anode has attracted widespread attention and has been hailed as the"Holy Grail"and"the ultimate anode material"in anode materials due to its ultra-high theoretical specific capacity(3860 mAh g^-1),the most negative electrode potential?-3.044 V vs SHE?,the smallest molar mass(6.938 g mol^-1)and extremely low density(0.59 g cm^-3).Unfortunately,the Li dendrite growth and low Coulombic efficiency?CE?are the key issues which still hinder the Li metal anode in commercial application.Both of these problems are closely related to the naturally formed solid electrolyte interphase?NSEI?film between Li anode and electrolyte for that the NSEI film is fragile,inhomogeneous and instable.On the one hand,the inhomogeneities and poor mechanical properties of the NSEI film lead to nucleation and growth of Li dendrites,eventually causing battery failure.On the other hand,the NSEI film is continuously destroyed and reconstructed during the long-term cycling,resulting in continuous consumption of Li and electrolyte and increase in battery impedance,and thereby reducing the CE of the battery.Therefore,it is one of the most effective methods to solve these problems by constructing an artificial solid electrolyte interphase?ASEI?with excellent properties on the surface of the electrode before assembly of the battery.Based on the above ideas,here we create an ASEI film on non-lithiumsubstratebycontrolledelectrochemicalreductionof LiTFSI/DME-LiNO₃ electrolyte,which can simultaneously inhibit dendrite formation and greatly improve electrochemical performance of Li metal anode.The main contents of this work are as follows:1.The electrochemical reduction characteristics of the components in LiTFSI/DME-LiNO₃ electrolyte and the formation mechanism of ASEI are investigated by LSV method.The reduction peak at 1.71 V?vs Li⁺/Li?on the LSV curves is correlated to the reduction ofLiNO₃;the one at 1.17 V is ascribed to the reduction of LiTFSI,and the reduction product ofLiNO₃ contributes to the complete decomposition ofTFSI^-ion;the one at 0.67 V is caused by the reduction of DME;when the potential is?0.30 V,there is no Li metal deposit on the Cu electrode.In addition,the preparation conditions of ASEI are optimized according to its electrochemical characterization.It is found that by galvanostatic polarization of Cu electrode in1 mol L^-1LiTFSI/DME+2 wt.%LiNO₃ electrolyte at0.015 mA cm^-2from open circuit potential?OCP?to 0.30 V at 25°C,the formed ASEI film achieves the best performance.2.The electrochemical properties,morphology,composition and composition distribution of ASEI prepared under the optimal conditions are characterized,and the mechanism of ASEI is discussed by combining the characterization results.Electrochemical characterization demonstrates that the ASEI has low self-impedance,excellent cycle stability and high Li⁺ion conductivity,which can effectively inhibit the growth of Li dendrites and significantly improve the electrochemical performance of Li metal anode.Under the protection of ASEI,the Li|Cu cell can be stably cycled for over 500 cycles with an average CE of 98.0%.The SEM observation proves that ASEI can help to improve the morphology of Li deposits,which can make Li deposits uniform,compact,dense and free of Li dendrites.At the same time,ASEI itself can maintain a uniform and intact morphology for a long time.The XPS analysis indicates that the ASEI with an average thickness of 40 nm shows a dual-layered structure:The inner layer contains abundant inorganic species?LiF,Li₂O,Li₃N and Li₂S?with LiF as the major component,and the outer layer is mainly composed of organic components?lithium alkylcarbonate and-?7?CH ₂CH₂ O?8?_n-?.The composition and composition distribution of the ASEI are well optimized,so that it can not only provide sufficient flexibility and elasticity to accommodate volume changes caused by Li deposition/stripping,but also effectively guide the uniform nucleation/growth of Li and inhibit the growth of Li dendrites.3.The ASEI is further applied to Li metal anode of Li-S full cell.It is found that the Li-S cell has higher discharge specific capacity,higher CE and better rate performance under the protection of ASEI,which further proves that ASEI has significant protective effect on Li metal anode.