Study On The Interface Regulation Of Magnesium Electrode And Magnesium Aluminum Chloride Complex Electrolyte In Magnesium Sulfur Battery

Magnesium metal offers a low reduction potential,high volumetric capacity,and high abundance,making it attractive for low-cost,high energy density electrochemical devices.Due to the low barrier of self-diffusion of metallic magnesium,the formation of dendrites during the electrochemical cycle is suppressed,and the safety problems caused by the internal short circuit of the rechargeable metal battery are avoided.The Magnesium sulfur(Mg-S)battery coupled anode Mg and cathode S has the advantages of high energy density,high safety and low cost.Research on magnesium-sulfur batteries is still in its infancy.One of the main obstacles to realizing commercial Mg-S is the poor compatibility between magnesium anode and existing electrolytes.Many solvents,salts and contaminants are readily reduced on the metallic magnesium anode to form a passivating layer that is ionically insulating,preventing reversible Mg stripping/plating.In this paper,by studying the polarization of Mg-S batteries,the existing problems of magnesium electrode/electrolyte interface are studied,and the methods to reduce the polarization on Mg electrode are proposed from the aspects of magnesium anode and electrolyte respectively.The TFSI^- can passivate the Mg electrode in the HMDS-Mg +LiTFSI and Mg (TFSI)₂/TEG electrolytes,resulting in the large overpotential of Mg stripping and the low discharge platform of Mg-S battery.Herein,the polarization on Mg electrode during cycling is analyzed,and it is found that the Mg electrode can be actived by electrochemical cycle.Further,a method of activating Mg-S batteries by cycling at high rate is proposed,which improves the discharge platform of the Mg-S batteries and maintains a high discharge specific capacity of S.The activation mechanism of the in-situ activation method is further explored,and the activation conditions are explored.In order to improve the compatibility between Mg electrode and the electrolytes,a method of preparing artificial SEI layer on the surface of Mg electrode is proposed.The artificial SEI layer is obtained by simply soaking Mg foil in a tetraethylene glycol dimethyl ether(TEG)solution containing LiTFSI and AlCl₃.The artificial SEI layer is found to mitigate Mg passivation in Mg (TFSI)₂/DME electrolytes.This approach can be extended to the cells with Mg (ClO₄)₂/DME and Mg (TFSI)₂/PC electrolytes.By comparing the differences between the artificial SEI layer and the passivated layer in terms of structure and chemical composition,it is found that the artificial SEI layer is rich in MgCl₂,MgF₂ and MgS,which is conducive to the migration of magnesium ions.In addition,there is no TFSI^- addorption on the surface of the Mg electrode containing SEI layer,which is also an important characteristic of the difference between the passivated layer and the artificial SEI layer.The MgCO₃,MgO and Mg(OH)₂ produced by TFSI^-decomposition on the surface of the Mg electrode are the main components that lead to the passivation of the magnesium electrode.In term of electrolyte,the magnesium aluminum chloride complex(MACC)electrolyte has the advantages of low polarization of Mg deposition/stripping and high Coulombic efficiency.However,the MACC electrolyte usually needs nearly one hundred cycles of voltammetry to reduce the overpotential of magnesium deposition and eliminate the co-deposition of aluminum during the cycle.Here,a novel conditioning-free electrolyte is synthesized by the reaction among Mg powder,AlCl₃and Ti Cl₄.The electrolyte can support Mg deposition and stripping for many cycles with a low overpotential,and the non-nucleophilic characteristics make it suitable for Mg-S batteries.The Mg-S battery with the electrolyte delivers a high discharge specific capacity nearly 1000 mAh/g and an impressive cycle life for 100 cycles.To sum up,in order to improve the issues that the electrolyte of Mg-S battery is easy to passivate the Mg electrode,this study focuses on the protection of Mg anode and the development electrolyte that does not passivate Mg electrode.It is suitable for in-situ activation at high rate for the electrolyte containing TFSI^-and AlCl₃,and the method of synthesized an artificial SEI film is effective for other electrolytes.For the bare Mg anode,a novel concept of electrolyte is developed.The above methods effectively improve the compatibility of electrolyte and Mg electrode,which is beneficial to the development of high-energy density Mg-S system energy storage devices.