Performance Study Of Magnesium Sulfur Battery Based On Interface Modified Magnesium Anode And Copper Modified Sulfur Cathode

Mg-S battery adopts sulfur as cathode with high theoretical specific capacity and low cost,and magnesium as negative electrode with low electrode potential,high specific capacity and good safety.This system can not only provide high theoretical energy density(1722 Wh kg^-1 or 3221 Wh L^-1),but also has the advantages of good safety and low cost.Therefore,the Mg-S battery has an extensive application prospect in the field of power and energy storage battery.The electrolyte is crucial to the development of Mg-S battery,and the Mg（TFSI）₂-based electrolytes have the advantages of good stability,high conductivity,wide electrochemical window,strong solvation ability and compatibility with sulfur cathode.Aiming at solving the problems of high polarization and poor reversibility of Mg-S battery in Mg（TFSI）₂-based electrolytes,this study mainly carries out the basic and application research on interface regulation for Mg electrode,electrolyte optimization and sulfur electrode design,in order to clarify action mechanism and improve the electrochemical performance of the battery.The Mg electrode faces the severe interface passivation in the Mg（TFSI）₂-based electrolyte,which leads to high Mg plating-stripping overpotentials and voltage hysteresis.The interface modification of Mg electrode is realized by the sequential reaction with H₃PO₄ and Si Cl₄.It is found that the interface film formed on Mg electrode is mainly composed of Mg Cl₂ and silicon phosphate compounds,and the compatibility between Mg（TFSI）₂-based electrolyte and Mg metal electrode can be improved.The interface impedance of the modified Mg electrode decreases from 10⁶Ωto 81Ω.At the current density of 0.1 m A cm^-2,the polarization volatage of the Mg symmetric battery decreases from 2.0 V to 0.25 V,and the stable cycling of 700 times of the battery indicates good interface stability.The discharge voltage platform of Mg-S battery using the modified Mg as anode in Mg（TFSI）₂-based electrolyte increases from 0.5 V to 1.5 V at 0.1 C,and the voltage hysteresis is significantly decreased.At the discharge cut-off voltage of 0.5 V,the specific discharge capacity of the battery can reach 1006 m Ah g^-1.The interface modification of Mg electrode is realized by the ion replacement reaction between CuCl₂ and Mg,and the introduction of lithium salt（Li TFSI）into Mg（TFSI）₂-based electrolyte further modify the interface properties.At the same time,the use of Cu mesh between cathode and separator can improve the cycling stability of Mg-S battery.It is found that the artificial interphase on the surface of Mg electrode is composed of CuCl/Cu/Mg Cl₂,and the Mg symmetric battery based on the modified Mg electrode shows low polarization volatage（0.25V）and interface impedance（48Ω）in 0.5 mol/L Mg（TFSI）₂/DGM electrolyte,as well as stable cycling of 800 times.When lithium salt is introduced into the electrolyte,the polarization volatage and interface impedance further decreases to 0.2 V and 7Ω,respectively.Using Cu mesh contributes to excellent electrochemical performance.At 0.1 C,the initial discharge specific capacity can reach 1500 m Ahg^-1,and the voltage hysteresis is only 0.39 V.After 50 cycles,the capacity can still maintain at 1371 m Ahg^-1.The poor reversibility of sulfur electrode in Mg-S battery leads to the rapid attenuation of capacity.The use of Cu metal can significantly improve the cycling stability of the battery,but the action mechanism is unclear.In addition to the interface modification of Mg electrode by virtue of substitution reaction between Zn Cl₂ and Mg,the mixed cathode of Cu powder and S/C composite material was prepared.The effect of Cu powder on the electrochemical performance of Mg-S battery is explored,and the reaction mechanism was deeply analyzed.The modified Mg electrode can show stable 1000 cycles with low polarization volatage（0.2 V）at 0.1 m A cm^-2.Combined with the application of Cu powders,the electrochemical performance of Mg-S battery is significantly improved,which is directly related to the added amount of Cu powder and particle size.By virtue of various characterization methods,the action mechanism of Cu is clarified.The results reveal that the chemical reaction can occur between Cu and intermediates Mg S₈at the initial stage of discharge process,and the reaction product of Cu₂S continues to participate in the electrochemical reduction reaction to regenerate Cu,and then the Cu can be re-oxidized to Cu₂S during the charging process.The change of redox couple from S/S^2-to Cu₂S/Cu⁰ completed during the cycling leading to the improvement in cycling stability of battery.