Studies On Passivation Behavior Of Anode/Electrolyte Interface And Its Inhibition Method In Magnesiuim-sulfur Batteries

Among all battery anode materials,magnesium metal anode has become the most alternative material for lithium metal anode due to its low standard electrode potential,high specific capacity,abundant reserves,and low cost.Particularly,dendrites rarely grow on their surface during the batteries charging/discharging,making it a high-safety anode material.Among the cathode materials,sulfur has the advantages of high theoretical specific capacity,high safety,non-toxicity,and low cost.Therefore,the Mg/S batteries has irreplaceable advantages in terms of safety,cost and energy density（1722 Wh/kg）.However,the Mg anode is prone to react with the components in organic electrolyte,such as the anions and solvents.As a result,passive film is formed on Mg anode surface,which hinders the migration of magnesium ions and increases the electrochemical polarization of the magnesium anode,resulting in a low discharge voltage platform of the battery.In this paper,the phenomenon of voltage hysteresis caused by the passivation of Mg anode in Mg/S batteries is studied,and two solutions to suppress the Mg anode passivation are proposed from the aspects of electrolyte and anode material.Besides,the change of anode/electrolyte interface and the different passivation inhibition mechanisms are analyzed.The voltage hysteresis of Mg/S batteries seriously affects the discharge voltage platform and energy density of the battery.However,there is a lack of analysis and research on voltage hysteresis currently in the field of Mg/S batteries.Through the three-electrode test,it is analyzed that the cause of the voltage hysteresis is the change in the polarization of the Mg anode.The effects of battery discharge continuity,current density and Cl O₄^-concentration in the electrolyte on the formation and rupture process of the passive film and the voltage hysteresis are further analyzed.The result shows that the formation and rupture of the passive film proceed simultaneously and compete with each other;the rupture rate of the passive film is affected by the current density on the anode;the formation rate of the passive film is affected by the anion concentration in the electrolyte which can passivate Mg anode.Li Br electrolyte additive is introduced to avoid the passivation of Mg anode from the Cl O₄^-in the electrolyte,by regulating the thickness and composition of the passive film.Consequently,the interface film impedance of the Mg/S battery is reduced,and the low-voltage platform stage of the voltage hysteresis is shortened.With regard to the problems that the current magnesium salts in Mg/S batteries are difficult to synthesis and with poor stability,and passive film is easy to formed on the Mg anode surface which need to be polished before use,a novel magnesium salt with the structures of[Mg Cl·TEG]⁺and[Mg Cl·2TEG]⁺（TEG:tetraethylene glycol dimethyl ether）prepared by an in-situ method is proposed for Mg/S batteries.The magnesium salts are synthesized by the chemical reaction between Al Cl₃,In Cl₃and Mg metal or the passive film components on its surface in situ in the batteries.The reaction mechanism is explained by a series of characterization and analysis,and it is proved that the reaction could in situ remove various passive films on the surface of Mg anode,and consequently inhibit its negative effect on the electrochemical performance of Mg anode.The electrolyte exhibits good oxidation stability and reversibility of magnesium deposition/dissolution.In this electrolyte,the polarization of the Mg/Mg symmetric battery is 0.065 V with a current density of 0.2 m A/cm².Besides,the discharge voltage platform of the Mg/S battery is 1.125 V,and the capacity retention rate after 50 cycles reaches 95.44%at 0.05 C.The in-situ electrolyte synthesis method of the novel electrolyte can be applied to various magnesium battery systems with Mg metal anode.Passive film is prone to be formed on the surface of Mg anode in organic electrolytes,resulting in a large polarization on Mg anode.Herein,design principles of a passivation-free magnesium alloy anode are proposed based on this.A Mg-Li alloy with a lithium content of 8.4 wt.%is screened out as a passivation-free anode,which hinders the passivation reaction on the anode in a halogen-free electrolyte by forming an active anode/electrolyte interface state through the substitution reaction between lithium in the alloy and the magnesium ions in the electrolyte.Compared with the Mg/S battery,the electrochemical performance of the Mg-Li alloy/S battery is significantly improved.The interface film impedance reduces by 5 orders of magnitude,the discharge voltage platform increases from 0.3 V to 1.5 V,and the energy density based on the cathode active material（sulfur）increases from 287Wh/kg to 1829 Wh/kg.In addition,the mechanism of passivation inhibition of the Mg-Li alloy anode is elucidated by various characterization and analysis methods,and the electrochemical reaction process of the Mg-Li alloy anode during the battery charging and discharging is clarified.