Failure Mechanism Of Magnesium Metal Anode- Electrolyte Interphase And Strategy For Improving Performance Of Electrolyte

Lithium ion batteries are widespread used in the fields of portable devices,electric vehicles,and so on.Their energy density and security,however,cannot keep pace with the development of society.Rechargeable magnesium（Mg）batteries,as one of the advanced energy storage technologies,possess the advantages of high theoretical energy density,good security,and resource-rich.But the development of rechargeable Mg battery is still in infant stage.Magnesium（II）bis（trifluoromethanesulfonyl）imide（Mg（TFSI）₂）/ether electrolytes usually show the strengths of high ionic conductivity and wide electrochemical windows.But the structure of Mg metal anode-electrolyte interphase in this case is complex,the passivating mechanism is not clear yet.Besides,Mg（TFSI）₂/ether electrolytes suffer from the high Mg plating/stripping overpotential（above 1.5 V）and low Coulombic efficiency（below 20%）.These issues hinder the further development of rechargeable Mg batteries.This work reveals that the formation of H₂O molecules competitively solvating Mg-ion solvation structures and their decomposition contribute to the Mg metal anode-electrolyte passivation.The Mg plating/stripping performance of Mg（TFSI）₂/ether electrolytes could be greatly improved by introducing isobutylamine–di-n-butylmagnesium or isobutylamine–cesium（I）bis（trifluoromethanesulfonyl）imide additives.The formulated electrolytes deliver extra-low overpotential below 0.2 V and high Coulombic efficiency nearly 90%.The details are as followed.（1）The investigation on the failure mechanism of Mg metal anode–electrolyte interphase.Mg（TFSI）₂ reagent and ether solvents have the strong hygroscopicity,which results in the electrolytes contain trace moisture from tens to thousands ppm,based on the Karl Fischer moisture titrator tests.The presence of H₂O molecules causes the side reaction between Mg metal and H₂O molecules to from passivating species,such as Mg（OH）₂ and Mg O,on electrode surface.Indeed,the existence form of H₂O molecules plays an important role in the reaction process.This work begins by the interaction among Mg salts,solvents and H₂O molecules,researches the evolution and decomposition of Mg ion solvation structures formed by the competitive coordination of H₂O molecules.This competitive coordination behavior of H₂O molecules not only results in the preferential decomposition of H₂O molecules and passivating the electrodes,but also forms the neutral Mg ion solvation structures which reduces the ion transference number in electrolyte and further causes the dendritic growth of Mg deposits.The morphology,phase and element valence of Mg dendrite have been detected by the cryogenic electron microscopy,X-ray photoelectron spectroscopy,and other technologies.The total mosaic-type model of Mg deposits has been built.Inspired from the above failure mechanism,the improved electrochemical performance is achieved by regulating the Mg ion solvation structures by introducing di-n-butylmagnesium and isobutylamine additives.（2）The investigation on the strategy for improving electrochemical performance of electrolytes.Previous reports reveal that the Mg ion solvation structures in Mg（TFSI）₂/ether electrolyte can be regulated by introducing amine solvents,and the energy barrier during charge transfer process could be greatly decreased,which further results in the reduced Mg plating/stripping overpotential.The influence of ionization/association behavior of amine solvents on Mg ion solvation structure and Mg plating/stripping performance,however,is not fully understood and researched.Our work discards the traditional idea that the amine in electrolytes is only existed as the form of molecules,and proposes the mechanism of action for amine solvents on active species in electrolytes based on the amine-based anions and cations formed by the autoionization.On the one hand,amine-based anions have the stronger interaction with Mg ions compared with amine molecules.It benefits the formation of active[Mg²⁺-（R-NH^–）]⁺species rather than[Mg²⁺-TFSI^–]⁺,which suppresses the decomposition of TFSI anion.On the other hand,amine-based cations could interact with TFSI anions and ether molecules,which weakens the interaction between them and Mg ions and further alleviates the decomposition of TFSI anions and ether molecules.Owing to above effects,the passivating process on Mg metal anode–electrolyte interphase is greatly suppressed.The extra-low Mg plating/stripping overpotential（below 0.2 V）and high Coulombic efficiency（nearly 90%）are achieved.The cycling span life is also prolonged by the cesium ion electrostatic shielding strategy.