(Keynote) Updates in Non-Aqueous Mg Electrochemistry, Biu Corner.
Authors: Aurbach Doron 
Published: February 2020

Developing practical rechargeable magnesium batteries (RMB) is very attractive. The first rechargeable magnesium battery prototype was demonstrated almost two decade ago. It comprised magnesium metal anode, Mo6S8 Chevrel phase cathode, and complex ethereal solutions that included THF or glymes and ionic organo-metallic species formed by complicated reactions between a RnMgCl2-n Lewis base and an AlClnR3-n Lewis acid (R = alkyl or aryl groups). These prototypes exhibited very prolonged cycle life at 100% cycling efficiency (no side reactions). However the low abundance of molybdenum and the low energy density (1.1V, 120mAh/g cathode capacity) avoided any practical development of these first systems. Unfortunately, we are not aware of any practically promising RMB that that could show better electrochemical performance that the 19 years old first prototype. A main challenge related to Mg anodes is that they cannot behave reversibly if they are covered by surface films (no relevance for the SEI model). This challenge was met quite well over the years, since several families of solutions where developed, possessing high anodic stability, in which Mg metal anodes are surface films free, behave quite reversibly. Matching appropriate, attractive enough cathodes to the reversible Mg/electrolyte solutions couples remained the big, unresolved yet, challenge. Hence, the main difficulty in the realization of RMB starts from the tendency of the reactive Mg anodes to form surface films on their surface that block reversible Mg ions migration through them. Therefore, RMB have to operate in a non-reducing environment, namely ethereal solutions and Mg salts which anions are not reduced by Mg metal to form passivating surface species. These limitations reduce considerably the choice of relevant cathode materials for high energy density RMB. These constraints force us to concentrate in recent years in fundamental aspects of non-aqueous magnesium electrochemistry. MgTFSI2 is currently one of the only readily ether soluble salts which anion is not readily reduced by Mg metal. However the electrochemical performance of ethers/MgTFSI2 is poor. Surprisingly we proved that in DME (1,2 di methoxy ethane), this salt fully disassociates and forms 3DME-Mg2+ cations with a rigid structure.1 These solutions exhibit a very unique thermodynamic behavior as will be described. The solvent-solvate bonds are too strong to allow reversible Mg deposition in DME/MgTFSI2 solutions. Adding MgCl2 to these solutions changes there structure and improves remarkably their electrochemical performance.2 The main ionic species therein are [Mg2(μ-Cl)2(DME)4] 2+ cations and completely un-coordinate 2TFSI- anions for the 1:1 MgCl2:MgTFSI2 ratio, and [Mg3(μ-Cl)4(DME)5] 2+ cations and un-coordinate 2TFSI- anions for the 1:2 MgCl2:MgTFSI2 ratio. The formation of "softer" complex cations in the presence of MgCl2 is the reason for the improved electrochemical activity.3 Turning to the cathode side - V2O5 is the most studied cathode material in connection to Mg batteries mainly due to its high theoretical specific capacity, of 441 mAh g1 (for 3e-/unit discharge) and its proven capability to intercalate Mg ions. However the vast majority of studies could not meet need to couple to this important cathode material electrolyte solutions in which Mg electrodes behave reversibly. We demonstrated that although Mg ions intercalation into V2O5 electrodes occurs from acetonitrile solutions (not suitable for Mg anodes), addition of an ether solvent such as DME to these solutions dramatically slows down the Mg ions insertion kinetics into V2O5 electrodes due to strong solvent-cation interactions.
4 Another important cathode material that was explored in connection with RMB is Sulfur. This cathode material is very attractive in terms of capacity and cost, thus a lot of efforts are invested in developing Mg-S batteries. We have shown that polysulfide species dissolution during discharge of sulfur cathodes in Mg-S cells leads to a severe blockage of the Mg anodes Due to formation of surface films comprising Mg sulfides moieties. Therefore in any practical Ng-S batteries, avoiding polysulfide species dissolution is mandatory.4

References:

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