Chloride‐Free Electrolytes for High‐Voltage Magnesium Metal Batteries: Challenges, Strategies, and Perspectives

Abstract

The demand for high‐energy‐density and safe energy storage devices has spurred increasing interest in high‐voltage rechargeable magnesium batteries (RMB). As electrolytes are the bridge connecting the cathode and anode materials, the development of high‐voltage electrolytes is the key factor in realizing high‐voltage RMBs. This concept presents an overview of three chloride‐free electrolyte systems with wide electrochemical windows, together with the degradation mechanisms and modification strategies at the anode/electrolyte interphase. Finally, future directions in stabilizing Mg anodes and realizing high‐voltage RMBs are highlighted.

Revealing the Structural Evolution of Electrode/Electrolyte Interphase Formation during Magnesium Plating and Stripping with operando EQCM‐D

“…[9,10] Besides other non-nucleophilic salts, Mg(TFSI) 2based electrolytes gained increasing attention within the last years as Mg(TFSI) 2 is commercially available and enables Mg deposition. But the overpotentials for Mg stripping are very high and there are signs that the TFSI anion is not stable towards Mg. [11][12][13] Especially the reduced Mg(I)TFSI shows instabilities and is susceptible to bond cleavage. [12,14,15] The TFSI salt is often used in combination with the chloride salt which leads to higher Coulombic efficiencies and lower overpotentials for plating and stripping.…”

“…[8,12] A better understanding is crucial to rationally design future electrolyte systems leading to higher cycling stability and higher Coulombic efficiencies. [13,18,30] One operando technique which is capable of revealing information about the existence and structural evolution of possibly formed interphases at the electrode/electrolyte interface is the electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D). [31][32][33][34][35] By recording the resonance frequencies f n (at multiple overtone orders n) of a metalcoated quartz crystal, which acts as the working electrode at the same time, it is possible to measure mass changes ~m of the electrode simultaneously to the electrochemical information.…”

“…This means that there is a lack of suitable operando investigations on the evolution of interphase layer formation during cycling [8,12] . A better understanding is crucial to rationally design future electrolyte systems leading to higher cycling stability and higher Coulombic efficiencies [13,18,30] …”

Borohydride-Based Interphase Enabling Reversible Magnesium Metal Anode in Conventional Electrolytes

“…Despite high capacity and reactivity endowed by the divalent electrochemistry and relatively low redox potential (−2.37 V vs SHE), 3−7 these features normally lead to the ready formation of undesirable solvation configuration and passivation layer in simple ionic salt electrolytes [e.g., Mg(TFSI) 2 ] (TFSI, bis(trifluoromethanesulfonyl)imide). 8,9 Additionally, the investigation surrounding Mg diffusion and growth kinetics theory 10−12 under specific electrolytes (e.g., all-phenyl complex, APC) or current thresholds 11,13,14 has also cast a veil over the dendrite-free characteristic of Mg anode. Specifically, the above challenges are interrelated and primarily concerned with the Mg/electrolyte interfaces, including in situ and catalytic reduction of electrolyte components (anions and solvents) with the inevitable impurities (like H 2 O, etc.…”

The Effect of Chlorides on the Performance of DME/Mg[B(HFIP)4]2 Solutions for Rechargeable Mg Batteries

“…10 To avoid those issues a lot of recent research has also focused on chloride-free electrolyte systems for Mg batteries. 11 Thereby, the most promising solution that is capable of reversible Mg deposition at room temperature was found to be magnesium tetrakis(hexafluoroisopropyloxy)borate (Mg[B(HFIP) 4 ] 2 ) in dimethoxyethane (DME) solution. 12 Yet, a special pre-treatment is required in order to bring these solutions to any practical importance.…”