Electrochemical Limits of Electrolytes for Mg-Based Batteries Containing MgxClyz+-Complexes

Summary:

The high capacity of Mg metal (2205 mAh g−1; 3833 mAh cm−3), its low standard potential of −2.36 V vs. SHE (Mg|Mg2+), as well as its high abundancy (Li: 18 ppm, Mg: 2.2% in the earth crust1) and resulting low costs render Mg-metal based batteries great alternatives to state-of-the-art Li ion batteries (LIB).2 Further on, the reduced tendency of Mg metal to form high surface area Mg during electrodeposition compared to Li metal, might enable batteries with higher safety.2 Still, rechargeable Mg metal batteries face many challenges which inhibits their commercial application. While graphite and Li metal form an ion-conductive but electronically passivating solid electrolyte interphase (SEI) upon reaction with the electrolyte, the high charge of Mg2+ reduces its mobility in solids, leading to a surface layer on Mg metal upon contact with most organic solvents which is not only electronically, but also ionically non-conductive. To avoid passivation of the Mg metal anode, electrolytes with a low reactivity towards Mg metal such as ether-based systems are therefore mandatory to allow for a (blocking) layer-free Mg surface. Since usage of simple Mg salts such as Mg(TFSI)2 in ethers leads to high overpotentials of Mg electrodeposition and dissolution, complex electrolytes, for example based on in situ formed cationic [MgxCly]z+-complexes are needed.2, 3 These electrolytes, however, have limited electrochemical stabilities (