We have conducted extensive theoretical and experimental investigations to unravel the origin of the electrochemical properties of hybrid Mg(2+)/Li(+) rechargeable batteries at the atomistic and macroscopic levels. By revealing the thermodynamics of Mg(2+) and Li(+) co-insertion into the Mo6S8 cathode host using density functional theory calculations, we show that there is a threshold Li(+) activity for the pristine Mo6S8 cathode to prefer lithiation instead of magnesiation. By precisely controlling the insertion chemistry using a dual-salt electrolyte, we have enabled ultrafast discharge of our battery by achieving 93.6% capacity retention at 20 C and 87.5% at 30 C, respectively, at room temperature.

Catalytic dissolution of Mg metal in THF leads to a highly stable electrolyte for rechargeable magnesium batteries which share many common features with MACC, but does not require an onerous conditioning process. The high Mg to Al ratio in the electrolyte promotes 100% coulombic efficiency for Mg in the first cycle.

The available capacity of Mg hybrid batteries is closely related to the number of charge carriers within electrolyte solutions. Therefore, in this study, a dual-salt composition capable of supplying high Li+ concentration was prepared. A dual-salt electrolyte consisting of a LiAlCl4 complex (LACC) and LiN­(SO2CF3)2 (LiTFSI) was found to be an excellent candidate, providing 2.2 M Li+ concentration along with anodic stability up to 3 V (vs Mg/Mg2+). However, the LACC moiety of the above composition first had to undergo a two-step modification procedure comprising “Mg powder treatment” and “conditioning process” to properly implement Mg deposition and stripping at the Mg anode. Spontaneous substitutions of oxidation states between the anionic Al3+ complex and metallic Mg induced by these processes resulted in the generation of Mg2+ complex species within the LACC solutions. The modified LACC was compatible even with 2 M of LiTFSI, the concentration with which we achieved 150 mA h g–1 capacity of a FePO4 cathode at 1.5 mg cm–2 loading density, when using an electrolyte volume of only 25.5 μL cm–2.

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