Strategic Design of Highly Concentrated Electrolyte Solutions for Mg ²⁺ /Li ⁺ Dual-Salt Hybrid Batteries

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 LiAlCl ₄ complex (LACC) and LiN(SO ₂ CF ₃ ) 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/Mg ²⁺ ). 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 Al ³⁺ complex and metallic Mg induced by these processes resulted in the generation of Mg ²⁺ 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 FePO ₄ cathode at 1.5 mg cm ^-2 loading density, when using an electrolyte volume of only 25.5 μL cm ^-2 .