Fast kinetics of Mg²⁺/Li⁺ hybrid ions in a polyanion Li₃V₂(PO₄)₃ cathode in a wide temperature range

Magnesium ion batteries (MIBs) have attracted significant research attention owing to their low cost, high energy density, and the natural abundance of magnesium. However, lack of compatible cathode materials hinders their further development. Herein, we present a magnesium–lithium hybrid ion battery (MLIB) comprising a Li₃V₂(PO₄)₃ (LVP) cathode and magnesium metal anode, where fast reaction kinetics of Li⁺ ions at the LVP cathode and that of Mg²⁺ ions at the anode led to high reversible capacities. As a result, a rechargeable MLIB shows excellent rate performance (147.8 and 65.3 mA h g⁻¹ at 50 and 2500 mA g⁻¹ respectively) and capacity retention (99% for 200 cycles), which are the highest values among the reported hybrid batteries using lithium cathodes. To improve temperature adaptability of the designed MLIB, two different kinds of magnesium electrolytes (i.e. all-phenyl-complex and magnesium borohydride in diglyme) and inorganic lithium additives were investigated. It is found that the all-phenyl-complex along with LiCl additives can suppress the freezing of electrolytes effectively even at −40 °C. While, high reversible capacities of 117, 93.4, and 63.1 mA h g⁻¹ can be obtained at 0, −20, and −40 °C respectively, at a current density of 100 mA g⁻¹, showing a very promising prospect for low temperature applications.