Rechargeable Mg–Na and Mg–K hybrid batteries based on a low-defect Co3[Co(CN)6]2 nanocube cathode

Rechargeable Mg-Li hybrid batteries have the merits of a Mg metal anode and a Li+ intercalation cathode, but the limited availability of Li resources hinders their large-scale application. Herein, rechargeable Mg-Na and Mg-K hybrid batteries are constructed with a low-defect Co3[Co(CN)6]2 nanocube cathode. The large redox active cavities of Co3[Co(CN)6]2 are highly favorable for reversible Na+ and K+ intercalations. The Mg-Na and Mg-K hybrid batteries provide reversible capacities of 123 and 126 mA h g-1, respectively, and a stable long-term cycling over 600 cycles. By contrast, the Mg-Li hybrid battery delivers a much lower capacity of 55 mA h g-1. It is also demonstrated that controlled crystallization of well-defined nanocubes with few lattice defects is important for these high-capacity Mg-Na and Mg-K hybrid battery cathodes. The low-defect Co3[Co(CN)6]2 nanocube cathode shows good rate capability, providing specific capacities of 45 and 53 mA h g-1 at 1000 mA g-1 in Mg-Na and Mg-K hybrid batteries, respectively. This study illustrates that low-defect Prussian blue analogs of Co3[Co(CN)6]2 are highly compatible and promising cathodes for Mg-Na and Mg-K hybrid batteries. The present study also provides a new method for the development of low-cost and high-performance batteries based on Mg metal anodes.