Li-salt mediated Mg-rhodizonate batteries based on ultra-large cathode grains enabled by K-ion pillaring

Mg-based batteries modulated by Li-salt additive can bypass the sluggish Mg solid diffusion at cathode side by kinetically better Li intercalation. However for this dual-salt system the highly electroactive cathodes endowed with high intrinsic conductivity and multi-electron transfer are still lacking, especially under large-sized grains and their high loading. Here a rhodizonate organic salt pre-pillared by K-ions (K 2 C 6 O 6 ) is proposed as high-rate cathode for Li-salt mediated Mg batteries. Under a high loading of 70 wt%, well-crystallized K 2 C 6 O 6 composed of ultra-large micro-sized grains (up to 30 μm) shows a reversible capacity more than 300 mA h/g at 0.1 C (based on three electron transfer), and up to 200, 130 and 60 mA h/g at 2 C, 5 C and 10 C respectively. The high bulk electron/ion conductivity and low solubility are responsible for such a superior performance based on fast Li-K electrochemical exchange and highly reversible Li intercalation without the assistance of nano-sizing, as a consequence of optimization and stabilization of desired electron structure and open framework by K-ion pre-supporting (instead of Li/Na-pillaring) and Mg-ion trapping between C 6 O 6 2- layers. These large monoliths of K 2 C 6 O 6 do not undergo serious grain fracture after long-term cycling, and at that time the Mg anode still remains compact and dendrite-free.