High-Capacity Mg–Organic Batteries Based on Nanostructured Rhodizonate Salts Activated by Mg–Li Dual-Salt Electrolyte

A magnesium battery is a promising candidate for large-scale transportation and stationary energy storage due to the security, low cost, abundance, and high volumetric energy density of a Mg anode. But there are still some obstacles retarding the wide application of Mg batteries, including poor kinetics of Mg-ion transport in lattices and low theoretical capacity in inorganic frameworks. A Mg–Li dual-salt electrolyte enables kinetic activation by dominant intercalation of Li-ions instead of Mg-ions in cathode lattices without the compromise of a stable Mg anode process. Here we propose a Mg–organic battery based on a renewable rhodizonate salt (<i>e</i>.<i>g</i>., Na₂C₆O₆) activated by a Mg–Li dual-salt electrolyte. The nanostructured organic system can achieve a high reversible capacity of 350–400 mAh/g due to the existence of high-density carbonyl groups (CO) as redox sites. Nanocrystalline Na₂C₆O₆ wired by reduced graphene oxide enables a high-rate performance of 200 and 175 mAh/g at 2.5 (5 C) and 5 A/g (10 C), respectively, which also benefits from a high intrinsic diffusion coefficient (10^–12–10^–11 cm²/s) and pesudocapacitance contribution (>60%) of Na₂C₆O₆ for Li–Mg co-intercalation. The suppressed exfoliation of C₆O₆ layers by a firmer non-Li pinning <i>via</i> Na–O–C or Mg–O–C and a dendrite-resistive Mg anode lead to a long-term cycling for at least 600 cycles. Such an extraordinary capacity/rate performance endows the Mg–Na₂C₆O₆ system with high energy and power densities up to 525 Wh/kg and 4490 W/kg (based on active cathode material), respectively, exceeding the level of high-voltage insertion cathodes with typical inorganic structures