Eco-Efficient Synthesis of Highly Porous CoCO₃ Anodes from Supercritical CO₂ for Li⁺ and Na⁺ Storage

An eco-efficient synthetic route for the preparation of high-performance carbonate anodes for Li⁺ and Na⁺ batteries is developed. With supercritical CO₂ (scCO₂) as the precursor, which has gas-like diffusivity, extremely low viscosity, and near-zero surface tension, CoCO₃ particles are uniformly formed and tightly connected on graphene nanosheets (GNSs). This synthesis can be conducted at 50 °C, which is considerably lower than the temperature required for conventional preparation methods, minimizing energy consumption. The obtained CoCO₃ particles (ca. 20 nm in diameter), which have a unique interpenetrating porous structure, can increase the number of electroactive sites, promote electrolyte accessibility, shorten ion diffusion length, and readily accommodate the strain generated upon charging/discharging. With a reversible capacity of 1105 mAh g⁻¹, the proposed CoCO₃/GNS anode shows an excellent rate capability, as it can deliver 745 mAh g⁻¹ in 7.5 min. More than 98 % of the initial capacity is retained after 200 cycles. These properties are clearly superior to those of previously reported CoCO₃-based electrodes for Li⁺ storage, indicating the merit of our scCO₂-based synthesis, which is facile, green, and can be easily scaled up for mass production.