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. I should CoCO3: An eco‐efficient synthetic route is used for the synthesis of metal carbonates for Li+ and Na+ battery applications by using supercritical CO2 as a green precursor at 50 °C. The method minimizes energy consumption and has a low environmental footprint. The obtained CoCO3/graphene composites with a unique architecture show excellent properties for reversible Li+ and Na+ storage.