Improved Adsorption and Migration of Divalent Ions Over C₄N Nanosheets: Potential Anode for Divalent Batteries

Divalent ion batteries are potential substitutes to existing rechargeable batteries because of the high-energy density, safety, and low cost. However, the applications of divalent batteries are strongly dependent on the availability of efficient electrode materials. We herein report the adsorption and migration mechanism of divalent ions like Mg⁺² and Ca⁺², on two-dimensional carbon nitride monolayer (C₄N). The adsorption of both Mg and Ca ions are much stronegr on C₄N saturated with functional groups like hydrogen (-H), hydroxyl (-OH), and carboxylic (-COOH) as compared to the pristine monolayers, which implies the improvement in metal storage caused by the functional groups. On functionalized C₄N, the first Mg binds within the binding energy range of 1.5-2.2 eV having migration barrier, at the saturated sites, of around 0.5 eV, which indicates desirable binding and robust diffuse. However, the functional groups tend to act as the trapping sites for Ca ions and the diffusion might get hindered as compared to Mg ions. In addition to binding and diffusion mechanism, charge transfer, electronic structures and open circuit volatges have also been calculated for both Mg and Ca on pristine and functionalized C₄N monolayers. We find that -H, -OH, and -COOH play important role for the cyclic performance of C₄N as the prospective anode material for divalent ion batteries.