Recent advances in Zr and Hf-based MXenes and their hetero-structure as novel anode materials for Ca-ion batteries: theoretical insights from DFT approach.

Recently, MXenes have been widely investigated for use as electrodes in various ion storage batteries. In this study, Zr 2 N, Hf 2 N and ZrHfN were explored as potential anode materials for Ca-ion batteries. AIMD simulations predict higher structural stability for our proposed MXenes at a temperature of 300 K. The adsorption energies at the most favourable adsorption sites are 1.31, 1.33 and 1.27 eV for Zr 2 N, Hf 2 N and ZrHfN, respectively. During the adsorption process, a significant amount of charge transfer occurs from the Ca atom to the nanosheets. DOS and PDOS analyses reveal that the adsorption of Ca atoms enhances the conductivity of the nanosheets. Moreover, the low diffusion barriers are found to be 0.076, 0.073 and 0.097 eV when the Ca atom migrates from its favourable adsorption site to a nearby site on Zr 2 N, Hf 2 N and ZrHfN nanosheets, resulting in high charging rates. The theoretical capacities of Zr 2 N, Hf 2 N and ZrHfN nanosheets are 1034, 561 and 707 mA h g -1 , respectively. All the results from this study suggest that our proposed nanosheets can be potential anode materials for Ca-ion batteries. Among them, the Zr 2 N nanosheet shows superior anodic properties for Ca-ion batteries, which is also confirmed by specific capacity, diffusion barrier and open circuit voltage calculations.