First-principles study on the regulation of vacancy defects in two-dimensional MoSi2N4 as an anode material for potassium-ion batteries.

CONTEXT: In this paper, the adsorption properties of K atoms on the surface of two-dimensional MoSi2N4 were investigated via density functional theory. It was found that the adsorption energies of K atom at the Mo-top, N-top and Si-top sites of pristine MoSi2N4 were - 0.03 eV, - 0.05 eV and - 0.08 eV, respectively. The diffusion barrier of K atom on the MoSi2N4 surface was calculated to be only 0.052 eV, indicating the excellent diffusion ability of K atom on MoSi2N4. With the increasing of adsorbed K atoms, stable adsorption of K atoms on the MoSi2N4 surface was no longer achievable. By introducing Si-vacancy defects, the adsorption energy of K atoms at the most favourable adsorption site on the MoSi2N4 surface reached - 3.92 eV. Compared with pristine MoSi2N4, the adsorption strength was enhanced by approximately 50 times. Meanwhile, the number of stably adsorbed K atoms increased from 1 to 6. Accordingly, a maximum theoretical K storage capacity of 773 mAh/g and an open-circuit voltage of 1.66 V were obtained. Moreover, the diffusion barrier of K atom on the VSi-MoSi2N4 surface was calculated to be 0.053 eV. The modified two-dimensional MoSi2N4 combined excellent K atom transport capability with moderate adsorption strength, demonstrating its potential application as an anode material for potassium-ion batteries.

METHODS

First-principles calculations based on density functional theory were performed to investigate the effects of vacancy defects on the potassium-storage properties of two-dimensional MoSi2N4. All calculations were performed using the Vienna ab initio Simulation Package. The generalized gradient approximation with the Perdew-Burke-Ernzerhof exchange-correlation functional was adopted for structural relaxation and energy calculation. The Heyd-Scuseria-Ernzerhof hybrid functional was utilized to calculate electronic band structures and acquire accurate band gap values. The projector augmented-wave method was employed to describe core-valence electronic interactions, and the DFT-D3(BJ) approach was used to account for van der Waals interactions. The CI-NEB method and AIMD simulations were separately adopted to evaluate the diffusion kinetics and thermal stability of the material system.