First-Principles Study of ScM2 (M = C and N) Monolayers as High-Performance Anode Materials for Potassium-Ion Storage

Abstract

The increasing demand for sustainable energy storage technologies has driven the exploration of alternatives to lithium-ion batteries, with potassium-ion batteries (PIBs) emerging as promising candidates owing to the natural abundance and favorable electrochemical properties of potassium. Two-dimensional (2D) scandium carbide (ScC2) and scandium nitride (ScN2) monolayers were systematically investigated as potential anode materials for PIBs through first-principles density functional theory calculations performed using the Vienna Ab Initio Simulation Package (VASP). The absence of imaginary phonon modes and ab initio molecular dynamics simulations confirm the dynamical and thermal stability of both monolayers, while electron localization function analysis reveals mixed ionic-metallic bonding in ScC2 and ionic-dominant charge transfer in ScN2, with both satisfying the Born mechanical stability criteria. Electronic band structure calculations using GGA-PBE and hybrid HSE06 functionals confirm an intrinsic metallic character in both materials, ensuring efficient electron transport during charge/discharge cycling. The most favorable K adsorption occurs at the Sc top site with adsorption energies of −0.86 eV and −1.32 eV for ScC2 and ScN2, respectively, with Bader charge analysis confirming a K+-like ionic character through charge transfers of 0.85e and 0.86e. The K-ion diffusion barriers calculated by the climbing image nudge elastic band method are 0.078 eV and 0.21 eV for ScC2 and ScN2, indicating ultralow barriers favorable for high-rate PIB performance. The theoretical storage capacities of 777.08 mAh g−1 and 734.57 mAh g−1 with average open circuit voltages (OCV) of 0.27 V and 0.46 V for ScC2 and ScN2, respectively, substantially exceed that of conventional graphite anodes. Small maximum lattice changes of 7.44% and 5.09% upon full K loading confirm mechanical resilience during cycling, while confirmed chemical stability against water adsorption establishes their suitability for ambient fabrication conditions, collectively identifying ScC2 and ScN2 as promising high-performance anode candidates for PIB applications.