Covalent bond chemistry enabling M2CN2 MXenes as anode materials for halide-ion batteries

Abstract

The development of halide-ion batteries is limited by the lack of efficient electrode materials. Two-dimensional M2CN2 MXenes are promising anode candidates due to their structural flexibility and low molar mass, yet their stability and storage mechanism remain unclear. Using first-principles calculations, we identify Ti2CN2, Nb2CN2, and Ta2CN2 as stable MXenes. Ti2CN2 exhibits excellent performance with low voltages (0.07 V for F−) and high specific capacities (394.8 mAh/g for F−). The storage mechanism involves covalent bonding between surface N and halide-ions, where adsorption strength is governed by the energy difference between occupied σ* and unoccupied π* orbitals and their electron overlap. Moreover, O or Zr doping significantly enhances halide-ion diffusion kinetics. This work elucidates the covalent bond-mediated storage in M2CN2 MXenes and guides the design of high-performance halide-ion battery electrodes.