Insights into Ti₃N₂T₂/VS₂ (T = F, O, OH) heterostructures as innovative anode materials for lithium/sodium/magnesium-ion batteries

To promote the development of anode materials for rechargeable batteries, we investigated the relationships between the structure characteristics and the electronic, mechanical, adsorption, diffusion and electrochemical properties of the designed Ti₃N₂T₂/VS₂ (T = F, O, OH) heterostructures for lithium/sodium/magnesium-ion batteries using density functional theory calculations. Three optimal heterostructures with different terminations, Ti₃N₂F₂/VS₂, Ti₃N₂O₂/VS₂ and Ti₃N₂(OH)₂/VS₂, were selected, and the mechanisms of charge transfer and ion diffusion were expounded clearly. These heterostructures can be used as novel anode materials based on excellent thermal stability, metallicity, mechanical stability, adsorption and diffusion performance. The results indicate that the main contributors to the metallicity are the d-orbital electronic states derived from Ti and V. The intrinsic nature of metallicity guarantees a good electrical conductivity that enables rapid charge migration and accelerates the redox process. The ion insertion not only affects the density of states distributions of the heterostructures, but also enhances their Fermi levels. The amount of charge transfer from a metal ion to VS₂ is more than that to Ti₃N₂T₂ at the respective interlayers of Ti₃N₂F₂/VS₂ and Ti₃N₂(OH)₂/VS₂, while the opposite trend appears for Ti₃N₂O₂/VS₂. The monolayer that gains more charges from the ion corresponds to the charge acceptor in the pristine heterostructure without ion adsorption. The most favorable ion-diffusion path between two optimal adsorption sites inevitably passes through the second optimal adsorption site with the second smallest adsorption energy. Considering the structural integrity, strong adsorption abilities, low diffusion energy barriers, high capacities and suitable open-circuit voltages, Ti₃N₂F₂/VS₂ and Ti₃N₂(OH)₂/VS₂ are proposed to be promising high-performance anode materials for SIBs and LIBs/MIBs, respectively, and Ti₃N₂O₂/VS₂ is an ideal anode material for LIBs/SIBs/MIBs.