Enhancement of Li and Na Storage in Janus HfSSe Monolayers by Co-Induced Electron-Deficient Chalcogen Sites: A First-Principles Study

The growing demand for high-capacity, fast-charging, and durable rechargeable batteries has intensified the search for advanced electrode materials, with two-dimensional materials offering large surface-to-mass ratios and short ion-diffusion pathways. Here, pristine and Co-doped Janus HfSSe monolayers were investigated as anode materials for Li-ion and Na-ion batteries using first-principles density functional theory. Pristine HfSSe was found to be dynamically stable and exhibited a 0.85 eV band gap, while Co substitution shifted the Fermi level toward the valence-band region. Adsorption of the first Li or Na layer induced metallic behavior, thereby improving charge transport. Co doping strengthened alkali-metal binding, with the most favorable Li and Na adsorption energies changing from -2.6125 and -1.9800 eV to -3.7405 and -3.0529 eV, respectively. Co doping created electron-deficient sites that enhanced Li/Na binding. Co-doped HfSSe accommodated up to four Li layers and three Na layers, increasing the theoretical capacities from 555.43 to 776.19 mAh/g for Li and from 370.29 to 582.14 mAh/g for Na. The average open-circuit voltages were 0.62 V for Li and 0.29-0.30 V for Na, with minimum migration barriers of 0.2153 and 0.1044 eV, respectively. These results identified Co-doped Janus HfSSe as a promising anode material for Li-ion and Na-ion batteries.