Activating lithium-ion storage performances of two-dimensional MSi₂N₄ (M = Mo, W) monolayers: a theoretical prediction

Two-dimensional materials MA₂Z₄ (where M = Mo, W, V, Cr, Nb, Ta, Ti, Zr or Hf; A = Si or Ge; and Z = N, P, or As) have attracted increasing interest for their exceptional stability and mechanical strength. Therefore, it is vital to explore their lithium-ion (Li-ion) energy storage behavior for next-generation energy storage technologies. Herein, the potential of two synthesized MSi₂N₄ (M = Mo, W) materials as Li-ion battery anodes was systematically investigated, and two effective ways were proposed to trigger their Li-ion storage performances through first-principles calculations. First-principles calculations reveal that the intrinsic vacancies can activate the Li-ion adsorbability and reduce the bandgap of MSi₂N₄ (M = Mo, W), especially the Si vacancies, which enhances Li-ion storage capacity. Moreover, after the introduction of 3d, 4d, and 5d transition metal doping, a linear relationship was found between the adsorption energy (<i>E</i>_ads) of Li-ions and the lowest unoccupied level or Fermi-level (<i>E</i>_LUS). This indicates that <i>E</i>_LUS might be an effective descriptor to predict <i>E</i>_ads. Among 122 subjects, five candidates (MoSi₂N₄@Pd_Si, MoSi₂N₄@Ag_Si, WSi₂N₄@Cu_Si, WSi₂N₄@Pd_Si, and WSi₂N₄@Ag_Si) were selected as potential Li-ion battery anodes with high specific capacity, low open-circuit voltage, good electronic conductivity and fast Li-ion dynamics. This work not only highlights a family of potential Li-ion battery anodes, but also provides feasible strategies for activating their Li-ion storage performances.