Sodium/Potassium Intercalation on the Cu₄S₄ Nanosheet Accompanied by a Surface Phase Transition and Their Competition with Protons

Sodium- and potassium-ion batteries (SIBs and PIBs) are promising for electrochemical energy storage and conversion to complement lithium-ion batteries owing to their unique characteristics including elemental abundance of Na/K, no formation of Al-Na/K intermetallic compounds, and fast ion diffusion. However, SIBs and PIBs still suffer from poor storage capacity and unfavorable ion-intercalation thermodynamics and kinetics. Herein, we identified the Na+, K+, and H+ intercalation on the Cu4S4 nanosheet using density functional theory (DFT) and ab initio molecular dynamics simulations. The calculated mechanical, thermodynamic, and vibrational properties demonstrated the stability of the Cu4S4 nanosheet. Both Na+ and K+ can intercalate on both sides of the Cu4S4 nanosheet with a full bilayer of adsorbed atoms and a specific capacity of 280 mA h g(-1). It was found that two-surface phase coexistence accompanies the Na/K-intercalation in the first adsorption layer, resulting in voltage plateaus of 0.78 and 0.68 eV for SIBs and PIBs, respectively. The first-order surface phase transition is always accompanied by abrupt changes of the lattice constant and work function. The Na and K migration barriers on the Cu4S4 nanosheet are 0.312 and 0.196 eV, respectively, ensuring adequately fast ion diffusion. The DFT-predicted results reveal the metallic character of the Cu4S4 nanosheet. Its in-plane structure and good electronic conductivity can be well retained during the charge/discharge cycles. The reversible Na/K storage system is not necessary to be the most thermodynamically stable, but it must be dynamically stable. Comparison of the intercalation voltages of H+, Na+, and K+ indicates that the Cu4S4 nanosheet could play a role as an electrode material for aqueous SIBs and PIBs in strong alkaline solutions, but the capacity will significantly reduce. This work provides an effective method to identify the usability of a two-dimensional material as an electrode of both organic and aqueous metal-ion batteries.