Theoretical insights into surface-phase transition and ion competition during alkali ion intercalation on the Cu₄Se₄ nanosheet

The development of stable and efficient electrode materials is imperative and also indispensable for further commercialization of sodium/potassium-ion batteries (SIBs/PIBs) and new detrimental issues such as proton intercalation arise when utilizing aqueous electrolytes. Herein the electrochemical performance of the Cu₄Se₄ nanosheet was determined for both organic and aqueous SIBs and PIBs. By means of density functional theory calculation, Na⁺, K⁺ and H⁺ intercalations onto both sides of the Cu₄Se₄ nanosheet were revealed. The Cu₄Se₄ nanosheet well maintains its metallic electronic conductivity and the changes in lateral lattice parameters are within 4.66% during the whole Na⁺/K⁺ intercalation process for both SIBS and PIBs. The theoretical maximum Na/K storage capacity of 188.07 mA h g⁻¹ can be achieved by stabilized second-layer adsorption of Na⁺/K⁺. The migration barriers of Na and K atoms on the Cu₄Se₄ nanosheet are 0.270 and 0.173 eV, respectively. It was discovered that Na/K- intercalation in the first layer is accompanied by a first-order surface phase transition, resulting in an intercalation voltage plateau of 0.659/0.756 V, respectively. The region of the two-surface phase coexistence for PIBs, is shifted toward a lower coverage when compared with that for SIBs. The partially protonated Cu₄Se₄ nanosheet (HxCu₄Se₄, x ≤ 10/9) was revealed to be structurally and thermodynamically stable. While the partially protonated Cu₄Se₄ nanosheet is favorable in acidic electrolytes (pH = 0) when protons and Na/K ions compete, we showed that Na⁺/K⁺ intercalated products may be preferred over H⁺ at low coverages in alkali electrolyte (pH = 14). However, the proton intercalation substantially decreases the battery capacity in aqueous SIBs and PIBs. Our work not only identifies the promising performance of Cu₄Se₄ nanosheets as an electrode material of SIBs and PIBs, but also provides a computational method for aqueous metal-ion batteries.