MoS₂, WS₂, and MoWS₂ Flakes as Reversible Host Materials for Sodium-Ion and Potassium-Ion Batteries

Transition-metal dichalcogenides (TMDs) and their alloys are vital for the development of sustainable and economical energy storage alternatives due to their large interlayer spacing and hosting ability for alkali-metal ions. Although the Li-ion chemically correlates with the Na-ion and K-ion, research on batteries with TMD anodes for K⁺ is still in its infancy. This research explores TMDs such as molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂) and TMD alloys such as molybdenum tungsten disulfide (MoWS₂) for both sodium-ion batteries (NIBs) and potassium-ion batteries (KIBs). The cyclic stability test analysis indicates that in the initial cycle, the MoS₂ NIB demonstrates exceptional performance, with a peak charge capacity of 1056 mAh g^-1, while retaining high Coulombic efficiency. However, the WS₂ KIB underperforms, with the least charge capacity of 130 mAh g^-1 in the first cycle and exceptionally low retention at a current density of 100 mA g^-1. The MoWS₂ TMD alloy exhibits a moderate charge capacity and cyclic efficiency for both NIBs and KIBs. This comparison study shows that decreasing sizes of alkali-metal ions and constituent elements in TMDs or TMD alloys leads to decreased resistance and slower degradation processes as indicated by cyclic voltammetry and electrochemical impedance spectroscopy after 10 cycles. Furthermore, the study of probable electrochemical intercalation and removal processes of Na-ions and K-ions demonstrates that large geometrically shaped TMD flakes are more responsive to intercalation for Na-ions than K-ions. These performance comparisons of different TMD materials for NIBs and KIBs may promote the future development of these batteries.