We systematically investigated the potential of single-layer VS2 polytypes as Na-battery anode materials via density functional theory calculations. We found that sodiation tends to inhibit the 1H-to-1T structural phase transition, in contrast to lithiation-induced transition on monolayer MoS2. Thus, VS2 can have better structural stability in the cycles of charging and discharging. Diffussion of Na atom was found to be very fast on both polytypes, with very small diffusion barriers of 0.085 eV (1H) and 0.088 eV (1T). Ab initio random structure searching was performed in order to explore stable configurations of Na on VS2. Our search found that both the V top and the hexagonal center sites are preferred adsorption sites for Na, with the 1H phase showing a relatively stronger binding. Notably, our random structures search revealed that Na clusters can form as a stacked second layer at full Na concentration, which is not reported in earlier works wherein uniform, single-layer Na adsorption phases were assumed. With reasonably high specific energy capacity (232.91 and 116.45 mAh/g for 1H and 1T phases, respectively) and open-circuit voltage (1.30 and 1.42 V for 1H and 1T phases, respectively), VS2 is a promising alternative material for Na-ion battery anodes with great structural sturdiness. Finally, we have shown the capability of the ab initio random structure searching in the assessment of potential materials for energy storage applications.

We investigated the application of 1T'-MX2 (M = Mo, W; X = S, Se, Te) 2D materials as hydrogen evolution reaction (HER) catalysts using density functional theory. Our results show that 1T'-MX2 have lower energies and are dynamically more stable than their 1T counterparts, therefore likely more relevant to previous experimental findings and applications. We found that sulfides are better catalysts, followed by selenides and tellurides. Specifically, 1T'-MoS2 and WS2 are the best HER catalysts among MX2. We proposed a mechanism, rather than the metallicity surmised previously, based on the calculated density of states. On the other hand, the effectively stretched (compressed) X site on the 1T' 2 × 1 reconstruction from 1T is shown to be more (less) active for the HER. We further exploited the application of external strain to tune and boost the HER performance. Our findings are of significance in the elucidation of previous experimental studies and exploration of potential materials for clean energy applications.