Atomic insight into the structural transformation and anionic/cationic redox reactions of VS2 nanosheets in sodium-ion batteries

Two-dimensional transition metal disulfides have attracted great attention as anode materials for sodium ion batteries (SIBs) due to their high capacities and long cycle life, but knowledge of the mechanisms for phase transitions, charge-transfer reactions, and ionic diffusion kinetics during Na+ insertion has been lacking. These properties were systematically investigated in this work via experimental testing and first-principles calculations using VS2 nanosheets as an example material. The material showed a stable discharge capacity of 386 mA h g(-1) in the 0.3-3.0 V voltage window which then increased to 657 mA h g(-1) with further discharging to 0.01 V. It was discovered that Na+ first intercalated into octahedral interstitial sites of NaxVS2, with 0 < x 1.0, accompanied by partial reduction of S anions. Afterwards, Na+ intercalated into tetrahedral interstitial sites of NaxVS2, with 1.0 < x 2.0, causing partial reduction of both V cations and S anions. The electrode was finally converted into a V/Na2S nanocomposite after insertion of 3.0 mol of Na+, giving rise to a large specific capacity. This work not only revealed the structural transformation and mixed anionic/cationic redox reactions of VS2 during Na+ intercalation, but also helped us to understand the electrochemical reaction mechanisms of transition metal disulfides in SIBs.