Abnormal Phenomena Of Multi-Way Sodium Storage In Selenide Electrode

Transition-metal chalcogenides have gained special attention as potential anodes for Na-ion batteries due to their high capacities to originate from complex charge storage mechanisms. Although the sodium storage process in chalcogenides is still unclear, it is common to assume that it can occur via one of the following routes: intercalation, conversion, or alloying. In this paper, an anomalous multi-way mechanism in MoSe2 electrode, including all three of the above scenarios, is reported. The intercyclic product of the discharge/charge process is a mixture of Se, Mo, and 1T-structured NaxMoSe2. An unexpected phenomena of Se precipitation leads to the additional alloying reaction, which is exclusive among all chalcogenides, and runs together with conversion and intercalation reactions in the same cycle. This new concept of sodium storage process includes two models, previously seemed to be mutually exclusive. Despite of complex electrode mechanisms, MoSe2 retains high capacity and coulombic efficiency even after 50 cycles.