Direct evidence of an unanticipated crystalline phase responsible for the high performance of few-layered-MoS2 anodes for Na-ion batteries

One of the major bottlenecks of Na-ion batteries (SIBs) is the lack of appropriate anode materials. The promising candidate is MoS2 exhibiting a very high specific capacity that unfortunately decreases after the first discharge due to structural reorganizations. Furthermore, the electrochemical reactions of MoS2 during subsequent cycles, which determine the performance of the cycle, remain unclear. This study elaborates the detailed phase evolution of few-layered 2H-MoS2 with an enhanced electrochemical performance by multiple operando/ex-situ techniques (XRD, XAS, XPS). Novel concept of MoS2 (de)sodiation mechanism is proposed based on repetitive disintegration and rearrangement of Na2S, Na2S2 and Mo accompanied by amorphization of the sulfur deficient NaxMoSy phase followed by NaxMo3S4 crystallization from an amorphous matrix through electrochemically driven congruent crystallization. The high Na-ion diffusion in the NaxMo3S4 phase and the fast propagation of the interface between the crystallized and the amorphous regions enable very high electrode reaction kinetics. The presented iterative conversion processes accompanied by amorphization and electrochemically driven congruent crystallization reactions provide new insight into the electrochemical (de)sodiation processes of MoS2, which contributes to the rational design of MoS2 electrodes and enriches the fundamental understanding of conversion reaction chemistry.