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 Na_xMoS_y phase followed by Na_xMo₃S₄ crystallization from an amorphous matrix through electrochemically driven congruent crystallization. The high Na-ion diffusion in the Na_xMo₃S₄ 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.