In Situ Atomic-Scale Deciphering of Multiple Dynamic Phase Transformations and Reversible Sodium Storage in Ternary Metal Sulfide Anode

Ternarymetal sulfides (TMSs), endowed with the synergistic effectof their respective binary counterparts, hold great promise as anodecandidates for boosting sodium storage performance. Their fundamentalsodium storage mechanisms associated with dynamic structural evolutionand reaction kinetics, however, have not been fully comprehended.To enhance the electrochemical performance of TMS anodes in sodium-ionbatteries (SIBs), it is of critical importance to gain a better mechanisticunderstanding of their dynamic electrochemical processes during live(de)sodiation cycling. Herein, taking BiSbS3 anode as arepresentative paradigm, its real-time sodium storage mechanisms downto the atomic scale during the (de)sodiation cycling are systematicallyelucidated through in situ transmission electronmicroscopy. Previously unexplored multiple phase transformations involvingintercalation, two-step conversion, and two-step alloying reactionsare explicitly revealed during sodiation, in which newly formed Na2BiSbS4 and Na2BiSb are respectivelyidentified as intermediate phases of the conversion and alloying reactions.Impressively, the final sodiation products of Na6BiSb andNa(2)S can recover to the original BiSbS3 phaseupon desodiation, and afterward, a reversible phase transformationcan be established between BiSbS3 and Na6BiSb,where the BiSb as an individual phase (rather than respective Bi andSb phases) participates in reactions. These findings are further verifiedby operando X-ray diffraction, density functionaltheory calculations, and electrochemical tests. Our work providesvaluable insights into the mechanistic understanding of sodium storagemechanisms in TMS anodes and important implications for their performanceoptimization toward high-performance SIBs.