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Ternary metal sulfides (TMSs), endowed with the synergistic effect of their respective binary counterparts, hold great promise as anode candidates for boosting sodium storage performance. Their fundamental sodium storage mechanisms associated with dynamic structural evolution and reaction kinetics, however, have not been fully comprehended. To enhance the electrochemical performance of TMS anodes in sodium-ion batteries (SIBs), it is of critical importance to gain a better mechanistic understanding of their dynamic electrochemical processes during live (de)sodiation cycling. Herein, taking BiSbS3anode as a representative paradigm, its real-time sodium storage mechanisms down to the atomic scale during the (de)sodiation cycling are systematically elucidated through in situtransmission electron microscopy. Previously unexplored multiple phase transformations involving intercalation, two-step conversion, and two-step alloying reactions are explicitly revealed during sodiation, in which newly formed Na2BiSbS4and Na2BiSb are respectively identified as intermediate phases of the conversion and alloying reactions. Impressively, the final sodiation products of Na6BiSb and Na2S can recover to the original BiSbS3phase upon desodiation, and afterward, a reversible phase transformation can be established between BiSbS3and Na6BiSb, where the BiSb as an individual phase (rather than respective Bi and Sb phases) participates in reactions. These findings are further verified by operandoX-ray diffraction, density functional theory calculations, and electrochemical tests. Our work provides valuable insights into the mechanistic understanding of sodium storage mechanisms in TMS anodes and important implications for their performance optimization toward high-performance SIBs.

The recent discovery of a fast-charging vanadium-based disordered rock-salt anode for lithium-ion batteries (Nature, 2020, 585, 63−67) has rekindled great interest to screen possible anode candidates from the existing cathodes by studying their underexplored low-voltage ion storage behavior, particularly for vanadium-based compounds. Among them, layered NH4V4O10(NVO) material is typically known as an intercalation-type cathode material, with large interlayer spacing for facile ion intercalation; however, to date, there is no investigation of its utilization as a potential anode material. Here, the vanadium redox and structural evolution of NVO nanobelts (NBs) under an anode voltage window (0.01−3.0 V vs. Na+/Na) are carefully studied by in-situtransmission electron microscopy (TEM) and electrochemical measurements. By in-situTEM tracking the full sodiation process in real-time, a stepwise Na-storage reaction mechanism is revealed, initiating with the interlaminar intercalation of Na ions accompanied with the appearance of NaxNVO phase and ending with the conversion reaction with the final formation of V2O3phase. While upon desodiation, the V2O3phase can only be oxidized to VO2phase, rather than the original NVO phase. Afterward, a reversible conversion reaction between VO2and V2O3phases is established upon the subsequent (de)sodiation cycles, which delivers a reversible cycling capacity of 148 mAh g–1at 1 C, as verified by electrochemical measurements. The in-situobservation also witnesses the emergence of nanopores in NBs that may alleviate significant structural strain and contribute to the long-term cycling stability during the following (de)sodiation cycles. This work has validated for the first time the practicability of NVO as an anode material in sodium-ion batteries and afforded a paradigm of revisiting existing cathodes to explore their possible anode utilization.