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 BiSbS3 anode 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 situ transmission 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 Na2BiSbS4 and 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 BiSbS3 phase
upon desodiation, and afterward, a reversible phase transformation
can be established between BiSbS3 and Na6BiSb,
where the BiSb as an individual phase (rather than respective Bi and
Sb phases) participates in reactions. These findings are further verified
by operando X-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.