Resolving the Origins of Superior Cycling Performance of Antimony Anode in Sodium-ion Batteries: A Comparison with Lithium-ion Batteries.

Alloying-type antimony (Sb) with high theoretical capacity is a promising anode candidate for both lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs). Given the larger radius of Na+ (1.02 Å) than Li+ (0.76 Å), it was generally believed that the Sb anode would experience even worse capacity degradation in SIBs due to more substantial volumetric variations during cycling when compared to LIBs. However, the Sb anode in SIBs unexpectedly exhibited both better electrochemical and structural stability than in LIBs, and the mechanistic reasons that underlie this performance discrepancy remain undiscovered. Here, using substantial in situ transmission electron microscopy, X-ray diffraction, and Raman techniques complemented by theoretical simulations, we explicitly reveal that compared to the lithiation/delithiation process, sodiation/desodiation process of Sb anode displays a previously unexplored two-stage alloying/dealloying mechanism with polycrystalline and amorphous phases as the intermediates featuring improved resilience to mechanical damage, contributing to superior cycling stability in SIBs. Additionally, the better mechanical properties and weaker atomic interaction of Na-Sb alloys than Li-Sb alloys favor enabling mitigated mechanical stress, accounting for enhanced structural stability as unveiled by theoretical simulations. Our finding delineates the mechanistic origins of enhanced cycling stability of Sb anode in SIBs with potential implications for other large-volume-change electrode materials.

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

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.