Unique Na5−xSbSe Phase Enables High-Rate Performance of Sb2Se3 Anodes in Na-Ion Batteries

Abstract

Na-ion batteries (NIBs) need new anode materials to improve energy density. Metal chalcogenides, such as Sb₂Se₃, represent a promising alternative to commonly used hard carbon materials, demonstrating high-rate performance up to 5 A g⁻¹ with minimal capacity losses. However, Sb₂Se₃ is believed to operate under the conversion/alloying mechanism, typically linked with large structural transformations and volumetric changes—quite contrary to its performance. Herein, by combining multiple operando techniques and atomistic simulations, a new fully sodiated phase, Na_5−xSbSe, is unambiguously revealed as the origin of the high-rate performance of Sb₂Se₃. Na_5−xSbSe is stable within 0.01–0.80 V versus Na/Na⁺ and crystallizes in I4/mmm. The remarkable structural flexibility of Na₅SbSe to changes in Na-content allows the anode to be (de)sodiated with minimal volumetric changes (≈3.4%). This unique “breathing effect” is intimately linked to high inherent vacancy concentration, disordered, and structurally flexible anion sublattice, providing a stable framework for fast Na diffusion, contributing to the fast-charging properties of Sb₂Se₃. The study showcases the power of operando methods for discovering new phases that are hidden in the mechanistic paths of well-studied reactions and underlines the intertwined nature of various characterization methods assisted by atomistic insights for a comprehensive understanding of complex (de)sodiation mechanisms.