Insights into electrolyte-induced temporal and spatial evolution of an ultrafast-charging Bi-based anode for sodium-ion batteries

The development of extremely fast-charging sodium-ion batteries is significantly impeded by the lack of appropriate electrode materials. Bismuth (Bi) is emerging as a potential fast-charging anode, while it still faces three major challenges: complex synthesis, mismatch of electrolytes and unclear evolution mechanisms under the fast-charging operation conditions. Here, we report an ultrafast Joule heating method in just 30 s to produce ultrafine Bi nanoparticles embedded in three-dimensional carbon frameworks (Bi@3D-CFs). Through physicochemical characterization and theoretical calculations, we propose that electrochemical kinetics dominates the fast-charging performance. We further reveal the temporal and spatial evolution of Bi@3D-CFs anode using the focused ion beam technique, in-depth X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry. Bi@3D-CFs in 1,2-dimethoxyethane (DME)-based electrolyte exhibit excellent sodium storage performance (349 mAh g−1 after 12,000 cycles at 5 A g−1 and 311 mAh g−1 at 100 A g−1 with ∼11 s to fully (dis)charge), attributed to the unique 3D porous structure formed during cycling, alongside the gradient organic/inorganic solid electrolyte interphase. When paired with Na3V2(PO4)3@C, the full cells demonstrate remarkable electrochemical performance. This work examines the underlying mechanism of alloying-type anodes in ether-based electrolytes and offers new insights into the development of fast-charging batteries.