Toad egg-like bismuth nanoparticles encapsulated in an N-doped carbon microrod via supercritical acetone as anodes in lithium-ion batteries

A toad egg-inspired structure comprising bismuth (Bi) nanoparticles (NPs) contained in a carbon microrod shell (Bi@C) was synthesized via the one-pot supercritical acetone (scAct) route and subsequent carbonization. During the formation of Bi NPs in scAct in the presence of nitric acid, a few decomposed acetone molecules acted as carbon sources, which generated an albumen-like N-doped carbon microrod with an average shell thickness of 38 nm and were embedded with yolk-like Bi NPs having size in the range of 30–200 nm. The densely packed Bi NPs inside the carbon micron shell resulted in a high Bi loading of 78 wt%. When utilized for Li storage, the Bi@C delivered a high reversible capacity of 337 mAh g⁻¹ after 70 cycles at 0.05 A g⁻¹, long-term cyclability of 0.04 decay per cycle for 1000 cycles at 1 A g⁻¹, and high volumetric energy density of 870 mAh cm⁻³. The use of a mixed ether- and ester-based electrolyte in the Bi@C cell reduced the resistivity and increased the capacitive contribution, thereby resulting in a better high-rate performance and long-term stability than those obtained using conventional ester-based electrolytes.