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Transitional metal oxides are promising anode materials for sodium ion batteries (SIBs) due to their high theoretical capacities and material abundance; however, their sodium storage capability is significantly hindered by the sluggish sodiation/desodiation reaction kinetics. Herein, towards achieving fast and durable sodiation/desodiation reaction, Fe₃O₄ and Co₃O₄ nanocrystals encapsulated in carbon micro-spheres are synthesized via a biochemistry approach using recombinant elastin-like polypeptides containing hexahistidine tag (ELP16-His) followed by annealing. Fe₃O₄ and Co₃O₄ nanocrystals of approximately 5 nm in size, which are uniformly dispersed in a carbon matrix, are obtained. The carbon-encapsulated metal oxides exhibit encouraging sodium storage capacities (657 and 246 mA h g⁻¹ at 0.1 and 2 A g⁻¹, respectively, for carbon-encapsulated Fe₃O₄; 583 and 183 mA h g⁻¹ at 0.1 and 2 A g⁻¹ …