Three-Dimensional Hierarchical Framework Assembled by Cobblestone-Like CoSe@C Nanospheres for Ultrastable Sodium-Ion Storage.

Sodium-ion batteries (SIBs), as the promising commercial energy system, are restricted by their sluggish kinetics and low sodium-ion storage. Metal selenide possesses good conductivity and capacity but still suffers from the stacked problem and volume expansion. Significantly, CoSe/C is successfully prepared with the assistance of citric acid as both a chelating agent and carbon precursor, displaying that cobblestone-like nanospheres with the radii (<25 nm) distribute uniformly in the carbon matrix. It is expected that the established Co-O-C bonds enhance the stability of the structure with faster ion shuttling. With the available electrolyte (NaCFSO/diethylene glycol dimethyl ether) in a potential window range from 0.5 to 3.0 V, the as-obtained sample shows the ultralong lifespan at 4.5 A g, retaining a capacity of 345 mA h g after 10 000 cycles. From the detailed kinetic analysis, it is clear that the surface-controlled electrochemical behavior mainly contributes to the excellent large-current cycling stability and Na storage capacity. The ex situ results support that the crystal and morphological structure remains stable. This work is anticipated to enhance the in-depth understanding of the CoSe/C anode and supply a facile manner to obtain electrode materials for SIBs.