Operando Sodiation Mechanistic Study of a New Antimony-Based Intermetallic CoSb as a High-Performance Sodium-Ion Battery Anode

Electrode materials based on antimony (Sb) are of growing interest because Sb achieves a balance between high capacity and good cycle performance in battery operation. The electrochemical performance of CoSb nanoparticles synthesized by a template-free assisted ball-milling method is reported for the first time. The first discharge (sodiation) profile is characterized by a gentle slope around 0.3 V. During subsequent electrochemical cycles, the reaction proceeds between 0.4 and 0.8 V. The obtained reversible capacity is 211 mAh/g. To get a mechanistic insight into the Na insertion into the CoSb lattice, suites of material analyses were performed in different stages of (dis)charging. Upon sodiation, the formation of the Na3Sb phase was evidenced, which converts to elemental Na and Sb following a two-step alloying pathway upon desodiation. The local environment probed by various ex situ tools (diffraction, spectroscopy, microscopy, and electrochemical) hinted at the formation of an Sb-deficient CoSb1-x (Co4Sb) phase. The structural change was evident from the structural analysis, confirming the formation of Na metal. CoSb is by far the best among the most reported fluoroethylene-carbonate-free Sb intermetallic anodes for sodium-ion batteries.