Electrochemical studies of a high voltage Na₄Co₃(PO₄)₂P₂O₇-MWCNT composite through a selected stable electrolyte

Cathode materials that operate at high voltages are required to realize the commercialization of high-energy-density sodium-ion batteries. In this study, we prepared different composites of sodium cobalt mixed-phosphate with multiwalled carbon nanotubes (Na₄Co₃(PO₄)₂P₂O₇-MWCNTs) by the sol-gel synthesis technique. The crystal structure and microstructure were characterized by using PXRD, TGA, Raman spectroscopy, SEM and TEM. The electrochemical properties of the Na₄Co₃(PO₄)₂P₂O₇-20 wt% MWCNT composite were explored using two different electrolytes. The composite electrode exhibited excellent cyclability and rate capabilities with the electrolyte composed of 1 M sodium hexafluorophosphate in ethylene carbonate:dimethyl carbonate (EC:DMC). The composite electrode delivered stable discharge capacities of 80 mA h g^-1 and 78 mA h g^-1 at room and elevated (55 °C) temperatures, respectively. The average discharge voltage was around 4.45 V versus Na⁺/Na, which corresponded to the Co^2+/3+ redox couple. The feasibility of the Na₄Co₃(PO₄)₂P₂O₇ cathode for sodium-ion batteries has been confirmed in real time using a full cell configuration vs. NaTi₂(PO₄)₃-20 wt% MWCNT, and it delivers an initial discharge capacity of 78 mA h g^-1 at 0.2C rate.