Na4Co3(PO4)2P2O7: A novel storage material for sodium-ion batteries

The electrochemical properties of Na4Co3(PO4)2P2O7 as a positive electrode for sodium-ion batteries were characterized by a cyclic voltammetry technique and galvanostatic charge/discharge tests. Interestingly, Na4Co3(PO4)2P2O7 has the multi redox couples in the highest potential region between 4.1 V and 4.7 V among ever reported sodium storage materials and can deliver the reversible capacity of ca. 95 mAhg−1, corresponding to ca. 2.2 Na+ extraction and insertion in Na4Co3(PO4)2P2O7, at the current density of 34 mAg−1 (0.2 C). Na4Co3(PO4)2P2O7 also exhibits the high rate capabilities and has a small polarization in the charge–discharge profile even at the high current densities of 4.25 Ag−1 (25 C), resulting in high energy efficiency of the battery. These performances indicate Na4Co3(PO4)2P2O7 can be one of the good candidates for a positive electrode material of sodium-ion batteries.