Evidence of CoFeOPO4 activity in Na-ion batteries

Phosphate materials became attractive electrode materials for sodium ion batteries due to their high capacity and safety properties. Cobalt iron (III) oxyphosphate CoFeOPO4@C was successfully prepared by conventional solid-state reaction at an annealing temperature of 1000 °C in air atmosphere. Our group previously reported the electrochemical properties of this material versus Li+/ Li. In this paper, we study for the first time the electrochemical activity of CoFeOPO4@C in sodium half-cells and evaluate its potential use as an anode material for sodium ion batteries. CoFeOPO4@C shows a high discharge capacity of 479 mAh g−1 at C/2 current rate over 0.01–3.0 V voltage range during the first cycle by accommodating around four sodium ions. Although the first irreversible capacity is relatively high resulting from a conversion mechanism, this phosphate can reversibly exchange about two sodium ions in the subsequent cycles, delivering a capacity of more than 200 mAh g−1 with a coulombic efficiency exceeding 97%.