Mixed polyanion NaCo_1−x (VO) _x PO₄ glass–ceramic cathode: role of ‘Co’ on structural behaviour and electrochemical performance

Glass samples with general formula NaCo_1−x (VO) _x PO₄ (x = 0.1, 0.3, 0.5 and 0.7) are synthesized via a simple melt quenching method followed by high-energy ball milling for 30 h to form the homogeneous nanoscaled glass powders. DTA traces of all the glass and glass–ceramic samples indicated exothermic processes confirming selective crystallization induced in the glass network. The formation of major crystalline phase [sodium cobalt pyrophosphate (Na₂CoP₂O₇)] with an ordered layered structure was monitored by X-ray diffraction and the same was justified by SEM images. Structural illustration of major crystalline Na₂CoP₂O₇ phase offered more intra-layer Co–Co distance (7.12 Å) than inter-layer Co–Co distance (5.37 Å) which facilitates two-dimensional Na-ion diffusion pathways to achieve the fast intercalation and de-intercalation phenomenon along the a and c directions. The ionic conductivity was monitored by Impedance analysis and achieved to be highest (6.41 × 10⁻⁷ S cm⁻¹) for the glass–ceramic cathode x = 0.3, NaCo_1−x (VO) _x PO₄. The initial discharge capacity for the highest conducting NaCo_0.7(VO)_0.3PO₄ cathode is obtained as 93 mA h g⁻¹ in 0.1 C and had 65% capacity retention even at high rate 10 C.