Electrical conductivity and charge/discharge profiles of mixed polyanion glass-ceramic cathodes for use in Na-ion batteries

Mixed polyanion glass and glass-ceramic cathodes can have the potential to overcome the disadvantages of achieving high theoretical capacities of promising crystalline network. Three targeted cathode compositions were chosen for the present study (NaFe0.5(VO)0.5PO4, NaMn0.7(VO)0.3PO4 and NaCo0.7(VO)0.3PO4) based on the superior performance over their crystalline counter parts with highest current density ~ 100 mAh/g. XRD results displayed that major crystalline phases (Na2FeP2O7 (COD ID: 4001802), Na2MnP2O7 (ICSD ID: 71229)) which are precipitated with an ordered triclinic, P1 structure. However, Na2CoP2O7 (ICSD ID: 71230) crystalline phase exhibits orthorhombic/P n a 21 structure. The highest conducting glass-ceramic cathode sample NaCo0.7(VO)0.3PO4 (σ = 6.41 × 10−7 S·cm−1) retains its discharge capacity retention as 84.5 mAh g−1 (91%) even after 50 cycles. At a high rate of 10C, NaFe0.5(VO)0.5PO4 cathode sample is achieved to be highest discharge capacity retention 69.7% than NaCo0.7(VO)0.3PO4 (53.76%) which is a significant feature of mixed polyanion family of glass-ceramic cathodes.