Sodium-ion transport, storage mechanisms, and structural evolution in Na₂S-MoO₃-P₂O₅ glass cathodes for sodium-ion...

The Na₂S-MoO₃-P₂O₅ glass is assessed as a positive electrode in a NaPF₆-based electrolyte for sodium-ion batteries. Impedance and DC polarization measurements indicate mixed conduction with a Na⁺ transference number of 0.90 and a bulk conductivity of approximately 1.20 × 10^-7 S m^-1 at room temperature. In the Na half-cells, the electrode initially delivers more than 170 mAh g^-1, followed by about 50% capacity decay after approximately 100 cycles. The remaining capacity is sustained for over 300 cycles. Ex situ SEM and EDS analyses highlight Na incorporation and partial extraction during cycling, with no evidence of microcrack formation. The Na/P atomic ratio increases upon discharge and partially returns to its initial value, consistent with the Na⁺ diffusion coefficient decreasing from 7.3 × 10^-12 to 1.2 × 10^-12 cm² s^-1 for the pristine sample and after the first cycle, respectively. This indicates the partial trapping of sodium ions within the disordered glass network structure, while ex situ XRD analysis reveals the formation of a stable crystalline sodium molybdate (Na₂MoO₄) phase on the cycled electrode surfaces. These findings suggest that the Na₂S-MoO₃-P₂O₅ glass is a cathode material with promising characteristics for the further development of sodium-ion batteries.