Enhanced Electrochemical Performance of the Na₃V₂(PO₄)₃/C Cathode Material upon Doping with Mn/Fe for Na-Ion...

Research studies on Na-ion batteries (NIBs) are receiving significant scientific and commercial attention recently owing to the availability of low-cost, safe, and abundant materials in comparison to the conventional Li-ion batteries. The cathode material in a battery plays a crucial role in determining its cell capacity and cycle life. NASICON-based Na₃V₂(PO₄)₃, NVP, is known to be a favorable cathode material for NIBs due to its structural stability with high Na-ion mobility. The present work shows the structural and electrochemical properties of bare NVP/C and NVP/C partially doped with low-cost and much abundant transition element Fe/Mn at the toxic and expensive V site. The bare NVP/C as well as the transition-metal ion-doped NVP/C materials are prepared by the sol-gel method. XRD and FTIR studies confirm the formation of materials exhibiting the rhombohedral NVP structure (R3̅c) without any trace of impurities. The presence of a carbon layer in the investigated cathode materials is confirmed by the HRTEM micrographs; furthermore, the oxidation states of different transition-metal elements present are evaluated by X-ray photoelectron spectroscopy. Electrochemical studies reveal that the moderate doping of Fe/Mn in NVP/C results in an enhancement in discharge capacities in the doped materials at different C rates compared to the bare NVP/C sample. The differences in their electrochemical results are explained with respect to their Na-ion diffusion coefficient values obtained using the Randles-Sevcik equation. A Mn-doped NVP/C material exhibits an enhanced discharge capacity of 107 mA h g^-1 at 0.1C with 90% capacity retention even after 100 cycles at 1C current rate. At the end, a Na-ion full cell (NVMP/C||HC) comprising a Mn-doped NVP/C cathode with the commercial hard carbon anode delivering a discharge capacity of 90 mA h g^-1 is demonstrated.