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

Abstract

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.

Enhancing Structure Stability by Mg/Cr Co‐Doped for High‐Voltage Sodium‐Ion Batteries

“…This observation suggests that excessive doping may cause some Cr and Mg elements that fail to enter the lattice of the bulk material during calcination to combine to form oxides or react with excess sodium to produce by-products. [21,22] The Rietveld refinement results of XRD data for P2-NaNM and P2-NaNMCM-0.01 are presented in Figure 1d,e as well as Tables S1 and S2, Supporting Information. In the doped material P2-NaNMCM-0.01, Cr, Mg occupy the Na site and transition metal site, respectively.…”

“…This observation suggests that excessive doping may cause some Cr and Mg elements that fail to enter the lattice of the bulk material during calcination to combine to form oxides or react with excess sodium to produce by‐products. [ 21,22 ]…”