Enhancement in Battery Performance of Na₂Ti₃O₇ Through Vanadium Doping for Sodium-Ion Batteries

Sodium-ion batteries are a next-generation and cost-effective alternative to rechargeable lithium-ion batteries. However, there is a substantial reduction in volumetric and gravimetric capacity using sodium ion as a charge carrier. Na₂Ti₃O₇ (Sodium titanate oxide-NTO) has a wide direct band gap of 3.07 eV leading to poor performance and high capacity fading after prolonged galvanostatic cycling, limiting the mobility of ions between vacant sites of the structure, which results in low intrinsic electrical conductivity across the material. The above drawback of NTO can be improved by incorporating vanadium into the NTO crystal lattice. This work investigates the varying percentage of doping of vanadium ions in NTO and its overall effect on cell performance with sodium ias a charge carrier. Various concentration of vanadium ions were substituted in sites of Ti⁴⁺ sites and the phases were well-characterized by powder XRD, TEM-EDX, HRTEM, ICP, and XPS. Likewise, electrochemical measurements, i.e., EIS, CV, galvanostatic cycling, etc, were carried out in the swagelok cell in two-electrode configuration. The charge-discharge profile has been comparatively analyzed with the varied amount of vanadium in Na₂Ti_3-xV_xO₇ at different C-rates to investigate performance and capacity retention. The initial discharge capacity of the V-doped NTO half-cell was improved to 119.46 mAh/g compared to NTO with 98.61mAh/g at a 0.5 C-rate. Also, there was a significant reduction of 57% in the internal resistance of V-doped NTO half-cell when EIS was recorded between 10 MHz-100 kHz. Cyclic voltammetry study exhibits an increase in the maximum peak current with increasing scan rate, leading to high diffusivity (D_Na⁺) in the case of Na₂Ti_3-xV_xO₇ when compared against Na₂Ti₃O₇. The very identical nature of CV profiles of both samples, from 0.1 mV/s to 2 mV/s, is indicative of the pseudocapacitive behaviour of the electrode. The presence of two anodic peaks-0.33V and 0.51V in the cyclic voltammetry of V-doped NTO suggests a biphasic mechanism similar to NTO associated with the transition of Na₂Ti₃O₇ to Na_3–yTi₃O₇ and then to Na₄Ti₃O₇. Thus, our study concludes that doping high valence transition metal ions like vanadium provides enhanced electrochemical performance attributed to the increased conductivity and formation of oxygen vacancies.