Pseudocapacitive Na ion storage in binder-less, carbon additive-free Nb2O5-x electrode synthesised via solvothermal-assisted electro-coating with enhanced areal capacitance and lowered impedance parameters

Negatrodes with wide negative operating voltage, high electrochemical storage capacity, and intrinsic metal ion intercalation abilities are vital to the continuous development of storage devices with simultaneous energy and power density improvement. Herein, we report binder-less coating of non-stoichiometric vacancy-implanted Nb₂O_5-x as carbon additive-free negatrode on FTO substrate for asymmetric supercapacitor and sodium ion capacitor applications. The negatrode was fabricated through vacuum-less and low-temperature solvothermal-assisted electro-coating technique and yielded several orders of enhancement in its areal capacitance and retained ca. 90 % of its capacity after 5000 cycles of charge-discharge in aqueous Na⁺ electrolyte. The solvothermal treated electro-coated electrode (STT_Nb₂O_5-x) achieved an areal capacitance of 22.58 mF/cm^2, which was far higher than those of hydrothermal-treated electro-coated HTT_Nb₂O₅ and Nb₂O₅ electrodes. The solvothermal treatment simultaneously enhanced the electro-coated samples' impedance properties and mass load through oxygen vacancy implantation and re-crystallization of the electro-coated Nb₂O₅ layer, respectively. This study presented a facile and energy-efficient technique of direct coating of defect-enhanced pseudocapacitive nanomaterials for the fabrication of electrochemical storage devices.