Amorphous VO₂ : A Pseudocapacitive Platform for High-Rate Symmetric Batteries

Among the various VO₂ polymorphs, the layered compound, VO₂ (B), has been the most widely investigated lithium-ion battery electrode material. For sodium-ion electrodes, however, an amorphous solid may be more advantageous as a result of the open framework to facilitate ion insertion and the ability to tolerate volumetric changes. Herein, it is shown that the Na⁺ insertion properties of amorphous VO₂ (a-VO₂ ) are superior to those of crystalline VO₂ (B). Amorphous VO₂ exhibits a linear voltage characteristic over a 3 V range (4.0 to 1.0 V vs Na/Na⁺ ) leading to a reversible capacity as high as 400 mAh g^-1 and rapid redox kinetics, which is attributed to its pseudocapacitive nature. The linear voltage characteristic over 3 V affords the opportunity of fabricating a symmetric Na-ion battery in which the a-VO₂ material serves as both the positive electrode and the negative electrode. Such a symmetric battery offers safer operation in terms of overcharging, overdischarging, polarity reversal, high charge/discharge current abuse, and long-term usage. The results suggest that amorphous transition metal oxides may offer advantageous attributes for rapid, safe, and energy-dense storage.