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

Abstract Among the various VO 2 polymorphs, the layered compound, VO 2 (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 VO 2 (a‐VO 2 ) are superior to those of crystalline VO 2 (B). Amorphous VO 2 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‐VO 2 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.