A high-voltage activated high-erformance cathode for aqueous Zn-ion batteries

Searching for high-capacity and stable cathodes is of paramount importance to the commercial development of high-energy-density, low-cost, and safe aqueous Zn-ion batteries (ZIBs). A technical challenge to this effort is the capacity-stability tradeoff encountered in all the oxide-based ZIB cathodes. Herein, we report a high-capacity and stable ZIB cathode enabled by in situ electrochemical oxidation of VO2 nanorods into bilayer V2O5•1.75H2O xerogel nano-grids at a high anodic potential (~1.55 V vs. Zn/Zn2+). VO2 is a better precursor than V2O3 to form V2O5•1.75H2O xerogel due to its high stability in aqueous solutions. The activated cathode exhibits a very high discharge capacity (610 mAh g−1 at 0.1 A g−1, a significant increase from 332 mAh g−1 of the pure VO2), superior high-rate performance (~410 mAh g−1 at 5 A g−1) and excellent cycling stability (~ 370 mAh g−1 at 5 A g−1 after 3000 cycles), which translates to an energy density/power density of 295 Wh kg−1/4,656 W kg−1 at 5 A g−1 and high round-trip efficiency of 85%. The activated VO2 is also found to possess low-crystallinity and nanowires morphology and exhibit a pronounced pseudocapacitive behavior and high ionic diffusion coefficient. Strong evidence has also been given to support simultaneous H+ and Zn2+ intercalation/extraction mechanisms in the activated VO2 and that the resultant excellent cycling performance is attributed to the high stability of V2O5•1.75H2O xerogel in the aqueous electrolytes and no formation of Zn3(OH)2V2O7•2H2O. Overall, the demonstrated electrochemically activated VO2 with both high capacity and stability is a promising cathode for practical ZIBs.