Valence and surface modulated vanadium oxide nanowires as new high-energy and durable negative electrode for flexible asymmetric supercapacitors

Designing high-energy and durable negative electrode materials is highly desirable for advancing the performance of asymmetric supercapacitors (ASCs). The feasibility of vanadium oxides (VOx) as negative electrodes in ASCs has been revealed, but most of them suffered from fast capacitance fading. Additionally, achieving high areal capacitance and gravimetric capacitance for an electrode simultaneously is very challenging. In this work, we solve well these issues through introducing low-valence-state V3+ and phosphate ions into vanadium oxides (PVO) nanowires by a simple and innovative strategy, which exhibits unprecedented capacitive properties and long-term stability in negative potential window. Because of the dramatically increased active sites, improved electrochemical reversibility, enhanced electron and Li+ ion transport rates, the as-prepared PVO electrode delivers ultrahigh areal and gravimetric capacitances of 1.57 F/cm2 and 1652.3 F/g at 2 mA/cm2. Furthermore, an impressive cyclic durability of 20000 cycles (almost 100% retention) is obtained for the PVO electrode, substantially superior to the untreated vanadium oxide electrode (22.8% retention). When employing this PVO as a negative electrode, a 1.8 V-flexible and stable ASC device with an admirable energy density of 1.93 mWh/cm3 is achieved.