The relatively low operating voltage window of aqueous energy storage devices is a key parameter that limits their energy density. Electrode materials with high electrochemical activities and a wide stable working potential range are crucially needed. Herein, we reported a strategy to control the working potential range of the negative electrode by optimizing the component proportion of molybdenum-tungsten-oxide solid-state solutions. The operating potential range of the molybdenum-tungsten-oxide solid-state solutions was tunable between −0.4 and −1.2 V. An asymmetric supercapacitor device was fabricated by using a Mo0.1W0.9O3-x/single-walled carbon nanotube film as the negative electrode and a commercial activated carbon film as the positive electrode. The optimized device showed a stable working voltage of 2.0 V in 1 M Li2SO4 aqueous electrolyte. This study opens up new avenues for developing high voltage window aqueous energy storage devices.