Creating oxygen-vacancies in MoO3-x nanobelts toward high volumetric energy-density asymmetric supercapacitors with long lifespan

Highlights: • Oxygen vacancies-rich MoO_3-x is prepared through a novel defect-engineering strategy. • Oxygen-vacancies greatly increase the interlayer spacing, electrical conductivity and electrochemical activity of MoO₃. • High specific capacity up to 306 C g⁻¹ (692 C cm⁻³) is obtained. • An ultrahigh volumetric energy density of 319 Wh L⁻¹ is obtained, the highest value for aqueous supercapacitors up to date. • ASC displays an ultra-long lifespan with 101% retention ratio after 30,000 cycles. -- Abstract: Herein, we present the synthesis of oxygen vacancies-rich α-MoO_3-x nanobelts through a novel defect-engineering strategy. The oxygen-vacancies could not only greatly increase the interlayer spacing and the electrical conductivity of MoO₃, but also significantly enhance the electrochemical activity, which promotes faster charge storage kinetics. Meanwhile, to further facilitate the electron transfer and ion transport, a graphene nanomesh-carbon nanotube/MoO_3-x (GC/MoO_3-x) nanocomposite with three-dimensional sandwiched structure was fabricated, which displays high specific capacity up to 306 C g⁻¹ as well as high volumetric capacity of 692 C cm⁻³. Our fabricated asymmetric supercapacitor (ASC) with the GC/MoO_3-x and GC/MnO₂ nanocomposites as anode and cathode, respectively, exhibits an ultrahigh energy of 150 Wh kg⁻¹, corresponding to an impressive volumetric energy density of 319 Wh L⁻¹. Notably, both the gravimetric and volumetric energy densities are much higher than most of the previously reported metal oxide based ASCs in aqueous electrolytes. Furthermore, the ASC displays an ultra-long lifespan with 101% retention ratio after 30,000 cycles. The outstanding performances of GC/MoO_3-x composite render it a highly promising candidate for next-generation supercapacitors with both high energy and power densities in future applications, especially in greatly limited space.