Enhancing pseudocapacitive behavior of MOF-derived TiO2-x@Carbon nanocubes via Mo-doping for high-performance sodium-ion capacitors.

Sodium-ion capacitors (SICs) have been viewed as promising energy storage devices because of their high power/energy density, cycling stability and cost-efficiency, but they are also restricted by the unmatched reaction kinetics between the battery-type anode and capacitor-type cathode. Herein, we present a novel way to enhance the pseudocapacitive storage behavior and reaction kinetics of TiO 2 -based anode via Mo-doping and carbon hybridization, using the Mo-doped titanium metal-organic framework (Ti-MOF, MIL-125) as the precursor. Appropriate amount of Mo-doping (Mo:Ti = 1:9) induces the shape evolution from the round MIL-125 nanotablets to square Mo-MIL-125 nanocubes, which can be readily converted to Mo-doped TiO 2-x @carbon composite with conformal morphology (namely, Mo 0.1 -TiO 2-x @C). Mo-doping increases the concentration of Ti3+/oxygen vacancy and decreases its crystallinity, which greatly enhances the reaction kinetics and sodium storage performance. When examined in half-cells, the Mo 0.1 -TiO 2-x @C anode exhibits higher pseudocapacitive contribution (∼85%), higher reversible capacity (216 mAh g−1 at 0.5 A g−1), and better cycling and rate capability (185 mAh g−1 even after 3000 cycles at 1 A g−1). When paired with commercial activated carbon (AC) as cathode, the Mo 0.1 -TiO 2-x @C//AC SICs deliver a maximum energy density of 269.37 Wh kg−1 at a power density of 80.4 W kg−1 and 61.75 Wh kg−1 even at a high power density of 5421.95 W kg−1. [Display omitted] • Approximate Mo-doping induces the morphology change of MIL-125 from round tablet-like to square cube-like shapes. • Mo-doping TiO 2-x @C nanocubes display more defective Ti3+ species and increased oxygen vacancies. • Mo 0.1- TiO 2-x @C nanocubes display predominantly pseudocapacitive sodium storage behavior. • Mo 0.1- TiO 2-x @C//AC SIC displays a high operating voltage of 4 V and a maximum energy density of 80.4 Wh kg−1. [ABSTRACT FROM AUTHOR]