Enhancing pseudocapacitive behavior of MOF-derived TiO_2-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 TiO₂-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.