Facile Synthesis of Nb₂O₅@Carbon Core-Shell Nanocrystals with Controlled Crystalline Structure for High-Power Anodes in Hybrid Supercapacitors

Hybrid supercapacitors (battery-supercapacitor hybrid devices, HSCs) deliver high energy within seconds (excellent rate capability) with stable cyclability. One of the key limitations in developing high-performance HSCs is imbalance in power capability between the sluggish Faradaic lithium-intercalation anode and rapid non-Faradaic capacitive cathode. To solve this problem, we synthesize Nb₂O₅@carbon core-shell nanocyrstals (Nb₂O₅@C NCs) as high-power anode materials with controlled crystalline phases (orthorhombic (T) and pseudohexagonal (TT)) via a facile one-pot synthesis method based on a water-in-oil microemulsion system. The synthesis of ideal T-Nb₂O₅ for fast Li⁺ diffusion is simply achieved by controlling the microemulsion parameter (e.g., pH control). The T-Nb₂O₅@C NCs shows a reversible specific capacity of ∼180 mA h g^-1 at 0.05 A g^-1 (1.1-3.0 V vs Li/Li⁺) with rapid rate capability compared to that of TT-Nb₂O₅@C and carbon shell-free Nb₂O₅ NCs, mainly due to synergistic effects of (i) the structural merit of T-Nb₂O₅ and (ii) the conductive carbon shell for high electron mobility. The highest energy (∼63 W h kg^-1) and power (16528 W kg^-1 achieved at ∼5 W h kg^-1) densities within the voltage range of 1.0-3.5 V of the HSC using T-Nb₂O₅@C anode and MSP-20 cathode are remarkable.