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 (<i>T</i>) and pseudohexagonal (<i>TT</i>)) <i>via</i> a facile one-pot synthesis method based on a water-in-oil microemulsion system. The synthesis of ideal <i>T</i>-Nb₂O₅ for fast Li⁺ diffusion is simply achieved by controlling the microemulsion parameter (<i>e.g.,</i> pH control). The <i>T</i>-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 <i>vs</i> Li/Li⁺) with rapid rate capability compared to that of <i>TT</i>-Nb₂O₅@C and carbon shell-free Nb₂O₅ NCs, mainly due to synergistic effects of (i) the structural merit of <i>T</i>-Nb₂O₅ and (ii) the conductive carbon shell for high electron mobility. The highest energy (∼63 W h kg^–1) and power (16 528 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 <i>T</i>-Nb₂O₅@C anode and MSP-20 cathode are remarkable