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

Abstract

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 Nb<inf>2</inf>O<inf>5</inf>@carbon core-shell nanocyrstals (Nb<inf>2</inf>O<inf>5</inf>@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-Nb<inf>2</inf>O<inf>5</inf> for fast Li+ diffusion is simply achieved by controlling the microemulsion parameter (e.g., pH control). The T-Nb<inf>2</inf>O<inf>5</inf>@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-Nb<inf>2</inf>O<inf>5</inf>@C and carbon shell-free Nb<inf>2</inf>O<inf>5</inf> NCs, mainly due to synergistic effects of (i) the structural merit of T-Nb<inf>2</inf>O<inf>5</inf> 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-Nb<inf>2</inf>O<inf>5</inf>@C anode and MSP-20 cathode are remarkable. © 2015 American Chemical Society