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 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 (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 T-Nb2O5@C anode and MSP-20 cathode are remarkable.

Electrochemical Transport Phenomena in Hybrid Pseudocapacitors under Galvanostatic Cycling

“…12 Indeed, it is common for hybrid pseudocapacitors to have electrodes with different loadings in order to match their capacitances. 9,[18][19][20][21][22] Figure 1b indicates that, for hybrid pseudocapacitors featuring MnO 2 positive electrodes, ψ s (t) = ψ P (t) − ψ C (t) varied approximately linearly through most of the cycle. There was a slightly larger slope |dψ s /dt| immediately after the charging/discharging transition.…”

Facile synthesis of T-Nb2O5 nanosheets/nitrogen and sulfur co-doped graphene for high performance lithium-ion hybrid supercapacitors

“…At the 6 th cycle, a first discharge capacity of 133 mA h g −1 is obtained and then a stable reversible capacity of 124.5 mA h g −1 (a low fading of 6.4%) is achieved at the 1000 th cycle. The cycling stability is evidently superior to that of the T-Nb 2 O 5 @C composites [1,45,46], demonstrating the excellent cycling stability of T-Nb 2 O 5 /NS-G. Additionally, the Coulombic efficiency maintains nearly 100%. The co-doped graphene plays vital roles in enhancing the rate performance and cycling stability.…”

“…Supercapacitors (SCs) and Li-ion batteries (LIBs) have been the focus of research and discussion during past decades [1,2]. However, SCs often suffer from the relatively limited energy density despite providing high power density and long cycling stability through fast accumulation of electric charges on the electrode/electrolyte interface [3][4][5][6].…”

“…5d shows that the T-Nb 2 O 5 /NS-G delivers the improved capacitive contribution with the scan rate increasing. Furthermore, compared with the pure T-Nb 2 O 5 [1], the TNb 2 O 5 /NS-G hybrid shows much higher capacitive contribution, which suggests that combining with highly conductive co-doped graphene can improve the inherent capacitive behavior of T-Nb 2 O 5.…”

“…The discharge and charge capacities in the following charge/discharge cycles are almost the same, keeping a reversible capacity of~168 mA h g −1 . Furthermore, the rate capacities of the T- [21], T-Nb 2 O 5 /graphene composite paper (53%) [22] and T-Nb 2 O 5 @C NCs (~50%) [1]. More detailed comparisons with T-Nb 2 O 5 -based composites were given in Table S1.…”