High-Energy-Density Sodium-Ion Hybrid Capacitors Enabled by Interface-Engineered Hierarchical TiO₂ Nanosheet Anodes

Abstract

Sodium-ion hybrid capacitors are known for their high power densities and superior cycle life compared to Na-ion batteries. However, low energy densities (<100 Wh kg–1) due to the lack of high-capacity (>150 mAh g–1) anodes capable of fast charging are delaying their practical implementation. Herein, we report a high-performance Na-ion hybrid capacitor based on an interface-engineered hierarchical TiO2 nanosheet anode consisting of bronze (∼15%) and anatase (∼85%) crystallites (∼10 nm). This pseudocapacitive dual-phase anode demonstrated exceptional specific capacity of 289 mAh g–1 at 0.025 A g–1 and excellent rate capability (110 mAh g–1 at 1.0 A g–1). The Na-ion hybrid capacitor integrating a dual-phase hierarchical TiO2 nanosheet anode and an activated carbon cathode exhibited a high energy density of 200 Wh kg–1 (based on the total mass of active materials in both electrodes) and power density of 6191 W kg–1. These values are in the energy and power density range of Li-ion batteries (100–300 Wh kg–1) and supercapacitors (5000–15 000 W kg–1), respectively. Furthermore, exceptional capacity retention of 80% is observed after 5000 charge–discharge cycles. Outstanding electrochemical performance of the demonstrated Na-ion hybrid capacitor is credited to the enhanced pseudocapacitive Na-ion intercalation of the two-dimensional TiO2 anode resulting from nanointerfaces between bronze and anatase crystallites. Mechanistic investigations evidenced Na-ion storage through intercalation pseudocapacitance with minimal structural changes. This approach of nanointerface-induced pseudocapacitance presents great opportunities toward developing advanced electrode materials for next-generation Na-ion hybrid capacitors.

In Situ Two‐Step Activation Strategy Boosting Hierarchical Porous Carbon Cathode for an Aqueous Zn‐Based Hybrid Energy Storage Device with High Capacity and Ultra‐Long Cycling Life

“…Figure S10, Supporting Information shows the galvanostatic charging/discharging profiles plotted against time, where the charging process of the aqueous Zn‐HESD can be accomplished in 20 s at 20 A g −1 , which is ascribable to the higher specific surface area and the reasonable size distribution of micropores and mesopores in the hierarchical HPAC, providing more effective active sites, more transfer paths for ions and large contact area with an aqueous electrolyte. Compared with previously reported HESDs based on non‐lithium ions, [ 8a,9c,e,23 ] this aqueous Zn‐HESD shows significant improvement in both the specific capacity and rate capability (Figure 3c). Moreover, the Zn‐HESD exhibits high energy densities (Figure 3d), particularly at high power densities (77.5 Wh kg −1 at 11.4 kW kg −1 ), among the best results of the reported Zn‐ion energy‐storage devices.…”

“…[ 8 ] Several alkali‐ and alkaline‐based HESDs have been developed to improve the electrochemical performances of the lithium‐free energy storage devices. [ 7d,8a,c–e,9 ] However, the application of flammable organic liquid electrolytes brings about serious safety concerns and increases production cost. [ 7d,10 ]…”

“…Proper pore size and pore volume allow the anion storage and, simultaneously, the smooth transfer for the following anions. As typical cathodes for HESDs, AC materials have been prepared by one‐step activation procedure at high temperatures of carbonizing or carbonized precursors, that is, asphalt, [ 15 ] biomass, [9d,e,16 ] coal, [ 17 ] lignin, [ 18 ] etc. However, the one‐step activation of asphalt at high temperatures generally causes the intensive volatilization of light constituent and the polymerization of polycyclic aromatic hydrocarbons, [ 15,19 ] and brings about excessively large pores of the resultant carbon materials.…”

“…At the Zn anode, the plating/stripping of Zn 2+ cations dominate the charging/discharging processes. Figure 2b shows the typical charge/discharge profile of the aqueous Zn‐HESD at 1 A g −1 in a voltage range of 0.01–1.8 V, which presents no obvious voltage plateau and non‐deal linear feature, denoting a hybrid energy storage mechanism [ 8a,9a ] of battery‐type plating/stripping of Zn 2+ ions on the anode and capacitor‐type adsorption/desorption of both CF 3 SO 3 − anions and Zn 2+ ions on the cathode. Figure 2c displays a typical electrochemical cyclic voltammetry (CV) curve of the HESD at a sweep rate of 20 mV s −1 , characterized by neither ideal rectangular shape nor obvious redox peaks, consistent with the hybrid reaction mechanism.…”

“…Moreover, the Zn‐HESD exhibits high energy densities (Figure 3d), particularly at high power densities (77.5 Wh kg −1 at 11.4 kW kg −1 ), among the best results of the reported Zn‐ion energy‐storage devices. [ 8e,9d,e,24 ]…”