Pseudocapacitive materials generally offer both high capacitance and high rate capability, which has stimulated great efforts in developing the materials system and related energy storage devices. In recent years, however, with the extensive use of nanomaterials in batteries, fast redox kinetics comparable to pseudocapacitive have been achieved in many kinds of battery materials due to the much shortened ion diffusion lengths and highly exposed surface/interface as a result of nanosize effect. Consequently, the terms “pseudocapacitive materials” and “battery materials” are becoming more and more confusing. In this review, different opinions on the definition of pseudocapacitive materials and the evolution of the definitions as well as the resulting confusion will be firstly reviewed. Then, to accurately distinguish pseudocapacitive and battery materials, method with the consideration of both the electrochemical signatures (CVs and GCD) and quantitative kinetics analysis as a supplement is proposed. Finally, we end this review by discussing the possible device configurations of asymmetric supercapacitors and hybrid supercapacitors. The present review will help understanding the differences between pseudocapacitive materials and battery materials, and thus avoiding the definition confusion.
One of the key challenges of aqueous supercapacitors is the relatively low voltage (0.8-2.0 V), which significantly limits the energy density and feasibility of practical applications of the device. Herein, this study reports a novel Ni-Mn-O solid-solution cathode to widen the supercapacitor device voltage, which can potentially suppress the oxygen evolution reaction and thus be operated stably within a quite wide potential window of 0-1.4 V (vs saturated calomel electrode) after a simple but unique phase-transformation electrochemical activation. The solid-solution structure is designed with an ordered array architecture and in situ nanocarbon modification to promote the charge/mass transfer kinetics. By paring with commercial activated carbon anode, an ultrahigh voltage asymmetric supercapacitor in neutral aqueous LiCl electrolyte is assembled (2.4 V; among the highest for single-cell supercapacitors). Moreover, by using a polyvinyl alcohol (PVA)-LiCl electrolyte, a 2.4 V hydrogel supercapacitor is further developed with an excellent Coulombic efficiency, good rate capability, and remarkable cycle life (>5000 cycles; 95.5% capacity retention). Only one cell can power the light-emitting diode indicator brightly. The resulting maximum volumetric energy density is 4.72 mWh cm , which is much superior to previous thin-film manganese-oxide-based supercapacitors and even battery-supercapacitor hybrid devices.
MnO2 is one of the most studied cathodes for aqueous neutral zinc‐ion batteries. However, the diverse reported crystal structures of MnO2 compared to δ‐MnO2 inevitably suffer a structural phase transition from tunneled to layered Zn‐buserite during the initial cycles, which is not as kinetically direct as the conventional intercalation electrochemistry in layered materials and thus poses great challenges to the performance and multifunctionality of devices. Here, a binder‐free δ‐MnO2 cathode is designed and a favorable “layered to layered” Zn2+ storage mechanism is revealed systematically using such a “noninterferencing” electrode platform in combination with ab initio calculation. A flexible quasi‐solid‐state Zn–Mn battery with an electrodeposited flexible Zn anode is further assembled, exhibiting high energy density (35.11 mWh cm−3; 432.05 Wh kg−1), high power density (676.92 mW cm−3; 8.33 kW kg−1), extremely low self‐discharge rate, and ultralong stability up to 10 000 cycles. Even with a relatively high δ‐MnO2 mass loading of 5 mg cm−2, significant energy and power densities are still achieved. The device also works well over a broad temperature range (0–40 °C) and can efficiently power different types of small electronics. This work provides an opportunity to develop high‐performance multivalent‐ion batteries via the design of a kinetically favorable host structure.
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