In Situ Orthorhombic to Amorphous Phase Transition of Nb2O5 and Its Temperature Effect on Pseudocapacitive Behavior

Niobium pentoxide (Nb₂O₅) represents an exquisite class of negative electrode materials with unique pseudocapacitive kinetics that engender superior power and energy densities for advanced electrical energy storage devices. Practical energy devices are expected to maintain stable performance under real-world conditions such as temperature fluctuations. However, the intercalation pseudocapacitive behavior of Nb₂O₅ at elevated temperatures remains unexplored because of the scarcity of suitable electrolytes. Thus, in this study, we investigate the effect of temperature on the pseudocapacitive behavior of submicron-sized Nb₂O₅ in a wide potential window of 0.01-2.3 V. Furthermore, ex situ X-ray diffraction and X-ray photoelectron spectroscopy reveal the amorphization of Nb₂O₅ accompanied by the formation of NbO via a conversion reaction during the initial cycle. Subsequent cycles yield enhanced performance attributed to a series of reversible Nb^V, IV/Nb^III redox reactions in the amorphous Li_xNb₂O₅ phase. Through cyclic voltammetry and symmetric cell electrochemical impedance spectroscopy, temperature elevation is noted to increase the pseudocapacitive contribution of the Nb₂O₅ electrode, resulting in a high rate capability of 131 mAh g^-1 at 20,000 mA g^-1 at 90 °C. The electrode further exhibits long-term cycling over 2000 cycles and high Coulombic efficiency ascribed to the formation of a robust, [FSA]^--originated solid-electrolyte interphase during cycling.

Nanostructure and Advanced Energy Storage: Elaborate Material Designs Lead to High-Rate Pseudocapacitive Ion Storage

The drastic need for development of power and electronic equipment has long been calling for energy storage materials that possess favorable energy and power densities simultaneously, yet neither capacitive nor battery-type materials can meet the aforementioned demand. By contrast, pseudocapacitive materials store ions through redox reactions with charge/discharge rates comparable to those of capacitors, holding the promise of serving as electrode materials in advanced electrochemical energy storage (EES) devices. Therefore, it is of vital importance to enhance pseudocapacitive responses of energy storage materials to obtain excellent energy and power densities at the same time. In this Review, we first present basic concepts and characteristics about pseudocapacitive behaviors for better guidance on material design researches. Second, we discuss several important and effective material design measures for boosting pseudocapacitive responses of materials to improve rate capabilities, which mainly include downsizing, heterostructure engineering, adding atom and vacancy dopants, expanding interlayer distance, exposing active facets, and designing nanosheets. Finally, we outline possible developing trends in the rational design of pseudocapacitive materials and EES devices toward high-performance energy storage.

Confined Selenium in N-Doped Mesoporous Carbon Nanospheres for Sodium-Ion Batteries

In this paper, we have adopted a simple and etching-free method to prepare mesoporous carbon spheres in one step. Selenium can be deposited in the internal cavity, which can avoid pulverization due to the combined effect of volume expansion and a solid-electrolyte interphase (SEI) film while charging and discharging. Therefore, the as-prepared selenium and nitrogen codoped mesoporous carbon nanosphere (Se@NMCS) composites can deliver an outstanding sodium-storage performance of 336.6 mAh g^-1 at a present density of 200 mA g^-1 and great long-cycling performance. For a further understanding of the Na⁺ storage mechanism of the Se@NMCS anode in sodium-ion batteries (SIBs), the phase evolution of the Se@NMCS anode has been explored during the charge/discharge process by conducting in situ Raman investigation.

True Meaning of Pseudocapacitors and Their Performance Metrics: Asymmetric versus Hybrid Supercapacitors

The development of pseudocapacitive materials for energy-oriented applications has stimulated considerable interest in recent years due to their high energy-storing capacity with high power outputs. Nevertheless, the utilization of nanosized active materials in batteries leads to fast redox kinetics due to the improved surface area and short diffusion pathways, which shifts their electrochemical signatures from battery-like to the pseudocapacitive-like behavior. As a result, it becomes challenging to distinguish "pseudocapacitive" and "battery" materials. Such misconceptions have further impacted on the final device configurations. This Review is an earnest effort to clarify the confusion between the battery and pseudocapacitive materials by providing their true meanings and correct performance metrics. A method to distinguish battery-type and pseudocapacitive materials using the electrochemical signatures and quantitative kinetics analysis is outlined. Taking solid-state supercapacitors (SSCs, only polymer gel electrolytes) as an example, the distinction between asymmetric and hybrid supercapacitors is discussed. The state-of-the-art progress in the engineering of active materials is summarized, which will guide for the development of real-pseudocapacitive energy storage systems.

Realizing an Asymmetric Supercapacitor Employing Carbon Nanotubes Anchored to Mn3O4 Cathode and Fe3O4 Anode

A facile route to anchor pseudocapacitive materials on multiwalled carbon nanotubes (CNTs) to realize high-performance electrode materials for asymmetric supercapacitors (ASCs) is reported. The anchoring process is developed after direct decomposition of metal-hexacyanoferrate complex on the CNT surface. Transmission electron microscopy (TEM) analysis reveals that the nanoparticles (NPs) are discretely attached over the CNT surface without forming a uniform layer, thus making most of the entire NP surface available for electrochemical reactions. Accordingly, CNT-Mn₃O₄ nanocomposite cathode shows significantly improved capacitive performance as compared to pristine CNT electrode, validating the efficacy of designing the composite electrode. With CNT-Fe₃O₄ nanocomposite as the paired anode, the hybrid ASC delivers a specific capacitance of 135.2 F/g at a scan rate of 10 mV/s within a potential window of 0-1.8 V in the aqueous electrolyte and retains almost 100% of its initial capacitance after 15,000 cycles. The serially connected ASCs can power commercial light-emitting diodes (LEDs) and mobile phones, reflecting their potential in next-generation storage applications.

Flexible Asymmetric Supercapacitors with Ultrahigh Energy Density through Synergistic Design of Electrodes

Despite being among the most researched energy storage devices, supercapacitors have often suffered from their relatively low operating voltage and energy density, which greatly limit their practical applications. In this work, asymmetric supercapacitors (ASCs) are developed by synergistically designing carbon nanotube composite electrodes with 3D porous structures. The resultant ASC devices exhibit an extended operating voltage of 1.8 V, much higher than that of symmetric supercapacitors (≤1.0 V). Significantly, the obtained ASC devices deliver ultrahigh volumetric energy density as high as 19.8 mWh cm-3 (corresponding to an areal energy density of 198 µWh cm-2), which is the highest value among reported ASC devices. In addition, the ASC devices not only possess outstanding cycling stability and long self-discharging time, but also exhibit excellent mechanical flexibility under any bending states, even over 5000 bending cycles. The demonstrated flexible ASC devices with high performance are promising to be used as power sources for next-generation portable and wearable electronics.

Hierarchically superstructured prussian blue analogues: spontaneous assembly synthesis and applications as pseudocapacitive materials

Hierarchically superstructured Prussian blue analogues (hexacyanoferrate, M=Ni(II) , Co(II) and Cu(II) ) are synthesized through a spontaneous assembly technique. In sharp contrast to macroporous-only Prussian blue analogues, the hierarchically superstructured porous Prussian blue materials are demonstrated to possess a high capacitance, which is similar to those of the conventional hybrid graphene/MnO2 nanostructured textiles. Because sodium or potassium ions are involved in energy storage processes, more environmentally neutral electrolytes can be utilized, making the superstructured porous Prussian blue analogues a great contender for applications as high-performance pseudocapacitors.