High entropy spinel oxide (HESO) nanoparticles were synthesized via a surfactant-assisted hydrothermal method and used as a novel anode material in a lithium-ion battery. The HESO consists of non-equimolar cations of Cr, Mn, Fe, Co, and Ni dispersed in two Wyckoff sites with various valence states. Due to a strong entropy-induced phase stabilization effect of the HESO, no inactive MgO structural pillars, which are exclusively present in the reported rock salt type high entropy oxides, are required to achieve high electrode cycling stability. A superior charge–discharge capacity of 1235 mA h g⁻¹, the highest among all known HEOs, is obtained with 90% capacity retention after 200 cycles. The unique HESO is also characterized by plenty of oxygen vacancies and three-dimensional Li⁺ transport pathways. Also, great high-rate performance, i.e., 500 mA h g⁻¹ @ 2000 mA g⁻¹, of the HESO electrode is demonstrated.

The electrochemical sodium-ion storage properties of graphene nanosheets (GNSs), carbon nanotubes (CNTs), mesocarbon microbeads (MCMBs), and activated carbon (AC) are investigated. An irreversible oxidation occurs for the AC electrode during desodiation, limiting its use in sodium-ion batteries. The MCMB electrode shows a negligible capacity (∼2 mA h g⁻¹), since the graphitic structure has a low surface area and is thus not capable of storing a sufficient amount of Na⁺. In contrast, the CNT and GNS electrodes exhibit reversible capacities of 82 and 220 mA h g⁻¹, respectively, at a charge–discharge rate of 30 mA g⁻¹. The high electro-adsorption/desorption area, large number of Na⁺ entrance/exit sites, and a large d-spacing of GNSs contribute to their superior Na⁺ storage capacity. At a high rate of 5 A g⁻¹, the GNS electrode still delivers a capacity of as high as 105 mA h g⁻¹, indicating great high-power ability. The charge storage mechanism of the electrode is examined using an ex situ X-ray diffraction technique.

Hydrous manganese oxide with promising pseudocapacitive behavior was deposited on a carbon substrate at anodic potentials of 0.5-0.95 V vs. saturated calomel electrode (SCE) in 0.25 M <mml:math display="inline"><mml:mi mathvariant="normal">Mn</mml:mi><mml:mo stretchy="false">(</mml:mo><mml:msub><mml:mo>CH</mml:mo><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub><mml:mo>COO</mml:mo><mml:msub><mml:mo stretchy="false">)</mml:mo><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:math>Mn(CH3COO)2 solution at 25°C. The effects of the deposition potential on the material characteristics and electrochemical performances of the hydrous manganese oxide prepared were investigated. Porous manganese oxide with higher crystallinity was formed at a lower deposition potential. When the deposition potential was 0.8 <mml:math display="inline"><mml:msub><mml:mo>V</mml:mo><mml:mrow><mml:mo>SCE</mml:mo></mml:mrow></mml:msub></mml:math>VSCE or higher, the deposited oxide consisted of an inner layer with a laminated structure and a rough outer layer with nodules on the surface. X-ray photoelectron spectroscopy was also carried out to examine the chemical state of the deposited oxide. Analytical results indicated that the oxide was composed of both trivalent and tetravalent manganese oxides at a deposition potential of 0.5 <mml:math display="inline"><mml:msub><mml:mo>V</mml:mo><mml:mrow><mml:mo>SCE</mml:mo></mml:mrow></mml:msub><mml:mo>.</mml:mo></mml:math>VSCE. However, the tetravalent manganese oxide became the dominant species in the film deposited at above 0.65 <mml:math display="inline"><mml:msub><mml:mo>V</mml:mo><mml:mrow><mml:mo>SCE</mml:mo></mml:mrow></mml:msub><mml:mo>.</mml:mo></mml:math>VSCE. The manganese oxide formed at 0.5 <mml:math display="inline"><mml:msub><mml:mo>V</mml:mo><mml:mrow><mml:mo>SCE</mml:mo></mml:mrow></mml:msub></mml:math>VSCE exhibited a specific capacitance as high as 240 F/g, as evaluated by cyclic voltammetry (CV) with a potential scan rate of 5 mV/s in 2 M KCl at 25°C. Increasing the CV scan rate reduced the specific capacitance. Only about 70% of the capacitance at 5 mV/s could be maintained when the CV scan rate was increased to 100 mV/s, for all the manganese oxide electrodes prepared. Moreover, a high deposition potential gave rise to a low specific capacitance of the manganese oxide formed. © 2003 The Electrochemical Society. All rights reserved.

A simple, low-cost, and efficient electrochemical strategy, which includes the co-deposition of a Ni–Cu layer, selective etching of Cu from the film (leaving nano-porous Ni), and electrodeposition of Co(OH)₂ nano-whiskers on the obtained Ni substrate, is used to construct a nano-structured electrode. This process can be conducted on many conductive surfaces, which can be cheap, flexible, and wearable, and can be integrated into advanced mobile micro-power systems. Due to its unique nano-architecture, the prepared Co(OH)₂ electrode shows exceptional energy storage performance as compared to that of the conventional version of the electrode. The optimum specific capacitance obtained in this study, evaluated using cyclic voltammetry (CV), was as high as 2800 F/g. When the CV scan rate was increased from 5 to 200 mV/s, only a 4% decay in the capacitance was found, indicating excellent high-power capability. These characteristics make the nano-structured Co(OH)₂ electrode a promising candidate for supercapacitor applications.