There are economic and environmental advantages by replacing Li with Na in energy storage. However, sluggishness in the charge/discharge reaction and low capacity are among the major obstacles to development of high-power sodium-ion batteries. Among the electrode materials recently developed for sodium-ion batteries, selenium shows considerable promise because of its high capacity and good cycling ability. Herein, we have investigated the mechanism and kinetics of both sodiation and lithiation reactions with selenium nanotubes, using in situ transmission electron microscopy. Sodiation of a selenium nanotube exhibits a three-step reaction mechanism: (1) the selenium single crystal transforms into an amorphous phase Na0.5Se; (2) the Na0.5Se amorphous phase crystallizes to form a polycrystalline Na2Se2 phase; and (3) Na2Se2 transforms into the Na2Se phase. Under similar conditions, the lithiation of Se exhibits a one-step reaction mechanism, with phase transformation from single-crystalline Se to a Li2Se. Intriguingly, sodiation kinetics is generally about 4-5 times faster than that of lithiation, and the kinetics during the different stages of sodiation is different. Na-based intermediate phases are found to have improved electronic and ionic conductivity compared to those of Li compounds by first-principles density functional theory calculations.
Nickel-cobalt oxides were prepared by coprecipitation of their hydroxides precursors and a following thermal treatment under a moderate temperature. The preformed nickel-cobalt bimetallic hydroxide exhibited a flower-like morphology with single crystalline nature and composed of many interconnected nanosheets. The ratio of Ni to Co in the oxides could easily be controlled by adjusting the composition of the original reactants for the preparation of hydroxide precursors. It was found that both the molecular ratio of Ni to Co and the annealing temperature had significant effects on their porous structure and electrochemical properties. The effect of the Ni/Co ratio on the pseudocapacitive properties of the binary oxide was investigated in this work. The binary metal oxide with the exact molar ratio of Ni:Co = 0.8:1 annealed at 300 °C, showing an optimum specific capacitance of 750 F/g. However, too high an annealing temperature would lead to a large crystal size, a low specific surface area, as well as a much lower pore volume. With the use of the binary metal oxide with Ni:Co = 0.8:1 and activated carbon as the positive and negative electrode, respectively, the assembled hybrid capacitor could exhibit a high-energy density of 34.9 Wh/kg at the power density of 875 W/kg and long cycling life (86.4% retention of the initial value after 10000 cycles).
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