Battery-type electrodes of three-dimensional (3D) hierarchical cobalt hydroxide carbonate arrays on Ni foam were fabricated using a hydrothermal method for use in supercapacitors. X-ray diffraction analysis, scanning electron microscopy, and transmission electron microscopy were used to characterize their structures and morphologies. The cobalt hydroxide carbonate synthesized with 10 h reaction time showed the highest specific capacitance (1381 F g−1 at a current density of 2 A g−1) and excellent cycling stability (92% capacitance retention after 5000 cycles). Moreover, its capacitance increased by 33% at 2 A g−1 and by 10% at 20 A g−1 after 5000 charge–discharge cycles. This cobalt hydroxide carbonate composite is a promising candidate for electrochemical energy-related applications
The effect of fluorine doping on the electrochemical performance of LiFePO 4 /C cathode material is investigated. The stoichiometric proportion of LiFe(PO 4 ) 1 −x F 3x /C (x=0.01, 0.05, 0.1, 0.2) materials was synthesized by a solid-state carbothermal reduction route at 650°C using NH 4 F as dopant. X-ray diffraction, scanning electron microscope, energy-dispersive X-ray, and X-ray photoelectron spectroscopy analyses demonstrate that fluorine can be incorporated into LiFePO 4 /C without altering the olivine structure, but slightly changing the lattice parameters and having little effect on the particle sizes. However, heavy fluorine doping can bring in impurities. Fluorine doping in LiFePO 4 /C results in good reversible capacity and rate capability. LiFe(PO 4 ) 0.95 F 0.15 /C exhibits highest initial capacity and best rate performance. Its discharge capacities at 0.1 and 5 C rates are 156.1 and 119.1 mAh g −1 , respectively. LiFe(PO 4 ) 0.95 F 0.15 /C also presents an obviously better cycle life than the other samples. We attribute the improvement of the electrochemical performance to the smaller charge transfer resistance (R ct ) and influence of fluorine on the PO 4 3− polyanion in LiFePO 4 /C.
LiFe0.9V0.1(PO4)0.95F0.15/C was prepared via solid-state carbothermal reaction (CTR). F and V codoping did not alter the olivine structure of LiFePO4 but reduced the particle size and improved the Li+ diffusion coefficient. The cells based on this material showed higher discharge capacity, working voltage, rate capability, and better cyclic performance than that of undoped and F-doped materials.
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