RESUMO
The Li1.17Ni0.17Mn0.50Co0.17O2 Li-rich NMC positive electrode (cathode) for lithium-ion batteries has been coated with nanocrystals of the LiMn1.5Co0.5O4 high-voltage spinel cathode material. The coating was applied through a single-source precursor approach by a deposition of the molecular precursor LiMn1.5Co0.5(thd)5 (thd = 2,2,6,6-tetramethyl-3,5-heptanedionate) dissolved in diethyl ether, followed by thermal decomposition at 400 °C inair resulting in a chemically homogeneous cubic spinel. The structure and chemical composition of the coatings, deposited on the model SiO2 spheres and Li-rich NMC crystallites, were analyzed using powder X-ray diffraction, electron diffraction, high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and energy-dispersive X-ray (EDX) mapping. The coated material containing 12 wt.% of spinel demonstrates a significantly improved first cycle Coulombic efficiency of 92% with a high first cycle discharge capacity of 290 mAhg-1. The coating also improves the capacity and voltage retention monitored over 25 galvanostatic charge-discharge cycles, although a complete suppression of the capacity and voltage fade is not achieved.
RESUMO
NASICON-structured Na3V2O2x (PO4)2F3-2x (0 < x ≤ 1) solid solutions have been prepared using a microwave-assisted hydrothermal (MW-HT) technique. Well-crystallized phases were obtained for x = 1 and 0.4 by reacting V2O5, NH4H2PO4, and NaF precursors at temperatures as low as 180-200 °C for less than 15 min. Various available and inexpensive reducing agents were used to control the vanadium oxidation state and final product morphology. The vanadium oxidation state and O/F ratios were assessed using electron energy loss spectroscopy and infrared spectroscopy. According to electron diffraction and powder X-ray diffraction, the Na3V2O2x (PO4)2F3-2x solid solutions crystallized in a metastable disordered I4/mmm structure (a = 6.38643(4) Å, c = 10.62375(8) Å for Na3V2O2(PO4)2F and a = 6.39455(5) Å, c = 10.6988(2) Å for Na3V2O0.8(PO4)2F2.2). With respect to electrochemical Na+ (de)insertion as positive electrodes (cathodes) for Na-ion batteries, the as-synthesized materials displayed two sloping plateaus upon charge and discharge, centered near 3.5-3.6 V and 4.0-4.1 V vs. Na+/Na, respectively, with a reversible capacity of â¼110 mA h g-1. The application of a conducting carbon coating through the surface polymerization of dopamine with subsequent annealing at 500 °C improved both the rate capability (â¼55 mA h g-1 at a discharge rate of 10C) and capacity retention (â¼93% after 50 cycles at a discharge rate of C/2).
