Na-ion batteries with lower cost than Li-ion batteries would be developed to large-scale energy-storage device to store solar and wind energies. However, large radius renders Na + ions to insert into/extract out the layered transition metal oxides (LTMOs) sluggishly. To improve the intercalation dynamics of Na + ions, the interlayer spacing of crystals has to be expanded for those LTMOs that are capable of fast lithiation and delithiation. Herein, a LTMO based on vanadium is firstly doped with larger K + ions to expand the interlayer spacing to yield K +-doped sodium vanadate (Na 5 K x V 12 O 32) cathode material by a hydrothermal method at 200 ℃ for 24 h followed by calcination at 500 ℃ for 3 h. The samples were characterized by scanning electron microscope (SEM)/transmission electron microscope (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) technologies. Effect of the doping amount of K + on the structure and sodium-storage performance of the sample was studied in detail. The synthesized materials display nanoplate morphology viewing from the TEM images. K + ions are doped into the interlayer of the sodium vanadate crystallites, which is proved by analysis of the XRD patterns and XPS spectra. Expanded interlayer spacing favors Na + ions' intercalation and deintercalation between the [V 3 O 8 ] layers, which is testified by the chemical diffusion coefficient of cathodes, thus enhancing the rate capability. On the other hand, the chemically pre-intercalated K + ions are pinned in the crystallites during insertion and extraction of Na + ions and act as pillars to stabilize the layered structure, improving the cycliability of the cathode. However, excessive doping of K + leads to a discounted rate capability of the cathode, suggesting an optimized amount of K + doping into the crystal. The results from galvanostatic charge-discharge tests indicate that the obtained NVO(3K) sample, in which 0.118 mol of K + ions are doped into per mol of Na 5 V 12 O 32 , presents the best electrochemical performance among the various samples. It can deliver the maximum capacities of 169, 160, 148, 132, 98 and 69 mAh•g-1 at the rates of 0.1C, 0.2C, 0.5C, 1C, 3C and 10C after activated for several times over the voltage window of 4.0~1.5 V (vs. Na + /Na), respectively. Even run at
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