Effect of K-Doping on the Sodium-storage Performance of Sodium Vanadate Nanoplates

Abstract

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

VOC Bonding of Heterointerface Boosting Kinetics of Free‐Standing Na5V12O32 Cathode for Ultralong Lifespan Sodium‐Ion Batteries

“…Vanadium-based compounds possess multivalent states of vanadium (from V 2+ to V 5+ ) and a wide interplanar spacing, which provides more possibilities to further extend their specific capacities by enabling the reversible Na + intercalation/deintercalation. [1][2][3][4][5][6][7][8] Among them, in numerous reports, sodium vanadium oxides have displayed satisfying capacities acting as cathode of SIBs when operated in a wider voltage window (1.5-4.0 V). [9][10][11][12][13][14][15][16] In our previous work, the sodium vanadium bronze (NaV 6 O 15 ) nanotubes of clew-like carbon-coated were designed, which could supply an exceptional capacity of 209 mAh g −1 at 25 mA g −1 .…”

“…[ 7 ] Furthermore, a K + ‐doped sodium vanadate (Na 5‐x K x V 12 O 32 ) cathode was synthesized with a reversible capacity of 169 mAh g −1 at 26 mA g −1 . [ 8 ] Dong et al also synthesized hierarchicalzigzag Na 1.25 V 3 O 8 nanowires that showed the capacity of 172.5 mA h g −1 at 100 mA g −1 . [ 9 ] Indeed, the Na 1.1 V 3 O 7.9 nanobelts assembled by Yuan et al showed the capacity of 173 mA h g −1 at 25 mA g −1 .…”