| Lithium-ion batteries（LIBs） have been the most promising devices for sustainable energy development because of their high safety, high energy density and long lifetime. Recently, the global demand for LIBs has grown quite rapidly owing to their applications in mobile products, electric vehicles（EV） and energy-storage systems. Considering the limitations of the lithium resources and the certain geographic locations, large-scale lithium energy storage systems could be inevitably restricted. Thus, alternative battery materials are actively sought. Because of the widely distribution and high abundance of sodium in the Earth’ crust, the sodium ion battery is now regarded a promising candidate. In addition, sodium is the second-lightest alkali metal next to lithium. Therefore, sodium ion batteries for large-scale energy storage have a favorable prospect of development. On the basis of elemental abundance and economic, high capacity P2-type Na2/3Fe1/2Mn1/2O2 layered material is regarded as potential cathode material for sodium-ion batteries.In this paper, P2-type Na2/3Fe1/2Mn1/2O2 layered material was synthesized by a chelating-agent-assisted sol-gel method with NH3?H2O as a chelating agent. Based on the addition of NH3?H2O and the controlling of the synthesis conditions, highly active material with good electrochemical performance was obtained. Compared the XRD patterns of the two samples NFMO-900 and NFMO-900 B, the particle size of these two samples are similar. And sample NFMO-900 is more homogeneous due to the NH3?H2O-assistant sol-gel procedure, which is result from the formation of the homogeneous precursor because of the using of chelating agent NH3?H2O. For samples NFMO-900 and NFMO-900B（a blank sample without NH3?H2O）, the initial capacities are 126.3 and 134.5 m Ah?g-1, respectively. After 30 cycles, the coulombic efficiency of sample NFMO-900 is significantly higher than sample NFMO-900 B. The higher capacity retention of sample NFMO-900 may be ascribed to the effects of NH3?H2O, which could adjust the potential of hydrogen of the mixed solution to a moderate site. Hence, the NH3?H2O could improve the chelation to form the material with better performance.Calcination temperature has an important effect on the electrochemical performance of the active materials for rechargeable batteries. Hence, we calcined the precursor at 700 oC, 800 oC, 900 oC, 1000 oC, respectively. And studied their electrochemical properties and physical characterization, all of the four samples have similar XRD patterns. With increasing temperature the intense of characteristic peak increased, and both a（a=b） and c have slight changes. Sample NFMO-900 shows a longest length of a-aix（a= b=2.927?）, which has the utmost in-plane atomic distance accompanying the minimum electrostatic repulsion between O-O atoms leading to the stability. And a slightly grown c-axis, which can be contribute to the ionic transfer. To further investigate the effect of temperature, EIS is conducted. The high DNa+ values of sample NFMO-900 with 1.28×10-13 cm2s-1 should be attributed to the stable structure and wider track of Na+ migration. The first discharge capacities of the four samples NFMO-700, NFMO-800, NFMO-900, NFMO-1000 are 130.7 m Ahg-1, 138.5 m Ahg-1, 132.2m Ahg-1 and 125 m Ahg-1. After 40 cycles, the capacity retention is less than 65% except sample NFMO-900. Apparently, sample NFMO-900 displays the best performance.It also reported that the voltage range has effects on the phase transitions and electrochemical performance. After being charged to 4.0V, the diffraction patterns still have no obvious change. However, the diffraction patterns are obviously changed from the P2-type when it charged to 4.2V because of the gliding of the Me O2 layers. And the XRD patterns are consistent with neither the P2-type nor the O2-type, it is an intermediate phase OP4-type with a stacking fault, in which the prismatic and octahedral are piled up along the c-axis direction alternately. And the crystal structure is gradually restored during the sodium intercalation process. Based on the effects of voltage range on the phase transitions, we also investigate the influence on cyclic voltammograms. Both of the two voltage range（1.5-4.0V and1.5-4.3V Na+/Na）, the redox reaction of Mn3+→Mn4+and Fe3+→Fe4+ taking place. Due to the energy for migration are different because of the different migration paths of Na+ in P2 or O2, the reversibility of sample NFMO-900 in the high voltage range is inferior to the relative narrow one. The initial capacities in the range of 1.5-4.0V and 1.5-4.3V are 135.7 and 177.5 m Ahg-1, respectively. After 30 cycles, the discharge capacities of sample NFMO-900 measured in the voltage ranges of 1.5-4.0V and 1.5-4.3V are 92 and 82.9 m Ahg-1, respectively.EIS tests are next conducted to investigate the difference in sodium ions diffusion coefficient between the as-prepared materials and the 10 th cycled. And the Na+ diffusion coefficients of the no cycling and 10 th cycled are 1.28×10-13cm2s-1, 4.85×10-14 cm2s-1, respectively. The decreased of the diffusion coefficient could be ascribed to the host structure distortion and expanded through sodium ions extraction and intercalation. |
PDF Full Text Request