The author of the paper: Professor Huang Haitao / Professor Hu Jinlian from Hong Kong University of technology have paid close attention to sodium ion batteries because of the uneven distribution and high price of lithium resources, and the rich and low price of sodium resources in the earth's crust
However, the radius of sodium ion is larger than that of lithium ion, so the kinetics is relatively slow, which leads to the unsatisfactory electrochemical performance, especially its rate performance
Recently, Professor Huang Haitao / Professor Hu Jinlian's research group of Hong Kong University of science and technology designed the fiber in the co9s8-c / Co9S8 tube, and the co9s8-c composite nanofiber was sheathed inside the Co9S8 nanotube to form the fiber structure in the tube
The hollow structure can effectively buffer the volume change of CO 9s 8 during the cycle to achieve long cycle life and high rate performance
The co 9s 8-c composite nanofibers in the nanotubes not only increase the density of the active materials in the composites, but also provide a conductive path for electrons
Experimental and simulation results show that the pseudo capacitance mechanism plays a leading role in the storage performance of high efficiency sodium ions
The design of abundant grain boundaries, three exposed layer interfaces and carbon wiring in the fiber structure is conducive to the pseudo capacitance mechanism
At the current density of 0.5 a · g-1, after 150 cycles, the mixed fiber anode in the tube shows a high specific capacity of 616 MAH · g-1
At 1 a · G - 1 ratio, the capacity after 500 cycles is about 451 MAH · G - 1
Relevant research results were published in angelw
Chem
Int.ed
(DOI: 10.1002/anie.201900076) under the title of "fiber ‐ in ‐ tube design of CO 9s 8 ‐ carbon/co 9s 8 enables efficient sodium storage", and were included as very important paper (VIP)
The first author of the article was Dr
Li Xiaoyan
Due to the strong coordination between cobalt and 2-methylimidazole, zif-67 was deposited on electrospun pan / CO (AC) 2 nanofibers
In the process of vulcanization, due to the dissolution of zif-67 and the reaction mechanism of diffusion control, the fiber structure in the tube is formed
After vulcanization, the material is further heat treated in nitrogen to obtain co 9s 8-c / CO 9s 8 composite nanomaterials (Figure 1)
The design of the hollow structure can effectively buffer the volume change of the active material co 9s 8 during the charging and discharging process; the co 9s 8-c composite nanofiber in the nanotube not only increases the density of the active material in the composite, but also provides a conductive path for electrons
In addition, the three exposed layer interfaces of the fibers in the tube are very conducive to the pseudocapacitance storage of sodium ions
Fig
1 the structure of the fibers in the co 9s 8-c / CO 9s 8 tube is revealed in the electron microscope picture of the fibers in the co 9s 8-c / CO 9s 8 tube
XPS and XRD characterization also confirmed the cubic system of CO 9s 8 (Fig
2)
Fig
2 A, b) SEM; C, d) tem; E-H) EDX element distribution; I, J) HRTEM; K) XRD; L, m) The results of XPS spectrum (source: angel
Chem
Int
ed.) show that the high specific capacity of 616 MAH · g-1 can be seen in the co 9s 8-c / CO 9s 8 tube after 150 cycles at the current density of 0.5 a · g-1
After 500 cycles at 1 a · G - 1 ratio, the capacity was maintained about 451 MAH · G - 1
In addition, at a high current density of 10 a · g-1, the reversible capacity of the device reaches 422 MAH · g-1
Its excellent rate performance is better than most of the reported cobalt sulfide electrode materials (Fig
3)
Fig
3 electrochemical performance diagram of fiber in CO 9 s 8-c / CO 9 s 8 tube: a) cyclic voltammetry; b) charge discharge curve; c) rate performance diagram; d) cycle curve; E) energy comparison diagram; F) rate performance comparison diagram (source: angelw
Chem
Int
ed.) for further study of CO 9 s 8-c / CO 9 s 8 The electrochemical kinetics of CO 9s 8-c / CO 9s 8 was analyzed by cyclic voltammetry at different scanning rates and impedance spectroscopy at different temperatures
The results show that pseudocapacitor storage is the dominant mechanism, which is conducive to the rapid transmission of electrons and ions
At the same time, the charge transfer activation energy obtained from Arrhenius plot also shows the fast transport state of sodium ions
Fig
4 electrochemical kinetic analysis: a) cyclic voltammetry at different scanning rates; b) peak current versus scanning rate; c) capacitance storage contribution at 0.2mv-1; d) capacitance storage contribution at different scanning rates; E) Nyquist diagram at different temperatures; F) relationship between electronic activation energy ln (R-1 · T) and 1000 / T (source: angelw
Chem
Int
ed.) in order to better reveal the mechanism of pseudocapacitance, the author conducted simulation analysis
The simulation results show that CO 9s 8 can hold up to eight sodium, and the adsorption binding energy is - 2.025ev, which also shows the rapid reaction process between CO 9s 8 and sodium (Figure 5)
Fig
5 a) schematic diagram of high rate sodium storage of fiber in CO 9s 8-c / CO 9s 8 tube; b) optimal structure diagram of adsorption of one sodium and C) adsorption of eight sodium (source: angelw
Chem
Int
ed.) related research results are published in angelw
Chem
Int
ed
(DOI: 10.1002 / anie
201900076)
The work was supported by the National Natural Science Foundation, the Hong Kong Innovation and Technology Fund and the Hong Kong Research Grants Council.
