Zhou Yongning research group of Fudan University has made new progress in the research of cathode materials for sodium ion batteries

Sodium ion battery is a research hotspot in the field of energy storage at present It has the same working principle as lithium ion battery Like lithium ion, sodium ion can move reversibly between the positive and negative poles to realize the storage and release of electric energy Due to the abundant reserves of sodium on the earth (three orders of magnitude higher than that of lithium), sodium ion batteries have great potential in the field of low-cost and large-scale energy storage However, at present, sodium ion battery is still in the laboratory stage, and its electrochemical performance is far from that of commercial lithium-ion battery, and the anode material is the key bottleneck to limit its performance Recently, Zhou Yongning group, Department of materials science, Fudan University, has made a breakthrough in the design and structural regulation of positive materials for sodium ion batteries The research results were published in J am Chem SOC under the title of "tuning p2-structured cathode material by Na site mg substitution for Na ion batteries"（ DOI: 10.1021/jacs.8b08638 ）。
P2 type layered metal oxide (P2 Na x Mo 2, M = transition metal) is the most potential system in the cathode materials of sodium ion batteries In the crystal structure of p2-na x Mo 2, sodium ion is located in the center of a triangular prism composed of six oxygen ions In the process of charge and discharge, sodium ions are reversible from this position The change of Na content in P2 Na x Mo 2 and the electrostatic interaction between Na Na and na-m will lead to the orderly arrangement of Na ions and vacancies in the layered structure The electrochemical behavior of the ordered array is represented by many separate charging and discharging platforms This super structure will hinder the migration of sodium ions in the layer and have a negative impact on the performance of sodium ion batteries In addition, due to the large ion radius of sodium ion, the crystal structure changes during the deblocking process are more obvious When p2-na x Mo 2 layered material is charged to a high voltage, because most of the sodium ions have come out, the layered structure cannot keep stable, and irreversible phase transformation and collapse in the c-axis direction will occur This seriously affects the cycle performance of P2 type layered cathode material In view of the above problems, Zhou Yongning group successfully introduced Mg 2 + into the Na layer of p2-na 0.7 [Mn 0.6 Ni 0.4] O 2 cathode material Because of the smaller radius and more positive charge of Mg2 +, the introduction of Mg2 + not only breaks the orderly arrangement of sodium ions and vacancy, but also effectively stabilizes the layer spacing of sodium layer in the process of charging and discharging, slows down the crystal structure change caused by the process of sodium ion embedding and disengaging, and inhibits the irreversible phase transition during high voltage charging Thus, the charge discharge curve is smoother and the cycle reversibility is higher They used synchrotron radiation X-ray in situ diffraction (XRD) and absorption spectroscopy (XAS) to track the phase transition behavior and charge compensation mechanism of the cathode material in the process of charge and discharge in real time The authors found that the introduction of Mg 2 + also changed the electrochemical reaction mechanism and induced the reversible redox of lattice oxygen In addition, they also used the synchrotron radiation time-resolved in-situ XRD technology to reveal the phase transformation behavior of the cathode materials of the sodium ion battery during the rapid charging process for the first time in the world The first author of this article is Dr Wang Qinchao, and the corresponding author is Zhou Yongning, a young researcher The work was supported by Shanghai light source, Brookhaven National Laboratory, Lawrence Berkeley National Laboratory and Argonne National Laboratory.