The interest in lithium and sodium ion batteries is rapidly increasing with the development of electrical vehicles and hybrid electrical vehicles. Not only due to the low cost of sodium but also due to its wide distribution and abundance, sodium ion batteries are more prospective for large-scale production of stationary storage devices compared with lithium ion batteries. Among the different types of Na based cathode materials, layered NaxTMO2 (TM = Mn, Ni, Co, and Fe) cathode materials are one of the top candidates, which behave similar to layered materials for Li ion batteries. However, the phase transformation of these sodium ion cathode materials during charge and discharge process is much more complicated than that of lithium ion batteries[1-4].

P3-type Na0.67Mn0.67Ni0.33O2 (NMNO) synthesized by solid-state reaction method at 700 ˚C for 12 hours is a promising cathode material for sodium ion batteries due to its low production temperature compared with P2-type NMNO. Additionally, the capacity of P3-type NMNO during the first 40 cycles is higher than that of P2-type NMNO. In order to understand the electrochemical performance of P3-type NMNO and its related practical use as cathode material, it is urgently necessary to investigate the details of the structure and redox reaction of P3-type NMNO. We, therefore, used X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) to investigate the structure and oxidation state, respectively, of Ni and Mn ions at different stages of charge/discharge. The XRD pattern of P3-type NMNO was refined with the small hexagonal cell (R3m space group, a=b=2.886 Å and c=16.780 Å).

Fig. 1 shows the normalized Mn (a) and Ni (b) K-edge X-ray absorption near edge spectra (XANES) at the first charge and discharge process. One can see an energy shift of Ni K-edge to higher energy after charge to 4.5 V and a shift back to the position of the pristine material after discharge to 1.5 V. This means the average valence of Ni increases during the charge process and then decreases to lower valence state during discharge. However, the energy shift of Mn K-edge is much smaller during the first cycle indicating Ni ions are the main part of redox reaction, while Mn ions participate only to minor extend in the redox reaction. This behavior can be also proved by the extended X-ray absorption fine spectra (EXAFS) after Fourier transform. The Ni-O bonding length decreases after charging to 4.5 V and then increases after discharge to 1.5 V indicating a change of average valence of Ni ions which affects the attraction between nickle ions and the coordinated oxygen ions. However, the Mn-O bonding length was almost immutably during the first cycle. The changes in disorder of P3-type NMNO and changes of the number of neighboring atoms surrounding Ni or Mn ions during charge and discharge process lead to the variation of peak intensities of the Ni and Mn EXAFS spectra.

References

[1] P Vassilaras, A J Toumar and G Ceder, Electrochemistry Communications. 2014, 38, 79-81

[2] M D Slater, D Kim, E Lee and C S Johnson, Advanced Functional Materials. 2013, 23 (8), 947-58

[3] J Billaud, G Singh, A R Armstrong, E Gonzalo, V Roddatis, M Armand, T Rojo and P G Bruce, Energy & Environmental Science. 2014, 7 (4), 1387-91

[4] D Yuan, X Hu, J Qian, F Pei, F Wu, R Mao, X Ai, H Yang and Y Cao, Electrochimica Acta. 2014, 116, 300-05