Operando X-ray absorption spectroscopy investigations on NaxNi1/3Fe1/3Mn1/3O2 positive electrode materials for sodium and sodium ion batteries

NaxMn1/3Fe1/3Ni1/3O2 (x = 2/3 and 1) layered oxides are synthesized and applied as positive electrode materials for sodium batteries. The crystal structure of the material changes from the O3 single phase to P2/O3 mixed phases as the Na content decreases from 1 to 2/3. The mixed-phases Na2/3Mn1/3Fe1/3Ni1/3O2 shows superior cycling performance compared to the single-phase NaMn1/3Fe1/3Ni1/3O2 due to different redox process and structural change that are demonstrated by operando X-ray absorption spectroscopy (XAS). The Na2/3Mn1/3Fe1/3Ni1/3O2 experiences redox reactions of Ni3+|Ni4+ and Fe3+|Fe4+ with inactive Mn ions during the charge/discharge processes, while the NaMn1/3Fe1/3Ni1/3O2 undergoes deeper redox reactions from Ni2+|Ni3+|Ni4+ and Fe3+|Fe4+ that Mn ions are irreversibly oxidized to Mn4+ in the 1st charge process. In addition, the mixed-phases material has smaller changes in the transition metal oxygen bond lengths during cycling, corresponding to less distortions of TMO6 units in the crystal structure. The better reversibility of the redox reactions and the occurrence of less structural changes are both responsible for the enhanced cycling performance obtained from the mixed-phases material compared to the single-phase material. These results strengthen the understanding of interactions of transition metals in the layered cathode and provide guidelines for designing positive electrode materials for sodium batteries.