Speaker
Description
Na$_2$Ni$_2$TeO$_6$ honeycomb layered oxide has suitable properties for use as a Na-ion battery cathode material. The substitution of Ni with Co has been shown to have a detrimental effect on the energy density of Na$_2$Ni$_2$$_-$$_x$Co$_x$TeO$_6$, whereas the plateau potential vs Na$^+$/Na increases. Thus, to ascertain the cause of the electrochemical properties change upon substituting Ni with Co Na-ion self-diffusion properties are investigated with the use of zero field and longitudinal field $μ^+$SR methods in Na$_2$Ni$_2$$_-$$_x$Co$_x$TeO$_6$ with $x$ of 0.0, 1.0 and, 1.5. Na-ion site occupancies and crystal structure was determined from neutron powder diffraction measurements and used for the determination of Na-ion jump paths. All measurements were performed in a temperature range from 50 K to 550 K. Three distinct Na-ion sites are determined from the neutron powder diffraction measurements. In addition, two distinct temperature regions for Na-ion self-diffusion, with different Na-ion diffusion pathways, are determined and analysed. The Na-ion diffusional pathway dependence on the substitution of Ni with Co is shown and discussed. Based on the obtained results we propose a cause for the decrease in the capacity, with the simultaneous increase in plateau potential vs Na$^+$/Na, with the increased substitution of Ni with Co. Based on the results, a roadmap on how to further improve Na$_2$Ni$_2$$_-$$_x$Co$_x$TeO$_6$-based Na-ion battery cathode materials is given.
