Honeycomb Compound Na₃Ni₂BiO₆ As Positive Electrode Material for Na-Ion Battery

Introduction Layered sodium transition metal oxide materials are promising candidates as positive electrode for sodium ion batteries. Partial substitution of the transition metal with other metal could modify the crystal structure and electrochemistry of the materials. Honeycomb oxides such as Na3Ni2SbO6 have been studied and showed high energy density.1,2 In this study, honeycomb compound Na3Ni2BiO6 was synthesized and characterized in Na cells. Experimental Stoichiometric amounts of NiO, NaBiO3 and Na2CO3 powders were mixed by ball milling and pelletized. The pellets were heated at 700 ºC for 8 hours first then 750 ºC for 12 hours under oxygen to obtain Na3Ni2BiO6. Heated samples were transferred to Ar-filled glovebox immediately. Electrodes were made with active material, PVDF binder, carbon black at a ratio of 8:1:1 and dried under vacuum at 120 ℃ overnight. 1M NaPF6 in a solution of EC, DEC, FEC (volume ratio 3:6:1) was used as electrolyte and Na foil was used as counter/reference electrode. X-ray diffraction (XRD) patterns were measured with a Rigaku Ultima IV X-ray diffractometer equipped with a Cu anode X-ray tube and a diffracted beam monochromator. Results and discussion Figure 1 shows the XRD pattern of synthesized Na3Ni2BiO6. The structure resembles that of Rm α-NaFeO2 (space group 166), except for the honeycomb ordering at low angles not described by the Rm α-NaFeO2 structure (indicated by stars). The honeycomb structure is generated by the 2:1 ordering of edge sharing NiO6 and BiO6 in the a-b plane. An impurity phase of NiO was present (indicated by solid circles). Figure 2 shows the voltage curve of Na3Ni2BiO6 for the first 2 cycles in the voltage range of 1.5 V – 3.8 V and 1.5 V – 4.5 V. Na3Ni2BiO6 was found to have a reversible capacity of ~80 mAh/g, corresponding to the reversible removal of ~0.5 Na per NaNi2/3Bi1/3O2. It should be noted that the heavy atomic weight of Bi lowers the gravimetric capacity. The voltage curve is characterized by two flat plateaus located at ~3.3 V and ~3.5 V during charging, which is typical for Ni-compounds. The two plateaus are both reversible during discharging with a hysteresis of ~0.2 V. When the cell was cycled between 1.5 V – 4.5 V, another plateau with a capacity of ~20 mAh/g appears at ~4.3 V upon charging. However this plateau is not reversible, as can be seen by the similar discharge curve to that of cycled between 1.5 V – 3.8 V. The cycling performance, rate capability and structural changes during charge/discharge will also be discussed. Acknowledgments The authors acknowledge funding from NSERC and 3M Canada under the auspices of the Industrial Research Chair and Discovery grant programs. References 1. D. Yuan et al., Adv. Mater., 26, 6301-6306 (2014) 2. J. Ma et al., Chem. Mater., 27, 2387-2399 (2015) Figure 1

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