Na 2 Ni 2 TeO 6 : Evaluation as a cathode for sodium battery

The sodium layered oxides Na2Ni2TeO6, Na2Co2TeO6 and Na1.6Sr0.2Ni2TeO6 have been synthesized by conventional solid-state method. The samples were characterized by X-ray diffractometry (XRD), cyclic voltammetry (CV), scanning electron microscopy (SEM) and charge–discharge measurements. The results show that all the samples crystallize in a hexagonal unit cell without impurity. The electrochemical Na+ extraction (i.e., the charge profile) for Na2Ni2TeO6 exhibits two voltage plateaus at 3.6 and 4.4 V, the first corresponds to Na+ ordering at extraction of 1/3 of the sodium ions per formula unit, the second to the onset of SEI formation. Detailed XPS analysis confirms that the Ni(III) does not disproportionate into Ni(III) and Ni(IV). The process of Na extraction/insertion in the interlayers of Na2Ni2TeO6 is highly reversible with a capacity of 100 mAh g−1 at slow cycling rate (0.03 C) in the 4.35 to 3 V voltage range versus Na+/Na. The lattice constant and the unit-cell volume increase with the substitution of Na+ by Sr2+. In the presence of Sr2+, the appearance of a step in the charge–discharge profile of Na1.6Sr0.2Ni2TeO6 is suppressed. Na2Co2TeO6 showed little Na extraction/insertion.