Synthesis and structure determination of novel honeycomb layered A2Ni2TeO6 (A = Na, K) type and other oxidotellurates(VI)

Abstract

Novel battery materials promise improvements in the efficiency and sustainability of energy storage. Recent research into A2M2TeO6 honeycomb layered oxidotellurates(VI) (for short: A2M2TeO6 honeycomb tellurates) has demonstrated them as potential electrode materials, owing to their high voltage (up to 4V for K2Ni2TeO6) vs. K/K+ and reversible K ion insertion. In this work, we investigate novel A2M 2TeO6 (A=Na, K; M =Ni) honeycomb tellurates and demonstrate the existence of novel phases in this system. Moreover, we show that stacking faults and superstructures occur in multiple A2M 2TeO6 type phases, as has already been shown for Na2Ni2TeO6. Several other novel oxidotellurates(VI) (including Ca3Cu3Te2O12), a mixed valence oxidotellurate Ca2Te2O7 and two calcium oxidotellurate(IV) chlorides (Ca2TeO3Cl2 and Ca5Te4O12Cl2) are described as well. Solid-state (600-800°C) and hydrothermal (210°C) synthesis routes were employed to obtain crystalline products. Samples were characterized with single crystal X-ray diffraction (single crystal XRD), powder diffraction (powder XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM) and electron diffraction (ED), depending on the crystal size present in the sample. Where suitable single crystals were present, single crystal XRD was the preferred method. A novel K4Ni5Te3O16 honeycomb tellurate was synthesized (space group Amm2, a = 12.0639(8) Å, b = 25.2528(16) Å, c = 10.4470(6) Å). In the honeycomb layer formed by [(Ni,Te)O6] octahedra, two edge sharing [TeO6]6- octahedra form a [Te2O10]8- subunit in the honeycomb layer surrounded by [NiO6]10- octahedra. This structural feature was previously unknown in A2M2TeO6 honeycomb tellurates. In the hitherto described honeycomb tellurate structures [TeO6]6- octahedra share edges only with [M O6]10- units. Furthermore, one-dimensional diffuse scattering in the form of diffuse rods was observed in the K4Ni5Te3O16 phase and the novel Na2(x–y)NixTeyO2(x+y) and K2(x–y)NixTeyO2(x+y) type phases. This is due to stacking faults of the honeycomb layers, which was also evidenced by high resolution TEM imaging. It clearly showed the presence of stacking faults in a K2Ni2TeO6 single crystal. Other novel tellurates such as Ca3Cu3Te2O12 were synthesized as byproducts. Finally, TEM and ED measurements revealed the presence of further novel phases in the Na2(x–y)NixTeyO2(x+y) and K2(x–y)NixTeyO2(x+y) systems. Single crystal XRD measurements could not be performed for these latter phases, since no suitable single crystals were found. Rb and Cs analogues could not be obtained, even after numerous systematic experiments. We found some indications for the presence of Rb honeycomb tellurates in few samples, which proved to be inconclusive. The results presented here improve the understanding of A2M 2TeO6 honeycomb tellurates which can be used to better gauge their suitability as battery materials in the future. Moreover, we demonstrated that multiple novel honeycomb tellurate phases exist in the already investigated systems. Further research into this direction seems promising.