Intertwined Crystal Structure and Electrical Properties in Layered Battery Materials A2Ni2TeO6 (A = Na/Li)

We report temperature evolution of the crystal structure and electrical properties, and their interconnection in the promising two-dimensional (2D) layered battery materials A(2)Ni(2)TeO(6) (A = Na/Li) by comprehensive neutron diffraction, Raman spectroscopy, and impedance spectroscopy studies. In this paper, we reveal that the crystal structures of Na- and Li-based compounds, with space group P6(3)/mcm and Cmca, respectively, have identical global crystal structures with alternating (Ni/Te)O-6 and alkali metal layers but with different internal structures of the alkali metal layers. Our soft bond valence sum analysis of the neutron diffraction patterns illustrates 2D conduction pathways within the ab planes for both compounds. Despite the Li ion having smaller size and lighter weight than that of the Na ion, our results reveal that the Na compound has higher ionic conductivity than the Li compound. By detailed crystal structural analysis, we divulge that the differences in the internal structures of the alkali metal ion layers, such as distance between the alkali metal ions, local crystal structural environment, and the relative site occupancies of the alkali metal ions, result in differences in the ionic conductivity. Additionally, in this paper, we demonstrate a correlation between crystal structure and electrical properties, where anomalies are found for both temperature-dependent electrical and local crystal structural parameters at similar to 475-575 K and similar to 200-300 K for Na- and Li-based compounds, respectively. In this comprehensive paper, we thus facilitate the understanding of the microscopic crystal structure, electrical properties, and their intercorrelations which are essential for the design and fabrication of highly efficient layered battery materials.