Lattice Vibrations to Identify the Li/Na Ratio in Li_xNa_2-xTi₆O₁₃ (x = 0 ... 2)

Li_xNa_2-xTi₆O₁₃ has received attention as 3d-metal oxide based anode material for possible battery application. Generally, titanium oxides represent excellent Li hosts due to their zero-strain behavior, cycling stability and high operating voltage. New developments choose Na as charge carrier, but less effort is put in the investigation of mixed cation conductors. Owing to the synthesis route of Li_xNa_2-xTi₆O₁₃(0 ≤ x ≤ 2) the coordination of Na and Li in the channels is different which had been investigated by means of X-ray and neutron diffraction. Up to now, no Raman spectroscopy has been applied. This oxide is highly Raman-active, thus the local structure can also be analyzed in terms of vibrational spectroscopy. Micro-Raman spectroscopy carried out at room temperature with different cation contents (x = 0, 0.33, 1, 2) shows the continuous change from Na to Li by a superposition of the modes for Na₂Ti₆O₁₃ and Li₂Ti₆O₁₃. The only exceptions are two distinct modes. They appear either for Li (118 cm^-1) or Na (135 cm^-1). The results confirm the high-temperature phase stability of Na₂Ti₆O₁₃ as well as the anisotropic thermal expansion of the unit cell seen by in situ X-ray powder diffraction under two different gas atmospheres. Combining these results, we suppose that the anisotropic thermal expansion of the lattice parameters is affected by the normal vectors of the corresponding bond vibrations in Na₂Ti₆O₁₃ and Li₂Ti₆O₁₃. Crystalline-orbital calculations of the Raman shifts of Li_xNa_2-xTi₆O₁₃ were carried out for the cation contents x = 0, 1, 2 and Raman modes were assigned to specific bond vibrations supported by theory. Besides, this gives additionally information about the Li/Na ratio in a new and simple way.