Isotropic negative thermal expansion of a Li1.3Al0.3Ti1.7(PO4)3 solid-state electrolyte

Abstract

Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) is a promising solid-electrolyte for Li-ion batteries with a favorable open structural framework made up of a corner shared LiO₆ and TiO₆ octahedra and PO₄ tetrahedra network. Here we report the temperature dependence of this interconnected polyhedral network of LATP, leading to an isotropic negative thermal expansion (NTE) with an estimated lattice thermal expansion along the a and c axes of −1.07 × 10⁻⁵(±0.08 × 10⁻⁵) K⁻¹ and −2.56 × 10⁻⁵(±0.25 × 10⁻⁵) K⁻¹ in the temperature range of 93 K and 303 K, respectively. The change in bond lengths and bond angles of LiO₆, TiO₆ octahedra and PO₄ tetrahedra are estimated as a function of temperature by Rietveld refining the X-ray diffraction patterns recorded in the temperature range of 93 K to 303 K. The overall shrinkage of the LiO₆ octahedra results in a contraction of the lattice along the c axis. Local distortion in the lattice is also observable through the temperature dependent change of the bond angles and bond lengths for TiO₆ octahedra and PO₄ tetrahedra. The existence of phosphate rotational/librational motion is identified to be the underlying reasons for the occurrence of NTE along the a-axis. The isotropic thermal contraction with temperature results in a decrease of the lattice volume with a volume expansion coefficient of −4.82 × 10⁻⁵(±0.37 × 10⁻⁵) K⁻¹. Estimation of the distortion indices, such as quadratic elongation and bond angle variance, suggests the Li-occupied 6b polyhedral site undergoes a greater degree of distortion than that of other polyhedral units in LATP. Quadratic elongation and bond angle variance parameters for all the polyhedral units also suggest a larger extent of distortion in LiO₆ and the absence of any Jahn–Teller distortion in the framework. With further increases in temperature beyond 80 °C, a gradual switch over to a positive thermal expansion behavior becomes observable.