Elucidating the local structure of Li1+xAlxTi2–x(PO4)3 and Li3AlxTi2–x(PO4)3 (x = 0, 0.3) via total scattering

Li_1+xAl_xTi_2–x(PO₄)₃ (LATP) and Li₃Al_xTi_2–x(PO4)3 (x = 0, 0.3) are promising candidates in all-solid-state batteries due to their high room temperature conductivity of 10^–3 S cm^–1 and air- and moisture-stability. They also exhibit unusual thermal expansion properties, with Li_1+xAl_xTi_2–x(PO₄)₃ showing near-zero thermal expansion along the a axis while Li₃Al_xTi_2–x(PO₄)₃ exhibits polynomial positive thermal expansion along the a axis and polynomial negative thermal expansion along the c axis. A crucial component to understanding these properties is understanding the local structure. Total scattering is a powerful analytical technique as it provides information on the long-range, average structure as well as the local structure. Here, we report the first X-ray and neutron total scattering experiments performed on Li_1+xAl_xTi_2–x(PO₄)₃ and Li₃Al_xTi_2–x(PO₄)₃ (x = 0, 0.3). We show that the PO₄ and TiO₆ polyhedra experience very little expansion of the P/Ti–O bonds up to 800 °C, nor is there much expansion when the Li content increases significantly. The minor thermal expansion of the nearest-neighbor bonds of the polyhedra is revealed to be the reason behind the unusual thermal expansion properties, causing the near-zero thermal expansion along a in Li_1+xAl_xTi_2–x(PO₄)₃ and moving as whole units in Li₃Al_xTi_2–x(PO₄)₃. The structural robustness of the framework is also the reason for the increased conductivity as Li content increases, as the framework remains undistorted as Li content increases, permitting Li-ion mobility as the number of charge carriers increases. This suggests that phosphate-based framework materials beyond LATP would also be a good material space to explore for new Li-ion (and other ion-) conducting materials.