Decoding the AlPO4 and LATP surface with a combined NMR-DFT approach

Abstract

A milestone in the development of next generation high-performance lithium ion batteries is the understanding and targeted engineering of hybrid electrolytes, consisting of a polymer and a ceramic component, and in particular their interfaces. Nuclear magnetic resonance (NMR) spectroscopy is a powerful non-destructive technique for unraveling the intricate interface structures and ion dynamics in these materials, yet data interpretation often relies on empirical rules that have been devised using data from the bulk of materials. By exploiting the synergies between advanced NMR experiments and density functional theory (DFT) simulations, AlPO₄ is studied as a model for the surface of the well-known solid ion conductor Li_1+xAl_xTi_2−x(PO₄)₃ with 0.3 ≤ x ≤ 0.5 (LATP), which is a promising candidate for the ceramic component of a hybrid electrolyte. By combining the multi-nuclear NMR techniques cross-polarization (CP) and transfer of populations in double resonance (TRAPDOR) on AlPO₄ powder with DFT calculations of NMR observables for a variety of surface models, the surface structure of commercial AlPO₄ is elucidated. It is shown that even after extended drying, the surface of AlPO₄ is hydroxylated, exhibiting a TRAPDOR-estimated ¹H–²⁷Al quadrupolar coupling constant, C_Q, of 5.8 ± 0.9 MHz. The joint theoretical-experimental approach also enables first insights into the bonding motifs of organic entities on functionalized AlPO₄ surfaces as a model for LATP surfaces. Surface interactions and the presence of functional groups upon silanization of hydroxylated surfaces are confirmed both on AlPO₄ and LATP. We demonstrate that observables, which are experimentally as well as theoretically accessible, provide information on interfacial bonding motifs, interatomic distances, and interactions, surpassing the capabilities of either NMR or DFT techniques alone.