Probing Structural Subtleties in Functional Materials using Multinuclear Solid-State NMR Spectroscopy
Karen Johnston a, Tavleen Attari a, Abby Haworth a, James Dawson b, Saiful Islam c, Sharon Ashbrook d, Phil Lightfoot d
a Department of Chemistry, University of Durham, DH1 3LE Durham, United Kingdom
b School of Natural and Environmental Sciences, Newcastle University, UK, Newcastle upon Tyne, Reino Unido, Newcastle upon Tyne, United Kingdom
c Department of Chemistry, University of Bath, Claverton Down, University of Bath, Bath,UK, BA2 7AY, United Kingdom
d School of Chemistry, University of St Andrews, UK
Proceedings of Atomic-level characterization of hybrid perovskites (HPATOM)
Online, Spain, 2021 January 26th - 28th
Organizers: Dominik Kubicki and Amita Ummadisingu
Invited Speaker, Karen Johnston, presentation 009
Publication date: 14th January 2021

Our research is focused on the structural characterisation of ordered and disordered functional materials in the solid state, using powder diffraction, solid-state NMR spectroscopy and first-principles DFT calculations. In particular, we are interested in perovskite-based systems and their use in a wide range of devices and applications, including electronics and energy storage devices.

The alkaline niobates, NaNbO3, KNbO3 and the solid-solution KxNa1−xNbO3 (KNN) are of considerable interest owing to reports of exceptional piezoelectric responses, believed to be comparable to those of the most widely used piezoelectric ceramic, Pb(ZrxTi1−x)O3 (PZT). The work presented here focuses on the room temperature phases of NaNbO3, which remain a subject of considerable discussion.[1] Several phases have been suggested and observed experimentally, including the antiferroelectric Pbcm and polar P21ma polymorphs of NaNbO3.[2] The relative quantities of these two phases are known to vary considerably depending on the precise synthesis conditions used, e.g., conventional solid-state techniques versus molten salt and sol-gel approaches. Here, high-resolution powder diffraction and solid-state NMR data will be presented comparing the different synthetic techniques used.[2]

All-solid-state lithium-ion (Li-ion) batteries are attracting considerable attention as possible alternatives to conventional liquid electrolyte-based devices as they present a viable opportunity for increased energy density and safety. In recent years, a number of candidate materials have been explored as possible solid electrolytes, including garnets, Li-stuffed garnets, Li-rich anti-perovskites (LiRAPs) and thio-LISICONs. In particular, LiRAPs, including Li3−xOHxCl, have generated considerable interest, based on their reported ionic conductivities (~10−3 S cm−1).[3,4] However, until recently, their lithium and proton transport capabilities as a function of composition were not fully understood. Hence, current research efforts have focused on the synthesis and structural characterisation of Li3−xOHxCl using a combination of ab initio molecular dynamics and variable-temperature 1H, 2H and 7Li solid-state NMR spectroscopy. We will demonstrate that Li-ion transport is highly correlated with the proton and Li-ion vacancy concentrations. In particular, we will show that the Li ions are free to move throughout the structure, whilst the protons are restricted to solely rotation of the OH groups. Based on these findings, and the strong correlation between long-range Li-ion transport and OH rotation, we have proposed a new Li-ion hopping mechanism, which suggests that the Li-rich anti-perovskite system is an excellent candidate electrolyte for all-solid-state batteries.[5] However, to fully understand the mechanism for conduction, multiple, complementary characterisation techniques are needed. 

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