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at Interfaces: Nuclear Magnetic Resonance Studies Paul Heitjans and Martin Wilkening

Abstract Interface engineering and the study of diffusion and transport processes through and along interfacial regions play important roles in materials science and energy research. For the latter, nanostructured materials are increasingly considered to act as powerful electrodes and solid electrolytes in sustainable energy systems, such as Li ion batteries. This is due to reduced diffusion lengths achieved when going to the nanometer scale and the fact that nanocrystalline materials with an average particle size of less than about 50 nm often show an enhanced diffusivity of their charge carriers. In this article, we show examples of how solid-state nuclear magnetic resonance (NMR) spectroscopy can be used to study the diffusion parameters of Li cations located in the interfacial regions separately from those in the interior of the grains. This article will demonstrate the future challenges and perspectives of Li NMR as a powerful tool of probing dynamic properties in functional materials.

Introduction Among the materials dominated by internal interfaces are well-ordered layered crystals such as intercalation compounds, where planes of the host lattice are sandwiched by mono- (or submono-) layers of guest atoms or molecules (the intercalate) being denoted as buried interfaces.1 There are also systems with highly complex internal interfaces such as nanocrystalline materials,2 which consist of an assembly of crystalline grains with a diameter of the order of 10 nm and a network of structurally disordered interfacial regions. In both intercalation compounds and nanocrystalline materials, interfaces are ubiquitous and have an area density of typically 109 m2/m3. Nuclear magnetic resonance (NMR) spectroscopy has long been applied to intercalation compounds and, in particular, light nuclei such as 1H, 2D, 7Li, and 19F have been used to study the interfacial dynamics of the intercalate (see Reference 3 for an early comprehensive review). Over the years up to the present day, a true playground for various NMR techniques has

been the intercalation compound LixTiS2 (0 < x ≤ 1),4–8 where Li interface diffusion was unambiguously shown to be two dimensional.5 The same is true for Ligraphite intercalation compounds (LiC6, LiC12), where besides 7 Li, also the betaradioactive nucleus 8Li (half-life 0.8 s) was used in studies by beta radiation– detected NMR (β-NMR).9,11–14 Concerning NMR studies of diffusion in nanocrystalline materials, the majority have up until now been done on ceramics, starting with an early work on 19F in CaF2.15 In nanocrystalline metals, very few investigations of diffusion by NMR have been reported, comprising studies on 63Cu in copper,16 27Al in aluminum coated with Al2O3,17 and 1H in vanadium-hydrogen systems.18 The present article is confined to nanocrystalline ceramics. Here, recent studies of diffusion dynamics by NMR spectroscopy were mostly performed on Li ion conductors being both model systems and potential fu