The Batteries' New Clothes: Li and H Dynamics in Poorly Conducting Li2OHCl Directly Probed by Nuclear Spin Relaxation

Li2OHCl is considered to act as a suitable Li+ ionic conductor that is, excluding Li, solely composed of rather abundant elements. Its low meting point allows rather easy synthesis methods to prepare Li2OHCl in large quantities. So far, only few studies tackled the problem to deliver a clear-cut picture of Li+ self-diffusion. Li+ hopping in Li2OHCl is suggested to be coupled to OH- rotational dynamics. Proving such an interdependent coupling beyond any doubt remains, however, challenging. Here, we observed diffusion-induced 7Li and 1H NMR spin-lattice relaxation in both the laboratory and rotating frame of reference to find out (i) whether Li+ displacements are caused by fast OH- motions or (ii) whether anisotropic rotational OH- dynamics is a consequence of the rapid Li+ translational processes. By considering 7Li and 1H NMR line shapes and comparing our results with those obtained from conductivity spectroscopy as well as from recent ab initio molecular dynamics simulations and 2H NMR by Dawson et al., Energy. Environ. Sci. 2018, 11, 2993-3002, we propose that OH- rotational dynamics could also be a consequence rather than the trigger for fast Li+ self-diffusion (0.41 eV) on the NMR time scale.

Automated summary

Li2OHCl is a suitable Li+ ionic conductor composed of abundant elements excluding Li. Its low melting point enables easy synthesis methods. However, the clear picture of Li+ self-diffusion is still challenging to prove, as it is suggested to be coupled to OH- rotational dynamics in Li2OHCl.