Slow ion exchange in crystalline Li2SO4 ⋅H2O: A 6Li 2D EXSY NMR investigation

Abstract

Diffusion of lithium ions in solids is an interesting topic for fundamental research as well as applied science. Fast moving Li ions are desirable for several energy storage applications, whereas materials with slow diffusing Li ions find use, for example, as blankets for tritium fusion reactors and substrates for semiconductor growth. Lithium sulfate monohydrate (Li2SO4 ⋅H2O) is a model system with slow Li ion motion under ambient conditions. Li2SO4 ⋅H2O has two crystallographically inequivalent Li positions, which can be seen from 6Li NMR experiment as two resolved signals. In the present work, we report solid-state NMR studies of ion dynamics and structure of Li2SO4 ⋅H2O using 6Li NMR spectroscopy. We employed two-dimensional 6Li exchange NMR experiments to probe the Li+ exchange process between the two sites. A series of 2D 6Li exchange experiments with mixing-time dependence under magic-angle spinning revealed the slow ion exchange process with an exchange rate of 0.05s−1 at 307K.