Lithium confinement and dynamics in hexagonal and monoclinic tungsten oxide nanocrystals: a 7 Li solid state NMR study.

Mixed-valence tungsten bronzes A x WO 3 (A = alkali metal, NH 4 + , etc .) are a series of compounds with adaptive structural and compositional features that make them a hot research topic in thermoelectrics, electrochromics, catalysis or energy applications in battery electrodes. The mixed hexagonal lithium ammonium bronze Li x (NH 4 ) y WO 3 is a new member of this structural family whose properties are compared to those of the pure hexagonal tungsten bronze (NH 4 ) x WO 3 . Surface and structural (nanoconfined) Li + cations were characterized by 7 Li single pulse excitation and 1 H- 7 Li cross-polarization (CP) NMR experiments. CP build-up curves and two-dimensional heteronuclear correlation solid-state NMR techniques provide information about the spatial connectivity between different proton and Li + species. At 500 °C the bronze structurally transforms from the hexagonal to a monoclinic phase, and defects are formed that are characterized through the Li + environment. 7 Li exchange spectroscopy (EXSY) NMR experiments provide information about the chemical exchange between the lithium species. The measured 7 Li T 1 and T 2 relaxation time constants and the T 1 / T 2 ratio allow characterizing the local strength of Li + binding.