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

Mixed-valence tungsten bronzes A(x)WO(3) (A = alkali metal, NH4+, 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(NH4)(y)WO3 is a new member of this structural family whose properties are compared to those of the pure hexagonal tungsten bronze (NH4)(x)WO3. Surface and structural (nanoconfined) Li+ cations were characterized by Li-7 single pulse excitation and H-1-Li-7 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 degrees C the bronze structurally transforms from the hexagonal to a monoclinic phase, and defects are formed that are characterized through the Li+ environment. Li-7 exchange spectroscopy (EXSY) NMR experiments provide information about the chemical exchange between the lithium species. The measured Li-7 T-1 and T-2 relaxation time constants and the T-1/T-2 ratio allow characterizing the local strength of Li+ binding.