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

Abstract

Mixed-valence tungsten bronzes A_xWO₃ (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)_yWO₃ is a new member of this structural family whose properties are compared to those of the pure hexagonal tungsten bronze (NH₄)_xWO₃. Surface and structural (nanoconfined) Li⁺ cations were characterized by ⁷Li single pulse excitation and ¹H–⁷Li 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. ⁷Li exchange spectroscopy (EXSY) NMR experiments provide information about the chemical exchange between the lithium species. The measured ⁷Li T₁ and T₂ relaxation time constants and the T₁/T₂ ratio allow characterizing the local strength of Li⁺ binding.