Insight into the channel ion distribution and influence on the lithium insertion properties of hexatitanates A2Ti6O13 (A = Na, Li, H) as candidates for anode materials in lithium-ion batteries

Abstract

Li₂Ti₆O₁₃ and H₂Ti₆O₁₃ were easily synthesized from Na₂Ti₆O₁₃ by successive Na⁺–Li⁺–H⁺ ion exchange. The crystal structures of Na₂Ti₆O₁₃, Li₂Ti₆O₁₃ and H₂Ti₆O₁₃ were investigated using neutron powder diffraction. Monovalent A⁺ cations (Na, Li and H) have been located using difference Fourier analysis. Although monoclinic lattice parameters (space group C2/m) of the three titanates remain almost unchanged with retention of the basic [Ti₆O₁₃²⁻] network, monovalent Na, Li and H cations occupy different sites in the tunnel space. By comparing the structural details concerning the A⁺ oxygen coordination, i.e. NaO₈ square prismatic coordination, LiO₄ square planar coordination and covalently bond H atoms, with results from ²³Na, ⁷Li and ¹H NMR spectroscopy we were able to obtain a more detailed insight into the respective local distortions and anharmonic motions. We were able to show that the site that the A⁺ cation occupies in the hexatitanate channel structure strongly influences the lithium insertion properties of these compounds and therefore their usefulness as electrode materials for energy storage.