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

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 N-23(a), Li-7 and H-1 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.