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

Li(2)Ti(6)O(13) and H(2)Ti(6)O(13) were easily synthesized from Na(2)Ti(6)O(13) by successive Na(+)-Li(+)-H(+) ion exchange. The crystal structures of Na(2)Ti(6)O(13), Li(2)Ti(6)O(13) and H(2)Ti(6)O(13) 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 [Ti(6)O(13)(2-)] 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. NaO(8) square prismatic coordination, LiO(4) square planar coordination and covalently bond H atoms, with results from (23)Na, (7)Li and (1)H 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.