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Structural features responsible for lithium conductivity in Li 1+x Ti 2−x Al x (PO 4 ) 3 (x = 0, 0.2, and 0.4) samples have been investigated by Rietveld analysis of high-resolution neutron diffraction (ND) patterns. From structural analysis, variation of the Li site occupancies and atomic thermal factors have been deduced as a function of aluminum doping in the temperature range 100−500 K. Fourier map differences deduced from ND patterns revealed that Li ions occupy M1 sites and, to a lower extent, M3 sites, disposed around ternary axes. The occupation of M1 sites by Li ions is responsible for the preferential expansion of the rhombohedral R3̅ c unit cell along the c axis with temperature. The occupation of less symmetric M3 sites decreases electrostatic repulsions among Li cations, favoring ion conductivity in Li 1+x Ti 2−x Al x (PO 4 ) 3 compounds. The variations detected on long-range lithium motions have been related to variations of the oxygen thermal factors with temperature. The information deduced by ND explains two lithium motion regimes deduced previously by 7 Li NMR and impedance spectroscopy.Article pubs.acs.org/IC

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VO2F with a ReO3-type structure has been synthesized at high pressures. It reversibly inserts up to 1 Li+ per vanadium above 2.15 V delivering a high specific capacity (250 mA h g−1).

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.

A detailed structural and electrochemical study of the ion exchanged Li(2)Ti(6)O(13) titanate as a new anode for Li-ion batteries is presented. Subtle structural differences between the parent Na(2)Ti(6)O(13), where Na is in an eightfold coordinated site, and the Li-derivative, where Li is fourfold coordinated, determine important differences in the electrochemical behaviour. While the Li insertion in Na(2)Ti(6)O(13) proceeds reversibly the reaction of lithium with Li(2)Ti(6)O(13) is accompanied by an irreversible phase transformation after the first discharge. Interestingly, this new phase undergoes reversible Li insertion reaction developing a capacity of 170 mAh g(-1) at an average voltage of 1.7 V vs. Li(+)/Li. Compared with other titanates this result is promising to develop a new anode material for lithium ion rechargeable batteries. Neutron powder diffraction revealed that Na in Na(2)Ti(6)O(13) and Li in Li(2)Ti(6)O(13) obtained by Na/Li ion exchange at 325 °C occupy different tunnel sites within the basically same (Ti(6)O(13))(2-) framework. On the other hand, electrochemical performance of Li(2)Ti(6)O(13) itself and the phase released after the first full discharge is strongly affected by the synthesis temperature. For example, heating Li(2)Ti(6)O(13) at 350 °C produces a drastic decrease of the reversible capacity of the phase obtained after full discharge, from 170 mAh g(-1) to ca. 90 mAh g(-1). This latter value has been reported for Li(2)Ti(6)O(13) prepared by ion exchange at higher temperature.