Lithium insertion properties of LixTiNb2O7 investigated by neutron diffraction and first-principles modelling

TiNb2O7, a good candidate as anode in lithium batteries, was treated with n-butyllithium to synthesize LixTiNb2O7 phases similar to those formed during electrochemical reactions. The Li2.67TiNb2O7 and Li3.33TiNb2O7 compounds, monoclinic C2/m, were studied by time-of-flight powder neutron diffraction. Their crystal structures, containing 3 x 3 blocks of (Ti,Nb)O-6 octahedra, were Rietveld refined including Li positions. The Li atoms are distributed in similar amounts over sites with (Li-O) Coordination Numbers 5 and 4, although CN=5 should be preferred for having lower energy. Quantum-mechanical calculations were also performed, determining the average charge-discharge voltages to be 1.415 and 1.571 V for Li3.33TiNb2O7 and Li2.67TiNb2O7, respectively, in good agreement with experimental results. An analysis of the theoretical charge distribution shows that, on lithiation, the chemical reduction of Ti and Nb atoms concentrates in the more condensed peripheral octahedra of the 3 x 3 block. This corresponds to electrons moving into a partly spin-polarized small band which gives rise to semi-metallic conductivity. (C) 2015 Elsevier Inc. All rights reserved.