Li-Ion Diffusion in the Equilibrium Nanomorphology of Spinel Li4+xTi5O12

Li4Ti5O12 spinel as Li-ion electrode material combines good capacity, excellent cycleability with a high rate capability. Although the potential of about 1.56 V vs Li is relatively high, these features make it the anode of choice for state of the art high power Li-ion batteries. Although the flat voltage profile reflects a two-phase reaction during lithiation, the small change in lattice parameters upon lithiation ("zero-strain" property) leads to a solid solution in equilibrium, as recently demonstrated with diffraction. In this study, the morphology and Li-ion mobility is studied by NMR spectroscopy leading to a more detailed picture, showing that the solid solution in Li4+xTi5O12 spinel should actually be described as domains with sizes less than 9 nm having either tetrahedral (8a) Li occupation or octahedral (16c) Li occupation. The abundant domain boundaries and the associated disorder appear to be responsible for the facile diffusion through the lattice, and hence these nm-sized domains are most likely the origin of the relative high rate capability of this material as electrode for Li-ion batteries. The small domain size, smaller than typical Debye lengths, makes that the material electrochemically behaves as a solid solution. As such, the results give insight in the fundamental properties of the "zero-strain" Li4Ti5O12 spinel material explaining the favorable Li-ion battery electrode properties on an atomic level.