Temperature-dependent insertion and adsorption of lithium on spinel Li4Ti5O12(111) thin films – an angle-resolved XPS study

Abstract

Aiming at a detailed understanding of the Li adsorption and insertion behavior on/into lithium titanate (Li₄Ti₅O₁₂, LTO), which is a promising anode material in Li-ion batteries, we have investigated the interaction of vapor deposited Li with LTO in the temperature range between 80 K and room temperature by angle-resolved X-ray photoelectron spectroscopy (ARXPS). The experiments were performed under ultrahigh vacuum (UHV) conditions, and the presence of additional Li species was detected by the formation of Ti³⁺ in the Ti 2p core level signal due to charge transfer from adsorbed/inserted Li to Ti. Even at 80 K most of the deposited Li diffuses into the bulk of LTO, reflecting the facile insertion and diffusion of Li into and in LTO. Deposition of up to 3 monolayers equivalent (MLE) of Li at 80 K results in an increase in Li concentration in the surface region (topmost 6 nm), up to a stoichiometry of Li_4+xTi₅O₁₂ with x > 0.3, and slightly lower values in the near-surface region (topmost 1 nm). For higher Li doses, the amount of Li in the near-surface region, including adsorbed Li, increases more than the concentration in the underlying surface region. This is attributed to the blocking of diffusion channels by inserted Li at a stoichiometry of Li_4.3Ti₅O₁₂ and above. Upon increasing the temperature, Li⁺ starts to diffuse into the LTO bulk at temperatures between 120 K and 175 K, depending on the concentration in the surface region. It has completely disappeared at 260 K. The consequences of these results for the understanding of physical Li insertion will be discussed.