Elucidating the Phase Transformation of Li4Ti5O12 Lithiation at the Nanoscale

This work provides insight regarding the fundamental lithiation and delithiation mechanism of the popular lithium ion battery anode material, Li4Ti5O12 (LTO). Our results quantify the extent of reaction between Li4Ti5O12 and Li7Ti5O12 at the nanoscale, during the first cycle. Lithium titanate's discharge (lithiation) and charge (delithiation) reactions are notoriously difficult to characterize due to the zero-strain transition occurring between the end members Li4Ti5O12 and Li7Ti5O12. Interestingly, however, the latter compound is electronically conductive, while the former is an insulator. We take advantage of this critical property difference by using conductive atomic force microscopy (c-AFM) to locally monitor the phase transition between the two structures at various states of charge. To do so, we perform ex situ characterization on electrochemically cycled LTO thin-films that are never exposed to air. We provide direct confirmation of the manner in which the reaction occurs, which proceeds via percolation channels within single grains. We complement scanning probe analyses with an X-ray photoelectron spectroscopy (XPS) study that identifies and explains changes in the LTO surface structure and composition. In addition, we provide a computational analysis to describe the unique electronic differences between LTO and its lithiated form.