Elucidating the Phase Transformation of Li₄Ti₅O₁₂ Lithiation at the Nanoscale

Abstract

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.

Advances in Understanding Materials for Rechargeable Lithium Batteries by Atomic Force Microscopy

“…d) 3D topographical AFM images of Li 4 Ti 5 O 12 electrode cycled to different states of charge. d 1 pristine, d 2 discharged 50%, d 3 discharged to 1.5 V, d 4 discharged to 1.0 V, d 5 charged 50%, and d 6 charged to 2.0 V. Copyright 2016, American Chemical Society . AFM, atomic force microscopy.…”

“…The AFM images present comprehensive current and topography maps at different states of charge to demonstrate how two‐phase transformation occurs. They conclude that the phase transition proceeds via percolation channels within single grains and surface chemistry occurs across grain boundaries by combining with XPS analysis . Due to unexpected severe gassing, LTO is still not widely used.…”

“…The dynamic strain response to applied electric bias to tip is detected by electrochemical strain microscopy (ESM) (Figure f) . The conductive atomic force microscopy (c‐AFM) is applied to measure local electronic conductivity properties of materials (Figure e) . The Kelvin probe force microscopy (KPFM) is useful to detect the surface potential of materials (Figure b) …”

“…d) AM – FM , Copyright 2017, Springer Nature . e) c‐ AFM , Copyright 2016, American Chemical Society . and f) ESM , Copyright 2012, American Chemical Society .…”

High‐Density Microporous Li4Ti5O12 Microbars with Superior Rate Performance for Lithium‐Ion Batteries

“…Although LTO suffers from a low intrinsic electronic conductivity (≈10 −13 S cm −1 ) and poor Li‐ion mobility,11, 12, 13, 14 intermediate compositions drastically increase both the electronic conductivity15, 16, 17, 18 and Li‐ion mobility11, 12, 13, 14 to exploit the intrinsic high rate LTO materials properties requiring excellent electronic and Li‐ion wiring throughout the electrodes. To address this issue, many approaches have been developed, mainly concentrating on adding a conductive coating layer, reducing the particle size, and shortening of the electron and Li‐ion diffusion distances.…”