Kinetic Pathways of Ionic Transport in Fast Charging Lithium Titanate

Fast-charging batteries typically employ electrodes capable of accommodating lithium continuously via solid-solution transformation because they have few kinetic barriers apart from Li+ diffusion. One exception is lithium titanate, an anode that exhibits extraordinary rate capability seemingly inconsistent with its two-phase reaction and slow diffusion within the two phases. Through real-time tracking of Li+ migration using operando electron energy-loss spectroscopy (EELS) along with simulation of the EELS spectra, we observe that the kinetic pathway that enables facile ionic transport in lithium titanate consists of distorted Li polyhedra in metastable intermediate states. Thus, fast-charging electrodes may not be controlled solely by the intrinsic ionic diffusivity of macroscopic phases, but also by the transport via kinetically accessible low-energy landscapes.

In order to understand the origin of various EELS spectra features, we simulate EELS spectra using the Vienna Ab initio Simulation (VASP) package. For a specific Li in a given configuration, this is done by calculating the DOS and integrated DOS considering a Li core-hole on the position of the specific Li and calculating the EELS based on the DOS. We also calculated the minimum energy paths (MEP) and migration energy of Li in various compositions, including Li4Ti5O12 with an additional Li carrier, Li5Ti5O12 with an additional Li carrier, and Li7Ti5O12 with a Li vacancy carrier. The calculation was performed using the nudged elastic band (NEB) method in VASP. Analysis of the VASP outputs was done using scripts from Transition State Tools for VASP (VTST, https://theory.cm.utexas.edu/vtsttools/index.html). We present here the relevant input and output files from the VASP calculations, the python code used to generate EELS spectra from VASP outputs, and the outputs of the VTST scripts that were used to generate MEP plots.