Kinetics of Anatase Electrodes:
The Role of Ordering,
Anisotropy, and Shape Memory Effects
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Abstract
We perform a comprehensive first-principles statistical
mechanical
study of the thermodynamic and kinetic properties of lithiated anatase
Li<sub><i>x</i></sub>TiO<sub>2</sub>. We establish that
the experimentally observed step in the voltage vs lithium composition
curve between <i>x</i> = 0.5 and 0.6 is due to Li ordering.
Furthermore, we predict that full lithiation of anatase TiO<sub>2</sub> is thermodynamically possible at positive voltages but that there
is an enormous difference in Li diffusion coefficients in the dilute
and fully lithiated forms of TiO<sub>2</sub>, providing an explanation
for the limited capacity in large electrode particles. We also predict
that Li diffusion in the ordered phase (Li<sub>0.5</sub>TiO<sub>2</sub>) is strictly one-dimensional. The TiO<sub>2</sub> to Li<sub>0.5</sub>TiO<sub>2</sub> phase transition has much in common with shape memory
alloys. Crystallographically, it can support strain invariant interfaces
separating TiO<sub>2</sub> and Li<sub>0.5</sub>TiO<sub>2</sub> within
the same particle. The strain invariant interfaces are parallel to
the one-dimensional diffusion direction in Li<sub>0.5</sub>TiO<sub>2</sub>, which, we argue, has important consequences for the role
of particle shape on achievable capacity, charge and discharge rates,
and hysteresis
