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anatase TiO2 is a robust model anode for Li-insertion in batteries.
The influence of nanocrystal size on the equilibrium potential and kinetics of
Li-insertion is investigated with in operando
spectroelectrochemistry of thin film electrodes. Distinct visible and infrared
responses correlate with Li-insertion and electron accumulation, respectively,
and these optical signals are used to deconvolute Li-insertion from other electrochemical
responses, such as double-layer capacitance and electrolyte leakage. Electrochemical
titration and phase-field simulations reveal that a difference in surface
energies between anatase and lithiated phases of TiO2 systematically
tunes Li-insertion potentials with particle size. However, particle size does
not affect the kinetics of Li-insertion in ensemble electrodes. Rather,
Li-insertion rates depend on applied overpotential, electrolyte concentration,
and initial state-of-charge. We conclude that Li diffusivity and phase
propagation are not rate-limiting during Li-insertion in TiO2
nanocrystals. Both of these processes occur rapidly once the transformation between
the low-Li anatase and high-Li orthorhombic phases begins in a particle. Instead,
discontinuous kinetics of Li accumulation in TiO2 particles prior to the phase transformations limits
(dis)charging rates. We demonstrate
a practical means to deconvolute non-equilibrium charging behavior in
nanocrystalline electrodes through a combination of colloidal synthesis, phase
field simulations andspectroelectrochemistry.