Imaging Phase Boundary Kinetics in Lithium Titanate Using Operando Electron Energy-Loss Spectroscopy

Lithium titanate accommodates and releases lithium ions through phase separation. The dynamics of the phase boundary movement are critical to battery performance, particularly for maximizing the charge/discharge rates. However, details of this boundary movement remain unclear. Here, we visualize the phase boundary movement by tracking the Li distribution using operando scanning transmission electron microscopy coupled with electron energy-loss spectroscopy. For Li insertion, the rate constants of the phase boundary movement were 3.6 ± 0.9 × 10 –13 cm 2 /s at 30 °C and 3.2 ± 0.3 × 10 –11 cm 2 /s at 105 °C, whereas for Li extraction they were 4.0 ± 0.7 × 10 –11 cm 2 /s at 30 °C and 1.9 ± 0.6 × 10 –9 cm 2 /s at 105 °C. The activation energies for Li-ion diffusion were 0.49 and 0.59 eV for Li 4 Ti 5 O 12 and Li 7 Ti 5 O 12, respectively. The relatively low activation energy of 0.49 eV is the reason lithium titanate exhibits a high-rate discharge performance.