Operando Monitoring the Insulator-Metal Transition of LiCoO2

LiCoO2 (LCO) is one of the most-widely used cathode active materials for Li-ion batteries. Even though the material undergoes an electronic two-phase transition upon Li-ion cell charging, LCO exhibits competitive performance in terms of rate capability. Herein the insulator-metal transition of LCO is investigated by operando Raman spectroscopy complemented with DFT calculations and a developed sampling volume model. We confirm the presence of a Mott insulator a-phase at dilute Li-vacancy concentrations (x > 0.87, x in LixCoO2), which gradually transitions to primarily a metallic beta-phase as x approaches 0.75. In addition, we find that the charge-discharge intensity trends of LCO Raman-active bands exhibit a characteristic hysteresis, which, unexpectedly, narrows at higher cycling rates. When comparing these trends to our numerical model of laser penetration into a spatially heterogeneous particle we provide compelling evidence that the insulator-metal transition of LCO follows a two-phase route at very low cycling rates, which is suppressed in favor of a solid-solution route at rates above 20 mA/gLCO (similar to C/10). The observations explain why LCO exhibits competitive rate capabilities despite being observed to undergo an intuitively slow two-phase transition route: a kinetically faster solid-solution transition route becomes available when the active material is cycled at rates >C/10. Operando Raman spectroscopy combined with sample volume modeling and DFT calculations is shown to provide unique insights into fundamental processes governing the performance of state-of-the-art cathode materials for Li-ion batteries.

Automated summary

In this study, the insulator-metal transition of LiCoO2 was investigated using operando Raman spectroscopy, DFT calculations, and a sample volume model. It was found that LCO follows a two-phase transition route at low cycling rates, but a solid-solution transition route is favored at higher rates, explaining its competitive rate capabilities. The research provides unique insights into fundamental processes governing the performance of state-of-the-art cathode materials for Li-ion batteries.