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 α-phase at dilute Li-vacancy concentrations (x > 0.87, x in LixCoO2), which gradually transitions to primarily a metallic ß-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 (∼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.

Direct Operando Observation of Double Layer Charging and Early Solid Electrolyte Interphase Formation in Li-Ion Battery Electrolytes.

The solid electrolyte interphase (SEI) is the most critical yet least understood component to guarantee stable and safe operation of a Li-ion cell. Herein, the early stages of SEI formation in a typical LiPF6 and organic carbonate-based Li-ion electrolyte are explored by operando surface-enhanced Raman spectroscopy, on-line electrochemical mass spectrometry, and electrochemical quartz crystal microbalance. The electric double layer is directly observed to charge as Li+ solvated by ethylene carbonate (EC) progressively accumulates at the negatively charged electrode surface. Further negative polarization triggers SEI formation, as evidenced by H2 evolution and electrode mass deposition. Electrolyte impurities, HF and H2O, are reduced early and contribute in a multistep (electro)chemical process to an inorganic SEI layer rich in LiF and Li2CO3. This study is a model example of how a combination of highly surface-sensitive operando characterization techniques offers a step forward to understand interfacial phenomena in Li-ion batteries.