Journal
ACS APPLIED MATERIALS & INTERFACES
Volume 13, Issue 19, Pages 22540-22548Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acsami.1c04383
Keywords
lithium cobalt oxide; lithium ion batteries; positive electrode; Mott insulator; Raman spectroscopy; Raman resonance; phase transition; sampling volume model
Funding
- Swiss National Science Foundation (SNSF) under the Ambizione Energy funding scheme [160540]
- SNF Professorship Grants [PP00P2_157615, PP00P2_187185]
- Knut and Alice Wallenberg (KAW) Foundation [2017.0204]
- StandUp for Energy
- Swiss National Science Foundation (SNF) [PP00P2_187185] Funding Source: Swiss National Science Foundation (SNF)
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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.
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.
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