Simultaneous migration mechanism of Co-O couple toward spinel-like Co3O4 formation on LixCoO2 surfaces

Abstract

Degradation of battery cathode has been a crucial issue for battery durability. For Li_xCoO₂, a representative cathode material, the formation of harmful Co₃O₄ spinel with Li deficiency induces capacity fading, resistivity increase, and safety concerns. Such degradation mode is usually initiated by the surface, whereas the role of surface O-ion migration on Co₃O₄ formation is an open question. Herein, we analyze the initial processes of the Co₃O₄ spinel formation, including O vacancy dynamics and Co-ion migration on representative surfaces (003), (104), and (110). We demonstrate that the O-ions migrate concurrently with Co-ion migration, accelerating the latter on the surfaces and playing a crucial role in the mechanism of Co₃O₄ formation. Overall, it is confirmed that capturing the O-ions from surfaces is key to preventing Co₃O₄ formation. To verify this, we examined the coating layer effect by using Li_0.25CoO₂||Li₃NbO₄ as a test model and found that the formation energy of O vacancy at the contact becomes comparable to that obtained within the bulk structure Li_xCoO₂, indicating that effective suppression of the cathode surface degradation is another role of the coating layer in addition to suppressing solid electrolyte oxidation.