Facet-Dependent Thermal Instability in LiCoO₂

Thermal runaways triggered by the oxygen release from oxide cathode materials pose a major safety concern for widespread application of lithium ion batteries. Utilizing in situ aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) at high temperatures, we show that oxygen release from Li_xCoO₂ cathode crystals is occurring at the surface of particles. We correlated this local oxygen evolution from the Li_xCoO₂ structure with local phase transitions spanning from layered to spinel and then to rock salt structure upon exposure to elevated temperatures. Ab initio molecular dynamics simulations (AIMD) results show that oxygen release is highly dependent on Li_xCoO₂ facet orientation. While the [001] facets are stable at 300 °C, oxygen release is observed from the [012] and [104] facets, where under-coordinated oxygen atoms from the delithiated structures can combine and eventually evolve as O₂. The novel understanding that emerges from the present study provides in-depth insights into the thermal runaway mechanism of Li-ion batteries and can assist the design and fabrication of cathode crystals with the most thermally stable facets.