Thermodynamic Stability Dynamics during Delithiation of LiCoO₂

Cathode cycling leads to critical instabilities of the structures, being one of the most critical causes for battery failure. Interfacial properties are critical in this scenario, but there is a lack of fundamental experimental studies concerning the evolution of surface energies during delithiation and how to control them using thermodynamic strategies to improve cathode performance. Here we report experimental data on the thermodynamics of surfaces in nanocrystalline LiCoO2 using microcalorimetry and how it relates to stability against dissolution/degradation. By testing chemically de-lithiated states using microcalorimetry and EELS/HRTEM, we were able to quantify the thermodynamic changes between lithiated and delithiated states for a direct evaluation of driving force for growth and degradation. Further, we demonstrate how a dopant (lanthanum) designed to segregate to the surface lowers the excess energy of the interface, causing nano-LiCoO2 to show greater resistance to dissolution/growth/degregation founded on the dopant impact interface energies. Although LiCoO2 was used a model material, we discuss here the design principle behind interface stabilization to allow greater performance in liquid and solid state batteries.