Enhanced Thermodynamic Stability of Delithiated Nano-LiCoO₂ by Lanthanum Doping

The dynamic environment within lithium-ion batteries induces significant changes in local thermodynamic functions, hampering the accurate prediction of the stability of the cathodes during cycling. While delithiation primarily affects the surface properties of the cathode structure, there is a lack of fundamental understanding concerning the evolution of interfacial energies with varying stoichiometry. Here, we used microcalorimetry to quantify the thermodynamic changes between the stoichiometric and partially delithiated nano-LiCoO₂ states for the first time. A mild delithiation from LiCoO₂ to Li_0.71CoO₂ caused a surface energy reduction, negatively affecting the adhesion between adjacent grains by ∼0.4J/m². The introduction of lanthanum at 1.0 atom % reduced the surface energy of the stoichiometric LiCoO₂ while forcing a constant surface energy state during delithiation down to Li_0.57CoO₂. This reduced the thermodynamic stress between grains during lithium cycling, mitigating degradation mechanisms. The lanthanum-induced surface stabilization also inhibited the coarsening and dissolution of the cathode particles. We used electron microscopy to propose an atomistic mechanism by which the lanthanum doping pins surface dissolution for improved cathode stability.