Enhanced Thermodynamic Stability of Delithiated Nano-LiCoO<sub>2</sub> by Lanthanum Doping

The dynamic environment\nwithin lithium-ion batteries induces significant\nchanges in local thermodynamic functions, hampering the accurate prediction\nof the stability of the cathodes during cycling. While delithiation\nprimarily affects the surface properties of the cathode structure,\nthere is a lack of fundamental understanding concerning the evolution\nof interfacial energies with varying stoichiometry. Here, we used\nmicrocalorimetry to quantify the thermodynamic changes between the\nstoichiometric and partially delithiated nano-LiCoO<sub>2</sub> states\nfor the first time. A mild delithiation from LiCoO<sub>2</sub> to\nLi<sub>0.71</sub>CoO<sub>2</sub> caused a surface energy reduction,\nnegatively affecting the adhesion between adjacent grains by ∼0.4J/m<sup>2</sup>. The introduction of lanthanum at 1.0 atom % reduced the\nsurface energy of the stoichiometric LiCoO<sub>2</sub> while forcing\na constant surface energy state during delithiation down to Li<sub>0.57</sub>CoO<sub>2</sub>. This reduced the thermodynamic stress\nbetween grains during lithium cycling, mitigating degradation mechanisms.\nThe lanthanum-induced surface stabilization also inhibited the coarsening\nand dissolution of the cathode particles. We used electron microscopy\nto propose an atomistic mechanism by which the lanthanum doping pins\nsurface dissolution for improved cathode stability.