Chemo-elastic behavior of reconstructed Li-ion battery cathode particles with phase transformation: a numerical and analytical investigation

Abstract

The chemo-mechanical response of 3-D reconstructed and idealistic Li[subscript x]CoO[subscript 2] cathode particles has been elucidated with the implementation of an isothermal microscale numerical and analytical solution. The electrode stress-strain state is delineated upon the contribution of: i) phase transformation; ii) anisotropy and crystallographic orientation; iii) Li-composition inhomogeneity; iv) particle morphology and size; and v) composition-dependent chemical-expansion coefficient, [beta]. Diffusion-induced stresses (DIS) can be quite extreme with increasing particle size, Li concentration and discharge rate. Peaks of DIS locally emerge in the vicinity of concave features and protuberances of the anisotropic-elastic actual morphologies. In some cases, the severity of DIS indicate proneness to particle fragmentation. It is shown that phase-transition-induced stresses detrimentally contribute to abrupt changes in the particle mechanical behavior. The strong anisotropic chemo-elastic field induces the evolution of bands of Li-composition, chemical strains, and high-peak stresses. These occurrences are considered to be irrespective of the particle morphology but are highly dependent on the grain crystallographic orientation. Deleterious phase-transformation-induced stress bands are also found within the particle structure and are closely related to bands of chemical-misfit strains. It is demonstrated that [beta] is a key parameter in demarcating the chemo-stress-strain state of the Li[subscript x]CoO[subscript 2] cathode material. Under the linearity of [beta], both the stress-induced diffusion (SID) and DIS are dramatically more affected than when [beta] is constant. Hence, non-linear volumetric changes in the cathode structure can undermine its mechanical integrity. Because the chemo-elastic phenomena emanate in a reciprocate fashion, the linear-[beta]-based hydrostatic-stress gradients significantly facilitate Li diffusion under both charge-and discharge conditions comparing to the classical-Fickian-diffusion case. Subsequently, the stress-decoupling model promotes greater DIS due to composition inhomogeneity.