On the fragmentation of active material secondary particles in lithium ion battery cathodes induced by charge cycling

The loss of connectivity within battery electrodes due to mechanical failure by decohesion and fracture between primary grains that form spheroidal secondary particles is one of the principal mechanisms responsible for the widely observed and reported capacity fading. In this study we focus our attention on the elucidation, via combined analytical and numerical modeling, of the coupled electrochemical and mechanical processes that occur during lithiation and delithiation. We run sequential diffusion and deformation analyses of polycrystalline aggregate, formulate conditions for crack initiation at the interfaces between primary particles, and obtain predictions for the distributed damage within the secondary particle. The discrete element method with cohesive crack modeling is employed as the simulation tool. The conclusions that can be drawn from the analysis can be summarized as follows: (1) anisotropic expansion of primary particle crystallites due to Li+ ion diffusion causes cracks to form at the interfaces and grain boundaries when stresses reach the cohesive strength limit; (2) Li+ ion concentration and its gradients have influence on crack formation, distribution and density, with high charging and steep gradients promoting rupture; (3) anisotropic particle expansion/contraction promotes interfacial fracture; (4) new crack appear and existing cracks extend under cyclic charging conditions.