Since their introduction 25 years ago, lithium-ion batteries have risen to become the primary power source for mobile devices such as electric tooth brushes and razors, but also for cellphones and tablet PCs. They have a high potential for wider application, e.g. in (hybrid) electric vehicles or for the storage of renewable energy. This is currently hindered by the gradual capacity fade over cycle lifetime. Main factors for this degradation are electrochemical side-effects such as SEI formation and mechanical damage. The cyclic insertion and extraction of Li ions results in high stress levels in and large deformation of the electrodes, leading to delamination of the electrode structure and to fracture of individual particles, respectively. The freshly cracked surface is then exposed to further chemical reactions. Experimental evidence however suggests, that nanostructured electrodes exhibit a higher resilience against this diffusion-induced mechanical degradation.

In this talk, we report on our efforts towards modeling the chemo-mechanical processes taking place in free-standing electrode particles at the micro- and the nano-scale. In particular, we describe two models used to study the interaction of phase segregation and crack growth, as well as the interaction of chemical reactions and surface tension in nanoparticles. Both models feature a direct coupling between diffusion and elasticity as well as concentration-dependent diffusivity. In particular, the Butler-Volmer relation for the description of electrochemical surface reactions has been modified to account for the effects of mechanical stress and phase formation.

We show that surface tension provides mechanical stabilization in nanoparticles, allowing, in principle, for higher charge/discharge rates. This effect is size-and shape-dependent. Due to surface tension, however, the usable capacity of a particle shrinks with its size. Regarding phase segregation, we demonstrate how the rate of surface reactions can determine the separation behavior, and that the evolution of the phase interface can drive the propagation of pre-existing cracks in an electrode particle.