Role of Plastic Deformation of Binder on Stress Evolution during Charging and Discharging in Lithium-Ion Battery Negative Electrodes

We studied the mechanical damage within a lithium-ion graphite-based porous electrode during electrochemical cycling. The effects of charging–discharging rate and the variation in graphite diffusivity on average stress in the electrode cell were investigated. In particular, differences between spatial and average stress evolution in graphite particles were explored. We considered two different microstructures: a) graphite particles connected together with binder bridges and b) graphite particles encased in binder shells. Electrochemical charging–discharging in a composite electrode was simulated by spatially resolving the electrode and electrolyte phases. As indicated by experimental measurements, the binder is assumed to follow an elastic–plastic stress–strain relation. Average stress developed in the electrode was calculated for different binder yield-stress levels and an appropriate yield-stress value was chosen on the basis of experimental findings of the literature. We find the stress in the particles can be of the order of 43 MPa, and can be particularly large in regions where the particles come in close contact with their neighbors. The average stress in the electrodes, however, is the range of 10 MPa and is largely determined by the mechanical properties, in particular the yield stress of the binder. Computed stress profiles were compared qualitatively with experimental measurements using the wafer-curvature method. Elastic stresses and plastic strains predicted by 3D models are shown to be close to those predicted using simpler 2D models of the microstructure.