Analysis of Mechanical Degradation in Lithium-Ion Battery Electrodes

Lithium-ion battery (LIB) electrodes consist of four different phases; active particle, binder, conductive additive and electrolyte. During operation, the active particles (e.g. graphite anode, lithium metal oxide cathode) are responsible for hosting the Li ions. Conductive additives help in increasing the electronic conductivity of the electrode. The binder, typically PVDF, holds the active particles and conductive additives together and gives mechanical stability to the electrode. The electrolyte occupies the pore-space and forms the electrochemically active interfacial area with the active particles and provides pathways for lithium-ion transport. Lithium ions intercalate and deintercalate into the host material through diffusion process. This can cause significant swelling in the active particles and induce diffusion induced stress. For high-capacity active materials, the swelling can be significant. Because of swelling, the active particles squeeze the binders and nearby conductive additives to make space for themselves. During delithiation, the active particles shrink. Because of its inherent plasticity, the binders cannot restore its initial configuration. As a result, delamination may occur and the contact between the active particles and the conductive additives may be lost, which eventually would lead to capacity fade and deterioration in performance of the lithium ion battery. Diffusion induced stress may lead to mechanical degradation in terms of formation of microcracks in the active particles. Nucleation of these microcracks results in formation of spanning cracks which eventually leads to catastrophic failure of the material.

We have demonstrated a random lattice spring based methodology to characterize the development of microscopic damage and their subsequent nucleation to form crack fronts. According the lattice spring method, breaking of each bond is simulated by solving the equilibrium equation. A discrete lattice based model will be developed to characterize the nonlinear deformation of binder materials. Delamination between the active particles and the binders will also be captured. Part (a), (b) and (c) in the figure provides a schematic demonstration of the delamination mechanism observed in lithium ion batteries. Part (d) of the figure shows mechanical degradation inside an active particle during delithiation. In this work, we will highlight the implications of mechanical degradation in terms of interfacial delamination and micro-crack formation in the LIB electrodes.