Solid-Electrolyte Interphase (SEI) Fracture: The Coupled Mechanical-Chemical Degradation of Lithium Ion Battery

The coupled mechanical-chemical degradation of electrodes upon electrochemical cycling is recognized as a major challenge in lithium ion batteries. Instability of commonly employed electrolytes at the operating potentials results in solid electrolyte interphase (SEI) formation. Although, a stable SEI layer is necessary, as it passivates the electrode-electrolyte interface from further solvent decomposition, SEI formation also contributes to irreversible capacity loss. During electrochemical cycling, due to the volume changes of the electrode particles and the associated stresses within the SEI layer, the SEI layer may fracture, exposing new electrode surfaces to the electrolyte. A new SEI is formed at these newly exposed surfaces resulting in loss of active lithium inventory. We develop a mathematical model to calculate stresses and strain energy for a particleencapsulated with SEI during lithiation and delithiation. We show that, loss of capacity of a battery due to SEI fracture actually occurs during lithiation since SEI experiences high tensile stress suring electrode lithiation. We demonstrate that for slow lithiation, excess strain energy in the particle is proportional to square of State of Lithiation (SOL) swing. A corelation is developed to calculate the cell capacity loss due to SEI fracture as a function of SOL swing, and is compared with the experimental data. We found that the model fits the capacity loss data very well. This is the first physics based model which correlates SEI cracking with the cell capacity loss.