Battery Cycle Life Prediction with Coupled Chemical Degradation and Fatigue Mechanics

Coupled mechanical-chemical degradation of electrodes upon charging and discharging has been recognized as a major failure mechanism in lithium ion batteries. The instability of commonly employed electrolytes results in solid electrolyte interphase (SEI) formation. Although the SEI layer is necessary, as it passivates the electrode-electrolyte interface from further solvent decomposition, SEI formation consumes lithium and thus contributes to irreversible capacity loss. In this paper, we study irreversible capacity loss in a graphite-LiFePO4 cell. Our results support the mechanism of irreversible capacity loss due to the consumption of lithium in forming SEI. We attribute irreversible capacity loss to diffusion induced stresses (DISs) that cause pre-existing cracks on the electrode surfaces to grow gradually upon cycling, leading to the growth of SEI on the newly exposed electrode surfaces. Because lithium is consumed in forming the new SEI, irreversible capacity loss continues with cycling. Along with the SEI formation upon newly exposed (cracked) surfaces, the existing SEI thickness also grows with cycling, resulting in additional loss of lithium. In this study, we provide, a simple mathematical model, based on the Paris' Law formulation of mechanical fatigue, in combination with chemical degradation to explain battery life. We compare the predicted capacity at different temperatures with the experimental data obtained from electrochemical measurements on graphite-LiFePO4 cells.