Li-ion batteries are the best available secondary batteries in the market currently. The life of the batteries plays a key role for increasing its penetration into the various market fields including EV’s. In order to prolong the battery life it is important to understand the degradation mechanisms occurring inside it which are highly sophisticated. Because of the various chemical and physical processes occurring during cycling the performance of the batteries degrade. The major chemical degradation occurring inside the batteries are the formation of the the Solid Electrolyte Interface (SEI) layer, at the surface of the graphite electrode. These layers are formed due to the electrolytic reduction at the surface, consuming the lithium ions resulting in capacity fading and also in power fading because of increased resistance. Alongside the formation of protective layers at the surface, the electrode also experiences stress and strain as battery undergoes cycling. Due to these stresses, the electrode material undergoes exfoliation and also loses contact with the current collector resulting in increased impedance and reduced capacity. These stresses also crackdown the SEI layers on the surface which causes further chemical degradation [1]. The SEI layers in fact provides an additional compressive strength to the electrode maximizing its life, as shown in Figure 1. Since the lithium ions are consumed in the side reaction, the actual particles diffusing into the active material are less compared to when there is no side reaction, therefore reducing stress although by small margin. In order to understand these inter-twined effects and its overall effect on the battery life a physics based mathematical model is built integrating both these mechanisms using a single particle battery model [2]. The current density at the graphite electrode is distributed into the intercalation current density and the side reaction current density for accounting for the chemical degradation [3] while stress-strain relations with the existing concentration gradients are established for both spherical particle and the SEI layers analogously to the thermal gradient [4]. This talk will present results from simulations performed for determining the effect of the SEI on stresses and vice-versa on cycling of the battery and its effect on the life and performance. The effects of C-rates and temperature are also explored and an optimized charging protocol is designed for minimizing the degradation thereby prolonging the battery life will be presented.

References:

1. I. Laresgoiti, S. Käbitz, M. Ecker, and D. U. Sauer, “Modeling mechanical degradation in lithium ion batteries during cycling: Solid electrolyte interphase fracture,”Journal of Power Sources, vol. 300, pp. 112–122, 2015.
2. M. Guo, G. Sikha, and R. E. White, “Single-Particle Model for a Lithium-Ion Cell: Thermal Behavior,” Journal of The Electrochemical Society, vol. 158, pp. A122–A132, 2011.
3. P. Ramadass, B. Haran, P. M. Gomadam, R. White, and B. N. Popov, “Development of First Principles Capacity Fade Model for Li-Ion Cells,” Journal of The Electrochemical Society, vol. 151, pp. A196–A203, 2004.
4. X. Zhang, W. Shyy, and A. Marie Sastry, “Numerical Simulation of Intercalation Induced Stress in Li-Ion Battery Electrode Particles,” Journal of The Electrochemical Society, vol. 154, pp. A910–A916, 2007.