Intercalation induced surface cracking in electrode particles

Degradation mechanisms in Li-ion batteries such as SEI formation, isolation of active material and reduction in electronic conductivity appear to correlate with the increase in surface area of electrode particles caused by particle fracture during charge/discharge cycles. The focus of this study is on the surface cracking of an electrode particle, as large tensile stresses operate on the surface during the delithiation process of charge/discharge cycling. The pre-existing surface flaws act as crack initiators under this scenario and we discuss the extension of these cracks under two different operating conditions. Approximate analytical expressions for the propensity for surface crack growth is derived in terms of stress intensity factor and fracture toughness of the material. Utilizing dimensional analysis, we arrive at fracture limit diagrams to determine fracture-free conditions as design guidelines for prescribed electrode particle size or diffusion boundary conditions related to the charging/discharging process. Another significant result of the fracture analysis is that smaller particles can withstand a wider range of fluctuations in concentration or flux at the boundary for surface fracture-free conditions. This result supplements the conventional understanding that smaller particles show higher structural integrity because of fewer pre-existing defects.