Inhomogeneous aging of cathode materials in commercial 18650 lithium ion battery cells

Abstract To understand the relationships among first life use case, battery state of health, and end-of-life condition, the materials in commercial 18650 Li-ion cells were aged using prolonged cycling under different conditions and subsequently harvested and analyzed. In particular, different charge termination protocols and temperatures were applied to potential-limited galvanostatic cycling over 1000 cycles. Relationships between observed aging phenomenon and cycling conditions were explored at the cell and cathode materials level. We noted area-specific inhomogeneities in cathode cycled under constant current charge protocol at 25 °C while such inhomogeneity was not observed in three other conditions. The mechanism behind the cathode inhomogeneity was discussed in the context of uneven growth of a secondary solid electrolyte interphase (SEI) on the anode surface occuring during constant-current charge protocol at 25 °C.