Impact of Electrolyte Volume on the Cycling Performance and Impedance Growth of 18650 Li-ion Cells

We provide compelling evidence that the cycling performance of 18650 Li-ion cells is adversely affected by excessive amounts of electrolyte volume, with a noticeable decline observed within the initial 30 cycles, particularly at higher discharge rates. This “high-volume effect” imposes additional constraints on the optimization of cell manufacturing, highlighting the importance of identifying its underlying causes. The electrochemical impedance of 3.5 Ah 18650 cylindrical cells with varying levels of electrolyte volume was extensively measured using PEIS and GEIS techniques. The results indicate that, in general, the ohmic and charge transfer resistance(s) of the cells increase at a faster rate when excess electrolyte volume (9% and 18%) is present. During high discharge rate cycling, relaxation periods can effectively recover the lost capacity, but when high discharge rate cycling resumes, the trend in the capacity loss reappears. We hypothesize that a salt segregation effect in the electrolyte may contribute to the growth of both ohmic and charge transfer resistance, leading to capacity loss when excess electrolyte is present.