On the discharge capability and its limiting factors of commercial 18650 Li-ion cell at low temperatures

We here study the discharge capability of commercial 18650 cylindrical lithium-ion cells at low temperatures. The discharge capacity at −20 °C ranges from 67 to 88% of the rated capacity at 0.2C rate, which is good. However, the cell discharge capacity varies substantially at −30 and −40 °C among the studied cells. It ranges from 2 to 70% of the rated capacity at −30 °C, and 0 to 30% at −40 °C at 0.2C rate. The cell impedance at 1 kHz increases very little from room temperature down to −20 or −30 °C in general, which does not correlate with the cell discharge capability. However, the dc impedance is increased by a factor of about ten at −30 °C and about twenty at −40 °C from room temperature. The discharge capability at low temperature correlates well with the dc resistance at both room and low temperatures. The limiting factors in the discharge capability at low temperatures and the direction for the future improvement are discussed according to the cell discharge capability, the electrode geometric area, the cell impedance at 1 kHz, and the dc impedance at various temperatures. It appears that the ionic conductivity of the electrolyte and lithium solid diffusion in the electrode do not limit the cell discharge capability, while the lithium diffusion in the SEI layer on the positive surface may be the limiting factor. Cell discharge capability at low temperature does not correlate with cycle life at room temperature.