The Effects of Internal Pressure Evolution on the Aging of Commercial Li-Ion Cells

Abstract

Utilizing a custom developed chamber, the internal pressure evolution of a commercial 2.6 Ah, LiFePO4//Graphite lithium-ion battery (LIB) was studied throughout elevated rate lifecycle testing. Verification tests confirmed the custom test chamber and puncture procedure resulted in minimal changes to the LIB performance. Capacity measurements and analysis confirmed the battery's accessible capacity faded 20% after 500 cycles as a result of the increase in DC Equivalent Series Resistance (ESR). Electrochemical Impedance Spectroscopy (EIS) measurements and modeling analysis also established that minimal material change occurred within the LIB during lifecycle testing and the decreased capacity is believed to primarily result from the increase in battery ESR. Analysis of the battery's internal pressure evolution during the baseline discharges yielded information elucidating apparent electrode volume changes during the 1 C discharge. A strong statistically significant correlation was identified between the 1C molar density delta and the battery capacity fade. Differential capacity analysis was used to examine the correlation of the behavior with the de-intercalation process. The pressure data and molar density analysis indicated a reduction in the amount of lithium de-intercalated during discharge as a result of the rise in ESR and thus a measurable shift in the volume as the LIB ages.