Lithium-ion batteries are widely used in many applications and it is important to monitor battery aging over time. However, it remains challenging to quickly and accurately measure aging. The correlation between battery mechanics and capacity fade can be a useful tool in battery diagnosis [1]. In this work, we present a method which uses the coupling between mechanics and electrochemistry to measure aging of a pouch cell. To quantify aging, state of health (SOH) measurements are often used. By measuring the initial peak stress and peak stress at later times, with the known stress-SOH relationship, the amount of aging can be determined [2]. We investigate the relationship between stress and SOH in different types of commercial batteries under various cycling ranges.

A lithium ion pouch cell was constrained inside a fixture along with a sensor that monitors stresses. Pouch cells were cycled at two different rates, C/2 and C/4, under Constant Current Constant Voltage charging scheme. Every 50 cycles, capacity was accurately measured through a slow discharge. In this presentation, we present the stress evolutions of pouch cells cycled at various ranges, shown in Figure 1 [3]. Using a model, we show that the observed stress-SOH relationship is composed of a long term linear film growth and a short term non-linear stress relaxation, as indicated in Figure 2 [3]. The various cycling ranges cause the average stress level inside the batteries to vary, which leads to different levels of softening. Such stress relaxation mechanism first dominates the overall stress behavior, but is gradually overtaken by the linear stress-increasing mechanism due to film growth. Commercial batteries with different compositions exhibit qualitatively similar results on stress-SOH, but vary quantitatively due to differences in their mechanical properties. We characterize the mechanical responses of individual components and compare their softening behaviors under low stress ranges.

Reference:

1. Cannarella, John, and Craig B. Arnold. "Stress evolution and capacity fade in constrained lithium-ion pouch cells." Journal of Power Sources 245 (2014): 745-751.
2. Cannarella, John, and Craig B. Arnold. "State of health and charge measurements in lithium-ion batteries using mechanical stress." Journal of Power Sources 269 (2014): 7-14.
3. Liu, Xinyi M., and Craig B. Arnold. "Effects of Cycling Ranges on Stress and Capacity Fade in Lithium-Ion Pouch Cells." Journal of The Electrochemical Society 163.13 (2016): A2501-A2507.

Figure Captions:

Figure 1. a) Representative stress evolution profiles plotted using maximum and minimum stresses during each cycle for batteries cycled between 1) 75-100% SOC 2) 50-100% SOC 3) 25-100% SOC 4) 0-100% SOC. For every 50 cycles, capacity is measured at a C/10 rate through a full discharge. b) A comparison between stress evolution profiles of batteries cycled from 0% to 100% SOC and from 75% to 100% SOC.

Figure 2. A comparison between experimental results and the fitted stress-SOH model.