Measurements of SOH Using Stack Stress

In previous ageing studies in which the stack stress of commercially available lithium-ion (LCO/C) pouch cells is monitored during cycling [1], we observe that stack stress (Figs 1a and 2a) is linearly related to cell SOH (Figs 1b and 2b) as shown in Figs 1c and 2c [2]. The increase in stack stress is attributed to irreversible volumetric expansion of the anode which we assume to be related to SOH. This suggests the possibility of using stress to measure SOH, a method whose simplicity provides a distinct advantage to conventional computationally intense methods which rely on complex physical models of the underlying cells. Stack stress could be used as a standalone SOH monitoring system, or integrated into existing battery management systems to increase management fidelity. In this talk we investigate the origins of the stress-SOH relationship.

We report the following data from ageing studies of cells cycled under different conditions: temporal stack stress evolution, capacity evolution, non-destructive differential voltage spectroscopy analysis, and destructive post-mortem analysis.  All of the temporal stress and capacity data show a t^1/2 dependence suggestive of a diffusion limited mechanism such as SEI growth, as seen in Figs. 1d and 2d. This assertion is corroborated by the differential voltage spectroscopy measurements and destructive post mortem analysis, which also show evidence of a SEI growth mechanism. Stack stress is also observed to increase in cells held at 4.2V in the absence of cycling as shown in Fig. 2a, supporting the notion of a chemical mechanism. Based on the assumptions of an SEI growth mechanism, we present simple scaling arguments which predict a linear relationship between stack stress and SOH. We discuss the implications for this method and model as it applies to battery management systems and fundamental ageing studies.

REFERENCES

[1]. J. Cannarella, C. B. Arnold, “Stress evolution and capacity fade in constrained lithium-ion pouch cells,” J. Power Sources 245 (2014) 745-751.

[2]. J. Cannarella, C. B. Arnold, "State of health and charge measurements in lithium-ion batteries using mechanical stress," submitted.

FIGURE CAPTIONS

Figure 1. Data from a cell aged by cycling under a C/2 CCCV scheme showing (a) stack stress as a function of cycle, (b) C/10 capacity as a function of accumulated cycles, (c) stack stress as a function of SOH, and (d) t^1/2 dependence of the C/10 capacity data.

Figure 2. Data from a cell aged by holding at 4.2V at room temperature and periodic cycling at C/10 to measure capacity showing (a) stack stress as a function of time, (b) C/10 capacity as a function of time, (c) stack stress as a function of SOH, and (d) t^1/2 dependence of the C/10 capacity data.

ACKNOWLEDGEMENTS

Support was provided by the DoD through the NDSEG Program and by the Siebel Energy Challenge. J. C. also acknowledges the Rutgers-Princeton IGERT in Nanotechnology for Clean Energy.