Measurements of SOH Using Stack Stress
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 t1/2 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.
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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) t1/2 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) t1/2 dependence of the C/10 capacity data.