Coupling Between Stack Stress and Chemical Degradation in Lithium-Ion Pouch Cells

Knowledge of how stack-level mechanical stress affects cell aging is important for the long term performance of practical li-ion cells which spend their entire service lives under compressive stress. This compressive stress is set by an initial manufacturing stack pressure, fluctuates with electrode expansion/contraction during charge/discharge, and increases over time due to irreversible electrode expansion. In a previous aging study in which we monitored the stack stress of constrained li-ion cells during cycling, we observed that higher levels of stack stress led to higher rates of capacity fade [1]. Through a post mortem analysis, we determined that the primary source of capacity fade was cycleable lithium loss, such that higher levels of stack stress led to an increase in parasitic side reactions. This degradation occurred in a spatially non-uniform manner such that localized regions of the cell’s graphite anode were covered with a surface film. We also observed similar distributions of localized pore closure in the separator leading us to hypothesize that stack stress and chemical degradation are coupled through separator deformation [2-3].

In this presentation we present a follow up study to the aforementioned aging study in which we present experimental evidence for the coupling of stack stress and chemical degradation through separator deformation. To isolate the effects of separator deformation on cell aging, coin cells are fabricated using conventional graphite and lithium cobalt oxide electrodes containing deformed separators. The deformed separators are deformed by applying high localized stresses to create controlled macroscopic patterns of pore closure as can be seen visually in Figure 1. Cells cycled with deformed separators show higher rates of capacity fade than cells cycled with pristine separators as seen from the capacity plot in Figure 2. Upon disassembly, localized visible surface films similar to those observed in [1] are found to be present on the graphite anode.

We explain the observed surface film patterns by considering the relative rates of lateral transport that occur in the presence of separator pore closure which restricts normal transport through the separator membrane. Because the electrochemical potential of lithium in lithium cobalt oxide varies more strongly with concentration than it does in graphite, lateral ion transport is enhanced in the cathode. However, sluggish lateral transport in the anode results in high overpotentials which can result in temporary lithium plating and consequential chemical degradation. The proposed explanation is supported by three electrode measurements of a full cell in which the graphite electrode exhibits a negative potential vs. Li/Li+ for cells constructed with deformed separators as seen in Figure 3. We anticipate these issues to become increasingly important in next generation lithium-ion cells which are expected to make use of higher expansion electrode materials.

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, “Ion transport restriction in mechanically strained separator membranes,” J. Power Sources 226, (2013) 149-155.

[3]. C. Peabody, C. B. Arnold, “The role of mechanically-induced separator creep in lithium-ion battery capacity fade,” J. Power Sources, 196 (2011) 8147-8153.

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.

FIGURE CAPTIONS

Figure 1. Photograph of a separator that has been deformed locally in a ring-shaped pattern.

Figure 2. Capacity evolution of a cells constructed with a deformed and pristine separator.

Figure 3. Three electrode measurements of a full cell cycled with a deformed separator showing anode voltage dropping below 0V vs. Li/Li+.