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