Characterizing the Microstructure of Separators in Lithium Batteries and Their Effects on Dendritic Growth
A Cannon and EM Ryan, ACS APPLIED ENERGY MATERIALS, 4, 7848-7861 (2021).
Porous separators are used to physically separate the electrodes in batteries while providing mechanical stability and improving the performance of lithium batteries. In this study, the effect of the battery separator microstructure on mass transport and lithium dendrite growth is investigated using pore-scale computational modeling. The microstructural characteristics of the separator, such as porosity, tortuosity, and constrictivity, directly alter diffusion paths for lithium ions during battery cycling. The accuracies of experimental relations, i.e., Bruggeman, MacMullin, used to determine these characteristics are unreliable. A pore-scale computational model is used to simulate mass transport and dendrite growth, utilizing an explicit representation of the separator microstructure. The simulation is compared to the experimental relations and shows that the experimental relations fail to adequately capture important physical characteristics in the microstructure of the separator. Tortuosity, a characteristic that is difficult to experimentally measure, is shown to significantly affect the growth rate of dendrites and can lead to a shorter lifetime in the battery. Additionally, the degree of heterogeneity in a battery separator is explored and shown to lead to different dendrite growth rates even when the bulk physical characteristics of separators are the same. Evidence provided in this paper suggests that neglecting local variation of these properties can lead to nonuniform diffusion and, in turn, problematic dendritic growth. The findings offer insight into properties not often considered in battery separator designs.
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