Stephen J. Harris, General Motors R&D
While battery performance is well predicted by the macrohomogeneous model of Newman and co-workers, predicting degradation and failure remains a challenge. It may be that, like most materials, failure depends on local imperfections and inhomogeneities. We use tomographic data to evaluate the homogeneity of the tortuosity of the electrode by directly integrating the transport equations through its pore space. The tortuosity of two halves of the electrode, each roughly 250 x 350 x 50 µm differ by about 30%. On a smaller scale, 80 x 100 x 50 µm, local tortuosity variations up to a factor of 4 are observed. The relationship between porosity and tortuosity is not well predicted by the Bruggeman relationship for this electrode, either locally or globally. We suggest that large local variations in tortuosity can lead to reduced capacity and life because of concomitant changes in current density. We also describe a study of SEI from a microscopic perspective using focused-ion-beam time-of-flight secondary ion mass spectrometry. Compound SEI films formed with LiClO4 followed by LiBF4 electrolyte are examined. Depth profiles of BF4 through the LiClO4 SEI allows us to visualize the porosity of the SEI as a function of depth, while depth profiles of Li through the LiClO4 SEI allows us to image Li transport through the SEI. The SEI formation process, the interaction of SEI with electrolyte, as well as the influence of these processes on cell operation performance will be discussed in detail.
