Local Tortuosity Inhomogeneities in a Lithium Battery Composite Electrode

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. In this work we use tomographic data previously obtained for a commercial lithium-ion battery graphite composite electrode to evaluate the homogeneity of the tortuosity of the electrode by directly integrating the transport equations through its pore space. We find that the tortuosity of two halves of the electrode, each roughly 250 × 350 × 50 μm differ by about 30%. On a smaller scale, 80 × 100 × 50 μm, local tortuosity variations up to a factor of 3 were observed. The Bruggeman relationship between porosity and tortuosity is also examined. We find that local porosity alone does not determine local tortuosity very well, and that the average relationship between porosity and tortuosity is not well predicted by the Bruggeman relationship for this electrode. We suggest that large local variations in tortuosity can lead to reduced capacity and life.