  Thick Li-ion battery electrodes with high ion transport rates could enable
batteries that cost less and that have higher gravimetric and volumetric energy
density, because they require fewer inactive cell-components. Finding ways to
increase ion transport rates in thick electrodes would be especially valuable
for electrodes made with graphite platelets, which have been shown to have
tortuosities in the thru-plane direction about 3 times higher than in the
in-plane direction. Here, we predict that bi-tortuous electrode structures
(containing electrolyte-filled macro-pores embedded in micro-porous graphite)
can enhance ion transport and can achieve double the discharge capacity
compared to an unstructured electrode at the same average porosity. We
introduce a new two-dimensional version of porous-electrode theory with
anisotropic ion transport to investigate these effects and to interpret the
mechanisms by which performance enhancements arise. From this analysis we
determine criteria for the design of bi-tortuous graphite anodes, including the
particular volume fraction of macro-pores that maximizes discharge capacity
(approximately 20 vol.%) and a threshold spacing interval (half the electrode's
thickness) below which only marginal enhancement in discharge capacity is
obtained. We also report the sensitivity of performance with respect to cycling
rate, electrode thickness, and average porosity/electroactive-material loading.
