On Graded Electrode Porosity as a Design Tool for Improving the Energy Density of Batteries

As the need for higher energy density batteries increases, there have been numerous attempts at tuning the design of the battery electrodes to improve performance. Increasing the electrode loading remains a straightforward method to increase the energy density. Li-ion batteries are typically liquid phase limited; therefore, battery designers attempt to increase the electrode thickness and decrease the porosity until polarization losses become significant. It is in this context that a graded porosity, with varying porosity across electrode, has been explored to reduce liquid phase limitations and thereby decrease the polarization losses. In the literature, mathematical models have been used to suggest that varying porosity designs can lead to improved energy density. In this paper we show that such an enhanced improvement is an artifact of the previous models using arbitrary base cases, and that a similar improvement in energy is readily achievable by judicious choice of a constant porosity. A more careful comparison, as shown in this paper, suggests only a marginal improvement in energy density. Here, we show the methodology used to reach this conclusion and detail the underlying reason for this result.