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

Abstract

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. Rechargeable batteries play an increasingly important role as energy-storage devices, especially as power sources for electric vehicles and for stationary storage of intermittent renewable energy. However, significant increase in energy density and reduction in cost is needed for widespread penetration of storage in these applications. 1 The two strategies that are commonly used to improve the energy density (thereby deceasing the cost) are to develop new materials and/or to develop better electrode/battery designs. The former approach has led to a big focus on next-generation cathodes and anodes for Li-ion batteries, 1-3 and on next generation "beyond Li-ion" systems such as Li-S. 1,4,5 The latter approach, wherein new electrode designs are used to improve the energy density/decrease cost, [6][7][8] is the focus of this paper.A main focus of the efforts related to electrode design is on increasing the thickness of the electrode. This has the effect of decreasing the fraction of inactive mass and volume in the cell (current collectors and separators) thereby providing a straightforward method to increase the energy density. However, as the electrode thickness increases, electrolyte-phase mass transfer limitations become more important impeding the power delivery, thereby leading to an inability to satisfy the requirement of the chosen application. [6][7][8] This interplay between energy and power has resulted in the use of thin (ca. 40 μm) electrodes for high-power applications like hybrid electric vehicles and thick (ca. 80 μm) electrodes for high-energy applications such as electric vehicles. 8 The thickness constraints are acute in Li-ion cells due to the low transport properties of the non-aqueous electrolytes, 8,9 with studies showing that the electrolyte is the main cause of the limitations in these batteri...

Is There a Benefit in Employing Graded Electrodes for Lithium-Ion Batteries?

“…However, the average porosities for 2-layer, 3-layer, 4-layer, and 5-layer graded electrode were 0.3214, 0.3152, 0.3135, and 0.3119 respectively, not far from the uniform optimal porosity 0.3435. With the same active material constraint, the minimal resistances for 2-layer, 3-layer, 4-layer, and 5-layer graded electrode are 5.1300 -cm 2 , 5.0976 -cm 2 , 5.0823 -cm 2 , and 5.0748 -cm 2 , which are slightly larger than the values in Table III. If a conclusion was to be made just based on the results so far, it would confirm the conclusion from Dai et al 13 that graded porosity is not very useful. However, the next section will show the cases where graded design in the electrode is needed.…”

“…Recently, Dai et al 13 carefully looked at the performance improvement by utilizing a full P2D model and recommended the manufacturers not to make the graded electrode due to the additional processes involved and very small improvement achieved. In this paper, we want to quantify the gain in terms of electrode resistance with the two optimization approaches.…”

Toward Low-Cost, High-Energy Density, and High-Power Density Lithium-Ion Batteries

“…65,66 Efforts have been put forth in a variable porosity electrode, but this has only led to marginal improvements in energy density compared with well-designed constant-porosity electrodes, suggesting it is more important to decrease the tortuosity. 67 Numerical simulation has also been applied to investigate the limiting factors of the energy-power density relationship in LiNi 0.8 Co 0.15 Al 0.05 O 2 (NCA)/ graphite cells with thick electrodes. 68 Lithium ion depletion in the electrolyte and lithium diffusion gradient in active material particles were found to attenuate the advantage of thick electrodes.…”

“…Nevertheless, the Li-ion transport limitations in liquid electrolytes become increasingly important as the electrode thickness increases 67,68,70,71 because the diffusion time in the liquid phase is no longer negligible as a result of the significant increase in diffusion length in the thick porous electrodes. Consequently, it is necessary to design thick electrode architectures to take advantage of the increased energy density provided by the higher active material volume ratio, while minimizing the tortuosity and transport limitations that can be detrimental to power performance.…”

“…Consequently, it is necessary to design thick electrode architectures to take advantage of the increased energy density provided by the higher active material volume ratio, while minimizing the tortuosity and transport limitations that can be detrimental to power performance. Many different designs have been suggested, including graded-porosity and hierarchical architectures, 67,[72][73][74][75][76] and several simulations have shown that smaller active material particles can help decrease the polarization and capacity loss observed at high discharge rates by shortening the Li-ion diffusion path in the particles. 68,71,77 Yet, particle size variations also affect the packing structure of the electrode and can result in different pore sizes and distributions, as well as in differences in the contact resistance between the electrode and the current collector.…”