Comparing Cell Energy Density of Two Chemistries

This case study will highlight the importance of reporting new electrode active material performance parameters in full cell architectures. It will also discuss the need to understand the limits of different cell chemistries, and why any extrapolation of future performance must take into account the physical realities of the materials.

The performance of electrode active materials is often reported using half-cell data, or as the theoretical energy density. Neither is representative of how the material will perform in a full cell setting. This is a point which continues to be raised in this field (see “A non-academic perspective on the future of lithium-based batteries” by James Frith, Matt Lacey and Ulderico Ullissi as a great example). However, this topic continues to be relevant today. The potential performance of new electrode active materials can be explored using the CAMS model. Lithium-Sulfur (Li-S) batteries will be used as an example.

Li-S is often presented as the next generation cell chemistry thanks to the very high theoretical energy density of Sulfur, an impressive 2,700 Wh/kg. However, prototype / near commercial Li-S cells currently sit at around ~400 Wh/kg, 7 times less than the theoretical value. So why is this the case? In short, you cannot have a battery just made of sulfur. Cathode, anode, electrolyte, and other key components must all come together to form a functioning cell, and these must all be considered when reporting on new materials.

The key is to maximize the proportion of active components in your cell to end up with the highest energy density possible, all while ensuring that your cell remains functional. These parameters will vary from one cell chemistry to another. Using the CAMS model we can model the expected energy density between three different cell chemistries: an NMC811||Graphite cell, an NMC811||Lithium cell and a Sulfur||Lithium cell.

From the data, we can analyse how the ratio of the modelled cell energy density and the theoretical cathode energy density varies between the three chemistries. Highly optimized NMC||Graphite cells reach 26% of the theoretical energy density thanks to decades of optimization. This can be increased to 42% for NMC||Lithium cells by using the “perfect” anode for lithium-ion batteries, lithium metal. However, Li-S cells currently achieve ~15% of the theoretical energy density. Granted, Li-S cells are still undergoing optimization, but its unlikely that Li-S cells will be reaching 42% of theoretical energy density any time soon due to inherent limitations on the cathode side.

First, the Sulfur cathode has a lower proportion of active material compared to NMC electrodes. As Sulfur is electronically insulating, it requires a lot of conductive carbon additive to be able to cycle efficiently. While the electrode formulation will continue to be improved, it will likely never reach the values that NMC cells can reach. The insulating properties of sulfur will also limit how thick the electrodes can be made, and thus the active material loading that can be achieved.

This example illustrates the importance of understanding how cell chemistries will differ from one another, and why these differences must be considered when considering future cell energy densities. A similar exercise can be done using CAMS for solid-state batteries, the energy density of which will be heavily influenced by the composition of the cathode electrode and the width of the solid electrolyte layer.

Below are the details of the cells which were modelled:

  1. All cells were modelled to have areal capacities close to 4.30mAh/cm2 by modifying the material loading.
  2. 20-micron thick separators were used in all cells.
  3. NMC 811 was the cathode material with a charge capacity of 220 mAh/g and a discharge capacity of 205 mAh/g.
  4. Default values for the A5 cell dimensions were used for all cells.

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