The cell stack comprises: anode electrode, anode active material, separator, cathode active material, cathode electrode and of course the electrolyte. The cell stack is the basic unit of the working electrochemistry.
Inside a cell is a wound or stacked electrochemical system.
However, if we were to take a section through that we would see a number of layers.
Within this what we see is a repeating element. This is the cell stack, the fundamental building block of the cell.
At a fundamental level we can consider this building block as the working unit.
| Anode Electrode | Anode Active | Separator | Electrolyte | Cathode Active | Cathode Electrode | |
| LG 18650 MJ1 | Graphite/SiOx 85 µm | NMC811 72 µm | ||||
| Tesla 4680 | Copper 6 to 8µm | Graphite ~121µm | NMC811 85 µm | Aluminium 9 to 10µm | ||
| CATL BTF0 Tesla LFP | Copper 5 µm +/-1 | Graphite 71 µm | 13 µm +/-3 | LFP 94 µm | Aluminium 12 µm +/-1 |
One temptation is to increase the thickness of the active layers, reducing the “overhead” of current collectors and separator and hence increasing the energy density.
Effect of Electrode Thickness on Cell Energy Density
In Li-ion batteries, the cathode thickness will heavily influence the energy density of the cell. A thicker cathode means there will be more cathode active material in the electrode. This increases the capacity of the electrode and hence increase the energy density of the cell.
In this case there will be a trade between energy and power density.
Power versus Energy Cells
Comparing power versus energy cells we see there are some fundamental differences. A high energy cell will have better volumetric and gravimetric energy density at the expense of the ability to deliver a high current. The power cell will have a low internal resistance and will be optimised to deliver current over energy density.
In the full working cell we have to go beyond this working unit to design the overall system. One aspect is that a cell normally has a larger active anode than cathode, hence immediately negating perhaps this idea of a building block.
Engineering Empty Space
The cathode layer in a lithium-ion battery is a composite of solid charge storing particles, a polymeric binder, and a conductive additive. Together, they are well dispersed in a solvent and spread like paint on a conductive substrate, an effective and pleasingly simple solution that works across various chemistries and cell designs.
References
- Sandro Stock, Jan Hagemeister, Sophie Grabmann, Johannes Kriegler, Josef Keilhofer, Manuel Ank, Jonas L.S. Dickmanns, Markus Schreiber, Fabian Konwitschny, Nikolaos Wassiliadis, Markus Lienkamp, Rüdiger Daub, Cell teardown and characterization of an automotive prismatic LFP battery, Electrochimica Acta, Volume 471, 2023
See also: Electrode, Cell Design, Lithium Iron Phosphate