Energy density vs Power density
The most important concept in battery design is that you can either be optimizing for energy density or power density but not both. (Unless you study flow batteries…but that’s another story)
Energy density is the energy per unit volume. Power density is colloquially defined as how quickly that energy can be accessed.
In order to optimize for energy density, one could design thick electrodes in their batteries to maximize capacity. However, thick electrodes would lower power density. Thin electrodes optimize the time required for electrochemistry to occur. Thin electrodes reduce energy density as more layers = more inactive materials. Thick electrodes are more “tortuous”, whereas thin electrodes are less so.
The measure of deviation from a straight line path is tortuosity. It is the ratio of the actual flow path length to the straight distance. I like to think of the term “as the crow flies”. Think about your driving directions from point A to point B. Then compare that to the distance as the crow flies (the shortest straight line) from point A to point B. This driving distance compared to crow flying distance is tortuosity. Less tortuosity translates to higher power density.
If you look at the units in the equations, there is a hidden time component in power density based on the units.
Energy [Wh] = Potential [V] x Capacity [Ah]
Power [W] = Potential [V] x Current [A]
The SI unit for 1 Ampere [A] = 1 Coulomb/s <= therefore the time component cancels in energy and power has a time component.
Electrode loading (g/cm²) can be optimized for energy or power density. If loading is high, the theoretical capacity of the cell is higher so higher energy. If we are optimizing for time to access the energy or power, low loading is better since the electrodes are smaller and the ions have to travel less distances.
Now that we have covered energy density and power density, what is press density?
Press density (g/cm³) is inversely related to porosity. A higher press density means lower porosity, which is crucial for optimizing the active material utilization in the electrode. Press density needs to be optimized based on your electrode recipe, mass loading (g/cm²), and solids loading. General knowledge would be that high press density increases energy density, and low press density is better for power density (decreased tortuosity) but this is an oversimplification. For batteries, achieving the right press density can impact charge/discharge efficiency, cycle life, and capacity. In lithium-ion batteries, the electrode density needs to be balanced to optimize for energy density (capacity) and power density (rate).
Designing for:
EV Range: Optimized for energy density
Phone Battery: Optimized for power density (specific power)
Fast Charging: Optimized for power density
Here is a brief graphic summary of the role energy density and power density play in battery design.