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.
Teardown Comparison of Energy versus Power Dense Cells
In reference 1 a teardown of a number of cells was done to understand the design versus the characteristic power to energy ratio. This gave the following table of design parameters:
| Cell Component | Energy Density | Power Density |
|---|---|---|
| Electrodes | High coat weights | Low coat weights |
| Low coating porosity | High coating porosity | |
| Medium + large particle sizes | Small + medium particle sizes | |
| Low conductive carbon content | High conductive carbon content | |
| Minimum possible binder content | ||
| Current Collectors | Thinner | Thicker |
| Coated to improve adhesion | Coated to reduce resistance | |
| Separator | Thin | Thin |
| Electrolyte | High conductivity | High conductivity |
| Connection Tags | Thin/narrow tags | Thick/wide tags |
| Single tag on each electrode | Multiple tags |
Thin separators benefit the design of both energy and power density cells as it reduces volume and resistance between the active layers.
The electrical tag configuration, thickness and number has an impact on both electrical and thermal conductivity. Pouch cells that are made up of multiple individual layers of electrodes with the positive and negative connections at opposite ends will have the lowest internal resistance. Cylindrical and wound prismatic cells will have less electrical tags per unit length/area of electrode and hence higher resistance.
In simple terms the energy cell has thicker layers of active material, thinner current collectors and less of them.
This means the energy cell will have a higher electrical internal resistance meaning it will generate more heat based on I2R heating.
The energy cell will have poorer thermal conductivity in-plane and through-plane. Thus, it will need a higher temperature gradient to reject the heat.
Performance Comparison Thick versus Thin Electrodes
A comparison of electrode thickness studied from test and modelling [Ref 2] gave the following fundamental results as a comparison:
- Under same discharge rates, the cell with thicker electrodes had a higher and more uneven temperature response. Both of the factors can lead to the depletion of active material and faster capacity fade.
- Longer diffusion distance in the thicker electrodes resulted in more serious concentration polarization, thicker electrode battery has relatively higher internal resistance, which can result in a lower power output and an earlier stopping of the discharge due to it reaching minimum voltage earlier.
- At high discharge rates the contribution of ohmic heating dominates, especially in thick electrode cells, leading to a faster deterioration in state of health.
- Due to the diffusion limitation, the electrochemical reactions are unevenly taking place inside the cell with thick electrodes, particularly under high rates, which will not only lead to the underutilization of active materials, but will also increase the thermal instability of the cell.
Data from commercial cell specification sheets shows that we have a trade between power and energy density at cell level.
References
- M. J. Lain, J. Brandon, E. Kendrick, “Design Strategies for High Power vs. High Energy Lithium Ion Cells“, Batteries 2019, 5(4), 64
- Rui Zhao, Jie Liu, Junjie Gu, “The effects of electrode thickness on the electrochemical and thermal characteristics of lithium ion battery“, Applied Energy, Volume 139, 2015, Pages 220-229
3D Electrodes
Another way to increase energy density is with 3D electrodes. Increasing the surface area and connection to the active materials can improve a number of features of the cell.
