In the battery application world everyone is always looking for power vs energy density. Wanting the maximum of both. However, as we know from a comparison of power vs energy cell design it is nearly impossible to have both.
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.
When searching for a high power cell we require a very low resistance. One cell resistance metric we can use is the conductance per unit of energy, thus normalising the DCIR vs energy. It is important to normalise this as the resistance scales with cell capacity and yet at pack level you can cluster cells in parallel, hence overcoming the limitation of a single cell capacity.
A higher conductance means a lower voltage drop for a given current demand and a lower joule heating. A key parameter for a high performance battery pack.
If we plot this cell conductance per unit of energy versus the gravimetric energy density of the cell we see a very clear relationship.
From Relationship Between DCIR and Peak Discharge C-Rate in Lithium-Ion Cells we see that the main parameters affecting DCIR are:
- Electrolyte Concentration: High electrolyte concentration increases ionic conductivity and can also affect viscosity.
- Tab Position and Thickness: Directly affects the length of the current path and resistance. Thicker tabs reduce internal resistance.
- Foil Thickness and Length: Thick foil provides lower resistance, while long current path leads to increased resistance.
- Foil-to-Tab Weld: More contact area in the weld = lower joint resistance
- Layer Count: More layers = more parallel elements = lower resistance (1/R1 + 1/R2 + 1/R3 + … = 1/Rtot)
- Conductive Agent in Anode and Cathode Muds: Conductive additives increase coating conductivity and reduce internal resistance.
- Separator Thickness: Thin separators increase ionic conductivity while requiring attention for safety.
One design approach is to improve the connection between the electrodes and the cell terminals. In the cylindrical cell a tabless approach is a significant change. From Tabbed vs Tabless Cylindrical Cells we see that the Tabless Design has a number of benefits:
- Lower internal resistance – current flows more evenly across the electrode
- Reduced heat generation – less I²R loss, better thermal performance under load
- Higher energy density – more room for active material due to removal of tabs
- Improved power capability – better suited for high current applications
- Potentially simpler thermal management – due to fewer hotspots
This post has been built based on the support of and sponsorship from: Quarto Technical Services, Eatron Technologies and About:Energy
Battery Cell Ragone Plot
The Ragone Plot is power density versus energy density. This can be volumetric or gravimetric.
Ragone Plot
When the Ragone plot is defined formally it is very powerful.
See also: Power