The layers of the cell are wound in a spiral. Normally these cells have the lower case as the negative terminal and the top centre as the positive terminal. However, a number of larger cylindrical cells have both +ve and -ve terminals on the top surface.
For this article we will concentrate on the 18650 and 21700 formats.
Perhaps the most famous of the cylindrical formats is the 18650 and 21700.
18650 => ~18mm in diameter and ~65.0mm long
21700 => ~21mm in diameter and ~70.0mm long
These dimensions vary between manufacturers.
However, the industry is advancing rapidly and looking for cost savings, hence larger cell formats.
Cylindrical Cell Comparison 4680 vs 21700 vs 18650
Tesla particularly uses Cylindrical cells in their Electric Vehicles. As per recent announcement Tesla is moving to 4680 from 21700 and the older 18650. Rivian and Lucid Motors are also using cylindrical cells 21700 in their vehicle models (R1T, R1S and AIR Dream, Air GT respectively). BMW along with CATL have announced that its NEUE KLASSE type models will use the 46mm diameter geometry cylindrical cells too.
Can Housing
The cans for the 18650 and 21700 are made from nickel plated steel and deep drawn in a two-stage process. The result is the base of the can is thicker than the cylindrical side wall.
- 18650
- Base thickness ~0.3mm
- Wall thickness ~0.22 to 0.28mm
- 21700
- Base thickness ~0.3 to 0.4mm
- Wall thickness ~0.22 to 0.34mm
- Mass ~8.5g (can, seal, cap)
- 4680
- wall thickness ~0.5 to 0.6mm
Aluminium Cell Housings for Cylindrical Lithium-ion Batteries
Thermal simulations reveal significant improvements in cooling performance at 3C fast-charging of the aluminium housing version compared to nickel-plated steel reference cell. The impact of the cell housing material is particularly pronounced in case of a sidewall cooling. In this case, simulation reveals differences in maximum temperature (hot spot) of 11°C after 10 minutes.
Packing
The cylindrical format limits the packing density to at best hexagonal close pack.
Electrical Connections
The outside can of most cylindrical cells is connected to the anode of the jelly roll. Hence this is the negative terminal.
This means that you can connect to the negative at the bottom and the positive on the centre button at the top. This is traditionally how we connect primary cells together when we use them in remote controls, torches etc.
Single Sided Busbars
However, as the can is the negative we can connect to the negative on the top edge of the can. This means we connect to both the +ve and -ve at one end of the cylindrical cell.
Cooling
Cylindrical cells are used in numerous applications and cooling varies from passive through to immersed dielectric cooling. The diameter, length and connection of the jelly roll to the outer case all have an impact on the cooling potential and resultant temperature gradient through the active material.
Base vs Side Cooling Cylindrical Cells
- For the dimensions of the 21700 cell base cooling gives ~12% greater heat flux, for the same temperature gradient, than side cooling.
- If we listen to Peter Rawlinson’s description of the Lucid Motors battery design he points out that the base cooling design gives a more consistent thermal connection to the cell.
- In the end though this probably comes down to the package dimensions and whether you can fit a slightly taller base cooled design or a wider/longer side cooled design.
- One point not explored here is that with a side cooled cell design the busbars that join the cells together will have to be slightly longer to accommodate thickness of the cooling plates.
Electrode Estimation
Knowing the outer and inner diameter of the spiral along with it’s thickness we can calculate the length of the material to create it.
D is the inner diameter of the cylindrical can.
The inner diameter is that of the mandrel around which we wind the spiral.
Alternative Electrode Estimation
Here we present a simple method for estimating electrode length in a cylindrical cell. The method is equally applicable to other formats since we make an estimation of the total active electrode area. Results require knowledge of one electrode Active Material (AM) chemistry, electrode porosity and thickness and cell capacity. We assume that 100% of the active material is utilised and contributes to the cell capacity. This is optimistic and will likely lead to an underestimate of the length.
Paper Review: Energy Density of Cylindrical Li-Ion Cells: A Comparison of Commercial 18650 to the 21700 Cells by Jason B. Quinn et al 2018 J. Electrochem. Soc. 165 A3284
Example Applications
Formula E Battery 2019-21
This was the second generation of the Formula E battery design. This pack used a Murata 18650 cylindrical cell and nearly doubled the energy capacity of the generation 1 battery pack. Thus allowing the cars to run a full race with one car and one charge.
Lucid Air Battery
The battery pack in the Lucid Air Dream is 188kWh and uses 6600 cylindrical cells in the 21700 format. Using a very similar design approach to the Tesla Model 3.
Tesla Model Y 4680 Battery
The first outing for the Tesla 4680 tabless cell design at pack level. An interesting approach that has made significant changes to optimise the cell, pack and vehicle integration.
Improving the Design
The Tesla 4680 cell is 5x the energy of the 21700 cell.
Perhaps the most important upgrade is not the larger cell, but the change to the engineering design and the manufacture of this cell.
The tabless jelly roll significantly improves the electrical and thermal connections. Tranter et al [1] have analysed this design and looked at the electrochemical and thermal behaviour.
The new design is found to mitigate the ohmic losses experienced around the “jelly-roll” current collectors which are significant for the traditional tabbed case, thus leading to higher efficiency and capacity and reduced heat production.
T. G. Tranter, R. Timms, P. R. Shearing and D. J. L. Brett, “Communication—Prediction of Thermal Issues for Larger Format 4680 Cylindrical Cells and Their Mitigation with Enhanced Current Collection“, Journal of The Electrochemical Society, Volume 167, Number 16
References:
- T. G. Tranter, R. Timms, P. R. Shearing and D. J. L. Brett, “Communication—Prediction of Thermal Issues for Larger Format 4680 Cylindrical Cells and Their Mitigation with Enhanced Current Collection“, Journal of The Electrochemical Society, Volume 167, Number 16