Cylindrical Cells

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.

18650 and 21700 cylindrical cell drawings

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
Tesla 4680

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

Section through a cylindrical cell

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.

Thermal conductivity of a cell

If we look at the active layers of a cell the thermal conductivity in the plane of the layers is approximately 10x to 100x that through the planes.

This should not be unexpected as the electrodes are made from sheets of aluminium and copper. Two of the best materials for thermal conductivity.

These values though have a large range:

  • 15 to 160 W/mK In-Plane
  • 0.2 to 8 W/mK Through-Plane

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.

Example Applications

gen 2 battery for formula e

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 module busbars

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 4680 battery cooling

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

Tesla 4680 cell

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:

  1. 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

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