What are the terminals of a cylindrical cell?

The active layers of the cell are wound in a spiral. Normally these cells have the lower case as the negative terminal and the top center as the positive terminal. However, some larger cylindrical cells feature both +ve and -ve terminals on the top surface.

For this article, we will concentrate on the 18650 and 21700 formats, but there is a growing trend towards the 46mm diameter 46xx class of cylindrical cells.

Among the most recognized cylindrical formats are 18650 and 21700.

18650 => ~18mm in diameter and ~65.0mm long

21700 => ~21mm in diameter and ~70.0mm long

These dimensions can vary between manufacturers.

However, the industry is rapidly evolving and aiming for cost efficiency, leading to the adoption of larger cell formats.

Cylindrical Cell Comparison 4680 vs 21700 vs 18650

Tesla predominantly uses cylindrical cells in their Electric Vehicles. Recently, Tesla announced a shift from 21700 and the older 18650 cells to the 4680 format. Rivian and Lucid Motors also employ cylindrical cells (21700) in their vehicle models (R1T, R1S, and AIR Dream, Air GT, respectively). BMW, together with CATL, has revealed that its NEUE KLASSE models will use cylindrical cells with a 46mm diameter.

Can Housing

The cans for the 18650 and 21700 cells are made from nickel-plated steel and undergo a two-stage deep drawing process, resulting in a thicker base compared to 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 aluminum housing cells compared to nickel-plated steel reference cells. The effect of the cell housing material is notably pronounced with sidewall cooling, showing temperature differences (hot spots) of 11°C after 10 minutes.

Aluminium 4680 Cell Can Structural Performance

We present a study on the structural performance of aluminum 4680 cell cans made from two different materials: Speira ION Cell 3-CB and Speira ION Cell 3-CS. These cans were produced through deep drawing and wall ironing, featuring a wall thickness of 0.75 mm and a bottom thickness of 0.9 mm. This manufacturing process allows the use of high-strength aluminum alloys and hard tempers.

Packing

The cylindrical format restricts the packing density to a maximum of hexagonal close packing.

Electrical Connections

The exterior can of most cylindrical cells is connected to the anode of the jelly roll, making it the negative terminal.

This design allows a connection to the negative terminal at the bottom and the positive terminal at the top center button. Traditionally, this is how primary cells are connected for use in devices like remote controls and torches.

Single Sided Busbars

Additionally, since the can itself is the negative terminal, connections can also be made to the negative terminal at the top edge of the can. This allows both the +ve and -ve terminals to be accessed from one end of the cylindrical cell.

Cooling

Cylindrical cells are employed in many applications, with cooling methods ranging from passive to immersed dielectric cooling. The cell's diameter, length, and the jelly roll's connection to the outer case all influence the cooling potential and resulting temperature gradient within the active material.

Base vs. Side Cooling Cylindrical Cells

• For dimensions equivalent to a 21700 cell, base cooling offers ~12% higher heat flux for the same temperature gradient compared to side cooling.
• Peter Rawlinson of Lucid Motors notes that base cooling design provides a more consistent thermal connection to the cell.
• This decision may ultimately depend on packaging dimensions and whether it is preferable to have a slightly taller base-cooled design or a wider/longer side-cooled design.
• One downside of side-cooled designs not explored here is that the busbars joining the cells need to be longer to accommodate the thickness of the cooling plates.

Electrode Estimation

By knowing the outer and inner diameter of the spiral along with its thickness, we can calculate the length of the material needed to create it.

D represents the inner diameter of the cylindrical can, around which we wind the spiral.

Alternative Electrode Estimation

This method provides a simplified approach for estimating electrode length in a cylindrical cell. This technique is also applicable to other formats by estimating the total active electrode area. Accurate results require knowledge of the electrode Active Material (AM) chemistry, electrode porosity, thickness, and cell capacity. We assume 100% utilization of the active material, which is optimistic and likely results in an underestimate in 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

Paper Link

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, allowing the cars to complete a full race on one charge.

If you want to learn more, please visit our website cylindrical lithium battery battery pack assembly.

Additional reading:
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Lucid Air Battery

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The battery pack in the Lucid Air Dream is 188kWh and uses 6600 cylindrical cells in the 21700 format. This design approach is quite similar to that of the Tesla Model 3.

Tesla Model Y 4680 Battery

The first appearance of Tesla's 4680 tabless cell design at the pack level represents a major innovation, optimizing cell, pack, and vehicle integration.

Improving the Design

The Tesla 4680 cell has 5 times the energy of the 21700 cell.

Arguably, the most significant improvement is not just the larger cell size but the enhanced engineering design and manufacturing process.

The tabless jelly roll design greatly enhances electrical and thermal connections. Research by Tranter et al. highlights the electrochemical and thermal benefits of this design.

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

   

1. What is a cylindrical battery?

Batteries come in three different shapes: cylindrical, square, and lipo. Among these, the cylindrical battery is the most commonly used type globally. Named for its cylindrical shape, this battery is housed in a metal can with the positive terminal on the cap and the negative terminal at the bottom. Due to lower costs and faster production, cylindrical batteries are preferred by most manufacturers.

2. Why are batteries cylindrical?

The cylindrical shape facilitates automated manufacturing. It also helps maintain mechanical stability and evenly distributes internal pressure, allowing the cell to withstand increased internal pressure without deforming. While the cylindrical shape does decrease packaging density, it leaves small gaps when these batteries are combined into a pack. These gaps aid in thermal management, allowing coolants to flow freely between the cells.

3. How do you make a cylindrical battery?

A cylindrical battery is composed of thin sheet-like anodes, cathodes, and separators. The manufacturing process begins by sintering raw cathode and anode materials to create electrode sheets, which are then milled for uniformity. These sheets are rolled to the desired thickness and sliced into strips. Current collectors and tabs are welded using an Ultrasonic Welding Machine. The strips are wound into a cell core, which is then dried in a vacuum oven. One tab is welded to the cell's bottom, and the other to its cap. The cell is filled with electrolytes, sealed, and activated by charging and discharging cycles. Finally, the metal case is wrapped with heat-shrinkable PVC to isolate the terminals, making the battery ready for use.

4. What are the three types of batteries? Which battery type is the best?

Batteries for consumer use primarily come in three types: lithium iron phosphate, lithium ternary, and lead-acid batteries.

Lithium iron phosphate (LFP) batteries use lithium iron phosphate as the cathode material and a graphitic carbon electrode for the anode. These batteries are favored for their low cost and toxicity, as well as their long life cycle of over 4000 cycles, making them ideal for vehicle use.

Lithium ternary batteries utilize lithium nickel cobalt manganese as the anode material. These batteries provide a high voltage of 3.7V, offering high specific energy and power. They also charge quickly and perform well at low temperatures.

Lead-acid batteries are the earliest rechargeable batteries, known for their high surge currents despite having low energy densities. They can be recycled, but the recycling process can lead to contamination. These batteries are heavy and have a shorter cycle life compared to lithium-ion batteries.

5. 18650 vs 21700 vs 20700 vs 4680? Which cylindrical lithium battery is the best?

The 18650 battery is a lithium-ion cylindrical battery measuring 18mm x 65mm. These batteries offer a voltage range of 3.2-3.7V and capacities of 2600mAh to 3500mAh. They have excellent longevity and are commonly used in flashlights, EVs, and electronics.

The 21700 battery is another lithium-ion cylindrical battery, named for its dimensions of 21mm diameter and 70mm length. These batteries are suitable for hybrid and electric vehicles. The 20700 batteries are larger than 18650 batteries, measuring 20mm x 70mm, and are ideal for high-drain devices and electronics. They can be stored for 3 to 5 years and recharged between 1500 to 4000 times.

The 4680 batteries offer significant advantages in capacity, production cost, and efficiency. With five times the capacity of a 2170 battery, the 4680 can improve mileage and reduce cost by decreasing the number of cells needed. Its design also enhances heat dissipation and reduces the consumption of non-energy materials like diaphragms and steel shells.

Among these types, the 18650 battery is the most classic and commonly used cylindrical battery. While it's challenging to declare one type as the best, the ideal cylindrical battery depends on specific usage scenarios.

6. What is a drawback of a cylindrical battery?

Compared to pouch batteries, cylindrical batteries are housed in a steel case, making them heavier and reducing specific energy. The electrodes inside can expand or contract, potentially leading to shape deformation and internal short circuits.

7. What are the advantages of the cylindrical battery?

Cylindrical batteries are one of the earliest industrialized lithium battery products. Their shape aids in handling high internal pressures without deformation. They offer better temperature control and are cost-effective compared to competitors.

8. Can solid-state batteries be cylindrical?

Solid-state batteries, which use solid electrolytes instead of gel electrolytes, can also be cylindrical. These batteries employ ceramics like oxides, sulfides, and phosphates as solid electrolytes. The solid electrolyte sets concave-convex inside the cylindrical barrel, with insulating fluid filling the spaces within.

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