When it comes to electric vehicles (EVs), one of the most important components is the battery. The battery is the heart of an electric vehicle, powering it for long drives and fast acceleration. But not all batteries are created equal—there are different types of lithium-ion (Li-ion) cells used in automotive applications, each offering its own set of advantages and disadvantages. In this blog post, we’ll take a deep dive into the most used types of Li-ion cells in the automotive industry and explore their benefits and limitations. 

1. Cylindrical Cells (e.g., 18650, 21700)

Cylindrical cells are among the most widely used types of Li-ion cells in electric vehicles, especially in early models of popular EVs like Tesla. They have a classic round shape and come in various sizes, with 18650 and 21700 being the most common. 

What are they? 

• 18650 cells measure 18mm in diameter and 65mm in height, while 21700 cells are slightly larger, measuring 21mm in diameter and 70mm in height. 

Where are they used? 

• Tesla, for instance, uses 18650 cells in earlier models like the Model S and Model X, while newer models like the Model 3 and Model Y feature 21700 cells.

Pros: 

• Cylindrical cells are known for their high energy density and reliable performance. 

• They are easy to handle and modular, meaning they can be arranged in large battery packs to meet different energy needs. 

• This form factor benefits from an efficient cooling system due to its round shape, which helps manage heat dissipation more effectively. 

Cons: 

• Despite their advantages, cylindrical cells have a lower energy density compared to prismatic or pouch cells. This means more cells are needed to provide the same energy capacity, which could lead to larger and heavier battery packs. 

2. Prismatic Cells

Prismatic cells are rectangular in shape and are commonly used in electric vehicles due to their efficient use of space. They are a popular choice for automakers looking for a balance between energy density, safety, and cost. 

What are they? 

• Prismatic cells are flat, rectangular batteries that can be stacked or arranged in tight configurations. They are typically used in mass-market EVs like the Nissan Leaf or BMW i3. 

Where are they used? 

• Automakers like Nissan and BMW have embraced prismatic cells for their ability to maximize space within the vehicle and optimize performance. 

Pros: 

• These cells make the best use of space, allowing them to be stacked closely together in the battery pack, resulting in a more compact battery. 

• Prismatic cells have a high energy density per unit volume, which means they can store more energy within a given space. 

• The technology behind prismatic cells is also more established, and they are considered relatively cost-effective compared to other formats. 

Cons: 

• One downside of prismatic cells is that they require a more advanced cooling system to manage heat, especially since their flat shape can sometimes trap heat. 

• While they are better than cylindrical cells in terms of space efficiency, they still don’t offer the same level of energy density as pouch cells. 

Pouch Cells 

• Pouch cells are flat, flexible batteries that resemble a pouch or bag. They have gained popularity in newer electric vehicles due to their high energy density and flexible form factor. 

What are they? 

• Pouch cells are flat and flexible, made with thin aluminum foil that wraps around the cell’s active material. This design helps maximize the battery’s capacity without needing as much structural material. 

Where are they used? 

• Automakers such as Chevrolet (with the Bolt) and various models from Hyundai and Kia use pouch cells in their EVs. 

Pros: 

• Pouch cells are incredibly space-efficient, making them ideal for vehicles with limited battery compartment space. 

• They offer the highest energy density per unit volume, meaning vehicles with pouch cells can achieve longer driving ranges without adding bulk or weight. 

• The flexibility of the form factor allows for unique battery pack designs that can be customized to fit specific vehicle shapes.

Cons: 

• Due to their lightweight and flexible design, pouch cells can be more prone to swelling from gas buildup inside the pouch, which could compromise their safety or lifespan. 

• They also require careful attention to thermal management to prevent overheating or degradation over time.   

4. Flat or Square Cells 

Flat or square cells are another option for electric vehicle manufacturers. While not as common as cylindrical or pouch cells, flat cells are used in certain applications where space is a premium and energy density is crucial. 

What are they? 

• Flat cells are typically rectangular, providing a compact, space-efficient way to pack a large amount of energy into a vehicle’s battery system. 

Where are they used? 

• Some hybrid vehicles, such as those produced by Toyota, incorporate flat cells into their battery packs for improved energy density.

Pros: 

• They offer high energy density, allowing for longer range in a more compact space. 

• The design of flat cells makes them more thermally stable compared to pouch cells. 

Cons: 

• They aren’t as flexible as pouch cells, which means they are less customizable for unique battery pack designs. 

• While their energy density is high, they are typically more expensive to manufacture compared to cylindrical cells. 

Which Type of Li-Ion Cell is Best for Automotive Use? 

Choosing the right type of Li-ion cell depends on several factors, including the vehicle’s performance requirements, cost, space constraints, and safety considerations. Here’s a quick comparison: 

• Cylindrical Cells: Best for applications requiring modularity, reliability, and relatively lower costs. Tesla, for instance, uses cylindrical cells in their vehicles. 

• Prismatic Cells: Ideal for space efficiency, especially in mass-market EVs where fitting a battery pack into a limited space is critical. 

• Pouch Cells: Perfect for high-performance EVs where energy density and pack flexibility are the priority. 

• Flat Cells: A good option when space is at a premium, offering high energy density but less flexibility than pouch cells. 

In most cases, automakers combine these different cell types within the same vehicle to balance performance, cost, and battery longevity. As battery technology continues to evolve, newer innovations like solid-state batteries may eventually lead to even more efficient, safer, and energy-dense cells for automotive use. 

Conclusion 

The choice of Li-ion cell type plays a crucial role in determining the performance, cost, and safety of electric vehicles. Whether it’s cylindrical, prismatic, pouch, or flat cells, each form factor has its advantages and disadvantages. By understanding how these cells work and their applications, we can better appreciate the technology driving the EV revolution and look forward to even more efficient batteries in the future!