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Cylindrical Battery Vs Prismatic battery Cell, Which Is Better For Electric Vehicles?

If you drive a Gasoline or Diesel fueled car, you would be familiar with the box-type battery sitting snugly in a corner, under the hood. Heavy and bulky Lead Acid batteries are not suited to electric cars, which accommodate thousands of lightweight batteries. To achieve compact stacking, thin slab- shaped batteries are a better choice. These come in two varieties, namely Pouch and Prismatic types, the difference being their flexible and hard casings respectively. By the year 2010, when the first Lithium ion battery powered cars were launched, the Pouch and Prismatic designs were well established in consumer electronics devices and their adoption in electric cars was a pragmatic choice. For a couple of years, these two varieties dominated the electric vehicle market.

The twist in the tale came in 2012, when Tesla introduced their Model S sedan using relatively cheap, Cylindrical Lithium ion batteries. Figure 1 shows that these three formats have been constantly competing for market share.

It can be seen from Figure 1 that between 2013 to 2019, Cylindrical batteries displaced the Prismatic type and achieved market dominance. This was primarily due to Tesla. But is the tide turning again? It seems that since 2021, the Prismatic format is once again pushing its way to the forefront.

Market Share Evolution Of EV Battery Formats

If you drive a Gasoline or Diesel fueled car, you would be familiar with the box-type battery sitting snugly in a corner, under the hood. Heavy and bulky Lead Acid batteries are not suited to electric cars, which accommodate thousands of lightweight batteries. To achieve compact stacking, thin slab- shaped batteries are a better choice. These come in two varieties, namely Pouch and Prismatic types, the difference being their flexible and hard casings respectively. By the year 2010, when the first Lithium ion battery powered cars were launched, the Pouch and Prismatic designs were well established in consumer electronics devices and their adoption in electric cars was a pragmatic choice. For a couple of years, these two varieties dominated the electric vehicle market.

The twist in the tale came in 2012, when Tesla introduced their Model S sedan using relatively cheap, Cylindrical Lithium ion batteries. Figure 1 shows that these three formats have been constantly competing for market share.

It can be seen from Figure 1 that between 2013 to 2019, Cylindrical batteries displaced the Prismatic type and achieved market dominance. This was primarily due to Tesla. But is the tide turning again? It seems that since 2021, the Prismatic format is once again pushing its way to the forefront.

Cylindrical Battery Vs Prismatic battery Cell

Choosing the appropriate type of battery is probably the biggest decision that any electric vehicle manufacturer will make. Apart from performance and cost, battery supply chains have to be secured. So, what are the key features of the Cylindrical and Prismatic batteries? How would one choose between them? Let’s dive into the details!

What’s The Cylindrical Battery Cell?

Cylindrical Li-ion battery cells are made of an electrolyte gel sandwiched between Anode and Cathode electrode sheets. A porous separator sheet is provided between the Anode and Cathode sheets, preventing short circuiting. Similar to how a Jelly Roll is made, the electrode, electrolyte gel and separator “sandwich” is rolled into a cylinder and placed inside a metallic housing which is then capped. The housing may be Nickel-plated Steel or Aluminium. Generally, the Cathodes are made from Lithium Oxide mixed with Nickel, Manganese and Cobalt coated onto Aluminium foil, whereas the Anode is made of Graphite coated onto Copper foil. The metallic foils act as current collectors, in addition to providing a base for the electrode coating. Current flows to and from the cell via metallic strips called Tabs welded to the electrodes.

Structure of Cylindrical EV Battery

What Are The Common Models Of Cylindrical Battery cells?

Tesla pioneered use of Cylindrical cells in their electric vehicles and are the world’s biggest buyers of these batteries. They also have their own manufacturing facilities. Apart from Tesla, on a much smaller scale, newcomers like Lucid and Rivian also use cylindrical batteries for some models. Major German Producer BMW will adopt Tesla’s big battery format for its new class of batteries. The most common cylindrical batteries are the 18650, 21700 and 4680 models, where the numbers represent the form factor of each battery.

Structure of Prismatic EV Battery

Pros And Cons Of The Cylindrical Cell

Pros

  • Cylindrical cells have better structural integrity compared to the other formats since the tubular shape can withstand higher mechanical stresses and there are fewer stress concentration points (sharp edges).
  • They have Higher gravimetric energy density compared to Prismatic cells.
  • The  cost of production is 15 to 10% lower than for Prismatic Cells.
  • Due to absence of multiple stacked layers, Cylindrical cells have lower electrical resistance compared to Prismatic cells.

Cons

  • Cylindrical cells can do only about 2000 to 3000 charge and discharge cycles before 20% degradation . This is about half of what Prismatic cells can achieve.
  • The packing density in a battery pack is lower than for Prism cells which make it disadvantageous for small cars.

What’s The Prismatic Battery Cell?

The name Prismatic cell is derived from its geometric shape, which is a rectangular Prism. In contrast to the spiral wound construction of Cylindrical cells, the Prismatic cell resembles a stacked sandwich. Each cell is made up of several layers, with each layer comprising electrolyte gel placed between Anode and Cathode sheets and intervening separators. The stacked layers are then enclosed within an Aluminium or Steel shell and metal tabs provided at the top for electrical connection to the external circuit.

Cylindrical Battery Form factors used by Tesla

What Are The common Models of Prismatic Battery cells?

Prismatic cells are used primarily by German car manufacturers and the batteries are made in accordance with standards specified by the German Automotive Association.  At present, Prismatic batteries for battery electric and hybrids are available in the following formats:

Cell Format Width [mm] Height [mm] Thickness [mm]
HEV 120 85 12.5
PHEV1 173 85 21
PHEV2 148 91 26.5
PHEV2+ 148 125 26.5
EV1 173 115 32
EV2 173 115 45

The abbreviations HEV, PHEV and EV stand for Hybrid electric vehicle, Plug-in hybrid electric vehicle and battery electric vehicle respectively.

Pros And Cons Of The Prismatic Cell

Pros

  • Prismatic cells can be stacked closely together without air gaps. This enables more cells to be packed into a smaller volume, which is useful for smaller cars.
  • Prismatic cells can last for a greater number of charge and discharge cycles (upto 5000), which is almost double that of cylindrical
  • Manufacturing is comparatively more standardized, due to compliance with VDA standards.

 

Cons

  • Lower gravimetric energy density compared to cylindrical cells.
  • Thermal management of battery stacks is more difficult due to the absence of air gap, which reduces radiative and convective air cooling.
  • The Prismatic format is more vulnerable to mechanical stresses, compared to the Cylindrical format.
  • Production costs are higher than for Cylindrical cells.

 

Electric Vehicle Battery Requirements

While batteries are the heart of any electric vehicle, they are part of a complex integrated battery system.

Components of An EV Battrey System

Multiple cells are stacked and connected together in series and parallel, to form a battery module. Many such battery modules are connected electrically make a battery pack. Accordingly, the batteries must be lightweight and mechanically strong, while maximizing energy content and available power. Heat transfer must be good, to ensure efficient cooling. From a customer perspective, long mileage, quick charge time, long lifespan and low cost are key considerations

Thus, a good electric vehicle battery must satisfy customer requirements, as well as engineering requirements. So, what are the key parameters and benchmarks to be met for an electric vehicle battery? The following table lists key electric vehicle battery requirements.

Battery Parameter Current Benchmarks Significance
Energy Densities >750 Wh/L & >350 Wh/kg for cells Determines the driving range
Cost <$100/kWh for cells Battery costs are a high proportion of EV price
Fast Charge and Power 80% ΔSOC in 15 min Customers want fast charge times.
Life 15 years The battery life must be at least vehicle life as replacement of individual batteries is not practicable.
Performance Minimum impact by environment Battery functionality must be good in cold weather, hot weather, rainy weather, desert conditions.
Safety No fire/flame/rupture/explosion Thermal runaway is a major concern with all Lithium ion batteries.

Which Is Better For Electric Vehicles?

The market for electric vehicle batteries is growing for both battery formats. However, the smaller cars prefer Prismatic cells as they require less space compared to Cylindrical cells.  On the other hand, when vehicles and battery packs become larger and powerful, the preference is for Cylindrical cells. The cost of production is also substantially lower for Cylindrical cells. That is why Tesla’s decision to adopt the 4680 format is being copied by other big players such as BMW.

Why Tesla Used Cylindrical Batteries From The Beginning

At the beginning, Tesla chose the cylindrical 18650 for their sedans, as it was the cheapest, technologically mature design available. They had already gained some experience with their earlier Roadster sports car.  Apart from lower battery costs, it is clear that Cylindrical batteries deliver more range compared to the Prismatic type. Now with the larger 4680, Tabless format, Tesla are changing the game for long range higher power electrical vehicles. Another important reason is the existence of top quality manufacturers of Cylindrical cells like Panasonic, helping Tesla to establish a stable supply chain at the beginning.

From a thermal management perspective, the air gap between adjacent cells helps in radiative cooling unlike closely packed Prismatic cells. The radial transfer of heat is more uniform than in the case of asymmetric Prismatic cells.

Conclusion

Both Cylindrical and Prismatic formats are important for the electric vehicle market. The Prismatic format is better for smaller vehicles. For larger vehicles, requiring more range and power, the Cylindrical format is better.

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