[Complete Battery Guide](35) What Are the Differences Between Prismatic and Pouch Types? ... Form Factor Debate Over All-Solid-State Batteries

Editor's NoteWe are currently in the battery era. Batteries are found in almost every device, including mobile phones, laptops, and electric vehicles. [Complete Battery Mastery] is a section created to easily convey the basics of batteries, their ecosystem, company information, industry trends, and outlook to general readers, students, and investors interested in the battery industry and related companies. We will meet you every Saturday.

China's top battery company, CATL, announced on the 28th of last month at the China International Battery Fair (CIBF) held in Shanghai that it will begin small-scale production of solid-state batteries by 2027.

According to local media, Wu Kai, a senior executive at the company, stated, "Currently, about 1,000 research and development (R&D) personnel are working on developing solid-state batteries." The solid-state batteries developed by CATL are expected to achieve an energy density of 500 watt-hours (Wh)/kg. CATL identified 'price' as the main challenge to overcome for mass production of solid-state batteries.

Earlier this year, Nikkei Asia reported that Chinese battery companies such as CATL, CALB, EVE Energy, SVOLT, Gotion High-Tech, and Findreams Battery (a BYD subsidiary) formed the China Solid-State Battery Cooperation Innovation Platform (CASIP) to develop solid-state batteries and build supply chains by 2030.

With China joining the solid-state battery market previously led by South Korea and Japan, market competition is intensifying. Samsung SDI has announced plans to mass-produce solid-state batteries with an energy density of 900 Wh/L (450 Wh/kg) by 2027. Toyota is also expected to commercialize solid-state batteries between 2027 and 2028.

As solid-state batteries become more tangible, one of the points attracting attention is what form factor the solid-state batteries will take. Signs of renewed debate over form factors are emerging ahead of their commercialization.

So far, most prototypes of solid-state batteries have been in pouch form. However, Samsung SDI unveiled a prismatic solid-state battery mock-up at InterBattery 2024 held last March.

Pouch-type all-solid-state battery showcased by SK On at InterBattery 2024. It is a prototype produced in collaboration with Solid Power. Photo by Kang Hee-jong

At the 2024 NGBS seminar hosted by SNE Research in March, Samsung SDI Vice President Ko Ju-young said about the form factor of solid-state batteries, "While prototypes utilize pouch types, we are considering prismatic types based on customer demands," adding, "We are contemplating whether to start commercialization with prismatic types or begin with pouch and later switch to prismatic."

This means that although pouch types were initially showcased for ease of sample production, ultimately, Samsung SDI intends to produce solid-state batteries in prismatic form. Notably, customers are reportedly favoring prismatic types. Chinese companies, which have strengths in prismatic batteries, also seem likely to release solid-state batteries in prismatic form.

Various types of batteries showcased by Samsung SDI at InterBattery 2024. At the very top, a prismatic solid-state battery mockup is visible. Photo by Kang Hee-jong

On the other hand, LG Energy Solution, which does not have prismatic production lines and only manufactures cylindrical and pouch types, is likely to release solid-state batteries in pouch form. Kim Je-young, CTO of LG Energy Solution, devoted considerable time to highlighting the advantages of pouch batteries during his keynote speech at InterBattery 2024 in March.

He stated, "Solid-state batteries require uniform pressure to reduce interfacial resistance," and "pouch form is suitable for accommodating new chemistries." Since solid-state batteries use solid electrolytes, maintaining pressure is necessary to enhance ion conductivity, and in this regard, pouch batteries have an advantage over prismatic types.

Kim also said, "The two form factors, cylindrical and pouch, are complementary and can support both current and future chemistries," indicating that there are no plans to produce prismatic types.

In contrast, SK On, which currently produces only pouch batteries, is developing prismatic and cylindrical batteries as well, judging that pouch types alone cannot satisfy all customer demands.

Prismatic and Pouch: Stacking Without Rolling

Prismatic and pouch batteries originated from efforts to overcome the drawbacks of cylindrical batteries. Cylindrical batteries have simple processes and high production efficiency but inevitably create dead space when modularized, which lowers volumetric energy density. Making batteries flatter improves space efficiency, allowing more batteries to fit in the same space and thus increasing energy density.

Initially, prismatic and pouch batteries used a flat winding method similar to the jelly roll of cylindrical batteries, winding the cathode, anode, and separator in a flat shape before placing them in a case. While winding offers simple and fast manufacturing, it also results in some dead space inside the case, similar to cylindrical batteries.

To improve space utilization, the best method is to produce cathode, separator, and anode as individual sheets and stack them neatly, but this reduces productivity. The stacking method, which stacks cathode-anode-separator sheets one by one, was unsuitable for mass production.

However, recently developed stacking processes that maintain productivity while sequentially stacking electrodes and separators have been widely adopted for manufacturing prismatic and pouch cells. Samsung SDI, for example, switched from flat winding to stacking for prismatic batteries starting with its 5th generation (Gen5).

Stacking methods vary slightly by manufacturer. SK On (pouch) and Samsung SDI (prismatic) apply a method stacking electrodes and separators in a zigzag pattern centered on the separator. SK On calls this Z-folding, while Samsung SDI refers to it as Z-stacking.

LG Energy Solution has been manufacturing pouch cells using its proprietary Lamination & Stacking (L&S) method but has recently introduced the Z-stacking method as well, calling it Advanced Z-Stacking (AZS).

SK On first applied the Z-folding method domestically in 2019. Samsung SDI adopted the Z-stacking method starting with its 5th generation prismatic batteries in 2021.

The Z-folding or Z-stacking method involves cutting cathode and anode sheets individually, then stacking separators in a zigzag manner while alternately inserting cathode and anode sheets between them. To apply this technology, the previous notching process must cut cathode and anode sheets first, then weld cathode and anode tabs.

Stacking method using Z-folding technique. Image source=SK On

The Z-folding (Z-stacking) method completely separates cathode and anode by wrapping the separator in a zigzag pattern between them, reducing the possibility of direct contact at the edges and enhancing fire safety. It also improves case space efficiency compared to the winding method, increasing energy density. However, production speed is reportedly slower than winding.

LG Energy Solution's Lamination & Stacking method starts by creating bi-cells, which are battery semi-finished products combining multiple electrodes and separators. The laminated structure has the same electrode at both ends, such as cathode-separator-anode-separator-cathode or anode-separator-cathode-separator-anode.

Image source=LG Energy Solution

Next, a lamination process aligns the bi-cell with a half-cell composed of separator and anode. Finally, electrodes are stacked sequentially based on the separator to complete the battery material.

Lamination & Stacking Method of LG Energy Solution. Image source=LG Energy Solution

The Lamination & Stacking method offers high productivity and efficient internal battery space utilization. However, misalignment of separators and electrodes during lamination increases process difficulty and requires careful management. LG Energy Solution also applies the AZS method, which combines lamination with Z-stacking. The AZS method was first applied at a joint venture LG Energy Solution established with Hyundai Motor in Indonesia in 2022.

Can or Pouch?

Prismatic and pouch batteries share similar electrode manufacturing processes with cylindrical batteries up to the electrode stage but differ in assembly.

Basically, battery materials made by winding or stacking become prismatic batteries when placed in a square metal can, and pouch batteries when placed in a thin film pouch. The choice depends on customer demands and company strategies.

Prismatic batteries are made by welding tabs to battery materials, connecting them to a cap (lid), and placing them in a metal can made of aluminum or steel. Electrolyte is injected through a small hole in the cap and then sealed. The formation process of prismatic batteries is similar to cylindrical batteries.

Structure of a prismatic battery. Image source=Grepow

Like cylindrical batteries, prismatic batteries can be equipped with safety devices such as vents to release gas and prevent heat transfer. Their rigid metal casing provides excellent durability and reliability. However, they are heavier and difficult to shape into various forms.

Samsung SDI and SK On have also introduced side-terminal prismatic batteries, which place cathode and anode tabs on the sides rather than the top. Side-terminal prismatic batteries allow cooling from both sides (top and bottom) and enable stacking cells in two layers within a pack.

SK On's bidirectional prismatic battery showcased at InterBattery 2024. Photo by Kang Hee-jong

Pouch battery cases consist of thin films made of aluminum foil, nylon, polypropylene, and other materials. The pouch case is divided into an electrode pocket to hold battery materials and an air pocket to inject electrolyte and store gas. After placing battery materials in the electrode pocket, electrolyte is injected and sealed. During formation, charging and discharging generate gas that collects in the air pocket. The process of removing this air pocket is called degassing.

Degassing process of pouch battery. Image source=LG Energy Solution

Pouch batteries are lightweight, allowing higher gravimetric energy density and can be made in various shapes. However, their cases are not rigid, requiring additional technology to compensate during module or pack assembly.

Curved pouch battery from LG Energy Solution exhibited at InterBattery 2024 and products applying it. Photo by Kang Hee-jong

Each of cylindrical, prismatic, and pouch batteries has its pros and cons, making it difficult to say one form is superior overall. Cylindrical batteries have simple manufacturing and good heat transfer prevention but lower space efficiency. Prismatic batteries offer good space efficiency and stacking advantages but are heavy and hard to shape diversely. Pouch batteries are light and versatile in shape but less resistant to impact and heat transfer.

Battery manufacturers are developing complementary technologies to maximize strengths and overcome weaknesses of each type. They also do not insist on a single type but release two to three types simultaneously to flexibly meet customer demands.

For prismatic batteries, technologies such as Cell To Pack (CTP), which omits the module process, or Cell To Chassis (CTC), which attaches cells directly to the vehicle body, are being introduced. Electric vehicle batteries increase volume and weight and reduce energy density due to added devices in module and pack stages. Simplifying these processes allows more batteries to be installed, increasing energy density and driving range.

CATL's CTP3.0 System Introduced in 2022

CTP technology was first applied by Chinese companies like CATL to prismatic batteries to overcome the low energy density of lithium iron phosphate (LFP) batteries. BYD's 'Blade' battery is also designed with CTP technology. Recently, domestic companies have begun adopting CTP technology as well. LG Energy Solution has introduced CTP technology for pouch batteries.

CTC technology omits modules and packs, attaching cells directly to the vehicle chassis. This greatly reduces parts and vehicle weight, extending driving range. However, it requires close collaboration with automakers, posing challenges.