Cylindrical cell？Prismatic cell？Pouch cell？Which is suitable for Solid-State Lithium-metal power battery?

Lithium-ion batteries are in everything from electric cars to mobile phones. However, different applications have different requirements for the characteristics of the battery form, and electric vehicles are a particularly challenging application case. Electric vehicle batteries are packaged in several ways -Cylindrical, Prismatic and Pouch. Each packaging method has its unique advantages, and there is no obvious winner for these three packaging methods.

Electric vehicle battery packs typically consist of hundreds or thousands of individual cells that are assembled into subunits called modules, which are then combined into the larger unit of the battery pack. There is a battery management system around the battery pack to monitor and regulate the status of the battery pack; A cooling system is also usually included to keep the battery pack operating safely at the proper temperature and extend its service life. Each different design has its advantages, but there are also specific disadvantages, especially for batteries using the new generation of solid-state lithium metal technology. The key difference between lithium metal batteries and traditional lithium-ion batteries is that lithium metal batteries expand during the process of lithium deposition. As a result, when fully charged, the entire battery thickens. This expansion is particularly evident in QuantumScape's negative-free design, which presents a challenge in choosing the right battery format because it expands more than traditional lithium-ion batteries. In this article, we will review the advantages and disadvantages of today's three major EV battery designs and examine the applicability of each design to solid-state lithium metal technology. The following chart compares the advantages and disadvantages of different battery forms.

Cylindrical cell

The cylindrical battery cell is probably the most familiar form of battery, found in everything from laptops to some of the most popular electric vehicles on the market. They are often expressed in terms of physical size: for example, the 2170 batteries currently used in many best-selling electric vehicles are 21 mm wide and 70 mm long. The battery consists of a negative layer, a polymer diaphragm and a positive layer, and is manufactured by rolling up a long sheet and inserting it into a metal shell, which is then injected with a liquid electrolyte. SONY first popularized this design for handheld camcorders in the 1990s. In electric vehicle battery packs, the cylindrical shape means that there will always be a gap between individual cells, which reduces the energy density of the battery pack and the range of the car. In modern electric vehicles, part of the gap can be used for cooling, but there is still wasted space.

Despite these drawbacks, cylindrical battery cell are still popular because of some unique advantages. For example, lithium-ion batteries produce gas when they age, resulting in increased pressure inside the battery. Cylindrical batteries are more robust than square batteries and can use thinner metal walls to withstand internal pressure, so the energy density of cylindrical batteries is slightly higher than that of batteries of other shapes. Is it possible to make cylindrical batteries out of lithium metal negatives? QS says no. When lithium is polarized on the negative electrode, the negative electrode layer will expand significantly. If the negative layer is rolled up, this expansion causes the negative layer to attempt to unfold, which causes stress and damage to the internal structure of the battery.

Prismatic cell

As the name suggests, a square-case battery is a rectangular metal box. The layers of the battery are either stacked together like playing cards, or wound together and then flattened into the battery housing. Prismatic batteries are particularly useful in electric vehicles because they can be packed close together without wasting space, and thus have excellent group efficiency.

For Prismatic cell, cooling is usually done through the bottom of the battery pack, with the metal housing acting as a radiator, absorbing heat and conducting it downward to the cooling element. However, the tight packaging of the battery makes keeping the battery cool during rapid charging itself more challenging, especially when the battery is larger - for example, a square-shell battery has a storage capacity of more than 50 Ah, compared to 4.8 Ah for a standard cylindrical battery. Prismatic cell also face other challenges. For example, because square batteries are not as rigid as cylindrical batteries, the battery walls must be thicker, which increases the weight and cost of the battery. The top of a square-shell battery, called the top cap, is also relatively complex to assemble, adding to manufacturing costs and time. When it comes to lithium metal cells, square is appropriate, but there are some caveats. The layers of the lithium metal battery can be laid out in a square shape. However, when the negative layer expands, there must be something else inside the battery (such as a spring or foam) to accommodate this expansion. Because the device is located between the lithium battery layer and the metal housing, it partially blocks the internal heat transfer, thereby limiting fast charging performance and taking up additional space, reducing energy density.

Pouch cell

The last type of battery is probably the simplest. Pouch cells is a series of negative, diaphragm and positive layers stacked together and encased in a flexible laminated material made of plastic and aluminum. Pouch cells have several advantages. First, since there is no hard shell around the battery, it can provide good battery-level energy density and keep the cost low. The soft pack material can be stretched to accommodate the expansion that occurs in conventional lithium-ion batteries during use. In addition, they are made of cheap materials and do not contain many complex components like prismatic batteries, which means they can be manufactured very quickly. However, they also have drawbacks. The soft bag itself is not strong and the material is easily damaged, which poses a safety risk to traditional lithium-ion batteries. In the case of EV battery packs, this means that they must be protected by a more robust (and heavier) modular construction, which negates the energy density advantage of the battery. The module must also act to remove heat from the battery and inhibit the expansion of the battery over time. Soft bags simplify battery design by transferring some of the mechanical requirements from the battery layer to the module layer, but at the cost of a heavier and more complex module design. In contrast, square or cylindrical cells are more self-sustaining and, in some cases, can even enable battery-to-battery pack integration, thus eliminating modules entirely. For lithium metal systems, Pouch cells have similar disadvantages to Prismatic cell . Any mechanism designed at the module level will expand and contract as the soft-pack battery is charged and discharged, which will become a less than ideal heat sink, adding cost and complexity, adding weight and taking up valuable space. Thermal management is particularly important for batteries that repeatedly provide a 15-minute fast charge, as the heat generated can be far greater than current batteries generate at lower charge rates.