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Working principle of lithium-ion batteries

Before introducing the shape of the battery, let's first briefly understand the working principle of lithium-ion batteries. Lithium ion batteries are a type of secondary battery (rechargeable battery) that relies heavily on the movement of lithium ions between the positive and negative electrodes for operation. During the charging and discharging process, lithium ions are intercalated and de intercalated back and forth between the two electrodes. During charging, lithium ions are removed from the positive electrode and embedded into the negative electrode through an electrolyte, leaving the negative electrode in a lithium rich state. During discharge, the opposite is true. Let's take a look at the advantages and disadvantages of each of these three different battery shapes. Cylindrical battery

The first thing we want to talk about is the most popular cylindrical battery currently. As early as 1992, Japanese company SONY developed 18650 batteries. The current cylindrical batteries can be divided into models such as 14650, 18650, 21700, etc., which are named after the battery size. I believe everyone has heard of 18650 and 21700 batteries. 18650 is currently the most common battery model on the market and the most mature technology, while 21700 is a new type of battery that Tesla has been promoting in recent years. The size of a single cylindrical battery has become larger and the battery capacity has also increased. In this way, the number of battery cells can be significantly reduced, making the vehicle more proficient in controlling the battery system.

Let's talk about the advantages of cylindrical batteries first. Firstly, after years of development, the standardization level of cylindrical batteries is relatively high, making it easy to achieve unified standards within the industry. In addition, cylindrical batteries also have inherent advantages in heat dissipation. When packaging, there is a good heat dissipation space between the cylinders, so some vehicles equipped with cylindrical batteries use low-cost air cooling technology.

Of course, cylindrical batteries also have disadvantages. For vehicles using cylindrical batteries, it is inevitable to face battery control issues caused by a large number of individual batteries. After all, even with 21700 batteries, thousands of cylindrical batteries need to be placed together to form a battery pack, which is very large. In addition, due to the use of steel shells when combining cylindrical batteries into battery packs, their weight is relatively high and their energy density is insufficient compared to the other two shapes of batteries.

In terms of battery shape, most cylindrical battery packs can only be arranged in a rectangular layout and installed below the vehicle, which is relatively simple because it can ensure that the battery pack does not affect the front and rear weight of the entire vehicle, but it will have an impact on the ground clearance of the vehicle and the height of the platform inside the carriage. Some hybrid models that choose to place the battery pack under the trunk, although achieving a balance in weight distribution, also sacrifice the trunk space.

prismatic cell

Next, let's talk about square batteries. Square batteries have stronger plasticity and can be customized according to the specific needs of the products they carry. This also leads to varying sizes, and currently there is no clear standard division as cylindrical batteries, regardless of manufacturing process or application standards. But precisely because of its high flexibility, it has been used for a long time in early new energy vehicles, and car companies can customize the size of square batteries according to the needs of the vehicle model. Square batteries were once considered the most suitable battery design for new energy vehicle applications. In fact, there are many square battery packs on the market, although they are nominally square batteries, the internal essence is still the soft pack batteries that will be discussed next.

Soft pack battery

Due to the use of stacked manufacturing methods, the soft pack battery has a thinner volume and the highest energy density, which is also in line with the development trend of China's new energy vehicle market. In addition, the soft pack battery can also be customized according to different needs. Because the controllability of its volume is also valued by car brands, especially for hybrid models, when considering both the overall layout and weight of the vehicle, the advantage of soft pack batteries is even more obvious. The soft pack battery pack also adopts an aluminum plastic film packaging that is lighter in weight than the aluminum shell, further improving the energy density of the entire battery pack.

In terms of battery pack design, soft pack batteries also have significant advantages. Similarly, customizable soft pack batteries have more imaginative space and variations in shape, not limited to rectangles. It can form a small battery pack and be placed under the trunk, or it can be made into a T-shaped layout.

Although soft pack batteries have many advantages, they also have obvious drawbacks, namely poor consistency, which is the overall performance of the battery pack. Because the heat rate of a battery varies at different temperatures, this is because the chemical reactions inside the battery are closely related to temperature. If the battery operates in an internal environment with insufficient heat transfer such as insulation or high temperature, the temperature of the battery will significantly increase, leading to the formation of hot spots inside the battery pack, which may eventually lead to loss of control. This also means that the soft pack battery requires a more complex battery control system.

The appearance of the soft pack battery is the same as the other two types of batteries, which is a hard shell large package and does not match the so-called soft pack. In fact, the interior of this hard shell large package is a stacked soft pack battery, with each pack of soft pack batteries stacked vertically together. Of course, what engineers do is not simply putting them together. A layer of cooling film needs to be added between every two batteries, and the cooling liquid is injected through the water inlet to fill the pipeline on the cooling film, circulating and taking away heat. In addition, the cooling liquid can also be heated by the coil to raise the temperature of the battery, ensuring that it is at the most suitable operating temperature even in extreme cold environments.

Another issue that soft pack batteries face is the use of aluminum plastic film. At present, the cost of aluminum plastic film technology is high and the level of domestic technology is limited. Most aluminum plastic films rely on imports. In addition, there are also significant difficulties in controlling the lamination technology process, with higher levels of difficulty posing a significant challenge to quality control. Once quality control issues arise, they can directly affect battery life and usage safety.

Summary: In the current new energy vehicle market, cylindrical, square, and soft pack batteries are all used by manufacturers, each occupying a certain position. Moreover, there is no absolute distinction between good and bad at this stage, and it can only be said that each has its own advantages and disadvantages. Cylindrical batteries have significant advantages in consistency due to their long development history, and their manufacturing process is also more mature with lower costs. However, there is still room for further improvement in battery layout and energy density for cylindrical batteries. As for square batteries and soft pack batteries, although they are difficult to compete with cylindrical batteries in terms of standardization, their customizable characteristics enable them to better coordinate with vehicles and meet the special needs of different manufacturers and models. It can be said that each of the three battery shapes has its own advantages.

From the current situation, it seems that the soft pack battery is more in line with policy requirements. With the higher requirements for battery energy density set by domestic subsidies for new energy vehicles, soft pack technology has become an important means of achieving lightweight and high-density batteries. At the beginning of this year, Ningde Times, Lishen, and Guoxuan High Tech, three companies undertaking new lithium-ion power lithium battery projects, all chose the soft pack battery technology route. Of course, this does not mean that it will become the world of soft pack batteries in the future. It can only be said that under current conditions, using soft pack batteries is the most convenient way to improve energy density. However, which type of battery is the ultimate solution in the future depends on the next technological development.

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