Design and Analysis of Prismatic Cell Structural Components

一. Overview of Prismatic Cell Structural Components
Prismatic cell structural components play a crucial role in lithium batteries. They primarily serve functions such as energy transmission, electrolyte containment, safety protection, battery support and fixation, and exterior decoration. These components directly impact the safety, sealing performance, and energy utilization efficiency of lithium batteries.

According to relevant data, the market size of lithium battery structural components in China reached 33.8 billion yuan in 2022, representing a year-on-year growth of 93.2%. Among them, prismatic battery structural components have long occupied the majority of the structural component market, with a market share as high as 90.7%, while cylindrical battery structural components account for only 9.3%. This dominance is mainly due to the rapid development of China's new energy vehicle market, driven by strong government policy support. The production capacity of battery manufacturers and the number of cells per order have increased significantly, and prismatic batteries are better suited to meet the demands of large-scale production.

Prismatic cell structural components are usually composed of a shell and a cover plate. The shell manufacturing process is relatively simple, primarily using continuous deep drawing processes, and is generally made of steel or aluminum. It offers high structural strength and strong resistance to mechanical loads. In contrast, the manufacturing process of the cover plate is usually far more complex than that of the shell. Its main functions include fixing/sealing, current conduction, pressure relief, fuse protection, and reducing electrical corrosion. For example, the top cover is laser-welded to the aluminum shell to encapsulate and secure the bare cell while ensuring a sealed structure. The top cover's terminals, busbars, and cell tabs are welded to ensure proper charge and discharge current conduction. When the battery encounters an abnormal situation and internal pressure increases, the top cover's safety valve opens to release pressure, reducing the risk of explosion.

Prismatic cell structural components play an indispensable role in lithium batteries, and their market prospects are becoming increasingly broad with the development of the new energy vehicle and energy storage markets.

二. Types and Functions of Structural Components

(a) Shell
As a crucial component of prismatic cell structural components, the shell plays key roles in fixation, protection, sealing, and heat dissipation. It serves as a barrier between the active materials inside the cell and the external environment throughout its entire lifecycle, providing structural stability to the internal electrochemical system and ensuring the cell maintains a stable structure under various conditions.

In terms of protection, the shell can withstand certain mechanical loads, preventing external impacts from damaging the cell. Its sealing function ensures that the electrolyte does not leak, maintaining the normal operating state of the battery. Additionally, the shell aids in heat dissipation by releasing heat generated during battery operation, thereby enhancing the battery's safety and extending its lifespan.

The production process of the shell mainly includes raw material slitting, precision continuous deep drawing, cutting, cleaning, drying, and inspection. Among these, precision continuous deep drawing technology is the most challenging aspect of shell production. During this process, it is essential to ensure uniform wall thickness and prevent fractures.

Compared to conventional single-step stamping, precision continuous deep drawing is more difficult. Its core barriers lie in the molds and drawing equipment. High-quality molds and advanced drawing equipment are critical to ensuring dimensional accuracy and performance stability of the shell.

(b) Cover Plate
The cover plate plays a crucial role in prismatic cell structural components, providing functions such as connection, isolation, sealing, and explosion protection.

The steel cap is located at the top of the cover plate and has a high strength, making it resistant to deformation under external forces. It serves to protect the explosion-proof aluminum sheet and is also a component for connecting batteries in the pack. The sealing ring is located on the outermost edge of the cover plate, isolating the internal metal parts of the combined cap from the battery steel shell. It provides insulation to prevent internal short circuits and also ensures sealing after the battery is sealed.

The explosion-proof component is primarily used for power cut-off and pressure relief during battery overload to prevent an explosion caused by excessive internal pressure. It consists of an isolation ring, explosion-proof aluminum sheet, and connecting aluminum sheet. The explosion-proof aluminum sheet is located in the middle of the cover plate and is the core component that determines the circuit cutoff and the release of critical pressure. When the internal pressure of the battery reaches a certain value, it automatically bursts to release pressure, ensuring the safety of the battery. The connecting aluminum sheet is located at the bottom of the cover plate and is connected to the explosion-proof aluminum sheet by laser welding. In the event of a dangerous situation, it disconnects from the explosion-proof aluminum sheet. The isolation ring is located at the connection between the connecting aluminum sheet and the explosion-proof aluminum sheet, providing isolation and insulation.

The production process of the cover plate is more complex than that of the shell and mainly includes stamping & injection molding, component inspection, gluing, asphalt immersion, edge wrapping and shaping, spot welding, component assembly, spot welding, final assembly, and inspection before storage. The testing stages include explosion-proof pressure testing, helium leak testing, internal resistance testing, and resistance testing. The more challenging stages in the production process are the stamping and welding parts, including steel cap stamping, explosion-proof aluminum sheet stamping, connecting aluminum sheet stamping, sealing ring stamping, isolation ring stamping, friction welding during terminal installation, and laser welding during assembly.

(c) Battery Module Connection Plate

The battery module connection plate plays an important role in connecting the components of the power battery module. It is mostly made using multi-layer composite materials, with one layer acting as the connecting layer between the connector and the terminal to ensure good welding performance. The multi-layer material stacking ensures the electrical conductivity of the connection plate. After processing the base plate with multiple layers of foil, it forms a flexible area to compensate for displacement caused by the expansion of the power battery cell, reducing the impact on low-strength interfaces. The connectors for power battery modules are generally in rectangular, trapezoidal, triangular, or stepped shapes. The connection surface is coated with 0.1mm thick nickel-plated copper foil, which is prone to oxidation and discoloration at high temperatures during welding, requiring polishing and cleaning without damaging the surface coating.

三. Design Case Analysis

(a) Design of New Explosion-Proof Valve

In a new type of prismatic cell structure, the explosion-proof valve is positioned on the opposite side of the positive and negative electrodes, facing the ground. This design offers several advantages. Firstly, with this layout, the upper space of the cell does not need to reserve space for the explosion-proof valve, greatly saving internal space in the cell shell. According to relevant research data, this design can increase the volumetric energy density by approximately [X]%. Secondly, in practical applications, if the product experiences thermal runaway due to excessive temperature, the explosion-proof valve will rupture without posing a hazard to the cockpit and cabin occupants, effectively eliminating personal safety risks.

For example, in practical applications in new energy vehicles, this new prismatic cell structure provides higher safety assurance for passengers.

(b) Integrated Design
In some cases of prismatic cell structure manufacturing, the liquid cooling plate, busbar, and sampling harness are designed in an integrated manner. This design has significant advantages. On one hand, the liquid cooling plate quickly reduces the cell temperature, ensuring the cell operates within an optimal temperature range, thereby improving cell performance and lifespan. For example, in practical tests, prismatic cells with integrated liquid cooling plates were able to lower their temperature by [X]°C under continuous high-load operation compared to traditional designs. On the other hand, the integrated design reduces the number of components, simplifies the assembly process, and improves production efficiency. At the same time, the integrated design helps reduce overall costs and enhances the product's market competitiveness.

(c) Full Tab Assembly Structure
The design of the spring clip in the full tab prismatic cell structure is unique. The spring clip consists of a first flat plate and a second flat plate, forming a V-shaped structure made of elastic metal. This design has significant advantages in connecting the tabs and the cover plate. Firstly, the elastic V-shaped spring clip uses its own rebound force to press against both the cover plate and the tab surfaces, achieving an electrical connection. The elastic force also improves the contact conductivity between the interfaces. As long as the elastic force exists, the conductivity will remain, eliminating the need for welded connections and reducing assembly difficulty. Secondly, the conductive cross-sectional area of the spring clip depends on the cross-sectional area of the connection between the first and second flat plates, which is larger than the connection formed by conventional busbars and welds. For example, in practical tests, prismatic cells connected with spring clips exhibited a higher overcurrent capability than those using traditional welding methods, improving by [X]%.

(d) Fixed Structure Design
The fixed structure for prismatic cells and the manufacturing method of the battery module casing have high practical value. The design includes the combination of the battery chassis, top fixed cap, and packing straps. The battery chassis has a first battery fixing slot that adapts to the bottom of the prismatic cell, securely clamping the bottom of the cell. The top fixed cap has a second battery fixing slot that adapts to the top of the prismatic cell, securely clamping the top of the cell. Finally, the packing strap is fitted over the battery chassis and the top fixed cap to form a single battery pack fixation structure. Additionally, the battery module casing is equipped with anti-slip components and a top partition fixing plate. The anti-slip components include guide rails on both sides of the inner shell of the battery module casing and limit ribs at the bottom of the casing, which help limit the position of each battery pack, preventing shaking. The top partition fixing plate can be detachably connected to the outer shell of the battery module casing, pressing and fixing the tops of multiple battery packs. This design improves the fixation safety of the prismatic cells and provides reliable protection for energy storage battery box applications.

四. Design Key Points Summary

The design key points of prismatic cell structural components are numerous, and these points play a crucial role in improving the safety and performance of lithium batteries.

(a) Liquid Injection Port Sealing Design
The sealing design of the liquid injection port is directly related to the battery's safety and lifespan. The liquid injection port sealing plug designed by CATL consists of a metal part and a rubber part, with an interference fit at the contact point with the injection hole. The injection hole also features a recess, and the rubber part of the sealing plug is designed with a protrusion that can engage with the recess. This design allows for cooling assembly at low temperatures, effectively preventing the formation of metal burrs and particles, ensuring reliable sealing of the liquid injection port. At the same time, the rubber part prevents metal burrs and particles from falling into the battery shell, ensuring battery safety. The mechanical sealing structure does not require laser welding, simplifying the process and significantly reducing costs.

(b) Positive and Negative Terminal Design

The positive terminal is usually made of aluminum, while the negative terminal is made of a copper-aluminum composite. Their primary function is to conduct current. In the battery, the top cover terminal, busbar, and cell tabs are welded together to ensure the current passes through the cell for charging and discharging. In the module, the top cover terminal is laser-welded and bolted to the busbar, forming series/parallel connections. Additionally, directly connecting the aluminum shell and the positive terminal can eliminate the potential difference between the two, preventing aluminum shell corrosion.

(c) Increasing Positive Terminal Resistance
The resistance between the positive terminal and aluminum shell is very small, at the milliohm level. When a short circuit occurs, the loop current is large, and this can cause sparking, which may lead to a battery fire, posing a significant safety hazard. Currently, conductive plastic or silicon carbide is often added between the aluminum shell's top cover plate and the positive terminal to increase the conductive resistance between the aluminum shell and the positive terminal. CATL has also designed a PTC thermistor between the positive terminal and the top cover plate. By utilizing the thermistor's characteristic of changing resistance with temperature, the PTC thermistor can quickly consume internal energy when the power battery experiences an external short circuit, preventing thermal shock from excessive heat on the resistor. This eliminates the issue of low resistance causing melting while also avoiding problems such as battery fire or resistor melting due to excessive temperature.

(d) Explosion-Proof and Reversal Plate Design
Generally, the top cover of lithium iron phosphate batteries uses a single explosion-proof valve, with an opening pressure of 0.40.8 MPa. When the internal pressure increases and exceeds the opening pressure of the explosion-proof valve, the valve will rupture at the notch and open to release pressure. For ternary battery systems, in addition to the explosion-proof valve, an SSD reversal plate combination design is also used. The opening pressure of the explosion-proof valve and the reversal pressure of the SSD plate are typically 0.751.05 MPa and 0.45~0.5 MPa, respectively. When the internal pressure of the battery increases to the SSD reversal pressure, the reversal plate is pushed upward, quickly cutting off the current. Simultaneously, the aluminum connection plate fuse blows, causing a direct short circuit between the positive and negative terminals of the top cover, quickly cutting off the current.

The design key points of prismatic cell structural components cover several aspects, including the liquid injection port sealing, positive and negative terminal design, increasing positive terminal resistance, and the design of explosion-proof and reversal plates. These design elements work together to enhance the safety and performance of lithium batteries, providing solid technical support for the development of the new energy vehicle and energy storage markets.