Structural Features of Cylindrical, Prismatic and Soft-Pack Lithium Batteries

Lithium-ion batteries dominate the new energy market due to their high energy density, stable cycle performance and low self-discharge rate, serving as core power sources for electric vehicles, consumer electronics, energy storage systems and portable industrial equipment. According to external packaging forms and internal structural designs, commercial lithium batteries are mainly divided into three categories: cylindrical, prismatic and soft-pack lithium batteries. Each type has unique structural characteristics, which directly determine their energy density, heat dissipation performance, safety, assembly flexibility and application scenarios. A systematic comparison of their structural features is essential to understand their performance differences and guide reasonable battery selection, design optimization and application promotion.

Cylindrical lithium batteries are the earliest and most mature commercialized battery type, featuring a standard cylindrical winding structure and rigid metal shell. The internal structure adopts a spiral winding process, where positive electrode sheets, negative electrode sheets and isolating films are tightly wound into a cylindrical core and encapsulated in a stainless steel or aluminum alloy shell. The top and bottom of the shell are equipped with sealing components, safety valves and overcurrent protection devices, forming a fully sealed integrated structure. Structurally, cylindrical batteries have highly standardized dimensions, fixed caliber and height specifications, and extremely high production automation, which effectively ensures consistent product quality.

Thanks to its special structural design, cylindrical batteries present prominent advantages. The circular arc shell can uniformly disperse internal pressure generated by charging and discharging expansion, effectively avoiding local stress concentration and reducing the risk of structural damage. Built-in safety mechanisms such as explosion-proof valves and PTC thermal resistors can timely release internal gas and cut off overcurrent during abnormal conditions, providing excellent basic safety performance. In addition, the standardized cylindrical structure is conducive to batch assembly and modular combination, making it suitable for large-scale battery pack formation. However, its structural limitations are also obvious. The circular shape leads to low space utilization during group assembly, with large gaps between single cells, resulting in poor overall heat dissipation consistency and low system-level energy density. Typical applications of cylindrical batteries include small electronic devices, power tools, and modular energy storage equipment.

Prismatic lithium batteries adopt a square rigid metal shell and laminated or semi-wound internal structure, featuring regular rectangular appearance and high structural compactness. The internal electrode core is encapsulated in a sealed aluminum or steel square shell, with independent sealing structures and pole layouts on the top. Different from the fixed specifications of cylindrical batteries, prismatic batteries support customized length, width and thickness dimensions according to application demands, with flexible structural design. The internal laminated structure makes electrode distribution more uniform, effectively improving the consistency of ion transmission and heat distribution inside the cell.

Structurally, prismatic batteries balance performance and applicability, occupying a dominant position in the new energy vehicle market. Their regular square shape greatly improves space utilization during assembly, reducing internal gaps of the battery pack and effectively promoting the overall energy density of the system. The rigid metal shell provides strong mechanical protection, enhancing the battery’s compression resistance and drop resistance. Meanwhile, the independent internal sealing design reduces electrolyte volatilization and improves battery cycle stability. Nevertheless, the structural defects of prismatic batteries cannot be ignored. Customized diverse specifications lead to low production standardization and poor versatility. In addition, the flat square shell has weak pressure dispersion ability; long-term charge-discharge expansion easily causes shell bulging and deformation, which may further trigger internal short circuits and safety hazards. They are widely used in new energy passenger vehicles, commercial vehicles and large-scale fixed energy storage devices.

Soft-pack lithium batteries, also known as polymer lithium batteries, adopt an aluminum-plastic composite film packaging structure, with no rigid metal shell. Different from cylindrical and prismatic rigid batteries, their internal core is packaged by flexible composite materials, featuring ultra-thin, lightweight and flexible structural characteristics. The internal structure mostly adopts laminated technology, with more uniform electrode arrangement and smaller internal resistance. The soft packaging material has strong ductility, which can adapt to the slight volume expansion of the internal core during charging and discharging, avoiding structural rupture caused by stress accumulation.

The unique flexible structure endows soft-pack batteries with unparalleled advantages in lightweight and high energy density. Without heavy metal shell, the overall weight is greatly reduced, and the battery thickness can be minimized, which is highly suitable for thin and portable electronic devices. The flexible shape design supports arbitrary cutting and bending within a certain range, adapting to diverse and irregular product assembly spaces. In terms of safety, the soft packaging film will only bulge slightly or crack for gas release under abnormal internal pressure, avoiding violent explosion accidents of rigid batteries. However, their structural weaknesses are prominent. The lack of rigid shell protection leads to poor mechanical strength, making them vulnerable to extrusion, piercing and structural damage. In addition, the flexible packaging structure has higher requirements for sealing technology; poor sealing easily causes air and moisture infiltration, leading to battery bulging and performance attenuation. Soft-pack batteries are mainly applied in smartphones, tablets, wearable devices and low-power portable electronic products.

In summary, the structural differences of the three types of lithium batteries fundamentally distinguish their performance characteristics and application boundaries. Cylindrical batteries feature standardized rigid structure, high safety and low cost but low assembly space utilization. Prismatic batteries have compact rigid structure, high system energy density and strong mechanical stability but suffer from poor versatility and bulging risks. Soft-pack batteries possess flexible lightweight structure and high single-cell energy density but have poor mechanical protection and high sealing process requirements. With the continuous upgrading of new energy technology, the structural design of lithium batteries is constantly optimized. Reasonable selection of battery types based on structural characteristics and continuous structural innovation will further improve the safety, energy efficiency and service life of lithium battery systems, promoting the sustainable development of the new energy industry.