The shell of lithium-ion cylindrical batteries (such as the common 18650 and 21700 models) is usually made of stainless steel, which is significantly different from the design idea of using aluminum shell or aluminum-plastic film for prismatic batteries or pouch batteries. The reason for choosing stainless steel needs to be considered from many aspects such as the structural characteristics, mechanical requirements, electrochemical environment and manufacturing process of cylindrical batteries.

1. Mechanical strength and compressive requirements

The shell of the cylindrical battery is directly used as the physical support of the electrode core (such as the winding structure of the positive electrode-separator-negative electrode). The tensile strength of stainless steel (300~600 MPa, depending on the model) is much higher than that of aluminum (about 40~50 MPa for pure aluminum, 200~400 MPa for aluminum alloy), and can better withstand the expansion stress of the internal core (especially when the expansion rate of the silicon-based anode can reach 300%) and external mechanical impact, and avoid internal short circuit caused by shell deformation. Cylindrical batteries may produce high-pressure gas when overcharging and thermal runaway, and the yield strength of stainless steel is higher, which can effectively resist the rise of internal pressure, delay or prevent the rupture of the shell, and buy time for the timely pressure relief of the safety valve.

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Material

2. Electrochemical compatibility and potential matching

In some cylindrical battery designs, the shell is connected as the negative current collector, and the steel shell is directly connected with the current collector (copper foil) of the negative electrode (graphite or silicon-based material) through welding or mechanical contact, which becomes the negative current path of the battery.

3. Adaptability of welding and sealing process

The sealing of cylindrical batteries usually adopts laser welding process (such as the welding of the top cover and the shell). The high melting point (about 1400~1450°C) and low thermal conductivity (about 15 W/m·K) of stainless steel make it have a small heat-affected zone during welding, high weld strength and good sealing. However, aluminum shell welding requires protective gas (such as argon) and is prone to porosity, which makes the process more difficult. Some cylindrical batteries are mechanically crimped (e.g., some low-cost models), the ductility and resilience of stainless steel are more suitable for this process, while aluminum shells may fail due to stress relaxation after crimping.

4. Thermal management characteristics

The coefficient of thermal expansion of stainless steel (~17×10⁻⁶/°C) is closer to that of internal electrode materials (e.g., copper, aluminum foil), reducing the risk of interfacial peeling due to thermal expansion and contraction during high temperature cycling. Aluminum, on the other hand, has a high coefficient of thermal expansion (~23×10⁻⁶/°C), which may exacerbate the internal structural stress. In addition, stainless steel has a lower thermal conductivity than aluminum (about 15 vs 237 W/m·K), which can slow down the transfer of heat to adjacent battery cells and reduce the risk of thermal propagation in extreme cases (but in combination with the overall design of the battery pack).

5. Trade-off between cost and large-scale production

The unit price of stainless steel is higher than that of aluminum, but the degree of standardization of cylindrical batteries is high (for example, the global annual production of more than billions of units in 18650) can significantly dilute the cost of large-scale production. In addition, the stainless steel case further reduces costs by omitting the additional anti-corrosion coating or composite layer required for the aluminum case, such as the nickel plating of some prismatic batteries. The stamping, winding and welding processes of stainless steel shells are highly mature, which is suitable for high-speed automated production (hundreds of batteries per minute can be processed), while aluminum shells are prone to deformation due to excessive ductility on high-speed production lines, which affects the yield.

6. Adaptability of application scenarios

Cylindrical batteries are mostly used in power tools, electric vehicles (such as Tesla's early adoption of the 18650 battery pack) and other scenarios, which need to withstand high-rate discharge (>3C) and vibration environments, and the mechanical strength of stainless steel shells has become a key advantage. The standardized design of the cylindrical battery (e.g., size, interface) facilitates modular integration, and the robustness of the steel case also supports multiple disassembly and maintenance of the battery pack.

7. Comparison with other materials

The density of stainless steel (7.8~8.0 g/cm³) is about 3 times that of aluminum, resulting in the energy density of cylindrical batteries being slightly lower than that of aluminum shell batteries of the same size, but it can be partially compensated by optimizing the internal design (such as high nickel cathode, silicon carbon anode).

The choice of stainless steel housing for lithium-ion cylindrical batteries is essentially the result of seeking the best balance between energy density, mechanical strength, process cost, and electrochemical compatibility. Although stainless steel is heavier, its high rigidity, excellent weld sealing, compatibility with negative electrode potential, and cost advantages of large-scale production make it an irreplaceable packaging material for cylindrical batteries. With the development of battery technology (such as all-tab design and 4680 large cylindrical battery), the process adaptability advantages of stainless steel shell have been further highlighted, consolidating its mainstream position in the field of cylindrical batteries.