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. A-CC-18650 18650 Battery Case with Anti-Explosive Cap and Insulation O-ring,Used for 18650 battery research, producing, especially for Powerful 18650 battery making,100set/package included Cylinder cell case, Cap and up/down spacer. Specification Case Dimensions 18.15mm(OD) x 17.6mm(ID) x 68.25mm(H) Top Cap 17.6mm(D) x 3.85mm(H) Top Cap with O ring 17.6mm(D) x 4mm(H) Top Insulator 17mm(D) x 0.12mm(H) with 6.5mm hole Bottom Insulator 17mm(D) x 0.12mm(H) with 6.5mm hole Material Case 304ss stainless steel cap Nickel plated steel Seal O-ring Nylon Insulating gasket PP 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 mechanical...