Why are steel-clad lithium batteries cylindrical?

The lithium battery was a steel shell core in the early days, and the shell material was steel. Since the metal expansion coefficient of the steel material was not as good as that of the aluminum shell material, when the battery was short-circuited, the probability of explosion was higher than that of the aluminum shell. Early lithium batteries were mainly used in consumer electronics fields such as laptops and mobile phones. Notebooks used 18650, and mobile phones first used square aluminum shell cores to replace steel shell cores(previously cell phone batteries used for a long time). Drum bag), Due to the development of ultra-thin smart phones, mobile phone lithium batteries have now developed to the third generation, aluminum plastic film polymer core.

The definition of a cylindrical lithium battery: For example, the 18650 type refers to a battery with a diameter of 18mm and a length of 65mm. In fact, the cylindrical steel shell core is not only 18650, but also 26650 and other sizes.

Why is it cylindrical? My understanding is that the circular side can share the pressure and improve the safety performance of the battery in the event of a heavy squeeze. Another advantage of cylindrical batteries is that mass production is simpler. When the winding machine winding the diaphragm and the positive and negative poles, it can be automated. A single size of 18650 can make batteries of the same size and different capacity formulas. Reduce the cost. As the technology of cylindrical batteries becomes more and more mature, for example, different types of lithium ion batteries can be used on 1,8650(lithium ferric phosphate, lithium cobalt acid, lithium titanate)

There is no doubt that Apple's MacBook battery is now the pioneer of polymer batteries. Now that the domestic lightweight laptops have turned to square polymers, few manufacturers are still using outdated 18650 designs unless the cost control is relatively low.

One might wonder, but isn't Tesla's electric car a 18650 battery brick? Isn't it advanced?

My answer: Tesla's specialty is circuit monitoring, and what he does best is to be able to effectively monitor every cell in the battery pack in real time, but not battery innovation, but car innovation. The battery of the TESLA electric vehicle uses the NCA series(nickel-cobalt aluminum system) 18650 cobalt acid lithium battery provided by Panasonic, with a single battery capacity of 3,100 mA. Or based on cost, the current polymer battery surface is aluminum plastic film, soft package is not resistant to impact, collision. If polymer is used, Tesla will not be able to pass the United States 'car safety inspection certification.

With the rise of smart wearable devices and the comprehensive development of smart phones, small-sized polymer batteries will become more and more widespread in the digital field. The aluminum shell core will be gradually eliminated, and the use of 18650 in electric bicycles and electric vehicles will continue until the safety problems such as impact resistance and collision of polymer batteries are effectively solved.

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