he 18650 battery was born in 1991 as a standard lithium-ion battery model designed to save costs. It was mass-produced by Sony and was initially used in fields such as laptops and power tools. The so-called 18650 refers to the specification and model of the battery, where 18 represents a battery diameter of 18mm, 65 represents a battery length of 65mm, and 0 represents a cylindrical battery. This design cleverly balances volume and energy density, but is only suitable for small electronic products.

In 2008, Tesla released the Roadster, which consisted of thousands of 18650 batteries connected in series to form a battery pack. Although 18650 battery technology is mature, it is not suitable for new energy vehicles in terms of performance, capacity, reliability, etc. due to its design for small electrical appliances, with low single cell capacity and the need for thousands of cells to be connected in series, resulting in low grouping efficiency.

In 2016, 21700 batteries were mass-produced and launched, with a significant increase in battery diameter of 21mm and a single cell capacity of 5Ah. By increasing the cell volume, the number of cells was reduced by 30%, effectively reducing the complexity of the battery pack. Compared to 18650 batteries, 21700 batteries significantly reduce the number of individual cells, effectively reduce production costs, and improve range.

Breakthrough in innovative performance of large cylindrical battery structure

In 2020, the 4680 battery was unveiled, also known as the large cylindrical battery due to its diameter reaching 46mm. At present, there are various large cylindrical batteries on the market, all with a diameter of 46mm and heights of 80mm, 95mm, 120mm and other specifications. The single cell energy is 5 times higher than the 21700 battery, the power is 6 times higher, the energy density is over 300Wh/kg, the single cell capacity is increased by over 27Ah, and the grouping efficiency is greatly improved.

Taking the BMW cylindrical battery as an example, compared with the previous square battery cells, the positive electrode contains higher nickel content, reduced cobalt content, and increased silicon content in the negative electrode. Thanks to this, the energy density of the large cylindrical battery cells has increased by 20% compared to the previous generation, the range has increased by 30%, and the charging speed has increased by 30%.

Of course, the improvement of various performance is not only achieved by increasing the size of the battery, but also requires the support of new battery technologies.

In small cylindrical and square batteries such as 18650 and 21700, the electrode ears are metal pieces welded out from the positive and negative electrode pieces to connect to external circuits. This design requires current to be collected from the edge of the pole piece through the pole ear, resulting in high local current density, and the battery is prone to heating up during fast charging. At the same time, the pole ear occupies the internal space of the battery and requires additional welding processes, increasing weight and cost. Finally, the welding point between the pole ear and the pole piece is prone to detachment due to vibration, expansion, or poor contact caused by oxidation, which affects the battery life.

The large cylindrical battery adopts electrodeless ear technology, in which the positive and negative electrode plates are directly extended to the top and bottom of the battery, with the top as the positive electrode and the bottom as the negative electrode. The conductive part changes from "point contact" to "surface contact", and the conductive area expands by tens of times, resulting in a more uniform current distribution and avoiding local overheating.

It is understood that the traditional 21700 battery has a single pole ear internal resistance of about 30m Ω, while the 4680 battery uses a full pole ear, which reduces the internal resistance to below 15m Ω and increases the charging current carrying capacity by at least 1 time. At the same time, the uniform distribution of current reduces the temperature difference between the electrodes from over 10 ℃ in traditional batteries to within 3 ℃, significantly reducing the risk of thermal runaway. After removing the pole ears, the utilization rate of internal space in the battery has been further improved, and the battery capacity has also increased under the same volume.