A comprehensive comparison of 18650 and 21700 batteries

The traditional 18650 battery has a mature supporting industrial chain and low cost. It is an ideal lithium-ion battery and has been widely used in the field of consumer electronics. However, the 18650 battery has a small capacity. When applied in the field of power batteries, it often requires tens of thousands of single cells to be combined into a battery pack. This has led to a significant increase in the management difficulty of the battery pack. Therefore, under the leadership of Tesla, it has set off 21700 replaces the 18650 battery storm. Compared with the 18650 battery, the 21700 battery has a higher capacity, reduces the number of single cells in the battery pack, reduces the complexity of the battery pack, and attracts a lot of attention. Recently, Thomas Waldmann (first author, corresponding author) of the Baden-Württemberg Solar Energy and Hydrogen Energy Research Center in Germany and others conducted a comprehensive comparison of the two batteries in terms of electrical and thermal performance. The study showed that the 21700 battery In the process of high-current charging and discharging, the heat generation is more obvious, so the polarization of the battery is correspondingly smaller. At the same time, the energy density can be increased by about 6% because the proportion of inactive materials such as the casing is reduced. In order to be able to compare the two batteries more accurately, the author prepared 18650 and 21700 batteries with the same formula of electrodes, separators and electrolyte. The formula design of the two batteries is shown in the table below, so the main components of the two batteries are The difference comes from the size. In order to obtain the temperature information inside the battery in the experiment, the author implanted a K-type thermocouple in the middle of the battery. Although the theoretical diameters of 21700 and 18650 batteries are 21mm and 18mm respectively, in fact, in order to be able to wind more electrodes and increase the capacity and energy density of the battery, the diameter of the energy-type battery is usually slightly larger than the theoretical value. For example, the author The measured outer diameter of a certain energy 21700 battery is actually about 21.15mm. Therefore, in the figure a below, the author analyzes the increased winding length of the battery when the outer diameter of the casing increases by 50-400um. From the figure, it can be seen that when the total thickness of the pole piece + diaphragm is 200um, when the diameter of the casing increases by 400um, the electrode is additionally increased. The length of the winding can reach 1 turn. Thomas Waldmann's research shows that the difference in structure will lead to differences in electrical and thermal characteristics of the two batteries. For example, the 21700 battery has more obvious thermal effects during the discharge process, so the 21700 battery has less polarization in the rate test. The 21700 battery can involve more electrodes, so the weight of inactive parts such as the casing is significantly reduced, so the energy density of the battery is increased by 6% compared to the 18650 battery.