From a battery performance level:
1.)21700 li-ion battery has larger capacity
Being that the 21700 battery is slightly larger, it will also provide more energy/power and runtime. Experimental studies by scholars have shown that the 21700 battery has about 5.3 circles of electrodes more than the 18650 battery. Taking into account the change in the height of the 21700 battery, the overall 21700 battery has about 51% more capacity than the 18650 battery.One thing to consider here is the mAh of these batteries. mAh’s on 18650 batteries will range from 2000 up to 3500. On 21700 batteries, you can expect mAh’s to range between 3000 up to 5000 mAh’s. You may see advertising for 18650 batteries that claim to have up to 9000 mAH.
2.)21700 battery has smaller inter resistance than 18650 battery
For cylindrical cells these parameters correlate with each other. This contributes to the lower performance of high energy cells. Electrode thickness has a possitive influence on cell resistance. 21700 battery is thick than 18650 battery.
Going from the 18650 to the 21700 format, the cell resistance decreases noticeably and shows a relatively flatter correlation to anode coating thickness.
The reason is the larger usable coated cathode area in the larger 21700-type cells, especially due to the outer windings of the jelly roll.
3.)21700 battery has lower discahrge rate than 18650 battery
Electrode thickness has a negative impact on the discharge rate capability. As we all known that 21700 battery is thick than 18650 battery.
Cells with thicker electrodes experience higher losses by limited transport, resulting in lower discharge energy and underutilized electrode active material. Thegeneral trend for all tested cell types was found that therate-capability is limited by the temperature on the cell surface due to current flow.
4.)21700 battery has higher energy density than 18650 battery
Change 18650 cell into 21700 cell, battery energy density can be increased by more than 20%. 21700 battery power density is better than 18650 batteries. According to the current disclosure of Tesla, under the existing conditions, the energy density of 21700 battery systems is approximately 20% higher than the 250WH to the original 18650-300 Wh battery system. At the same time, according to the Panasonic lithium battery power test in monomer batteries, the bulk density of its 21,700 battery is significantly higher than that of the 18650 cell monomer.