Why cannot supercapacitors replace batteries?

Why cannot supercapacitors replace batteries?

Supercapacitors can already replace batteries as energy storage devices in specific situations, such as the tram in Wuhan, which can be quickly charged at every station. The disadvantage is that the energy density is still relatively low. After all, the chemical energy storage of batteries is much higher than the energy density. Additionally, the output voltage of a capacitor decreases linearly. Unlike batteries, the voltage is relatively stable throughout the cycle, making the circuit corresponding to supercapacitors more complex. Of course, the most crucial issue is still the energy density.

Two supercapacitors, connected in series, are charged to 5V and discharged to around 0.5V, with a discharge capacity of around 1700mAh. Due to the linear discharge of the capacitor, calculated based on the average voltage (5-0.5)/2=2.25V, the discharge energy is only about 3.9Wh, which is much smaller than a 18650 lithium battery (calculated based on the entry-level 2000mAh) 2Wh. And this capacitor size is dozens of times larger than the 18650 battery.

Predicting that supercapacitors can destroy the lithium-ion battery market. It is believed that the progress of supercapacitors is much faster than that of lithium-ion batteries.

Supercapacitors do not need to reach the energy density of lithium-ion batteries to devour the battery market. Perhaps one percent of the battery market has been replaced because this energy density has a longer duration, is safe, and has a power density of 10 times. On some buses in China, supercapacitors have replaced lithium-ion batteries, but due to high initial prices, the sales of supercapacitors are 3% lower than lithium-ion batteries. 

Supercapacitors, as a power source with special performance that lies between traditional capacitors and batteries, have advantages such as fast charging and discharging speed, and long cycle usage. Due to environmental reasons, buses equipped with supercapacitors and braided trams are being planned for large-scale promotion in multiple cities. There are many opinions that believe that with the advancement of technology, supercapacitors will be able to replace batteries.

Supercapacitor experts say that there is a misunderstanding of the ability of supercapacitors by the outside world. Taking new energy vehicles as an example, supercapacitors are usually able to compensate for peak power during the start-up and acceleration stages of the vehicle. But the key to determining the range of electric vehicles is still the lithium battery. In the current situation, there is a complementary relationship between supercapacitors and lithium batteries, and no one can replace anyone. For a considerable period of time in the future, the energy density of supercapacitors will not be comparable to that of lithium batteries.