As early as more than ten years ago, there were endless talks about supercapacitors replacing lithium batteries. With the current rise of lithium batteries in the electric vehicle market, many people who insist on this argument have begun to focus on this market. But why hasn’t this viewpoint, which has been touted for more than a decade, come true?
Supercapacitors vs. Lithium Batteries
As an energy storage device, supercapacitors have inherent advantages. The energy storage of lithium batteries is that electricity is converted to chemical energy and then converted to electric energy. There is a certain energy loss. The energy of supercapacitors is from beginning to end during the charging and discharging process. For electrical energy, the charging and discharging efficiency is extremely high. Because its energy storage process has no chemical reaction and is reversible, its charge and discharge life is much longer than that of ordinary lithium batteries.
Secondly, the power density of supercapacitors is very high. Its instantaneous dynamic characteristics and torque performance are better, which is very suitable for solving the problem of starting and accelerating large electric vehicles. Moreover, with high power meter density, it is easier to achieve high-power fast charging. It can reach more than 95% of the rated capacity in just a few seconds to ten minutes.
In terms of safety, supercapacitors are also better than lithium batteries. Because the supercapacitor is physical energy storage, even in the case of a short circuit, there will be no liquid leakage, smoke, fire, rupture, or explosion. The temperature rise during charging and discharging is very small, so there is no need to worry about overheating. Moreover, the raw materials of supercapacitors are pollution-free and are green energy storage devices that can easily meet environmental protection standards.
Disadvantages of supercapacitors
So with so many advantages, why haven’t supercapacitors replaced lithium batteries? The key is energy density. The primary task of energy storage equipment is “energy storage”. If it cannot store large-capacity electrical energy, then the requirements for fast charging and longevity are false propositions, or they can only be used in specific scenarios.
SkelCap Supercapacitors/Skeleton Technologies
The energy density of supercapacitors is not high, even generally lower than 10Wh/kg. This is a fatal disadvantage compared with lithium batteries that are frequently higher than 150Wh/kg. Take the supercapacitor launched by the Estonian company Skeleton in the picture above, the highest energy density is only 6.8Wh/kg.
Not only the mass-energy density, but the volume energy density of supercapacitors is also lower than that of lithium batteries. That is to say, under the same power capacity, the volume and mass required by the supercapacitor are greater than that of the lithium battery. For passenger cars with strict structural and volume requirements, supercapacitors are out in the first round.
Public transportation: Weakening the disadvantages of supercapacitors
Though energy density is low, supercapacitors can find a place in the electric vehicle market, that is, public transportation.
It might as well choose a public transportation market that doesn’t have strict requirements for the battery size. As early as 2006, Shanghai introduced supercapacitor buses. It can run 5 kilometers in 30 seconds to 1 minute during loading and unloading. However, the equipment installation and manufacturing costs are too high. Although this kind of supercapacitor car has low energy consumption, it has not been widely promoted.
Not only that, the energy stored on a charge is too little, especially under the large passenger flow and the high power consumption of the air conditioner in summer. If you want to take into account the mileage, the charging time is difficult to meet the fast and convenient characteristics of public transportation. Passengers do not want to encounter the situation where the bus stops charging after two stops, and the driver does not want to delay the mileage and time due to charging.
In order to solve this problem, many supercapacitor vehicles have begun to adopt the technology of supercapacitor + lithium battery hybrid. The supercapacitor is responsible for solving problems such as charging speed, acceleration torque, and braking. And the lithium battery is responsible for assisting in solving the cruising range problem. If the total travel distance is not long, the charging bow can be erected only at the head and tail stations. If it is necessary to charge in the middle, a charging bow with low charging power can also be erected at the intermediate station to solve the problem of charging time.
How to break through the energy density?
Although the energy density of supercapacitors at this stage is not comparable to that of lithium batteries. Relevant research is still exploring how to break this limitation from the perspective of materials.
The Technical University of Munich, Germany, developed an “asymmetric” supercapacitor this year. It combines chemically modified graphene materials and nanostructured metal-organic frameworks. Its energy density can reach 73Wh/kg, which can basically replace the current lead. Acid battery. Moreover, the supercapacitor is very stable, and can still maintain 90% of the capacity after 10,000 cycles.
University College London and the Chinese Academy of Sciences also jointly released a study last year. They increased the volumetric energy density of supercapacitors to 88.1Wh/L through porous graphene electrode materials, while the volumetric energy density of traditional lead-acid batteries is usually 50 to 90Wh/L. Supercapacitors using new materials also have extremely high flexibility. They can be bent 180 degrees without affecting their performance, so they can also be used in folding mobile phones and wearable products.
CRRC is also continuing to develop graphene supercapacitors. Their current 60,000F supercapacitors can achieve an energy density of 40Wh/kg and a power density of 2314W/kg. They not only intend to continue to make breakthroughs in energy density but also hope to achieve higher power density to meet the requirements of some defense applications.
The breakthrough in the energy density of supercapacitors is closely related to graphene. But graphene is still facing the problem of large-scale mass production, which also imposes shackles on supercapacitors with high energy density. If we cannot solve it, supercapacitors can only be used in fields such as public transportation and wind power generation. There is still a long way to go before they can replace lithium batteries.