As the most important energy storage medium at present, batteries and capacitors are indispensable in the development of new energy, and their technological innovation is also particularly important for the new energy revolution. Lithium ion capacitors (LICs) combine the advantages of lithium batteries and supercapacitors with high power density and can be charged and discharged above 100C. Lithium ion capacitors have a long cycle life of more than 100,000 times, which is 100 times higher than that of lithium batteries, and have high energy density, which is 3 to 5 times that of traditional supercapacitor.
1. Applications of lithium ion capacitors
Lithium ion capacitors take into account high specific power and high specific energy, and have fast charging and long life characteristics, representing the forefront of the development of capacitor technology for manufacturers. With the mature development of supercapacitor technology and industry, a broader application market space has been created. The main application markets for lithium ion capacitors include:
● Electric vehicles
Lithium ion capacitors have huge market value in electric vehicles such as hybrid cars and buses, as well as pure electric buses. Many well-known automobile companies have systematically tested and evaluated lithium-ion capacitor products, and are very optimistic about the application prospects of lithium-ion capacitors in the follow-up development of hybrid cars. With the continuous development of technology, lithium ion capacitors will develop in the direction of electric vehicles, which will meet the car battery voltage, performance and other parameters, and will occupy an important market share in the future.
● Rail transit
Off-line operation is one of the technical development directions of modern trams, and global companies attach great importance to this. Using lithium ion capacitors as the power source, on the premise of maintaining a reasonable weight and cost, the terminal can be quickly charged to meet the requirements of the entire off-line operation, which saves a lot of infrastructure costs and has significant cost and performance advantages.
● Wind pitch
In order to ensure the reliability of the back-up power supply of the wind pitch system, supercapacitors have been widely used in the back-up power supply of wind pitch systems instead of batteries due to their advantages of high power, long life, wide operating temperature range, and maintenance-free.
● Energy-saving elevator
Use supercapacitors to recover the electrical energy converted from stored potential energy, and use it first in the next working cycle to achieve energy saving. At the same time, it can be used as an emergency backup power supply for elevators, which can greatly improve the safety of elevator use.
● Golf cart and AGV
Compared with the use of golf cart batteries, golf carts and AGVs (Automated Guided Vehicles) use lithium-ion capacitors, which have lower volume, weight and cost of the power supply, longer life, more convenient use, and can open up new application modes.
● DC circuit and smart three meters
Supercapacitors are widely used in smart meters, car DVRs, driving recorders, handheld GPRS devices, smart phones, tablet computers and other electrical appliances.
2. The difference between lithium ion capacitors, lithium-ion batteries, and supercapacitors
① Lithium-ion batteries
Lithium ion batteries are the fastest-growing secondary batteries after nickel-cadmium and nickel-hydrogen batteries. Lithium-ion batteries use graphite as the anode and LiCoO2 as the cathode. During charging, lithium ions are extracted from the cathode material and migrate to the anode through the electrolyte driven by the electrochemical potential gradient. Charge balance requires that the same amount of electrons flow from the cathode to the anode under the external circuit, and the lithium ions that obtain electrons after reaching the anode are then embedded into the lattice of the anode material.
During discharge, the reverse process is performed, that is, lithium ions leave the anode lattice and are embedded in the cathode to form LiCoO2 again. Compared lithium vs alkaline batteries and other batteries, lithium-ion batteries have the advantages of high energy density, high average output voltage, high charging efficiency, low self-discharge efficiency, good safety performance, and long cycle and service life.
Supercapacitors are generally mainly composed of electrodes, electrolytes, current collectors and separators. During charging, electrons are transferred from the cathode to the anode through the external power supply, so that the cathode and the anode are respectively positively and negatively charged, and the cathode and anode ions in the electrolyte solution body are separated and moved to the electrode surface to confront the charge layer on the electrode surface to form an electric double layer.
During discharge, electrons flow from the anode to the cathode through the load, cathode and anode ions are released from the electrode surface and return to the body of the electrolyte solution, and the electric double layer disappears at the same time. It can be seen that the electric double layer capacitor uses the electric double layer at the interface between the electrode and the electrolyte to store the charge, and the charging and discharging process is always a physical process, and no electrochemical reaction occurs. Therefore, it has the advantages of stable performance, short charge and discharge time, long cycle life, high power density, and good high and low temperature performance.
③ Lithium ion capacitors
The cathode material of the lithium ion capacitor is an activated carbon material with electric double layer energy storage, the anode material is an intercalated carbon material with a lithium ion deintercalation function, and the electrolyte is a lithium salt electrolyte. When the battery is charged, lithium ions are detached from the surface of the cathode material and inserted into the lattice of the anode material after passing through the electrolyte and the separator.
During discharge, lithium ions are extracted from the lattice of the anode material, returned to the surface of the cathode material through the electrolyte, and form an electric double layer with the charge of the cathode. The anode potential after lithium insertion is low, and it has the characteristics of high operating voltage, energy density and power density between lithium-ion batteries and supercapacitors.
3. Advantages of lithium ion capacitors over lithium-ion batteries and supercapacitors
● Comparison of capacity, voltage and self-discharge
Lithium ion capacitors have lower energy density than lithium-ion batteries, but high output density. The energy density of the single volume of the lithium ion capacitor is 10~15Wh/L, which is much larger than the 2~8Wh/L capacity of the electric double layer capacitor, which is twice the latter. In terms of voltage, the highest voltage of lithium ion capacitors can reach 4V, which is similar to that of lithium-ion batteries, much higher than that of electric double-layer capacitors, and smaller than both in terms of self-discharge.
Due to the use of lithium oxide, the cathode of lithium-ion batteries not only contains a large amount of lithium, which can form lithium dendrites and pierce the separator, but also contains oxygen, an important ignition element. Once the battery is short-circuited, the overall thermal decomposition can develop, and the reaction with the electrolyte can cause combustion. The cathode of lithium ion capacitors is activated carbon. Even if the internal short circuit will react with the anode, it will not react with the electrolyte. In theory, it will be much safer than lithium batteries.
● Long life
In order to achieve long life, lithium-ion batteries have a certain range of charge and discharge depth limits, which reduces the capacity that can be substantially utilized. The principle of charge and discharge of electric double layer capacitors is simply to adsorb or desorb ions in the electrolyte to have a long life, and it is difficult to prolong the actual life based on this alone. However, in lithium ion capacitors, even if the cathode potential is lowered, the voltage of the cell itself does not drop significantly, so the capacity can be secured.
● High temperature resistance
Under high temperature conditions, the electrolyte and the cathode are prone to oxidative decomposition. Therefore, it may be necessary to reduce the potential of the cathode under high temperature conditions. However, when the potential decreases, the overall voltage of the electric double layer capacitor will drop, and the capacity cannot be ensured. Lithium-ion batteries, on the other hand, cannot reduce pressure, which is prone to safety problems. Only lithium-ion capacitors can be used at locations where the cathode potential is far from the oxidative decomposition region, resulting in excellent high-temperature performance.
Status of industrialization of lithium ion capacitors
The lithium ion capacitor industry includes:
- The upstream mainly includes: cathode and anode raw materials, electrolytes, separators, perforated current collectors and elemental metal lithium electrodes, etc.;
- The midstream mainly includes lithium ion capacitor monomers of various shapes and specifications, and lithium ion capacitor monomer system integrated modules;
- The downstream is mainly the application demand of the end market. At present, the Japanese market has initially opened, and then it will radiate in the international market, such as: wind power generation, LED street lighting, solar power generation and hybrid electric vehicles.
Lithium ion capacitors usually consist of a battery-type anode and a capacitive porous carbon cathode, which can output twice the energy density of conventional electric double-layer capacitors. However, lithium ion capacitors have low energy density and short cycle life under high rate conditions, so their wide application still faces challenges.