The difference between lithium-ion capacitors, lithium-ion batteries, and supercapacitors

The difference between lithium-ion capacitors , lithium-ion batteries, and supercapacitors

       As a new type of energy storage device, lithium-ion capacitors have the advantages of high power density, high electrostatic capacity and relatively long cycle life, and are expected to be widely used in new energy vehicles, solar energy, wind energy and other fields. Its working principle is different from that of lithium-ion batteries and supercapacitors .

1. The working principle of

lithium-ion batteries Lithium-ion batteries are the fastest-growing secondary batteries after nickel-cadmium and nickel-hydrogen batteries. The positive and negative active materials of lithium-ion batteries are compounds that can reversibly intercalate and deintercalate lithium. At least one electrode material is in a lithium intercalation state before assembly, such as transition metal oxides LiCoO2, LiNiO2 and LiMn2O4 as the positive electrode. Various carbon materials, metal oxides or alloys, etc. are used as negative electrode materials.

The working principle of lithium-ion batteries is that graphite is used as the negative electrode and LiCoO2 is used as the positive electrode. During charging, lithium ions are extracted from the positive electrode material, and migrated to the negative electrode through the electrolyte under the driving of the electrochemical potential gradient. The charge balance requires that the same amount of electrons flow from the positive electrode to the negative electrode under the external circuit. It is embedded into the lattice of the negative electrode material; during discharge, the reverse process is performed, that is, the lithium ions leave the negative electrode lattice and are embedded in the positive electrode to form LiCoO2 again.

The reaction formula of the charging and discharging process is:

positive electrode reaction: LiCoO2?Li1-xCoO2+xe+xLi+

negative electrode reaction: xLi++xe+nC?LixCn

battery reaction: LiCoO2+nC?Li1-xCoO2+LixCn

The working principle of lithium-ion batteries is based on electrochemical intercalation-extraction in addition to "oxidation-reduction" Reaction, that is, during the charging and discharging process of lithium-ion batteries, lithium is used as the carrier of energy exchange in the form of ions (Li+), and through the electrolyte, the insertion and extraction of lithium ions are used to swing between the positive and negative electrodes to achieve the purpose of energy exchange. . Compared with 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.

2. The working principle of supercapacitors

When charging, electrons are transferred from the positive electrode to the negative electrode through the external power supply, so that the positive electrode and the negative electrode are respectively positively and negatively charged, and the positive and negative ions in the electrolyte solution body are separated and moved to the electrode surface and the charge on the electrode surface. The layers confront each other to form an electric double layer; during discharge, electrons flow from the negative electrode to the positive electrode through the load, and the positive and negative 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. The charging and discharging process is always a physical process and no electrochemical reaction occurs, so it has stable performance, short charging and discharging time, long cycle life, high power It has the advantages of high density and good high and low temperature performance.

3. The working principle of lithium ion capacitors The

positive electrode material is an activated carbon material with electric double layer energy storage, the negative electrode material is an intercalated carbon material with lithium ion deintercalation function, and the electrolyte is a lithium salt electrolyte. When the battery is charging, the lithium ions are separated from the surface of the positive electrode material and inserted into the lattice of the negative electrode material after passing through the electrolyte and the separator; when discharging, the lithium ions are detached from the lattice of the negative electrode material and return to the positive electrode material through the electrolyte. On the surface, an electric double layer is formed with the charge of the positive electrode. The negative electrode 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.

4. The advantages of lithium-ion capacitors compared to lithium-ion batteries and supercapacitors

(1) Comparison of capacity, voltage, and self-discharge The

energy density of lithium-ion capacitors is lower than that of lithium-ion batteries, but the output density is high; the energy density of a single volume It is 10~15Wh/L, which is much larger than the 2~8Wh/L capacity of the electric double layer capacitor, which is twice that of the latter.

In terms of voltage, the high 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.

(2) Security

Due to the use of lithium oxide, the positive electrode 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 positive electrode of lithium-ion capacitors is activated carbon. Even if the internal short circuit will react with the negative electrode, it will not react with the electrolyte. In theory, it will be much safer than lithium-ion batteries.

(3) Long life

Lithium-ion batteries have a certain range of charge and discharge depth limits in order to achieve long life, which reduces the capacity that can be used in essence. The charge and discharge principle of electric double layer capacitors is simply to adsorb or discharge It is difficult to prolong the practical life only by removing the ions in the electrolyte and having a long life. However, even if the positive electrode potential of the lithium ion capacitor is lowered, the voltage of the cell itself does not drop significantly, so the capacity can be ensured.

(4) High temperature resistance

Under high temperature conditions, the electrolyte and positive electrode are prone to oxidative decomposition. Therefore, it may be necessary to reduce the potential of the positive electrode under high temperature conditions. However, when the potential is reduced, the overall voltage of the electric double layer capacitor will drop, which cannot be ensure capacity. Lithium-ion batteries, on the other hand, cannot reduce pressure, which is prone to safety problems. Only lithium-ion capacitors can be used in locations where the positive electrode potential is far from the oxidative decomposition region, so they have excellent high-temperature performance.

5. The application and industrialization status of

lithium ion capacitors The upstream of the lithium ion capacitor industry mainly includes: positive and negative raw materials, electrolytes, separators, perforated current collectors and elemental metal lithium electrodes, etc.; the midstream mainly includes various shapes and specifications of lithium Ion capacitor monomer and lithium ion capacitor monomer system integrated module; the downstream is mainly the application demand of the end market. At present, the Japanese market has initially opened, and will then radiate in the international market, such as: wind power, LED street lighting, Solar power generation and hybrid electric vehicles, etc.

At present, these industries are mainly controlled by foreign companies. For example, Japan's Kuraray and Japan's ACT have mastered activated carbon and nano-gate carbon technologies respectively; Japan's Kanebo Company, Japan's Kureha Chemical, and Japan's ATEC Company have mastered polyacene, hard carbon, etc. The technology of anode materials such as FERRO in the United States and HONEYWELL in Germany dominate the electrolyte; the diaphragm is monopolized by NKK of Japan, and the porous current collector is monopolized by three Japanese metal companies. At present, only a few companies in China are developing positive electrode activated carbon and negative electrode hard carbon materials for lithium ion capacitors.