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

As a new type of energy storage device, lithium ion capacitor has the advantages of high power density, high electrostatic capacity and long cycle life. It is expected to be widely used in new energy vehicles, solar energy, wind energy and other fields. It works differently from lithium-ion batteries and supercapacitors.

1. Working principle of lithium ion battery

Lithium ion battery is the fastest developing secondary battery after nickel cadmium and nickel hydrogen battery. The active materials of the positive and negative electrodes of lithium ion batteries are all compounds that can be reversibly embedded into and out of lithium, among which at least one electrode material is in the lithium embedded state before assembly, such as transition metal oxides LiCoO2, LiNiO2 and LiMn2O4 as the positive electrode, and various carbon materials, metal oxides or alloys as the negative electrode materials.

The working principle of lithium ion battery is shown in figure 1, with graphite as the negative electrode and LiCoO2 as the positive electrode. When charging, lithium ions are separated from the anode material and migrate to the negative electrode through the electrolyte driven by the electrochemical potential gradient. Charge balance requires that equal amount of electrons flow from the positive electrode to the negative electrode in the external circuit. After reaching the negative electrode, lithium ions that obtain electrons are embedded in the lattice of the negative material. When discharging, the process goes in the opposite direction, that is, lithium ions leave the negative electrode lattice and embed the positive electrode to form LiCoO2 again.

Equation of charging and discharging process is:

Cathode reaction: LiCoO2? Li1 - xCoO2 + + xLi + xe

Negative reaction: xLi++xe+nC? LixCn

Battery response: LiCoO2+nC? Li1 - xCoO2 + LixCn

The working principle of lithium-ion batteries in addition to the oxidation - reduction, also based on electrochemical embedded - emergence response, namely the lithium-ion batteries in charging and discharging process, lithium (Li +) as an energy in the form of ion exchange of the carrier, through the electrolyte, the use of lithium ion embedded and emergence, swing between is negative, achieve the goal of energy exchange. Compared with other batteries, lithium ion battery has the advantages of high energy density, high average output voltage, high charging efficiency, low self-discharge efficiency, good safety performance, cycle and long service life.

2. Working principle of supercapacitors

During charging, electrons are transferred from the positive electrode to the negative electrode through an external power supply, so that the positive electrode and the negative electrode are positively and negatively charged respectively. Meanwhile, the positive and negative ions in the electrolyte solution body are separated and moved to the electrode surface and the charge layer on the electrode surface to face each other, forming a double layer. When discharging, 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 electrolyte solution body, and the double layer disappears at the same time. It can be seen that the double-layer capacitor USES the double-layer of electrode and electrolyte interface to store charge. The charging and discharging process is always a physical process without electrochemical reaction, so it has the advantages of stable performance, short charging and discharging time, long cycle life, high power density, good performance at high and low temperature and so on.

3. Working principle of lithium ion capacitor

The positive electrode material is the activated carbon material with double electric layer energy storage, the negative electrode material is the intercalated carbon material with lithium ion delocking function, and the electrolyte is the lithium salt electrolyte. When the battery is charged, the lithium ions are separated from the surface of the anode material and inserted into the lattice of the anode material after passing through the electrolyte and the membrane. When discharging, lithium ions come out of the lattice of the negative material and return to the surface of the positive material through the electrolyte, forming a double layer with the charge of the positive electrode. The negative electrode potential after lithium implantation is low, which has the characteristics of high service voltage, high energy density and power density between lithium ion battery and supercapacitor.

4. Advantages of lithium-ion capacitors over lithium-ion batteries and supercapacitors

(1) comparison of capacity, voltage and self-discharge

The energy density of li-ion capacitor is less than that of li-ion battery, but the output density is high. The energy density of the monomer volume is 10~15Wh/L, which is much larger than the capacity of 2~8Wh/L of the double-layer capacitor and twice of the latter.

In terms of voltage, the maximum voltage of a lithium ion capacitor can reach 4V, which is similar to that of a lithium ion battery, but much higher than that of a double-layer capacitor, and smaller than both in terms of self-discharge.

(2) security

The positive electrode of lithium ion battery is made of lithium oxide, which not only contains a large amount of lithium which can form lithium dendrites and Pierce the membrane, but also contains oxygen, an important element of ignition. Once the battery short circuit can be developed for the thermal decomposition of the whole, and the reaction with the electrolyte can cause combustion. But the positive pole of the lithium ion capacitor is activated carbon, even if the internal short circuit will react with the negative pole, but will not react with the electrolyte, in theory, will be much safer than the lithium battery.

(3) long life

In order to achieve a long life, lithium ion battery has a certain range limit on its charging and discharging depth, so as to reduce the capacity that can be used in essence. The charging and discharging principle of double-layer capacitor is that it has a long life simply by absorbing or removing the ions in the electrolyte, which is difficult to extend the actual life. However, even if the cathode potential of a lithium ion capacitor is reduced, the voltage of the unit itself will not drop significantly, so the capacity can be guaranteed.

(4) high temperature resistance

Under the condition of high temperature, the electrolyte and positive electrode are prone to oxidation and decomposition. Therefore, the positive electrode potential may need to be reduced under the condition of high temperature. But the lithium ion battery cannot reduce the voltage, easy to produce the safety question. Only lithium ion capacitors can be used in the positive electrode potential away from the oxidation decomposition area, so high temperature performance is excellent.

5. Application and industrialization status of lithium ion capacitors

The upstream of lithium ion capacitor industry mainly includes: anode and cathode raw materials, electrolyte, diaphragm, perforated collector fluid and simple metal lithium electrode, etc. The middle reaches mainly include lithium ion capacitor monomer of various shapes and specifications, and lithium ion capacitor monomer system integration module; The downstream is mainly the application demand of the terminal market. At present, the Japanese market is 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.

At present, these industries are mainly controlled by foreign companies. For example, Japan colorie and Japan ACT have mastered activated carbon and nano-carbon technology respectively. Japan kanebo company, Japan kureha chemical, Japan ATEC company, etc. FERRO from the United States and HONEYWELL from Germany dominate the electrolyte industry. The membrane was monopolized by NKK of Japan, and the porous collecting fluid was monopolized by 3 metal companies of Japan. At present, only a few companies in China are developing positive activated carbon and negative hard carbon materials for lithium ion capacitors.

References:

Yuan meirong, wang Chen, xu yongjin, et al. Research progress of lithium ion capacitor [J]. Materials guide, 2013,27 (21):140-149.

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