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 and wind energy. Its working principle is different from lithium ion batteries and super capacitors.
1, The working principle of lithium-ion battery
Lithium-ion batteries are the fastest-growing secondary batteries after cadmium-nickel and nickel-hydrogen batteries. The positive and negative active materials of the lithium ion battery are all compounds capable of reversibly intercalating-deintercalating lithium, and at least one of the electrode materials is in a lithium intercalation state before assembly, such as transition metal oxides LiCoO2, LiNiO2, and LiMn2O4 as positive electrodes. Various carbon materials, metal oxides or alloys are used as the negative electrode material.
The working principle of the lithium ion battery is shown in Fig. 1, with graphite as the negative electrode and LiCoO2 as the positive electrode. During charging, lithium ions are removed from the positive electrode material and migrated to the negative electrode through the electrolyte under the gradient of the electrochemical potential. The charge balance requires that the same amount of electrons flow from the positive electrode to the negative electrode under the external circuit, and the lithium ion is obtained after reaching the negative electrode. It is embedded in the lattice of the negative electrode material; when discharging, it is carried out in the opposite process, that is, lithium ions leave the negative electrode lattice, and the positive electrode is embedded to re-form LiCoO2.
The charging and discharging process is:
Positive electrode reaction: LiCoO2?Li1-xCoO2+xe+xLi+
Negative reaction: xLi++xe+nC?LixCn
Battery reaction: LiCoO2+nC?Li1-xCoO2+LixCn
In addition to "oxidation-reduction", the working principle of lithium-ion battery is also based on electrochemical insertion-extraction reaction, that is, lithium ion battery in the form of ion exchange (Li+) as a carrier for energy exchange during charge and discharge, through the electrolyte, The use of lithium ion insertion and extraction, 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, long cycle and long service life.
2, The working principle of super capacitor
During charging, electrons pass from the positive electrode to the negative electrode through the external power source, so that the positive electrode and the negative electrode are positively and negatively charged, respectively, and the positive and negative ions in the body of the electrolyte solution are separated and moved to the surface of the electrode and the charge layer on the surface of the electrode to form an electric double layer. When discharging, electrons flow from the negative electrode to the positive electrode through the load, and positive and negative ions are released from the surface of the electrode and return to the body of the electrolyte solution, while the electric double layer disappears. It can be seen that the electric double layer capacitor uses the electric double layer of the electrode and the electrolyte interface to store the electric charge. The charging and discharging process is always a physical process, and no electrochemical reaction occurs, so the performance is stable, the charging and discharging time is short, the cycle life is long, and the power is high. The advantages of high density, good high and low temperature performance, etc.
3. Working principle of lithium ion capacitor
The positive electrode material is an activated carbon material having an electric double layer energy storage, the negative electrode material is an intercalated carbon material having a lithium ion deintercalation function, and the electrolyte is a lithium salt electrolyte. When the battery is being charged, the lithium ions are separated from the surface of the positive electrode material and inserted into the crystal lattice of the negative electrode material after passing through the electrolyte and the separator; when discharging, lithium ions are extracted from the crystal lattice of the negative electrode material, and returned to the positive electrode material through the electrolyte. The surface forms an electric double layer with the charge of the positive electrode. The lithium-ion negative electrode has a low potential and has a high use voltage, energy density and power density between the lithium ion battery and the super capacitor.