The structure of supercapacitors is similar to that of lithium-ion batteries, and they are assembled in the order of positive electrode/separator/negative electrode. The positive and negative electrodes are all formed by coating the active material on the current collector and then drying and compacting. The separator is placed between the positive and negative electrodes to avoid short-circuit between the positive and negative electrodes, and the electrolyte serves as a carrier for ion transmission.
With the interdisciplinary research of supercapacitors and the widening of application scenarios, the types of supercapacitors are increasing.
According to the principle of energy storage: electric double layer capacitors, Faraday pseudocapacitors, and hybrid capacitors; according to electrode materials: carbon-based capacitors, metal oxide capacitors and conductive polymer capacitors; according to whether the electrode materials are the same electrode material : Symmetrical capacitors and hybrid capacitors; distinguished according to electrolyte types: there are water-based electrolyte capacitors and non-aqueous electrolyte capacitors, among which water-based electrolyte capacitors are divided into acidic, neutral and alkaline water-based electrolyte capacitors.
Unlike lithium-ion batteries that allow lithium ions to be repeatedly inserted and extracted from the positive and negative electrodes to store and release energy through redox reactions, the basis of the energy storage mechanism of supercapacitors is the electric double layer theory.
The external electric field causes the positive and negative ions in the electrolyte to be aligned on the solid-liquid interface between the negative electrode and the positive electrode of the capacitor. When charging, the surface of the positive and negative plates of the capacitor accumulates excess charge, and the opposite charge in the electrolyte will be on the positive and negative electrodes. The solid-liquid interface is oriented and arranged to form an electric double layer; during discharge, the positive and negative plates undergo charge transfer through the conductive external circuit, and the excess charge is reduced, and the corresponding opposite charges at the solid-liquid interface return to the electrolyte, thereby realizing energy Storage and release.
The energy storage mechanism of Faraday pseudocapacitors is not only the energy storage method of electric double layer capacitors, but also the redox energy storage method, that is, the ions are adsorbed on the surface of the active material on the positive and negative plates or embedded in the active material, and the surrounding material Redox reactions occur to achieve energy storage. This method has no phase change of materials, which is different from the insertion and extraction of lithium ions between the positive and negative electrodes in a lithium ion battery.
Based on the above principles, compared to traditional rechargeable batteries, supercapacitors have their own unique characteristics in terms of electrode material requirements, including high specific energy, high specific power, and long cycle life.
In terms of the characteristics of the material itself, it is required to have a high specific surface area and good electrical conductivity and adsorption performance. Carbon-based electrode materials have the above advantages, and are rich in raw materials, simple in preparation process, easy to adjust the pore state, stable chemical properties, good thermal and electrical conductivity, high specific surface area, excellent cycle performance, and can be used as a substrate with metal oxides and metal oxides. Conductive polymer composites are currently the most widely used and commercialized electrode materials for supercapacitors.