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2018

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Application of Organic Electrode Materials in Rechargeable Sodium Ion Batteries

Sodium ion batteries are considered as potential large-scale energy storage battery systems due to their abundant resources and low cost. Compared with the inorganic electrode materials reported extensively at present, organic materials have the following advantages. Firstly, organic electrode materials are usually prepared by mild methods, such as substitution or polymerization at room temperature or below 200 C, which can reduce energy consumption and carbon dioxide emissions in the process of electrode preparation; secondly, most organic electrode materials can be derived from natural products or their derivatives. Moreover, organic electrode materials are based on charge transfer reaction of redox centers and can withstand large radius of sodium ions. Most importantly, through reasonable design, the specific capacity of organic cathode materials can be close to 500 amperes per kilogram, which is much higher than the reported inorganic cathode materials. Despite these advantages, organic electrode materials also have many problems to be solved, such as the solubility of active materials in organic electrolytes, poor conductivity and low voltage.
 
At present, organic electrode materials used in sodium ion batteries are mainly based on carbon-oxygen double bond reaction, doping reaction and carbon-nitrogen double bond reaction. Among them, the electrode materials based on carbon-oxygen double bond reaction mainly include quinones, carboxylates, anhydrides and amides. These compounds have high capacity and stable cycling performance, and are currently the most widely studied. The electrode materials based on doping reaction mainly include organic radical compounds, conductive polymers, microporous polymers, organic metal compounds and so on. The p-doping reaction is usually participated by anions in the electrolyte, and the working potential is generally higher than 3 V. The N-doping reaction is participated by cations in the electrolyte, and the working potential is generally below 2V. Compounds based on Carbon-Nitrogen double bond reaction mainly include Schiff base, pyridine derivatives and so on. At present, there are few studies on this kind of electrode material, and its working principle needs further exploration. In addition, through a series of design, the voltage, specific capacity, solubility, conductivity and other parameters of organic electrode materials can be reasonably regulated. For example, the theoretical specific capacity of electrode materials can be increased by increasing the proportion of active functional groups in molecules. By adjusting the energy of the lowest unoccupied orbital level of organic molecules, the voltage of materials can be effectively regulated. Electron-withdrawing groups can increase the working potential of materials, while electron-donating groups can reduce the working potential of materials. Polymerization of organic molecules can effectively inhibit the dissolution of electrode materials in electrolyte. Compound with carbon material can improve the conductivity of the material and promote the increase of the rate performance of the electrode material.

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