Main components of electrolyte in sodium ion battery

Main components of sodium ion battery electrolyte: Sodium ion battery electrolyte can be divided into liquid and solid according to its existing state. Similar to lithium ion batteries, the liquid electrolyte used in sodium ion batteries is also dissolved in organic solvents through sodium salts, which can generally be NaPF6, Na-ClO4, NaAIClh, NaFeClh, NaSO, CF3, NaBF4, NaBClh, NaNO3, NaPOFA, NaSCN, NaCN, NaAsF6, NaCF3CO2, NaSbF6, NAC. Organic solvents have the following requirements: high dielectric constant, low melting point (liquid at room temperature) and strong conductivity of sodium ions. In order to meet the above requirements, the electrolyte solvent is generally an anhydrous binary component, and its components can be ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), 1, 2- dimethoxyethane (DME), tetrahydrofuran (THF), 2- methyltetrahydrofuran (2-MTHF) and so on. In the final electrolyte, the molar concentration of Na+ is about 1 mole/liter ... The liquid electrolyte configuration of sodium ion battery is high (anhydrous), which is easy to leak and unsafe (for example, dendrite is generated at the negative electrode of simple metal, which leads to short circuit and explosion). Especially when sodium is used as the negative electrode material, the reaction between sodium and liquid electrolyte makes the development of this battery difficult. It is a scheme to use alloy anode, but it is difficult for sodium ions to diffuse in the alloy, and the volume changes obviously after repeated cycles. Another solution is to improve the electrolyte, that is, add additives while selecting suitable solvents. But people are also looking for new electrolyte materials, and polymer electrolyte, which has developed rapidly in recent years, is a typical example. Generally speaking, the so-called polymer electrolyte is to mix salt substances into polymers in the form of doping to make conductive (mainly ionic) polymers. Common polymers used as solid polymer electrolyte (SPE) for sodium ion batteries include polyethylene oxide, polyaniline, polypyrrole, ethylene acrylic polymer, polytetrafluoroethylene and so on. According to different polymer configurations, they can form different kinds of polymer electrolytes, such as linear polymer electrolytes, comb polymer electrolytes and cross-linked network polymer electrolytes. The alkali metal salt is Nal, NaBH4, NaBF4, sodium polyphosphate, etc. Generally, they are large-volume anions with negative charges. When developing new salts in the future, we can consider: ① having a wide electrochemical window; (2) forming a low melting point composite material with a polymer matrix; ③ It has symmetrical or supple anionic structure and plasticizing effect. The conductivity of this kind of polymer composites may be the result of competition among three mechanisms: conductive channel, tunneling effect and field emission. In the discovered PEO- sodium borohydride system, the ionic conductivity decreases due to the hindrance of anion pairing. In order to meet the conductive needs of rechargeable batteries, the ionic conductivity of SPE should be above 10-3S/cm. However, the ionic conductivity of SPE should be above 10-3S/cm to meet the needs of salt pond conductivity. However, the ionic conductivity of the solid polymer electrolyte obtained after salt doping can not reach this level. Therefore, the future research work in this field should focus on developing solid polymer electrolytes with high ionic conductivity and stable anode and cathode materials. Nasicon is also a sodium ion conductor developed in recent ten years. It is a composite electrolyte composed of five elements: sodium, copper, silicon, phosphorus and oxygen. U.S. patent reports that a very thin solid electrolyte can be prepared by mixing Na3Zr2Si2PO 12 powder with Teflon. Common sodium sulfate-based solid electrolytes and na3x+2y+zpcoyclz (0 ≤ x, y, z ≤1; Only one of x, y and z is 0) is also a fast ion conductor used at medium and high temperatures. In order to be used in a new type of secondary sodium ion battery, this solid electrolyte should have high ionic conductivity at room temperature and be easy to prepare. The development of three-dimensional sodium ion conductor with SiO2 _ 2 skeleton is close to this goal, but it has not been applied in sodium ion batteries.