Why should we improve the ion conductivity of polymer battery electrolytes?

　　Lithium polymer batteries have been widely developed and studied as a rechargeable, high specific energy device. These batteries are composed of lithium or carbon negative electrodes (anodes), corresponding high potential embedded positive electrodes (cathodes), a polymer electrolyte. Due to the application of polymer electrolytes, it is possible to develop a battery with higher safety and the ability to twist and deform at will. The main requirements for the performance of polymer electrolytes in lithium-ion batteries are high ion conductivity, stable electrochemical and thermal properties, flexible mechanical properties, and high mechanical strength. The main parameters that characterize the performance of electrolytes include ion conductivity, electrochemical stability window, and lithium ion migration number.

　　To improve the ion conductivity of polymer battery electrolytes, it can be achieved by increasing the number of charged particles in the polymer and the migration rate of charged particles. Lithium salts such as LiCIO, LiBF4, LiN (CFaSOz) 2, LiSCN, LiC, and Coa generally have high ionic conductivity in polymers with high dielectric constants due to their low dissociation energy. The use of composite carbon electrodes and AMS based colloidal polymers as electrolytes in lithium-ion batteries has shown excellent performance.

　　Increasing the migration rate of charged particles can also improve the ion conductivity of polymers while maintaining their dielectric constant. The more lithium salts dissociate into free ions in the electrolyte, the faster the ion migration, the higher the conductivity, the larger the dielectric constant of the solvent, the smaller the electrostatic interaction between lithium ions and anions, and the more free ions there are. But solvents with high dielectric constants also have high viscosity, which can actually slow down the migration rate of ions.

　　For solutes, as the concentration of lithium salts increases, the conductivity increases, but the electrolyte viscosity also increases accordingly; In addition, the larger the radius of anions in lithium salts, the smaller the lattice energy, and the easier the lithium salts are to dissociate, but the viscosity also increases accordingly. Due to the interaction of the above factors, the maximum value of conductivity in a specific electrolyte is generally between 1.1-1.2 mol • I lithium salt concentration. Therefore, a solvent with a high dielectric constant can be mixed with one or several solvents with low viscosity, and the electrolyte with higher conductivity in polymer lithium-ion batteries can be obtained by adjusting the ratio (volume ratio) of each component.

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