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Systematic Characterization of Ionic Liquid Electrolyte Systems for Lithium Ion Batteries
Conventional lithium ion batteries are multi-component systems with the anode and cathode active materials determining the voltage and theoretical energy content. In addition, current electrolyte systems of lithium ion batteries comprise a lithium salt and a mixture of carbonate solvents which to date, have allowed for the use of cathodes with 4V operating windows. These electrolyte systems are relatively conductive at 5-10 mS/cm, and many of them form stable solid-electrolyte interphase layers to promote effective battery operation. However, carbonate-based electrolytes suffer from poor thermal stability and an upper limit of electrochemical stability of 4.5V vs. Li/Li+. Expansion of the voltage window is needed in order to allow use of higher voltage cathodes, above 4.5 V.
Results and Discussion
The study of ionic liquids for use in energy storage applications is a relatively recent occurrence; however, they are prospective candidates because of their potential for increased electrochemical stability and thermal safety. Many ionic liquids have been investigated in neat and mixed solutions to find an optimal balance of viscosity, conductivity, electrochemical stability, and thermal safety. Mixtures with carbonate based organic solvents have also proved promising for incrementally changing electrolyte systems toward more favorable performance using currently studied ionic liquids.
In recent work physical and electrochemical properties of ionic liquids were systematically investigated.[1, 2] Cation type, anion type, and substituent chain length effects on conductivity, viscosity, electrochemical stability, and wettability as determined by cyclic voltammetry, impedance, and contact angle were explored. In addition, mixtures of ionic liquids with organic carbonate solvents similar to those used in conventional liquid electrolytes were prepared and similar characterization was completed. Further characterization via calorimetry and contact angle measurements can support the understanding of ionic liquid behavior and give insight into continued optimization of electrolyte systems using these materials.
References
1. Di Leo, R.A.; Marschilok, A.C.; Takeuchi, K.J. and Takeuchi, E.S., "Battery electrolytes based on saturated ring ionic liquids: Physical and electrochemical properties," Electrochimica Acta, 2013, 109(0),27-32.
2. Di Leo, R.A.; Marschilok, A.C.; Takeuchi, K.J. and Takeuchi, E.S., "Battery Electrolytes Based on Unsaturated Ring Ionic Liquids: Conductivity and Electrochemical Stability," Journal of The Electrochemical Society, 2013, 160(9), A1399-A1405.