Spectroscopic and optical diagnostics for investigations of liquid electrolytes

Abstract

In this work, optical diagnostics are developed to investigate the electrochemical reactions and materials limitations of new battery technologies through in-situ and in-operando identification of species and measurements of species concentrations in liquid electrolytes. Room Temperature Ionic Liquids (RTILs) are a promising type of new electrolyte material. RTILs have large electrochemical windows, low vapor pressures, and higher thermal stabilities than many organic electrolytes. The physical and electrochemical properties of RTILs are highly dependent on purity of the RTIL, and as a result considerable expense is added to the synthesis process by purification steps. Here, the first quantitative optical diagnostics are presented for heated RTILs. These quantitative spectra are used to make in-situ spectroscopic measurements of the rate of decomposition of heated [EMIM][EtSO4]. This study is the first in-situ optical investigation of the thermal breakdown of RTILs. Quantitative UV-vis spectral data of [EMIM][TFSI] and a common impurity, MIM, were applied to develop an low-cost optical sensor to provide process control for the industrial production of [EMIM][TFSI]. Custom hardware was constructed and tested in order to establish a calibration curve and limit of detection for the sensor. Quantitative spectroscopic techniques were also applied to measurements in a Na-Cu-I- battery. Quantum chemical calculations were performed to estimate the UV-vis absorption spectrum of the CuI2- ion. A custom optically accessible transmission cell has been designed and tested using both electrochemical methods and UV-vis spectroscopy. The quantitative spectral data are used to develop optical diagnostics for in-operando quantitative measurements of species concentrations in the cell. Comparison of the optical concentration measurements and the cell current reveals the formation of CuI2- is slow compared to the formation of CuI.