Investigating Ion Solvation in Electrolyte Solutions via X-ray Absorption Spectroscopy
X-ray Absorption Spectroscopy (XAS) is a powerful atom-selective probe of the local chemical environment of a target atom. Streaming liquid microjets, created by forcing liquid at high pressures (ca. 20-100 atm.) through a capillary tube of inner diameter 30-100 µm, allow for liquids to be introduced into a vacuum chamber and interrogated with soft X-rays generated by synchrotron light sources. This technique has enabled the study of many liquids and solutions. In this dissertation, the use of soft X-ray XAS for the study of the solvation of ions in electrolyte solutions is discussed. The majority of chemical reactions occur in the solution phase. The thermodynamics and kinetics of these reactions are explicitly dependent upon the local chemical environments of the reactants. Consequently, knowledge of the solvation of ions in solution is critical to understanding their chemistry in this phase. By interpreting XAS measurements with ab initio Density Functional Theory (DFT)-based electronic structure calculations, it is possible to acquire valuable knowledge about the local chemical environment of ions and molecules in solution, such as the solvation number and geometry and propensity to form ion pairs.
Commercial lithium ion batteries typically contain a liquid electrolyte comprising a lithium salt dissolved in a mixture of alkyl carbonates such as propylene carbonate, dimethyl carbonate, and ethylene carbonate. The solvation environment of the lithium ion in these solutions is thought to direct the formation of the Solid Electrolyte Interphase (SEI), which in turn is believed to play a critical role in determining essential cell properties including power output, recharging rate, and cycle life. Consequently, understanding this solvation environment is of critical importance. Here I report the a study of the solvation of Li+ in solutions of LiBF4 in alkyl carbonates via XAS on the carbon and oxygen K-edges. In collaboration with Dr. David Prendergast, we have performed electronic structure calculations within the eXcited electron and Core Hole (XCH) approximation and extracted a solvation number for Li+ in propylene carbonate.
I also report the study of aqueous solutions sodium nitrate and nitrite on the nitrogen K-edge utilizing similar experimental and theoretical methods. Aqueous nitrate and nitrite salts are important commercial reagents and play a critical role in the global nitrogen cycle. A detailed understanding of their chemical environment of solution would be beneficial in understanding and modeling reactivity. Unfortunately, the nitrogen K-edge XAS spectra of these species are found to be largely insensitive to ion pairing and solvation geometry around the anion, such that limited chemical information was extracted from this study.