Infrared Investigation of Lithium Ion Electrolytes: Characterization of Structure and Dynamics via Linear and Nonlinear Spectroscopy

Lithium ion batteries are widely employed in energy storage, but the connection between the molecular interactions in their electrolytes and the macroscopic properties remains elusive. Across three vastly different electrolytes, speciation and dynamics were studied via linear and nonlinear infrared spectroscopy to shed light on this relationship. The impact of mixed solvation on ionic speciation was studied from the perspective of the anion, which revealed a significant energetic favorability for the formation of contact ion pairs in linear carbonate solvents over cyclic carbonates. Infrared spectroscopy and density functional theory calculations described a complete inversion of the speciation due to solvent composition from 74% free anion in the cyclic carbonate to 95% contact ion pair in the linear carbonate. The impact of adding a highly-fluorinated additive on the speciation of two different electrolytes was studied via linear infrared spectroscopy. These fluorinated co-solvents are typically considered inert species with no impact on the speciation, but the research presented opposite trends in infrared spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, electrochemistry, and conductivity for the two different electrolytes, indicating a change in speciation with co-solvent concentration. To explain the interesting dependence of the change in speciation on the anion, density function theory (DFT) calculations showed the formation of weak hydrogen bonds between the TFSI- anion and the co-solvent; this interaction is not observed with the PF6- anion. Finally, a polyacrylonitrile polymer gel electrolyte was investigated. The results from linear spectroscopy showed different interactions in the electrolyte with increasing polymer concentration; polymer addition also raised the viscosity of the sample by orders of magnitude, changing the sample composition from a liquid electrolyte to a room-temperature gel. Interestingly, the dynamics determined from 2DIR spectroscopy are similar across sample. A molecular picture was proposed of the nitrile side- vii chains interacting at the highest polymer concentration, forming channels to facilitate the flow of ions; these interactions were confirmed via differential scanning calorimetry. This synergistic approach enabled the complete characterization of these complex systems from multiple perspectives to fully understanding the unintuitive way the molecular interactions can alter the macroscopic properties