Probing Interfacial Processes of Lithium Ion Batteries

In the last decade, lithium ion batteries held a major role in the path towards personal electronics due to being lightweight and providing a high energy density. However, several problems have been identified with lithium ion batteries. Due to inherent instability, lithium ion batteries are known to have issues with safety and capacity loss. Our goal is to advance the understanding of the electrochemical processes, specifically the interfacial processes at the anode, to continue their advancement in our electronic age. At the interface of the electrolyte and anode, during the first several charging and discharging cycles, appears a protective layer by interaction of decomposed electrolyte at the electrode surface. This protective layer, termed the solid electrolyte interphase, is of particular importance as it increases the stability, impeding dendrite growth, and ultimately leading to improved capacity and safety. Our electrolyte is a lithium salt (LiClO4) with ethylene carbonate (EC) in a tetrahydrofuran (THF) solvent, leading, primarily, to one of the main SEI contributors, lithium ethylene dicarbonate (LiEDC). By spectroscopically probing the interface with sumfrequency generation and simultaneously scanning with cyclic voltammetry, we are able to see the SEI contribution formation in real time.Ope

Modifying Vibrational Energy Flow in Aromatic Molecules: Effects of Ortho Substitution

Ultrafast infrared (IR) Raman spectroscopy was used to measure vibrational energy transfer between nitrobenzene nitro and phenyl groups, in the liquid state at ambient temperature, when ortho substituents (−CH₃, −F) were introduced. Quantum chemical calculations were used to assign the vibrations of these molecules to three classes, phenyl, nitro, or global. Combining transient anti-Stokes and Stokes Raman spectra determined the energies of multiple molecular vibrational modes, which were summed to determine the aggregate energies in the phenyl, nitro, or global modes. In a previous study (Pein, B. C. ; Sun, Y.; Dlott, D. D., J. Phys. Chem. A 2013, 117, 6066−6072) it was shown that, in nitrobenzene, there was no energy transfer from nitro to phenyl or from nitro to global modes, but there was some transfer from phenyl to nitro and phenyl to global. The ortho substituents activated energy flow from nitro-to-phenyl and nitro-to-global and reduced phenyl-to-nitro flow. The −CH₃ substituent entirely shut down the phenyl-to-nitro pathway, presumably by efficiently directing some of the phenyl energy into methyl bending excitations. There is (inefficient) unidirectional vibrational energy flow in nitrobenzene only in the nitro-to-phenyl direction, whereas in <i>o</i>-nitrotoluene, vibrational energy flows only in the nitro-to-phenyl direction

Structural Transition in an Ionic Liquid Controls CO₂ Electrochemical Reduction

Broad-band multiplex vibrational sum-frequency generation spectroscopy (SFG) was used to study CO₂ reduction on a polycrystalline Ag electrode with a room-temperature ionic liquid (RTIL) electrolyte, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF₄), with 0.3 mol % water. The Ag/RTIL/H₂O system has been shown to reduce CO₂ with low overpotential and, depending on water concentration, with Faradaic efficiency of nearly 100% (Rosen, B. A.; Salehi-Khojin, A.; Thorson, M. R.; Zhu, W.; Whipple, D. T.; Kenis, P. J. A.; Masel, R. I. <i>Science</i> <b>2011</b>, <i>334</i>, 643–644). The adsorbed CO created by CO₂ reduction was probed with infrared (IR) pulses tuned to the CO stretch. Nonresonant (NR) SFG was used to probe the double layer. SFG showed that CO binds weakly to Ag at the CO₂ reduction threshold of −1.33 V (vs Ag/AgCl), so CO does not poison the surface. At potentials equal to or more negative than the threshold, the curvature of the parabolic potential-dependent NR intensity significantly increased, and the Stark shift of adsorbed CO, a measure of the surface field, more than doubled. The curvature increase indicates a potential-driven structural transition in the RTIL within the double layer. This transition was a property of the RTIL itself since it occurred whether or not CO₂ was present. Significantly, the RTIL transition and the increased surface field occurred precisely at the CO₂ reduction threshold. Thus, we have demonstrated a close association between an electrochemically driven structural transition of the RTIL and low overpotential CO₂ reduction

SURFACES AND INTERFACES OF HIGH EXPLOSIVES PROBED BY NONLINEAR OPTICAL SPECTROSCOPY

Author Institution: SCHOOL OF CHEMICAL SCIENCES, UNIVERSITY OF ILLINOIS URBANA CHAMPAIGN, 600 S MATTHEWS, URBANA, IL 61801The surfaces and interfaces of plastic explosives (PBX 9501) were studied using vibrational sum-frequency generation spectroscopy (SFG). SFG is a surface and interface selective nonlinear optical spectroscopy technique with which we study the interfaces within plastic explosive. We are interested in the explosive crystal to plastic binder interface and also the explosive crystal to explosive crystal interface. We initially studied the surfaces of the components of plastic explosives i.e. HMX crystals and a plastic binder, Estane. Our initial results showed that solution grown $\beta$-HMX crystals has small deposits of the $\delta$-HMX isomorphs on its surface. We also found that rapid evaporation from droplets of HMX solution produces nanocrystals of only $\delta$-HMX; presumably because the polar boat conformation of $\delta$-HMX is stabilize by polar solvents. A detail study of the surface vibrational modes of $\beta$-HMX was also carried out on a cleaved HMX crystal