Lynn Trahey

Investigation of fluoroethylene carbonate effects on tin-based lithium-ion battery electrodes.

Electroless plating of tin on copper foil (2-D) and foams (3-D) was used to create carbon- and binder-free thin films for solid electrolyte interphase (SEI) property investigation. When electrochemically cycled vs lithium metal in coin cells, the foam electrodes exhibited better cycling performance than the planar electrodes due to electrode curvature. The effect of the additive/cosolvent fluoroethylene carbonate (FEC) was found to drastically improve the capacity retention and Coulombic efficiency of the cells. The additive amount of 2% FEC is enough to derive the benefits in the cells at a slow (C/9) cycling rate. The interfacial properties of Sn thin film electrodes in electrolyte with/without FEC additive were investigated using in situ electrochemical quartz crystal microbalance with dissipation (EQCM-D). The processes of the decomposition of the electrolyte on the electrode surface and Li alloying/dealloying with Sn were characterized quantitatively by surface mass change at the molecular level. FEC-containing electrolytes deposited less than electrolyte without FEC on the initial reduction sweep, yet increased the overall thickness/mass of SEI after several cyclic voltammetry cycles. EQCM-D studies demonstrate that the mass accumulated per mole of electrons (mpe) was varied in different voltage ranges, which reveals that the reduction products of the electrolyte with/without FEC are different.

In situ high-energy synchrotron X-ray diffraction studies and first principles modeling of α-MnO2 electrodes in Li–O2 and Li-ion coin cells

Despite their technological challenges, non-aqueous rechargeable lithium–oxygen cells offer extremely high theoretical energy densities and are therefore attracting much attention in a rapidly emerging area of electrochemical research. Early results have suggested that, among the transition metal oxides, alpha manganese dioxide (α-MnO2) appears to offer electrocatalytic properties that can enhance the electrochemical properties of Li–O2 cells, particularly during the early cycles. In this study, we have investigated the hybrid Li-ion/Li–O2 character of α-MnO2 electrodes in Li–O2 coin cells by in situ high-energy synchrotron X-ray diffraction, and compared the results with conventional Li/α-MnO2 coin cells assembled under argon. Complementary first principles density functional theory calculations have been used to shed light on competing lithium insertion and lithium and oxygen insertion reactions within the α-MnO2 tunnel structure during discharge, relative to lithium peroxide or lithium oxide formation.

In situ synchrotron X-ray diffraction studies of lithium oxygen batteries

Lithium oxygen batteries were studied in situ by synchrotron X-ray diffraction. Crystalline Li2O2 was shown to form reversibly on a plain carbon cathode under normal cycling conditions. However, if the cell was polarized to induce electrolyte decomposition before the oxygen reduction reaction was initiated, LiOH was found to cycle reversibly on the cathode instead. A mechanism linking the LiOH production to the electrolyte decomposition was proposed.

Conjugated Polymer Energy Level Shifts in Lithium-Ion Battery Electrolytes

The ionization potentials (IPs) and electron affinities (EAs) of widely used conjugated polymers are evaluated by cyclic voltammetry (CV) in conventional electrochemical and lithium-ion battery media, and also by ultraviolet photoelectron spectroscopy (UPS) in vacuo. By comparing the data obtained in the different systems, it is found that the IPs of the conjugated polymer films determined by conventional CV (IPC) can be correlated with UPS-measured HOMO energy levels (EH,UPS) by the relationship EH,UPS = (1.14 ± 0.23) × qIPC + (4.62 ± 0.10) eV, where q is the electron charge. It is also found that the EAs of the conjugated polymer films measured via CV in conventional (EAC) and Li(+) battery (EAB) media can be linearly correlated by the relationship EAB = (1.07 ± 0.13) × EAC + (2.84 ± 0.22) V. The slopes and intercepts of these equations can be correlated with the dielectric constants of the polymer film environments and the redox potentials of the reference electrodes, as modified by the surrounding electrolyte, respectively.