Showing papers by "Carl V. Thompson published in 2014"

Chemical Instability of Dimethyl Sulfoxide in Lithium-Air Batteries.

Abstract: Although dimethyl sulfoxide (DMSO) has emerged as a promising solvent for Li-air batteries, enabling reversible oxygen reduction and evolution (2Li + O2 ⇔ Li2O2), DMSO is well known to react with superoxide-like species, which are intermediates in the Li-O2 reaction, and LiOH has been detected upon discharge in addition to Li2O2. Here we show that toroidal Li2O2 particles formed upon discharge gradually convert into flake-like LiOH particles upon prolonged exposure to a DMSO-based electrolyte, and the amount of LiOH detectable increases with increasing rest time in the electrolyte. Such time-dependent electrode changes upon and after discharge are not typically monitored and can explain vastly different amounts of Li2O2 and LiOH reported in oxygen cathodes discharged in DMSO-based electrolytes. The formation of LiOH is attributable to the chemical reactivity of DMSO with Li2O2 and superoxide-like species, which is supported by our findings that commercial Li2O2 powder can decompose DMSO to DMSO2, and that the presence of KO2 accelerates both DMSO decomposition and conversion of Li2O2 into LiOH.

Impact of Water-Assisted Electrochemical Reactions on the OFF-State Degradation of AlGaN/GaN HEMTs

Abstract: The origin of structural and electrical degradation in AlGaN/GaN high-electron mobility transistors (HEMTs) under OFF-state stress was systematically studied. Hydroxyl groups (OH-) from the environment and/or adsorbed water on the III-N surface, were found to play an important role in the formation of surface pits during OFF-state electrical stress. The mechanism of this water-related structural degradation is explained by an electrochemical cell formed at the gate edge where gate metal, the III-N surface, and the passivation layer meet. The relationship between structural and electrical degradation in AlGaN/GaN HEMTs under OFF-state stress is discussed. Specifically, the permanent decrease in the drain current is directly linked with the formation of the surface pits, while the permanent increase in the gate current is found to be uncorrelated with the structural degradation.

Correlation of shape changes of grain surfaces and reversible stress evolution during interruptions of polycrystalline film growth

Abstract: Short interruptions of the growth of polycrystalline films often lead to stress evolution that is reversed when growth is resumed. Correlated in situ stress measurements and ex situ transmission electron microscopy and atomic force microscopy characterizations of grain boundary surface grooves as a function of the interruption time are reported for films deposited at different temperatures and held for different times before quenching to room temperature. These studies suggest that during film deposition surface grooves at grain boundaries are kinetically constrained to be shallow, while during a growth interruption surface diffusion allows grain boundary grooves to deepen and approach their equilibrium depth. The latter relieves a component of the compressive stress associated with trapped atoms in the grain boundaries. When growth is resumed, the non-equilibrium surface morphology is reestablished and the compressive stress increases to its pre-interruption value.

Fast and slow stress evolution mechanisms during interruptions of Volmer-Weber growth

Abstract: The evolution of mechanical stress during Volmer-Weber growth of thin films is complex, often including a reversible stress evolution during interruptions of film deposition. The underlying mechanism for stress evolution during growth interruptions has been extensively debated, but remains unclear. In this work, in situ measurements of stress evolution during growth interruptions of various time scales, film thicknesses, and substrate temperatures were made during deposition of gold and nickel films. It was found that at least two mechanisms lead to the observed stress evolution, one fast (time constant ∼102 s) and one slow (time constant ∼104 s). The fast process is reversible and weakly dependent on the film thickness, while the slow process is irreversible and strongly dependent on the film thickness. It is shown that grain growth during growth interruptions can account for a significant portion of the stress change associated with the slow process. The fast reversible process is likely to be associated with reversible changes of the surface structure.

On the redox origin of surface trapping in AlGaN/GaN high electron mobility transistors

Abstract: Water-related redox couples in ambient air are identified as an important source of the surface trapping states, dynamic on-resistance, and drain current collapse in AlGaN/GaN high electron mobility transistors (HEMTs). Through in-situ X-ray photoelectron spectroscopy (XPS), direct signature of the water-related species—hydroxyl groups (OH) was found at the AlGaN surface at room temperature. It was also found that these species, as well as the current collapse, can be thermally removed above 200 °C in vacuum conditions. An electron trapping mechanism based on the H2O/H2 and H2O/O2 redox couples is proposed to explain the 0.5 eV energy level commonly attributed to the surface trapping states. Finally, the role of silicon nitride passivation in successfully removing current collapse in these devices is explained by blocking the water molecules away from the AlGaN surface.

The Kinetics and Product Characteristics of Oxygen Reduction and Evolution in LiO2 Batteries

Abstract: Understanding the origin of substantial performance challenges limiting the practical development of Li–O2 batteries, such as low rate capability, limited cycle life (<100 cycles), and the large voltage polarization (0.6–1 V) on charge, requires improved understanding of chemical, electrochemical, morphological, and electronic processes occurring in the electrode. This chapter highlights current understanding of how the kinetics and reaction product characteristics in Li–O2 batteries during discharge and charge influence performance characteristics at the cell level. First, a brief overview of energy and power of various Li–O2 electrodes reported in the literature to date is presented for a range of O2 electrode materials and designs as a benchmark for what has been achieved at the laboratory scale. Next, we review chemical and morphological understanding of the oxygen reduction (discharge) process, with a particular focus on nanostructured carbon electrodes in 1,2-dimethoxyethane (DME) electrolyte. The kinetics of oxygen reduction and the influence of kinetics on the morphology and shape evolution of Li2O2 are discussed, including recent insights into the microscale structure and proposed growth mechanisms of “toroidal” crystalline Li2O2 at low currents or overpotentials. We next discuss the surface chemistry of discharged oxygen electrodes, including the morphology-dependent surface chemistry of Li2O2, reactivity between Li2O2 and the carbon electrode, reactivity between Li2O2 and ether-based electrolytes, and resulting parasitic products that form upon discharge and during subsequent cycling. In light of chemical instabilities present nearly universally in liquid cells, we highlight recent work utilizing in situ ambient pressure XPS (APXPS) to examine Li–O2 electrochemistry during battery operation in an all-solid-state cell. Finally, we discuss the influence of morphology and surface chemistry of the discharge product on the charging kinetics in carbon-nanostructured electrodes, where morphology-dependent Li2O2 surface chemistry and structure are found to significantly influence the overpotential required during oxidation. Combined chemical, electrochemical, morphological, and electronic understanding is increasingly important as researchers seek to develop improved O2 electrodes with increased round-trip efficiency and improved chemical/electrochemical reversibility approaching what is needed for practical devices.