A. C. Luntz

Solvents' Critical Role in Nonaqueous Lithium-Oxygen Battery Electrochemistry.

Among the many important challenges facing the development of Li-air batteries, understanding the electrolytes role in producing the appropriate reversible electrochemistry (i.e., 2Li(+) + O2 + 2e(-) ↔ Li2O2) is critical. Quantitative differential electrochemical mass spectrometry (DEMS), coupled with isotopic labeling of oxygen gas, was used to study Li-O2 electrochemistry in various solvents, including carbonates (typical Li ion battery solvents) and dimethoxyethane (DME). In conjunction with the gas-phase DEMS analysis, electrodeposits formed during discharge on Li-O2 cell cathodes were characterized using ex situ analytical techniques, such as X-ray diffraction and Raman spectroscopy. Carbonate-based solvents were found to irreversibly decompose upon cell discharge. DME-based cells, however, produced mainly lithium peroxide on discharge. Upon cell charge, the lithium peroxide both decomposed to evolve oxygen and oxidized DME at high potentials. Our results lead to two conclusions; (1) coulometry has to be coupled with quantitative gas consumption and evolution data to properly characterize the rechargeability of Li-air batteries, and (2) chemical and electrochemical electrolyte stability in the presence of lithium peroxide and its intermediates is essential to produce a truly reversible Li-O2 electrochemistry.

On the efficacy of electrocatalysis in nonaqueous Li-O2 batteries.

Heterogeneous electrocatalysis has become a focal point in rechargeable Li-air battery research to reduce overpotentials in both the oxygen reduction (discharge) and especially oxygen evolution (charge) reactions. In this study, we show that past reports of traditional cathode electrocatalysis in nonaqueous Li-O(2) batteries were indeed true, but that gas evolution related to electrolyte solvent decomposition was the dominant process being catalyzed. In dimethoxyethane, where Li(2)O(2) formation is the dominant product of the electrochemistry, no catalytic activity (compared to pure carbon) is observed using the same (Au, Pt, MnO(2)) nanoparticles. Nevertheless, the onset potential of oxygen evolution is only slightly higher than the open circuit potential of the cell, indicating conventional oxygen evolution electrocatalysis may be unnecessary.

Limitations in Rechargeability of Li-O2 Batteries and Possible Origins

Quantitative differential electrochemical mass spectrometry (DEMS) is used to measure the Coulombic efficiency of discharge and charge [(e(-)/O2)dis and (e(-)/O2)chg] and chemical rechargeability (characterized by the O2 recovery efficiency, OER/ORR) for Li-O2 electrochemistry in a variety of nonaqueous electrolytes. We find that none of the electrolytes studied are truly rechargeable, with OER/ORR <90% for all. Our findings emphasize that neither the overpotential for recharge nor capacity fade during cycling are adequate to assess rechargeability. Coulometry has to be coupled to quantitative measurements of the chemistry to measure the rechargeability truly. We show that rechargeability in the various electrolytes is limited both by chemical reaction of Li2O2 with the solvent and by electrochemical oxidation reactions during charging at potentials below the onset of electrolyte oxidation on an inert electrode. Possible mechanisms are suggested for electrolyte decomposition, which taken together, impose stringent conditions on the liquid electrolyte in Li-O2 batteries.

CH4 dissociation on metals: a quantum dynamics model

The dissociation probability of CH4 on metal surfaces displays striking, interrelated dependencies on several parameters such as the incident kinetic energy (or the gas-temperature) and the surface temperature, as well as a large CH4/CD4 isotope effect. Many models have been proposed to account for one aspect or another of this behavior, but none has yet succeeded in explaining all aspects in a coherent way. In this paper we will show that all experimental results to date are consistent with a mechanism based on a concerted direct dissociation breaking a single CH bond on impact. A theoretical model describing this mechanism is developed by treating nuclear dynamics on a reasonable reduced dimensionality potential energy surface, with the CH4 or CD4 behaving essentially like a quasidiatomic molecule RH. We demonstrate explicitly that this dissociation mechanism is dominated by quantum mechanical tunneling. We resolve previous controversies about tunneling models by showing that the tunnel process cannot be viewed in terms of a static one-dimensional barrier, but must be interpreted as a quantum dynamics problem involving a potential energy surface with a minimum of three degrees of freedom; the distance of the molecule to the surface Z, the RH bond distance D and a coordinate representing lattice vibrations. In particular, a dynamic coupling to the lattice during the tunneling process is a crucial element and gives rise to the phenomenon of thermally assisted tunneling, which dominates the rate of dissociation under conditions that obtain in all experiments and in industrial catalysis. Via explicit quantum dynamical calculations using a model interaction we show that a wide variety of results from previous molecular beam and thermal “activation” experiments can readily be understood, in many cases semi-quantitatively. Since this model explains satisfactorily the full range of dramatic interrelated dependencies on controllable parameters for CH4 and CD4 dissociation on metals, we believe the basic mechanism whereby this important dissociation reaction occurs to be established.

The sticking of O2 on a Pt(111) surface

This paper reports detailed molecular beam measurements of the sticking coefficient at zero coverage for O2 on a Pt(111) surface as a function of initial energy (Ei), angle of incidence (θi), and surface temperature (Ts). Under most conditions the sticking coefficient measures the probability for dissociative chemisorption. These results demonstrate that both precursor mediated and quasi‐direct dissociation can be observed, depending upon the initial conditions. The quasi‐direct process is revealed by a step increase in the sticking with Ei. This feature scales intermediately between Ei and the normal component En, and is weakly dependent on Ts. The precursor mediated sticking is well described by standard precursor kinetic models. At low Ei and Ts, sticking measures trapping into a molecularly adsorbed state. This trapping decreases more rapidly with Ei than anticipated from simple models and scales intermediately between Ei and En. The sticking results are discussed in terms of likely dynamic processes ...

Identifying Capacity Limitations in the Li/Oxygen Battery Using Experiments and Modeling

The Li/oxygen battery may achieve a high practical specific energy as its theoretical specific energy is 11,400 Wh/kg Li assuming Li 2 O 2 is the product. To help understand the physics of the Li/oxygen battery we present the first physics-based model that incorporates the major thermodynamic, transport, and kinetic processes. We obtain a good match between porous-electrode experiments and simulations by using an empirical fit to the resistance of the discharge products (which include carbonates and oxides when using carbonate solvents) as a function of thickness that is obtained from flat-electrode experiments. The experiments and model indicate that the discharge products are electronically resistive, limiting their thickness to tens of nanometers and their volume fraction in one of our discharged porous electrodes to a few percent. Flat-electrode experiments, where pore clogging is impossible, show passivation similar to porous-electrode experiments and allow us to conclude that electrical passivation is the dominant capacity-limiting mechanism in our cells. Although in carbonate solvents Li 2 O 2 is not the dominant discharge product, we argue that the implications of this model, (i.e., electrical passivation by the discharge products limits the capacity) also apply if Li 2 O 2 is the discharge product, as it is an intrinsic electronic insulator.

Activation of methane dissociation on a Pt(111) surface

This paper reports detailed molecular beam measurements of the dissociative chemisorption probability for methane on a Pt(111) surface. We find large increases in the dissociative chemisorption probability S0 with increases in Ei cos2 θi (the normal component of translational energy), Ev (the vibrational energy of the incident methane), and Ts (surface thermal energy). The comparable activation of the reaction by addition of any of these three forms of energy cannot be accounted for by any single model for C–H bond activation proposed to date. A large kinetic isotope effect is also observed, with S0 decreasing significantly for CD4 relative to CH4.

Chemical dynamics of the reactions of O(1D2) with saturated hydrocarbons

Nascent OH internal state distributions produced in the reactions of O(1D2) with saturated hydrocarbons have been measured by combining laser photolysis initiation of the reactions with laser induced fluorescence detection of the OH product. The OH rotational distributions are bimodal. One component corresponds to a broad distribution of high rotational states characterized by a linear surprisal. The other corresponds to population of only the lowest few OH rotational states. These are interpreted as due to insertion and abstraction, respectively. The insertion component dominates for small hydrocarbons (CH4, C2H6), while the abstraction component is the dominant mechanism for production of OH from larger hydrocarbons [C3H8, C(CH3)4]. The large vibrational and rotational surprisals for the insertion component imply that OH is produced by a prompt, non‐RRKM decay of the alcohol collision complex. For the smaller hydrocarbons, insertion preferentially populates the lower Λ‐doublet component of OH. Since the...

The chemical dynamics of the reactions of O(3P) with saturated hydrocarbons. I. Experiment

Molecular beam‐laser induced fluorescence experiments have probed the nascent internal state distributions and excitation functions of OH formed in the technologically important reactions O(3P)+RH→OH+R⋅. RH is a saturated hydrocarbon and R⋅ is an alkyl radical. A variety of RH have been investigated corresponding to abstraction of primary, secondary, and tertiary hydrogens. The OH rotational state distribution is nearly identical for all RH and decrease rapidly from its peak at the lowest rotational level. This demonstrates that reaction occurs when O(3P) is collinear to a C–H bond in the hydrocarbons. The vibrational state distribution of OH depends markedly on the type of hydrogen abstracted, with vibrational excitation increasing dramatically across the series primary to secondary to tertiary. This is interpreted as a shift from a repulsive towards a more attractive surface across the series. Partitioning into the OH spin doublets shows that these reactions are midway between the adiabatic and diabatic...

Theoretical evidence for low kinetic overpotentials in Li-O2 electrochemistry

We develop a density functional theory model for the electrochemical growth and dissolution of Li(2)O(2) on various facets, terminations, and sites (terrace, steps, and kinks) of a Li(2)O(2) surface. We argue that this is a reasonable model to describe discharge and charge of Li-O(2) batteries over most of the discharge-charge cycle. Because non-stoichiometric surfaces are potential dependent and since the potential varies during discharge and charge, we study the thermodynamic stability of facets, terminations, and steps as a function of potential. This suggests that different facets, terminations, and sites may dominate in charge relative to those for discharge. We find very low thermodynamic overpotentials (<0.2 V) for both discharge and charge at many sites on the facets studied. These low thermodynamic overpotentials for both discharge and charge are in very good agreement with the low kinetic overpotentials observed in recent experiments. However, there are other predicted paths for discharge/charge that have higher overpotentials, so the phase space available for the electrochemistry opens up with overpotential.