Katharine Lee Harrison

Nanostructured Metal Carbides for Aprotic Li-O2 Batteries: New Insights into Interfacial Reactions and Cathode Stability.

The development of nonaqueous Li-oxygen batteries, which relies on the reversible reaction of Li + O2 to give lithium peroxide (Li2O2), is challenged by several factors, not the least being the high charging voltage that results when carbon is typically employed as the cathode host. We report here on the remarkably low 3.2 V potential for Li2O2 oxidation on a passivated nanostructured metallic carbide (Mo2C), carbon-free cathode host. Online mass spectrometry coupled with X-ray photoelectron spectroscopy unequivocally demonstrates that lithium peroxide is simultaneously oxidized together with the Li(x)MoO3-passivated conductive interface formed on the carbide, owing to their close redox potentials. The process rejuvenates the surface on each cycle upon electrochemical charge by releasing Li(x)MoO3 into the electrolyte, explaining the low charging potential.

Mechanical properties of water-assembled graphene oxide Langmuir monolayers: Guiding controlled transfer

Liquid-phase transfer of graphene oxide (GO) and reduced graphene oxide (RGO) monolayers is investigated from the perspective of the mechanical properties of these films. Monolayers are assembled in a Langmuir-Blodgett trough, and oscillatory barrier measurements are used to characterize the resulting compressive and shear moduli as a function of surface pressure. GO monolayers are shown to develop a significant shear modulus (10-25 mN/m) at relevant surface pressures while RGO monolayers do not. The existence of a shear modulus indicates that GO is acting as a two-dimensional solid driven by strong interaction between the individual GO sheets. The absence of such behavior in RGO is attributed to the decrease in oxygen moieties on the sheet basal plane, permitting RGO sheets to slide across one another with minimum energy dissipation. Knowledge of this two-dimensional solid behavior is exploited to successfully transfer large-area, continuous GO films to hydrophobic Au substrates. The key to successful transfer is the use of shallow-angle dipping designed to minimize tensile stress present during the insertion or extraction of the substrate. A shallow dip angle on hydrophobic Au does not impart a beneficial effect for RGO monolayers, as these monolayers do not behave as two-dimensional solids and do not remain coherent during the transfer process. We hypothesize that this observed correlation between monolayer mechanical properties and continuous film transfer success is more universally applicable across substrate hydrophobicities and could be exploited to control the transfer of films composed of two-dimensional materials.

Na intercalation in Fe-MIL-100 for aqueous Na-ion batteries

Here we report for the first time the feasibility of using metal–organic frameworks (MOFs) as electrodes for aqueous Na-ion batteries. We show that Fe-MIL-100, a known redox-active MOF, is electrochemically active in a Na aqueous electrolyte, under various compositions. Emphasis was placed on investigating the electrode–electrolyte interface, with a focus on identifying the relationship between additives in the composition of the working electrode, particle size and overall performance. We found that the energy storage capacity is primarily dependent on the binder additive in the composite; the best activity for this MOF is obtained with Nafion as a binder, owing to its hydrophilic and ion conducting nature. Kynar-bound electrodes are clearly less effective, due to their hydrophobic character, which impedes wetting of the electrode. The binder-free systems show the poorest electrochemical activity. There is little difference in the overall performance as function of particle size (micro vs. nano), implying the storage capacities in this study are not limited by ionic and/or electronic conductivity. Excellent reversibility and high coulombic efficiency are achieved at higher potential ranges, observed after cycle 20. That is despite progressive capacity decay observed in the initial cycles. Importantly, structural analyses of cycled working electrodes confirm that the long range crystallinity remains mainly unaltered with cycling. These findings suggest that limited reversibility of the intercalated Na ions in the lower potential range, together with the gradual lack of available active sites in subsequent cycles is responsible for the rapid decay in capacity retention.

Lithium Self-Discharge and Its Prevention: Direct Visualization through In Situ Electrochemical Scanning Transmission Electron Microscopy

To understand the mechanism that controls low-aspect-ratio lithium deposition morphologies for Li-metal anodes in batteries, we conducted direct visualization of Li-metal deposition and stripping behavior through nanoscale in situ electrochemical scanning transmission electron microscopy (EC-STEM) and macroscale-cell electrochemistry experiments in a recently developed and promising solvate electrolyte, 4 M lithium bis(fluorosulfonyl)imide in 1,2-dimethoxyethane. In contrast to published coin cell studies in the same electrolyte, our experiments revealed low Coulombic efficiencies and inhomogeneous Li morphology during in situ observation. We conclude that this discrepancy in Coulombic efficiency and morphology of the Li deposits was dependent on the presence of a compressed lithium separator interface, as we have confirmed through macroscale (not in the transmission electron microscope) electrochemical experiments. Our data suggests that cell compression changed how the solid-electrolyte interphase formed, which is likely responsible for improved morphology and Coulombic efficiency with compression. Furthermore, during the in situ EC-STEM experiments, we observed direct evidence of nanoscale self-discharge in the solvate electrolyte (in the state of electrical isolation). This self-discharge was duplicated in the macroscale, but it was less severe with electrode compression, likely due to a more passivating and corrosion-resistant solid-electrolyte interphase formed in the presence of compression. By combining the solvate electrolyte with a protective LiAl0.3S coating, we show that the Li nucleation density increased during deposition, leading to improved morphological uniformity. Furthermore, self-discharge was suppressed during rest periods in the cycling profile with coatings present, as evidenced through EC-STEM and confirmed with coin cell data.

Understanding Reaction Mechanisms in Electrochemistry and Corrosion: Liquid-Cell S/TEM

Electrochemical and corrosion studies have greatly benefited from using liquid-cell S/TEM techniques for providing real-time information on the nanoscale mechanisms occurring at solid-liquid interfaces [1]. Within a liquid environment, nanoscale electrodes and metals undergo reactions with the solution creating surface layers and films at the interface. In batteries, these interface layers are known as solidelectrolyte interfaces (SEI). In corrosion experiments, the surface layers are known as scale materials. These systems have related domination of the surface film composition and structure impacting the overall behavior of the electrode and the rate of reaction during corrosion. Therefore, to better understand these material systems and determinant mechanisms, we are investigating using real-time imaging and spectroscopy to characterize these interfaces for initial structural identification, in-situ monitoring of interfacial processes, and post-mortem analysis of electrode/material surfaces.

Liquid-Cell TEM Observations of Sn Lithiation reactions: A Temperature Case Study

In recent years, the increasing use of portable electronics and the rise of eco-friendly electric vehicles put tremendous pressure on high-energy rechargeable lithium-ion batteries [1,2]. As a result, significant research has been focused on more advanced, high-capacity electrode materials such as Si, Sn, Ge and Li as anode and S and O2 as cathode materials [2-4]. These electrode materials are attractive due to their higher specific capacities than present commercial electrode materials such graphite (anode) and lithium metal oxides (cathode). However, issues like volume expansion, capacity fading and unwanted electrolyte reactions upon cycling prevent the commercialization of these electrode materials. Over the last decade, in order to address the degradation of high-capacity electrode materials, significant investigations have been carried out with in-situ/ operando S/TEM, X-ray diffraction, NMR, Raman and Mass spectroscopes [5]. In fact, to diagnose the effect of size and morphology of electrode materials at or near working electrochemical conditions, S/TEM can be used to visualize the lithium insertion and de-insertion of electrode materials at atomic resolution in some commercial volatile electrolytes such as organic carbonates. In addition, the electrode/electrolyte interface reactions can also be visualized while cycling. We are using a highly customized TEM liquid cell that successfully performs quantitative electrochemical control on ultramicroelectrodes for testing nanomaterials in volatile electrolytes during nanoscale imaging [6,7]. This design allows up to 10 electrodes incorporated into an even liquid gap of ~ 150-200 nm, where controlled assembly of nanomaterials onto the custom patterned electrodes allows for an ideally designed working cell inside the TEM.

Controlled Electrochemical Li Cycling in a TEM to Observe Li Morphology Evolution

To meet the increased demand for high power energy storage for grid and transportation applications, a stable highly efficient Li metal battery electrode is being investigated. The system is dependent on the formation of a solid electrolyte interphase that is capable of suppressing Li dendrite formation, which is the limiting characteristic that prevents application of high capacity Li metal anodes in current lithium ion batteries (LIBs). The SEI layer that is formed between the electrolyte and the Li metal electrode is dependent on the breakdown of the Li containing electrolyte. We investigated lithium bis(fluorosulfonyl)imide (LiFSI) in dimethoxyethane (DME) for suppressed Li dendrite formation [1]. This electrolyte is targeted for stable stripping of lithium at current densities up to 10 mA/cm 2 and Coulombic efficiencies above 99.1%. The morphological evolution of the Li deposition and stripping on copper electrodes was monitored using quantitative in situ scanning transmission electron microscopy (STEM) in a custom fabricated electrochemical cell.