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Dive into the research topics where Robert R. Mitchell is active.

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Featured researches published by Robert R. Mitchell.


Energy and Environmental Science | 2013

Lithium–oxygen batteries: bridging mechanistic understanding and battery performance

Yi-Chun Lu; Betar M. Gallant; David G. Kwabi; Jonathon R. Harding; Robert R. Mitchell; M. Stanley Whittingham; Yang Shao-Horn

Rechargeable energy storage systems with high energy density and round-trip efficiency are urgently needed to capture and deliver renewable energy for applications such as electric transportation. Lithium–air/lithium–oxygen (Li–O2) batteries have received extraordinary research attention recently owing to their potential to provide positive electrode gravimetric energies considerably higher (∼3 to 5×) than Li-ion positive electrodes, although the packaged device energy density advantage will be lower (∼2×). In light of the major technological challenges of Li–O2 batteries, we discuss current understanding developed in non-carbonate electrolytes of Li–O2 redox chemistry upon discharge and charge, oxygen reduction reaction product characteristics upon discharge, and the chemical instability of electrolytes and carbon commonly used in the oxygen electrode. We show that the kinetics of oxygen reduction reaction are influenced by catalysts at small discharge capacities (Li2O2 thickness less than ∼1 nm), but not at large Li2O2 thicknesses, yielding insights into the governing processes during discharge. In addition, we discuss the characteristics of discharge products (mainly Li2O2) including morphological, electronic and surface features and parasitic reactivity with carbon. On charge, we examine the reaction mechanism of the oxygen evolution reaction from Li2O2 and the influence of catalysts on bulk Li2O2 decomposition. These analyses provide insights into major discrepancies regarding Li–O2 charge kinetics and the role of catalyst. In light of these findings, we highlight open questions and challenges in the Li–O2 field relevant to developing practical, reversible batteries that achieve the anticipated energy density advantage with a long cycle life.


Energy and Environmental Science | 2011

All-carbon-nanofiber electrodes for high-energy rechargeable Li–O2 batteries

Robert R. Mitchell; Betar M. Gallant; Carl V. Thompson; Yang Shao-Horn

Hollow carbon fibers with diameters on the order of 30 nm were grown on a ceramic porous substrate, which was used as the oxygen electrode in lithium-oxygen (Li–O2) batteries. These all-carbon-fiber (binder-free) electrodes were found to yield high gravimetric energies (up to 2500 W h kgdischarged−1) in Li–O2cells, translating to an energy enhancement ∼4 times greater than the state-of-the-art lithium intercalation compounds such as LiCoO2 (∼600 W h kgelectrode−1). The high gravimetric energy achieved in this study can be attributed to low carbon packing in the grown carbon-fiber electrodes and highly efficient utilization of the available carbon mass and void volume for Li2O2 formation. The nanofiber structure allowed for the clear visualization of Li2O2 formation and morphological evolution during discharge and its disappearance upon charge, where Li2O2 particles grown on the sidewalls of the aligned carbon fibers were found to be toroids, having particle sizes increasing (up to ∼1 μm) with increasing depth-of-discharge. The visualization of Li2O2 morphologies upon discharge and disappearance upon charge represents a critical step toward understanding key processes that limit the rate capability and low round-trip efficiencies of Li–O2 batteries, which are not currently understood within the field.


Energy and Environmental Science | 2013

Influence of Li2O2 morphology on oxygen reduction and evolution kinetics in Li–O2 batteries

Betar M. Gallant; David G. Kwabi; Robert R. Mitchell; Jigang Zhou; Carl V. Thompson; Yang Shao-Horn

Understanding the origins of high overpotentials required for Li2O2 oxidation in Li–O2 batteries is critical for developing practical devices with improved round-trip efficiency. While a number of studies have reported different Li2O2 morphologies formed during discharge, the influence of the morphology and structure of Li2O2 on the oxygen evolution reaction (OER) kinetics and pathways is not known. Here, we show that two characteristic Li2O2 morphologies are formed in carbon nanotube (CNT) electrodes in a 1,2-dimethoxyethane (DME) electrolyte: discs/toroids (50–200 nm) at low rates/overpotentials (10 mA gC−1 or E > 2.7 V vs. Li), or small particles (<20 nm) at higher rates/overpotentials. Upon galvanostatic charging, small particles exhibit a sloping profile with low overpotential (<4 V) while discs exhibit a two-stage process involving an initially sloping region followed by a voltage plateau. Potentiostatic intermittent titration technique (PITT) measurements reveal that charging in the sloping region corresponds to solid solution-like delithiation, whereas the voltage plateau (E = 3.4 V vs. Li) corresponds to two-phase oxidation. The marked differences in charging profiles are attributed to differences in surface structure, as supported by X-ray absorption near edge structure (XANES) data showing that oxygen anions on disc surfaces have LiO2-like electronic features while those on the particle surfaces are more bulk Li2O2-like with modified electronic structure compared to commercial Li2O2. Such an integrated structural, chemical, and morphological approach to understanding the OER kinetics provides new insights into the desirable discharge product structure for charging at lower overpotentials.


Journal of Physical Chemistry Letters | 2013

Mechanisms of Morphological Evolution of Li2O2 Particles during Electrochemical Growth

Robert R. Mitchell; Betar M. Gallant; Yang Shao-Horn; Carl V. Thompson

Li-O2 batteries, wherein solid Li2O2 is formed at the porous air cathode during discharge, are candidates for high gravimetric energy (3212 Wh/kgLi2O2) storage for electric vehicles. Understanding and controlling the nucleation and morphological evolution of Li2O2 particles upon discharge is key to achieving high volumetric energy densities. Scanning and transmission electron microscopy were used to characterize the discharge product formed in Li-O2 batteries on electrodes composed of carpets of aligned carbon nanotubes. At low discharge rates, Li2O2 particles form first as stacked thin plates, ∼10 nm in thickness, which spontaneously splay so that secondary nucleation of new plates eventually leads to the development of a particle with a toroidal shape. Li2O2 crystallites have large (001) crystal faces consistent with the theoretical Wulff shape and appear to grow by a layer-by-layer mechanism. In contrast, at high discharge rates, copious nucleation of equiaxed Li2O2 particles precedes growth of discs and toroids.


Nano Letters | 2013

In Situ Transmission Electron Microscopy Observations of Electrochemical Oxidation of Li2O2

Li Zhong; Robert R. Mitchell; Yang Liu; Betar M. Gallant; Carl V. Thompson; Jian Yu Huang; Scott X. Mao; Yang Shao-Horn

In this Letter, we report the first in situ transmission electron microscopy observation of electrochemical oxidation of Li2O2, providing insights into the rate limiting processes that govern charge in Li-O2 cells. In these studies, oxidation of electrochemically formed Li2O2 particles, supported on multiwall carbon nanotutubes (MWCNTs), was found to occur preferentially at the MWCNT/Li2O2 interface, suggesting that electron transport in Li2O2 ultimately limits the oxidation kinetics at high rates or overpotentials.


Nano Letters | 2009

Low temperature synthesis of vertically aligned carbon nanotubes with electrical contact to metallic substrates enabled by thermal decomposition of the carbon feedstock.

Gilbert D. Nessim; Matteo Seita; A. John Hart; Ryan K. Bonaparte; Robert R. Mitchell; Carl V. Thompson

Growth of vertically aligned carbon nanotube (CNT) carpets on metallic substrates at low temperatures was achieved by controlled thermal treatment of ethylene and hydrogen at a temperature higher than the substrate temperature. High-resolution transmission electron microscopy showed that nanotubes were crystalline for a preheating temperature of 770 degrees C and a substrate temperature of 500 degrees C. Conductive atomic force microscopy measurements indicated electrical contact through the CNT carpet to the metallic substrate with an approximate resistance of 35 kOmega for multiwall carpets taller than two micrometers. An analysis of the activation energies indicated that thermal decomposition of the hydrocarbon/hydrogen gas mixture was the rate-limiting step for low-temperature chemical vapor deposition growth of CNTs. These results represent a significant advance toward the goal of replacing copper interconnects with nanotubes using CMOS-compatible processes.


Journal of Physical Chemistry Letters | 2013

Rate-Dependent Morphology of Li2O2 Growth in Li–O2 Batteries

Birger Horstmann; Betar M. Gallant; Robert R. Mitchell; Wolfgang G. Bessler; Yang Shao-Horn; Martin Z. Bazant

Compact solid discharge products enable energy storage devices with high gravimetric and volumetric energy densities, but solid deposits on active surfaces can disturb charge transport and induce mechanical stress. In this Letter, we develop a nanoscale continuum model for the growth of Li2O2 crystals in lithium-oxygen batteries with organic electrolytes, based on a theory of electrochemical nonequilibrium thermodynamics originally applied to Li-ion batteries. As in the case of lithium insertion in phase-separating LiFePO4 nanoparticles, the theory predicts a transition from complex to uniform morphologies of Li2O2 with increasing current. Discrete particle growth at low discharge rates becomes suppressed at high rates, resulting in a film of electronically insulating Li2O2 that limits cell performance. We predict that the transition between these surface growth modes occurs at current densities close to the exchange current density of the cathode reaction, consistent with experimental observations.


Archive | 2014

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

Betar M. Gallant; Yi-Chun Lu; Robert R. Mitchell; David G. Kwabi; Thomas J. Carney; Carl V. Thompson; Yang Shao-Horn

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.


Journal of Physical Chemistry C | 2012

Chemical and Morphological Changes of Li–O2 Battery Electrodes upon Cycling

Betar M. Gallant; Robert R. Mitchell; David G. Kwabi; Jigang Zhou; Lucia Zuin; Carl V. Thompson; Yang Shao-Horn


Composites Science and Technology | 2013

A technique for spatially-resolved contact resistance-free electrical conductivity measurements of aligned-carbon nanotube/polymer nanocomposites

Robert R. Mitchell; Namiko Yamamoto; Hulya Cebeci; Brian L. Wardle; Carl V. Thompson

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Betar M. Gallant

Massachusetts Institute of Technology

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Yang Shao-Horn

Massachusetts Institute of Technology

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Carl V. Thompson

Massachusetts Institute of Technology

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David G. Kwabi

Massachusetts Institute of Technology

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Martin Z. Bazant

Massachusetts Institute of Technology

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Wolfgang G. Bessler

University of Applied Sciences Offenburg

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Yi-Chun Lu

The Chinese University of Hong Kong

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