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One of the most promising means to increase the energy density of state-of-the-art lithium Li-ion batteries is to replace the graphite anode with a Li metal anode. While the direct use of Li metal may be highly advantageous, at present its practical application is limited by issues related to dendrite growth and low Coulombic efficiency, CE. Here operando electrochemical scanning transmission electron microscopy (STEM) is used to directly image the deposition/stripping of Li at the anode-electrolyte interface in a Li-based battery. A non-aqueous electrolyte containing small amounts of H2O as an additive results in remarkably different deposition/stripping properties as compared to the “dry” electrolyte when operated under identical electrochemical conditions. The electrolyte with the additive deposits more Li during the first cycle, with the grain sizes of the Li deposits being significantly larger and more variable. The stripping of the Li upon discharge is also more complete, i.e., there is a higher cycling CE. This suggests that larger grain sizes are indicative of better performance by leading to more uniform Li deposition and an overall decrease in the formation of Li dendrites and side reactions with electrolyte components, thus potentially paving the way for the direct use of Li metal in battery technologies.

The rational design of improved electrode–electrolyte interfaces (EEI) for energy storage is critically dependent on a molecular-level understanding of ionic interactions and nanoscale phenomena. The presence of non-redox active species at EEI has been shown to strongly influence Faradaic efficiency and long-term operational stability during energy storage processes. Herein, we achieve substantially higher performance and long-term stability of EEI prepared with highly dispersed discrete redox-active cluster anions (50 ng of pure ∼0.75 nm size molybdenum polyoxometalate (POM) anions on 25 μg (∼0.2 wt%) carbon nanotube (CNT) electrodes) by complete elimination of strongly coordinating non-redox species through ion soft landing (SL). Electron microscopy provides atomically resolved images of a uniform distribution of individual POM species soft landed directly on complex technologically relevant CNT electrodes. In this context, SL is established as a versatile approach for the controlled design of novel surfaces for both fundamental and applied research in energy storage., The design and understanding of electrode–electrolyte interfaces is important for the development of improved energy storage devices. Here, the authors study the controlled deposition of molybdenum polyoxometalate anions onto carbon nanotube electrodes, and show this can result in increased specific capacitance.

Recently, there has been a transition from fully carbonaceous positive electrodes for the aprotic lithium oxygen battery to alternative materials and the use of redox mediator additives, in an attempt to lower the large electrochemical overpotentials associated with the charge reaction. However, the stabilizing or catalytic effect of these materials can become complicated due to the presence of major side-reactions observed during dis(charge). Here, we isolate the charge reaction from the discharge by utilizing electrodes prefilled with commercial lithium peroxide with a crystallite size of about 200-800 nm. Using a combination of S/TEM, online mass spectrometry, XPS, and electrochemical methods to probe the nature of surface films on carbon and conductive Ti-based nanoparticles, we show that oxygen evolution from lithium peroxide is strongly dependent on their surface properties. Insulating TiO2 surface layers on TiC and TiN - even as thin as 3 nm-can completely inhibit the charge reaction under these conditions. On the other hand, TiC, which lacks this oxide film, readily facilitates oxidation of the bulk Li2O2 crystallites, at a much lower overpotential relative to carbon. Since oxidation of lithium oxygen battery cathodes is inevitable in these systems, precise control of the surface chemistry at the nanoscale becomes of upmost importance.

The long-term operation of Li-O2 batteries under full discharge/charge conditions is investigated in a glyme-based electrolyte. The formation of stable interfacial layer on the electrode surface during the initial cycling stabilizes reaction products at subsequent cycling stages as demonstrated by quantitative analyses of the discharge products and the gases released during charging. There is a quick switch from the predominant formation of Li2O2 to the predominant formation of side products during the first few cycles. However, after the formation of the stable interfacial layer, the yield of Li2O2 in the reaction products is stabilized at about 33-40%. Extended cycling under full discharge/charge conditions is achievable upon selection of appropriate electrode materials (carbon source and catalyst) and cycling protocol. Further investigation on the interfacial layer, which in situ forms on air electrode, may increase the long-term yield of Li2O2 during the cycling and enable highly reversible Li-O2 batteries required for practical applications.

The recent development of in-situ liquid stages for (scanning) transmission electron microscopes now makes it possible for us to study the details of electrochemical processes under operando conditions. As electrochemical processes are complex, care must be taken to calibrate the system before any in-situ/operando observations. In addition, as the electron beam can cause effects that look similar to electrochemical processes at the electrolyte/electrode interface, an understanding of the role of the electron beam in modifying the operando observations must also be understood. In this paper we describe the design, assembly, and operation of an in-situ electrochemical cell, paying particular attention to the method for controlling and quantifying the experimental parameters. The use of this system is then demonstrated for the lithiation/delithiation of silicon nanowires.

Electrochemical deposition of crosslinked oxo-cyanoruthenate, Ru-O/CN-O, from a mixture of RuCl 3 and K 4 Ru(CN) 6 is known to yield a film on glassy carbon that promotes oxidations by a combination of electron and oxygen transfer. Layer-by-layer (LbL) deposition of this species at a film formed by cycling of the electrode potential in a ZrO 2 solution systematically increases the number of catalytically active sites of the Ru-O/CN-O on the electrode. The evaluation of the electrocatalytic activity was by cyclic voltammetric oxidation of cysteine at pH 2. Plots of the anodic peak current vs. the square root of scan rate were indicative of linear diffusion control of this oxidation, even in the absence of ZrO 2 , but the slopes of these linear plots increased with bilayer number, n , of (ZrO 2 | Ru-O/CN-O) n . The latter observation is hypothesized to be due to an increased number of active sites for a given geometric electrode area, but proof requires further study. To optimize utilization of the catalyst and to provide a size-exclusion characteristic to the electrode, the study was extended to LbL deposition of the composite in 50-nm pores of an organically modified silica film deposited by electrochemically assisted sol-gel processing using surface-bound poly(styrene sulfonate) nanospheres as a templating agent.

Precise analysis of both (S)TEM images and video are time and labor intensive processes. As an example, determining when crystal growth and shrinkage occurs during the dynamic process of Li dendrite deposition and stripping involves manually scanning through each frame in the video to extract a specific set of frames/images. For large numbers of images, this process can be very time consuming, so a fast and accurate automated method is desirable. Given this need, we developed software that uses analysis of video compression statistics for detecting and characterizing events in large data sets. This software works by converting the data into a series of images which it compresses into an MPEG-2 video using the open source “avconv” utility [1]. The software does not use the video itself, but rather analyzes the video statistics from the first pass of the video encoding that avconv records in the log file. This file contains statistics for each frame of the video including the frame quality, intra-texture and predicted texture bits, forward and backward motion vector resolution, among others. In all, avconv records 15 statistics for each frame. By combining different statistics, we have been able to detect events in various types of data.more » We have developed an interactive tool for exploring the data and the statistics that aids the analyst in selecting useful statistics for each analysis. Going forward, an algorithm for detecting and possibly describing events automatically can be written based on statistic(s) for each data type.« less

Over the past few years, in situ transmission electron microscopy (TEM) studies of lithium ion batteries using an open-cell configuration have helped us to gain fundamental insights into the structural and chemical evolution of the electrode materials in real time. In the standard open-cell configuration, the electrolyte is either solid lithium oxide or an ionic liquid, which is point-contacted with the electrode. This cell design is inherently different from a real battery, where liquid electrolyte forms conformal contact with electrode materials. The knowledge learnt from open cells can deviate significantly from the real battery, calling for operando TEM technique with conformal liquid electrolyte contact. In this paper, we developed an operando TEM electrochemical liquid cell to meet this need, providing the configuration of a real battery and in a relevant liquid electrolyte. To demonstrate this novel technique, we studied the lithiation/delithiation behavior of single Si nanowires. Some of lithiation/delithation behaviors of Si obtained using the liquid cell are consistent with the results from the open-cell studies. However, we also discovered new insights different from the open cell configuration-the dynamics of the electrolyte and, potentially, a future quantitative characterization of the solid electrolyte interphase layer formation and structural and chemical evolution.

Modification of electrodes with nanometer-scale organically modified silica films with pore diameters controlled at 10- and 50-nm is described. An oxidation catalyst, mixed-valence ruthenium oxide with cyano cross-links or gold nanoparticles protected by dirhodium-substituted phosphomolybdate (AuNP-Rh2PMo11), was immobilized in the pores. These systems comprise size-exclusion films at which the biological compounds, phosphatidylcholine and cardiolipin, were electrocatalytically oxidized without interference from surface-active concomitants such as bovine serum albumin. Ten-nanometer pores were obtained by adding generation-4 poly(amidoamine) dendrimer, G4-PAMAM, to a (CH3)3SiOCH3 sol. Fifty-nanometer pores were obtained by modifying a glassy carbon electrode (GC) with a sub-monolayer film of aminopropyltriethoxylsilane, attaching 50-nm diameter poly(styrene sulfonate), PSS, spheres to the protonated amine, transferring this electrode to a (CH3)3SiOCH3 sol, and electrochemically generating hydronium at uncoated GC sites, which catalyzed ormosil growth around the PSS. Voltammetry of Fe(CN)6 3− and Ru(NH3)6 3+ demonstrated the absence of residual charge after removal of the templating agents. With the 50-nm system, the pore structure was sufficiently defined to use layer-by-layer electrostatic assembly of AuNP-Rh2PMo11 therein. Flow injection amperometry of phosphatidylcholine and cardiolipin demonstrated analytical utility of these electrodes.

Silica sol-gel (SG) films with templated pores were deposited on glassy carbon (GC) electrodes by an electrochemically initiated process. Generation-4 poly(amidoamine), PAMAM, dendrimer was included in the tetraethoxysilane precursor to facilitate pore formation. The PAMAM adsorbs to the GC, which blocks SG formation at those sites on the electrode. The pore size was 10 ± 5 nm. After removal of the PAMAM, cyclic voltammetry of Fe(CN)6 3− and Ru(NH3)6 3+ at pH 6.2 showed that the residual negative charge on the silica attenuated the current for the former and increased the current for the latter, presumably by electrostatic repulsion and ion-exchange preconcentration, respectively. This premise was supported by repeating the measurements at the isoelectric point. Methylation of the silanol sites was used to eliminate the charge of the SG. At the end-capped SG, the voltammetry of Fe(CN)6 3− and Ru(NH3)6 3+ yielded currents that were independent of pH over the range 2.1 to 7.2. Circumventing the need for the silanization by using (3-glycidyloxypropyl)trimethoxysilane as the sol-gel precursor failed because the oxygen plasma treatment to remove the PAMAM attacked the organically modified sol-gel backbone. The resulting modified electrode mitigated the influence of proteins on the voltammetry of test species and stabilized functionalize nanoparticle catalysts under hydrodynamic conditions.

1. Fundamental and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, USA 2. Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, USA 3. Environmental and Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, USA 4. Department of Chemical and Biological Engineering, University of WisconsinMadison, Madison, USA

1. Fundamental and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, WA, USA. 2. Environmental Molecular Science Laboratory, Pacific Northwest National Laboratory, Richland, WA, USA 3. Department of Industrial and Manufacturing Engineering, Florida State University, Tallahassee, FL, USA 4. Energy and Environmental Directorate, Pacific Northwest National Laboratory, Richland, WA, USA. 5. Center for Condensed Matter Science and Technology, Department of Physics, Harbin Institute of Technology, PR China.

Lithium (Li)-ion batteries are currently used for a wide variety of portable electronic devices, electric vehicles and renewable energy applications [1,2]. In addition, extensive worldwide research efforts are now being devoted to more advanced “beyond Li-ion” battery chemistries - such as lithium-sulfur (Li-S) [3] and lithium-air (Li-O2) [4] - in which the carbon anode is replaced with Li metal. However, the practical application of Li metal anode systems has been highly problematic. The main challenges involve controlling the formation of a solid-electrolyte interphase (SEI) layer and the suppression of Li dendrite growth during the charge/discharge process (achieving “dendrite-free” cycling). The SEI layer formation continuously consumes the electrolyte components creating highly resistive layer, which leads to the rapid decrease of cycling performance and degradation of the Li anode [5]. The growth of Li metal dendrites at the anode contributes to rapid capacity fading (the presence of “dead Li” created during the discharge leads to an increased overpotential) and, in the case of continuous growth, leads to internal short circuits and extreme safety issues [6]. Here we demonstrate the application of an operando electrochemical scanning transmission electron microscopy (ec-(S)TEM) cell to study the SEI layer formation and the initial stages of Li dendrite growth - the goal is to develop a mechanism for mitigating the degradation processes and increasing safety. Bright field (BF) STEM images in Figure 1 A-C show Li metal deposition and dissolution processes at the interface between the Pt working electrode and the lithium hexafluorophosphate (LiPF6) in propylene carbonate (PC) electrolyte during three charge/discharge cycles. A contrast reversal caused by Li metal being lighter/less dense than surrounding electrolyte (Li appears brighter than the background in BF STEM images) allows Li to be uniquely identified from the other components in the system - the only solid material that is less dense than the electrolyte is Li metal. Using these images, we can precisely quantify the total volume of Li deposition, the thickness of the SEI layer (observed as a ring of positive contrast around the electrode) and alloy formation due to Li+ ion insertion during each cycle. Furthermore, at the end of each discharge cycle we can quantify the presence of “dead Li” detached from the Pt electrode, thereby demonstrating the degree of irreversibility (and degradation of Pt electrode) associated with insertion/removal of Li+during this process with direct correlation to electrochemical performance. Such analyses provide significant insights into Li metal dendrite growth, which is critical to understand the complex interfacial reactions needed to be controlled for future Li-based and next generation energy storage systems. [7] References: [1] J. M. Tarascon, M. Armand, Nature, 414, (2001), 414, 359-367 [2] J. B. Goodenough, Y. Kim, Chem. Mater.,22, (2010), 587-603 [3] X. L. Jie, L. F. Nazar, J. Mater. Chem., 20, (2010), 9821-9826 [4] P.G. Bruce, S. A. Freunberger, L. J. Hardwick, J. M. Tarascon, Nat. Mater., 11, (2012), 19-29 [5] P. Verma, P. Maire, P. Novak, Electrochim. Acta, 55, (2010), 6332-6341 [6] J. Wen, Y. Yu, C. Chen, Mater. Express, 2, (2012), 197-212 [7] This work was primarily supported by JCESR, an Energy Innovation Hub funded by DOE-BES. The development of the operando stage was supported by the Chemical Imaging LDRD Initiative at PNNL. PNNL is a multi-program national laboratory operated by Battelle for the U.S. DOE under Contract DE-AC05-76RL01830. A portion of the research was performed at the EMSL user facility sponsored by DOE-BER and located at PNNL. The multi-target tracking algorithm is supported by NSF-1334012. Figure 1

Many processes in materials science, chemistry and biology take place in a liquid environment – such as the synthesis of nanoparticles, biological cellular functions and the operation of Li-ion/next generation batteries. In these cases, the overall process/function of the system is a result of a series of complicated transients, where a change in the order, magnitude or location of any of the individual steps can lead to a radically different result. Understanding and subsequently controlling the final system outcome can therefore be greatly aided by the ability to directly observe these fundamental transient processes as and where they happen. Aberration corrected (scanning) transmission electron microscopy ((S)TEM) has the spatial resolution (typically < 0.1 nm) to directly visualize the atomic scale structural and chemical variations taking place in materials. Historically, such high resolution microscopy has been used to analyze materials before and after a process takes place to infer the dynamics of what happened in between. While there are still great advances that can be made with such analyses (at the very least in providing benchmark structures for nanoscale systems), a major breakthrough in recent years has been the design and implementation of in-situ gas and liquid stages that allow (S)TEM images to be obtained while the transient processes are actually taking place. For battery systems in particular, this means that we can now observe structural and chemical variations that occur in and around the interfaces between the electrodes and a liquid electrolyte during the charge/discharge process. Here we will discuss the implementation of an operando electrochemical stage within the (S)TEM that has been configured to form a “Li battery” (the same general configuration can be used for Li-ion, Li-air and Li-S batteries, as well as for any other novel battery architecture). One of the first experiments that has been performed is a quantification of the electrochemical processes that occur at the anode during charge/discharge cycling. Of particular importance for these observations is the identification of an image contrast reversal that originates from solid Li being less dense than the surrounding liquid electrolyte and electrode surface. This contrast allows Li to be identified from Li containing compounds that make up the solid-electrolyte interphase (SEI) layer. By correlating images showing the sequence of Li electrodeposition and the evolution of the SEI layer with simultaneously acquired cyclic voltammograms (CV), electrodeposition and electrolyte breakdown processes can be quantified directly on the nanoscale. In addition, changes in the morphology of the Li deposits (dendrites) obtained by introducing additives into the electrolyte can also be readily quantified from these (S)TEM observations. The experimental conditions needed to obtain these results (including identifying and eliminating the electron beam effect) will be discussed in detail. Preliminary results from Li-air, Li-S and Mg battery systems will be presented to highlight the development of stage technologies for future electrochemical experiments. In addition, the potential for microsecond time resolution experiments in the newly developed dynamic transmission electron microscope (DTEM) and the implementation of Compressive Sensing (CS) methods to identify the initial stages of SEI/dendrite formation will be explored.

The high demand for new energy storage materials has created a need for experimental techniques that can provide real-time information on the dynamic structural changes/processes that occur locally at the electrode/electrolyte interface during battery operation. In this regard, in-situ electrochemical stages for (scanning) transmission electron microscopes ((S)TEM) enable the fabrication of a “nano-battery” to study the fundamentals of electrochemical processes under operando conditions with the high spatial and temporal resolution of an electron microscope. Here, we describe quantitative operando observations using an in-situ liquid electrochemical (S)TEM cell to study lithium dendrite formation (results were obtained from a range of electrodes/electrolytes combinations including a Pt microelectrode and LiPF6 in PC electrolyte). Images in the STEM usually have mass thickness contrast, meaning that something thicker, heavier/more dense appears darker in the bright field image and lighter in the dark field image. As Li metal is lighter and less dense than the surrounding electrolyte, the formation of dendrites appears to be light in the bright field image and dark in the dark field image – the contrast is effectively “reversed”. Such contrast is unique to Li metal and provides a very straightforward way to identify Li metal (and therefore dendrites) during the nano-battery operation in the microscope. From the individual STEM images (that are combined to form video rate movie of the dynamic process), the amount of Li deposited/incorporated into the electrolyte during the charge/discharge cycle can be directly quantified and correlated with the standard ex-situbulk scale cyclic voltammetry measurements, thereby providing a direct nanoscale view of the whole electrochemical process. Results show that the amount/morphology of Li deposited changes after the first cycle due to the differences in initial electroactive surface area of the bare Pt electrode and the surface roughness of Pt electrode after Li dendrite deposition in subsequent cycles. Furthermore, this morphology and the amount of “dead” Li can be controlled by application of various types of additives that can drastically suppress the growth or change the morphology of the Li dendrites. These enables for direct link between the STEM images and the electrochemical behavior can be extended to any combination of next generation battery systems (Na, Mg, Zn, Al etc in any aqueous or none-aqueous electrolytes) to provide significant insights into the electrochemistry of conventional battery systems on the nanometer scale. Acknowledgments This work was supported as part of the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the US Department of Energy, Office of Science, and Basic Energy Sciences. The research is also part of the Chemical Imaging Initiative at Pacific Northwest National Laboratory under Contract DE-AC05-76RL01830 operated for DOE by Battelle. A portion of the research was performed using EMSL, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory.

Understanding the fundamental processes taking place at the electrode-electrolyte interface in batteries will play a key role in the development of next generation energy storage technologies. One of the most fundamental aspects of the electrode-electrolyte interface is the electrical double layer (EDL). Given the recent development of high spatial resolution in-situ electrochemical fluid cells for scanning transmission electron microscopy (STEM), there now exists the possibility that we can directly observe the formation and dynamics of the EDL. In this paper we predict electrolyte structure within the EDL using classical models and atomistic Molecular Dynamics (MD) simulations. Classical models are found to greatly differ from MD in predicted concentration profiles. It is thus suggested that MD must be used in order to accurately predict STEM images of the electrode-electrolyte interface. Using MD and image simulation together for a high contrast electrolyte (the high atomic number CsCl electrolyte), it is determined that, for a smooth interface, concentration profiles within the EDL should be visible experimentally. When normal experimental parameters such as rough interfaces and low-Z electrolytes (like those used in Li-ion batteries) are considered, observation of the EDL appears to be more difficult.