Joint Center For Energy Storage Research Research Articles

Investigation of Surface Adsorption and Electrode Passivation during Electrochemical Oxidation of Redox-Active Organics

Redox-active organic molecules (ROMs) have gained interest as electroactive materials for redox flow batteries (RFBs). Electrochemical properties of ROMs, i.e., their solubility and redox potential, can be tuned through molecular engineering of the base molecule. Previously, we reported values of the apparent anodic charge transfer coefficient α (values approaching 1) associated with the electrochemical oxidation of a model ROM, 4-hydroxy-TEMPO.1 Also, we have proposed an adsorption-mediated mechanism to explain this unusual value of α, and have presented evidence to support the occurrence of adsorbed intermediates. Here, we extend this investigation to gain further insights using techniques of slow-scan voltammetry and electrode cycling, and apply them to other ROMs. We will discuss results of spectroscopic and micro-gravimetric studies, which provide further insights into the physicochemical properties of the adsorbed surface-passivating films. Ramifications of these to implementation of ROMs in flow batteries will be discussed too.[1] N. A. Shaheen, M. Ijjada, M. B. Vukmirovic, R. Akolkar. J. Electrochem. Soc., 167 (2020) 143505. Acknowledgements: Investigation of the model ROM was supported by Breakthrough Electrolytes for Energy Storage (BEES)—an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award #DE-SC0019409. Work completed at Argonne was supported by the Joint Center for Energy Storage Research (JCESR), a U.S. Department of Energy, Energy Innovation Hub, and by the U.S. Department of Energy, Office of Science, Office of Workforce Development for Teachers and Scientists, Office of Science Graduate Student Research (SCGSR) program. The SCGSR program is administered by the Oak Ridge Institute for Science and Education (ORISE) for the DOE. ORISE is managed by ORAU under contract #DE‐SC0014664. All opinions expressed in this paper are the author’s and do not necessarily reflect the policies and views of DOE, ORAU, or ORISE.

How Halide Sub-Lattice Affects Li Ion Transport in Antiperovskites

Li-based antiperovskites (LiAP, Li3-x OH x X, X = Cl, Br) are an emergent class of Li-ion conductors that are potential candidates for electrolytes in all-solid-state batteries. As a material class, pLiAP shows vast compositional design freedom; however, the resulting properties are susceptible to synthesis and processing methodologies. For example, proton incorporation and halide mixing stabilize the perovskite cubic phase near room temperature, and halides mixtures near the eutectic points drive the solid-state reaction temperature down, allowing for faster synthesis and processing conditions (< 1 h). The mixed halogen compositions, such as Li2OHCl0.37Br0.63, also show a 30-fold improvement in room temperature ionic conductivity of a single halide structure, 1.5 x 10-6 vs. 4.9 x 10-8 S cm-1 (Li2OHCl). Despite the growing interest in these materials, important questions remain about LiAPs on the structure-property correlation upon halide substitution and the correlations between the OH/halide dynamics and the Li-ion transport. We thus attempted to deconvolute how proton dynamics and halide substitution enhance or impede ionic conduction in pLiAP at compositions near the halide salts' eutectic points.We combined infrared spectroscopy and nuclear magnetic resonance (NMR) with first-principles density functional theory (DFT) calculations to deconvolute halide mixing effects from local proton dynamics on Li-ion transport. The NMR results and ab initio molecular dynamics suggest that Li+ transport is more strongly correlated with halide dynamics. While the hydroxide does stabilize the highly conductive cubic structure, it limits correlative ionic transport and thus lowers Li+ conductivity.Experiment design, data analysis, and manuscript preparation (RLS) were supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Science and Engineering. Synthesis (THB and JN) were supported by Asst. Secretary, Energy Efficiency and Renewable Energy (EERE), Vehicle Technologies Office (VTO) through the Advanced Battery Materials Research (BMR) Program. P. J. acknowledges partial support by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award DE-FG02-96ER45579. H. F. was supported from U.S. Department of Energy (Award No. DE-EE0008865). This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The NMR characterization part of the work is supported by the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, and Basic Energy Sciences. The NMR work was performed at the W. R. Wiley Environmental Molecular Sciences Laboratory, a DOE User Facility sponsored by the Office of Biological and Environmental Research, located at Pacific Northwest National Laboratory. Figure 1

(Battery Division Postdoctoral Associate Research Award Address Sponsored by MTI Corporation and the Jiang Family Foundation) Tailoring Bulk and Interfacial Electrolyte Properties to Design Electrochemical Interphases and Enable Highly Reversible Zn Anode

As an important alternative for Li-based battery technology, rechargeable zinc metal batteries promise attractive advantages including safety, high volumetric energy density and low cost; however, such benefits cannot be unlocked unless Zn reversibility meets stringent commercial viability1-2.As part of the Joint Center for Energy Storage Research (JCESR), a DOE Energy Innovation Hub led by Argonne National Laboratory and focused on advancing battery science and technology, ARL team is building new understanding and improvement strategies for Zn metal anode reversibility. In this presentation, we will revisit the quantification of Zn reversibility and report a few remarkable improvements on Zn reversibility with novel electrolyte chemistries. The microscopic characterizations combined with molecular dynamics simulation and density functional theory calculations were used to identify the electrolyte bulk structures, correlated interphasial chemistries and their functionalities as the key factors responsible for dictating reversible Zn chemistry.Reference E. Blanc, D. Kundu, L. F. Nazar, Joule 2020, 4, 771–799.Ma, M. A. Schroeder, O. Borodin, T. P. Pollard, M. S. Ding, C. Wang, K. Xu, Nature Energy 2020, 5, 743-749.

Natural Product Based Small Molecule Redoxmer for Redox Flow Batteries

Renewable energy sources such as solar and wind energy are making substantial inroads into the global energy supply as environment-friendly alternatives. To accommodate those renewable resources, grid-scale energy storage technologies have drawn enormous amounts of interest, with redox flow batteries (RFBs) among the leaders in performance. RFBs uses liquid solutions of redox-active chemicals (redoxmers) as energy storage media, rather than the solid-state electrode materials found in conventional batteries. Due to the liquid nature, RFBs feature remarkable flexibilities, including decoupled energy and power that is highly desired for adapting various scales of energy storage requirements. Non-aqueous RFB systems are extensively studied to achieve high voltage and high energy dense batteries. To that end, developing redoxmers with low molecular weight, high solubility in non-aqueous solvents and high stability are actively pursed. Design of redox-active materials based on natural products allows for the development of sustainable and potentially environment-friendly redoxmers for RFBs. Here, we describe the development of a small molecule bipolar redoxmer based on a widely available natural product. We show that a simple modification of the redoxmer leads to high stability in galvanostatic H-cell cycling and high solubility in non-aqueous solvents. The identification and molecular engineering of non-aqueous compatible natural product based redoxmers represent a promising strategy for developing sustainable redoxmers for RFBs. Acknowledgement The research was financially supported by the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.

Self-Reporting Redoxmers: State of Health Metrics for Redox Flow Batteries

Redox-active organic molecules (redoxmers) are popular for nonaqueous redox flow batteries (NRFBs). However, many parameters need to be achieved for their success. These include high solubility, high conductivity, little to no crossover, wide voltage windows, and high stability in all states of charge. Metrics are needed to track these properties and the overall state of health (SOH) of the battery. Here, we describe the concept of self-reporting redoxmers, where an intrinsic property of the molecule can be used to trace a SOH property. In this case, 2,1,3-benzothiadiazole, an anolyte redoxmer, was functionalized with a pi-extending acetamide groups, which initiated fluorescence, a highly sensitive property. In turn, the fluorescence of the molecules was used to detect species crossover in a simplified H-cell in different electrolyte conditions. Using these methods, we were able to correlate permeability with electrolyte environment. Further, changes in fluorescence emission with different electrolyte salts were direct indicators of solvation environment. Importantly, the synthetic design of the molecule substantially affected electrochemical performance, with a simple methylation of the pi-extending acetamide group giving high stability. The fluorescence also served as a handle to probe and report on solvation environment. In acetonitrile, electrolyte salts containing small lithium cations quenched the redoxmer fluorescence due to a strong chelation effects that were not observed for larger cations like tetraalkylammonium cations. However, this chelation ability was removed when solvents like N,N-dimethylformamide that also have strong chelation effects were used as supported by computational binding calculations. These chelation effects may also correspond with changes in half-wave potential in cyclic voltammetry experiments. Finally, we extend these studies to larger, more complex molecular architectures, which we hypothesize will lead to improved battery performance. Our studies represent the fundamental design of new organic redoxmers with an applied basis to electrolyte fluids while providing molecular-level understanding and SOH diagnosis, which together will provide successful implementation in a flow battery environment.The research was financially supported by the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.

Study of the Effects of Thermal Annealing Temperature to Electrochemical Cycling of Mg2+ and Structure of Spinel MgV2O4 Crystals

Spinel vanadium oxides are a promising candidate material for multivalent battery cathodes due to the various possible valence states of V within the spinel framework, which allows for pathways in Mg2+ ion deintercalation [1]. In recent work, Hu et al. experimentally demonstrated that spinel vanadium oxides nanocrystals have a high capacity for Mg2+ electrochemical cycling [2]. While they reported capacities above 200mAh/g at 110 °C, thus outperforming counterparts such as Cr and Mn oxide spinels, it was noted that these capacities are limited by complex defect structures, as suggested by peak pair distances that cannot be explained in the spinel framework in X-ray diffraction measurements.Here, we will utilize atomic-resolution scanning transmission electron microscopy to characterize the structural defects in spinel V2O4 nanocrystals and their possible effect on Mg2+ intercalation. Additionally, we will report the effects of thermal annealing on the structure as a function of temperature and explore the stability of the MgV2O4 nanocrystals. We find that the effects of electron beam exposure can be used as a proxy for damage incurred on the crystals during electrochemical cycling, on the structure of the crystals. The atomic-resolution characterization will be conducted with an aberration-corrected cold field emission JEOL ARM200CF operated at 200kV primary electron energy and emission current of 15μA. The electron probe will be operated at 24mrad convergence semi-angle. High angle annular dark field, low angle annular dark field and annular bright field detectors will be set to the following inner angles: 75 mrad, 30 mrad, 11mrad. X-ray energy dispersive spectroscopy measurements will be conducted using a large solid-angle Oxford XMAX100TLE detector. Additionally, the ARM200CF is equipped with a post-column Gatan Continuum GIF spectrometer providing capabilities for electron energy loss spectroscopy measurements.In this contribution, we will demonstrate that thermal annealing at higher temperatures increases the crystal size of the MgV2O4 crystals while simultaneously decreasing the presence of defect structures. Additionally, we will characterize a variety of defect structures, similar to those shown in Figure 1, present in crystals annealed at lower temperatures using atomic resolved images and electron spectroscopy techniques. [3]Figure 1. LAADF Image showing a grain boundary defect in a spinel MgV2O4 nanocrystal thermally anneal at 400 degrees Celsius.[1] Computer Modeling Investigation of MgV2O4 for Mg-ion Batteries. Navaratnarajah Kuganathan, Konstantinos Davazolglou, Alexander Chroneos; Journal of Applied Physics 2020 127, 035106.[2] High Capacity for Mg2+ Deintercalation in Spinel Vanadium Oxide Nanocrystals. Linhua Hu, Jacob R. Jokisaari, Bob Jin Kwon, Liang Yin, Soojeong Kim, Haesun Park, Saul H. Lapidus, Robert F. Klie, Baris Key,Peter Zapol, Brian J. Ingram, John T. Vaughey, and Jordi Cabana; ACS Energy Letters 2020 5 (8), 2721-2727.[3] This work is solely supported by the Joint Center for Energy Storage Research (JCESR) and Energy Innovation Hub funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences. Figure 1

High-Performance Structural Batteries

Download : Download high-res image (193KB) Download : Download full-size image Brandon J. Hopkins is a National Research Council postdoctoral fellow at the U.S. Naval Research Laboratory (NRL) focused on advancing next-generation electrochemical energy systems. Hopkins received his PhD at the Massachusetts Institute of Technology (MIT) where he worked on aqueous metal–air batteries. As a master’s candidate at MIT, he was part of the Joint Center for Energy Storage Research (JCESR) where he created gravity-driven flow batteries using semi-solid suspensions. He received his bachelor’s degree from Harvard University and interned at Akamai Technologies, Inc. and Lawrence Livermore National Laboratory. Download : Download high-res image (181KB) Download : Download full-size image Debra Rolison (left) heads and Jeffrey Long (center) and Joseph Parker (right) are members of the Advanced Electrochemical Materials Section at NRL. They design, synthesize, characterize, and prototype 3D-structured, ultraporous, multifunctional, hold-in-your-hand nanoarchitectures for such rate-critical applications as catalysis, energy storage and conversion, ultrafiltration, and sensors. They recently demonstrated that reformulating zinc into a monolithic sponge form-factor allows Zn-based batteries to be cycled at high rate to high specific energy without forming cell-shorting dendrites. They received their PhDs in Chemistry from the University of North Carolina at Chapel Hill in 1980, 1997, and 2010, respectively.

Next-generation battery research and development: Non-politicized science at the Joint Center for Energy Storage Research

Research on the politicization of science often highlights the role of the media or the effects of the public's prior beliefs. Less prominent are case studies addressing the direct communications from scientists working on a narrowly defined project. This paper introduces for consideration the United States Department of Energy-funded Joint Center for Energy Storage Research (JCESR), which fuses together basic research, battery design, and pathways to market. The central point is to assess whether, given its proximity to the climate change issue, JCESR's research has been politicized. Based on the results from interviews, observations, measures of public interest, and a survey of scientists working at JCESR, it is concluded that JCESR's next-generation battery research has avoided politicization and even thrived. This is attributed in part to bipartisan support among policy makers, the absence of any ideology-based impacts on beliefs about energy storage, and a perception among key actors that next-generation battery technology is not a pressing threat. JCESR also presents a collective ability to protect its scientific credibility while enhancing its political relevance. JCESR's battery storage research and development has been effectively buffered from the volatility of climate change.

Energy storage emerging: A perspective from the Joint Center for Energy Storage Research

Energy storage is an integral part of modern society. A contemporary example is the lithium (Li)-ion battery, which enabled the launch of the personal electronics revolution in 1991 and the first commercial electric vehicles in 2010. Most recently, Li-ion batteries have expanded into the electricity grid to firm variable renewable generation, increasing the efficiency and effectiveness of transmission and distribution. Important applications continue to emerge including decarbonization of heavy-duty vehicles, rail, maritime shipping, and aviation and the growth of renewable electricity and storage on the grid. This perspective compares energy storage needs and priorities in 2010 with those now and those emerging over the next few decades. The diversity of demands for energy storage requires a diversity of purpose-built batteries designed to meet disparate applications. Advances in the frontier of battery research to achieve transformative performance spanning energy and power density, capacity, charge/discharge times, cost, lifetime, and safety are highlighted, along with strategic research refinements made by the Joint Center for Energy Storage Research (JCESR) and the broader community to accommodate the changing storage needs and priorities. Innovative experimental tools with higher spatial and temporal resolution, in situ and operando characterization, first-principles simulation, high throughput computation, machine learning, and artificial intelligence work collectively to reveal the origins of the electrochemical phenomena that enable new means of energy storage. This knowledge allows a constructionist approach to materials, chemistries, and architectures, where each atom or molecule plays a prescribed role in realizing batteries with unique performance profiles suitable for emergent demands.

(Invited) Nonaqueous Redoxmer Flow Batteries: New Opportunities for Size-Exclusion Using Nanoporous Separators

Nonaqueous redox flow batteries (NRFBs) are attractive devices for energy storage due to an increased potential window and the manifold possibilities they offer in terms of redox-active architectures and chemistries available for increasing energy density and battery performance. Our collaborations in the context of the Joint Center for Energy Storage Research (JCESR) have shown that pairing commercial nanoporous separators with redoxmer architectures[1] such as Redox-Active Polymers (RAPs),[2] Redox-Active Colloids (RACs),[3] and Redox-Active Oligomers (RAOs),[4] effectively prevent crossover of the active component within the flow system. The choice of redoxmer size and chemical makeup impact profoundly the electrochemical dynamics of the NRFB, and it is clear that optimal performance is only attained with a balanced pairing between redoxmer and separator membrane. In this presentation, I will discuss the design principles underlying size-exclusion NRFBs and some current directions on how nanoporous separators enable new strategies for long-term performance in the flow battery. I will highlight how the progression from high polymers to oligomers modifies the charge transfer characteristics of NRFB electrolytes and the implications of the dynamics of these redoxmer solutions on the size-exclusion approach.[5] Unlike small molecules, the electrochemical reactivity of solution-phase redoxmers is strongly modulated by the properties of both surface confined and freely diffusing species. Building from this knowledge, the recent introduction of Polymers of Intrinsic Microporosity (PIM) separators[4] into the NRFB scene area allows new opportunities in the design of redoxmers with improved charge transfer characteristics and diffusivity without sacrificing their size-exclusion performance. Finally, the use of a size-exclusion approach enables advanced concepts for long-term maintenance of a battery system. I will discuss recent results where controlled de-polymerization reactions are used for maintaining the state-of-health of NRFB components, highlighting the role of component separation using our strategy. Altogether, the combination of novel structural motifs and the use of new nanoporous materials, results in new directions to make better NRFBs that exploit the versatility of a new type of reacting redoxmer fluids for energy storage.

Quantifying Concentration Distribution in an Organic Redox Flow Battery Using in-Plane Neutron Radiography

Redox flow batteries (RFBs) are promising rechargeable electrochemical devices for grid-scale energy storage, but further cost reductions are needed for ubiquitous adoption of this technology [1]. While recent research trends have focused on molecular discovery [2], advances in the science and engineering of electrochemical reactors can lead to cost reductions via increased power density and/or decreased component cost. Key to such efforts is deconvolution of coupled transport and kinetic processes within operating device and, to this end, imaging diagnostics, such as X-ray tomographic microscopy [3], infrared thermography [4], and electrochemiluminescence imaging [5], have been employed to visualize relevant properties (e.g., fluid distribution, temperature) in RFBs. However, obtaining quantitative property descriptions at high resolution and under representative operation conditions remains a challenge. In this presentation, we introduce neutron radiography as an operando characterization method for RFBs which is particularly sensitivity to organic materials with high hydrogen content. To demonstrate the potential of this diagnostic tool, we characterize active species concentration distribution within a redox flow cell in a single electrolyte configuration with a nonaqueous electrolyte containing TEMPO/TEMPO+. We use subtractive neutron imaging [6] in combination with isotropic labelling to deconvolute the concentration of different electrolyte materials (i.e. supporting salts, active species). To resolve the concentration profiles across the different layers, we employ the in-plane imaging configuration [7] and correlate these profiles to cell performance via polarization and electrochemical impedance spectroscopy. References Darling et al., Energy & Environmental Science, 7, 3459 (2014).A. Kowalski et al., Current Opinion in Chemical Engineering, 13, 45 (2016).Tariq et al., Sustainable Energy & Fuels, 2, 2068 (2018).Tanaka et al., Journal of Energy Storage, 19, 67 (2018).Rubio-Garcia et al., Electrochemistry Communications, 93, 128 (2018).Boillat et al., Journal of The Electrochemical Society, 162(6), F531 (2015).Boillat et al., Electrochemistry Communications, 10(4), 546 (2008). Acknowledgments We gratefully acknowledge the financial support of the Swiss National Science Foundation (P2EZP2_172183) and the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the United States Department of Energy.

Towards Bottom-up Engineered Electrode Microstructures for Redox Flow Batteries

Redox flow batteries (RFBs) are promising rechargeable electrochemical devices for grid-scale energy storage, but further cost reductions are needed for widespread implementation. While research efforts have primarily focused on molecular discovery, there has been significantly less attention paid to the engineering aspects (i.e. transport, reactor design) of the battery. Of particular importance are the porous electrodes used in the electrochemical stack. Today’s electrodes largely draw from the fuel cell material set, but, within a RFB, the porous electrode must perform a number of different roles including providing active surfaces for electrochemical reactions, facilitating uniform liquid electrolyte distribution, and supporting low pressure drops. Accordingly, before purpose-built electrodes can be developed for RFB applications, performance-limiting factors for the present materials set must be quantified. Unambiguous analysis is challenging in an operating RFB due to the complex coupling of transport and reactions, which vary as a function of state of charge during cell cycling. Deconvoluting the role of electrode properties on flow battery performance requires the development of diagnostic techniques that enable electrode characterization under well-controlled but application-relevant conditions. To this end, we systematically evaluate the performance of several different carbon paper, felt, and cloth electrodes using the single-electrolyte cell configuration [1] and a model organic redox couple (TEMPO/TEMPO+) [2]. Using polarization and electrochemical impedance spectroscopy, we quantify the impact of electrode microstructure on battery performance. To further understand the role of electrode microstructure on the performance, we use tomography, and numerical simulation in tandem with the aforementioned electrochemical diagnostics. We find that woven electrodes, which feature a bimodal pore size distribution, provide an outstanding electrochemical performance and low pressure drop [3]. Inspired on these learnings, our most recent work focuses on developing bottom-up synthetic methods to fabricate porous electrodes with engineered microstructures, with a particular interest in bimodal pore size distributions and porosity gradients across the electrode thickness. In this talk, I will discuss our most recent experimental strategies to engineer electrodes with highly-controlled microstructures tailored for specific RFB chemistries. Darling et al., J. Electrochem. Soc., 161, A1381 (2014).D. Milshtein et al., J. Power Sources, 327, 151 (2016).Forner-Cuenca et al., Under Review at Electrochem. Soc. Acknowledgments We gratefully acknowledge the financial support of the Swiss National Science Foundation (P2EZP2_172183) and the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the United States Department of Energy.

Neutron Imaging as a Method for Quantifying Concentration Distribution within an Organic Redox Flow Battery

Redox flow batteries (RFBs) are promising rechargeable electrochemical devices for grid-scale energy storage, but further cost reductions are needed for ubiquitous adoption of this technology [1]. While recent research trends have focused on molecular discovery [2], advances in the science and engineering of electrochemical reactors can lead to cost reductions via increased power density and/or decreased component cost. Key to such efforts is deconvolution of coupled transport and kinetic processes within operating device and, to this end, imaging diagnostics, such as X-ray tomographic microscopy [3], infrared thermography [4], and electrochemiluminescence imaging [5], have been employed to visualize relevant properties (e.g., fluid distribution, temperature) in RFBs. However, obtaining quantitative property descriptions at high resolution and under representative operation conditions remains a challenge. In this presentation, we introduce neutron radiography as an operando characterization method for RFBs which is particularly sensitivity to organic materials with high hydrogen content. To demonstrate the potential of this diagnostic tool, we characterize active species concentration distribution within a redox flow cell in a single electrolyte configuration with a nonaqueous electrolyte containing TEMPO/TEMPO+. Specifically, we employ the in-plane imaging configuration [6] to visualize concentration gradients across the flow fields, electrodes, and membrane and to correlate these profiles to cell performance via polarization and electrochemical impedance spectroscopy. References Darling et al., Energy & Environmental Science, 7, 3459 (2014).A. Kowalski et al., Current Opinion in Chemical Engineering, 13, 45 (2016).Tariq et al., Sustainable Energy & Fuels, 2, 2068 (2018).Tanaka et al., Journal of Energy Storage, 19, 67 (2018).Rubio-Garcia et al., Electrochemistry Communications, 93, 128 (2018).Boillat et al., Electrochemistry Communications, 10(4), 546 (2008). Acknowledgments We gratefully acknowledge the financial support of the Swiss National Science Foundation (P2EZP2_172183) and the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the United States Department of Energy.

(Keynote) Where are Batteries Going?

Lithium-ion batteries are expanding well beyond their initial market in personal electronics, to encompass electric vehicles, the electricity grid and some forms of electric flight. These new applications require new standards for higher performance, lower cost and greater safety. Batteries have several promising options to meet these challenges, including incremental improvements to the basic lithium-ion platform, substantial modifications of the lithium-ion platform, (often called advanced lithium-ion) and entirely new concepts beyond the lithium-ion platform. The challenges, opportunities and applications of each option will be considered. 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, Basic Energy Sciences.

Critical Role of Supporting Electrolyte Selection on Flow Battery Cost: Combined Effects of Cell Materials and Operation

With commercial development of redox flow batteries (RFBs) underway and given the importance of capital cost, we analyze RFB supporting electrolyte charge carrier, membrane, open-circuit voltage (OCV), cell performance, and active species size as possible major cost drivers. Physics-based and techno-economic modeling includes a porous electrode model and accounts for chemical and material selection and amounts, stack and tank sizing, and flow-dependent pumping power.1 The 1D porous electrode model, validated in a prior study,2 couples mass transfer, charge transfer, and ohmic transport to estimate the electrode contribution to full cell area specific resistance (ASR). Primary findings for a 5-h discharge of an aqueous RFB are that present costs are in general $10 ~ 100/kWh lower with 1.4 V versus 1.0 V OCV, size selective separators (SSS) versus ion exchange membranes (IEM), as shown in Figure 1a. Large active species size alone would increase costs slightly due to higher viscosity and thus higher pumping cost, reduced mass transfer, and higher mass-based electrolyte cost versus lower molecular weight actives. However, large actives enable SSS use, and the slight incremental cost is more than offset by cost reductions from the high conductivity and inherent low cost of SSS membranes. In the future, the costs of different scenarios converge assuming significant cost reductions in the stack, membranes, and unit cost less materials. In Figure 1b, multiple options, led by H+/IEM with small active species and OCV > 1.3 V, can meet a Joint Center for Energy Storage Research target adjusted to $100/kWh for 5-h storage. Implications of the work here are: 1) recommendations on the path to enable low cost aqueous RFBs, and 2) a modeling tool and opportunity to quantify cost implications of additional factors held constant in the present study. One recommendation is the further exploration of IEMs using H+ or Na+ and SSS, as these membranes have relatively favorable likelihoods to enable system cost targets. We also suggest development work towards redox couples and conditions for high OCV, which enhances electrolyte energy content and stack power. Subsequently, engineering effort and adoption of RFBs is needed to enable significant reduction in cost through high volume manufacturing of stacks, membranes, and overall RFB systems. Lastly, one can use the present model to explore the effect of various additional parameters on cost. Such parameters include: the cell components (e.g., electrode properties, membrane thickness) affecting ASR, the operation (e.g., flow rate, V-I operating point) affecting ASR, and the application (e.g., discharge duration) affecting power-related cost (e.g., stack) versus energy-related cost (e.g., electrolyte volume, tank size). Acknowledgments This work was supported by the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the United States Department of Energy. JDM acknowledges additional support from the NSF Graduate Research Fellowship.

A Mechanistic Investigation of Crossover-Induced Capacity Fade in Redox Flow Batteries with Non-Selective Separators

The independent control of power capability, which is affected by reactor design, and energy capacity, which is affected by stored electrolyte volume, enables redox flow batteries (RFBs) to be potential candidates for large-scale energy storage systems. Typical RFBs consist of two large tanks with positive and negative electrolyte pumped to an electrochemical reactor through a network of pipes, while the reactor consists of high surface area porous electrodes (such as carbon felt) separated by an ion exchange membrane (IEM) or a non-selective separator (NSS). Previous research has established inorganic (such as vanadium) and organic/semi-organic (such as TEMPO) molecules for use in RFBs with aqueous or non-aqueous electrolytes. IEMs are used to electronically insulate the two electrodes and reduce crossover of the redox species by charge selectivity. IEMs are expensive and can contribute as much as 20% to the total battery cost depending on the size of the reactor [1]. Reduction of RFB capital costs could incentivize the adoption of RFBs for grid-energy arbitrage [2], and, hence, the development of RFBs with inexpensive NSSs with minimal crossover could enhance RFB use. In this regard, redox active polymers (RAPs) have been developed in both aqueous and non-aqueous systems to replace IEMs with NSSs. Here, NSSs mitigate crossover by size exclusion of RAPs [3,4]. However, the viscosity of such polymer based redox molecules increases with concentration leading to higher pumping pressures required to maintain the flow rate, thus adding to the pumping costs. In addition, higher pumping pressures increase electrolyte crossover which induces RFB capacity fade from cycle-to-cycle. The objectives of the present work are (1) to model multi-component transport during simultaneous electrochemical reactions within RFBs using NSSs, (2) to assess the tradeoffs in rate capability and cycle life incurred when replacing IEMs with NSSs, and (3) to predict performance as a function NSS design (including pore size and thickness). In this context, we are developing a two-dimensional model using porous electrode theory that explicitly captures porous-media flow, electronic current conservation, and conservation of molecular species (typically four redox-active species and two supporting ionic species that are inert) with simultaneous electrochemical reactions. These processes produce coordinated molecular fluxes can result in crossover and shuttling of redox species. In this model, molecular fluxes arise due to (1) pressure differences across the membrane (advection) (2) concentration gradient between the two electrolytes in the reactor (diffusion) and (3) migration due to the gradient in the electrolyte potential. In addition, we observe electrolyte volume losses which arises due to bulk electrolyte flow through the porous membrane because of viscosity (and therefore pressure) difference between the two electrolytes. Each electrode experiences primary reactions due to the major redox couple in the electrolyte and secondary reactions arising from the species that crossover from the counter electrode. We find that the secondary reactions experience a much higher overpotential than the primary reactions. The shuttling process occurs when the products of the secondary reactions diffuse back into the parent electrode. This shuttling process is used in bio sensing devices to amplify and detect the amount of catechol in human nervous system [5] and for overcharge protection in Li-ion batteries [6]. In the context of RFBs, this must be controlled to minimize capacity fade. The present porous-electrode model and associated reduced-order models will enable mechanistic interpretation of experimental data from which performance will be correlated as a function of several non-dimensional parameters that will aid in the design of NSSs. Further, we will validate our porous electrode model with experiments using aqueous RFBs having interdigitated flow fields using various NSSs and IEMs. We gratefully acknowledge the financial support of the Joint Center for Energy Storage Research (JCESR). References Arora, P., Zhang, Z. M., Chem. Rev., 2004, 104, 4419−4462.Darling et al., Energy Environ. Sci., 7 ,2014.Gavvalapalli et al., Am. Chem. Soc., 136 (46),2014Janoschka et al., Nature 527, 78–81, 2015Bernhard Wolfrum et al., Analytical Chemistry, Vol. 80, No. 4, 2008Jun Chen et al., Solid-State Lett., volume 8, issue 1, A59-A62, 2005

The Influence of Electrode Microstructure on the Performance of Non-Aqueous Redox Flow Batteries

Redox flow batteries (RFBs) are promising rechargeable electrochemical devices for grid-scale energy storage but further cost reductions are needed for ubiquitous adoption of this technology [1]. While current state-of-the-art RFB systems employ aqueous chemistries, transitioning to nonaqueous electrolytes offer a new pathway to reduced cost through increased energy density. While research efforts have primarily focused on molecular discovery [2], there has been significantly less attention paid to the development of other critical system components. Of particular importance are the porous electrodes used in the RFB’s electrochemical stack. Today’s electrodes largely draw from the fuel cell material set, but, within a RFB, the porous electrode must perform a range of different roles including serving as active surfaces for electrochemical reactions, enabling excellent liquid electrolyte distribution, and maintaining low pressure drops [3]. Thus, to enable the development of advanced electrodes tailored for RFB applications, it is necessary to quantify the performance-limiting factors for the present materials set. However, unambiguous analysis is challenging in an operating RFB due to the complex coupling of transport and reactions which varies as a function of state of charge during cell cycling. Deconvoluting the role of electrode properties on flow battery performance requires the development of diagnostic techniques that enable electrode characterization under well-controlled but application-relevant conditions. To this end, we systematically compare the operando performance of RFBs containing selected carbon paper, felt, and cloth electrodes using the single-electrolyte cell configuration (Figure 1a) [4] and a model organic redox couple (TEMPO/TEMPO+) [5]. Using polarization and electrochemical impedance spectroscopy, we quantify the impact of electrode microstructure on battery performance (Figure 1b). We find that, depending on the electrode choice and flow conditions, current densities as high as 450 mA cm-2 can be achieved at an overpotential of 0.3 V with a cell area specific resistance as low as 0.7 Ω cm2. This result suggests that, through appropriate cell engineering and materials selection, high power performance is possible in nonaqueous flow batteries. Finally, a simple convection-diffusion mass transport model is developed to explain the observed experimental behavior and provide guidance for the bottom-up design of next generation RFB electrodes. Darlin et al., Energy Environ. Sci., 7, 3459 (2014).A. Kowalski et al., Curr. Op. Chem. En., 13, 45 (2016).Weber et al., J. Appl. Electrochem., 41(10), 1137 (2011).Darling et al., J. Electrochem. Soc., 161, A1381 (2014).D. Milshtein et al., J. Power Sources, 327, 151 (2016). Acknowledgments We gratefully acknowledge the financial support of the Swiss National Science Foundation (P2EZP2_172183) and the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the United States Department of Energy. Figure 1

The Critical Role of Supporting Electrolyte Selection on Flow Battery Cost

Redox flow batteries (RFBs) are promising electrochemical devices for grid-scale energy storage due to their many favorable inherent attributes, such as decoupled power and energy ratings, as well as excellent performance and capacity retention. Recently, significant improvements in cell performance have made RFB systems more viable than ever. However, to meet the U.S. Department of Energy long range capital-cost target of $150 kWh-1, additional cost reductions are needed. One promising route is new active species for RFBs designed for lower cost or improved performance. These next-generation RFB chemistries are likely to be engineered molecules or complexes, such as redox-active organic or organometallic compounds, which offer a multitude of possibilities. A key consideration in the selection of any new RFB chemistry is the supporting electrolyte, which has a significant influence on the membrane resistance, electrolyte conductivity, and electrolyte viscosity. All of these physical parameters have a major impact on RFB performance and cost. Therefore, it is useful to quantify changes in economic viability for various RFB chemistry options with different aqueous supporting electrolytes paired with different types of membranes, which is the focus of this work. A techno-economic model is used to estimate RFB-system costs for the different membrane and supporting electrolyte options considered herein. Variations in cell performance due to the working ion selection and electrolyte viscosity can yield battery cost differences in the $100’s kWh-1, and this analysis allows for quantification of cost performance changes by selecting certain electrolyte characteristics. Beyond the conventional RFB design incorporating small active species and an ion-exchange membrane (IEM), this work also considers size-selective separators (SSS) as a cost-effective alternative to IEMs. The SSS concept utilizes nano-porous materials with no functionalization for ion selectivity, and active species that are too large to pass through the pores. Across the entire design space, SSS separators with low viscosity electrolytes offer the lowest RFB costs. Acknowledgements This work was supported by the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the United States Department of Energy. J.D.M. acknowledges additional financial support from the National Science Foundation Graduate Research Fellowship Program.

Modeling of Redox-Active Polymer Flow Batteries: Effects of Flow Rate on Tank Mixing

Redox flow batteries (RFBs) allow independent control of energy capacity (system size) and power density (reactor design) making them suitable candidates for grid-scale energy storage. Such large-scale applications necessitate high performance and cost effective designs. Aqueous vanadium redox flow batteries are largely studied at laboratory scale, but their application to the grid is restricted by the poor electrochemical stability window of aqueous based system, high costs associated with the metal ions such as vanadium and ion selective membranes. Redox-active polymers (RAPs) are potential redox active molecules which mitigate the parasitic crossover phenomenon. Previous work [G. Nagarjuna et al., J. Am. Chem. Soc., 136, 16309 (2014)] has shown that RAP polymers of molecular weights ranging between 0.56kDa to 300kDa showed increasing rejection rates at the Celguard separators with molecular weight, the largest molecules being rejected 86% relative to supporting salt species in the electrolyte solution. Although large RAP molecules have low trans-separator permeability, the viscosity of the electrolytes increases drastically with RAP concentration. The viscosity of the electrolytes is also sensitive to the supporting salt concentration [ T. Janoschka et al., Polym. Chem., 6, 7801 (2015)]. Rheological properties of the electrolyte influence the flow distribution inside the reactor eventually effecting the reaction distribution. Therefore, it is important to study the flow rate effects on performance of RFBs and estimate the pressure drops inside the reactor since high pressures risk the mechanical integrity of the system and can enhance crossover. A two-dimensional model based on porous electrode theory is developed to simulate the performance of a RAP based RFB with inter digitated flow field. The tanks are modeled as perfectly mixed units and we show that the mixing in the tanks causes significant loss in capacity particularly at low flow rates. In addition to this, we developed a simplified model which predicts the charge utilization (%) and polarization of an RFB as a function of two dimensionless numbers: (1) tank-to-electrode volume ratio, and (2) ratio of flow rate to the stoichiometric flow rate,. With the help of these two models, we identify certain irreversibilities inherent to the conventional RFB design, apart from ohmic resistances, which arise due to thermodynamics of mixing in the tanks and non-uniform reaction current distribution within the reactor. Operating the RFB at high flow rates would enable high utilization and low polarization. Fig 1 (a) shows the development of charge/discharge voltages as a function of the cell’s average state-of-charge at low () and high () flow rates. The polarization, which is measured by the difference in average charge and discharge voltages, is high at low flow rates and low at high flow rates. The spatial variation of the pore scale reaction current density at halfway of charge/discharge process reveal that the reaction current is localized at the flow inlets of the electrodes leading to high polarization. At high flow rates, the reaction current is more uniformly distributed through the volume of the electrode leading to greater utilization and low polarization. Figure 1(b) shows the variation of the magnitude of the charge utilization (Fig 1(b)(i)) and polarization (Fig 1(b)(i)) with the non-dimensional flow rate, as predicted by the porous electrode model and the simplified model. For flow rates greater than twice the stoichiometric flow rate, , the utilization increases and polarization decreases with increasing flow rate. For flow rates close to stoichiometric value, , the RFB approaches a non-flowing battery, therefore polarization decreases with decreasing flow rate in that regime. A simplified transient model developed based on species conservation equation, agrees well with the porous electrode model and therefore can be used as a tool for performance predictions. On-going work is aimed at modeling crossover and determining the losses associated with it. The modeling results highlight the importance of flow rate and tank design. Results confirm that the RFBs should be operated at high flow rates from the perspective of electrochemical performance, but high pressures associated with high flow rates, particularly for viscous electrolytes such as RAPs, would require bulky reactor designs to ensure mechanical integrity and also the high pumping costs associated would reduce the effective energy stored by the entire system. This motivates optimization of RFB design, operating conditions and cost, the relative importance of which is dependent upon the scale of demonstration (lab or grid). We gratefully acknowledge the financial support of the Joint Center for Energy Storage Research (JCESR). Figure 1

(Keynote) Electrolyte Design for High Energy Density Li-S Batteries

Creating a cost effective, high energy density, long cycle life transportation battery based on the lithium – sulfur electrochemical couple requires overcoming a series of scientific and technical challenges (1). These challenges include reducing the electrolyte volume fraction within the cell, increasing the sulfur loading in the cathode, reducing sulfur loss from the cathode and the subsequent redox shuttle of lithium polysulfides, and reducing excess lithium in the anode, all while maintaining the necessary kinetics to support facile charge transport. The Joint Center for Energy Storage Research (JCESR) is redesigning the lithium-sulfur battery using a combination of novel materials concepts to address these challenges. An example is the use of electrolytes that are sparingly solvating of the polysulfide reaction intermediates and that constrain sulfur at the cathode, reducing the quantity of electrolyte necessary within the cell (2). Focus will be placed on the design of a select class of sparingly solvating electrolytes, their structure-function relationship, and their impact on cell performance. Solvent-in-salt electrolytes, loosely referred to as solvates, represent a class of electrolytes in which the saturated lithium polysufide concentration can be tailored to greatly reduce redox shuttling between cathode and anode. Polysulfide solubility restriction has been demonstrated to redirect sulfur reduction from a solution mediated to a semi-solid state path. A number of solvates are shown to exhibit this behavior. Experimental methods are developed to describe the extent of the solvate regime (solvent-to-salt ratio) in these electrolytes using NMR, Raman spectroscopy, and conductivity measurements. Solvates are found to function below target compositions expected for the standard coordination of the lithium cation, raising questions as to details of the local structure and impact on kinetics. Experimental data is complemented by molecular dynamics simulations to gain insight into electrolyte structure at low solvent levels. The benefit of a semi-solid reaction pathway is the possibility of using the electrolyte in concert with the cathode structure to regulate how lithium sulfide is redistributed. The challenge of integrating both electrolyte and cathode design to take advantage of the unique properties of these electrolytes will be discussed. The author acknowledges the contribution of the Joint Center for Energy Storage Research Li-S team in this presentation. This work was supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science. Sandia is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the U.S. DOE’s NNSA under contract DE-AC04-94AL85000. 1. Eroglu, D.; Zavadil, K. R.; Gallagher, K. G., J Electrochem Soc 2015, 162, A982-A990. 2. Cheng, L.; Curtiss, L. A.; Zavadil, K. R.; Gewirth, A. A.; Shao, Y.; Gallagher, K. G. ACS Energy Lett. 2016, 1, 503−509.