Transition of lithium growth mechanisms in liquid electrolytes

Next-generation high-energy batteries will require a rechargeable lithium metal anode, but lithium dendrites tend to form during recharging, causing short-circuit risk and capacity loss, by mechanisms that still remain elusive. Here, we visualize lithium growth in a glass capillary cell and demonstrate a change of mechanism from root-growing mossy lithium to tip-growing dendritic lithium at the onset of electrolyte diffusion limitation. In sandwich cells, we further demonstrate that mossy lithium can be blocked by nanoporous ceramic separators, while dendritic lithium can easily penetrate nanopores and short the cell. Our results imply a fundamental design constraint for metal batteries (“Sand's capacity”), which can be increased by using concentrated electrolytes with stiff, permeable, nanoporous separators for improved safety.MIT Energy Initiative (Robert Bosch GmbH)National Science Foundation (U.S.) (Grant DMR-1410636)Stanford University. Global Climate and Energy ProjectUnited States. Dept. of Energy. Office of Basic Energy Sciences (Stanford University. SUNCAT Center for Interface Science and Catalysis

Transport Property Requirements for Flow Battery Separators

Flow batteries are a promising technology for storing and discharging megawatt hours of electrical energy on the time scale of hours. The separator between the positive and negative electrodes strongly affects technical and economic performance. However, requirements for separators have not been reported in a general manner that enables quantitative evaluation of new systems such as nonaqueous flow batteries. This gap is addressed by deriving specifications for transport properties that are chemistry agnostic and align with aggressive capital cost targets. Three key transport characteristics are identified: area-specific resistance RΩ, crossover current density ix, and the coupling between crossover and capacity loss Ψ. Suggested maximum area-specific resistances are 0.29 and 2.3 Ω·cm[superscript 2] for aqueous and nonaqueous batteries, respectively. Allowable crossover rates are derived by considering the possible fates of active molecules that cross the separator and the coupling between Coulombic efficiency (CE) and capacity decline. The CE must exceed 99.992% when active species are unstable at the opposing electrode, while a CE of 97% can be tolerated when active molecules can be recovered from the opposing electrode. The contributions of diffusion, migration, and convection are discussed, quantified, and related to the physical properties of the separator and the active materials.United States. Department of Energy. Office of Basic Energy Sciences (Joint Center for Energy Storage Research

Exploring the role of electrode microstructure on the performance of non-aqueous redox flow batteries

Redox flow batteries are an emerging technology for long-duration grid energy storage, but further cost reductions are needed to accelerate adoption. Improving electrode performance within the electrochemical stack offers a pathway to reduced system cost through decreased resistance and increased power density. To date, most research efforts have focused on modifying the surface chemistry of carbon electrodes to enhance reaction kinetics, electrochemically active surface area, and wettability. Less attention has been given to electrode microstructure, which has a significant impact on reactant distribution and pressure drop within the flow cell. Here, drawing from commonly used carbon-based diffusion media (paper, felt, cloth), we systematically investigate the influence of electrode microstructure on electrochemical performance. We employ a range of techniques to characterize the microstructure, pressure drop, and electrochemically active surface area in combination with in-operando diagnostics performed in a single electrolyte flow cell using a kinetically facile redox couple dissolved in a non-aqueous electrolyte. Of the materials tested, the cloth electrode shows the best performance; the highest current density at a set overpotential accompanied by the lowest hydraulic resistance. We hypothesize that the bimodal pore size distribution and periodic, well-defined microstructure of the cloth are key to lowering mass transport resistance

Investigating the factors that influence resistance rise of PIM-1 membranes in nonaqueous electrolytes

As redox active macromolecules are introduced to the materials repertoire of redox flow batteries (RFBs), nanoporous membranes, such as polymers of intrinsic microporosity (PIMs), are emerging as a viable separation strategy. Although their selectivity has been demonstrated, PIM-based membranes suffer from time-dependent resistance rise in nonaqueous electrolytes. Here, we study this phenomenon as a function of membrane thickness, electrolyte flow rate, and solvent washing using a diagnostic flow cell configuration. We find that the rate and magnitude of resistance rise can be significantly reduced through the combination of low electrolyte flow rate and solvent prewash. Further, our results indicate that, since the increase is not associated with irreversible chemical and structural changes, the membrane performance can be recovered via ex-situ or in-situ solvent washes. Keywords: Energy storage, Redox flow battery, Polymer of intrinsic microporosity, Size-exclusion membranes, Performance recovery, Cell resistanc

Feasibility of a Supporting‐Salt‐Free Nonaqueous Redox Flow Battery Utilizing Ionic Active Materials

Nonaqueous redox flow batteries (NAqRFBs) are promising devices for grid‐scale energy storage, but high projected prices could limit commercial prospects. One route to reduced prices is to minimize or eliminate the expensive supporting salts typically employed in NAqRFBs. Herein, the feasibility of a flow cell operating in the absence of supporting salt by utilizing ionic active species is demonstrated. These ionic species have high conductivities in acetonitrile (12–19 mS cm−1) and cycle at 20 mA cm−2 with energy efficiencies (>75 %) comparable to those of state‐of‐the‐art NAqRFBs employing high concentrations of supporting salt. A chemistry‐agnostic techno‐economic analysis highlights the possible cost savings of minimizing salt content in a NAqRFB. This work offers the first demonstration of a NAqRFB operating without supporting salt. The associated design principles can guide the development of future active species and could make NAqRFBs competitive with their aqueous counterparts.Salt‐free cell: Decreasing the contribution of salt costs to the total electrolyte cost for nonaqueous redox flow batteries is essential for economic viability. A nonaqueous flow battery utilizing ionic active materials completely removes the need for a supporting salt. The cell cycling performance and area‐specific specific resistance are comparable to those of state‐of‐the‐art nonaqueous flow cells with high salt concentrations.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/137469/1/cssc201700028-sup-0001-misc_information.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137469/2/cssc201700028.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137469/3/cssc201700028_am.pd

Engineering Redox Flow Battery Electrodes with Spatially Varying Porosity Using Non-Solvent-Induced Phase Separation

Redox flow batteries (RFBs) are a promising electrochemical platform for efficiently and reliably delivering electricity to the grid. Within the RFB, porous carbonaceous electrodes facilitate electrochemical reactions and distribute the flowing electrolyte. Tailoring electrode microstructure and surface area can improve RFB performance, lowering costs. Electrodes with spatially varying porosity may increase electrode utilization and provide surface area in reaction-limited zones; however, the efficacy of such designs remains an open area of research. Herein, a non-solvent-induced phase-separation (NIPS) technique that enables the reproducible synthesis of macrovoid-free electrodes with well-defined across-thickness porosity gradients is described. The monotonically varying porosity profile is quantified and the physical properties and surface chemistries of porosity-gradient electrodes are compared with macrovoid-containing electrode, also synthesized by NIPS. Then, the electrochemical and fluid dynamic performance of the porosity-gradient electrodes is evaluated, exploring the effect of changing the direction of the porosity gradient and benchmarking against the macrovoid-containing electrode. Lastly, the performance is examined in a vanadium RFB, finding that the porosity-gradient electrode outperforms the macrovoid electrode, is independent of gradient direction, and performs favorably compared to advanced electrodes in the contemporary literature. It is anticipated that the approach motivates further exploration of microstructurally tailored electrodes in electrochemical systems.</p

Evaluation of Electrospun Fibrous Mats Targeted for Use as Flow Battery Electrodes

Electrospinning was used to create custom-made fibrous electrode materials for redox flow batteries with targeted structural properties. The aim was to increase the available surface area for electrochemical reaction without diminishing the transport properties of the electrode. Electrospinning conditions were identified that could produce fibers several times larger than those typically yielded by the technique, yet much smaller than in commercially available electrodes. These materials were subsequently carbonized using widely reported protocols. The resultant materials were subjected to a range of characterization tests to confirm that the feasibility of the target material, including surface area, pore and fiber sizes, porosity, conductivity, and permeability. The most promising material to emerge from this selection processes was then tested for electrochemical performance in a flow cell. The produced material performed markedly better than a commercially available material. Further optimizations such as improved consistency in the production and some surface activation treatments could provide significant advancements.NSERC Discovery grant Post-Graduate Scholarship program Eugenie-Ulmer Lamothe Fund of Department of Chemical Engineering at McGill Flow cell testing funded by the Joint Center for Energy Storage Research (JCESR) managed by Argonne National Laborator

An Investigation of the Ionic Conductivity and Species Crossover of Lithiated Nafion 117 in Nonaqueous Electrolytes

Nonaqueous redox flow batteries are a fast-growing area of research and development motivated by the need to develop low-cost energy storage systems. The identification of a highly conductive, yet selective membrane, is of paramount importance to enabling such a technology. Herein, we report the swelling behavior, ionic conductivity, and species crossover of lithiated Nafion 117 membranes immersed in three nonaqueous electrolytes (PC, PC : EC, and DMSO). Our results show that solvent volume fraction within the membrane has the greatest effect on both conductivity and crossover. An approximate linear relationship between diffusive crossover of neutral redox species (ferrocene) and the ionic conductivity of membrane was observed. As a secondary effect, the charge on redox species modifies crossover rates in accordance with Donnan exclusion. The selectivity of membrane is derived mathematically and compared to experimental results reported here. The relatively low selectivity for lithiated Nafion 117 in nonaqueous conditions suggests that new membranes are required for competitive nonaqueous redox flow batteries to be realized. Potential design rules are suggested for the future membrane engineering work.United States. Dept. of Energy. Office of Basic Energy Sciences. Joint Center for Energy Storage Researc

The lightest organic radical cation for charge storage in redox flow batteries

In advanced electrical grids of the future, electrochemically rechargeable fluids of high energy density will capture the power generated from intermittent sources like solar and wind. To meet this outstanding technological demand there is a need to understand the fundamental limits and interplay of electrochemical potential, stability, and solubility in low-weight redox-active molecules. By generating a combinatorial set of 1,4-dimethoxybenzene derivatives with different arrangements of substituents, we discovered a minimalistic structure that combines exceptional long-term stability in its oxidized form and a record-breaking intrinsic capacity of 161 mAh/g. The nonaqueous redox flow battery has been demonstrated that uses this molecule as a catholyte material and operated stably for 100 charge/discharge cycles. The observed stability trends are rationalized by mechanistic considerations of the reaction pathways.United States. Dept. of Energy. Office of Basic Energy Sciences. Chemical Sciences, Geosciences, & Biosciences Division (Contract DE-AC02-06CH11357

Microfluidic platforms for the investigation of fuel cell catalysts and electrodes

A clear need exists for novel approaches to producing and utilizing energy in more efficient ways, in light of society’s ever increasing demand as well as growing concerns with respect to climate change related to CO2 emissions. The development of low temperature fuel cell technologies will continue to play an important role in many alternative energy conversion strategies, especially for portable electronics and automotive applications. However, widespread commercialization of fuel cell technologies has yet to be achieved due to a combination of high costs, poor durability and, system performance limitations (Chapter 1). Developing a better understanding of the complex interplay of electrochemical, transport, and degradation processes that govern the performance and durability of novel fuel cell components, particularly catalysts and electrodes, within operating fuel cells is critical to designing robust, inexpensive configurations that are required for commercial introduction. Such detailed in-situ investigations of individual electrode processes are complicated by other factors such as water management, uneven performance across electrodes, and temperature gradients. Indeed, too many processes are interdependent on the same few variable parameters, necessitating the development of novel analytical platforms with more degrees of freedom. Previously, membraneless microfluidic fuel cells have been developed to address some of the aforementioned fuel cell challenges (Chapter 2). At the microscale, the laminar nature of fluid flow eliminates the need for a physical barrier, such as a stationary membrane, while still allowing ionic transport between electrodes. This enables the development of many unique and innovative fuel cell designs. In addition to addressing water management and fuel crossover issues, these laminar flow-based systems allow for the independent specification of individual stream compositions (e.g., pH). Furthermore, the use of a liquid electrolyte enables the simple in-situ analysis of individual electrode performance using an off-the-shelf reference electrode. These advantages can be leveraged to develop microfluidic fuel cells as versatile electro-analytical platforms for the characterization and optimization of catalysts and electrodes for both membrane- and membraneless fuel cells applications. To this end, a microfluidic hydrogen-oxygen (H2/O2) fuel cell has been developed which utilizes a flowing liquid electrolyte instead of a stationary polymeric membrane. For analytical investigations, the flowing stream (i) enables autonomous control over electrolyte parameters (i.e., pH, composition) and consequently the local electrode environments, as well as (ii) allows for the independent in-situ analyses of catalyst and/or electrode performance and degradation characteristics via an external reference electrode (e.g., Ag/AgCl). Thus, this microfluidic analytical platform enables a high number of experimental degrees of freedom, previously limited to a three-electrode electrochemical cell, to be employed in the construct of working fuel cell. Using this microfluidic H2/O2 fuel cell as a versatile analytical platform, the focus of this work is to provide critical insight into the following research areas: • Identify the key processes that govern the electrode performance and durability in alkaline fuel cells as a function of preparation methods and operating parameters (Chapter 3). • Determine the suitability of a novel Pt-free oxygen reduction reaction catalyst embedded in gas diffusion electrodes for acidic and alkaline fuel cell applications (Chapter 4). • Establish electrode structure-activity relationships by aligning in-situ electrochemical analyses with ex-situ microtomographic (MicroCT) structural analyses (Chapter 5). • Investigate the feasibility and utility of a microfluidic-based vapor feed direct methanol fuel cell (VF-DMFC) configuration as a power source for portable applications (Chapter 6). In all these areas, the information garnered from these in-situ analytical platforms will advance the development of more robust and cost-effective electrode configurations and thus more durable and commercially-viable fuel cell systems (both membrane-based and membraneless)