Abstract:Storage of photovoltaic and wind electricity in batteries could solve the mismatch problem between the intermittent supply of these renewable resources and variable demand. Flow batteries permit more economical long-duration discharge than solid-electrode batteries by using liquid electrolytes stored outside of the battery. We report an alkaline flow battery based on redox-active organic molecules that are composed entirely of earth-abundant elements and are non-toxic, non-flammable, and safe for use in residential and commercial environments. The battery operates efficiently with high power density near room temperature. These results demonstrate the stability and performance of redox-active organic molecules in alkaline flow batteries, potentially enabling cost-effective stationary storage of renewable energy. Main Text:The cost of photovoltaic (PV) and wind electricity has dropped so much that one of the largest barriers to getting the vast majority of our electricity from these renewable sources is their intermittency (1-3). Batteries provide a means to store electrical energy; however, traditional, enclosed batteries maintain discharge at peak power for far too short a duration to adequately regulate wind or solar power output (1, 2). In contrast, flow batteries can independently scale the power and energy components of the system by storing the electro-active species outside the battery container itself (3)(4)(5). In a flow battery, the power is generated in a device resembling a fuel cell, which contains electrodes separated by an ion-permeable membrane. Liquid solutions of redox-active species are pumped into the cell where they can be charged and discharged, before being returned to storage in an external storage tank. Scaling the amount of energy to be stored thus involves simply making larger tanks ( Fig 1A). Existing flow batteries are based on metal ions in acidic solution but there are challenges with corrosivity, hydrogen evolution, kinetics, material cost and abundance, and efficiency that thus far have prevented large-scale commercialization. The use of anthraquinones in an acidic aqueous flow battery can dramatically reduce battery costs (6, 7); however, the use of bromine in the other half of the system precludes deployment in residential communities due to toxicity concerns.We demonstrate that quinone-based flow batteries can be adapted to alkaline solutions, where hydroxylated anthraquinones are highly soluble and bromine can be replaced with the non-toxic ferricyanide ion (8, 9) -a food additive (10). Functionalization of 9,10-anthraquinone (AQ) with electron-donating groups such as OH has been shown to lower the reduction potential and expand the battery voltage (6). In alkaline solution, these OH groups are deprotonated to provide solubility and greater electron donation capability, which results in an increase in the open circuit voltage of 47% over the previously reported system. Because functionalization away from the ketone group provides molecules with the highest solubility...

Rational Design of Electrolyte Material 84Designing an appropriate organic molecule as electrolyte material starts from 85 identifying redox-active cores followed by functionalization of the core structure to 86 achieve a practical reduction potential and solubility. We observed that riboflavin 5' (Fig. 1c). The larger separation between its oxidation 102 and reduction peaks than those of FMN and lumichrome is likely due to slower kinetics. 103From our rotating disk electrode (RDE) measurement, the reduction rate constant was 104 measured to be 1.2±0.2 × 10 −5 cm s −1 ( Supplementary Fig. 3). Nevertheless, this value is 105 still an order of magnitude higher than that of the slower side of all-vanadium RFBs. 6 106Besides the large shift in reduction potential moving from isoalloxazine to 107 alloxazine, we also observed a significant increase in chemical stability in alkaline 10% to 90% SOC (Fig 2a). Polarization studies conducted at room temperature showed a 128 peak power density of 0.35 W cm −2 at a current density of 0.58 A cm −2 . The linearity of 129 the polarization curves allows us to derive a polarization area-specific resistance (ASR), 130 which is 1.03 Ω cm 2 at 50% SOC. About 70% of this cell ASR is contributed by the 131 membrane ( Supplementary Fig. 6), similar to our previous observation. 10 Note that ACA ACA was evaluated based on an extended charge-discharge study over 400 cycles (Fig. 136 2c). The current efficiency exceeded 99.7% at 0.1 A cm -2 , which is indicative of Fig. 7). From this result, the measured capacity retention from 400 cycles was 95%, i.e. 144the loss rate was 0.013% per cycle. We believe the discrepancy between this 145 measurement and the capacity retention observed during constant-current cycling was 146 due to an increase of system resistance (which we infer from decreasing energy 147 efficiency with cycle number); this effect moved the charging and discharging curves 148 closer to the cutoff voltages, resulting in less complete charging and discharging with 149 increasing cycle count ( Supplementary Fig. 8). We expect further cell development, 150including variations in pH, membrane and sealing method, to lead to further improvement 151 of capacity retention. By increasing the concentration of ACA to 1 M, we increased the 152 electrolyte charge density by almost two-fold ( Supplementary Fig. 9a). Together with 153 adjusted cell compression and higher ACA concentration, we were able to improve 154 round-trip energy efficiency to 74%, while retaining the same level of current efficiency 155 (99.7%) and capacity retention per cycle (99.95%) (Supplementary Fig. 9b). 156 Theoretical Modeling and Screening 157One useful feature of organic electrolyte materials is the ability to optimize their 158 properties through chemical modification, a process that can be accelerated by virtual 159 testing with computational methods. 8,20 We assayed the chemical landscape around the (Fig 3c and d). (Fig. 3b) 210Chemical synthesis and characterization 3,4-diaminophenol was purcha...

This work demonstrates a new, organic redox-flow battery (RFB) that outlives its predecessors, offering the longest-lived high-performance organic flow battery to date. It appears to be the first aqueous-soluble organic RFB chemistry to meet all the technical criteria for commercialization. The potential low reactant and membrane costs of this chemistry offer the potential for RFBs of this type to be used cost effectively at the gigawatt scale in order to enable massive penetration of intermittent renewable electricity.

dendrites, [6] guided lithium plating, [7] and nanostructured electrode design. [8] Among all the methods, the focus on solidelectrolyte interphase (SEI) between anode materials and electrolyte is one of the most critical issues. During LMB operation, the SEI that primarily originated from electrolyte decomposition, is easily cracked. This will locally enhance ion flux and promote nonuniform lithium depositing/stripping, [9] resulting in Li dendrites that can trigger internal short circuit and compromise battery safety. Repeated breakdown and repair of SEI during cycling create a vicious cycle which alternates between "uneven stripping/plating and SEI fracture," brings about continuous loss of active materials and limited battery cycle life. Therefore, an ideal SEI should continuously passivate the anode and prevent the parasitic reactions between reactive anode and electrolyte to address the aforementioned problems in principle. [3] Previous studies have demonstrated several effective artificial SEI to protect lithium metal anode such as polymer, [10] inorganic conductive compounds, [11,12] electrolyte additives, [13,14] and carbonbased materials. [7,15] However, the evolution of SEI during cycling and key mechanisms such as impact of SEI quality on its stability need to be further explored. [16] Herein, we demonstrate a "simultaneous homogeneous and high ionic conductivity" strategy by developing a method of forming a uniform lithium sulfide (Li 2 S) protective layer for suppressing dendrite growth and stabilizing the lithium metal anode. Although Li 2 S interfacial layers through soluble electrolyte additives have been studied before, [14,[17][18][19][20] the work here demonstrates that the elevated temperature (170 °C) and gas phase reaction are critical for the synthesis of a homogenous Li 2 S coating, which importantly can be used as SEI in carbonate electrolyte system. We reveal the evolution of thus formed Li 2 S artificial SEI component distribution during battery operation: the uniform and high ionic conductivity protective layer turns into a layered SEI that preserves protective function, rather than into a disordered, broken SEI mainly made up of parasitic reaction products. Simulation results also confirm the critical importance of compositional homogeneity and high ionic conductivity in stabilizing SEI. With this strategy, stable cycles in both high capacity symmetric cells and Li-LiFePO 4 full cells were realized. We believe that this practical fabrication method, fundamental design strategy, and understanding on Artificial solid-electrolyte interphase (SEI) is one of the key approaches in addressing the low reversibility and dendritic growth problems of lithium metal anode, yet its current effect is still insufficient due to insufficient stability. Here, a new principle of "simultaneous high ionic conductivity and homogeneity" is proposed for stabilizing SEI and lithium metal anodes. Fabricated by a facile, environmentally friendly, and low-cost lithium solidsulfur vapor reaction at elevated tempe...

Approaches for regulated fluid secretion, which typically rely on fluid encapsulation and release from a shelled compartment, do not usually allow for a fine, continuous modulation of secretion, and can be difficult to adapt for monitoring or functionintegration purposes. 1-5 Here, we report self-regulated, self-reporting secretion systems consisting of liquid-storage compartments in a supramolecular polymer-gel matrix with a thin liquid layer on top, and demonstrate that dynamic liquid exchange between the compartments, matrix and surface layer allows for repeated, responsive self-lubrication of the surface layer and for cooperative healing of the matrix. Depletion of the surface liquid or local material damage induces self-regulated secretion of the stored liquid via a dynamic feedback between polymer crosslinking, droplet shrinkage and liquid transport that can be read out through changes in the system's optical transparency. We envision diverse applications in fluid delivery, wetting and adhesion control, and material self-repair.Nearly every form of living tissue autonomously packages, transports, and secretes fluids, mediating defense, adhesion, wound healing, temperature -often several of these at once -through tightly self-regulated release systems. [6][7][8][9] Fundamental to these systems, fluid storage is itself an active, finely regulated balance. Storage droplets or vesicles continuously adjust their size, shape and contents through ongoing exchange with the surroundings, creating intrinsically responsive control mechanisms that tie secretion to a wide range of chemical and physical stimuli and feedback signals. [10][11][12][13] At the same time, collective changes in the stores are reported to the organism, alerting it that it needs to drink or eat to replenish the limited supply. Many synthetic approaches have been developed to enable triggered release from microcapsules, hydrogels, nanoparticles, vesicles, micelles, mesoporous carriers and other containers. [1][2][3][4][5][14][15][16][17] While these systems can secrete fluid in response to various stimuli, it remains a challenge to design a synthetic approach that displays finely tuned, continuous self-adjustment, integrated functionalities, and continuous liquid supply monitoring.2 Figure 1. Schematic of the self-regulated, liquid secretion system. Secretion liquid is stored as shell-less droplets inside a gel matrix composed of dynamic polymers, with ongoing liquid exchange between droplet and gel phases. If S = γ ga -(γ la + γ gl ) > 0, the matrix surface will be coated with a thin liquid overlayer. When this layer is removed, the disjoining pressure will trigger secretion of the stored liquid to restore the original film thickness, while the supramolecular gel matrix reconfigures through reversible bond disassembly and reassembly to release any buildup of mechanical stress due to shrinking droplets. With successive removal/restoring cycles, the liquid droplets will continuously shrink and the gel will become progressively transparent.Inspire...