An Aqueous Rechargeable Fluoride Ion Battery with Dual Fluoride Electrodes

Aqueous redox flow batteries are potentially well suited for grid scale energy storage for their uncoupled power and energy, safety, cost-effectiveness and longevity.[1] Although organic aqueous flow batteries with quinone redox active species have demonstrated promising results, including high stability, > 1 V open-circuit potential and high solubility, during operation they tend to be solubility-limited at moderate pH and, due to proton-coupled electron transfer, tend to swing the electrolyte pH to extreme values during cycling.[2-5]Viologens, another class of redox active organic molecules, are soluble regardless of solution pH and their redox reactions do not involve coupled protons or hydroxides, thus enabling stable pH during cycling. However, previously reported viologen-based flow batteries suffer from high capacity fade rates, high active material permeability, or low power density.[6-9] Here we present a highly stable phosphonate-functionalized viologen as the redox-active species in or aqueous redox flow batteries (ARFBs) operating at nearly neutral pH. The solubility is 1.23 M and the reduction potential is the lowest of any substituted viologen utilized in a flow battery, reaching -0.462 V vs. SHE at pH 9. The negative charges in both the oxidized and the reduced states of 1,1'-bis(3-phosphonopropyl)-[4,4'-bipyridine]-1,1'-diium dibromide (BPP - Vi) effect low permeability in cation exchange membranes and suppress a bimolecular mechanism of viologen decomposition. A flow battery pairing BPP-Vi with a ferrocyanide-based posolyte across an inexpensive, non-fluorinated cation exchange membrane at pH = 9 exhibits an open-circuit voltage of 0.9 V and an extremely low capacity fade rate of 0.016%/day or 0.00069%/cycle. Overcharging leads to viologen decomposition, causing irreversible capacity fade. This work introduces extremely stable, extremely low-permeating and low reduction potential redox active materials into near neutral ARFBs. Figure Inserted Here Figure Caption: Extended cycling at 40 mA cm-2 of a 1 m BPP-Vi | Fe(CN)6 flow cell. Electrolytes comprised 6.2 mL of 1.0 m BPP - Vi titrated with 14 m NH4OH to pH = 9 (negolyte) and 40 mL of 0.3 m K4Fe(CN)6 and 0.3 m K3Fe(CN)6 in 2 m NH4Cl at pH = 9 (posolyte). The average coulombic efficiency was >99.95%. The fitted discharge fade rate was 0.016%/day or 0.00069%/cycle. References 1T. Nguyen and R.F. Savinell, "Flow Batteries", The Electrochemical Society Interface 54 (2010). 2B. Huskinson, M.P. Marshak, C. Suh, S. Er, M.R. Gerhardt, C.J. Galvin, X. Chen, A. Aspuru-Guzik, R.G. Gordon, and M.J. Aziz, "A Metal-Free Organic-Inorganic Aqueous Flow Battery", Nature 505, 195 (2014). 3D.G. Kwabi, K. Lin, Y. Ji, E.F. Kerr, M.-A. Goulet, D. De Porcellinis, D.P. Tabor, D.A. Pollack, A. Aspuru-Guzik, R.G. Gordon, and M.J. Aziz, "Alkaline Quinone Flow Battery with Long Lifetime at pH 12", Joule 2, 1907 (2018). 4Y. Ji, M.A. Goulet, D.A. Pollack, D.G. Kwabi, S. Jin, D. Porcellinis, E.F. Kerr, R.G. Gordon, and M.J. Aziz, "A Phosphonate‐Functionalized Quinone Redox Flow Battery at near‐Neutral pH with Record Capacity Retention Rate", Advanced Energy Materials 9, 1900039 (2019). 5S. Jin, Y. Jing, D.G. Kwabi, Y.L. Ji, L.C. Tong, D. De Porcellinis, M.A. Goulet, D.A. Pollack, R.G. Gordon, and M.J. Aziz, "A Water-Miscible Quinone Flow Battery with High Volumetric Capacity and Energy Density", ACS Energy Letters 4, 1342 (2019). 6E.S. Beh, D. De Porcellinis, R.L. Gracia, K.T. Xia, R.G. Gordon, and M.J. Aziz, "A Neutral pH Aqueous Organic-Organometallic Redox Flow Battery with Extremely High Capacity Retention", ACS Energy Letters 2, 639 (2017). 7B.H. Camden DeBruler, Jared Moss, Jian Luo, T. Leo Liu, "A Sulfonate-Functionalized Viologen Enabling Neutral Cation Exchange, Aqueous Organic Redox Flow Batteries toward Renewable Energy Storage", ACS Energy Letters 3, 663 (2018). 8J. Luo, B. Hu, C. Debruler, Y.J. Bi, Y. Zhao, B. Yuan, M.W. Hu, W.D. Wu, and T.L. Liu, "Unprecedented Capacity and Stability of Ammonium Ferrocyanide Catholyte in pH Neutral Aqueous Redox Flow Batteries", Joule 3, 149 (2019). 9T. Janoschka, N. Martin, M.D. Hager, and U.S. Schubert, "An Aqueous Redox-Flow Battery with High Capacity and Power: The TEMPTMA/MV System", Angew Chem Int Ed Engl 55, 14427 (2016). Figure 1

Electrochemical Energy Storage with Mediator-Ion Solid Electrolytes

Rechargeable batteries with high energy density have been required for recent applications such as battery electric vehicles. Fluoride ion batteries (FIBs) can achieve high energy density by using monovalent fluoride ions as charge carriers and multi-electron reactions of electrode active materials, and have attracted much attention as a candidate to surpass the performance of the current lithium-ion battery (LIB). However, FIB using the metal/metal fluoride conversion reaction, which has a high potential for energy density, has not been fabricated with good reversibility as the current LIB. On the other hand, the FIB using a crystal capable of intercalation (insertion-extraction) reaction of fluoride ions as the active material has been developed [1]. Recently, an all-solid-state fluoride ion battery using an oxyfluorosulfide (Sr2F2Fe2OS2; SFFOS) as the intercalation-type positive active material has been reported, and the battery has operated with high capacity and good reversibility at 413 K [2]. In this study, SFFOS was evaluated as an active material for liquid-based FIB. In particular, the thermal effect on the intercalation reaction of SFFOS in ionic liquid-based electrolyte solution was measured.The SFFOS was synthesized from four kinds of powder, SrF2, SrO, Fe and S, with molar ratio of 1:1:2:2 referring to the previous papers [2, 3]. These powders were mechanically ground under argon and pressed at 6 MPa to form a pellet. The resulting pellet was vacuum sealed in a quartz tube and sintered at 1073 K for 36 hours to obtain the SFFOS. PbSnF4 as the counter electrode material was prepared from PbF2 and SnF2 by mechanical milling at 600 rpm for 6 h using a planetary ball mill, and then calcined at 673 K for 1 h under an argon atmosphere. The electrolyte solution was prepared by dissolving anhydrous tetramethylammonium fluoride (TMAF) in an ionic liquid, N,N,N-trimethyl-N-propylammonium-bis(trifluoromethanesulfonyl)amide (TMPA-TFSA), at various concentrations such as 0.075 mol/dm3 (the molar ratio of TMAF:TMPA-TFSA=1:50). The working and counter electrodes were prepared by mixing of SFFOS or PbSnF4 with acetylene black and PVdF, respectively. The galvanostatic intercalation reaction of fluoride ions into SFFOS was measured using a three-electrode cell with a Pb|PbF2 reference electrode at 298, 373, and 423 K.The galvanostatic intercalation reaction (corresponding to a charging reaction) of the SFFOS/AB/PVdF composite electrode showed the charging plateau at about 0.5 V vs. Pb|PbF2. The following deintercalation (discharging) reaction, the discharge plateau was observed around 0 V. The intercalation/deintercalation plateaus were similar to those of the previously reported all-solid-state FIB, well [2].

A fluoride-ion battery (FIB) is a novel type of energy storage system that has a higher volumetric energy density and low cost. However, the high working temperature (>150 °C) and unsatisfactory cycling performance of cathode materials are not favorable for their practical application. Herein, fluoride ion-intercalated CoFe layered double hydroxide (LDH) (CoFe-F LDH) was prepared by a facile co-precipitation approach combined with ion-exchange. The CoFe-F LDH shows a reversible capacity of ∼50 mAh g-1 after 100 cycles at room temperature. Although there is still a big gap between FIBs and lithium-ion batteries, the CoFe-F LDH is superior to most cathode materials for FIBs. Another important advantage of CoFe-F LDH FIBs is that they can work at room temperature, which has been rarely achieved in previous reports. The superior performance stems from the unique topochemical transformation property and small volume change (∼0.82%) of LDH in electrochemical cycles. Such a tiny volume change makes LDH a zero-strain cathode material for FIBs. The 2D diffusion pathways and weak interaction between fluoride ions and host layers facilitate the de/intercalation of fluoride ions, accompanied by the chemical state changes of Co2+/Co3+ and Fe2+/Fe3+ couples. First-principles calculations also reveal a low F- diffusion barrier during the cyclic process. These findings expand the application field of LDH materials and propose a novel avenue for the designs of cathode materials toward room-temperature FIBs.

The ability to store large amounts of electrical energy is of increasing importance with the growing fraction of electricity generation from intermittent renewable sources such as wind and solar. We have developed high performance flow batteries based on the aqueous redox behavior of small organic and metalorganic molecules, e.g. [1-9]. These redox active materials can be inexpensive and exhibit rapid redox kinetics, high solubilities, and long lifetimes, although short lifetimes are more common [7, 10]. We will discuss the economic tradeoff between upfront capital cost and periodic chemical replacement cost [11]. We discuss the very few chemistries with long enough calendar life for practical application in stationary storage [3-6, 9], and on the prospects for reversing capacity fade by recomposing decomposed molecules [8, 12].[1] B. Huskinson, M.P. Marshak, C. Suh, S. Er, M.R. Gerhardt, C.J. Galvin, X. Chen, A. Aspuru-Guzik, R.G. Gordon and M.J. Aziz, "A metal-free organic-inorganic aqueous flow battery", Nature 505, 195 (2014), http://dx.doi.org/10.1038/nature12909 [2] K. Lin, Q. Chen, M.R. Gerhardt, L. Tong, S.B. Kim, L. Eisenach, A.W. Valle, D. Hardee, R.G. Gordon, M.J. Aziz and M.P. Marshak, "Alkaline Quinone Flow Battery", Science 349, 1529 (2015), http://dx.doi.org/10.1126/science.aab3033 [3] E.S. Beh, D. De Porcellinis, R.L. Gracia, K.T. Xia, R.G. Gordon and M.J. Aziz, "A Neutral pH Aqueous Organic/Organometallic Redox Flow Battery with Extremely High Capacity Retention", ACS Energy Letters 2, 639 (2017). http://dx.doi.org/10.1021/acsenergylett.7b00019 [4] D.G. Kwabi, K. Lin, Y. Ji, E.F. Kerr, M.-A. Goulet, D. DePorcellinis, D.P. Tabor, D.A. Pollack, A. Aspuru-Guzik, R.G. Gordon, and M.J. Aziz, “Alkaline Quinone Flow Battery with Long Lifetime at pH 12” Joule 2, 1907 (2018). https://doi.org/10.1016/j.joule.2018.07.005 [5] Y. Ji, M.-A. Goulet, D.A. Pollack, D.G. Kwabi, S. Jin, D. DePorcellinis, E.F. Kerr, R.G. Gordon, and M.J. Aziz, “A phosphonate-functionalized quinone redox flow battery at near-neutral pH with record capacity retention rate” Advanced Energy Materials 2019, 1900039; https://doi.org/10.1002/aenm.201900039 [6] M. Wu, Y. Jing, A.A. Wong, E.M. Fell, S. Jin, Z. Tang, R.G. Gordon and M.J. Aziz, “Extremely Stable Anthraquinone Negolytes Synthesized from Common Precursors” Chem 6, 1432 (2020); https://doi.org/10.1016/j.chempr.2020.03.021 [7] M.-A. Goulet & M.J. Aziz, “Flow Battery Molecular Reactant Stability Determined by Symmetric Cell Cycling Methods”, J. Electrochem. Soc. 165, A1466 (2018). http://dx.doi.org/10.1149/2.0891807jes [8] M.-A. Goulet, L. Tong, D.A. Pollack, D.P. Tabor, S.A. Odom, A. Aspuru-Guzik, E.E. Kwan, R.G. Gordon, and M.J. Aziz, “Extending the lifetime of organic flow batteries via redox state management” J. Am. Chem. Soc. 141, 8014 (2019); https://doi.org/10.1021/jacs.8b13295 [9] M. Wu, M. Bahari, E.M. Fell, R.G. Gordon and M.J. Aziz, “High-performance anthraquinone with potentially low cost for aqueous redox flow batteries” J. Mater. Chem. A 9, 26709-26716 (2021). https://doi.org/10.1039/D1TA08900E [10] D.G. Kwabi, Y.L. Ji and M.J. Aziz, “Electrolyte Lifetime in Aqueous Organic Redox Flow Batteries: A Critical Review” Chem. Rev. 120, in press (2020); https://doi.org/10.1021/acs.chemrev.9b00599 [11] F.R. Brushett, M.J. Aziz and K.E. Rodby, “On lifetime and cost of redox-active organics for aqueous flow batteries” Invited Viewpoint article for ACS Energy Letters. 5, 879 (2020); https://doi.org/10.1021/acsenergylett.0c00140 [12] Y. Jing, E.W. Zhao, M.-A. Goulet, M. Bahari, E. Fell, S. Jin, A. Davoodi, Erlendur Jónsson, M. Wu, C.P. Grey, R.G. Gordon and M.J. Aziz, “Closing the Molecular Decomposition-Recomposition Loop in Aqueous Organic Flow Batteries” Nature Chemistry, in press (2022). Preprint: http://dx.doi.org/10.33774/chemrxiv-2021-x05x1

Recently reversible batteries based on fluoride ions as a charge carriers have attracted some attentions as an alternative electrochemical energy storage system to conventional lithium ion batteries (LIBs). Fluoride is the most stable anion with a high mobility and therefore, fluoride ion batteries (FIBs) can theoretically provide a wide electrochemical potential window. Moreover, FIBs are capable of being built in an all solid-state modification. Previously, electrochemical fluoride ion cells based on conversion-based electrode materials have been built. However, the state of the art of the FIBs suffer from poor cycling performance in lack of well-developed cell components including the electrode materials. In the current study, intercalation-based cathode materials have been investigated as an alternative approach to make electrode materials for FIBs. In this respect, various compounds with mainly Ruddlesden-Popper-type structure including LaSrMO4 (M = Mn, Co, Fe) and La2MO4+d (M = Co, Ni) as well as Schafarzikite-type compounds of Fe0.5M0.5Sb2O4 (M = Mg, Co) have been subjected to electrochemical measurements including galvanostatic cycling, cyclic voltammetry and electrochemical impedance spectroscopy and the structural changes upon electrochemical fluorination/de-fluorination were analyzed by X-ray Diffraction (XRD). LaSrMnO4 has been fluorinated/de-fluorinated via electrochemical method confirming successful intercalation/de-intercalation of the fluoride ions, but showed problems for long-term operation. In contrast, La2NiO4+d showed to be the most promising intercalation-based cathode material (for FIB) in terms of cycling stability (>220 cycles and 60 cycles for cutoff capacities of 30 and 50 mAh/g, respectivly) with a nearly 100% Coulombic efficiency (average Coulombic efficiency of 97.68% and 95.44% for cutoff capacities of 30 and 50 mAh/g, respectively). This is the highest cycle life that has been reported so far for a FIB. One of the major challenges of the proposed FIB systems was found in avoiding oxidation of the conductive carbon which has been mixed with the electrodes to improve the electronic conductivity. This decomposition of the carbon matrix results in a remarkable increase in the impedance of the cell and can significantly impair the cycle life and discharge capacity. However, the critical charging conditions which could be determined by cyclic voltammetry and electrochemical impedance spectroscopy have a major impact on preserving the conductivity of the cell. In addition, the effect of volume change in the conversion-based anode materials has been studied showing that the overpotentials arising from the volume change can significantly influence the cycling behavior of the battery system (due to absence of well-developed intercalation-based anode materials for FIBs, conversion-based counter electrodes have been used as anode materials).

Fluoride ion conductors are developed for use as solid-state electrolytes in fluoride ion batteries which are one of promising candidates for next-generation storage batteries. Ba-doped LaF3 (La0.9Ba0.1F2.9: LBF) is mainly used as a solid-state electrolyte in fluoride ion batteries. However, room temperature conductivity of LBF is considerably low, on the order of 10−6 S cm−1 and it is still unclear the optimal elements to be doped to LaF3. In this study, we have explored La0.9Sr x Ba0.1−x F2.9 system (x = 0, 0.01, 0.025, 0.05, 0.1), in which Ba in LBF is substituted for Sr and investigated the composition dependence of ionic conductivity. We elucidate that the higher concentration of Sr without Ba can significantly improve the ionic conductivity, and the maximum ionic conductivity of La0.9Sr0.1F2.9 is 1.5 × 10−5 S cm−1 at room temperature, which is one order of magnitude larger than that of LBF. The higher ionic conductivity of LSF is due to the larger grain size and higher sintering density of LSF compared to LBF, which results in lower grain boundary resistance. The LSF total ionic conductivity of 10−4 S cm−1 can be achieved at 350 K, which significantly lowers operating temperature of fluoride ion batteries down to 350 K.

Fluoride ion batteries (FIBs) have recently gained much attention as a next generation battery system because of their potential to surpass lithium-ion batteries (LIBs) in many aspects such as high energy density and low cost. FIBs with solid or liquid electrolyte have been studied intensively, amongst liquid-based FIBs has an advantage of good interfacial contact between the electrolyte and electrode, and low-temperature operation over all-solid-state FIBs. However, stability has been a major concern for liquid electrolytes. The instability caused by the strong basicity of F− ion attacking most of the commonly used organic solvents leading to side reactions.1 So far bis(2,2,2-trifluoroethyl) ether (BTFE), which has good base resistance, has been proposed as an effective organic solvent for electrolyte.2 However, the boiling point of BTFE is relatively low (64 °C). While looking beyond organic solvents, ionic liquids (ILs) can be a choice as an electrolyte for FIBs owing to their good properties such as nonvolatility and wide electrochemical window.3 It is commonly known that quaternary ammonium cations undergo Hofmann elimination, during which β-hydrogen is withdrawn by strong bases such as OH− and F− ions.4,5 Therefore, to use ILs as an electrolyte for FIB, it is necessary to improve their stability against F− ion. One way to weaken the basicity of F- ion is to solvate them by a complexing agent. Hagiwara et al. reported that imidazolium-based ILs containing ethylene glycol are stable despite the presence of F− ion.6 Bond formation between F− ion and hydrogen in the hydroxy groups of ethylene glycol and imidazolium-based cation prevents the F− ion from attacking the β-hydrogen of the cation's alkyl chain. However, there have been no reports of FIBs using ILs containing solvated F− ion as electrolytes. In this study, we investigated the thermal stability of choline bis(trifluoromethanesulfonyl)amide (N111(2OH) TFSA), an IL with hydrogen bond donor functional group, against F− ions. Furthermore, we performed charge-discharge tests on liquid-based FIBs using N111(2OH) TFSA containing fluoride salt as an electrolyte.N111(2OH) TFSA (Fig. 1 (a)) was synthesized by ion exchange of N111(2OH) Cl with K TFSA. The melting point of the synthesized N111(2OH) TFSA was estimated to be 38 °C from DSC measurement and it was solid at room temperature. 0.4 mol kg−1 tetramethylammonium fluoride (TMAF) in N111(2OH) TFSA was found to be stable up to about 150 °C (Fig. 1(c)). TGA measurements of N,N,N-trimethyl-N-propylammonium (N1113 TFSA) (Fig. 1(b)) without hydroxy groups were also performed under the same conditions for comparison. A solution of 0.4 mol kg−1 TMAF in N1113 TFSA showed a gradual weight loss right after the measurement was started, with a significant weight loss at about 120 °C (Fig. 1(c)). These results suggest that N111(2OH) TFSA with the hydroxy group exhibits higher thermal stability toward F− ion. This was attributed to the hydrogen bonding of the hydroxy group to F− ion, which weakened the basicity of F− ion. Finally, charge-discharge tests were performed in a three-electrode cell using BiF3 working electrode, Pb counter electrode, and Ag/Ag+ reference electrode. Figure 1 (d) shows charge-discharge curves of BiF3 electrode with 0.4 mol kg−1 TMAF in N111(2OH) TFSA as an electrolyte at 60 °C. The first discharge capacity of the BiF3 electrode was 300 mAh g−1, which represents 99 % of the theoretical capacity of BiF3 (302 mAh g−1) with a columbic efficiency of 85%. This result suggests that the BiF3 was reversibly defluorinated/fluorinated with 0.4 mol kg−1 TMAF in N111(2OH) TFSA.AcknowledgmentThis presentation is based on results obtained from a project, JPNP21006, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).References(1) V. K. Davis, et al., Mater. Chem. Front., 2019, 3, 2721-2727.(2) V. K. Davis, et al., Science, 2018, 362, 1144–1148.(3) K. Okazaki, et al., ACS Energy Lett., 2017, 2, 1460–1464(4) S. Raiguel, et al., Green Chem., 2020, 22, 5225-5252.(5) H. Sun, and S. G. Dimagno, J. Am. Chem. Soc., 2005, 127, 2050-2051.(6) Z. Chen, et al., J. Phys. Chem. Lett., 2018, 9, 6662−6667 Figure 1

Aqueous fluoride ion batteries (FIBs) have garnered attention for their high theoretical energy density, yet they are challenged by sluggish fluorination kinetics, active material dissolution, and electrolyte instability. Here, we present a room temperature rocking-chair aqueous FIBs featuring KOAc-KF binary salt electrolytes, enabling concurrent fluorination and defluorination reactions at both cathode and anode electrodes. Experimental and theoretical results reveal that acetate ions in the electrolyte compete with fluoride ions in hydrogen bonding formation, weakening the excessively strong solvation between H2O and F- ions. This results in the suppression of detrimental HF formation and a reduced desolvation energy of F- ions, enhancing the electrochemical reaction kinetics. The bismuth-based cathode exhibits direct conversion in the optimized electrolyte, effectively suppressing the detrimental disproportionation reactions from Bi2+ intermediates. Additionally, zinc anode undergoes a typical fluorination process, forming solid KZnF3 as the electrode product, minimizing the risks of hydrogen evolution. The proposed aqueous FIBs with the optimized electrolyte demonstrate high discharge capacity, long-term cycling stability and excellent rate capabilities.

Solid-state synthesis and ion transport characteristics of the β-KSbF4 for all-solid-state fluoride-ion batteries

Aqueous batteries are expected to significantly impact future grid energy storage systems due to their abundant raw materials, low commodity pricing, and non-flammable electrolyte formulations. While these attributes meet basic techno-economic expectations, optimizing materials and their interfaces is crucial to fully utilizing active material content, enable fast recharging, and achieve cycle life requirements for systems lasting 30 years or more. This talk will focus on the traditional lead-acid battery technology and the emerging organic redox aqueous electrolyte flow batteries by examining the fundamentals of the electrochemical interface in each system and identifying new avenues for accelerating their development.Despite being one of the oldest electrochemical technologies, current lead-acid batteries utilize only 50% of their materials for energy storage, are slow to charge, and have a shorter cycle life compared to Li-ion. To unlock their full capabilities, we must address fundamental limits of the discharge reaction on both negative and positive electrodes. By developing high-purity, well-defined Pb and PbO2 surfaces with nanometer-scale roughness, we will discuss how the discharge capacity is limited by surface passivation due to the insoluble discharge product, PbSO4. By examining the relationship between discharge rates and capacity, the Peukert relationship, under various conditions, we derive the Peukert equation from first principles, connecting thermodynamics, kinetics, and mass transport properties. This provides insights into controlling active material thickness for higher utilization at both electrodes. Importantly, we demonstrate how intrinsic properties of a flat interface relate to commercial electrode types, highlighting the relevance of fundamental knowledge to practical battery design.For recharging, size-selected and shape-controlled PbSO4 nanoparticles serve as an ideal platform to demonstrate the role of particle size and interfacial energy in determining lead sulfate practical charging rates in lead-acid electrodes. By understanding discharge and charge processes and the impact of additives on nucleation, growth dynamics, and dissolution kinetics, we will discuss the development of a high-throughput autonomous-ready electrolyte design platform. The Electrochemical Small Molecule Accelerated Reaction Testing platform (E-SMART) allows simultaneous mixing of up to six electrolyte streams in a microfluidic platform connected to an electrochemical flow cell, enabling evaluation of ~50 new molecules per week. This accelerates the creation of a structure-property database necessary for predicting functional groups relevant for enhancing discharge and charge properties. Insights from lead-acid batteries and modern tools can be applied to other dissolution/precipitation energy storage chemistries.In the second part, we explore electrode interface processes using nitroxide radicals like 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy (HT) as redox-active materials in aqueous electrolytes for redox flow batteries (RFBs). While low concentrations and fast charge rates show HT as highly reversible, high concentrations and slow rates reveal a surface passivation process limiting HT reversibility. Extensive characterization using SEM, XPS, and EQCM reveals a polymeric layer formation under specific conditions. We will discuss how surface reactions on the current collect can remove this passive film, restoring the electrode's ability to oxidize HT, offering a solution to secondary processes that may limit battery lifetime.Lastly, multiple factors influence the electrochemical interface properties, such as electrolyte solvent, co-solvents, anion and cation nature, and pH, thus requiring an accelerated experimental platform. We discuss the FLOW-AIDE: Flexible Laboratory for Optimizing Wide-ranging Autonomous Investigations and Discoveries in Electrochemistry. This system includes sixteen electrochemical testing channels, each with four peristaltic pump channels and selector valves, all controlled by software for electrolyte design and delivery, enabling complete automation and close-loop capabilities. FLOW-AIDE is chemistry agnostic, suitable for exploring various redox-active materials across different electrochemical systems, including aqueous and non-aqueous. Thus, combining fundamental interfacial investigations with accelerated electrochemical platforms move us closer to deploying electrochemical energy technologies. Acknowledgements: The research was conducted at Argonne National Laboratory, a U.S. Department of Energy Office of Science laboratory, operated by UChicago Argonne, LLC under Contract no. DE-AC02-06CH11357. The authors acknowledge the support from the Lead Battery Science Research Program and the American Battery Research Group via a Collaborative Research and Development Agreement. The submitted abstract 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. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan. http://energy.gov/downloads/doe-public-access-plan

Due to the energy intermittency of renewable energy sources and the imbalance of supply-demand, energy storage systems are necessary for the continued deployment of renewable energy. Redox flow batteries (RFBs) has been suggested as a promising technology for long term energy storage utilizing relatively low cost, aqueous organic redox species.1–3 Some aqueous organic negolytes have been developed to improve the capacity, voltage and stability of RFBs using common organic redox species e.g., anthraquinones4,5, phenazines6,7 and viologens8,9. However, finding a suitable organic redox species for the positive side is still challenging due to difficulties increasing the standard potential toward positive voltages. Ferri-/ferrocyanide is the most common species used as posolyte, but is difficult to modify apart from counterion exchange10. Another common organic posolyte, utilizes TEMPOL (4-Hydroxy-TEMPO) in neutral conditions 11. Although these posolytes are commonly utilized, they are limited to certain pH ranges.One of the most significant limitations for posolytes, which are electron-poor compounds by nature, is the difficulty in maintaining structural integrity as they become increasingly electrophilic during charging i.e., oxidation.In this work, development of imidazole-based posolytes will be presented concerning reversibility, solubility and stability. A substrate scope has been synthesized and assessed via voltammetry studies in dimethyl formamide and aqueous conditions. A model substrate (DPIP) was synthesized at a multigram scale, showing reversible potential in 1 M KOH with a potential of + 0.24 V vs. SHE. Additionally, DPIP was quasi-reversible in 1 M AcOH. Solubility was assessed in various aqueous conditions. Degradation mechanisms during cycling were quantified using 2D NMR methods, providing insight into the electrophilicity of these compounds. Since all substrates in this scope were tested without further functionalization, many variations can be explored to improve potential and solubility. Skyllas-Kazacos, M. & Menictas, C. Vanadium Redox Flow Batteries. in Encyclopedia of Energy Storage: Volume 1-4 vols 1–4 407–422 (Elsevier, 2022).González-González, J. M., Parrilla, P. & Aguado, J. A. Chemical energy storage technologies. in Encyclopedia of Electrical and Electronic Power Engineering: Volumes 1-3 vol. 1 V1-426-V1-439 (Elsevier, 2022).Wedege, K., Dražević, E., Konya, D. & Bentien, A. Organic Redox Species in Aqueous Flow Batteries: Redox Potentials, Chemical Stability and Solubility. Sci Rep 6, (2016).Wu, M. et al. Extremely Stable Anthraquinone Negolytes Synthesized from Common Precursors. Chem 6, 1432–1442 (2020).Hu, B., Luo, J., Hu, M., Yuan, B. & Liu, T. L. A pH‐Neutral, Metal‐Free Aqueous Organic Redox Flow Battery Employing an Ammonium Anthraquinone Anolyte. Angewandte Chemie 131, 16782–16789 (2019).Hollas, A. et al. A biomimetic high-capacity phenazine-based anolyte for aqueous organic redox flow batteries. Nat Energy 3, 508–514 (2018).Li, L., Su, Y., Ji, Y. & Wang, P. A Long-Lived Water-Soluble Phenazine Radical Cation. J Am Chem Soc (2022) doi:10.1021/jacs.2c12683.Hu, B. et al. Improved radical stability of viologen anolytes in aqueous organic redox flow batteries. Chemical Communications 54, 6871–6874 (2018).Luo, J., Hu, B., Debruler, C. & Liu, T. L. A π-Conjugation Extended Viologen as a Two-Electron Storage Anolyte for Total Organic Aqueous Redox Flow Batteries. Angewandte Chemie 130, 237–241 (2018).Luo, J. et al. Unprecedented Capacity and Stability of Ammonium Ferrocyanide Catholyte in pH Neutral Aqueous Redox Flow Batteries. Joule 3, 149–163 (2019).Liu, Y. et al. A Long-Lifetime All-Organic Aqueous Flow Battery Utilizing TMAP-TEMPO Radical. Chem 5, 1861–1870 (2019).

A flexible tysonite-type La0.95Ba0.05F2.95@PEO-based composite electrolyte for the application of advanced fluoride ion battery

A key aspect of any future battery technology development is safety. Although lithium-based batteries are ubiquitous, there are still challenges related to their energy density, cycle life, cost and safety. In regard to safety, compared with organic electrolyte, aqueous rechargeable batteries may provide a safer alternative for reliable, low-cost and large-scale energy storage systems. As seen from the penetration test in Fig. 1a-1b, the battery with organic electrolyte catches fire, yet the battery with aqueous electrolyte is relatively safe. Moreover, aqueous batteries have high ion conductivity and cost effectiveness. Generally, the cell voltage and energy density of aqueous batteries are lower than those of organic-based batteries (e.g. Li-ion) because of the relatively smaller electrochemical stability window of water. Among all the metals that are stable in water, zinc is the most active and has the lowest possible operating potential. This means using Zn anode can increase overall cell voltage of aqueous batteries. Moreover, zinc is globally available, inexpensive (3.19 USD kg-1), and has high capacity (820 Ah kg-1 and 5854 Ah L-1). Zinc-based aqueous batteries also possess the stability to be operated in ambient air. Accordingly, Zn aqueous rechargeable batteries are promising to become a safer energy storage system. Among zinc-based aqueous batteries, Zn-air batteries have high theoretical volumetric energy density, which is around three times that of conventional Li-ion batteries (LIB). Zn anodes have been investigated in neutral/mild acidic aqueous electrolytes. Yet in order to pair them with oxygen cathode to reach the highest energy density, alkaline aqueous electrolyte is ideal, in which the oxygen electrode has low polarization. In alkaline aqueous electrolyte, Zn anode undergoes a Zn (s) ↔ Zn(OH)4 2- (aq) ↔ ZnO (s) conversion. This solid-solute-solid transformation and insulating discharge product ZnO lead to three vital challenges: 1) ZnO passivates Zn surface and prevents further discharging, leading to low Zn utilization; 2) ZnO is insulating and can hardly be charged back to Zn; 3) diffusion of Zn(OH)4 2- causes the loss of active material and change of electrode morphology. Thus, anode modification and protection are needed to alleviate the passivation and dissolution. We firstly designed a Zn mesh@GO anode (Fig 1c). Graphene oxide (GO) layers on the Zn mesh surface deliver electrons across insulating ZnO and can slow down the Zn dissolution. However, the utilization of zinc is still low because passivation problem is not completely solved. Through SEM investigation, critical passivation size was found to be ~ 2 µm. Thus, we further designed a lasagna-inspired ZnO@GO anode (Fig 1d). ZnO nanoparticles are encapsulated by GO. ZnO lasagna structure has three features: 1) the size of ZnO nanoparticles is smaller than the critical size of passivation; 2) the fabrication of ZnO lasagna anode starts with commercially available ZnO nanoparticles (~100 nm), and is compatible with the roll-to-roll process, which is ideal for large-scale manufacturing; 3) GO allows permeation of OH- and water, and prevents loss of Zn active material through blocking bigger Zn(OH)4 2-. As a result, such lasagna anode achieves a high volumetric capacity of 2308 Ah/L and a remarkable capacity retention of 86% after 150 cycles. In contrast, the open-structured ZnO nanoparticle anode, without the protection of GO, completely died after 90 cycles. Figure 1

Fluoride ion batteries (FIBs), as a promising next-generation high-energy-density storage technology, have attracted significant attention. However, developing an ideal fluoride-ion electrolyte that suppresses the β-H abstraction (caused by strong Lewis-basicity F-) and electrolyte decomposition remains challenging. To address this bottleneck, we design an electrolyte system based on commercial tetrabutylammonium fluoride (TBAF) salt and 1-butyl-3-methylimidazolium tetrafluoroborate (BMImBF4) ionic liquid solvent through anion-cation coordination engineering and hard-soft-acid-base (HSAB) balance modulation, unveiling its multiscale mechanisms for mitigating interfacial parasitic reaction and enhancing metal anode stability. Experimental and theoretical analyses reveal that the soft-acid BMIm⁺ participates in the solvation structure of hard-base fluoride ions, effectively blocking the β-H elimination pathway and expanding the electrochemical window to 4.5V. The ionic conductivity of this ionic liquid based electrolyte reaches 5.0×10-3S cm-1 at 60°C even after in situ polymerization. The Cu2O cathode coupling insertion and conversion reactions can alleviate the volume deformation and capacity decay of Cu2O||Li-LiF high-voltage FIBs, with a high resting voltage (2.91V) and a high initial capacity of 589.9mAh g-1. The Cu2O||Pb-PbF2 FIBs maintain a high reversible capacity of 243.6mAh g-1 even after 800 cycles under 200mA g-1. The work establishes a novel electrolyte design paradigm for high-voltage reversible FIBs.