Salt Concentration Research Articles

Considerations for Aqueous Electrochemistry Towards Sustainable Iron Production: From Dilute to Highly Concentrated Electrolytes

Iron (Fe) metal is used to make steel, which is a critical component in manufacturing and transportation in the industrialized world. Close to 1.2 billion tons of ‘primary’ Fe is produced globally using carbothermic reduction of iron ore to Fe metal in a blast furnace. This process is a major source of greenhouse gas (GHG) emissions. During carbothermic reduction, about 0.6 kg of CO2 is emitted per kg of Fe, corresponding to 0.7 billion tons of CO2 emitted annually by iron producers around the world. Furthermore, the steelmaking industry is highly energy intensive, accounting for about 2% of the global energy consumption.(1) Due to the scale of this environmental burden, energy-efficient and sustainable (GHG-free) Fe metal production that eliminates CO2 emission has been the holy grail of the steelmaking industry for decades.Recently, electrochemical methods for iron production have been of particular interest for their ability to convert iron oxides to oxygen and iron metal in a low temperature carbon free process.(2, 3). However, electrochemical production of iron from aqueous systems poses several challenges. First the deposition potential of iron is -0.447 V vs SHE meaning that at any potential in which iron could be plated, hydrogen evolution will also occur as a parasitic side reaction. Minimizing or eliminating the hydrogen side reaction is critical to efficient deposition of iron and several pathways to eliminating this challenge have been investigated.(4) Literature suggest that that water-in-salt electrolytes at a wide variety of salt composition and concentrations have the ability to enhance or suppress hydrogen evolution rates depending on the choice of salt, concentration and electrode material.(5, 6)While hydrogen evolution as a parasitic side reaction can be a major drawback when developing aqueous based electrochemical metal production methods, minimizing this effect is but one of many considerations that must be addressed in the development of a viable process. In this work, we investigate several convoluting factors that affect the energy requirements and rate capability of iron electrodeposition such as conductivity, solubility, deposition kinetics, and efficiency as a function of the electrolyte pH and salt composition from relatively dilute systems such as 0.1 M NaCl to highly concentrated salt systems such as 7 M choline chloride that approach water-in-salt type electrolytes. We use careful measurement techniques such IR-free RDE polarizations, gas collection and impedance spectroscopy to quantify these changes on parasitic side reaction rates on an iron surface at potentials negative of the iron plating potential to ensure a stable substrate and to characterize typical operating potentials for an iron plating process. We then discuss the tradeoffs between this Coulombic efficiency improvement and other critical parameters to gain a holistic view of the capability of aqueous electrolytes to enable sustainable iron production.This work was supported as part of the Center for Steel Electrification by Electrosynthesis (C-STEEL), an Energy Earthshot Research Center funded by the U.S. Department of Energy, Office of Science. J. Suer, M. Traverso and N. Jäger, Sustainability, 14, 14131 (2022).D. V. Lopes, M. J. Quina, J. R. Frade and A. V. Kovalevsky, Frontiers in Materials, 9 (2022).A. Q. Pham, S. Nijhawan, A. Alvarez, C. Wallace and S. FATUR, 2-step iron conversion system, in, USPTO Editor, Electrasteel Inc, USA (2022).K. L. Hawthorne, T. J. Petek, M. A. Miller, J. S. Wainright and R. F. Savinell, Journal of The Electrochemical Society, 162, A108 (2015).A. Guha and T. N. Narayanan, Journal of Physics: Energy, 2, 034001 (2020).L. Chen, J. Zhang, Q. Li, J. Vatamanu, X. Ji, T. P. Pollard, C. Cui, S. Hou, J. Chen, C. Yang, L. Ma, M. S. Ding, M. Garaga, S. Greenbaum, H.-S. Lee, O. Borodin, K. Xu and C. Wang, ACS Energy Letters, 5, 968 (2020).

Mitigating Anodic Reactions in Water-in-Salt Electrolytes for Refractory Metal Electrodeposition

The use of water-in-salt electrolytes has been a transformational advancement in electrochemistry. Water-in-salt electrolytes rely on a super high concentration of salts in aqueous electrolytes to lower the activity of water. The water is occupied in the solvation of the salts, leaving minimal ‘free water’ in the system. In the context of electrodeposition, this results in a widened cathodic window as the limited quantity of protons suppresses the hydrogen evolution reaction (HER).Seldom discussed is the impact on anodic reactions that are also limited by the transport of water for the oxygen evolution reaction (OER). Lithium chloride (LiCl) has been used as a cost effective and readily available support salt in water-in-salt electrolytes. However, in this system OER competes with chlorine evolution. At low pH chlorine evolution is more thermodynamically and kinetically favorable. This reaction depletes the system of chlorine atoms required to solvate excess water and saturates the electrolyte with dissolved chlorine. As a result, the electrolyte has a limited lifetime.One solution to this problem is the use of proton exchange membranes to separate the anode from the cathode. The use of these membranes will be discussed focused on our interest in rhenium (Re) electrodeposition. Rhenium has traditionally been difficult to deposit by electrolysis from aqueous solutions due to the over potential for hydrogen evolution and complex electrochemical reactions. Further, many intermediate rhenium oxides catalyze HER, so maintaining a stable water-in-salt electrolyte is crucial.Herein, we present systematic studies investigating various proton exchange membranes in an H-cell confirmation on the electrodeposition of rhenium. Perfluorosulfonic acid and hydrocarbon polymer backbones are explored, both with sulfonic acid functional groups. As these membranes are semipermeable, membrane thickness is also explored. Both volume changes in the two H-cell components and pH are monitored as a function of time. The impact on the morphology and electrodeposition kinetics of rhenium will be presented as a function of membrane properties.

Realization of a Long-Lifespan Thin Li Metal Batteries Capable of Self-Recovery with a Functional Separator

Due to the rapid expansion of the electric vehicles (EVs) market, the demand for large-scale batteries is increasing. Particularly, EVs face several limitations in terms of battery volume and weight. Therefore, achieving high energy density in limited spaces requires a significant reduction in the volume of the anode. Recently, the research is underway on batteries that employ thin Li metal anodes, typically around 20-25μm thick, as the next generation of lithium-ion batteries. However, thin Li anodes present various challenges regarding their cycle life. Compared to Li metal, thin Li metal faces scarcity in lithium sources. Furthermore, due to the unstable formation and destruction of the Solid Electrolyte Interphase (SEI) during the plating-stripping process in Li metal anode charging and discharging, lithium ions and electrolyte are constantly consumed. These side reactions lead to decreased capacity and deteriorated lifespan characteristics. To maintain stable charging and discharging properties, it is necessary to minimize electrolyte decomposition by forming a stable SEI in the early cycle and continuously supplying consumed Li ions throughout the cycling process. Commonly used methods to supplement the limited lithium source of the thin Li anode and replenish consumed Li ions during charging and discharging represent the employing the high-concentration electrolytes (HCE). However, HCE have limits on the dissolved concentration of salts in the electrolyte, and the method of dissolving salts in the entire electrolyte is not attractive a cost perspective. Additionally, as the electrolyte concentration increases, viscosity also increases, leading to a decrease in ion mobility.In this study, high-concentration lithium bis(fluoromethanesulfonyl)imide (LiFSI) salts are coated onto commercial separators to provide a locally high-concentration environment at the electrode/electrolyte interface. This coating layer dissolves as Li ions are consumed, replenishing the depleted Li ions. Furthermore, the presence of high-concentration Li ions in the electrolyte eliminates free solvent. Reducing free solvent suppresses electrolyte consumption further. Additionally, by adopting LiFSI as the salt, rapid decomposition of anions in the initial cycle facilitates the formation of a stable LiF layer. The densely formed LiF layer exhibits high mechanical strength and electrochemical stability, aiding in achieving long-term characteristics.Li||Li symmetric cells were tested using commercial electrolytes, including 1M LiFSI in diethylene glycol dimethyl ether (DME) and 1M lithium hexafluorophosphate (LiPF6) in ethylene carbonate/ diethylene carbonate (EC/DEC). Both electrolytes showed improved cycle stability and lower polarization in cells using an ionically conductive salt-coated separator (SDL) compared to polyethylene (PE) separators. Particularly, the nucleation polarization observed during the initial charging process, when Li ions are deposited onto bare lithium metal, significantly decreased in SDL cells, indicating better Li ion affinity on the lithium surface of SDL cells. Tafel plots were used to analyze the exchange current at the electrode/electrolyte interface. The exchange current for PE was 1.09 mA/cm2, while for SDL, it was 1.52 mA/cm2, suggesting faster interfacial kinetics in SDL compared to PE. The results were also evident in rate capability tests, with the largest capacity difference observed at 5C. Scanning electron microscope (SEM) images revealed distinctly different Li ion deposits on the Cu foil. PE showed vigorous dendrite growth with many voids, while SDL exhibited dense Li deposition, attributed to the presence of high-concentration salts at the interphase, suppressing dendrite growth and inducing uniform Li ion flux. The stable interfacial characteristics were analyzed using X-ray photoelectron spectroscopy (XPS). After 100 cycles, PE showed a high proportion of electrolyte decomposition components, while SDL exhibited a higher proportion of LiF formation. These results demonstrate coated separator with LiFSI effectively suppressed electrolyte decomposition. Additionally, boehmite was pre-coated on the PE separator to endow with coating adhesion, resulting in improved surface morphology. When various commercial electrolytes were applied after salt coating, significantly enhanced wettability was observed compared to PE. Ultimately, half-cells were constructed using thin-film Li metal for galvanostatic charge-discharge tests, achieving superior cycle stability of over 200 cycles with SDL. Figure 1

The Effect of Surface pH on Rhenium Electrodeposition

Electrodeposition is considered as a cost-effective, controllable, scalable, and facile process for material deposition[1] and has been widely used in the device fabrication[2-4]. Electrodeposition of high quality rhenium films with improved morphology is enabled in the so called water-in-salt (WIS) electrolyte, where a superhigh concentration of salt, such as LiCl, is present to deplete the free water molecules, reduce the proton diffusion, and suppress the hydrogen evolution side reactions[5]. Yet, inclusion of oxygen and chlorine is observed for the films deposited from such electrolytes. While Cl incorporation may result from high concentration of LiCl, the presence of oxygen can relate to oxide formation due to surface pH increase expected during electrodeposition. Therefore, understanding the surface pH during deposition might provide insight on the Re deposition mechanism and a guideline to further develop the chemistry and process for improved film property.This work aims to measure the electrolyte pH at the electrode surface during Re electrodeposition and to determine the effects of salt speciation and deposition potential on the hydrogen evolution. How the surface pH impacts the impurity incorporation will also be investigated. A customized rotating ring disk electrode (RRDE) is utilized to perform the measurement. The Iridium Oxide pH sensing layer is fabricated using electrodeposition method on the Au ring and Re deposition is carried out on the Pt disk. The electrolyte pH during deposition is recorded as the open circuit potential (OCP) on the IrOx ring electrode. The interference of the anions or cations in the solution (other than proton or hydroxide) on the OCP reading is studied and calibrated. In-situ pH change under different solute conditions will be investigated to evaluate the efficacy of salts in suppressing hydrogen evolution. Furthermore, a correlation between surface pH and the incorporation of impurities, especially the oxides, will be studied under different solute environments. References Zhang, Q., et al., Electrodeposition in ionic liquids. ChemPhysChem, 2016. 17(3): p. 335-351.Romankiw, L., Compositional Profiles in Electroplated Permalloy and Copper Permalloy: a Methodology Toward Laminated Permalloy for Thin Film Head Applications. Magnetic Materials, Processes, and Devices, 1991: p. 367-379.Andricacos, P.C., et al., Damascene copper electroplating for chip interconnections. IBM Journal of Research and Development, 1998. 42(5): p. 567-574.Moffat, T.P. and D. Josell, Superconformal Electrodeposition for 3-Dimensional Interconnects. Israel Journal of Chemistry, 2010. 50(3): p. 312-320.Huang, Q. and T.W. Lyons, Electrodeposition of rhenium with suppressed hydrogen evolution from water-in-salt electrolyte. Electrochemistry Communications, 2018. 93: p. 53-56.

Insights into the Lithium Solvation Environments of Localized High Concentration Electrolytes by NMR and Effects on NMC811/Gr Cell Performance

Advanced battery technologies require advanced electrolytes to support high voltage and long cycle life at a wide range of temperatures and charging rates, and enhanced safety. The lithium solvation environment within the electrolyte can have a large impact on cell performance, influencing ion transport in the electrolyte as well as ion transfer at the electrode-electrolyte interfaces.1 By optimizing lithium-ion solvation, cell kinetics can be improved to facilitate low temperature and fast charge performance, while also promoting stable electrode interfaces during high temperature operation. Localized high concentration electrolytes (LHCEs)- composed of lithium salt(s), solvents, and a non- or minimally-solvating diluent -are designed to enhance the local concentration of salt around the lithium ion. These anion-involved lithium solvation environments can increase ion transport in the electrolyte and lead to more stable, inorganic-rich interfacial species.2 While LHCEs have shown promise in improving performance for lithium metal, high voltage, fast charge, and low temperature applications,2,3 further understanding of solvation in LHCEs and development of new diluent molecules is needed. The development of diluents should include solvation and physical property characterization, as well as performance testing in battery cells.Previous studies have demonstrated that nuclear magnetic resonance spectroscopy (NMR) can be used to provide insight into the solvation environments of electrolytes with standard carbonate solvents as well as novel solvent species.4,5 In this work, we utilized NMR to build understanding of the cation (Li: 7Li-NMR), anion (PF6: 19F-NMR), and solvent (carbonates: 13C-NMR) solvation environments of standard electrolytes as a function of salt concentration. We then applied this understanding to NMR analysis of electrolytes with a known diluent to establish a baseline of how solvation changes for LHCEs. Finally, this body of knowledge was used to investigate a novel fluorinated ether species, KDC-403, proposed as a diluent for LHCEs. Furthermore, we demonstrate improved low temperature cycling and more stable fast charge cycling with reduced lithium plating in electrolytes containing KDC-403, supporting previous studies that conclude LHCEs can provide superior battery performance.

Understanding the Kinetic Origins of Voltage Relaxation and Low-Frequency EIS Measurements in Porous Electrodes

Overpotentials in porous electrodes lead to undesirable behavior during battery cycling and understanding the origins of these overpotentials is crucial. In Li-ion batteries, a significant fraction of the electrode overpotentials in layered cathodes like NMC's arise from low-frequency sources. These low-frequency overpotential sources and the impedances they lead to are traditionally analyzed in the time domain using galvanostatic intermittent titration technique (GITT) or the frequency domain using electrochemical impedance spectroscopy (EIS). Currently in the standard porous electrode model, solid-state diffusion of Li in the cathode material is the only mechanism that is considered to lead to the electrochemical behavior seen at low frequencies and longer timescales. However, there are various experimental inconsistencies that emerge when attributing voltage relaxation behavior and low-frequency impedance behavior solely to solid-state Li diffusion. Thus, it is necessary to further investigate and understand the true nature of the impedances that arise in these timescale and frequency regimes.In this work, we investigate the origins of the voltage relaxation behavior and low-frequency impedance in porous electrodes. Using a 3-electrode coin cell system, we vary the kinetics of the electrode/electrolyte interface by systematically changing the salt concentration of the electrolyte used. We perform GITT and EIS measurements and find that the relaxation behavior and low-frequency impedance changes as the kinetics of the electrode/electrolyte interface changes. These results contradict the experimental dependencies that are expected to arise from a system that is only diffusion controlled. Instead of a solid-state diffusion limitation, we propose an alternative mechanism that could also contribute to the observed GITT and EIS behavior: a distribution of reaction constants. We demonstrate the validity of this hypothesis with a multi-particle simulation. Furthermore, we characterize the hierarchical morphology of porous electrodes through particle physisorption and nano-computed tomography and investigate how their complex structures could lead to inherently broad distributions of electrochemical reaction times. Figure 1

Development of Localized High-Concentration Electrolytes Containing DME Solvent for Long-Life and High-Power Lithium-Ion Batteries

1.IntroductionWe are promoting electrification towards carbon neutrality, since the evolution of batteries is indispensable, we are developing high energy, high power and long-life lithium-ion batteries.While Ni-rich layered cathodes such as NCM811 have high capacity, the cycle life using NCM811 at high temperatures decrease. To overcome poor cycle performance, reported localized high concentration electrolytes (LHCE) which contains DMC solvent [1] was adapted to our chemical system and evaluated. Although better cycle performance at high temperature was observed, the resistance of cells using the LHCE should be lowered toward practical application.In this study, the types and concentrations of solvents were investigated for long-life and high-power battery. 2.ExperimentalThe LiFSI salt concentration was set to 1.7 mol/L, and mixtures was prepared using 1,2-Dimethoxyethane (DME) and Ethylene carbonate (EC) as solvents, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) as a diluent as shown in Figure 1 by weight ratio of solvents.350mAh-class laminate cells were fabricated with NCM811 as the positive electrode active material and a mixture of silicon oxide and graphite as the negative electrode active material. The electrolyte amount was set to 11g/Ah. Resistance measurements were performed at 25°C with a SOC of 50%, applying a 10-second pulse from 0.2 to 2.5C for six points. Cycle tests were carried out at 60°C, charging at 0.3 C rate discharging at 0.8 C rate with cut-off voltage of 3.0-4.15 V.３. Results and DiscussionThe area specific DCR (ASD) and the absolute value of the slope of the capacity retention against the cumulative discharge energy were illustrated on the ternary diagram shown in Figure 2. The weight of EC to 15-20wt.% reduced ASD, and favorable cycle performances were obtained with a weight of TTE at 55wt.% or higher. By optimizing the electrolyte composition using DME, high-temperature cycle performance was improved by 3% and ASD resistance was reduced by 15% compared with reported LHCE using DMC which was shown as “ref” in Figure 2.The effects of electrolyte compositions and the mechanisms for improving performance will be discussed based on data such as initial charging curves, energy levels of Li-FSI state by DFT calculation, coordination number of Li-FSI by molecular dynamics simulation.*Since there was the potential risk of the corrosion on an aluminum current collector with our LHCE, investigations were conducted, and results were reported by our colleague.

Quantitative Electrolyte Analysis from Large Format Cylindrical Lithium-Ion Cells after Electrolyte Extraction

In 2022, Europe’s carbon dioxide (CO2) emissions made up 11% of the worldwide output and a quarter of it was produced by the transportation sector.[1] To reduce these CO2 emissions the sector is shifting to electric vehicles with lithium-ion cells as energy storage. As lithium-ion cells gain popularity, topics such as cell lifetime and fast charging ability increase significantly. Both are closely linked to electrolyte degradation reactions and can be investigated by analyzing the electrolyte and its decomposition. In industrially produced large format cylindrical lithium-ion cells (46950) the electrolyte amount added to the cell is monitored carefully by the cell manufacturer. Almost no excess electrolyte can be found in commercially produced 46950 cells after the formation process. Few methods are described in literature to extract the electrolyte from the cell materials if no excess electrolyte is present. Thompson et al. outlined a centrifugation method of the cell materials to separate electrolyte from the cell materials.[2] Stockhausen et.al. presented a liquid-liquid extraction for pouch cells by adding solvent to the cell.[3] Grützke et al. performed a supercritical CO2 extraction on cylindrical 18650 cells.[4] Drawbacks to the methods known in literature are that these are either not transferable to a large format cylindrical lithium-ion cell or do not enable a representative recovery of all electrolyte components, such as lithium salts, carbonates and additives.In this research we present a liquid-liquid electrolyte extraction method to achieve representative electrolyte extraction in large format cylindrical lithium-ion cells (46950). The electrolyte extraction was performed in a prototype electrolyte extraction chamber designed at BMW Group.The cells are based on an NMC811/Si-graphite material and produced at the prototype plant at BMW Group in Munich, Germany. In comparison to commercially available lithium-ion cells the composition of all components, such as electrode and electrolyte materials are known. A variation of electrolytes is investigated with conductive salt concentrations of 1-1.5M LiPF6 and solvent mixtures of different carbonates to ensure a variety of applications. Quantitative analysis of the conductive salt is performed with ion chromatography–mass spectrometry (IC-MS). The electrolyte solvents are quantified with a high performance liquid chromatography-Orbitrap-mass spectrometry (HPLC-Orbitrap-MS). These extensive analysis will enable a better understanding of the function, changes and importance of the electrolyte in lithium-ion cells.

The Impact of Microstructure Variation on Lithium-Ion Battery Charging Capability

As electrification of the automotive industry progresses, there is an increasing consumer desire to reduce charge times and increase power and energy density of electric vehicles1. However, fast charge rates induce large gradients across the electrochemical components of these battery electrodes and can lead to adverse impacts2 at the active material and electrode level. Particularly at high states of charge, local anode potentials can drop significantly below the equilibrium potential3, and due to the proximity of the graphite equilibrium potential4,5 to 0V vs metallic Li at high states of lithiation, a risk that the anode potential drops below 0.0 V vs lithium metal exists, which can drive lithium plating on the active material2, reducing the electrochemically active surface area, impacting performance and heat generation6, and impacting lithium salt concentration present in the electrolyte. By using electrochemical simulations, the onset of lithium plating can be predicted and used to set charging guidelines to reduce the risk of lithium plating. The pseudo-two-dimensional (P2D) model7–9 is a popular modeling method for capturing this behavior. With many effective media assumptions present translated to an average continuum impact, the P2D model can simulate rapid charge and capture a global equivalent onset of lithium plating for the anode. However, the P2D model lacks the ability to resolve any localized behaviors across the individual components due to local non-uniformities, as variation in the through plane direction only represents an average value across a slice of the full domain. Lopata10 developed a three-dimensional microstructure based (3DMS) modeling method, we employ here to simulate rapid charge events and capture what occurs locally across the electrochemical components while broadly agreeing with the established P2D’s model results. With the 3DMS model, we are able to better predict the onset of local lithium plating and can use this knowledge to design more conservative charging conditions to delay the onset of lithium plating and improve the performance of these electrochemical systems. In this work, several similar microstructures are evaluated to determine the variation of the lithium plating onset time during fast charge operation. A small normal distribution for particle sizes are explored to drive variation in performance and are compared to a uniform particle size structure. References T. R. Garrick, Y. Zeng, J. B. Siegel, and V. R. Subramanian, J. Electrochem. Soc., 170, 113502 (2023). U. Janakiraman, T. R. Garrick, and M. E. Fortier, J. Electrochem. Soc., 167, 160552 (2020). T. R. Garrick, J. Gao, X. Yang, and B. J. Koch, J. Electrochem. Soc., 168, 010530 (2021). T. R. Garrick et al., J. Electrochem. Soc., 170, 060548 (2023). A. Paul et al., J. Electrochem. Soc., 171, 023501 (2024). M. Song, Y. Hu, S.-Y. Choe, and T. R. Garrick, J. Electrochem. Soc., 167, 120503 (2020). S. T. Dix, J. S. Lowe, M. R. Avei, and T. R. Garrick, J. Electrochem. Soc., 170, 083503 (2023). T. F. Fuller, M. Doyle, and J. Newman, Journal of the electrochemical society, 141, 1 (1994). M. Doyle, T. F. Fuller, and J. Newman, Journal of the Electrochemical society, 140, 1526 (1993). J. S. Lopata et al., J. Electrochem. Soc., 170, 020530 (2023).

Molecular Dynamics Characterization of the Free and Encapsidated RNA2 of CCMV with the oxRNA Model.

The cowpea chlorotic mottle virus (CCMV) has emerged as a model system to assess the balance between electrostatic and topological features of single-stranded RNA viruses, specifically in the context of the viral self-assembly. Yet, despite its biophysical significance, little structural data on the RNA content of the CCMV virion is available. Here, the conformational dynamics of the RNA2 fragment of CCMV wasassessed via coarse-grained molecular dynamics simulations, employing the oxRNA2 force field. The behavior of RNA2 wascharacterized both as a freely-folding molecule and within a mean-field depiction of the capsid. For the former, the role of the salt concentration, the temperature and of ad hoc constraints on the RNA termini wasverified on the equilibrium properties of RNA2. For the latter, a multi-scale approach wasemployed to derive a potential profile of the viral cavity from atomistic structures of the CCMV capsid in solution. The conformational ensembles of the encapsidated RNA2 weresignificantly altered with respect to the freely-folding counterparts, as shown by the emergence of long-range motifs and pseudoknots. Finally, the role of the N-terminal tails of the CCMV subunits is highlighted as a critical feature in the construction of a proper electrostatic model of the CCMVcapsid.

Weakly Coordinating Solvents with FSI-Inspired Structures for Highly Efficient Lithium Metal Batteries

１．Introduction Low Coulombic efficiency (CE) of lithium (Li) plating/stripping due to electrolyte decomposition on Li metal negative electrodes is a critical issue in the practical application of Li metal batteries. Our previous study[1] revealed that increasing the concentration of lithium bis(fluorosulfonyl)imide (LiFSI) in organic electrolytes leads to a weakly coordinated environment of Li+ with FSI−, which increases Li+ chemical potential (µ Li +) and Li electrode potential (E Li), thus minimizing electrolyte reductive decomposition and achieving high CEs. However, concentrated electrolytes have disadvantages such as high viscosity and high cost.Herein we focused on FSI-inspired solvents with a weakly Li+-coordinating structure of -NSO2F.[2][3] This class of solvents have been studied with a focus on their solid electrolyte interphase (SEI) forming ability similar to LiFSI,[2][3] but there has been no understanding on the µ Li + and E Li in such solvents. We studied the E Li and Li plating/stripping CE in various FSI-inspired solvents and discuss their correlation. ２．Experimental The electrolytes were prepared by dissolving LiFSI in FSI-inspired solvents (diMeFSI; N,N-dimethylsulfamoyl fluoride, EtMeFSI; N-ethyl, N-methylsulfamoyl fluoride, diEtFSI; N,N-diethylsulfamoyl fluoride), FEC; fluoroethylene carbonate, and DME; 1,2-dimethoxyethane at a LiFSI:solvent molar ratio of 1/8 (Figure 1.). A three-electrode cell was used for cyclic voltammetry (CV) with a Pt plate as the working electrode and Li foil as the counter and reference electrodes. Ferrocene (Fc, 1 mM) was dissolved in the electrolytes. E Li was evaluated with reference to the redox potential of Fc as an internal standard. Cu|Li coin cells were used to evaluate Li plating/stripping CEs. Li was plated on Cu for 1 hour and stripped up to 0.5 V at a current density of 0.5 mA cm- 2. ３．Results & Discussion Figure 2. shows E Li in various electrolytes with reference to Fc/Fc+. The E Li values of FSI-inspired solvents were notably higher (-2.9 V vs. Fc/Fc+) than FEC or DME even at the same LiFSI:solvent molar ratios (1/8). This is due to the weakly Li+-solvating nature of the FSI--inspired solvents, which was also supported by DFT calculations.Figure 3. shows the average Li plating/stripping CEs of 2~20 cycles in various electrolytes. For diMeFSI, EtMeFSI, diEtFSI, DME, and FEC, the LiFSI/solvent ratios were 1/8. The data of other various electrolytes (limited to salt concentrations below 2 M) are reproduced from our previous publication[1] and shown as black dots. Among such dilute electrolytes, the highest E Li (-2.92 V) and the highest CE can be both achieved in diMeFSI electrolytes. This suggests that introducing such an FSI structure into solvents is a promising way to not only forming a better FSI-derived SEI but also upshifting the E Li for highly efficient Li metal anodes. 4 ．References [1] S. Ko et al., Nat. Energy, 7, 1217 (2022).[2] W. Xue et al., Energy Environ. Sci., 13, 212 (2020).[3] K. Jiang et al., ACS Energy Lett. 7, 3378 (2022). 5.Acknowledgement This work was partially supported by JSPS KAKENHI Grant-in-Aid for Transformative Research Areas (B) (23H03824, 23H03827). Figure 1

Macroscopic Access Resistances Hinders the Measurement of Ion-Exchange-Membrane Performances for Electro-Dialysis Processes

Ion exchange membranes (IEMs) are widely used in industrial sectors from batteries and energy harvesting, to water treatment and desalination. They are designed with a high selectivity and a low electrical resistance. However, the way to measure such performances independently from interpretation and device parameters (geometrical sizes, water flux, electrodes, measurement type, etc.) remains a challenge. Therefore, many performances announced in the current literature are not reliable. This study focuses on membranes prepared for reverse electro-dialysis applications, but could be generalized to other membrane based processes.Due to the rising energy demand and the severe environmental crisis, worldwide reduction of conventional fossil fuel consummation is needed. As a renewable and clean energy resource, the Blue energy is an osmotic energy released during the mixing of solutions of different salinity. Naturally, an irreversible mixing of river water and sea water happens at the estuary. A precise and engineering control of this mixing enables the blue energy harvesting. The worldwide salinity gradient energy could reach 70 GW, which is not negligeable.Since the beginning of blue-energy harvesting research, a lot of advances have been made about membrane design and nanofluidic diffusio-osmotic transport, but the efficiency gap of 6 orders of magnitude between nano-scale designs and industrial power plants remains unsolved.Just after synthesis, selective membranes are benchmarked on very small samples on the basis of the output power. It is usually calculated based on the membrane resistance per unit area, and the measured Donnan potential. This method of power per unit area based on membrane resistance is therefore a key point to compare several devices and their efficiency.A classical electrochemical cell has been used to place a Nafion selective membrane of varying area between two electrodes. The membrane resistance and power density have been experimentally measured. Homogeneous salt concentration or gradient of concentration have been used to measure different parameters.This systematic study shows that for classical measurement cells, the membrane resistance is inversely proportional to the square root of its area. Whereas it's currently assumed in the literature that the resistance and the area of a membrane are inversely proportional. Based on this assumption, the power density is believed to be independent of the membrane area (reason why it is used as a comparison criteria), which we prove to be wrong. This result has a big impact on the reliability of the power density announced in the literature for different devices: the smaller the area of the tested membrane the higher is the output power density: from 0,5 W/m² up to 150 W/m² for the same membrane, same salt ratio (100) different membrane areas. This leads to overly optimistic extrapolation of electrical performances for membranes engineered and tested at sub millimeter sizes that will be industrially used at meter sizes.To explain these results, we show that for small membranes (compared to the electrode size and distance) there is an access resistance at the scale of the membrane. It results that the area mismatch between the membrane and the electrodes add a new resistance in the system that does not scale with the membrane area and that is taken into account in membrane characterizations. Experimental data and theoretical modeling are corroborating without any adjustment parameters.Consequently, the power per unit area obtained at micro scale cannot be extrapolated a priori to larger membranes. So, the comparison is biased between large scale commercially available membranes, and nano-engineered microscale membranes. This raises the urgent need for new indicators or standard measurement protocols.Based on these conclusions, we formulate a few recommendations to perform scalable and meaningful measurements of membrane resistance and power density.

Redox Reaction of Some Nitroxyl Radicals in 1-Butyl-1-Methylpyrrolidinium Bis(fluorosulfonyl)amide in the Presence and Absence of Lithium Ion

Ionic liquids composed of bis(fluorosulfonyl)amide (FSA–) have been investigated as the promising electrolytes for rechargeable lithium batteries due to their high conductivity and low viscosity dependence on the Li salt concentration. Despite many reports on FSA–-based ionic liquids as the electrolytes for rechargeable lithium and sodium batteries, a few studies have been reported on the electrode reactions of redox species. The redox reactions of Ag+/Ag and ferrocenium (Fc+)/ferrocene (Fc) were investigated in BMPFSA (BMP+ = 1-butyl-1-methylpyrrolidinium)[1]. It was reported that the donor ability of FSA– was found to be lower than that of bis(trifluoromethylsulfonyl)amide (TFSA–) from the potential difference between Ag+/Ag and Fc+/Fc. 2,2,6,6,-Tetramethylpiperidine-1-oxyl (TEMPO) is one of the typical niroxyl radicals with redox activity. In the present study, the redox reactions of TEMPO and 4-oxo-TEMPO were investigated in BMPFSA in the presence and absence of lithium ion.The redox reactions of TEMPO+/TEMPO and 4-oxo-TEMPO+/4-oxo-TEMPO were observed on a Pt electrode in BMPFSA by cyclic voltammetry. The formal potential of TEMPO+/TEMPO was lower than that of 4-oxo-TEMPO+/4-oxo-TEMPO. The formal potential of TEMPO+/TEMPO in the presence of Li+ was higher than that in the absence of Li+, suggesting that the interaction between TEMPO and Li+ affected the thermodynamic properties of the redox species. The diffusion coefficients and the standard rate constants were evaluated by chronoamperometry and electrochemical impedance spectroscopy, respectively. The diffusion coefficient and the standard rate constant were suggested to be controlled not only by the viscosity but also by the interaction between the nitroxyl radicals and Li+.Reference[1] S. Kato, N. Serizawa, and Y. Katayama, J. Electrochem. Soc., 170, 046504 (2023).

Deep Supercooling of Li Salts By Trace Polymer Additives: An Approach to Single Li Ion Conducting Liquid Electrolytes

As a solution to global warming in recent years, the conversion from gasoline-powered vehicles to EVs is being attempted around the world, but the long charging time of the batteries in EVs has delayed their widespread adoption. For realizing the rapid charging of Li secondary batteries, also equipped with EVs, it is necessary to improve the transport properties of electrolytes such as ionic conductivity (σ) and Li+ transference number (t Li) 1. Therefore, all-solid-batteries with solid electrolytes, which has high σ and t Li ~ 1, are considered as a promising next-generation rechargeable battery, even though the designing of the electrolyte-electrode interface still remains a challenge 2. On the other hand, in liquid electrolytes (LEs), which has an advantage in interface design, t Li is still low. Although various studies have reported an enhancement of in LEs by means of changes to anion structures, the use of polyanions, and highly concentrated electrolytes, the reports of t Li ~ 1 are limited to the use of molten salts in a battery cell 3, 4. However, most of Li salt have both a high melting point and crystallinity, so that it crystallizes easily near room temperature, which is unsuitable for battery adaptation.To overcome this high crystallinity of Li salt at room temperature, we herein report anti-crystallization of Li salt by adding a small amount of polymers as obstacles to pack crystal structure. As shown in figure, we mixed lithium (fluorosulfonyl)(trifluoromethanesulfonyl)amide (Li[FTA]) and poly(methyl methacrylate) (PMMA) at the molar ratio of 9:1 (95.5 wt% of Li salt), thereby we could obtain the liquid mixture at room temperature. The result of differential scanning calorimetry shows that it remains supercooled state at ambient temperature (deep supercooling), even though 95 wt% of mixture is composed of Li salts. Additionally, the experimental value of 95.5 wt% Li[FTA]/ PMMA mixture was −25 °C and lower than that of Li[FTA] (−5.3 °C) and PMMA (106 °C), respectively (super-plasticizing). This indicates that the presence of slight polymer is not only an obstacle for packing into crystal lattice but also a plasticizer for Li salts. Therefore, this mixture, composed almost entirely of Li salt and liquified by deep supercooling and super-plasticizing effect, is considered as a deeply supercooled salt (DSS).DSS has ionic conduction at ambient temperature and show t Li ~ 1 under anion blocking conditions at 60 °C due to the absence of neutral solvents and only a small amount of polymer in electrolytes, which cannot cause valuable salt concentration gradients in measurement time scale. We will report ionic transport and electrochemical properties of DSS for batteries. Acknowledgement: This study was supported by the Advanced Low Carbon Technology Research and Development Program (ALCA-Next) of the Japan Science and Technology Agency (JST). Reference: 1. M. Diederichsen, E. J. McShane and B. D. McCloskey, ACS Energy Letters, 2017, 2, 2563-2575.2. Kato, S. Hori, T. Saito, K. Suzuki, M. Hirayama, A. Mitsui, M. Yonemura, H. Iba and R. Kanno, Nature Energy, 2016, 1, 16030.3. Kubota and H. Matsumoto, The Journal of Physical Chemistry C, 2013, 117, 18829-18836.4. Shigenobu, F. Philippi, S. Tsuzuki, H. Kokubo, K. Dokko, M. Watanabe and K. Ueno, Physical Chemistry Chemical Physics, 2023, 25, 6970-6978. Figure 1

Advancements in Property Predictions and Optimization of Highly Complex Electrolytes

The world-wide endeavor to improve battery energy storage carries the need for expanded exploratory research of electrolyte systems. Such research requires considerable resource and time commitments, resulting in delayed market entries of advanced cell chemistries. One answer to this need is the computational tool Advanced Electrolyte Model (AEM) developed at the Idaho National Laboratory, which has been used most prominently for Li-ion, Li metal and Na-ion battery systems. AEM has a molecular chemical physics (statistical mechanical) basis that enables fast property predictions, yielding over 100 property metrics per run over broad conditions of salt concentrations and temperatures with runs that span only seconds. More than 60 solvents and about 40 salts reside in the AEM library, although recent expanded AEM capability accommodates the input of arbitrarily-chosen chemical compounds (ACCC) that do not exist in the library. As such, AEM is equipped to consider a massive list of candidate solvents and salts. AEM has demonstrated excellent efficacy in predicting properties for complex electrolyte systems, defined by mixtures containing a high number of possibly chemically-diverse compounds. Examples of AEM outputs will be given in terms of supporting design of fast-charge electrolytes, quantifying solution microstructure, and implementation of ACCC options.

Regulating the Solvation Shell Structure of Eutectic Electrolytes for Stable and High Performance Anode-Free Zn-Li Hybrid Batteries

With the development of technology, various related policies have emerged wherein the application of batteries plays a crucial role. Presently, lithium-ion batteries widely used in the market suffer from poor safety, high cost, and temperature usage limitations. To solve these challenges, this study employs Zinc ion batteries (ZIBs) as the electrolyte. Zinc-based electrolytes exhibit high energy density, low cost, high safety, and abundant resources. However, the application of Zinc ion batteries (ZIBs) is accompanied by specific electrochemical issues, such as hydrogen evolution reaction (HER), dendritic growth, and the formation of passivation layers, which subsequently affect battery lifespan, Coulombic efficiency (CE), and energy efficiency (EE). Therefore, addressing these side reactions can lead to superior performance and benefits across various aspects of the electrolyte.Deep Eutectic Solvent (DES), formed through intricate hydrogen bond networks, divides salt compositions into hydrogen bond acceptors (HBA) and hydrogen bond donors (HBD), thereby altering the total melting point. DES electrolytes can form unique structures, thus improving electrochemical side reactions. Leveraging these characteristics, this experiment designs a novel deep electronic solution (DES) electrolyte and assembles a pouch cell to measure its electrochemical performance.This electrolyte is prepared by initially heating and stirring a mixture of 1 mol of zinc chloride and 4 mol of acetamide until fully dissolved, followed by adding 1 mol of lithium acetate. Due to the high viscosity of the resulting electrolyte, 5 mol of acetonitrile is added after cooling to reduce viscosity. Additionally, the impact of adding water to the electrolyte on battery lifespan, overpotential, Coulombic efficiency, and energy efficiency is investigated. Therefore, another electrolyte is prepared for comparison, maintaining the same primary salt concentration but including 1 mol of water and 4 mol of acetonitrile.Using the positive electrode material lithium manganese oxide (LiMn2O4, LMO), a Zn//LMO anode-free cell is constructed for electrochemical testing. Initially, the electrolyte is tested for the electrochemical performance of Zn//Zn symmetric cells, Zn//Cu asymmetric cells, and Zn//LMO anode-free cells (with or without co-solvent). Various electrochemical tests are then conducted, including Electrochemical Impedance Spectroscopy (EIS), cyclic voltammetry (CV), linear sweep voltammetry (LSV), Tafel plots, and C-Rate cycling. Furthermore, Raman spectroscopy and Fourier Transform Infrared (FTIR) spectroscopy techniques are utilized to investigate the solvation structure of the electrolyte. Lastly, viscosity and conductivity measurements are performed.The results indicate that in Zn//Zn symmetric cells, electrolytes with added co-solvent exhibit a 0.36V reduction in overpotential during Zn plating/stripping compared to those without co-solvent. Additionally, in Zn//Cu asymmetric cells, electrolytes with added co-solvent demonstrate relatively stable Coulombic efficiency, maintaining an average of over 99.7% efficiency at a current density of 0.5 mA/cm2. In Zn//LMO anode-free cell testing, it is observed that electrolytes with added 1 mol of water and 4 mol of acetonitrile initially exhibit the highest specific capacity (75mAh/g), maintaining good specific capacity even after cycling at a current of 100mA/g. In C-Rate testing, the Zn//Cu asymmetric cell with added co-solvent shows better stability, with Coulombic efficiency maintaining above 99.9% when cycling back to low current density. the electrolyte with added 1 mol of water and 4 mol of acetonitrile in the Zn//LMO half-cell maintains a Coulombic efficiency of 99.4% and a specific capacity of 50 mAh/g when cycling back to low current density.Electrolyte analysis reveals the presence of [ZnCl(C2H5NO)2(CH3COO-)]+ at the peak of 282 cm-1 in the Raman spectrum. Upon adding acetonitrile, a characteristic peak of C-C≡N at 381 cm-1 appears, which decreases with adding water. Additionally, at the peak of 944 cm-1, a characteristic peak of lithium acetate C-C is observed, which shifts upon adding acetonitrile. This suggests a change in the coordination of zinc ions in the solvation shell, from one chloride ion and three acetamide molecules to one chloride ion, one acetate ion, and two acetamide molecules upon adding acetonitrile. In the FTIR spectrum, a blue shift is observed in the C=O characteristic peaks between 1640 cm-1~1670 cm-1 upon adding acetonitrile, indicating the penetration of acetonitrile into the solvation shell. Additionally, a blue shift is observed between 1360 cm-1~1420 cm-1, attributed to the C=O characteristic peaks of acetonitrile and acetate ions, confirming the penetration of acetate ions into the solvation shell, forming a unique Deep Eutectic Solvent (DES) structure. Figure 1

Observing the Nanoaggregates in the Electrolytes by Small-Angle X-Ray Scattering

The solution solvation structure of the liquid electrolyte directly affects transport properties, which ultimately determines the performance of batteries. It has been predicted that the nanoscale units such as clusters and aggregates in the liquid electrolytes, especially as the salt concentrations increase. However, more is needed to know about these structures due to the limitations of the traditional methods. Currently, there are no direct methods that can be used to investigate the cluster or nanoaggregates in the electrolytes. Herein, we demonstrate that small-angle X-ray scattering-wide angle X-ray scattering (SAXS-WAXS), and even ultra-small-angle X-ray scattering (USAXS) could be applied to capture these larger aggregates in the electrolytes, including conventional organic electrolytes, high concentration electrolytes, water-in-slat electrolytes, and redox-active electrolytes. Raman, neutron scattering, IR, and molecular dynamics (MD) simulations further support the proposed structures. This work highlights the fundamental structure analysis in the nanoscale, which will prompt better electrolytes in the future.

Bipolar Membrane Polarization Governed By Interfacial Ionic Species

Bipolar membranes (BPMs), consisting of an anion exchange polymer membrane layer, a cation exchange polymer membrane layer, with a catalytic interfacial layer in between, enable electrolysis cells with large difference of pH between adjacent chambers. This pH gradient is a working principle of a number of devices for energy and sustainability: acid/base batteries store and release energy by building and depleting a pH difference; BPM electrodialysis reactors capture CO2 in alkaline electrolytes and release CO2 by acidifying; CO2 and H2O electrolysis devices may employ pH-decoupled electrodes to avoid precious metal electrocatalysts while maintaining low electrode overpotentials. For all of these devices the most efficient operation - in terms of extracting maximum voltage from a battery or driving electrolysis with minimum overpotentials - is achieved when the BPM selectively transports H+ and OH- ions in the cation and anion exchange layers respectively. But in practical devices, salt impurities may be present in the electrolytes, or supporting salt may be intentionally added to improve electrolyte conductivity. In this work, we explore how ion exchange processes in bipolar membranes in impure electrolytes affect the ion composition at the bipolar junction - the interfacial layer where the two membrane layers meet. We propose that the ion composition at this interface determines the voltage measured across the membrane at open-circuit, as well as the polarization behavior of the membrane under applied current.We investigate the proposed mechanism using a model system comprising electrolytes with KOH, HCl, and KCl at various relative concentrations with a commercially available BPM (Fumasep FBM). We combine analysis of polarization experiments in a four-electrode setup with a 1D continuum model of water dissociation and multi-ion transport. We report that increased relative salt concentration in the electrolytes erodes the open-circuit voltage across the membrane, yielding voltages significantly lower than expected based on the measured pH difference, indicating that H+ and OH- ions within the BPM exchange with ions from the added salt to occupy membrane fixed charges at the bipolar junction. This phenomenon penalizes the energy efficiency of BPM acid/base batteries. Additionally, we find that during polarization, impure electrolytes exhibit salt crossover at low current density and that the magnitude of this crossover depends on the relative concentration of the salt versus H+ and OH- ions. We utilize the 1D continuum model to visualize concentration profiles of ions in the BPM at both open-circuit and applied current conditions.This work illustrates the importance of electrolyte species on interfacial BPM phenomena and provides guidance both for designing new BPM materials and for choosing electrolytes and operating conditions in electrochemical cells. Figure 1

Lithium, Ether, and Graphite: A Retrospective Trilogy of the Anode-Electrolyte Interface

Ethers represent a family of solvents widely applied in various battery chemistries, except in the most successful battery—Li-ion batteries (LIBs). It is believed that ether electrolytes are incompatible with graphite anodes in LIBs due to detrimental solvent co-intercalation and exfoliation. Here, we demonstrate that ether electrolytes at standard salt concentration can enable outstanding reversibility and fast-charge capability of graphite anodes in LIBs. In short, for ether electrolytes, the anion and the solid-electrolyte interphase (SEI) govern the electrochemical behaviors of graphite anodes. Firstly, we point out that exfoliation and co-intercalation are not always the root causes of the documented cell failures using graphite anodes. Matching the reductive stability between solvents and salts is key to resolving the interphase problem. Reductively unstable lithium bis(fluorosulfonyl)imide (LiFSI) is the optimal salt paired with 1,3-dioxolane (DOL), enabling weakened Li-solvent interaction, preferential reduction of the FSI anion, and a homogeneous SEI enriched with inorganic species. Consequently, electrolyte reduction and Li-DOL co-intercalation are inhibited. Secondly, reductively stable LiBF4 paired with dimethoxyethane (DME) realizes improved cyclability of graphite based on a cointercalation mechanism. We conducted a holistic investigation into the nature, function, and formation of the heterogeneous SEI. Specifically, we revealed a self-terminating, heterogeneous interphase based on grainy LiF located on the catalytically active edge plane of natural graphite. Thirdly, we discovered a novel cointercalation mechanism, namely, in-situ synthesis of ternary graphite-intercalation compounds in a new electrolyte. We characterize the progressive cointercalation via operando synchrotron X-ray analyses, supporting reduced volume expansion and high structural integrity of the cointercalated graphite. The resulting t-GIC chemistry delivers unprecedented fast charging (1 minute) and ultralong lifetimes exceeding 10,000 cycles. The full cells based on cointercalated graphite display remarkable cycling stability upon a 15-minute charging and excellent rate capability even at -40°C. In these three case studies combined as a trilogy with strong correlation, we clarify a long-standing confusing issue in the LIBs community—that is, the compatibility between ether electrolytes and graphite anodes. This trilogy summarizes and reflects on past experiences with the anode/electrolyte interface (mostly failures of ether electrolytes when cycling graphite anodes) dating back to the 1970s. Learning from history, we exemplify different intercalation chemistries of graphite based on DOL, DME, and another common ether solvent, paired with 1M lithium salts, all showing approximately 99.9% Coulombic efficiency, high capacity retention, and rapid interphase kinetics. Our findings not only shed light on the enigmatic interphase formed by ether electrolytes but also offer critical insights into future electrolyte design for graphite anodes operated under extreme conditions.

Operando FTIR Measurements of Li-Ion Concentration in a Graphite/NMC Battery During Fast Charging

When cycled at high charging rates, Li-ion batteries can suffer from poor performance and cell degradation, especially for thicker batteries intended for electric vehicles (> 4 mAh/cm2). Such degradation in these energy-dense cells can be attributed to heterogeneous electrode utilization due to Li-ion concentration gradients in thick electrodes. Researchers have used pseudo-2D (P2D) models to predict concentration polarization phenomena during fast charging, but experimental measurements have been minimal. In the present research, we developed an optically accessible operando cell for FTIR (Fourier transform infrared spectroscopy) and ATR (attenuated total reflection) to measure Li-ion concentration at the back of the graphite anode during fast charging (> 2C). The graphite/NMC811 full cells with 1.2M LiPF6 in 3/7 EC/EMC (ethylene carbonate/ethyl methyl carbonate) (wt./wt.). had cathode loadings ranging from 2.6 mAh/cm2 to 3.9 mAh/cm2 and N/P ratios of ≈1.1. Changes in Li-ion concentration are measured and compared with a P2D model at C-rates of C/2, 1C, 2C, 3C, 4C, and 5C. Large salt-depletion events were observed at the back of the anode in both the model and experiment, especially at charging rates greater than 2C. The model and experiments agreed most closely for thinner electrodes and at lower charging rates. However, at higher rates and for thicker loadings model/experiment disparities existed during periods in which the P2D model predicted minimal salt concentrations at the back of the anode. The disparity between measured and predicted Li-ion concentrations suggests a gap in understanding of electrolyte physics under extreme polarization.