Nonaqueous Electrolyte Solutions Research Articles

Differentiable modeling and optimization of non-aqueous Li-based battery electrolyte solutions using geometric deep learning

Electrolytes play a critical role in designing next-generation battery systems, by allowing efficient ion transfer, preventing charge transfer, and stabilizing electrode-electrolyte interfaces. In this work, we develop a differentiable geometric deep learning (GDL) model for chemical mixtures, DiffMix, which is applied in guiding robotic experimentation and optimization towards fast-charging battery electrolytes. In particular, we extend mixture thermodynamic and transport laws by creating GDL-learnable physical coefficients. We evaluate our model with mixture thermodynamics and ion transport properties, where we show improved prediction accuracy and model robustness of DiffMix than its purely data-driven variants. Furthermore, with a robotic experimentation setup, Clio, we improve ionic conductivity of electrolytes by over 18.8% within 10 experimental steps, via differentiable optimization built on DiffMix gradients. By combining GDL, mixture physics laws, and robotic experimentation, DiffMix expands the predictive modeling methods for chemical mixtures and enables efficient optimization in large chemical spaces.

First Isolation of Solvated MgCl+ species as the Sole Cations in Electrolyte Solutions for Rechargeable Mg Batteries

Synthesis of complex magnesium cations in ethereal solutions, is receiving a lot of attention due to their potential utilization in rechargeable magnesium batteries (RMB). The simplest complex cation, namely, solvated MgCl+, was hypothesized and reported as the most important cation in nonaqueous magnesium electrolyte solutions chemistry. However, such ions have never isolated as the only cationic species in ethereal solutions developed for RMB. In this study, we report on a successful isolation of the pure electrolyte MgCl(THF)5 + - PhAlCl3 - , and on the electrochemical behaviour of its ethereal solutions. The structure of this compound was proved by single-crystal X-ray diffraction, Raman, and, NMR spectroscopies. The novel MgCl(THF)5PhAlCl3/THF electrolyte solutions exhibit reversible Mg deposition/dissolution processes with anodic stability up to 2.7 V. The application of electrochemical cleaning pre-treatment for these solutions enabled to profoundly increase the coulombic efficiency of their Mg deposition/dissolution processes. Examining Mg prototype batteries with Mg foil anodes, composite MgxMo6S8Chevrel phase (CP) cathodes containing these solutions exhibited a viable battery performance, indicating that upon a further optimization, electrolyte solutions based on the complex MgCl(THF)5 + - PhAlCl3 - electrolyte may be suitable for practical RMB. Figure 1

Anodic dissolution of aluminum in non-aqueous electrolyte solutions for sodium-ion batteries

The respective impact of the electrolyte and solvent, during anodic dissolution or passivation of aluminum was investigated in electrolyte solutions developed for sodium-ion batteries.

Thin Reinforced Anion-Exchange Membranes for Non-Aqueous Redox Flow Battery Employing Fe/Co-Metal Complex Redox Species

Non-aqueous redox flow batteries (NARFBs) have been attracting much attention because they can significantly increase power and energy density compared to conventional RFBs. In this study, novel pore-filled anion-exchange membranes (PFAEMs) for application to a NAPFB employing metal polypyridyl complexes (i.e., Fe(bpy)32+/Fe(bpy)33+ and Co(bpy)32+/Co(bpy)33+) as the redox species are successfully developed. A porous polyethylene support with excellent solvent resistance and mechanical strength is used for membrane fabrication. The PFAEMs are prepared by filling an ionic liquid monomer containing an imidazolium group and a crosslinking agent into the pores of the support film and then performing in situ photopolymerization. As a result, the prepared membranes exhibit excellent mechanical strength and stability in a non-aqueous medium as well as high ion conductivity. In addition, a low crossover rate for redox ion species is observed for the prepared membranes because they have relatively low swelling characteristics in non-aqueous electrolyte solutions and low affinity for the metal-complex redox species compared to a commercial membrane. Consequently, the PFAEM is revealed to possess superior battery performance than a commercial membrane in the NARFB tests, showing high energy efficiency of about 85% and stable operation for 100 cycles.

(Digital Presentation) Hybrid Membranes Engineered from Sustainable Sources Exhibit Improved Ionic Conductivity

Ion-conducting membranes are crucial in electrochemical storage and energy conversion technologies such as fuel cells, alkaline ion batteries, and redox-flow batteries. The purpose of the membrane is to prevent direct contact between the electrodes while allowing selective ions to pass through. The characteristics of the ion-conducting membranes include excellent mechanical and thermal stability, chemical resistance, high ionic conductivity, and selectivity. Polymers and polymer-based precursors make up the majority of the materials that have been used as ion-conducting membranes. However, most polymer-based membranes have issues related to poor chemical resistance, recyclability, and thermal and mechanical stability. In addition, as more sustainable materials are being sought to create a sustainable society, new environmentally friendly and economically viable materials are in high demand. The clay-based material could be an alternative for ion-conducting applications due to its porous structure, high surface area, tunable interlayer spacing, excellent thermal and mechanical stability, and high ion conductivity. Clay minerals are primarily used in water treatment, pharmaceutics, ceramics, paints, and coating industries. Furthermore, clay minerals have been used as a filler material in energy storage and conversion system to improve the thermal stability and ion conductivity in electrodes, electrolytes, and separators.Herein, we report a novel approach to making non-polymeric hybrid clay membranes using environmentally benign materials such as clay and zwitterion (i.e., sulfobetaine). These membranes with improved ionic conductivity will find immense potential in applications in energy storage and conversion systems. Such membranes have been synthesized by intercalating sulfobetaine molecules into the interlayer spacing of the bentonite clay matrix. Sulfobetaine is a zwitterion composed of quaternary ammonium cations and sulfonate anions. Leveraging the unique structure-property features that exist in zwitterions helps to alter the interlayer spacings in the clay matrix, thereby enabling ion diffusion through the silicate layers. Furthermore, it helps in the formation of a self-supporting membrane. The functionalized membrane's crystal structure and chemical composition were investigated using XRD, XPS, and ATR-FTIR. XRD was used to identify the layered structure of the silicates in clay and the increased interlayer spacing (1.15 nm to 1.8 nm) of the hybrid clay membrane. Data from XPS and ATR-FTIR confirmed the successful functionalization of clay with sulfobetaine. Furthermore, the ionic conductivity of the hybrid clay membrane was characterized using a non-aqueous lithium electrolyte solution, and found a conductivity of . This is one of the first reports showcasing how zwitterions can impart conductivity in clay silicate motifs. It suggests an easy and novel approach to transport lithium-ion through silicate layers, thereby making it suitable for use as membranes in batteries and supercapacitors. The current study demonstrates an easy and versatile approach to developing cost-effective and eco-friendly membranes from sustainable materials such as clay, which, when functionalized, has the potential to be a highly efficient ion-conducting material with applications in energy storage, devices, and technologies.

Indium Alloy as Anodes for Rechargeable Calcium Ion Batteries at Room Temperature

Rechargeable multivalent ion batteries are poised to deliver the necessary breakthroughs in energy density and stability required for extended-range electric vehicles and large-scale grid energy storage. Calcium ion batteries are of interest owing to their high theoretical energy densities, natural abundance, and safety, yet are hindered by calcium’s strong interactions with non-aqueous electrolyte solutions and a general lack of suitable intercalation hosts. In order to realize the high voltage and theoretical capacity of a calcium-ion battery, replacement of the Ca metal anode with new electrode materials that can operate reversibly in conventional electrolyte systems is necessary, beginning with fundamental studies to identify novel intercalation or alloy-type hosts which can provide high-Ca content while avoiding considerable volume expansion and capacity loss. In this work, research into a novel indium anode as a host for Ca2+ ions in conventional electrolytes at room temperature is presented.

Modeling Charge Transport in Flowable Suspension Electrolytes

Flowable suspension electrolytes (FSEs) consist of electrically conductive, redox-active, and/or energy-storing particles suspended in aqueous or non-aqueous electrolyte solutions (supporting salt + solvent). These particles, which can range in size from nanometers to micrometers, are subjected to Brownian forces, hydrodynamic forces, hydrophobic interactions, and van der Walls interactions among others. FSEs may enable electrochemical processes where solid reactants can be integrate with flow cells potentially unlocking new approaches to materials processing (e.g., synthesis, extraction/recovery). For example, FSEs has been demonstrated in redox flow batteries, facilitating higher energy densities without sacrificing independent power and energy scaling However, understanding and controlling charge transport through the deformable particle microstructures, especially under shear, is key to enable desirable current distributions and thus effective electrode utilization.In this presentation, we investigate the impact of interparticle interactions, particle volume fractions, and applied shear rate on charge transport through FSEs. At sufficiently large volume fractions above the percolation threshold and stationary conditions, particle interactions in an FSE can drive the formation of networks that span the cell dimensions and provide electronic transport similar to stationary freestanding porous electrodes. However, under flowing conditions, shear forces can break these networks, hampering charge transport. Conversely, increasing attractive particle interactions or particle concentration can strengthen the particle networks and fortifying charge transport, albeit at the expense of fluid viscosity. To better understand these processes, we model the charge transport through these dynamic particle networks, using a combination of local charge transport models and discrete particle simulations, considering the Peclet number (ratio of shearing to Brownian forces) and Mason number (ratio of shearing to attractive forces). Our results contribute to the development of continuum models for charge transport in FSEs, which, in turn, can facilitate efficient and accurate system-level analysis. Acknowledgments This work was funded by the Skoltech – MIT Next Generation Program.

Electrochemistry of Polyfluorinated Organic Molecules in Nonaqueous Electrolyte Solutions

Fluoroorganic molecules provide significant benefit for many applications including pharmaceuticals, agrichemicals, and advanced functional materials due to its highly stable carbon fluorine bonds. Fluorination can have profound effects upon tuning the electronic and optical properties of organic molecules. Thus, fluorine is found in many state-of-the-art materials. For example, fluorinated liquid crystals are used for active-matrix liquid crystal displays; fluorinated membranes are utilized as proton transfer elements necessary for the function of fuel cells; fluorinated electrolyte solutions are tested in lithium batteries to improve their long-term stability and cell voltage. Lately, fluorinated and perfluoroalkylated organic semiconductors were reported for its application in organic field effect transistors (OFETs). Fluorination can tune a p-type organic semiconductor into an n-type semiconductor because of the strong electron withdrawing effect of fluorine, though use of multiple fluorine substituents directly on the aromatics or use of trifluoromethyl and perfluoroalkyl substituents on the pi system is essential for such conversion.The high electronegativity of fluorine results in exceptionally strong bonds to carbon (BDE = 110-126 kcal/mol). Moreover, fluorine substitution results in an increase in the C-C bond energy in fluorocarbons. Strong C-F and corresponding strong C-C bonds in fluorocarbons make them less polarizable than typical hydrocarbons. The highly polar, weakly polarizable, non-hydrogen bonding, non-coordinating properties of perfluoroalkylated materials and perfluoropolyether materials, underlie the motto “Nothing sticks to Teflon® and Teflon®-coated products”. Though all these profound properties of fluoroorganics paved the road for applications exemplified above, their superior stability creates significant challenges for their end-of-life recycling and repurposing, as well as remediation of environmental persistent fluoroorganics such as PFAS. Furthermore, the chemical stability of many these materials often refers to the stability under oxidizing conditions. For fluorinated n-type organic semiconductor materials and fluorinated solvents (e.g. fluoroethylene carbonate) in lithium battery applications, access the stability of these materials and understand its decomposition mechanism under reductive conditions are necessary for rationally optimize the conditions for such applications.In this presentation, we will present and discuss our latest electrochemical study of several different types of polyfluorinated organic molecules involving both fluorinated and perfluoroalkylated aromatics, as well as inert hydrofluorocarbons where only sp3-hybridized carbon presents. As an example, our electrochemical and computational chemistry results indicate that aromatic trifluoromethlation is likely a more stable option against reduction than pentafluoroethylation of the same aromatic core, though the electronic effects of trifluoromethyl and pentafluoroethyl substituents on aromatics are almost identical. We hope that these electrochemical results will shine some lights on searching new materials for electronic and energy storage applications and repurposing inert hydrofluorocarbons, for example, pentafluoroethane, a refrigerant with high global warming potential to be phased out in the next 10-20 years.

An automated and lightweight framework for electrolyte diagnostics using quantitative microelectrode voltammetry

Voltammetry is a ubiquitous electroanalytical method that can be used to help probe sustainable electrochemical technologies. When conducted with a microelectrode (radius ca. μm), voltammetry enables special interrogation of electrolyte solutions by minimizing distortions and facilitating near-steady-state measurements. Methodologies aimed to evaluate the behavior of redox-active species often leverage well-established, physically-grounded expressions that can be extended to examine electrolyte solutions under non-ideal conditions (e.g., signal convolution from multiple redox events) by simulating the entire voltammogram. To characterize these analyte systems, we first develop closed-form expressions—building on previous work that utilizes oblate spheroidal coordinates—and establish a framework for rapidly evaluating electrolyte composition. We subsequently apply finite difference transient voltammogram models to assess the performance of this workflow. We then validate our findings using model, deterministically-prepared nonaqueous electrolyte solutions containing N-[2-(methoxyethoxy)ethyl]phenothiazine. Overall, we show the toolkit is particularly adept at rapidly (< 1 min) estimating the degree to which an electrolyte solution is charged (its “state-of-charge”) and remains intact (its “state-of-health”). Finally, we highlight potential extensions of this method towards advancing in situ or operando diagnostic methods within operating electrochemical devices.

Aluminum foil negative electrodes with multiphase microstructure for all-solid-state Li-ion batteries

Metal negative electrodes that alloy with lithium have high theoretical charge storage capacity and are ideal candidates for developing high-energy rechargeable batteries. However, such electrode materials show limited reversibility in Li-ion batteries with standard non-aqueous liquid electrolyte solutions. To circumvent this issue, here we report the use of non-pre-lithiated aluminum-foil-based negative electrodes with engineered microstructures in an all-solid-state Li-ion cell configuration. When a 30-μm-thick Al94.5In5.5 negative electrode is combined with a Li6PS5Cl solid-state electrolyte and a LiNi0.6Mn0.2Co0.2O2-based positive electrode, lab-scale cells deliver hundreds of stable cycles with practically relevant areal capacities at high current densities (6.5 mA cm−2). We also demonstrate that the multiphase Al-In microstructure enables improved rate behavior and enhanced reversibility due to the distributed LiIn network within the aluminum matrix. These results demonstrate the possibility of improved all-solid-state batteries via metallurgical design of negative electrodes while simplifying manufacturing processes.

Fundamental investigations on the ionic transport and thermodynamic properties of non-aqueous potassium-ion electrolytes

Non-aqueous potassium-ion batteries (KIBs) represent a promising complementary technology to lithium-ion batteries due to the availability and low cost of potassium. Moreover, the lower charge density of K+ compared to Li+ favours the ion-transport properties in liquid electrolyte solutions, thus, making KIBs potentially capable of improved rate capability and low-temperature performance. However, a comprehensive study of the ionic transport and thermodynamic properties of non-aqueous K-ion electrolyte solutions is not available. Here we report the full characterisation of the ionic transport and thermodynamic properties of a model non-aqueous K-ion electrolyte solution system comprising potassium bis(fluorosulfonyl)imide (KFSI) salt and 1,2-dimethoxyethane (DME) solvent and compare it with its Li-ion equivalent (i.e., LiFSI:DME), over the concentration range 0.25–2 molal. Using tailored K metal electrodes, we demonstrate that KFSI:DME electrolyte solutions show higher salt diffusion coefficients and cation transference numbers than LiFSI:DME solutions. Finally, via Doyle-Fuller-Newman (DFN) simulations, we investigate the K-ion and Li-ion storage properties for K∣∣graphite and Li∣∣graphite cells.

Ultrasonic, transport and vibrational frequency analysis of non-aqueous peptide + electrolyte solutions

Ultrasonic method finds extensive application in investigating various physicochemical parameters involving molecular interactions in liquid mixtures. The density, viscosity and ultrasonic velocity measurements were carried out in non-aqueous peptide and electrolyte solutions at different concentrations and temperatures to illustrate the intermolecular interactions prevailing in the solutions. The main transport parameters such as adiabatic compressibility, internal pressure and free volume have been computed using the measured values. These parameters were utilized to interpret the intermolecular interactions existing in the mixture of glycyl-glycine and potassium ascorbate in non-aqueous formamide medium. The ultrasonic results are corroborated with the vibrational frequency analysis of the peptide and ascorbate solutions.

Non-polar ether-based electrolyte solutions for stable high-voltage non-aqueous lithium metal batteries

The electrochemical instability of ether-based electrolyte solutions hinders their practical applications in high-voltage Li metal batteries. To circumvent this issue, here, we propose a dilution strategy to lose the Li+/solvent interaction and use the dilute non-aqueous electrolyte solution in high-voltage lithium metal batteries. We demonstrate that in a non-polar dipropyl ether (DPE)-based electrolyte solution with lithium bis(fluorosulfonyl) imide salt, the decomposition order of solvated species can be adjusted to promote the Li+/salt-derived anion clusters decomposition over free ether solvent molecules. This selective mechanism favors the formation of a robust cathode electrolyte interphase (CEI) and a solvent-deficient electric double-layer structure at the positive electrode interface. When the DPE-based electrolyte is tested in combination with a Li metal negative electrode (50 μm thick) and a LiNi0.8Co0.1Mn0.1O2-based positive electrode (3.3 mAh/cm2) in pouch cell configuration at 25 °C, a specific discharge capacity retention of about 74% after 150 cycles (0.33 and 1 mA/cm2 charge and discharge, respectively) is obtained.

Investigating microstructure evolution of lithium metal during plating and stripping via operando X-ray tomographic microscopy

Efficient lithium metal stripping and plating operation capable of maintaining electronic and ionic conductivity is crucial to develop safe lithium metal batteries. However, monitoring lithium metal microstructure evolution during cell cycling is challenging. Here, we report the development of an operando synchrotron X-ray tomographic microscopy method capable of probing in real-time the formation, growth, and dissolution of Li microstructures during the cycling of a Li||Cu cell containing a standard non-aqueous liquid electrolyte solution. The analyses of the operando X-ray tomographic microscopy measurements enable tracking the evolution of deposited Li metal as a function of time and applied current density and distinguishing the formation of electrochemically inactive Li from the active bulk of Li microstructures. Furthermore, in-depth analyses of the Li microstructures shed some light on the structural connectivity of deposited Li at different current densities as well as the formation mechanism of fast-growing fractal Li microstructures, which are ultimately responsible for cell failure.

Electrolyte engineering via ether solvent fluorination for developing stable non-aqueous lithium metal batteries

Fluorination of ether solvents is an effective strategy to improve the electrochemical stability of non-aqueous electrolyte solutions in lithium metal batteries. However, excessive fluorination detrimentally impacts the ionic conductivity of the electrolyte, thus limiting the battery performance. Here, to maximize the electrolyte ionic conductivity and electrochemical stability, we introduce the targeted trifluoromethylation of 1,2-dimethoxyethane to produce 1,1,1-trifluoro-2,3-dimethoxypropane (TFDMP). TFDMP is used as a solvent to prepare a 2 M non-aqueous electrolyte solution comprising bis(fluorosulfonyl)imide salt. This electrolyte solution shows an ionic conductivity of 7.4 mS cm–1 at 25 °C, an oxidation stability up to 4.8 V and an efficient suppression of Al corrosion. When tested in a coin cell configuration at 25 °C using a 20 μm Li metal negative electrode, a high mass loading LiNi0.8Co0.1Mn0.1O2-based positive electrode (20 mg cm–2) with a negative/positive (N/P) capacity ratio of 1, discharge capacity retentions (calculated excluding the initial formation cycles) of 81% after 200 cycles at 0.1 A g–1 and 88% after 142 cycles at 0.2 A g–1 are achieved.

Lithium hexamethyldisilazide as electrolyte additive for efficient cycling of high-voltage non-aqueous lithium metal batteries

High-voltage lithium metal batteries suffer from poor cycling stability caused by the detrimental effect on the cathode of the water moisture present in the non-aqueous liquid electrolyte solution, especially at high operating temperatures (e.g., ≥60 °C). To circumvent this issue, here we report lithium hexamethyldisilazide (LiHMDS) as an electrolyte additive. We demonstrate that the addition of a 0.6 wt% of LiHMDS in a typical fluorine-containing carbonate-based non-aqueous electrolyte solution enables a stable Li||LiNi0.8Co0.1Mn0.1O2 (NCM811) coin cell operation up to 1000 or 500 cycles applying a high cut-off cell voltage of 4.5 V in the 25 °C−60 °C temperature range. The LiHMDS acts as a scavenger for hydrofluoric acid and water and facilitates the formation of an (electro)chemical robust cathode|electrolyte interphase (CEI). The LiHMDS-derived CEI prevents the Ni dissolution of NCM811, mitigates the irreversible phase transformation from layered structure to rock-salt phase and suppresses the side reactions with the electrolyte solution.

Two-steps electrochemical polishing of laser powder bed fusion 316l stainless steel

Laser Powder Bed Fusion fabricated 316L stainless steel test components were electrochemically polished in a non-aqueous electrolytic solution consisting of 1M sodium chloride, ethylene glycol, and 10% ethanol, and in an aqueous commercial electrolyte A2. The influence of the high current densities ranging between 250 and 2000 mA/cm2 on the surface roughness (Psa, Wsa and Ssa), materials removal weight and thickness reduction with various morphological characteristics were investigated. It is confirmed that polishing at the tranpassive region was feasible in non-aqueous electrolytes where little pitting occurred. A two-step electrochemical process was proposed based on the characterisations to enhance the polishing effect, which consisted of two processes with different electrolytes and current densities. The experimental results indicated that the surface roughness of two-step polished steels with 1500 and 250 mA/cm2 current densities was reduced by 11.25% than the optimum result of the one-step EP with the non-aqueous electrolyte solution. The weight and thickness reduction were reduced by 3.39% and 9.02%, respectively, more than the optimum results of the one-step EP with the aqueous commercial electrolyte.

On the nanoscale structural evolution of solid discharge products in lithium-sulfur batteries using operando scattering

The inadequate understanding of the mechanisms that reversibly convert molecular sulfur (S) into lithium sulfide (Li2S) via soluble polysulfides (PSs) formation impedes the development of high-performance lithium-sulfur (Li-S) batteries with non-aqueous electrolyte solutions. Here, we use operando small and wide angle X-ray scattering and operando small angle neutron scattering (SANS) measurements to track the nucleation, growth and dissolution of solid deposits from atomic to sub-micron scales during real-time Li-S cell operation. In particular, stochastic modelling based on the SANS data allows quantifying the nanoscale phase evolution during battery cycling. We show that next to nano-crystalline Li2S the deposit comprises solid short-chain PSs particles. The analysis of the experimental data suggests that initially, Li2S2 precipitates from the solution and then is partially converted via solid-state electroreduction to Li2S. We further demonstrate that mass transport, rather than electron transport through a thin passivating film, limits the discharge capacity and rate performance in Li-S cells.

Autonomous optimization of non-aqueous Li-ion battery electrolytes via robotic experimentation and machine learning coupling

Developing high-energy and efficient battery technologies is a crucial aspect of advancing the electrification of transportation and aviation. However, battery innovations can take years to deliver. In the case of non-aqueous battery electrolyte solutions, the many design variables in selecting multiple solvents, salts and their relative ratios make electrolyte optimization time-consuming and laborious. To overcome these issues, we propose in this work an experimental design that couples robotics (a custom-built automated experiment named "Clio”) to machine-learning (a Bayesian optimization-based experiment planner named "Dragonfly”). An autonomous optimization of the electrolyte conductivity over a single-salt and ternary solvent design space identifies six fast-charging non-aqueous electrolyte solutions in two work-days and forty-two experiments. This result represents a six-fold time acceleration compared to a random search performed by the same automated experiment. To validate the practical use of these electrolytes, we tested them in a 220 mAh graphite∣∣LiNi0.5Mn0.3Co0.2O2 pouch cell configuration. All the pouch cells containing the robot-developed electrolytes demonstrate improved fast-charging capability against a baseline experiment that uses a non-aqueous electrolyte solution selected a priori from the design space.

Extension of the eSAFT-VR Mie equation of state from aqueous to non-aqueous electrolyte solutions

In this work, the eSAFT-VR Mie equation of state (EoS) is extended to low relative permittivity, non-aqueous solutions. The effect of using different relative permittivity relations for the electrolyte solutions is studied, ranging from experimentally measured values to a salt-composition independent relative permittivity. Furthermore, the effect of using different approaches for the characteristic diameters in the Debye-Hückel and Born terms is presented. The eSAFT-VR Mie EoS is reparametrized using aqueous mean ionic activity coefficients, individual ion activity coefficients and densities with different relations for the relative permittivity. Afterwards, the performance of these models on non-aqueous solutions is evaluated based on the Mean Ionic Activity Coefficients of salts in non-aqueous solutions. The conclusion is that a mole fraction based mixing rule for the relative permittivity yields the best extrapolation from aqueous to non-aqueous solutions, and achieves quantitative predictions for the mean ionic activity coefficients of monovalent salts in methanol and ethanol without additional adjustable parameters.