Glyme Solvents Research Articles

Mechanism Analysis of Solution Li Pre-Doping Technique Toward Si-Based Anodes for Next-Generation Batteries

In recent years, Li metal anode has attracted much attention to increase the energy density for next-generation batteries. However, there are still some problems to be solved such as extra electrolyte decomposition and Li dendrite deposition. Therefore, Si is expected as a promising alternative to Li especially for suppression of the Li dendrite. However, Li pre-doping toward Si anode is necessary because some of cathode materials for next-generation batteries such as Li-S and Li-O2 do not contain Li. Therefore, the Li pre-doping technique is a quite important for Si-based anodes. We have also researched a Li pre-doping technique using Li-naphthalenide (Li-NTL) solutions and reported that the pre-doped capacities depended on the kinds of solvents [1] and Li+ concentration of the Li-NTL solution. In this study, we investigated the mechanism from the viewpoint from kinds of the generated NTL anions by the solvents and discussed on the relationship among them, equilibrium potential of the Li-NTL solutions and the solvation structure including NTL anions and Li+. Si electrode was prepared by coating the slurry of Si powder (f 40-50 nm), Ketjen Black conductive agent, and polyimide binder in a mass ratio of 80: 5: 15. Li-NTL pre-doping solutions were prepared with 5.0 mL of 2-methyltetrahydrofuran (MeTHF), monoglyme (G1), diglyme (G2) or triglyme (G3) solvents, 2.5 mmol naphthalene, and “saturated” Li. Moreover, extra Li foil immersing in the Li-NTL solution was also used during the Li pre-doping toward the Si anode. The equilibrium potentials of the Li-NTL solutions were measured up to 12 h. The amount of generated radical monoanion NTL· - and dianion NTL2 -was evaluated by UV-Vis spectra for the Li-NTL solutions and the solvation structures was also analyzed by a DFT calculation. Fig. 1 shows a change in the equilibrium potentials for the Li-NTL solutions with different solvents. The order of magnitude by the solvents was MeTHF < G1< G3 < G2, which is a good agreement with the pre-doped capacities toward the Si anode expect for the influence of some reduction decomposition of G1 and G2 on the surface of Li alloyed Si anode. Fig. 2 shows the UV-Vis spectra of Li-NTL solutions using different solvents. For the MeTHF solvent, a spectrum derived from NTL2- dianion was clearly appeared at 540 nm, which leads to lowering the equilibrium potential of the Li-NTL solution. From this point, G1 also exhibited the NTL2- dianion signal together with radical monoanion NTL· - at 320 nm. The other glyme solvents exhibited only smaller monoanion signal than that for the G1. DFT calculation also supported the ease to form the dianion for the MeTHF solvent from the solvation structure including NTL2- dianion and Li+. The detail mechanism and the effect of Li+ concentration in the Li-NTL solutions on the relationship between the solvation structure and equilibrium potential will be also discussed in the meeting.

Optimizing Nonaqueous Sodium–Polysulfide Redox-Flow Batteries: The Role of Solvation Effects with Glyme Solvents

Optimizing Nonaqueous Sodium–Polysulfide Redox-Flow Batteries: The Role of Solvation Effects with Glyme Solvents

Solvent-Mediated, Reversible Ternary Graphite Intercalation Compounds for Extreme-Condition Li-Ion Batteries.

Traditional Li-ion intercalation chemistry into graphite anodes exclusively utilizes the cointercalation-free or cointercalation mechanism. The latter mechanism is based on ternary graphite intercalation compounds (t-GICs), where glyme solvents were explored and proved to deliver unsatisfactory cyclability in LIBs. Herein, we report a novel intercalation mechanism, that is, in situ synthesis of t-GIC in the tetrahydrofuran (THF) electrolyte via a spontaneous, controllable reaction between binary-GIC (b-GIC) and free THF molecules during initial graphite lithiation. The spontaneous transformation from b-GIC to t-GIC, which is different from conventional cointercalation chemistry, is characterized and quantified via operando synchrotron X-ray and electrochemical analyses. The resulting t-GIC chemistry obviates the necessity for complete Li-ion desolvation, facilitating rapid kinetics and synchronous charge/discharge of graphite particles, even under high current densities. Consequently, the graphite anode demonstrates unprecedented fast charging (1 min), dendrite-free low-temperature performance, and ultralong lifetimes exceeding 10 000 cycles. Full cells coupled with a layered cathode display remarkable cycling stability upon a 15 min charging and excellent rate capability even at -40 °C. Furthermore, our chemical strategies are shown to extend beyond Li-ion batteries to encompass Na-ion and K-ion batteries, underscoring their broad applicability. Our work contributes to the advancement of graphite intercalation chemistry and presents a low-cost, adaptable approach for achieving fast-charging and low-temperature batteries.

Unraveling the solvent stability on the cathode surface of Li-O2 batteries by using in situ vibrational spectroscopies.

In aprotic lithium-oxygen (Li-O2) batteries, solvent properties are crucial in the charge/discharge processes. Therefore, a thorough understanding of the solvent stability at the cathode surface during the oxygen reduction/evolution reactions (ORR/OER) is essential for the rational design of high-performance electrolytes. In this study, the stability of typical solvents, a series of glyme solvents with different chain lengths, has been investigated during the ORR/OER by in situ vibrational spectroscopy measurements of sum frequency generation (SFG) spectroscopy and infrared reflection absorption spectroscopy (IRRAS). The structural evolution and decomposition mechanism of the solvents during ORR/OER have been discussed based on the observations. Our results demonstrate that superoxide (O2-) generated during the ORR plays a critical role in the stability of the solvents.

Toward Sustainable Li-S Battery Using Scalable Cathode and Safe Glyme-Based Electrolyte.

The search for safe electrolytes to promote the application of lithium-sulfur (Li-S) batteries may be supported by the investigation of viscous glyme solvents. Hence, electrolytes using nonflammable tetraethylene glycol dimethyl ether added by lowly viscous 1,3-dioxolane (DOL) are herein thoroughly investigated for sustainable Li-S cells. The electrolytes are characterized by low flammability, a thermal stability of ∼200 °C, ionic conductivity exceeding 10-3 S cm-1 at 25 °C, a Li+ transference number of ∼0.5, electrochemical stability window from 0 to ∼4.4 V vs Li+/Li, and a Li stripping-deposition overpotential of ∼0.02 V. The progressive increase of the DOL content from 5 to 15 wt % raises the activation energy for Li+ motion, lowers the transference number, slightly limits the anodic stability, and decreases the Li/electrolyte resistance. The electrolytes are used in Li-S cells with a composite consisting of sulfur and multiwalled carbon nanotubes mixed in the 90:10 weight ratio, exploiting an optimized current collector. The cathode is preliminarily studied in terms of structure, thermal behavior, and morphology and exploited in a cell using standard electrolyte. This cell performs over 200 cycles, with sulfur loading increased to 5.2 mg cm-2 and the electrolyte/sulfur (E/S) ratio decreased to 6 μL mg-1. The above sulfur cathode and the glyme-based electrolytes are subsequently combined in safe Li-S batteries, which exhibit cycle life and delivered capacity relevantly influenced by the DOL content within the studied concentration range.

Sodium-ion conducting pseudosolid electrolyte for energy-dense, sodium-metal batteries

A liquid electrolyte based on the addition of a sodium salt in glyme solvent is encapsulated within a polymer-modified mesoporous silica matrix enabling the fabrication of a pseudosolid, sodium-ion electrolyte. This ionogel electrolyte is stable in contact with sodium metal leading to low overpotentials for sodium metal plating/stripping along with high Na+ conductivity and > 4 V electrochemical stability window. Sodium-metal batteries with sodium vanadium fluorophosphate positive electrode achieve over 400 Wh kg−1 at 0.5C.

Electrolytes and Interfaces Driven Thermal Stability of Sodium-Ion Batteries

Recognizing the mandates in sustainability and material abundance, sodium-ion batteries hold great potential in being alternate chemistry for applications such as grid storage systems. Along with other performance matrices, the safety problem known as “thermal runaway” must be understood and overcome for the practical realization of sodium-ion batteries in countless applications. While the physiochemical properties of the model electrode materials play a major role in determining overall thermal stability, electrolyte-derived unstable solid electrolyte interphases (SEI) can also trigger early thermal instability. Since the sodium-ion battery has a highly soluble SEI layer, especially in carbonate electrolytes, it is essential to understand the thermal instability signatures of electrode-electrolyte interactions. So far, the interfacial driven thermal stability of sodium-ion battery electrodes remains largely unexplored. Herein, we aim to investigate the intrinsically stochastic and heterogeneous nature of the SEI layer, illustrating the role of carbonate and glyme solvents in determining the overall thermal stability of the cell. The fundamentals of the thermal failure issues explored in this work can pave the way toward building a safer sodium-ion battery.

Toward practical issues: Identification and mitigation of the impurity effect in glyme solvents on the reversibility of Mg plating/stripping in Mg batteries.

Reversible electrochemical magnesium plating/stripping processes are important for the development of high-energy-density Mg batteries based on Mg anodes. Ether glyme solutions such as monoglyme (G1), diglyme (G2), and triglyme (G3) with the MgTFSI2 salt are one of the conventional and commonly used electrolytes that can obtain the reversible behavior of Mg electrodes. However, the electrolyte cathodic efficiency is argued to be limited due to the enormous parasitic reductive decomposition and passivation, which is governed by impurities. In this work, a systematic identification of the impurities in these systems and their effect on the Mg deposition–dissolution processes is reported. The mitigation methods generally used for eliminating impurities are evaluated, and their beneficial effects on the improved reactivity are also discussed. By comparing the performances, we proposed a necessary conditioning protocol that can be easy to handle and much safer toward the practical application of MgTFSI2/glyme electrolytes containing impurities.

Continuous-Flow Synthesis of Perfluoroalkyl Ketones via Perfluoroalkylation of Esters Using HFC-23 and HFC-125 under a KHMDS–Triglyme System

Abstract Hydrofluorocarbons (HFCs) are widely used as cooling agents in refrigerators and air conditioners and as solvents in industrial processes. However, their application has been restricted by their high global warming potential. Thus, strategies for HFC decomposition and effective utilization are urgently required. Herein, we describe a method for the chemical transformation of two HFCs, viz. HFC-23 and HFC-125, based on the continuous-flow perfluoroalkylation of esters to synthesize the pharmaceutically and agrochemically vital trifluoromethyl and pentafluoroethyl ketones. The combination of a potassium base and a glyme solvent system is found to be the most effective. The proposed method is attractive for industrial use because it allows the consumption of a large volume of HFCs, promotes the synthesis of high-value medicinal compounds, and serves as an ideal alternative to the current HFC decomposition processes like thermal plasma treatment.

The effects of solvents on the physical and electrochemical properties of potassium-ion conducting polymer gel electrolytes

The aim of present work is to investigate the effect of different solvent mixed within poly (vinylidiene fluoride-hexafluoropropylene) (PVDF-HFP) based Potassium ion conducting polymer gel electrolytes. The samples are prepared using solution casting method and the comparative behavior of adding carbonate and glyme in PVDF-HFP/KClO4 matrix is investigated. Subsequently, polymer gel electrolytes are characterized using X-ray diffractometer to analyze the structural features of the electrolyte films. The XRD results reveal that the prepared electrolyte films using both solvents demonstrate semi-crystalline nature. The surface morphology is studied using scanning electron microscopy and significant changes in surface morphology of the polymer gel electrolyte films is observed on introducing carbonate and glyme solvents. Thermal properties and effects of temperature on the prepared electrolytes is examined using differential scanning calorimetry and investigations reveal significant effect of temperature on the heat flow into the polymer gel electrolytes samples. The weight loss of the electrolyte samples with temperature is studied using Thermogravimetric analysis and it is observed that carbonate and glyme based electrolyte offer weight loss of about 7–8 %. The ionic conductivity of 2.53×10−7 Scm−1 and 3.67×10−4 Scm−1 while electrochemical stability window of ∼2.5 V is obtained for both polymer gel electrolytes with carbonate and glyme based solvents. The reversibility of K-ion has been established using cyclic voltammetry measurements.

Factors Influencing Preferential Anion Interactions during Solvation of Multivalent Cations in Ethereal Solvents

Most multivalent secondary batteries have employed electrolytes composed of cyclic ether solvents such as tetrahydrofuran or linear glycol ether solvents (glymes) such as 1,2-dimethoxyethane (G1). A robust understanding of multivalent cation solvation tendencies in these classes of solvents provides insight into corresponding structure–property relationships which, in turn, promotes the design and discovery of improved electrolytes. In this work, our goal is to systematically address how electrolyte constituent properties, namely, ether solvent structure and dication size, direct the solvation interactions of divalent electrolytes and their resultant properties. This study utilizes pulsed-field gradient (PFG) nuclear magnetic resonance (NMR) spectroscopy in conjunction with Raman spectroscopy and ionic conductivity measurements to elucidate the preferential interactions between multivalent cations, anions, and solvent molecules along with their correlated ion dynamics. These investigations incorporate two representative divalent cations (Ca2+ and Zn2+) as well as two ethereal solvent representatives from both the cyclic ether and glyme structural classes. The results reveal that anions coordinate more readily with divalent cations in cyclic ethers than in glymes. Furthermore, the coordination of the anions with Ca2+, i.e., contact-ion pair (CIP) formation is more pronounced than with Zn2+ in a glyme solvent of limited chain length (G1), providing insight into cation size effects that are important for translating solvation behavior across various multivalent electrolytes. Importantly, we find that specific anion coordination is more strongly controlled by solvent structure than by salt concentration in the practical range of 0.1–0.5 M. However, simply reducing these inner-sphere inter-ionic interactions by changing solvent structure does not necessarily de-correlate ionic motion. Instead, concentration-dependent changes in molar ionic conductivity suggest that second-shell interactions, i.e., solvent separated ion pairs (SSIPs), are prevalent in these electrolytes and that the solution dielectric constant, which is increased by the presence of dipolar ion pairs, is critical for controlling these interactions. These findings thus provide a basis for understanding the physical chemistry of multivalent battery electrolytes.

Performance and mechanism of CO2 absorption in 2-ethylhexan-1-amine + glyme non-aqueous solutions

In this work, novel non-aqueous absorbents composed of 2-ethylhexan-1-amine (EHA) and glyme were proposed for CO2 capture. The absorption performance of CO2 in EHA + diglyme, EHA + triglyme and EHA + tetraglyme non-aqueous solutions was investigated and the viscosities (η) of the CO2-saturated absorbents were measured. Besides the experiments, kinetic models were applied to correlate the CO2 absorption. The activation energy (Ea) was obtained from Arrhenius equation, and the absorption mechanism was deduced. The results showed that both Lagergren model and Avrami model can accurately correlate and predict the time-dependent absorption amount, thus an optimized composition under which excellent absorption performance and relatively low η and Ea can be simultaneously achieved was determined. Compared to water-based absorbents like MEA, the optimized non-aqueous absorbents take the advantages of better absorption performance and lower activation energy. Moreover, the glyme solvents have about 50% lower specific heat capacities and much higher boiling points (≥435 K) than water, which is expected to greatly reduce the sensible heat and the latent heat of the solvent during regeneration. Therefore, the proposed novel absorbents have promising industrial application potential in the CO2 capture process.

Characteristics of Gel Electrolyte Composed of PEO-Based Polymer / Solvated Ionic Liquid for High Performance Li-S Batteries

IntroductionLithium-sulfur (Li-S) batteries as new secondly battery system type are attracted attention. The sulfur positive electrode of this system have theoretical capacity of 1672 mAhg-1, which shows approximately 10 times higher than conventional Li-ion battery positive electrode material (LiCoO2). The problem of this Li-S batteries is dissolution of sulfur component of positive electrode which reached a state of reaction intermediate (Li2S m : m=2~8) into electrolyte during charge / discharge processes. The dissolution and diffusion of Li2S m into electrolyte solution lead significant degradation of cycle characteristics and irreversible side reaction of Li-S batteries. Currently, as functional electrolyte solution that suppresses dissolution of Li2S m , “Solvate Ionic Liquid (SIL)” has been proposed [1]. SIL is equimolar mixed electrolyte of glyme solvent and Li salt, and has strongly interaction between glyme / Li+ to form complex cation. SIL was suppressed dissolution of Li2S m because of weak Lewis acidity / basesity. By application of SIL electrolyte, Li-S battery exhibited stable charge / discharge reaction more than 800 cycles [2]. However, equimolar composition of SIL was destroyed at electrolyte / electrode interface during charge and discharge, and free glyme which have not coordinated Li+ is temporary formed. Thus, it is dynamically possible that Li2S m dissolution into generated free glyme [3]. To solve this problem, we attempted increasing carrier cation (suppression of free glyme generation), solidification of SIL electrolyte by introduction of polymer matrix (diffusion suppression of dissolved chemical species).ExperimentalIn this study, all procedures of sample preparation were conducted in argon-filled glovebox. At first, as highly concentrated SILs, [Li a (G4)1]TFSA a (a = 1 and 1.25) (G4 : CH3-O-(C2H4O)4-CH3, LiTFSA: LiN(SO2CF3)2 = 1 : a) were prepared [4]. Gel electrolyte were prepared by mixing of SILs (a = 1, 1.25), polyether-based P(EO/PO) macromonomer and photo initiator. Composition ratio of gel electrolyte were SILs : polymer = 7 : 3, 8 : 2, 9 : 1 (wt%), respectively. After stirring, samples casted to glass substrate and polymerized by UV irradiation to obtain thin-films, which was transparent and self-standing gel electrolyte films. Prepared electrolyte films ware evaluated by differential scanning calorimetry (DSC) and AC impedance measurements.Result and discussionFig.1(a) show DSC thermograms of prepared liquid and solid electrolytes. All electrolytes exhibited glass transition temperatures (T gs). Gel electrolyte showed lower T g than that neat polymer electrolyte (without SIL). From the results, it was considered that mobility of PEO chain was increased by SIL electrolyte solutions. With increasing SIL amount into the gel electrolyte, T g appeared low temperature side, and therefore, the gel electrolyte with high SIL composition should indicate high ionic conductivity.Fig.1(b) shows temperature dependence of the ionic conductivity (σ) of gel electrolytes, respectively. Owing to increase of viscosity with high Li salt concentration, σ value of high Li salt composition SIL electrolyte (a=1.25) was lower than equimolar SIL (a=1). At 303 K, σ of [Li1(G4)1]TFSA1 : polymer = 8 : 2 electrolyte was 1.5×10−3 Scm−1, which was almost same σ value with SIL electrolyte solution.Fig.1(c) shows charge/discharge curves of Li-S cell using gel electrolyte of [Li1(G4)1]TFSA1 : polymer = 8 : 2. The Li-S battery using the equimolar SIL of gel electrolyte was confirmed first discharge capacity of 1100 mAhg-1. However, this system showed unstable voltage profiles during charge process, and this charge capacity exhibited larger than that discharging capacity. From the unstable and large voltage in this charging process, Li2S m was dissolved into gel electrolyte and side reactions occurred, was considered. This result correlates with the report that the co-migration of glyme molecule and Li cation is destroyed in when PEO matrix introduction by NMR spectroscopy method measurement [5].

Highly Reversible Sodiation of Tin in Glyme Electrolytes: The Critical Role of the Solid Electrolyte Interphase and Its Formation Mechanism.

Utilization of high-capacity alloying anodes is a promising yet extremely challenging strategy in building high energy density alkali-ion batteries (AIBs). Excitingly, it was very recently found that the (de-)sodiation of tin (Sn) can be a highly reversible process in specific glyme electrolytes, enabling high specific capacities close to the theoretical value of 847 mA h g-1. The unique solid electrolyte interphase (SEI) formed on Sn electrodes, which allows highly reversible sodiation regardless of the huge volume expansion, is herein demonstrated according to a series of in situ and ex situ characterization techniques. The SEI formation process mainly involves NaPF6 decomposition and the polymerization/oligomerization of the glyme solvent, which is induced by the catalytic effect of tin, specifically. This work provides a paradigm showing how solvent, salt, and electrode materials synergistically mediate the SEI formation process and obtains new insights into the unique interfacial chemistry between Na-alloying electrodes and glyme electrolytes, which is highly enlightening in building high energy density AIBs.

Investigation of Sodium Plating and Stripping on a Bare Current Collector with Different Electrolytes and Cycling Protocols.

In the present study, stable sodium plating/stripping has been achieved on a bare aluminum current collector, without any surface modifications or artificial SEI deposition. The crucial role of predeposited sodium using cyclic voltammetry on bare aluminum as a matrix for plating/stripping has been highlighted using different protocols for cycling. The predeposition strategy ensures stable and efficient cycling of sodium in anode-free sodium batteries without dendritic formations. The study highlights the difference of sodium plating/stripping in carbonate and glyme solvent electrolytes on the bare aluminum current collector. Contrary to the carbonate solvent electrolyte, the cell with the tetraglyme solvent electrolyte and sodium loading of 1 mA h/cm2 has an overpotential under 20 mV during the sodium plating/stripping cycles at 0.5 mA/cm2 for a testing period of 650 h. Overpotentials under 40 and 100 mV have been achieved at current densities up to 1 and 2 mA/cm2 for loadings up to 5 and 10 mA h/cm2, respectively, for a testing time up to 1500 h. Density functional theory simulations have been performed to obtain the solvation energies, and the highest occupied molecular orbital-lowest unoccupied molecular orbital band gap of the solvent-sodium ion complexes for the glyme solvent electrolytes and their trends have been correlated with the experimental observations.

Ion transport and association study of glyme-based electrolytes with lithium and sodium salts

Glyme solvents are a promising avenue of research in novel electrolytes, due to their thermal and chemical stability, large electrochemical window, and tunable properties. This work investigates the transport properties of LiPF6 and NaPF6 salts in Monoglyme (G1), Diglyme (G2), and Tetraglyme (G4). Self-Diffusion coefficients, and degrees of ion association were found through spectroscopic techniques. Raman and Fourier-Transform Infrared (FTIR) spectroscopy exhibited an absorbance peak at 740 cm−1, corresponding to the a1g mode; known to signify the presence of solvent-separated and contact ion pairs. The increased intensity of this peak for G1 compared to G4 suggests that the electrolyte exhibits stronger association with decreasing solvent size. Nuclear Magnetic Resonance Diffusometry measurements yield self-diffusion coefficients for 7Li, 23Na, and 19F across all samples to be on the order of 10−9 m2s−1 in G1, to 10−11 m2s−1 in G4. Comparison of conductivities calculated from the measured diffusivities and the Nernst-Einstein equation with conductivity measurements deduced from Electrochemical Impedance Spectroscopy (EIS) determined the ion association degree; the electrolytes were shown to exhibit stronger ion pairing with increased temperature which is attributed to a decrease in the dielectric constant of the solvents with increasing temperature. Additionally, the ion association was shown to decrease with increasing solvent molecular size, consistent with FTIR findings.

(Keynote) Important and Less Important Challenges of Metal Sulfur Batteries

Metal (Li, Mg) sulfur batteries can be considered as important energy storage devices in the future due to abundance of sulfur and high theoretical gravimetric energy density. In the last decade different groups showed progress in the electrochemical stability during cycling and in the understanding of complicated conversion reactions connected with unpredictable equilibrium states of sulfur in different solvents. Vast of reports was connected with application of different types of host matrices for sulfur impregnation and recently also with additives which can adsorb polysulfides. Much less work has been performed on development of novel electrolytes, ion selective separators and on protection of the negative electrode. Sulfur and polysulfides are soluble in the classical electrolytes based on glyme solvents and this future enables reduction of sulfur to metal sulfide and its reoxidation. But on the same time, in the liquid cells with high excess of electrolyte, soluble sulfur species diffuse/migrate to the negative electrode where can be further reduced and even deposited on the surface of the negative electrode. Although sulfur conversion in Li-S and Mg-S batteries is very similar, their electrochemical stability is completely different what can be connected with passivation of the negative electrode. Recent progress in understanding of both systems and their electrochemical stability will be discussed in this presentation. Acknowledgement: This work is supported by the HELiS project which receives funding from the European Union‘s Horizon 2020 research and innovation program under Grant Agreement No 666221. The financial support from the Slovenian Research Agency (research core funding No. P2-0393) is fully acknowledged. Reference s : [1] R. Dominko, A. Vizintin, G. Aquilanti, L. Stievano, M.J. Helen, A.R. Munnangi, M. Fichtner, I. Arcong, J. Electrochem. Soc., 2018,165, A5014-A5019. [2] S. Drvarič Talian, S. Jeschke, A. Vizintin, K. Pirnat, I. Arčon, G. Aquilanti, P. Johansson, R. Dominko, Chem. Mater., 2017, 29, 10037–10044. [3] A. Vizintin, M. Lozinšek, R. K. Chellappan, D. Foix, A. Krajnc, G. Mali, G. Drazic, B. Genorio, R. Dedryvère and R. Dominko, Chem. Mater., 2015, 27, 7070–7081. [4] A. Robba, A. Vizintin, J. Bitenc, G. Mali, I. Arčon, M. Kavčič, M. Žitnik, K. Bučar, G. Aquilanti, C. Martineau-Corcos, A. Randon-Vitanova, R. Dominko, Chem. Mater.,2015, 29, 9555-9564. [5] R. Dominko, M. U. M. Patel, V. Lapornik, A. Vizintin, M. Koželj, N. Novak Tušar, I. Arcon, L. Stievano, and G. Aquilanti, J. Phys. Chem. C, 2015, 119, 19001–19010.

(Invited) Paving the Way for K-Ion Batteries: P-Group Elements as Negative Electrodes

In the last years, many efforts have been devoted to the development of batteries based on Earth abundant elements such as sodium, magnesium or more recently potassium. Many questions arise concerning the latter of these metals: are K-ion batteries (KIB) competitive with Li-ion batteries (LIB) and Na-ion batteries (NIB)? Are the benefits of K in terms of ionic conductivity in organic electrolytes, energy density (originating from its low standard potential), abundance and cost, higher than the limitations expected from the high atomic mass and Shannon ionic radius of K+ compared to Li+ and Na+?[1] Similarly to LIB and differently from NIB, graphite can be used as negative electrode in KIB. Indeed, K+ ions can be electrochemically inserted in graphite up to the formation of KC8, corresponding to a specific capacity of 279 mAh/g.[2] Other forms of carbon such as hard carbon or carbon fibres can also be used in KIB providing interesting performance.[3] Moreover, as for LIB and NIB, other p-block elements form binary compounds with potassium with general formula K3M for Bi, Sb, P and KM for Sn and Ge, leading to theoretical gravimetric and volumetric capacities much higher than that of carbon based materials.[4],[5] Tin might be an interesting choice for its sustainability and low toxicity, even though its availability could be limited in the future. Unfortunately, limited cycling performance are reported even with Sn/C composite electrodes.[6]As a proof of concept, the performance and the detailed electrochemical mechanism of different electrode materials based on C, Sn and/or Sb will be presented and compared to those of the same materials in LIB and NIB. The advantages and drawbacks of such materials in KIB will be highlighted, without forgetting that the development of performing KIB has remained a challenge so far because of the lack of efficient electrolytes and the high reactivity of potassium metal. These effects may alter typical test in half-cells influencing both the electrochemical performance and the nature of the solid electrolyte interphase (SEI). The impact of the electrode/electrolyte reactivity on the formation of the SEI in K/Sb half-cells using electrolytes prepared with carbonate or glyme solvents, and KFP6 or KFSI salts will be discussed (Fig. 1).[7] [1] M. Okoshi, Y. Yamada, S. Komaba, A. Yamada and H. Nakai, J. Electrochem. Soc., 2017, 164, A54–A60 [2] Komaba, S.; Hasegawa, T.; Dahbi, M.; Kubota, K. Electrochem. Commun. 2015, 60, 172−175. [3] W. Wang, J. Zhou, Z. Wang, L. Zhao, P. Li, Y. Yang, C. Yang, H. Huang and S. Guo, Adv. Energy Mater., 2017, 1701648, 1701648 [4] V. Gabaudan, R. Berthelot, L. Stievano and L. Monconduit, J. Phys. Chem. C, 2018, 122, 18266−18273 [5] J. Huang, X. Lin, H. Tan and B. Zhang, Adv. Energy Mater., 2018, 8, 1703496 [6] Q. Wang, X. Zhao, C. Ni, H. Tian, J. Li, Z. Zhang, S. X. Mao, J. Wang and Y. Xu, J. Phys. Chem. C, 2017, 121 (23), 12652–12657 [7] L. Madec, V. Gabaudan, G. Gachot, L. Stievano, L. Monconduit, H. Martinez, ACS Applied Materials & Interfaces, 2018, 10 (40) 34116-34122. Figure 1

NMR Transport Study of LiPF6 and NaPF6 in Glymes for Use in Supercapacitors

Electric double-layer capacitors (EDLC) store ionic charge electrostatically at the interface of high surface area electrodes such as carbon in a liquid electrolyte. Despite significant improvements in electrode and materials design, virtually all state-of-the-art nonaqueous electrochemical capacitors use the same electrolyte: tetraethylammonium tetrafluoroborate (TEABF4) in acetonitrile (ACN). This electrolyte has an exceptionally high conductivity which minimizes resistive losses and enables capacitors to operate at very high power. However, this electrolyte has a practical voltage window around only 2.7 V, beyond which the capacitor lifetime is significantly shortened. Since the energy stored in a capacitor increases quadratically with voltage, extending this electrochemical window could significantly improve the energy density. In this study we report on electrolytes consisting of LiPF6 or NaPF6 in tetraethylene glycol dimethyl ether (TEGDME or tetraglyme or G4), bis(2-methoxyethyl) ether (diglyme, or G2) and dimethoxyethane (DME or G1) which clearly show a 4 V electrochemical window. The extended voltage window doubles the energy density that can be achieved with any given capacitor electrode. Electrolyte conductivities range from 1-10 mS/cm, which are comparable to ionic liquids and adequate for moderate power applications. Futhermore, glyme-based electrolytes have good chemical stability and low flammability. Importantly, all salts and solvents are commercially available, which should enable these electrolytes to be as cost-effective and scalable as the current generation of commercial supercapacitor electrolytes. NMR experiments were performed on 0.1m LiPF6 and 0.1m/0.8m NaPF6, each dissolved in G1, G2, and G4. Using Pulsed Field Gradient (PFG) NMR, the solvent and ionic self-diffusion coefficients, D, of 1H, 7Li, 19F, and 23Na were measured, at both 25°C and 60°C. As NMR does not distinguish between the movement of single ions and ion pairs, it can be difficult to accurately determine the true conductivity of the material. However, the diffusion measurements were used to find the limiting conductivity, σNMR , of each material via the Nernst-Einstein equation: σNMR = F2[C]/(RT)*(Dcation+Danion) Direct comparison with conductivity values obtained by electrochemical impedance spectroscopy measurements yields the degree of ion association, α: α = 1 - (σdirect/σNMR) Both diffusion and conductivity decrease with increasing solvent size, primarily attributed to viscosity effects. However the degree of ion association is significant, determined to be 85% for both 0.1m LiPF6 and 0.1m NaPF6 in G2. In glyme solvents, ion association tends to decrease with increasing solvent molecular mass, and increase with increasing temperature. These results will be discussed in the context of practical use of glyme-based electrolytes for electrochemical capacitors.

Electrochemical Stability of Closo-Carborane Anions and Their Impact on Cathode Interfacial Properties

Electrochemically stable, charge delocalized anions are essential electrolyte constituents for rechargeable, high voltage magnesium batteries. Within this class of anions, magnesium closo-carborane salts were demonstrated to support reversible magnesium electrodeposition/dissolution in tetraethylene glycol dimethyl ether (tetraglyme, G4) highlighting their cathodic stability.1,2 The corresponding anodic stability of these anions remains undetermined due to the lower anodic stability of the glyme solvents preferred for increased salt solubility, ionic conductivity, and stability against magnesium metal. In this paper, we explore the anodic stability of the carba-closo-dodecaborate anion (CB11H12 -) and select functionalized derivatives thereof relative to the more typical battery electrolyte anions bis(trifluoromethylsulfonyl)imide (TFSI) and tetrafluoroborate (BF4). In solvents of higher anodic stability than the glymes, including 3-methylsulfolane and 1,1,3,3-hexafluoroisopropanol, we demonstrate an oxidation potential of 4.5 V vs. Mg/Mg(II) for the base CB11H12 - anion, which is surprisingly lower than that measured for TFSI or BF4. Computational screening of the adiabatic ionization potential and electron affinity of RCB11H11 - (where R is a functional group that replaces the H-C proton) indicates that substitution of the proton at the carbon vertex with an electron withdrawing group can yield a several hundred millivolt increase in oxidative stability without decreasing the cathodic stability. Synthesis and electrochemical evaluation of the fluoro-carba-closo-dodecaborate validates this prediction and offers a rational design strategy for improved electrolyte stability. Oxidation of either the parent or substituted carba-closo-dodecaborate anion is irreversible, resulting in a neutral radical that incorporates within a surface film. The surface film formed inhibits further electron transfer leading to a varying degree of passivation toward further electrolyte decomposition depending on the solvent employed. The composition and structure of this metastable interface and its Mg cation transport properties will be discussed. O. Tutusaus, R. Mohtadi, T.S. Arthur, F. Mizuno, E.G. Nelson, Y.V. Sevryugina, “An Efficient Halogen-Free Electrolyte for Use in Rechargeable Magnesium Batteries,” Angew. Chem. Int. Ed. 2015, 54, 7900.S.G. McArthur, R. Jay, L. Geng, J. Guo, V. Lavallo, “Below the 12-vertex: 10-vertex carborane anions as non-corrosive, halide free, electrolytes for rechargeable Mg batteries,” Chem. Comm. 2017, 53, 4453.