Phase behavior of binary mixtures of tetraglyme (G4) and Mg­[TFSA]2 (TFSA: bis­(trifluoro­methane­sulfonyl)­amide) was investigated. In a 1:1 molar ratio, G4 and Mg­[TFSA]2 formed a stable complex with a melting point of 137 °C. X-ray crystallography of a single crystal of the complex grown from a G4-Mg­[TFSA]2 binary mixture revealed that the G4 molecule wraps around Mg2+ to form a complex [Mg­(G4)]2+ cation, and the two [TFSA]− anions also participate in the Mg2+ coordination in the crystal. The thermal stability of [Mg­(G4)]­[TFSA]2 was examined by thermogravimetry, and it was found that the complex is stable up to 250 °C. Above 250 °C, desolvation of the Mg2+ ion takes place and G4 evaporates. On the other hand, the weight loss starts at around 140 °C in solutions containing excess G4 (n > 1 in Mg­[TFSA]2:G4 = 1:n) due to the evaporation of free (uncoordinated) G4. The suppression of G4 volatility in the [Mg­(G4)]­[TFSA]2 complex is attributed to strong electrostatic and induction interactions between divalent Mg2+ and G4. In addition, complexation of G4 with Mg2+ is effective in enhancing the oxidative stability of G4. Linear sweep voltammetry revealed that the oxidative decomposition of [Mg­(G4)]­[TFSA]2 occurs at electrode potentials >5 V vs Li/Li+, while the oxidation of uncoordinated G4 occurs at around 4.0 V. This oxidative stability enhancement occurs because the HOMO energy level of G4 is reduced by complexation with Mg2+, which is supported by the ab initio calculations.

The physicochemical properties of pentaglyme (G5) and sodium bis(trifluoromethanesulfonyl)amide (Na[TFSA]) binary mixtures were investigated with respect to salt concentration and temperature. The density, viscosity, ionic conductivity, self-diffusion coefficient, and oxidative stability of a series of binary mixtures were measured, and the mixtures were examined as electrolytes for Na secondary batteries. An equimolar mixture of G5 and Na[TFSA] formed a low melting solvate, [Na(G5)1][TFSA], which exhibited an ionic conductivity of 0.61 mS cm(-1) at 30 °C. The ionicity (Λimp/Λideal) of the glyme-Na[TFSA] mixture was estimated from the molar conductivity of electrochemical impedance measurements (Λimp) and the Walden plot (Λideal). [Na(G5)1][TFSA] possessed a high ionicity of 0.63 at 30 °C, suggesting that [Na(G5)1][TFSA] is highly dissociated into a [Na(G5)1](+) cation and a [TFSA](-) anion, regardless of the extreme salt concentration in the liquid. The oxidative stabilities of G5-Na[TFSA] mixtures were investigated by linear sweep voltammetry, and the higher concentration resulted in higher oxidative stability due to the lowering of the HOMO energy level of G5 by complexation with the Na(+) ion. In addition, battery tests were performed using the mixtures as electrolytes. The [Na|[Na(G5)1][TFSA]|Na0.44MnO2] cell showed good charge-discharge cycle stability, with a discharge capacity of ca. 100 mA h g(-1), while the [Na(G5)1.25][TFSA] system, containing excess G5, showed poor stability.

The physicochemical and electrochemical properties of an equimolar complex of pentaglyme (G5) and sodium bis­(fluorosulfonyl)­amide (NaFSA), [Na­(G5)]­[FSA], mixed with a hydrofluoroether (HFE) were investigated. Ab initio calculations and Raman spectroscopy showed that the coordination structure of [Na­(G5)]­[FSA] was similar to that of [Na­(G5)]­[TFSA] (TFSA: bis­(trifluoromethanesulfonyl)­amide). The ligand G5 remained coordinated to Na+ and was not liberated from the cationic [Na­(G5)]+ complex even in the presence of HFE. The charge transport property was greater in [Na­(G5)]­[FSA]/HFE than in [Na­(G5)]­[TFSA]/HFE. A prominent difference was found in the Na metal deposition/dissolution behavior. Highly reversible Na deposition/dissolution with a Coulombic efficiency (95%) could be achieved in [Na­(G5)]­[FSA]/HFE; however, the reversibility in [Na­(G5)]­[TFSA]/HFE was very low. X-ray photoelectron spectroscopy (XPS) of the deposited Na metal in each electrolyte revealed that a thin and compact layer of electrolyte decomposition products was formed on the Na deposits in [Na­(G5)]­[FSA]/HFE. The FSA-derived thin layer can effectively inhibit the further decomposition of the electrolyte. By contrast, a thick electrolyte decomposition product found for [Na­(G5)]­[TFSA]/HFE suggested continuous decomposition of the electrolyte during Na deposition-dissolution. Highly stable charge and discharge of a hard carbon (HC) electrode was accomplished in [Na­(G5)]­[FSA]/HFE, with high Coulombic efficiency over 99.9% and negligible capacity decrease over 300 cycles. Electrochemical impedance measurements of a symmetrical cell for Na, HC, and Na0.44MnO2 electrodes with the above electrolytes verified that a stable electrode | electrolyte interface was formed on the HC and Na0.44MnO2 electrodes in [Na­(G5)]­[FSA]/HFE.

Understanding the kinetics of electrochemical oxygen reduction reaction (ORR) and controlling the chemistry, morphology, and size of discharge products are critical to realize reversible operation of metal−air batteries. Here we show that increasing Na + activity and free DME (not coordinated to Na + ) activity in the solution increases the solubility of NaO 2 and size of NaO 2 cubes in Na−O 2 cells. With increasing Na salt concentration, Raman spectroscopy revealed that Na + activity increased while free DME activity decreased. NaO 2 solubility and NaO 2 cube size were found to exhibit a maximum at a medium concentration of Na + , which was accompanied by the highest full discharge capacity. This trend was attributed to two competing effects that stabilize NaO 2 in solution; both higher Na + activity and higher free DME activity can enhance NaO 2 solubility. These results highlight immense opportunities in the design of discharge/charge characteristics such as reaction product sizes and discharge capacity through the manipulation of the chemical physics of electrolytes as well as the solvation of reaction intermediates in the electrolytes.

Lithium-ion sulfur batteries with a [graphite|solvate ionic liquid electrolyte|lithium sulfide (Li2S)] structure are developed to realize high performance batteries without the issue of lithium anode. Li2S has recently emerged as a promising cathode material, due to its high theoretical specific capacity of 1166 mAh/g and its great potential in the development of lithium-ion sulfur batteries with a lithium-free anode such as graphite. Unfortunately, the electrochemical Li(+) intercalation/deintercalation in graphite is highly electrolyte-selective: whereas the process works well in the carbonate electrolytes inherited from Li-ion batteries, it cannot take place in the ether electrolytes commonly used for Li-S batteries, because the cointercalation of the solvent destroys the crystalline structure of graphite. Thus, only very few studies have focused on graphite-based Li-S full cells. In this work, simple graphite-based Li-S full cells were fabricated employing electrolytes beyond the conventional carbonates, in combination with highly loaded Li2S/graphene composite cathodes (Li2S loading: 2.2 mg/cm(2)). In particular, solvate ionic liquids can act as a single-phase electrolyte simultaneously compatible with both the Li2S cathode and the graphite anode and can further improve the battery performance by suppressing the shuttle effect. Consequently, these lithium-ion sulfur batteries show a stable and reversible charge-discharge behavior, along with a very high Coulombic efficiency.