Lithium Bis Trifluoromethanesulfonyl Amide Research Articles

Linear ether-based highly concentrated electrolytes for Li-sulfur batteries.

Li-S batteries have attracted attention as next-generation rechargeable batteries owing to their high theoretical capacity and cost-effectiveness. Sparingly solvating electrolytes hold promise because they suppress the dissolution and shuttling of polysulfide intermediates to increase the coulombic efficiency and extend the cycle life. This study investigated the solubility of polysulfide (Li2S8) in a range of liquid electrolytes, including organic electrolytes, highly concentrated electrolytes, and ionic liquids. The Li2S8 solubility was well correlated with the donor number (DNNMR), estimated via23Na-NMR, and was lower than 100 mM_(elemental sulfur) in electrolytes with DNNMR < 14, regardless of the type of electrolyte. Highly concentrated electrolytes comprising lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and linear chain dialkyl ethers such as methyl propyl ether (MPE), n-butyl methyl ether (BME), and ethyl propyl ether (EPE) were studied as sparingly solvating electrolytes for Li-S batteries. Monomethyl ethers, such as BME, showed more pronounced Li-ion coordination and higher ionic conductivity, whereas the steric hindrance of the longer alkyl chains in EPE lowered the solvation number, enhanced ion association, and lowered the ionic conductivity despite the solvents having similar dielectric constants. The charge-discharge rate capabilities of Li-S cells with dialkyl ether-based electrolytes were more impressive than those of cells with a localized high-concentration electrolyte using sulfolane (SL) and hydrofluoroether (HFE), [Li(SL)2][TFSA]-2HFE. The higher rate performance was attributed to the superior Li-ion transport properties of the dialkyl ether-based electrolytes. A pouch-type cell using lightweight [Li(BME)3][TFSA] demonstrated an energy density exceeding 300 W h kg-1 under lean electrolyte conditions.

Electrolyte/Electrode Interface Reaction in Polysulfides Insoluble Lithium-Sulfur Battery Using Li-Glyme Solvate Ionic Liquid

Lithium-Sulfur (Li-S) battery is attracting attention as a new generation secondary battery. However, one of the major problems is dissolution of discharge products of lithium polysulfides (Li2Sn: n = 1, 2, 4, 6 and 8) that cause the self-discharge. Solvate ionic liquids (SIL), which consist of a solvate ion and its counter ion, are a subclass of ionic liquids. Li-Gn SIL is yielded by equimolar mixing lithium bis-(trifluoromethanesulfonyl)amide (LiTFSA) and glyme, an oligoether such as triglyme and tetraglyme (Gn : CH3O-(CH2CH2O) n -CH3, n = 3, 4). Watanabe and Dokko et al. [1] reported that the Li-glyme SIL had low solubility of the polysulfides, and proposed polysulfides insoluble Li-S batteries using these electrolyte solutions. Recently, a sulfolane-based super-concentrated electrolyte solution (SCES) also exhibits low solubility of the polysulfides and has been applied to Li-S battery [2]. Both of Li-Gn SIL and the SCES are polysulfide insoluble electrolytes, however, the Li-S batteries with the SCES has better battery performance than that with Li-Gn SIL, implying that the different discharge reaction paths. In this contribution, in situ electrochemical impedance spectroscopy (EIS), which can acquire charge-discharge curves and impedance spectra simultaneously, is applied to the Li-S battery using the Li-Gn SIL to obtain further insight into the discharge behavior.A positive electrode slurry was obtained by mixing a sulfur S8, a Ketjen black (KB, EC600JD, Lion Corporation) and a carboxymethyl cellulose (CMC2200, Daicel) at a weight ratio of 60 : 30 : 10. The slurry was spread on an Al foil to obtain a KB-S composite electrode. The typical loading of the sulfur was 1.0 mg cm-1. A negative electrode and a separator were used a lithium metal foil (Honjo Metal) and a glass separator (GA-55, Advantec), respectively. The Li-Gn SIL electrolyte solution was prepared by mixing LiTFSA, G4 and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE) as the molar ratio of 1 : 1 : 4. The Li-S batteries were assembled using the CR 2032 coin-type cell in an Ar-filled glove box. In situ EIS measurements were conducted at different C-rate under constant-current discharging/charging condition using an electrochemical measurement system (SP-150, BioLogic). The current values were 336 and 84 μA. The DC current was superimposed with a small AC current adjusted so that the response voltage amplitude did not exceed 10 mV. The measurements were carried out in the frequency range from 500 kHz to 10 mHz.The voltage drop is observed at the DOD = 20-35 %, in the discharge profile measured at the high C-rate (336 μA), whereas that measured at the low C-rate (84 μA) has no voltage drop. The compensation for the changes over time [3,4] was performed on the obtained impedance spectra to determine instantaneous impedance spectra at a specific DOD (depth of discharge). Two capacitive semicircles and a straight line are observed for all of the investigated systems. The semicircle in the high frequency range (< 10 Hz) is asymmetrical, as if two semicircles in the high frequency range from 500 kHz to 1 kHz and in the middle frequency range from 1 kHz to 10 Hz are overlapped. For the high C-rate, the diameter of the semicircle in the middle frequency range increases with increasing DOD and remarkably increases at DOD = 20-35 %. By contrast, in the low C-rate, the diameter does not change during discharging. This semicircle is assigned to the charge transfer resistance of the negative electrode. According to the previous studies [5, 6], when the charge transfer reaction of lithium is faster than the diffusion rate of G4, the solvation structure changes due to the lack of G4 coordinating to Li+. This solvation structure change causes suppression of Li+ elution to increase the charge transfer resistance of the negative electrode. References 1) K. Dokko et al., J. Electrochem. Soc., 160, A1304-A1310 (2013).2) A. Nakanishi et al., J. Phys. Chem. C, 123, 14229-14238 (2019).3) Z. B. Stoynov et al., J. Electroanal. Chem., 183, 133-144 (1985).4) M. Itagaki et al., J. Power Sources, 135, 255-261 (2004).5) N. Serizawa et al., J. Electrochem. Soc., 160(9), A1529–A1533, (2013).6) A. Miki et al., J. Mater. Chem. A, 9, 14700–14709, (2021).

Phase Behaviors and Ion Transport Properties of LiN(SO&lt;sub&gt;2&lt;/sub&gt;CF&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;2&lt;/sub&gt;/Sulfone Binary Mixtures

Highly concentrated Li salt/aprotic solvent solutions are promising electrolytes for next-generation batteries. Understanding the Li+ ion transport process is crucial for designing novel battery electrolytes. In this study, we systematically investigated the phase behavior, solvate structures, and Li+ transport properties of binary mixtures comprising lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and various sulfones, such as sulfolane (SL), 3-methyl sulfolane (MSL), dimethyl sulfone (DMS), ethyl methyl sulfone (EMS), and ethyl isopropyl sulfone (EiPS). Except for the MSL system, the [LiTFSA]/[sulfone] = 1/2 mixtures remained in a liquid state at room temperature, thus enabling a systematic comparison of the Li+ transport properties in the highly concentrated electrolytes. In highly concentrated liquid electrolytes, Li+ ions diffuse by exchanging ligands (sulfone and TFSA). Li+ ions diffuse faster than TFSA in all electrolytes except the EiPS-based electrolyte at a composition of [LiTFSA]/[sulfone] = 1/2, resulting in high Li+ transference numbers. SL-based electrolytes show higher ionic conductivity and Li+ transference numbers than other sulfone-based electrolytes. Consequently, sulfone solvents with compact molecular sizes and low energy barriers of conformational change are favorable for enhancing the Li+ ion transport in the electrolytes.

(Electro)Chemical Processes of Poly(Ethylene Oxide)‐Based Electrolyte on Cu Surface during Lithium Secondary Battery Operation

Understanding the surface electrochemistry of polymer electrolytes under battery operating conditions is of great importance for tuning the solid electrolyte interphase (SEI) in lithium batteries with polymer electrolytes. Herein, the surface (electro)chemical process of poly(ethylene oxide) (PEO) lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) (PEO10LiTFSA) electrolyte on the Cu electrode is studied by operando attenuated total reflection infrared (ATR‐IR) and ex situ X‐ray photoelectron spectroscopy (XPS) techniques coupled with the interfacial resistance obtained from electrochemical impedance spectroscopy (EIS). Linear sweep voltammogram of the Cu electrode in PEO10LiTFSA electrolyte suggests the SEI‐like layer formation involves three reduction steps. Operando ATR‐IR, ex situ XPS, and EIS reveal the corresponding (electro)chemical processes and their effect on the interfacial resistance; LiTFSA and residual H2O and CO2 are first reduced, forming an inorganic SEI‐like layer without affecting the interfacial resistance. Subsequently, the PEO matrix reduces into several decomposed products, including CC components, responsible for the most resistive SEI‐like layer. Finally, PEO‐decomposed species with CC components further reduce into alkyl‐related species, leading significant reduction of the resistance of the SEI‐like layer. The study successfully reveals the complex surface (electro)chemical process at the PEO electrolyte–Cu electrode interface, and may serve as a model system for understanding SEI component and interfacial resistance relationship in the polymer electrolyte system.

Lithium salt-concentrated organogels prepared via one-step polymer network formation in acetonitrile-based solutions

We report lithium (Li) salt-concentrated organogel prepared via one-step gelation conducted in an acetonitrile (AN)-based solution containing tetra-arm poly (ethylene glycol) (TetraPEG) and the lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) salt at extremely high concentration. The polymer network formation via a Michael addition reaction between thiol-terminated and maleimide-terminated TetraPEG prepolymers successfully proceeded in concentrated LiTFSA/AN solutions under noncatalytic conditions at room temperature to yield a homogeneous polymer network with high reaction efficiency (>95%). We investigated the electrochemical properties of the concentrated organogel to apply it as a quasi-solid electrolyte material to the Li-ion capacitors comprising a graphite anode and activated carbon (AC) cathode. The concentrated organogel afforded the reversible Li-ion insertion/desertion into/from graphite; however, reactivity depended on the polymer concentration in the organogels. The reversible adsorption/desorption of TFSA anions into/from the AC was observed in the concentrated organogel, which was comparable to that of the corresponding electrolyte solution.

Thermally durable electrolyte for lithium-ion battery

For ensuring the safety of lithium-ion batteries (LIBs), we extensively investigated a composite electrolyte composed of a less volatile Li conductive electrolyte solution, silica (SiO2) particles, and fluorine-based binder as a thermally durable electrolyte. In this article, we review the roles of the electrolyte constituents. For the less volatile solution, we applied an equimolar complex of lithium bis(trifluoromethanesulfonyl)amide (LiTFSI) and tetraethylene glycol dimethyl ether (G4), Li(G4)TFSI, which was diluted with a low-viscosity solvent (propylene carbonate, PC) to enhance the ionic conductivity. The resultant electrolyte exhibited a favorable volatilization temperature higher than 373 K, and was applied to a 100-Wh-class laminated LIB. The cell generated neither fire nor smoke in a nail-penetration test, where the composite of SiO2 and fluorine-based binder had evidently covered the electrodes at the short-circuit region. This insulation phenomena effectively suppressed the joule heat generation and thereby kept the battery safe. One of the challenges with Li(G4)TFSI-based electrolytes is how to improve the durability during the charge and discharge cycles. The continuous decompositions of G4 and PC were a major reason for the growth of a solid-electrolyte interphase, which caused the capacity fade during charge-discharge cycles. We therefore replaced the G4-based liquid with a highly concentrated sulfolane-based liquid featuring a high Li+ transport number and a lower-viscosity solvent and found that it demonstrated a higher capacity retention after a charge-discharge cycle test.

Local Lithium-Ion Transport of a Ternary Sulfolane-Lithium Bis(trifluoromethanesulfonyl)amide-Carbonate Electrolyte: Experimental and First-Principles Molecular Dynamics Analysis toward Quasi-Solid-State Lithium-Ion Battery

The ion coordination and local ion transport of the ternary electrolyte containing sulfolane (SL), lithium bis(trifluoromethanesulfonyl)amide (LiTFSA), and a carbonate solvent were investigated by combining the electrochemical measurement and first-principles molecular dynamics (FPMD) simulation. Li(SL)3TFSA solution with the low-viscosity carbonate solvents, propylene carbonate (PC) and butylene carbonate (BC) was quasi-solidified by nanosized SiO2 and PTFE binder. Electrochemical impedance spectroscopy revealed that the quasi-solidified electrolyte sheets exhibited the Li+ transport number of 0.5, which was higher than that of Li(G4)TFSA containing PC and the conventional organic liquid electrolyte. The higher Li+ transport number contributed to an enhanced discharge capacity of the LIB at a high rate, and the LIB with Li(SL)3TFSA-PC and Li(SL)3TFSA-BC based electrolyte exhibited higher capacity retention in cycle test compared with Li(G4)TFSA-PC. The FPMD analysis revealed that a bridging coordination by SL and TFSA, where a single SL or TFSA coordinates two Li+, was observed even under coexistence with the carbonate solvent. It was also confirmed that some Li+ tend to transfer independently to the other molecules/anions via the bridging coordination, leading to the high Li+ transport number. In conclusion, the composition design of the electrolyte to keep the bridging coordination is desirable for a high Li+ transport number and superior LIB performances.

Li+ transference number and dynamic ion correlations in glyme-Li salt solvate ionic liquids diluted with molecular solvents.

Highly concentrated electrolytes (HCEs) have attracted significant interest as promising liquid electrolytes for next-generation Li secondary batteries, owing to various beneficial properties both in the bulk and at the electrode/electrolyte interface. One particular class of HCEs consists of binary mixtures of lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and oligoethers that behave like ionic liquids. [Li(G4)][TFSA], which comprises an equimolar mixture of LiTFSA and tetraglyme (G4), is an example. In our previous works, the addition of low-polarity molecular solvents to [Li(G4)][TFSA] was found to effectively enhance the conductivity while retaining the unique Li-ion solvation structure. However, it remains unclear how the diluents affect another key electrolyte parameter-the Li+ transference number-despite its critical importance for achieving the fast charging/discharging of Li secondary batteries. Thus, in this study, the effects of diluents on the extremely low Li+ transference number under anion-blocking conditions in [Li(G4)][TFSA] were elucidated, with a special focus on the polarity of the additional solvents. The concentration dependence of the dynamic ion correlations was further studied in the framework of the concentrated electrolyte theory. The results revealed that a non-coordinating diluent is not involved in the modification of the ion transport mechanism, and therefore the low Li+ transference number is inherited by the diluted electrolytes. In contrast, a coordinating diluent effectively reduces the anti-correlated ion motions of [Li(G4)][TFSA], thereby improving the Li+ transference number. This is the first time that the significant effects of the coordination properties of the diluting solvents on the dynamic ion correlations and Li+ transference numbers have been reported for diluted solvate ionic liquids.

Polymer‐Induced Inversion of the Li+ Drift Direction in Ionic Liquid‐Based Ternary Polymer Electrolytes

AbstractWhile salt‐in‐ionic liquid (IL) electrolytes provide a promising alternative to organic electrolytes, they suffer from large ionic clusters impeding lithium ion transport. Here, a lithium‐coordinating polymer is added and the transition from salt‐in‐IL to IL‐based ternary polymer gel electrolytes is followed, investigating the influence of the composition on Li transport mechanisms. The electrolyte is composed of the salt lithium bis(trifluoromethanesulfonyl)amide (LiTFSA), the IL 1‐butyl‐1‐methylpyrrolidinium (Pyr14) TFSA, and a variable amount of poly(ethylene oxide) (PEO). Impedance spectroscopy, multinuclear (1H, 7Li, and 19F) diffusion NMR and electrophoretic NMR (eNMR) yield electrophoretic mobilities, transference numbers, partial conductivities, and effective charges of all ionic species. As the polymer fraction increases, the lithium ion drift direction is inverted. The sign reversal of Li+ mobility and transference number occurs at the optimum coordination of 5–6 ethylene oxide units per Li+ ion. The dominant Li+ transport mechanism shifts from a vehicular transport via anionic clusters to chain‐dominated transport mechanisms. It is concluded that the lithium transport properties of IL‐based electrolytes can be vastly improved by the addition of PEO. Tuning the composition of the ternary electrolyte reveals optimal Li+ transport properties at moderately high polymer contents as well as high salt concentrations.

Electrochemical Properties of Poly(vinylidene fluoride-&lt;i&gt;co&lt;/i&gt;-hexafluoropropylene) Gel Electrolytes with High-Concentration Li Salt/Sulfolane for Lithium Batteries

Combining highly concentrated electrolytes with a polymer network is a valid approach to simultaneously achieve fast Li+ ion transport, high thermal stability, and a wide electrochemical window in a quasi-solid-state form. In this work, flexible gel electrolytes comprising commercially available poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF–HFP) and highly concentrated electrolytes of Li salts/sulfolane (SL) were prepared by a simple solution casting method. The anionic effects of the gel electrolytes on the Li-ion conductivity and charge transfer kinetics at the gel/electrode interface were investigated. The SL-based gel electrolyte with lithium bis(fluorosulfonyl)amide (LiFSA) showed an ionic conductivity of 0.7 mS cm−1 and a high Li transference number (>0.5) at 30 °C. The charge transfer resistance in a [Li/gel/LiCoO2] cell with LiFSA was lower than that of the cells with lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) or LiBF4, indicating faster interfacial charge transfer kinetics in the gel electrolyte with FSA. The Li/LiCoO2 cell with the LiFSA/SL gel electrolyte exhibited a higher capacity than that of the cells with the LiTFSA/SL and LiBF4/SL gel electrolytes. Hence, rationally designed gel electrolytes containing highly concentrated SL-based electrolytes enable the high rate performance of Li batteries.

Research and Development of Thermally Durable Electrolyte for Lithium Ion Battery

For ensuring safety of lithium ion batteries (LIBs), we have extensively investigated the quasi-solid electrolyte where lithium ion conducive liquid is quasi-solidified at silica surfaces as thermally durable electrolyte, and applied it to high capacity and high energy density LIB. For the liquid phase, a solvate ionic liquid, which is an equimolar complex of lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and tetraethylene glycol dimethyl ether (G4), Li(G4)TFSA, was used. For enhancing discharge capability at a higher rate, Li(G4)TFSA was diluted by low viscos solvent such as propylene carbonate (PC). The developed electrolyte possessed a favorable volatilization temperature higher than 373 K. A 100-Wh-class laminated LIB with energy density of 363 Wh L−1 was fabricated by employing the electrolyte to graphite-LiNixCoyMnzO2 chemistry, and it generated neither fire nor smoke in a nail-penetration test. The result suggest that the developed LIB has high safety compared to a LIB comprised of a conventional organic liquid electrolyte. In addition, to enhance the cycle life of the LIB, the formation and growth mechanism of a solid-electrolyte interphase on a graphite-based negative electrode was investigated. Nuclear magnetic resonance and hard x-ray photoelectron spectroscopy revealed that the decompositions of LiTFSA, PC, and G4 contributed to the SEI formation at the initial charge, and that continuous decompositions of G4 and PC were a major reason for the SEI growth during charge-discharge cycles. Based on these analysis, we have substituted a highly concentrated sulfolane based liquid which exhibits a high Li ion conductivity with less amount of the low viscos solvent, for the G4 based liquid. The modification effectively improved the electrochemical durability of the electrolyte, leading to a higher capacity retention after charge-discharge cycle test.

In Situ Impedance Spectra Analysis of Lithium-Sulfur Battery Using Sulfolane-Based Super-Concentrated Electrolyte Solution

Lithium-Sulfur (Li-S) battery has attracted attention as a new generation secondary battery. However, the Li-S battery has some problems toward its realization, one of which is the dissolution of discharge products of lithium polysulfides. Recently, Watanabe and Dokko et al. [1,2] reported that a solvate ionic liquid and a sulfolane-based super-concentrated electrolyte solution had low solubility of the polysulfides, and proposed polysulfides insoluble Li-S batteries using these electrolyte solutions. The discharge reaction paths of the Li-S batteries are currently unclear due to the strong dependence on the electrolyte solution In this contribution, in situ electrochemical impedance spectroscopy (EIS) was applied to the Li-S battery using the sulfolane-based super-concentrated electrolyte solution to obtain further insight into the reaction paths during the discharge. A positive electrode slurry was obtained by mixing a sulfur S8, a Ketjen black (KB) and a carboxymethyl cellulose (CMC2200) at a weight ratio of 60 : 30 : 10. The Slurry was spread on an Al foil to obtain a KB-S composite electrode. The typical loading of the sulfur was 2.0 mg cm-1. A negative electrode and a separator were used a lithium metal foil and a glass filter (GA-50, Advantec), respectively. The sulfolane (SL)-based super-concentrated electrolyte solution was prepared by dissolving lithium bis-(trifluoromethanesulfonyl)amide (LiTFSA) into the SL as the molar ratio of 1 : 2. 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE) was added to the SL-based electrolyte solution as a diluent and finally the molar ratio of LiTFSA : SL : HFE was prepared at 1 : 2 : 2. The Li-S battery was assembled using the CR 2032 coin-type cell in an Ar-filled glove box. The galvanostatic charge-discharge measurement was carried out at the C-rate of 1/12 C. The measurement was stopped at the SOC (state of charge) = 50 % in the second charge step. Then, after relaxing to an equilibrium state, the in situ EIS measurements were performed using an electrochemical system (HZ-7000) in the frequency range from 100 mHz to 100 kHz. The in situ EIS measurement was carried out while charging at the C-rate of 1/8 C for 30 minutes. Afterward, the discharge process was measured under the same condition after keeping open circuit potential for 5 minutes. The amplitude of the AC current was determined to make the AC potential response 10 mV. In the charge-discharge curves for the Li-S battery using the SL-based electrolyte solution, the first and the second plateaus appeared in the range of 16 to 90 mAhg-1 and 180 to 700 mAhg-1, respectively. The charging curve during the in situ EIS measurement was almost identical to the charging curves during the 1st and the 2nd cycles. On the other hand, the discharging curve during the in situ EIS measurement was quite different from one in the steady state of discharging. That is, it took time to achieve the steady state of the discharge. Instantaneous impedance spectra [3, 4] were obtained with compensating the time of the measured impedance spectra. Two capacitive semicircles were observed in the instantaneous impedance spectra. The semicircle appeared in the low frequency region was attributable to the charge transfer resistance and constant phase element related to double-layer capacitance. Curve fittings using an equivalent circuit were performed on the instantaneous impedance spectra. Although the charge transfer resistance was almost constant during the charge, it remarkably increased during the discharge. This suggests that resistances for an increase of Li ion concentration at the positive electrode/electrolyte interface and the Li ion insertion to the active material are quite large. These resistances probably cause to take much time to reach the steady state of discharge. References 1) K. Dokko et al., J. Electrochem. Soc., 160, A1304-A1310 (2013).2) A. Nakanishi et al., J. Phys. Chem. C, 123, 14229-14238 (2019).3) Z. B. Stoynov et al., J. Electroanal. Chem., 183, 133-144 (1985).4) M. Itagaki et al., J. Power Sources, 135, 255-261 (2004).

High Transference Number of Li Ion in Highly Concentrated Lithium Bis(trifluoromethanesulfonyl)Amide/Dinitrile Liquid Electrolytes for Lithium Sulfur Batteries

Liquid electrolytes with high concentration of Li salts are receiving increasing attention owing to various unusual physicochemical and electrochemical properties.1 Recently, our group reported that Li+ ion hopping conduction in highly concentrated solutions of lithium bis(fluorosulfonyl)amide (LiFSA) dissolved in dinitrile solvents, namely, succinonitrile (SN), glutaronitrile (GN), and adiponitrile (ADN), is involved in the ionic conduction.2 In the liquids with composition [LiFSA]/[dinitrile] > 1, the two cyano groups of the dinitrile coordinate to two different Li+ ions and form solvent-bridged structures of Li+−dinitrile−Li+. In addition to the solvent-bridged structures, ionic aggregates (Li+−FSA−−Li+) are formed in these liquids. We report here the phase behaviors, solvate structures and transport properties of dinitrile-based highly concentrated electrolytes composed of lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) with higher thermal and chemical stability. LiTFSA/SN and LiTFSA/ADN binary mixtures form stable solvates at the composition of LiTFSA-(Dinitrile)1.5 and keep solid state at room temperature. On the other hands, LiTFSA/GN mixtures become glass forming liquids and maintains liquid state in a wide range of composition. Raman spectra for the liquids of LiTFSA/GN revealed that the average Li+ ion solvation number in the range of 1/8 ≤ [LiTFSA]/[GN] ≤ 1/3 is 3.1. In the highly concentrated solutions with compositions of [LiTFSA]/[GN] ≥ 1/3, the coordination of TFSA− anion to Li+ ion becomes remarkable due to the deficiency of GN and contact ion pairs and ionic aggregates are formed. The self-diffusion coefficients of Li+, TFSA−, and GN measured with pulsed field gradient (PFG) NMR suggested that Li+ ion diffuses faster than the anion in the concentrated electrolytes. This suggests that Li+ ion exchanges the anions in the ionic aggregates and moves forward leaving the anions. In other words, the ion-exchange conduction mode emerges in the high concentration region. We estimated the Li+ transference number in an electrolyte of [LiTFSA]/[GN] = 1/1.5 using the Bruce-Vincent method3 and found that the electrolyte possesses a high transference number of ca. 0.7. Finally, we applied the [LiTFSA]/[GN] = 1/1.5 electrolyte to a lithium sulfur cell. Thanks to the high transference number, the cell showed a relatively high rate capability, regardless of its low ionic conductivity (0.21 mS cm−1) at room temperature. Acknowledgements This study was supported in part by JSPS KAKENHI (Grant Nos. 16H06368, 18H03926, and 19H05813) from the Japan Society for the Promotion of Science (JSPS) and the JST ALCA-SPRING Grant Number JPMJAL1301. References 1 Y. Yamada and A. Yamada, Review—Superconcentrated Electrolytes for Lithium Batteries, J. Electrochem. Soc., 2015, 162, A2406–A2423.2 Y. Ugata, M. L. Thomas, T. Mandai, K. Ueno, K. Dokko and M. Watanabe, Li-ion hopping conduction in highly concentrated lithium bis(fluorosulfonyl)amide/dinitrile liquid electrolytes, Phys. Chem. Chem. Phys., 2019, 21, 9759–9768.3 J. Evans, C. A. Vincent and P. G. Bruce, Electrochemical measurement of transference numbers in polymer electrolytes, Polymer, 1987, 28, 2324–2328.

Synthesis and Electrochemical Studies of Ionic Liquid Electrolytes for Lithium-Air Batteries

Lithium-air battery (LAB) is one candidate of energy sources beyond conventional lithium ion batteries for overcoming future energy need without worsening environmental issues. Presently, LABs have attracted interest due to its high energy density, while maintaining a long operational life. Although, significant progress has been made in developing electrochemical performance of these batteries, many technical challenges still remain unsolved. For example, the electrolytes used in rechargeable LABs have inferior conductivity, high viscosity, high volatility and flammability, decomposition on Li, and overpotential during charging [1]. To overcome these limitations, it is crucial to replace the conventional electrolyte with an ionic liquid (IL) because of its non-volatility, non-flammability, high ion density and chemical stability [2], [3]. However, there are a few issues that need to be solved when using ILs in LABs; they have high viscosity, low oxygen solubility, and low cyclability. Therefore, synthesis of an appropriate IL electrolyte is necessary due to improve the overall performance of LABs.In order to synthesis of ILs, bis(trifluoromethanesulfonyl)amide (TFSA) anion was used for its anionic similarity with Li salts. Five different types of cations such as ammonium, phosphonium, piperidinium, sulfonium, and pirolidinium were used. Subsequently, Lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) was completely dissolved at a concentration of 1.0 mol dm−3 in these ILs, and then stirred for 8 h at 60 ºC. The viscosities of prepared IL electrolytes were measured by viscometer. Then, Ketjen black (KB1600) and polyvinylidene difluoride (weight ratio = 9:1, respectively) were mixed in N-methylpyrrolidone solution. After stirring at RT for 5 min, the slurry was coated on the carbon paper followed by drying in a vacuum oven at 80 ºC for 12 h to obtain the cathode. The electrode area was set to 2.0 cm2 in which carbon loading was adjusted to 0.5 mg cm−2. Approximately 50 µl of IL electrolyte drop was deposited on the surface of as-prepared cathode (2.0 cm2 area), and subsequently anode was placed onto the center. The coin cells were assembled inside an Ar filled glove box in which moisture content was less than 1.00 ppm. To observe micro structures on the electrode surface, FE-SEM was used. The electrochemical performances were evaluated in coin cells using mainly cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) tests.Fig. 1 shows the result of viscosity of IL electrolytes. The viscosities of these IL electrolytes are different, and depend strongly on temperatures. The viscosity decreased with increasing temperature. It can be attributed to the reduced forces of cohesion among surfactant molecules. The viscosities of the IL electrolytes at specific temperature decrease in the order: piperidinium> pirolidinium> ammonium> phosophonium> sulfonium. The result of CV analysis has confirmed similar pattern of chemical reactions in which composition and subsequent decomposition of Li2O2 occurred. However, the current density obtained for the case of sulfonium IL electrolyte is higher than other electrolytes. GCD tests were performed to determine the cycle performance and capacity. The LAB which used low viscosity IL electrolyte, especially sulfonium IL electrolyte showed better cycling performance and capacity than that of high viscosity IL electrolytes. We investigated the temperature effects on sulfonium IL electrolyte to determine optimum performance characteristics. In this regard, the Li metal was immersed in the sulfonium IL electrolyte for 1 week at 25 ºC, 50 ºC, and 75 ºC. Subsequently, the SEM observation was performed to evaluate the morphologies of the Li metal surface. No significant change is observed in the Li metal surface after immersion when compared with the pristine lithium. This result indicates that the Li metal is highly stable in the sulfonium IL electrolyte. Then GCD measurements were performed in dry air at 25 ºC, 50 ºC, and 75 ºC. The discharge/charge capacities of LAB were found to increase with increase in temperature, and a lower overvoltage was obtained at high temperature. However, an initial discharge/charge cycle was retained up to 2nd cycles. To further examine the cycle performance, the measurement atmosphere has been changed to O2 instead of dry air. Fig. 2 shows the discharge/charge curves obtain at 75 ºC under O2 and dry air. As shown in figure, the initial cycle performance is retained up to 10th cycles, and a lower overvoltage is obtained in O2 as compared with dry air. These results indicate a notable improvement in cycle performance. In summary, the sulfonium IL electrolyte has exhibited very high performance in electrochemical tests, even worked at a high temperature under O2 atmosphere. Therefore, the proposed electrolyte design concept has great potential for not only LAB, but also next-generation batteries for various applications. Figure 1

Composite Electrolytes Based on Sulfide-Based Solid Electrolytes and Highly Concentrated Electrolytes

Lithium-ion batteries (LIBs) have been widely used as power sources for electronic devices because of their high energy densities. LIBs are typically composed of a transition metal oxide cathode, a graphite anode, and an organic electrolyte. Flammability, volatility and liquid leakage issues of conventional organic electrolytes lead to safety concerns. As an alternative to organic liquid electrolytes, inorganic solid electrolytes are considered to be promising since they impart nonflammablity, and single ion conducting property [1],[2]. Among inorganic solid electrolytes, sulfide-type solid electrolytes show high ionic conductivity and softer mechanical properties compared to oxide-type solid electrolytes [3]. However, there is still large contact loss at the interfaces unlike liquid electrolyte systems, which results in high interfacial resistance between the electrode and electrolyte. Additionally, it is well-known that sulfides react with trace moisture in the atmosphere, resulting in generation of toxic H2S gas. The contact between the electrode active material and the solid electrolyte interface can be improved by mixing a small amount of electrolyte solution or gel electrolyte with the solid electrolyte. Among the liquid electrolytes, highly concentrated electrolytes with a high concentration of lithium salt show properties such as non-flammability and non-volatility. In our previous works, the concentrated electrolyte using sulfolane as a solvent shows a high lithium ion transference number of ~0.7. Therefore, it can be expected that a good interface is formed without sacrificing the intrinsic high Li ion transport property of the solid electrolyte such as lithium ion transference number of unity when a small amount of the concentrated electrolyte is impregnated.In this study, we prepared a composite electrolyte by mixing an Li7P3S11 (LPS) and a concentrated liquid electrolyte [Li(SL)2][TFSA] which is a 1:2 molar mixture of lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and sulfolane (SL), with different weight ratios of [Li(SL)2][TFSA]. After compression molding of the obtained composite, the ionic conductivity was determined by AC impedance measurement using an electrochemical cell.Figure 1 shows the ionic conductivity of the composite electrolytes with different weight ratios of the concentrated electrolyte added. As the ratio of concentrated electrolyte increased, the ionic conductivity in the composite electrolyte decreased. For the eletrolytes with the weight ratio of [Li(SL)2][TFSA] less than 20% ( i.e., LPS-rich composite electrolytes), the conductivity value was ~ 1 mS cm-1, which is close to the ionic conductivity of LPS (1.4 mS cm-1). On the other hand, when the weight fraction of concentrated electrolyte increases up to 40%, the conductivity value approached 0.42 mS cm-1 which is the ionic conductivity of [Li(SL)2][TFSA]. This result shows that Li ion conduction between LPS particles is a dominant route for the LPS-rich composite electrolyte, whereas the liquid phase appears to be the main conduction pathway for [Li(SL)2][TFSA]-rich composite electrolytes. An the intermediate weight ratios of the concentrated electrolyte of ~30%, the ionic conductivity significantly decreased. These results suggest that both LPS and the concentrated electrolyte serve as ion conducting pathway depending on the weight ratios and the interfacial resistance between the solid electrolyte and the liquid electrolyte is not a fatal problem in these composite electrolytes. Reference [1] N. Riphaus, P. Strobl, B. Stiaszny, T. Zinkevich, M. Yavuz, J. Schenll, S. Indris, H. A. Gasteiger, S. J. Sedlmaier, J. Electrochem. Soc., 2018, 165, A3993-A3999.[2] Y. Wang, D. Lu, M. Bowden, P. Khoury, K. Han, Z. Deng, J. Zhang, and J. Liu, Chem. Mater. 2018, 30, 990−997.[3] T. Ates, M. Keller, J. Kulisch, T.Adermann, S. Passerini, Energy Storage Mater., 2019, 17, 204–210.[4] A. Nakanishi, K. Ueno, D. Watanabe, Y. Ugata, Y. Matsumae, J. Liu, M. L. Thomas, K. Dokko, M. Watanabe, J. Phys. Chem. C, 2019, 123, 14229-14238. Figure 1

Analysis of Solid-Electrolyte Interphase at the Interface between a Graphite Negative Electrode and a Diluted Solvate Ionic Liquid-Based Quasi-Solid-State Electrolyte

The formation and growth mechanism of a solid-electrolyte interphase (SEI) on a graphite-based negative electrode was investigated to enhance the cycle life of lithium ion batteries with a quasi-solid state electrolyte (QSE). In a QSE, liquid constituents including solvate ionic liquid (SIL), diluting solvent, and additives are quasi-solidified on surface of silica particles, which ensures the safety of a 100 Wh class laminated cell. For the SIL, an equimolar complex composed of tetraethylene glycol dimethyl ether (G4) and lithium bis(trifluoromethanesulfonyl)amide (LiTFSA), was utilized. Propylene carbonate (PC) was used as diluting solvent to enhance the ionic conductivity of the SIL. Vinylene carbonate (VC) additive was introduced to form a robust SEI for inhibiting the reductive decomposition of G4 and PC. Nuclear magnetic resonance and hard X-ray photoelectron spectroscopy revealed that the decompositions of LiTFSA, PC, and G4 contributed to the SEI formation at the initial charge. During charge-discharge cycles, continuous decompositions of G4 and PC were a major reason for the SEI growth. To suppress the decomposition, charging at a low rate was introduced at beginning of the initial charge to enhance the VC decomposition and the robust SEI formation. Consequently, the decomposition of the QSE was inhibited, which enhanced its cycle life.

Effects of fluoroethylene carbonate addition to Li-glyme solvate ionic liquids on their ionic transport properties and Si composite electrode performance

The design of battery electrolytes with superior transport, thermal, and electrochemical properties is a key consideration in the development of high-energy-density batteries. To this end, an equimolar mixture of lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and tetraglyme (G4) is of interest because it forms a thermally and electrochemically stable solvate ionic liquid (SIL), [Li(G4)][TFSA]. In this study, fluoroethylene carbonate (FEC) was added to [Li(G4)][TFSA] in an attempt to improve transport properties of the SIL and to modify the chemical composition of the solid electrolyte interphase (SEI) formed on Si electrodes. The addition of FEC had a negligible impact on the Li ion coordination structure in [Li(G4)][TFSA] while enhancing its ionic conductivity nearly twofold. The Si electrode in [Li(G4)][TFSA] with the FEC additive showed a stable and reversible capacity in long-term cycling with high Coulombic efficiency. X-ray photoelectron spectroscopy revealed that the chemical composition of the SEI formed in [Li(G4)][TFSA] with the FEC additive was significantly different from that in the electrolyte without FEC. The FEC-addition-derived SEI most likely consists of carbonates and LiF, retarding electrolyte decomposition and, thus, improving the electrochemical performance of the Si electrode therein.

High Transference Number of Li Ion in Highly Concentrated Lithium Bis(trifluoromethanesulfonyl)Amide/Dinitrile Liquid Electrolytes

Liquid electrolytes with high concentration of Li salts are receiving increasing attention owing to various unusual physicochemical and electrochemical properties.1 Recently, our group reported that Li+ ion hopping conduction in highly concentrated solutions of lithium bis(fluorosulfonyl)amide (LiFSA) dissolved in dinitrile solvents, namely, succinonitrile (SN), glutaronitrile (GN), and adiponitrile (ADN), is involved in the ionic conduction.2 In the liquids with composition [LiFSA]/[dinitrile] > 1, the two cyano groups of the dinitrile coordinate to two different Li+ ions and form solvent-bridged structures of Li+−dinitrile−Li+. In addition to the solvent-bridged structures, ionic aggregates (Li+−FSA−−Li+) are formed in these liquids. We report here the phase behaviors, solvate structures and transport properties of dinitrile-based highly concentrated electrolytes composed of lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) with higher thermal and chemical stability. LiTFSA/SN and LiTFSA/ADN binary mixtures form stable solvates at the composition of LiTFSA-(Dinitrile)1.5 and keep solid state at room temperature. On the other hands, LiTFSA/GN mixtures become glass forming liquids and maintains liquid state in a wide range of composition. Raman spectra for the liquids of LiTFSA/GN revealed that the average Li+ ion solvation number in the range of 1/8 ≤ [LiTFSA]/[GN] ≤ 1/3 is 3.1. In the highly concentrated solutions with compositions of [LiTFSA]/[GN] ≥ 1/3, the coordination of TFSA− anion to Li+ ion becomes remarkable due to the deficiency of GN and contact ion pairs and ionic aggregates are formed. The self-diffusion coefficients of Li+, TFSA−, and GN measured with pulsed field gradient (PFG) NMR suggested that Li+ ion diffuses faster than the anion in the concentrated electrolytes. This suggests that Li+ ion exchanges the anions in the ionic aggregates and moves forward leaving the anions. In other words, the ion-exchange conduction mode emerges in the high concentration region. We estimated the Li+ transference number in an electrolyte of [LiTFSA]/[GN] = 1/1.5 using the Bruce-Vincent method3 and found that the electrolyte possesses a high transference number of ca. 0.7. Finally, we applied the [LiTFSA]/[GN] = 1/1.5 electrolyte to a lithium sulfur cell. Thanks to the high transference number, the cell showed a relatively high rate capability, regardless of its low ionic conductivity (0.21 mS cm−1) at room temperature. References 1 Y. Yamada and A. Yamada, Review—Superconcentrated Electrolytes for Lithium Batteries, J. Electrochem. Soc., 2015, 162, A2406–A2423.2 Y. Ugata, M. L. Thomas, T. Mandai, K. Ueno, K. Dokko and M. Watanabe, Li-ion hopping conduction in highly concentrated lithium bis(fluorosulfonyl)amide/dinitrile liquid electrolytes, Phys. Chem. Chem. Phys., 2019, 21, 9759–9768.3 J. Evans, C. A. Vincent and P. G. Bruce, Electrochemical measurement of transference numbers in polymer electrolytes, Polymer, 1987, 28, 2324–2328. Acknowledgements This study was supported in part by JSPS KAKENHI (Grant Nos. 16H06368, 18H03926, and 19H05813) from the Japan Society for the Promotion of Science (JSPS) and the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST).

Highly concentrated LiN(SO2CF3)2/dinitrile electrolytes: Liquid structures, transport properties, and electrochemistry.

Liquid structures, transport properties, and electrochemical properties of binary mixtures of lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and dinitrile solvents [succinonitrile (SN), glutaronitrile (GN), and adiponitrile (ADN)] were investigated. In the LiTFSA/SN and LiTFSA/ADN systems, the stable crystalline solvates of LiTFSA-(SN)1.5 [melting point (Tm): 59 °C] and LiTFSA-(ADN)1.5 (Tm: 50 °C) were formed, respectively. In contrast, the LiTFSA/GN mixtures of a wide range of compositions were found to be glass-forming liquids at room temperature. Raman spectroscopy of LiTFSA/GN liquid mixtures revealed that increasing the LiTFSA concentration results in the formation of the solvent-bridged network structure Li+-GN-Li+. In addition, the considerable formation of contact ion pairs and ionic aggregates was observed in highly concentrated electrolytes. In the liquids, the Li+ ion dynamically exchanged ligands (GN and TFSA) and higher LiTFSA concentrations led to an increase in the ratio of the self-diffusion coefficients of Li+ and TFSA-, DLi/DTFSA, as determined by pulsed field gradient NMR spectroscopy. The Li+ transference number (tLi+ ) of the [LiTFSA]/[GN] = 1/1.5 electrolyte in an electrochemical cell under anion-blocking conditions was estimated to be as high as 0.74. Furthermore, electrochemical measurements revealed that the reductive stability of the LiTFSA/GN electrolyte increases with increasing LiTFSA concentration. A [LiTFSA]/[GN] = 1/1.5 electrolyte is stable against the Li metal electrode, provided that the polarization is relatively small. Owing to high tLi+ , a Li-S battery with the [LiTFSA]/[GN] = 1/1.5 electrolyte showed a high rate discharge capability despite its low ionic conductivity (0.21 mS cm-1) at room temperature.

Solvate Cation Migration and Ion Correlations in Solvate Ionic Liquids.

Lithium salt-glyme mixtures are interesting candidates as electrolytes for battery applications. Depending on the type of glyme or anion and the salt concentration, they either show ionic liquid-like behavior with stable lithium-glyme complex cations or concentrated salt solution-like behavior. Here, we apply electrophoretic NMR (eNMR) to elucidate transport mechanisms by observing the migration of the molecular species in an electric field. We investigate two solvate ionic liquids, i.e., lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and lithium tetrafluoroborate (LiBF4), in tetraglyme (G4) at different glyme-salt molar ratios X. A field-induced migration of neutral glyme molecules is directly observed, which is due to stable solvate-Li complex formation. Transference numbers, effective charges, and ionicities are derived from electrophoretic mobilities and self-diffusion coefficients, respectively, for the nuclei 1H, 7Li, and 19F. The effective charges are the highest at the equimolar mixture, X = 1, they differ strongly for lithium and anion, and they show large differences between both systems. These findings are qualitatively interpreted in a speciation model, suggesting anionic clusters and solvate cations as the species dominating charge transport. The resulting effective charges can only be explained taking into account ion-ion anticorrelations in the framework of the Onsager formalism, where anticorrelations between the solvate cation and the anionic complexes arise due to momentum conservation. The contributions to the anticorrelation are most dominant at high salt concentrations and in the system with the LiBF4- anion due to its lower mass and ability to form larger asymmetric clusters with Li+. Thus, in either system, also the lithium transference number is influenced to a different extent by ion-ion anticorrelations.