Lithium Bis Fluorosulfonyl Amide Research Articles

Entropy-Driven 60mol% Li Electrolyte for Li Metal-Free Batteries.

Highly Li-concentrated electrolytes are acknowledged for their compatibility with Li metal negative electrodes and high voltage positive electrodes to achieve high-energy Li metal batteries, showcasing stable and facile interfaces for Li deposition/dissolution and high anodic stability. This study aims to explore a highly concentrated electrolyte by adopting entropy-driven chemistry for Li metal-free (so-called anode-free) batteries. The combination of lithium bis(fluorosulfonyl)amide (LiFSA) and lithium trifluoromethanesulfonate (LiOTf)salts in a pyrrolidinium-based ionic liquid is found to significantly modify the coordination structure, resulting in an unprecedented 60mol% Li concentration and a low solvent-to-salt ratio of 0.67:1 in the electrolyte system. This novel 60mol% Li electrolyte demonstrates unique coordination stricture, featuring a high ratio of monodentate-anion structures and aggregates, which facilitates an enhanced Li+ transference number and improved anodic stability. Moreover, the developed electrolyte provides a facile de-coordination process and leads to the formation of an anion-based solid electrolyte interface, which enables stable Li deposition/dissolution properties and demonstrates excellent cycling stability in the Li metal-free full cell with a Li[Ni0.8Co0.1Mn0.1]O2 (NCM811) positive electrode.

Dimensionality Control of Li Transport by MOFs Based Quasi‐Solid to Solid Electrolyte (Q‐SSEs) for Li−Metal Batteries

AbstractNowadays, lithium‐ion batteries (LIBs) are widely used in all walks of life and play a very important role. As complex systems composed of multiple materials with diverse chemical compositions, where different electrochemical reactions take place, battery interfaces are essential for determining the operation, performance, durability and safety of the battery. This work, set out to study the incorporation of lithium bis(fluorosulfonyl)amide (LiFSI) doped 1‐ethyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIm][TFSI]) ionic liquid into an archetype Ti‐based Metal Organic Framework (MOF) ((Ti) MIL125−NH2) to create a solid to quasi‐solid (depending on the amount of IL in the system), and how it affects not only ionic transport but also the structural properties of the IL/MOF electrolyte. Remarkably high ionic conductivity values (2.13×10−3 S ⋅ cm−1 at room temperature) as well as a lithium transference number (tLi=0.58) were achieved, supported by pulsed field gradient (PFG) NMR experiments. Electrochemical characterization revealed reversible plating‐stripping of lithium and lower overpotential after 750 h at 50 °C. Additionally, a proof‐of‐concept solid state battery was fabricated resulting in a discharge capacity of 160 mAh ⋅ g−1 at 50 °C and 0.1 C rate after 50 cycles. This work presents a suitable strategy to dendrite suppression capability, allowing its implementation as interface modifiers in next‐generation solid‐state batteries.

Boosting the Ionic Conductivity of Pyrrolidinium-Based Ionic Plastic Crystals by LLZO Fillers.

Organic ionic plastic crystals (OIPCs) have attracted attention as novel organic solid electrolyte materials, but their insufficient mechanical strength and ionic conductivity have prevented their application. In this study, a lithium salt, lithium bis(fluorosulfonyl)amide (LiFSA), and an inorganic solid electrolyte, Li7La3Zr2O12 (LLZO), were added to an OIPC, N,N-diethylpyrrolidinium bis(fluorosulfonyl)amide ([C2epyr][FSA]). The fabricated organic-inorganic hybrid solid electrolytes were evaluated thermally, mechanically, and electrochemically to reveal which factors affect the properties of the electrolytes. All samples showed excellent thermal stability regardless of LiFSA or LLZO concentration, and they were found to be highly plastic and ion-conductive solids at a wide range of temperatures. It was also revealed that the addition of LLZO raised the nanoindentation stiffness (HIT) of the [C2epyr][FSA]/LiFSA composites. The ionic conductivity of the hybrid electrolytes was higher than that of the pristine OIPC, reaching a value of 2.1 × 10-4 S cm-1 at 25 °C upon addition of appropriate amounts of LiFSA and LLZO. Overall, samples with higher LiFSA concentration and moderate LLZO concentration exhibited higher ionic conductivity. Cyclic voltammetry results showed that the [C2epyr][FSA]/LiFSA/LLZO composites were lithium-ion conductors. These findings indicate that by optimizing the concentrations of lithium salt and LLZO, it would be possible to realize their applications as solid electrolytes.

Elucidation of Mass Transport Phenomena in Highly Concentrated Electrolytes during Current Cycling Using In-Situ Interferometry and Finite Difference Method

Understanding electrolyte mass transfer during charge–discharge reactions is essential for developing next-generation storage batteries with high energy densities. In this study, we investigated Li+ transport in a highly concentrated electrolyte (HCE) consisting of an equimolar mixture of lithium bis(fluorosulfonyl)amide (LiFSA) and tetraglyme (G4) under current reversal and re-reversal. Concentration profiles of the electrolyte at a distance of 0–600 μm from the Li electrodes were obtained using in situ laser interferometry. The Li+ transference numbers and LiFSA diffusion coefficients were calculated from these profiles. Raman spectroscopy suggested that the coordination structure surrounding Li+ ions in the electrolytes mainly contributed to the transference number. A one-dimensional unsteady diffusion equation and the finite difference method were employed to simulate the concentration profiles. The maximum error percentage between the measured and simulated values was only 3%, confirming the accuracy and validity of the interferometric measurements. Our findings on Li-ion transfer in HCEs could promote the rational design of high-energy-density Li-ion batteries with higher cation transference numbers of electrolytes and charge–discharge rates.

New Liquid Electrolytes Consisting of Li Salts and Urea Derivatives for Li-Ion Batteries

Deep eutectic solvents are mixtures that exhibit melting points much lower than those of the endmembers due to the molecular interactions. Deep eutectic solvents attracted attention because high melting point compounds are available as flame-resistance solvents.1 However, the stable operation of Li-ion batteries (LIBs) with deep eutectic electrolytes (DEEs) is still challenging probably due to their high protonic nature and consequent narrow potential window.2 To develop Li-ion DEEs with a wide potential window, we investigated DEEs based on lithium bis(fluorosulfonyl)amide (LiFSA) and a series of urea derivatives to clarify the correlation between compound structure and electrolyte properties. Furthermore, we examined the electrode performances of the Li4Ti5O12 (LTO) negative electrode and LiNi1/3Co1/3Mn1/3O2 (NCM) positive electrode in the DEEs.To prepare DEEs, urea derivatives, such as urea, 1-methylurea, 1,3-dimethylurea, 1,1-dimethylurea, and tetramethyl urea are mixed with LiFSA at molar ratios of 1:2 or 1:4 at room temperature. The oxidation and reduction stability was evaluated by linear sweep voltammetry (LSV) in three-electrode cells with Pt foil as the working electrode. Galvanostatic charge-discharge tests were performed in three-electrode cells with LTO or NCM working electrodes and activated carbon counter electrodes.The prepared LiFSA:urea derivative (1:4) mixtures (LiFSA:urea, LiFSA:1,3-dimethylurea, and LiFSA:tetramethylurea) became transparent liquids at room temperature. Moreover, LiFSA:1,3-dimethylurea and LiFSA:tetramethylurea were clear liquid at even in 1:2 mixtures. Using the five DEEs, we evaluated their electrolyte properties. Figure 1 shows the LSV curves of the DEEs. Among the five deep eutectic liquids, LiFSA:1,3-dimethylurea (1:2) exhibited the widest potential window of 4.49 V, showing an even wider potential window than electrolytes with non-protonic tetramethyl urea.Figure 2a displays the charge-discharge curves of LTO electrodes in the DEEs. The LiFSA:urea (1:4) electrolyte cell exhibits a large irreversible capacity, and its initial reversible capacity is 228 mAh g-1, significantly larger than the theoretical capacity. This indicates that side reactions possibly takes place in addition to the LTO charge-discharge reaction. On the other hand, the LiFSA:1,3-dimethylurea (1:2) electrolyte demonstrates stable and reversible charge-discharge for 50 cycles, with an initial discharge capacity of 179 mAh g-1, which is close to the theoretical capacity. The LiFSA:tetramethylurea cell showed lower cycle stability than the LiFSA:1,3-dimethylurea cell, possibly due to electrolyte precipitation during cycling because LiFSA:tetramethylurea (1:2) electrolyte is an almost saturated solution.Using the LiFSA:1,3-dimethylurea (1:2) DEE, we further evaluated the electrochemical performance of the NCM electrode as shown in Figure 2b. The NCM electrode showed reversible charge-discharge behavior over 10 cycles with moderate capacity of 75.7 mAh g-1. These results demonstrate the potential applicability of the DEE to 3 V-class LIBs. In the presentation, we will also discuss the optimization of LiFSA concentration to improve battery properties and the observation of the formation of surface films on the electrodes in DEEs.

In-Situ Observation of the Formation and Relaxation Processes of Concentration Gradients in a Lithium Bis(fluorosulfonyl) Amide–Tetraglyme Solvate Ionic Liquid Using Digital Holographic Interference Microscopy

Introduction Next-generation Lithium-ion batteries (LIBs) require fast charge–discharge operations, during which a steep concentration gradient (CG) is formed between the two electrodes. The formation of a concentration profile between the electrodes in an LIB with a solvate ionic liquid (SIL) is observed at a distance of less than 1 mm using holographic interferometry. This in situ technique enables the visualization of concentration profile formation near both electrodes during electrolysis, which relaxes after the electrolysis is stopped. The diffusion coefficients near both the electrodes are calculated from the concentrations of transient species near the electrode surfaces. The diffusion coefficient is smaller on the anode side of the cell than that on the cathode side owing to the viscosity of the electrolyte in the diffusion layer. This viscosity effect may have caused the concentration profile to become asymmetrical during the relaxation of the CG. Experimental Li metal was used for the anode and cathode. The distance between the electrodes was 600 μm. The electrolyte was a SIL mixture of LiFSA (lithium bisfluorosulfonylamide) and G4 (tetraglyme) at a molar ratio of 1:1. After 100 seconds of Li electrodeposition and dissolution at a constant current density (3 mA cm-2), the current was stopped, and the relaxation process of the concentration gradient generated by electrolysis was observed with a laser interference microscope (Lyncee Tec) for 1700 seconds. The Li+ concentration near the electrode was calculated from the optical phase difference based on the bulk electrolyte outside the diffusion layer. The electrode surface concentration and transference number of Li+ were used to obtain the diffusion coefficients of Li+ near both electrodes. The time variation of the diffusion layer thickness during and after electrolysis was measured to study the mass transfer phenomenon during the formation-relaxation process of the concentration gradient.

Demystifying the Salt-Induced Li Loss: A Universal Procedure for the Electrolyte Design of Lithium-Metal Batteries

HighlightsThe loss mechanisms of irreversible Li in electrolytes with various salts (e.g., lithium hexafluorophosphate (LiPF6), lithium difluoro(oxalato)borate (LiDFOB), and lithium bis(fluorosulfonyl)amide (LiFSI)) are systemically revealed.A universal procedure for the electrolyte design of Li metal batteries is proposed: (i) decouple and find the main reason for the irreversible Li loss; (ii) add the corresponding electrolyte additive.

Promise of dual carbon batteries with graphene-like graphite as both electrodes

Graphene-like graphite (GLG) exhibits a higher capacity for intercalation/deintercalation of lithium ions and anions compared to graphite, making it a promising candidate for both electrodes for dual carbon batteries, also known as dual-ion batteries (DIBs). In this study, we constructed DIB full cells with GLG electrodes to address specific issues related to the full cell while optimizing the cell configuration and charge-discharge conditions to improve its performance. With 3 mol dm−3 LiPF6/ethyl methyl carbonate (EMC) electrolyte solution, the reversible capacity was very small, suggesting that this was due to a cross-talk reaction where decomposition products generated at the cathode migrated to the anode and were reductively decomposed. Conversely, the use of 3 mol dm−3 lithium bis(fluorosulfonyl)amide (LiFSA)/EMC suppressed the cross-talk reaction, resulting in improved charge-discharge performance. In addition, the pre-cycling of the anode to form solid electrolyte interphase significantly suppressed the cross-talk reaction to greatly increase the reversible capacity. Consequently, we achieved a relatively high capacity of 66 mAh g−1 for the GLG cathode, even with a low cutoff voltage at 4.6 V, surpassing the capacity of a dual graphite battery under the same conditions. This outcome clearly demonstrates the promise of GLG as active materials for DIBs.

Elucidation of Liquid Structures and Transport Properties of Highly Concentrated LiN(SO2F)2/Ethylene Carbonate Electrolytes

The Li+ conduction mechanism in highly concentrated ethylene carbonate (EC) solution of lithium bis(fluorosulfonyl)amide (LiFSA) was studied based on the measurements of thermal properties, density, viscosity, ionic conductivity, self-diffusion coefficients, Raman spectra, and proton nuclear magnetic resonance relaxation time. The compositions of EC and LiFSA were varied from dilute to concentrated states ([EC]/[LiFSA] = 9.2–0.80). For compositions of [EC]/[LiFSA] ≤ 1.5, the free solvents decrease, and contact ion pairs and aggregates become dominant, causing a significant change in the transport properties. Faster diffusion of Li+ compared with solvent (Li+ hopping conduction) was observed when the molar composition of [EC]/[LiFSA] ≤ 1.0 as in the case of sulfolane.

All-Solid-State High-Voltage Supercapacitors Using an Ionic Plastic Crystal-Based Electrolyte

Recently, ionic plastic crystals (IPCs) have been actively investigated to develop all-solid-state rechargeable batteries, such as lithium-ion batteries. Herein, we report supercapacitors assembled with mesoporous carbon electrodes and an IPC electrolyte, N-ethyl-N-methylpyrrolidinium bis(fluorosulfonyl)amide ([C2mpyr][FSA]). [C2mpyr][FSA] with a 10 mol% lithium bis(fluorosulfonyl)amide (LiFSA) dopant was used as the solid electrolyte in the supercapacitors. The charge–discharge tests of the supercapacitors were performed at various C-rates in the voltage range of 0–2.5 V at 25°C. The capacitance of the cells was 12.3 Fg−1 at a lower C rate (1 C = 8.9 mA g−1). The capacitance retention of the supercapacitors was maintained at approximately 100% up to 20 C, which was comparable to that of the cells containing organic electrolyte solutions. The advantage of using solid electrolytes was the fabrication of bipolar cells using two pairs of mesoporous carbon electrodes and a [C2mpyr][FSA]/LiFSA composite. The charge–discharge tests of the bipolar cells were also performed in the voltage range of 0–5.0 V at 25°C. The capacitance of the bipolar cells was 6.4 Fg−1 at a lower C rate. The bipolar cells exhibited a typical charge–discharge profile for 1,000 cycles, confirming their stable cyclic performance. Thus, IPC electrolytes are interesting materials for developing all-solid-state high-voltage supercapacitors.

Electrochemical Properties of Poly(vinylidene fluoride-<i>co</i>-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.

Anodic Stabilities of Various Metals as the Current Collector in High Concentration Electrolytes for Lithium Batteries

Lithium bis(fluorosulfonyl)amide (LiFSI)-based electrolytes not only deliver improved ionic conductivity, but also provide high thermal stability and water tolerance. However, the notorious corrosion of sulfonamide anions against aluminum (Al) current collector restricts their application. High concentration electrolytes (HCEs) have been proposed as alternative electrolytes for lithium batteries owning to their merits of good electrochemical performance and less flammability. Although HCEs have been proved stable to many alkaline cathodes and anodes in the past reports, their compatibility with other battery components, such as the current collector, has not been clearly reported. Herein, electrochemical stabilities of various metals in LiFSI/dimethyl carbonate (DMC) electrolytes were characterized by linear sweep voltammetry and cyclic voltammogram. According to results, when the concentration of the LiFSI/DMC electrolyte is higher than 1:3 (salt/solvent) by molar for Al, and 1:1.2 for titanium (Ti) and stainless steel (SS304), the three materials can be used as the cathode current collector or the other battery components. Moreover, Al that exhibits the best electrochemical stability is still the most ideal current collector compared to other metals. Notably, in the LiFSI:DMC (1:1.5) electrolyte with an Al current collector, stable battery performance with a remaining capacity of 98.8% after 50 cycles can be obtained.

Observation of lithium stripping in super-concentrated electrolyte at potentials lower than regular Li stripping.

Lithium plating/stripping was investigated under constant current mode using a copper powder electrode in a super-concentrated electrolyte of lithium bis(fluorosulfonyl)amide (LiFSA) with methylphenylamino-di(trifluoroethyl) phosphate (PNMePh) and vinylene carbonate (VC) as additives. Typical Li plating/stripping for Cu electrodes in organic electrolytes of conventional lithium batteries proceeds at potentials of several millivolts versus a Li counter electrode. In contrast, a large overpotential of hundreds of millivolts was observed for Li plating/stripping with the super-concentrated electrolyte. When Li stripping started immediately after Li plating and with no rest time between plating and stripping, two potential plateaus, i.e., two-step Li stripping, was observed. The potential plateau for the 1st stripping step appeared at −0.2 V versus a Li metal counter electrode. The electrical capacity for the 1st stripping step was 0.04 mA h cm−2, which indicates irregular Li stripping. Two-step Li stripping was also recorded using cyclic voltammetry. The electrochemical impedance spectroscopy (EIS) studies indicated that the two-step Li stripping behaviour reflected two different solid electrolyte interphases (SEIs) on electrodeposited Li in a Cu electrode. The SEI for the 1st-step stripping was in a transition period of the SEI formation. The open circuit voltage (OCV) relaxation with an order of tens of hours was detected after Li plating and before Li stripping. The in operando EIS study suggested a decrease of the charge transfer resistance in the Cu powder electrode during the OCV relaxation. Since the capacitance for the voltage relaxation was a dozen microfarads, it had a slight contribution to the 1st-step Li stripping behaviour. The voltage relaxation indicated the possibility that it is difficult for Li ions to be electrodeposited or that the Li plating is in a quasi-stable state.

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.

Li-Ion Transport in Three-Layer Electrolyte of Ionic Liquid/Solid-State Electrolyte (SSE)/Ionic Liquid

In conventional Li-ion batteries, organic liquid electrolytes are widely used due to their high ionic conductivity and wettability to the porous composite electrodes. On the other hand, future high-energy battery may require utilizing inorganic solid-state electrolyte (SSE) to obtain superior safety and higher Li+ transport property (as Li+ transference number of inorganic solid electrolyte is theoretically unity). [1] However, since the oxide-based inorganic solid electrolyte is hard and brittle, it is difficult to achieve the intimate contact between the electrode and electrolyte. The poor contact causes a high interfacial resistance, resulting in the slow electrochemical reaction rate in a solid-state battery. To address this issue, the use of an organic electrolyte as an interlayer between an electrode and an SSE is a promising approach. [2] In this case, in addition to the interface between the electrode and the organic electrolyte, the interfacial phenomena at the organic electrolyte and SSE should be considered. In the present study, we prepared a three-layer electrolyte of organic electrolyte/SSE/organic electrolyte, and the Li+ ion transport property of the three-layer electrolyte was elucidated. We used an ionic liquid (IL) as the organic electrolyte, and the IL was prepared by mixing 1-ethyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)amide (P12TFSA) and lithium bis(fluorosulfonyl)amide (LiFSA) in a molar ratio of 1:1. A NASICON-type Li-ion conducting glass ceramic of Li1+x+yAlx(Ti, Ge)2-xSiyP3-yO12 (LATP) was purchased from OHARA and used as the SSE layer. The three-layer electrolyte was assembled by sandwiching a LATP glass ceramic plate with cellulose separators (20 μm thick, TBL46-20, NKK) impregnated with the IL. Li metal electrodes (2 cm2) were attached to both sides of the three-layer electrolyte to fabricate a symmetric cell of Li / three-layer electrolyte / Li. Electrochemical impedance spectroscopy (EIS) was performed for the symmetric cell, and the interfacial phenomena were analyzed. Nyquist plots of the symmetric cell are shown in Figure 1(a). The total interfacial resistance in the cell was estimated to be 123 ohm. This resistance is originated from the Li/IL interface (R Li/IL) and the IL/LATP interface (R IL/LATP). To evaluate the interfacial resistance of R Li/IL, EIS measurement was performed for another symmetric cell of Li / IL / Li, and the R Li/IL was estimated to be 34 ohm. Therefore, R IL/LATP in the Li / three-layer electrolyte / Li was calculated to be 123-34 = 90 ohm. We also evaluated the apparent Li+ transference number (t Li+) of the three-layer electrolyte using a DC polarization method. [3] Figure 1(b) shows the chronoamperogram of the Li / three-layer electrolyte / Li. From the steady-state current and R Li/IL, the apparent t Li+ was estimated to be 0.52. Although t Li+ of LATP should be close to 1, t Li+ of IL is as low as 0.24. In addition, Li+ ion should pass through the interface of IL/LATP. Therefore, apparent t Li+ of the three-layer electrolyte is affected by t Li+ of IL, the thickness of the IL layer, and the R IL/LATP. We will also report the battery performance of the three-layer electrolyte.AcknowledgementThis study was supported in part by the JSPS KAKENHI (Grant Nos. 16H06368, 18H03926, and 19H05813) from the Japan Society for the Promotion of Science (JSPS).

Electrochemical Performance of Nickel Silicide Electrodes for Lithium-Ion Batteries in an Ionic-Liquid Electrolyte

Developing lithium-ion batteries (LIBs) with a high energy density, long cycle, and high safety is essential for establishing a sustainable society because LIBs are used as electric vehicle batteries and stationary batteries for utilizing renewables. Silicon (Si) has great potential as an anode active material for next-generation LIBs due to its high theoretical capacity (3580 mA h g- 1 for Li15Si4). However, Si electrodes show poor cycle stability, which is mainly caused by a large volume change in Si during the lithiation (charge) and delithiation (discharge) reactions. Additionally, Si has other disadvantages, such as high electrical resistivity and a low Li+ diffusion coefficient. We have previously investigated the lithiation and delithiation properties of pure binary silicide (M ySiz, M : transition metal) electrodes. Some electrodes maintained a high reversible capacity in an ionic-liquid electrolyte, whereas they showed a poor cycling performance in a conventional organic-liquid electrolyte. In contrast, the effect of difference in crystal phase of silicides composed of the same elements on the lithiation and delithiation properties has not yet been clarified. Herein, we synthesized various pure nickel silicide (Ni-Si) powders. and investigated their lithiation and delithiation properties in an ionic-liquid electrolyte. Ni-Si powders (NiSi2, NiSi, Ni2Si, and Ni3Si) were synthesized by a mechanical alloying (MA) method. The obtained powders were characterized by X-ray diffraction (XRD) and Raman spectroscopy. XRD patterns were identified compared with patterns in the Inorganic Crystal Structure Database (ICSD). Each silicide electrode was prepared by a gas deposition (GD) method, which does not require a binder or conductive agent. We assembled 2032-type coin cells, which consisted of the silicide electrode as the working electrode, Li metal foil as the counter electrode and a glass fiber filter as the separator. 1 mol dm- 3 (M) lithium bis(fluorosulfonyl)amide (LiFSA) in N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)amide (Py13-FSA) was used as an ionic-liquid electrolyte. A galvanostatic charge-discharge test was carried out in the potential range between 0.005 and 2.000 V vs. Li+/Li at 303 K. The current density was set at 50 mA g- 1. To discuss the charge density of each element in Li x Ni y Si z , a first-principle calculation based on density functional theory (DFT) was performed using the projector augmented wave (PAW) method as implemented in the plane wave code of the Vienna Ab initio Simulation Package (VASP). A generalized gradient approximation (GGA) was used as the term exchange correlation with a kinetic cutoff of 350 eV. Brillouin zone sampling was performed with an 8×8×8 k point mesh within a Gamma point centered mesh scheme. We synthesized various Ni-Si powders by a MA method. Comparing the XRD pattern of the prepared samples and the corresponding ICSD pattern, all peaks were assigned to pure Ni-Si phase. Additionally, there were no peaks arising from the raw materials (Ni and Si). In Raman spectra, peaks assigned to crystalline Si (c-Si) and amorphous Si (a-Si) appear at approximately 520 and 490 cm- 1, respectively. Each silicide also gives no Raman peaks of c-Si and/or a-Si which indicates that neither c-Si nor a-Si is included in the synthesized powders. Therefore, the MA treatment successfully produced a pure Ni-Si phase. Fig.1 shows the dependence of the gravimetric discharge capacity of various nickel silicide electrodes on the cycle number in 1 M LiFSA/Py13-FSA.The NiSi2 electrode exhibited the highest initial capacity of approximately 800 mA h g- 1, and the Ni3Si electrode showed the second highest capacity. The results demonstrated that there was no correlation between initial capacity and silicon content in Ni-Si. To reveal the difference in the initial capacity, we determined the charge density of each element in Li x Ni y Si z based on computational chemistry. Li and Si have positive charges in all silicides, whereas Ni has a negative charge in Li x Ni y Si z . These results indicate that Li has a high affinity with Ni in Li x Ni y Si z . In addition, it is suggested that the higher the affinity of Ni with Li is, the higher the initial capacity is. Furthermore, we investigated the distance between Li and close atoms (Ni or Si) in Li x Ni y Si z . The initial capacity increased with an increase in the distance. Therefore, both the affinity of Ni with Li and the distance between Li and neighboring atom should influence on the initial capacity. Figure 1

Competition between Conversion Reaction with Cerium Dioxide and Lithium Plating in Superconcentrated Electrolyte.

Li-ion insertion into cerium dioxide (CeO2) and its subsequent conversion reaction were studied using a CeO2/copper composite electrode in a superconcentrated electrolyte of lithium bis(fluorosulfonyl)amide (LiFSA) and methylphenylamino-di(trifluoroethyl) phosphate (PNMePh) under conditions promoting Li plating/stripping. Since the conversion reaction potential with CeO2 generally lies above the Li plating/stripping level, the conversion ideally occurs first in the cathodic scan. However, the conversion reaction was delayed until after the Li plating in the superconcentrated electrolyte contrary to expectations, whereas this phenomenon was unobserved in a dilute LiFSA/PNMePh electrolyte. Energy-dispersive X-ray spectroscopy and electrochemical impedance analysis indicated that the reversed order of the electrochemical behaviors was caused by the solid electrolyte interphase (SEI) on the CeO2, which had a different material composition and a higher interfacial resistance than the SEI on electrodeposited metallic lithium.

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).

Microscopic Origin of the Solid Electrolyte Interphase Formation in Fire-Extinguishing Electrolyte: Formation of Pure Inorganic Layer in High Salt Concentration.

A highly salt-concentrated (HC) electrolyte based on the nonflammable solvent trimethyl-phosphate (TMP) has recently shown an attractive self-extinguishing property in addition to an excellent charge-discharge performance. However, the microscopic understanding of its solid electrolyte interphase (SEI) layer remains an open question. In this Letter, the red moon (RM) method was used to investigate the molecular mechanism of SEI layer formation depending on lithium bis(fluorosulfonyl)amide (LiFSA) salt concentration in a TMP-based electrolyte and was able to reproduce successfully the experimental observations, i.e., the "bottom-up" formation mechanism with a thinner and denser SEI layer mainly based on salt reduction in the HC electrolyte. The results showed that a pure dense inorganic layer is formed in the HC electrolyte, which should considerably improve the SEI layer stability leading to a longer lifetime in charge-discharge performance. This new microscopic finding should provide an important guide in designing an effective nonflammable electrolyte to develop advanced, safe secondary batteries.

Solvate Structures and Transport Properties in Highly Concentrated Li[FSA]/Succinonitrile Liquid Electrolytes

Highly concentrated Li salt electrolyte solutions have attracted attention recently due to the unique physicochemical and electrochemical properties.1 Recently, our group reported that Li+ ion hopping conduction is involved in the ionic conduction in highly concentrated Li salt/sulfolane electrolytes.2 Similar Li+ transport was also observed for the highly concentrated lithium bis(fluorosulfonyl)amide (LiFSA)/keto ester electrolytes.3 We found that the solvent molecules, having two coordinating sites, bridge different Li+ ions, resulting in the formation of the solvent-bridged network structures of Li+−solvent−Li+ in these highly concentrated electrolytes. Therefore, highly concentrated electrolytes with certain solvents having multiple coordinating sites are anticipated to exhibit Li+ ion hopping conduction. We report here the phase behaviors, solvate structures and transport properties of highly concentrated electrolytes composed of LiFSA and succinonitrile (SN) solvent, which has two cyano groups in the molecular structure. LiFSA and SN form a stable complex at 1:2 molar ratio, and the melting point of the solvate of LiFSA-(SN)2 is 63.4 °C. X-ray crystallography for the solvate of LiFSA-(SN)2 revealed that the SN molecule bridges two different Li+ ions in the crystal. At a molar ratio higher than [LiFSA]/[SN] = 1/1, the mixture becomes a glass forming liquid, and the [LiFSA]/[SN] = 1/0.8 mixture maintains a liquid state at room temperature. Raman spectra for [LiFSA]/[SN] = 1/0.8 suggested that solvent-bridged structure of Li+−SN−Li+ is still maintained in the liquid. In addition to the solvent-bridged structure, anion bridged structure of Li+−FSA−−Li+ is also formed in the solvent-deficient mixture of [LiFSA]/[SN] = 1/0.8. The self-diffusion coefficients of Li+, FSA−, and SN measured with pulsed field gradient (PFG) NMR suggested that Li+ ion diffuses faster than anion and SN (Table 1), and the Li+ ion hopping conduction emerges in these liquid electrolytes having the polymeric network structures. The [LiFSA]/[SN] = 1/0.8 electrolyte possesses a wide electrochemical window, and the graphite anode and LiNi1/3Mn1/3Co1/3O2 cathode of Li-ion batteries can be charged and discharged reversibly in this electrolyte. References 1 Y. Yamada and A. Yamada, Review—Superconcentrated Electrolytes for Lithium Batteries, J. Electrochem. Soc., 2015, 162, A2406–A2423. 2 K. Dokko, D. Watanabe, Y. Ugata, M. L. Thomas, S. Tsuzuki, W. Shinoda, K. Hashimoto, K. Ueno, Y. Umebayashi and M. Watanabe, Direct Evidence for Li Ion Hopping Conduction in Highly Concentrated Sulfolane-Based Liquid Electrolytes, J. Phys. Chem. B, 2018, 122, 10736–10745. 3 S. Kondou, M. L. Thomas, T. Mandai, K. Ueno, K. Dokko and M. Watanabe, Ionic transport in highly concentrated lithium bis(fluorosulfonyl)amide electrolytes with keto ester solvents: Structural implications for ion hopping conduction in liquid electrolytes, Phys. Chem. Chem. Phys., 2019, 21, 5097–5105. Table caption Table 1. Viscosity (η), density (ρ), concentration of LiFSA (c), ionic conductivity (σ), and self-diffusion coefficients of electrolytes of LiFSA/SN at 30 °C. Figure 1