Lithium Bis Trifluoromethanesulfonyl Amide Research Articles

Mixture of monoglyme-based solvent and lithium Bis(trifluoromethanesulfonyl)amide as electrolyte for lithium ion battery using silicon electrode

Abstract The electrochemical property of a silicon electrode in a half-cell was investigated using the mixture of monoglyme-based solvents and lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) with 2:1 stoichiometry. The solvents used in this study are monoglyme (G1), 1,2-diethoxyethane (DEE), 1,2-dibutoxyethane (DBE), and 1,2-dimethoxypropane (P-G1). When Li (DEE)2 or Li(P-G1)2 was used, the cell retained ca. 20% capacity at a rate of 3 C, while the other samples showed almost no capacity under the same conditions. Raman spectroscopy measurement revealed that Li+ exists as two forms in the mixture, that is, a solvent shared ion pair (SSIP) [ Li+ ··· solvent ··· TFSA– ] and a contact ion pair (CIP) [ Li+ ··· TFSA– ]. According to the spectroscopic analysis, SSIP is the dominant species for Li (DEE)2 and Li(P-G1)2, and CIP for Li (G1)2 and Li (DBE)2. The order of the rate property was Li(P-G1)2 ∼ Li (DEE)2 > Li (DBE)2 ∼ Li (G1)2, which is almost consistent with the stability of free energy levels of Li+ in the respective monoglymes against CIP Li+ ··· TFSA– evaluated from the Raman spectra.

Solid polymer electrolytes based on polystyrene‐polyether block copolymers having branched ether structure

To obtain solid polymer electrolytes (SPEs) having high ionic conductivity together with mechanical integrity, we have synthesized polystyrene (PSt)‐polyether (PE) diblock copolymers via one‐pot anionic polymerization. The PSt block is expected to aggregate to act as hard fillers in the SPE to enhance the mechanical property. The PE block consists of random copolymer (P(EO‐r‐MEEGE)) of ethylene oxide (EO) and 2‐(2‐methoxyethoxy) ethyl glycidyl ether (MEEGE) in different molar ratios ([EO]/[MEEGE] = 100/0, 86/14, 75/25, 68/32, and 41/59). The introduction of the MEEGE moiety in PEO reduced the crystallinity of PEO, and the fast motion of the MEEGE side chain caused plasticization of the PE block, thereby contributing to the fast ion transport. SPEs were fabricated by mixing the obtained diblock copolymer (PSEx) and lithium bis(trifluoromethanesulfonyl) amide (LiTFSA) with [Li]/[O] = 0.05. Ionic conductivity of the obtained SPEs was dependent on the molar ratio of EO in the PE block (x) as well as the weight fraction of PE block (fPE) in the block copolymer. PSE0.86 (fPE = 0.65) exhibited high ionic conductivity (3.3 × 10−5 S cm−1 at 30°C; 1.1 × 10−4 S cm−1 at 60°C) comparable with that of P(EO‐r‐MEEGE) (PE0.85; fPE = 1.00) (9.8 × 10−5 S cm−1 at 30°C; 4.0 × 10−4 S cm−1 at 60°C).

Anion Coordination Characteristics of Ion-pair Complexes in Highly Concentrated Aqueous Lithium Bis(trifluoromethanesulfonyl)amide Electrolytes.

We report on the structures of Li-ion complexes in salt-concentrated aqueous electrolytes based on lithium bis(trifluoromethanesulfonyl)amide (LiTFSA), particularly focusing on the anion coordination behavior of the ion-pair complexes in the high concentration region cLi > 3.0 mol dm-3. Quantitative data analysis of the Raman spectra revealed the following. (1) Li ions do not coordinate with TFSA anions at lower cLi (<3.0 mol dm-3) to exist as ion pair-free ions. (2) In the concentrated region (cLi = 3.0 - 4.0 mol dm-3), the TFSA anions coordinate as monodentate ligands (mono-TFSA) with Li ions to form ion-pair complexes and coexist with free TFSA in the bulk. (3) Further increasing the cLi (4.0 - 5.2 mol dm-3) results in both monodentate and bidentate coordination (bi-TFSA) modes of TFSA anions to Li ions, yielding complicated ion-pair complexes in the first coordination sphere. The Walden plots, based on ionic conductivity and viscosity data, implied that the ion-conducting mechanism in the highly salt-concentrated region was considerably different from that in the dilute region (i.e., vehicle mechanism).

Lithium-Ion Intercalation by Calcium-Ion Addition in Propylene Carbonate-Trimethyl Phosphate Electrolyte Solution

Electrochemical intercalation of lithium ions was investigated in propylene carbonate (PC)-trimethyl phosphate (TMP) electrolyte solution. Intercalation of lithium ions took place in 1 mol dm−3 lithium bis(trifluoromethanesulfonyl)amide (LiTFSA)/PC:TMP by adding a certain amount of calcium ions. When the molar ratio of PC:TMP:Li+ was 6:6:1, lithium-ion intercalation took place when the molar ratio of Ca2+/Li+ was 1.2. The criteria was Ca2+/Li+ = 0.8 when the molar ratio of the electrolyte was PC:TMP:Li+ = 2:10:1. It was suggested from Raman spectra that the solvation structure of lithium ions were altered from the four coordinated tetrahedral configuration in the case that intercalation reaction occurred. This change in the lithium-ion solvation structure was caused by adding calcium ions, which has stronger Lewis acidity than lithium ions. Different criteria of Ca2+/Li+ molar ratio for lithium-ion intercalation by the difference of PC:TMP:Li+ molar ratio shows that the lithium-ion solvation structure was also affected by the electrolyte composition.

Ionic Liquid-Based Electrolytes Containing Surface-Functionalized Inorganic Nanofibers for Quasisolid Lithium Batteries

In the present study, surface amino-functionalized silica nanofibers (f-SiO2NFs, average diameter = 400 and 1000 nm) are used as one-dimensional (1-D) fillers of ionic liquid (IL)-based quasisolid electrolytes. On adding f-SiO2NFs to an IL (1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide, EMITFSA) containing lithium bis(trifluoromethanesulfonyl)-amide (LiTFSA), the well-dispersed 1-D nanofillers easily form a three-dimensional network structure in the IL, function as physical cross-linkers, and increase the viscosity of the composites, consequently providing a quasisolid state at a 3.5 wt % fraction of the NFs. Rheological measurements demonstrate that the prepared composites exhibit “gel-like” characteristics at 40–150 °C. All prepared composites show high ionic conductivities, on the order of 10–3 S cm–1, around room temperature. To investigate the additive effect of f-SiO2NFs in the composites, the lithium transference numbers are also evaluated. It is found that thinner NFs enhance the transference numbers of the composites. In addition, quasisolid lithium-ion cells containing the prepared composites demonstrate relatively high rate characteristics and good cycling performance at high temperature (125 °C).

In Situ AFM Observation of Surface Morphology of Highly Oriented Pyrolytic Graphite in Propylene Carbonate-Based Electrolyte Solutions Containing Lithium and Bivalent Cations

Propylene carbonate (PC) is one of the promising solvents of lithium-ion batteries for use in cold area. Lithium salt concentrated PC and both lithium salt and bivalent salt dissolved PC enabled lithium ion to intercalate at graphite negative electrodes. The key to use PC is a solid electrolyte interphase (SEI). In this study, the SEI formation process on a highly oriented pyrolytic graphite was investigated by in-situ atomic force microscopy (AFM) in 4 mol dm−3 lithium bis(trifluoromethanesulfonyl)amide (LiTFSA)/PC and 1 mol dm−3 LiTFSA + 1.2 mol dm−3 M(TFSA)2 (M = Ca or Mg)/PC. Based on the in-situ AFM observation, the SEI formation processes in 4 mol dm−3 LiTFSA/PC and 1 mol dm−3 LiTFSA + 1.2 mol dm−3 Ca(TFSA)2/PC were the co-intercalation type and that in 1 mol dm−3 LiTFSA + 1.2 mol dm−3 Mg(TFSA)2/PC was the surface decomposition type. Relation between the SEI formation process and the charge–discharge properties was also discussed.

Elucidation of Reaction Behavior of Phosphorus-Doped Si Negative Electrode for Lithium-Ion Batteries

Silicon (Si) is a promising active material as a negative electrode in next-generation lithium-ion batteries (LIBs) due to its high theoretical capacity of 3580 mA h g− 1 (Li15Si4). It has been reported that crystalline Si (c-Si) forms an amorphous Li-Si alloy phase (a-Li x Si) in the first charge process and that the resulting a-Li x Si subsequently alloys with Li to form a c-Li15Si4 phase.1 The volumetric change ratio per Si atom from c-Si to c-Li15Si4 corresponds to 380%, which generates high stresses and large strains in the active materials. The strains that accumulate under repeated charge−discharge cycling cause disintegration of the Si negative electrode, leading to a rapid decrease in capacity. Si also has disadvantages of a low electrical conductivity and a low diffusion coefficient of Li+ within it. To address these issues, doping of Si with impurities has been attempted to increase the electrical conductivity of Si. However, the reaction behavior of the doped Si has not yet been clarified. In this study, the relationship between the electrochemical performance of phosphorus (P)-doped Si negative electrode and the reaction behavior was investigated by X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FE-SEM), and so on. P-doped Si (P concentration; 0, 50, 124, 633, and 1000 ppm) powders were supplied by Elkem (Sigrain e-Si). Electrochemical measurements were carried out with a laboratory-made three-electrode cell. The working electrode was fabricated by gas-deposition method, which is without any binder or conductive agent. Both the counter and reference electrodes consisted of Li metal. The electrolyte solution used was 1 M lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) in propylene carbonate (PC). XRD and Raman spectroscopy were used to investigate the phase transition and change in the surface crystallinity of P-doped Si electrodes before and after a charge-discharge cycle. The surface morphology and cross-sectional surface of the P-doped Si electrodes were observed by FE-SEM. A focused ion-beam was used to fabricate the cross-sectional surface. Li insertion energy (E insertion) into Si was also calculated based on computational chemistry. The electric resistivity and the first charge-discharge capacity of a Si electrode decreased under P-doping; an increase in electrical conductivity does not always correspond to an increase in the initial charge-discharge capacity. In addition, a very low P concentration of 124 ppm has a significant influence on the electrochemical properties of a Si negative electrode; P-doping of Si suppressed the rapid fading of the discharge capacity. After the charge-discharge cycle, the undoped Si electrode was full of cracks and the active material of Si was deteriorated. On the other hand, the surface morphology of P-doped Si was almost the same as that before cycle, even though a slight crack was observed. Therefore, P-doping suppressed the change in the surface morphology of a Si negative electrode after charge-discharge cycles. It is well-known that c-Si has a triply degenerated Raman active F2g-mode at around 520 cm-1, and that this peak shifts toward a lower wavenumber at around 490 cm-1. The peak assigned to F2g-mode of P-doped Si electrode was shifted to lower wavenumber, and the degree of the peak shift increased with a decrease in the P concentration; P-doping suppressed the change from c-Si to amorphous Si (a-Si). While the peaks assigned to c-Si phase disappeared in XRD patterns of 0 and 50 ppm P-doped Si electrodes after the first lithiation at 0.005 V vs. Li+/Li in LiTFSA/PC, these peaks remained in XRD patterns of 124, 633, and 1000 ppm P-doped Si electrodes. In addition, the intensity of peaks which attribute to c-Li15Si4 phase decreased with an increase in the P concentration. Hence, P-doping controlled the formation of a c-Li15Si4 phase during charge process. This suppression of the phase transition led to a decrease in the large change in volume of Si and prevented the Si negative electrode from disintegrating. E insertion increased with an increase in the P concentration; Li insertion into P-doped Si is energetically unfavorable, which indicates that the crystal lattice of Si shrinks as a result of the replacement of some Si atoms with smaller P atoms. Hence, it is more difficult to insert into P-doped Si. These effects of P-doping help to improve the otherwise poor cycle performance of a Si electrode.2 References1) J. R. Dahn, et al., J. Alloy Compd., 2010, 496, 25-36.2) Y. Domi, H. Usui, H. Sakaguchi, et al., ACS Appl. Mater. Interfaces, 2016, 8, 7125-7132. AcknowledgmentsThis work was partially supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant numbers 24350094 and 15K21166.

Sulfurized Polyethylene Glycol As Electrode Material for Li-S Battery

The battery of safe, low cost, durable and high capacity is strongly demanded for vehicles, mobile phones, personal computers, power backup systems and so on. Lithium-sulfur batteries gather attentions because sulfur as a cathode material has a high capacity of 1672 mAhg-1, is abandon, cheap and benign to environment. The sulfur cathode, however, has disadvantages such as high resistance and dissolution into electrolyte in the form of Li2Sx, which degrade the charge/discharge efficiency and capacity [1]. In order to mitigate the dissolution of sulfur species, several kinds of nano carbon materials such as graphene [2], carbon nanotube [3], and nano carbon architectures [4] have been investigated as the container for sulfur. Polymer and sulfur composites also have been examined. Among them, a material prepared through heat treatment of polyacrylonitrile and sulfur mixture showed good capacity and cyclability [5-7]. We found that heat-treated mixture of cheap polyethylene glycol and sulfur, sulfurized polyethylene glycol (SPEG) also showed good capacity and relatively good cycle stability as Li-S battery cathode. SPEG was synthesized by heat treatment 25.0 g of polyethylene glycol (PEG) (Kishida Chemical) and 51.6 g sulfur (Kanto Chemical) in an alumina tube (Nikkato), which was set in an electric furnace. The mixture was heated-up to 723 K under N2 flow then cooled. The reaction of SPEG production proceeded above 523 K with refluxing both of sulfur and PEG releasing H2S. This kind of production reaction for sulfur containing material is quite unique because not only sulfur but also PEG exist both liquid and vapor. The obtained crude SPEG was pulverized and heat-treated again at 573 K under N2 flow for more than 2h in order to take rid of residual molecular sulfur. Black powder was obtained by 8.9 g. The production of SPEG was easily scaled-up to 112 g in an experimental batch. SPEG was characterized using elemental analysis, XRD, and TG-DTA. For electrochemical measurement, cathode was prepared by mixing 50 wt% SPEG, 40 wt% acetylene black (AB), and 10 wt% polytetrafluoroethylene (PTFE), attached to aluminum mesh. The cathode was assembled into coin-type cells (CR2032), with the use of polypropylene sheet as separator, lithium foil as counter electrode, and lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) : tetraglyme (1:1 molar ratio) as electrolyte. Galvanostatic charge-discharge cycle tests were performed at a current density of 25 mA g-1 (C/20 rate) in the potential range of 1.0 – 3.0 V vs. Li/Li+. From elemental analysis of SPEG, it consist of C: S: H: O = 35.3: 61.4: 0.4: 2.9 wt%. The content ratio of sulfur in SPEG is relatively higher than that of the other sulfur containing cathode materials. The XRD profiles of SPEG showed only a halo profile typical for such as low-crystalline or amorphous material without reflection of crystalline sulfur. TG-DTA result showed that there was only 17% weight-loss of SPEG until 773 K. This showed SPEG was a heat-resistive material and strongly held sulfur in it. The electrochemical properties of SPEG were investigated in a half cell. The capacity was calculated based on the mass of SPEG not sulfur. SPEG cathode showed large first discharge capacity of 910 mAh/g, and first charge one 550 mAh/g, therefore this material had irreversible capacity of 360 mAh/g. The cathode showed second discharge capacity 488 mAh/g, and kept 433 mAh/g at 10th cycle. The average discharge voltage of the cathode was 1.8 V vs. Li/Li+. Because relatively large capacity cathodes like one of Li-S battery changes its volume during charge/discharge, cathode structure is apt to be destroyed. A suitable binder would improve SPEG cathode cyclability. As a conclusion, the cheap law materials of PEG and sulfur were changed through new method to novel cathode material SPEG with good productivity up to 112 g. SPEG has high content ratio of sulfur with heat-resistive up to 773 K, and has large capacity with good cycle stability.

Bacterial-Cellulose-Derived Carbon As Air Electrode Material for Lithium Air Secondary Batteries

Introduction Lithium air secondary batteries exhibit higher theoretical energy density than lithium ion batteries and are expected as next-generation secondary batteries. However, there remain technical challenges related to their poor cycle properties.Since the first report by K. M. Abraham et al. [1], various oxygen reduction/evolution catalyst [1-3] and electrolyte [1, 4] materials have been intensively investigated for air batteries to improve their electrochemical properties, such as their cyclability. However, there have only been a few studies on support materials for air electrodes, such as nanoporous gold [5].We consider that one of the reasons for the poor cyclability is a decrease in the electrical contact of the air electrode during Li2O2 deposition (discharge) and oxygen generation (charge), which results from large the volume change. The purpose of our work is to improve the cyclability by using a new air electrode structure and material. As an elastic support material for air electrodes, we focus on carbon fabricated from bacterial cellulose (BC), which has a continuous interconnected network of carbon. Such an elastic carbon support would prevent the large volume change of the air electrodes and improve the cycle performance. Here we report the preparation conditions for BC-derived carbon supports materials and the performance of lithium air secondary batteries incorporating them. Experimental A BC sheet (Fujicco Co., Ltd.) was frozen at -40 °C and then freeze-dried [6]. The freeze-dried BC sheet was carbonized in N2 atmosphere at 600, 900, and 1200 °C to obtain BC-derived carbon. It was cut into a circle (diameter: 5 mm) to obtain the binder-free air electrode. The lithium air secondary battery consisted of the air electrode, a lithium metal sheet, and 1.0 mol/l lithium bis(trifluoromethanesulfonyl)amide (LiTFSA)/tetraethylen glycol dimethyl ether (TEGDME) as the positive electrode, negative electrode, and electrolyte solution, respectively. The battery was assembled with an ECC-Air Cell (EL-CELL GmbH). Electrochemical measurements were carried out under a galvanostatic condition of 0.1 mA/cm2 in an O2 atmosphere. The discharge and charge capacities were normalized by the weight of the BC-derived carbon (the air electrode). Results and discussion Figure 1 shows an SEM image of the BC-derived carbon prepared at 1200 °C. The continuous interconnected network of the carbon fibers with diameter of less than 10 nm can be seen in this image. These carbon fibers build a 3-D disordered macroporous framework. The BC-derived carbon prepared at 600 and 900 °C was confirmed to have morphologies similar to that at 1200 °C. Moreover, in a preliminary compression test, the BC-derived carbon showed a highly elastic compressibility and almost completely recovered its original volume after the compression was released.Figure 2 shows the first discharge/charge curves of the air batteries incorporating the BC-derived carbon prepared at 600, 900, and 1200 °C. The air batteries with the carbon prepared at 900 °C showed the largest discharge capacities of 8800 mAh/g and the average discharge voltage of 2.68 V. This is because it has the largest number of active sites among the carbons tested. The batteries incorporating BC-derived carbon prepared at 900 and 1200 °C showed almost the same charge capacities of about 8000 mAh/g. These charge properties have a tendency similar to the discharge properties even though the air electrodes have no catalysts. These results indicate that the BC-derived carbon can be used as support material for binder-free air electrodes. Figure 3 shows the correlation between the discharge capacities, D/G ratio, and carbonization temperature. In the Raman spectra of the carbon as shown in the inset, the peaks of D- and G-bands are derived from the edge-plane and basal-plane in the crystal structure of carbon, respectively. The D/G ratio was obtained from the results of wave analyses with the spectra in the inset. It correlates roughly with the discharge capacities as shown in Fig. 3. In particular, the battery incorporating the carbon prepared at 900 °C, which has the largest discharge capacity, has the highest D/G ratio. This indicates the importance of the number of carbon edge planes in relation to the discharge property, because the D/G ratio corresponds to a ratio of number of carbon edge planes to that of basal planes. These results indicate that the carbon edge planes would play an important role as active sites in the reaction mechanism with the air electrode.

Structural and Electrochemical Properties of Li Ion Solvation Complexes in the Salt-Concentrated Electrolytes Using an Aprotic Donor Solvent, N,N-Dimethylformamide

We report the relation between the structural and electrochemical properties of N,N-dimethylformamide (DMF)-based electrolytes containing lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) in the concentration range cLi = 0–3.2 mol dm–3. Raman spectroscopy and DFT calculations indicate that Li+ ions are solvated by DMF molecules in the form of [Li(DMF)4]+ complexes at low cLi ( 2.5 mol dm–3. The high cLi solutions, in which all the DMF molecules solvate to Li+ ions (i.e., no DMF remains in the bulk), make the electrochemical window wider; the oxidative stability is enhanced owing to lower HOMO energy le...

Li+ Local Structure in Li–Tetraglyme Solvate Ionic Liquid Revealed by Neutron Total Scattering Experiments with the 6/7Li Isotopic Substitution Technique

Equimolar mixtures of lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and tetraglyme (G4: CH3O-(CH2CH2O)4-CH3) yield the solvate (or chelate) ionic liquid [Li(G4)][TFSA], which is a homogeneous transparent solution at room temperature. Solvate ionic liquids (SILs) are currently attracting increasing research interest, especially as new electrolytes for Li-sulfur batteries. Here, we performed neutron total scattering experiments with (6/7)Li isotopic substitution to reveal the Li(+) solvation/local structure in [Li(G4)][TFSA] SILs. The experimental interference function and radial distribution function around Li(+) agree well with predictions from ab initio calculations and MD simulations. The model solvation/local structure was optimized with nonlinear least-squares analysis to yield structural parameters. The refined Li(+) solvation/local structure in the [Li(G4)][TFSA] SIL shows that lithium cations are not coordinated to all five oxygen atoms of the G4 molecule (deficient five-coordination) but only to four of them (actual four-coordination). The solvate cation is thus considerably distorted, which can be ascribed to the limited phase space of the ethylene oxide chain and competition for coordination sites from the TFSA anion.

Analysis of Surface Deposit Formed on the Binder-Free Si Nanoparticle Electrode in Room-Temperature Ionic Liquid Containing Lithium Bis(trifluoromethanesulfonyl)Amide

1. Introduction Si is expected to be a promising alternative negative electrode material for lithium secondary batteries. Despite that, it suffers from poor cyclability due to the cracks caused by the large volume change during a charge-discharge reaction. On the other hand, room-temperature ionic liquids (RTILs) have attracted much attentions as an electrolyte for lithium-ion secondary batteries due to their non-flammability and negligible volatility. However, the electrochemical behavior of the Si electrode in many types of RTILs is not enough in spite of many researchers' vigorous efforts. As one reason, the SEI film on the Si electrode in RTILs may be less effective than that on the graphite electrode in the conventional electrolyte. To solve the problem, the analysis of the surface deposit formed on the Si electrode during a charge-discharge reaction is desired. To analyze it in more detail, we focus on an electrophoretic deposition (EPD) method because the binder-free (BF) electrode not containing conductive additives and a binder can be prepared. In this study, we prepared the BF nano-Si film electrode by using the EPD method and then investigated the electrochemical behavior in the RTILs. By employing the BF nano-Si film electrode not containing carbon, the surface deposit on the Si electrode was analyzed. 2. Experimental An EPD bath was prepared as follows: a concentration of nano-Si (average particle size: 50 nm) in acetone bath of 1.0 g L-1. Nano-Si was deposited on a Cu foil as a current collector substrate. The electrolyte was prepared by dissolving lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) in N, N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium bis(trifluoromethanesulfonyl)amide (DEME-TFSA). The R2032 coin type cell, which consisted of the BF nano-Si film electrode (W.E.), a pressed lithium metal foil (C.E.), a 1 mol dm−3 solution of LiTFSA/DEME-TFSA (electrolyte), and celgard#2500 (separator), was used for the electrochemical measurements such as cyclic voltammetry (CV) and the charge-discharge cycle tests. For comparison, a 1 mol dm−3 solution of LiPF6/EC + DEC (50:50 vol. %) was used as a conventional electrolyte. The surface analysis of the BF nano-Si film electrode was carried out by energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). 3. Results and Discussion The CVs of the BF nano-Si film electrodes in the above RTILs (DEME type) and above organic solvent electrolyte (organic solvent type) were measured. The reduction wave at the 1st cycle was observed in the cell voltage from 0.25 V to 5 mV regardless of the electrolyte type. Similarly, the oxidation wave at the 1st cycle was observed in the potential range from 0.20 V to 0.85 V in both electrolyte types. The reduction and oxidation currents would be due to the insertion and extraction reactions of Li+ ion into and from Si. Figure 1 shows the initial charge-discharge curves of the BF nano-Si film electrodes (ca. 0.23 mg cm-2) (a) in the DEME type and (b) in the organic solvent type at 25 oC. The 1st discharge capacity and the 1st charge-discharge efficiency of the BF nano-Si film electrode in the DEME type are 1507 mAh g-1 and 67.5%, respectively. In contrast, the 1st discharge capacity and the 1st charge-discharge efficiency in the organic solvent type are 1968 mAh g-1 and 73.2%, respectively. After the 2 cycle, the charge-discharge efficiency of the BF nano-Si film electrode increased and achieved to 98% regardless of the electrolyte type. These results suggest that the SEI film would be mostly formed on the BF nano-Si film electrode during the 1st charge. About the DEME type, the surface of the BF nano-Si film electrode was analyzed by the EDX mappings after charging down to 5 mV and after discharging up to 2.0 V, respectively. The EDX mapping results showed that C, O, F, and S were detected on the surface of the BF nano-Si film electrode. The surface deposit was uniformly formed on the nano-Si particle. The high resolution XPS spectra of F 1s region were measured. The peaks were assigned to the fluorine of a Li-F bond derived from LiF, suggesting that the layer of LiF was present on the surface. It was clarified that the LiF-based compound as the surface deposit was formed on the BF nano-Si film electrode. Based on these results, it was found that the compound containing LiF, C, O, and S as a surface deposit was formed on the BF nano-Si film electrode due to the cathodic decomposition of a DEME+ cation as well as a TFSA- anion as a side reaction after the 1st charging. Figure 1

First-Principles Molecular Dynamics Simulation on Glyme-Based Electrolyte

Introduction: Rechargeable Li-air battery has attracted much attention as a candidate of the potential next-generation energy-storage devices, because of its significantly high theoretical capacity, which potentially extend the one-charge cursing range of electric vehicles to the comparable level with internal combustion engine. There are, however, several technical issues in realizing the practical application of rechargeable Li-air battery, such as, the large over-voltage in the charge process, irreversible unwanted side reactions, the low cycleability due to electrolyte deterioration, and the dendrite growth of Li metal on the anode surface during charge/discharge cycles. A choice of appropriate electrolytes and/or Li-salts and the development of them is a promising approach to solve above issues. Recently, the combination of tetraethylene glycol dimethyl ether (TEGDME) solvate and lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) salt has been demonstrated to exhibit excellent improvement: the over-voltage in charge process was significantly lowered, the irreversible Li2CO3production reaction at cathode-electrolyte interface was suppressed. Thus, understanding details of this system, such as, solution structures, mechanism of ion conduction in bulk electrolytes, and electrochemical reactions on electrode-electrolyte interfaces, is important to gain information on designing better electrolyte solutions. In this study, we therefore investigated molecular-level structure and mechanism of ion conduction in the electrolyte, in TEGDME + LiTFSA solution by using a large-scale first-principles molecular dynamics (FPMD) calculation. Methodology: Calculation model: Equimolar (2.75 M) TEGDME + LiTFSA electrolyte model was constructed with 27 TEGDME and 27 LiTFSA molecules (in total 1,484 atoms) in a cell of 25.67 × 25.67 × 25.67 Å3 size (see Figure), which is corresponding to experimentally obtained density, 1.40 g/ml. FPMD calculation: First-principles calculation code OpenMX[1] was applied to our simulation. DFT calculation was implemented with O(N) (linear-scaling) method based on Krylov-subspace method.[2] O(N)-DFT calculation was performed using a generalized gradient approximation (GGA-PBE) and norm-conserving type pseudopotentials. Cutoff energy was set at 230 Ry. Double-numerical + single polarization type localized basis sets were adopted (cutoff radii (Bohr): Li: 10.0; C, H, O, N. F: 7.0; S: 9.0). The MD calculation was carried out in an NVT ensemble at 500 K. Velocities of atoms were scaled every 20 MD steps to keep the temperature constant with the velocity-scaling method. The time step width was 1.2 fs with substitution of the deuterium mass for hydrogen atoms. Results and Discussion: First, we show the optimized coordination structures of Li+ in 2.75 M TEGDME + LiTFSA electrolyte. The obtained ratio of Li+[TEGDME][TFSA−], Li+[TEGDME] and Li+[TFSA−] complexes were 72, 4 and 24 %, relatively. This indicates that Li+ has a strong tendency of interacting with both TEGDME and TFSA−, and forming the stable Li+[TEGDME][TFSA−] complex. This result is consistent with experimental analysis that in 2.75 M TEGDME + LiTFSA electrolyte, the self-diffusion coefficients of Li+, TEGDME and TFSA− were almost identical (1.23, 1.33 and 1.16 × 10− 7 cm2/s, respectively). Then, we investigated the structural details of TEGDME molecules in Li+[TEGDME][TFSA−] and Li+[TEGDME] complexes. We define two different angles consisting of oxygen atoms in TEGDME molecule, O2-O1-O2 and O3-O1-O3 (see inset of Figure) as a structure index of TEGDME molecule. The average values of angles O2-O1-O2 and O3-O1-O3 were 118.1 and 102.7 degree, respectively. The radial distribution function analyses on Li-O2 and Li-O3 has shown that the coordination numbers of Li+ for O2 and O3 were 1.89 and 1.77, respectively, which are similar to each other and close to 2. These results indicate that in 2.75 M TEGDME + LiTFSA electrolyte, TEGDE molecules constructing complexes with Li+ have strongly-curved structures wrapping around Li+. Such unique solvation structure could be a key to exhibit excellent battery properties. Results of ion conduction mechanism analysis will also be presented and discussed combined with our other experimental results. Acknowledgement: This work was supported by the Strategic Programs for Innovative Research (SPIRE), MEXT, and the Computational Materials Science Initiative (CMSI), Japan. The computations in this work were performed using the computer “K” at the Advanced Institute for Computational Science, RIKEN.

Development of Organic-Inorganic Hybrid Electrolytes Based on Ionic Liquid-Nanoparticles-Composite

[Introduction] The development of high-performance all-solid-state Li-ion batteries are strongly required for high safety and reliability. The properties of solid-state electrolytes are crucial to the performance of all-solid-state Li-ion batteries. However, the conventional solid-state electrolytes with sufficient ion conductivity and high stabilities during battery operation are limited to few cases. Here, we propose a new idea to fabricate new type of solid-state hybrid electrolytes using mixture of room temperature ionic liquids (RTILs)-Li-salts and oxide nanoparticles. Presently, RTILs-Li salts mixture are considered as a potential candidate electrolyte material because of its promising properties, such as high ion conductivity, high stability and wide potential window. Recently, some researches revealed that RTILs-Li-salts are quasi-solidified incorporating with oxide particles attributed to the strong interaction between the oxide particles surfaces and RTILs-Li-salts. These composite materials maintain the liquid-like properties of RTILs-Li-salts. Our group have reported about a favorable soft sheet hybrid electrolyte based on RTILs-Li-salt and 7 nm-SiO2 nanoparticles. The liquid-like properties of hybrid electrolytes was proved to be beneficial. However, the insufficient power density of hybrid electrolytes became the problem. To solve this problem, we seek to sufficiently increase the power density of hybrid electrolytes incorporating with different nanoparticles. Therefore, the outstanding point of this study is that various commercial oxide nanoparticles were utilized to fabricate quasi-solid-state hybrid electrolytes with RTILs-Li-salt.[Experimental] The quasi-solid-state powders were prepared by mixing nanoparticles (CeO2: 15-30 nm, ZrO2: 30-60 nm and gamma-Al2O3: 5 nm) with RTIL-Li-salt mixture. Nanoparticles were preheated to 80℃ for 24h to rule out effects due to physicsorbed water. RTIL-Li-salt mixture was prepared by mixing equimolar concentration of tetraethylene glycol dimethyl ether (tetraglyme, G4) and lithium bis(trifluoromethanesulfonyl)amide (Li-TFSA) powder. The resultant solution was mixed with the above-mentioned nanoparticles in methanol for 3h. The quasi-solid-state powders were achieved by evaporating the mixture on a hot plate. The volume ration of nanoparticles reached 75, 60 and 75 vol%, respectively. Self-assembled hybrid electrolyte sheets were obtained by further mixing the powder of polytetrafluoroethylene (PTFE) with the quasi-solid-state powder at 5 wt%. All of the raw materials were used without any further treatment, but stored in the argon-filled glove box. The ion conductivities of quasi-solid-state hybrid electrolytes were measured by the ac impedance method over the frequency range from 1 × 106 to 1 Hz.[Results and discussion] White quasi-solid-state powders were successfully achieved incorporating various nanoparticles with G4/LiTFSA solution. Self-assembled electrolyte sheets were obtained mixing quasi-solid-state powders with 5 wt.% PTFE as final samples. The ionic conductivity of CeO2, Al2O3, ZrO2 – x vol% G4/LiTFSA composites were measured as a function of inverse temperature, respectively. Considered with the ionic conductivity of SiO2, it was found that all the samples had liquid-like high ion conductivities although the conduction paths, i.e. RTIL-Li-salts, are quasi-solidified. Additionally, CeO2, Al2O3 and ZrO2-based quasi-solid-state electrolytes displayed higher ion conductivities than that of SiO2-based one. The reason can be attributed to the enhancement of interaction between RTIL and nanoparticle surfaces. Thus, the higher performance of the Li-ion battery composed of the above-mentioned quasi-solid-state electrolytes can be expected. From applications point of view, the hybrid electrolytes might be potentially applied to various electrochemical devices, such as bipolar all-solid-state Li-ion batteries with wider electrical potential ranges.

Solvate Ionic Liquids as Electrolyte for Li-Oxygen Battery

Rechargeable Li-air (Li-O2) batteries have recently attracted enormous attention due to their prospective high energy density. To realize reversible and stable operation of Li-air batteries, however, many serious problems, such as volatilization of electrolyte and electrolyte decomposition by O2 - radical, have to be solved.[1] Our group recently reported that equimolar mixtures of glymes and Li salts behave as ionic liquids (ILs) rather than as ordinary concentrated electrolyte solutions.[2] For instance, as shown in Fig. 1, triglyme (G3) can coordinate to a Li+ ion, forming a 1:1 complex cation, [Li(G3)1]+; it can also behave as an independent cation similar to the cations of typical ILs. Thus, the glyme-Li salt complex has been categorized as a new subclass of ILs called “solvate ILs”.[3] The solvate ILs have many desirable properties as lithium conducting electrolyte[4]-[5] as well as low volatility and thermal- and electrochemical stabilities, which are crucial for Li-air batteries. In the present study, we applied solvate ILs to electrolyte of Li-air battery.[6] The solvate IL, [Li(G3)1][TFSA], was prepared by simply mixing purified triglyme (G3) and lithiumbis(trifluoromethanesulfonyl)amide (LiTFSA) in a 1:1 molar ratio. The electrochemical properties of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in the solvate ILs were investigated by cyclic voltammetry (CV), rotating ring-disk electrode voltammetry (RRDE) and galvanostatic charge-discharge measurement. Fig. 2 shows CVs for a glassy carbon (GC) electrode in [Li(G3)1][TFSA] saturated with O2 or Ar (as background). A large reduction peak emerges when the electrode is polarized to negative potential under the O2 atmosphere, and a clear anodic peak can be seen on reversing the sweep from 1.5 to 4.2 V, although no appreciable current is observed under the Ar atmosphere. It suggests that reversible ORR-OER takes place on the GC electrode in [Li(G3)1][TFSA]. The results of RRDE and charge-discharge test in this solvate ILs will also be reported. Acknowledgement This study was partly supported by RISING program from the New Energy and Industrial Technology Development Organization (NEDO) of Japan, for which the authors are grateful.

Development of lithium-sulfur batteries using room temperature ionic liquid-based quasi-solid-state electrolytes

Quasi-solid-state electrolytes (QSEs), consisting of pseudo-ternary system, i.e., room temperature ionic liquid (RTIL)-lithium bis(trifluoromethanesulfonyl)amide (Li-TFSA)-fumed silica nano particles, were prepared for use as electrolytes in bulk-type all-solid-state cell configuration lithium-sulfur rechargeable batteries. Regardless of the high concentration of RTIL-Li-salt mixtures with fumed silica nano particles, i.e., 80-90% volume ratio, the composites were white powders. Further blending with 5wt% polytetrafluoroethylene (PTFE) powder caused the composite powders to become transparent sheets of 30-200μm in thickness. The electrolyte sheets exhibited liquid-like high apparent conductivities regardless of their solid-like appearance. Sulfur utilization ratios were enhanced using conductive additives with high specific surface areas. Cells with notable performance were successfully assembled after the optimization of the RTIL solvent composition, electrolyte thickness, and conductive additive species. The initial discharge capacity was 1100mAh g−1 at 0.05 C. After 45 discharge-charge cycles, the discharge capacity was still 690mAhg−1. This performance is comparable to lithium-sulfur cells that use the RTIL-Li-salt liquid electrolytes.

Development of high capacity all-solid-state lithium battery using quasi-solid-state electrolyte containing tetraglyme–Li-TFSA equimolar complexes

Abstract Polyacrylonitrile sulfide (PAN-S) was synthesized for use as a positive electrode of a lithium rechargeable battery. To assemble the lithium rechargeable battery with an all-solid-state cell configuration, we also prepared a quasi-solid-state electrolyte consisting of a room temperature ionic liquid (RTIL)-like equimolar complex of tetraethylene glycol dimethyl ether (tetraglyme, G4)–lithium bis(trifluoromethanesulfonyl)amide (Li-TFSA) and fumed silica nanoparticles. The lithium rechargeable battery with the PAN-S positive electrode, quasi-solid-state electrolyte and lithium negative electrode showed high capacity and good cycle performance. The 2nd and 50th discharge capacities were 450 and 390 mAh g − 1 , respectively, with nearly 100% Coulombic efficiency without remarkable capacity fade during the operation.

Electrochemical preparation of a lithium–graphite-intercalation compound in a dimethyl sulfoxide-based electrolyte containing calcium ions

Electrochemical preparation of lithium–graphite-intercalation compound in dimethyl sulfoxide (DMSO)-based electrolytes containing calcium salt was studied. Intercalation of DMSO-solvated cation took place in 1.0moldm−3 lithium bis(trifluoromethanesulfonyl)amide (LiTFSA)+1.5moldm−3 calcium bis(trifluoromethanesulfonyl)amide (Ca(TFSA)2)/DMSO, whereas intercalation of Li+ ions without solvent took place in 1.0moldm−3 LiTFSA+2.5moldm−3 Ca(TFSA)2/DMSO. Raman spectroscopic study suggests absence of free DMSO in 1.0moldm−3 LiTFSA+2.5moldm−3 Ca(TFSA)2/DMSO, which can lead to different solvation structure of Li+ from the one in 1.0moldm−3 LiTFSA+1.5moldm−3 Ca(TFSA)2/DMSO. Factors that are responsible for co-intercalation and only Li+ ion intercalation are discussed based on the Li+ ion solvation structures.

Development of all-solid-state lithium battery using quasi-solidified tetraglyme–lithium bis(trifluoromethanesulfonyl)amide–fumed silica nano-composites as electrolytes

A quasi-solid-state electrolyte consisting of tetraethylene glycol dimethyl ether (tetraglyme, G4)/lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) mixtures together with fumed silica nanoparticles was prepared for use as the electrolyte of an all-solid-state lithium rechargeable battery. Via the liquid route, we have succeeded in fabricating a quasi-solid-state electrolyte regardless of the high concentration of G4/Li-TFSA such as 75 vol%. It was found that the conductivities, the self-diffusion coefficients and the structure of the G4/Li-TFSA complex in the quasi-solid-state composite are essentially similar to those of the bulks regardless of the solid-like appearance. Compared to the quasi-solid-state electrolytes containing the conventional room temperature ionic liquids (RTILs), the ones containing the equimolar G4/LiTFSA complex exhibited a higher stability to the lithium electrodes. The all-solid-state lithium rechargeable battery was assembled using LiCoO2 and lithium metal as the cathode active material and anode, respectively. The all-solid-state cells showed noticeable charge–discharge and cycle performances. In the cut-off voltage range of 4.3–3.4 V, it achieved high initial discharge capacities of 130, 110 and 90 mAh g−1 at 0.05, 0.1 and 0.2 C, respectively. Even after one hundred charge–discharge cycles, the discharge capacities remained high at 105, 100 and 95 mAh g−1, respectively.

Electrical conductivity and dynamics of quasi-solidified lithium-ion conducting ionic liquid at oxide particle surfaces

Lithium ion conducting solid-state composites consisting of lithium ion conducting ionic liquid, lithium bis(trifluoromethanesulfonyl)amide (Li-TFSA) dissolved 1-ethyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)amide (EMI-TFSA), denoted by [ yMLi +][EMI +][TFSA −] in this study, and various oxide particles such as SiO 2, Al 2O 3, TiO 2 (anatase and rutile) and 3YSZ are synthesized via a liquid route for the molar concentration of lithium, y, to be 1. The composite consists of SiO 2 and the ionic liquid with y = 0.2 was also prepared. The ionic liquid are quasi-solidified at the above oxide particle surfaces when x is below 40 for y = 1 and x is below 30 for y = 0.2, corresponding to the confinable thickness of the ionic liquid at the oxides' surfaces to be approximately 5–10 nm regardless of the oxide compositions. The electrical conductivities of x vol.%[ yMLi +][EMI +][TFSA -]–SiO 2, Al 2O 3, TiO 2s or 3YSZ composites are evaluated by ac impedance measurements. The quasi-solid-state composites exhibited liquid-like high apparent conductivity, e.g. 10 − 3.3 –10 − 2.0 S cm − 1 in the temperature range of 323–538 K for SiO 2–ionic liquid composites with y = 1. The self-diffusion coefficients of the constituent species of x vol.% [ yMLi +][EMI +][TFSA −] ( x is below 40, y = 0.2 and 1) − SiO 2 are evaluated by the Pulse Gradient Spin Echo (PGSE)-NMR technique in the temperature range of 298–348 K. By the quasi-solidification of the ionic liquid at SiO 2 particle surfaces, the absolute values of the diffusion coefficients of all constituent species decreased. The SiO 2 surfaces work to promote ionization of ion pair, [EMI +][TFSA −], while significant influence on the solvation coordination, [Li(TFSA) n + 1 ] n− , was not observed. The apparent transport numbers of Li-containing species both in the bulk and the quasi-solidified ionic liquid showed similar values with each other, which was evaluated to be in the range of 0.010–0.017 for y = 0.2 and 0.051–0.093 for y = 1 in the abovementioned temperature range.