Electrolytes For Li Batteries Research Articles

Radical Decomposition of Ether-Based Electrolytes for Li-S Batteries

In this work, the stability of ether-based electrolytes for Li-S batteries is investigated with particular regard to the effect of dissolved oxygen. Specifically, the performance of two different electrolyte solvents, i.e., 1,2-dimethoxyethane and its mixture with 1,3-dioxolane (DME:DOL, 1:1 v/v), is characterized in cells assembled in dry air environment, which would substantially lower production costs with respect to inert atmosphere (Ar). Although stability of all the components would suggest that Li-S batteries built in both the environments should behave similarly, it is found that cells containing the DME:DOL-based electrolyte are rather unstable in the presence of O2 in contrast to those employing DME-based electrolyte, which show a relatively good performance. The different sensitivity toward O2 of these electrolytes is associated to the ring-opening reaction of DOL, which happens to a greater extent when O2 is present, but occurs also in its absence. Based on these results a mechanism for electrolyte degradation in Li-S cells, and its reaction with dissolved polysulfides is proposed, which rationally explain for the first time the behavior already reported in literature for these kind of batteries. These findings are also relevant to the field of Li-O2 batteries, where these ether-based electrolytes are also used.

Contribution of I−/I3 − and I3 −/I2 Redox Couples to Charge and Discharge Reactions of Li-Oxygen Battery with Solvate Ionic Liquid Electrolytes

Introduction Rechargeable Li-Air batteries have recently attracted great attention due to their much higher theoretical energy density than widespread Li-ion batteries. However, to realize reversible and stable operation, many serious problems must be overcome such as volatility of electrolyte, electrolyte decomposition by O2 − radicals, and high overpotential especially during charging[1]. We recently reported that certain equimolar mixtures of glymes and Li salts behave like ionic liquids (ILs), consisting of solvate cations (Li[glyme]+) and the anions, and thus they are categorized as ‘‘solvate’’ ILs[2]. They have desirable properties as electrolytes for Li batteries, such as low volatility, thermal and electrochemical stability, and high Li+ transference number. In our recent study, we applied solvate ILs as electrolytes for Li-Air batteries[3].However, as mentioned above, extremely high overpotential during charging is a serious problem. A way to reduce the charge overpotential and improve cycle capability is the use of soluble redox mediators such as TTF/TTF+, I−/I3 −, and TEMPO/TEMPO+ redox couples for the facile oxidation of lithium peroxide[4][5][6][7]. In this study, we adopted [Li(G4)]I as a mediator for Li-Air batteries and elucidated the mediation mechanism. Iodide (I−) can be electrochemically oxidized to triiodide (I3 −) and iodine (I2) according to the following equations. 3 I− ⇆ I3 − + 2 e− (1)I3 − ⇆ 3/2 I2 + e− (2)However, it is unclear which of these redox couples (I−/I3 − and I3 −/I2) is active as a mediator in the Li-air cells. Possible mediation mechanisms during charging of the cell are shown below.Li2O2 + I3 − → 2 Li+ + O2 + 3 I− (3)Li2O2 + I2 → 2 Li+ + O2 + 2/3 I3 − (4) Experimental The solvate IL ([Li(G4)][TFSA]; Fig.1) and a mediator ([Li(G4)]I) were prepared by simply mixing purified tetraglyme (G4) and lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA]) or lithium iodide (LiI) in a 1:1 molar ratio in an Ar-filled glove box. [Li(G4)]I was mixed with [Li(G4)][TFSA] to obtain several electrolytes containing different concentrations of the mediator. In order to prepare the carbon cathode, 75 wt% Ketjen black (KB) and 25 wt% polytetrafluoroethylene (PTFE) were mixed in N-methyl-2-pyrrolidone (NMP) and then spread onto carbon paper and dried. Galvanostatic discharge-charge tests were performed under dry O2 atmosphere using a coin cell (CR2032-type) consisting of Li metal, a glass separator, the electrolyte, and carbon cathode. Results and Discussion Fig. 2 shows the discharge-charge curves of [Li | 100 or 200 mM [Li(G4)]I in [Li(G4)][TFSA] | KB/O2 ] cells. During charging at the 1st cycle in each electrolyte, there were two plateaus around 3.0 V and 3.3 V which were not observed when a mediator was not added in electrolyte. These plateaus were tentatively assigned to eq. (1) and eq. (2). In this figure, vertical red lines indicate the theoretical capacity calculated from the amount of electrolyte and the concentration of iodide when I− completely oxidizes to I3 − unaccompanied with a mediation reaction. Since the length of the first plateau (~3.0 V, eq. (1)) was less than the theoretical value (did not reach the red line), I−/I3 − did not effectively operate as a mediator and eq. (3) did not proceed after the formation of I3 −. On the other hand, without a subsequent mediation reaction, the length of the second plateau (~3.3 V, eq. (2)) should have been half that of the first plateau, considering the stoichiometry of eq. (1) and (2). In fact, the length of the second plateau was much longer than that of the first plateau. This suggests that I3 −/I2 operated as a mediator and eq. (4) proceeded. Assuming that the mediation due to the combination of eq. (2) and eq. (4) proceeded, at the end of the charge (limited capacity), I3 − must be present in the electrolyte. During discharging at the 2nd cycle, there was a new plateau around 2.9 V, notably different from the oxygen reduction reaction. This plateau is tentatively assigned to the reduction of I3 − (formed during charging) to I− (reverse of eq.(1)).In conclusion, these results suggest that the mediation reaction using LiI in [Li(G4)][TFSA] (SIL) occurred not via the I−/I3 − redox couple, but via the I3 −/I2 redox couple. Acknowledgement This study was partly supported by the RISING program from the New Energy and Industrial Technology Development Organization (NEDO) of Japan, for which the authors are grateful. Figure 1

Solvent-Dictated Lithium Sulfur Redox Reactions: An Operando UV-vis Spectroscopic Study.

Fundamental understanding of solvent's influence on Li-S redox reactions is required for rational design of electrolyte for Li-S batteries. Here we employ operando UV-vis spectroscopy to reveal that Li-S redox reactions in high-donor-number solvents, for example, dimethyl sulfoxide (DMSO), undergo multiple electrochemical and chemical reactions involving S8(2-), S6(2-), S4(2-), and S3(•-), where S3(•-) is the most stable and dominant reaction intermediate. In low-donor-number solvents, for example, 1,3-dioxolane:1,2-dimethoxyethane, the dominant reaction intermediate, is found to be S4(2-). The stability of these main polysulfide intermediates determines the reaction rates of the disproportionation/dissociation/recombination of polysulfides and thereby affects the reaction rates of the Li-S batteries. As an example, we show that dimethylformamide, a high-donor-number solvent, which exhibits stronger stabilization of S3(•-) compared with DMSO, significantly reduces Li-S cell polarization compared with DMSO. Our study reveals solvent-dependent Li-S reaction pathways and highlights the role of polysulfide stability in the efficiency of Li-S batteries.

Electrochemical and Analytical Characterizations of Li-S Batteries

During the last decades more attention has been paid to developing new batteries based on the lithium-sulphur electrochemical system, having high theoretical specific energy (2600 Wh kg-1). However, lithium-sulphur batteries are still far from reaching the market place due to several limitations. Among them are: (i) the use of a Li metal anode with liquid electrolyte, (ii) the insulating character of sulphur and the ending product Li2S and (iii) the soluble polysulphide species generated during the battery operation, which diffuses throughout the separator and deposits on the Li side, resulting in a loss of active material. Despite numerous reports on the mechanism of polysulfide formation the direct information on the magnitude of polysulfide shuttle (diffusion coefficients, solubility) is still missing. Thus reliable quantification of dissolved S in the electrolyte is a necessity to investigate the role of soluble S species in the S redox reaction. This might of particular interest for theoretical help in the search for the optimized composition of the battery components. Thus within these studies we focused on the development of the new simple analytical techniques/procedures that could give such information. Firstly, the electrochemistry of elemental sulphur S8 and synthesized polysulfides (Li2S4, Li2S6 and Li2S8) has been studied for the first time in the non-aqueous electrolyte 1M LiTFSIa in 1:1 (v/v) TEGDME/Dioxolaneb and in the ionic liquid electrolytes (ILs) such as 1M LiTFSI in DEME TFSIc, 1M LiTFSI in EMIm TFSId and 1M LiTFSI in BMP TFSIe. The cyclic voltammetry of S8 in the electrolytes used and mentioned above is different to the behavior reported in some organic solvents (such as THF, DMF, DMSO, or CH3CN), with two reduction and one oxidation peaks obtained. The cyclic voltammograms (CVs) in different liquids were obtained at constant temperature (25 °C) using a PC controlled Autolab PGSTAT30 potentiostat/galvanostat and a three-electrode cell. UV_Vis spectroscopy and spectro-electrochemical studies have been carried out to help the identification of the various sulphur species formed in the solutions. The main reduction products of S8 in the electrolytes used have been identified as S6 2- and S4 2- and plausible pathways for the formation of these species are proposed. The most important results of this study will be presented in more details. This study has allowed us to understand the solid sulphur-electrolyte interaction (S solubility in electrolyte, Kinetic of dissolution). Secondly, we will present our last results using HPLC - UV Visible technique to show how we were able to determine the solubility limit of sulphur, Li2S and polysulfides in different non-aqueous electrolytes for Li-S batteries. By using high performance liquid chromatography with a UV detector, the solubility of S in the different pure solvents and in the different electrolytes mentioned above, was determined. In addition, the S content in the electrolyte recovered from a discharged Li-S battery was successfully determined by the proposed HPLC/UV method. Thus, the feasibility of the method to the online analysis for a Li-S battery is demonstrated. It’s worth to emphasize that the S was found super-saturated in the electrolyte recovered from a discharged Li-S cell. Finally, we will present the results obtained for cycling using coin cells. In order to understand the influence of dissolved sulphur on cycling properties, we were interested in the study of the influence of the nature and the amount of electrolyte on the OCP self discharge. In all cases, the cycling of batteries was done in GCPL mode. In conclusion, these results are crucial for applications since they demonstrate the important effect of the electrolyte versus sulphur ratio for the good functioning of Li/S batteries. Abbreviations a Bis(trifluoromethane)sulfonimide lithium b Tetraethylene glycol dimethyl ether/1,3-Dioxalane c N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium) d Ethyl Méthyl Imidazolium Tétra fluorosulfonyl imide e 1-Butyl-1-Methylpyrrolidinium Bis(trifluoromethylsulfonyl)imide Keywords: Li-S battery, Electrochemistry, S8, Polysulfides, Ionic liquid, HPLC-UV, Self discharge.

Single Lithium‐Ion Conducting Polymer Electrolytes Based on a Super‐Delocalized Polyanion

AbstractA novel single lithium‐ion (Li‐ion) conducting polymer electrolyte is presented that is composed of the lithium salt of a polyanion, poly[(4‐styrenesulfonyl)(trifluoromethyl(S‐trifluoromethylsulfonylimino)sulfonyl)imide] (PSsTFSI−), and high‐molecular‐weight poly(ethylene oxide) (PEO). The neat LiPSsTFSI ionomer displays a low glass‐transition temperature (44.3 °C; that is, strongly plasticizing effect). The complex of LiPSsTFSI/PEO exhibits a high Li‐ion transference number (tLi+=0.91) and is thermally stable up to 300 °C. Meanwhile, it exhibits a Li‐ion conductivity as high as 1.35×10−4 S cm−1 at 90 °C, which is comparable to that for the classic ambipolar LiTFSI/PEO SPEs at the same temperature. These outstanding properties of the LiPSsTFSI/PEO blended polymer electrolyte would make it promising as solid polymer electrolytes for Li batteries.

Single Lithium-Ion Conducting Polymer Electrolytes Based on a Super-Delocalized Polyanion.

A novel single lithium-ion (Li-ion) conducting polymer electrolyte is presented that is composed of the lithium salt of a polyanion, poly[(4-styrenesulfonyl)(trifluoromethyl(S-trifluoromethylsulfonylimino)sulfonyl)imide] (PSsTFSI(-)), and high-molecular-weight poly(ethylene oxide) (PEO). The neat LiPSsTFSI ionomer displays a low glass-transition temperature (44.3 °C; that is, strongly plasticizing effect). The complex of LiPSsTFSI/PEO exhibits a high Li-ion transference number (tLi (+) =0.91) and is thermally stable up to 300 °C. Meanwhile, it exhibits a Li-ion conductivity as high as 1.35×10(-4) S cm(-1) at 90 °C, which is comparable to that for the classic ambipolar LiTFSI/PEO SPEs at the same temperature. These outstanding properties of the LiPSsTFSI/PEO blended polymer electrolyte would make it promising as solid polymer electrolytes for Li batteries.

Covalently functionalized carbon nanotubes as stable cathode materials of lithium/organic batteries

Anthraquinone molecules are grafted onto carbon nanotubes in order to avoid their dissolution in the Li battery electrolyte.

Polymer-ionic liquid ternary systems for Li-battery electrolytes: Molecular dynamics studies of LiTFSI in a EMIm-TFSI and PEO blend.

This paper presents atomistic molecular dynamics simulation studies of lithium bis(trifluoromethane)sulfonylimide (LiTFSI) in a blend of 1-ethyl-3-methylimidazolium (EMIm)-TFSI and poly(ethylene oxide) (PEO), which is a promising electrolyte material for Li- and Li-ion batteries. Simulations of 100 ns were performed for temperatures between 303 K and 423 K, for a Li:ether oxygen ratio of 1:16, and for PEO chains with 26 EO repeating units. Li(+) coordination and transportation were studied in the ternary electrolyte system, i.e., PEO16LiTFSI⋅1.0 EMImTFSI, by applying three different force field models and are here compared to relevant simulation and experimental data. The force fields generated significantly different results, where a scaled charge model displayed the most reasonable comparisons with previous work and overall consistency. It is generally seen that the Li cations are primarily coordinated to polymer chains and less coupled to TFSI anion. The addition of EMImTFSI in the electrolyte system enhances Li diffusion, associated to the enhanced TFSI dynamics observed when increasing the overall TFSI anion concentration in the polymer matrix.

Quantitative Chromatographic Determination of Dissolved Elemental Sulfur in the Non-Aqueous Electrolyte for Lithium-Sulfur Batteries

A fast and reliable analytical method is reported for the quantitative determination of dissolved elemental sulfur in non-aqueous electrolytes for Li-S batteries. By using high performance liquid chromatography with a UV detector, the solubility of S in 12 different pure solvents and in 22 different electrolytes was determined. It was found that the solubility of elemental sulfur is dependent on the Lewis basicity, the polarity of solvents and the salt concentration in the electrolytes. In addition, the S content in the electrolyte recovered from a discharged Li-S battery was successfully determined by the proposed HPLC/UV method. Thus, the feasibility of the method to the online analysis for a Li-S battery is demonstrated. Interestingly, the S was found super-saturated in the electrolyte recovered from a discharged Li-S cell.

Invited: Lithiated Fluorinated TiO2 Nps Doped with Imidazolium ILs As Electrolytes for Lithium Batteries

The development of new materials for secondary batteries is nowadays one of the most active fields in the international scientific panorama [1].The state of the art for liquid electrolytes in a lithium-ion cell is typically a mixture of organic carbonates such as ethylene carbonate (EC) or dimethyl carbonate (DMC). The mixture ratio varies depending upon the desired cell properties. These solvents contain solvated lithium ions. The latter are provided by lithium salts, most commonly lithium hexafluorophosphate (LiPF6)[2]. Despite the high conductivity of these electrolytes (s >10-3 S·cm-1 at RT), important drawbacks are associated with the volatility and flammability of liquid solvents and the chemical instability of the LiPF6 salt. The latter undergoes hydrolysis in the presence of water traces, releasing fluoride anions. [3]Ionic liquids (ILs) are salts with melting temperatures lower than 100°C. When they are liquid at room temperature, ILs are classified as room temperature ILs (RTILs). Some typical properties of ILs are: a) a low volatility, a high thermal stability and a negligible flammability; b) a high ion density and conductivity; c) a wide electrochemical stability window; and d) a simple synthesis, carried out by choosing the cation and anion which best comply with the requirements of the intended application. These features make ILs very suitable as solvents in electrolytes for Li batteries[4].Recently, a class of solid-state single-ion conducting materials based on lithiated fluorinated-TiO2 (LiFT) was proposed, which demonstrated its applicability as a nanofiller to obtain nanocomposite polymer electrolytes [5,6]. LiFT consists of fluorinated anatase nanoparticles that are directly surface-functionalized with Li+ through an innovative one-step reaction with molten metallic lithium[7]. The conductivity of LiFT is due to Li+hopping processes between coordination sites present at the nanoparticle grain boundaries. These latter events take place in a very effective way.The use of LiFT nanoparticles as the source of Li+ ions and ILs as the plasticizing agents allowed us to obtain innovative composite electrolytes for application in lithium batteries with a high thermal stability and conductivity. Indeed, the use of imidazolium ILs based on TFSI- and BF4 - anions allowed us to investigate the different ability of these anions to coordinate and dissociate the Li+cations present on the surface of LiFT nanoparticles. For these reasons it has to be highlighted that no conventional Li salts are used in the electrolytes here proposed.In this report, composite electrolytes based on LiFT nanopowder, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMImTFSI) ILs are proposed. In details, LiFT nanopowder is doped with a known amount of both ILs to obtain two types of electrolytes with formula LiFT/EMImTFSI0.118 and LiFT/EMImBF4 0.200. The resulting electrolytes contain around 25% wt of ILs and show a powder-like consistence.The correlation between structure, thermal properties and conductivity mechanisms of the resulting LiFT/EMImTFSI0.118 and LiFT/EMImBF4 0.200electrolytes is investigated by a variety of techniques: (a) FT-MIR and FIR at different temperatures; (b) Differential Scanning Calorimetry (DSC); (c) Thermogravimetric Analysis (TGA); and (d) Broadband Electrical Spectroscopy (BES).The materials are thermally stable up to 250°C and their conductivities at 30 and 130°C are, respectively, of 1.2x10-2 cm-1 and 4.1x10-2 Scm-1 for LiFT/EMImTFSI0.118 and 1.7x10-3 Scm-1 and 1.7x10-2 Scm-1, for LiFT/EMImBF4 0.200. Finally, the performance of the proposed materials, tested in operating CR2032 coin cells by galvanostatic cycling in a Li4Ti5O12/{LiFT/EMImTFSI0.118}/LiCoO2configuration is shown in Figure 1. Figure 1 Discharge profiles (inset) and capacity vs. cycle number for a Li4Ti5O12/ {LiFT/EMImTFSI0.118}/LiCoO2, CR2032, coin cell (discharge plotted as negative current).

Chelate Effects in Glyme/Lithium Bis(trifluoromethanesulfonyl)amide Solvate Ionic Liquids, Part 2: Importance of Solvate-Structure Stability for Electrolytes of Lithium Batteries

Highly concentrated, molten mixtures of lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA]) and ether solvents (tetrahydrofuran (THF), monoglyme (G1), diglyme (G2), and triglyme (G3)) were investigated as electrolytes for Li batteries. To compare the electrochemical reactions in the electrolytes with different solvents, the ratio of ether–oxygen atoms and Li+ ([O]/[Li]) in the electrolytes was fixed at four. The capacity of a Li–LiCoO2 cell with [Li(THF)4][TFSA] dramatically decreased upon charge/discharge cycling, whereas [Li(G3)1][TFSA] allowed the cell to have a stable charge–discharge cycles and a Coulombic efficiency of greater than 99% over 100 cycles. Corrosion of the Al current collector of the cathode was also affected by the composition of the electrolytes. Persistent Al corrosion took place in [Li(THF)4][TFSA] and [Li(G1)2][TFSA], which contain shorter ethers, but the corrosion was effectively suppressed in [Li(G3)1][TFSA]. Furthermore, lithium polysulfides, which are formed as discharge inte...

Microscopic Origin of the Ionic Transport Behaviors in Solid Electrolytes for Li Batteries

Recently Li-ion-conducting solid electrolytes have received intensive research interest, as they provide solutions for issues associated with the organic liquid electrolytes in conventional Li-ion batteries. However, despite the relatively high bulk conductivity achieved in many solid electrolytes, the grain-boundary conductivity is frequently low and limits the application. Detailed atomic structure and elemental distribution on the grain boundaries in these materials must be revealed in order to fundamentally understand the origin of large grain-boundary resistance. Such studies, however, are currently very rare. Here we present a detailed atomic scale study on the conduction behaviors of an extensively studied solid electrolyte (Li3x La2/3-x )TiO3 (LLTO). Using aberration-corrected scanning transmission electron microscopy (STEM) and the associated electron energy loss spectroscopy (EELS) analysis, we found that the large grain-boundary resistance of LLTO arose from the intrinsic local structural reconstruction, which is energetically not favorable for Li accommodation. Therefore, large flux of charge carriers was unlikely to occur both along and across the grain boundary, leading to the poor grain-boundary conductivity. The relationship among the processing condition, microstructure, and ionic conductivity was established. And the ionic transport behavior in LLTO is further compared to that in garnet-type Li7La3Zr2O12. These results paved the way for further improvement of the Li-ion-conducting solid electrolytes.

Electrochemical and Physicochemical Properties of PY14FSI -Based Electrolytes with LiFSI

We report here the characterization of Li battery electrolytes based upon the -butyl--methylpyrrolidinium bis(fluorosulfonyl)imide ionic liquid with lithium bis(fluorosulfonyl)imide (LiFSI) as a support salt. These electrolytes show low viscosity relative to other pyrrolidinium-based ionic liquids (ILs) and corresponding higher conductivity at subambient temperatures. The melting point of the IL decreases with the addition of LiFSI and concentrated samples remain totally amorphous. The electrolytes exhibit decreased thermal stability and increased parasitic cathodic reactions with increasing LiFSI fraction relative to the pure IL, probably due to a higher impurity level for the commercial LiFSI. Despite this, the electrolytes have excellent lithium cycling behavior at .

Identification of Li Battery Electrolyte Degradation Products Through Direct Synthesis and Characterization of Alkyl Carbonate Salts

Aiming toward the identification of carbonate-based electrolyte degradation species formed during high-temperature cycling of cells, we have embarked in the synthesis and characterization of lithium-based alkyl carbonates. Through the reaction of commercial or synthesized lithium alkoxides with carbon dioxide, we succeeded in preparing lithium methyl, ethyl, propyl mono carbonates, and therefore we have extended our work to the synthesis of lithium ethandiol-bis carbonate. Their analytical characterization ( and nuclear magnetic resonance, electrospray ionization-mass spectroscopy, Fourier transform: infrared/attenuated total reflection) is described. Furthermore, to our surprise, we managed to demonstrate that these well-known alkyl carbonates show some electrochemical reactivity toward .

On Li-chelating additives to electrolytes for Li batteries

The relative affinity of several electrochemically stable bi- and polydentate organic ligands (containing ether, amino, carbonate or phosphonate moieties) to Li ion was estimated by ab initio calculations at the MP2/6-31G(d) level comparing their calculated binding energies (BEs). Polyether 12-crown-4 and a bisphosphonate, which is covalently locked in a cyclic “chelating” conformation, are stronger Li+ chelators than isosparteine and 1,2-methylenebisphosphonate (an open-chain compound). Organic biscarbonates are the weakest complexing agents among the ligands considered. Although BE values were calculated for the gas phase, the order of ligand complexation ability obtained is also considered relevant to solutions since the difference between the BEs for these three groups of ligands is significant. The same calculations that were performed for complexes of Li ion and different organic carbonate diesters (solvents usually used in Li-ion batteries) allowed singling out of conformationally locked bisphosphonates as new battery life-prolonging additives to carbonate electrolytes.

Improved Li-Battery Electrolytes by Heterogeneous Doping of Nonaqueous Li-Salt Solutions

The concept of heterogeneous doping is applied to liquid lithium battery electrolytes. For that purpose, surface-acidic oxides (viz. SiO 2 ) are dispersed in LiPF 6 containing ethylene carbonate/dimethyl carbonate (w/w 1:1). If the volume fraction (φ) is so high that the particles touch, the overall conductivity is significantly enhanced, in particular if the grain size is small. The results are explained by adsorption of the anion and therefore dissociating Li + out of the ion pairs. This is in line with the zeta potential measurements. At lower φ, the effect is negligible owing to the repulsion of the colloids. At higher φ the conductivity is lowered because of inhomogeneity effects and eventually by the insulating nature of the admixtures. In addition to the enhanced overall conductivity, these composites show the favorable mechanical properties of soft matter (soggy sand electrolytes) and exhibit high effective viscosities. A significant advantage is that the SiO 2 content can make the use of polymer separators dispensable. Performance tests in Li/Li x C 6 cells do not show any perceptible interaction of SiO 2 with high Li activity materials.

Viscosity Changes of Li Battery Electrolytes and Their Long-Term Effect on the Frequency of EQCM Electrodes

The resonant frequency of electrochemical quartz crystal microbalance (EQCM) electrodes (carbon, nickel, and aluminum) decreases gradually with time in 1 M LiClO 4 /ethylene carbonate (EC) + dimethyl carbonate (DMC) after the immediate frequency shift due to the presence of liquid column on the quartz crystals. It is found that the viscosity change of electrolyte is the main reason for the frequency decrease. The viscosities of five liquids (1 M LiClO 4 /EC + DMC, 1 M LiPF 6 /EC + DMC, EC + DMC, DMC, and propylene carbonate) have been measured. Systems with EC + DMC as the solvent of the lithium salt or with the solvent alone show viscosity increase in contact with the electrode, whereas none of the above liquids show viscosity change if they are stored in glass vials.

Glass Formation, Ionic Conductivity, and Conductivity/Viscosity Decoupling, in LiAlCl4 + LiClO4 and LiAlCl4 + LiAlCl3·Imide Solutions

As part of a search for chemically and electrochemically stable ambient temperature molten lithium salt systems, we have investigated the properties of solutions of LiAlCl4 with various second components. In this paper we review the factors which determine the ambient temperature conductivity and report results for two systems, one of which satisfies the stability requirements although failing to provide the high conductivities which are needed for a successful ambient temperature Li battery electrolyte. These ionic solutions appear to be very fragile liquids. Evidence is found for a mixing incompatibility of polarizable and nonpolarizable components of binary melts.

A Study of Lithium Deposition‐Dissolution Processes in a Few Selected Electrolyte Solutions by Electrochemical Quartz Crystal Microbalance

In this work, the electrochemical quartz crystal microbalance (EQCM) technique was applied to the study of the charge‐discharge cycling of lithium electrodes in a few important Li battery electrolyte solutions. These included ethylene and dimethyl carbonates (EC‐DMC) mixtures containing or as the electrolyte, tetrahydrofuran (THF)‐EC/ solutions of two different solvent ratios, and 1–3‐dioxolane/ solutions. The substrate electrode was nickel plated on the quartz crystal. All the experiments were conducted in potentiostatic mode. After an initial step in which the electrode was polarized from open‐circuit potential to 0 V , repeated deposition‐dissolution steps were conducted within predetermined potential limits. These experiments proved the superiority of 1–3‐dioxolane/ solutions as a selected electrolyte system for rechargeable Li batteries with Li metal anode. The EQCM studies revealed that in both EC‐DMC or EC‐THF solutions, Li deposition is accompanied by a pronounced corrosion and accumulation of surface species that further partially dissolve during the anodic steps. These studies also reflected the difference in the surface chemistry developed on lithium in EC‐THF solutions of different EC concentrations. At low EC concentration the Li passivity is better, and thus cycling efficiency is higher. This correlates well with previous studies which revealed that at low EC concentrations in ethereal solutions, is an important component in the Li surface films.

The Reaction of Clean Li Surfaces with Small Molecules in Ultrahigh Vacuum: II. Water

Reactions at the Li/ interface were studied at 160 to 290 K in ultrahigh vacuum by a combination of spectroscopic ellipsometry and Auger electron spectroscopy. Ice multilayers, ca. 100 ML thick, were deposited on clean Li at 160 K. The evaporation rate of water at 160 K is sufficiently slow that the ice layer remains on the surface for about 1 h. After 10 min at 160 k, a pure LiOH layer ca. 70 Aå thick is produced, and after 1 h there is evidence of a slow conversion of LiOH to in the layer, probably at the Li/LiOH interface. Raising the temperature to 240 K results in desorption of the adsorbed water and conversion of all the LiOH to a porous (60% void) layer composed mostly of (35%) with some metallic Li mixed in. Raising the temperature further to 290 K results in densification of the layer by both collapse of the voids and by diffusion of Li into the interstices of the , increasing the Li content to 27% and shrinking the film thickness to 26 Aå. Based on these results, a model for the behavior of small amounts of water in Li battery electrolyte is presented.