Yair Ein-Eli

The Study of Electrolyte Solutions Based on Ethylene and Diethyl Carbonates for Rechargeable Li Batteries II . Graphite Electrodes

The electrochemical behavior of Li-graphite intercalation anodes in ethylene and diethyl carbonates (EC-DEC) solutions was studied using surface sensitive Fourier transform infrared spectroscopy (FTIR) and impedance spectroscopy in conjunction with standard electrochemical techniques. Three different solvent combinations, four different salts: LiBF{sub 4}, LiPF{sub 6}, LiClO{sub 4}, and LiAsF{sub 6}, and the influence of the presence of CO{sub 2} were investigated. Graphite electrodes could be cycled hundreds of times obtaining a reasonable reversible capacity. The best electrolyte was found to be LiAsF{sub 6} and the presence of CO{sub 2} in solutions considerably increased the reversible capacity upon cycling. This improved performance is due to precipitation of the ethylene carbonate reduction product, (CH{sub 2}OCO{sub 2}Li){sub 2}, which is an excellent passivating agent, on the electrode surface. Aging processes of these surface films and their influence on the electrode properties are discussed.

The Study of Electrolyte Solutions Based on Ethylene and Diethyl Carbonates for Rechargeable Li Batteries I . Li Metal Anodes

The behavior of Li electrodes was studied in ethylene and diethyl carbonates (EC-DEC) solutions of LiAsF{sub 6}, LiClO{sub 4}, LiBF{sub 4}, and LiPF{sub 6}. The correlation of the surface chemistry to the interfacial properties, morphology, and Li cycling efficiency was investigated using surface sensitive Fourier transform infrared spectroscopy and impedance spectroscopy, scanning electron microscopy, X-ray energy dispersive microanalysis, and standard electrochemical techniques. The Li surface chemistry is initially dominated by EC reduction to an insoluble species, probably (CH{sub 2}OCO{sub 2}Li){sub 2}. Upon storage, several aging processes may take place, depending on the salt used. Their mechanisms are discussed. Although EC-DEC solutions were found to be adequate for Li ion rechargeable batteries, this work indicates that they are not suitable as electrolyte solutions for batteries with Li metal electrodes. This is mostly because Li electrodes cannot be considered stable in these systems and Li deposition is highly dendritic.

LiMn2 − x Cu x O 4 Spinels (0.1 ⩽ x ⩽ 0.5): A new Class of 5 V Cathode Materials for Li Batteries I. Electrochemical, Structural, and Spectroscopic Studies

A series of electroactive spinel compounds, LiMn 2-x Cu x O 4 (0.1 ≤ x ≤ 0.5), has been studied by crystallographic, spectroscopic, and electrochemical methods and by electron microscopy. These LiMn 2-x Cu x O 4 spinels are nearly identical in structure to cubic LiMn 2 O 4 and successfully undergo reversible Li intercalation. The electrochemical data show a remarkable reversible electrochemical process at 4.9 V which is attributed to the oxidation of Cu 2+ to Cu 3+ . The inclusion of Cu in the spinel structure enhances the electrochemical stability of these materials upon cycling. The initial capacity of LiMn 2-x Cu x O 4 spinels decreases with increasing x from 130 mAh/g in LiMn 2 O 4 (x = 0) to 70 mAh/g in LiMn 1.5 Cu 0.5 O 4 (x = 0.5). The data also show slight shifts to higher voltage for the delithiation reaction that normally occurs at 4.1 V in standard Li 1-x Mn 2 O 4 electrodes (1 ≥ x ≥ 0) corresponding to the oxidation of Mn 3+ to Mn 4+ . Although the powder X-ray diffraction pattern of LiMn 1.5 Cu 0.5 O 4 shows a single-phase spinel product, neutron diffraction data show a small but significant quantity of an impurity phase, the composition and structure of which could not be identified. X-ray absorption spectroscopy was used to gather information about the oxidation states of the manganese and copper ions. The composition of the spinel component in the LiMn 1.5 Cu 0.5 O 4 was determined from X-ray diffraction and X-ray absorption near-edge spectroscopy to be Li 1.01 Mn 1.67 Cu 0.32 O 4 , suggesting to a best approximation that the impurity in the sample was a lithium-copper-oxide phase. The substitution of manganese by copper enhances the reactivity of the spinel structure toward hydrogen: the compounds are more easily reduced at moderate temperature (∼200°C) than LiMn 2 O 4 .

A New Perspective on the Formation and Structure of the Solid Electrolyte Interface at the Graphite Anode of Li ‐ Ion Cells

A new model describing the interactions between the graphite anode in batteries and the liquid electrolyte reduction products [the solid electrolyte interface (SEI)] is presented. According to this model, certain solvents and additives produce efficient passive films which mimic a double‐layer capacitor, wherein local, fixed positive charges in the SEI serve to counteract the negatively charged graphite anode. The model also suggests that internal passive layer interactions are of a multilayered capacitor type. Previously published experimental data in support of this model is presented. ©1999 The Electrochemical Society

Chemical Oxidation: A Route to Enhanced Capacity in Li‐Ion Graphite Anodes

Chemical oxidation of graphite powder by the strong oxidative agents ammonium peroxysulfate and hot, concentrated nitric acid was examined as a way to enhance the capacity of graphite anodes in Li-ion cells. Chemical oxidation increased the reversible capacity obtained during cycling from 370 to 430 mAh/g, whereas the irreversible capacity accompanying the first intercalation cycle was reduced. Fourier transform infrared spectroscopy was used to investigate the effects of chemical oxidation on the surface of the graphite. Slow-scan cyclic voltammetry was applied in order to detect the origin of the extra capacity (where x > 1 in Li{sub x}C{sub 6}) associated with oxidized graphite powder.

The Role of SO 2 as an Additive to Organic Li‐Ion Battery Electrolytes

Previous work has shown that the addition of a large amount of sulfur dioxide (SO 2 ) (∼20 weight percent) promotes the reversible intercalation-deintercalation of Li ions into graphite in selected nonaqueous electrolytes. These electrolytes were previously considered to be incompatible with graphite negative electrodes because of solvent-graphite interaction, which led to catastrophic graphite exfoliation of the graphitic structure. We have performed a series of conductivity studies along with electrochemical experiments at varying SO 2 concentrations. The electrolyte solutions were composed of either 1 M LiAsF 6 or 1 M LiPF 6 . We found that the specific conductance values of the organic electrolytes containing SO 2 were increased dramatically. Cyclic voltammetry and Fourier transform infrared measurements show that the use of SO 2 as an additive to the organic solutions, even at very low levels, offers the advantage of forming fully developed passive films on the graphite electrode at potentials much higher than that of the electrolyte reduction itself. These graphite surface films are composed of mixtures of SO 2 and solvent reduction products. The SO 2 reduction products are primarily responsible for the improved characteristics of the Li-ion cells containing these SO 2 -based electrolytes.

Silicon-air batteries

A new ‘‘metal”–air battery based on silicon–oxygen couple is described. Silicon–air battery employing EMI 2.3HF F room temperature ionic liquid (RTIL) as an electrolyte and highly-doped silicon wafers as anodes (fuels) has an undetectable self-discharge rate and high tolerance to the environment (extreme moisture/dry conditions). Such a battery yields an effectively infinite shelf life with an average working voltage of 1–1.2 V. Silicon–air battery can support relatively high current densities (up to 0.3 mA/cm) drawn from flat polished silicon wafers anodes. Such batteries may find immediate applications, as they can provide an internal, built-in autonomous and self sustained energy source. 2009 Elsevier B.V. All rights reserved.

Ethylmethylcarbonate, a Promising Solvent for Li‐Ion Rechargeable Batteries

Ethylmethylcarbonate (EMC) has been found to be a promising solvent for rechargeable Li-ion batteries. Graphite electrodes, which are usually sensitive to the composition of the electrolyte solution, can be successfully cycled at high reversible capacities in several Li salt solutions in this solvent (LiAsF{sub 6}, LiPF{sub 6}, etc.). These results are interesting because lithium ions cannot intercalate into graphite in diethyl carbonate solutions and cycle poorly in dimethyl carbonate solutions. To understand the high compatibility of EMC for Li-ion battery systems as compared with the other two open-chain alkyl carbonates mentioned above, the surface chemistry developed in both Li and carbon electrodes in EMC solution was studied and compared with that developed on these electrodes in other alkyl carbonate solutions. Basically, the major surface species formed on both electrodes in EMC include ROLi, ROCO{sub 2}Li, and Li{sub 2}CO{sub 3} species. The uniqueness of EMC as a battery solvent is discussed in light of these studies.

Methyl Propyl Carbonate: A Promising Single Solvent for Li‐Ion Battery Electrolytes

Methyl propyl carbonate (MPC) solutions containing Li salts can be used as a single-solvent electrolyte with addition of ethylene carbonate (EC). Graphite electrodes can be cycled at high reversible capacity in MPC solutions containing LiPF{sub 6} and LiAsF{sub 6}. The use of acyclic, unsymmetric alkyl carbonate solvents, such as ethyl methyl carbonate (EMC) and MPC in Li-ion based electrolytes, increases the stability of the graphite electrode. Whereas a small amount of EC is still needed as cosolvent in EMC solutions to obtain stable surface films on graphite electrodes, the authors show here that the surface films produced on graphite in MPC solutions (without added EC) are highly stable, allowing reversible Li-ion intercalation. To understand this trend, they investigated the surface chemistry developed on lithium and carbon electrodes in MPC solutions in conjugation with electrochemical studies.

Li‐Ion Battery Electrolyte Formulated for Low‐Temperature Applications

Low-temperature (<0 C) applications of Li-ion batteries have prompted the search for improved, high-conductivity electrolytes. Because the performance of the carbonaceous anode is highly sensitive to changes in electrolyte composition, the authors focused their efforts on this electrode. Electrolytes containing LiAsF{sub 6}, LiPF{sub 6}, LiN(SO{sub 2}CF{sub 3}){sub 2}[lithium bis(trifluoromethanesulfonyl)imide], or LiIm, and LiC(SO{sub 2}CF{sub 3}){sub 3} [lithium tris(trifluoromethanesulfonyl)methide], or LiMe, in methyl formate (MF)-ethylene carbonate (EC) solvent mixtures were tested in lithium-graphite half-cells. The graphite electrodes could be cycled at ambient temperature with high reversible capacity. The best supporting electrolyte was found to be LiAsF{sub 6}, and the presence of a high concentration of ethylene carbonate and up to 300 ppm H{sub 2}O in the solution considerably increased the reversible capacity upon cycling. The conductivity values of a binary solvent mixture of methyl formate and ethylene carbonate containing LiAsF{sub 6} or LiMe were measured between {minus}40 C and room temperature. Graphite electrodes cycled at {minus}2 C in these electrolytes obtained reasonable reversible capacity, approaching 50%.