Yosef Gofer

Prototype systems for rechargeable magnesium batteries.

The thermodynamic properties of magnesium make it a natural choice for use as an anode material in rechargeable batteries, because it may provide a considerably higher energy density than the commonly used lead–acid and nickel–cadmium systems. Moreover, in contrast to lead and cadmium, magnesium is inexpensive, environmentally friendly and safe to handle. But the development of Mg batteries has been hindered by two problems. First, owing to the chemical activity of Mg, only solutions that neither donate nor accept protons are suitable as electrolytes; but most of these solutions allow the growth of passivating surface films, which inhibit any electrochemical reaction. Second, the choice of cathode materials has been limited by the difficulty of intercalating Mg ions in many hosts. Following previous studies of the electrochemistry of Mg electrodes in various non-aqueous solutions, and of a variety of intercalation electrodes, we have now developed rechargeable Mg battery systems that show promise for applications. The systems comprise electrolyte solutions based on Mg organohaloaluminate salts, and MgxMo 3S4 cathodes, into which Mg ions can be intercalated reversibly, and with relatively fast kinetics. We expect that further improvements in the energy density will make these batteries a viable alternative to existing systems.

On the use of vinylene carbonate (VC) as an additive to electrolyte solutions for Li-ion batteries

Vinylene carbonate (VC) was tested as an additive to electrolyte solutions for Li-ion batteries. For the model electrodes, synthetic graphite was chosen as the anode material, while LiMn2O4 spinel and LiNiO2 were chosen as the cathode materials. The test solution was 1 M LiAsF6 in a 1:1 mixture of ethylene and dimethyl carbonates (EC–DMC). Cyclic voltammetry (CV), chronopotentiometry, impedance spectroscopy, electrochemical quartz crystal microbalance (EQCM), FTIR and X-ray photoelectron spectroscopies have been used in this study. It was found that VC is a reactive additive that reacts on both the anode and the cathode surfaces. The influence of this additive on the behavior of Li–graphite anodes is very positive, since it improves their cyclability, especially at elevated temperatures, and reduces the irreversible capacity. The spectroscopic studies indicate that VC polymerizes on the lithiated graphite surfaces, thus forming poly alkyl Li-carbonate species that suppress both solvent and salt anion reduction. The presence of VC in solutions reduces the impedance of the LiMn2O4 and LiNiO2 cathodes at room temperature. However, we have not yet found any pronounced impact of VC on the cycling behavior of the cathodes, either at room temperature or at elevated temperatures. Thus, VC can be considered as a desirable additive for the anode side in Li-ion batteries, one which has no adverse effect on the cathode side.

Mg rechargeable batteries: an on-going challenge

The first working Mg rechargeable battery prototypes were ready for presentation about 13 years ago after two breakthroughs. The first was the development of non-Grignard Mg complex electrolyte solutions with reasonably wide electrochemical windows in which Mg electrodes are fully reversible. The second breakthrough was attained by demonstrating high-rate Mg cathodes based on Chevrel phases. These prototypes could compete with lead–acid or Ni–Cd batteries in terms of energy density, very low self-discharge, a wide temperature range of operation, and an impressive prolonged cycle life. However, the energy density and rate capability of these Mg battery prototypes were not attractive enough to commercialize them. Since then we have seen gradual progress in the development of better electrolyte solutions, as well as suggestions of new cathodes. In this article we review the recent accumulated experience, understandings, new strategies and materials, in the continuous R&D process of non-aqueous Mg batteries. This paper provides a road-map of this field during the last decade.

The behaviour of lithium electrodes in propylene and ethylene carbonate: Te major factors that influence Li cycling efficiency☆

Abstract The Li cycling efficiency surface chemistry and Li morphology in ethylene carbonate (EC) and propylene carbonate (PC) based electrolyte solutions were investigated and correlated. Surface sensitive ex situ FTIR spectroscopy, X-ray microanalysis and scanning electron microscopy were used in conjunction with standard electrochemical techniques. EC is more reactive than PC in electroreduction processes and is reduced on noble metals to ethylene dicarbonate. The difference in reactivity between the two solvents is discussed, based on MO ab initio calculations of their radical anions (and Li+ stabilized radical anions). In spite of the high reactivity of these systems to lithium, the Li cycling efficiency is strongly dependent on the presence of additives and contaminants at the ppm level that modify the Li surface chemistry in solutions. The two alkyl carbonate solvents decompose when stored over activated Al2O3 and CO2 is formed. The presence of CO2 in solutions increases the Li cycling efficiency considerably due to the formation of Li2CO3 on the Li surfaces.

The Correlation Between Surface Chemistry, Surface Morphology, and Cycling Efficiency of Lithium Electrodes in a Few Polar Aprotic Systems

Lithium electrodes in a few selected polar aprotic electrolyte systems were investigated using electrochemical techniques in conjunction with surface - sensitive Fourier transform infrared spectroscopy and scanning electron microscopy. The solvents used were {gamma} butyrolactone (BL), propylene carbonate (PC), and tetrahydrofuran (THF), and the salts included LiClO{sub 4} and LiAsF{sub 6}. Cycling efficiency of lithium electrodes was correlated to their surface chemistry and morphology in the various solvent systems. The effects of both water and oxygen contamination were rigorously studied. It was found that the presence of oxygen in solutions considerably increased the cycling efficiency of the lithium electrode. This effect correlates well with the influence of the presence of oxygen on the surface morphology of lithium electrodes in solutions. The presence of water increases cycling efficiency of Li electrodes in PC, and decreases cycling efficiency of Li electrodes in ethers. These results are discussed in light of the surface chemistry of lithium in the various solvent systems.

Recent studies of the lithium-liquid electrolyte interface Electrochemical, morphological and spectral studies of a few important systems

Our recent studies on the correlation between Li-cycling efficiency, morphology, interfacial properties and surface chemistry in a variety of Li battery electrolyte solutions are reviewed. The solvent systems include alkyl carbonate mixtures, ether and ether alkyl carbonate mixtures, and methyl formate solutions. The techniques include surface sensitive Fourier-transform infrared spectroscopy and standard electrochemical techniques. The principal points are: (i) the surface chemistry of Li is determined by a delicate balance between reduction processes of the solvents, salts and common contaminants; (ii) the surface films initially formed are subjected to ageing processes which gradually change their structure and properties; (iii) the heterogeneous chemical structure of the Li electrodes surface films induces non-uniform Li deposition; (iv) the cycling efficiency is high in systems where Li deposition is smooth and/or the Li deposited is efficiently passivated by the surface species instantaneously formed on it, and (v) it is evident that less hygroscopic surface species passivate the active metal in solution (e.g., Li2CO3, LiF) more effectively.

Electrochemical and Spectroscopic Analysis of Mg2+ Intercalation into Thin Film Electrodes of Layered Oxides: V2O5 and MoO3

Electrochemical, surface, and structural studies related to rechargeable Mg batteries were carried out with monolithic thin-film cathodes comprising layered V2O5 and MoO3. The reversible intercalation reactions of these electrodes with Mg ion in nonaqueous Mg salt solutions were explored using a variety of analytical tools. These included slow-scan rate cyclic voltammetry (SSCV), chrono-potentiometry (galvanostatic cycling), Raman and photoelectron spectroscopies, high-resolution microscopy, and XRD. The V2O5 electrodes exhibited reversible Mg-ion intercalation at capacities around 150-180 mAh g(-1) with 100% efficiency. A capacity of 220 mAh g(-1) at >95% efficiency was obtained with MoO3 electrodes. By applying the electrochemical driving force sufficiently slowly it was possible to measure the electrodes at equilibrium conditions and verify by spectroscopy, microscopy, and diffractometry that these electrodes undergo fully reversible structural changes upon Mg-ion insertion/deinsertion cycling.

The electrochemical behaviour of 1,3-dioxolane—LiClO4 solutions—I. Uncontaminated solutions

The electrochemical behavior of contaminated 1,3-dioxolane (DN)—LiClO4 solutions with lithium and noble metals (e.g gold, platinum) electrodes was investigated using surface sensitive FT-ir, scanning electron microscopy, (SEM), X-ray microanalysis, linear sweep voltammetry and other electrochemical techniques. The contaminants included products of polymerization of DN, water and oxygen. It was found that the above contaminants considerably influence surface chemistry of lithium or noble metal electrodes in DN solution. The presence of polymeric species (resulting from solvent reactions) in solutions increases the solubility of solvent, water or oxygen reduction products and therefore reduces the “natural” passivation of lithium or noble metals at low potentials in solutions. Consequently, water or solvent reduction at low potentials is more feasible compared to other polar aprotic systems. Thus, the voltammetric behavior of contaminated DN solutions is quite different compared to other ethereal solutions. The presence of the above contaminants in DN solutions is detrimental to the performance of Li electrodes. This is in contrast to several polar aprotic systems where the presence of O2 or even H2O increases Li cycling efficiency.

Improved Electrolyte Solutions for Rechargeable Magnesium Batteries

Electrolyte solutions of magnesium organo-halo-aluminates in ethers are suitable for rechargeable magnesium batteries as they enable highly reversible electrodeposition for magnesium while they possess a wide electrochemical window (>2.2 V). Adding LiCI or tetrabutylammonium chloride to these solutions considerably improves their ionic conductivity, the kinetics of the Mg deposition-dissolution processes, and the intercalation behavior of Mg x MO 6 S 8 Chevrel cathodes. The dissolution of both salts in the electrolytic solutions involves acid-base reactions with complex species. Multinuclei nuclear magnetic resonance and Raman spectroscopy were used in conjunction with electrochemical techniques to study these systems. The nature of these reactions, their products, and the way they influence the various properties of these solutions, are discussed herein.

Investigation of the Electrochemical Windows of Aprotic Alkali Metal (Li, Na, K) Salt Solutions

This work is a comparative study of the electrochemical windows and the basic processes on gold electrodes in LiClO 4 , NaClO 4 , and KClO 4 solutions in propylene carbonate (PC). The analytical tools included cyclic voltammetry, electrochemical quartz crystal microbalance, surface-sensitive Fourier transform infrared spectroscopy (ex situ, external reflectance mode), and X-ray photoelectron spectroscopy. The apparent electrochemical windows of these systems are anodically limited at potentials above 1.3 V (vs. Ag pseudoreference electrode corresponding to 4.3 vs. Li/Li 1 ) due to solvent oxidation. The apparent cathodic side is limited due to the reversible bulk active metal deposition occurring at approximately -3 and < -2.7 V vs Ag pseudoreference electrode for Li and Na, respectively. In the case of the potassium salt solution, the electrochemical window is limited by a pronounced cathodic process below -2 V (vs Ag reference electrode), which is attributed to irreversible reduction of solution species. Irreversible potassium deposition occurs at potentials below - 2.5 V. This process cannot be separated from the reduction processes of the solution starting below -2 V. The study revealed that irreversible trace O 2 , trace H 2 O. and PC reduction form passivating surface films on these electrodes, These films act as a solid electrolyte interphase, i.e., they allow transport of the alkali metal ions through them. The study also found that the major constituent in the surface films is the PC reduction product CH 3 CH(OCO 2 M)CH 2 OCO 2 M. In general, the surface films formed on the noble metal electrodes in the Li and K salt solutions are more stable than those formed in the Na salt solutions, because the sodium oxides, hydroxide, and carbonates thus formed are more soluble in PC than the corresponding Li and K compounds.