M. Moshkovich

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.

New insights into the interactions between electrode materials and electrolyte solutions for advanced nonaqueous batteries

Abstract In this paper we review some recent work with Li metal and Li–graphite anodes and Li x MO y cathodes (M=transition metals such as Ni, Co, Mn). The emphasis was on the study of surface phenomena using in situ and ex situ FTIR spectroscopy, atomic force microscopy (in situ AFM), electrochemical quartz crystal microbalance (EQCM) and impedance spectroscopy (EIS). The performance of Li metal and Li–carbon anodes in secondary batteries depends on the nature of the surface films that cover them. The use of Li metal anodes requires the formation of highly uniform and elastic surface films. Thus, most of the commonly used liquid electrolyte solutions are not suitable for Li metal-based rechargeable batteries. In the case of Li–C-based batteries, the passivating films need not be elastic. Channeling the Li–C electrode surface chemistry towards the formation of Li 2 CO 3 surface films provides adequate passivation for these electrodes. This can be achieved through the use of EC-based solutions of low EC concentration (cosolvents should be less reactive than EC). An interesting finding is that the behavior of many commonly used cathodes also depends on their surface chemistry, and that their overall Li insertion processes include the step of Li ion migration through surface films. Their origin is discussed herein, as well as possible oxidation processes of the relevant solutions.

On the electrochemical behavior of magnesium electrodes in polar aprotic electrolyte solutions

Abstract Mg electrodes were studied in a variety of polar aprotic electrolyte solutions, using cyclic voltammetry (CV), impedance spectroscopy (EIS), surface sensitive FTIR spectroscopy, element analysis by dispersive X-rays (EDAX), scanning electron microscopy (SEM), and electrochemical quartz crystal microbalance (EQCM) studies. The solutions included Mg, Li, Na, K and Bu 4 N + salt solutions in acetonitrile (AN), propylene carbonate (PC) and tetrahydrofuran (THF). In addition, THF+RMgX (R=alkyl, X=Cl, Br) solutions were studied. This paper aims at providing a general description of the electrochemical behavior of Mg electrodes in different types of polar aprotic systems. It appears that Mg electrodes are spontaneously covered by surface films in most of the solutions studied. In AN and PC, solvent reduction seems to dominate surface film formation, while in THF, the solvent is inactive and, thus, reduction of salt anions such as ClO 4 − and BF 4 − leads to the precipitation of surface films. The impedance of Mg electrodes is very high, due to these surface films (several orders of magnitude higher than that of Li electrodes in the same solutions). However, the above difference in the surface chemistry is clearly reflected by the electrode’s impedance. Consequently, Mg dissolution in these solutions occurs via a breakdown of the surface films. However, it is possible to reduce the overpotential of Mg dissolution considerably by the presence of acidic species in solutions, which remove part of the surface films chemically. Reversible Mg deposition and dissolution are obtained in THF+RMgX solution due to the fact that in these solutions, irreversible formation of stable surface films on the Mg electrodes is avoided largely. However, EQCM studies showed that these processes are not just a simple two-electron transfer to Mg ions and are complicated by adsorption–desorption processes of the solution species.

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.

Magnesium Deposition and Dissolution Processes in Ethereal Grignard Salt Solutions Using Simultaneous EQCM‐EIS and In Situ FTIR Spectroscopy

Magnesium deposition and dissolution processes in ethereal Grignard salt solutions using tetrahydrofuran as the solvent and Grignard salt which included , , and (R = methyl, ethyl, butyl, or benzyl) are reported. dissolution and deposition in these solutions are basically reversible with an overall mass balance close to zero for complete cycles. In prolonged deposition processes, the mass accumulated per mole of electron transferred was similar to , the equivalent weight of magnesium. However, deposition‐dissolution is not a simple two‐electron process, but involves adsorption‐desorption processes of species such as and/or . These adsorption processes lead the electrodes to devel p high impedance upon storage in these solutions (up to hundreds of thousands of ). However, this passivation is not stable, and breaks down as the electrochemical processes proceed. ©2000 The Electrochemical Society

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 LiAsF 6 or LiAsF 6 as the electrolyte, tetrahydrofuran (THF)-EC/LiAsF 6 solutions of two different solvent ratios, and 1-3-dioxolane/LiAsF 1 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 (Li/Li + ), repeated deposition-dissolution steps were conducted within predetermined potential limits. These experiments proved the superiority of 1-3-dioxolane/LiAsF 6 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, Li 2 CO 3 is an important component in the Li surface films.

A comparison between the electrochemical behavior of reversible magnesium and lithium electrodes

Abstract This paper describes briefly the difference between reversible lithium and magnesium electrodes. In the case of lithium, the active metal is always covered by surface films. Li dissolution–deposition is reversible only when the surface films contain elastomers and are flexible. Hence, they can accommodate the morphological changes of the electrode during the electrochemical processes without breaking down. In an ideal situation, lithium is deposited beneath the surface films, while being constantly protected in a way that prevents reactions between freshly deposited lithium and solution species. In contrast to lithium, magnesium electrodes are reversible only in solutions where surface film free conditions exist. Mg does not react with ethers, and thus, in ethereal solutions of Grignard reagents (RMgX, where R=alkyl, aryl, X=halide) and complexes of the following type: Mg(AlX 4− n R n ′ R n ″ ′) 2 , R and R′=alkyl groups, X=halide, A=Al, 0 n n ′+ n ′′= n , magnesium electrodes behave reversibly. However, it should be noted that the above stoichiometry of the Mg salts does not reflect the true structure of the active ions in solutions. Mg deposition does not occur via electron transfer to simply solvated Mg 2+ ions. The behavior of Mg electrodes in these solutions is discussed in light of studies by EQCM, EIS, FTIR, XPS, STM and standard electrochemical techniques.