Orit Chusid

The Correlation Between the Surface Chemistry and the Performance of Li‐Carbon Intercalation Anodes for Rechargeable ‘Rocking‐Chair’ Type Batteries

The correlation between the electrochemical properties of Li carbon intercalation electrodes and their surface chemistry in solutions was investigated. The carbons investigated were primarily graphite and petroleum coke, and the solvent systems included methyl formate (MF), propylene and ethylene carbonates, ethers and their mixtures. The surface chemistry of the electrodes was studied using mainly diffuse reflectance Fourier transform infrared spectroscopy. The following aspects were studied: (1) the effect of temperature on the buildup of the surface films; (2) the effect of additives (e.g., CO[sub 2], crown ethers), (3) the behavior when the passive layer is built in one solution followed by cycling in another; and (4) the effect of cosolvent in MF solutions. The results obtained further prove that the electrochemical behavior of these systems is surface film controlled. An understanding of the surface chemistry of these electrodes enables judicious optimization of carbon-solution systems for use in rechargeable Li batteries.

Recent studies on the correlation between surface chemistry, morphology, three-dimensional structures and performance of Li and Li-C intercalation anodes in several important electrolyte systems

Abstract This paper reviews some advances in the comparative study of lithium and graphite electrodes in a large matrix of solvents, salts and additives. The major purpose of this work was to support RD (ii) successful and useful application of AFM and EQCM in order to study the surface film formation and Li-deposition processes; (iii) understanding the correlation between the reversibility and stability of graphite electrodes in Li-intercalation processes and their surface chemistry, and (iv) finding an interesting correlation between the three-dimensional structure of graphite electrodes, the diffusion coefficient of Li + and their voltammetric behaviour in Li-intercalation processes.

Electrolyte Solutions with a Wide Electrochemical Window for Rechargeable Magnesium Batteries

Electrolyte solutions for rechargeable Mg batteries were developed, based on reaction products of phenyl magnesium chloride (PhMgCl) Lewis base and Alcl 3 Lewis acid in ethers. The transmetallation of these ligands forms solutions with Mg x Cl + y and AlCl 4-n Ph n - ions as the major ionic species, as analyzed by multinuclei nuclear magnetic resonance spectroscopy. Tetrahydrofuran (THF) solutions of (PhMgCl) 2 -Alcl 3 exhibit optimal properties: highly reversible Mg deposition (100% cycling efficiency) with low overvoltage: <0.2 V and electrochemical windows wider than 3 V. A specific conductivity of 2-5 X 10 -3 Ω -1 cm -1 could be measured between -10 and 30°C for these solutions, similar to that of standard electrolyte solutions for Li batteries. Mg ions intercalate reversibly with Chevrel phase (Mg x Mo 6 S 8 ) cathodes in these solutions. These systems exhibit high thermal stability. The solutions may enable the use of high voltage, high-capacity Mg insertion materials as cathodes and hence open the door for research and development of high-energy density, rechargeable Mg batteries.

Correlation between surface chemistry, morphology, cycling efficiency and interfacial properties of Li electrodes in solutions containing different Li salts

The influence of the Li salt used on the behaviour of Li electrodes in tetrahydrofurane (THF) and propylene carbonate (PC) solutions was investigated. The salts studied included Li halides (LiBr, LiI), LiBF4, LiPF6, LiSO3CF3 and LiN(SO2CF3)2. The correlation between the electrochemical properties, surface chemistry and morphology of Li electrodes in the above systems was studied using impedance spectroscopy, surface sensitive in situ and ex situ FTIR, X-ray microanalysis, electron microscopy and standard experiments of charge discharge cycling. It was found that all the salt anions explored have strong effects on all the above aspects, eg they strongly affect Li surface chemistry in solutions and participate in the build-up of surface films. The electrical properties of the Li-solution interphase formed in the different salt solutions are remarkably dependent on the salt anion. Consequently, the morphology and Li utility in repeated charge-discharge cycling are also strongly influenced by the salt used. Except for the Li halides, all the salts studied seem to be more reactive to lithium than LiClO4 and LiAsF6. They are worse than the commonly used LiAsF6 for rechargeable Li battery systems because their strong involvement in the Li surface chemistry adversely affects Li cycling efficiency.

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 studies of carbon electrodes in lithium battery electrolyte systems

Abstract In this work we studied several parameters that influence the intercalation of lithium ions into carbons (e.g. carbon type, binder and solution composition). The carbons investigated included carbon blacks (e.g. acetylene black, Ketjen black), graphite and carbon fibers. The solvents used in this study include methyl formate, propylene and ethylene carbonate, ethers (e.g. tetrahydrofuran) and their mixtures. The salts included LiClO4, LiAsF6 and LiBF4. CO2 was tested as an additive. The electrochemical behavior of the electrodes in solutions was followed by chronopotentiometry in galvanostatic charge/discharge cycling and their surface chemistry in solutions was investigated using surface sensitive Fourier-transform infrared spectroscopy (FT-IR) in transmittance, attenuated total reflectance and diffuse reflectance modes. It was found that the solvents and salts are reduced on the carbon electrodes at low potentials to form surface films. In general, their surface chemistry is quite similar to that of lithium or noble metal electrodes at low potential (in the same solutions). The electrochemical behavior of the carbon electrodes in terms of degree of intercalation and its reversibility is strongly affected by their surface chemistry. Reversible intercalation was obtained with graphite in methyl formate solutions containing CO2. Some degree of reversible intercalation was also obtained with graphite in ethers. The presence of propylene carbonate in solution is detrimental for lithium intercalation in graphite. Reversible lithium-carbon intercalation was also obtained with acetylene black and carbonized polyacrylonitrile. The binder types have a strong impact on the electrodes performance. Preliminary guidelines for optimizing the performance of carbon electrodes as anodes in rechargeable lithium battery are discussed.

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.

In situ FTIR spectroelectrochemical studies of surface films formed on Li and nonactive electrodes at low potentials in Li salt solutions containing CO[sub 2]

In recent studies it was found that the presence of CO[sub 2] in Li battery electrolyte solutions considerably improves the cycling efficiency of Li electrodes. Surface films formed on Li and noble metal electrodes at low potentials in salt solutions saturated with CO[sub 2] were investigated using in situ Fourier transform infrared spectroscopy (FTIRS). Two methods have been used: subtractively normalized interfacial FTIRS (SNIFTIRS)-type measurements, external reflectance mode (thin layer cell configuration) and internal reflectance mode. The solutions used included LiAsF[sub 6] in propylene carbonate, tetrahydrofuran, and methylformate. It was found that Li[sub 2]CO[sub 3] is a major surface species formed, and its precipitation suppresses solvent-related surface reactions on the active electrode surfaces. A coproduct of Li carbonate seems to be CO gas.

Spectroelectrochemical studies of magnesium deposition by in situ FTIR spectroscopy

Abstract Magnesium can be reversibly deposited electrochemically from solutions of ethereal solvents, with Grignard reagents (RMgX) or complexes of Mg(AX 3− n R n +1 ) 2 stoichiometry as the electrolytes (A=Al, B; X=Cl, Br; R=alkyl or aryl groups). These processes are far from being simple reactions of the Mg/Mg ++ couple, since the above electrolytes in solutions have complicated structures in which the ether molecules play an important stabilization role. In addition, Mg deposition processes in all of the above solutions are accompanied by adsorption phenomena. The surface chemistry of magnesium electrodes was studied in situ by FTIR spectroscopy, using an internal reflectance mode. The electrolyte solutions studied included tetrahydrofuran (THF) solutions of the RMgX electrolytes (R=butyl, ethyl, methyl benzyl, and X=Cl, Br); Mg(AlCl 2 BuEt) 2 ; Mg(AlCl 3 Bu) 2 and Mg(BPh 2 Bu 2 ); Bu, Et, Ph=butyl, ethyl and phenyl groups, respectively. It was clear from these studies that Mg electrodes do not develop stable passivation in these solutions (i.e. formation of surface films). The nature of the adsorbed species in the above systems is discussed, based on the spectral results.

LiC(SO2CF3)3, a new salt for Li battery systems. A comparative study of Li and non-active metal electrodes in its ethereal solutions using in situ FTIR spectroscopy

Abstract The surface chemistry of lithium electrodes and non-active electrodes polarized to low potentials in LiC(SO 2 CF 3 ) 3 solutions in 1-3-dioxolane (DN) and tetrahydrofuran (THF) was rigorously investigated using three different modes of in situ FTIR measurements. One method is based on external reflectance (SNIFTIRS type) and two methods are based on internal reflectance modes. In addition, Li electrodes treated in these solutions were also studied using ex situ FTIR external reflectance mode. For a comparison, the surface chemistry of these electrodes in LiAsF 6 and LiN(SO 2 CF 3 ) 2 solutions in the same solvents was also investigated using in situ and ex situ FTIR spectroscopy. It was found that while in LiAsF 6 solutions the surface chemistry developed is dominated mostly by the solvent reduction, in the other two salt solutions, the salt anion reduction products are the major constituents in the surface films formed. The LiN(SO 2 CF 3 ) 2 is more reactive towards Li than LiC(SO 2 CF 3 ) in these systems. The differences in Li cycling efficiency and morphology observed in the three salt solutions are discussed in light of the difference in the surface chemistry developed on lithium.