Elena Levi

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 correlation between surface chemistry and performance of graphite negative electrodes for Li ion batteries

This paper discusses some important aspects of the correlation between surface chemistry, 3D structure, and the electrochemical behavior of lithiated graphite electrodes. By reviewing results obtained with diAerent electrolyte solutions (e.g. ethylene carbonate-based solutions, propylene carbonate solutions, and ether-based systems), we describe the stabilization and capacity fading mechanisms of graphite electrodes. One of the failure mechanisms occurs at potentials >0.5 V Li/Li + , and relates to an increase in the electrode’s impedance due to improper passivation and a simultaneous change in the electrode’s morphology, probably due to gas formation. At low potentials (depending on the electrolyte solution involved), phenomena such as exfoliation and amorphization of the graphite electrodes can be observed. Stabilization mechanisms are also discussed. In general, surface stabilization of the graphite is essential for obtaining reversible lithiation and a long electrode cycle life. The latter usually relates to precipitation of highly compact and insoluble surface species, which adhere well, and irreversibly, to the active surface. Hence, the choice of appropriate electrolyte solutions in terms of solvents, salts and additives is very critical for the use of graphite anodes in Li batteries. The major analytical tools for this study included FTIR and impedance spectroscopies, XPS, and in situ and ex situ XRD in conjunction with standard electrochemical techniques. # 1999 Elsevier Science Ltd. All rights reserved.

Common electroanalytical behavior of Li intercalation processes into graphite and transition metal oxides

This paper compares the electroanalytical behavior of lithiated graphite, Li x CoO 2 , Li x NiO 2 , and Li x Mn 2 O 4 spinel electrodes. Slow scan rate cyclic voltammetry (SSCV), potentiostatic intermittent titration (PITT), and electrochemical impedance spectroscopy (EIS) were applied in order to study the potentiodynamic behavior, the variation of the solid-state diffusion coefficient, and the impedance of these electrodes. In addition, X-ray diffractometry and Fourier transform infrared (FTIR) spectroscopy were used in order to follow structural and surface chemical changes of these electrodes upon cycling. It was found that all four types of electrodes behave very similarly. Their SSCV are characterized by narrow peaks which may reflect phase transition between intercalation stages, and the potential-dependent Li chemical diffusion coefficient is a function with sharp minima in the vicinity of the CV peak potentials, in which the differential electrode capacity is maximal. The impedance spectra of these electrodes reflect an overall process of various steps in series. These include Li + ion migration through surface films, charge transfer which depends strongly on the potential, solid-state diffusion and, finally, accumulation of the intercalants in their sites in the bulk of the active mass, which appears as a strongly potential-dependent, low-frequency capacitive element. It is demonstrated that the above electroanalytical response, which can be considered as the electrochemical fingerprint of these electrodes, may serve as a good in situ tool for the study of capacity fading mechanisms.

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.

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.

The mechanism of lithium intercalation in graphite film electrodes in aprotic media. Part 2. Potentiostatic intermittent titration and in situ XRD studies of the solid-state ionic diffusion

The potentiostatic intermittent titration technique (PITT) and in situ XRD have been used to measure the dependence of the diffusion coefficient Do of lithium ions in graphite on the intercalation level X. Thin graphite electrodes provide an excellent opportunity to obtain highly-resolved data (with respect to X). A non-monotonic peak-shape dependence of log Do vs. X has been observed, which correlates well with the corresponding dependence of the XRD peak intensity and the cyclic voltammetric peaks. The results thus obtained have been explained tentatively within a framework similar to the Daumas and Herold cluster model for the staged phase transition. Problems connected with the determination of the true values of D0 for porous electrodes are also discussed.

Capacity fading of LixMn2O4 spinel electrodes studied by XRD and electroanalytical techniques

Abstract Li x Mn 2 O 4 spinels were synthesized in different ways, leading to different particle morphologies and different electrochemical behavior. Two types of Li x Mn 2 O 4 electrodes comprised of active mass synthesized in two different ways were investigated in a standard solution (ethylene carbonate–dimethyl carbonate 1:3/LiAsF 6 1 M) using X-ray diffraction technique (XRD) in conjunction with a variety of electroanalytical techniques. These included slow scan rate cyclic voltammetry, chronopotentiometry, impedance spectroscopy and potentiostatic intermittent titration. We discovered two types of capacity fading mechanisms. One involves the formation of a new, less symmetric and more disordered phase (compared with the pristine Li x Mn 2 O 4 materials) during the first Li deinsertion reaction of a pristine electrode in the 3.5–4.2 V (Li/Li + ) potential range. This new phase, although inactive, has no detrimental effect on the kinetics of the remaining active mass. Another capacity fading mechanism occurs at >4.4 V (Li/Li + ) potential and involves dissolution of Mn into the solution, and a pronounced increase in the electrodes impedance. It appears that dissolution of Mn at elevated potentials is connected with degradation of the solution, which also occurs at these potentials at low rates.

Comparison Between the Electrochemical Behavior of Disordered Carbons and Graphite Electrodes in Connection with Their Structure

This work relates to a rigorous study of the surface chemistry (Fourier transform infrared, X-ray photoelectron spectroscopy), crystal structure (X-ray diffraction), galvanostatic, cyclic voltammetric, and impedance behavior of lithiated carbon electrodes in commonly used liquid electrolyte solutions. Two different types of disordered carbons and graphite as a reference system, were explored in a single study. All three types of carbons develop a similar surface chemistry in alkyl carbonate solutions, which are dominated by reduction of solvent molecules and anions from the electrolyte. The differences in the crystal structure of these carbons lead to pronounced differences in the mechanisms of Li insertion into them Whereas Li-ion intercalation into graphite is a staged process, Li-ion insertion into the disordered carbons occurs in the form of adsorption on both sides of the elementary graphene flakes and on their edges. The electroanalytical behavior of the disordered carbons was found to correlate well with their unique structure described in terms of the butterfly model. Both types of the disordered carbons reveal exceptionally good cyclability in coin-type cells (vs Li counter electrodes), with only moderate capacity fading. Highly resolved plots of the chemical diffusion coefficient of Li-ions. D vs. potential E, for the disordered carbon electrodes were obtained. Surprisingly, a maximum in D appears on these plots at intermediate levels of Li-ion insertion corresponding to ca. 0.4-0.5 V (vs. Li/Li + ). We propose that these maxima may originate from a combination of two effects, (i) repulsive interactions between the inserted species, and (ii) pronounced heterogeneity of Li insertion sites in terms of carbon-Li interactions and Li-ion mobility.

The study of capacity fading processes of Li-ion batteries: major factors that play a role

In this work, we studied the impact of some factors on the behavior of practical electrodes of Li-ion batteries. These included elevated temperatures (45–80 8C), prolonged storage of Li-ion cells, and additives in the electrolyte solution. The Li-ion battery systems studied included negative electrodes (anodes) comprising of mesocarbon microbeads (MCMB) and mesocarbon fibers (MCF), and LixCoO2 positive electrodes (cathodes) in an ethylene carbonate (EC)/ethyl-methyl carbonate (EMC) (1:2)/LiPF6 1 M solution. Vinylene carbonate (VC) and a Li-organo-borate complex (Li-OBC) were tested as additives. It is shown that the electrochemical response of Li–C negative electrodes depends on the structure of the surface films controlling their behavior, which change upon storage, temperature, and cycling. We established that impedance of these electrodes increased with storage time due to the enrichment of the surface films by LiF and other fluorine-containing species. The capacity fading of the LixCoO2 electrodes in cycling/storage processes at elevated temperatures relates mostly to surface phenomena, whereas the bulk structural characteristics of the electrodes do not change. # 2003 Elsevier Science B.V. All rights reserved.

Development and testing of nanomaterials for rechargeable lithium batteries

Abstract The use of nanoparticles in composite electrodes for Li batteries may have considerable kinetic advantages due to the reduction of the diffusion length for lithium insertion in the active mass, and also because of the reduction of the overall charge transfer resistance of the electrodes. We report herein on the synthesis of various types of nanomaterials for rechargeable lithium batteries and their testing as active mass in anodes and cathodes. These include SnO, VO x , Li x MnO 2 , and various types of carbon nanotubes. Sonochemistry was applied for the synthesis of part of the nanophases. The tools for this study included X-ray diffraction (XRD), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and standard electrochemical techniques (CV, SSCV, chronopotentiometry and impedance spectroscopy).