M. D. Levi

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.

Solid‐State Electrochemical Kinetics of Li‐Ion Intercalation into Li1 − x CoO2: Simultaneous Application of Electroanalytical Techniques SSCV, PITT, and EIS

The electroanalytical behavior of thin electrodes is elucidated by the simultaneous application of three electroanalytical techniques: slow‐scan‐rate cyclic voltammetry (SSCV), potentiostatic intermittent titration technique, and electrochemical impedance spectroscopy. The data were treated within the framework of a simple model expressed by a Frumkin‐type sorption isotherm. The experimental SSCV curves were well described by an equation combining such an isotherm with the Butler‐Volmer equation for slow interfacial Li‐ion transfer. The apparent attraction constant was −4.2, which is characteristic of a quasi‐equilibrium, first‐order phase transition. Impedance spectra reflected a process with the following steps: ion migration in solution, ion migration through surface films, strongly potential‐dependent charge‐transfer resistance, solid‐state diffusion, and accumulation of the intercalants into the host materials. An excellent fit was found between these spectra and an equivalent circuit, including a Voigt‐type analog ( migration through multilayer surface films and charge transfer) in series with a finite‐length Warburg‐type element ( solid‐state diffusion), and a capacitor (Li accumulation). In this paper, we compare the solid‐state diffusion time constants and the differential intercalation capacities obtained by the three electroanalytical techniques.

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 1. High resolution slow scan rate cyclic voltammetric studies and modeling

Using slow scan rate (4 to 80 μVs−1) cyclic voltammetry for thin graphite electrodes (8 to 10 μm thick), two limiting cases for the intercalation mechanism of Li ion in graphite in aprotic solvents have been observed: (i) quasi-equilibrium, capacitive-like step at very slow potential scan rates and (ii) semi-infinite diffusion of Li+ ions inside the graphite matrix at higher scan rates. Each of these two limiting types of behavior has been appropriately modeled, and from the comparison of experimental and simulated voltammetric curves quantitative information has been extracted, including (a) the effective heterogeneous rate constants for Li+ ion transfer through the graphite|solution interface; (b) the lateral attraction parameter for the intercalated species; (c) half-peak width and peak potential separation; and (d) diffusion coefficients of the intercalated ions. The features of the experimental CV curves are in qualitative agreement with the island model of the staging process proposed in the literature. The diffusion coefficients of Li+ ions in graphite evaluated from the voltammetric data were found to be close to those obtained from a potentiostatic intermittent titration technique applied to the same electrodes.

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.

Attractive forces in microporous carbon electrodes for capacitive deionization

The recently developed modified Donnan (mD) model provides a simple and useful description of the electrical double layer in microporous carbon electrodes, suitable for incorporation in porous electrode theory. By postulating an attractive excess chemical potential for each ion in the micropores that is inversely proportional to the total ion concentration, we show that experimental data for capacitive deionization (CDI) can be accurately predicted over a wide range of applied voltages and salt concentrations. Since the ion spacing and Bjerrum length are each comparable to the micropore size (few nanometers), we postulate that the attraction results from fluctuating bare Coulomb interactions between individual ions and the metallic pore surfaces (image forces) that are not captured by mean-field theories, such as the Poisson-Boltzmann-Stern model or its mathematical limit for overlapping double layers, the Donnan model. Using reasonable estimates of the micropore permittivity and mean size (and no other fitting parameters), we propose a simple theory that predicts the attractive chemical potential inferred from experiments. As additional evidence for attractive forces, we present data for salt adsorption in uncharged microporous carbons, also predicted by the theory.

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.

LiPF3 ( CF 2 CF 3 ) 3 : A Salt for Rechargeable Lithium Ion Batteries

LiPF 3 (CF 2 CF 3 ), from Merck KGaA (LiFAP) was tested as a new electrolyte for Li-ion batteries that can replace the commonly used LiPF 6 . The latter salt is known to be unstable, to decompose thermally to LiF and PF 5 , and to readily undergo hydrolysis with protic species to form HF contamination in solutions. The latter contamination may have a detrimental impact on the performance of both anodes and cathodes for Li-ion batteries. Solutions comprising LiFAP, LiPF 6 , and LiN(SO 2 CF 5 CF 3 ) 2 (LiBETI) in mixtures of ethylene, dimethyl, and diethyl carbonates were tested with composite graphite and LiMn 2 O 4 electrodes. The tools for this study included voltammetry (fast and slow scan rates), chronopotentiometry, impedance spectroscopy, Fourier transform infrared, and X-ray and photoelectron spectroscopies. It was found that LiFAP is superior to LiPF 6 as an electrolyte for both graphite anodes and LiMn 5 O 4 cathodes. This should be attributed to the different surface chemistry developed on these electrodes when LiPF 6 is replaced by LiFAP. An important impact of such a replacement is probably the absence of possible pronounced HF contamination in LiFAP solutions.