G. Salitra

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.

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.

Application of a quartz-crystal microbalance to measure ionic fluxes in microporous carbons for energy storage

Fast ionic transport in microporous activated-carbon electrodes is a prerequisite for the effective energy storage in electrochemical supercapacitors. However, the quartz-crystal microbalance (QCM), a direct tool to measure ionic fluxes in electrochemical systems, has not yet been used for studying transport phenomena in activated carbons (except for an early report on carbon nanotubes). Conventional electroanalytical and suitable surface and structure-analysis techniques provide limited prognostic information on this matter. It has been demonstrated herein that the QCM response of typical microporous activated carbons can serve as a gravimetric probe of the concentration and compositional changes in their pore volume. This allowed direct monitoring of the ionic fluxes, which depended strongly on the electrodes point of zero change, pore width, ion size and cycling conditions (polarization amplitude, charge/discharge depth and so on). The information on the nature of ionic fluxes into activated carbons is critical for promoting improvements in the performance of electrochemical supercapacitors, membrane technologies and (electro/bio)chemical sensors.

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.

Attempts to Improve the Behavior of Li Electrodes in Rechargeable Lithium Batteries

In this work we studied properties of modified lithium electrodes in an attempt to improve the high rate performance of rechargeable Li (metal) batteries containing liquid electrolyte solutions. Li (metal)-Li 0.3 MnO 2 AA batteries with solutions containing 1,3-dioxolane (DN), LiAsF 6 , and a basic stabilizer became commercial several years ago but failed to compete with Li-ion battery technology because of a very limited cycle life at high charging rates. The problem relates to intensive reactions between Li deposited at high rates and the electrolyte solutions, which dry the batteries. The lithium-solution reactivity was modified through several approaches. Li anodes doped by Li 3 N, Al, and Mg were tested, as well as solutions containing derivatives of DN that are expected to be less reactive toward lithium than DN. It was concluded that reduction of the Li anode-solution reactivity by these approaches cannot solve the problem, because it is impossible to modify the rough morphology, high surface of lithium electrodes when charging (Li deposition) rates are high (>1 mA/cm 2 ). Since there is no hermetic passivation of any Li surface in liquid electrolyte solutions, the high-surface-area Li deposits react with solution components. Therefore, upon charge-discharge cycling of practical Li (metal) batteries, the electrolyte solution is consumed in these reactions. Hence, the future of Li (metal) rechargeable batteries lies either in the use of solid electrolyte matrices instead of the liquid solutions, or in applications where low charging rates are tolerable.

Basic electroanalytical characterization of lithium insertion into thin, well-crystallized V2O5 films

Slow-scan rate cyclic voltammetry (SSCV), potentiostatic intermittent titration (PITT) and electrochemical impedance spectroscopy (EIS) have been applied simultaneously to study Li ion intercalation into V2O5 films prepared by evaporative vacuum-deposition on Pt foils. Two different films, 1600 and 3600 A thick, were used to study the influence of the films thickness on the major electroanalytical characteristics of these intercalation electrodes. Modeling of the impedance spectrum related to the thin V2O5 film was performed using an equivalent circuit analog including the following elements: three R ∣∣ C semicircles (covering the high-frequency domain) and finite-length Warburg in sequence with the intercalation capacity (a straight line of unit slope at intermediate frequencies, and a sloping capacitive line at the very low frequencies). Sharp minima on D versus E plots, which are observed in the vicinity of the cyclic voltammetric peaks, present further evidence of very high, attractive electron–ion interactions during Li ion intercalation into the V2O5 electrode, as was already described for similar processes in graphite and some transition metal oxides: LixCoO2, LixNiO2, LixCoyNi1−yO2 and LixMn2O4. The diffusion length in these electrodes related to the V2O5 films thickness.

Behavior of Graphite Electrodes in Solutions Based on Ionic Liquids in In Situ Raman Studies

In this work, the behavior of composite graphite electrodes comprising synthetic graphite flakes in solutions based on a 1-methyl-1-propylpiperidinium [bis(trifluoromethylsulfonyl)] imide (MPP p TFSI) ionic liquid (IL) was investigated, using in situ Raman spectroscopy with microscopic lateral resolution, in conjunction with cyclic voltammetry. Both pure IL and IL solutions containing a LiN(SO 2 CF 3 ) 2 (LiTFSI) salt were studied. Upon cathodic polarization, the IL cations (MPP + p are intercalated. This process is irreversible in a pure IL solution. When the solution comprises both IL and a Li salt (LiTFSI), the graphite electrodes can intercalate simultaneously the IL cations MPP + p and the Li cations at potentials ∼0.5 V and below 0.3 V vs Li/Li + , respectively. The graphite electrodes become passivated due to the presence of the Li salt by the formation of surface films, which are Li-ion conducting, but electronically insulating. Hence, upon consecutive voltammetric cycling, the IL cation-intercalation is suppressed, while reversible Li intercalation becomes the dominant process. Raman spectroscopy enables one to distinguish among the various processes in these systems.

In Situ Raman Spectroscopy Study of Different Kinds of Graphite Electrodes in Ionic Liquid Electrolytes

In this paper, the study of three types of graphite electrodes in two types of ionic liquid solutions (ILs) using in situ Raman spectroscopy and X-ray diffraction in conjunction with electrochemical techniques such as voltammetry is described. The graphite materials included two types of synthetic flakes, differing from each other in their average particle size, and natural graphite flakes. The ILs included 1-methyl-1-propylpiperidinium bis(trifluoromethyl sulfonyl)imide (MPPp-TFSI) and 1-methyl-1-butyl pyrrolidinium bis(trifluoromethyl sulfonyl)imide (BMP-TFSI). The Li salt was Li TFSI. The graphite electrodes can intercalate with both Li ions and IL cations simultaneously. The latter intercalate with graphite at higher potentials (the onset potential is >0.7 V). The graphite electrodes develop passivation in the above Li TFSI/Li solutions upon their cathodic polarization, which blocks the intercalation of the IL cations but allows highly reversible intercalation with lithium. In situ Raman spectroscopy proved to be a very useful tool for studying both Li and IL cation intercalation processes with graphite electrodes and for determining their onset and reversibility. The effectiveness of the passivation of graphite electrodes in these solutions depends on both the type of graphite used and the structure of the IL cations. The most effective passivation, developed during a first cathodic polarization of the electrodes, was found for natural graphite electrodes and for MPPp + -based solutions. The important factors that may determine the performance of graphite electrodes in these systems are discussed.

A new approach for the preparation of anodes for Li-ion batteries based on activated hard carbon cloth with pore design

We demonstrate herein the possibility to prepare carbon anodes for Li-ion batteries using simple carbonized polymeric precursors such as cotton and phenolic cloths. Activation by controlled oxidation forms highly porous carbons whose electrochemical activity in Li salt solutions is mostly an irreversible reduction of solution species and double layer charging. Treating these porous carbons by chemical vapor deposition (CVD) of carbon on their surfaces, closes the pores in a way that they can insert Li-ions, but not solution species. These general carbon engineering processes form new carbons with nanoscopic, selectively closed pores, which can serve as highly reversible anode materials for Li-ion batteries, with relatively low irreversible capacity. The capacity of these electrodes depends on the nature of the carbon CVD process. This paper describes the scheme for carbon engineering, gas adsorption measurements that demonstrate the impact of the carbon CVD process, and the relevant changes in the structure of the pores and some preliminary electrochemical measurements in non-aqueous Li salt solutions.

The effect of a solid electrolyte interphase on the mechanism of operation of lithium–sulfur batteries

Composite sulfur–carbon electrodes were prepared by encapsulating sulfur into the micropores of highly disordered microporous carbon with micrometer-sized particles. The galvanostatic cycling performance of the obtained electrodes was studied in 0.5 M Li bis(fluorosulfonyl)imide (FSI) in methylpropyl pyrrolidinium (MPP) FSI ionic-liquid (IL) electrolyte solution. We demonstrated that the performance of Li–S cells is governed by the formation of a solid electrolyte interphase (SEI) during the initial discharge at potentials lower than 1.5 V vs. Li/Li+. Subsequent galvanostatic cycling is characterized by a one plateau voltage profile specific to the quasi-solid-state reaction of Li ions with sulfur encapsulated in the micropores under solvent deficient conditions. The stability of the SEI thus formed is critically important for the effective desolvation of Li ions participating in quasi-solid-state reactions. We proved that realization of the quasi-solid-state mechanism is controlled not by the porous structure of the carbon host but rather by the nature of the electrolyte solution composition and the discharge cut off voltage value. The cycling behavior of these cathodes is highly dependent on sulfur loading. The best performance at 30 °C can be achieved with electrodes in which the sulfur loading was 60% by weight, when sulfur filled micropores are not accessible for N2 molecules according to gas adsorption isotherm data. A limited contact of the confined sulfur with the electrolyte solution results in the highest reversible capacity and initial coulombic efficiency. This insight into the mechanism provides a new approach to the development of new electrolyte solutions and additives for Li–S cells.