E. Markevich

Fluoroethylene carbonate as an important component in electrolyte solutions for high-voltage lithium batteries: role of surface chemistry on the cathode.

The effect of fluorinated ethylene carbonate (FEC) as a cosolvent in alkyl carbonates/LiPF6 on the cycling performance of high-voltage (5 V) cathodes for Li-ion batteries was investigated using electrochemical tools, X-ray photoelectron spectroscopy (XPS), and high-resolution scanning electron microscopy (HRSEM). An excellent cycling stability of LiCoPO4/Li, LiNi0.5Mn1.5O4/Si, and LiCoPO4/Si cells and a reasonable cycling of LiCoPO4/Si cells was achieved by replacing the commonly used cosolvent ethylene carbonate (EC) by FEC in electrolyte solutions for high-voltage Li-ion batteries. The roles of FEC in the improvement of the cycling performance of high-voltage Li-ion cells and of surface chemistry on the cathode are discussed.

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.

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.

The effect of dimethyl pyrocarbonate on electroanalytical behavior and cycling of graphite electrodes

The addition of small concentrations of dimethyl pyrocarbonate (DMPC), about 5% by volume, to a standard solution of 1 M LiPF 6 /ethylene carbonate + dimethyl carbonate (1:1) was shown to improve substantially the cycling behavior of graphite electrodes. A combination of fast and slow scan rate cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), Fourier transform infrared, 1 H, and 13 C nuclear magnetic resonance spectroscopy were applied to understand the nature of the stabilizing effect caused by the presence of DMPC. DMPC was shown to increase impregnation of the active mass of the porous electrodes with solution (thus increasing the specific electrode capacity), and facilitates the rate of Li-ion migration across the surface films around the graphite particles and across the surface film/particle interface. Based on a combination of fast- and slow-scan rate CV and EIS, a convenient procedure allowing for an intermittent monitoring of the kinetic and thermodynamic characteristics of the Li-insertion process into graphite in the course of long-term electrode cycling was elaborated. We show that a simple procedure consisting of the application of a series of consecutive fast- and slow-scan rate CVs can be conveniently used for a systematic search and optimization of electrolyte solutions suitable for long-term cycling of graphite electrodes in lithium-ion cells.

New Insight into Studies of the Cycling Performance of Li-Graphite Electrodes A Combination of Cyclic Voltammetry, Electrochemical Impedance, and Differential Self-Discharge Measurements

Based on a combination of cyclic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy techniques, a new method of self-discharge characterization of lithiated graphite electrodes as a function of cycle number has been elaborated. Different domains were identified in the plot of the self-discharge current, I s d vs. cycle number. During the beginning of cycling, the electrodes impedance and I s d , both decrease as cycling proceeds. This is due to reorganization and reformation of the surface films on the electrode surface. After several tens of consecutive cycles, I s d start to increase. In parallel, the impedance of the deintercalated graphite electrode increases as well. Both I s d and the electrodes impedance reach steady value upon prolonged cycling. This relates to the semiconducting properties of the surface films formed on the electrode surface which change upon cycling toward an increasingly better electronic conductivity of the surface films (their passivity decreases). This, in turn, creates favorable conditions for electroadsorption of Li ions on the surface films of the long-term cycled electrode reflected as an increase in R c t . A porous electrode model was adopted in order to qualitatively describe the evolution of the impedance spectra of the fully lithiated graphite electrodes with cycle number, at room temperature.

In Situ FTIR Spectroscopy Study of Li / LiNi0.8Co0.15Al0.05O2 Cells with Ionic Liquid-Based Electrolytes in Overcharge Condition

We developed a methodology of in situ Fourier transform infrared (FTIR) measurements of gaseous products formed in an electrochemical cell upon polarization. LiNi 0.8 Co 0.15 Al 0.05 O 2 (NCA) cathodes were explored at potentials of up to 5.5 V vs Li in the ionic liquid (IL)-based electrolyte solution, LiTFSI/N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)amide. The polarization of the NCA electrodes formed CO 2 and CO due to the liberation of oxygen and the parallel dissolution of nickel ions, which oxidizes the carbon black in the composite electrode. The oxygen was mostly liberated from the NCA and also due to minor contribution from the surface groups on the carbon black additive.

High performance of thick amorphous columnar monolithic film silicon anodes in ionic liquid electrolytes at elevated temperature

The cycling performance of thick (about 7 μm) amorphous columnar monolithic film silicon anodes was studied in ionic liquid based electrolyte solutions. Cycling results obtained for these Si anodes in 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide-based electrolyte solutions are superior to those demonstrated in LiPF6/fluoroethylene carbonate/dimethyl carbonate electrolyte solution under identical conditions.

Unusually high stability of a poly(alkylquaterthiophene-alt-oxadiazole) conjugated copolymer in its n and p-doped states

Incorporation of electron accepting units (oxadiazole) into the 2,5-thienylene conjugated chain leads to a significant improvement in the n-doping-undoping redox stability of the resulting polymer.

On Li-chelating additives to electrolytes for Li batteries

The relative affinity of several electrochemically stable bi- and polydentate organic ligands (containing ether, amino, carbonate or phosphonate moieties) to Li ion was estimated by ab initio calculations at the MP2/6-31G(d) level comparing their calculated binding energies (BEs). Polyether 12-crown-4 and a bisphosphonate, which is covalently locked in a cyclic “chelating” conformation, are stronger Li+ chelators than isosparteine and 1,2-methylenebisphosphonate (an open-chain compound). Organic biscarbonates are the weakest complexing agents among the ligands considered. Although BE values were calculated for the gas phase, the order of ligand complexation ability obtained is also considered relevant to solutions since the difference between the BEs for these three groups of ligands is significant. The same calculations that were performed for complexes of Li ion and different organic carbonate diesters (solvents usually used in Li-ion batteries) allowed singling out of conformationally locked bisphosphonates as new battery life-prolonging additives to carbonate electrolytes.