Ethylene Carbonate-based Solutions Research Articles

Investigation of the electrochemical properties of a propylene carbonate-derived SEI in an ethylene carbonate-based solution

Investigation of the electrochemical properties of a propylene carbonate-derived SEI in an ethylene carbonate-based solution

Interfacial Reactions Between Graphite and Diethyl Carbonate Solutions Containing Different Concentrations of Li.

The electrochemical reactions occurring at graphite electrodes in lithium-ion batteries using diethyl carbonate (DEC) electrolytes containing different concentrations of LiClO4 (0.85 and 2.82 mol kg-1) were examined. The poor charge-discharge performance of the electrode in the 0.85 mol kg-1 LiClO4 in DEC was improved considerably when employing 2.82 mol kg-1 LiClO4 in DEC as the electrolyte. In situ electrochemical atomic force microscopy analysis revealed that an effective film had formed on the graphite surface after the first potential cycle when using the more concentrated solution. Charge-discharge analysis revealed that the nature of the film was significantly different from that formed in ethylene carbonate-based solutions.

Effects of Electrolyte Concentration on Surface Film Formation on Graphite in Ethylene Carbonate-Based Solutions.

The physicochemical properties of a surface film generated on a graphite surface were greatly dependent on the concentration of the electrolyte solution. Cyclic voltammetry results obtained using four ethylene carbonate-based solutions of different electrolyte (i.e., LiN(SO2C2F5)2) concentrations showed that the amount of irreversible current used to generate the surface film between 0.8 and 0.1 V decreased upon increasing the electrolyte concentration. Transmission electron microscopy results revealed that the thickness and morphology of the surface film were greatly affected by the concentration of the electrolyte solution. In addition, electrochemical impedance spectroscopy revealed that the resistance of the surface film was also affected by the electrolyte concentration. These results therefore indicated that both the chemical and physical properties of the surface film were affected by the concentration of the electrolyte solution.

Effects of Lithium Salt on Interfacial Reactions between SiC and EC-Based Solutions in Lithium Secondary Batteries

Electrochemical reactions occurring at a SiC electrode were investigated to gain insight into the effects of lithium salts, such as LiPF6, LiClO4, LiCF3SO3, and LiBF4, on the interfacial resistance. Lithium salts were found to exert little effect on the magnitude of the resistance of the solid-electrolyte interphase (SEI). In contrast, the charge-transfer reactions observed in the LiCF3SO3-containing solution exhibited the smallest resistance. Charge-discharge analysis revealed that the nature of the SEI was significantly different from that formed in ethylene carbonate-based solutions containing different lithium salts. The SiC electrode exhibited large discharge capacity and low coulombic efficiency in the LiCF3SO3-containing solution. This might be closely related to the smallest charge-transfer resistance.

Effects of Dimethoxyethane as a Co-Solvent on Electrochemical Formation of a Surface Film on Graphite in an Ethylene Carbonate-Based Solution

The electrochemical processes occurring at the surface of a highly ordered pyrolytic graphite electrode were investigated, to understand the effects of dimethoxyethane as a co-solvent on the formation of a surface film. In-situ electrochemical atomic force microscopy revealed that a thin film of ~5 nm thickness was formed on the graphite surface after the first potential cycling. There was no evidence of co-intercalation of the solvent molecules. The cyclic voltammetry analysis revealed that a irreversible reduction peak closely related to the film formation was present at ~1.7 V vs. Li+/Li.

Thermal and oxidation stability of organo-fluorine compound-mixed electrolyte solutions for lithium ion batteries

Thermal and oxidation stability of fluorine compound-mixed electrolyte solutions have been investigated. Charge/discharge behavior of natural graphite electrode has been also examined in the same electrolyte solutions. Fluorine compounds demonstrate much lower reactivity with metallic Li than ethylene carbonate/dimethyl carbonate. Fluorine compound-mixed electrolyte solutions show the lower reactivity with LiC6 and the smaller exothermic peaks due to decomposition of electrolyte solutions and surface films than original solutions without fluorine compound. Oxidation currents are also smaller in fluorine compound-mixed electrolyte solutions than in original ones. First coulombic efficiencies in fluorine compound-mixed electrolyte solutions are similar to those in original ethylene carbonate-based solutions except one case. Mixing of fluorine compounds highly increase first coulombic efficiencies of natural graphite electrode in propylene carbonate-containing solution.

TiO 2(B) as a promising high potential negative electrode for large-size lithium-ion batteries

Needle-like TiO 2(B) powder was obtained from K 2Ti 4O 9 precursor by ion exchange to protons, followed by dehydration. The charge and discharge characteristics of the TiO 2(B) powder were investigated as a high potential negative electrode in lithium-ion batteries. It had a high discharge capacity of 200–250 mAh g −1 at around 1.6 V vs. Li/Li +, which was comparable with that of TiO 2(B) nanowires and nanotubes prepared via a hydrothermal reaction in alkaline solution. It showed very good cycleability, and gave a discharge capacity of 170 mAh g −1 even in the 650th cycle. It also had a high rate capability, and gave a discharge capacity of 106 mAh g −1 even at 10 °C. In most of ethylene carbonate-based solutions, the TiO 2(B) powder exhibited good charge and discharge characteristics. However, it showed a poor compatibility with LiBF 4, propylene carbonate, and γ-butyrolactone. The TiO 2(B) powder showed good cycle performance in the presence of a non-flammable additive, trimethyl phosphate, up to 20 vol.%, and a high tolerance to water up to 1000 ppm. It was also found that inexpensive Al foil can be used as a current collector of the TiO 2(B) powder instead of Cu foil without sacrificing the performance.

Morphology/Behavior Relationship in Reversible Electrochemical Lithium Insertion into Graphitic Materials

Different types of graphite particles, including synthetic flakes, natural graphite flakes, graphite fibers, round-shaped graphite particles (mesocarbon microbeads), and combinations of graphite flakes and disordered carbon particles were studied as electrode materials in lithium insertion/deinsertion processes. We suggest that the morphology of the graphite particles plays an important role in their stability. One of the key factors is the structure of the facets, perpendicular to the basal planes, through which Li insertion takes place. The smoother these facets are, and which contain fewer crevices, the more stable are the graphite particles in Li insertion reactions. This is attributed to the fact that the surface reactions of the cathodically polarized graphite electrodes in the nonaqueous Li salt solutions involve gas formation, which may have a detrimental effect on the integrity of the graphite particles. Another important factor is the existence of some degree of disorder and distortion in the graphite structure. More distortion in the ordered graphite structure may lead to higher stability. A clue to the understanding of the factors that determine stability or destruction of lithiated graphite electrodes is the pronounced detrimental effect of the presence of propylene carbonate in ethylene carbonate-based solutions, as discussed herein. Another important factor dealt with in this paper is the correlation between the morphology of the graphite particles and their orientation in practical electrodes. The major tools for this study included chronopotentiometry, electron microscopy, and X-ray diffraction. © 2002 The Electrochemical Society. All rights reserved.

Surface film formation on a graphite negative electrode in lithium-ion batteries: AFM study on the effects of co-solvents in ethylene carbonate-based solutions

In situ AFM observation of the basal plane of highly oriented pyrolytic graphite (HOPG) was performed before and after cyclic voltammetry in 1 mol dm −3 LiClO 4 dissolved in ethylene carbonate (EC), EC+diethyl carbonate (DEC), and EC+dimethyl carbonate (DMC) to clarify the effects of co-solvents in EC-based solutions on surface film formation on graphite negative electrodes in lithium-ion cells. In each solution, surface film formation involved the following two different processes: (i) intercalation of solvated lithium ions and their decomposition beneath the surface; and (ii) direct decomposition of solvent molecules on the basal plane to form a precipitate layer. The most remarkable difference among these solvent systems was that solvent co-intercalation took place more extensively in EC+DEC than in EC+DMC or EC. Raman analysis of ion–solvent interactions revealed that a lithium ion is solvated by three EC molecules and one DEC molecule in EC+DEC, whereas it is solvated exclusively by EC in EC+DMC and in EC, which suggested that the presence of linear alkyl carbonates in the solvation shell of lithium ion enhance the degree of solvent co-intercalation that occurs in the initial stage of the surface film formation.

Surface Film Formation on Graphite Negative Electrode in Lithium-Ion Batteries: AFM Study in an Ethylene Carbonate-Based Solution

In situ atomic force microscopic (AFM) observation of the basal plane of highly oriented pyrolytic graphite was performed during cyclic voltammetry at a slow scan rate of 0.5 mV in 1 mol dissolved in a mixture of ethylene carbonate and diethyl carbonate. In the potential range 1.0-0.8 V, atomically flat areas of 1 or 2 nm height (hill-like structures) and large swellings of 15-20 nm height (blisters) appeared on the surface. These two features were formed by the intercalation of solvated lithium ions and their decomposition beneath the surface, respectively, and may have a role in suppressing further solvent cointercalation. At potentials more negative than 0.65 V, particle-like precipitates appeared on the basal plane surface. After the first cycle, the thickness of the precipitate layer was 40 nm, and increased to 70 nm after the second cycle. The precipitates were considered to be mainly organic compounds that are formed by the decomposition of solvent molecules, and they have an important role in suppressing further solvent decomposition on the basal plane. © 2001 The Electrochemical Society. All rights reserved.

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 different 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.

STM study on graphite/electrolyte interface in lithium-ion batteries: solid electrolyte interface formation in trifluoropropylene carbonate solution

Lithium intercalation within graphite was studied in an electrolyte system, 1 M LiClO 4 dissolved in trifluoropropylene carbonate (TFPC). Lithium was intercalated within graphite in TFPC. The reversible capacity obtained (275 mAh g −1) was smaller than that in ethylene carbonate-based solutions while the irreversible capacity was larger (335 mAh g −1). The morphology change of the basal plane of highly oriented pyrolytic graphite (HOPG) was observed by electrochemical scanning tunneling microscopy (STM) to obtain information about passivating film (solid electrolyte interface, SEI) formation in this solvent system. The exfoliation of graphite layers was observed at 1.1 and 1.0 V vs. Li +/Li, and then swelling of graphite layers appeared along step edges at 0.5 V. The feature observed at 0.5 V was considered as SEI itself in this solvent system.