Electrochemical Behavior Of Graphite Research Articles

Kinetically Determined Phase Transition from Stage II (LiC12) to Stage I (LiC6) in a Graphite Anode for Li-Ion Batteries.

The electrochemical insertion of Li into graphite initiates a series of thermodynamic and kinetic processes. An in-depth understanding of this phenomenon will deepen the knowledge of electrode material design and optimize rechargeable Li batteries. In this context, the phase transition from dense stage II (LiC12) to stage I (LiC6) was comprehensively elucidated in a graphite anode via both experimental characterizations and first-principles calculations. The results indicate that, although the transition from stage II to stage I is thermodynamically allowed, the process is kinetically prohibited because Li ions tend to cluster into stage compounds rather than form a solid solution. Additionally, the phase transitions involve at least three intermediate structures (1T, 2H, and 3R) before reaching the LiC6 (stage I) phase. These findings provide new insights into the electrochemical behavior of graphite and the electrode process kinetics for rechargeable Li batteries.

A Study of Graphite as Anode in the Electro-Deoxidation of Solid UO2 in LiCl-Li2O Melt

Electrochemical behavior of graphite in LiCl and LiCl melts containing up to 1 wt% Li2O was studied by using cyclic voltammetry and electrolysis, in the context of its probable use as anode in the electrochemical reduction of solid UO2 in the molten salt at 650°C. The lithium deposition and chlorine evolution potentials of LiCl melt were determined by CV of tantalum (cathodic polarization) and graphite (anodic polarization) working electrodes as −2.19 V and +1.21 V vs Ni/NiO reference electrode respectively. In LiCl-Li2O melts, carbon dioxide was formed on the graphite WE from > +0.75 V onwards and its current and potential range of formation increased with increase in the Li2O content of the melt. Cathodic polarization of a UO2 disc WE in LiCl-1 wt% Li2O melt showed that the reduction of UO2 to U occurs at −2.18 V and the lithium metal deposition on it at −2.36 V. Electro-reduction experiments carried out with a dense UO2 pellet cathode and graphite anode in both LiCl and LiCl-Li2O melts proved that the surface of the pellet was reduced to U metal at ≥ −2.2 V. The reduction became increasingly difficult with increase in the Li2O content of the melt.

Influence of Fe (II) Species in Electrolyte on Performance of Graphite Anode for Lithium-Ion Batteries

Iron dissolution into an electrolyte from the olivine LiFePO4 in the lithium-ion cell has been recently investigated. In order to study the influence of the dissolved Fe2+ species in electrolyte on the lithium intercalation/deintercalation into an artificial graphite electrode, the electrochemical behavior of graphite was investigated in 1 mol L−1 LiPF6 electrolyte solution containing Fe2+ species. The cells containing Fe2+ species showed larger reversible capacity loss than the Fe-free cells. Electrochemical impedance spectroscopy and ex-situ techniques, such as scanning electron microscopy (SEM), Raman spectroscopy, Fourier Transform Infrared Spectrometer (FTIR) were employed to investigate the aging mechanism. The Fe2+ species were reduced on the graphite anode and formed iron deposits. The iron deposits affected the performance of graphite on two approaches. On the one hand, the deposits hindered intercalation of Li ions into graphite. On the other hand, the deposits accelerated decomposition of electrolyte and aggravated surface structural disorder in graphite, resulting in thicker solid-electrolyte interface SEI layer. Both effects caused capacity fading upon cycling.

Electrochemical Behavior of Graphite in KF – AlF3-Based Melt with Low Cryolite Ratio

The electrochemical behavior of graphite in -based melt with a cryolite ratio (molar ratio of KF to ) of 1.3 as well as a low oxygen content at was investigated by cyclic voltammetry, potentiostatic electrolysis, sampled-current voltammetry, and chronopotentiometry. Aluminum deposited on graphite at a potential of ca. 0.25 V (vs Al) and the electrochemical intercalation of potassium into graphite occurred at ca. −0.1 V (vs Al). The intercalation of potassium into graphite enhanced the formation of . The deposition of aluminum on graphite was a linear diffusion-controlled process, while its reverse reaction involved kinetic limitations. Sampled-current voltammetry results indicated that the aluminum deposition on graphite was depressed in the potential region extending from about 0.1 to 0.03 V (vs Al). Chronopotentiometry results revealed that the critical cathodic current density for potassium intercalation was about .

Graphite intercalation in the graphite-H2SO4-R (R = H2O, C2H5OH, C2H5COOH) systems

The electrochemical behavior of graphite in polar solvent-H2SO4 electrolytes is studied in a wide range of H2SO4 concentrations. The results demonstrate that, with decreasing H2SO4 concentration, the charging curves become smoother and shift to higher potentials, the stage index increases, and intercalation compounds are more difficult to obtain. At H2SO4 concentrations of 50% and lower, graphite polarization is accompanied by a significant overoxidation, as evidenced by the anomalously small intercalate layer thicknesses: 7.75–7.85 A. Anodic polarization of graphite in electrolytes consisting of H2SO4 and a polar solvent (H2O and C2H5OH) follows the same mechanism as in the case of the formation of graphite bisulfate. In going from water to C2H5OH, a less polar solvent, the intercalation threshold increases from 30 to 70% H2SO4. It is shown using a set of characterization techniques that, in the graphite-H2SO4-R (R = H2O, C2H5OH) systems, the solvent is not intercalated into graphite. Stage I–III ternary graphite intercalation compounds (TGICs) are synthesized for the first time in the graphite-H2SO4-C2H5COOH system: stage I TGICs at H2SO4 concentrations above 70%, stage II in the range 30–70% H2SO4, and stage III at H2SO4 concentrations down to 10%. The intercalate layer thickness in the TGICs is 7.94 A. The mechanism of TGIC formation in this system is shown to differ from those in mixtures of H2SO4 and other organic acids. Thermal analysis in combination with spectroscopic analysis of gaseous products provides clear evidence for intercalation of propionic acid into the TGIC and indicates that the thermal stability of this compound is lower than that of graphite bisulfate.

Electrochemical behavior of graphite in nonaqueous FeCl3 solutions

The electrochemical synthesis of ternary graphite intercalation compounds (GICs) with ferric chloride and an organic solvent is described. Stage IV and V ternary GICs with identity periods I c = 19.19 A (d i = 9.14 A) and 22.70 A (d i = 9.30 A), respectively, are obtained in solutions of FeCl3 in acetyl chloride. In the graphite-FeCl3-CH3COOH-HCl system, a ternary GIC with a transition-metal chloride (FeCl3) and weak aprotic acid (CH3COOH) (stage III, I c = 16.15 A, d i = 9.45 A) is synthesized for the first time. Using radiotracer analysis, the solvent molecules are shown to be evenly distributed over the graphite host. Thermal analysis results demonstrate that GICs with FeCl3 and ternary GICs with FeCl3 and acetic acid differ markedly in thermal properties. The synthesized GICs offer record large expansion on flash heating. The resulting exfoliated graphite contains residual Fe2O3.

Influence of manganese(II), cobalt(II), and nickel(II) additives in electrolyte on performance of graphite anode for lithium-ion batteries

Manganese dissolution into an electrolyte from the spinel LiMn 2O 4 in the lithium-ion cell has been recently investigated. In order to study the influence of the dissolved manganese species on the lithium intercalation/deintercalation into a natural graphite electrode, the electrochemical behavior of graphite was investigated in 1 mol dm −3 LiClO 4 electrolyte solution containing a small amount of Mn(II) by the addition of manganese(II) perchlorate. During the charging process, Mn(II) ions were firstly electroreduced on the electrode around 1.0 V versus Li/Li + followed by irreversible decomposition of the electrolyte and lithium intercalation into the graphite. By microscopic observation of the graphite surface, manganese deposition was confirmed after the charge/discharge test. Due to the manganese deposition, the reversible capacity of the graphite electrode was drastically decreased. Furthermore, the cyclability of the anode was degraded with the amount of the manganese additive increasing. We compared these results with those of the cobalt(II) and nickel(II) additives by dissolving the corresponding perchlorates. Furthermore, we discussed the influence in practical cells based on the consideration of electrochemistry of the deposited metals.

Electrochemical behavior of graphite in electrolyte of sulfuric and acetic acid

Electrochemical formation of graphite intercalation compounds (GICs) has been studied in the electrolyte with mixed sulfuric acid and acetic acid. The results show that, with addition of acetic acid, GICs can be synthesized in the electrolyte with sulfuric acid concentration as low as 3.6M, even though no GIC is formed in pure acetic acid. The stage structure of the synthesized GICs varies with the concentration of sulfuric acid, and stage 3 can be obtained in the electrolyte with 3.6M of sulfuric acid. The onset potential and current efficiency for intercalation changes linearly from 0.54 to 1.10 V (versus Hg/Hg 2SO 4) and from 0.90 to 0.68, respectively, with acetic acid addition. Two paths of graphite oxidation in an electrochemical reaction have been observed. Commonly the graphite oxide (GO) is formed via a stage 1 GIC, however, GO can be formed via a stage 2 GIC directly at elevated temperature when acetic acid is added in the electrolyte. The reduction of sulfur content in the GIC and efficiency of GIC synthesis might be balanced with an appropriate combination of sulfuric acid and acetic acid.

ChemInform Abstract: The Electrochemical Behavior of Graphite and Ni‐Cr Electrodes in Sodium Polysulfide in the Absence and Presence of Hydrogen Sulfide

Abstractis studied by voltammetric, chronoamperometric, and chronopotentiometric techniques in order to investigate the suitability of the sodium polysulfide melt as electrolyte for the decomposition of H2S. During cathodic polarization in sodium tetrasulfide the formation of two continuous phases on the electrodes, apparently Na2S2 and Na2S, is observed at ‐0.28 and ‐0.4 V vs.

Electrochemical Behavior of Graphite and Ni‐Cr Electrodes in Sodium Polysulfide in the Absence and Presence of Hydrogen Sulfide

The electrochemical behavior of graphite and Ni‐Cr electrodes in sodium tetrasulfide melt has been investigated using voltammetric, chronoamperometric, and chronopotentiometric techniques in the presence and absence of hydrogen sulfide. Two continuous phases, namely, and , are apparently formed at different potentials during the cathodic polarization. The blocking effect by these layers was much less significant on the Ni‐Cr electrode than on graphite. The presence of did not appear to influence the electrode reactions, but it significantly reduced the rate of formation of the continuous phases ( or ) during cathodic polarization. A small amount of hydrogen gas was formed, possibly by chemical reactions between polysulfides and hydrogen sulfide.