Graphite Electrode

The lithiated graphite electrode operates by a series of staging reactions, each of which constitutes a two-component, two-phase electrode that provides a constant potential a few tens of millivolts above that of metallic lithium.

From: Encyclopedia of Energy, 2004

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Coal use in iron and steel metallurgy

A. Babich, D. Senk, in The Coal Handbook: Towards Cleaner Production: Coal Utilisation, 2013

Graphite electrodes

Graphite electrodes serve to transfer the electrical energy from the power supply to the steel melt in the EAF bath. They are typically made using premium petroleum needle coke, coal tar pitch, and some additives (Fruehan, 1998). Specification of needle coke for the manufacture of large diameter graphite electrodes is shown in Table 12.12.

Table 12.12. Typical needle coke specification for graphite electrodes

Specification Units Amounts
Ash wt. −% 0.2
Apparent density g/cm3 1.6–1.72
Porosity % 22–28
Transverse strength N/mm2 9–13.8
Young Modulus × 103 N/mm2 6.21–9.66
Electrical resistance × 106 Ω.mm 45.7–83.8
Coefficient of thermal expansion (CTE) × 10 − 6/°C 2.16–3.24

Source: Parkash, 2010.

Electrode consumption varies between 1.8 and 9.9 kg/t of liquid steel (Parkash, 2010) depending on the process characteristics and electrode quality. Ameling et al. (2011) reported that the electrode consumption in Germany in 2010 was approximately 1.1 kg per ton as a result of the reduction of time between the taps to 40 min and consequently the lower electricity consumption (345 kWh/t). Electrodes are classified as regular grade or premium grade on the basis of their physical properties (International Iron and Steel Institute, 1983).

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MEASUREMENT METHODS | Structural Properties: Atomic Force Microscopy

R. Hiesgen, J. Haiber, in Encyclopedia of Electrochemical Power Sources, 2009

Examples

Composite graphite electrodes were studied by in situ AFM working in EC-dimethyl carbonate (EC-DMC) and EC-PC solutions of lithium bis(oxalato)borate (LiBOB), lithium hexafluorophosphate (LiPF6), and lithium perchlorate (LiClO4) in an attempt to follow any pronounced morphological changes in the graphite particles owing to the surface film formation in the course of the first cathodic polarization and during lithium insertion and desertion processes. Figure 22 gives AFM images obtained in situ from composite graphite electrodes at open-circuit voltage (∼3 V) and after polarization to 0.3 V (Li/Li+) in 1 mol L−1 LiBOB and 1 mol L−1 LiPF6 solutions. They show morphological changes owing to cathodic polarization of both electrodes connected with the formation of surface films. A rougher surface film was formed in the case of a LiPF6 solution.

Figure 22. Atomic force microscopy images (2 mm×2 mm) of composite graphite electrodes obtained in situ in 1 mol L−1 LiBOB and 1 mol L−1 LiPF6 solution in ethylene carbonate:propylene carbonate 2:3 at open-circuit potential and after polarization to 0.3 V (vs Li/Li+). Reproduced with permission from Larush-Asraf L, Biton M, Teller H, Zinigrad E, and Aurbach D (2007) On the electrochemical and thermal behavior of lithium bis(oxalato)borate (LiBOB) solutions. Journal of Power Sources 174(2): 400–407.

More examples of different studies for determination of surface morphology and on the development of novel measuring techniques are given in (Table 2).

Table 2. Examples of atomic force microscopy (AFM) studies on different components for Li-based batteries

Examples of studies using AFM at components for Li-based batteries
AFM has been performed
During the formation of a passivation film (solid electrolyte interphase, SEI) at the surface of the negative electrode of full LiCoO2/graphite lithium-ion cells using LiPF6 (1 M) in carbonate solvents as electrolyte, the formation of a passivation layer what appears to be crystallites at the lithium/poly(ethylene oxide)-lithium triflate electrolyte interface.
On a LiCoO2 thin film cathodes, prepared by RF magnetron sputtering and post-annealing, and a difference of micro-structural evolution after post-annealing was observed, which related to the thin film properties.
On amorphous lithium ion conductor films, prepared by pulsed laser deposition (PLD), for the determination of the nominal composition of 0.6(Li4SiO4-0.4(Li3VO4)) for a good amorphous thin film.
On spinel-based (LiMn2O4, Li4Ti5O12) foil electrodes with combined additive of graphite, carbon black and polyvinyl difluoride compacted by a magnetic pulsed laser.
Nanocrystalline lithium manganate thin films (LixMn2O4; x=1.0–1.4).
On Lithium iron phosphate (LiFePO4) thin film electrodes, prepared by pulsed laser deposition (PLD), the film annealed at 773 K (773 K-film) consisted of small grains with 50 nm in thickness, and the grain size increased with an increase of annealing temperature.
To measure the thickness of thin-film electrodes of LiNi0.8Co0.2O2, deposited by pulsed laser ablation.
Of the basal plane of highly oriented pyrolytic graphite (HOPG), before and after cyclic voltammetry in 1 mol dm−3 LiClO4 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.
Of the interfacial phenomena between graphite (mesocarbon-microbeads (MCMB)) electrode and organic electrolyte solution.
The nucleation and growth mechanism for the electropolymerization of aniline was investigated at higher potentials on highly oriented pyrolytic graphite.
Of performances and morphology of the lead foam.
To measure the step height change during the intercalation/de-intercalation of Li+ into the graphene layers of highly oriented pyrolytic graphite (HOPG).
Of the microstructure and morphology of Si thin films, deposited on stainless steel substrates that act as current collectors using the pulsed laser deposition (PLD) technique.
Of the growth of nano-size particles are deposited in LiPF6 solutions at the boundaries of the V2O5 grains, thus slowing the insertion of lithium ions into the layered matrix, while in the presence of the ClO4− anions, there were only negligible changes in the morphology, leading to intercalation of lithium ions only.
Of influence of substrate temperature on the growth of V2O5 films, prepared by the pulsed laser deposition technique, in order to understand the growth mechanism.
An in situ electrochemical atomic force microscopy (EC-AFM) cell was developed to study surface and dimensional changes of individual LixCoO2 crystals during lithium de-intercalation.
Discrete Li2CO3 particles having 50–250 nm in diameter and 5–15 nm in height were observed on the surface of stoichiometric LiCoO2 crystals and they were shown to gradually dissolve into the LiPF6-containing electrolyte. The dimensional change of individual LixCoO2 crystals along the chex axis was monitored in situ during lithium de-intercalation.
Development of novel techniques
Current-sensing atomic force microscopy was used for imaging the cathode surface which revealed that the cathode of a pouch-type lithium-ion cell, with graphite anode and LiNi0.8Co0.15Al0.05O2 cathode. The surface electronic conductance diminished significantly in the tested cells. Loss of contact of active material particles with the carbon matrix and thin film formation via electrolyte decomposition not only led to LiNi0.8Co0.15Al0.05O2 particle isolation and contributed to cathode interfacial charge-transfer impedance but also accounted for the observed cell power and capacity loss.
Surface morphology in 3.5×3.5 mm2 area of spinel LiMn2O4, which is a typical cathode material for Li ion secondary batteries, is studied using an AFM with a conductive probe. Negative bias voltage is applied to the probe to attract Li+ ions toward LiMn2O4 surface during the AFM observation. Before applying the voltage (0 V), the whole LiMn2O4 surface is covered with scale-shaped grains. Under the negative voltage of 5.5 V, electric current abruptly increases, indicating Li+ ionic conduction. Simultaneously, part of the scale-shaped grains expand and flatten.
A thin-film solid-state battery was prepared with a vanadium pentoxide cathode and a lithium phosphate electrolyte and studied in situ by ultrahigh vacuum scanning tunneling microscope/atomic force microscopy (STM/AFM). Orientation of the (001) plane of V2O5 parallel to the substrate was detected via observation of the periodicity of 11.7±0.5 Å, which is consistent with the unit cell spacing in the [100] direction. Conductance of the battery was studied locally with the probe tip of the STM/AFM in the regime of mechanical contact with a constant repulsive force. Lateral variation of contact conductance from 0.4 to 2.2 nA was detected as a function of position of the tip in contact with the cathode. The device revealed an extremely high current density of 1 A cm−1(2) due to the low thickness of the electrolyte and the cathode and the concentration of electric field under the scanning probe microscope tip. Transformation of cathode structure due to Li ion intercalation was observed in real time.
The volume changes of continuous and patterned films of crystalline Al, crystalline Sn, amorphous Si (a-Si), and a-Si0.64Sn0.36 as they reversibly react with Li. Although these materials all undergo large volume expansions, the amorphous phases undergo reversible shape and volume changes, the crystalline materials do not.
In situ atomic force microscopy measurements of patterned amorphous Sn–Co–C sputtered films reacting with Li in an electrochemical cell have been made. Prismatic-shaped patches of Sn0.34Co0.19C0.47 were found to undergo reversible volume expansion of 175±5% [(Vfinal−Vinitial)/Vinitial] without fracture.
AFM analyses prove that after storage at room temperature for a month, PEO–LiTFSI forms large dendrites while only a small amount of tiny crystals can be observed in the PEO–LiTFSI–ZnO(PEGME) film. In contrast, ZnO(Ac) particles agglomerate around the PEO–LiTFSI dendrites and separate from the original phase. This direct observation on the micromorphology of the SPE films after long-term storage elucidates why the PEO–LiTFSI–ZnO(PEGME) electrolyte is much more stable than its counterparts.
For thickness measurements without a reference in situ AFM roughness measurements have been performed on alloy film electrodes on rigid substrates as they react with lithium electrochemically. The addition (or removal) of lithium to (or from) the alloy causes the latter to expand (or contract) reversibly in the direction perpendicular to the substrate and, in principle, the change in the overall height of these materials is directly proportional to the change in roughness.
In situ 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 s−1 in 1 mol dm−3 LiClO4 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 co-intercalation. 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.
The surface morphology changes on polyaniline films as well as polyaniline (PANI)-LiNi0.5La0.02Fe1.98O4 nanocomposites was investigated by in-situ AFM and the surface morphology changes and surface roughening occurring during doping and de-doping cyles was investigated under different conditions and with different additives.
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Electric Furnace Steelmaking

Jorge Madias, in Treatise on Process Metallurgy: Industrial Processes, 2014

1.5.3.2 Electrodes

Artificial graphite electrodes are currently a standard in EAF operations. Raw materials are petroleum coke (needle type is preferred) and coal tar pitch. They are mixed and processed at high temperature in several steps. Milestones of electrode technology are shown in Table 1.5.3.

Table 1.5.3. Milestones in the Development of Artificial Graphite Electrodes [15]

1898 Invention of amorphous carbon electrodes for the Hérault EAF
1900 Acheson furnace for electrode graphitization
1931 Introduction of connection nipple
1963 Massive use of conical connection nipple
1963 Massive use of impregnation with coal tar pitch
1980 Massive use of needle coke for electrode production
1990 Massive use of digital electrode regulation systems
1990 Introduction of robot for automatic joining of electrodes
1998 Patenting of electrode cooling with water ring
2004 First commercial use of electrodes with male–female union
2009 Patenting of the use of carbon fibers for electrode reinforcement

Electrode water cooling was first adopted by Nippon Steel Corporation, and then most companies followed. Main advantage is to decrease side consumption. Large DC furnaces with only one electrode obliged to the development of very large electrodes, 800 mm diameter. Some development work has been carried out recently to avoid some drawbacks of the nipple system joining two electrodes.

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Anion Intercalation in Graphite Studied by Electrochemical-Scanning Probe Microscopy: State of the Art and Perspectives

G. Bussetti, L. Duò, in Encyclopedia of Interfacial Chemistry, 2018

Role of Graphite Surface Defects in the Intercalation Mechanism

If the HOPG electrode undergoes a CV cycle in H2SO4 or HClO4 electrolyte, the pristine surface is significantly affected by blisters, as reported in Fig. 2.

Fig. 2. EC-AFM acquired on (A) HOPG immersed in H2SO4 at 0.3 V, after a CV cycle up to 1.3 V (A region of Fig. 1A, see earlier); (B) HOPG immersed in HClO4 electrolyte at 0.3 V, after the anion intercalation and blister formation (A region of Fig. 1B, see earlier).

Adapted with permission from Bussetti, G.; Yivlialin, R.; Alliata, D.; Li Bassi, A.; Castiglioni, C.; Tommasini, M.; Casari, C. S.; Passoni, M.; Biagioni, P.; Ciccacci, F.; Duò, L. Disclosing the Early Stages of Electrochemical Anion Intercalation in Graphite by a Combined Atomic Force Microscopy/Scanning Tunneling Microscopy Approach. J. Phys. Chem. C 2016, 120, 6088–6093. Copyright 2016 American Chemical Society.

The Goss’ model predicts that steps are preferential intercalation sites and, consequently, areas close to the steps could be favored in surface swelling. This picture is partially true. For example, when the HOPG electrode is set to an EC potential greater than 1.0 V in diluted perchloric electrolyte, some fractures appear on the surface and blisters decorate this regions, where the density of steps is higher, as reported in Fig. 3.

Fig. 3. Scanning electron microscopy image of an HOPG electrode after a CV above 1.0 V in diluted perchloric electrolyte. A fracture and a blister are marked in the image.

Nonetheless, the chemical stability of the graphite basal plane in HClO4 electrolyte even just below 0.9 V (B region) is critical and we observe carbon dissolution as a function of time, as reported in Fig. 4.

Fig. 4. EC-STM images (Itunnel = 0.7 nA; Vbias = 0.8 V) acquired on graphite in HClO4 solution at VEC just below 0.9 V. The acquisition time for each image was 150 s. The reported Δt refers to the elapsed time computed from the scanning start of the first image (A) to the scanning start of the 150 s image (B) or the 300 s image (C). The formation of damages (holes) on the graphite surface is marked by dashed circles. Pre-existing damages increase their sizes (see the dashed straight line). In addition, we observe that the terrace edges are smoothed and the corner eroded, as marked by the dashed squares.

Reproduced with permission from Bussetti, G.; Yivlialin, R.; Alliata, D.; Li Bassi, A.; Castiglioni, C.; Tommasini, M.; Casari, C. S.; Passoni, M.; Biagioni, P.; Ciccacci, F.; Duò, L. Disclosing the Early Stages of Electrochemical Anion Intercalation in Graphite by a Combined Atomic Force Microscopy/Scanning Tunneling Microscopy Approach. J. Phys. Chem. C 2016, 120, 6088–6093. Copyright 2016 American Chemical Society.

In the image, the step is clearly eroded (see the area inside the dashed rectangle) and new defects in the surface basal plane are created due to the carbon dissolution in perchloric acid. Thus, even if the best quality of the HOPG crystal (e.g., α-type) is used as WE, the electrode surface is not preserved from a massive intercalation process, due to the carbon dissolution process that starts just below the A region in the CV (see Fig. 1).

A possible stratagem consists in covering the overall graphite surface with an ultrathin film, which can protect the carbon atoms, the crystal defects (steps, kinks, adatoms, etc.), and preserve a good EC exchange with the electrolyte. We found that free-base tetraphenyl porphyrin (H2TPP) forms a single molecule wetting layer on HOPG, when deposited in vacuum.54 The latter protects the HOPG from both dissolution and blistering damages during a CV in H2SO4, as reported in Fig. 5.

Fig. 5. AFM images (2 × 4 μm2) of a sample covered with a 0.5 Å thick layer of H2TPP. (A) Topography acquired in air in tapping mode before the EC process in H2SO4. The white dashed line represents the profile cross-section. (B) Phase-contrast image. The blue areas indicate the porphyrin wetting layer, while the yellow ones represent HOPG regions. (C) Image of the 0.5 Å-thick H2TPP sample after anion intercalation acquired on a sample area (labeled as “region A”) where graphite is covered by porphyrins and no blisters are observed.

Adapted with permission from Yivlialin, R.; Bussetti, G.; Penconi, M.; Bossi, A.; Ciccacci, F.; Finazzi, M.; Duò, L. Vacuum-Deposited Porphyrin Protective Films on Graphite: Electrochemical Atomic Force Microscopy Investigation During Anion Intercalation. ACS Appl. Mater. Interfaces 2017, 9, 4100–4105. Copyright 2017 American Chemical Society.

The ultrathin porphyrin film is hardly visible from the topography image (panel A), while it appears in the phase-contrast figure (panel B). After a CV sweep, the H2TPP film undergoes a molecular reassembling, but the HOPG substrate does not show any blister (panel C). The protective role of the porphyrin depends on the used electrolyte. Even if H2SO4 and HClO4 shows comparable effects on the pristine HOPG electrode, when the latter is covered by porpyrin, the perchloric acid is able to dissolve the organic film, intercalating inside the graphite and swelling the surface,55 strongly in contrast to what was previously observed in the sulfuric electrolyte (Fig. 5).

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Synthesis, Characterization, and Applications of Carbon Nanotubes

Filipe V. Ferreira, ... Luciana S. Cividanes, in Carbon-Based Nanofillers and Their Rubber Nanocomposites, 2019

1.2.1.1 Arc Discharge

Arc discharge between graphite electrodes was the first method to produce CNTs [42]. This technique involves establishing a direct current (DC) between a pair of graphite electrodes under an inert gas (helium or argon) at about 500 torr [40,43]. MWCNTs are produced by arc discharge without any metal catalyst, while mixed metals catalysts (Fe, Co, and Ni) are required for the production of SWNCTs [44]. Usually, CNTs synthesized by arc discharge show a high degree of structural perfection [45]; however, a number of variables (temperature of the chamber, the composition and concentration of the catalyst, the presence of hydrogen, etc.) influence their size and structure [46]. Recently, other electrodes [47] and other chemicals [48–50] have been employed for the synthesis of CNTs using arc discharge. Belgacem et al. [48] were able to produce MWCNTs doped with boron and nitrogen using the arc discharge method. Other research groups [49] have synthesized SWCNT–SWCNT hybrids by arc discharge in open air at less cost.

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Safety Improvement of Lithium Ion Battery by Organofluorine Compounds

Tsuyoshi Nakajima, in Advanced Fluoride-Based Materials for Energy Conversion, 2015

7.5 Charge/Discharge Behavior of Natural Graphite Electrodes in Fluorine Compound-Mixed Electrolyte Solutions

Charge/discharge characteristics of natural graphite electrodes were investigated because fluorine compounds are electrochemically reduced at higher potentials than EC, PC, DMC, and DEC. Electrochemical reduction of fluorine compounds used in the study starts between 1.9 and 2.7 V versus Li/Li+ [42,43]. These potentials are higher than the reduction potentials of EC (1.4 V), PC (1.0–1.6 V), DMC (1.3 V), and DEC (1.3 V) [56,62]. EC-based solvents should be used for high crystalline graphite such as natural graphite for the smooth formation of SEI on the electrode. Many fluorine compounds can be used for EC/DMC or EC/DEC electrolytes because EC easily forms SEI on natural graphite electrodes [42,44–46]. First coulombic efficiencies obtained in fluorocarbonate-mixed electrolyte solutions are nearly the same as or slightly higher than those obtained in EC/DMC and EC/DEC electrolytes without fluorine compounds. If decomposed products of fluorine compounds facilitate SEI formation on graphite electrode, PC-containing electrolytes with low melting points can be also used. Figure 7.16 shows the first charge/discharge curves obtained using a natural graphite electrode (NG15 μm) in 0.90, 0.78, and 0.67 molL−1 LiClO4-EC/DEC/PC (1:1:1 vol) and -EC/DEC/PC/(A, B, D, E, or F) (1:1:1:0.33, 0.83 or 1.5 in vol, or 10.0, 21.7, or 33.3 vol%, respectively) electrolytes as functions of the concentration of fluorocarbonate and current density [44]. The potential plateaus at 0.8 V versus Li/Li+ indicate the reductive decomposition of PC. In EC/DEC/PC electrolyte without the fluorine compound, the potential plateau was prolonged with decreasing concentrations of LiClO4 and increasing current densities. According to these changes, the first columbic efficiency in the EC/DEC/PC electrolyte also decreased. On the other hand, in the fluorocarbonate-mixed solutions, the potential plateau was shortened with increasing concentrations of fluorocarbonate and current density. The difference in the EC/DEC/PC electrolytes with and without fluorine compound was clearly observed when fluorocarbonate was mixed by 33.3 vol%. First coulombic efficiency in EC/DEC/PC/(A, B, D, E, or F) electrolyte increased, that is, irreversible capacity decreased with increasing concentrations of fluorocarbonate and current density. Fluorocarbonate B is the best among five fluorocarbonates, giving much higher first coulombic efficiencies, that is, lower irreversible capacities than others. For other fluorocarbonates except for B, much higher first coulombic efficiencies than those in the EC/DEC/PC electrolyte were also obtained by mixing of fluorocarbonates by 33.3 vol%. Charge capacities are nearly the same as for each other in the electrolyte solutions with and without fluorocarbonates at 60 mAg−1.

Figure 7.16. First charge/discharge curves of an NG15 μm in 0.90, 0.78, and 0.67 molL−1 LiClO4-EC/DEC/PC (1:1:1 vol) and 0.90, 0.78, and 0.67 molL−1 LiClO4-EC/DEC/PC/(A, B, D, E, or F 1:1:1:0.33, 0.83 or 1.5 vol, 10.0, 21.7 or 33.3 vol%, respectively) as functions of the concentration of fluorocarbonate and current density [44].

: EC/DEC/PC,
: EC/DEC/PC/A,
: EC/DEC/PC/B,
: EC/DEC/PC/D,
: EC/DEC/PC/E,
: EC/DEC/PC/F.

Figure 7.17 shows charge/discharge potential curves at the first cycle in 1 molL−1 LiPF6-EC/DMC (1:1:1 vol) and 1 molL−1 LiPF6-EC/DMC/(A, B, G, or J) (1:1:1 vol) at 60 mAg−1 [46]. Electrode potentials quickly decreased except that in 1 molL−1 LiPF6-EC/DMC/G (1:1:1 vol), in which a short potential plateau indicating the reduction of G was observed. First coulombic efficiencies obtained in fluorine compound-mixed electrolyte solutions are similar to those obtained in 1 molL−1 LiPF6-EC/DMC (1:1:1 vol) without fluorine compounds except that for 1 molL−1 LiPF6-EC/DMC/G (1:1:1 vol). Contribution of G to SEI formation is slightly lower than others. The results indicate that fluorine compounds A, B, and J can be used for 1 molL−1 LiPF6-EC/DMC (1:1:1 vol). The effect of mixing of fluorine compounds was also examined using PC-containing electrolytes. Figure 7.18 shows charge/discharge potential curves at the first cycle in 1 molL−1 LiPF6-EC/EMC/PC/(A, B, or J) (1:1:1:0.33 or 1:1:1:1.5 vol) [46]. The potential plateau observed in 1 molL−1 LiPF6-EC/EMC/PC (1:1:1 vol) indicates the electrochemical reduction of PC. However, the potential plateau almost disappeared in fluorine compound-mixed solutions. Table 7.4 gives electrochemical data obtained in PC-containing electrolytes. First coulombic efficiencies were largely increased by mixing of fluorine compounds without any decrease in capacities [46]. First coulombic efficiencies were in the range of 69–74% when A, B, and J were mixed by 10 vol%, but it reached 81% when J was mixed by 33 vol%. The increments of first coulombic efficiencies were in the range of 16–20% in 1 molL−1 LiPF6-EC/EMC/PC/(A, B, or J) (1:1:1:0.33 vol), and 28% in 1 molL−1 LiPF6-EC/EMC/PC/J (1:1:1:1.5 vol) at 60 mAg−1. This means that fluorine compounds A, B, and J effectively facilitate SEI formation in PC-containing electrolyte solutions.

Figure 7.17. First charge/discharge curves of an NG15 μm in 1 molL−1 LiPF6-EC/DMC (1:1:1 vol) and 1 molL−1 LiPF6-EC/DMC/(A, B, G, or J) (1:1:1 vol) at 60 mAg−1 [46].

: EC/DMC,
: EC/DMC/A,
: EC/DMC/B,
: EC/DMC/G,
: EC/DMC/J.

Figure 7.18. First charge/discharge curves of an NG15 μm at 60 mAg−1 in 1 molL−1 LiPF6-EC/EMC/PC (1:1:1 vol) and 1 molL−1 LiPF6-EC/EMC/PC/(A, B, or J) (1:1:1:0.33 vol) (a), and in 1 molL−1 LiPF6-EC/EMC/PC (1:1:1 vol) and 1 molL−1 LiPF6-EC/EMC/PC/J (1:1:1:1.5 vol) (b) [46].

: EC/EMC/PC,
: EC/EMC/PC/A,
: EC/EMC/PC/B,
: EC/EMC/PC/J.

Table 7.4. Charge/Discharge Capacities and Coulombic Efficiencies of NG15 μm in 1 molL−1 LiPF6-EC/EMC/PC (1:1:1 vol), 1 molL−1 LiPF6-EC/EMC/PC/(A, B or J) (1:1:1:0.33 vol), and 1 molL−1 LiPF6-EC/EMC/PC/J (1:1:1:1.5 vol) at 60 mAg−1 [46]

Electrolyte Solution Cycle Number Discharge Capacity (mAhg−1) Charge Capacity (mAhg−1) Coulombic Efficiency (%)
EC/EMC/PC (1:1:1:0.33) 1st 630 337 53.5
10th 337 333 98.6
EC/EMC/PC/A (1:1:1:0.33) 1st 476 326 69.3
10th 315 311 98.7
EC/EMC/PC/B (1:1:1:0.33) 1st 456 335 73.5
10th 313 309 98.7
EC/EMC/PC/J (1:1:1:0.33) 1st 486 342 70.3
10th 335 331 98.8
EC/EMC/PC/J (1:1:1:1.5) 1st 419 340 81.2
10th 327 324 99.2

Figure 7.19 shows charge/discharge potential curves at the first cycle in 0.90, 0.78, and 0.67 molL−1 LiClO4-EC/DEC/PC with or without fluoroether [45]. In all cases, potential plateaus at 0.8 V versus Li/Li+ indicating the electrochemical reduction of PC are reduced by mixing of fluoroethers. This trend becomes more distinct with increasing fluoroethers. Coulombic efficiencies are shown in Figure 7.20 as a function of cycle number [45]. Mixing of fluoroethers largely increases first coulombic efficiencies, which indicates that fluoroethers effectively facilitate SEI formation on natural graphite powder because electrochemical reduction of fluoroethers H and I starts at 2.1 and 2.3 V versus Li/Li+, respectively [43], higher than 1.3–1.6 V for PC, EC, and DEC [56,62]. The increments in first coulombic efficiencies by mixing of fluoroethers were approximately 10–30%, 20–40%, and 10–50% at 60, 150, and 300 mAg−1, respectively. The charge capacities obtained in fluoroether-mixed solutions are nearly the same as those in original electrolyte solutions without fluorine compounds at 60 mAg−1; however, they slightly decrease at higher current densities.

Figure 7.19. Charge/discharge potential curves of an NG15 μm at the first cycle, obtained in 0.90, 0.78, and 0.67 molL−1 LiClO4-EC/DEC/PC (1:1:1 vol), and 0.90, 0.78, and 0.67 molL−1 LiClO4-EC/DEC/PC/(H or I) (1:1:1:0.33, 0.83, or 1.5 vol) at current densities of 60, 150, and 300 mAg−1 [45].

: EC/DEC/PC,
: EC/DEC/PC/H,
: EC/DEC/PC/I.

Figure 7.20. Coulombic efficiencies for an NG15 μm as a function of cycle number, obtained in 0.90, 0.78, and 0.67 molL−1 LiClO4-EC/DEC/PC (1:1:1 vol), and 0.90, 0.78, and 0.67 molL−1 LiClO4-EC/DEC/PC/(H or I) (1:1:1:0.33, 0.83, or 1.5 vol) at current densities of 60, 150, and 300 mAg−1 [45].

: EC/DEC/PC,
: EC/DEC/PC/H,
: EC/DEC/PC/I.

In LiPF6-containing electrolyte solutions, the first coulombic efficiencies are high in most of the cases. Figure 7.21 shows charge/discharge curves at the first cycle obtained in 1 molL−1 LiPF6-EC/EMC/PC (1:1:1 vol) and 1 molL−1 LiPF6-EC/EMC/PC/(H or I) (1:1:1:1.5 vol) [45]. In the PC-containing electrolytes, SEI formation is faster in fluoroether-mixed solutions than in the original one. First coulombic efficiencies obtained in fluoroethers H- and I-mixed solutions were 78 and 74%, respectively, >68% in the original solution. The results indicate that fluoroethers H and I also facilitate SEI formation in PC-containing electrolytes.

Figure 7.21. First charge/discharge curves of an NG15 μm in 1 molL−1 LiPF6-EC/EMC/PC (1:1:1 vol) and 1 molL−1 LiPF6-EC/EMC/PC/(H or I) (1:1:1:1.5 vol.) at 60 mAg−1 [45].

: EC/EMC/PC,
: EC/EMC/PC/H,
: EC/EMC/PC/I.

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Preparation and properties of manipulated carbon nanotube composites and applications

R.B. Rakhi, in Nanocarbon and its Composites, 2019

16.3.1 Carbon arc discharge

In the arc discharge method, two graphite electrodes separated by a distance of nearly 1 mm are kept in an inert He atmosphere and a direct current is passed through them. The anode is consumed due to arcing and a cigar-like deposit is formed on the cathode. The outer shell of this deposit is gray and hard, with a black soft inner core that contains MWCNTs, polyhedral particles, and amorphous carbon [29]. SWCNTs may also be obtained but the synthesis of which requires mixed metal catalysts, such as Fe:Co and Ni:Y [30] that are inserted into the anode. SWCNTs are found distributed in the chamber as a fluffy web-like material [30]. In 2006, Ando and Zhao reported the synthesis of SWCNT nets of up to 20–30 cm in length by arc evaporation of a graphite rod containing a pure Fe catalyst in the chamber filled with a mixture of hydrogen and inert gas. Replacement of H2 by He resulted in the formation of MWCNTs with a very thin innermost tube of < 0.4 nm [31]. MWCNTs synthesized by arc discharge are highly crystalline and typically 20 μm long. They have an average outer diameter of around 10 nm and have 20–30 concentric graphitic walls. MWCNTs produced by the arc discharge method exhibit fewer defects than those produced by other methods. SWCNTs occur in bundles with diameters ranging from 1 to 2 nm and due to the entanglement of the SWCNT bundles, it is really difficult to measure the SWCNT length accurately. The SWCNTs in the bundles exhibit a collection of different chiralities [30]. Along with the CNTs, the as-prepared material by arc discharge also contains substantial amounts of byproducts such as polyhedral carbon and amorphous carbon. Encapsulated metal catalyst particles may also be present in SWCNT samples [32, 33] (Fig. 16.4).

Fig. 16.4. Arc discharge scheme for the growth of CNTs.

The carbon arc discharge evaporation method was used by Iijima in the discovery of CNTs [10]. The first obtained CNTs had diameters ranging from 4 to 30 nm and a length up to 1 mm. TEM analysis revealed that on each of these tubes, the carbon-atom hexagons were arranged in a helical fashion about the tube axis. The tips of the tubes were closed by curved polygonal or cone-shaped caps. The TEM study of the growth morphology revealed that there were many variations in shape, especially near the tube tips [7]. A topological model was constructed in which pentagons and heptagons played a key role in the tube tip shapes. For an open-ended growth, Iijima et al. proposed a model in which the carbon atoms are captured by the dangling bonds, resulting in layer-by-layer growth [13]. The large-scale synthesis of MWCNTs by the arc discharge technique was reported by Ebbesen and Ajayan [29].

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Thermal Stability of Materials in Lithium-Ion Cells

Jun-ichi Yamaki, in Lithium-Ion Batteries, 2014

3.1 Graphite Electrode [6]

The results reported here refer to a graphite electrode prepared by mixing 95 wt.% of natural graphite with 5 wt.% of poly(vinylidene fluoride) (PVdF)-binder. Graphite electrodes without PVdF-binder were also fabricated. The electrolyte was 1 M LiPF6/ethylene carbonate (EC) + dimethyl carbonate (DMC) (1:1 v/v) and the counter electrode a Li metal sheet.

The cells were cycled between 0.01 and 1.5 V with a relaxation period of 60 min at the end of charge, at a constant current of 0.2 mA/cm2. After two cycles in this condition, the cells were charged to 0 V with the time limit of 372 mAh/g to obtain a fully charged negative electrode.

Figure 20.2 shows DSC curves for fully lithiated or delithiated graphite (a–d) and the electrolyte (e). Sample (a) shows a mild heat generation starting at 130 °C with a small peak at 140 °C. The mild heat generation continued until a sharp exothermic peak appeared at 280 °C. From our experiments, the small peak at 140 °C is caused by SEI formation. There is already SEI on the sample (lithiated graphite), which is formed during cycling for sample preparation. This original SEI protects the reaction of the electrolyte and Li in graphite at a lower temperature, and there is no heat generation. However, at ∼140 °C, the protection effect of the original SEI is not sufficient, and a new, thicker SEI is formed. When this becomes thick enough, its formation speed decreases, and a small exothermic heat peak is observed at 140 °C. The mild heat generation continued until a sharp exothermic peak appeared at 280 °C, because the SEI formation continues with increase in temperature even if there is a protection effect of the SEI. If the original SEI formed during cycling is thick enough, the small peak at 140 °C does not appear because the protection effect of the original SEI is enough even at this temperature. Sample (b) is charged at a very low current density because PVdF-binder is not used to make the electrode. Therefore, SEI of sample (b) is very thick, and no peak appeared at 140 °C. Samples (c) and (d) did not show the small peak at 140 °C. Therefore, the lithiated graphite and the electrolyte are necessary to show the small peak at 140 °C. This fact also supports that the small peak at 140 °C is the formation of SEI. This peak is sometimes large and the peak temperature is different from 140 °C, because the thickness of original SEI is different (the charge current density is different).

FIGURE 20.2. DSC curves of (a) fully lithiated graphite with the electrolyte and PVdF (the usual graphite anode), (b) fully lithiated graphite with the electrolyte, (c) fully delithiated graphite with the electrolyte and PVdF (the usual graphite electrode), (d) fully lithiated graphite with PVdF (the usual graphite electrode), and (e) the electrolyte (1 M LiPF6/EC + DMC) [6].

There is evidence [7] that the peak at 280 °C in Figure 20.2 is caused by decomposition of SEI with reaction of Li in graphite. DSC curves of charged electrode powder (without electrolyte) obtained after the 2nd charge are shown in Figure 20.3, together with that of discharged electrode powder (without electrolyte) after the 2nd discharge. No exothermic peak was seen at around 100–130 °C. The heat values, which were evaluated by integrating DSC curves, were proportional to the amount of charged electrode powder. These results suggest that SEI formed on graphite during charging would react with charged graphite at ∼280 °C accompanied by exothermic heat.

FIGURE 20.3. DSC curves of (a–c) charged and (d) discharged graphite-electrode powder. The weight of the graphite-electrode powder in the pan was (a) 1 mg, (b) 2 mg, and (c, d) 4 mg [7].

As shown in Figure 20.3 no exothermic peak was visible at 100–160 °C for charged graphite only, thus the electrolyte should be directly involved in the exothermic reaction at this temperature. To identify the effect of solvent and LiPF6 in the electrolyte separately, the thermal behavior of the charged graphite in solvent was studied firstly [8]. Figure 20.4(a) shows DSC curves for 4 mg of Li0.92C6 mixed with a given amount of the EC + DMC solvent (from 0.25 to 4 μl). When the amount of the solvent was 0.25 μl, an exothermic peak was observed at ∼160 °C. When the amount of solvent increased from 0.25 to 2 μl, the heat values of the peak increased significantly. However, the heat value was almost constant when the amount of the coexisting solvent increased to 3 and 4 μl. Therefore, the exothermic peak at ∼160 °C is caused by the reaction between solvent and intercalated Li. The protection effect of original SEI, which was formed during sample preparation, has to be considered. The heat value of the peak increased with the increase of the solvent until the amount of solvent became to 3 μl. All the coexisting solvent was used for the reaction, and some of intercalated Li remained, because the reaction was limited by the amount of the solvent. With 4 μl, the heat value did not increase too much from that of 3 μl solvent, because the reaction was limited by the amount of intercalated Li. All the intercalated Li was consumed, and excess solvent remained after the reaction.

FIGURE 20.4. (a) DSC curves for mixtures of 4 mg Li0.92C6 and given amounts of EC + DMC solvent; (b) DSC curves for mixtures of 4 mg Li0.48C6 and given amounts of EC + DMC solvent [8]. (For color version of this figure, the reader is referred to the online version of this book.)

To confirm the above supposition of exothermic peak at around 160 °C, half-charged graphite (Li0.48C6) with solvent was also quantitatively studied by DSC. Figure 20.4(b) shows DSC curves for 4 mg of Li0.48C6 mixed with a given amount of EC + DMC solvent (from 0.25 to 4 μl). Compared with the DSC curves of the mixtures of Li0.92C6 and solvent (Figure 20.4(a)), it was easy to find that the dominant peak was quite similar to that obtained for Li0.92C6 in the solvent, including peak position and peak shape. At the same time, similar tendency of the heat value was visible. The heat values in both cases increased with the increasing of the amount of solvent, and then remained almost constant when all the intercalated Li was consumed with excess solvent. The heat value became almost constant when the solvent was about 3 μl with 4 mg of Li0.92C6, and the solvent was from 1 to 2 μl with 4 mg of Li0.48C6. Furthermore, the largest heat value of Li0.48C6 was almost half the value of Li0.92C6. Based on these results, it was clear that the amount of solvent limits the reaction when its amount is small, and the amount of intercalated Li limits the reaction when the amount of solvent is large. LiPF6 in the electrolyte is needed to form SEI (with protection effect) on the charged graphite.

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Lanthanide materials as chemosensors

Farnoush Faridbod, ... Morteza Hosseini, in Lanthanide-Based Multifunctional Materials, 2018

12.2.2 Ln materials for electrochemical molecular sensing

In 1998 [6], a graphite electrode, which was pretreated by lanthanum nitrate, was modified with 2,6-dichlorophenolindophenol (DCPI) through physical adsorption (DCPI-La). This electrode was then applied for the voltammetric detection of nicotinamide coenzyme (NADH) in a flow injection analysis system. NADH oxidized at bare electrodes at high potentials (from 450 to 1100 mV based on the electrode materials). The oxidation reaction was irreversible and was affected from interferences from other oxidizable species at these high voltages. Hence, modifiers were needed to promote NADH oxidation at lower potentials. The role of La in the modified electrode was to enhance the analytic properties of the DCPI-modified electrode in terms of linear range and detection limit. Since the solubility of the DCPI-La was low (about Ksp < 10− 10), DCPI remained better on the electrode surface, and the stability of the electrode was increased (about 20% higher after 6 h of continuous operation in the flow injection system).

In 2000, [7] synthesized a lanthanide porphyrin complex and used it for the modification of an electrode to make an ethacrynic acid (EA) potentiometric sensor. The complex pentane-2,4-dionato(meso-tetraphenylporphinato)terbium [TbTPP(acac)] was applied as a sensing material in a polymeric membrane of the potentiometric sensor. Nernstian response to EA− ion in the concentration range from 7.4 × 10− 6 to 1.0 × 10− 1 mol L− 1, in a pH range from 3.2 to 6.8, with a fast response time of 30 s was achieved. Lanthanide porphyrin complexes performed better than copper porphyrin complexes in the potentiometric sensor. The sensor was then successfully used in the analysis of EA in human urine samples.

In 2001, an electronic nose was introduced for discrimination among diverse virgin olive oils. In that work, a sensor array based on Langmuir-Blodgett (LB) films of lanthanide bisphthalocyanines (LnPc2) was used as a sensing material. A LB film consists of one or more monolayers of an organic material, deposited from the surface of a liquid onto a solid by immersing (or emersing) the solid substrate into (or from) the liquid. Bisphthalocyanines composed of unsubstituted bisphthalocyanines with a distinct central Ln atom (PrPc2 and LuPc2) and an octa-tert-butyl-substituted bisphthalocyanine (PrPc2t) were synthesized. The interaction of the sensors in the designed electronic nose (composed of an array five sensors) with the headspace of olive oil samples caused the chemisorption of the volatile organic compounds (VOCs) in the Ln-biphthalocyanine LB films and led to the formation of complexes, which changed the conductivity in the LB films. Other studies by the same group, related to a tobacco smoke sensor using LB films of Pr, Gd, and Yb diphthalocyanines and an octa-tert-butyl praseodymium diphthalocyanine [8] or to the application of lutetium bisphthalocyanine thin films as sensors for organic volatile components of aromas [9], have also contributed to evince the feasibility of using thin LB films or evaporated films of lanthanide bisphthalocyanines for the determination of volatile VOCs (e.g., alcohols, aldehydes, esters, and acids).

In 2004, a series of anion-selective potentiometric sensors were introduced by the application of lipophilic lanthanide tris(β-diketonates) as sensing materials in a plasticized poly(vinyl chloride) membrane [10]. The designed sensors possessed high selectivity toward the chloride anion in the 1.0 × 10− 5 − 10− 1 mol L− 1 concentration range with near-Nernstian slopes. A non-Hofmeister anion selectivity in the determination of Cl− anion from NO3−, ClO4−, and other anions was observed, which was due to a 1:1 complex formation of lanthanide tris(β-diketonates) with chloride.

In a study in 2009, a Ln-containing ionic liquid of [(C4H9)2-bim]3[La(NO3)6] (bim = benzimidazole) was synthesized and utilized as a modifier in a carbon paste electrode [11]. The results showed that the modified electrode had excellent electrocatalytic activities toward the reduction of H2O2, nitrite, bromate, and trichloroacetic acid.

In 2014 [12], Selvaraju and Ramaraj reported an electroactive sodium lanthanum hexacyanoferrate complex that was deposited on a glassy carbon electrode (GCE). The modified electrode was an excellent transducer for oxidation of neurotransmitter molecules: it enhanced the oxidation peak of dopamine by a factor of 50 as compared with the bare GCE.

In a study in 2016, a novel LB-film-based taste sensor for evaluating the quality of Japanese sake was developed by Hiroki et al. [13]. LB films were made of lanthanides coordinated to stearic acid (Tb-SA and Eu-SA). Three different kinds of Japanese sake were assessed by the sensors. The Ln-SA films increased the sensitivity of taste sensors.

A lanthanum MOF, [La(BTC)(H2O)(DMF)] (H3BTC = 1,3,5-benzenetricarboxylic acid), was prepared through mild hydrothermal conditions [14]. The synthesized MOF showed an electrocatalytic activity toward H2O2 reduction in acidic media at ca. − 0.7 V. The modified electrode showed a linear range from 5 μM to 2.67 mM with a detection limit of 0.73 μM.

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Carbon nanotube for targeted drug delivery

Jignesh Priyakant Raval, ... Dharmesh R. Chejara, in Applications of Nanocomposite Materials in Drug Delivery, 2018

9.4.1 Arc-discharge

In arc discharge method, two high purity graphite electrodes as anode and cathode are held at short distance apart under a helium atmosphere. Under these conditions, some of the carbon evaporated from the anode, re-condensed as a hard cylindrical deposit on the cathodic rod. The key point in the arc–evaporation method is the current applied. Higher current application will result in a hard, sintered material with few free nanotubes. Therefore, the current should be kept as low as possible. Using arc-discharge method, individual carbon nanotubes could be achieved in generally several hundred microns long.

Arc discharge process has scale up limitations and also sometimes requires the addition of a small amount of metal catalysts, which increases the yield of nanotubes. So the resulting products contain some catalyst particles, amorphous carbons, and non-tubular fullerenes. Therefore, subsequent purification steps are required. High temperatures are also necessary for this technique. Arc discharge technique needs 600–1000°C, so, the differences in lattice arrangements could exist in the tubes and also there may be a difficulty in the control of chirality and diameter of the nanotubes.

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