Surface modification of carbon anodes for secondary lithium battery by fluorination

Recent results on the surface modification of petroleum cokes and their electrochemical properties as anodes of secondary lithium batteries are summarized. The surface of petroleum coke and those heat-treated at 1860-2800 °C were fluorinated by elemental fluorine (F₂), chlorine trifluoride (ClF₃) and nitrogen trifluoride (NF₃). No surface fluorine was found except only one sample when ClF₃ and NF₃ were used as fluorinating agents while surface region of petroleum coke was fluorinated when F₂ was used. Transmission electron microscopic (TEM) observation revealed that closed edge of graphitized petroleum coke was destroyed and opened by surface fluorination. Raman spectra showed that surface fluorination increased the surface disorder of petroleum cokes. Main effect of surface fluorination with F₂ is the increase in the first coulombic efficiencies of petroleum cokes graphitized at 2300-2800 °C by 12.1-18.2% at 60 mA/g and by 13.3-25.8% at 150 mA/g in 1 mol/dm³ LiClO₄-ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1, v/v). On the other hand, main effect of the fluorination with ClF₃ and NF₃ is the increase in the first discharge capacities of graphitized petroleum cokes by ∼63 mAh/g (∼29.5%) at 150 mA/g in 1 mol/dm³ LiClO₄-EC/DEC.     

Modification of surface properties of carbon-based and polymeric materials through fluorination routes: From fundamental research to industrial applications

The outstanding characteristics of fluorine gas, e.g., extreme reactivity and oxidizing power, and the utmost electronegativity of F⁻ ion, lead to very strong bonds between fluorine and most of the other elements of the periodical table. Treatments involving F₂, fluorinated gases and rf plasma-enhanced fluorination (PEF) constitute exceptional tools for modifying the surface properties of materials. Many advantages of these techniques can be indeed outlined, when compared to more conventional methods: low-temperature reactions (even at room temperature), chemical modifications limited to surface only without changing the bulk properties, possible non-equilibrium reactions. Depending on the type of starting materials and employed techniques, the improved properties may concern wettability, adhesion, chemical stability, barrier properties, biocompatibility, grafting, mechanical behavior. Several examples of surface fluorination will be given on various types of carbon-based materials, elastomers and polymers.     

Surface and bulk properties of severely fluorinated carbon fibres

The development of ultra-inert composites using fluorinated carbon fibres as the reinforcement requires fluorinated carbon fibres with a durable surface composition. Here we report the effect of direct fluorination using an F₂/N₂ mixture at 653 K on the surface and bulk properties of two types of high strength carbon fibres. These were treated up to a surface fluorine content of ∼64 at.% and a bulk fluorine content of ∼15 mass%. A colour change was observed after fluorination caused by the changes in the graphitic band structure of the carbon fibres by the introduction of carbon sp³ hybridisation. The tensile strength and Young's modulus decrease after fluorination by up to 33 and 22%, respectively. XRD shows marginal changes in the interlayer distance but the crystallite size increases. Changes in the electrical conductivity of the fluorinated carbon fibres indicate that the modification is confined to the near surface volume. Predominantly covalent C-F bonds are formed as shown by X-ray photoelectron spectroscopy (XPS) and measured zeta (ζ)-potentials. Hence the fluorinated fibres are hydrophobic and have low surface tensions. This and the large increase in fibre surface area, as determined by nitrogen adsorption, is expected to facilitate interfacial interaction between fluorinated carbon fibres and fluoropolymers.     

Fluorination of polyethylene films

Fluorination of HDPE films (100 and 1800 μm thick) and LDPE films (100 μm thick) with elemental fluorine is described. The films were fluorinated at temperatures between 40°C and 100°C. The degree of fluorination was increased by increasing the concentration of fluorine in the F₂/N₂ gas mixture. A diffusion-controlled process was indicated by plotting the measured depth of fluorination against the square root of fluorination time. The chemistry of the process was studied by ESCA measurements or by energy dispersive analyses in the scanning electron microscope.     

Carbon Electrode Material with High Densities of Energy and Power

Activated carbon (AC) samples as electrode materials were prepared by means of simultaneous physical-chemical activation using walnut shells as precursors. The porosity and surface chemistry of the resultant AC samples were studied by the nitrogen adsorption at 77 K, and FTIR spectrum. The testing supercapacitors were assembled with resultant carbon electrode and electrolyte of 6 mol·L⁻¹ KOH solution. Their electrochemical properties were investigated by charge-discharge of constant current, cyclic voltammogram, impedance spectrum and so on. The results showed that the capacitor had low inner resistance, low leakage current, high stability, and capacitance retainability. The specific capacitance of AC increased with increasing BET specific surface area. The specific capacitance of the AC sample with a specific area of 1197 m²·g⁻¹ could be as high as 292 F·g⁻¹. At a discharge current of 80 mA, the corresponding specific energy density, power density, and maximum power of the supercapacitor are 7.3 Wh·kg⁻¹, 770 W·kg⁻¹, and 5.1 W·g⁻¹, respectively.     

Direct fluorination—Useful tool to enhance commercial properties of polymer articles

Fundamental features and industrial applications of the direct fluorination of polymers are reviewed. Fundamental features of the direct fluorination of a set of polymers, such as polystyrene, polyethyleneterephthalate, poly(2,6-dimethyl-1,4-phenylene oxide), PMMA, LDPE (two types), HDPE (six types), polyvinyltrimethylsilane, poly(4-methyl-pentene-1), polyimide Matrimid 5216^®, polysulfones, polyetheretherketone, polycarbonatesiloxane, polysulphone-polybuthadiene block-copolymers, polypropylene, PVF, PVDF, etc. are described. Influence of composition of the fluorinating mixture (F₂-He-N₂-O₂-HF), fluorine partial pressure, temperature and fluorination duration, on the rate of formation of the fluorinated layer and the chemical composition, density, refraction index, surface energy, gas separation properties and friction coefficient of fluorinated layer have been investigated. Processes of formation and termination of long-living and short-living radicals and grafting of acrylonitrile to fluorinated polymers have been studied. Industrial applications of the direct fluorination to enhance the commercial properties of polymeric goods, such as separation factor of polymeric membranes for gas separation, barrier properties of polymer vessels, pipes and packagings, adhesion, printability, wetting and transparency in visible and IR are reviewed.     

Fluorine‐Doped Fe2O3 as High Energy Density Electroactive Material for Hybrid Supercapacitor Applications

Nanostructured α‐Fe₂O₃ with and without fluorine substitution were successfully obtained by a green route, that is, microwave irradiation. The hematite phase materials were evaluated as a high‐performance electrode material in a hybrid supercapacitor configuration along with activated carbon (AC). The presence of fluorine was confirmed through X‐ray photoelectron spectroscopy and transmission electron microscopy. Fluorine‐doped Fe₂O₃ (F‐Fe₂O₃) exhibits an enhanced pseudocapacitive performance compared to that of the bare hematite phase. The F‐Fe₂O₃/AC cell delivered a specific capacitance of 71 F g^?1 at a current density of 2.25 A g^?1 and retained approximately 90 % of its initial capacitance after 15 000 cycles. Furthermore, the F‐Fe₂O₃/AC cell showed a very high energy density of about 28 W h kg^?1 compared to bare hematite phase (～9 W h kg^?1). These data clearly reveal that the electrochemical performance of Fe₂O₃ can be improved by fluorine doping, thereby dramatically improving the energy density of the system.     

Surface fluorination of the cathode active materials for lithium secondary battery

LiMn₂O₄ was treated with F₂ at room temperature (RT), 373 and 473 K under 1.3, 6.6 and 13.2 kPa-F₂. XPS data indicate that two kinds of fluorine species may exist on the sample surface and the ratio of these fluorines is affected by choosing the reaction condition. The peak indicating Mnⁿ⁺ bonded to fluorine appeared in the XPS spectra of Mn2p3/2 electron. From the results of the charge/discharge measurements, the efficiency of charge/discharge process for the sample fluorinated under 1.3, 6.6 and 13.2 kPa-F₂ below 373 K was larger than that of untreated one. The discharge capacity of the fluorinated sample was also larger than that of untreated one. The discharge capacity, the loss of discharge capacity during 50 charge/discharge cycles, F/O ratio measured from XPS data and the intensity of the peak indicating Mnⁿ⁺ bonded to fluorine in the XPS spectra were closely related to each other. The optimal fluorination condition was under 1.3 kPa-F₂ at RT for 1 h.     

Surface fluorination and electrochemical behavior of petroleum cokes graphitized at medium and high temperatures for secondary lithium battery

The surface structure and electrochemical performance have been investigated of petroleum cokes heat-treated at 2100 and 2600 °C (abbreviated to PC2100 and PC2600) and those fluorinated by elemental fluorine at 200 and 300 °C. XPS study indicated that surface fluorine was covalently bonded to carbon and surface fluorine contents were in the range of 4.9-17.8 at.%. Surface oxygen was reduced by fluorination. BET surface areas were nearly the same before and after fluorination. Fluorination enhanced D-band intensity in two Raman shifts observed at 1580 cm⁻¹ (G-band) and 1360 cm⁻¹ (D-band), indicating the increase in the surface disordering. At a high current density of 150 mA/g, the capacity increase was observed for PC2100 fluorinated at 200 °C and for PC2600 fluorinated at 200 and 300 °C. The most interesting result was the increase in first coulombic efficiencies by surface fluorination. First columbic efficiencies for PC2600 fluorinated at 300 °C were increased by 12.1% at 60 mA/g and by 25.8% at 150 mA/g, respectively. The impedance measurements showed that the resistances of surface films on carbon electrodes were increased by fluorination, however, the charge transfer resistances were decreased by 12.3% for PC2100 fluorinated at 200 °C, and by 27.5 and 6.4% for PC2600 fluorinated at 200 and 300 °C, respectively. The reduction of the charge transfer resistances was consistent with increase in the charge capacities for PC2100 fluorinated at 200 °C and PC2600 fluorinated at 200 and 300 °C.     

Improving the electrochemical performances of active carbon-based supercapacitors through the combination of introducing functional groups and using redox additive electrolyte

High-performance and low-cost electrochemical capacitors (ECs) are essential for large-scale applications in energy storage. In this work, the specific capacitance of active carbon (AC) electrode was significantly improved through the combination of introducing functional groups on the surface of AC and adding redox-active molecules (K₃Fe(CN)₆) into 2?M KOH aqueous electrolytes. The surface-oxygen functionalized AC (FAC) was synthesized using HNO₃ echoed as the electrode and 2?M KOH with 0.1?M K₃Fe(CN)₆ as the electrolyte. The surface functional groups of the AC not only contribute to the pseudocapacitance but also increase the active sites of the electrode/electrolyte interface, which enhances the electrochemical activity of the Fe(CN)₆^3?/Fe(CN)₆^4? redox pair, thus leading to high capacitance. In the redox electrolyte, the specific capacitance was much higher in 229.17?F?g^?1 (1?A?g^?1) achieved for those FAC than in raw AC (only 147.06?F?g^?1). Similarly, the FAC electrode suggested high energy density and extended cycling stability in the KOH?+?K₃Fe(CN)₆ electrolyte.     

Surface Modification and Performance of Activated Carbon Electrode Material

A commercial activated carbon was modified by surface treatment using three chemicals, nitric acid, hydrogen peroxide, and ammonia, respectively. The modified carbons were characterized by N₂ adsorption-desorption isotherms and FTIR spectroscopy. The resultant carbon electrode-based electric double-layer capacitors (EDLCs) were assembled with 6 mol·L⁻¹ KOH as the electrolyte. The influence of surface modification on the performance of EDLCs was studied by galvanostatic charge-discharge, cyclic voltammetry, and alternating current impedance. The surface modification resulted in no big decrease in specific surface area and little decrease in average pore size, and introduced functional groups, such as hydroxyl, carbonyl, and amidogen, on the carbon surface. These functional groups significantly improved the wettability and reduced the resistance of the activated carbon. As a result, the specific capacitance of the carbon modified with 65% HNO₃ reached 250 F·g⁻¹, 72.4% higher than that of original carbon. The leakage current of testing EDLCs decreased unexpectedly to 3-18 μA, only 0.8%-4.9% that of the original carbon electrode-based EDLC (371 μA).     

Enhanced Electrochemical Performance of Supercapacitors via Atomic Layer Deposition of ZnO on the Activated Carbon Electrode Material

Fabricating electrical double-layer capacitors (EDLCs) with high energy density for various applications has been of great interest in recent years. However, activated carbon (AC) electrodes are restricted to a lower operating voltage because they suffer from instability above a threshold potential window. Thus, they are limited in their energy storage. The deposition of inorganic compounds’ atomic layer deposition (ALD) aiming to enhance cycling performance of supercapacitors and battery electrodes can be applied to the AC electrode materials. Here, we report on the investigation of zinc oxide (ZnO) coating strategy in terms of different pulse times of precursors, ALD cycles, and deposition temperatures to ensure high electrical conductivity and capacitance retention without blocking the micropores of the AC electrode. Crystalline ZnO phase with its optimal forming condition is obtained preferably using a longer precursor pulse time. Supercapacitors comprising AC electrodes coated with 20 cycles of ALD ZnO at 70 °C and operated in TEABF₄/acetonitrile organic electrolyte show a specific capacitance of 23.13 F g⁻¹ at 5 mA cm⁻² and enhanced capacitance retention at 3.2 V, which well exceeds the normal working voltage of a commercial EDLC product (2.7 V). This work delivers an additional feasible approach of using ZnO ALD modification of AC materials, enhancing and promoting stable EDLC cells under high working voltages.     

Improved thermal stability of crosslinked PTFE using fluorine gas treatment

Crosslinked PTFE (XF) samples were fluorinated at 293-593 K under 0.7-101 kPa F₂ and for 1 h to 7 days to improve its thermal stability. Because the weight uptake which may be caused by the fluorine addition was detected at room temperature, CC bonds in XF can be fluorinated and the fluorine content was saturated after 72 h. Weights of all samples increased more than that of original XF through additional fluorination of CC bonds, whereas it decreased by the chain-scission to form gaseous fluorocarbons such as CF₄. The intensity ratio in IR spectra of the peaks correspond to the double bond (CFCF₂) at 1785 cm⁻¹ and the characteristic peaks of PTFE at 1794 cm⁻¹, I_PTFE/I_PTFE was smaller for the fluorinated XF rather than that for XF. Average values of heat of crystallization (ΔH_c) for all fluorinated XF samples were about 2 J/g higher than that of the original XF. The decomposition temperature calculated from the TG curves increased with increasing reaction temperature and reaction time up to 72 h. Thermal stability of XF was improved through fluorine gas treatment.     

An acidic cellulose–chitin hybrid gel as novel electrolyte for an electric double layer capacitor

A novel acidic cellulose–chitin hybrid gel electrolyte including binary ionic liquids (ILs) with an aqueous H₂SO₄ solution was prepared for an electric double layer capacitor (EDLC). Its electrochemical characteristics were investigated by galvanostatic charge–discharge measurements. The test cell with a hybrid gel electrolyte shows a specific capacitance of 162 F g^?1 at room temperature, which is higher than that for a cell with an H₂SO₄ electrolyte, 155 F g^?1. This hybrid gel electrolyte exhibits excellent high-rate discharge capability in a wide range of current densities as well as an aqueous H₂SO₄ solution. The discharge capacitance of the test cell can retain over 80% of its initial value in 100,000 cycles even at a high current density of 5000 mA g^?1.     

Plasma fluorination of organosilicon polymeric films for gas separation applications

SF₆ plasma treatment using an RF discharge was carried out for the surface fluorination of polytrimethylsilylpropyne (PTMSP) and polyvinyltrimethylsilane (PVTMS) films. Gas permeation of the fluorinated and untreated films for O₂, N₂, He, H₂, CH₄ and CO₂ gases has been measured. Plasma fluorination increases the ideal selectivities of the PTMSP films decreasing their permeances for all the gases measured, and does not affect the permeances and selectivities of the PVTMS films. The composition and chemical structure of the fluorinated polymer surface were investigated using X-ray photoelectron spectroscopy (XPS) and ¹⁹F nuclear magnetic resonance (NMR) spectroscopy. Within the range of the treatment parameters studied, permselectivity and surface composition of the fluorinated PTMSP films depend slightly on the treatment time and the density of the fluorine atom flux on the modified surface. The trimethylsilyl substituents are detached and carbon atoms are partially fluorinated during modification. The structure of the fluorinated layer contains crosslinks and unsaturated bonds.     

Electrodeposited PbO2 thin film on Ti electrode for application in hybrid supercapacitor

PbO₂ thin films were prepared by pulse current technique on Ti substrate from Pb(NO₃)₂ plating solution. The hybrid supercapacitor was designed with PbO₂ thin film as positive electrode and activated carbon (AC) as negative electrode in the 5.3 M H₂SO₄ solution. Its electrochemical properties were determined by cyclic voltammetry (CV), charge–discharge test and electrochemical impedance spectroscopy (EIS). The results revealed that the PbO₂/AC hybrid supercapacitor exhibited large specific capacitance, high-power and stable cycle performance. In the potential range of 0.8–1.8 V, the hybrid supercapacitor can deliver a specific capacitance of 71.5 F g^?1 at a discharge current density of 200 mA g^?1(4 mA cm^?2) when the mass ratio of AC to PbO₂ was three, and after 4500 deep cycles, the specific capacitance remains at 64.4 F g^?1, or 32.2 Wh Kg^?1 in specific energy, and the capacity only fades 10% from its initial value.     

High Energy Density Heteroatom (O,N and S) Enriched Activated Carbon for Rational Design of Symmetric Supercapacitors

Carbon-based symmetric supercapacitors (SCs) are known for their high power density and long cyclability, making them an ideal candidate for power sources in new-generation electronic devices. To boost their electrochemical performances, deriving activated carbon doped with heteroatoms such as N, O, and S are highly desirable for increasing the specific capacitance. In this regard, activated carbon (AC) self-doped with heteroatoms is directly derived from bio-waste (lima-bean shell) using different KOH activation processes. The heteroatom-enriched AC synthesized using a pretreated carbon-to-KOH ratio of 1:2 (ONS@AC-2) shows excellent surface morphology with a large surface area of 1508 m² g⁻¹. As an SC electrode material, the presence of heteroatoms (N and S) reduces the interfacial charge-transfer resistance and increases the ion-accessible surface area, which inherently provides additional pseudocapacitance. The ONS@AC-2 electrode attains a maximum specific capacitance (C_sp) of 342 F g⁻¹ at a specific current of 1 Ag⁻¹ in 1 m NaClO₄ electrolyte at the wide potential window of 1.8 V. Moreover, as symmetric SCs the ONS@AC-2 electrode delivers a maximum specific capacitance (C_sc) of 191 F g⁻¹ with a maximum specific energy of 21.48 Wh kg⁻¹ and high specific power of 14 000 W kg⁻¹ and excellent retention of its initial capacitance (98 %) even after 10000 charge/discharge cycles. In addition, a flexible supercapacitor fabricated utilizing ONS@AC-2 electrodes and a LiCl/polyvinyl alcohol (PVA)-based polymer electrolyte shows a maximum C_sc of 119 F g⁻¹ with considerable specific energy and power.     

Property control of new forms of carbon materials by fluorination

Multiwall carbon nanotubes (MWNTs) based on the template carbonization technique were fluorinated in a temperature range 323-473 K by elemental fluorine. The fluorination of the carbon nanotubes results in functionalization and modification of pristine nanotubes with respect to adsorption and electrochemical properties. Selective fluorination of the inner surface of the carbon nanotubes, brings about a decrease in the surface free energy of the inner surface of the tubes and an increase in colombic efficiency of Li/nanotubes rechargeable cells in an aprotic medium. Electrochemical fluoride-ion doping of fullerene C₆₀ thin films (250-450 nm) was carried out in a fluoride-ion conductive solution, MeCN solution of 1 M Et₄NF·4HF. Galvanostatic oxidation yielded C₆₀F_ca.1-3 where fluorine exists as a semi-ionic species in the cavity surrounded by C₆₀ molecules without forming covalent CF bonds     

Solid-state NMR and EPR study of fluorinated carbon nanofibers

Carbon nanofibers were fluorinated in two manners, in pure fluorine gas (direct fluorination) and with a fluorinating agent (TbF₄ during the so-called controlled fluorination). The resulting fluorinated nanofibers have been investigated by solid-state nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR). This underlines that the fluorination mechanisms differ since a (CF)_n structural type is obtained, whatever the temperature, with the controlled reaction, whereas, during the direct process, a (C₂F)_n type is formed over a wide temperature range. Through a careful characterization of the products, i.e. density of dangling bonds (as internal paramagnetic centers), structural type (acting on molecular motion) and specific surface area (related to the amount of physisorbed O₂), the effect of atmospheric oxygen molecules on the spin-lattice nuclear relaxation has been underlined.     

Surface modification of several carbon-based materials: comparison between CF4 rf plasma and direct F2-gas fluorination routes

Due to their extreme reactivity, fluorine and fluorinated gases may be used to modify the surface properties of numerous materials. In the following, the surface fluorination of some carbon-based compounds (graphite, graphitised carbon fibres, carbon blacks and elastomers) using CF₄ rf plasma technique and direct F₂-gas fluorination is proposed. From XPS studies, the different types of CF bonding obtained in the materials after treatment have been correlated either to the physico-chemical characteristics of the pristine material or to the experimental parameters of the fluorination. Reaction mechanisms are proposed.