Physical characteristics and rate performance of (CFx)n (0.33 < x < 0.66) in lithium batteries

Sub-fluorinated graphite fluorides (CF_x)_n compounds, 0.33 < x < 0.63 were prepared from natural graphite and characterized by TGA, SEM-EDX and XRD. Their cathode behavior in lithium batteries was investigated under different discharge rates and compared to commercial petroleum coke based (CF)_n. At low discharge rate, the energy density increases with the fluorine content x. However, at higher rates, sub-fluorinated compounds performed better than commercial (CF)_n. The result is discussed with relation to higher electrical conductivity of sub-fluorinated compounds.     

Carbon-coated fluorinated graphite for high energy and high power densities primary lithium batteries

The electrochemical performances of fluorinated graphite have been improved by coating a uniform carbon layer on commercial CF_x (x = 1) powder used as cathode material in lithium battery. In comparison with the cell using un-coated CF_x as cathode, the cell using carbon coated CF_x cathode has a higher energy density and higher power density, particularly at higher discharge current rates (1C above). This is because the conductive carbon coating provides the exterior connectivity between particles for facile electron conduction, resulting in high rate performance.     

Study and optimisation of manganese oxide-based electrodes for electrochemical supercapacitors

A manganese oxide material was synthesised by an easy precipitation method based on reduction of potassium permanganate(VII) with a manganese(II) salt. The material was treated at different temperatures to study the effect of thermal treatment on capacitive property. The best capacitive performance was obtained with the material treated at 200 °C. This material was used to prepare electrodes with different amounts of polymer binder, carbon black and graphite fibres to individuate the optimal composition that gave the best electrochemical performances. It was found that graphite fibres improve the electrochemical performance of electrodes. The highest specific capacitance (267 F g⁻¹ MnO_x) was obtained with an electrode containing 70% of MnO_x, 15% of carbon black, 10% of graphite fibres and 5% of PVDF. This electrode, with CB/GF ratio of 1.5, showed a higher utilization of manganese oxide. The results reported in the present paper further confirmed that manganese oxide is a very interesting material for supercapacitor application.     

Lithium intercalation in heat-treated petroleum cokes

Petroleum needle cokes were processed by air-milling and heat treatment at three temperatures 1800, 2100 and 2350 °C, to produce a final average particle size of 10 p.m. The effects of air-milling (before and after heat treatment) on the physical and microstructural properties of the petroleum coke particles were examined. The results obtained for electrochemical lithium intercalation/de-intercalation in 0.5 M LiN(CF₃SO₂)₂/EC:DMC electrolyte using these petroleum cokes after the different processing conditions are reported.     

Improved electrochemical properties of amorphous Mg65Ni27La8 electrodes: Surface modification using graphite

Amorphous Mg₆₅Ni₂₇La₈ alloy is prepared by melt-spinning. The alloy surface is modified using different contents of graphite to improve the performances of the Mg₆₅Ni₂₇La₈ electrodes. In detail, the electrochemical properties of (Mg₆₅Ni₂₇La₈) + xC (x = 0–0.4) electrodes are studied systematically, where x is the mass ratio of graphite to alloy. Experimental results reveal that the discharge capacity, cycle life, discharge potential characteristics and electrochemical kinetics of the electrodes are all improved. The surface modification enhances the electrocatalytic activity of the alloy, reduces the contact resistance of the electrodes and obstructs the formation of Mg(OH)₂ on the alloy surface. An optimal content of graphite has been obtained. The (Mg₆₅Ni₂₇La₈) + 0.25 C electrode has the largest discharge capacity of 827 mA h g⁻¹, which is 1.47 times as large as that of the electrode without graphite, and the best electrochemical kinetics. Further increasing of graphite content will lead to the increase of contact resistance and activation energy for charge-transfer reaction of the electrode, resulting in the degradation of electrode performance.     

Carbothermal treatment for the improved discharge performance of primary Li/CFx battery

Carbothermal treatment was used to improve the discharge rate performance of primary lithium/carbon monofluoride (Li/CF_x with x = 1) batteries. The treatment was carried out by heating a mixture of CF_x and carbon black (CB) just below the decomposition temperature of CF_x under nitrogen for 2 h. In the treatment, poly(vinylidene fluoride-co-hexafluoropropylene) (Kynar) was used as a fluorinated polymer binder to press the CF_x/CB mixture into pellets. It was shown that the content of Kynar significantly affected the discharge performance of the resulting treated-CF_x (T-CF_x). This can be attributed to the catalytic effect of HF formed by the pyrolysis of Kynar on the decomposition of CF_x and on the reaction of CB with the volatile fluorocarbons formed by the decomposition of CF_x. The discharge performance of T-CF_x cathode was also affected by the temperature of carbothermal treatment and by the ratio of CF_x to CB. In this work the best result was obtained from a treatment conducted at 470 °C on a 87CF_x/10CB/3Kynar (by weight) mixture. In the discharge condition of C/5 and 20 °C, the Li/CF_x cell with such-obtained T-CF_x cathode showed about 95 mV higher voltage than the control cell while retaining nearly the same specific capacity. Impedance analyses indicate that the improved discharge performance is mainly attributed to a reduction in the cell reaction resistance (R_cr) that includes an ohmic resistance related to the ionic conductivity of the discharge product shell and a Faradic resistance related to the processes of charge-transfer and Li⁺ ion diffusion in the CF_x reaction zone.     

Enhancement of discharge performance of Li/CFx cell by thermal treatment of CFx cathode material

In this work we demonstrate that the thermal treatment of CF_x cathode material just below the decomposition temperature can enhance discharge performance of Li/CF_x cells. The performance enhancement becomes more effective when heating a mixture of CF_x and citric acid (CA) since CA serves as an extra carbon source. Discharge experiments show that the thermal treatment not only reduces initial voltage delay, but also raises discharge voltage. Whereas the measurement of powder impedance indicates the thermal treatment does not increase electronic conductivity of CF_x material. Based on these facts, we propose that the thermal treatment results in a limited decomposition of CF_x, which yields a subfluorinated carbon (CF_x−δ), instead of a highly conductive carbon. In the case of CF_x/AC mixture, the AC provides extra carbon that reacts with F₂ and fluorocarbon radicals generated by the thermal decomposition of CF_x to form subfluorinated carbon. The process of thermal treatment is studied by thermogravimetric analysis and X-ray diffraction, and the effect of treatment conditions such as heating temperature, heating time and CF_x/CA ratio on the discharge performance of CF_x cathode is discussed. As an example, a Li/CF_x cell using CF_x treated with CA at 500 °C under nitrogen for 2 h achieved theretical specific capacity when being discharged at C/5. Impedance analysis indicates that the enhanced performance is attributed to a significant reduction in the cell reaction resistance.     

The improved discharge performance of Li/CFx batteries by using multi-walled carbon nanotubes as conductive additive

The discharge performance of Li/CF_x (x = 1) battery is improved by using multi-walled carbon nanotubes (MWCNTs) as an alternative conductive additive. Compared with the battery using acetylene black as conductive additive at the same amount, the Li/CF_x battery using MWCNTs as conductive additive has higher specific capacity and energy density as well as smoother voltage plateau, especially at higher discharge rate. The specific capacity at discharge rate of 1 C is improved by nearly 26% when MWCNTs are employed as conductive additive. Meanwhile, it is also found that the discharge performance is able to be tuned by the amount of MWCNTs and the battery containing more MWCNTs is favorable to be discharged at higher rates. The specific capacity of Li/CF_x battery with 11.09 wt.% MWCNTs is approximately 712 mAh g⁻¹ at the discharge rate of 1 C. It is proposed that the formed three-dimensional networks of MWCNTs in cathode, which enlarges the contact area of interphase and facilitates electrons delivery, accelerates the rates of lithium ion diffusion into the fluorinated layers and electrons transport in cathode at the same time, which improves the discharge performance of Li/CF_x battery subsequently, especially at higher rates.     

Performance enhancement at low temperatures and in situ X-ray analyses of discharge reaction of Li/(CFx)n cells

In the Sandia National Laboratories internally funded Laboratory Directed Research and Development (LDRD) project we are studying the fundamental limitation(s) of the discharge reaction that reduces the operating voltage of the Li/(CF_x)_n cells at moderate discharge rates. As a subset of this effort, we are evaluating the electrochemical properties of (CF_x)_n electrodes prepared with materials from different vendors at different temperatures and in two different electrolytes in order to provide an optimized system to the above study. The temperatures studied span the range −51 to 72 °C. The electrolytes consist of EC:EMC (3:7 wt.%)–1.2 M LiPF₆ denoted as HCE (Highly Conductive Electrolyte) and EC:PC:EMC (1:1:3 wt.%)–1 M LiBF₄ denoted as SNL-E (Sandia National Laboratories Electrolyte). The four different (CF_x)_n materials studied showed comparable capacity at 0 °C and above in the two electrolytes. However, at sub-ambient temperatures the SNL-E performed better than the HCE. The performance improvement with SNL-E comes mainly from a lower interfacial resistance compared to HCE.     

Electrochemical characteristic and discharge mechanism of a primary Li/CFx cell

dc-polarization and ac-impedance techniques were used to analyze the discharge characteristic of a primary Li/CF_x cell. In most cases, impedance spectrum of a Li/CF_x cell shows a suppressed semicircle followed by a sloping straight line. The semicircle is shown to present a cell reaction resistance (R_cr), which reflects an ohmic resistance (mainly, ionic conductivity of the discharge product shell) and a charge-transfer process. It is shown that the overall resistance of a Li/CF_x cell is dominated by the CF_x cathode, whose resistance is further dominated by the R_cr that is found to be extremely sensitive to the temperature. Therefore, the low temperature performance and rate capability of a Li/CF_x cell are mainly determined by the CF_x cathode. In addition, based on the discharge curve and open circuit voltage (OCV) recovery of a Li/CF_x cell, we proposed a “core-shell” model consisting of a shrinking “CF_x core” and a growing “product shell” for the discharge process of CF_x cathode. The “product shell” plays an important role in the discharge performance of Li/CF_x cells.     

Cresyl diphenyl phosphate effect on the thermal stabilities and electrochemical performances of electrodes in lithium ion battery

To improve the safety of lithium ion battery, cresyl diphenyl phosphate (CDP) is used as a flame-retardant additive in a LiPF₆ based electrolyte. The electrochemical performances of LiCoO₂/CDP-electrolyte/Li and Li/CDP-electrolyte/C half cells are evaluated. The thermal behaviors of Li_0.5CoO₂ and Li_0.5CoO₂-CDP-electrolyte, and Li_xC₆ and Li_xC₆-CDP-electrolyte are examined using a C80 micro-calorimeter. For the LiCoO₂/CDP-electrolyte/Li cells, the onset temperature of single Li_0.5CoO₂ is put off and the heat generation is decreased greatly except the one corresponding to 5% CDP-containing electrolyte. When Li_0.5CoO₂ coexists with CDP-electrolyte, the thermal stability is enhanced. CDP improves the thermal stability of lithiated graphite anode effectively and the addition of 5% CDP inhibits the decomposition of solid electrolyte interphase (SEI) films significantly. The electrochemical tests on LiCoO₂/CDP-electrolyte/Li and Li/CDP-electrolyte/C cells show that when less than 15% CDP is added to the electrolyte, the electrochemical performances are not worsen too much. Therefore, the addition of 5-15% CDP to the electrolyte almost does not worsen the electrochemical performance of LiCoO₂ cathode and graphite anode, and improves theirs thermal stability significantly; thus, it is a possible choice for electrolyte additive.     

Characterization and electrochemical properties of Cx(VOF3)F as positive material for primary lithium batteries

Vanadium oxide fluoride-graphite intercalation compounds, i.e. C_x(VOF₃)F with 17.2 ≤ × ≤ 38.8, have been prepared from V₂O₅ and graphite in a fluorine atmosphere at 130°C. The structural characteristics of these compounds have been deduced from X-ray diffraction and X-ray photoelectron spectroscopy measurements. Intercalation of VOF₃ and fluorine was pointed out. The electrochemical insertion of lithium into C_17.7(VOF₃)F was investigated by chronopotentiometry and a.c. impedence spectroscopy in propylene carbonate-1 M LiClO₄. The chemical diffusion coefficient, D_Li, was estimated to be close to 4.0 × 10⁻¹⁰cm²s⁻¹ for all the values of the intercalation ratio of lithium under study (0 < y < 1.5).     

Experimental validation of the “EasyTest Cell” operational principle for autonomous MEA characterization

The paper presents the experimental validation of the “EasyTest Cell” operational principle via comparative electrochemical tests on MEAs carried out in three types of electrochemical hydrogen energy conversion (EHEC) testing cells: conventional polymer electrolyte membrane fuel cells (PEMFC) and polymer electrolyte membrane water electrolyzers (PEMWE), properly equipped with all the required auxiliaries (products conditioning and supplying, reagents removal, etc.), and the simple, autonomous EasyTest Cell. Along with EasyTest Cell validation and demonstration of its advantages, the influence of argon pressure during sputtering on the electrode characteristics, including gas diffusion limitations was investigated. The electrodes under investigation were magnetron sputtered C/Ti/IrO_x (IrO_x loading in the range 0.12–0.4 mg cm⁻²), C/Ti/IrO_x/Pt/IrO_x (IrO_x 0.08/Pt 0.06/IrO_x 0.08 mg cm⁻²), sputtered at various argon pressure C/Ti/Pt (0.15 and 0.25 mg cm⁻²), and commercial ELAT electrode (V.21, Lot # MB030105-1, Pt loading 0.5 mg cm⁻², E-TEK). The results obtained proved the reliability, simplicity (running-periphery-free) and broadened experimental possibilities of EasyTest Cell over PEMFC and PEMWE single cell testing. Thus, significant cost reduction and resource saving in R&D laboratory can be achieved. Moreover, validation of EasyTest Cell contributes not only to testing facilitations, but potentially to standardization of MEA testing since it gives possibilities for precise control and more uniform distribution of the working parameters applied to the testing object, which are both compulsory for performance comparison and qualifying.     

Synthesis and properties of Sm-deficient Sm1−xBaCo2O5+δ perovskite oxides as cathode materials

Double-layered perovskite oxides of Sm_1−xBaCo₂O_5+δ (S_1−xBCO) with various A-site Sm³⁺-deficiencies (x = 0.00–0.08) were synthesized and evaluated as cathode materials of intermediate-temperature solid oxide fuel cells (IT-SOFCs). The Sm³⁺-deficiency content in S_1−xBCO was limited up to x = 0.05, and higher content x = 0.08 caused impurity phase. S_1−xBCO oxides were chemically stable with GDC electrolyte at 1050 °C and below. Introduction of Sm³⁺-deficiency caused decreased oxygen content and increased concentration of oxygen vacancy in S_1−xBCO. Electrical conductivities of S_1−xBCO decreased with increasing temperature in air, and also changed with the Sm³⁺-deficiency content. Electrochemical performance of S_1−xBCO cathodes were characterized by impedance spectra measurement based on symmetric cells. Higher Sm³⁺ deficiency content has resulted in decreased area specific resistances (ASRs) and activation energy (Ea), i.e. enhanced electrochemical reaction reactivity for the S_1−xBCO cathodes. Among the studied samples, the S_0.95BCO (x = 0.05) oxide showed the best electrochemical performance with ASR values of 0.316 Ω cm² at 600 °C, 0.137 Ω cm² at 650 °C, 0.068 Ω cm² at 700 °C and 0.038 Ω cm² at 750 °C respectively, thus it's a promising cathode material of IT-SOFCs.     

NiOx nanoparticles supported on polyethylenimine functionalized CNTs as efficient electrocatalysts for supercapacitor and oxygen evolution reaction

Ni oxide based nanoparticles (NPs) have been widely used as electrocatalysts in the electrochemical energy storage and conversion applications. In this paper, NiO_x NPs are successfully synthesized by the self-assembly of Ni precursor onto polyethylenimine functionalized carbon nanotubes (PEI-CNTs) assisted with microwave radiation. NiO_x NPs with size around 2–3 nm are homogenously dispersed on the PEI-CNTs supports with no aggregation. The electrochemical activity of NiO_x NPs on PEI-CNTs, NiO_x/PEI-CNTs, as effective electrocatalysts is studied for supercapacitor and oxygen evolution reaction in alkaline solutions. NiO_x/PEI-CNTs show a capacitance of 1728 and 1576 F g⁻¹ based on active material, and 221 and 394 F g⁻¹ based on total catalyst loading on 12.5% and 25% NiO_x/PEI-CNTs, respectively, which is substantially higher than 152 F g⁻¹ of unsupported NiO. The NiO_x/PEI-CNTs electrodes exhibit reversible and stale capacitance of ∼1200 F g⁻¹ based on active materials after 2000 cycles at a high current density of 10 A g⁻¹. NiO_x/PEI-CNTs also exhibit significantly higher activities for oxygen evolution reaction (OER) of water electrolysis, achieving a current density of 100 A g⁻¹ at an overpotential of 0.35 V for 25% NiO_x/PEI-CNTs. It is believed that the uniformly dispersed nano-sized NiO_x NPs and synergistic effect between the NiO_x NPs and PEI-CNTs is attributed to the high electrocatalytic performance of NiO_x/PEI-CNTs electrocatalysts. The results demonstrate that NiO_x NPs supported on PEI-CNTs are highly effective electrocatalysts for electrochemical energy storage and conversion applications.     

CO tolerant PtRu–MoOx nanoparticles supported on carbon nanofibers for direct methanol fuel cells

Novel nanostructured catalysts based on PtRu–MoO_x nanoparticles supported on carbon nanofibers have been investigated for CO and methanol electrooxidation. Carbon nanofibers are prepared by thermocatalytic decomposition of methane (NF), and functionalized with HNO₃ (NF.F). Electrocatalysts are obtained using a two-step procedure: (1) Pt and Ru are incorporated on the carbon substrates (Vulcan XC 72R, NF and NF.F), and (2) Mo is loaded on the PtRu/C samples. Differential electrochemical mass spectrometry (DEMS) analyses establish that the incorporation of Mo increases significantly the CO tolerance than respective binary counterparts. The nature of the carbon support affects considerably the stabilization of MoO_x nanoparticles and also the performance in methanol electrooxidation. Accordingly, a significant increase of methanol oxidation is obtained in PtRu–MoO_x nanoparticles supported on non-functionalized carbon nanofiber, in parallel with a large reduction of the Pt amount in comparison with binary counterparts and commercial catalyst.     

Effects of nitrogen doping on the electrochemical performance of graphite felts for vanadium redox flow batteries

Vanadium redox flow batteries (VRFB), originally proposed by Skyllas‐Kazacos et al., have been considered as one of the most promising energy storage systems for intermittently renewable energy. However, the poor electrochemical activity and hydrophobicity of graphite felt electrode greatly limit energy storage efficiency of VRFB system. In this paper, two nitrogen‐doped (N‐doped) graphite felts, obtained by heat‐treating in an NH₃ atmosphere at 600 °C and 900 °C, have been investigated as electrodes with high electrochemical performance for vanadium redox flow batteries. In particular, the one obtained at 900 °C exhibits an excellent electrochemical activity for both V²⁺/V³⁺ and VO²⁺/VO₂⁺ redox couples. The cells with different graphite felt electrodes were assembled, and the charge–discharge performance was evaluated. The cell with the N‐doped graphite felts has larger discharge capacity, discharge capacity retention, and energy efficiency, especially with the sample treated at 900 °C. The average energy efficiency of the cell with the 900 °C treated N‐doped graphite felts is 86.47%, 5.44% higher than that of the cell with the pristine graphite felt electrodes. These enhanced electrochemical properties of the N‐doped graphite felt electrodes are attributed to the increased electrical conductivity, more active sites, and better wettability provided by the introduction of the nitrogenous groups on the surface of graphite felts. It indicates that N‐doped graphite felts have promising application prospect in VRFB. Copyright © 2015 John Wiley & Sons, Ltd.     

Electro-synthesis of tungsten carbide containing catalysts in molten salt for efficiently electrolytic hydrogen generation assisted by urea oxidation

Energy-efficient production of hydrogen through urea electrolysis is still challenging due to the lack of satisfactory catalysts for urea oxidation reaction (UOR) and hydrogen evolution reaction (HER) in urea containing solution. In this study, Ni–W_xC/C (x = 1,2) composite with high activity for urea electrocatalysis was prepared by direct electro-reduction of affordable feedstock of NiO–CaWO₄–C in molten CaCl₂–NaCl at 873–973 K. The addition of graphite in precursor decreases the particle size of Ni. Introducing W_xC into Ni particles can reduce the overpotential for UOR. As a result, the obtained Ni-W_xC/graphite composite exhibits high current density for urea oxidation, which is about 11-folds and 52-folds higher than that of Ni/graphite and Ni (@1.53 V vs. RHE), respectively. After changing the carbon source from graphite to CNTs, the anodic current density was further increased by 43%, reaching 50.31 mA cm^?2. Moreover, the cathodic catalyst W_xC/CNTs obtained by the same preparation process exhibits high performance towards HER, with a low onset potential of 131.5 mV and a Tafel slope of 69.5 mV dec^?1. Assembling an electrolyzer using Ni-W_xC/CNTs as anode and W_xC/CNTs as cathode can yield a current density of 10 mA cm^?2 at merely 1.65 V in 1 M KOH/0.33 M urea aqueous solution, with excellent long-term electrochemical durability. The environmental-friendly production process uses affordable feedstocks for the synthesis of efficient catalysts toward urea electrolysis, promising an energy-saving hydrogen production as well as waste treatment.     

Electrochemical behaviour of carbon electrodes in some electrolyte solutions

The electrochemical properties of coke and natural graphite in some electrolyte solutions containing diethylcarbonate (DEC) are studied. It is found that natural graphite exhibits am excellent performance, such as high discharge capacity (370 mAh⁻¹ g), when a mixed solvent composed of ethylene carbonate (EC) and DEC is used. The charge/discharge characteristics of the coke electrode are mot influenced by the species of the electrolyte solution, but those of the natural graphite electrode are very much influenced by the species of the electrolyte solution. It is confirmed that there are three patterns in the behaviour of the graphite electrode in the electrolyte solutions tested in this investigation. In the first pattern, natural graphite can be charged to C₆Li and them discharged. In the second pattern, the charging and discharging of the natural graphite electrode is impossible and destruction of the natural graphite crystal structure is observed. In the third pattern, lithium is intercalated into the graphite layer but the de-intercalation of lithium does not take place.     

Scandium-doped PrBaCo2−xScxO6−δ oxides as cathode material for intermediate-temperature solid oxide fuel cells

Scandium-doped PrBaCo_2−xSc_xO_6−δ(PBCS-x, x = 0.00–1.00) oxides have been evaluated as cathode materials of intermediate-temperature solid oxide fuel cells (IT-SOFCs) with respect to phase structure, oxygen content, thermal expansion behavior and electrical and electrochemical properties. The XRD results have demonstrated a phase transition in PBCS-x due to Sc³⁺ doping from tetragonal double-layered perovskite structure at x = 0.00–0.20, bi-phase mixtures at x = 0.30–0.40, to cubic perovskite structure at x = 0.50–0.90. The oxygen contents (6-δ) and average valences of cobalt ions in PBCS-x decrease with the higher Sc³⁺ content and increasing temperatures in air. Sc³⁺ doping has also led to decreased thermal expansion coefficients, lowered electrical conductivities and enhanced electrochemical reaction activities for PBCS-x characterized by decreased area-specific resistances (ASRs) and smaller reaction activation energies. Among the studied samples, the PBCS-0.50 oxide with Sc³⁺-doping content of x = 0.50 exhibits the best electrochemical performance on Ce_0.9Gd_0.1O_1.95 electrolyte. Its ASR values range from 0.123 Ω cm² at 600 °C to 0.022 Ω cm² at 750 °C, which are much lower than the related cathode materials. These results have demonstrated that the PBCS-0.50 oxide is a promising cathode material for IT-SOFCs.