Cathode materials based on carbon nanotubes for high-energy-density lithium–sulfur batteries

The rational integration of conductive nanocarbon scaffolds and insulative sulfur is an efficient method to build composite cathodes for high-energy-density lithium–sulfur batteries. The full demonstration of the high-energy-density electrodes is a key issue towards full utilization of sulfur in a lithium–sulfur cell. Herein, carbon nanotubes (CNTs) that possess robust mechanical properties, excellent electrical conductivities, and hierarchical porous structures were employed to fabricate carbon/sulfur composite cathode. A family of electrodes with areal sulfur loading densities ranging from 0.32 to 4.77 mg cm⁻² were fabricated to reveal the relationship between sulfur loading density and their electrochemical behavior. At a low sulfur loading amount of 0.32 mg cm⁻², a high sulfur utilization of 77% can be achieved for the initial discharge capacity of 1288 mAh g_S⁻¹, while the specific capacity based on the whole electrode was quite low as 84 mAh g_{C/S+binder+Al}⁻¹ at 0.2 C. Moderate increase in the areal sulfur loading to 2.02 mg cm⁻² greatly improved the initial discharge capacity based on the whole electrode (280 mAh g_{C/S+binder+Al}⁻¹) without the sacrifice of sulfur utilization. When sulfur loading amount further increased to 3.77 mg cm⁻², a high initial areal discharge capacity of 3.21 mAh cm⁻² (864 mAh g_S⁻¹) was achieved on the composite cathode.     

Hierarchical carbon nanotube membrane with high packing density and tunable porous structure for high voltage supercapacitors

We reported the fabrication of a hierarchical carbon nanotube (CNT) membrane by using the 90% granulated double- or triple-walled CNTs and 10% 100 μm long multiwalled CNTs as the linker. The membrane with packing density of 420 kg/m³, excellent electrical conductance and good mechanical strength, functioned as both the electrode and current collector and allowed the weight ratio of CNTs increased up to 45–50% based on the weight of CNT, electrolyte and separator. The granulated double or triple walled CNTs, by the aggregation at high temperature etching using CO₂, simultaneously exhibited high surface area and tunable pore structure and high pore volume, and were favorable for the ion transport of organic electrolyte, due to the effect of opening cap or side wall by the CO₂. The CNT membrane electrode, exhibited the capacitance of 57.9 F/g and the energy density of 35 W h/kg, as operated at 4 V.     

On the manufacturing and electrical and mechanical properties of ultra-high wt.% fraction aligned MWCNT and randomly oriented CNT epoxy composites

The effect of CNT orientation on electrical and mechanical properties is presented on the example of an ultra-high filler loaded multi-walled carbon nanotube (68 wt.% MWCNTs) epoxy-based nanocomposite. A novel manufacturing method based on hot-press infiltration through a semi-permeable membrane allows to obtain both, nanocomposites with aligned and randomly oriented CNTs (APNCs and RPNCs) over a broad filler loading range of ≈10–68 wt.%. APNCs are based on low-defected, mm-long aligned MWCNT arrays grown in chemical vapour deposition (CVD) process. Electrical conductivity and mechanical properties were measured parallel and perpendicular to the direction of CNTs. RPNCs are based on both, aligned mm-long MWCNTs and randomly oriented commercial μm-long and entangled MWCNTs (Baytube C150P, and exemplarily Arkema Graphistrength C100). The piezoresistive strain sensing capability of these high-wt.% APNCs and RPNCs had been investigated towards the influence of CNT orientations. For the highest CNT fraction of 68 wt.% of unidirectional aligned CNTs a Young’s modulus of E_|| ≈ 36 GPa and maximum electrical conductivity of σ_|| ≈ 37·10⁴ S/m were achieved.     

A high-capacity lithium–air battery with Pd modified carbon nanotube sponge cathode working in regular air

We report a lithium–air battery with a free-standing, highly porous Pd-modified carbon nanotube (Pd–CNT) sponge cathode. The Pd-CNT sponge was synthesized through a chemical vapor deposition growth followed with an electrochemical deposition process. To build a whole lithium–air battery, the air cathode is integrated with a ceramic electrolyte-protected lithium metal anode and non-volatile ionic liquid electrolyte. The lithium anode is stable during the operation and long-time storage and the ionic liquid is chemically inert. By controlling the amount of ionic liquid electrolyte, the sponge is wet but not fulfilled by the electrolyte. Such configuration offers a tricontinuous passage for lithium ions, oxygen and electrons, which is propitious to the discharge reaction. In addition, the existence of Pd nanoparticles improves the catalytic reactivity of the oxygen reduction reaction. The battery is durable to any humidity level and delivers a capacity as high as 9092 mA h g⁻¹.     

Preparation and characterization of manganese oxide/CNT composites as supercapacitive materials

Manganese oxide was synthesized and dispersed on carbon nanotube (CNT) matrix by thermally decomposing manganese nitrates. CNTs used in this paper were grown directly on graphite disk by chemical vapor deposition technique. The capacitive behavior of manganese oxide/CNT composites was investigated by cyclic voltammetry and galvanostatic charge–discharge method in 1 M Na₂SO₄ aqueous solutions. When the loading mass of MnO₂ is 36.9 μg cm^− 2, the specific capacitance of manganese oxide/CNT composite (based on MnO₂) at the charge–discharge current density of 1 mA cm^− 2 equals 568 F g^− 1. Additionally, excellent charge–discharge cycle stability (ca. 88% value of specific capacitance remained after 2500 charge–discharge cycles) and power characteristics of the manganese oxide/CNT composite electrode can be observed. The effect of loading mass of MnO₂ on specific capacitance of the electrode has also been investigated.     

Lithium–sulfur batteries with activated carbons derived from olive stones

A lithium–sulfur battery, using activated carbon obtained from olive stones as the sulfur host, is reported. The microporous texture allows large amounts of sulfur to be infiltrated into the host (sulfur loading 80%). The resulting composite material possesses a high capacity, about 670 mA h g⁻¹, excellent capacity retention on cycling and good rate capability. We believe that activated carbons derived from biomass could be an alternative source for the preparation of the cathode for Li–S batteries.     

Large-scale synthesis of ordered mesoporous carbon fiber and its application as cathode material for lithium–sulfur batteries

A novel type of one-dimensional ordered mesoporous carbon fiber has been prepared via the electrospinning technique by using resol as the carbon source and triblock copolymer Pluronic F127 as the template. Sulfur is then encapsulated in this ordered mesoporous carbon fibers by a simple thermal treatment. The interwoven fibrous nanostructure has favorably mechanical stability and can provide an effective conductive network for sulfur and polysulfides during cycling. The ordered mesopores can also restrain the diffusion of long-chain polysulfides. The resulting ordered mesoporous carbon fiber sulfur (OMCF-S) composite with 63% S exhibits high reversible capacity, good capacity retention and enhanced rate capacity when used as cathode in rechargeable lithium–sulfur batteries. The resulting OMCF-S electrode maintains a stable discharge capacity of 690 mAh/g at 0.3 C, even after 300 cycles.     

Nano-sized LiNi0.5Mn1.5O4 cathode powders with good electrochemical properties prepared by high temperature flame spray pyrolysis

LiNi_0.5Mn_1.5O₄ cathode powders with a mean particle size of 140 nm are prepared by high-temperature flame spray pyrolysis. Li/LiNi_0.5Mn_1.5O₄ cells show two plateaus at approximately 4.1 and 4.7 V during discharge, irrespective of any excess of the lithium component in the spray solution, although the 4.1 V plateau decreases when the spray solution contained 20% excess lithium. The discharge capacity of the powder prepared from a spray solution with 20% excess lithium decreases from 133 to 126 mAh g^?1 by the 50th cycle at a current density of 0.1 C, which is a capacity retention of 95%.     

Dry micro-electro-discharge machining of carbon-nanotube forests using sulphur-hexafluoride

The effect of using sulphur hexafluoride (SF₆), a high-dielectric-strength gas, for dry micro-electro-discharge machining (μEDM) of carbon-nanotube (CNT) forests is investigated. It is found that SF₆ enables μEDM of CNTs without O₂, which is known to be essential for CNT machining in N₂. The process in the SF₆ ambient at a discharge voltage of 25 V is found to lead to a smaller discharge gap, i.e., tighter tolerance as well as higher machining quality compared with the N₂ case at the same voltage. The N₂ environment produces smaller discharge gap when 10 V is used; however, both the quality and rate of machining are somewhat lower in this case. The mixture with 20% O₂ in SF₆ is revealed to be an optimum condition for machining tolerance and quality. CNT forests are used as the cathode in the process, as opposed to conventional μEDM where the workpiece forms the anode. This configuration in the SF₆–O₂ mixture is observed to generate higher discharge currents at low voltages, presumably due to effective field-emission by the CNTs, leading to finer and cleaner machining. Energy-dispersive X-ray analysis reveals that the optimal conditions result in less contamination by the electrode element on the processed forest surfaces.     

Hydrolysis lignin: Electrochemical properties of the organic cathode material for primary lithium battery

Electrochemical energy production has been extensively used in large scale applications. At present, organic compounds are considered as efficient and environmentally friendly electrode materials. The paper describes the study of the possibility of using hydrolysis lignin as the lithium battery cathode material. Hydrolysis lignin features have been investigated by the impedance spectroscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The discharge performance of hydrolysis lignin-based lithium battery was investigated at room temperature using 1 M LiBF₄ in γ-butyrolacton electrolyte system. It was found that the specific capacity of hydrolysis lignin was equal to 450 mAh g⁻¹ at a discharge current density of 25 μA/cm². Two main voltage plateaus located at ∼1.8 and ∼1.1 V were observed. The chemical composition of cathode materials upon battery discharge down to 0.9 V was studied by the X-ray photoelectron spectroscopy and infrared spectroscopy. The suggestions on possible electrochemical reactions occurring in the lithium/hydrolysis lignin system were made on the basis of the products composition analysis. The results demonstrate the potential of hydrolysis lignin based batteries to be used as low-rate power sources.     

Growth of carbon nanotube forests between a bi-metallic catalyst layer and a SiO2 substrate to form a self-assembled carbon–metal heterostructure

A special nanostructure was formed by the growth of carbon nanotubes (CNTs) between a substrate and a thin bi-metallic catalyst layer using a thermal chemical vapor deposition process. The catalyst layer is composed of adjacently disposed Cr and Ni phases formed prior to CNT growth. The Cr/Ni layer serves as a bi-metallic catalyst layer, which is pushed away from the substrate as a thin and continuous nanomembrane with the growth of CNTs. The self-assembled CNT–catalyst heterostructure possesses a smooth surface (RMS = 2.9 nm) with a metallic shine. Directly interlinked to the Cr/Ni layer, dense and vertically aligned multi-walled CNTs are found. Compared to conventional CNT films, the structure has significant advantages for CNT integration. From technology point of view, the structure allows further processing without impact on the CNTs as well as transfer of pristine vertically aligned CNTs to arbitrary substrates. Moreover, the as-grown CNT films provide an interface ideal for further electrical, thermal and mechanical contacting of CNT films. We present structural investigations of this special CNT–metal heterostructure. Furthermore, we discuss possible interface mechanisms during catalyst layer formation and CNT growth.     

Extraction of europium and electrodeposition of Al–Li–Eu alloy from Eu2O3 assisted by AlCl3 in LiCl–KCl melt

The work presents an electrochemical study on preparation of Al–Li–Eu alloys on a tungsten electrode in molten LiCl–KCl–AlCl₃–Eu₂O₃ system at 753 K and 953 K. Gibbs energy shows that AlCl₃ can chloridize Eu₂O₃, with a discharge in the form of Eu(III) ions on the cathode. The electrochemical behavior of Al(III), Li(I) and Eu(III) and alloy formation processes were investigated by cyclic voltammetry, square wave voltammetry, and chronopotentiometry. Cyclic voltammetry indicated that the underpotential deposition of europium on pre-deposited Al forms two Al–Eu intermetallic compounds at electrode potentials around ?2.00 V and ?2.34 V, respectively. And the underpotential deposition of lithium on Al surface at about ?2.24 V leads to a formation of Al–Li alloy. X-ray diffraction (XRD) indicated that Al–Li–Eu alloys with different phases were obtained via galvanostatic electrolysis. The microstructure and micro-zone chemical analysis of Al–Li–Eu alloy were characterized by scanning electron microscopy (SEM) with energy dispersive spectrometry (EDS), respectively. The analysis of EDS showed that element Eu mainly distributes on needle-like precipitate, and not homogeneously in the Al–Li–Eu alloy. Composition of the alloys was analyzed by inductive coupled plasma analysis, and current efficiency was also determined with respect to the alloy composition.     

Very large cathode current and long term stability of vacuum sealed tubes with engrafted-carbon-nanotube emitters

For the future commercial applications of carbon nanotubes (CNTs) in high power vacuum microwave amplifiers or compact X-ray tubes, we have attempted to fabricate engrafted CNT field emitters on a metallic substrate using both screen printing and chemical vapor deposition. Cobalt nano-grains are doped in the printed CNT paste and act as the catalyst for the engrafted growth of CNTs by the cold wall chemical vapor deposition. Stable cathode current (~ 30 mA) from a small area (~ 1.5 mm²) of engrafted CNT emitters was measured in a vacuum-sealed diode tube. High current density (> 1.6A/cm²) has also gotten in the vacuum sealed tube in which the emitters spread about 0.78 mm² after an aging process that lasts more than 12 h in DC mode with the water cooling of the anode.     

Pressureless sintering of carbon nanotube–Al2O3 composites

Alumina ceramics reinforced with 1, 3, or 5 vol.% multi-walled carbon nanotubes (CNTs) were densified by pressureless sintering. Commercial CNTs were purified by acid treatment and then dispersed in water at pH 12. The dispersed CNTs were mixed with Al₂O₃ powder, which was also dispersed in water at pH 12. The mixture was freeze dried to prevent segregation by differential sedimentation during solvent evaporation. Cylindrical pellets were formed by uniaxial pressing and then densified by heating in flowing argon. The resulting pellets had relative densities as high as ～99% after sintering at 1500 °C for 2 h. Higher temperatures or longer times resulted in lower densities and weight loss due to degradation of the CNTs by reaction with the Al₂O₃. A CNT/Al₂O₃ composite containing 1 vol.% CNT had a higher flexure strength (～540 MPa) than pure Al₂O₃ densified under similar conditions (～400 MPa). Improved fracture toughness of CNT–Al₂O₃ composites was attributed to CNT pullout. This study has shown, for the first time, that CNT/Al₂O₃ composites can be densified by pressureless sintering without damage to the CNTs.     

High capacity Li storage in sulfur and nitrogen dual-doped graphene networks

Three-dimensional (3D) networks composing of S and N dual-doped graphene (SNG) were synthesized by a chemical vapor deposition approach using MgSO₄-containing whiskers as templates and S source and NH₃ as N source. Energy dispersive spectrometer mapping and X-ray photoelectron spectroscopy coupled with Raman analysis have revealed that S and N atoms with concentrations of 5.2 and 1.8 atom%, respectively, have been substitutionally incorporated into the graphene networks via covalent bonds. The SNG, as an anode material for lithium ion batteries (LIBs), exhibits extremely high capacity (3525 mAh/g at the current density of 50 mA/g) and superior rate capability (870 mAh/g at 1000 mA/g) with excellent cycling stability (remaining a reversible capacity of 400 mAh/g at 10 A/g after 2500 cycles). The enhanced conductivity, the 3D porous network with many disorders and the intrinsically high Li storage capacity of S and N-doped carbon segments have led to the excellent electrode performance of the SNG networks. The effects of binder content and calendaring pressure on the electrode performance have been investigated. The full LIB with SNG as anode and LiCoO₂ as cathode can afford a high reversible capability (164 mAh/g at 0.2 C) and good cycling stability.     

A review on integrating nano-carbons into polyanion phosphates and silicates for rechargeable lithium batteries

Lithium metal phosphate (Li₂MPO₄) and silicates (Li₂MSiO₄) (where M = Fe, Mn, and Co) are promising polyanion cathodes for rechargeable lithium batteries, owing to the inherent merits such as low cost, decent electrochemical property, and high stability. However, these merits have often been undermined by insufficient energy and power delivery due to poor Li extraction/insertion kinetics. It is generally believed that the extremely low conductivity, i.e. ∼10⁻⁹ s cm⁻¹ for phosphates and 10⁻¹²–10⁻¹⁶ s cm⁻¹ for silicates at room temperature, in combination with slow Li ion diffusion could account for such sluggish Li cycling kinetics. To address this critical issue, it is essential to integrate well-defined nano-carbons such as one-dimensional (1D) carbon nanotube (CNT), two-dimensional (2D) graphene, and their three-dimensional (3D) assembly into polyanion materials. By constructing hybrid architectures, integrated composites could afford much improved activity towards Li storage versus the bare ones. In this short review, we summarize recent advance in integrating CNT, graphene, and their 3D assemblies into LiMPO₄ and Li₂MSiO₄ cathodes, with particular emphasis on how the cathodes interact with carbon materials and their mechanism. We also conclude some general rules to engineer such integration structures to maximize their utilization towards Li storage.     

Overcoming the quality–quantity tradeoff in dispersion and printing of carbon nanotubes by a repetitive dispersion–extraction process

Dispersion-printing processes are essential for the fabrication of various devices using carbon nanotubes (CNTs). Insufficient dispersion results in CNT aggregates, while excessive dispersion results in the shortening of individual CNTs. To overcome this tradeoff, we propose here a repetitive dispersion–extraction process for CNTs. Long-duration ultrasonication (for 100 min) produced an aqueous dispersion of CNTs with sodium dodecylbenzene sulfonate with a high yield of 64%, but with short CNT lengths (a few μm), and poor conductivity in the printed films (∼450 S cm⁻¹). Short-duration ultrasonication (for 3 min) yielded a CNT dispersion with a very small yield of 2.4%, but with long CNTs (up to 20–40 μm), and improved conductivity in the printed films (2200 S cm⁻¹). The remaining sediment was used for the next cycle after the addition of the surfactant solution. 90% of the CNT aggregates were converted into conductive CNT films within 13 cycles (i.e., within 39 min), demonstrating the improved conductivity and reduced energy/time requirements for ultrasonication. CNT lines with conductivities of 1400–2300 S cm⁻¹ without doping and sub-100 μm width, and uniform CNT films with 80% optical transmittance and 50 Ω/sq sheet resistance with nitric acid doping were obtained on polyethylene terephthalate films.     

The growth of carbon nanotubes in aluminum powders by the catalytic pyrolysis of polyethylene glycol

A simple approach based on the catalytic pyrolysis of polyethylene glycol (PEG) was developed to grow a uniform dispersion of carbon nanotubes (CNTs) in Al powders and thus supply raw material for the powder metallurgy fabrication of CNT/Al composites. Al nanoflake powders with quite a large surface area were used to adsorb a homogeneous PEG and citric acid film, and then were impregnated with a cobalt nitrate solution to anchor Co(II) by complexation with citric acid. Then the Al nanoflake powders were heated to 230 °C to form Co oxide nanoparticles, and then to 570 °C to induce the thermal decomposition of PEG. The pyrolytic products of PEG not only served as the reducing agent to reduce Co oxide to Co nanoparticle catalyst, but also as the carbon source for CNT growth. As a result, 2.13 wt.% graphitic CNTs, with diameters of 10–20 nm and length ranging from sub-micron to a few micrometers, were homogeneously grown in 500 nm thick Al nanoflakes. And the as-obtained CNT/Al composites fabricated by hot-pressing exhibited enhanced strength, which was almost two times that of the matrix.     

Enhanced field emission properties from titanium-coated carbon nanotubes

The effect of titanium (Ti) coating over the surface of carbon nanotubes (CNTs) on field emission characteristics was investigated. Vertically aligned CNTs were grown by inductively-coupled plasma-enhanced chemical vapor deposition (ICP-CVD). In order to reduce the screening effect of electric field due to densely packed CNTs, as-grown CNTs were partly etched back by DC plasma of N₂. Ti with various thicknesses from 5 nm to 150 nm was coated on CNTs by a sputtering method. Since thick Ti coating with thickness of 100 nm or more resulted in the shape of a metal post by merging an individual CNT in a bundle, it was inadequate to a field emission application. On the other hand, thin Ti-coated CNTs with thickness of 10 nm or less showed a lower turn-on field, a higher emission current density, and improved emission uniformity compared with pristine CNTs. The improved emission performance was mainly attributed to the low work function of Ti and the reliable and lower resistance contact between CNTs and substrates.     

Improved mechanical performance of solution-processed MWCNT/Ag nanoparticle composite films with oxygen-pressure-controlled annealing

A method for the synthesis of solution process-based MWCNT/Ag nanoparticle composite thin films as electrode or interconnect materials for flexible electronic devices is presented. The method produces homogeneously-dispersed CNT networks and increases the density of the Ag matrix, which are major factors in determining the mechanical performance of CNT/Ag films. By introducing nanometer-sized Ag particles as a matrix material, the agglomeration of CNTs is suppressed. In addition, the generation of pores during the synthesis procedure is effectively restrained by oxygen-pressure-controlled annealing. The elastic modulus of the pristine Ag films was observed to increase by 34% by adding 5 wt% CNTs. An improvement in the fatigue resistance of the CNTs under cyclic tensile deformation was confirmed. The normalized resistance change ((R ? R_o)/R_o) of the Ag films containing 5 wt% CNTs after fatigue testing was reduced by about 27% compared to that of the pristine Ag films. For industrial application the process has the advantage of relatively low-temperature processing without any high pressure compaction compared to the conventional powder metallurgy techniques normally used.