The electrochemical performance of rechargeable Li-air batteries containing a reduced graphene oxide (rGO)/alpha-MnO2 composite and neat alpha-MnO2 electrode is studied. The rGO/alpha-MnO2 composite exhibits a specific capacity as high as 558.4 mA h g(-1) at a current density of 100 mA g(-1), indicating its potential to make a good cathode material. The composite electrode also presents a relatively moderate degradation of capacities with increasing cycles, compared to the neat alpha-MnO2 electrode. rGO functions as the conducting medium to connect the alpha-MnO2 nanorods, thus improving the Li ion transfer. The mechanisms responsible for the capacity degradation in the composite electrodes are studied after a series of interrupted charge/discharge cycles, detecting Li2O2 and LiF as the main reaction products formed on the electrode surface. In particular, the LiF layer is identified to be an important component of reaction products, which serves as a barrier to reactions between the Li ions and electrons with the electrode, giving rise to detrimental effects on the cyclic and capacity performance of Li-air batteries.

Tri-layer composite microspheres consisting of a carbon shell, a porous LiFePO4 layer and a carbon core with a hierarchical conductive structure are synthesized as cathode materials for Li ion batteries. The electrodes made from the core-shell microspheres deliver remarkable specific capacities of 160.0, 121.0, 101.5, 77.0 and 59.7 mA h g(-1) at 1C, 10C, 20C, 30C and 40C, respectively.

A facile, low temperature polyol method is developed to synthesize graphene-wrapped, rod-shaped LiFePO4 (LFP) cathode materials. The effects of reaction temperature and reduced graphene oxide (RGO) on the morphology and electrochemical performance of RGO/LFP composites are studied. The RGO/LFP containing 10 wt% precursor GO and synthesized at 220 degrees C presents the best electrochemical properties, namely discharge specific capacities of 164.1, 156.7 and 121.5 mA h g(-1) after 100 cycles with capacity retention ratios of 99.0, 97.9 and 98.6% at 0.1 C, 1 C and 10 C, respectively. These values are considered among the highest reported in the literature for LFP cathodes with similar carbon additives. The improved electronic conduction and reduced charge transfer resistance arising from the RGO sheets are responsible for the excellent electrochemical properties.

The ever-increasing demands for higher energy density and higher power capacity of Li-ion secondary batteries have led to search for electrode materials whose capacities and performance are better than those available today. Carbon nanotubes (CNTs), because of their unique 1D tubular structure, high electrical and thermal conductivities and extremely large surface area, have been considered as ideal additive materials to improve the electrochemical characteristics of both the anode and cathode of Li-ion batteries with much enhanced energy conversion and storage capacities. Recent development of electrode materials for LIBs has been driven mainly by hybrid nanostructures consisting of Li storage compounds and CNTs. In this paper, recent advances are reviewed of the use of CNTs and the methodologies developed to synthesize CNT-based composites for electrode materials. The physical, transport and electrochemical behaviors of the electrodes made from composites containing CNTs are discussed. The electrochemical performance of LIBs affected by the presence of CNTs in terms of energy and power densities, rate capacity, cyclic life and safety are highlighted in comparison with those without or containing other types of carbonaceous materials. The challenges that remain in using CNTs and CNT-based composites, as well as the prospects for exploiting them in the future are discussed.

This paper reports the preparation and characterization of LiNi1/3Co1/3Mn1/3O2 (NCM) cathodes containing multi-walled carbon nanotubes (MWCNTs) with different types of functionalization, including acid treatment, amino functionalization and silanization. Among these MWCNTs, the silane-functionalized CNTs give rise to significant reduction in inner electronic resistance and improvements in both capacity retention and rate capability of the nanocomposite cathodes. This finding is attributed to the synergy to form 3D spatial conductive networks among the active NCM particles by the well-dispersed MWCNTs and carbon black. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.013301jes] All rights reserved.

Graphene oxide/carbon nanotube (GO/CNT) hybrid films are self-assembled on a Ti substrate via simple casting of aqueous dispersion. The amphiphilic nature of graphene oxide sheets allows adsorption of CNTs onto their surface in water, capable of forming a highly stable dispersion. Binder-free electrodes are prepared using the annealed GO/CNT films for high performance supercapacitors. The hybrid film electrodes with a moderate CNT content, typically 12.5 wt%, give rise to remarkable electrochemical performance with extremely high specific capacitances of 428 and 145 F g(-1) at current densities of 0.5 and 100 A g(-1), respectively, as well as a remarkable retention rate of 98% of the initial value after 10 000 charge/discharge cycles. The synergistic effects arising from (i) the enlarged surface area of electrodes due to the intercalation of CNTs between the stacked GO sheets with associated large electrochemical active sites and (ii) the improved conductivity through the formation of a 3D network aided by CNTs are mainly responsible for these findings.

The effects of the reduction process and carbon nanotube (CNT) content on the supercapacitive behavior of electrodes made from flexible, binder-free thick graphene oxide (GO) papers are studied. It is found that the supercapacitive performance depends on several factors, including the presence of oxygenated functional groups after reduction, the interlayer spacing of the GO papers and their wettability with electrolyte. A moderate reduction of GO papers using hydrazine or annealing at a low temperature of 220°C in air is proven to be more beneficial to achieve a high capacitance than the heavy reduction using a hydrazine vapor or a high temperature thermal treatment. The addition of a small amount of CNT, typically 12.5wt.%, to form thick GO/CNT sandwich papers gives rise to an excellent specific capacitance of 151Fg−1 at a current density of 0.5Ag−1, as well as a retention ratio of 86% of the initial value after 6000 charge/discharge cycles at 5Ag−1. These improvements arise from the synergistic effects of the increased electronic conductivity and effective surface area associated with large electrochemical active sites due to the presence of intercalated CNT.

A facile strategy is developed based on a sol-gel method to prepare lithium titanate (Li4Ti5O12, LTO)-carbon nanofiber (CNF) composites as anode in Li-ion batteries (LIBs). Depending on the conductive CNF and carbon black (CB) additive contents added in the electrode, the resultant composite particles present either an urchin-like or a corn-dog structure with largely different electronic conductivities and associated electrochemical properties. When small amounts of both one-dimensional CNF and zero-dimensional CB particles, 5.0 wt% each, are present, conductive networks are established within the urchin-like LTO secondary particles by the penetrating multiple CNFs, whereas the CB particles attached onto the surface of the LTO particles connect the gaps between them, being able to form extensive three-dimensional conductive networks across the whole composites. The electrodes made from this composite deliver a remarkable capacity of 123 mA h g(-1) when charged/discharged at 15 C, which is much higher than 91 mA h g(-1) for those made from the neat LTO powders, a reflection of significant improvements in both the conductivity and Li ion diffusion coefficient in the composite electrode. When the CNF content is increased to 10 wt%, corn-dog shaped composites are formed consisting of individual CNFs penetrating the elongated LTO secondary particles along the axial direction with limited conductive networks. The electrodes made from this composite present much poorer capacities at all current rates than those with an urchin-like structure. These intriguing observations verify that both the structure of the active material and the conductivity of the electrode play important roles in delivering high capacities and rate capabilities.

A porous cathode material consisting of interconnected single crystal LiNi1/3Co1/3Mn1/3O2 (NCM) nanoparticles has been synthesized for lithium ion batteries. Trace nitric acid is used as pH value adjuster to form honeycomb-shaped foam, and a novel stepwise crystallization process is employed to obtain NCM particles. The modified sol-gel process followed by an optimized crystallization process results in significant improvements in chemical and physical characteristics of the NCM particles. They consist of a fully-developed single crystal NCM with uniform composition and a porous NCM architecture with a reduced degree of fusion and a large specific surface area. These structural modifications in turn significantly enhance the electrochemical properties of the NCM cathode material, in terms of improved charge/discharge capacity, cyclic stability and rate capability.

A new sol–gel process is developed to modify the Li4Ti5O12 anode material for improved rate capability. The new process brings about the following effects, namely (i) doping of Sn2+ to form Li3.9Sn0.1Ti5O12, (ii) carbon coating and (iii) creation of a porous structure. The doping of Sn2+ results in the lattice distortion without changing the phase composition. A thin layer of amorphous carbon is coated on the doped particles that contain numerous nanopores. The rate capability of the anode material made from the modified powder is significantly improved when discharged at high current rates due to the reduced charge transfer resistance.

SnO2–graphene–carbon nanotube (SnO2–G–CNT) mixture is synthesized using graphene oxide as precursor for application as anode material in rechargeable Li ion batteries. It is shown that the SnO2 nanoparticles of 3–6nm in diameter are not only attached onto the surface of graphene sheets by anchoring with surface functional groups, but they also are encapsulated in pore channels formed by entangled graphene sheets. The incorporation of carbon nanotubes reduces the charge transfer resistance of the anode made from the mixture through the formation of 3D electronic conductive networks. The SnO2–G–CNT anodes deliver remarkable capacities of 345 and 635mAhg−1 at 1.5 and 0.25Ag−1, respectively. Flexible electrodes consisting of highly-aligned SnO2–G–CNT papers are also prepared using a simple vacuum filtration technique. They present a stable capacity of 387mAhg−1 at 0.1Ag−1 after 50 cycles through the synergy of the high specific capacity of SnO2 nanoparticles and the excellent cycleability of G–CNT paper.

This study reports the development of multiwalled carbon nanotube (MWCNT)-LiMn2O4 nanocomposites by a facile sol–gel method. The elemental compositions, surface morphologies and structures of the nanocomposites are characterized with a view to their use as cathode materials for Li-ion batteries. The results indicate that the nanocomposite consists of LiMn2O4 nanoparticles containing undamaged MWCNTs. The nanocomposites show high cycle performance with a remarkable capacity retention of 99% after 20 cycles, compared with LiMn2O4 nanoparticles with a 9% loss of the initial capacity after 20 cycles. Measurements of a.c. impedance show that the charge-transfer resistance of the nanocomposites is much lower than that of spinel LiMn2O4. A cyclic voltammetry study further confirms higher reversibility of the nanocomposites compared with LiMn2O4 particles. The enhanced electrochemical performance of the nanocomposites is attributed to the formation of conductive networks by MWCNTs that act as intra-electrode wires, thereby facilitating charge-transfer among the spinel LiMn2O4 particles.

A new method for preparing black birnessite nanowires is introduced. Layer-structured manganese oxide nanowires were synthesized by a facile hydrothermal method, and using both NaMnO4 and CH3CH2OH as the precursors in a concentrated NaOH solution. The structure, composition, appearance and electrochemical performance of the product were characterized by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), energy diffraction (ED), high-resolution transmission electron microscopy (HRTEM), thermogravimetric analysis (TGA) and constant current charge/discharge. The XRD patterns showed a single phase corresponding to a crystalline birnessite-based manganese oxide. TEM studies suggested their wire-like structures. The TGA measurement demonstrated that they possessed an excellent thermal stability up to 400 degrees C. In the potential window of 2.0-4.3 V, the product exhibited excellent cyclic stability and rapid charge-discharge performance.