Microemulsion-mediated hydrothermal growth of pagoda-like Fe3O4 microstructures and their application in a lithium–air battery

This paper reported the growth of novel pagoda-like Fe₃O₄ particles via a facile microemulsion-mediated hydrothermal procedure. The chemical compositions and morphologies of the as-grown Fe₃O₄ particles were characterized by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), and field emission scanning electron microscopy (FE-SEM). The morphologies of the as-prepared sample evolved from pagoda-like to pinwheel-like to flower-like shapes with increasing reaction time. In addition, the NaOH concentration and polyethylene glycol (PEG)-2000 had key effects on the formation of the final product. The electrocatalytic properties of the prepared pagoda-like micro-Fe₃O₄, as catalytic materials for a lithium–air battery, were further evaluated by galvanostatic charge/discharge cycling and electrochemical impedance spectrometry (EIS). Results showed that the cell displayed an initial discharge capacity of 1429 mA h g⁻¹ at a voltage of 1.5–4.5 V at 100 mA g⁻¹.     

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.     

Improved activity of a graphene–TiO2 hybrid electrode in an electrochemical supercapacitor

We present a simple and fast approach for the synthesis of a graphene–TiO₂ hybrid nanostructure using a microwave-assisted technique. The microstructure, composition, and morphology were characterized by X-ray diffraction, Fourier-transform infrared spectroscopy, Raman microscopy, X-ray photoelectron spectroscopy, and field-emission scanning electron microscopy. The electrochemical properties were evaluated using cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge–discharge tests. Structural analysis revealed a homogeneous distribution of nanosized TiO₂ particles on graphene nanosheets. The material exhibited a high specific capacitance of 165 F g⁻¹ at a scan rate of 5 mV s⁻¹ in 1 M Na₂SO₄ electrolyte solution. Theenhanced supercapacitance property of these materials could be ascribed to the increased conductivity of TiO₂ and better utilization of graphene. Moreover, the material exhibited long-term cycle stability, retaining ∼90% specific capacitance after 5000 cycles, which suggests that it has potential as an electrode material for high-performance electrochemical supercapacitors.     

Preparation of porous-structured LiFePO4/C composite by vacuum sintering for lithium-ion battery

The LiFePO₄/C (LFP/C) composite as a cathode material for lithium-ion battery was synthesized by solid-state reaction under vacuum sintering condition (20–5 Pa). The effects of vacuum sintering temperature and time on the phase composition, morphological structure, and electrochemical performance of LFP/C composite were investigated by X-ray diffraction, scanning electron microscopy, galvanostatic charge–discharge cycling test, and electrochemical impedance spectroscopy. The synthetic LFP/C composite possessed uniform particle-size distribution with porous architecture upon sintering at 650 °C for 12 h and thus exhibited the highest discharge capacity and best cycle performance. The complete decomposition of citric acid at a suitable temperature under vacuum condition resulted in the formation of porous structure. Compared with atmospheric argon sintering, vacuum sintering method led to the formation of porous architecture, the porous sample showed excellent cycle performance with less than 2% capacity loss after 80 cycles at 0.2 C, and reached the discharge specific capacity of 87.6 mAh g⁻¹ at 10 C rate, these are better than that of atmospheric argon sintering. The LFP/C composite prepared under vacuum sintering also reduced the optimum sintering temperature by nearly 100 °C compared with that prepared under atmospheric argon sintering.     

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%.     

One-step synthesis of Li-doped NiO as high-performance anode material for lithium ion batteries

The poor electronic conductivity and huge volume expansion of NiO are the vital barriers when used as anode for lithium ion batteries. In order to solve above issues, Li-doped NiO are prepared by a facile one-step ultrasonic spray pyrolysis method. The effects of Li doping on the morphology, structure and chemical composition of the Li-doped NiO powders are extensively studied. When used as lithium ion batteries anode, it is demonstrated that the doping of Li has significant positive effect on improving the electrochemical performance. After 100 cycles at 400 mA g⁻¹, The Li-doped NiO samples deliver a discharge capacity of 907 mAh g⁻¹, much more than that of un-doped sample (736 mAh g⁻¹). The improved electrochemical performances can be ascribed to the improved p-type conductivity and lower impedance, which are confirmed by Rietveld refinement, X-ray photoelectron spectroscopy and electron impedance spectroscopy.     

Understanding the influence of conductive carbon additives surface area on the rate performance of LiFePO4 cathodes for lithium ion batteries

Conductive carbon additives with different surface area and particle size, alone or in different combinations, were tested as conductive additives for LiFePO₄ cathode materials in lithium ion batteries. Their influence on the conductivity, rate capability as well as the structure of the resulting electrodes was investigated. Mercury porosimetry was carried out to define the porosity and pore size distribution of electrodes, and scanning electron microscopy was used to image their morphology. By comparing the discharge capacity, especially at higher rates, it can be concluded that the electrochemical performance of LiFePO₄ cathode material is significantly affected by the surface area, particle size and morphology of the used carbon additives. The best rate performance is achieved with the electrode containing a carbon additive with a specific surface area of 180 m² g⁻¹. This work reveals that the choice of conductive additive influences discharge capacity of LiFePO₄ Li-ion battery cells by as much as 20–30%. This is due to conductive additive’s influence on both electronic conductivity and porosity (which determines ionic conductivity) of LiFePO₄ electrodes. A system approach to lithium ion battery material research should always consider inactive materials, such as conductive additives and binders, in addition to active materials.     

Influence of different lithium sources on the morphology,structure and electrochemical performances of lithium-rich layered oxides

Lithium-rich layered oxides were synthesized via co-precipitation by using different lithium sources (LiOH, Li₂CO₃ and CH₃COOLi). Scanning electron microscope (SEM), Thermo gravimetric analysis (TGA), Brunauer-Emmett-Teller (BET), Inductively coupled plasma atomic emission spectrometry (ICP-AES), X-ray diffraction (XRD) and electrochemical measurements were used to investigate the morphology, reaction process, specific surface area, composition, structure and electrochemical performance of the lithium-rich oxides, respectively. The use of different lithium sources mainly affects the primary particle size and secondary particle morphology of the final product. Using LiOH as the lithium source, the maximum discharge capacity of sample can reach to 272.1 mA h g^–1 in the voltage range of 2.0–4.6 V at room temperature, even after 50 cycles, the retention rate is still reach 91.4%. The electrochemical impedance spectroscopy (EIS) results show that lithium-rich oxides using LiOH as the lithium source have the minimum value of impedance after 50 cycles. Therefore, the choice of appropriate lithium source is an effective way to improve the electrochemical properties of lithium-rich layered oxides.     

LiMgxMn2−xO4 (x≤0.10) cathode materials with high rate performance prepared by molten-salt combustion at low temperature

LiMg_xMn_2−xO₄ (x≤0.10) cathode materials for lithium-ion batteries were prepared by molten-salt combustion and then structurally characterized by powder X-ray diffraction. All the cathode materials were identified as the spinel structure of LiMn₂O₄ and the lattice parameter decreased as the Mg content of LiMg_xMn_2−xO₄ increased. Scanning electron microscopy revealed that the average particle size and agglomeration decreased with increasing Mg content. Galvanostatic charge–discharge experiments showed that Mg doping could effectively enhance the cycling performance of the cathode materials. LiMg_0.05Mn_1.95O₄ demonstrated excellent electrochemical performance with an initial discharge specific capacity of 122.0 mA h g⁻¹ and capacity retention of 86.4% after 100 cycles at 0.5 C (1 C=148 mA g⁻¹). Rate performance, cyclic voltammetry and electrochemical impedance spectroscopy measurements showed that the Mg-doped spinels had high rate capability and reversible cycling performance.     

SnO2 nano-spheres/graphene hybrid for high-performance lithium ion battery anodes

SnO₂ nano-spheres/graphene composite was fabricated via a simple one-step hydrothermal method with graphene oxide and SnCl₄·5H₂O as the precursors. The composite was characterized by X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy and surface area measurement. It is shown that fine SnO₂ nano-spheres with an average size of 50–100 nm could be homogeneously deposited on graphene nano-sheets layer by layer. The structural feature enabled SnO₂ nano-spheres/graphene hybird as an excellent anode material in lithium ion battery. The composite possesses 1306 mA h g^?1 of initial discharge capacity and good capacity retention of 594 mA h g^?1 up to the 50th cycle at a current density of 100 mA g^?1. These results indicate that the composite is a promising anode material in high-performance lithium ion batteries.     

Reduced graphene oxide anchored with δ-MnO2 nanoscrolls as anode materials for enhanced Li-ion storage

We developed a one-pot in situ synthesis procedure to form nanocomposite of reduced graphene oxide (RGO) sheets anchored with 1D δ-MnO₂ nanoscrolls for Li-ion batteries. The as-prepared products were characterized by X-ray diffraction (XRD), Raman spectra, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM). The electrochemical performance of the δ-MnO₂ nanoscrolls/RGO composite was measured by galvanostatic charge/discharge cycling and electrochemical impedance spectroscopy. The results show that the δ-MnO₂ nanoscrolls/RGO composite displays superior Li-ion battery performance with large reversible capacity and high rate capability. The first discharge and charge capacities are 1520 and 810 mAh g⁻¹, respectively. After 50 cycles, the reversible discharge capacity is still maintained at 528 mAh g⁻¹ at the current density of 100 mAh g⁻¹. The excellent electrochemical performance is attributed to the unique nanostructure of the δ-MnO₂ nanoscrolls/RGO composite, the high capacity of MnO₂ and superior electrical conductivity of RGO.     

Aligned sulfur-coated carbon nanotubes with a polyethylene glycol barrier at one end for use as a high efficiency sulfur cathode

High efficient sulfur cathode materials were constructed by the incorporation of aligned sulfur-coated carbon nanotubes (CNTs) and a polyethylene glycol (PEG) barrier at one end. During the charge and discharge of lithium sulfur batteries, high Li ion storage performance can be achieved on the composite electrode, which was benefited from both the aligned CNT structure and the polymer barrier. Aligned CNT framework afforded high conductivity for electron transportation and ordered pores for lithium ion transportation. Meanwhile, the PEG barrier layer greatly suppressed the shuttle of polysulfides. Therefore, this aligned sulfur-coated CNTs with a PEG barrier showed a high initial discharge capacity of 920 and 1128 mAh g⁻¹ in lithium bis(trifluoromethanesulfonyl)imide/1,3-dioxolane/1,2-dimethoxyethane and electrolyte with LiNO₃ additives, respectively. The PEG coated cathode showed high cycle stability that a low degradation with 0.38% per cycle during the 100 cycles at 0.1 C was achieved in LiNO₃-free electrolytes. These Li storage performance was superior to the aligned sulfur-coated CNT electrode without PEG barrier.     

High-surface area carbons from renewable sources with a bimodal micro-mesoporosity for high-performance ionic liquid-based supercapacitors

Highly porous materials with a bimodal pore size distribution in the micro-mesopore range have been produced from biomass by adding melamine to the hydrochar/KOH mixture used in the activation process. These carbons are characterized by BET surface areas in excess of ∼3300 m² g⁻¹ and a porosity equally distributed between micropores and mesopores. The use of melamine in the synthesis process not only extends the pore size distribution into the mesopore region, but leads to the incorporation of a certain amount of nitrogen atoms into the carbon framework. These materials combine high ion adsorption capacities (micropores) and enhanced ion-transport kinetics (mesopores) leading to an outstanding capacitive performance in ionic liquid-based supercapacitors. Thus, they have specific capacitances >160 F g⁻¹ at 1 A g⁻¹ and >140 F g⁻¹ at 60 A g⁻¹ in both pure ionic liquid and in acetonitrile-diluted ionic liquid, enabling these materials to store up to a maximum of ca. 60 W h kg⁻¹ in both kinds of electrolytes and deliver ca. 20 W h kg⁻¹ at ∼42 kW kg⁻¹ (discharge time ca. 2 s) in pure ionic liquid and ∼25–30 W h kg⁻¹ at ∼97–100 kW kg⁻¹ (discharge time ∼1 s) in acetonitrile-diluted ionic liquid.     

Effect of molybdenum substitution on electrochemical performance of Li[Li0.2Mn0.54Co0.13Ni0.13]O2 cathode material

Molybdenum doping is introduced to improve the electrochemical performance of lithium-rich manganese-based cathode material. X-ray diffraction (XRD) results illustrate that the crystallographic parameters a, c and lattice volume V become larger with the increase of Mo content. The scanning electron microscope (SEM) shows that the molybdenum substitution increases the crystallinity of the primary particles. When evaluated as cathode material, the as-prepared Li[Li_0.2Mn_0.54-x/3Ni_0.13-x/3Co_0.13-x/3Mo_x]O₂ (x = 0.007) delivers a discharge capacity of 155.5 mA h g⁻¹ at 5 C (1 C = 250 mA g⁻¹) and exhibits the capacity retention of 81.8% at 1 C after 200 cycles. The results of cyclic voltammetry (CV) and electronic impedance spectroscopy (EIS) tests reflect that the molybdenum substitution is able to significantly reduce the electrode polarization and lower the charge-transfer resistance. Within appropriate amount of Mo doping, the lithium ion diffusion coefficient of the material can reach to 8.92 × 10^–15 cm² s⁻¹, which is ~ 30 times higher than that of pristine materials (2.65 × 10^–16 cm² s⁻¹).     

Effect of substrate heating and microwave attenuation on the catalyst free growth and field emission of carbon nanotubes

Carbon nanotubes (CNTs) have been directly grown on Inconel 600 substrates by microwave plasma enhanced chemical vapor deposition without using any external catalyst. Grown CNTs were characterized by field emission scanning electron microscopy, high resolution transmission electron microscopy, X-ray diffraction, Raman spectroscopy and field emission measurements. Characterization results show that field emission current density increases from 200 μA/cm² at ∼5.5 V/μm to 14.5 mA/cm² at ∼1.6 V/μA when substrate is heat-treated and incident microwave is attenuated before reaching it. Detailed characterization reveals that heat-treatment results in migration of Cr and Fe oxides towards the top surface which completely changes substrate morphology also. Microwave attenuation reduces reflection of microwaves from the substrate and increases residence time of the precursor over the substrate promoting high density growth of CNTs. The combination of these two process parameters resulted in growth of long, dense CNTs with bamboo-like defects that contributes to enhanced current density at lower applied field.     

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.     

From hydrated layered-spinel lithium manganate composite to high-performance spinel LiMn2O4: A novel synthesis tuned by the concentration of LiOH

To obtain high-performance spinel LiMn₂O₄, various types of hydrated layered-spinel lithium manganate composites have been controllably synthesized through the hydrothermal process. It is found that the composition and morphology of these intermediate products can be tuned by the concentration of LiOH: Li⁺ act as the template and OH^- provide the required alkaline environment. In particular, the nanostructure varies from nanowires to nanosheets at different levels, depending on the phase ratio of the spinel phase ranging from 0% to 100%. Phase purity and the corresponding electrochemical properties of the as-prepared LiMn₂O₄ products are further tailored through the subsequent heat treatment. With the optimized LiOH concentration of 0.08 M, the resulting LiMn₂O₄ cathode material exhibits the best electrochemical performance with the initial discharge capacity of 121.7 mA h g⁻¹ at 1 C and 117.8 mA h g⁻¹ at 30 C, while a retention over 90% can be achieved after 1500 cycles. This study will help deepen understanding of the function mechanisms and further direct the novel synthesis from hydrated layered-spinel lithium manganate composites to high-performance spinel LiMn₂O₄ cathode materials.     

High rate capability of LiFePO4 cathodes doped with a high amount of Ti

A Li-ion battery cathode with target stoichiometry LiTi_0.08Fe_0.92PO₄ was prepared by doping a high amount of Ti⁴⁺ into the olivine structure using solid-state reaction at 700 °C. Synchrotron X-ray diffraction studies confirmed the presence of a major olivine phase and NASICON-type ion-conducting Li₃Fe₂(PO)₄ and Li₂TiFe(PO₄)₃ impurities. Further, the calculated lattice parameter values appear to confirm Ti-doping in LiFePO₄. Electron microscopy and particle distribution studies not only revealed a slightly increased average particle-size and particle fragmentation but also confirmed a fair distribution of Ti⁴⁺ ions in the prepared sample. The LiTi_0.08Fe_0.92PO₄ cathode delivered high specific capacities and good capacity retentions (≤2% capacity loss after 50 cycles) for lithium battery applications. Besides, the cathode delivered impressive rate capabilities as specific capacities of 160 and 110 mA h g⁻¹ at 0.2 and 11.4 C, respectively, were retained. Although the average particle-size was slightly higher, the presence of ion-conducting NASICON-type species appear to contribute to the enhanced electrical conductivity and hence to the cathode performance of LiTi_0.08Fe_0.92PO₄.     

Simple fabrication of a Fe2O3/carbon composite for use in a high-performance lithium ion battery

A simple approach was developed for the fabrication of a Fe₂O₃/carbon composite by impregnating activated carbon with a ferric nitrate solution and calcinating it. The composite contains graphitic layers and 10 wt.% Fe₂O₃ particles of 20–50 nm in diameter. The composite has a high specific surface area of ∼828 m² g⁻¹ and when used as the anode in a lithium ion battery (LIB), it showed a reversible capacity of 623 mAh g⁻¹ for the first 100 cycles at 50 mA g⁻¹. A discharge capacity higher than 450 mAh g⁻¹ at 1000 mA g⁻¹ was recorded in rate performance testing. This highly improved reversible capacity and rate performance is attributed to the combination of (i) the formation of graphitic layers in the composite, which possibly improves the matrix electrical conductivity, (ii) the interconnected porous channels whose diameters ranges from the macro- to meso- pore, which increases lithium-ion mobility, and (iii) the Fe₂O₃ nanoparticles that facilitate the transport of electrons and shorten the distance for Li⁺ diffusion. This study provides a cost-effective, highly efficient means to fabricate materials which combine conducting carbon with nanoparticles of metal or metal oxide for the development of a high-performance LIB.     

Effects of acid vapour mediated oxidization on the electrochemical performance of thermally exfoliated graphene

The surface modification of thermally exfoliated graphene (TEG) is an important technique for alteration of its hydrophobic nature and the resolution of its limited dispersibility. We have developed an easy acid-vapour-mediated method to functionalize the inert TEG surface with oxygen functional groups. The effects of oxygen functional groups on the capacitive performances of TEG were investigated with various reaction times. Ultraviolet–visible, Fourier transform infrared and Raman spectroscopy analyses demonstrated that the dispersibility of TEG was improved due to defect augmenting as the extent of oxidation progressed. Quantitative analyses of functional groups of the oxidized TEG samples (O-TEGs) were performed by thermogravimetric analysis and X-ray photoelectron spectroscopic studies. Physisorption surface analysis showed that the pore volumes of O-TEGs were greater than that of the pristine TEG, whereas the specific surface areas of O-TEGs were lower than that of pristine TEG. Electrochemical performances of the O-TEG samples were measured through cyclic voltammetry, galvanostatic charge–discharge, and electrochemical impedance spectroscopy analysis. A maximum specific capacitance of 175.2 F g⁻¹ was recorded at a current density of 1 A g⁻¹ for the O-TEG oxidized for 2 h. Retention of specific capacitance for the sample was ∼97% after 5000 charge–discharge cycles.