Fabrication and electrochemical behavior of flower-like ZnO-CoO-C nanowall arrays as anodes for lithium-ion batteries

This study reported the electrochemical performance of flower-like ZnO-CoO-C nanowall arrays as anodes of lithium-ion batteries. The arrays were fabricated through solution-immersion steps and subsequent calcination at 400 °C. At a rate of 0.5 C, the arrays exhibited a delithiation capacity of 438 mA h g⁻¹ at the 50th cycle. The arrays still delivered a reversible capacity of 224 mA h g⁻¹ at 2.0 C rate, much higher than those of the flower-like ZnO and ZnO-C nanowall arrays. The mechanism for the high capacity of flower-like ZnO-CoO-C nanowall arrays mainly resulted from the catalytic effect of Co phase on the decomposition of Li₂O and the conducting carbon layer formed on ZnO nanowalls. The present finding also provides a kind of nanostructured films that might be applied in solar cells and sensors, etc.     

Synthesis and characterization of in situ carbon-coated Li2FeSiO4 cathode materials for lithium ion battery

Li₂FeSiO₄/C composites with in situ carbon coating were synthesized via sol-gel method based on acid-catalyzed hydrolysis/condensation of tetraethoxysilane (TEOS) with sucrose and l-ascorbic acid as carbon additives, respectively. As-obtained Li₂FeSiO₄/C composites prepared with l-ascorbic acid as a carbon additive are composed of nanoparticulate Li₂FeSiO₄ in an intimate contact with a continuous thin layer of residual carbon and exhibit large specific surface area up to 395.7 m² g⁻¹. The results indicate that structure of the residual carbon is graphene-rich with obviously lower disordered/graphene (D/G) ratio. These as-obtained Li₂FeSiO₄/C composites exhibit first discharge capacity of 135.3 mAh g⁻¹ at C/16 and perform cycling stability, which are superior to those of Li₂FeSiO₄/C composites synthesized with sucrose as a carbon additive.     

Investigation of structural and electrical properties of (1 − x) Bi0.5Mg0.5TiO3–(x) PbTiO3 ceramic system

A 3 V cathode material for lithium ion batteries, Li_0.33MnO₂, was synthesized by solid-state reaction. Two Mn crystallographic positions, Mn₍₁₎ and Mn₍₂₎, were determined by X-ray diffraction analysis. The [Mn₍₂₎O₆] octahedron had a lower symmetrical degree than that of [Mn₍₁₎O₆], which was attributed to the geometrical effects of the non-symmetrical environment around Mn₍₂₎. Li_0.33MnO₂ delivered a reversible discharge capacity 140 mA h g⁻¹. In situ synchrotron diffraction clearly showed a reversible phase transition of Li_0.33MnO₂ during electrochemical process. The analysis of X-ray absorption near edge spectroscopy observed the conversion of Mn⁴⁺ to Mn³⁺ with Li⁺ intercalation into Li_0.33MnO₂, accompanied by the formation of more severely distorted [MnO₆] octahedron.     

A novel method to synthesize anatase TiO2 nanowires as an anode material for lithium-ion batteries

We demonstrate a simple and novel approach for the synthesis of a kind of anatase TiO₂ nanowires. The method is based on a hydrothermal method under normal atmosphere without using the complex Teflon-lined autoclave, high concentrations NaOH solution and long react time. The as-prepared materials are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical measurements. The obtained anatase TiO₂ nanowires show excellent performance. There is a potential plateau at 1.77 and 1.88 V in the process of Li insertion and extraction, and the initial Li insertion/extraction capacities are 283 and 236 mAh g⁻¹ at the density of 20 mA g⁻¹, respectively. In the 20th cycle, the reversible capacities still remain about 216 and 159 mAh g⁻¹ at the current densities of 20 and 200 mA g⁻¹, respectively, and the coulombic efficiency is more than 98%, exhibiting excellent electrochemical performance.     

Synthesis of in situ network-like vapor-grown carbon fiber improved LiFePO4 cathode materials by microwave pyrolysis chemical vapor deposition

The low electronic conductivity of LiFePO₄ currently limits its use in lithium ion batteries. In order to solve the problem, in situ network-like vapor-grown carbon fiber (VGCF) improved LiFePO₄ cathode materials have been prepared in one step by microwave pyrolysis chemical vapor deposition. The phase, microstructure and electrochemical performances of the composites were investigated. Compared with the cathodes without in situ VGCF, the initial discharge capacity of the composite electrode increases from 84 mAh g⁻¹ to 123 mAh g⁻¹ at 3.0 C rate, and the charge transfer resistance varies from 420 Ω to 75 Ω. The possible reasons of those are proposed.     

Low temperature synthesis of Fe3O4 micro-spheres and its application in lithium ion battery

Fe₃O₄ micro-spheres with nanoparticles close-packed architectures were synthesized via a simple chemical method using (NH₄)₂Fe(SO₄)₂·6H₂O, hexamethylenetetramine, and NaF as reaction materials. This chemical synthesis took place in a vitreous jar under low temperature (90 °C) and atmospheric pressure. The morphology and structure of the as-synthesized products were characterized by field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), and Raman spectrum. Electrochemical properties of the as-synthesized Fe₃O₄ micro-spheres as anode electrode of lithium ion batteries were studied by conventional charge/discharge tests, which exhibit steady charge/discharge platforms at different current densities. The as-prepared Fe₃O₄ electrode shows high initial discharge capacity of 1166 and 1082 mAh g⁻¹ at current density of 0.05 and 0.1 mA cm⁻², respectively.     

One-pot hydrothermal synthesis of ruthenium oxide nanodots on reduced graphene oxide sheets for supercapacitors

Ruthenium oxide nanodots have been deposited on reduced graphene oxide (RGO) sheets homogeneously by hydrothermal and annealing methods. Adding NaOH solution in GO colloids prevents the restack and agglomeration of GO sheets when mixed with ruthenium chloride solution. Local crystallization of RuO₂ in the composites is revealed by X-ray diffraction and transmission electron microscopy. The element mapping image demonstrates the uniform distribution of Ru on RGO sheets. Unlike the pure crystalline RuO₂ exhibiting poor electrochemical performance, the composites present superior capacitive properties. The hydrothermal time is optimized and a maximum of 471 F g⁻¹ is measured in the composites at 0.5 A g⁻¹ when loaded with 45 wt% of RuO₂. After 3000 cycles, its specific capacitance remains 92% of the maximum capacitance. Our results suggest potential application of the reduced graphene oxide/ruthenium oxide composites to supercapacitors.     

Synthesis and electrochemical properties of porous LiV3O8 as cathode materials for lithium-ion batteries

In this paper, we report on the synthesis of porous LiV₃O₈ by using a tartaric acid-assisted sol-gel process and their enhanced electrochemical properties for reversible lithium storage. The crystal structure, morphology and pore texture of the as-synthesized samples are characterized by means of XRD, SEM, TEM/HRTEM and N₂ adsorption/desorption measurements. The results show that the tartaric acid plays a pore-making function and the calcination temperature is an important influential factor to the pore texture. In particular, the porous LiV₃O₈ calcined at 300 °C (LiV₃O₈-300) exhibits hierarchical porous structure with high surface area of 152.4 m² g⁻¹. The electrochemical performance of the as-prepared porous LiV₃O₈ as cathode materials for lithium ion batteries is investigated by galvanostatic charge-discharge cycling and electrochemical impedance spectroscopy. The porous LiV₃O₈-300 displays a maximum discharge capacity of 320 mAh g⁻¹ and remains 96.3% of its initial discharge capacity after 50 charge/discharge cycles at the current density of 40 mA g⁻¹ due to the enhanced charge transfer kinetics with a low apparent activity energy of 35.2 kJ mol⁻¹, suggesting its promising application as the cathode material of Li-ion batteries.     

Preparation of Fe2O3/graphene composite and its electrochemical performance as an anode material for lithium ion batteries

The micro-sized sphere Fe₂O₃ particles doped with graphene nanosheets were prepared by a facile hydrothermal method. The obtained Fe₂O₃/graphene composite as the anode material for lithium ion batteries showed a high discharge capacity of 660 mAh g⁻¹ during up to 100 cycles at the current density of 160 mA g⁻¹ and good rate capability. The excellent electrochemical performance of the composite can be attributed to that graphene served as dispersing medium to prevent Fe₂O₃ microparticles from agglomeration and provide an excellent electronic conduction pathway.     

Synthesis of layered birnessite-type manganese oxide thin films on plastic substrates by chemical bath deposition for flexible transparent supercapacitors

Layered birnessite-type manganese oxide thin films are successfully fabricated on indium tin oxide coated polyethylene terephthalate substrates for flexible transparent supercapacitors by a facile, effective and inexpensive chemical bath deposition technology from an alkaline KMnO₄ aqueous solution at room temperature. The effects of deposition conditions, including KMnO₄ concentration, initial molar ratio of NH₃·H₂O and KMnO₄, bath temperature, and reaction time, on the electrochemical properties of MnO₂ thin films are investigated. Layered birnessite-type MnO₂ thin films deposited under optimum conditions display three-dimensional porous morphology, high hydrophilicity, and a transmittance of 77.4% at 550 nm. A special capacitance of 229.2 F g⁻¹ and a capacitance retention ratio of 83% are obtained from the films after 1000 cycles at 10 mV s⁻¹ in 1 M Na₂SO₄. Compressive and tensile bending tests show that as-prepared MnO₂ thin film electrodes possess excellent mechanical flexibility and electrochemical stability.     

Investigation on 3-dimensional carbon foams/LiFePO4 composites as function of the annealing time under inert atmosphere

The morphological and electrochemical investigation of 3-dimensional (3D) carbon foams coated with olivine structured lithium iron phosphate as function of the annealing time under nitrogen atmosphere is reported. The LiFePO₄ as cathode material for lithium ion batteries was prepared by a Pechini-assisted sol-gel process. The coating has been successfully performed on commercially available 3D-carbon foams by soaking in aqueous solution containing lithium, iron salts and phosphates at 70 °C for 2-4 h. After drying-out, the composites were annealed at 600 °C for different times ranging from 0.4 to 10 h under nitrogen. The formation of the olivine-like structured LiFePO₄ was confirmed by X-ray diffraction analysis performed on the powder prepared under similar conditions. The surface investigation of the prepared composites showed the formation of a homogeneous coating by LiFePO₄ on the foams. The cyclic voltammetry curves of the composites show an enhancement of electrode reaction reversibility by increasing the annealing time. The electrochemical measurements on the composites showed good performances delivering a discharge specific capacity of 85 mAh g⁻¹ at a discharging rate of C/25 at room temperature after annealing for 0.4 h and 105 mAh g⁻¹ after annealing for 5 h.     

Surfactant assisted electrodeposition of MnO2 thin films: Improved supercapacitive properties

In order to obtain a high specific capacitance, MnO₂ thin films have been electrodeposited in the presence of a neutral surfactant (Triton X-100). These films were further characterized by means of X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, field emission scanning electron microscopy (FESEM) and contact angle measurement. The XRD studies revealed that the electrodeposited MnO₂ films are amorphous and addition of Triton X-100 does not change its amorphous nature. The electrodeposited films of MnO₂ in the presence of the Triton X-100 possess greater porosity and hence greater surface area in relation to the films prepared in the absence of the surfactant. Wettability test showed that the MnO₂ film becomes superhydrophilic from hydrophilic due to Triton X-100. Supercapacitance properties of MnO₂ thin films studied by cyclic voltammetry, galvanostatic charge-discharge cycling and impedance spectroscopy showed maximum supercapacitance for MnO₂ films deposited in presence of Triton X-100 is 345 F g⁻¹.     

LiFePO4/C composite cathode material with a continuous porous carbon network for high power lithium-ion battery

A series of LiFePO₄/porous carbon composites with different LiFePO₄ loading amounts were prepared by impregnation from ethanol solution of the LiFePO₄ precursors. The samples were characterized using X-ray powder diffraction (XRD), thermogravimetry (TG), differential scanning calorimetric (DSC), transmission electron microscopy (TEM) and nitrogen sorption prior to the electrochemical testing. The size and morphology of the porous carbon supported LiFePO₄ nanoparticles depended strongly on the LiFePO₄ loading amounts. The impact of LiFePO₄ loading on the electrochemical performance of the composites was discussed in detail. Among all the samples, the LiFePO₄/microporous carbon composites with the LiFePO₄ loading amount of 19.10 wt.% and 35.58 wt.%, respectively, demonstrated high rate performance with discharge capacity of 60 mAh g⁻¹ and 66 mAh g⁻¹ at 50 C.     

Effects of annealing conditions on the properties of TiO2/ITO-based photoanode and the photovoltaic performance of dye-sensitized solar cells

Photovoltaic performance of dye-sensitized solar cell (DSSC) is enhanced by a two-step annealing process of the photoanode. The 1st-step of annealing is performed in oxygen at 450 °C for 30 min which effectively removes the residual organics originated from the TiO₂ precursor pastes. This enhances the dye adsorption on the TiO₂ nanoparticles and raises the short-circuit current density (J_SC). The 2nd-step of annealing is performed in nitrogen at 450 °C for 10 min which removes extra oxygen atoms resulted from the incorporation of oxygen atoms into the tin-doped indium oxide (ITO) film during the 1st-step of annealing. This reduces the sheet resistance of ITO and thereby enhances the fill factor (FF). With the enhanced J_SC of 15.9mAcm⁻² and FF of 0.65, the AM1.5 solar to electric conversion efficiency (η) of DSSC reaches 6.7% which is better than that based on the conventional one-step air annealing (η = 5.53%, J_SC = 14.08 mA cm⁻², FF = 0.6).     

TiO2 nanotube-supported amorphous Ni-B electrode for electrocatalytic oxidation of methanol

Amorphous Ni-B/TiO₂ electrodes were successfully prepared by electroless plating. Highly ordered TiO₂ nanotube arrays fabricated by anodic oxidation were employed as substrate and loaded with Ni-B alloy by electroless plating. The phase formation, microstructures and catalytic activity of electrodes were investigated by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and electrochemistry analyzer, respectively. The results show that Ni-B/TiO₂ electrodes with an average particle size of 200 nm present a typical amorphous structure of Ni and B, and have high catalytic activity for methanol electrooxidation in alkaline medium. The peak current density in cyclic voltammetry (CV) curves reaches 360 mA cm⁻² in the solution with 0.5 mol L⁻¹ methanol, much higher than that of Ni-B/Ti electrode. With the methanol concentration increasing to 1.5 mol L⁻¹, the peak current density increases to 488 mA cm⁻², after which it remains almost constant. The Ni-B/TiO₂ electrodes are relatively stable according to catalytic lifetime test; the peak current density remains 72.1% of the original value after 1300 times cycles. The amorphous Ni-B/TiO₂ electrode should be a promising candidate for direct methanol fuel cell.     

Synthesis and electrochemical properties of Li4Ti5O12

The spinel compound Li₄Ti₅O₁₂ was synthesized by a solid state method. In this synthesizing process, anatase TiO₂ and Li₂CO₃ were used as reactants. The influences of reaction temperature and calcination time on the properties of products were studied. When calcination temperature was 750 °C and calcination temperature was 24 h, the products exhibited good electrochemical properties. Its discharge capacity reached 160 mAh g⁻¹ and its capacity retention was 97% at the 50th cycle when the current rate was 1 C. When current rate increased to 10 C, its first discharge capacity could reach 136 mAh g⁻¹, and its capacity retention was 85% at the 50th cycle.     

Room-temperature synthesis of crystallized LiCoO2 thin films by electrochemical technique

LiCoO₂ thin films have been directly synthesized on cobalt substrate in LiOH solution at room temperature by electrochemical method. The obtained LiCoO₂ thin films were characterized by X-ray diffraction (XRD), scanning electronic microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The influence of electrochemical reaction time, current density and concentration of LiOH solution on the crystal structure and morphology of the obtained LiCoO₂ thin films was discussed emphatically. Our results show that the as-synthesized LiCoO₂ films all are pure hexagonal structure. The crystallinity, densification and uniformity of the films increase with increasing electrochemical reaction time, current density as well as concentration of LiOH solution and then decrease. The preferable electrochemical reaction conditions were optimized as: electrochemical reaction time is 50 h, current density is 1 mA cm⁻² and concentration of LiOH solution is 3 mol dm⁻³.     

Synthesis and rate performance of lithium vanadium phosphate as cathode material for Li-ion batteries

A sphere-like carbon-coated Li₃V₂(PO₄)₃ composite was synthesized by carbothermal reduction method with two sessions of ball milling followed by spray-drying with the dispersant of polyethylene glycol added. The structure, particle size, and surface morphology of the cathode material were investigated via X-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy. Results indicate that the Li₃V₂(PO₄)₃/C composite has a sphere-like morphology composed of a large number of carbon-coated ultrafine particles linked together with a monoclinic structure. In the voltage range of 3.0-4.3 V, it exhibits the discharge capacities of 130 mAh g⁻¹ and 100 mAh g⁻¹ at 0.2 C and 20 C rates, respectively. This behavior indicates that the obtained Li₃V₂(PO₄)₃/C material has excellent rate capability.     

Synthesis and electrochemical performance of Li(Ni0.8Co0.15Al0.05)0.8(Ni0.5Mn0.5)0.2O2 with core-shell structure as cathode material for Li-ion batteries

The core-shell structure cathode material Li(Ni_0.8Co_0.15Al_0.05)_0.8(Ni_0.5Mn_0.5)_0.2O₂ (LNCANMO) was synthesized via a co-precipitation method. Its applicability as a cathode material for lithium ion batteries was investigated. The core-shell particle consists of LiNi_0.8Co_0.15Al_0.05O₂ (LNCAO) as the core and a LiNi_0.5Mn_0.5O₂ as the shell. The thickness of the LiNi_0.5Mn_0.5O₂ layer is approximately 1.25 μm, as estimated by field emission scanning electron microscopy (FE-SEM). The cycling behavior between 2.8 and 4.3 V at a current rate of 18 mA g⁻¹ shows a reversible capacity of about 195 mAh g⁻¹ with little capacity loss after 50 cycles. High-rate capability testing shows that even at a rate of 5 C, a stable capacity of approximately 127 mAh g⁻¹ is retained. In contrast, the capacity of LNCAO rapidly decreases in cyclic and high rate tests. The observed higher current rate capability and cycle stability of LNCANMO can be attributed to the lower impedance including charge transfer resistance and surface film resistance. Differential scanning calorimetry (DSC) indicates that LNCANMO had a much improved oxygen evolution onset temperature of approximately 251 °C, and a much lower level of exothermic-heat release compared to LNCAO. The improved thermal stability of the LNCANMO can be ascribed to the thermally stable outer shell of LiNi_0.5Mn_0.5O₂, which suppresses oxygen release from the host lattice and not directly come into contact with the electrolyte solution. In particular, LNCANMO is shown to exhibit improved electrochemical performance and is a safe material for use as an electrode for lithium ion batteries.     

Structural and electrochemical hydrogen storage properties of Mg2Ni-based alloys

Four different methods, i.e. hydriding combustion synthesis + mechanical milling (HCS + MM), induction melting (followed by hydriding) + mechanical milling (IM(Hyd) + MM), combustion synthesis + mechanical milling (CS + MM) and induction melting + mechanical milling (IM + MM), were used to prepare Mg₂Ni-based hydrogen storage alloys used as the negative electrode material in a nickel-metal hydride (Ni/MH) battery. The structural and electrochemical hydrogen storage properties of the Mg₂Ni-based alloys have been investigated systematically. The XRD results indicate that the as-milled products show nanocrystalline or amorphous-like structures. Electrochemical measurements show that the as-milled hydrides exhibit higher discharge capacity and better electrochemical kinetic property than the as-milled alloys. Among the four different methods, the HCS + MM product possesses the highest discharge capacity (578 mAh g⁻¹), the best high rate dischargeability (HRD) and the highest exchange current density (58.8 mA g⁻¹). It is suggested that the novel method of HCS + MM is promising to prepare Mg-based hydrogen storage electrode alloy with high discharge capacity and activity.