Morphology and electrochemistry of LiMn2O4 optimized by using different Mn-sources

Spinel-typed LiMn₂O₄ cathode active materials have been prepared for different microstructures by the melt-impregnation method using different forms of manganese. The effect of the starting materials on the microstructure and electrochemical properties of LiMn₂O₄ is investigated by X-ray diffraction, scanning electron microscopy, and electrochemical measurements. The powder prepared from nanostructured γ-MnOOH, with good crystallinity and a regular cubic spinel shape, provided an initial discharge capacity of 114 mAh g⁻¹ with excellent rate and high capacity retention. These advantages render LiMn₂O₄ attractive for practical and large-scale applications in mobile equipment.     

Electrochemical properties of helical carbon nanomaterials formed on LiCoO2 by chemical vapor deposition

We fabricated LiCoO₂/carbon composites by forming helical carbon nanomaterials (HCNs) on the surface of LiCoO₂ particles by chemical vapor deposition (CVD). The aim was to inhibit the degradation of the conductive path between the cathode active materials and carbon by mitigating the expansion of the electrode through cycling. We estimated the electrochemical properties of the composites as cathodes of rechargeable lithium cells. Using scanning electron microscopy measurements, we observed HCNs formed firmly on the surface of LiCoO₂ particles. X-ray diffraction measurements indicated decomposition of LiCoO₂, which was the main reason for the inferiority of the electrochemical properties. We found that the electrochemical properties of cells with the HCNs were not as good as those of cells with acetylene black (AB). Successful use of these composites requires inhibition of cathode active material decomposition and improvement of HCN conductivity.     

Effect of binder content on the cycle performance of nano-sized Fe2O3-loaded carbon for use as a lithium battery negative electrode

Nano-sized Fe₂O₃-loaded carbon material was prepared by loading Fe₂O₃ on carbon using various carbonaceous materials. Carbonaceous materials strongly affected the electrochemical behavior of nano-sized Fe₂O₃-loaded carbon. In addition, the binder content also significantly affected the cycle performance of nano-sized Fe₂O₃-loaded carbon. The content of binder depended on the type of carbon used. In the optimal condition for binder content, nano-carbons such as acetylene black (AB), tubular carbon nanofibers (CNF), and platelet CNF provided larger capacities than graphite, and tubular CNF showed the greatest capacity after long-term cycling.     

Enhanced electrochemical hydrogen storage capacity of multi-walled carbon nanotubes by TiO2 decoration

Electrochemical hydrogen storage of multi-walled carbon nanotubes (MWCNTs) decorated by TiO₂ nanoparticles (NPs) has been studied by the galvanostatic charge and discharge method. The TiO₂ NPs are deposited on the surface of MWCNTs by sol-gel method. Structural and morphological characterizations have been carried out using XRD, SEM and TEM, respectively. TiO₂ NPs can significantly enhance the discharge capacity of MWCNTs. The cyclic voltammograms analysis indicates that the electrical double layer contributes little to the discharge capacity of TiO₂-decorated MWCNTs. The MWCNTs modified with a certain amount of TiO₂ NPs have a discharge capacity of 540 mAh/g, corresponding to an electrochemical hydrogen storage capacity of about 2.02 wt%, which is quite interesting for the battery applications. The enhancement effect of TiO₂ NPs on the discharge capacity of MWCNTs could be related to the increased effective area for the adsorption of hydrogen atoms in the presence of TiO₂ NPs on MWCNTs and the preferable redox ability of TiO₂ NPs.     

The study of Mg2Si/carbon composites as anode materials for lithium ion batteries

The study of Mg₂Si/C composites as anode materials for lithium ion batteries is reported in this paper. Firstly, Mg₂Si was synthesized by mechanically activated annealing (MAA) technique and the preparing conditions for pure Mg₂Si alloy were investigated and optimized. Then the composite materials of Mg₂Si and carbon materials such as CNTs and CMS with different ratios were prepared by the followed ball-milling techniques. Their electrochemical performances were compared by the galvanostatically charge/discharge and EIS experiments. The pure Mg₂Si alloy delivers a large initial capacity, but the capacity decreases rapidly with cycling. In contrast, the composites show good cyclic stability and deliver a reversible capacity of about 400 mAh g⁻¹ with 40% carbon in the composite. The results of EIS indicate that the composite of Mg₂Si/CMS has better interface stability than that of pure Mg₂Si materials.     

Study of the surface modification of LiNi1/3Co1/3Mn1/3O2 cathode material for lithium ion battery

The surface of LiNi_1/3Co_1/3Mn_1/3O₂ (LNMCO) particles has been studied for material synthesized at 900 °C by a two-step process from a mixture of LiOH·H₂O and metal oxalate [(Ni_1/3Co_1/3Mn_1/3)C₂O₄] obtained by co-precipitation. Samples have been characterized by X-ray diffraction (XRD), high-resolution transmission electron microscope (HRTEM), Raman scattering (RS) spectroscopy, and magnetic measurements. We have investigated the effect of the heat treatment of particles at 600 °C with organic substances such as sucrose and starch. HRTEM images and RS spectra indicate that the surface of particles has been modified. The annealing does not lead to any carbon coating but it leads to the crystallization of the thin disordered layer on the surface of LiNi_1/3Co_1/3Mn_1/3O₂. The beneficial effect has been tested on the electrochemical properties of the LiNi_1/3Co_1/3Mn_1/3O₂ cathode materials. The capacity at 10C-rate is enhanced by 20% for post-treated LNMCO particles at 600 °C for half-an-hour.     

Electrochemical properties of Mg-based hydrogen storage materials modified with carbonaceous materials prepared by hydriding combustion synthesis and subsequent mechanical milling (HCS + MM)

Mg₂Ni-based hydride was prepared by hydriding combustion synthesis (HCS), and subsequently modified with various carbonaceous materials including graphite, multi-walled carbon nanotubes (MWCNTs), carbon aerogels (CAs) and carbon nanofibers (CNFs) by mechanical milling (MM) for 5 h. The structural properties of the modified hydrides were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). All of the modified hydrides show amorphous or nanocrystalline-like phases. The hydride modified with graphite exhibits the most homogenous distribution of particles and the smallest particle size. The effects of the modifications on electrochemical properties of the hydride were investigated by galvanostatic charge/discharge, linear polarization, Tafel polarization, electrochemical impedance spectroscopy and potentiostatic discharge measurements. The results show that the maximum discharge capacity, the high rate dischargeability (HRD), the exchange current density and the hydrogen diffusion ability of the hydride modified with the carbonaceous materials are all increased. Especially, the hydride modified with graphite possesses the highest discharge capacity of 531 mAh/g and the best electrochemical kinetics property.     

Electrochemical properties of ball-milled LaMg12–Ni composites containing carbon nanotubes

Nanocrystalline LaMg₁₂–Ni composites containing carbon nanotubes (CNTs) were prepared by two ball-milling ways, and the resulting microstructure and electrochemical characteristics were investigated. It is found that the discharge capacities and high-rate dischargeabilities (HRDs) of the CNT-containing composites prepared by ball-milling as-prepared nanocrystalline LaMg₁₂–Ni composite and CNTs for 1 h (denoted as Composite-CNT1-1) were obviously higher than that by ball-milling LaMg₁₂ alloy, Ni powder and CNT1 together for 12 h (denoted as Composite-CNT1-2). The highest discharge capacity reaches 999.8 mA h/g. Raman spectra and X-ray diffraction (XRD) patterns show that the structure of the CNTs still exists and the defect increases in Composite-CNT1-1. However, in Composite-CNT1-2, due to the overlong ball-milling time, the crystalline structure of the CNTs has been destroyed and amorphous carbons have formed. Cyclic voltammetry and electrochemical impedance spectra measurements indicate that the CNT modification in Composite-CNT1-1 increases the electrocatalytic activity and surface area, which leads to its higher discharge capacity and HRD. The larger electrochemical reaction resistance caused by amorphous carbon in Composite-CNT1-2 results in its lower discharge capacity and HRD. The CNT modification has negligible effect on the diffusion process of hydrogen from the surface to the bulk of the composites.     

Oxygen-doped activated carbon fiber cloth as electrode material for electrochemical capacitor

Novel oxygen-doped activated carbon fiber cloths (OACFC), with different compositions of surface oxygen functionalities, have been prepared by direct electrooxidative/reductive methods in an undivided electrolytic cell filled with high purity water without a supporting electrolyte under high voltage conditions. The morphology and surface chemical composition of the materials have been investigated by SEM, Raman and XPS spectroscopies. They revealed an electrochemical erosion of the CF surface upon activation, concomitant with a strong change of the D/G ratio of characteristic Raman bands and the surface O/C atomic ratio, respectively. Thus pretreated material was tested as electrodes for an electrochemical capacitor by cyclic voltammetry, galvanostatic charge/discharge, and electrochemical impedance spectroscopy measurements in 3.75 M H₂SO₄. The performance of the electrochemical capacitor based on modified carbon electrodes was compared to that of an analogous device with unmodified carbon. The measurements revealed altered electrochemical behavior of the OACFC in terms of the determined capacitances. The proposed activation method is also superior to other electrochemical activation procedures, since it uses much less energy per CF surface or mass.     

Preparation and electrochemical properties of indium- and sulfur-doped LiMnO2 with orthorhombic structure as cathode materials

The indium- and sulfur-doped LiMnO₂ samples with orthorhombic structure as cathode materials for Li-ion batteries are synthesized via hydrothermal method. The microstructure and composition of the samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), inductively coupled plasma atom emission spectroscopy (ICP-AES), and X-ray photoelectron spectroscopy (XPS) analysis. It is shown that these samples with the orthorhombic structure have irregular shapes with a grain size of about 100–200 nm. The electrochemical performance of these samples as cathode materials was studied by galvanostatic method. All doped materials can offer improved cycling stability and high rate discharge ability as compared with the un-doped Li_0.99MnO₂. Moreover, dual In/S doping can slow down the capacity decay to a great extent, although the transformation to spinel occurs undesirably for all the doped samples during electrochemical cycling.     

Effect of carbonaceous materials on electrochemical properties of nano-sized Fe2O3-loaded carbon as a lithium battery negative electrode

Nano-sized Fe₂O₃-loaded carbon material was prepared by loading Fe₂O₃ on carbon using various carbonaceous materials. Carbonaceous materials strongly affected the electrochemical behavior of nano-sized Fe₂O₃-loaded carbon. Among the carbons used, nano-carbons such as acetylene black (AB), tubular carbon nanofibers (CNF), and platelet CNF provided larger capacities than other carbons. This may be due to the greater surface area of nano-carbon, which gives a greater distribution of nano-sized Fe₂O₃ particles than other carbons and delivers a greater capacity than other carbons. Investigation of the first-cycle materials by X-ray photoelectron spectroscopy (XPS) revealed that Fe₂O₃ was reduced to Fe metal in the charge process (reduction of Fe₂O₃), and, conversely, Fe metal was not completely oxidized to Fe₂O₃ during discharge (oxidation of Fe). This result may be due to the covering of non-conductive Li₂O formed during charging.     

Synthesis and electrochemical characterization of carbon-coated nanocrystalline LiFePO4 prepared by polyacrylates-pyrolysis route

A carbon-coated nanocrystalline LiFePO₄ cathode material was synthesized by pyrolysis of polyacrylate precursor containing Li⁺, Fe³⁺ and PO₄⁻. The powder X-ray diffraction (XRD) and high-resolution TEM micrographs revealed that the LiFePO₄/C composite as prepared has a core-shell structure with pure olivine LiFePO₄ crystallites as cores and intimate carbon coating as a shell layer. Between the composite particulates, there exists a carbon matrix binding the nanocrystallites together into micrometer particles. The electrochemical measurements demonstrated that the LiFePO₄/C composite with an appropriate carbon content can deliver a very high discharge capacity of 157 mAh g⁻¹ (>92% of the theoretical capacity of LiFePO₄) with 95% of its initial capacity after 30 cycles. Since this preparation method uses less costly materials and operates in mild synthetic conditions, it may provide a feasible way for industrial production of the LiFePO₄/C cathode materials for the lithium-ion batteries.     

Novel Pd/β-MnO2 nanotubes composites as catalysts for methanol oxidation in alkaline solution

In this work, we demonstrated a completely new, simple and effective strategy for preparing catalysts by using β-MnO₂ nanotubes as the supporting materials, and the Pd nanoparticles were coated onto β-MnO₂ nanotubes through a simple reductive process firstly. The as-prepared materials were characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and electrochemical measurements. The results indicated that the Pd nanoparticles were homogeneously dispersed and well separated from one another on the β-MnO₂ nanotubes surfaces, which makes it have a potential application in catalysts. In this study, we mainly tested the electrochemical performance of Pd/β-MnO₂ for methanol oxidation in alkaline solution. Further research to optimize the synthesis condition, particularly to develop β-MnO₂ nanotubes as supporting materials of other noble metal catalysts is currently in progress.     

Synthesis and electrochemical properties of olivine LiFePO4 prepared by a carbothermal reduction method

LiFePO₄/C composite cathode material was prepared by carbothermal reduction method, which uses NH₄H₂PO₄, Li₂CO₃ and cheap Fe₂O₃ as starting materials, acetylene black and glucose as carbon sources. The precursor of LiFePO₄/C was characterized by differential thermal analysis and thermogravimetry. X-ray diffraction (XRD), scanning electron microscopy (SEM) micrographs showed that the LiFePO₄/C is olivine-type phase, and the addition of the carbon reduced the LiFePO₄ grain size. The carbon is dispersed between the grains, ensuring a good electronic contact. The products sintered at 700 °C for 8 h with glucose as carbon source possessed excellent electrochemical performance. The synthesized LiFePO₄ composites showed a high electrochemical capacity of 159.3 mAh g⁻¹ at 0.1 C rate, and the capacity fading is only 2.2% after 30 cycles.     

Investigation on capacity fading of aqueous MnO2·nH2O electrochemical capacitor

Cycle stability of MnO₂·nH₂O electrochemical capacitors (ECs) has been studied by using galvanostatic tests and electrochemical impedance spectroscopy (EIS). The extent of capacity fading, ranging from 5 to 30% in 1000 cycles, increases with current-rate, and is markedly reduced with increasing binder content. Two fading mechanisms have been identified. With low binder content, and at high current-rate, capacity fading occurs in conjunction with appreciable increase in transmission resistance, suggesting progressively deteriorating electric contacts among the pseudocapacitive oxide particles and conductive carbon. The mechanical failure of the electrode structure may arise from the cyclic volumetric variation of the pseudocapacitive oxide particles as previously reported. With high binder content or at low current-rate, capacity fading is associated with increasing interfacial charge-transfer resistance upon cycling, which has a less pronounced effect than the mechanical failure mechanism.     

Effect of synthesis temperature on the properties of LiFePO4/C composites prepared by carbothermal reduction

LiFePO₄/C composite cathode materials were synthesized by carbothermal reduction method using inexpensive FePO₄ as raw materials and glucose as conductive additive and reducing agent. The precursor of LiFePO₄/C was characterized by differential thermal analysis and thermogravimetry. The microstructure and morphology of the samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM) and particle size analysis. Cyclic voltammetry (CV) and charge/discharge cycling performance were used to characterize their electrochemical properties. The results showed that the LiFePO₄/C composite synthesized at 650 °C for 9 h exhibited the most homogeneous particle size distribution. Residual carbon during processing was coated on LiFePO₄, resulting in the enhancement of the material's electronic properties. Electrochemical measurements showed that the discharge capacity first increased and then decreased with the increase of synthesis temperature. The optimal sample synthesized at 650 °C for 9 h exhibited a highest initial discharge capacity of 151.2 mA h g⁻¹ at 0.2 C rate and 144.1 mA h g⁻¹ at 1 C rate with satisfactory capacity retention rate.     

Impact of synthesis conditions on the structure and performance of Li2FeSiO4

Three different synthesis techniques (hydrothermal synthesis, modified Pechini synthesis and Pechini synthesis) were successfully used for preparation of Li₂FeSiO₄ samples. The obtained samples possess some differences in the morphology and in the particle size, as well as in the presence of in situ formed carbon. The best electrochemical performance has been obtained with the smallest particles embedded into carbon matrix. Such a Li₂FeSiO₄/C composite contains the highest amounts of impurities (Fe₂O₃, SiO₂ and Li₂SiO₃) and only 68.8 at.% of iron is in the form of Fe^II as detected by Mössbauer spectroscopy, respectively. Despite the highest amount of impurities, the sample shows the highest reversible capacity (approximately 100 mAh g⁻¹ based on whole silicate-derived material). With the proper structuring of Li₂FeSiO₄/C composites, utilisation of large part of capacity is also possible at current densities corresponding to C/5 and C/2 cycling rate. A lower amount of impurities was found in the samples that do not contain any in situ carbon after synthesis. Among them, the highest purity is possessed by the sample prepared at 900 °C, as determined using Mössbauer spectroscopy. The results obtained by Mössbauer spectroscopy and XRD analysis indicate on the differences in the crystal structure between the thermally treated samples and the sample prepared by hydrothermal synthesis.     

Effect of carbon nanotube on the electrochemical performance of C-LiFePO4/graphite battery

This paper describes the fabrication and testing of C-LiFePO₄/graphite battery with different conductive carbon additives: carbon nanotube (CNT) or carbon black (CB). The discharge capacity, rate capability and cyclic performance of the battery were investigated. Compared with the batteries with CB additive, those with CNT additive show better electrochemical performances with capacity retention ratio of 99.2% after 50 cycles, and the ratio of discharge capacity at 0.1 C rate to that at 1 C rate is 94.6%. The reason for the difference in electrochemical property was studied with cyclic voltammagrams and AC impedance. It was found that, with CNT additive, the polarization voltage was decreased from 0.3 to 0.2 V, and the impedance was decreased from 423.2 to 36.88 Ω. The structures of active materials after cycling were characterized using XRD. The better crystal retaining of LiFePO₄ was found in the active materials with CNT added.     

Synthesis and characterization of Nd doped LiMn2O4 cathode for Li-ion rechargeable batteries

Spinel powders of LiMn_1.99Nd_0.01O₄ have been synthesized by chemical synthesis route to prepare cathodes for Li-ion coin cells. The structural and electrochemical properties of these cathodes were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, cyclic voltammetry, and charge-discharge studies. The cyclic voltammetry of the cathodes revealed the reversible nature of Li-ion intercalation and deintercalation in the electrochemical cell. The charge-discharge characteristics for LiMn_1.99Nd_0.01O₄ cathode materials were obtained in 3.4–4.3 V voltage range and the initial discharge capacity of this material were found to be about 149 mAh g⁻¹. The coin cells were tested for up to 25 charge-discharge cycles. The results show that by doping with small concentration of rare-earth element Nd, the capacity fading is considerably reduced as compared to the pure LiMn₂O₄ cathodes, making it suitable for Li-ion battery applications.     

Morphological characterization of LiFePO4/C composite cathode materials synthesized via a carboxylic acid route

A new type of LiFePO₄/C composite surrounded by a web containing both amorphous and crystalline carbon phases was synthesized by incorporating malonic acid as a carbon source using a high temperature solid-state method. SEM, TEM/SAED/EDS and HRTEM were used to analyze surface morphology and confirmed for the first time that crystalline carbon was present in LiFePO₄/C composites. The composite was effective in enhancing the electrochemical properties such as capacity and rate capability, because its active component consists of nanometer-sized particles containing pores with a wide range of sizes. An EDS elemental map showed that carbon was uniformly distributed on the surface of the composite crystalline particles. TEM/EDS results clearly show a dark region that is LiFePO₄ with a trace of carbon and a gray region that is carbon only. To evaluate the materials’ electrochemical properties, galvanostatic cycling and conductivity measurements were performed. The best cell performance was delivered by the material coated with 60 wt.% malonic acid, which delivered first cycle discharge capacity of 149 mAh g⁻¹ at a C/5 rate and sustained 222 cycles at 80% of capacity retention. When carboxylic acid was used as a carbon source to produce LiFePO₄, overall conductivity increased from 10⁻⁵ to 10⁻⁴ S cm⁻¹, since particle growth was prevented during the final sintering process.