Synthesis of LiFePO<Subscript>4</Subscript> nanoparticles and electrochemical studies at full-cell level

LiFePO₄ nanoparticles have been successfully obtained by a solid-phase synthesis method using nanoscaled FePO₄ as starting materials. Three-electrode system using as-prepared LiFePO₄ as cathode was assembled to monitor the variation of voltage and impedance in the anode or cathode. The pouch-typed cells with prepared LiFePO₄ were assembled to investigate electrochemical performance at level of full-cell. The results show that the assembled pouch-typed full-cells present higher power density and favorable cycle life.     

Pouch-typed cell from Li-rich LiFePO<Subscript>4</Subscript> prepared by low-cost carbothermal reduction method

Lithium iron phospho-olivine cathode material with optimized lithium amount for lithium-ion batteries was successfully prepared from low cost Fe₂O₃ as raw materials by thermal reduction method. The as-obtained material showed a reversible discharge capacity of 153.8 mAh g^–1 in the voltage window of 2.0–4.2 V at half-cell level. The pouch-typed cells with prepared Li_1.05FePO₄ were assembled to investigate electrochemical performance at level of full-cell. The results show that the assembled pouch-typed full-cells present better rate capability and cycle life. The low-cost approach reported here firstly sheds light on application of mass production of olivinestructured LiFePO₄ at level of full-cell.     

Effect of Li excess content and Ti dopants on electrochemical properties of LiFePO<Subscript>4</Subscript> prepared by thermal reduction method

Carbon coated Li_{1 + x} FePO₄ (x = 0, 0.01, 0.02, 0.03, 0.04) and doped compositions Li_1.03Fe_0.99Ti_0.01PO₄ have been synthesized by thermal reduction method in this paper. The results showed that increasing the content in Li_{1 + x} FePO₄ result in better electrochemical properties and cyclic performances until x = 0.03, which had similar change law with the particle size of samples; and the initial discharge capacity and cycle life of Li_1.03Fe_0.9Ti_0.01PO₄ was better than other samples under 1 C rate. When the Li_1.03Fe_0.99Ti_0.01PO₄/C sample cycled before 60 times, this sample exhibited a trend of increased capacity, and reached the highest discharging rate capacity of 156 mA h g⁻¹ at 60 cycles. The electrochemical performances of LiFePO₄ compositions synthesized by thermal reduction method, to some extent, can be improved by Li excess content and Ti doping.     

Preparation of LiFePO<Subscript>4</Subscript>/graphene composites by microwave-assisted solvothermal method

Well-crystallized and nano-sized LiFePO₄/graphene composite have been successfully synthesized by in-situ disperse graphene oxide (GO) in precursor via a rapid microwave-solvothermal process at 200°C within 10 min. In spite of the low synthesis temperature, the structural and morphological properties of as-prepared composites present high specific capacity, an excellent high rate capability, and stable cycling performance.The short reaction times of just 10 min show the basis for an efficient and time-saving synthesis of LiFePO₄ρaphene composite.     

Electrochemical and magnetic characterization of LiFePO<Subscript>4</Subscript> and Li<Subscript>0.95</Subscript>Mg<Subscript>0.05</Subscript>FePO<Subscript>4</Subscript> cathode materials

A series of lithium iron phosphates was synthesized via the sol–gel route. Iron phosphides, which are electronic conductors, were formed when sintered at 850°C. Magnetic susceptibility measurements on the samples show antiferromagnetic behaviour with T _N=50±2 K for LiFePO₄ and Li_0.95Mg_0.05PO₄ sintered at temperatures below 850°C. The LiFePO₄ and Li_0.95Mg_0.05FePO4 cathodes show a stable electrochemical capacity in the range of 150–160 mA h/g on cycling. The cyclability deteriorates with increasing sample sintering temperature due to the increased crystal size and impurities.     

Chelation-assisted method for the preparation of cathode material LiFePO<Subscript>4</Subscript>

LiFePO₄/C composite cathode material is prepared by ball milling with the assistance of EDTA chelation with using water as the media of ball mill procedure. FePO₄ and LiOH are used as starting materials; a certain amount of glucose is used as carbon sources and reduction agent. The structure and morphology of the composite are characterized by X-ray diffraction and scanning electron microscopy. Cyclic voltammetry, AC impedance measurements, and galvanostatic charge–discharge and cycling performances are used to characterize its electrochemical properties. The results indicate that the performances of composites prepared by chelation-assisted method are much better than common ball milling method which using alcohol or acetone as the media of ball mill procedure. The stable discharge capacity of the prepared composite is 150 and 105 mAh g⁻¹ at 1 and 10 C rate, respectively.     

Particle size effects of carbon sources on electrochemical properties of LiFePO<Subscript>4</Subscript>/C composites

Carbon-coated LiFePO₄ cathode materials were prepared by a solid-state method incorporating different sizes of polystyrene (PS) spheres as carbon sources. In scanning electron microscope images, small PS spheres appear more effective at preventing aggregation of LiFePO₄ particles. From transmission electron microscopy images, it was found that the LiFePO₄ particles were completely uniformly coated with 5-nm carbon layer when the carbon source was 0.22 μm PS spheres. When the size of PS sphere was increased to 2.75 μm, a network of carbon was formed and wrapped around the LiFePO₄ to create a conductive web. Raman spectroscopy and four-point probe conductivity measurement showed that using larger sizes of PS spheres as carbon sources leads to greater conductivity of LiFePO₄/C. The LiFePO₄ precursor sintered with 0.22 μm PS spheres delivered an initial discharge capacity of 145 mAh g^?1 at a 0.2 C rate, but it only sustained 289 cycles at 80% capacity. When the diameter of PS spheres was increased to 2.75 μm, the discharge capacity of LiFePO₄/C decreased, but the cycle life reached 755 cycles, the highest number in this work probably due to the network formation of carbon wrapping around LiFePO₄ particles.     

Temperature characterization analysis of LiFePO<Subscript>4</Subscript>/C power battery during charging and discharging

In order to study the surface temperature change and distribution during charging and discharging and in the simulation working condition of LiFePO₄/C power battery at normal temperature, the surface temperature is tested by placing the battery in the incubator and fixing 10 temperature probes on the battery surface. Results show that the temperature of the upper part is higher, and the temperature at the bottom is the lowest, while around the positive electrode is the highest during charging and discharging. The maximum temperature rising rate is reached at the moment of constant current charging transforming to the constant voltage charging during charging, and at the end moment during discharging. During charging in a certain range and discharging, the relations between the maximum temperature, the average temperature rising rate, and the maximum temperature difference of all the measurement points at the same time and the current are approximately linear, respectively. In the simulation working condition, the moment of the maximum temperature is consistent with the large current discharging instantaneous in each stage.     

Effects of Ni-ion doping on electrochemical characteristics of spinel LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> powders prepared by a spray-drying method

Spinel LiMn_2−x Ni _x O₄ compounds doped with a range of Ni (x=0–0.06) were synthesized by a spray-drying method. The structure and morphology characteristics of the powders were studied in detail by means of X-ray diffraction (XRD), scanning electron microscopy, and transmission electron microscopy. The XRD data reveal that all the samples have well-defined spinel structure, but, with the increase in Ni content, the doped lithium manganese spinels have smaller lattice constant. The undoped and doped spinel LiMn₂O₄ particles are fine, narrowly distributed, and well crystallized. The electrochemical characteristics of the samples are measured in the coin-type cells in a potential range of 3.2–4.35 V vs Li/Li⁺. All cyclic voltammogram curves exhibit two pairs of redox reaction peaks, but, among them, there are some differences about the peak split. With the increase in the Ni content, the specific capacities of the samples decrease slightly, but their cyclic ability increases.     

Preparation and characterization of LiFePO<Subscript>4</Subscript> from NH<Subscript>4</Subscript>FePO<Subscript>4</Subscript>·H<Subscript>2</Subscript>O under different microwave heating conditions

Olivine-type LiFePO₄ is a very promising polyanion-type cathode material for lithium-ion batteries. In this work, LiFePO₄ with high specificity capacity is obtained from a novel precursor NH₄FePO₄·H₂O via microwave processing. The grains grow up in the duration of sintering until they reach the decomposition temperature. The apparent conductivity of the samples rises rapidly with the irradiation time and influences the electrochemical performance of the material greatly at high current density. As a result, the LiFePO₄ cathode material obtained with a sintering time of 15 min has good electrochemical performance. Between 2.5 and 4.2 V versus Li, a reversible capacity is as high as 156 mAh g⁻¹ at 0.05 C.     

A novel method with low-cost Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> to synthesize small size LiFePO<Subscript>4</Subscript> by carbothermal reduction method

The phase-pure LiFePO₄/C cathode materials have been successfully fabricated by precursor containing Fe₂O₃ coated with polyaniline through carbothermal reduction method. The polyaniline coating at Fe₂O₃ could restrict the growth of crystal and subsequently become electric carbon at higher synthesis temperature. Compared with conventional carbothermal reduction method, the sample showed a better capacity and less than 3% capacity fading after 30 cycles at various charge and discharge rate.     

Synthesis of LiFePO<Subscript>4</Subscript> by In Situ Polymerization Restriction Method for Pouch-Typed Cell

We report a two-step approach to synthesize nanosized LiFePO₄C composite by an in situ polymerization restriction method. First step, the Fe₂O₃@polyaniline is fabricated with in situ polymerization method. Secondly, the as-obtained Fe₂O₃ coated with polyaniline is mixed with LiH₂PO₄, followed by carbothermal reduction process. The outer polyaniline shell coating Fe₂O₃ can restrict the aggregation of particles and serves as carbon-containing precursor. The studies of electrochemical performances were carried out at full cell level. The results show that the assembled pouch-typed full-cells with as-obtained LiFePO₄ present excellent rate capability and cycle life.     

Electrochemical performance of LiFePO<Subscript>4</Subscript>/C doped with F synthesized by carbothermal reduction method using NH<Subscript>4</Subscript>F as dopant

The effect of fluorine doping on the electrochemical performance of LiFePO₄/C cathode material is investigated. The stoichiometric proportion of LiFe(PO₄)_1−x F_3x /C (x = 0.01, 0.05, 0.1, 0.2) materials was synthesized by a solid-state carbothermal reduction route at 650 °C using NH₄F as dopant. X-ray diffraction, scanning electron microscope, energy-dispersive X-ray, and X-ray photoelectron spectroscopy analyses demonstrate that fluorine can be incorporated into LiFePO₄/C without altering the olivine structure, but slightly changing the lattice parameters and having little effect on the particle sizes. However, heavy fluorine doping can bring in impurities. Fluorine doping in LiFePO₄/C results in good reversible capacity and rate capability. LiFe(PO₄)_0.95 F_0.15/C exhibits highest initial capacity and best rate performance. Its discharge capacities at 0.1 and 5 C rates are 156.1 and 119.1 mAh g⁻¹, respectively. LiFe(PO₄)_0.95 F_0.15/C also presents an obviously better cycle life than the other samples. We attribute the improvement of the electrochemical performance to the smaller charge transfer resistance (R _ct) and influence of fluorine on the PO₄³⁻ polyanion in LiFePO₄/C.     

Comparing the electrochemical properties of pure phase Zn<Subscript>2</Subscript>SnO<Subscript>4</Subscript> and composite Zn<Subscript>2</Subscript>SnO<Subscript>4</Subscript>/C

Compound Zn₂SnO₄ was synthesized by a hydrothermal method in which SnCl₄ · 5H₂O, ZnCl₂ and N₂H₄ · H₂O were used as reactants. Composite Zn₂SnO₄/C was then synthesized through a carbothermic reduction process using the as-prepared Zn₂SnO₄ and glucose as reactants. Comparing to the pure Zn₂SnO₄, some improved electrochemical properties were obtained for composite Zn₂SnO₄/C. When doped with 15% glucose, the composite Zn₂SnO₄/C showed the best electrochemical performance. Its first discharge capacity was about 1500 mA h g⁻¹, with a capacity retain of 500 mA h g⁻¹ in the 40th cycle at a constant current density of 100 mA/g in the voltage range of 0.05–3.0 V. There were also some differences displayed in their cyclic voltammogram.     

Enhanced electrochemical performance of LiFePO<Subscript>4</Subscript>/C nanocomposites due to in situ formation of Fe<Subscript>2</Subscript>P impurities

We have studied LiFePO₄/C nanocomposites prepared by sol-gel method using lauric acid as a surfactant and calcined at different temperatures between 600 and 900 °C. In addition to the major LiFePO₄ phase, all the samples show a varying amount of in situ Fe₂P impurity phase characterized by x-ray diffraction, magnetic measurements, and Mössbauer spectroscopy. The amount of Fe₂P impurity phase increases with increasing calcination temperature. Of all the samples studied, the LiFePO₄/C sample calcined at 700 °C which contains ～15 wt% Fe₂P shows the least charge transfer resistance and a better electrochemical performance with a discharge capacity of 136 mA h g^?1 at a rate of 1 C, 121 mA h g^?1 at 10 C (～70 % of the theoretical capacity of LiFePO₄), and excellent cycleability. Although further increase in the amount of Fe₂P reduces the overall capacity, frequency-dependent Warburg impedance analyses show that all samples calcined at temperatures ≥700 °C have an order of magnitude higher Li⁺ diffusion coefficient (～1.3?×?10^?13 cm² s^?1) compared to the one calcined at 600 °C, as well as the values reported in literature. This work suggests that controlling the reduction environment and the temperature during the synthesis process can be used to optimize the amount of conducting Fe₂P for obtaining the best capacity for the high power batteries.     

Structure of Ir-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> coatings obtained by chemical vapor deposition in the hydrogen atmosphere

Chemical vapor deposition is used to obtain Ir and Ir-Al₂O₃ coatings with a thickness of up to 5 μm and growth rate of 2.5 μm/h on steel substrates previously covered with an alumina layer. Tris-acetylacetonates of Ir(III) and Al(III) are used as precursors. The deposition process is carried out at the atmospheric pressure in the presence of hydrogen. The coatings obtained are studied by X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy. A dependence of the structure and composition of coatings on their preparation conditions is found.     

Preparation and characterization of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>composite particles

Using Fe₃O₄ nano-particles as seeds, a new type of Fe₃O₄/Au composite particles with core/shell structure and diameter of about 170 nm was prepared by reduction of Au³⁺ with hydroxylamine in an aqueous solution. Particle size analyzer and transmission electron microscope were used to analyze the size distribution and microstructure of the particles in different conditions. The result showed that the magnetically responsive property and suspension stability of Fe₃O₄ seeds as well as reduction conditions of Au³⁺to Au⁰are the main factors which are crucial for obtaining a colloid of the Fe₃O₄/Au composite particles with uniform particle dispersion, excellent stability, homogeneity in particle sizes, and effective response to an external magnet in aqueous suspension solutions. UV-Vis analysis revealed that there is a characteristic peak of Fe₃O₄/Au fluid. For particles with d(0.5)=168 nm, the λmax is 625 nm.     

Preparation and electrochemical properties of Li<Subscript>0.97</Subscript>Er<Subscript>0.01</Subscript>FePO<Subscript>4</Subscript>/C composite for lithium-ion batteries

Li_0.97Er_0.01FePO₄/C composite was prepared by solid-state reaction, using particle modification with amorphous carbon from the decomposition of glucose and lattice doping with supervalent cation Er³⁺. All samples were characterized by X-ray diffraction, scanning electron microscopy, multi-point Brunauer Emmett and Teller methodes. The electrochemical tests show Li_0.97Er_0.01FePO₄/C composite obtains the highest discharge specific capacity of 154 mAh g⁻¹ at C/10 rate and the best rate capability. Its specific capacity reaches 131 mAh g⁻¹ at 2C rate. Its capacity loss is only 14.9 % when the rate varies from C/10 to 2C.     

Magnetic,optical and electrical properties of Mn-doped CuCrO<Subscript>2</Subscript> thin films prepared by chemical solution deposition method

CuCrO₂ and CuCrO₂:Mn thin films were prepared on sapphire substrates by chemical solution deposition method. The effects of the annealing temperatures and Mn concentration on the structural, electrical and optical properties were investigated. The X-ray diffraction measurement was used to confirm the c-axis orientation of CuCrO₂ and CuCrO₂:Mn thin films. The maximum transmittances of the films in the visible region are about 65% with direct band gaps of 3.25 eV. All films showed the p-type conduction and semiconductor behavior. The electrical conductivity decreases rapidly with the increase of Mn content, the maximum of the electrical conductivity of 1.35 × 10⁻² S cm⁻¹ is CuCrO₂ film deposited at 600 °C temperature in 10⁻³ Torr vacuum, which is about four orders of magnitude higher than that of the Mn-doped CuCrO₂ thin film. The energy band of the samples is constructed based on the grain-boundary scattering in order to investigate the conduction mechanism. Moreover, the samples exhibit a clear ferromagnetism, which was likely ascribed to originating from the double-exchange interaction between the Mn³⁺ and Cr³⁺ ions.     

Effect of aerosol chemical vapor deposition on characteristics of MoS<Subscript>2</Subscript> particles

Influence exerted by the main technological parameters in the process in which nano- and microparticles of molybdenum disulfide are formed by the aerosol chemical vapor deposition method from a gas phase containing aerosol particles of (NH₄)₂MoS₄?C₃H₇NO solutions on the dimension characteristics, structure, and composition of the products being formed was studied. It was shown that the shape, size, and structure of the particles being formed are determined by the processes occurring in the first, streamwise, reactor zone. The temperature of this zone is the most important technological parameter. The concentration of ammonium thiomolybdate in solution makes it possible to gradually vary the size of disulfide particles in a wide range (from tens of nanometers to micrometers). In the conditions under study, the technological conditions have no effect on the chemical composition of the products being synthesized, which is always described by the formula MoS₂. The results obtained can be used in development of industrial apparatus and technology for synthesis of molybdenum disulfide nano- and microparticles to be used as the antifriction component of lubricating materials.