Facile synthesis of carbon coated LiFePO4 nanocomposite with excellent electrochemical performance through in situ formed lithium stearate pyrolysis route

Carbon coated LiFePO₄ (LiFePO₄/C) nanocomposite is successfully synthesized at a comparatively low temperature (400 °C) via a pyrolysis process of in situ formed lithium stearate. The obtained products are characterized by X-ray diffraction, electron microscopy, thermogravimetry, infrared and X-ray photoelectron spectroscopy. Experimental results indicate that the in situ formed lithium stearate can decompose at ∼290 °C, which is beneficial for the formation of carbon coating and reduction of Fe³⁺ species, and then the crystallized LiFePO₄/C nanocomposite can be formed at 400 °C without other intermediate products. As cathode material of Li-ion battery, the obtained LiFePO₄/C nanocomposite exhibits a good rate and cycling performance with a high discharge capacity of ∼160 mAh g⁻¹ (>94% theoretical capacity of LiFePO₄) at a current density of 1 C (170 mA g⁻¹), and ∼96% of its initial capacity can be retained after 200 charging/discharging cycles. Even at a high current density (10 C), the LiFePO₄/C nanocomposite still presents a discharge capacity as high as ∼100 mAh g⁻¹. The excellent electrochemical performances of the present LiFePO₄/C nanocomposite mainly originate from the good crystallinity, small particles and enhanced electronic conductivity of the materials coated and linked by carbon layers.     

High rate electrochemical performances of nanosized ZnO and carbon co-coated LiFePO4 cathode

The high rate electrochemical performances of ZnO and carbon co-coated LiFePO₄ have been studied by X-ray diffraction, high-resolution transmission electron microscope, electrochemical impedance spectroscopy, cyclic voltammetry and galvanostatic measurements. The carbon coated LiFePO₄ material was prepared by a freeze-drying method, and the diffusion coefficient and exchange current of these materials were calculated from their electrochemical impedance spectroscopy. The electrode delivered a reversible capacity of about 90% of the theoretical capacity when cycled between 2.5 and 4.2 V and showed stable cycle performance at high charge/discharge rates. This study showed that the co-coating process and freeze-drying method can effectively improve the electrochemical performances of LiFePO₄ materials.     

Effect of pH value on particle morphology and electrochemical properties of LiFePO4 by hydrothermal method

Lithium iron phosphate was prepared by hydrothermal synthesis using LiOH·H₂O, FeSO₄·7H₂O and H₃PO₄ as raw materials. The effects of pH value of reaction solution on particle morphology and electrochemical property were investigated. The pH value of the reaction solution was adjusted in the range of 2.5-8.8 by dilute sulfuric acid and ammonia water. The samples were characterized by field-emission scanning electronic microscope (FE-SEM), X-ray powder diffraction (XRD), constant-current charge/discharge cycling tests and chemical analysis. The results indicated that the particles exhibited acute angle diamond flake-like morphology at pH = 2.5, and as the pH value increased, the particle became hexagon flake-like, round flake-like and irregular flake-like morphology gradually. The optimal sample synthesized at pH = 6.4 exhibited discharge capacities of 151.8 mAh g⁻¹ at 0.2 C rate and 129.3 mAh g⁻¹ at 3 C rate. It was found that pH value affected the morphologies and properties of the product by means of different crystal growth rates.     

Influence of isovalent ion substitution on the electrochemical performance of LiCoPO4

LiCo_1−xM_xPO₄ (M = Mg²⁺, Mn²⁺ and Ni²⁺; 0 ≤ x ≤ 0.2) compounds have been synthesized by solid-state reaction method and studied as cathode materials for secondary lithium batteries. LiCoPO₄ exhibits a discharge plateau at ∼4.7 V with an initial discharge capacity of 125 mAh/g and on cycling capacity falls. Substitution of Co²⁺ with Mg²⁺/Mn²⁺/Ni²⁺ in LiCoPO₄ has an influence on the initial discharge capacity and on cycling behaviour. The capacity retention of LiCoPO₄ is improved by manganese substitution. Among the manganese substituted phases, LiCo_0.95Mn_0.05PO₄ shows good reversible capacity of ∼50 mAh/g.     

Synthesis and electrochemical performances of spinel LiCr0.1Ni0.4Mn1.5O4 compound

The spinel compound LiCr_0.1Ni_0.4Mn_1.5O₄ was synthesized by a solid reaction method and a sol-gel method using citric acid as chelating agent. The pure phase LiCr_0.1Ni_0.4Mn_1.5O₄ was obtained by the wet method. The electrochemical performances of the pure phase sample were measured at different current rates. There were three voltage plateaus at about 4.9, 4.7 and 4.0 V in the charge-discharge curves, which were attributed to the oxidation/reduction of chromium, nickel and manganese respectively. In the range of 3.5-5.0 V, its first discharge capacity was 143, 118 and 111 mAh/g corresponding to current densities of 1.0, 4.0 and 5.0 mA/cm², respectively. After 50 cycles, the capacity retention remained well at the current densities of 1.0, 4.0 and 5.0 mA/cm². The electrochemical performances of pure phase LiCr_0.1Ni_0.4Mn_1.5O₄ at 55 °C was also measured, and the results were discussed.     

Preparation of LiFePO4/C composite powders by ultrasonic spray pyrolysis followed by heat treatment and their electrochemical properties

LiFePO₄ powders could be successfully prepared from a precursor solution, which was composed of Li(HCOO)·H₂O, FeCl₂·4H₂O and H₃PO₄ stoichiometrically dissolved in distilled water, by ultrasonic spray pyrolysis at 500 °C followed by heat treatment at sintering temperatures ranging from 500 to 800 °C in N₂ + 3% H₂ gas atmosphere. Raman spectroscopy revealed that α-Fe₂O₃ thin layers were formed on the surface of as-prepared LiFePO₄ powders during spray pyrolysis, and they disappeared after sintering above 600 °C. The LiFePO₄ powders prepared at 500 °C and then sintered at 600 °C exhibited a first discharge capacity of 100 mAh g⁻¹ at a 0.1 C charge-discharge rate. To improve the electrochemical properties of the LiFePO₄ powders, LiFePO₄/C composite powders with various amounts of citric acid added were prepared by the present method. The LiFePO₄/C (1.87 wt.%) composite powders prepared at 500 °C and then sintered at 800 °C exhibited first-discharge capacities of 140 mAh g⁻¹ at 0.1 C and 84 mAh g⁻¹ at 5 C with excellent cycle performance. In this study, the optimum amount of carbon for the LiFePO₄/C composite powders was 1.87 wt.%. From the cyclic voltammetry (CV) and AC impedance spectroscopy measurements, the effects of carbon addition on the electrochemical properties of LiFePO₄ powders were also discussed.     

Synthesis and electrochemical characterizations of nano-La2O3-coated nanostructure LiMn2O4 cathode materials for rechargeable lithium batteries

LiMn₂O₄ spinel cathode materials were coated with 1.0, 2.0 and 3.0 wt.% of La₂O₃ by polymeric process, followed by calcinations at 850 °C for 6 h in air. The surface coated LiMn₂O₄ cathode materials were physically characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and XPS. XRD patterns of La₂O₃-coated LiMn₂O₄ revealed that the coating did not affect the crystal structure and space group Fd3m of the cathode materials, compared to the uncoated LiMn₂O₄. The surface morphology and particle agglomeration were investigated using scanning electron microscopy and the TEM image showed a compact coating layer on the surface of the core materials that had average thickness of about 100 nm. XPS data illustrated that the La₂O₃ was completely coated over the surface of the LiMn₂O₄ core cathode materials. The galvanostatic charge and discharge of the uncoated and La₂O₃-coated LiMn₂O₄ cathode materials were carried out in the potential range of 3.0 and 4.5 V at 30 °C and 60 °C. Among them, 2.0 wt.% of La₂O₃-coated spinel LiMn₂O₄ cathode has improved the structural stability, high reversible capacity and excellent electrochemical performances of the rechargeable lithium batteries.     

Preparation and electrochemical properties of Li-rich spinel-type lithium manganate coated LiMn2O4

Li-rich spinel-type lithium manganate (SC) coated LiMn₂O₄ composites were prepared and characterized by XRD, SEM, FT-IR, ICP, etc. Their charge/discharge behaviors were studied between 3.0 and 4.3 V at 40 mA g⁻¹ under room temperature, and the results showed that SC coated on surface of LiMn₂O₄ could improve cycling stability of composite electrodes. The composite (S1) containing 4.8 wt% of SC exhibited noticeably improved cycling stability, whereas the initial specific capacity was very close to that of LiMn₂O₄.     

Intermittent microwave heating synthesized high performance spherical LiFePO4/C for Li-ion batteries

An intermittent microwave heating method was used to synthesize spherical LiFePO₄/C in the presence of glucose as reductive agent and carbon source without the use of the inert gas in the oven processes. The FePO₄ was used as iron precursor to reduce the cost and three lithium salts of Li₂CO₃, LiOH and CH₃COOLi were chosen for comparison of the resulting materials. The materials can be alternatively heated by this method at a temperature controllable mode for crystallization and phase transformation and to provide relaxation time for protecting particles growth. The X-ray diffraction and scanning electron microscope measurements confirmed that the LiFePO₄/C is olivine structured with the average particle size of 50-100 nm. The spherical LiFePO₄/C as cathode material showed better electrochemical performance in terms of the specific capacity and the cycling stability, which might be attributed to the highly crystallized phase, small particle distribution and improved conductivity by carbon connection.     

Electrochemical lithium ion storage property of C60 encapsulated single-walled carbon nanotubes

This is the first study to investigate the electrochemical Li ion insertion/deinsertion property of C₆₀ encapsulated single-walled carbon nanotubes (SWCNTs) (C₆₀-peapods). It was found that the reversible Li ion storage capacity of the C₆₀-peapod per unit weight is about 1.2 times greater than that of the empty tubes. This suggests that one peapod tube can store almost 1.7 times more reversible Li ions compared to one empty SWCNT tube.     

Synthesis of spherical LiMnPO4/C composite microparticles

Spherical LiMnPO₄/C composite microparticles were prepared by a combination of spray pyrolysis and spray drying followed by heat treatment and examined as a cathode material for lithium batteries. The structure, morphology and electrochemical performance of the resulting spherical LiMnPO₄/C microparticles were characterized by X-ray diffraction, field-emission scanning electron microscopy, transmission electronic microscopy and standard electrochemical techniques. The final sample was identified as a single phase orthorhombic structure of LiMnPO₄ and spherical powders with a geometric mean diameter of 3.65 μm and a geometric standard deviation of 1.34. The electrochemical cells contained the spherical LiMnPO₄/C microparticles exhibited first discharge capacities of 112 and 130 mAh g⁻¹ at 0.05 C at room temperature and 55 °C, respectively. These also showed a good rate capability up to 5 C at room temperature and 55 °C.     

Synthesis of layered Li1.2+x[Ni0.25Mn0.75]0.8−xO2 materials (0 ≤ x ≤ 4/55) via a new simple microwave heating method and their electrochemical properties

Li_1.2+x[Ni_0.25Mn_0.75]_0.8−xO₂ (0 ≤ x ≤ 4/55) was prepared by a new simple microwave heating method and the effect of extra Li⁺ content on electrochemistry of Li_1.2Ni_0.2Mn_0.6O₂ (x = 0) was firstly revealed. X-ray diffraction identified that they had layered α-NaFeO₂ structure (space group R-3m). Linear variation of lattice constant as a function of x value supported the formation of solid solution, that is, extra Li⁺ is possibly incorporated in structure of layered Li_1.2Ni_0.2Mn_0.6O₂ (x = 0), accompanying oxidization of Ni²⁺ to Ni³⁺ to form Li_1.2+x[Ni_0.25Mn_0.75]_0.8−xO₂ (0 ≤ x ≤ 4/55). This was confirmed by X-ray photoelectron spectroscopy that Ni³⁺ appeared and increased in content with increasing x value. Charge–discharge tests showed that Li_1.2+x[Ni_0.25Mn_0.75]_0.8−xO₂ (0 ≤ x ≤ 4/55) truly displayed different electrochemical properties (different initial charge–discharge plots, capacities and cycleability). Li_1.2Ni_0.2Mn_0.6O₂ (x = 0) in this work delivered the highest discharge capacity of 219 mAh g⁻¹ between 4.8 and 2.0 V. Increasing Li content (x value in Li_1.2+x[Ni_0.25Mn_0.75]_0.8−xO₂) reduced charge–discharge capacities, but significantly enhancing cycleability.     

Growth of single and double walled carbon nanotubes over Co/V/MgO catalysts

High quality single walled carbon nanotubes (SWCNTs) and double walled carbon nanotubes (DWCNTs) were synthesized on Co/V/MgO catalysts by catalytic decomposition of CH₄ in H₂. Raman spectroscopy data revealed that the diameters of as-prepared SWCNTs are 1.28 and 0.73 nm. The diameter value of DWCNTs from Raman analysis also showed a narrow diameter distribution. Using field emission transmission electron microscopy (TEM), it was found that the diameter of carbon nanotubes can be controlled mainly by adjusting the molar ratio of Co–V versus the MgO support. The structure properties of catalysts were examined by X-ray diffraction (XRD). The formation of C₇V₈ may play an important role in preserving carbon in the catalyst particle and favoring the dissociation balance of CH₄.     

A PEG assisted sol-gel synthesis of LiFePO4 as cathodic material for lithium ion cells

In order to obtain fine-particle LiFePO₄ with excellent electrochemical performance, LiFePO₄/C powders were synthesized by a poly(ethylene glycol) (PEG) assisted sol-gel method. All samples were characterized by X-ray powder diffraction and scanning electron microscopy, and their electrochemical properties were investigated by cycle voltammograms and charge-discharge tests. The sample, synthesized with the n_PEG/n_LFP = 1:1 under sintering temperature of 600 °C, possesses the global morphology and particle size of about 100 nm. This sample delivers the first discharge capacity of 162 mAh g⁻¹, i.e. 95.3% of the theoretical capacity, at the 15 mA g⁻¹ discharge current between 2.5 and 4.0 V (versus Li/Li⁺). The sample also displays a robust rate capability and stable cycle-life. The improved electrochemical performance originates mainly from the fine particle of nanometric dimension, regular global morphology and uniform dispersing in the product as well as the increased electronic conductivity by carbon coating.     

Microwave synthesis and electrochemical characterization of mesoporous carbon@Bi2O3 composites

An efficient and quick microwave method has been employed to prepare worm-like mesoporous carbon@Bi₂O₃ composites for the first time. As-prepared products have been characterized by X-ray diffraction, N₂ adsorption-desorption, scanning electron microscopy, transmission electron microscopy and inductive coupled plasma atomic emission spectroscopy. The electrochemical measurement shows the worm-like mesoporous carbon@Bi₂O₃ composites exhibits excellent capacitance performance and the maximum specific capacitance reaches 386 F g⁻¹, three times more than the pure worm-like mesoporous carbon.     

Microwave synthesis of nanocrystalline Sb2S3 and its electrochemical properties

Nanocrystalline antimony trisulfide (Sb₂S₃) was successfully synthesized via microwave irradiation by the reaction of antimony trichloride (SbCl₃) and thiourea (CS(NH₂)₂) with PVP as the surfactant. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and high resolution TEM (HRTEM). XRD results show that the as-prepared sample is orthorhombic-phase Sb₂S₃. TEM image of the as-prepared Sb₂S₃ shows the rod-like structure. HRTEM image indicates that rodbundles of Sb₂S₃ consists of a number nanorods with the diameter ranging from 30 nm to 50 nm. Detailed HRTEM image demonstrates the preferential direction growth of the Sb₂S₃ nanorods. The electrochemical properties of Sb₂S₃ were primarily investigated by constant current charge/discharge cycling tests in lithium hexafluorophosphate (LiPF₆) solution. The possible electrochemical reaction mechanism was explained. The results indicate that the nanocrystalline Sb₂S₃ shows potential application in the field of the electrode materials.     

Synthesis and performance of LiMn0.7Fe0.3PO4 cathode material for lithium ion batteries

Pure and carbon-containing olivine LiMn_0.7Fe_0.3O₄ were synthesized at 600 °C by the method of solid-state reaction. Structure, surface morphology and charge/discharge performance of LiMn_0.7Fe_0.3O₄ were characterized by X-ray diffraction, scanning electron microscopy, and electrochemical measurement, respectively. The prepared materials with and without carbon both show the single olivine structure. The morphologies of primary particles are greatly affected by the addition of carbon. Large particles (500-1000 nm) and densely sintered blocks were observed in pure LiMn_0.7Fe_0.3PO₄, which made the insertion and extraction of lithium ions difficult. Battery made from this sample can not charge and discharge effectively. The carbon-containing LiMn_0.7Fe_0.3PO₄ has a small particle size (100-200 nm) and a regular appearance. This material demonstrates high reversible capacity of about 120 mAh g⁻¹, perfect cycling performance, and excellent rate capability. It is obvious that the addition of carbon plays an important role in restricting the particle size of the material, which helps to prepare LiMn_0.7Fe_0.3PO₄ with excellent electrochemical performance. The electrochemical reaction resistance is much lower in the partly discharged state than in the fully charged or fully discharged state by the measurement of ac impedance for carbon-containing LiMn_0.7Fe_0.3PO₄. It is indicated that the mixed-valence of Fe³⁺/Fe²⁺ or Mn³⁺/Mn²⁺ is beneficial to the transfer of electron which happens between the interface.     

Synthesis of LiFePO4 with fine particle by co-precipitation method

LiFePO₄ is a potential candidate for the cathode material of the lithium secondary batteries. A co-precipitation method was adopted to prepare LiFePO₄ because it is simple and cheap. Nitrogen gas was needed to prevent oxidation of Fe²⁺ in the aqueous solution. The co-precipitated precursor shows the high reactivity with the reductive gas, and the single phase of LiFePO₄ is successfully synthesized with the aid of carbon under less reductive conditions. LiFePO₄ fine powder prepared by co-precipitation method shows high rate capability, impressive specific capacity and cycle property.     

Synthesis, structure, and electrochemical characteristics of LiNi1/3Co1/3Mn1/3O2 prepared by thermal polymerization

A thermal polymerization route was adopted to synthesize layered LiNi_1/3Co_1/3Mn_1/3O₂ materials. After annealing the polymer gel containing metal salts at different temperatures from 850 to 1000 °C for different time between 6 and 25 h, powders of pure α-NaFeO₂ phase were obtained. The crystal structure, morphology and electrochemical properties of the products were investigated by XRD, SEM, electrochemical cell cycling and AC impedance spectroscopy. It is found that the powder annealed at 950 °C for 15 h shows the best electrochemical property with the first specific discharge capacity of 188 mAh/g at C/10 and 87% retention after 100 cycles. It exhibits good rate capability with the specific capacity of 169 mAh/g at 1 C and 110 mAh/g at 6 C. Adopting a slowly cooling procedure during the powder annealing can improve the electrochemical performance of the LiNi_1/3Co_1/3Mn_1/3O₂ powder.     

Preparation of hexagonal-MoO3 and electrochemical properties of lithium intercalation into the oxide

Metastable hexagonal molybdenum trioxide has been synthesized by chemical precipitation and hydrothermal treatment at low temperature. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) were used to characterize the product, which has a unique hexagonal prism-like morphology. Excellent electrochemical performances were exhibited: the first reversible discharge specific capacity can reach 402 mAh g⁻¹ versus Li metal at 0.1 mA cm⁻² (voltage range 1.2-4.0 V).