High power performance of nano-LiFePO4/C cathode material synthesized via lauric acid-assisted solid-state reaction

A nano-LiFePO₄/C composite has been directly synthesized from micrometer-sized Li₂CO₃, NH₄H₂PO₄, and FeC₂O₄·2H₂O by the lauric acid-assisted solid-state reaction method. The SEM and TEM observations demonstrate that the synthesized nano-LiFePO₄/C composite has well-dispersed particles with a size of about 100–200 nm and an in situ carbon layer with thickness of about 2 nm. The prepared nano-LiFePO₄/C composite has superior rate capability, delivering a discharge capacity of 141.2 mAh g⁻¹ at 5 °C, 130.9 mAh g⁻¹ at 10 C, 121.7 mAh g⁻¹ at 20 °C, and 112.4 mAh g⁻¹ at 30 °C. At −20 °C, this cathode material still exhibits good rate capability with a discharge capacity of 91.9 mAh g⁻¹ at 1 °C. The nano-LiFePO₄/C composite also shows excellent cycling ability with good capacity retention, up to 100 cycles at a high current density of 30 °C. Furthermore, the effect of lauric acid in the preparation of nano-LiFePO₄/C composite was investigated by comparing it with that of citric acid. The SEM images reveal that the morphology of the LiFePO₄/C composite transformed from the porous structure to fine particles as the molar ratio of lauric acid/citric acid increased.     

Electrochemical properties of bare and Ta-substituted Nb2O5 nanostructures

In this work, bare and Ta-substituted Nb₂O₅ nanofibers are prepared by electrospinning followed by sintering at temperatures in the 800–1100 °C range for 1 h in air. Obtained bare and Ta-substituted Nb₂O₅ polymorphs are characterized by X-ray diffraction, scanning electron microscopy, density measurement, and Brunauer, Emmett and Teller surface area. Electrochemical properties are evaluated by cyclic voltammetry and galvanostatic techniques. Cycling performance of Nb₂O₅ structures prepared at temperature 800 °C, 900 °C, and 1100 °C shows following discharge capacity at the end of 10th cycle: 123, 140, and 164 (±3) mAh g⁻¹, respectively, in the voltage range 1.2–3.0 V and at current rate of 150 mA g⁻¹ (1.5 C rate). Heat treated composite electrode based on M-Nb₂O₅ (1100 °C) in argon atmosphere at 220 °C, shows an improved discharge capacity of 192 (±3) mAh g⁻¹ at the end of 10th cycle. The discharge capacity of Ta-substituted Nb₂O₅ prepared at 900 °C and 1100 °C showed a reversible capacity of 150, 202 (±3) mAh g⁻¹, respectively, in the voltage range 1.2–3.0 V and at current rate of 150 mA g⁻¹. Anodic electrochemical properties of M-Nb₂O₅ deliver a reversible capacity of 382 (±5) mAh g⁻¹ at the end of 25th cycle and Ta-substituted Nb₂O₅ prepared at 900 °C, 1000 °C and 1100 °C shows a reversible capacity of 205, 130 and 200 (±3) mAh g⁻¹ (at 25th cycle) in the range, 0.005–2.6 V, at current rate of 100 mA g⁻¹.     

A large capacity of LiV3O8 cathode material for rechargeable lithium-based batteries

A cathode material, LiV₃O₈, for rechargeable lithium-based batteries has been prepared via a combined freeze-drying method and appropriately post-treated in argon atmosphere. It was found that the sample post-treated at 300 °C in Ar has the optimal performance. Electrochemical tests performed on this material demonstrated its very high insertion capacity of 347 mAh g⁻¹ (3.7 Li⁺ per LiV₃O₈) at a current density of 50 mA g⁻¹ (C/6). Most important is that it displayed an excellent cycling stability and after 60 cycles, a discharge capacity with 351 mAh g⁻¹ was obtained. It was proposed that a short-range crystallographic order had a stronger influence on the electrochemical performance of an electrode material in this work, instead of the surface area, particle size and crystalline degree, etc.     

Sulfur–carbon composite electrode for all-solid-state Li/S battery with Li2S–P2S5 solid electrolyte

All-solid-state Li/S batteries with Li₂S–P₂S₅ glass–ceramic electrolytes were fabricated and their electrochemical performance was examined. Sulfur–carbon composite electrodes were prepared by grinding with a mortar and milling with a planetary ball-mill apparatus. Milling of a mixture of sulfur, acetylene black and the Li₂S–P₂S₅ glass–ceramic electrolyte resulted in the amorphization of sulfur and a reduction in the particle size of the mixture. The charge–discharge properties of all-solid-state cells with the composite electrode were investigated at temperatures from −20 °C to 80 °C. The cells retained a reversible capacity higher than 850 mAh g⁻¹ for 200 cycles under 1.3 mA cm⁻² (333 mA g⁻¹) at 25 °C. The cell performance was influenced by the crystallinity of sulfur and the particle size of the electrode material, whereby improved contact among the electrode components achieved by milling contributed to enhancement of the capacity of an all-solid-state Li/S cell.     

Synthesis and electrochemical characterization of LiCoxMn1−xPO4/C nanocomposites

LiCo_xMn_1−xPO₄/C nanocomposites (0 ≤ x ≤ 1.0) were prepared by a combination of spray pyrolysis at 300 °C and wet ball-milling followed by heat treatment at 500 °C for 4 h in 3% H₂ + N₂ atmosphere. X-ray diffraction analysis indicated that all samples had the single phase olivine structures indexed by orthorhombic Pmna. The lattice parameters linearly decreased with increasing cobalt content, which confirmed the existence of solid solutions. It was clearly seen from the scanning electron microscopy observation that the LiCo_xMn_1−xPO₄/C samples were agglomerates with approximately 100 nm primary particles. The LiCo_xMn_1−xPO₄/C nanocomposites were used as cathode materials for lithium batteries, and electrochemical performance was comparatively investigated with cyclic voltammetry and galvanostatic charge–discharge test using the Li?1 M LiPF₆ in EC:DMC = 1:1?LiCo_xMn_1−xPO₄/C cells at room temperature. The cells at 0.05 C charge–discharge rate delivered first discharge capacities of 165 mAh g⁻¹ (96% of theoretical capacity) at x = 0, 136 mAh g⁻¹ at x = 0.2, 132 mAh g⁻¹ at x = 0.5, 125 mAh g⁻¹ at x = 0.8 and 132 mAh g⁻¹ (79% of theoretical capacity) at x = 1.0, respectively. While the first discharge capacity increased with the cobalt content at high charge–discharge rates more than 0.5 C due to higher electronic conductivity of LiCoPO₄ in comparison with LiMnPO₄, the cycleability of cell became worse with increasing the amount of cobalt. The existence of Mn²⁺ seemed to enhance the cycleability of LiCo_xMn_1−xPO₄/C nanocomposite cathode.     

Synthesis of LiFePO4/C cathode material from ferric oxide and organic lithium salts

LiFePO₄/C cathode material has been simply synthesized via a modified in situ solid-state reaction route using the raw materials of Fe₂O₃, NH₄H₂PO₄, Li₂C₂O₄ and lithium polyacrylate (PAALi). The sintering temperature of LiFePO₄/C precursor is studied by thermo-gravimetric (TG)/differential thermal analysis (DTA). The physical properties of LiFePO₄/C are then investigated through analysis using by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM) and the electrochemical properties are investigated by electrochemical impedance spectra (EIS), cyclic voltammogram (CV) and constant current charge/discharge test. The LiFePO₄/C composite with the particle size of ∼200 nm shows better discharge capacity (156.4 mAh g⁻¹) than bare LiFePO₄ (52.3 mAh g⁻¹) at 0.2 C due to the improved electronic conductivity which is demonstrated by EIS. The as-prepared LiFePO₄/C through this method also shows excellent high-rate characteristic and cycle performance. The initial discharge capacity of the sample is 120.5 mAh g⁻¹ and the capacity retention rate is 100.6% after 50 cycles at 5 C rate. The results prove that the using of organic lithium salts can obtain a high performance LiFePO₄/C composite.     

Effects of Na and Cl co-doping on electrochemical performance in LiFePO4/C

Na⁺ and Cl⁻ co-doped LiFePO₄/C composites were prepared via a simple solid state reaction. The structure, valence state and electrochemical performance were carefully investigated. Rietveld refinement on X-ray diffractions reveals that Na⁺ and Cl⁻ have successfully been introduced into the lattice of LiFePO₄. X-ray photoelectron spectroscopy proves that the co-doping of Na⁺ and Cl⁻ does not change the chemical state of Fe(II). Experimental results further show that the co-doping contributes to induce the lattice distortion, modify the particle morphology, and increase the electronic conductivity. Considerably enhanced capacity, coulombic efficiency and rate capability were obtained in the co-doped LiFePO₄. The specific capacities are 157 mAh g⁻¹ at 0.2 C, 115 mAh g⁻¹ at 10 C and 98 mAh g⁻¹ at 20 C for the (Na⁺, Cl⁻) co-doped LiFePO₄/C cathode material. The improvement can be ascribed to the enhanced electronic conductivity and electrode kinetics due to the micro-structural modification promoted by co-doping.     

Preparation and characterization of core–shell structure Fe3O4/C nanoparticles with unique stability and high electrochemical performance for lithium-ion battery anode material

Core–shell structure carbon coating Fe₃O₄ nanoparticles are prepared by a two-step method. The crystalline structure and the electrochemical performance of the prepared samples are investigated. The results indicate that a uniform and continuous carbon layer is formed on the surface of Fe₃O₄ nanoparticles. The core–shell structure Fe₃O₄/C nanoparticles show a high initial discharge capacity of 1546 mAh g⁻¹ and a specific stable discharge capacity of about 800 mAh g⁻¹ at 0.5 C with no noticeable capacity fading up to 100 cycles.     

Synthesis and electrochemical properties of mixed-phase αx/β1-x-LiVOPO4/C composites

Mixed-phase α_x/β_1-x-LiVOPO₄/C (x = 0.1, 0.3, 0.5, 0.7, 0.9) composites were synthesized by the solid-phase sintering method. X-ray diffraction (XRD) analyses revealed that a series of composites were comprised of pure triclinic α-LiVOPO₄ and orthorhombic β-LiVOPO₄ components. The mixed-phase materials exhibit better rate performance than whether pure triclinic α-LiVOPO₄/C or orthorhombic β-LiVOPO₄/C. The α_0.5/β_0.5-LiVOPO₄/C (α:β = 1:1) composite exhibit superior high-rate capability. When cycled at 2C and 5C, the initial discharge capacities of the α_0.5/β_0.5-LiVOPO₄/C composite are 81.1 mAh g^?1 and 69.8 mAh g^?1 respectively, which are significantly higher than those of the pure α-LiVOPO₄/C (30.6 mAh g^?1 and 18.6 mAh g^?1, respectively) and β-LiVOPO₄/C (56.8 mAh g^?1 and 36.9 mAh g^?1, respectively). The improved electrochemical performance could be attributed to the mixed phase possess an open framework and stable structure.     

Fabrication and electrochemical capacitance of hierarchical graphene/polyaniline/carbon nanotube ternary composite film

A film composed of graphene (GN) sheets, polyaniline (PANI) and carbon nanotubes (CNTs) has been fabricated by reducing a graphite oxide (GO)/PANI/CNT precursor prepared by flow-directed assembly from a complex dispersion of GO and PANI/CNT, followed by reoxidation and redoping of the reduced PANI in the composite to restore the conducting PANI structure. Scanning electron microscope images indicate that the ternary composite film is a layered structure with coaxial PANI/CNT nanocables uniformly sandwiched between the GN sheets. Such novel hierarchical structure with high electrical conductivity perfectly facilitates contact between electrolyte ions and PANI for faradaic energy storage and efficiently utilizes the double-layer capacitance at the electrode–electrolyte interfaces. The specific capacitance of the GN/PANI/CNT estimated by galvanostatic charge/discharge measurement is 569 F g⁻¹ (or 188 F cm⁻³ for volumetric capacitance) at a current density of 0.1 A g⁻¹. In addition, the GN/PANI/CNT exhibits good rate capability (60% capacity retention at 10 A g⁻¹) and superior cycling stability (4% fade after 5000 continuous charge/discharge cycles).     

Electrochemical performance of Li3V2(PO4)3/C cathode materials using stearic acid as a carbon source

The Li₃V₂(PO₄)₃/C cathode materials are synthesized by a simple solid-state reaction process using stearic acid as both reduction agent and carbon source. Scanning electron microscopy and transmission electron microscopy observations show that the Li₃V₂(PO₄)₃/C composite synthesized at 700 °C has uniform particle size distribution and fine carbon coating. The Li₃V₂(PO₄)₃/C shows a high initial discharge capacity of 130.6 and 124.4 mAh g⁻¹ between 3.0 and 4.3 V, and 185.9 and 140.9 mAh g⁻¹ between 3.0 and 4.8 V at 0.1 and 5 C, respectively. Even at a charge–discharge rate of 15 C, the Li₃V₂(PO₄)₃/C still can deliver a discharge capacity of 103.3 and 112.1 mAh g⁻¹ in the potential region of 3.0–4.3 V and 3.0–4.8 V, respectively. Based on the analysis of cyclic voltammograms and electrochemical impedance spectra, the apparent diffusion coefficients of Li ions in the composites are in the region of 1.09 × 10⁻⁹ and 4.95 × 10⁻⁸ cm² s⁻¹.     

Preparation of a graphene nanosheet/polyaniline composite with high specific capacitance

A graphene nanosheet (GNS)/polyaniline (PANI) composite was synthesized using in situ polymerization. The morphology and microstructure of samples were examined by scanning electron microscopy (SEM), transition electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy. Electrochemical properties were characterized by cyclic voltammetry (CV) and galvanostatic charge/discharge. GNS as a support material could provide more active sites for nucleation of PANI as well as excellent electron transfer path. The GNS was homogeneously coated on both surfaces with PANI nanoparticles (∼2 nm), and a high specific capacitance of 1046 F g⁻¹ (based on GNS/PANI composite) was obtained at a scan rate of 1 mV s⁻¹ compared to 115 F g⁻¹ for pure PANI. In addition, the energy density of GNS/PANI composite could reach 39 W h kg⁻¹ at a power density of 70 kW kg⁻¹.     

Electrochemical properties of Cu2O/Cu composite particles prepared by a novel and facile method

Cu₂O/Cu composite particles were synthesized by a novel and facile chemical reduction method without any template or surfactant. X-ray diffraction (XRD) results showed that the product mainly consisted of the Cu₂O phase coexisting with a few Cu phases. Typical FE-SEM images indicated that the particles with an octahedral shape were Cu₂O. In addition, the electrochemical performance of the Cu₂O/Cu particles as the working electrode material in alkaline solution was systematically investigated. The particles showed a maximum discharge capacity of 222.9 mAh g⁻¹ at a discharge current density of 60 mA g⁻¹ and a high value of 109.1 mAh g⁻¹ after 50 charge–discharge cycles. The results of cyclic voltammetry demonstrated that the reaction between Cu₂O and Cu is the major electrochemical reaction during the charging and discharging process. The results of electrochemical impedance spectroscopy indicated that the formation of Cu₂O on the surface of Cu particles significantly increased the contact resistance and the charge transfer resistance of the electrode during the discharging process.     

Synthesis and electrochemical performance of Li2FeSiO4/carbon/carbon nano-tubes for lithium ion battery

Li₂FeSiO₄/carbon/carbon nano-tubes (Li₂FeSiO₄/C/CNTs) and Li₂FeSiO₄/carbon (Li₂FeSiO₄/C) composites were synthesized by a traditional solid-state reaction method and characterized comparatively by X-ray diffraction, scanning electron microscopy, BET surface area measurement, galvanostatic charge-discharge and AC impedance spectroscopy, respectively. The results revealed that the Li₂FeSiO₄/C/CNT composite exhibited much better rate performance in comparison with the Li₂FeSiO₄/C composite. At 0.2 C, 5 C and 10 C, the former composite electrode delivered a discharge capacity of 142 mAh g⁻¹, 95 mAh g⁻¹, 80 mAh g⁻¹, respectively, and after 100 cycles at 1 C, the discharge capacity remained 95.1% of its initial value.     

Enhanced rate performance of nano–micro structured LiFePO4/C by improved process for high-power Li-ion batteries

A spherical carbon-coated nano–micro structured LiFePO₄ composite is synthesized for use as a cathode material in high-power lithium-ion batteries. The composites are synthesized through carbothermal reduction with two sessions of ball milling (before and after pre-sintering of precursor) followed by spray-drying with the dispersant of polyethylene glycol added. The structure, particle size, and surface morphology of the cathode active material and the properties of the coated carbon are investigated by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and high-resolution transmission electron microscopy. Results indicate that the LiFePO₄/C composite has a spherical micro-porous morphology composed of a large number of carbon-coated nano-spheres linked together with an ordered olivine structure. The carbon on the surface of LiFePO₄ effectively reduces inter-particle agglomeration of the LiFePO₄ particles. A galvanostatic charge–discharge test indicates that the LiFePO₄/C composites exhibit initial discharge capacities of 155 mAh g⁻¹ and 88 mAh g⁻¹ at 0.2 C and 20 C rates with the end of discharge voltage of 2.5 V, respectively. This behavior is ascribed to the unique spherical structure, which shortens lithium ions diffusion length and improves the electric contact between LiFePO₄ particles.     

High capacity ZnFe2O4 anode material for lithium ion batteries

A nanostructured ternary transition metal oxide, ZnFe₂O₄, is synthesized via the simple polymer pyrolysis method. The characteristics of the material are examined by thermogravimetry, Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The electrochemical test results show that this method of ZnFe₂O₄ synthesis produces high specific capacities and good cycling performance, with an initial specific capacity as high as 1419.6 mAh g⁻¹ at first discharge that is maintained at over 800 mAh g⁻¹ even after 50 charge–discharge cycles. The electrode also presents a good rate capability, with a high rate of 4C (1C = 928 mA g⁻¹), a reversible specific capacity that can be as high as 400 mAh g⁻¹. ZnFe₂O₄ is a potential alternative to high-performance nanostructured anode material in lithium ion batteries.     

Boosted sodium storage of GeS2/GeO2/ZnS composite via heterostructure engineering

Transition metal dichalcogenides exhibit tremendous potential for sodium ion batteries (SIBs), owing to the outstanding specific capacity and aboundant reserves. However, the large ionic radius of sodium and poor conductivity often result in the fast decaying performance and inferior reaction kinetics. Herein, the GeS₂/GeO₂/ZnS@rGO (GGZ/C) ternary metal-based composite is fabricated as an anode material for SIBs. Notably, the GGZ/C composite is derived from the phase transformation of Zn₂GeO₄ precursor, which is beneficial for the heterostructure engineering. In this hierarchical structure, the metal phases ZnS and GeO₂ are used to form the heterogeneous framework, while graphene is applied to build a conductive network and anchor the host nanoparticles. Therefore, the great Na⁺ diffusion channels are achieved by the rational design of the huge exquisite interfaces among the heterogeneous mixed phases. Notably, it can almost completely relieve the volume expansion and restrain the internal stress of GGZ/C composite, providing the excellent structural tolerance. As expected, the GGZ/C composite exhibits excellent rate capability, with an impressive reversible capacity of 548 mAh g⁻¹ at a high rate of 5.0 A g⁻¹. Meanwhile, the GGZ/C also displays outstanding cycling performance with a specific capacity of 519 mAh g⁻¹ after 650 cycles at high rate of 5.0 A g⁻¹. This strategy offers the inspiration for rational heterostructure engineering for the energy storage materials with excellent reversible capacity and large volume variation.     

The electrochemical reduction of PdCl4 and PdCl6 in polyaniline: Influence of Pd deposit morphology on methanol oxidation in alkaline solution

The controlled uptake and electrochemical reduction of metal precursors PdCl₄²⁻ and PdCl₆²⁻ in polyaniline (PANI) is demonstrated. The formation of PANI/Pd composites is achieved with a reduction in proton doping and an increase in the oxidation of the polymer with Pd deposits physically blocking the nitrogen groups. High surface area filaments (PdCl₄²⁻) or a rough encapsulation (PdCl₆²⁻) of Pd metal on PANI are obtained. The structural differences highlight the influence of the metal precursor oxidation state on the morphology of the Pd deposits in PANI. Thermal gravimetric analysis provides an estimate of the Pd content for each composite of ∼40%. X-ray Photoelectron Spectroscopy and X-ray-excited Auger Electron Spectroscopy analyses confirm the deposition of Pd metal. The catalytic oxidation of methanol was demonstrated for both PANI/Pd composites in alkaline solutions that prohibit proton doping of the polymer. The data indicates that Pd metal acts as a solid-state dopant that may delocalize the charge on the polymer backbone to maintain conductivity. Methanol oxidation at PANI/Pd composites produced using PdCl₄²⁻ was enhanced relative to the composite produced using PdCl₆²⁻ and a planar Pd electrode. Comparison of PANI/Pd composite produced using PdCl₄²⁻ with other Pd catalysts from the literature indicates surface poisoning is reduced when Pd is coupled with the polymer. The composite is robust and stable in alkaline solution with the charge density decreasing by 5% on the positive scan and 13% on the negative scan after 200 voltammetric cycles. The data also indicates that the reductive desorption of surface contaminants is possible, minimizing the catalytic loss due to surface poisoning.     

Performance evaluation of CNT/polypyrrole/MnO2 composite electrodes for electrochemical capacitors

A ternary composite of CNT/polypyrrole/hydrous MnO₂ is prepared by in situ chemical method and its electrochemical performance is evaluated by using cyclic voltammetry (CV), impedance measurement and constant-current charge/discharge cycling techniques. For comparative purpose, binary composites such as CNT/hydrous MnO₂ and polypyrrole/hydrous MnO₂ are prepared and also investigated for their physical and electrochemical performances. The specific capacitance (SC) values of the ternary composite, CNT/hydrous MnO₂ and polypyrrole/hydrous MnO₂ binary composites estimated by CV technique in 1.0 M Na₂SO₄ electrolyte are 281, 150 and 35 F g⁻¹ at 20 mV s⁻¹ and 209, 75 and 7 F g⁻¹ at 200 mV s⁻¹, respectively. The electrochemical stability of ternary composite electrode is investigated by switching the electrode back and forth for 10,000 times between 0.1 and 0.9 V versus Ag/AgCl at 100 mV s⁻¹. The electrode exhibits good cycling stability, retaining up to 88% of its initial charge at 10,000th cycle. A full cell assembled with the ternary composite electrodes shows a SC value of 149 F g⁻¹ at a current loading of 1.0 mA cm⁻² during initial cycling, which decreased drastically to a value of 35 F g⁻¹ at 2000th cycle. Analytical techniques such as scanning electron microscopy (SEM), X-ray diffraction spectroscopy (XRD), Brunauer-Emmet-Teller (BET) surface area measurement and inductively coupled plasma-atomic emission spectrometry (ICP-AES) are also used to characterize the composite materials.     

Effects of the loading and polymerization temperature on the capacitive performance of polyaniline in NaNO3

Polyaniline (PANI) synthesized by a potentiostatic method at 4 °C in 1 M HNO₃ with the polymerization charge density equal to/less than 0.45 C cm⁻² was demonstrated to exhibit ideally capacitive characteristics (i.e. high reversibility and high-power property) with a high specific capacitance of 210 F g⁻¹ for the application of electrochemical supercapacitors in NaNO₃. Influences of the polymerization charge density (i.e. the polymer loading) and the polymerization temperature on the capacitive characteristics of PANI films compared by both cyclic voltammetry and charge-discharge technique were reasonably correlated with their structural properties examined by X-ray photoelectron spectroscopy (XPS). The highest specific capacitance of a PANI film polymerized at 4 °C was attributed to its lowest density of structure defects. The surface morphology of these PANI films was examined by a scanning electron microscope (SEM).