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.     

Insight into the improvement of rate capability and cyclability in LiFePO4/polyaniline composite cathode

A simple and efficient strategy was employed to enhance high-rate performance for carbon-coated LiFePO₄ (C-LFP) by incorporating with conducting polymer polyaniline (PANI). C-LFP was synthesized via a solid state reaction whereas PANI was formed in situ by chemical oxidative polymerization of aniline with ammonium persulfate as an oxidizer to achieve the C-LFP/PANI composites. Specific capacities as high as 165 mAh g⁻¹ at 0.2 C, 133 mAh g⁻¹ at 7 C and 123 mAh g⁻¹ at 10 C were obtained in C-LFP/7 wt.% PANI composite. Moreover, the composite exhibits remarkably improved cyclability as compared with the parent C-LFP. The mechanism has been carefully investigated for the improvement in the electrochemical performance. Experimental results show that the charge transfer impedance decreases significantly and the cathode surface becomes much smooth over cycling with modification of conductive PANI. The incorporated PANI can work not only as an additional host for Li⁺-ion insertion/extraction, but also as a binder to modify the electrode surface and a container for electrolyte to penetrate into C-LFP particles.     

Preparation and electrochemical properties of a LiFePO4/C composite cathode material by a polymer-pyrolysis–reduction method

A LiFePO₄/C composite was successfully prepared by a polymer-pyrolysis–reduction method, using FePO₄·2H₂O and lithium polyacrylate (PAALi) as raw materials. The structure of the LiFePO₄/C composites was investigated by X-ray diffraction (XRD). The micromorphology of the precursor and LiFePO₄/C powders was observed using scanning electron microscopy (SEM), and the in situ coating of carbon on the particles was observed by transmission electron microscopy (TEM). Furthermore, the electrochemical properties were evaluated by cyclic voltammograms (CVs), electrochemical impedance spectra (EIS) and constant current charge/discharge cycling tests. The results showed that the sample synthesized at 700 °C had the best electrochemical performance, exhibiting initial discharge capacities of 157, 139 and 109 mAh g⁻¹ at rates of 0.1, 1 and 5 C, respectively. Moreover, the sample presented excellent capacity retention as there was no significant capacity fade after 50 cycles.     

Enhanced performance of LiFePO4 through hydrothermal synthesis coupled with carbon coating and cupric ion doping

A hydrothermal reaction has been adopted to synthesize pure LiFePO₄ first, which was then modified with carbon coating and cupric ion (Cu²⁺) doping simultaneously through a post-heat treatment. X-ray diffraction patterns, transmission electron microscopy and scanning electron microscopy images along with energy dispersive spectroscopy mappings have verified the homogeneous existence of coated carbon and doped Cu²⁺ in LiFePO₄ particles with phospho-olivine structure and an average size of 400 nm. The electrochemical performances of the material have been studied by cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge–discharge measurements. The carbon-coated and Cu²⁺-doped LiFePO₄ sample (LFCu5/C) exhibited an enhanced electronic conductivity of 2.05 × 10⁻³ S cm⁻¹, a specific discharge capacity of 158 mAh g⁻¹ at 50 mA g⁻¹, a capacity retention of 96.4% after 50 cycles, a decreased charge transfer resistance of 79.4 Ω and superior electrode reaction reversibility. The present synthesis route is promising in making the hydrothermal method more practical for preparation of the LiFePO₄ material and enhancement of electrochemical properties.     

Synthesis of spindle-shaped nanoporous C–LiFePO4 composite and its application as a cathodes material in lithium ion batteries

In this work, LiFePO₄/C composites were prepared in hydrothermal system by using iron gluconate as iron source, and two feeding sequences during the preparation were comparatively studied. The morphology, crystal structure and charge–discharge performance of the prepared samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and galvanostatic charge–discharge testing. The results showed that the feeding sequences and iron gluconate seriously affected the microstructures and electrochemical properties of the resulting LiFePO₄ cathodes in lithium ion batteries. The spindle-shaped LiFePO₄ with hierarchical microporous structure self-assembled by nanoparticles has been successfully synthesized by synthesis route B. In addition, the cell performance of the synthesized LiFePO₄ by synthesis route B was better than that of LiFePO₄ by synthesis route A. Specially at high rates, the superior rate performance of the spindle-shaped LiFePO₄/C microstructure (LFP/C-B) was revealed. And special reversible capacities of ∼118 and ∼95 mAh g⁻¹ were obtained at rates of 2 C and 5 C, comparing to ∼96 and ∼68 mAh g⁻¹ for LFP/C-A.     

Effect of carbon coating on low temperature electrochemical performance of LiFePO4/C by using polystyrene sphere as carbon source

Carbon perfectly coated LiFePO₄ cathode materials are synthesized by carbon-thermal reduction method using polystyrene (PS) spheres as carbon source. The PS spheres with diameters of 150–300 nm used for the pyrolysis reaction not only inhibit the particle growth but also lead to uniform distribution of carbon coating on the surface of LiFePO₄ particles. Rate capability and cycling stability of LiFePO₄/C with the carbon contents ranging from 1.4 wt% to 3.7 wt% are investigated at −20 °C. The LiFePO₄/C with 3.0 wt% C exhibits excellent electrochemical capability at low temperature, which delivers 147 mAh g⁻¹ at 0.1 C. After 100 cycles at a charge–discharge rate of 1 C, there is still 100% of initial capacity retained for the LiFePO₄/C electrode at −20 °C. According to the transmission electron microscope analysis and cyclic voltammetry measurement, this can be attributed to the good carbon coating morphology and optimal carbon coating thickness.     

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.     

A novel synthesis of spherical LiFePO4/C composite using Fe1.5P and mixed lithium salts via oxygen permeation

A novel route was designed to synthesize LiFePO₄/C composites by using the Fe_1.5P byproduct, mixed lithium salts, and permeated oxygen from air via a rheological phase method. The reaction process was investigated with various techniques. When the calcining time was increased from 10 to 30 h, the gradual formation of olivine structure was observed. The growth kinetics of the crystals was analyzed. SEM and TEM results indicated the as-synthesized LiFePO₄ was constituted of small spheres covered with carbon particles. The discharge capacity of the LiFePO₄/C composite prepared at ??700 °C for ??25 h could reach 139.7 mAh g^?1 and still remained 130.2 mAh g^?1 after 15 cycles at 0.2 C rate, comparable to that of the reported LiFePO₄/C composite using conventional methods. Cyclic voltammogram confirmed the LiFePO₄/C composite had a high purity and good lithium ion insertion/desertion redox behavior.     

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.     

Synthesis and characterization of high-density LiFePO4/C composites as cathode materials for lithium-ion batteries

To achieve a high-energy-density lithium electrode, high-density LiFePO₄/C composite cathode material for a lithium-ion battery was synthesized using self-produced high-density FePO₄ as a precursor, glucose as a C source, and Li₂CO₃ as a Li source, in a pipe furnace under an atmosphere of 5% H₂-95% N₂. The structure of the synthesized material was analyzed and characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The electrochemical properties of the synthesized LiFePO₄/carbon composite were investigated by cyclic voltammetry (CV) and the charge/discharge process. The tap-density of the synthesized LiFePO₄/carbon composite powder with a carbon content of 7% reached 1.80 g m⁻³. The charge/discharge tests show that the cathode material has initial charge/discharge capacities of 190.5 and 167.0 mAh g⁻¹, respectively, with a volume capacity of 300.6 mAh cm⁻³, at a 0.1C rate. At a rate of 5C, the LiFePO₄/carbon composite shows a high discharge capacity of 98.3 mAh g⁻¹ and a volume capacity of 176.94 mAh cm⁻³.     

One-step hydrothermal synthesis of 3D porous microspherical LiFePO4/graphene aerogel composite for lithium-ion batteries

Three-dimensional (3D) porous LiFePO₄/graphene aerogel (LFP/GA) composite was successfully prepared by an in-situ hydrothermal process. In this composite, the LiFePO₄ microspheres assembled by nanoparticles were embedded in a three-dimensional framework intertwined with the graphene sheets, which acts as a bridge for transfer of electron and diffusion of lithium ion. The large specific surface of the composite structure enables the increased infiltration area and utilization of the active material. The content of the graphene sheet is analyzed and is found important for the Li-storage characteristics of LiFePO₄. An aerogel composite with 10% of graphene displays the best electrochemical performance, with the specific discharge capacities of 168 mAh g⁻¹ and 155 mAh g⁻¹ at respectively 0.1C and 1C, and the capacity retains 96.3% for up to 800 cycles. This novel 3D porous aerogel composite is identified as a promising cathode material for the rechargeable Li battery, and the simple strategy may be applied to construct other high performing composite structure and materials.     

Investigation on a core–shell nano-structural LiFePO4/C and its interfacial CO interaction

Core–shell LiFePO₄/C nanocomposite has been prepared by a sol–gel method. The mean size of the spherical core LiFePO₄ is about 30 nm, and thickness of carbon shell is about 3 nm. The bonding character on the interface of core–shell was revealed by soft X-ray absorption spectroscopy (XAS). The as-prepared sample was characterized by X-ray diffraction (XRD), Raman spectrum, Transmission electron microscope (TEM) and High-resolution transmission electron microscopy (HRTEM). Charge-discharge tests show the core–shell LiFePO₄/C demonstrates high rate capability (106 mAh g⁻¹ at 20 C) and good cycling performance (negligible capacity loss after 250 cycles).     

Preparation and characterization of nano-particle LiFePO4 and LiFePO4/C by spray-drying and post-annealing method

Pure, nano-sized LiFePO₄ and LiFePO₄/C cathode materials are synthesized by spray-drying and post-annealing method. The influence of the sintering temperature and carbon coating on the structure, particle size, morphology and electrochemical performance of LiFePO₄ cathode material is investigated. The optimum processing conditions are found to be thermal treatment for 10 h at 600 °C. Compared with LiFePO₄, LiFePO₄/C particles are smaller in size due to the inhibition of crystal growth to a great extent by the presence of carbon in the reaction mixture. And that the LiFePO₄/C composite coated with 3.81 wt.% carbon exhibits the best electrode properties with discharge capacities of 139.4, 137.2, 133.5 and 127.3 mAh g⁻¹ at C/5, 1C, 5C and 10C rates, respectively. In addition, it shows excellent cycle stability at different current densities. Even after 50 cycles at the high current density of 10C, a discharge capacity of 117.7 mAh g⁻¹ is obtained (92.4% of its initial value) with only a low capacity fading of 0.15% per cycle.     

Small capacity decay of lithium iron phosphate (LiFePO4) synthesized continuously in supercritical water: Comparison with solid-state method

Nanosize lithium iron phosphate (LiFePO₄) particles are synthesized using a continuous supercritical hydrothermal synthesis method at 25 MPa and 400 °C under various flow rates. The properties of LiFePO₄ synthesized in supercritical water including purity, crystallinity, atomic composition, particle size, surface area and thermal stability are compared with those of particles synthesized using a conventional solid-state method. Smaller size particles ranging 200-800 nm, higher BET surface area ranging 6.3-15.9 m² g⁻¹ and higher crystallinity are produced in supercritical water compared to those of the solid-state synthesized particles (3-15 μm; 2.4 m² g⁻¹). LiFePO₄ synthesized in supercritical water exhibit higher discharge capacity of 70-80 mAh g⁻¹ at 0.1 C after 30 cycles than that of the solid-state synthesized LiFePO₄ (60 mAh g⁻¹), which is attributed to the smaller size particles and the higher crystallinity. Smaller capacity decay at from 135 to 125 mAh g⁻¹ is observed during the 30 cycles in carbon-coated LiFePO₄ synthesized using supercritical water while rapid capacity decay from 158 to 140 mAh g⁻¹ is observed in the carbon-coated LiFePO₄ synthesized using the solid-state method.     

Synthesis of FePO4·2H2O nanoplates and their usage for fabricating superior high-rate performance LiFePO4

Monoclinic phase FePO₄·2H₂O nanoplates are synthesized very easily in a waterbath and are lithiated to LiFePO₄/C nanoparticles by a simple rheological phase method. The thickness of the nanoplates can be tuned simply by changing the concentrations of the reactants. The LiFePO₄/C nanoparticles lithiated from the thin FePO₄·2H₂O nanoplates, with the sizes about 50 nm and the carbon coating layer at the surface 1–2 nm, show excellent high-rate performance and long-term cyclability as the cathode for lithium ion batteries, delivering discharge capacities of more than 150, 120, 110, 100, and 75 mAh g⁻¹ at rates of 5 C, 10 C, 15 C, 20 C and 30 C, respectively.     

In situ construction of carbon nano-interconnects between the LiFePO4 grains using ultra low-cost asphalt

LiFePO₄/C composite cathode materials with carbon nano-interconnect structures were synthesized by one-step solid state reaction using low-cost asphalt as both carbon source and reducing agent. Based on the thermogravimetry, differential scanning calorimetry, transmission electron microscopy and high-resolution transmission electron microscopy, a growth model was proposed to illustrate the formation of the carbon nano-interconnect between the LiFePO₄ grains. The LiFePO₄/C composite shows enhanced discharge capacity (150 mAh g⁻¹) with excellent capacity retention compared with the bare LiFePO₄ (41 mAh g⁻¹) due to the electronically conductive nanoscale networking provided by the asphalt-based carbon. The results prove that the asphalt is a perfect carbon source and reduction agent for cost-effective production of high performance LiFePO₄/C composite.     

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.     

Effects of synthetic route on structure and electrochemical performance of Li3V2(PO4)3/C cathode materials

Three different synthetic routes, including solid-state reaction, sol–gel and hydrothermal methods are successfully used for preparation of Li₃V₂(PO₄)₃/C. Ascorbic acid is used as a reducing agent and/or as a chelating agent. The Li₃V₂(PO₄)₃/C synthesized by hydrothermal method with fine particles exhibits lower impedance and smaller potential difference values between oxidation and reduction peaks than those by solid-state reaction and sol–gel methods. Thus as cathode material for Li-ion batteries, the Li₃V₂(PO₄)₃/C synthesized by hydrothermal method shows higher discharge capacity, better rate capability and cyclic performance. Even at a high charge–discharge rate of 10 C, it still can deliver a discharge capacity of 101.4 mAh g⁻¹ and 106.6 mAh g⁻¹ in the potential range of 3.0–4.3 V and 3.0–4.8 V, respectively. The hydrothermal synthesis has been considered to be a competitive process to prepare Li₃V₂(PO₄)₃/C cathode materials with excellent electrochemical performances.     

Electrochemical performance of Li[Co0.1Ni0.15Li0.2Mn0.55]O2 modified by carbons as cathode materials

To enhance specific capacity, cycle performance and rate-capability of lithium-ion battery cathode materials, the Li[Co_0.1Ni_0.15Li_0.2Mn_0.55]O₂ (LCMNO) is modified by coating them with amorphous carbons and by preparing nanocomposites with nanostructured carbons (carbon nanotube and graphene). The carbon-treated LCMNO powders and their cathodes are characterized by morphological observation, crystalline property analysis, galvanostatic charge–discharge, and electrochemical impedance spectroscopy. The LCMNO nanocomposite shows a superior discharge capacity of ca. 290 mAh g⁻¹ at low C-rates, due to a greater number of active sites embedded by nanostructured carbon species. In contrast, the carbon-coated LCMNO shows higher discharge capacity in high rate regions due to the carbon-coated layer in the carbon-coated LCMNO, suppressing the side reactions and enhancing the electrical conductivity.