Preparation and characterization of Ti-doped LiFePO4 cathode materials for lithium-ion batteries

Olivine structured LiFePO₄ (lithium iron phosphate) and Ti⁴⁺-doped LiFe_1−xTi_xPO₄ (0.01 ≤ x ≤ 0.09) powders were synthesized via a solution route followed by heat-treatment at 700 °C for 8 h under N₂ flowing atmosphere. The compositions, crystalline structure, morphology, carbon content, and specific surface area of the prepared powders were investigated with ICP-OES, XRD, TEM, SEM, EA, and BET. Capacity retention study was used to investigate the effects of Ti⁴⁺ partial substitution on the intercalation/de-intercalation of Li⁺ ions in the olivine structured cathode materials. Among the prepared powders, LiFe_0.97Ti_0.03PO₄ manifests the most promising cycling performance as it was cycled with C/10, C/5, C/2, 1C, 2C, and 3C rate. It showed initial discharge capacity of 135 mAh g⁻¹ at 30 °C with C/10 rate. From the results of GSAS refinement for the prepared samples, the doped-Ti⁴⁺ ions did not occupy the Fe²⁺ sites as expected. However, the occupancy of the doped Ti⁴⁺ ions are still not clear, and theoretical calculations are needed for further studies. From the variation of lattice parameters calculated by the least square method without refinement, it suggested that Ti⁴⁺-doped LiFePO₄ samples formed solid solutions at low doping levels while TiO₂ was also observed with TEM in samples prepared with doping level higher than 5 mol%.     

High-power LiFePO4 cathode materials with a continuous nano carbon network for lithium-ion batteries

To meet the requirements of high-power products (ex. electric scooters, hybrid electric vehicles, pure electric vehicles and robots), high-energy safe lithium-ion batteries need to be developed in the future. This research will focus on the microstructures and electrochemical properties of olivine-type LiFePO₄ cathode materials. The morphologies of LiFePO₄/C composite materials show spherical-type particles and have good carbon conductive networks. From the TEM bright field image and EELS mapping, the LiFePO₄/C powder shows continuous, dispersive nano-carbon network. These structures will improve electron transfer and lithium-ion diffusion for LiFePO₄ cathode materials, and increase their conductivity from 10⁻⁹ S cm⁻¹ to 10⁻³ S cm⁻¹. The electrochemical properties of LiFePO₄/C cathode material in this work demonstrated high rate capability (≥12 C) and long cycle life (≥700 cycles at a 3 C discharge rate).     

Self-assembled mesoporous LiFePO4 with hierarchical spindle-like architectures for high-performance lithium-ion batteries

Self-assembled mesoporous LiFePO₄ (LFP) with hierarchical spindle-like architectures has been successfully synthesized via the hydrothermal method. Time dependent X-ray diffraction, scanning electron microscopy, and cross section high resolution transmission electron microscopy are used to investigate the detailed growth mechanism of these unique architectures. Reaction time and pH value play multifold roles in controlling the microstructures of LFP. The LFP particles are uniform mesoporous spindles, which are comprised of numerous single-crystal LFP nanocrystals. As the cathode material for lithium batteries, LFP exhibits high initial discharge capacity (163 mAh g⁻¹, 0.1 C), excellent high-rate discharge capability (111 mAh g⁻¹, 5 C), and cycling stability. These enhanced electrochemical properties can be attributed to this unique microstructure, which will remain structural stability for long-term cycling. Furthermore, nanosizing of LFP nanocrystals can increase the electrochemical reaction surface, enhance the electronic conductivity, and promote lithium ion diffusion.     

Effects of Fe2P and Li3PO4 additives on the cycling performance of LiFePO4/C composite cathode materials

In this study, a solution method was employed to synthesize LiFePO₄-based powders with Li₃PO₄ and Fe₂P additives. The composition, crystalline structure, and morphology of the synthesized powders were investigated by using ICP-OES, XRD, TEM, and SEM, respectively. The electrochemical properties of the powders were investigated with cyclic voltammetric and capacity retention studies. The capacity retention studies were carried out with LiFePO₄/Li cells and LiFePO₄/MCMB cells comprised LiFePO₄-based materials prepared at various temperatures from a stoichiometric precursor. Among all of the synthesized powders, the samples synthesized at 750 and 775 °C demonstrate the most promising cycling performance with C/10, C/5, C/2, and 1C rates. The sample synthesized at 775 °C shows initial discharge capacity of 155 mAh g⁻¹ at 30 °C with C/10 rate. From the results of the cycling performance of LiFePO₄/MCMB cells, it is found that 800 °C sample exhibited higher polarization growth rate than 700 °C sample, though it shows lower capacity fading rate than 700 °C sample. For Fe₂P containing samples, the diffusion coefficient of Li⁺ ion increases with increasing amount of Fe₂P, however, the sample synthesized at 900 °C shows much lower Li⁺ ion diffusion coefficient due to the hindrance of Fe₂P layer on the surface of LiFePO₄ particles.     

PVDF-HFP/PET/PVDF-HFP composite membrane for lithium-ion power batteries

In this study, a novel polymer electrolyte composite membrane is successfully fabricated using electrospinning and solution casting. The composite membrane comprises two microporous poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) layers plus an intermediate quaternary ammonium-containing SiO₂ nanoparticles modified polyethylene terephthalate (PET) nanofibrous nonwoven to form a sandwiched PVDF-HFP/PET/PVDF-HFP composite, which is employed as a separator for lithium-ion batteries (LIBs). The properties of the PET composite membrane are compared with those of commercial PE separator, such as the morphologies, physical properties, and electrochemical performances. According to our results, the composite membrane demonstrates superior thermal stability (thermal shrinkage ～8%), electrolyte-philicity (contact angle ～2.9°), electrolyte uptake and retention (282%, 74%), and ionic conductivity (～10^?3 S cm^?1). The separators are assembled into Li/LiFePO₄ cells for electrochemical tests, showing that the PET composite membrane cells exhibit higher capacities than those with the PE separator at 0.2–10C both at 25 °C and 55 °C. The discharge capacity retention and coulombic efficiency of the PET composite membrane cells at 1C/1C for 200 cycles can be respectively enhanced about 20% and 2% at 55 °C as compared to the PE separator cells. These results demonstrate that our prepared PET composite membrane is highly promising for LIB applications.     

V2O3 modified LiFePO4/C composite with improved electrochemical performance

In addition to lattice doping and carbon-coating, surface modification with other metal oxides can also improve the electrochemical performance of LiFePO₄ powders. In this work, highly conductive vanadium oxide (V₂O₃) is in situ produced during the synthesis of carbon-coated LiFePO₄ (LiFePO₄/C) powders by a solid state reaction process and acts as a surface modifier. The structures and compositions of LiFePO₄/C samples containing 0-10 mol% vanadium are analyzed by X-ray diffraction, Raman spectroscopy, scanning electron microscopy and transmission electron microscopy. Their electrochemical properties are also characterized with galvanostatic cell cycling and cyclic voltammetry. It is found that vanadium is present in the form of V₂O₃ that is incorporated in the carbon phase. The vanadium-modified LiFePO₄/C samples show improved rate capability and low-temperature performance. Their apparent lithium diffusion coefficient is in the range of 10⁻¹² to 10⁻¹⁰ cm² s⁻¹ depending on the vanadium content. Among the investigated samples, the one with 5 mol% vanadium exhibits the best electrochemical performance.     

Solvothermal synthesis of hierarchical LiFePO4 microflowers as cathode materials for lithium ion batteries

Hierarchical LiFePO₄ microflowers have been successfully synthesized via a solvothermal reaction in ethanol solvent with the self-prepared ammonium iron phosphate rectangular nanoplates as a precursor, which is obtained by a simple water evaporation method beforehand. The hierarchical LiFePO₄ microflowers are self-assemblies of a number of stacked rectangular nanoplates with length of 6-8 μm, width of 1-2 μm and thickness of around 50 nm. When ethanol is replaced with the water-ethanol mixed solvent in the solvothermal reaction, LiFePO₄ micro-octahedrons instead of hierarchical microflowers can be prepared. Then both of them are respectively modified with carbon coating through a post-heat treatment and their morphologies are retained. As a cathode material for rechargeable lithium ion batteries, the carbon-coated hierarchical LiFePO₄ microflowers deliver high initial discharge capacity (162 mAh g⁻¹ at 0.1 C), excellent high-rate discharge capability (101 mAh g⁻¹ at 10 C), and cycling stability, which exhibits better electrochemical performances than carbon-coated LiFePO₄ micro-octahedrons. These enhanced electrochemical properties can be attributed to the hierarchical micro/nanostructures, which can take advantage of structure stability of micromaterials for long-term cycling. Furthermore the rectangular nanoplates as the building blocks can improve the electrochemical reaction kinetics and finally promote the rate performance.     

A novel sol-gel method based on FePO4·2H2O to synthesize submicrometer structured LiFePO4/C cathode material

Carbon coated LiFePO₄/C cathode material is synthesized with a novel sol-gel method, using cheap FePO₄·2H₂O as both iron and phosphorus sources and oxalic acid (H₂C₂O₄·2H₂O) as both complexant and reductant. In H₂C₂O₄ solution, FePO₄·2H₂O is very simple to form transparent sols without controlling the pH value. Pure submicrometer structured LiFePO₄ crystal is obtained with a particle size ranging from 100 to 500 nm, which is also uniformly coated with a carbon layer, about 2.6 nm in thickness. The as-synthesized LiFePO₄/C sample exhibits high initial discharge capacity 160.5 mAh g⁻¹ at 0.1 C rate, with a capacity retention of 98.7% after 50th cycle. The material also shows good high-rate discharge performances, about 106 mAh g⁻¹ at 10 C rate. The improved electrochemical properties of as-synthesized LiFePO₄/C are ascribed to its submicrometer scale particles and low electrochemical impedance. The sol-gel method may be of great interest in the practical application of LiFePO₄/C cathode material.     

Li3V2(PO4)3/C composite as an intercalation-type anode material for lithium-ion batteries

The carbon coated monoclinic Li₃V₂(PO₄)₃ (LVP/C) powder is successfully synthesized by a carbothermal reduction method using crystal sugar as the carbon source. Its structure and physicochemical properties are investigated using X-ray diffraction (XRD), scanning electron microscopy, high-resolution transmission electron microscopy and electrochemical methods. The LVP/C electrode exhibits stable reversible capacities of 203 and 102 mAh g⁻¹ in the potential ranges of 3.0-0.0 V and 3.0-1.0 V versus Li⁺/Li, respectively. It is identified that the insertion/extraction of Li⁺ undergoes a series of two-phase transition processes between 3.0 and 1.6 V and a single phase process between 1.6 and 0.0 V. The ex situ XRD patterns of the electrodes at various lithiated states indicate that the monoclinic structure can still be retained during charge-discharge process and the insertion/deinsertion of lithium ions occur reversibly, which provides an excellent cycling stability with high energy efficiency.     

LiPF6 and lithium oxalyldifluoroborate blend salts electrolyte for LiFePO4/artificial graphite lithium-ion cells

The electrochemical behaviors of LiPF₆ and lithium oxalyldifluoroborate (LiODFB) blend salts in ethylene carbonate + propylene carbonate + dimethyl carbonate (EC + PC + DMC, 1:1:3, v/v/v) for LiFePO₄/artificial graphite (AG) lithium-ion cells have been investigated in this work. It is demonstrated by conductivity test that LiPF₆ and LiODFB blend salts electrolytes have superior conductivity to pure LiODFB-based electrolyte. The results show that the performances of LiFePO₄/Li half cells with LiPF₆ and LiODFB blend salts electrolytes are inferior to pure LiPF₆-based electrolyte, the capacity and cycling efficiency of Li/AG half cells are distinctly improved by blend salts electrolytes, and the optimum LiODFB/LiPF₆ molar ratio is around 4:1. A reduction peak is observed around 1.5 V in LiODFB containing electrolyte systems by means of CV tests for Li/AG cells. Excellent capacity and cycling performance are obtained on LiFePO₄/AG 063048-type cells tests with blend salts electrolytes. A plateau near 1.7-2.0 V is shown in electrolytes containing LiODFB salt, and extends with increasing LiODFB concentration in charge curve of LiFePO₄/AG cells. At 1C discharge current rate, the initial discharge capacity of 063048-type cell with the optimum electrolyte is 376.0 mAh, and the capacity retention is 90.8% after 100 cycles at 25 °C. When at 65 °C, the capacity and capacity retention after 100 cycles are 351.3 mAh and 88.7%, respectively. The performances of LiFePO₄/AG cells are remarkably improved by blending LiODFB and LiPF₆ salts compared to those of pure LiPF₆-based electrolyte system, especially at elevated temperature to 65 °C.     

MW-assisted synthesis of LiFePO4 for high power applications

LiFePO₄/C was prepared by solid-state reaction from Li₃PO₄, Fe₃(PO₄)₂·8H₂O, carbon and glucose in a few minutes in a scientific MW (microwave) oven with temperature and power control. The material was characterized by X-ray diffraction, scanning electron microscopy and by TGA analysis to evaluate carbon content. The electrochemical characterization as positive electrode in EC (ethylene carbonate)–DMC (dimethylcarbonate) 1 M LiPF₆ was performed by galvanostatic charge–discharge cycles at C/10 to evaluate specific capacity and by sequences of 10 s discharge–charge pulses, at different high C-rates (5–45C) to evaluate pulse-specific power in simulate operative conditions for full-HEV application. The maximum pulse-specific power and, particularly, pulse efficiency values are quite high and make MW synthesis a very promising route for mass production of LiFePO₄/C for full-HEV batteries at low energy costs.     

Li4Ti5O12/Sn composite anodes for lithium-ion batteries: Synthesis and electrochemical performance

Li₄Ti₅O₁₂/tin phase composites are successfully prepared by cellulose-assisted combustion synthesis of Li₄Ti₅O₁₂ matrix and precipitation of the tin phase. The effect of firing temperature on the particulate morphologies, particle size, specific surface area and electrochemical performance of Li₄Ti₅O₁₂/tin oxide composites is systematically investigated by SEM, XRD, TG, BET and charge-discharge characterizations. The grain growth of tin phase is suppressed by forming composite with Li₄Ti₅O₁₂ at a calcination of 500 °C, due to the steric effect of Li₄Ti₅O₁₂ and chemical interaction between Li₄Ti₅O₁₂ and tin oxide. The experimental results indicate that Li₄Ti₅O₁₂/tin phase composite fired at 500 °C has the best electrochemical performance. A capacity of 224 mAh g⁻¹ is maintained after 50 cycles at 100 mA g⁻¹ current density, which is still higher than 195 mAh g⁻¹ for the pure Li₄Ti₅O₁₂ after the same charge/discharge cycles. It suggests Li₄Ti₅O₁₂/tin phase composite may be a potential anode of lithium-ion batteries through optimizing the synthesis process.     

Synthesis, characterization and application of Li3Fe2(PO4)3 nanoparticles as cathode of lithium-ion rechargeable batteries

This work introduces a new method to synthesize Li₃Fe₂(PO₄)₃ nanoparticles in the nanopowder form and study its electrochemical performance by cyclic voltammetry and battery tests. Li₃Fe₂(PO₄)₃ is synthesized by the gel combustion method based on polyvinyl alcohol (PVA) as gel making agent. The optimum conditions of the synthesis include 8 wt% PVA, 0.34 wt% lithium slat, 1 wt% iron salt, 0.57 wt% ammonium dihydrogen phosphate, ethanol-water 50:50 as solvent, 675 °C combustion temperature and 4 h combustion time. Characterization of the samples is performed by the scanning electron microscopy (SEM), transmission electron microscopy (TEM), EDX analysis, XRD patterns, BET specific surface area and DSL size distribution. In the optimum conditions, a nanopowder is obtained that consisting of uniform nanoparticles with an average diameter of 70 nm. The optimized sample shows 12.5 m² g⁻¹ specific surface areas. Cyclic voltammetry (CV) studies show that the synthesized compound has good reversibility and high cyclic stability. The CV results are confirmed by the battery tests. The obtained results show that the synthesized cathodic material has high practical discharge capacity (average 125.5 mAh g⁻¹ approximately same with its theoretical capacity 128.2 mA h⁻¹) and long cycle life.     

Electrospinning synthesis of C/Fe3O4 composite nanofibers and their application for high performance lithium-ion batteries

Carbon-based nanofibers can be used as anode materials for lithium-ion batteries. Both pure carbon nanofiber and C/Fe₃O₄ composite nanofibers were prepared by electrospinning and subsequent carbonization processes. The composition and structures were characterized by Fourier transformation infrared spectroscopy, X-ray diffraction, scanning and transmission electron microscopy. The electrochemical properties were evaluated in coin-type cells versus metallic lithium. It is found that after an annealing temperature of 500–700 °C, the carbon has disordered structure while Fe₃O₄ is nanocrystalline with a particle size from 8.5 to 52 nm. Compared with the pure carbon nanofiber, the 600 °C-carbonized C/Fe₃O₄ composite nanofiber exhibits much better electrochemical performance with a high reversible capacity of 1007 mAh g⁻¹ at the 80th cycle and excellent rate capability. A beneficial powderization phenomenon is discovered during the electrochemical cycling. This study suggests that the optimized C/Fe₃O₄ composite nanofiber is a promising anode material for high performance lithium-ion batteries.     

Facile synthesis and electrochemical characterization of porous and dense TiO2 nanospheres for lithium-ion battery applications

We present a two-step method to optimize the nanoporous characteristics of TiO₂ samples thus enabling a higher charge and discharge capacity and a larger rate capability compared to dense TiO₂ materials. We use a simple sol-gel process to fabricate spherical titanium glycolates precursors followed by subsequent hydrothermal or annealing treatments resulting, respectively, in highly porous or dense TiO₂ nanospheres. These processes enable control of the grain size, pore structure, and specific surface area of the TiO₂. The fabricated TiO₂ nanostructures have been subsequently used to assemble lithium-ion batteries. Galvanostatic discharge-charge tests indicate that the porous TiO₂ nanospheres possess high and stable reversible capacity of 229, 133, and 56 mAh g⁻¹ at 0.06, 0.6 and 6 C, respectively; whereas the corresponding values for dense TiO₂ nanospheres are 217, 45, and ∼1 mAh g⁻¹. Such considerable improvement of the electrochemical activity is attributed to the porous TiO₂ nanostructures, and subsequent change in diffusion length, and enables the possibility to optimize the high rate capability in TiO₂-based lithium-ion batteries.     

Preparation and characterization of Na-doped LiFePO4/C composites as cathode materials for lithium-ion batteries

To improve the performance of LiFePO₄, single phase Li_1−xNa_xFePO₄/C (x = 0, 0.01, 0.03, 0.05) samples are synthesized by in situ polymerization restriction-carbonthermal reduction method. The effects of Na doping are studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results indicate that doped Na ion does not destroy the lattice structure of LiFePO₄, while enlarges the lattice volume. Electrochemical test results show that the Li_0.97Na_0.03FePO₄/C sample exhibits the best electrochemical performance with initial special discharge capacity of 158 mAh g⁻¹ at 0.1 C. EIS results demonstrate that the charge transfer resistance of the sample decreases greatly by doping an appropriate amount of Na.     

Improved electrochemical performances of 9LiFePO4·Li3V2(PO4)/C composite prepared by a simple solid-state method

9LiFePO₄·Li₃V₂(PO₄)₃/C is synthesized via a carbon thermal reaction using petroleum coke as both reduction agent and carbon source. The as-prepared material is not a simple mixture of LiFePO₄ (LFP) and Li₃V₂(PO₄)₃ (LVP), but a composite possessing two phases: one is V-doped LFP and the other is Fe-doped LVP. The typical structure enhances the electrical conductivity of the composite and improves the electrochemical performances. The first discharge capacity of 9LFP·LVP/C in 18650 type cells is 168 mAh g⁻¹ at 1 C (1 C_9LFP·LVP/C = 166 mA g⁻¹), and exhibits high reversible discharge capacity of 125 mAh g⁻¹ at 10 C even after 150 cycles. At the temperature of −20 °C, the reversible capacity of 9LFP·LVP/C can maintain 75% of that at room temperature.     

Synthesis and improved electrochemical performances of porous Li3V2(PO4)3/C spheres as cathode material for lithium-ion batteries

Spherical Li₃V₂(PO₄)₃/C composites are synthesized by a soft chemistry route using hydrazine hydrate as the spheroidizing medium. The electrochemical properties of the materials are investigated by galvanostatic charge-discharge tests, cyclic voltammograms and electrochemical impedance spectrum. The porous Li₃V₂(PO₄)₃/C spheres exhibit better electrochemical performances than the solid ones. The spherical porous Li₃V₂(PO₄)₃/C electrode shows a high discharge capacity of 129.1 and 125.6 mAh g⁻¹ between 3.0 and 4.3 V, and 183.8 and 160.9 mAh g⁻¹ between 3.0 and 4.8 V at 0.2 and 1 C, respectively. Even at a charge-discharge rate of 15 C, this material can still deliver a discharge capacity of 100.5 and 121.5 mAh g⁻¹ in the potential regions of 3.0-4.3 V and 3.0-4.8 V, respectively. The excellent electrochemical performance can be attributed to the porous structure, which can make the lithium ion diffusion and electron transfer more easily across the Li₃V₂(PO₄)₃/electrolyte interfaces, thus resulting in enhanced electrode reaction kinetics and improved electrochemical performance.     

A polyethylene glycol-assisted carbothermal reduction method to synthesize LiFePO4 using industrial raw materials

Olivine LiFePO₄ is synthesized by a carbothermal reduction method (CTR) using industrial raw materials with polyethylene glycol (PEG) as a reductive agent and carbon source. A required amount of acetone is added to the starting materials for the ball milling process and the precursor is sintered at 973 K for 8 h to form crystalline phase LiFePO₄. The structure and morphology of the LiFePO₄/C composite samples have been characterized by X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, energy-dispersive spectroscopy, differential scanning calorimetry and magnetic susceptibility. Electrochemical measurements show that the LiFePO₄/C composite cathode delivers an initial discharge capacity of 150 mAh g⁻¹ at a 0.2C-rate between 4.0 and 2.8 V, and almost no capacity loss is observed for up to 50 cycles. Remarkably, the cell can sustain a 30C-rate between 4.6 and 2.0 V, and this rate capability is equivalent to charge or discharge in 2 min. The simple technique, low-cost starting materials, and excellent electrochemical performance make this process easier to commercialize than other synthesized methods.