Synthesis of LiFePO4/C composite with high-rate performance by starch sol assisted rheological phase method

LiFePO₄/C composite was synthesized at 600 °C in an Ar atmosphere by a soluble starch sol assisted rheological phase method using home-made amorphous nano-FePO₄ as the iron source. XRD, SEM and TEM observations show that the LiFePO₄/C composite has good crystallinity, ultrafine sphere-like particles of 100-200 nm size and in situ carbon. The synthesized LiFePO₄ could inherit the morphology of FePO₄ precursor. The electrochemical performance of the LiFePO₄ by galvanostatic cycling studies demonstrates excellent high-rate cycle stability. The Li/LiFePO₄ cell displays a high initial discharge capacity of more than 157 mAh g⁻¹ at 0.2C and a little discharge capacity decreases from the first to the 80th cycle (>98.3%). Remarkably, even at a high current density of 30C, the cell still presents good cycle retention.     

Effects of carbon coating and iron phosphides on the electrochemical properties of LiFePO4/C

Carbon coated LiFePO₄ (LiFePO₄/C) with different contents of high electron conductive iron phosphide phase was synthesized by an aqueous sol–gel method in a reductive sintering atmosphere. Different synthesis parameters were used for adjusting the microstructure and phase compositions of the products. The effects of the carbon coating and iron phosphides on the electrochemical properties of the LiFePO₄/C electrodes were studied by means of testing the discharge capacities at rates of 0.1–5C (1C = 170 mAh g⁻¹) and analyzing the CV curves. The results show that carbon coating in a content of 1.5 wt.% derived from the carbon source of ethylene glycol greatly decreases the particle size of LiFePO₄ in one order in the specific surface area, and significantly improves the rate capability of LiFePO₄. The effect of the content of FeP on the capacity of the carbon coated LiFePO₄ was different at different discharge rates. Increasing the content of FeP from 1.2 to 3.7 wt.% slightly decreases the capacity of LiFePO₄/C at low discharge rate (0.1C and 1C), but obviously increases the capacity of LiFePO₄/C when the discharge rate is increased to 5C. For the carbon free sample, even it also has 1.8 wt.% FeP, it still possesses poor capacity due to the large particle size of LiFePO₄ and the lack of conductivity. And too much iron phosphides lowers the discharge capacity of the electrode since they are inert for the deinsertion/insertion of lithium ion.     

Morphology and electrical properties of carbon coated LiFePO4 cathode materials

Core-shell LiFePO₄@C composites were synthesized successfully from FePO₄/C precursor using the polyvinyl alcohol (PVA) as the reducing agent, followed by a chemical vapor deposition (CVD) assisted solid-state reaction in the presence of Li₂CO₃. Some physical and chemical properties of the products were characterized by X-ray powder diffraction (XRD), Raman, SEM, TEM techniques. The effect of morphology and electrochemical properties of the composites were thoroughly investigated. XRD patterns showed that LiFePO₄ has an order olivine structure with space group of Pnma. TEM micrographs exhibited that the LiFePO₄ particles encapsulated with 3-nm thick carbon shells. The powders were homogeneous with grain size of about 0.8 μm. Compared with those synthesized by traditional organic carbon source mixed method, LiFePO₄@C composite synthesized by CVD method exhibited better discharge capacity at initial 155.4 and 135.8 mAh g⁻¹ at 0.1C and 1C rate, respectively. It is revealed that the carbon layer coated on the surface of LiFePO₄ and the amorphous carbon wrapping and connecting the particles enhanced the electronic conductivity and rate performances of the cathode materials.     

Synthesises,characterizations and electrochemical properties of spherical-like LiFePO4 by hydrothermal method

Spherical-like LiFePO₄ was synthesized by hydrothermal synthesis method using Phenanthroline as a complexing-agent to avoid the Fe(II) ions from oxidation and control the growth of the crystal. Structural, electron valence state, morphology and particle size were investigated by X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), Mössbauer spectra, scanning electron microscopy (SEM) and laser particle sizer. Charge–discharge cycling performances were used to characterize its electrochemical properties. The sample possesses uniformly distributed spherical-like particles with an average size of 0.5–1 μm. Test shows that the reversible capacity of spherical-like LiFePO₄ is about 140 mAh g⁻¹ at 0.1 C. The capacity fading is neglectable.     

Electrochemical properties of LiFe0.9Mn0.1PO4/Fe2P cathode material by mechanical alloying

LiFePO₄, olivine-type LiFe_0.9Mn_0.1PO₄/Fe₂P composite was synthesized by mechanical alloying of carbon (acetylene back), M₂O₃ (M = Fe, Mn) and LiOH·H₂O for 2 h followed by a short-time firing at 900 °C for only 30 min. By varying the carbon excess different amounts of Fe₂P second phase was achieved. The short firing time prevented grain growth, improving the high-rate charge/discharge capacity. The electrochemical performance was tested at various C/x-rate. The discharge capacity at 1C rate was increased up to 120 mAh g⁻¹ for the LiFe_0.9Mn_0.1PO₄/Fe₂P composite, while that of the unsubstituted LiFePO₄/Fe₂P and LiFePO₄ showed only 110 and 60 mAh g⁻¹, respectively. Electronic conductivity and ionic diffusion constant were measured. The LiFe_0.9Mn_0.1PO₄/Fe₂P composite showed higher conductivity and the highest diffusion coefficient (3.90 × 10⁻¹⁴ cm² s⁻¹). Thus the improvement of the electrochemical performance can be attributed to (1) higher electronic conductivity by the formation of conductive Fe₂P together with (2) an increase of Li⁺ ion mobility obtained by the substitution of Mn²⁺ for Fe²⁺.     

Novel synthesis of LiFePO4-Li3V2(PO4)3 composite cathode material by aqueous precipitation and lithiation

LiFePO₄-Li₃V₂(PO₄)₃ composite cathode material is synthesized by aqueous precipitation of FeVO₄·xH₂O from Fe(NO₃)₃ and NH₄VO₃, following chemical reduction and lithiation with oxalic acid as the reducer and carbon source. Samples are characterized by XRD, SEM and TEM. XRD pattern of the compound synthesized at 700 °C indicates olivine-type LiFePO₄ and monoclinic Li₃V₂(PO₄)₃ are co-existed. TEM image exhibits that LiFePO₄-Li₃V₂(PO₄)₃ particles are encapsulated with a carbon shell 5-10 nm in thickness. The LiFePO₄-Li₃V₂(PO₄)₃ compound cathode shows good electrochemical performance, and its discharge capacity is about 139.1 at 0.1 C, 135.5 at 1 C and 116 mA h g⁻¹ at 3 C after 30 cycles.     

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.     

Study of LiFePO4 cathode materials coated with high surface area carbon

LiFePO₄ is a potential cathode material for 4 V lithium-ion batteries. Carbon-coated lithium iron phosphates were prepared using a high surface area carbon to react precursors through a solid-state process, during which LiFePO₄ particles were embedded in amorphous carbon. The carbonaceous materials were synthesized by the pyrolysis of peanut shells under argon, where they were carbonized in a two-step process that occurred between 573 and 873 K. The shells were also treated with a proprietary porogenic agent with the goal of altering the pore structure and surface area of the pyrolysis products. The electrochemical properties of the as-prepared LiFePO₄/C composite cathode materials were systematically characterized by X-ray diffraction, scanning electron microscope, element mapping, energy dispersive spectroscopy, Raman spectroscopy, and total organic carbon (TOC) analysis. In LiFePO₄/C composites, the carbon not only increases rate capability, but also stabilizes capacity. In fact, the capacity of the composites increased with the specific surface area of carbon. The best result was observed with a composite made of 8.0 wt.% with a specific surface area of 2099 m² g⁻¹. When high surface area carbon was used as a carbon source to produce LiFePO₄, overall conductivity increased from 10⁻⁸ to 10⁻⁴ S cm⁻¹, because the inhibition of particle growth during the final sintering process led to greater specific capacity, improved cycling properties and better rate capability compared to a pure olivine LiFePO₄ material.     

Effect of synthesis temperature on the properties of LiFePO4/C composites prepared by carbothermal reduction

LiFePO₄/C composite cathode materials were synthesized by carbothermal reduction method using inexpensive FePO₄ as raw materials and glucose as conductive additive and reducing agent. The precursor of LiFePO₄/C was characterized by differential thermal analysis and thermogravimetry. The microstructure and morphology of the samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM) and particle size analysis. Cyclic voltammetry (CV) and charge/discharge cycling performance were used to characterize their electrochemical properties. The results showed that the LiFePO₄/C composite synthesized at 650 °C for 9 h exhibited the most homogeneous particle size distribution. Residual carbon during processing was coated on LiFePO₄, resulting in the enhancement of the material's electronic properties. Electrochemical measurements showed that the discharge capacity first increased and then decreased with the increase of synthesis temperature. The optimal sample synthesized at 650 °C for 9 h exhibited a highest initial discharge capacity of 151.2 mA h g⁻¹ at 0.2 C rate and 144.1 mA h g⁻¹ at 1 C rate with satisfactory capacity retention rate.     

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.     

Physical and electrochemical properties of LiFePO4/C composite cathode prepared from various polymer-containing precursors

The goal of this research was to study the effect of various polymer-containing precursors on the performance of LiFePO₄/C composite. A coprecipitation method was applied to prepare a series of LiFePO₄/C materials by calcinating amorphous LiFePO₄ with various polymer compounds at 600 °C. The materials were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, particle size analysis, thermal analysis, BET specific surface area, Raman spectral analysis and electrochemical methods. The results showed that the structure of polymer precursors played an important role in improving the performance of LiFePO₄/C composites. The residual carbon produced by the pyrolysis of polymers with functionalized aromatic groups exhibited a better capacity in the LiFePO₄/C composites. A polyaromatic compound, e.g. polystyrene, with more functionalized aromatic groups displayed improved performance because its decomposition temperature was close to the temperature of the LiFePO₄ phase transformation, which resulted in fine particle size and uniform carbon distribution on the composite surface. According to Raman spectral analysis, polystyrene with more aromatic groups has a lower I_D/I_G and sp³/sp² peak ratio indicating more highly graphite-like carbon formation during polymer pyrolysis and exhibited a better capacity.     

Synthesis and electrochemical properties of olivine LiFePO4 prepared by a carbothermal reduction method

LiFePO₄/C composite cathode material was prepared by carbothermal reduction method, which uses NH₄H₂PO₄, Li₂CO₃ and cheap Fe₂O₃ as starting materials, acetylene black and glucose as carbon sources. The precursor of LiFePO₄/C was characterized by differential thermal analysis and thermogravimetry. X-ray diffraction (XRD), scanning electron microscopy (SEM) micrographs showed that the LiFePO₄/C is olivine-type phase, and the addition of the carbon reduced the LiFePO₄ grain size. The carbon is dispersed between the grains, ensuring a good electronic contact. The products sintered at 700 °C for 8 h with glucose as carbon source possessed excellent electrochemical performance. The synthesized LiFePO₄ composites showed a high electrochemical capacity of 159.3 mAh g⁻¹ at 0.1 C rate, and the capacity fading is only 2.2% after 30 cycles.     

Morphological characterization of LiFePO4/C composite cathode materials synthesized via a carboxylic acid route

A new type of LiFePO₄/C composite surrounded by a web containing both amorphous and crystalline carbon phases was synthesized by incorporating malonic acid as a carbon source using a high temperature solid-state method. SEM, TEM/SAED/EDS and HRTEM were used to analyze surface morphology and confirmed for the first time that crystalline carbon was present in LiFePO₄/C composites. The composite was effective in enhancing the electrochemical properties such as capacity and rate capability, because its active component consists of nanometer-sized particles containing pores with a wide range of sizes. An EDS elemental map showed that carbon was uniformly distributed on the surface of the composite crystalline particles. TEM/EDS results clearly show a dark region that is LiFePO₄ with a trace of carbon and a gray region that is carbon only. To evaluate the materials’ electrochemical properties, galvanostatic cycling and conductivity measurements were performed. The best cell performance was delivered by the material coated with 60 wt.% malonic acid, which delivered first cycle discharge capacity of 149 mAh g⁻¹ at a C/5 rate and sustained 222 cycles at 80% of capacity retention. When carboxylic acid was used as a carbon source to produce LiFePO₄, overall conductivity increased from 10⁻⁵ to 10⁻⁴ S cm⁻¹, since particle growth was prevented during the final sintering process.     

LiFePO4 with enhanced performance synthesized by a novel synthetic route

Pure LiFePO₄ was synthesized by heating an amorphous LiFePO₄. The amorphous LiFePO₄ obtained through lithiation of FePO₄·xH₂O by using oxalic acid as a novel reducing agent at room temperature. FePO₄·xH₂O was prepared through co-precipitation by employing FeSO₄·7H₂O and H₃PO₄ as raw materials. X-ray diffraction (XRD), scanning electron microscopy (SEM) observations showed that LiFePO₄ composites with fine particle sizes between 100 nm and 200 nm, and with homogenous sizes distribution. The electrochemical performance of LiFePO₄ powder synthesized at 500 °C were evaluated using coin cells by galvanostatic charge/discharge. The synthesized LiFePO₄ composites showed a high electrochemical capacity of 166 mAh g⁻¹ at the 0.1C rate, and possessed a favorable capacity cycling maintenance at the 0.1C, 0.2C, 0.5C and 1C rate.     

Effects of TiO2 coating on high-temperature cycle performance of LiFePO4-based lithium-ion batteries

LiFePO₄ particles were coated with TiO₂ (molar ratio = 3%) via a sol–gel process, and the effects of the coating on cycle performance of LiFePO₄ cathode at 55 °C against either a Li or a C (mesocarbon microbead) anode were investigated. It was found that, while the coating reduces capacity fading of the LiFePO₄/Li cell, it imposes a deteriorating effect on the LiFePO₄/C cell. Analyses on cell impedance and electrode surface morphology and composition showed that the oxide coating reduced Fe dissolution from the LiFePO₄ cathode and hence alleviated the impedance increase associated with the erosion process. This leads to reduced capacity fading as observed for the LiFePO₄/Li cell. However, the oxide coating itself was eroded upon cycling, and the dissolved Ti ions were subsequently reduced at the anode surface. Ti deposit on the C anode was found to be more active than Fe in catalyzing the formation of the solid-electrolyte interphase (SEI) layer, causing accelerated capacity decay for the LiFePO₄/C cell. The results point out the importance of evaluating the effect of cathode coating material on the anode side, which has generally been overlooked in the past studies.     

Structural and electrochemical properties of Cl-doped LiFePO4/C

Cl-doped LiFePO₄/C cathode materials were synthesized through a carbothermal reduction route, and the microstructure and electrochemical performances were systematically studied. Cl-doped LiFePO₄/C cathode materials presented a high discharge capacity of ∼90 mAh g⁻¹ at the rate of 20 C (3400 mA g⁻¹) at room temperature. Electrochemical impedance spectroscopy and cyclic voltamperometry indicated the optimized electrochemical reaction and Li⁺ diffusion in the bulk of LiFePO₄ due to Cl-doping. The improved Li⁺ diffusion capability is attributed to the microstructure modification of LiFePO₄ via Cl-doping.     

Effect of mechanical activation process parameters on the properties of LiFePO4 cathode material

Pure, nano-sized LiFePO₄ and carbon-coated LiFePO₄ (LiFePO₄/C) positive electrode (cathode) materials are synthesized by a mechanical activation process that consists of high-energy ball milling and firing steps. The influence of the processing parameters such as firing temperature, firing time and ball-milling time on the structure, particle size, morphology and electrochemical performance of the active material is investigated. An increase in firing temperature causes a pronounced growth in particle size, especially above 600 °C. A firing time longer than 10 h at 600 °C results in particle agglomeration; whereas, a ball milling time longer than 15 h does not further reduce the particle size. The electrochemical properties also vary considerably depending on these parameters and the highest initial discharge capacity is obtained with a LiFePO₄/C sample prepared by ball milling for 15 h and firing for 10 h at 600 °C. Comparison of the cyclic voltammograms of LiFePO₄ and LiFePO₄/C shows enhanced reaction kinetics and reversibility for the carbon-coated sample. Good cycle performance is exhibited by LiFePO₄/C in lithium batteries cycled at room temperature. At the high current density of 2C, an initial discharge capacity of 125 mAh g⁻¹ (73.5% of theoretical capacity) is obtained with a low capacity fading of 0.18% per cycle over 55 cycles.     

In situ high-resolution transmission electron microscopy synthesis observation of nanostructured carbon coated LiFePO4

In situ high-resolution transmission electron microscopy (HRTEM) studies of the structural transformations that occur during the synthesis of carbon-coated LiFePO₄ (C-LiFePO₄) and heat treatment to elevated temperatures were conducted in two different electron microscopes. Both microscopes have sample holders that are capable of heating up to 1500 °C, with one working under high vacuum and the other capable of operating with the sample surrounded by a low gaseous environment. The C-LiFePO₄ samples were prepared using three different compositions of precursor materials with Fe(0), Fe(II) or Fe(III), a Li-containing salt and a polyethylene-block-poly(ethylene glycol)-50% ethylene oxide or lactose. The in situ TEM studies suggest that low-cost Fe(0) and a low-cost carbon-containing compound such as lactose are very attractive precursors for mass production of C-LiFePO₄, and that 700 °C is the optimum synthesis temperature. At temperatures higher than 800 °C, LiFePO₄ has a tendency to decompose. The same in situ measurements have been made on particles without carbon coat. The results show that the homogeneous deposit of the carbon deposit at 700 °C is the result of the annealing that cures the disorder of the surface layer of bare LiFePO₄. Electrochemical tests supported the conclusion that the C-LiFePO₄ derived from Fe(0) is the most attractive for mass production.     

Synthesis and electrochemical characterization of carbon-coated nanocrystalline LiFePO4 prepared by polyacrylates-pyrolysis route

A carbon-coated nanocrystalline LiFePO₄ cathode material was synthesized by pyrolysis of polyacrylate precursor containing Li⁺, Fe³⁺ and PO₄⁻. The powder X-ray diffraction (XRD) and high-resolution TEM micrographs revealed that the LiFePO₄/C composite as prepared has a core-shell structure with pure olivine LiFePO₄ crystallites as cores and intimate carbon coating as a shell layer. Between the composite particulates, there exists a carbon matrix binding the nanocrystallites together into micrometer particles. The electrochemical measurements demonstrated that the LiFePO₄/C composite with an appropriate carbon content can deliver a very high discharge capacity of 157 mAh g⁻¹ (>92% of the theoretical capacity of LiFePO₄) with 95% of its initial capacity after 30 cycles. Since this preparation method uses less costly materials and operates in mild synthetic conditions, it may provide a feasible way for industrial production of the LiFePO₄/C cathode materials for the lithium-ion batteries.     

Cathode performance of olivine-type LiFePO4 synthesized by chemical lithiation

Chemical lithiation with LiI in acetonitrile was performed for amorphous FePO₄ synthesized from an equimolar aqueous suspension of iron powder and an aqueous solution of P₂O₅. An orthorhombic LiFePO₄ olivine structure was obtained by annealing a chemically lithiated sample at 550 °C for 5 h in Ar atmosphere. The average particle size remained at approximately 250 nm even after annealing. The lithium content in the sample was quantitatively confirmed by Li atomic absorption analysis and ⁵⁷Fe Mössbauer spectroscopy. While an amorphous FePO₄/carbon composite cathode has a monotonously decreasing charge–discharge profile with a reversible capacity of more than 140 mAh g⁻¹, the crystallized LiFePO₄/carbon composite shows a 3.4 V plateau corresponding to a two-phase reaction. This means that the lithium in the chemically lithiated sample is electrochemically active. Both amorphous FePO₄ and the chemically lithiated and annealed crystalline LiFePO₄ cathode materials showed good cyclability (more than 140 mAh g⁻¹ at the 40th cycle) and good discharge rate capability (more than 100 mAh g⁻¹ at 5.0 mA cm⁻²). In addition, the fast-charge performance was found to be comparable to that with LiCoO₂.