A study on the electrochemical characteristics of LiFePO4 cathode for lithium polymer batteries by hydrothermal method

Phospho-olivine LiFePO₄ cathode materials were prepared by hydrothermal reaction at 150 °C. Carbon black was added to enhance the electrical conductivity of LiFePO₄. LiFePO₄-C powders (0, 3, 5 and 10 wt.%) were characterized by X-ray diffraction (XRD) and transmission electron microscope (TEM). LiFePO₄-C/solid polymer electrolyte (SPE)/Li cells were characterized electrochemically by charge/discharge experiments at a constant current density of 0.1 mA cm⁻² in a range between 2.5 and 4.3 V vs. Li/Li⁺, cyclic voltammetry (CV) and ac impedance spectroscopy. The results showed that initial discharge capacity of LiFePO₄ was 104 mAh g⁻¹. The discharge capacity of LiFePO₄-C/SPE/Li cell with 5 wt.% carbon black was 128 mAh g⁻¹ at the first cycle and 127 mAh g⁻¹ after 30 cycles, respectively. It was demonstrated that cycling performance of LiFePO₄-C/SPE/Li cells was better than that of LiFePO₄/SPE/Li cells.     

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.     

Cycling degradation of an automotive LiFePO4 lithium-ion battery

Degradation of a high capacity prismatic LiFePO₄ cell with deep cycling at elevated temperature of 50 °C is studied by electrochemical impedance spectroscopy as well as capacity and power fading characterization at different test temperatures (45, 25, 0 and −10 °C). Capacity fade after 600 cycles is 14.3% at 45 °C and 25.8% at −10 °C. There is little power fade at 45 °C after 600 cycles, whereas the power fade after 600 cycles is 61.6% and 77.2%, respectively, at 0 and −10 °C. The capacity and power fade evidently becomes more severe at lower temperature. In particular, the power fade at low temperatures (e.g., 0 and −10 °C) rather than capacity loss is a major limitation of the LiFePO₄ cell. The primary mechanism for capacity fade is loss of cyclable lithium in the cell resulting from lithium-consuming solid electrolyte interphase (SEI) layer growth and side reactions. The increased interfacial resistance (R_w) due to the catalytic growth of SEI layer on the graphite anode and increased electrolyte resistance are the main sources for power fade.     

Improved high-rate charge/discharge performances of LiFePO4/C via V-doping

V-doped LiFePO₄/C cathode materials were prepared through a carbothermal reduction route. The microstructure was characterized by X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy. The electrochemical Li⁺ intercalation performances of V-doped LiFePO₄/C were compared with those of undoped one through galvanostatic intermittent titration technique, cyclic voltamperometry, and electrochemical impedance spectrum. V-doped LiFePO₄/C showed a high discharge capacity of ∼70 mAh g⁻¹ at the rate of 20 C (3400 mA g⁻¹) at room temperature. The significantly improved high-rate charge/discharge capacity is attributed to the increase of Li⁺ ion “effective” diffusion capability.     

Optimized solid-state synthesis of LiFePO4 cathode materials using ball-milling

Olivine-type LiFePO₄ cathode materials were synthesized by a solid-state reaction method and ball-milling. The ball-milling time, heating time and heating temperature are optimized. A heating temperature higher than 700 °C resulted in the appearance of impurity phase Fe₂P and growth of large particle, which was shown by high resolution X-ray diffraction and field emission scanning electron microscopy. The impurity phase Fe₂P exhibited a considerable capacity loss at the 1st cycle and a gradual increase in discharge capacity upon cycling. Moreover, it exhibited an excellent high-rate capacity of 104 mAh g⁻¹ at 3 C in spite of the large particle size. The optimum synthesis conditions for LiFePO₄ were ball-milling for 24 h and heat-treatment at 600 °C for 3 h. LiFePO₄/Li cells showed an enhanced cycling performance and a high discharge capacity of 160 mAh g⁻¹ at 0.1 C.     

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.     

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.     

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.     

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.     

Electrochemical properties of LiFePO4 prepared via hydrothermal route

LiFePO₄ as a cathode material for rechargeable lithium batteries was prepared by hydrothermal process at 170 °C under inert atmosphere. The starting materials were LiOH, FeSO₄, and (NH₄)₂HPO₄. The particle size of the obtained LiFePO₄ was 0.5 μm. The electrochemical properties of LiFePO₄ were characterized in a mixed solvent of ethylene carbonate and diethyl carbonate (1:1 in volume) containing 1.0 mol dm⁻³ LiClO₄. The hydrothermally synthesized LiFePO₄ exhibited a discharge capacity of 130 mA h g⁻¹, which was smaller than theoretical capacity (170 mA h g⁻¹). The annealing of LiFePO₄ at 400 °C in argon atmosphere was effective in increasing the discharge capacity. The discharge capacity of the annealed LiFePO₄ was 150 mA h g⁻¹.     

Electrochemical properties of LiFePO4 prepared via ball-milling

LiFePO₄ cathode materials with distinct particle sizes were prepared by a planetary ball-milling method. The effects of particle size on the morphology, thermal stability and electrochemical performance of LiFePO₄ cathode materials were investigated. The ball-milling method decreased particle size, thereby reducing the length of diffusion and improving the reversibility of the lithium ion intercalation/deintercalation. It is worth noting that the small particle sample prepared using malonic acid as a carbon source achieved a high capacity of 161 mAh g⁻¹ at a 0.1 C rate and had a very flat capacity curve during the early 50 cycles. However, the big particle samples (∼400 nm) decayed more dramatically in capacity than the small particle size samples (∼200 nm) at high current densities. The improvement in electrode performance was mainly due to the fine particles, the small size distribution, and the increase in electronic conductivity as a result of carbon coating. The structure and morphology of the ground LiFePO₄ samples were characterized with XRD, FE-SEM, TEM, EDS, and DSC techniques.     

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.     

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.     

Lithium oxalyldifluoroborate/carbonate electrolytes for LiFePO4/artificial graphite lithium-ion cells

The electrolytes based on lithium oxalyldifluoroborate (LiODFB) and carbonates have been systematically investigated for LiFePO₄/artificial graphite (AG) cells, by ionic conductivity test and various electrochemical tests, such as cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and charge-discharge test. The conductivity of nine electrolytes as a function of solvent composition and LiODFB salt concentration has been studied. The coulombic efficiency of LiFePO₄/Li and AG/Li half cells with these electrolytes have also been compared. The results show that 1 M LiODFB EC/PC/DMC (1:1:3, v/v) electrolyte has a relatively higher conductivity (8.25 mS cm⁻¹) at 25 °C, with high coulombic efficiency, good kinetics characteristics and low interface resistance. With 1 M LiODFB EC/PC/DMC (1:1:3, v/v) electrolyte, LiFePO₄/AG cells exhibit excellent capacity retention ∼92% and ∼88% after 100 cycles at 25 °C and at elevated temperatures up to 65 °C, respectively; The LiFePO₄/AG cells also have good rate capability, the discharge capacity is 324.8 mAh at 4 C, which is about 89% of the discharge capacity at 0.5 C. However, at −10 °C, the capacity is relatively lower. Compared with 1 M LiPF₆ EC/PC/DMC (1:1:3, v/v), LiFePO₄/AG cells with 1 M LiODFB EC/PC/DMC (1:1:3, v/v) exhibited better capacity utilization at both room temperature and 65 °C. The capacity retention of the cells with LiODFB-based electrolyte was much higher than that of LiPF₆-based electrolyte at 65 °C, while the capacity retention and the rate capacity of the cells is closed to that of LiPF₆-based electrolyte at 25 °C. In summary, 1 M LiODFB EC/PC/DMC (1:1:3, v/v) is a promising electrolyte for LiFePO₄/AG cells.     

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.     

Synthesis of carbon-coated LiFePO4 nanoparticles with high rate performance in lithium secondary batteries

A novel preparation technique was developed for synthesizing carbon-coated LiFePO₄ nanoparticles through a combination of spray pyrolysis (SP) with wet ball milling (WBM) followed by heat treatment. Using this technique, the preparation of carbon-coated LiFePO₄ nanoparticles was investigated for a wide range of process parameters such as ball-milling time and ball-to-powder ratio. The effect of process parameters on the physical and electrochemical properties of the LiFePO₄/C composite was then discussed through the results of X-ray diffraction (XRD) analysis, field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), the Brunauer-Emmet-Teller (BET) method and the use of an electrochemical cell of Li|1 M LiClO₄ in EC:DEC = 1:1|LiFePO₄. The carbon-coated LiFePO₄ nanoparticles were prepared at 500 °C by SP and then milled at a rotating speed of 800 rpm, a ball-to-powder ratio of 40/0.5 and a ball-milling time of 3 h in an Ar atmosphere followed by heat treatment at 600 °C for 4 h in a N₂ + 3% H₂ atmosphere. SEM observation revealed that the particle size of LiFePO₄ was significantly affected by the process parameters. Furthermore, TEM observation revealed that the LiFePO₄ nanoparticles with a geometric mean diameter of 146 nm were coated with a thin carbon layer of several nanometers by the present method. Electrochemical measurement demonstrated that cells containing carbon-coated LiFePO₄ nanoparticles could deliver markedly improved battery performance in terms of discharge capacity, cycling stability and rate capability. The cells exhibited first discharge capacities of 165 mAh g⁻¹ at 0.1 C, 130 mAh g⁻¹ at 5 C, 105 mAh g⁻¹ at 20 C and 75 mAh g⁻¹ at 60 C with no capacity fading after 100 cycles.     

Enhancement of electrochemical performance of lithium dry polymer battery with LiFePO4/carbon composite cathode

LiFePO₄/carbon composite electrode was prepared and applied to the dry polymer electrolyte. Enhanced low-temperature performance of LiFePO₄ was achieved by modifying the interface between LiFePO₄ and polymer electrolyte. The molecular weight of the polymer and the salt concentration as the Li/O ratio were optimized at 3 × 10⁵ and 1/10, respectively. Impedance analysis revealed that a small resistive component occurred in the frequency range of the charge transfer process. The reversible capacity of the laminate cell was 140 mAh g⁻¹ (C/20) and 110 mAh g⁻¹ (C/2) at 40 °C, which is comparable to the performance in the liquid electrolyte system.     

LiFePO4 as an optimum power cell material

Nano-crystallized LiFePO₄ has been synthesized with a simple three-step-synthesis technology in the presence of nano-ferric oxide as iron source and polyacence (PAS) as a reductive agent and high conductive carbon source. The use of PAS increases the conductivity and prevents the particles growth. The most feasible calcined temperature and time was investigated and the best cell performance was delivered by the sample calcined at 700 °C for 4 h. This material shows excellent specific capacity and cycle efficiency at high current rates, almost no capacity loss can be observed up to 100 cycles which make it more superior as an optimum power cell cathode material.     

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.     

La0.4Ce0.6O1.8–La0.8Sr0.2MnO3–8 mol% yttria-stabilized zirconia composite cathode for anode-supported solid oxide fuel cells

The composite cathodes of La_0.4Ce_0.6O_1.8 (LDC)–La_0.8Sr_0.2MnO₃ (LSM)–8 mol% yttria-stabilized zirconia (YSZ) with different LDC contents were investigated for anode-supported solid oxide fuel cells with thin film YSZ electrolyte. The oxygen temperature-programmed desorption profiles of the LDC–LSM–YSZ composites indicate that the addition of LDC increases surface oxygen vacancies. The cell performance was improved largely after the addition of LDC, and the best cell performance was achieved on the cells with the composite cathodes containing 10 wt% or 15 wt% LDC. The electrode polarization resistance was reduced significantly after the addition of LDC. At 800 °C and 650 °C, the polarization resistances of the cell with a 10 wt% LDC composite cathode are 70% and 40% of those of the cell with a LSM–YSZ composite cathode, respectively. The impedance spectra show that the processes associated with the dissociative adsorption of oxygen and diffusion of oxygen intermediates and/or oxygen ions on LSM surface and transfer of oxygen species at triple phase boundaries are accelerated after the addition of LDC.