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.     

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.     

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.     

Fabrication and electrochemical characteristics of electrospun LiFePO4/carbon composite fibers for lithium-ion batteries

LiFePO₄/C composite fibers were synthesized by using a combination of electrospinning and sol-gel techniques. Polyacrylonitrile (PAN) was used as an electrospinning media and a carbon source. LiFePO₄ precursor materials and PAN were dissolved in N,N-dimethylformamide separately and they were mixed before electrospinning. LiFePO₄ precursor/PAN fibers were heat treated, during which LiFePO₄ precursor transformed to energy-storage LiFePO₄ material and PAN was converted to carbon. The surface morphology and microstructure of the obtained LiFePO₄/C composite fibers were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and elemental dispersive spectroscopy (EDS). XRD measurements were also carried out in order to determine the structure of LiFePO₄/C composite fibers. Electrochemical performance of LiFePO₄/carbon composite fibers was evaluated in coin-type cells. Carbon content and heat treatment conditions (such as stabilization temperature, calcination/carbonization temperature, calcination/carbonization time, etc.) were optimized in terms of electrochemical performance.     

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.     

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.     

More on the reactivity of olivine LiFePO4 nano-particles with atmosphere at moderate temperature

The changes appearing for LiFePO₄-C nano-composites exposed to atmosphere at 120 °C have been structurally and chemically examined by the use of TGA, XRD, XPS, Mössbauer, ⁷Li MAS NMR and electrochemical methods. The results conclude that a highly disordered phase resulting from the aging of LiFePO₄ appears on the surface of the grains of the material, is assigned to a phosphate phase and can insert lithium around 2.6 V with poor reversibility. The essential role of water has been investigated and clearly demonstrated. Thus, the aging mechanism occurring in hot humid air is completely different from a simple oxidation as well as from the aging process observed above 150 °C and involves the incorporation of hydroxyl groups. In addition, Fe₂O₃ formation has not been observed for such an aging in mild conditions.     

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.     

High temperature electrochemical performance of nanosized LiFePO4

The electrochemical performance of LiFePO₄ was tested at temperatures up to 150 °C for micrometric and nanometric size samples. Among the latter, both highly defective samples obtained by direct precipitation and annealed samples were tested. The comparison of voltage composition profiles for these samples coupled to GITT experiments allowed to conclude that defects seem to be the major factor in inducing the solid solution behaviour at room temperature. Good capacity retention is exhibited upon prolonged cycling at 100 °C in EC LiBOB electrolyte, also for nanosized samples that still maintain 75% of the initial capacity after 170 cycles. These results prove that the enhanced thermal stability of such electrolytes can be extended to temperatures much higher than those usually tested.     

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.     

Physical and electrochemical properties of LiFePO4 nanoparticles synthesized by a combination of spray pyrolysis with wet ball-milling

A novel preparation technique was developed to synthesize LiFePO₄ nanoparticles through a combination of spray pyrolysis (SP) with wet ball-milling (WBM). Using this technique, the preparation of LiFePO₄ nanoparticles was investigated for a wide range of process parameters such as ball-milling time and sintering temperature. The effect of process parameters on the physical and electrochemical properties of LiFePO₄ was then discussed through analysis using by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), the Brunauer–Emmet–Teller (BET) method, Raman spectroscopy and using an electrochemical cell of Li|1 M LiClO₄ in EC:DEC = 1:1|LiFePO₄. LiFePO₄ nanoparticles with a geometric mean diameter of 58 nm were prepared at a rotating speed of 800 rpm and a ball-milling time of 12 h in an Ar atmosphere followed by heat treatment at 500 °C for 4 h in a N₂ + 3% H₂ atmosphere. The sample delivered first discharge capacities of 164 and 100 mAh g⁻¹ at charge–discharge rates of 0.1 and 10 C in the test cells, respectively. The electrochemical properties of LiFePO₄ nanoparticles were strongly affected by the formation of Fe₂P, Fe₃P and α-Fe₂O₃ at higher charge–discharge rates.     

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.     

Thermal characteristics of a FeF3 cathode via conversion reaction in comparison with LiFePO4

The thermal stability of a FeF₃ cathode via a conversion reaction was quantitatively studied using differential scanning calorimetry (DSC). Mixtures of charged and discharged FeF₃ electrodes and electrolyte were measured by changing the ratio of electrode to electrolyte. A mild exothermic peak was observed at temperatures ranging from 210 to 380 °C for the mixtures of charged electrode and electrolyte even if the electrode/electrolyte ratio was changed. Moreover, the cycling depth had no effect on the thermal stability of the charged electrode in the electrolyte. For the mixtures of discharged electrode and electrolyte, exothermic reactions occurred in the range of 250-350 °C, which varied with the electrode/electrolyte ratio. Although the exothermic reactions of the mixtures varied with the electrode/electrolyte ratio, the thermal risk for both charged and discharged electrodes coexisted with the electrolyte appeared to be mainly due to electrolyte decomposition. By comparing the heat values of mixtures of the charged and discharged electrodes and electrolyte, the FeF₃ electrodes in the electrolyte demonstrated better thermal stability than LiFePO₄ electrodes at elevated temperatures.     

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.     

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.     

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.     

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.     

New freeze-drying method for LiFePO4 synthesis

The freeze-drying method is proposed as an effective synthesis process for the obtaining of LiFePO₄/C composites. The citric acid is used as a complexing agent and carbon source. After the low temperature annealing, the freeze-dried solution leads to a homogeneous carbon covered LiFePO₄ sample. The chemical characterization of the material included ICP and elemental analysis, infrared spectroscopy, X-ray diffraction, magnetic measurements and thermal analysis. SEM and TEM microscopies indicate an aggregate morphology with tiny particles of lithium iron phosphate inside a carbon matrix. Impedance spectroscopy showed a 8.0 × 10⁻⁷ S cm⁻¹ conductivity value. Cyclic voltammetry graphics displayed the two peaks corresponding to the Fe(II)/Fe(III) reaction and demonstrated the good reversibility of the material. The specific capacity value obtained at C/40 rate was 164 mAh g⁻¹, with a slight decrease on greater C-rates reaching 146 mAh g⁻¹ at C/1. The capacity retention study has evidenced good properties, with retention over 97% of the maximum values in the first 50 cycles, which allows an effective performance of the freeze-dried sample as cathodic material in lithium-ion batteries.     

PPy doped PEG conducting polymer films synthesized on LiFePO4 particles

In this work, polyethyleneglycole (PEG) is introduced into polypyrrole (PPy) film coated on LiFePO₄ powder particles to promote the properties of cathode material for lithium-ion batteries. The enhancement of the electrochemical activity by the substitution of a carbon with electrochemically active polymer is investigated. Films of the PPy doped with the PEG were prepared by the chemical oxidative polymerization of pyrrole (Py) monomer. PEG has been added as an additive during polymerization process to improve mechanical and structural properties of the PPy in final PPy/PEG-LiFePO₄ cathode material. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge/discharge measurements were employed to characterize the electrochemical properties of PPy/PEG-LiFePO₄ material. The electrochemical performance of PPy-LiFePO₄ electrodes was greatly improved by introduction of PEG into the PPy films. Charge/discharge measurements confirmed the increase in capacity when applying PEG in PPy. The morphology and particle sizes of the prepared cathode powder material were investigated by scanning electron microscopy (SEM) and particle size analysis (PSA). Distribution of PPy and PPy/PEG films onto the LiFePO₄ particles surface was studied by time of flight secondary ion mass spectrometry (TOF-SIMS). In addition to polymeric coating layer on the surface of PPy-LiFePO₄ composite particles, some PPy unequally distributed between the particles was found. The median diameter value is 4.92 μm for PPy-LiFePO₄ sample. TOF-SIMS measurements and SEM images confirmed that thickness of polypyrrole coating on LiFePO₄ particles is about 100 nm.