Electrical conductivity and reaction with lithium of LiFe1−yMnyPO4 olivine-type cathode materials

Structural, electrical and electrochemical properties of Mn-substituted phospho-olivines LiFe_1−yMn_yPO₄ were investigated and compared to those of LiFePO₄. Rietvield refined XRD patterns taken in the course of delithiation process showed apparent difference between phase compositions of these cathode materials upon lithium extraction. Contrary to the LiFePO₄ and LiMnPO₄ compositions for which a two-phase mechanism of electrochemical lithium extraction/insertion is observed, in case of Mn-substituted LiFe_1−yMn_yPO₄ samples a single-phase mechanism of deintercalation was observed in the studied range of lithium concentration. Electrochemical characterization of the cathode materials were performed in Li/Li⁺/Li_xFe_1−yMn_yPO₄-type cells for y = 0.0, 0.25, 0.55, 0.75 and 1.0 compositions. Voltammery studies showed low reversibility of the lithium extraction process in the high-voltage “manganese” range, while in the “iron” range the reversibility of lithium extraction is high. Impedance measurements of the LiFe_1−yMn_yPO₄ cathode materials, which enabled separation of the ionic and electronic components of their entire electrical conductivity, showed distinct influence of Mn content on the electronic part of conductivity. EIS measurements performed at different states of cell charge revealed that the charge-transfer impedance in Li_xFe_1−yMn_yPO₄ is much lower than that of Li_xFePO₄.     

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.     

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.     

Effects of roasting temperature and modification on properties of Li2FeSiO4/C cathode

Li₂FeSiO₄/C cathodes were synthesized by combination of wet-process method and solid-state reaction at high temperature, and effects of roasting temperature and modification on properties of the Li₂FeSiO₄/C cathode were investigated. The XRD patterns of the Li₂FeSiO₄/C samples indicate that all the samples are of good crystallinity, and a little Fe₃O₄ impurity was observed in them. The primary particle size rises as the roasting temperature increases from 600 to 750 °C. The Li₂FeSiO₄/C sample synthesized at 650 °C has good electrochemical performances with an initial discharge capacity of 144.9 mAh g⁻¹ and the discharge capacity remains 136.5 mAh g⁻¹ after 10 cycles. The performance of Li₂FeSiO₄/C cathode is further improved by modification of Ni substitution. The Li₂Fe_0.9Ni_0.1SiO₄/C composite cathode has an initial discharge capacity of 160.1 mAh g⁻¹, and the discharge capacity remains 153.9 mAh g⁻¹ after 10 cycles. The diffusion coefficient of lithium in Li₂FeSiO₄/C is 1.38 × 10⁻¹² cm² s⁻¹ while that in Li₂Fe_0.9Ni_0.1SiO₄/C reaches 3.34 × 10⁻¹² cm² s⁻¹.     

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.     

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.     

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.     

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-throughput studies of Li1−xMgx/2FePO4 and LiFe1−yMgyPO4 and the effect of carbon coating

A two-dimensional sample array synthesis has been used to screen carbon-coated Li_(1−x)Mg_x/2FePO₄ and LiFe_(1−y)Mg_yPO₄ powders as potential positive electrode materials in lithium ion batteries with respect to x, y and carbon content. The synthesis route, using sucrose as a carbon source as well as a viscosity-enhancing additive, allowed introduction of the Mg dopant from solution into the sol–gel pyrolysis precursor. High-throughput XRD and cyclic voltammetry confirmed the formation of the olivine phase and percolation of the electronic conduction path at sucrose to phosphate ratios between 0.15 and 0.20. Measurements of the charge passed per discharge cycle showed that the capacity deteriorated on increasing magnesium in Li_(1−x)Mg_x/2FePO₄, but improved with increasing magnesium in LiFe_(1−y) Mg_yPO₄, especially at high scan rates. Rietveld-refined XRD results on samples of LiFe_(1−y)Mg_yPO₄ prepared by a solid-state route showed a single phase up to y = 0.1 according to progressive increases in unit cell volume with increases in y. Carbon-free samples of the same materials showed conductivity increases from 10⁻¹⁰ to 10⁻⁸ S cm⁻¹ and a decrease of activation energy from 0.62 to 0.51 eV. Galvanostatic cycling showed near theoretical capacity for y = 0.1 compared with only 80% capacity for undoped material under the same conditions.     

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.     

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.     

Effect of firing temperature on the electrochemical performance of LiMn0.4Fe0.6PO4/C materials prepared by mechanical activation

Carbon-coated LiMn_0.4Fe_0.6PO₄ composites (LiMn_0.4Fe_0.6PO₄/C) were synthesized for use as cathode materials in lithium batteries. The composites were synthesized by a mechanical activation process that consists of high-energy ball milling for 10 h, followed by thermal treatment at different temperatures. The structure, particle size and surface morphology of these cathode active materials were investigated by inductively coupled plasma (ICP) analysis, energy dispersive spectrometry (EDS), high-resolution Raman spectroscopy, X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and high-resolution transmission electron microscopy (HR-TEM). The firing temperature was observed to affect morphology, particle size, elemental distribution, structure of the residual carbon, and consequently the electrochemical properties of the composites. LiMn_0.4Fe_0.6PO₄/C synthesized at 600 °C possessed the most desirable properties and it exhibited the best performance when used as cathode in lithium batteries at room temperature. The cell, comprising cathode of this composite, exhibited the initial discharge capacities of 144.5 mAh g⁻¹ (85.0% of theoretical capacity) and 122.0 mAh g⁻¹ (71.8%), respectively, at 0.1 and 1 C-rates. The cathode showed good cycle stability without substantial capacity fade up to 50 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.     

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.     

Surfactant based sol–gel approach to nanostructured LiFePO4 for high rate Li-ion batteries

Porous nanostructured LiFePO₄ powder with a narrow particle size distribution (100–300 nm) for high rate lithium-ion battery cathode application was obtained using an ethanol based sol–gel route employing lauric acid as a surfactant. The synthesized LiFePO₄ powders comprised of agglomerates of crystallites <65 nm in diameter exhibiting a specific surface area ranging from 8 m² g⁻¹ to 36 m² g⁻¹ depending on the absence or presence of the surfactant. The LiFePO₄ obtained using lauric acid resulted in a specific capacity of 123 mAh g⁻¹ and 157 mAh g⁻¹ at discharge rates of 10C and 1C with less than 0.08% fade per cycle, respectively. Structural and microstructural characterization were performed using X-ray diffraction (XRD), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) with energy dispersive X-ray (EDX) analysis while electronic conductivity and specific surface area were determined using four-point probe and N₂ adsorption techniques.     

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.     

LiFexMn1−xPO4: A cathode for lithium-ion batteries

The high redox potential of LiMnPO₄, ∼4.0 vs. (Li⁺/Li), and its high theoretical capacity of 170 mAh g⁻¹ makes it a promising candidate to replace LiCoO₂ as the cathode in Li-ion batteries. However, it has attracted little attention because of its severe kinetic problems during cycling. Introducing iron into crystalline LiMnPO₄ generates a solid solution of LiFe_xMn_1−xPO₄ and increases kinetics; hence, there is much interest in determining the Fe-to-Mn ratio that will optimize electrochemical performance. To this end, we synthesized a series of nanoporous LiFe_xMn_1−xPO₄ compounds (with x = 0, 0.05, 0.1, 0.15, and 0.2), using an inexpensive solid-state reaction. The electrodes were characterized using X-ray diffraction and energy-dispersive spectroscopy to examine their crystal structure and elemental distribution. Scanning-, tunneling-, and transmission-electron microscopy (viz., SEM, STEM, and TEM) were employed to characterize the micromorphology of these materials; the carbon content was analyzed by thermogravimetric analyses (TGAs). We demonstrate that the electrochemical performance of LiFe_xMn_1−xPO₄ rises continuously with increasing iron content. In situ synchrotron studies during cycling revealed a reversible structural change when lithium is inserted and extracted from the crystal structure. Further, introducing 20% iron (e.g., LiFe_0.2Mn_0.8PO₄) resulted in a promising capacity (138 mAh g⁻¹ at C/10), comparable to that previously reported for nano-LiMnPO₄.     

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.     

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.     

Synthesis and characterization of Carbon Nano Fiber/LiFePO4 composites for Li-ion batteries

Carbon Nano Fibers (CNFs) coated with LiFePO₄ particles have been prepared by a non-aqueous sol–gel technique. The functionalization of the CNFs by HNO₃ acid treatment has been confirmed by Raman and XPS analyses. The samples pure LiFePO₄ and LiFePO₄–CNF have been characterized by XRD, SEM, RAMAN, XPS and electrochemical analysis. The LiFePO₄–CNF sample shows better electrochemical performance compared to as-prepared LiFePO₄. LiFePO₄–CNF (10 wt.%) delivers a higher specific capacity (∼140 mAh g⁻¹) than LiFePO₄ with carbon black (25 wt.%) added after synthesis (∼120 mAh g⁻¹) at 0.1C.