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

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

A simple,cheap soft synthesis routine for LiFePO4 using iron(III) raw material

An order olivine structure LiFePO₄ was synthesized with a simple rheological phase reaction (RPR) of LiOH·H₂O and FePO₄·4H₂O in the presence of PEG as a reductive agent and carbon source. A required amount of water was added to the starting materials to form the rheological precursor and decomposed at 700 °C to form the crystalline phase LiFePO₄ directly, without ball-milling, preparation of intermediates, pre-sintering and post-deposition treatment. Fine particles with an average particle size about 216 nm are examined by scanning electron microscopy (SEM) and optical particle size analyzer. An initial discharge capacity of 157 mAh g⁻¹ was achieved for the as-prepared LiFePO₄ material with a rate of 0.1C (17 mA g⁻¹), what's more, this material shows excellent specific capacity, charge–discharge efficiency and cycle efficiency at high current rates, almost no capacity loss can be observed up to 40 cycles with the rate of 1, 2 and 3C at room temperature. The simple, cheap process as well as the excellent high-rate performance makes this RPR method feasible commercially.     

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.     

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.     

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.     

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.     

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⁺.     

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.     

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.     

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.     

Solid state synthesis of hydrous ruthenium oxide for supercapacitors

A novel solid state route has been successfully developed for the synthesis of nano-scale hydrous ruthenium oxide (denoted as RuO₂·xH₂O). The procedure involves directly mixing RuCl₂·xH₂O with alkali to form RuO₂·xH₂O in a mortar at room temperature. Transmission electron microscopy (TEM) and N₂ adsorption–desorption measurement indicate that the RuO₂·xH₂O particle is approximately 30–40 nm with mesoporous structure. The crystalline structure and the electrochemical properties of RuO₂·xH₂O have been systematically explored as a function of annealing temperature. At lower temperatures, the RuO₂·xH₂O powder was found in an amorphous phase and the maximum capacitance of 655 F g⁻¹ was obtained by annealing at 150 °C. Higher temperatures (exceeding 175 °C) presumably converted amorphous phase into crystalline one and the corresponding specific capacitance dropped rapidly from 547 F g⁻¹ at 175 °C to 87 F g⁻¹ at 400 °C. Also, the dependence of electrochemical performance on annealing conditions of RuO₂·xH₂O was investigated by electrical impedance spectroscopy (EIS) study.     

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.     

Understanding RuO2·xH2O/carbon nanofibre composites as supercapacitor electrodes

Composites made from RuO₂·xH₂O particles supported on carbon nanofibres (CNF) have been prepared for supercapacitor electrodes. CNF, produced by Grupo Antolin Ing. SA. using a floating catalyst procedure was treated either in HCl or in HNO₃. Then the composites were obtained by impregnation of CNF with an aqueous RuCl₃·0.5H₂O solution followed by filtering and alkali solution treatment. Heat treatment at 150 °C for 2 h was done. Specific capacitance of the composites has been measured and discussed on the basis of their RuO₂·xH₂O content and RuO₂·xH₂O particle size. The composites having RuO₂·xH₂O contents below 11 wt% show RuO₂·xH₂O particles, which grow from 2 to 4 nm as the RuO₂·xH₂O content increases. The specific capacitance of supported RuO₂·xH₂O, which can be very high (up to 840 F g⁻¹), decreases as the RuO₂·xH₂O content increases and RuO₂·xH₂O particles grow. The composites having RuO₂·xH₂O contents above 11 wt% show RuO₂·xH₂O particles of nearly constant size (4 nm); the effect of increasing the RuO₂·xH₂O content is to increase the amount of particles but not the size of the particles. In these composites the specific capacitance of supported RuO₂·xH₂O is nearly constant (440 F g⁻¹) and close to bare RuO₂·xH₂O (460 F g⁻¹).     

Electrochemical intercalation of lithium in the titanium hydrogeno phosphate Ti(HPO4)2·H2O

The electrochemical reactivity of the layered titanium hydrogeno phosphate Ti(HPO₄)₂·H₂O versus lithium has been studied. Lithium intercalation occurs at ∼2.5 V with low polarization, leading to a new lithiated Ti(III) phase, LiTi(HPO₄)₂·H₂O. Ti(HPO₄)₂·H₂O exhibits a reversible capacity of 80 mAh g⁻¹ in the voltage window 1.8–3.5 V at C/10 rate. The stable reversible capacity reveals that the presence of H₂O lattice is not affecting the electrochemical reaction. The reversibility of the reaction is demonstrated by extracting lithium from LiTi(HPO₄)₂·H₂O and the host structure is intact. The electrochemical behaviour of dehydrated phases Ti(HPO₄)₂ and TiP₂O₇ has also been investigated.     

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.     

Li1+xFePO4 (0 ≤ x ≤ 3) as anode material for lithium ion batteries: From ab initio studies

Li_1+xFePO₄ (0 ≤ x ≤ 3) as anode material for lithium ion batteries has been studied using ab initio calculations. Results show that large amount of lithium ions can be intercalated into LiFePO₄ host. The structure changes continuously when the first two Moles of lithium ions (x ≤ 2) are intercalated into the LiFePO₄ host, accompanied by large volume expansion (37.4% and 25.4% for the first and second Mole). The final product of Li₃FePO₄ possesses a stable chained structure, which is favorable for storing even more lithium. In the same time, lithium ion diffuses in a three-dimension pathway within the chained structure. The unit cell volume increases only by 4.9% from Li₃FePO₄ to Li₄FePO₄, and the chained structure keeps unchanged.     

Synthesis and characterization of submicron-sized LiNi1/3Co1/3Mn1/3O2 by a simple self-propagating solid-state metathesis method

Submicron-sized LiNi_1/3Co_1/3Mn_1/3O₂ cathode materials were synthesized using a simple self-propagating solid-state metathesis method with the help of ball milling and the following calcination. A mixture of Li(ac)·2H₂O, Ni(ac)₂·4H₂O, Co(ac)₂·4H₂O, Mn(ac)₂·4H₂O (ac = acetate) and excess H₂C₂O₄·2H₂O was used as starting material without any solvent. XRD analyses indicate that the LiNi_1/3Co_1/3Mn_1/3O₂ materials were formed with typical hexagonal structure. The FESEM images show that the primary particle size of the LiNi_1/3Co_1/3Mn_1/3O₂ materials gradually increases from about 100 nm at 700 °C to 200–500 nm at 950 °C with increasing calcination temperature. Among the synthesized materials, the LiNi_1/3Co_1/3Mn_1/3O₂ material calcined at 900 °C exhibits excellent electrochemical performance. The steady discharge capacities of the material cycled at 1 C (160 mA g⁻¹) rate are at about 140 mAh g⁻¹ after 100 cycles in the voltage range 3–4.5 V (versus Li⁺/Li) and the capacity retention is about 87% at the 350th cycle.     

Carbon nanofibre/hydrous RuO2 nanocomposite electrodes for supercapacitors

Amorphous RuO₂·xH₂O and a VGCF/RuO₂·xH₂O nanocomposite (VGCF = vapour-grown carbon fibre) are prepared by thermal decomposition. The morphology of the materials is investigated by means of scanning electron microscopy. The electrochemical characteristics of the materials, such as specific capacitance and rate capability, are investigated by cyclic voltammetry over a voltage range of 0–1.0 V at various scan rates and with an electrolyte solution of 1.0 M H₂SO₄. The specific capacitance of RuO₂·xH₂O and VGCF/RuO₂·xH₂O nanocomposite electrodes at a scan rate of 10 mV s⁻¹ is 410 and 1017 F g⁻¹, respectively, and at 1000 mV s⁻¹ are 258 and 824 F g⁻¹, respectively. Measurements of ac impedance spectra are made on both the electrodes at various bias potentials to obtain a more detailed understanding of their electrochemical behaviour. Long-term cycle-life tests for 10⁴ cycles shows that the RuO₂·xH₂O and VGCF/RuO₂·xH₂O electrodes retain 90 and 97% capacity, respectively. These encouraging results warrant further development of these electrode materials towards practical application.     

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.     

Preparation and characterization of sub-micro LiNi0.5−xMn1.5+xO4 for 5 V cathode materials synthesized by an ultrasonic-assisted co-precipitation method

Sub-micro spinel LiNi_0.5−xMn_1.5+xO₄ (x < 0.1) cathode materials powder was successfully synthesized by the ultrasonic-assisted co-precipitation (UACP) method. The structure and electrochemical performance of this as-prepared powder were characterized by powder XRD, SEM, XPS, CV and the galvanostatic charge–discharge test in detail. XRD shows that there is a small Li_yNi_1−yO impurity peak placed close to the (4 0 0) line of the spinel LiNi_0.5−xMn_1.5+xO₄, and the powders are well crystallized. XPS exhibits that the Mn oxidation state is between +3 and +4, and Ni oxidation state is +2 in LiNi_0.5−xMn_1.5+xO₄. SEM shows that the prepared powders (UACP) have the uniform and narrow size distribution which is less than 200 nm. Galvanostatic charge–discharge test indicates that the initial discharge capacities for the LiNi_0.5−xMn_1.5+xO₄ (UACP) at C/3, 1C and 2C, are 130.2, 119.0 and 110.0 mAh g⁻¹, respectively. After 100 cycles, their capacity retentions are 99.8%, 88.2%, and 73.5%, respectively. LiNi_0.5−xMn_1.5+xO₄ (UACP) at C/3 discharge rate exhibits superior capacity retention upon cycling, and it also shows well high current discharge performance. CV curve implies that LiNi_0.5−xMn_1.5+xO₄ (x < 0.1) spinel synthesized by ultrasonic-assisted co-precipitation method has both reversibility and cycle capability because of the ultrasonic-catalysis.