Process investigation, electrochemical characterization and optimization of LiFePO4/C composite from mechanical activation using sucrose as carbon source

LiFePO₄/C composite was synthesized by mechanical activation using sucrose as carbon source. High-energy ball milling facilitated phase formation during thermal treatment. TG-DSC and TPR experiments demonstrated sucrose was converted to CH_x intermediate before completely decomposed to carbon. Ball milling time, calcination temperature and dwelling time all had significant impact on the discharge capacity and rate performance of the resulted power. The optimal process parameters are high-energy ball milling for 2-4 h followed by thermal treatment at 700 °C for 20 h. The product showed a capacity of 174 mAh/g at 0.1C rate and around 117 mAh/g at 20C rate with the capacity fade less than 10% after 50 cycles. Too low calcination temperature or insufficient calcination time, however, could result in the residual of CH_x in the electrode and led to a decrease of electrode performance.     

Synthesis and characterization of LiFePO4 cathode material dispersed with nano-structured carbon

Carbon-dispersed LiFePO₄ materials were routinely prepared by heating metal-salt-containing pastes of organogels to temperatures at 300 and 700 °C to benefit the intrinsic conductivity, and we ultimately discerned the optimized carbon content, 4.55 wt%. Carbon doping will decrease tap density of prepared cathode material and then bring about electrode preparing difficulty, so we tried different kinds of organogels to make out the densest carbon composite. They were polyacrylamide (PAM), sugar and phenolic resin. The most excellent pyrolyzed PAM paste was found increasingly electrochemical reversible, exhibiting 113.2 mAh/g at C/6 and 95 mAh/g at C/3. And we found a good cycliability of 95 mAh/g at 0.2 mA cm⁻² at room temperature. Seen from atomic force microscopy, this composite was far more different from other pyrolyzed pastes in morphology, which contained judicious designed hiberarchy and highly dispersed nanoparticles. Decreased 2θ in XRD spectra also showed the varied cell parameters, though no exact figures of the varied cell parameters could be given due to a potential existence of an unknown second phase with electrochemical activity.     

Electrochemical properties of carbon-mixed LiFePO4 cathode material synthesized by the ceramic granulation method

LiFePO₄/carbon composite cathode material was prepared using polyvinyl alcohol (PVA) as carbon source by pelleting and subsequent pyrolysis in N₂. The samples were characterized by XRD, SEM and TGA. Their electrochemical performance was investigated in terms of charge–discharge cycling behavior. It consists of a single LiFePO₄ phase and amorphous carbon. The special micro-morphology via the process is favorable for electrochemical properties. The discharge capacity of the LiFePO₄/C composite was 145 mAh/g, closer to the theoretical specific capacity of 170 mAh/g at 0.1 C low current density. At 3 C modest current density, the specific capacity was about 80 mAh/g, which can satisfy for transportation applications if having a more planar discharge flat.     

Influence of carbon sources on LiFePO4/C composites synthesized by the high-temperature high-energy ball milling method

Carbon-coated LiFePO₄ composites were synthesized by a new method of high-temperature high-energy ball milling (HTHEBM). Fe₂O₃ and LiH₂PO₄ were used as raw materials. Glucose, sucrose, citric acid and active carbon were used as reducing agents and carbon sources, respectively. In this method, high-energy ball milling and carbon coating worked together and, therefore, fine and homogeneous LiFePO₄/C particles with excellent properties were obtained in a relatively short synthesis time of 9 h. Moreover, the synthesis process could be completely finished at a relatively lower temperature of 600 °C for high-energy ball milling transforming mechanical energy into thermal energy. The results of X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electrochemical performance tests indicated that carbon source had an important influence on the properties of LiFePO₄/C composites synthesized by the HTHEBM method. It was proved that the LiFePO₄ composites coated with glucose had the best properties with 1 μm geometric mean diameter and 150.3 mA h g⁻¹ initial discharge capacity at a current rate of 0.1 C. After the 20th cycle test, the reversible capacity was 148 mA h g⁻¹ at 0.1 C, showing a retention ratio to the initial capacity of 98.5%.     

Synthesis of LiFePO4/carbon composite from nano-FePO4 by a novel stearic acid assisted rheological phase method

LiFePO₄/carbon composite was synthesized at 600 °C for 4 h in an Ar atmosphere by a stearic acid assisted rheological phase method using amorphous nano-FePO₄ as the iron source. XRD, SEM and TEM observations show that the LiFePO₄/carbon composite has good crystallinity, ultrafine and well-dispersed particles of 60–150 nm size and in situ carbon coated on the surface of LiFePO₄ crystallites. The synthesized LiFePO₄/carbon composite shows a high discharge capacity of 160 mAh g⁻¹ and 155 mAh g⁻¹ at rates of 0.5 C and 1 C, respectively. Even at a high current density of 30 C, the material still presents a discharge capacity of 93 mAh g⁻¹ and exhibits an excellent cycling performance.     

Design and synthesis of high-rate micron-sized, spherical LiFePO4/C composites containing clusters of nano/microspheres

A micron-sized LiFePO₄/C composite with a spherical morphology was reduced carbothermally from precursor particles prepared by ball milling-assisted spray-drying. The specific capacity of the electrode at a 10 C (1700 mA/g) rate was 110 mAh/g and a high voltage plateau was achieved. The high-rate performance of the composite electrode was due to its unique spherical structure, comprising clusters of nano- or sub-micron-sized spherical particles. This morphology increases the effective conductive surface area, reduces the charge-transfer reaction resistance and improves the diffusion of lithium ions.     

Effect of CeO2-coating on the electrochemical performances of LiFePO4/C cathode material

The effect of CeO₂ coating on LiFePO₄/C cathode material has been investigated. The crystalline structure and morphology of the synthesized powders have been characterized by XRD, SEM, TEM and their electrochemical performances both at room temperature and low temperature are evaluated by CV, EIS and galvanostatic charge/discharge tests. It is found that, nano-CeO₂ particles distribute on the surface of LiFePO₄ without destroying the crystal structure of the bulk material. The CeO₂-coated LiFePO₄/C cathode material shows improved lithium insertion/extraction capacity and electrode kinetics, especially at high rates and low temperature. At −20 °C, the CeO₂-coated material delivers discharge capacity of 99.7 mAh/g at 0.1C rate and the capacity retention of 98.6% is obtained after 30 cycles at various charge/discharge rates. The results indicate that the surface treatment should be an effective way to improve the comprehensive properties of the cathode materials for lithium ion batteries.     

In situ construction of carbon nano-interconnects between the LiFePO4 grains using ultra low-cost asphalt

LiFePO₄/C composite cathode materials with carbon nano-interconnect structures were synthesized by one-step solid state reaction using low-cost asphalt as both carbon source and reducing agent. Based on the thermogravimetry, differential scanning calorimetry, transmission electron microscopy and high-resolution transmission electron microscopy, a growth model was proposed to illustrate the formation of the carbon nano-interconnect between the LiFePO₄ grains. The LiFePO₄/C composite shows enhanced discharge capacity (150 mAh g⁻¹) with excellent capacity retention compared with the bare LiFePO₄ (41 mAh g⁻¹) due to the electronically conductive nanoscale networking provided by the asphalt-based carbon. The results prove that the asphalt is a perfect carbon source and reduction agent for cost-effective production of high performance LiFePO₄/C composite.     

Preparation and characterization of nano-particle LiFePO4 and LiFePO4/C by spray-drying and post-annealing method

Pure, nano-sized LiFePO₄ and LiFePO₄/C cathode materials are synthesized by spray-drying and post-annealing method. The influence of the sintering temperature and carbon coating on the structure, particle size, morphology and electrochemical performance of LiFePO₄ cathode material is investigated. The optimum processing conditions are found to be thermal treatment for 10 h at 600 °C. Compared with LiFePO₄, LiFePO₄/C particles are smaller in size due to the inhibition of crystal growth to a great extent by the presence of carbon in the reaction mixture. And that the LiFePO₄/C composite coated with 3.81 wt.% carbon exhibits the best electrode properties with discharge capacities of 139.4, 137.2, 133.5 and 127.3 mAh g⁻¹ at C/5, 1C, 5C and 10C rates, respectively. In addition, it shows excellent cycle stability at different current densities. Even after 50 cycles at the high current density of 10C, a discharge capacity of 117.7 mAh g⁻¹ is obtained (92.4% of its initial value) with only a low capacity fading of 0.15% per cycle.     

Synthesis of spindle-shaped nanoporous C–LiFePO4 composite and its application as a cathodes material in lithium ion batteries

In this work, LiFePO₄/C composites were prepared in hydrothermal system by using iron gluconate as iron source, and two feeding sequences during the preparation were comparatively studied. The morphology, crystal structure and charge–discharge performance of the prepared samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and galvanostatic charge–discharge testing. The results showed that the feeding sequences and iron gluconate seriously affected the microstructures and electrochemical properties of the resulting LiFePO₄ cathodes in lithium ion batteries. The spindle-shaped LiFePO₄ with hierarchical microporous structure self-assembled by nanoparticles has been successfully synthesized by synthesis route B. In addition, the cell performance of the synthesized LiFePO₄ by synthesis route B was better than that of LiFePO₄ by synthesis route A. Specially at high rates, the superior rate performance of the spindle-shaped LiFePO₄/C microstructure (LFP/C-B) was revealed. And special reversible capacities of ∼118 and ∼95 mAh g⁻¹ were obtained at rates of 2 C and 5 C, comparing to ∼96 and ∼68 mAh g⁻¹ for LFP/C-A.     

Properties of Li-graphite and LiFePO4 electrodes in LiPF6–sulfolane electrolyte

Sulfolane (also referred to as tetramethylene sulfone, TMS) containing LiPF₆ and vinylene carbonate (VC) was tested as a non-flammable electrolyte for a graphite |LiFePO₄ lithium-ion battery. Charging/discharging capacity of the LiFePO₄ electrode was ca. 150 mAh g⁻¹ (VC content 5 wt%). The capacity of the graphite electrode after 10 cycles establishes at the level of ca. 350 mAh g⁻¹ (C/10 rate). In the case of the full graphite |1 M LiPF₆ + TMS + VC 10 wt% |LiFePO₄ cell, both charging and discharging capacity (referred to cathode mass) stabilized at a value of ca. 120 mAh g⁻¹. Exchange current density for Li⁺ reduction on metallic lithium, estimated from electrochemical impedance spectroscopy (EIS) experiments, was j_o(Li/Li⁺) = 8.15 × 10⁻⁴ A cm⁻². Moreover, EIS suggests formation of the solid electrolyte interface (SEI) on lithium, lithiated graphite and LiFePO₄ electrodes, protecting them from further corrosion in contact with the liquid electrolyte. Scanning electron microscopy (SEM) images of pristine electrodes and those taken after electrochemical cycling showed changes which may be interpreted as a result of SEI formation. No graphite exfoliation was observed. The main decomposition peak of the LiPF₆ + TMS + VC electrolyte (TG/DTA experiment) was present at ca. 275 °C. The LiFePO₄(solid) + 1 M LiPF₆ + TMS + 10 wt% VC system shows a flash point of ca. 150 °C. This was much higher in comparison to that characteristic of a classical LiFePO₄ (solid) + 1 M LiPF₆ + 50 wt% EC + 50 wt% DMC system (T_f ≈ 37 °C).     

Enhanced electrochemical performance of nano-sized LiFePO4/C synthesized by an ultrasonic-assisted co-precipitation method

A simple and effective method, the ultrasonic-assisted co-precipitation method, was employed to synthesize nano-sized LiFePO₄/C. A glucose solution was used as the carbon source to produce in situ carbon to improve the conductivity of LiFePO₄. Ultrasonic irradiation was adopted to control the size and homogenize the LiFePO₄/C particles. The sample was characterized by X-ray powder diffraction, field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). FE-SEM and TEM show that the as-prepared sample has a reduced particle size with a uniform size distribution, which is around 50 nm. A uniform amorphous carbon layer with a thickness of about 4-6 nm on the particle surface was observed, as shown in the HRTEM image. The electrochemical performance was demonstrated by the charge-discharge test and electrochemical impedance spectra measurements. The results indicate that the nano-sized LiFePO₄/C presents enhanced discharge capacities (159, 147 and 135 mAh g⁻¹ at 0.1, 0.5 and 2 C-rate, respectively) and stable cycling performance. This study offers a simple method to design and synthesis nano-sized cathode materials for lithium-ion batteries.     

Long-term cyclability of LiFePO4/carbon composite cathode material for lithium-ion battery applications

A simple high-energy ball milling combined with spray-drying method has been developed to synthesize LiFePO₄/carbon composite. This material delivers an improved tap density of 1.3 g/cm³ and a high electronic conductivity of 10⁻² to 10⁻³ S/cm. The electrochemical performance, which is especially notable for its high-rate performance, is excellent. The discharge capacities are as high as 109 mAh/g at the current density of 1100 mA/g (about 6.5C rate) and 94 mAh/g at the current density of 1900 mA/g (about 11C rate). At the high current density of 1700 mA/g (10C rate), it exhibits a long-term cyclability, retaining over 92% of its original discharge capacity beyond 2400 cycles. Therefore, the as-prepared LiFePO₄/carbon composite cathode material is capable of such large-scale applications as hybrid and plug-in hybrid electric vehicles.     

Synthesis of FePO4·xH2O for fabricating submicrometer structured LiFePO4/C by a co-precipitation method

Amorphous hydrated iron (III) phosphate has been synthesized by a coordinate precipitation method from equimolecular Fe(NO₃)₃ and (NH₄)₂HPO₄ solutions at an elevated temperature. Hydrated iron (III) phosphate samples and the corresponding LiFePO₄/C products were characterized by XRD and SEM. The electrochemical behavior was studied by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The LiFePO₄/C fabricated from as-synthesized FePO₄ delivered discharge capacities of 162.5, 147.3, 133.0, 114.7, 97.2, 91.3 and 88.5 mAh g⁻¹ at rates of 0.1C, 0.2C, 0.5C, 1C, 2C, 3C and 4C with satisfactory capacity retention, respectively.     

Synthesis of FePO4·2H2O nanoplates and their usage for fabricating superior high-rate performance LiFePO4

Monoclinic phase FePO₄·2H₂O nanoplates are synthesized very easily in a waterbath and are lithiated to LiFePO₄/C nanoparticles by a simple rheological phase method. The thickness of the nanoplates can be tuned simply by changing the concentrations of the reactants. The LiFePO₄/C nanoparticles lithiated from the thin FePO₄·2H₂O nanoplates, with the sizes about 50 nm and the carbon coating layer at the surface 1–2 nm, show excellent high-rate performance and long-term cyclability as the cathode for lithium ion batteries, delivering discharge capacities of more than 150, 120, 110, 100, and 75 mAh g⁻¹ at rates of 5 C, 10 C, 15 C, 20 C and 30 C, respectively.     

Preparation of synthetic rutile and metal-doped LiFePO4 from ilmenite

The synthetic rutile and metal-doped LiFePO₄ are prepared from the high-titanium residue and iron-rich lixivium, which are obtained from the ilmenite by a mechanical activation and leaching process. ICP results show that the rutile contains 92.01% TiO₂, 1.59% Fe₂O₃, 0.034% MnO₂ and 0.60% (MgO + CaO), which meet the requirement of the titanium dioxide chlorination process. The results also reveal that small amounts of Al³⁺, Ca²⁺ and Ti⁴⁺ precipitate in the FePO₄·xH₂O precursor. XRD and Rietveld-refine results show that the metal-doped LiFePO₄ is single olivine-type phase and well crystallized, and Ti⁴⁺ occupy M1 site, Ca²⁺ occupy M2 site and Al³⁺ occupy both sites, which indicates the formation of cation-deficient solid solution. The sample exhibits a capacity of 123 mAh g⁻¹ at 5C rate, and retains 94.3% of the capacity after 100 cycles.     

Emulsion drying synthesis of olivine LiFePO4/C composite and its electrochemical properties as lithium intercalation material

The electroactive LiFePO₄/C nano-composite has been synthesized by an emulsion drying method. During burning-out the oily emulsion precipitates in an air-limited atmosphere at 300 °C, amorphous or low crystalline carbon was generated along with releasing carbon oxide gases, and trivalent iron as a cheap starting material was immediately reduced to the divalent one at this stage as confirmed by X-ray photoelectron spectroscopy, leading to a low crystalline LiFePO₄/C composite. Heat-treatment of the low crystalline LiFePO₄/C in an Ar atmosphere resulted in a well-ordered olivine structure, as refined by Rietveld refinement of the X-ray diffraction pattern. From secondary electron microscopic and scanning transmission electron microscopic observations with the corresponding elemental mapping images of iron and phosphorous, it was found that the LiFePO₄ powders are modified by fine carbon. The in situ formation of the nano-sized carbon during crystallization of LiFePO₄ brought about two advantages: (i) an optimized particle size of LiFePO₄, and (ii) a uniform distribution of fine carbon in the product. These effects of the fine carbon on LiFePO₄/C composite led to high capacity retention upon cycling at 25 and 50 °C and high rate capability, resulting from a great enhancement of electric conductivity as high as 10⁻⁴ S cm⁻¹. That is, the obtained capacity was higher than 90 mAh (g-phosphate)⁻¹ by applying a higher current density of about 1000 mA g⁻¹ (11 C) at 50 °C.     

Synthesis of LiFePO4/C by solid-liquid reaction milling method

LiFePO₄/C was synthesized by the method of solid-liquid reaction milling, using FeCl₃·6H₂O, Li₂CO₃ and (NH₄)₂HPO₄ and glucose, which was used as reductant (carbon source). The samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), TG-DTA analysis, infrared absorption carbon-sulfur analysis and electrochemical performance test. The sample synthesized at 680 °C for 8 h showed, at initial discharge, a capacity of 155.8, 153.2, 148.5, 132.7 mAh g^− 1 at 0.2 °C, 0.5 °C, 1 °C and 3 °C rate respectively. The sample also showed an excellent capacity retention as there was no significant capacity fade after 10 cycles.     

Rate performance and structural change of Cr-doped LiFePO4/C during cycling

Structural change of Cr-doped LiFePO₄/C during cycling is investigated using conventional and synchrotron-based in situ X-ray diffraction techniques. The carbon-coated and Cr-doped LiFePO₄ particles are synthesized by a mechanochemical process followed by a one-step heat treatment. The LiFe_0.97Cr_0.03PO₄/C has shown excellent rate performance, delivering the discharge capacity up to 120 mAh/g at 10 C rate. The results suggest that the improvement of the rate performance is attributed to the chromium doping, which facilitates the phase transformation between triphylite and heterosite during cycling, and conductivity improvement by carbon coating. Structural analysis using the synchrotron source also indicates that the doped Cr replaces Fe and/or Li sites in LiFePO₄.     

Synthesis and characterization of high-density LiFePO4/C composites as cathode materials for lithium-ion batteries

To achieve a high-energy-density lithium electrode, high-density LiFePO₄/C composite cathode material for a lithium-ion battery was synthesized using self-produced high-density FePO₄ as a precursor, glucose as a C source, and Li₂CO₃ as a Li source, in a pipe furnace under an atmosphere of 5% H₂-95% N₂. The structure of the synthesized material was analyzed and characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The electrochemical properties of the synthesized LiFePO₄/carbon composite were investigated by cyclic voltammetry (CV) and the charge/discharge process. The tap-density of the synthesized LiFePO₄/carbon composite powder with a carbon content of 7% reached 1.80 g m⁻³. The charge/discharge tests show that the cathode material has initial charge/discharge capacities of 190.5 and 167.0 mAh g⁻¹, respectively, with a volume capacity of 300.6 mAh cm⁻³, at a 0.1C rate. At a rate of 5C, the LiFePO₄/carbon composite shows a high discharge capacity of 98.3 mAh g⁻¹ and a volume capacity of 176.94 mAh cm⁻³.