Bare LiFePO Research Articles

A new route for synthesizing C/LiFePO4/multi-walled carbon nanotube secondary particles for lithium ion batteries

Abstract Nanosized C/LiFePO 4 /MWCNTs secondary particles were synthesized by a combination of hydrothermal progress and a facile electro-polymerization polyaniline process with simultaneous calcinations. In combination with the continuous three-dimensional (3D) networks and high electronic conduction facilitating the kinetics of both electron transport and lithium ion diffusion within the particles, the optimized electrodes exhibit an ultrahigh rate capacity with a tap density of 1.78 g cm − 3 , stable charge/discharge cycle ability. The synthesized LiFePO 4 composite demonstrated an increased reversible capacity and better cycling ability compared to the bare LiFePO 4 , offering a discharge capacity of 169.9 mAh g − 1 (nearly to its the theoretical capability 170 mAh g − 1 ) at the C/10 rate and delivering a good rate performance with a capacity of 143.4 mAh g − 1 at a high rate of 20 C, and stable charge/discharge cycle ability (> 95% capacity retention after 200 charge/discharge cycles).This non-organic facile synthesize avenue can be highly desirable to prepare next-generation high-power lithium ion batteries.

Synthesis and characterization of LiFePO 4 and LiFePO 4/C cathode material from lithium carboxylic acid and Fe 3+

Olivine LiFePO 4 and LiFePO 4/C are successfully prepared by a simple solid-state reaction using FePO 4·2H 2O, lithium oxalate and glucose (bare LiFePO 4 without adding glucose) as raw materials. The structure of the LiFePO 4 and LiFePO 4/C is investigated by X-ray diffraction (XRD). The micromorphology of LiFePO 4 and LiFePO 4/C is observed using scanning electron microscopy (SEM) and BET, and the in situ coating of carbon on the particles is observed by transmission electron microscopy (TEM) and Raman spectrum. Furthermore, the electrochemical properties are evaluated by cyclic voltammograms (CVs), electrochemical impedance spectra (EIS) and constant current charge/discharge cycling tests. The results show that carbon coated LiFePO 4 can deliver better battery performance than the bare LiFePO 4. It exhibits initial discharge capacities of 162, 142 and 112 mA h g −1 at rates of 0.1, 1 and 10 C, respectively, and it presents excellent capacity retention as there is tiny capacity fade after 100 cycles. Moreover, the reductive mechanism of using lithium carboxylic acid to synthesize LiFePO 4 is firstly mentioned.

In situ high-resolution transmission electron microscopy synthesis observation of nanostructured carbon coated LiFePO 4

In situ high-resolution transmission electron microscopy (HRTEM) studies of the structural transformations that occur during the synthesis of carbon-coated LiFePO 4 (C–LiFePO 4) and heat treatment to elevated temperatures were conducted in two different electron microscopes. Both microscopes have sample holders that are capable of heating up to 1500 °C, with one working under high vacuum and the other capable of operating with the sample surrounded by a low gaseous environment. The C–LiFePO 4 samples were prepared using three different compositions of precursor materials with Fe(0), Fe(II) or Fe(III), a Li-containing salt and a polyethylene- block-poly(ethylene glycol)-50% ethylene oxide or lactose. The in situ TEM studies suggest that low-cost Fe(0) and a low-cost carbon-containing compound such as lactose are very attractive precursors for mass production of C–LiFePO 4, and that 700 °C is the optimum synthesis temperature. At temperatures higher than 800 °C, LiFePO 4 has a tendency to decompose. The same in situ measurements have been made on particles without carbon coat. The results show that the homogeneous deposit of the carbon deposit at 700 °C is the result of the annealing that cures the disorder of the surface layer of bare LiFePO 4. Electrochemical tests supported the conclusion that the C–LiFePO 4 derived from Fe(0) is the most attractive for mass production.

Synthesis of LiFePO 4/C cathode material from ferric oxide and organic lithium salts

LiFePO 4/C cathode material has been simply synthesized via a modified in situ solid-state reaction route using the raw materials of Fe 2O 3, NH 4H 2PO 4, Li 2C 2O 4 and lithium polyacrylate (PAALi). The sintering temperature of LiFePO 4/C precursor is studied by thermo-gravimetric (TG)/differential thermal analysis (DTA). The physical properties of LiFePO 4/C are then investigated through analysis using by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM) and the electrochemical properties are investigated by electrochemical impedance spectra (EIS), cyclic voltammogram (CV) and constant current charge/discharge test. The LiFePO 4/C composite with the particle size of ∼200 nm shows better discharge capacity (156.4 mAh g −1) than bare LiFePO 4 (52.3 mAh g −1) at 0.2 C due to the improved electronic conductivity which is demonstrated by EIS. The as-prepared LiFePO 4/C through this method also shows excellent high-rate characteristic and cycle performance. The initial discharge capacity of the sample is 120.5 mAh g −1 and the capacity retention rate is 100.6% after 50 cycles at 5 C rate. The results prove that the using of organic lithium salts can obtain a high performance LiFePO 4/C composite.

Improvement of electrochemical performances of LiFePO 4 cathode materials by coating of polythiophene

A series LiFePO 4/polythiophene (LiFePO 4/PTh) composites were synthesized by in situ polymerizing thiophene monomers on the surface of LiFePO 4 particles. Electrochemical impedance spectra (EIS) measurements show that the coating of polythiophene significantly decreases the charge transfer resistance of LiFePO 4 electrodes. Transmission electron microscopy (TEM) tests show that LiFePO 4 could be completely coated by addition of proper amount of polythiophene. The electrochemical performance of polythiophene and LiFePO 4/PTh for lithium insertion and extraction was examined by charge/discharge testing. The composites demonstrated an increased reversible capacity and better cycling ability compared to the bare LiFePO 4.

In situ construction of carbon nano-interconnects between the LiFePO 4 grains using ultra low-cost asphalt

LiFePO 4/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 LiFePO 4 grains. The LiFePO 4/C composite shows enhanced discharge capacity (150 mAh g −1) with excellent capacity retention compared with the bare LiFePO 4 (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 LiFePO 4/C composite.

Superior high-rate cycling performance of LiFePO 4/C-PPy composite at 55 °C

A facile chemical polymerization method was applied to prepare LiFePO 4/C-PPy composite using Fe(III)tosylate as oxidant. The as-prepared LiFePO 4/C-PPy sample with PPy content of approximately 4 wt% showed great rate capability with a discharge capacity of 115 mAh/g at 20C. High temperate cycling performance of the LiFePO 4/C-PPy sample was compared with bare LiFePO 4/C at 5C charge–discharge rate at 55 °C. The LiFePO 4/C-PPy cathode showed superior cycling stability with an initial capacity of 155 mAh/g. Ninety percentage of this initial capacity was retained after 300 cycles, compared to 40% of that of bare LiFePO 4/C. The LiFePO 4/C-PPy electrode showed stable discharge plateau voltage of 3.35–3.25 V vs. Li +/Li during long term cycling. The superior performance of the LiFePO 4/C-PPy electrode was due to the enhanced electrical conductivity, negligible iron dissolution and alleviated electrode cracking contributed by PPy coating.

Preparation of carbon coated LiFePO 4 by a combination of spray pyrolysis with planetary ball-milling followed by heat treatment and their electrochemical properties

Carbon coated LiFePO 4 could be successfully prepared by a combination of spray pyrolysis (SP) with planetary ball-milling (BM) followed by heat treatment at 500 °C. SEM and TEM observations revealed that the carbon coated LiFePO 4 were much smaller in size than the bare LiFePO 4 prepared by the SP followed by heat treatment at 600 °C and the ball-milled bare LiFePO 4 prepared by the combination of SP with BM followed by heat treatment at 500 °C. Furthermore, TEM observations also suggested that conductive carbon could be distributed well on the surface of LiFePO 4. The electrochemical measurements demonstrated that the carbon coated LiFePO 4 could deliver better battery performance in terms of the discharge capacity, cycling stability and rate capability than the bare LiFePO 4 and the ball-milled bare LiFePO 4. It exhibited first-discharge capacities of 158 mAh g − 1 at 0.1 C and 114 mAh g − 1 at 5 C with excellent cycle performances at 25 °C. The cell tested at 60 °C delivered the theoretical capacity (170 mAh g − 1 ) at 0.1 C and 78% (133 mAh g − 1 ) of theoretical capacity at 5 C, respectively.

Preparation and electrochemical properties of LiFePO 4/C composite with network structure for lithium ion batteries

The bare LiFePO 4 and LiFePO 4/C composites with network structure were prepared by solid-state reaction. The crystalline structures, morphologies and specific surface areas of the materials were investigated by X-ray difrractometry(XRD), scanning electron microscopy(SEM) and multi-point brunauer emmett and teller(BET) method. The results show that the LiFePO 4/C composite with the best network structure is obtained by adding 10% phenolic resin carbon. Its electronic conductivity increases to 2.86 × 10 −2 S/cm. It possesses the highest specific surface area of 115.65 m 2/g, which exhibits the highest discharge specific capacity of 164.33 mA·h/g at C/10 rate and 149.12 rnA·h/g at 1 C rate. The discharge capacity is completely recovered when C/10 rate is applied again.

Electrochemical properties of carbon-coated LiFePO 4 cathode using graphite, carbon black, and acetylene black

Electrochemical properties of LiFePO 4 were investigated by incorporating conductive carbon from three different carbon sources (graphite, carbon black, acetylene black). SEM observations revealed that the carbon-coated LiFePO 4 were smaller than the bare LiFePO 4 particles. The carbon-coated LiFePO 4 showed much better performance in terms of the discharge capacity and cycling stability than the bare LiFePO 4. Among carbon-coated LiFePO 4, the particles coated with graphite exhibited better electrochemical properties than others coated with carbon black or acetylene black.

Electrochemical properties of the carbon-coated LiFePO 4 as a cathode material for lithium-ion secondary batteries

In this study the effect of the carbon coating on the electrochemical properties of LiFePO 4 as a cathode for Li-ion batteries were investigated. The carbon-coated LiFePO 4 particles were synthesized by the mechanochemical process and one-step heat treatment. Microscopic observations using SEM and TEM revealed that the carbon coating reduced the particle size of the LiFePO 4. The carbon-coated LiFePO 4 showed much better performances in terms of the discharge capacity and cycle stability than bare LiFePO 4. It was confirmed that the carbon coating decreased the migration distance of Li-ion and enhanced the charge transfer from CV and ac impedance measurements. The improved electrochemical properties of the carbon-coated LiFePO 4 were, therefore, attributed to the reduced particle size and enhanced electrical contacts by carbon.

An investigation of polypyrrole-LiFePO 4 composite cathode materials for lithium-ion batteries

A series of polypyrrole-LiFePO 4 (PPy-LiFePO 4) composites were synthesised by polymerising pyrrole monomers on the surface of LiFePO 4 particles. AC impedance measurements show that the coating of polypyrrole significantly decreases the charge-transfer resistance of LiFePO 4 electrodes. The electrochemical reactivity of polypyrrole and PPy-LiFePO 4 composites for lithium insertion and extraction was examined by charge/discharge testing. The PPy-LiFePO 4 composite electrodes demonstrated an increased reversible capacity and better cyclability, compared to the bare LiFePO 4 electrode.