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.     

High power performance of nano-LiFePO4/C cathode material synthesized via lauric acid-assisted solid-state reaction

A nano-LiFePO₄/C composite has been directly synthesized from micrometer-sized Li₂CO₃, NH₄H₂PO₄, and FeC₂O₄·2H₂O by the lauric acid-assisted solid-state reaction method. The SEM and TEM observations demonstrate that the synthesized nano-LiFePO₄/C composite has well-dispersed particles with a size of about 100–200 nm and an in situ carbon layer with thickness of about 2 nm. The prepared nano-LiFePO₄/C composite has superior rate capability, delivering a discharge capacity of 141.2 mAh g⁻¹ at 5 °C, 130.9 mAh g⁻¹ at 10 C, 121.7 mAh g⁻¹ at 20 °C, and 112.4 mAh g⁻¹ at 30 °C. At −20 °C, this cathode material still exhibits good rate capability with a discharge capacity of 91.9 mAh g⁻¹ at 1 °C. The nano-LiFePO₄/C composite also shows excellent cycling ability with good capacity retention, up to 100 cycles at a high current density of 30 °C. Furthermore, the effect of lauric acid in the preparation of nano-LiFePO₄/C composite was investigated by comparing it with that of citric acid. The SEM images reveal that the morphology of the LiFePO₄/C composite transformed from the porous structure to fine particles as the molar ratio of lauric acid/citric acid increased.     

Improved electrochemical performance of LiFePO4 by increasing its specific surface area

Cathode material LiFePO₄ with an excellent rate capability has been successfully prepared by a simple solid state reaction method using LiCH₃COO·2H₂O, FeC₂O₄·2H₂O and (NH₄)₂HPO₄ as the starting materials. We have investigated the effects of the sintering temperature and mixing time of the starting materials on the physical properties and electrochemical performance of LiFePO₄. It was found that the rate capability of LiFePO₄ is mainly controlled by its specific surface area and it is an effective way to improve the rate capability of the sample by increasing its specific surface area. In the present study, our prepared LiFePO₄ with a high specific surface area of 24.1 m² g⁻¹ has an excellent rate capability and can deliver 115 mAh g⁻¹ of reversible capacity even at the 5 C rate. Moreover, we have prepared lithium ion batteries based on LiFePO₄ as the cathode material and MCMB as the anode material, which showed an excellent cycling performance.     

The structure and electrochemical performance of LiFeBO3 as a novel Li-battery cathode material

LiFeBO₃ cathode material has been synthesized successfully by solid-state reaction using Li₂CO₃, H₃BO₃ and FeC₂O₄·2H₂O as starting materials. The crystal structure has been determined by the X-ray diffraction. Electrochemical tests show that an initial discharge capacity of about 125.8 mAh/g can be obtained at the discharge current density of 5 mA/g. When the discharge current density is increased to 50 mA/g, the specific capacity of 88.6 mAh/g can still be held. In order to further improve the electrochemical properties, the carbon-coated LiFeBO₃, C-LiFeBO₃, are also prepared. The amount of carbon coated on LiFeBO₃ particles was determined to be around 5% by TG analysis. In comparison with the pure LiFeBO₃, a higher discharge capacity, 158.3 mAh/g at 5 mA/g and 122.9 mAh/g at 50 mA/g, was obtained for C-LiFeBO₃. Based on its low cost and reasonable electrochemical properties obtained in this work, LiFeBO₃ may be an attractive cathode for lithium-ion batteries.     

Enhanced electrochemical performance of carbon nanospheres-LiFePO4 composite by PEG based sol-gel synthesis

The carbon nanospheres-LiFePO₄ (CNSs-LiFePO₄) composite has been synthesized by PEG (polyethylene glycol, mean molecular weight of 30,000) based sol-gel route. Highly conductive CNSs (30-40 nm) were adopted to improve the electronic conductivity of LiFePO₄. PEG was used to promote the dispersion of CNSs with the surface functionalization of CNSs, which could facilitate the coating of CNSs on the surface of the LiFePO₄ particles. The sample was characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, and Raman scattering. Electrochemical performance of the CNSs-LiFePO₄ composite was characterized by the charge-discharge test and electrochemical impedance spectra measurement. The results indicated that LiFePO₄ particles were well coated with the conductive CNSs to overcome the intrinsic low electronic conductivity problem of LiFePO₄. The CNSs-LiFePO₄ composite delivered an enhanced rate capability (146, 128 and 113 mAh g⁻¹ at 0.1 C, 1 C and 5 C rate). The PEG based sol-gel route enables LiFePO₄ networked with CNSs, which offered a higher electrochemical performance.     

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.     

Improving the electrochemical performance of Li4Ti5O12/Ag composite by an electroless deposition method

Nano-sized silver particle (<20 nm) was highly dispersed on the surface of Li₄Ti₅O₁₂ particles by an electroless deposition method. The Ag additive played a positive role in improving the electrical contact between Li₄Ti₅O₁₂ particles and the current collector and therefore improved the high rate capacity of Li₄Ti₅O₁₂, but it did not take part in the electrochemical reactions with Li⁺ in Li₄Ti₅O₁₂/Ag composite during the cycling. The experimental results showed that the smaller the silver particles and the more homogeneous dispersion of silver particles in the Li₄Ti₅O₁₂ matrix, the better the cycling performance we obtained.     

Preparation and electrochemical properties of LiMn2O4 by the microwave-assisted rheological phase method

Pure-phase and well-crystallized spinel LiMn₂O₄ powders as cathode materials for lithium-ion batteries were successfully synthesized by a new simple microwave-assisted rheological phase method, which was a timesaving and efficient method. The physical properties of the as-synthesized samples compared with the pristine LiMn₂O₄ obtained from the rheological phase method were investigated by thermogravimetry analysis (TGA), X-ray diffraction (XRD) and scanning electronic microscope (SEM). The as-prepared powders were used as positive materials for lithium-ion battery, whose charge/discharge properties and cycle performance were examined in detail. The powders resulting from the microwave-assisted rheological phase method were pure, spinel structure LiMn₂O₄ particles of regular shapes with distribution uniformly, and exhibited promising electrochemical properties for battery. Furthermore, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were employed to characterize the reactions of Li-ion insertion into and extraction from LiMn₂O₄ electrode.     

Preparation and electrochemical performance of Li4Ti5O12/carbon/carbon nano-tubes for lithium ion battery

A Li₄Ti₅O₁₂/carbon/carbon nano-tubes (Li₄Ti₅O₁₂/C/CNTs) composite was synthesized by using a solid-state method. For comparison, a Li₄Ti₅O₁₂/carbon (Li₄Ti₅O₁₂/C) composite and a pristine Li₄Ti₅O₁₂ were also synthesized in the present study. The microstructure and morphology of the prepared samples are characterized by XRD and SEM. Electrochemical properties of the samples are evaluated by using galvanostatic discharge/charge tests and AC impedance spectroscopy. The results reveal that the Li₄Ti₅O₁₂/C/CNTs composite exhibits the best rate capability and cycling stability among the samples of Li₄Ti₅O₁₂, Li₄Ti₅O₁₂/C and Li₄Ti₅O₁₂/C/CNTs. At the charge-discharge rate of 0.5 C, 5.0 C and 10.0 C, its discharge capacities were 163 mAh/g, 148 mAh/g and 143 mAh/g, respectively. After 100 cycles at 5.0 C, it remained at 146 mAh/g.     

Synthesis and electrochemical performance of Li2FeSiO4/carbon/carbon nano-tubes for lithium ion battery

Li₂FeSiO₄/carbon/carbon nano-tubes (Li₂FeSiO₄/C/CNTs) and Li₂FeSiO₄/carbon (Li₂FeSiO₄/C) composites were synthesized by a traditional solid-state reaction method and characterized comparatively by X-ray diffraction, scanning electron microscopy, BET surface area measurement, galvanostatic charge-discharge and AC impedance spectroscopy, respectively. The results revealed that the Li₂FeSiO₄/C/CNT composite exhibited much better rate performance in comparison with the Li₂FeSiO₄/C composite. At 0.2 C, 5 C and 10 C, the former composite electrode delivered a discharge capacity of 142 mAh g⁻¹, 95 mAh g⁻¹, 80 mAh g⁻¹, respectively, and after 100 cycles at 1 C, the discharge capacity remained 95.1% of its initial value.     

Structure optimization and the structural factors for the discharge rate performance of LiFePO4/C cathode materials

Carbon coating and iron phosphides of high electron conductivity were introduced into the LiFePO₄ materials which were derived via a sol-gel method in order to improve the high discharge rate performance. The start constituents were FeC₂O₄·2H₂O, LiOH·H₂O, NH₄H₂PO₄ and ethylene glycol. Effects of the calcination temperature and the ethylene glycol on the structure and the electrochemical performance of the LiFePO₄ materials were investigated. Structure analyses showed that the addition of ethylene glycol caused an obvious decrease in the particle size of LiFePO₄. Calcination temperature and ethylene glycol jointly affected the formation of iron phosphides. Combining the electrochemical testing, it was found that, at low discharge rate, small particle size and high content of LiFePO₄ were much important for the capacity rather than the iron phosphides, and relative high content of Fe₂P (e.g. 8 wt.%) even worsened the capacity. However, with the increase of the discharge rate, the high electron conductive iron phosphides turned to play important role on the capacity. Fe₂P effectively increased both the reaction and diffusion kinetics and hence enhanced the utilization efficiency of the LiFePO₄ at high discharge rate. Combining relative small particle size, even 2 wt.% of iron phosphides could improve the high rate performance of LiFePO₄/C significantly.     

Synthesis and electrochemical properties of Co-doped Li3V2(PO4)3 cathode materials for lithium-ion batteries

Co-doped Li₃V_2−xCo_x(PO₄)₃/C (x = 0.00, 0.03, 0.05, 0.10, 0.13 or 0.15) compounds were prepared via a solid-state reaction. The Rietveld refinement results indicated that single-phase Li₃V_2−xCo_x(PO₄)₃/C (0 ≤ x ≤ 0.15) with a monoclinic structure was obtained. The X-ray photoelectron spectroscopy (XPS) analysis revealed that the cobalt is present in the +2 oxidation state in Li₃V_2−xCo_x(PO₄)₃. XPS studies also revealed that V⁴⁺ and V³⁺ ions were present in the Co²⁺-doped system. The initial specific capacity decreased as the Co-doping content increased, increasing monotonically with Co content for x > 0.10. Differential capacity curves of Li₃V_2−xCo_x(PO₄)₃/C compounds showed that the voltage peaks associated with the extraction of three Li⁺ ions shifted to higher voltages with an increase in Co content, and when the Co²⁺-doping content reached 0.15, the peak positions returned to those of the unsubstituted Li₃V₂(PO₄)₃ phase. For the Li₃V_1.85Co_0.15(PO₄)₃/C compound, the initial capacity was 163.3 mAh/g (109.4% of the initial capacity of the undoped Li₃V₂(PO₄)₃) and 73.4% capacity retention was observed after 50 cycles at a 0.1 C charge/discharge rate. The doping of Co²⁺into V sites should be favorable for the structural stability of Li₃V_2−xCo_x(PO₄)₃/C compounds and so moderate the volume changes (expansion/contraction) seen during the reversible Li⁺ extraction/insertion, thus resulting in the improvement of cell cycling ability.     

Synthesis of SnO2/Mg2SnO4 nanoparticles and their electrochemical performance for use in Li-ion battery electrodes

Uniform crystalline MgSn(OH)₆ nanocubes were synthesized by a hydrothermal method. The influences of reaction conditions were investigated and the results showed that the solvent constituents significantly affected the shape and size of MgSn(OH)₆·SnO₂/Mg₂SnO₄ has been obtained by thermal treatment at 850 °C for 8 h under a nitrogen atmosphere using MgSn(OH)₆ as the precursor. The electrochemical tests of SnO₂/Mg₂SnO₄ revealed that SnO₂/Mg₂SnO₄ has a higher capacity and better cyclability compared to pure SnO₂ or Mg₂SnO₄. The electrochemical performance of SnO₂/Mg₂SnO₄ was sensitive to the size of the nanoparticles. The lithium-driven structural and morphological changes of the electrode after cycling were also studied by the ex-situ XRD pattern and TEM tests. This work indicates that SnO₂/Mg₂SnO₄ is a promising anode material candidate for application in Li-ion batteries.     

Hydrothermal synthesis and rate capacity studies of Li3V2(PO4)3 nanorods as cathode material for lithium-ion batteries

It is an effective method by synthesizing one-dimensional nanostructure to improve the rate performances of cathode materials for Li-ion batteries. In this paper, Li₃V₂(PO₄)₃ nanorods were successfully prepared by hydrothermal reaction method. The structure, composition and shape of the prepared were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scan electron microscope (SEM) and transmission electron microscope (TEM), respectively. The data indicate the as-synthesis powders are defect-rich nanorods and the sizes are the length of several hundreds of nanometers to 1 μm and the diameter of about 60 nm. The preferential growth direction of the prepared material was the [1 2 0]. The electrodes consisting of the Li₃V₂(PO₄)₃ nanorods show the better discharge capacities at high rates over a potential range of 3.0-4.6 V. These results can be attributed to the shorter distance of electron transport and the fact that ion diffusion in the electrode material is limited by the nanorod radius. All these results indicate that the resulting Li₃V₂(PO₄)₃ nanorods are promising cathode materials in lithium-ion batteries.     

Synthesis of highly crystalline spinel LiMn2O4 by a soft chemical route and its electrochemical performance

Highly crystalline spinel LiMn₂O₄ was successfully synthesized by annealing lithiated MnO₂ at a relative low temperature of 600 °C, in which the lithiated MnO₂ was prepared by chemical lithiation of the electrolytic manganese dioxide (EMD) and LiI. The LiI/MnO₂ ratio and the annealing temperature were optimized to obtain the pure phase LiMn₂O₄. With the LiI/MnO₂ molar ratio of 0.75, and annealing temperature of 600 °C, the resulting compounds showed a high initial discharge capacity of 127 mAh g⁻¹ at a current rate of 40 mAh g⁻¹. Moreover, it exhibited excellent cycling and high rate capability, maintaining 90% of its initial capacity after 100 charge-discharge cycles, at a discharge rate of 5 C, it kept more than 85% of the reversible capacity compared with that of 0.1 C.     

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.     

Effects of Na and Cl co-doping on electrochemical performance in LiFePO4/C

Na⁺ and Cl⁻ co-doped LiFePO₄/C composites were prepared via a simple solid state reaction. The structure, valence state and electrochemical performance were carefully investigated. Rietveld refinement on X-ray diffractions reveals that Na⁺ and Cl⁻ have successfully been introduced into the lattice of LiFePO₄. X-ray photoelectron spectroscopy proves that the co-doping of Na⁺ and Cl⁻ does not change the chemical state of Fe(II). Experimental results further show that the co-doping contributes to induce the lattice distortion, modify the particle morphology, and increase the electronic conductivity. Considerably enhanced capacity, coulombic efficiency and rate capability were obtained in the co-doped LiFePO₄. The specific capacities are 157 mAh g⁻¹ at 0.2 C, 115 mAh g⁻¹ at 10 C and 98 mAh g⁻¹ at 20 C for the (Na⁺, Cl⁻) co-doped LiFePO₄/C cathode material. The improvement can be ascribed to the enhanced electronic conductivity and electrode kinetics due to the micro-structural modification promoted by co-doping.     

Spray-drying synthesized LiNi0.6Co0.2Mn0.2O2 and its electrochemical performance as cathode materials for lithium ion batteries

Layered LiNi_0.6Co_0.2Mn_0.2O₂ materials were synthesized at different sintering temperatures using spray-drying precursor with molar ratio of Li/Me = 1.04 (Me = transition metals). The influences of sintering temperature on crystal structure, morphology and electrochemical performance of LiNi_0.6Co_0.2Mn_0.2O₂ materials have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and charge-discharge test. As a result, material synthesized at 850 °C has excellent electrochemical performance, delivering an initial discharge capacity of 173.1 mAh g^− 1 between 2.8 and 4.3 V at a current density of 16 mA g^− 1 and exhibiting good cycling performance.     

High rate capability and long-term cyclability of Li4Ti4.9V0.1O12 as anode material in lithium ion battery

Li₄Ti_4.9V_0.1O₁₂ nanometric powders were synthesized via a facile solid-state reaction method under inert atmosphere. XRD analyses demonstrated that the V-ions successfully entered the structure of cubic spinel-type Li₄Ti₅O₁₂ (LTO), reduced the lattice parameter and no impurities appeared. Compared with the pristine LTO, the electronic conductivity of Li₄Ti_4.9V_0.1O₁₂ powders is as high as 2.9 × 10⁻¹ S cm⁻¹, which should be attributed to the transformation of some Ti³⁺ from Ti⁴⁺ induced by the efficient V-ions doping and the deficient oxygen condition. Meanwhile, the results of XPS and EDS further proved the coexistence of V⁵⁺ and Ti³⁺ ions. This mixed Ti⁴⁺/Ti³⁺ ions can remarkably improve its cycle stability at high discharge–charge rates because of the enhancement of the electronic conductivity. The images of SEM showed that Li₄Ti_4.9V_0.1O₁₂ powders have smaller particles and narrower particle size distribution under 330 nm. And EIS indicates that Li₄Ti_4.9V_0.1O₁₂ has a faster lithium-ion diffusivity than LTO. Between 1.0 and 2.5 V, the electrochemical performance, especially at high rates, is excellent. The discharge capacities are as high as 166 mAh g⁻¹ at 0.5C and 117.3 mAh g⁻¹ at 5C. At the rate of 2C, it exhibits a long-term cyclability, retaining over 97.9% of its initial discharge capacity beyond 1713 cycles. These outstanding electrochemical performances should be ascribed to its nanometric particle size and high conductivity (both electron and lithium ion). Therefore, the as-prepared material is promising for such extensive applications as plug-in hybrid electric vehicles and electric vehicles.     

Improved electrochemical performance of La0.7Sr0.3MnO3 and carbon co-coated LiFePO4 synthesized by freeze-drying process

Carbon coated LiFePO₄ particles were first synthesized by sol-gel and freeze-drying method. These particles were then coated with La_0.7Sr_0.3MnO₃ nanolayer by a suspension mixing process. The La_0.7Sr_0.3MnO₃ and carbon co-coated LiFePO₄ particles were calcined at 400 °C for 2 h in a reducing atmosphere (5% of hydrogen in nitrogen). Nanolayer structured La_0.7Sr_0.3MnO₃ together with the amorphous carbon layer forms an integrate network arranged on the bare surface of LiFePO₄ as corroborated by high-resolution transmission electron microscopy. X-ray diffraction results proved that the co-coated composite still retained the structure of the LiFePO₄ substrate. The twin coatings can remarkably improve the electrochemical performance at high charge/discharge rates. This improvement may be attributed to the lower charge transfer resistance and higher electronic conductivity resulted from the twin nanolayer coatings compared with the carbon coated LiFePO₄.