Enhanced electrochemical properties of LiFePO4/C synthesized with two kinds of carbon sources,PEG-4000 (organic) and Super p (inorganic)

Olivine structured LiFePO₄/C cathode was synthesized via a freeze-drying route and followed by microwave heating with two kinds of carbon sources: PEG-4000 (organic) and Super p (inorganic). XRD patterns indicate that the as-prepared sample has an olivine structure and carbon modification does not affect the structure of the sample. Image of SEM shows a uniform and optimized particles size, which greatly improves the electrochemical properties. TEM result reveals the amorphous carbon around the surface of the particles. At a low rate of 0.1 C, the LiFePO₄/C sample presents a high discharge capacity of 157.8 mAh g⁻¹ which is near the theoretical capacity (170 mAh g⁻¹), and it still attains to 149.1 mAh g⁻¹ after 200 cycles. It also exhibits an excellent rate capacity with high discharge capacities of 143.2 mAh g⁻¹, 137.5 mAh g⁻¹, 123.7 mAh g⁻¹ and 101.6 mAh g⁻¹ at 0.5 C, 1.0 C, 2.0 C and 5.0 C, respectively. EIS results indicate that the charge transfer resistance of LiFePO₄ decreases greatly after carbon coating.     

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.     

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.     

LiFePO4 cathode power with high energy density synthesized by water quenching treatment

A water quenching (WQ) method was developed to synthesize LiFePO₄ and C-LiFePO₄. Our results indicate that this synthesis method ensures improved electrochemical activity and small crystal grain size. The synthetic conditions were optimized using orthogonal experiments. The LiFePO₄ sample prepared at the optimized condition showed a maximum discharge capacity of 149.8 mAh g⁻¹ at a C/10 rate. C-LiFePO₄ with a low carbon content of 0.93% and a high discharge specific capacity of 163.8 mAh g⁻¹ has also been obtained using this method. Water quenching treatment shows outstanding improvement of the electrochemical performance of LiFePO₄.     

Electrochemical performance of the carbon coated Li3V2(PO4)3 cathode material synthesized by a sol-gel method

A carbon coated Li₃V₂(PO₄)₃ cathode material for lithium ion batteries was synthesized by a sol-gel method using V₂O₅, H₂O₂, NH₄H₂PO₄, LiOH and citric acid as starting materials, and its physicochemical properties were investigated using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) spectroscopy, scanning electron microscopy (SEM), energy dispersive analysis of X-ray (EDAX), transmission electron microscope (TEM), and electrochemical methods. The sample prepared displays a monoclinic structure with a space group of P2₁/n, and its surface is covered with a rough and porous carbon layer. In the voltage range of 3.0-4.3 V, the Li₃V₂(PO₄)₃ electrode displays a large reversible capacity, good rate capability and excellent cyclic stability at both 25 and 55 °C. The largest reversible capacity of 130 mAh g⁻¹ was obtained at 0.1C and 55 °C, nearly equivalent to the reversible cycling of two lithium ions per Li₃V₂(PO₄)₃ formula unit (133 mAh g⁻¹). It was found that the increase in total carbon content can improve the discharge performance of the Li₃V₂(PO₄)₃ electrode. In the voltage range of 3.0-4.8 V, the extraction and reinsertion of the third lithium ion in the carbon coated Li₃V₂(PO₄)₃ host are almost reversible, exhibiting a reversible capacity of 177 mAh g⁻¹ and good cyclic performance. The reasons for the excellent electrochemical performance of the carbon coated Li₃V₂(PO₄)₃ cathode material were also discussed.     

A novel micro-spherical CoSn2/Sn alloy composite as high capacity anode materials for Li-ion rechargeable batteries

Micro-scaled spherical CoSn₂/Sn alloy powders synthesized from oxides of Sn and Co via carbothermal reduction at 800 °C were examined for use as anode materials in Li-ion battery. The phase composition and particle morphology of the CoSn₂/Sn alloy composite powders were investigated by XRD, SEM and TEM. The prepared CoSn₂/Sn alloy composite electrode exhibits a low initial irreversible capacity of ca. 140 mAh g⁻¹, a high specific capacity of ca. 600 mAh g⁻¹ at constant current density of 50 mA g⁻¹, and a good rate capability. The stable discharge capacities of 500-515 mAh g⁻¹ and the columbic efficiencies of 95.8-98.1% were obtained at current density of 500 mA g⁻¹. The relatively large particle size of CoSn₂/Sn alloy composite powder is apparently favorable for the lowering of initial capacity loss of electrode, while the loose particle structural characteristic and the Co addition in Sn matrix should be responsible for the improvement of cycling stability of CoSn₂/Sn electrode.     

Enhanced electrochemical performances of LiNi0.5Mn1.5O4 spinel via ethylene glycol-assisted synthesis

A simple and effective method, ethylene glycol-assisted co-precipitation method, has been employed to synthesize LiNi_0.5Mn_1.5O₄ spinel. As a chelating agent, ethylene glycol can realize the homogenous distributions of metal ions at the atomic scale and prevent the growth of LiNi_0.5Mn_1.5O₄ particles. XRD reveals that the prepared material is a pure-phase cubic spinel structure (Fd3m) without any impurities. SEM images show that it has an agglomerate structure with the primary particle size of less than 100 nm. Electrochemical tests demonstrate that the as-prepared LiNi_0.5Mn_1.5O₄ possesses high capacity and excellent rate capability. At 0.1 C rate, it shows a discharge capacity of 137 mAh g⁻¹ which is about 93.4% of the theoretical capacity (146.7 mAh g⁻¹). At the high rate of 5 C, it can still deliver a discharge capacity of 117 mAh g⁻¹ with excellent capacity retention rate of more than 95% after 50 cycles. These results show that the as-prepared LiNi_0.5Mn_1.5O₄ is a promising cathode material for high power Li-ion batteries.     

Rheological phase reaction synthesis and electrochemical performance of Li3V2(PO4)3/carbon cathode for lithium ion batteries

Monoclinic lithium vanadium phosphate/carbon (Li₃V₂(PO₄)₃/C) cathode has been synthesized for applications in lithium ion batteries, via a rheological phase reaction (RPR) method. The sample is characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). This material exhibits high initial discharge capacity of 189 and 177 mAh g⁻¹ at 0.1 and 0.2 C between 3.0 and 4.8 V, respectively. Moreover, it displays good fast rate performance, which discharge capacities of 140, 133, 129 and 124 mAh g⁻¹ can be delivered after 100 cycles between 3.0 and 4.8 V vs. Li at a different rate of 0.5, 1, 2 and 5 C, respectively. The electrochemical impedance spectroscopy (EIS) is also investigated.     

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.     

Synthesis of spinel LiMn2O4 with manganese carbonate prepared by micro-emulsion method

Micro-spherical particle of MnCO₃ has been successfully synthesized in CTAB-C₈H₁₈-C₄H₉OH-H₂O micro-emulsion system. Mn₂O₃ decomposed from the MnCO₃ is mixed with Li₂CO₃ and sintered at 800 °C for 12 h, and the pure spinel LiMn₂O₄ in sub-micrometer size is obtained. The LiMn₂O₄ has initial discharge specific capacity of 124 mAh g⁻¹ at discharge current of 120 mA g⁻¹ between 3 and 4.2 V, and retains 118 mAh g⁻¹ after 110 cycles. High-rate capability test shows that even at a current density of 16 C, capacity about 103 mAh g⁻¹ is delivered, whose power is 57 times of that at 0.2 C. The capacity loss rate at 55 °C is 0.27% per cycle.     

Effect of synthesis condition on the structural and electrochemical properties of Li[Ni1/3Mn1/3Co1/3]O2 prepared by the metal acetates decomposition method

In order to get homogeneous layered oxide Li[Ni_1/3Mn_1/3Co_1/3]O₂ as a lithium insertion positive electrode material, we applied the metal acetates decomposition method. The oxide compounds were calcined at various temperatures, which results in greater difference in morphological (shape, particle size and specific surface area) and the electrochemical (first charge profile, reversible capacity and rate capability) differences. The Li[Ni_1/3Mn_1/3Co_1/3]O₂ powders were characterized by means of X-ray diffraction (XRD), charge/discharge cycling, cyclic voltammetry and SEM. XRD experiment revealed that the layered Li[Ni_1/3Mn_1/3Co_1/3]O₂ material can be best synthesized at temperature of 800 °C. In that synthesized temperature, the sample showed high discharge capacity of 190 mAh g⁻¹ as well as stable cycling performance at a current density of 0.2 mA cm⁻² in the voltage range 2.3-4.6 V. The reversible capacity after 100 cycles is more than 190 mAh g⁻¹ at room temperature.     

Mesoporous polyaniline or polypyrrole/anatase TiO2 nanocomposite as anode materials for lithium-ion batteries

A functional composite as anode materials for lithium-ion batteries, which contains highly dispersed TiO₂ nanocrystals in polyaniline matrix and well-defined mesopores, is fabricated by employing a novel one-step approach. The as-prepared mesoporous polyaniline/anatase TiO₂ nanocomposite has a high specific surface area of 224 m² g⁻¹ and a predominant pore size of 3.6 nm. The electrochemical performance of the as-prepared composite as anode material is investigated by cyclic voltammograms and galvanostatic method. The results demonstrate that the polyaniline/anatase nanocomposite provides larger initial discharge capacity of 233 mAh g⁻¹ and good cycle stability at the high current density of 2000 mA g⁻¹. After 70th cycles, the discharge capacity is maintained at 140 mAh g⁻¹. The excellent electrochemical performance of the polyaniline/TiO₂ nanocomposite is mainly attributed to its special structure. Furthermore, it is accessible to extend the novel strategy to other polymer/TiO₂ composites, and the mesoporous polypyrrole/anatase TiO₂ is also successfully fabricated.     

TiO2 nanotube arrays annealed in CO exhibiting high performance for lithium ion intercalation

Anatase titania nanotube arrays were fabricated by means of anodization of Ti foil and annealed at 400 °C in respective CO and N₂ gases for 3 h. Electrochemical impendence spectroscopy study showed that CO annealed arrays possessed a noticeably lower charge-transfer resistance as compared with arrays annealed in N₂ gas under otherwise the same conditions. TiO₂ nanotube arrays annealed in CO possessed much improved lithium ion intercalation capacity and rate capability than N₂ annealed samples. At a high charge/discharge current density of 320 mA g⁻¹, the initial discharge capacity in CO annealed arrays was found to be as high as 223 mAh g⁻¹, 30% higher than N₂ annealed arrays, ∼164 mAh g⁻¹. After 50 charge/discharge cycles, the discharge capacity in CO annealed arrays remained at ∼179 mAh g⁻¹. The improved intercalation capacity and rate capability could be attributed to the presence of surface defects like Ti-C species and Ti³⁺ groups with oxygen vacancies, which not only improved the charge-transfer conductivity of the arrays but also possibly promoted phase transition.     

1-Alkyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide ionic liquids as highly safe electrolyte for Li/LiFePO4 battery

Several 1-alkyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide ionic liquids (alkyl-DMimTFSI) were prepared by changing carbon chain lengths and configuration of the alkyl group, and their electrochemical properties and compatibility with Li/LiFePO₄ battery electrodes were investigated in detail. Experiments indicated the type of ionic liquid has a wide electrochemical window (−0.16 to 5.2 V vs. Li⁺/Li) and are theoretically feasible as an electrolyte for batteries with metallic lithium as anode. Addition of vinylene carbonate (VC) improves the compatibility of alkyl-DMimTFSI-based electrolytes towards lithium anode and LiFePO₄ cathode, and enhanced the formation of solid electrolyte interface to protect lithium anodes from corrosion. The electrochemical properties of the ionic liquids obviously depend on carbon chain length and configuration of the alkyl, including ionic conductivity, viscosity, and charge/discharge capacity etc. Among five alkyl-DMimTFSI-LiTFSI-VC electrolytes, Li/LiFePO₄ battery with the electrolyte-based on amyl-DMimTFSI shows best charge/discharge capacity and reversibility due to relatively high conductivity and low viscosity, its initial discharge capacity is about 152.6 mAh g⁻¹, which the value is near to theoretical specific capacity (170 mAh g⁻¹). Although the battery with electrolyte-based isooctyl-DMimTFSI has lowest initial discharge capacity (8.1 mAh g⁻¹) due to relatively poor conductivity and high viscosity, the value will be dramatically added to 129.6 mAh g⁻¹ when 10% propylene carbonate was introduced into the ternary electrolyte as diluent. These results clearly indicates this type of ionic liquids have fine application prospect for lithium batteries as highly safety electrolytes in the future.     

Synthesis and characterization of high-density non-spherical Li(Ni1/3Co1/3Mn1/3)O2 cathode material for lithium ion batteries by two-step drying method

Non-spherical Li(Ni_1/3Co_1/3Mn_1/3)O₂ powders have been synthesized using a two-step drying method with 5% excess LiOH at 800 °C for 20 h. The tap-density of the powder obtained is 2.95 g cm⁻³. This value is remarkably higher than that of the Li(Ni_1/3Co_1/3Mn_1/3)O₂ powders obtained by other methods, which range from 1.50 g cm⁻³ to 2.40 g cm⁻³. The precursor and Li(Ni_1/3Co_1/3Mn_1/3)O₂ are characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscope (SEM). XPS studies show that the predominant oxidation states of Ni, Co and Mn in the precursor are 2⁺, 3⁺ and 4⁺, respectively. XRD results show that the Li(Ni_1/3Co_1/3Mn_1/3)O₂ material obtained by the two-step drying method has a well-layered structure with a small amount of cation mixing. SEM confirms that the Li(Ni_1/3Co_1/3Mn_1/3)O₂ particles obtained by this method are uniform. The initial discharge capacity of 167 mAh g⁻¹ is obtained between 3 V and 4.3 V at a current of 0.2 C rate. The capacity of 159 mAh g⁻¹ is retained at the end of 30 charge-discharge cycle with a capacity retention of 95%.     

Niobium doped lithium titanate as a high rate anode material for Li-ion batteries

Niobium doped lithium titanate with the composition of Li₄Ti_4.95Nb_0.05O₁₂ has been prepared by a sol-gel method. X-ray diffraction (XRD) and scanning electron microscope (SEM) are employed to characterize the structure and morphology of Li₄Ti_4.95Nb_0.05O₁₂. The Li₄Ti_4.95Nb_0.05O₁₂ electrode presents a higher specific capacity and better cycling performance than the Li₄Ti₅O₁₂ electrode prepared by the similar process. The Li₄Ti_4.95Nb_0.05O₁₂ exhibits an excellent rate capability with a reversible capacity of 135 mAh g⁻¹ at 10 C, 127 mAh g⁻¹ at 20 C and even 80 mAh g⁻¹ at 40 C. Electrical resistance measurement and electrochemical impedance spectra (EIS) reveal that the Li₄Ti_4.95Nb_0.05O₁₂ exhibits a higher electronic conductivity and faster lithium-ion diffusivity than the Li₄Ti₅O₁₂, which indicates that niobium doped lithium titanate (Li₄Ti_4.95Nb_0.05O₁₂) is promising as a high rate anode for the lithium-ion batteries.     

One-step microwave synthesis and characterization of carbon-modified nanocrystalline LiFePO4

The precursors of LiFePO₄ were prepared by a sol-gel method using lithium acetate dihydrate, ferrous sulfate, phosphoric acid, citric acid and polyethylene glycol as raw materials, and then the carbon-modified nanocrystalline LiFePO₄ (LiFePO₄/C) cathode material was synthesized by a one-step microwave method with the domestic microwave oven. The effect of microwave time and carbon content on the performance of the resulting LiFePO₄/C material was investigated. Structural characterization by X-ray diffraction and scanning electron microscopy proved that the olivine phase LiFePO₄ was synthesized and the grain size of the samples was several hundred nanometers. Under the optimal conditions of microwave time and carbon content, the charge-discharge performance indicated that the nanosized LiFePO₄/C had a high electrochemical capacity at 0.2 C (152 mAh g⁻¹) and improved capacity retention; the exchange current density was 1.6977 mA cm⁻². Furthermore, the rate capability was improved effectively after LiFePO₄ was modified with carbon, with 59 mAh g⁻¹ being obtained at 20 C.     

Rapid preparation and electrochemical behavior of carbon-coated Li3V2(PO4)3 from wet coordination

An improved solid-state coordination method namely wet coordination has been developed to synthesize carbon-coated monoclinic Li₃V₂(PO₄)₃ rapidly at a low temperature of 600 °C in 1 h. The structure of the sample was characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM) and energy dispersive analysis of X-rays (EDAX). The diffusion coefficient of the lithium ions was measured by cyclic voltammetry (CV). The electrochemical behavior of the sample exhibits a high initial discharge capacity of 130 mAh g⁻¹, which is very close to its theoretical capacity of 132 mAh g⁻¹ under 95 mA g⁻¹ (0.67 C) in the range of 3.3-4.3 V (vs. Li/Li⁺). These results suggest that wet coordination is a promising method for large-scale production of carbon-coated Li₃V₂(PO₄)₃.     

Crystal structure and electrochemical performance of Li3V2(PO4)3 synthesized by optimized microwave solid-state synthesis route

To investigate the crystal structure and electrochemical performance of samples synthesized under different microwave solid-state synthesis condition, a series of Li₃V₂(PO₄)₃ samples has been synthesized at five different temperatures for 3-5 min and at 750 °C for various time. The as-synthesized Li₃V₂(PO₄)₃ samples are characterized and studied by ICP-AES analysis, X-ray diffraction (XRD), Rietveld analysis, scanning and transmission electron microcopy (SEM and TEM). At relatively lower temperature (650 °C) and very short reaction time (3 min), pure phase of Li₃V₂(PO₄)₃ could be synthesized in microwave irradiation field. The crystal structure and Li atomic fractional coordinate present a significant deviation upon the change of microwave irradiation temperature and time. Relatively, the diffusion ability of lithium cations and the electrochemical performance are affected. Under the proper reaction temperature and time, the carbon-free samples MW750C5m and MW850C3m show the best specific discharge capacity 126.4 and 132 mAh g⁻¹ at the voltage range of 3.0-4.3 V, near the reversible cycling of two lithium ions per Li₃V₂(PO₄)₃ formula unit (133 mAh g⁻¹). At the voltage range of 3-4.8 V, the sample MW750C5m presents the best initial specific charge capacity of 197 mAh g⁻¹, equivalent to the reversible cycling of three lithium ions per Li₃V₂(PO₄)₃ formula unit (197 mAh g⁻¹). The initial discharge capacity, the samples MW750C5m and MW850C3m present high specific discharge capacity 183.4 and 175.7 mAh g⁻¹, respectively. The relationship among microwave irradiation condition, crystal structure, lithium atomic fractional coordinates and the electrochemical performance have been discussed in detail.     

Synthesis of uniform polycrystalline tin dioxide nanofibers and electrochemical application in lithium-ion batteries

Under optimized synthesis conditions, very large area uniform SnO₂ nanofibers consisting of orderly bonded nanoparticles have been obtained for the first time by thermal pyrolysis and oxidization of electrospun tin(II)2-ethylhexanoate/polyacrylonitrile (PAN) polymer nanofibers in air. The structure and morphology were elaborated by X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The SnO₂ nanofibers delivered a reversible capacity of 446 mAh g⁻¹ after 50 cycles at the 100 mA g⁻¹ rate and excellent rate capability of 477.7 mAh g⁻¹ at 10.0 C. Owing to the improved electrochemical performance, this electrospun SnO₂ nanofiber could be one of the most promising candidate anode materials for the lithium-ion battery.