Synthesis and characterization of Li<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>·LiMn<Subscript>0.33</Subscript>Fe<Subscript>0.67</Subscript>PO<Subscript>4</Subscript>/C cathode materials

A new polyanionic cathode material, Li₃V₂(PO₄)₃·LiMn_0.33Fe_0.67PO₄/C for lithium-ion batteries, was synthesized using a sol-gel method and with N,N-dimethyl formamide as a dispersion agent. The analysis of electron transmission spectroscopy and X-ray diffraction revealed that the composite contained two phases. The material has high crystallinity with a grain size of 20–50 nm. The valence states of Mn, V, and Fe in the composite were analyzed by X-ray photoelectron spectroscopy. The electrochemical kinetics in Li₃V₂(PO₄)₃ is effectively enhanced by the incorporation of LiMnPO₄ and LiFePO₄, via structure modification and reduced Li diffusion length. The Li₃V₂(PO₄)₃·LiMn_0.33Fe_0.67PO₄/C materials displayed high rate capacity and steady cycle performance with discharge capacity remained 148 mAh g^?1 after 50 cycles at the rate of 0.2C. In particular, the composite exhibited excellent reversible capacities, with the values of 157, 134, 120, 102, and 94 mAh g^?1 at charge/discharge 0.2, 0.5, 1, 2, and 5C rates, respectively.     

Structural properties of composite cathode material LiFePO<Subscript>4</Subscript>–Li<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>

Composite cathode material LiFePO₄–Li₃V₂(PO₄)₃ is synthesized through a chemical reduction and lithiation using FeVO₄·xH₂O as both iron and vanadium sources. The structural properties of LiFePO₄–Li₃V₂(PO₄)₃ are investigated. X-ray diffraction results show the composite material containing olivine type LiFePO₄ and monoclinic Li₃V₂(PO₄)₃ phases. High-resolution transmission electron microscopy and energy-dispersive X-ray spectrometry results indicate that mutual doping effects take place between the LiFePO₄ and Li₃V₂(PO₄)₃ particles with V³⁺ doping the LiFePO₄ while Fe²⁺ dopes the Li₃V₂(PO₄)₃. LiFePO₄–Li₃V₂(PO₄)₃ nanocomposites are formed in the carbon webs. There is no structural compatibility between monoclinic (Li₃V₂(PO₄)₃) and olivine (LiFePO₄) domains in composite material LiFePO₄–Li₃V₂(PO₄)₃.     

Preparation of Li1.5Al0.5Ti1.5(PO4)3 solid electrolyte via a co-precipitation method

The co-precipitation method can make the materials react uniformly at molecular level and has the advantages of lower polycrystalline synthesized temperature and shorter sintering time. Therefore, it is expected that the mass production of Li_1.5Al_0.5Ti_1.5(PO₄)₃ (LATP) solid electrolyte would be possible by application of the co-precipitation method for LATP preparation. In this study, an application of the co-precipitation method for a preparation of LATP solid electrolyte is attempted. Crystallized LATP powder is obtained by heating precipitant containing Li, Al, Ti, and PO₄ at 800 °C for 30 min. The LATP bulk sintered pellet is successfully prepared using the crystallized LATP powder by calcinating at 1,050 °C. The cross-sectional SEM images show that many crystal grains exist, and the grains are in good contact with each other, i.e., there is no void space. All diffraction peaks of the pellet are attributed to LATP in XRD pattern. The sintered pellet is obtained by calcinating at 1,050 °C, which is more than 150 °C lower than that of conventional method. The LATP solid electrolyte shows a good conductivity which is 1.4?×?10^?3 S cm^?1 for bulk and 1.5?×?10^?4 S cm^?1 for total conductivities, respectively.     

Structural and electrochemical properties of LiFe1 − 3x/2Bi x PO4/C synthesized by sol–gel

In this study, LiFe_1???3x/2Bi _x PO₄/C cathode material was synthesized by sol–gel method. From XRD patterns, it was found that the Bi-doped LiFePO₄/C cathode material had the same olivine structure with LiFePO₄/C. SEM studies revealed that Bi doping can effectively decrease the particle sizes. It shortened Li⁺ diffusion distance between LiFePO₄ phase and FePO₄ phase. The LiFe_0.94Bi_0.04PO₄/C powder exhibited a specific initial discharge capacity of about 149.6 mAh g^?1 at 0.1 rate as compared to 123.5 mAh g^?1 of LiFePO₄/C. EIS results indicated that the charge-transfer resistance of LiFePO₄/C decreased greatly after Bi doping.     

Three-dimensionally ordered composite electrode between LiMn2O4 and Li1.5Al0.5Ti1.5(PO4)3

A novel electrode system composed of three-dimensionally ordered macroporous (3DOM) Li_1.5Al_0.5Ti_1.5(PO₄)₃ (LATP) and LiMn₂O₄ was fabricated by the colloidal crystal templating method and sol–gel process. A LATP nanoparticle for the fabrication of 3DOM-LATP was prepared by a sol–gel process. A suspension containing polystyrene (PS) beads and the LATP nanoparticles was filtrated by using a polycarbonate filter to accumulate PS beads and LATP. The accumulated PS beads had a close-packing structure, and the void between PS beads was filled with LATP nanoparticles. 3DOM-LATP was obtained by heat treatment of the accumulated composite. Li–Mn–O sol was injected by a vacuum impregnation process into the macropores of 3DOM-LATP and then was heated to form three-dimensionally ordered composite materials consisting of LiMn₂O₄ and LATP. The formation of the composite between 3DOM-LATP and LiMn₂O₄ were confirmed with scanning electron microscopy and X-ray diffraction method. The prepared composite electrode system exhibited a good electrochemical performance. Paper presented at the 11th EuroConference on the Science and Technology of Ionics, Batz-sur-Mer, Sept. 9–15, 2007.     

Laser‐induced phase changes in olivine FePO4: a warning on characterizing LiFePO4‐based cathodes with Raman spectroscopy

Raman spectroscopy is an excellent technique for probing lithium intercalation reactions of many diverse lithium ion battery electrode materials. The technique is especially useful for probing LiFePO₄‐based cathodes because the intramolecular vibrational modes of the PO₄³⁻ anions yield intense bands in the Raman spectrum, which are sensitive to the presence of Li⁺ ions. However, the high power lasers typically used in Raman spectroscopy can induce phase transitions in solid‐state materials. These phase transitions may appear as changes in the spectroscopic data and could lead to erroneous conclusions concerning the delithiation mechanism of LiFePO₄. Therefore, we examine the effect of exposing olivine FePO₄ to a range of power settings of a 532‐nm laser. Laser power settings higher than 1.3 W/mm² are sufficient to destroy the FePO₄ crystal structure and result in the formation of disordered FePO₄. After the laser is turned off, the amorphous FePO₄ compound crystallizes in the electrochemically inactive α‐FePO₄ phase. The present experimental results strongly suggest that the power setting of the excitation laser should be carefully controlled when using Raman spectroscopy to characterize fundamental lithium ion intercalation processes of olivine materials. In addition, Raman spectra of the amorphous intermediate might provide insight into the α‐FePO₄ to olivine FePO₄ phase transition that is known to occur at temperatures higher than 450 °C. Copyright © 2008 John Wiley & Sons, Ltd.     

Effect of carbon coating on the electrochemical performance of LiFePO<Subscript>4</Subscript>/C as cathode materials for aqueous electrolyte lithium-ion battery

Organic electrolyte is widely used for lithium-ion rechargeable batteries but might cause flammable fumes or fire due to improper use such as overcharge or short circuit. That weakness encourages the development of tools and materials which are cheap and environmental friendly for rechargeable lithium-ion batteries with aqueous electrolyte. Lithium iron phosphate (LiFePO₄) with olivine structure is a potential candidate to be used as the cathode in aqueous electrolyte lithium-ion battery. However, LiFePO₄ has a low electronic conductivity compared to other cathodes. Conductive coating of LiFePO₄ was applied to improve the conductivity using sucrose as carbon source by heating to 600 °C for 3 h on an Argon atmosphere. The carbon-coated LiFePO₄ (LiFePO₄/C) was successfully prepared with three variations of the weight percentage of carbon. From the cyclic voltammetry, the addition of carbon coatings could improve the stability of cell battery in aqueous electrolyte. The result of galvanostatic charge/discharge shows that 9 % carbon exhibits the best result with the first specific discharge capacity of 13.3 mAh g^?1 and capacity fading by 2.2 % after 100 cycles. Although carbon coating enhances the conductivity of LiFePO₄, excessive addition of carbon could degrade the capacity of LiFePO₄.     

Electrical and electrochemical behaviour of several LiFe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>PO<Subscript>4</Subscript> solid solutions as cathode materials for lithium ion batteries

Several olivine phosphates were investigated in the last years as cathode materials for secondary lithium ion batteries. Among these compounds, LiFe _x Co_1 − x PO₄ solid solutions might be interesting candidates because they should combine the high potential value of Co³⁺/Co²⁺ (higher than 4.5 V vs Li⁺/Li) with the relatively high charge–discharge rate of LiFePO₄. Solid solutions were prepared by solid-state route and characterised by X-ray powder diffraction, cyclic voltammetry, impedance spectroscopy and the Hebb–Wagner method. The results show that also low amount of iron induces high electronic conductivity in the solid solutions.     

Characterization of the surface of positive electrodes for Li-ion batteries using 7Li MAS NMR

The growth and evolution of the interphase, due to contact with the ambient atmosphere or electrolyte, are followed using ⁷Li magic-angle spinning nuclear magnetic resonance (MAS NMR) in the case of two materials amongst the most promising candidates for positive electrodes for lithium batteries: LiFePO₄ and LiMn_0.5Ni_0.5O₂. The use of appropriate experimental conditions to acquire the NMR signal allows observing only the «diamagnetic» lithium species at the surface of the grains of active material. The reaction of LiMn_0.5Ni_0.5O₂ with the ambient atmosphere or LiPF₆ (1 M in Ethylene Carbonated/DiMéthyl Carbonate (EC/DMC)) electrolyte is extremely fast and leads to an important amount of lithium-containing diamagnetic species compared to what can be observed in the case of LiFePO₄. The two studied materials display a completely different surface chemistry in terms of reactivity and/or kinetics of the surface towards electrolyte. Moreover, these results show that MAS NMR is a very promising tool to monitor phenomena taking place at the interface between electrode and electrolyte.     

Structural characteristics of olivine Li(Mg0.5Ni0.5)PO4 via TEM analysis

The structural characteristics of olivine-type lithium orthophosphate Li(Mg_0.5Ni_0.5)PO₄ synthesized via solid-state reaction have been studied using X-ray diffraction, ion beam technique, scanning electron microscopy, infrared spectroscopy, transmission electron microscopy and energy dispersive X-ray analysis. The parent LiNiPO₄ compound can be synthesized in olivine structure without any evidence of secondary phases as impurities. The structural quality of the parent LiNiPO₄ in the absence of secondary component phases resulted in the formation of hexagonal closed packed structure. The olivine analogue compound containing mixed M (M?=?Mg, Ni) cations, Li(Mg_0.5Ni_0.5)PO₄ contained Li₃PO₄ as a second phase upon synthesis, however a carbothermal reduction method produced a single-phase compound. The redox behaviour of carbon-coated Li(Mg_0.5Ni_0.5)PO₄ cathode in aqueous lithium hydroxide as the electrolyte showed reversible lithium intercalation.     

Characteristics of xLiFePO4·y Li3V2(PO4)3 electrodes for lithium batteries

A series of mechanical mixture of lithium–iron–vanadium–phosphate compositions that can be represented in two-component notation, xLiFePO₄·y Li₃V₂(PO₄)₃ (LFVP), has been evaluated as electrodes in lithium cells for x:y = 0:1, 1:1, 5:1, 10:1, and 1:0, in which an olivine component, LiFePO₄ (LFP), and a monoclinic component, Li₃V₂(PO₄)₃ (LVP), coexisted. Powder X-ray diffraction (XRD) patterns show that the end members and the electrochemical profiles of cells with these electrodes are consistent with those expected for the olivine LiFePO₄(x = 1, y = 0) and for monoclinic Li₃V₂(PO₄)₃ (x = 0, y = 1). XRD data and the changes of cell parameters indicate that there existed some V- and Fe-doping in the composite xLiFePO₄·y Li₃V₂(PO₄)₃, resulting with a good performance compared with single LiFePO₄ and Li₃V₂(PO₄)₃. Electrochemical characteristics were evaluated by using cyclic voltammetry and electrochemical impedance spectroscopy. The results show that the electron transfer activity and the lithium ion diffusion rate in LFVP are better than single LFP and LVP.     

Recent progress in the electrolytes for improving the cycling stability of LiNi<Subscript>0.5</Subscript>Mn<Subscript>1.5</Subscript>O<Subscript>4</Subscript> high-voltage cathode

High-voltage spinel LiNi_0.5Mn_1.5O₄ has been considered one of the most promising cathode materials for lithium-ion power batteries used in electrical vehicles (EVs) or hybrid electrical vehicles (HEVs) because the high voltage plateau at around 4.7 V makes its energy density (658 Wh kg^?1) 30 and 25 % higher than that of conventional pristine spinel LiMn₂O₄ (440 Wh kg^?1) or olivine LiFePO₄ (500 Wh kg^?1) materials, respectively. Unfortunately, LiNi_0.5Mn_1.5O₄-based batteries with LiPF₆-based carbonate electrolytes always suffer from severe capacity deterioration and poor thermostability because of the oxidization of organic carbonate solvents and decomposition of LiPF₆, especially at elevated temperatures and water-containing environment. The major goal of this review is to highlight the recent advancements in the development of advanced electrolytes for improving the cycling stability and rate capacity of LiNi_0.5Mn_1.5O₄-based batteries. Finally, an insight into the future research and further development of advanced electrolytes for LiNi_0.5Mn_1.5O₄-based batteries is discussed.     

Electrochemical behavior of Mg-doped 7LiFePO4–Li3V2(PO4)3 composite cathode material for lithium-ion batteries

Mg-doping effects on the electrochemical property of LiFePO₄–Li₃V₂(PO₄)₃ composite materials, a mutual-doping system, are investigated. X-ray diffraction study indicates that Mg doping decreases the cell volume of LiFePO₄ in the composite. The cyclic voltammograms reveal that the reversibility of the electrode reaction and the diffusion of lithium ion is enhanced by Mg doping. Mg doping also improves the conductivity and rate capacity of 7LiFePO₄–Li₃V₂(PO₄)₃ composite material and decreases the polarization of the electrode reaction. The discharge capacity of the Mg-doped composite was 93 mAh?g^?1 at the current density of 1,500 mA?g^?1, and Mg-doped composite has better discharge performance than the original 7LiFePO₄–Li₃V₂(PO₄)₃ composite at low temperature, too. At ?30 °C, the discharge capacity of Mg-doped LFVP is 89 mAh?g^?1, higher than that of the original composite. Electrochemical impedance spectroscopy study shows that Mg²⁺ doping could enhance the electrochemical activity of 7LiFePO₄–Li₃V₂(PO₄)₃ composite. Mg doping has a positive influence on the electrochemical performance of the LiFePO₄–Li₃V₂(PO₄)₃ composite material.     

Synthesis and characterization of LiFePO4/3-dimensional carbon nanostructure composites as possible cathode materials for Li-ion batteries

Composites of three-dimensional (3D) carbon nanostructures coated with olivine-structured lithium iron phosphates (LiFePO₄) as cathode materials for lithium ion batteries have been prepared through a Pechini-assisted reversed polyol process for the first time. The coating has been successfully performed on nonfunctionalized commercially available 3D carbon used as catalysts. Thermal analysis revealed no phase transitions till crystallization occurred at 579 °C. Morphological investigation of the prepared composites showed a very good quality of the coating on the 3D carbon structures. A great enhancement of the crystallinity of the olivine structure and of the composites was revealed by the structural investigation performed on pure LiFePO₄ and composites after annealing at 600 °C for 10 h under nitrogen atmosphere. The cyclic voltammetry curves of the composites show well-defined peaks and smaller value of the polarization overpotential indicating an enhancement of electrode reaction reversibility compared to the LiFePO₄ phase.     

Electrochemical performance of Mo-doped LiFePO4/C composites prepared by two-step solid-state reaction

To improve the performance of LiFePO₄, LiFe_1?x Mo _x PO₄/C (x?=?0, 0.005, 0.010, 0.015, 0.020, 0.025) cathode materials were synthesized via two-step ball milling solid-state reaction. The prepared samples were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy, cyclic voltammetry, electrochemical impedance spectra, and galvanostatic charge–discharge test. It is apparent from XRD analysis that Mo doping enlarges the interplanar distance of crystal plane parallel to [010] direction in LiFePO₄. In other words, it widens one-dimensional diffusion channels of Li⁺ along the [010] direction. The results of electrochemical test indicate that the LiFe_0.99Mo_0.01PO₄/C composite exhibits a discharge capacity of 144.8 mAh g^?1 at 1 C rate, a decreased charge transfer resistance of 162.4 Ω and better reversibility of electrode reactions. The present synthesis route is promising and practical for the preparation of LiFePO₄ materials.     

In-situ synchrotron X-ray absorption studies of LiMn0.25Fe0.75PO4 as a cathode material for lithium ion batteries

Recently, lithium bi-metal phosphates (LiM′M″PO₄) have been synthesized for use as cathode materials in order to increase cell voltages and electrical performances. In this work, we have substituted Mn²⁺ at the 4c site of LiFePO₄ to prepare the lithium bi-metal phosphate LiMn_0.25Fe_0.75PO₄ and have found that it greatly enhances the cell voltage. At a 0.05 C discharge rate, the cell capacity was about 153 mAhg^− 1 and the average working voltage rose to 3.53 V due to the Mn substitution. However, the capacity and working voltage both decrease as the discharge rate increases. By in-situ metal K-edge absorption analysis, it reveals that the substituted metal Mn²⁺ does not work completely at a higher discharge rate, due to poor electrical conductivity and a serious Jahn–Teller effect.     

Enhancement of capacity at high charge/discharge rate and cyclic stability of LiFePO4/C by nickel doping

By introducing nickel chemical into the precursor sol of LiFePO₄, a series of Ni-doped LiFePO₄ composite cathode materials, denoted as LiFe_1???x Ni _x PO₄/C (x?=?0, 0.01, 0.03, 0.05 and 0.10) were prepared by a spray drying–carbothermal approach. The materials were characterized with X-ray diffraction (XRD), scanning electron microscope (SEM), and electrochemical impedance spectrum etc. It is found that the doping of nickel with appropriate amount caused a slight shift of diffraction peaks towards higher angles and enhanced the dispersion of nanoprimary particles, which could be observed from their XRD patterns and SEM images. For the sample with 3 mol% Ni doing, the charge transfer resistance reduced from 52.4?Ω of LiFePO₄ to 18.7?Ω of LiFe_0.97Ni_0.3PO₄/C, and the potential interval of the redox peaks reduced from 0.51 to 0.40 V, indicating the better reversible of Ni-doped materials. For the sample LiFe_0.97Ni_0.03PO₄/C, its initial discharge capacities at various rates are 169.2 (0.2 C), 156.2 (1.0 C), 147.9 (2.0 C), 135.5 (5.0 C), and 94.0 (10.0 C)?mAh g^?1, respectively, enhanced by 55.2 % (at 5.0 C) and 82.1 % (at 10.0 C) compared with LiFePO₄. Furthermore, after 200 cycles of charge/discharge at 0.5 C, the capacity of LiFe_0.97Ni_0.03PO₄/C only decreased 8.8 %, but over 25 % decrease was observed for LiFePO₄/C.     

Mechanism of LiFePO4 solid-phase synthesis using iron (II) oxalate and ammonium dihydrophosphate as precursors

An experimental study of the thermolysis mechanism of FeC₂O₄, NH₄H₂PO₄, Li₂CO₃, and citric acid from the viewpoint of the usage of a mixture of these compounds in lithium power engineering for the solid-state synthesis of LiFePO₄ and its composite with carbon LiFePO₄/C as well as comparison of experimental data with thermodynamic calculations were made in the temperature range from 25 up to 1,000 °C. The oxides Fe₃O₄, Fe₂O₃, and FeO were detected as the intermediate products of thermolysis of ferrous oxalate in these conditions. Various paths of oxalate decomposition may well proceed concurrently with the predomination of this or that path under slight changes in the experimental conditions. The formation of orthorhombic lithium phosphate Li₃PO₄ is detected just in a blend grinded at room temperature, and Li₃PO₄ and NH₄PO₃ are the basis of triphylite synthesis at increased temperatures (up to 800 °C). A new phase of single-substituted anhydrous lithium citrate C₆H₇O₇Li is formed at room temperature if citric acid C₆H₈O₇?H₂O is used as an organic precursor. The thermal treatment, at which citric acid can form a carbon coating with a maximum conductivity, was estimated experimentally. To identify the products of chemical reactions, structural characterization, and comparative analysis of samples synthesized at several temperatures, a set of techniques was used, namely TG with gas release analysis, Mossbauer spectroscopy, X-ray diffraction, transmission electron microscopy, scanning electron microscopy, surface microanalysis, laser diffraction analyses. Galvanostatic cycling was used to study the electrochemical properties of the LiFePO₄/C electrode material.     

Effects of heating rate on morphology and performance of lithium iron phosphate synthesized by hydrothermal route in organic-free solution

Flake-like LiFePO₄ were hydrothermally synthesized in an organic-free solution at heating rates of 0.5, 1.5, 3, and 5 °C min^?1. The heating rate has a marked influence on crystal morphology but scarcely on phase purity. The reason for morphology variations is discussed based upon the solubility of precursors Li₃PO₄ and Fe₃(PO₄)₂·8H₂O. The optimum heating rate for hydrothermal synthesis of LiFePO₄ is 3 °C min^?1. The as-synthesized material exhibits a high specific capacity, excellent rate capability, good low-temperature performance and Li⁺ diffusivity after carbon coating, all of which could be ascribed to shortened Li⁺ diffusion distance and higher crystallization degree of the crystalline.     

Effects of Li2CO3 as a secondary lithium source on the LiFePO4/C composites prepared via solid-state method

Li₂CO₃ was used as the secondary lithium source for the synthesis of LiFePO₄/C composites via a solid-state reaction method by adopting Li₃PO₄ as the main lithium source. The main purpose of using Li₂CO₃ is to compensate for the partial lithium loss during the sintering while reducing the usage of excess Li₃PO₄. In this study, the effects of Li₂CO₃ amount on the phase, structural and electrochemical properties of LiFePO₄/C material were systematically investigated. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), constant-current charge–discharge test and cyclic voltammetry (CV). The results showed that by adding an appropriate amount of Li₂CO₃, the impurities, e.g. Li₃PO₄, normally appearing in the final product, could be excluded. It was found that LiFePO₄/C with Li₂CO₃ in 6% excess (vs. stoichiometric LiFePO₄) exhibited the best electrochemical performance, which delivered initial discharge capacities of 141.7, 125.2, 119.9 and 108.9 mAh g^?1, respectively, at 0.5, 1, 2 and 5C rates. The capacity was reduced to 113.4 mAh g^?1 after 50 cycles at 2C rate, with capacity retention rate of 94.6%.