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.     

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.     

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.     

Li3V2(PO4)3 modified LiFePO4/C cathode materials with improved high-rate and low-temperature properties

LiFePO₄/C surface modified with Li₃V₂(PO₄)₃ is prepared with a sol–gel combustion method. The structure and electrochemical behavior of the material are studied using a wide range of techniques such as X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope, galvanostatic charge–discharge, and electrochemical impedance spectroscopy. It is found that LiFePO₄/C surface modified with Li₃V₂(PO₄)₃ has the better electrochemical performance. The discharge capacity of the as-prepared material can reach up to 153.1, 137.7, 113.6, and 93.3 mAh g^?1 at 1, 2, 5, and 10 C, respectively. The capacitance of the LiFePO₄/C modified by Li₃V₂(PO₄)₃ is higher under lower discharging rate at ?20 °C, and the initial discharge capacity of 0.2 C is 131.4 mAh g^?1. It is also demonstrated that the presence of Li₃V₂(PO₄)₃ in the sample can reduce the charge transfer resistance in the range of ?20 to 25 °C, resulting in the enhanced electrochemical catalytic activity.     

A PEG-assisted rheological phase reaction synthesis of 5LiFePO<Subscript>4</Subscript>⋅Li<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>/C as cathode material for lithium ion cells

5LiFePO₄⋅Li₃V₂(PO₄)₃/C composite cathode material is synthesized by a polyethylene glycol (PEG)-assisted rheological phase method. As a surfactant and dispersing agent, PEG can effectively inhabit the aggregation of colloidal particles during the formation of the gel. Meanwhile, PEG will coat on the particles to play the role of carbon source during the sintering. The samples are characterized by X-ray diffraction (XRD), scanning electron microscopy, and electrochemical methods. XRD results indicate that the 5LiFePO₄⋅Li₃V₂(PO₄)₃/C composites are well crystallized and contain olivine-type LiFePO₄ and monoclinic Li₃V₂(PO₄)₃ phases. The composite synthesized at 650 °C exhibits the initial discharge capacities of 134.8 and 129.9 mAh g⁻¹ and the capacity retentions of 96.2 and 97.1 % after 50 cycles at 1C and 2C rates, respectively.     

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.     

Investigations on the synthesis and electrochemical performance of Li<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>/C by different methods

Li₃V₂(PO₄)₃/C samples were synthesized by two different synthesis methods. Their influence on electrochemical performances of Li₃V₂(PO₄)₃/C as cathode materials for lithium-ion batteries was investigated. The structure and morphology of Li₃V₂(PO₄)₃/C samples were characterized by X-ray diffraction and scanning electron microscopy. Electrochemical performance was characterized by charge/discharge, cyclic voltammetry, and alternating current (AC) impedance measurements. Li₃V₂(PO₄)₃/C with smaller grain size showed better performances in terms of the discharge capacity and cycle stability. The improved electrochemical properties of the Li₃V₂(PO₄)₃/C were attributed to the decreasing grain size and enhanced electrical conductivity produced via low temperature route. AC impedance measurements also showed that the Li₃V₂(PO₄)₃/C synthesized by low temperature route significantly decreased the charge-transfer resistance and shortened the migration distance of lithium ion.     

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%.     

Electrochemical performance of surfactant-processed LiFePO4 as a cathode material for lithium-ion rechargeable batteries

This study focuses on the effect of addition of surfactant as a dispersing agent during vibratory ball milling of LiFePO₄ (LFP) precursor materials on the electrochemical performance of solid-state reaction synthesized LFP for lithium-ion battery cathode material. LFP particles formed after calcinations of ball milled LFP precursors (Li₂CO₃, FeC₂O₄, and NH₄H₂PO₄) showed better size uniformity, morphology control, and reduced particle size when anionic surfactant (Avanel S-150) was used. The specific surface area of LFP particles increased by approximately twofold on addition of surfactant during milling. These particles showed significantly enhanced cyclic performance during charge/discharge due to a reduced polarization of electrode material. Electrodes fabricated from LFP particles by conventional milling process showed a 22 % decrease in capacity after 50 cycles, whereas the performance of electrode prepared by surfactant processed LFP showed only 3 % loss in capacity. The LFP particles were characterized using XRD, FE-SEM, particle size distribution, density measurement, and BET-specific surface area measurement. Electrochemical impedance spectra and galvanostatic charge/discharge test were performed for the electrochemical performance using coin-type cell.     

Electrochemical properties of Li<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>/C cathode materials synthesized via ethylene glycol-assisted solvothermal method

The Li₃V₂(PO₄)₃/C (LVP/C) cathode materials for lithium-ion batteries were synthesized via ethylene glycol-assisted solvothermal method. The phase composition, phase transition temperature, morphology, and fined microstructure were studied using X-ray diffraction (XRD), differential thermal analyzer (DTA), scanning electron microscope (SEM), and transmission electron microscope (TEM), respectively. The electrochemical properties, impedance, and electrical conductivity of LVP/C cathode materials were tested by channel battery analyzer, the electrochemical workstation, and the Hall test system, respectively. The results shown that the appropriate amount of water added to ethylene glycol solvent contributes to the synthesis of pure phase LVP. The LVP₁₀/C cathode material can exhibit discharge capacities of 128, 126, 126, 123, 124, and 114 mAh g^?1 at 0.1, 0.5, 2, 5, 10, and 20 C in the voltage range of 3.0–4.3 V, respectively. Meanwhile, it shows also a stable cycling performance with the capacity retention of 89.6% after 180 cycles at 20 C.     

Synthesis of flower-like Li3V2(PO4)3/C cathode with mixed morphology for advanced lithium-ion batteries

A rheological phase-assisted ball milling method was developed to synthesize of flower-like Li₃V₂(PO₄)₃/C composites consisting of nanofibers and nanoplate porous microstructure. The flower-like Li₃V₂(PO₄)₃/C composite delivered specific capacities of 120 and 108 mAh g^?1 at 0.5 and 10 C rates, respectively. A capacity retention of 99.5 % was sustained after 100 cycles at a 10-C cycling rate. The remarkable performance was attributed to the porous nanostructures that provide short electron/ion diffusion distance and large electrode/electrolyte contact area.     

Preparation and characterization of Li3V2(PO4)3/MWCNTs cathode material for lithium-ion batteries

The Li₃V₂(PO₄)₃/multiwalled carbon nanotubes (LVP/MWCNTs) composite is successfully synthesized by a sol?Cgel route using oxalic acid as the chelating reagent. Its structure and physicochemical properties are investigated using X-ray diffraction, field-emission scanning electron microscopy, and electrochemical methods. LVP particles are well mixed with MWCNTs, and most of them are around 100?nm. The galvanostatic charge?Cdischarge tests show that LVP/MWCNTs electrode owns an initial discharge capacity of 126?mAh?g^?1 at 0.5 C with capacity retention of 94% during the 100th cycle in the voltage range of 3.0?C4.3?V. A superior rate capability is also achieved, e.g., exhibiting discharge capacities of 75 and 58?mAh?g^?1 at high C rates of 10 and 15 C, respectively.     

Synthesis of high performance Li<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>/C cathode material by ultrasonic-assisted sol–gel method

A novel ultrasonic-assisted sol–gel method is proposed to prepare Li₃V₂(PO₄)₃/C cathode material. X-ray diffraction analyses show that both Li₃V₂(PO₄)₃/C(A) synthesized by the ultrasonic-assisted sol–gel method and Li₃V₂(PO₄)₃/C(B) synthesized by a traditional sol–gel method have monoclinic structure. Scanning electron microscopy images indicate that the Li₃V₂(PO₄)₃/C(A) composite has a more uniform morphology than that of the Li₃V₂(PO₄)₃/C(B) composite. In the voltage range of 3.0–4.3 V (vs. Li/Li⁺), the initial specific discharge capacities of the Li₃V₂(PO₄)₃/C(A) and Li₃V₂(PO₄)₃/C(B) samples are 129.8 and 125.9 mAh g⁻¹ at 1C rate (1C = 133 mA g⁻¹), respectively. Furthermore, at 2-C charge/10-C discharge rate, the specific discharge capacity of the Li₃V₂(PO₄)₃/C(A) composite retains 113.2 mAh g⁻¹ after 50 cycles, but the Li₃V₂(PO₄)₃/C(B) composite only presents a capacity of 94.8 mAh g⁻¹.     

热聚合法制备纳米级Li3V2(PO4)3/C作为锂离子电池正极材料的性能研究

通过热聚合法成功制备出纳米级Li₃V₂（PO₄）₃/C正极材料,中间产物和最终材料的Li₃V₂（PO₄）₃/C颗粒均小于200 nm,无定形碳的含量为4.6%,处于Li₃V₂（PO₄）₃颗粒表面和颗粒与颗粒之间.该材料在3.0sim4.3 V和0.1 C电流下放电比容量为124 mAh/g,100次循环之后无衰减,表现出较好的循环性能.其倍率性能优异,在3.0sim4.3 V和20 C的条件下放电比容量达到80 mAh/g,在3.0sim4.8 V和10 C的条件下放电比容量达到100 mAh/g.     

Synthesis of Li3V2(PO4)3/reduced graphene oxide cathode material with high-rate capability

The Li₃V₂(PO₄)₃/reduced graphene oxide (LVP/rGO) composite is successfully synthesized by a conventional solid-state reaction with a high yield of 10 g, which is suitable for large-scale production. Its structure and physicochemical properties are investigated using X-ray diffraction, Raman spectra, field-emission scanning electron microscopy, transmission electron microscopy, and electrochemical methods. The rGO content is as low as ~3 wt%, and LVP particles are strongly adhered to the surface of the rGO layer and/or enwrapped into the rGO sheets, which can facilitate the fast charge transfer within the whole electrode and to the current collector. The galvanostatic charge–discharge tests show that the LVP/rGO electrode delivers an initial discharge capacity of 177 mAh g^?1 at 0.5 C with capacity retention of 88 % during the 50th cycle in a wide voltage range of 3.0–4.8 V. A superior rate capability is also achieved, e.g., exhibiting discharge capacities of 137 and 117 mAh g^?1 during the 50th cycle at high C rates of 2 and 5 C, respectively.     

Structural and electrical characterization of the NASICON-type Li2FeZr(PO4)3 and Li2FeTi(PO4)3 compounds

In order to develop new electrolytes for all-solid-state rocking chair lithium batteries, the NASICON-type compounds Li₂FeZr(PO₄)₃ and Li₂FeTi(PO₄)₃ were investigated by powder X-ray diffraction technique and impedance spectroscopy. Li₂FeZr(PO₄)₃ is orthorhombic Pbna (a=8.706(3), b=8.786(2), c=12.220(5) Å) and Li₂FeTi(PO₄)₃ is orthorhombic Pbca (a=8.557(3), b=8.624(3), c=23.919(6) Å). They show no phase transitions from RT to 800 °C. In the same temperature range logσT vs. 1/T show no slope variations. The activation energies for the ionic conductivity were 0.62 and 0.64 eV for Li₂FeTi(PO₄)₃ and Li₂FeTi(PO₄)₃, respectively. In order to better evaluate the present results they were compared with those of α and β-LiZr₂(PO₄)₃ phases, which were also prepared and characterised. A change of activation energy from 0.47 eV to 1.03 eV was observed in the case of β phase, at about 300 °C; attributed to the β (orthorhombic) ? β′ (monoclinic) phase transition. In the α phase the activation energy 0.47 eV in the temperature range 150 – 850 °C. The Li₂FeZr(PO₄)₃ and Li₂FeTi(PO₄)₃ compounds can be interesting for applications as solid electrolytes in high temperature (>300 °C) lithium batteries.     

Enhanced electrochemical performance of lithium vanadium phosphate as cathode by LiBOB/LiTFSI with additive in EC/EMC

A series of Li₃V₂(PO₄)₃/C (LVP/C) samples with monoclinic structure indexed to P2₁/n space group were synthesized using V₂O₃ as vanadium source by solid state reaction method by different sintering temperatures. It was found that the LVP/C sintered at 750 °C with a carbon content 3 wt.% was the optimum condition for this synthesis. The structural, morphological, superficial, and textural properties of LVP/C were characterized by XRD, SEM, TEM, and XPS. The electrochemical performance was evaluated by galvanostatic charge–discharge cycling using new high voltage electrolyte. The optimized cell delivered an initial discharge capacity of 187 mAh g^?1 in the higher cut-off voltage of 3.0–4.8 V vs. Li⁺/Li⁰ at 0.2 C rate, with a capacity retention of 88 %, 89 %, and 61 % after 50 cycles discharging at 1 C, 2 C, and 4 C, respectively. The capacity can be almost recovered at 0.5 C after long cycles. The excellent stability is contributed to the new high-voltage electrolyte.     

Enhanced electrochemical performance of lithium ion battery cathode Li<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>/C

Li-ion battery cathode material lithium-vanadium-phosphate Li₃V₂(PO₄)₃ was synthesized by a carbon-thermal reduction method, using stearic acid, LiH₂PO₄, and V₂O₅ as raw materials. And stearic acid acted as reductant, carbon source, and surface active agent. The effect of its content on the crystal structure and electrochemical performance of Li₃V₂(PO₄)₃/C were characterized by XRD and electrochemical performance testing, respectively. The results showed that the content of carbon source has no significant effect on the crystal structure of lithium vanadium phosphate. Lihtium vanadium phosphate obtained with 12.3% stearic acid demonstrated the best electrochemical properties with a typical discharge capacity of 119.4 mAh/g at 0.1 C and capacity retention behavior of 98.5% after 50 cycles. And it has high reversible discharge capacity of 83 mAh/g at 5 C with the voltage window of 3 to 4.3 V.     

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.     

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.