Electrochemical performance of carbon-coated Li3V2(PO4)3 cathode materials derived from polystyrene-based carbon-thermal reduction synthesis

Li₃V₂(PO₄)₃ cathode materials were synthesized by a simple carbon-thermal reduction method using polystyrene as a carbon source. The residual carbon produced by the pyrolysis of polystyrene produced fine particle sizes and uniform carbon distribution on the Li₃V₂(PO₄)₃ particle surface. By increasing the amount of polystyrene added in the range of 0-16 wt.%, the thickness of the carbon coating increased, and the coating thickness was found to influence the electrochemical performance of the Li₃V₂(PO₄)₃ significantly. Our results indicate that the 6 wt.% polystyrene added Li₃V₂(PO₄)₃ with a 0.5-1 nm thick carbon coating possesses the highest initial discharge capacity of 132.7 mAh g⁻¹ between 3.0 and 4.3 V at 0.1 C. However, at high current densities, the higher polystyrene added Li₃V₂(PO₄)₃/C with a thicker carbon coating shows better performance in terms of the discharge capacity and cycling stability than that with the thin carbon coating. The improved cycling performance at higher current densities is attributed to the relatively small particle size and the suppressed impedance increase because of the thicker carbon coating.     

Synthesis and characterization of carbon-coated Li3V2(PO4)3 cathode materials with different carbon sources

The carbon-coated monoclinic Li₃V₂(PO₄)₃ (LVP) cathode materials were synthesized by a solid-state reaction process under the same conditions using citric acid, glucose, PVDF and starch, respectively, as both reduction agents and carbon coating sources. The carbon coating can enhance the conductivity of the composite materials and hinder the growth of Li₃V₂(PO₄)₃ particles. Their structures and physicochemical properties were investigated using X-ray diffraction (XRD), thermogravimetric (TG), scanning electron microscopy (SEM) and electrochemical methods. In the voltage region of 3.0-4.3 V, the electrochemical cycling of these LVP/C electrodes all presents good rate capability and excellent cycle stability. It is found that the citric acid-derived LVP owns the largest reversible capacity of 118 mAh g⁻¹ with no capacity fading during 100 cycles at the rate of 0.2C, and the PVDF-derived LVP possesses a capacity of 95 mAh g⁻¹ even at the rate of 5C. While in the voltage region of 3.0-4.8 V, all samples exhibit a slightly poorer cycle performance with the capacity retention of about 86% after 50 cycles at the rate of 0.2C. The reasons for electrochemical performance of the carbon coated Li₃V₂(PO₄)₃ composites are also discussed. The solid-state reaction is feasible for the preparation of the carbon coated Li₃V₂(PO₄)₃ composites which can offer favorable properties for commercial applications.     

Effects of synthetic route on structure and electrochemical performance of Li3V2(PO4)3/C cathode materials

Three different synthetic routes, including solid-state reaction, sol–gel and hydrothermal methods are successfully used for preparation of Li₃V₂(PO₄)₃/C. Ascorbic acid is used as a reducing agent and/or as a chelating agent. The Li₃V₂(PO₄)₃/C synthesized by hydrothermal method with fine particles exhibits lower impedance and smaller potential difference values between oxidation and reduction peaks than those by solid-state reaction and sol–gel methods. Thus as cathode material for Li-ion batteries, the Li₃V₂(PO₄)₃/C synthesized by hydrothermal method shows higher discharge capacity, better rate capability and cyclic performance. Even at a high charge–discharge rate of 10 C, it still can deliver a discharge capacity of 101.4 mAh g⁻¹ and 106.6 mAh g⁻¹ in the potential range of 3.0–4.3 V and 3.0–4.8 V, respectively. The hydrothermal synthesis has been considered to be a competitive process to prepare Li₃V₂(PO₄)₃/C cathode materials with excellent electrochemical performances.     

Effect of reduction agent on the performance of Li3V2(PO4)3/C positive material by one-step solid-state reaction

The effects of reduction agent on the electrochemical properties of Li₃V₂(PO₄)₃/C positive materials were studied. An one-step solid-state reaction route was applied to synthesize Li₃V₂(PO₄)₃/C samples. The humic acid, glucose and carbon were used as reduction agent. SEM images show that the particles of sample with humic acid as reduction agent merge with each other and form a porous structure, and yet the particles of sample synthesized using carbon as reduction agent are wrapped with small carbon particles and separate each other. Electrochemical tests show that the samples using humic acid and glucose as reduction agents have better cyclic performance than the one using carbon as reduction agent. At the 200th cycle, the sample using humic acid as reduction agent still keeps 145.2 mAh g⁻¹ at 1 C charge and discharge rates. However, the sample using carbon as reduction agent shows a fast decline in capacity during cycling and has 54.5% capacity loss of the initial value after 200 cycles. The results indicate that different reduction agents can obviously affect the morphologies and electrochemical properties of the products.     

Layered monodiphosphate Li9V3(P2O7)3(PO4)2: A novel cathode material for lithium-ion batteries

Single phase Li₉V₃(P₂O₇)₃(PO₄)₂ is synthesized at 750 °C via solid-state reaction method for the first time. The Rietveld refinement results show that the trigonal system (space group: ) with the lattice parameters a = 0.9724 nm, c = 1.3596 nm are obtained. Its intrinsic electrical conductivity of 1.43 × 10⁻⁸ S cm⁻¹ is higher than that of LiFePO₄ and as the same order of Li₃V₂(PO₃)₄. The electrochemical measurement results show that there are two plateaus (3.77 V and 4.51 V) and three plateaus (3.77 V, 4.51 V and 4.75 V) in the potential ranges of 2.0–4.6 V and 2.0–4.8 V, respectively. In the range of 2.0–4.6 V, two discharge plateaus (4.46 V and 3.74 V) can be observed and 110 mAh g⁻¹ of discharge capacity is achieved. The Rietveld refinement result of the X-ray diffraction (XRD) data at the end of discharge after the first cycle suggests that the structural reversibility can be retained during electrochemical reactions in Li₉V₃(P₂O₇)₃(PO₄)_2. In the range of 2.0–4.8 V, almost six lithium ions are extracted and the trigonal structure is still recovered after 30 cycles. Therefore, this novel layered vanadium monodiphosphate offers a promising candidate as cathode material for lithium-ion batteries.     

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.     

Structure and electrochemical properties of nanocarbon-coated Li3V2(PO4)3 prepared by sol-gel method

A liquid-based sol-gel method was developed to synthesize nanocarbon-coated Li₃V₂(PO₄)₃. The products were characterized by XRD, SEM and electrochemical measurements. The results of Rietveld refinement analysis indicate that single-phase Li₃V₂(PO₄)₃ with monoclinic structure can be obtained in our experimental process. The discharge capacity of carbon-coated Li₃V₂(PO₄)₃ was 152.6 mAh/g at the 50th cycle under 1C rate, with 95.4% retention rate of initial capacity. A high discharge capacity of 184.1 mAh/g can be obtained under 0.12C rate, and a capacity of 140.0 mAh/g can still be held at 3C rate. The cyclic voltammetric measurements indicate that the electrode reaction reversibility is enhanced due to the carbon-coating. SEM images show that the reduced particle size and well-dispersed carbon-coating can be responsible for the good electrochemical performance obtained in our experiments.     

Analysis of the chemical diffusion coefficient of lithium ions in Li3V2(PO4)3 cathode material

The chemical diffusion coefficients of lithium ions (DLi⁺) in Li₃V₂(PO₄)₃ between 3.0 and 4.8 V are systematically determined by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT). The DLi⁺ values are found to be dependent on the voltage state of charge and discharge. Based on the results from all the three techniques, the true diffusion coefficients measured in single-phase region are in the range of 10⁻⁹ to 10⁻¹⁰ cm² s⁻¹. Its apparent diffusion coefficients measured in two-phase regions by CV and GITT range from 10⁻¹⁰ to 10⁻¹¹ cm² s⁻¹ and 10⁻⁸ to 10⁻¹³ cm² s⁻¹, respectively, depending on the potentials. By the GITT, the DLi⁺ varies non-linearly in a “W” shape with the charge-discharge voltage, which is ascribed to the strong interactions of Li⁺ with surrounding ions. Finally, the chemical diffusion coefficients of lithium ions measured by CV, EIS and GITT are compared to each other.     

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.     

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.     

Electrochemical performance of Li3V2(PO4)3/C cathode materials using stearic acid as a carbon source

The Li₃V₂(PO₄)₃/C cathode materials are synthesized by a simple solid-state reaction process using stearic acid as both reduction agent and carbon source. Scanning electron microscopy and transmission electron microscopy observations show that the Li₃V₂(PO₄)₃/C composite synthesized at 700 °C has uniform particle size distribution and fine carbon coating. The Li₃V₂(PO₄)₃/C shows a high initial discharge capacity of 130.6 and 124.4 mAh g⁻¹ between 3.0 and 4.3 V, and 185.9 and 140.9 mAh g⁻¹ between 3.0 and 4.8 V at 0.1 and 5 C, respectively. Even at a charge–discharge rate of 15 C, the Li₃V₂(PO₄)₃/C still can deliver a discharge capacity of 103.3 and 112.1 mAh g⁻¹ in the potential region of 3.0–4.3 V and 3.0–4.8 V, respectively. Based on the analysis of cyclic voltammograms and electrochemical impedance spectra, the apparent diffusion coefficients of Li ions in the composites are in the region of 1.09 × 10⁻⁹ and 4.95 × 10⁻⁸ cm² s⁻¹.     

Effect of Sn doping on the electrochemical performance of NaTi2(PO4)3/C composite

In this paper, NaTi_2-xSn_x(PO₄)₃/C (x?=?0.0, 0.2, 0.3, and 0.4) composites were fabricated via facile sol-gel method, and employed as anodes for aqueous lithium ion batteries. Effect of Sn doping with various content on electrochemical properties of NaTi₂(PO₄)₃/C was investigated systematically. Sn doping on Ti site has no obvious effect on the lattice structure and morphology of NaTi₂(PO₄)₃/C. Among all samples, NaTi_1.7Sn_0.3(PO₄)₃/C (NC-Sn-3) demonstrates the best electrochemical properties. NC-Sn-3 exhibits the outstanding rate performance, delivering a discharge capacity of 103.3, 95.2, and 87.4?mAh?g^?1 at 0.5, 7, and 20?°C, respectively, 1.7, 30.5, and 56.2?mAh?g^?1 larger than those of pristine NaTi₂(PO₄)₃/C. In addition, NC-Sn-3 shows excellent cycling performance with the capacity retention of 80.6% after 1000 cycles at 5?°C. This work reveals that Sn doped NaTi₂(PO₄)₃/C with outstanding electrochemical properties are potential anode for aqueous lithium ion batteries.     

Low temperature solid-state synthesis routine and mechanism for Li3V2(PO4)3 using LiF as lithium precursor

Monoclinic lithium vanadium phosphate, Li₃V₂(PO₄)₃, has been successfully synthesized using LiF as lithium source. The one-step reaction with stoichiometric composition and relative lower sintering temperature (700 °C) has been used in our experimental processes. The solid-state reaction mechanism using LiF as lithium precursor has been studied by X-ray diffraction and Fourier transform infrared spectra. The Rietveld refinement results show that in our product sintered at 700 °C no impurity phases of VPO₄, Li₅V(PO₄)₂F₂, or LiVPO₄F can be detected. The solid-state reaction using Li₂CO₃ as Li-precursor has also been carried out for comparison. X-ray diffraction patterns indicate that impurities as Li₃PO₄ can be found in the product using Li₂CO₃ as Li-precursor unless the sintering temperatures are higher than 850 °C. An abrupt particle growth (about 2 μm) has also been observed by scanning electron microscope for the samples sintered at higher temperatures, which can result in a poor cycle performance. The product obtained using LiF as Li-precursor with the uniform flake-like particles and smaller particle size (about 300 nm) exhibits the better performance. At the 50th cycle, the reversible specific capacities for Li₃V₂(PO₄)₃ measured between 3 and 4.8 V at 1C rate are found to approach 147.1 mAh/g (93.8% of initial capacity). The specific capacity of 123.6 mAh/g can even be hold between 3 and 4.8 V at 5C rate.     

Preparation and electrochemical performance studies on Cr-doped Li3V2(PO4)3 as cathode materials for lithium-ion batteries

Cr-doped Li₃V_2−xCr_x(PO₄)₃/C (x = 0, 0.05, 0.1, 0.2, 0.5, 1) compounds have been prepared using sol–gel method. The Rietveld refinement results indicate that single-phase Li₃V_2−xCr_x(PO₄)₃/C with monoclinic structure can be obtained. Although the initial specific capacity decreased with Cr content at a lower current rate, both cycle performance and rate capability have excited improvement with moderate Cr-doping content in Li₃V_2−xCr_x(PO₄)₃/C. Li₃V_1.9Cr_0.1(PO₄)₃/C compound presents an initial capacity of 171.4 mAh g⁻¹ and 78.6% capacity retention after 100 cycles at 0.2C rate. At 4C rate, the Li₃V_1.9Cr_0.1(PO₄)₃/C can give an initial capacity of 130.2 mAh g⁻¹ and 10.8% capacity loss after 100 cycles where the Li₃V₂(PO₄)₃/C presents the initial capacity of 127.4 mAh g⁻¹ and capacity loss of 14.9%. Enhanced rate and cyclic capability may be attributed to the optimizing particle size, carbon coating quality, and structural stability during the proper amount of Cr-doping (x = 0.1) in V sites.     

Aluminothermal synthesis and characterization of Li3V2−xAlx(PO4)3 cathode materials for lithium ion batteries

Monoclinic Li₃V_2−xAl_x(PO₄)₃ with different Al³⁺ doping contents (x = 0, 0.05, 0.08, 0.10 and 0.12) have been prepared by a facile aluminothermal reaction. Aluminum nanoparticles have been used as source for Al³⁺ and nucleus for Li₃V_2−xAl_x(PO₄)₃ nucleation as well as reducing agent in the aluminothermal strategy. The products were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and electrochemical methods. The XRD results show that the as-obtained Li₃V_2−xAl_x(PO₄)₃ has a phase-pure monoclinic structure, irrespective of the Al³⁺ doping concentration. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) results reveal that the charge-transfer resistance of the Li₃V₂(PO₄)₃ is reduced and the reversibility is enhanced after V³⁺ substituted by Al³⁺. In addition, The Li₃V_2−xAl_x(PO₄)₃ phases exhibit better cycling stability than the pristine Li₃V₂(PO₄)₃.     

Microwave solid-state synthesis and electrochemical properties of carbon-free Li3V2(PO4)3 as cathode materials for lithium batteries

Monoclinic Li₃V₂(PO₄)₃ can be rapidly synthesized at 750 °C for 5 min (MW5m) by using microwave solid-state synthesis method. The refined cell parameters and atomic coordination of the sample MW5m show some deviations compared with those of the sample synthesized in conventional solid-state synthesis method, especially the coordinate of Li atoms. Compared with the electrochemical properties of the carbon-coating sample Li₃V₂(PO₄)₃, the carbon-free sample MW5m presents well electrochemical properties. In the cut-off voltage of 3.0-4.3 V, MW5m sample presents a specific charge capacity of 132 mAh g⁻¹, almost equivalent to the reversible cycling of two lithium ions per Li₃V₂(PO₄)₃ formula unit (133 mAh g⁻¹), and specific discharge capacity of 126.4 mAh g⁻¹. In the cut-off voltage of 3.0-4.8 V, MW5m shows an initial specific discharge capacity of 183.4 mAh g⁻¹ at 0.1 C, near the theoretical discharge capacity. In the cycle performance, the capacity fade of Li₃V₂(PO₄)₃ is dependent on the cut-off voltage and the preparation method, more capacity lost at relatively higher charge/discharge voltage. The reasons for the excellent electrochemical properties of Li₃V₂(PO₄)₃ rapidly synthesized in microwave field are discussed in detail.     

Electrochemical performance of nanocrystalline Li3V2(PO4)3/carbon composite material synthesized by a novel sol–gel method

A novel sol–gel method based on V₂O₅·nH₂O hydro-gel was developed to synthesize nanocrystalline Li₃V₂(PO₄)₃/carbon composite material. In this route, V₂O₅·nH₂O hydro-gel, NH₄H₂PO₄, Li₂CO₃ and high-surface-area carbon were used as starting materials to prepare precursor, and the Li₃V₂(PO₄)₃/carbon was obtained by sintering precursor at 750 °C for 4 h in flowing argon. The sol–gel synthesis ensures homogeneity of the precursors and improved reactivity. The sample was characterized by XRD, SEM and TEM. X-ray diffraction results show Li₃V₂(PO₄)₃ sample is monoclinic structure with the space group of P2₁/n. The TEM image indicates that the Li₃V₂(PO₄)₃ particles modified by conductive carbon are about 70 nm in diameter. The Li₃V₂(PO₄)₃/carbon system showed that the discharge capacities in the first and 50th cycle are about 155.3 and 143.6 mAh/g, respectively, in the range of 3.0–4.8 V. The sol–gel method is fit for the preparation of Li₃V₂(PO₄)₃/carbon composite material which may offer some favorable properties for commercial application.     

Mn influence on the electrochemical behaviour of Li3V2(PO4)3 cathode material

The role played by the substitution of Mn on the electrochemical behaviour of Li₃V₂(PO₄)₃ has been investigated. Independently of the synthesis route, the Mn doping improves the electrochemical features with respect to the undoped samples. Different reasons can be taken into consideration to explain the electrochemical enhancement. In the sol–gel synthesis the capacity slightly enhances due to the Mn substitution on both the V sites, within the solubility limit x = 0.124 in Li₃V_2−xMn_x(PO₄)₃. In the solid state synthesis the significant capacity enhancement is preferentially due to the microstructural features of the crystallites and to the LiMnPO₄ phase formation.     

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.     

The Li3V2(PO4)3/C composites with high-rate capability prepared by a maltose-based sol-gel route

The carbon coated monoclinic Li₃V₂(PO₄)₃ (LVP/C) cathode materials are synthesized via a sol-gel method using oxalic acid as a chelating reagent and maltose as a carbon source. The effect of carbon content on the synthesis of LVP/C composites is investigated using X-ray diffraction, scanning electron microscopy, galvanostatic charge/discharge and DC resistance measurements. The results show that, among the LVP/C powders with different carbon content (5.7, 9.6, 11.6 and 15.3 wt.%), the sample with 11.6 wt.% carbon content gives rise to the corresponding (LVP/C) ∥Li half cell with a low DC resistance and superior electrochemical performance, especially with excellent rate capability. Its discharge capacity decreases by only 7.2% from 125 mAh g⁻¹ at 0.5 C to 116 mAh g⁻¹ at 5 C between 3.0 and 4.3 V. The maltose-based sol-gel method is feasible for the preparation of LVP/C composites for high power lithium ion batteries.