Electrochemical performance of novel Li<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> glass-ceramic nanocomposites as electrodes for energy storage devices

The novel Li₃V₂(PO₄)₃ glass-ceramic nanocomposites were synthesized and investigated as electrodes for energy storage devices. They were fabricated by heat treatment (HT) of 37.5Li₂O–25V₂O₅–37.5P₂O₅?mol% glass at 450 °C for different times in the air. XRD, SEM, and electrochemical methods were used to study the effect of HT time on the nanostructure and electrochemical performance for Li₃V₂(PO₄)₃ glass-ceramic nanocomposites electrodes. XRD patterns showed forming Li₃V₂(PO₄)₃ NASICON type with monoclinic structure. The crystalline sizes were found to be in the range of 32–56 nm. SEM morphologies exhibited non-uniform grains and changed with variation of HT time. The electrochemical performance of Li₃V₂(PO₄)₃ glass-ceramic nanocomposites was investigated by using galvanostatic charge/discharge methods, cyclic voltammetry, and electrochemical impedance spectroscopy in 1 M H₂SO₄ aqueous electrolyte. The glass-ceramic nanocomposites annealed for 4 h, which had a lower crystalline size, exhibited the best electrochemical performance with a specific capacity of 116.4 F g^?1 at 0.5 A g^?1. Small crystalline size supported the lithium ion mobility in the electrode by decreasing the ion diffusion pathway. Therefore, the Li₃V₂(PO₄)₃ glass-ceramic nanocomposites can be promising candidates for large-scale industrial applications in high-performance energy storage devices.     

Facile synthesis of Li3V2(Po4)3/C nano-flakes with high-rate performance as cathode material for Li-ion battery

The flake-like Li₃V₂(PO₄)₃/C has been successfully synthesized by rheological phase method using polyvinyl alcohol (PVA) as template; the Li₃V₂(PO₄)₃/C without PVA assistance has been prepared for comparison. X-ray diffraction analysis shows that the two samples are well crystallized, and no impurity phases are detected. The scanning electron microscopy results reveal that there is a significant difference in morphologies between PVA-assisted sample and sample without PVA; the former shows a flake-like morphology, while the latter presents regular granular shape with some agglomeration. Transmission electron microscopy images reveal that Li₃V₂(PO₄)₃ particles are coated with a uniform surface carbon layer. The lattice fringes with a spacing of 0.428 nm can be clearly seen from the high-resolution transmission electron microscopy image. The PVA-assisted sample shows a discharge capacity of 120, 110, and 96 mAh g^?1 at 1 C, 20 C, and 50 C, respectively; however, the sample without PVA exhibits a lower discharge capacity. Based on the analysis of electrochemical impedance spectroscopy, the lithium ion diffusion coefficients of Li₃V₂(PO₄)₃/C and PVA-assisted Li₃V₂(PO₄)₃/C are 4.19?×?10^?9 and 4.99?×?10^?8 cm² s^?1, respectively. In summary, it is demonstrated that using PVA as a template can obtain flake-like morphology and significantly improve the comprehensive electrochemical performances of Li₃V₂(PO₄)₃/C cathode material.     

Synthesis and electrochemical properties of Li3V2(PO4)3/C with one-dimensional (1D) nanorod structure for cathode materials of lithium-ion batteries

A series of Li₃V₂(PO₄)₃/C cathode materials with different morphologies were successfully prepared by controlling temperatures using maleic acid as carbon source via a simple sol–gel reaction method. The Li₃V₂(PO₄)₃/C nanorods synthesized at 700 °C with diameters of about 30–50 nm and lengths of about 800 nm show the highest initial discharge capacity of 179.8 and 154.6 mA h g⁻¹ between 3.0 and 4.8 V at 0.1 and 0.5 C, respectively. Even at a discharge rate of 0.5 C over 50 cycles, the products still can deliver a discharge capacity of 140.2 mA h g⁻¹ in the potential region of 3.0–4.8 V. The excellent electrochemical performance can be attributed to one-dimensional nanorod structure and uniform particle size distribution. All these results indicate that the resulting Li₃V₂(PO₄)₃/C is a very strong candidate to be a cathode in a next-generation Li-ion battery for electric-vehicle applications.     

Natural graphite enhanced the electrochemical performance of Li<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> cathode material for lithium ion batteries

Natural graphite treated by mechanical activation can be directly applied to the preparation of Li₃V₂(PO₄)₃. The carbon-coated Li₃V₂(PO₄)₃ with monoclinic structure was successfully synthesized by using natural graphite as carbon source and reducing agent. The amount of activated graphite is optimized by X-ray diffraction, scanning electron microscope, transmission electron microscope, Raman spectrum, galvanostatic charge/discharge measurements, cyclic voltammetry, and electrochemical impedance spectroscopy tests. Our results show that Li₃V₂(PO₄)₃ (LVP)-10G exhibits the highest initial discharge capacity of 189 mAh g^?1 at 0.1 C and 162.9 mAh g^?1 at 1 C in the voltage range of 3.0–4.8 V. Therefore, natural graphite is a promising carbon source for LVP cathode material in lithium ion batteries.     

LiVPO4F/Li3V2(PO4)3 nanostructured composite cathode materials prepared via mechanochemical way

Single-phase LiVPO₄F and LiVPO₄F/Li₃V₂(PO₄)₃ nanostructured composite cathode materials were prepared by heating of the VPO₄?+?LiF mechanochemically activated mixture to 700 °C and subsequent quick or slow cooling to room temperature, respectively. The formation of the composites was proved by a combination of different physico-chemical methods, including XRD, FTIR, ⁶Li and ³¹P NMR, SEM, TEM, and HRTEM. It has been shown that in the composites LiVPO₄F and Li₃V₂(PO₄)₃ nanocrystals well inset into each other resulting in the nanodomain composite formation. Charge–discharge curves of the composites have a sloping profile both in the high-voltage (3.0–4.5 V) and in the low-voltage (1.3–2.5 V) ranges, noticeably different from plateaus for a phase-pure LiVPO₄F, thus indicating a probable change of a two-phase regime of lithium intercalation for a single-phase one. Enhanced rate capability of the LiVPO₄F/Li₃V₂(PO₄)₃ composites is associated with their microstructure and high ionic conductivity of Li₃V₂(PO₄)₃.     

Effects of Cr doping on the electrochemical performance of Li3V2(PO4)3 cathode material for lithium ion batteries

A series of Cr-doped Li₃V_2 − x Cr _x (PO₄)₃ (x = 0, 0.1, 0.25, and 0.5) samples are prepared by a sol–gel method. The effects of Cr doping on the physical and chemical characteristics of Li₃V₂(PO₄)₃ are investigated. Compared with the XRD pattern of the undoped sample, the XRD patterns of the Cr-doped samples have no extra reflections, which indicates that Cr enters the structure of Li₃V₂(PO₄)₃. As indicated by the charge–discharge measurements, the Cr-doped Li₃V_2 − x Cr _x (PO₄)₃ (x = 0.1, 0.25, and 0.5) samples exhibit lower initial capacities than the undoped sample at the 0.2 C rate. However, both the discharge capacity and cycling performance at high rates (e.g., 1 and 2 C) are enhanced with proper amount of Cr doping (x = 0.1). The highest discharge capacity and capacity retention at the rates of 1 and 2 C are obtained for Li₃V_1.9Cr_0.1(PO₄)₃. The improvement of the electrochemical performance can be attributed to the higher crystal stability and smaller particle size induced by Cr doping.

     

Effects of phenolic resin on the electrochemical performance of Li3V2(PO4)3/C cathode materials

As a kind of lithium-ion battery cathode material, monoclinic lithium vanadium phosphate/carbon Li₃V₂(PO₄)₃/C was synthesized by adopting phenolic resin as carbon source, both for reducing agent and coating material. The crystal structure and morphology of the samples were characterized through X-ray diffraction (XRD) and scanning electron microscope (SEM). Galvanostatic charge-discharging experiments and electrochemical impedance spectrum (EIS) were utilized to determine the electrochemical insertion properties of the samples. XRD data revealed that phenolic resin does not change the crystal structure of Li₃V₂(PO₄)₃/C. Furthermore, the morphology of grains and the electronic conductivity of Li₃V₂(PO₄)₃/C were improved. Galvanostatic charge-discharging and EIS results showed that the optimal electrochemical properties and the minimum charge-transfer resistance of Li₃V₂(PO₄)₃/C can be reached when added by 5 wt.% of redundant carbon (except the carbon needed to reduce V⁵⁺ to V³⁺). The initial discharge capacity is 128.4 mAh g^?1 at 0.2 C rate and 101.2 mAh g^?1 at 5 C in the voltage range of 3.0～4.3 V.     

Impact of carbon coating thickness on the electrochemical properties of Li<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>/C composites

A series of Li₃V₂(PO₄)₃/C composites with different amounts of carbon are synthesized by a combustion method. The physical and electrochemical properties of the Li₃V₂(PO₄)₃/C composites are investigated by X-ray diffraction, element analysis, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy and electrochemical measurements. The effects of carbon content of Li₃V₂(PO₄)₃/C composites on its electrochemical properties are conducted with cyclic voltammetry and electrochemical impedance. The experiment results clearly show that the optimal carbon content is 4.3 wt %, and more or less amount of carbon would be unfavorable to electrochemical properties of the Li₃V₂(PO₄)₃/C electrode materials. The results would provide some basis for further improvement on the Li₃V₂(PO₄)₃ electrode materials.     

Synthesis of Li3Ni x V2−x (PO4)3/C cathode materials and their electrochemical performance for lithium ion batteries

Li₃Ni _x V_2?x (PO₄)₃/C (x?=?0, 0.02, 0.04 and 0.06) samples have been synthesized via an improved sol–gel method. X-ray diffraction patterns indicate that the structure of the prepared samples retains monoclinic, and the single phase has not been changed with Ni doping. From the analysis of electrochemical performance, the Li₃Ni_0.04?V_1.96(PO₄)₃/C sample exhibits the best electrochemical property. It delivers a discharge capacity of 112.1 mAh?g^?1 with capacity retention of 95.2 % over 300 cycles at 10 C rate in the range of 3.0–4.8 V; cyclic voltammetry and electrochemical impedance spectra testing further prove that the electrochemical reversibility and lithium ion diffusion behavior of Li₃V₂(PO₄)₃ have also been effectively improved through Ni doping.     

Comparative investigations of LiVPO4F/C and Li3V2(PO4)3/C synthesized in similar soft chemical route

Triclinic LiVPO₄F and monoclinic Li₃V₂(PO₄)₃ are synthesized through a soft chemical process with mechanical activation assist, followed by annealing. In this process, ascorbic acid is used as reducing agent as well as carbon source. The as-prepared samples are coated with amorphous carbon. XPS analysis results show the expected valency states of ions in LiVPO₄F and Li₃V₂(PO₄)₃. The electrochemical properties of the prepared LiVPO₄F/C and Li₃V₂(PO₄)₃/C cathodes are evaluated. The as-prepared LiVPO₄F/C cathode shows an initial discharge specific capacity of 140?±?3 mAh?g^?1 at 30 mA?g^?1 in the voltage range of 3.0~4.4 V, compared with that of 138?±?3 mAh?g^?1 possessed by Li₃V₂(PO₄)₃/C. Both samples exhibit good cycle performance at different current densities. The capacity delivered by LiVPO₄F remains 95.5 and 91.7 % of its initial discharge capacity after 50 cycles at 150 and 750 mA?g^?1, respectively, while 97.4 and 90.6 % for Li₃V₂(PO₄)₃/C. But the rate capability of LiVPO₄F/C is not so good compared with as-prepared Li₃V₂(PO₄)₃/C.     

Effects of Cr doping on the electrochemical performance of Li<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> cathode material for lithium ion batteries

A series of Cr-doped Li₃V_2???x Cr _x (PO₄)₃ (x?=?0, 0.1, 0.25, and 0.5) samples are prepared by a sol–gel method. The effects of Cr doping on the physical and chemical characteristics of Li₃V₂(PO₄)₃ are investigated. Compared with the XRD pattern of the undoped sample, the XRD patterns of the Cr-doped samples have no extra reflections, which indicates that Cr enters the structure of Li₃V₂(PO₄)₃. As indicated by the charge–discharge measurements, the Cr-doped Li₃V_2???x Cr _x (PO₄)₃ (x?=?0.1, 0.25, and 0.5) samples exhibit lower initial capacities than the undoped sample at the 0.2 C rate. However, both the discharge capacity and cycling performance at high rates (e.g., 1 and 2 C) are enhanced with proper amount of Cr doping (x?=?0.1). The highest discharge capacity and capacity retention at the rates of 1 and 2 C are obtained for Li₃V_1.9Cr_0.1(PO₄)₃. The improvement of the electrochemical performance can be attributed to the higher crystal stability and smaller particle size induced by Cr doping.     

Effects of carbon coating on the temperature-dependent electrochemical properties of Li3V2(PO4)3

Carbon coated and carbon free Li₃V₂(PO₄)₃ cathode materials were prepared by carbothermal reduction and H₂ reduction methods, respectively. The carbon free material had a grain size about 1 μm whereas the carbon coated material was less than 100 nm. The surface carbon layer enhanced the electronic conductivity of Li₃V₂(PO₄)₃ by five orders of magnitude. In addition, the surface carbon layer also prevented the formation of SEI film, decreased the charge transfer resistance and increased the chemical diffusion coefficient of Li⁺ ions. All of these advantages improved the electrochemical performance of Li₃V₂(PO₄)₃. As most of intercalation materials, the low temperature performance of Li₃V₂(PO₄)₃ was poorer than that at room temperature. This was attributed to the electrochemical sluggish kinetics which caused higher charge transfer resistance and smaller chemical diffusion coefficient. The carbon coating technique was effective to eliminate these sluggish kinetics, and then improved the low temperature performance of Li₃V₂(PO₄)₃.     

Studies of xLiFePO4·yLi3V2(PO4)3/C composite cathode materials with high tap density and high performance prepared by sol spray drying method

The xLiFePO₄·yLi₃V₂(PO₄)₃/C cathode materials are synthesized by a sol spray drying method. X-ray diffraction results reveal that the xLiFePO₄·yLi₃V₂(PO₄)₃/C (x,y?≠?0) composites are composed of LiFePO₄ and Li₃V₂(PO₄)₃ phases, and no impurities are detected. The samples show spherical particles with the size of 0.5–5 μm, and the tap densities of all the samples are higher than 1.5 g cm^?3. Electrochemical tests show that the xLiFePO₄·Li₃V₂(PO₄)₃/C (x,y?≠?0) composites exhibit much better performance than the single LiFePO₄/C or Li₃V₂(PO₄)₃/C. Among all the samples, 3LiFePO₄·Li₃V₂(PO₄)₃/C possesses the best comprehensive performance in terms of the discharge capacity, average working voltage, and rate capability. At 1, 5, and 10 C rates, the sample shows first discharge capacities of 152.0, 134.3, and 116.8 mAh g^?1 and capacity retentions of 99.2, 98.2, and 97.7 % after 100 cycles, respectively. The excellent electrochemical performance of micron-sized xLiFePO₄·Li₃V₂(PO₄)₃/C (x,y?≠?0) powders is owing to the homogeneous mixing of reactants at a molecular level by sol spray drying, the incorporation of fast ion conductor Li₃V₂(PO₄)₃, and the mutual doping in LiFePO₄ and Li₃V₂(PO₄)₃.     

Li3V2(PO4)3/C和Li2.5Na0.5V2(PO4)3/C的结构和电化学性能的比较研究

采用溶胶-凝胶法合成了锂离子正极材料Li3V2(PO4)3/C(LVP/C)及Li2.5Na0.5V2(PO4)3/C,并用XRD、循环伏安及交流阻抗等方法,研究了大量Na+掺杂对材料结构和电化学性能影响。结果表明,大量钠离子的掺杂会使LVP结构由单斜向菱方转变。掺杂化合物Li2.5Na0.5V2(PO4)3/C在0.5 C充电1 C放电时,首次放电容量为118 mAh.g-1,50次循环后容量保持率为92.4%,并发现与单斜LVP存在多个放电平台不同,Li2.5Na0.5V2(PO4)3/C仅在3.7 V处有一个放电平台。     

Mechanism studies and electrochemical performance optimization on xLiFePO4·(1 ? x)Li3V2(PO4)3 hybrid materials

Hybrid materials xLiFePO₄·(1 − x)Li₃V₂(PO₄)₃ were synthesized by sol–gel method, with phenolic resin as carbon source and chelating agent, methylglycol as surfactant. The crystal structure, morphology and electrochemical performance of the prepared samples were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), cyclic voltammetry (CV), galvanostatic charge–discharge test and particle size analysis. The results show that LiFePO₄ and Li₃V₂(PO₄)₃ co-exist in hybrid materials, but react in single phase. Compared with individual LiFePO₄ and Li₃V₂(PO₄)₃ samples, hybrid materials have smaller particle size and more uniform grain distribution. This structure can facilitate Li ions extraction and insertion, which greatly improves the electrochemical properties. The sample 0.7LiFePO₄·0.3Li₃V₂(PO₄)₃ retains the advantages of LiFePO₄ and Li₃V₂(PO₄)₃, obtaining an initial discharge capacity of 166 mA h/g at 0.1 C rate and 109 mA h/g at 20 C rate, with a capacity retention rate of 73.3% and an excellent cycle stability.     

Multistep sintering preparation and electrochemical performances of LiFe0.7 V0.2PO4/C cathode material for lithium-ion batteries

Cathode material LiFe_0.7?V_0.2PO₄/C is successfully synthesized by multistep sintering through carbon thermal reaction including 650 °C for 10 h and 750 °C for 6 h. The crystal structure and surface morphology of the synthesized materials are characterized by X-ray diffractometer and scanning electron microscope, respectively. Cycle voltammetry, electrochemical impedance spectroscopy, and charge–discharge test are used to investigate the electrochemical performances of these samples. The results revealed that the synthesized LiFe_0.7?V_0.2PO₄/C material simultaneously contains olivine structure LiFePO₄ and monoclinic structure Li₃V₂(PO₄)₃. It shows improved conductivity, Li-ion diffusion coefficient, excellent charge/discharge performance, and reversibility due to both the incorporation of Li₃V₂(PO₄)₃ fast ion conductor and the employed multistep sintering. The initial discharge specific capacities of LiFe_0.7?V_0.2PO₄/C by multistep sintering are 167.8, 154.7, and 140.8 mAh g^?1 at 0.5, 1, and 2 C, respectively. After a total of 230 cycles at different rates, the sample still shows good performances. After 100 cycles at 2 C, the capacity retention is 99.1 %, and the capacity is 139.6 mAh g^?1. The LiFe_0.7?V_0.2PO₄/C material synthesized by this method can be used as a cathode material for advanced lithium-ion batteries.     

Effect of Al2O3 -coating on the electrochemical performances of Li3V2(PO4)3/C cathode material

The effect of Al₂O₃ -coating on Li₃V₂(PO₄)₃/C cathode material for lithium-ion batteries has been investigated. The crystalline structure and morphology of the synthesized powders have been characterized by XRD, SEM, and HRTEM, and their electrochemical performances are evaluated by CV, EIS, and galvanostatic charge/discharge tests. It is found that Al₂O₃ -coating modification stabilizes the structure of the cathode material, decreases the polarization of electrode and suppresses the rise of the surface film resistance. Electrochemical tests indicate that cycling performance and rate capability of Al₂O₃-coated Li₃V₂(PO₄)₃/C are enhanced, especially at high rates. The Al₂O₃-coated material delivers discharge capacity of 123.03 mAh g^?1 at 4 C rate, and the capacity retention of 94.15 % is obtained after 5 cycles. The results indicate that Al₂O₃ -coating should be an effective way to improve the comprehensive properties of the cathode materials for lithium-ion batteries.     

Conducting Graphene Decorated Li3V2(PO4)3/C for Lithium‐Ion Battery Cathode with Superior Rate Capability and Cycling Stability

In this study, core‐shell structured Li₃V₂(PO₄)₃/C wrapped in graphene nanosheets has been successfully prepared. The reduction of graphene oxide and the synthesis of Li₃V₂(PO₄)₃/C are carried out simultaneously using a chemical route followed by a solid‐state reaction. The effects of conducting graphene are studied by X‐ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectra and electrochemical measurements. The results reveal that the graphene sheets not only form a compact and uniform coating layer throughout the Li₃V₂(PO₄)₃/C, but also stretch out and cross‐link into a conducting network around the Li₃V₂(PO₄)₃/C particles. Thus, the graphene decorated Li₃V₂(PO₄)₃/C electrode exhibits superior high‐rate capability and long‐cycle stability. It delivers a reversible discharge capacity of 178.2 mAh·g^?1 after 60 cycles at a current density of 0.1 C, and the rate performances of 176, 169.3, 156.1 and 135.7 mAh·g^?1 at 1, 2, 5 and 10 C, respectively. The superior electrochemical properties make the graphene decorated Li₃V₂(PO₄)₃/C composite a promising cathode material for high‐performance lithium‐ion battery.     

Electrospun Li3V2(PO4)3 Nanobelts: Synthesis and Electrochemical Properties as Cathode Materials of Lithium‐Ion Batteries

Novel Li₃V₂ (PO₄)₃ nanobelts, which was confirmed by the peaks of X‐ray diffraction, were prepared by a facile and environmentally friendly electrospinning method. A distinct nanobelt structure, with an average width of 2.5 µm and a thickness of 200 nm, is observed by scanning electron microscopy (SEM), while the specific surface area of 140.8 m²/g is estimated by a specific surface area analyzer. Moreover, the unique Li₃V₂(PO₄)₃ nanobelts exhibited a specific discharge capacity of 155.6 mAh/g at 0.2 C rate when they were used as cathode material in lithium‐ion batteries, on testing from 3.0 to 4.8 V. Remarkably, the batteries containing Li₃V₂(PO₄)₃ nanobelts displayed excellent cycling performance, with only a 0.02% fading rate per cycle after 50 cycles in the range 30–4.3 V. These outstanding electrochemical performances could be ascribed to the particular morphology, large surface area, homogeneous particle size distribution, and the one‐dimensional microstructure of Li₃V₂(PO₄)₃ nanobelts.     

Wet coordination method to prepare carbon-coated Li3V2(PO4)3 cathode material for lithium ion batteries

A convenient method named wet coordination is used to prepare the sample or carbon-coated Li₃V₂(PO₄)₃ in the furnace with a flowing argon atmosphere at 600 °C for 1 h. The sample is characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM) and energy dispersive analysis of X-rays (EDAX). Galvanostatic charge–discharge between 3.3 and 4.3 V (vs. Li/Li⁺) shows that the sample exhibits a high discharge capacity of 128 mAh g^?1 with a good reversible performance under a current density of 95 mA g^?1. It suggests that carbon-coated Li₃V₂(PO₄)₃ with good electrochemical performance can be obtained via this method, which is suitable for large-scale production.