Sc3+-doping effects on porous Li3V2(PO4)3/C cathode with superior rate performance and cyclic stability

An enhanced sol-gel combustion method was used to synthesize different porous Sc³⁺-doped Li₃V_2-xSc_x(PO₄)₃/C (x = 0.00, 0.05, 0.10 and 0.15) compounds. The substitution of Sc³⁺ into the V³⁺ sites of Li₃V_2-xSc_x(PO₄)₃/C expands the lattice volume along with the enlargement of Li⁺ diffusion channel, which is beneficial for Li⁺ transportation and ionic conductivity improvement. Besides, the Sc³⁺ doping content exhibits a great impact on the morphology of Li₃V_2-xSc_x(PO₄)₃/C composite. The pristine Li₃V₂(PO₄)₃/C are constituted of porous particles and nanorods, and the ratio of nanorods to particles can be controlled by adjusting the amount of Sc³⁺ doping since the ratio of nanorods to particles decreases with increasing Sc³⁺ doping content. When Sc³⁺ doping content increases to a certain level (x = 0.15, Li₃V_1.85Sc_0.15(PO₄)₃/C), the nanorods are hardly seen. Li₃V_1.90Sc_0.10(PO₄)₃/C with higher tapped density, better reversibility, smaller resistance and larger Li⁺ diffusion coefficient demonstrates outstanding rate performance and cyclic stability, together with high specific discharge capacities of 130.2 and 92.9 mAh g⁻¹ at 0.5 and 20 C, respectively. Furthermore, a superior specific discharge capacity of 85.8 mAh g⁻¹ was retained at 20 C following 1000 cycles. Overall, a novel approach for the preparation of high-performance Li₃V_2-xSc_x(PO₄)₃/C cathodes with different morphologies for lithium-ion batteries is provided.     

Electrochemical performance of Li3V2(PO4)3/C cathode material using a novel carbon source

Polyethylene glycol (PEG, mean molecular weight of 10,000) has been used to prepare a Li₃V₂(PO₄)₃/C cathode material by a simple solid-state reaction. The Raman spectra shows that the coating carbon has a good structure with a low I_D/I_G ratio. The images of SEM and TEM show that the carbon is dispersed between the Li₃V₂(PO₄)₃ particles, which improves the electrical contact between the corresponding particles. The electronic conductivity of Li₃V₂(PO₄)₃/C composite is 7.0 × 10⁻¹ S/cm, increased by seven orders of magnitude compared with the pristine Li₃V₂(PO₄)₃ (2.3 × 10⁻⁸ S/cm). At a low discharge rate of 0.28C, the sample presents a high discharge capacity of 131.2 mAh/g, almost achieving the theoretical capacity (132 mAh/g) for the reversible cycling of two lithium. After 500 cycles, the discharge capacity is 123.9 mAh/g with only 5.6% fading of the initial specific capacity. The Li₃V₂(PO₄)₃/C material also exhibits an excellent rate capability with high discharge capacities of 115.2 mAh/g at 1C and 106.4 mAh/g at 5C.     

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.     

Fast Li-ion conductor Li1+yTi2-yAly(PO4)3 modified Li1.2[Mn0.54Ni0.13Co0.13]O2 as high performance cathode material for Li-ion battery

In the material of xLi₂MnO₃ ·(1-x) LiMO₂ (0 < x < 1), the Li₂MnO₃ component is used to stabilize the layered LiMO₂ structure. However, the electrochemical inactive Li₂MnO₃ makes Li-ion diffusion difficult, leading to a sluggish rate capability. In this work, Li_1.3Ti_1.7Al_0.3(PO₄)₃ (LTA0.3), a NASICON-type Li-ion conductor, is applied to modified Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ to overcome the above shortcoming. Additionally, the Li-ion conductivity of LiTi₂(PO₄)₃ can be improved effectively by replacing tetravalent cation Ti⁴⁺ with trivalent Al³⁺ at the optimal ratio. At 1C rate, the LR cathode with 3 wt% LTA0.3 delivers 200 mAh g^?1 after 170 cycles and maintains 140 mAh g^?1 after 500 cycles. Moreover, the modified cathode shows an enhanced rate performance of 169.7 mAh g^?1 at 5C. Enhanced cycle durability and rate capability are aroused by the 3D skeletal framework of LTA0.3, which is suitable for Li-ion diffusion. The LTA0.3 coating layer displays a robust shell which not only avoids the corrosion of electrode materials but also effectively facilitates Li-ion diffusion.     

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.     

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.     

Aluminum-doping effects on three-dimensional Li3V2(PO4)3@C/CNTs microspheres for electrochemical energy storage

A series of three-dimensional Al³⁺-doped Li₃V₂(PO₄)₃@C/CNTs microspheres have been fabricated for the first time using a facile spray drying route followed through a solid-state reaction process. The crystalline structure, morphology, microstructure and lithium storage performance for the fabricated composites have been researched using Raman spectrum, XRD, XPS, SEM, TEM, EDS and various electrochemical tests. Benefiting from the Al³⁺ doping and formed three-dimensional networks by the carbon film and CNTs, the Li ⁺ diffusion coefficient and electrical conductivity of Li₃V₂(PO₄)₃ are significantly enhanced. All the Al³⁺-doped composites possess superior lithium storage properties including high capacity and good cyclic-life. Thus, Al³⁺ doping is a prospective strategy to promote the rate properties of Li₃V₂(PO₄)₃ for lithium energy storage.     

Construction of simultaneous modified Na3V2(PO4)3/C cathode with K/Zr substitution and carbon nanotubes enwrapping for high performance sodium ion battery

Na₃V₂(PO₄)₃ (NVP) has been deemed to be a prospective cathode material due to the unique NASICON-type framework for sodium ion battery (SIB). Nevertheless, the inferior intrinsic conductive property seriously impedes the development of NVP. Herein, the K/Zr co-substituted and carbon nanotubes (CNTs) enwrapped NVP/C composite is successfully synthesized through a facile sol-gel route. Notably, the introduced K⁺ in Na1 site possesses a pillar effect on the crystal structure to efficiently stabilize the framework. Meanwhile, Zr⁴⁺ with larger ionic radius successfully replaces of V³⁺, which is beneficial to expanding the interplanar spacing to facilitate the migration of Na⁺. Moreover, the enwrapped tubular CNTs can restrict the agglomerations of active grains to diminish the pathways for ionic and electronic transportation. Synthetically, the CNTs and amorphous coated carbon layers jointly construct a cross-linked 3D network to provide accelerated channels for electronic transportation. Consequently, the modified Na_2.96K_0.04V_1.93Zr_0.0525(PO₄)₃/C@CNTs composite exhibits superior electrochemical performance with excellent kinetic properties. Accordingly, it delivers a great capacity value of 110.8 mAh g^?1 at 0.1 C. Besides, it exhibits a reversible capacity of 102 mAh g^?1 at 2 C and maintains 89.7% after 300 cycles. As for a higher rate of 5 C, it releases an initial capacity of 99 mAh g^?1 and a high retention of 90.9% can be obtained after 1300 cycles. Significantly, the optimized sample delivers a high capacity of 91.2 mAh g^?1 at an ultra-high rate of 60 C and sustains 78.3% after 3000 cycles. Furthermore, the symmetric full cell is successfully fabricated and reveals superior high-rate capability with excellent stability. Therefore, this modified Na_2.96K_0.04V_1.93Zr_0.0525(PO₄)₃/C@CNTs composite would be a promising cathode material for practical applications in SIB.     

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

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

Synthesis and electrochemical performance of LiVOPO4–Li3V2(PO4)3 cathode materials for lithium ion batteries

Lithium-ion batteries (LIBs) present the advantages of long cycle life, high voltage, and energy density and are widely made in the field of energy storage. LiVOPO₄ (LVOP), a cathode material used in LIBs, has a high conceptual capacity of 159 mAh g⁻¹ and high operating voltage of 3.9 V. However, its low electrical conductivity and cycle performance limit its commercial applications. According to the X-ray diffraction results, orthogonal crystal LVOP and monoclinic crystal Li₃V₂(PO₄)₃ (LVP) coexisted in the synthesised composite material. The transmission electron microscopy results also indicated that the LVOP and LVP phases coexist, which were coated by carbon layer of about 2.5 nm. The discharge of LVOP–LVP composite material initially was 143.2 mAh g⁻¹, and that after 120 cycles was 132.2 mAh g⁻¹ (at 0.1 C and 3–4.5 V). Thus, the electronic conductivity and first discharge specific capacity of the material enhanced due to the introduction of fast ion conductor LVP into LVOP. Electrochemical performance improved because the introduction of LVP led to an increase in Li⁺ pervasion channels in the original material and the acceleration of the Li⁺ transmission speed.     

Synthesis and electrochemical properties of a composite LiMn0.63Fe0.37PO4 with Li3V2(PO4)3 for lithium-ion battery cathode materials

A composite materials LiMn_0.63Fe_0.37PO₄ with Li₃V₂(PO₄)₃ can be synthesized by a sol-gel method using N,N-dimethylformamide (DMF) as a dispersing agent. The structures, characteristics of the appearance, and electrochemical properties of the composites have been studied by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), charge/discharge tests, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The composites contained LiMnPO₄/C (LMP/C), LiFePO₄/C (LFP/C), and Li₃V₂(PO₄)₃/C (LVP/C) phases with a nano-sized dispersion. The TEM images showed that the composites are crystalline with a grain size of 10–50 nm. The Mn2p, V2p, and Fe2p valence states were analyzed by X-ray photoelectron spectroscopy (XPS). The incorporation of LVP and LFP with LMP effectively enhanced the electrochemical kinetics of the LMP phase by a structural modification and shortened the lithium diffusion length in LMP. The capacity of the composite 0.79LiMn_0.63Fe_0.37PO₄·0.21Li₃V₂(PO₄)₃/C remained at 152.3 mAh g⁻¹ (94.7%) after 50 cycles at a 0.05 C rate. The composite exhibited excellent reversible capacities 159.4, 150, 140.1, 133.7 and 123.6 mAh g⁻¹ at charge-discharge rates of 0.05, 0.1, 0.2, 0.5 and 1 C, respectively.     

Enhanced electrochemical performance of Li3V2(PO4)3 structurally converted from LiVOPO4 by graphite nanofiber addition

In this work, the structural conversion of LiVOPO₄ to Li₃V₂(PO₄)₃ due to the addition of graphene nanofiber (GNF) was investigated, and the resulting materials were found to exhibit enhanced capacity and cyclability. First, LiVOPO₄ was synthesized using a solid-state method followed by annealing at 900 °C for 12 h under nitrogen atmosphere. Then, the conversion from the triclinic LiVOPO₄ structure to the monoclinic Li₃V₂(PO₄)₃ structure due to the GNF addition was observed. No impurity peak was observed in the X-ray diffraction patterns of LiVOPO₄ or Li₃V₂(PO₄)₃, and the structural conversion caused no defects to form in the resulting Li₃V₂(PO₄)₃ crystallite. Field emission-scanning electron microscope studies clearly demonstrate that larger corroded-structure-like particles formed which were mixed with GNF. This provided both a large active area and fast transport of lithium ions, which afforded enough active sites for simultaneous intercalation of many lithium ions, leading to improved electrochemical properties of the material. Compared with LiVOPO₄, the Li₃V₂(PO₄)₃–GNF showed better properties, such as an improved lithium ion diffusion coefficient, improved cyclability, and smaller impedance. Furthermore, the optimized Li₃V₂(PO₄)₃–GNF (7%) battery showed the best discharge capacity of 181 mA h g⁻¹ at 0.1 C and lithium ion diffusion coefficient of 6.01×10⁻⁹ cm² s⁻¹.     

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.     

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.     

Na3V2(PO4)3/C·Na3V2(PO4)2F3/C@rGO blended cathode material with elevated energy density for sodium ion batteries

Na₃V₂(PO₄)₃ (NVP), a promising cathode material of sodium ion batteries, has gained much attention due to its competitive capability of sodium storage. However, the relatively low discharge potential of ~3.4 V (vs. Na⁺/Na) needs to be improved and the current energy density cannot satisfy the practical applications. Herein, the introduction of Na₃V₂(PO₄)₂F₃ with high voltage plateaus is adopted to elevate the energy density. Therefore, the mixed composite seems to be more attractive because of the prospective high energy character. Nevertheless, this mixture still suffers from the intrinsic poor electronic conductivity. Accordingly, a novel Na₃V₂(PO₄)₃/C·Na₃V₂(PO₄)₂F₃/C @reduced graphene oxide (rGO) (denoted as B@rGO) blended cathode material is successfully synthesized through a facile sol-gel method. The coated carbon layers and rGO are employed to construct a conductive network for promoting the ionic and electronic diffusion efficiently. Detailed kinetic analyses demonstrate that the NVP phase dominates the kinetic reaction in the blended composite. With respect to the electrochemical properties, this modified B@rGO composite can deliver a discharge capacity of 115.1 mAh g^-1 at 0.1 C, corresponding to a high energy density of 418.3 Wh kg^-1, which is far more than the theoretical one of pure NVP (399.8 Wh kg^-1). Moreover, the cyclic characteristics are significantly improved, resulting from the boosted structural stability. Even at a high rate of 30 C, it can release a reversible capacity of 85.9 mAh g^-1, and a high capacity retention of 81.6% is obtained after 200 cycles. Furthermore, the superior electrochemical properties are determined and demonstrated by the improved kinetics, including the large capacitive contribution (79.6% at 0.5 mV s^-1) and rapid ionic migration. Therefore, this novel co-modification strategy with high-voltage material and conductive rGO can be an efficient way for Na₃V₂(PO₄)₃ to optimize the electrochemical performance and push its application in sodium ion batteries.     

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.     

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.     

Evaluation of the electrochemical behavior and structural reversibility of Li3−xNaxV2(PO4)3/C (0≤x≤3) as anode materials for Na-ion batteries

A series of Li_3−xNa_xV₂(PO₄)₃/C (0≤x≤3) materials are successfully prepared by a simple solid-state reaction method and used for the first time as anode materials for Na-ion batteries. Powder X-ray diffraction (XRD) results show that the phase structures of Li_3−xNa_xV₂(PO₄)₃/C evolve along with the change of Li/Na atomic ratio (0≤x≤3). With increasing x in Li_3−xNa_xV₂(PO₄)₃/C from 0.0 to 3.0, the main phase in as-prepared sample transforms from monoclinic Li₃V₂(PO₄)₃ to rhombohedral Li₃V₂(PO₄)₃, and finally to rhombohedral Na₃V₂(PO₄)₃, which results in different sodium storage behavior and performance between Li_3−xNa_xV₂(PO₄)₃/C (0≤x≤3) materials. Electrochemical results show that Li_3−xNa_xV₂(PO₄)₃/C (x=0.0, 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0) can deliver the initial charge capacities of 21.1, 35.9, 33.8, 41.7, 43.3, 43.9 and 47.7 mAh g⁻¹ at a current density of 10 mA g⁻¹, respectively. After 45 cycles, the reversible capacities can be kept at 16.9, 45.1, 32.6, 44.6, 43.7, 37.8 and 27.3 mAh g⁻¹ for Li₃V₂(PO₄)₃/C, Li_2.5Na_0.5V₂(PO₄)₃/C, Li₂NaV₂(PO₄)₃/C, Li_1.5Na_1.5V₂(PO₄)₃/C, LiNa₂V₂(PO₄)₃/C, Li_0.5Na_2.5V₂(PO₄)₃/C and Na₃V₂(PO₄)₃/C, respectively. Furthermore, the structural reversibility of Li_3−xNa_xV₂(PO₄)₃/C (x=1.0, 2.0, and 3.0) is also observed by in-situ XRD observation during sodiation/de-sodiation process. All these observed evidences indicate that only some of Li_3−xNa_xV₂(PO₄)₃/C (0≤x≤3) can be used as possible sodium storage materials.     

High performance Li3V2(PO4)3/C composite cathode material for lithium ion batteries studied in pilot scale test

Li₃V₂(PO₄)₃/C composite cathode material was synthesized via carbothermal reduction process in a pilot scale production test using battery grade raw materials with the aim of studying the feasibility for their practical applications. XRD, FT-IR, XPS, CV, EIS and battery charge-discharge tests were used to characterize the as-prepared material. The XRD and FT-IR data suggested that the as-prepared Li₃V₂(PO₄)₃/C material exhibits an orderly monoclinic structure based on the connectivity of PO₄ tetrahedra and VO₆ octahedra. Half cell tests indicated that an excellent high-rate cyclic performance was achieved on the Li₃V₂(PO₄)₃/C cathodes in the voltage range of 3.0-4.3 V, retaining a capacity of 95% (96 mAh/g) after 100 cycles at 20C discharge rate. The low-temperature performance of the cathode was further evaluated, showing 0.5C discharge capacity of 122 and 119 mAh/g at −25 and −40 °C, respectively. The discharge capacity of graphite//Li₃V₂(PO₄)₃ batteries with a designed battery capacity of 14 Ah is as high as 109 mAh/g with a capacity retention of 92% after 224 cycles at 2C discharge rates. The promising high-rate and low-temperature performance observed in this work suggests that Li₃V₂(PO₄)₃/C is a very strong candidate to be a cathode in a next-generation Li-ion battery for electric vehicle applications.     

Porous spherical LiFePO4·LiMnPO4·Li3V2(PO4)3@C@rGO composites as a high-rate and long-cycle cathode for lithium ion batteries

The porous spherical LiFePO₄·LiMnPO₄·Li₃V₂(PO₄)₃@C@rGO (Sample-G) composites are prepared via a spray drying process. The results show that the composites consist of orthorhombic olivine-type LiFe_0.5Mn_0.5PO₄ and monoclinic Li₃V₂(PO₄)₃, which are evenly distributed. In particular, nanoparticles are embedded in graphene nanosheets, which are interconnected and stacked to form a porous sphere structure with an interior three-dimensional conductive network, resulting in the huge improvement on electrochemical performance and structural stability. Due to the increased Li⁺ diffusion coefficient, the composite possesses 98.6 and 82.9 mAh g⁻¹ with capacities retention of 81.6% and 71.8% at 10 and 20C after 1000 cycles, respectively. The mutual cross-doping effect between LFP·LMP·LVP and a porous sphere structure with a 3D conductive network inside provides a practical method for improving the cycling and rate performance.