Modification of the morphology of Na3V2(PO4)2F3 as cathode material for sodium-ion batteries by polyvinylpyrrolidone

NASICON-type Na₃V₂(PO₄)₂F₃ (NVPF) is proposed to be a potential cathode material for sodium-ion battery because of its good structural stability, relatively high capacity and voltage platform. Nonetheless, the poor-rate performance, resulting from its low conductivity, has become a massive obstacle to its practical application. In this work, carbon coating together with morphology controlling were introduced to solve the issue of NVPF. This experiment used a hydrothermal method to prepare Na₃V₂(PO₄)₂F₃@C (NVPF@C) and explored the impact of surfactants (polyvinylpyrrolidone (PVP)) on the positive material performance of sodium-ion battery. Through various characterisation, NVPF@C compared its performance with that of untreated products, and verified that appropriate surfactant modification could enhance the performance of the electron conduction and sodium ion diffusion, thus effectively improved the performance of NVPF. Through comparison, it was found that appropriate surface modification with PVP can achieve the effects of specific crystal surface exposure and clusters of porous micron ball structure, and improve the electrochemical performance of NVPF best. Under the charge and discharge ratio of 0.2C, its initial reversible capacity was 127.8 mA h g^?1. After 100 cycles, its discharge capacity was 106.1 mA h g^?1, and the cycling retention rate reached 82.8%. Compared to the original NVPF, its performance has been dramatically improved.     

Effect of Fe-doping followed by C+SiO2 hybrid layer coating on Li3V2(PO4)3 cathode material for lithium-ion batteries

A novel Li₃V₂(PO₄)₃ composite modified with Fe-doping followed by C+SiO₂ hybrid layer coating (LVFP/C-Si) is successfully synthesized via an ultrasonic-assisted solid-state method, and characterized by XRD, XPS, TEM, galvanostatic charge/discharge measurements, CV and EIS. This LVFP/C-Si electrode shows a significantly improved electrochemical performance. It presents an initial discharge capacity as high as 170.8 mA h g⁻¹ at 1 C, and even delivers an excellent initial capacity of 153.6 mA h g⁻¹ with capacity retention of 82.3% after 100 cycles at 5 C. The results demonstrate that this novel modification with doping followed by hybrid layer coating is an ideal design to obtain both high capacity and long cycle performance for Li₃V₂(PO₄)₃ and other polyanion cathode materials in lithium ion batteries.     

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.     

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.     

Effects of nitrogen and sulfur atom regulation on electrochemical properties of Na3V2(PO4)2F3 cathode material for Na-ion batteries

The cathode material Na₃V₂(PO₄)₂F₃ of sodium-ion battery is well-known for its large number of ion migration channels and high working voltage. However, the electrochemical performance of Na₃V₂(PO₄)₂F₃ is not very outstanding. Thus, in the present study, Na₃V₂(PO₄)₂F₃ cathode materials were successfully synthesized by using the sol-gel method and mechanical milling method to enhance the electrochemical performance. The physicochemical properties of synthesized Na₃V₂(PO₄)₂F₃ were investigated by using X-ray diffraction spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, transition electron microscopy. X-ray diffraction spectroscopy indicates that the doping of nitrogen and sulfur did not alter the crystal form of Na₃V₂(PO₄)₂F₃. Transition electron microscopy image shows that Na₃V₂(PO₄)₂F₃ has a thin carbon layer, and x-ray photoelectron spectroscopy illustrates the successful doping of nitrogen and sulfur into the carbon layer. The cyclic voltammetry curves show that the nitrogen and sulfur co-doped Na₃V₂(PO₄)₂F₃ samples have good reversibility and low polarization. Materials with 15% thiourea has a high discharge specific capacity (126.9 mA h g^?1 at 0.2 C) at the first cycle and excellent cycle stability (126.3 mA h g^?1 after 100 cycles, a capacity retention of 99.5%) among the synthesized cathode materials. In the present study, the electrochemical performance of the Na₃V₂(PO₄)₂F₃ cathode material was enhanced by regulation of co-doping of nitrogen and sulfur atoms.     

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.     

Reversible Na+-extraction/insertion in nitrogen-doped graphene-encapsulated Na3V2(PO4)2F3@C electrode for advanced Na-ion battery

NASICON-structured sodium vanadium fluorophosphate has caused widespread concern for sodium energy conversion and storage because of its high voltage platform and high theoretical energy density. However, the inferior electrical conductivity is still a big problem, which greatly prevent the applications of Na₃V₂(PO₄)₂F₃ material. Herein, the nitrogen-doped graphene-encapsulated Na₃V₂(PO₄)₂F₃@C (NG-NVPF@C) has been prepared using the sol-gel approach. The physical and electrochemical performances for the resulted NG-NVPF@C composite have been systematically characterized and compared with that of Na₃V₂(PO₄)₂F₃@C (NVPF@C) in this study. The electrochemical tests demonstrate that the as-fabricated NG-NVPF@C displays higher capacity, superior rate property and better cyclic life than NVPF@C. It displays the discharge capacity of 108.6 mAh g⁻¹ at 5C. Moreover, it also possesses the high capacity of 101.6 mAh g⁻¹ at 10C over 300 cycles with the capacity retention of about 96.5%. The improved properties of NG-NVPF@C electrode are assigned to the constructed conductive network by nitrogen-doped graphene, which can modify the conductivity of Na₃V₂(PO₄)₂F₃.     

Sn substitution endows NaTi2(PO4)3/C with remarkable sodium storage performances

The further development of clean energy requires the use of more stable and reliable energy storage system. In addition to lithium ion battery power supplies, sodium ion batteries also have prospects for application and development thanks to the low cost and abundant resource. NaTi₂(PO₄)₃ has attracted much attention due to its three-dimensional channels for sodium ion transfer. In order to meliorate sodium storage properties of NaTi₂(PO₄)₃ electrode, a facile strategy of Sn substitution at Ti sites was employed, and a series of electrodes were successfully synthesized through sol-gel route. The electrochemical performances of Sn substituted composites are significantly improved compared with bare NaTi₂(PO₄)₃/C. And it was found that NaSn_0.2Ti_1.8(PO₄)₃ (NTP/C-Sn-2) delivers the largest capacity, and it also demonstrates the outstanding cycling performances. NTP/C-Sn-2 has discharge capacity of 131.1 mAh g⁻¹ at 4 A g⁻¹ in rate test and 121.4 mAh g⁻¹ at 1 A g⁻¹ after 1000 cycles in cycling test. The experimental results show that NaTi₂(PO₄)₃/C with Sn substitution with proper content exhibits the great potential in anode for sodium ion batteries, and can further provide reference for next generation electrode materials and battery systems.     

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.     

Hydrothermal synthesis of Na4Mn9O18 nanowires for sodium ion batteries

Na₄Mn₉O₁₈ was recognized as the most interesting material for sodium ion batteries due to its low cost, high specific capacity and good cycle performance. The excellent electrochemical performance of Na₄Mn₉O₁₈ nanostructures was shown in literature. In this work, Na₄Mn₉O₁₈ nanowires were synthesized by hydrothermal reactions of Mn₂O₃ powder and NaOH solution at the temperatures of 185–205 °C for 48–96 h. The investigation of composition and structure of the synthesized products via scanning electron microscopy and X-ray diffraction analyses showed that major intermediate products at the low and high temperatures were Mn₃O₄ and birnessite Na_0.55Mn₂O₄1.5H₂O, respectively. The synthesized Na₄Mn₉O₁₈ nanowires showed a good electrochemical performance with discharge capacities of over 90 mAhg⁻¹, and Coulombic efficiencies of more than 91% at a rate of 0.2C during 30 cycles of charge/discharge.     

A new high-performance cathode material for rechargeable lithium-ion batteries: Polypyrrole/vanadium oxide nanotubes

Polypyrrole/vanadium oxide nanotubes (PPy/VOx-NTs) as a new high-performance cathode material for rechargeable lithium-ion batteries are synthesized by a combination of hydrothermal treatment and cationic exchange technique. The morphologies and structures of the as-prepared samples are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, thermogravimetry and differential scanning calorimeter (TG-DSC) and X-ray powder diffraction (XRD). The results indicate that the organic templates are mainly substituted by the conducting polymer polypyrrole without destroying the previous nanotube structure. Their electrochemical properties are evaluated via galvanostatic charge/discharge cycling, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). It is found that PPy/VOx-NTs exhibit high discharge capacity and excellent cycling performance at different current densities compared to vanadium oxide nanotubes (VOx-NTs). After 20 cycles, the reversible capacity of PPy/VOx-NTs (159.5 mAh g⁻¹) at the current density of 80 mA g⁻¹ is about four times of magnitude higher than that of VOx-NTs (37.5 mAh g⁻¹). The improved electrochemical performance could be attributed to the enhanced electronic conductivity and the improved structural flexibility resulted from the incorporation of the conducting polymer polypyrrole.     

Three-dimensional (3D) LiMn0.8Fe0.2PO4 nanoflowers assembled from interconnected nanoflakes as cathode materials for lithium ion batteries

Three-dimensional (3D) olivine LiMn_0.8Fe_0.2PO₄ nanoflowers constructed by two-dimensional (2D) nanoflakes have been successfully synthesized through an easy liquid phase method. Hierarchical LiMn_0.8Fe_0.2PO₄/C could be easily formed via a liquid coating technology and subsequent calcination treatment. When acting as cathode materials for lithium ion batteries, the LiMn_0.8Fe_0.2PO₄/C nanoflowers show excellent rate performance and cycle stability. The unique flower-like hierarchical structured LiMn_0.8Fe_0.2PO₄ and thin carbon coating outside make this composite a promising candidate as cathode materials for lithium ion batteries.     

MoO3/reduced graphene oxide composites as anode material for sodium ion batteries

MoO₃/reduced graphene oxide (MoO₃/RGO) composites were successfully prepared via a facile one-step hydrothermal method, and evaluated as anode materials for sodium ion batteries (SIBs). The crystal structures, morphologies and electrochemical properties of the as-prepared samples were characterized by X-ray diffraction, field-emission scanning electron microscopy, cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge/discharge tests, respectively. The results show that the introduction of RGO can enhance the electrochemical performances of MoO₃/RGO composites. MoO₃/RGO composite with 6 wt% RGO delivers the highest reversible capacity of ~208 mA h g⁻¹ at 50 mA g⁻¹ after 50 cycles with good cycling stability and excellent rate performance for SIBs. The excellent sodium storage performance of MoO₃/RGO should be attributed to the synergistic effect between MoO₃ and RGO, which offers the increased electrical conductivity, the facilitated electron transfer ability and the buffering of volume expansion.     

锂离子电池Li2Fe1-xVxSiO4/C复合正极材料的合成、结构形貌和电化学性能

采用液相预处理和固相反应相结合的方法合成了聚阴离子型锂离子电池Li2Fe1-xVxSiO4/C（x=0,0.1,0.3,0.5）复合正极材料,采用XRD、SEM、EDX和电化学测试对材料进行了表征,研究了钒掺杂对材料结构、形貌和电化学性能的影响。结果表明所有样品均为正交晶系,属P21mn空间群,钒掺杂量x=0.1时样品晶格常数发生明显变化,晶胞呈c轴方向拉长的立方体,导致了材料的比容量和循环性能下降;所有样品均为纳米至亚微米尺寸颗粒,且随钒掺杂量的增加而增大,但x=0.5时样品的电化学性能最优。在常温和0.2C倍率下初始放电容量为124.3mAh/g,循环10次后容量无衰减,可逆放电容量仍高达126.2mAh/g,显示了良好的循环性能和应用前景。     

Microwave-assisted synthesis of LiNi0.5Co0.5O2 cathode material for lithium batteries using PAM as template

Layered cathode materials LiNi_0.5Co_0.5O₂ were successfully synthesized by microwave-assisted method using polyacrylamide (PAM) as template. Effects of the PAM concentration, sintering temperature and time on the morphology, microstructure and electrochemical performance of the materials were systematically investigated. X-ray diffraction (XRD) patterns reveal that the sample prepared with 8 wt.% PAM and sintered at 1023 K for 4 h shows the best ordering layered structure with the maximum I( 0 0 3)/I(1 0 4) ratio and the largest distance of splitting diffraction peaks of the crystal plants (0 0 6) and (0 1 2 ), (0 1 8) and (1 1 0). It can be seen that the above sample is composed of sphere-like particle from the scanning electronic microscopies (SEM) observation. The charge-discharge experiments indicate that the sample, compared with the samples prepared under other conditions, also has the best electrochemical properties, with the largest discharge capacity of 154 mAh/g and the capacity retention of 145 mAh/g after 20 cycles at a 0.2C rate between 3.0 and 4.3 V. The study confirmed that the application of microwave is in favor of the formation of nuclei, which plays a key role in shortening the synthetic time and reducing the sintering temperature.     

锂离子电池正极材料LiFePO4的研究进展

从LiFePO4的结构出发,分析了该材料所特有的优越性能以及存在的缺陷,阐述了物理掺杂和体相掺杂两类改性方法的特点和取得的成效。在此基础上,介绍了高温固相法、共沉淀法等方法合成LiFePO4的最新研究进展,探讨了各种制备方法的优缺点,并简要评述了LiFePO4未来发展的前景以及为使该材料走向实用化应注重的研究方向。     

Scalable synthesis of novel V2O3/carbon composite as advanced cathode material for aqueous zinc-ion batteries

Aqueous zinc-ion batteries (AZIBs) are now receiving incremental attention because of their inherent security and reduced cost of metal zinc. As one type of promising cathode candidates for AZIBs, V₂O₃-based materials have been widely investigated due to the special tunnel structure and high energy density. Nevertheless, the wide application of V₂O₃-based materials is still limited by the weak reaction kinetics, inferior cycling stability as well as unsatisfying strategies for large-scale synthesis. Herein, we designed and synthesized V₂O₃/carbon composite with V₂O₃ coated with a thin carbon layer (denoted as B–V₂O₃@C) via a facile ball-milling route as cathode material for AZIBs. Benefiting from the desirable structural and process features, the bottlenecks above can be effectively addressed. As a result, the as-synthesized B–V₂O₃@C delivers a considerable reversible capacity (as high as 430 mAh g^?1 at 1000 mA g^?1) and enhanced cycling stability (84 mAh g^?1 after 2000 cycles at 5000 mA g^?1), which are much superior than the those of the commercial V₂O₃ (C–V₂O₃). Besides, the Zn-storage mechanism and application in full battery based on B–V₂O₃@C were successively investigated. This work might contribute to the possible large scale application of high-performance V₂O₃-based cathode materials for AZIBs.     

Electrospinning synthesis of Na2MnPO4F/C nanofibers as a high voltage cathode material for Na-ion batteries

Three-dimensional carbon nanofibers embedded with Na₂MnPO₄F nanoparticles are fabricated via electrospinning method and investigated as cathode material for sodium ion batteries. The Na₂MnPO₄F nanoparticles with a size of about 10–30?nm are well-crystallized and the diameter of the carbon nanofibers are about 100?nm. Due to the ultrafine particle size of Na₂MnPO₄F together with high conductivity of the three-dimensional electron/ion hybrid network of carbon nanofibers, the material synthesized at 650?°C exhibit good electrochemical performance at room temperature. It is found that an obvious potential platform as high as 3.6?V during charge/discharge processes occurs and there is an initial specific capacity of 122.4?mAh?g^?1 at 0.05C rate, which is close to the theoretic capacity (one Na⁺ extracted) of Na₂MnPO₄F. This work suggests a new design strategy for high-performance Na₂MnPO₄F cathodes of sodium-ion batteries.