Design of Ti4+-doped Li3V2(PO4)3/C fibers for lithium energy storage

In this work, we propose a facile approach to fabricate Ti⁴⁺-doped Li₃V₂(PO₄)₃/C (abbreviated as C-LVTP) nanofibers using an electrospinning route followed by a high temperature treatment. In this designed nanocomposite, the ultrafine LVTP dots are homogeneously dispersed into one-dimensional carbon nanofibers and the Ti⁴⁺ doping does not destroy the crystal structure of monoclinic Li₃V₂(PO₄)₃. Compared to the undoped Li₃V₂(PO₄)₃/C (abbreviated as C-LVP), the as-fabricated C-LVTP fibers present higher reversible capacity, superior high-rate capability as well as better cyclic property. Especially, the C-LVT_7%P cathode delivers not only high capacities of 187.2 and 160.3 mAh g^?1 at 0.5 and 10 C respectively, but also stable cyclic property with the reversible capacity of 135.8 mAh g^?1 at 20 C following 500-cycle spans. The good battery characteristics of C-LVT_7%P can be mainly ascribed to Ti⁴⁺ doping, which can increase the electrical conductivity and Li⁺ diffusion coefficient.     

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.     

High performance Na3V2(PO4)3 cathode prepared by a facile solution evaporation method for sodium-ion batteries

Based on its abundance and low cost, sodium based batteries have aroused extensive attention for large scale energy-storage systems. In the current work, Na₃V₂(PO₄)₃ prepared by a facile solution evaporation method (denoted as NVP-SE) is used as cathode materials for sodium ion battery, with a control sample by solid state method. Raman spectrum and TEM are used to study the carbon layer coated on NVP-SE. The results show a highly graphitization and well-coated carbon layer, which is predominant by sp² carbon. Graphitized carbon leads to high electrical conductivity, which can improve the rate performance of Na₃V₂(PO₄)₃ materials. Besides, GITT tests show high Na-ion diffusion coefficient. Even at 30 C, the NVP-SE cathode still delivers a capacity of 70 mAh g⁻¹. Moreover, the material also shows great long term cycling performance. After 500 cycles at 1 C rate and 1000 cycles at 5 C, its discharge capacities are still 103.3 mAh g⁻¹ and 85.4 mAh g⁻¹, which maintain 92.6% and 85.0% of its initial capacity. Thus, simple preparation process and excellent electrochemical performance for Na₃V₂(PO₄)₃/C extend it as a potential material for high power applications.     

Design and fabrication of N-doped graphene decorated LiFePO4@C composite as a potential cathode for electrochemical energy storage

Olivine-structured LiFePO₄@C nanoparticles grown on N-doped graphene (NG) sheets have been fabricated by using a simple sol-gel method with post calcinations. During the synthesis process, both the carbon film and NG sheets can inhibit the growth of LiFePO₄ particles. Meanwhile, the constructed conductive network between the NG and carbon film can greatly enhance the electronic conductivity of LiFePO₄ material. These unique properties lead to markedly improved lithium storage performance. The NG-decorated LiFePO₄@C (NG-LiFePO₄@C) composite presents high specific capacity (163.1 mAh g^?1, 0.1 C) and excellent rate capability (118.6 mAh g^?1, 10 C). Therefore, this NG-LiFePO₄@C composite can be regarded as a potential electrode for electrochemical energy storage.     

ZrO2-coated Li3V2(PO4)3/C nanocomposite: A high-voltage cathode for rechargeable lithium-ion batteries with remarkable cycling performance

In this study, we show that the poor cycling performance which seriously hinders the application of Li₃V₂(PO₄)₃/C for rechargeable lithium-ion batteries is overcome by amorphous ZrO₂ nano-coating. The ZrO₂-coated Li₃V₂(PO₄)₃/C was synthesized via a conventional solid-state method followed by the application of wet coating. The crystalline structure, morphology and electrochemical performance of the as-synthesized samples were investigated by XRD, SEM, TEM, EDS, galvanostatic charge/discharge and EIS measurements. Compared with the pristine Li₃V₂(PO₄)₃/C, the structure of ZrO₂-coated Li₃V₂(PO₄)₃/C sample had no change, and the existence of ZrO₂ nano-coating effectively enhanced the cycling performance. From the above results, it is believed that the improved cycling performance is attributed to the ability of ZrO₂ layer in preventing direct contact of the active material with the electrolyte resulting in a decrease of electrolyte decomposition reactions.     

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.     

焙烧条件对Na3V2(PO4)3/C的制备及储锌性能影响

采用喷雾干燥法合成了Na₃V₂(PO₄)₃(NVP)前驱体,然后经过高温煅烧得到水系锌离子电池正极复合材料Na₃V₂(PO₄)₃/C(NVP/C),考察了煅烧温度和煅烧时间对NVP/C性能的影响。通过XRD、SEM和BET对样品结构和形貌进行了表征,通过循环伏安和充放电测试了样品的电化学性能。结果表明,不同煅烧温度和煅烧时间制备样品均为纯相的NVP/C,且并没有改变NVP/C的晶体结构;煅烧温度过高或煅烧时间过长会导致晶粒尺寸增大,性能迅速衰减。NVP/C制备最佳条件为煅烧温度700℃、煅烧时间8 h,在该条件下所制备的NVP/C(记为NVP/C-700-8)形貌更为规整,结晶性良好,具有较小的阻抗以及更好的离子扩散能力,进而表现出最佳的电化学性能。在0.1 A/g电流密度下表现出最佳的放电比容量(122.4 mA·h/g)。在1.0 A/g电流密度下经过200圈循环后放电比容量仍高达103.9 mA·h/g。     

Scalable synthesis of Na3V2(PO4)3/C with high safety and ultrahigh-rate performance for sodium-ion batteries

NASICON-type Na₃V₂(PO₄)₃ is a promising electrode material for developing advanced sodium-ion batteries. Preparing Na₃V₂(PO₄)₃ with good performance by a cost-effective and large-scale method is significant for industrial applications. In this work, a porous Na₃V₂(PO₄)₃/C cathode material with excellent electrochemical performance is successfully prepared by an agar-gel combined with freeze-drying method. The Na₃V₂(PO₄)₃/C cathode displayed specific capacities of 113.4 mAh·g^-1, 107.0 mAh·g^-1 and 87.1 mAh·g^-1 at 0.1 C, 1 C and 10 C, respectively. For the first time, the 500-mAh soft-packed symmetrical sodium-ion batteries based on Na₃V₂(PO₄)₃/C electrodes are successfully fabricated. The 500-mAh symmetrical batteries exhibit outstanding low temperature performance with a capacity retention of 83% at 0 ℃ owing to the rapid sodium ion migration ability and structural stability of Na₃V₂(PO₄)₃/C. Moreover, the thermal runaway features are revealed by accelerating rate calorimetry (ARC) test for the first time. Thermal stability and safety of the symmetrical batteries are demonstrated to be better than lithium-ion batteries and some reported sodium-ion batteries. Our work makes it clear that the soft-packed symmetrical sodium ion batteries based on Na₃V₂(PO₄)₃/C have a prospect of practical application in high safety requirement fields.     

Optimal synthesis and new understanding of P2-type Na2/3Mn1/2Fe1/4Co1/4O2 as an advanced cathode material in sodium-ion batteries with improved cycle stability

A sol-gel method with ethylene diamine tetraacetic acid and citric acid as co-chelates is employed for the synthesis of P2-type Na_2/3Mn_1/2Fe_1/4Co_1/4O₂ as cathode material for sodium-ion batteries. Among the various calcination temperatures, the Na_2/3Mn_1/2Fe_1/4Co_1/4O₂ with a pure P2-type phase calcined at 900 °C demonstrates the best cycle capacity, with a first discharge capacity of 157 mA h g^?1 and a capacity retention of 91 mA h g^?1 after 100 cycles. For comparison, the classic P2-type Na_2/3Mn_1/2Fe_1/2O₂ cathode prepared under the same conditions shows a comparable first discharge capacity of 150 mA h g^?1 but poorer cycling stability, with a capacity retention of only 42 mA h g^?1 after 100 cycles. Based on X-ray photoelectron spectroscopy, the introduction of cobalt together with sol-gel synthesis solves the severe capacity decay problem of P2-type Na_2/3Mn_1/2Fe_1/2O₂ by reducing the content of Mn and slowing down the loss of Mn on the surface of the Na_2/3Mn_1/2Fe_1/4Co_1/4O₂, as well as by improving the activity of Fe³⁺ and the stability of Fe⁴⁺ in the electrode. This research is the first to demonstrate the origin of the excellent cycle stability of Na_2/3Mn_1/2Fe_1/4Co_1/4O₂, which may provide a new strategy for the development of electrode materials for use in sodium-ion batteries.     

One-step microwave-assisted synthesis of Sb2O3/reduced graphene oxide composites as advanced anode materials for sodium-ion batteries

Sb₂O₃/reduced graphene oxide (RGO) composites were prepared through a facile microwave-assisted reduction of graphite oxide in SbCl₃ precursor solution, and investigated as anode material for sodium-ion batteries (SIBs). The experimental results show that a maximum specific capacity of 503 mA h g⁻¹ is achieved after 50 galvanostatic charge/discharge cycles at a current density of 100 mA g⁻¹ by optimizing the RGO content in the composites and an excellent rate performance is also obtained due to the synergistic effect between Sb₂O₃ and RGO. The high capacity, superior rate capability and excellent cycling performance of Sb₂O₃/RGO composites demonstrate their excellent sodium-ion storage ability and show their great potential as electrode materials for SIBs.     

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.     

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 of nanosheet-structured Na3V2(PO4)3/C as high-performance cathode material for sodium ion batteries using anthracite as carbon source

Recently, Na₃V₂(PO₄)₃ has shown great promise as cathode material for sodium-ion batteries. In this study, a series of carbon-modified Na₃V₂(PO₄)₃ (NVP/C) composites have been synthesized using anthracite as the carbon source. The NVP/C composite shows a nanosheet shape with a 3D continuously conductive network composed of carbon layer and carbon bump. The effect of anthracite dosage on the electrochemical performance of NVP/C has also been investigated. The results show that the NVP/C composite prepared with 10 wt% anthracite (NVP/C-10) exhibits the highest rate capability and a great cycle stability. Especially the NVP/C-10 electrode behaves an average capacity as high as 97 mAh g⁻¹ at a high current rate of 10 C. Moreover, NVP/C-10 still delivers a high specific capacity of 97.5 mAh g⁻¹ even after 800 cycles at 5 C, showing a very low capacity fading ratio of 0.012% per cycle. The excellent rate capability and cycle stability of NVP/C-10 can be ascribed to the synergistic effects of the nanosheet structure and the 3D continuously conductive network. Our results demonstrate that anthracite can be a promising carbon source for the preparation of NVP/C and other polyanion cathode materials as well.     

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.     

Dual-functional C-composited Na3.16Fe2.42(P2O7)2 cathode toward superior electrochemical performance for sodium-ion batteries

The development of a polyanion cathode for sodium-ion batteries is expected to accelerate the application of sodium-ion batteries in large-scale energy storage systems. However, the poor electrode conductivity is still a great challenge. In this paper, the novel carbon composite polyanion compound Na_3.16Fe_2.42(P₂O₇)₂@C (NFP@C) is developed through high-energy ball milling followed by annealing. The porous NFP nanoparticles modified with dual-functional C-composited for amorphous carbon coating and carbon nanofibers interpenetrating deliver excellent capacity retention of 85.3% after 1000 cycles at 5 C, which is more outstanding than pure NFP. X-ray diffraction, in situ galvanostatic intermittent titration techniques, electrochemical impedance spectroscopy, and cyclic voltammetry were performed to investigate the stability and sodium diffusion of NFP@C. The results show that the systematic and comprehensive dual-functional conductive network modification enables NFP exhibit excellent electronic and ionic conductivities, thereby improving the rate capability and cycling stability. Furthermore, a soft package sodium-ion full battery assembled based on NFP@C reveals a high-capacity retention of 95.2% for 150 cycles at 0.5C. This carbon composite strategy is simple and efficient and could be easily and widely extended to other cathodes in grid-scale energy storage applications.     

Na3V2(PO4)3 with specially designed carbon framework as high performance cathode for sodium-ion batteries

The structures of materials have great influence on their properties. For materials with low electron conductivity, fast electron transport pathway can be constructed through carbon structure design. Here we report a simple but effective method to improve the electrochemical performances of Na₃V₂(PO₄)₃. Polyvinyl Pyrrolidone (PVP) can improve the viscosity of the precursor solution, thus forming aggregate structured material. In Na₃V₂(PO₄)₃, primary particles with a diameter of approximately 300?nm are aggregated through a special carbon network to form micro-sized secondary particles. This kind of structure will provide easy access for electron transportation, thereby improving electrochemical performance of the material. As a cathode material for sodium-ion batteries, Na₃V₂(PO₄)₃ delivers excellent rate (86.6 mAh g^?1 at 30?C) and cycling performance (capacity retention of 88.4% after 2000 cycles at 10?C). The material also exhibits a specific capacity of 100.2 mAh g^?1 at 5?C under 55?°C. The above-mentioned performance is far better than the control sample without PVP. The special carbon network provides electron transport channels which improves the electrochemical performance of the material. This method may provide new ideas for the preparation of phosphate materials.     

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.     

One-pot pyro-synthesis of a high energy density LiFePO4-Li3V2(PO4)3 nanocomposite cathode for lithium-ion battery applications

A highly crystalline carbon-coated 0.66LiFePO₄•0.33Li₃V₂(PO₄)₃ (LFP-LVP) nanocomposite was synthesized by a one-pot pyro-synthetic strategy using a polyol medium at low temperature. Prior to any additional heat treatment, electron microscopy confirmed the as-synthesized composite to consist of spherical particles with average diameters in the range of 30–60 nm. A crystal growth phenomenon and particle aggregation was observed upon heat treatment at 800 °C, thus resulting in an increase in the average particle size to 200–300 nm. When tested for a lithium-ion cell, the nanocomposite electrode demonstrated impressive electrochemical properties with higher operating potentials hence enhanced energy densities. Specifically, the composite cathode delivered a high reversible capacity of 156 mAh g⁻¹ at 0.1 C and exhibited a remarkable reversible capacity of 119 mAh g⁻¹, corresponding to an energy density of 46.88 Wh Kg⁻¹ at 6.4 C. When cycling was performed at 6.4 C, the electrode could recover up to 85% of the capacity observed at low current density of 0.1 C, which indicates the excellent rate capability of the nanocomposite electrode. The enhanced performance was attributed to the inclusion of the high potential LVP phase constituent in the present cathode by a simple one-pot polyol-assisted pyro strategy.     

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.