硅酸盐学报

钒基磷酸盐复合材料(VPC)凭借其优异的结构稳定性和灵活的电压可调性，成为钠离子电池正极材料的候选者之一。然而，VPC本征电导率低和离子扩散速率慢等固有缺陷导致VPC存在倍率性能和循环稳定性不足的问题。采用简单的溶胶–凝胶辅助固相法，结合类石墨烯纳米片(GNS)协同改性策略，成功制备了一种由高容量Na_3V₂(PO₄)_2F₃(NVPF)和高稳定性Na_3V₂(PO₄)₃(NVP)组成的新型双相VPC(Bis-VPC)。GNS在Bis-VPC颗粒间构建起三维多孔网络结构，有效抑制了颗粒的无序生长与团聚，同时促进了离子与电子的传输；同时，GNS还通过减少合成过程中的氟损失，实现了NVPF与NVP相比例的调控。优化后的Bis-VPC在0.2 C倍率下表现出118.7 mA·h·g^–1的可逆容量，在20 C高倍率下循环5000次后仍具有90.4%的容量保持率。上述结果揭示了GNS碳材料在调控VPC正极相组成中的关键作用，为设计兼具高倍率和长寿命钠离子电池正极材料提供了新的研究思路。

Introduction Vanadium-based phosphate composite(VPC) can be used as a highly promising candidate for sodium-ion battery cathode materials due to its excellent structure stability and flexible voltage adjustability. However, some inherent limitations such as low intrinsic conductivity and low ion diffusion, lead to an inferior rate capability and an insufficient cycle stability in VPC materials. Efforts are dedicated to the improvement of cathode materials, such as bulk structure regulation and surface coating modification. Among them, carbon coating is usually applied to modify the inherently low conductivity of polyanion-type cathodes. The graphitization degree of pyrolytic carbon is restricted due to the limitation of VPC sintering temperature. This restriction results in a marginal enhancement of the electronic conductivity in carbon-coated VPC. To compensate for this deficiency, highly conductive carbon materials such as GO and CNTs are often incorporated into the fabrication process of VPC materials. Nevertheless, the utilization of these carbon materials can lead to a significant increase in production costs. It is thus crucial for improving the conductivity of VPC composite and simultaneously regulating the local electronic structure of vanadium ions to develop carbon materials with high cost-effectiveness and conductivity. Methods A certain amount of sodium citrate(Na_3C_6H_5O₇) was subjected to a sintering treatment in N₂ atmosphere at 800 ℃ for 2 h with a heating speed of 5 ℃·min^–1. After natural cooling, the resultant black powder was then immersed in a dilute solution of HCl(2 mol·L^–1) for 24 h to remove all sodium-containing compounds. Subsequently, graphene-like nanosheets(marked as GNS) as the product were collected via filtering, repeated rinsing with ultrapure water and ethanol, and finally drying at 80 ℃ for overnight. Vanadium pentoxide(V_2O₅, 1 mmol) and oxalic acid dihydrate(H_2C_2O₄·2H_2O, 2 mmol) were dissolved in 30 m L ultrapure water. After being stirred at 60 ℃ for 30 min, the initially orange turbid solution gradually transformed into a clear green solution. Subsequently, ammonium dihydrogen phosphate(NH_4H_2PO₄, 2 mmol) and sodium fluoride(NaF, 3 mmol) were added into the green solution above. Meanwhile, different specific quantities(i.e., 0.025, 0.033 g and 0.041 g) of GNS powder were individually dispersed into 10 mL ethyl alcohol under ultrasonic agitation for 5 min to obtain a uniform dispersion solution. The green solution was then added to the dispersion solution above. The mixture solution was subjected to continuous stirring at 60 ℃ until the gel was formed. Afterwards, the obtained gel was dried in an oven at 80 ℃ for overnight, and then ground into a fine powder. Finally, the resultant powder was first sintered at 350 ℃ for 5 h and subsequently in N₂ atmosphere at 600 ℃ for 8 h with a heating rate of 3 ℃·min^–1. The vanadium-base phosphate composites prepared with 0.025, 0.033 g and 0.041 g GNS powder were denoted as VPC-GN6, VPC-GN8 and VPC-GN10, respectively. For comparison purpose, a pristine VPC without GNS was prepared by the same method as for GNS modified VPC samples and labeled as P-VPC. Results and discussion A novel biphasic-VPC composite(Bis-VPC), composed of high-capacity Na_3V₂(PO₄)_2F₃(NVPF) and high-stability Na_3V₂(PO₄)₃(NVP), is effectively prepared by a facile sol-gel assisted solid-state method, and facilitated by the incorporation of GNS. The results obtained from the XRD, XPS and Rietveld refinement demonstrate that the incorporation of an appropriate amount of GNS effectively modulates the content of the NVP phase in VPC composites, while simultaneously broadening the transport channels of Na⁺. Furthermore, The results of Raman spectra, SEM/TEM images, and EIS measurements indicate that GNS can suppress the uncontrolled growth and agglomeration of particles, and facilitate the construction of an efficient ion/electron conductive network. As a result, the electronic conductivity of the optimized sample is significantly enhanced by several orders of magnitude from the 1.25×10^–9 S·cm^–1 to 1.06×10^–4 S·cm^–1. Simultaneously, the VPC-GN8 shows a D_Na+ value of 1.20×10^–12 cm²·s^–1, surpassing that of the P-VPC(5.43×10^–13 cm²·s^–1). The optimized sample also exhibits a high reversible specific capacity of 118.7 mAh·g^–1 at 0.2 C and maintains an excellent capacity retention of 90.4% after 5000 cycles at 20 C. These results indicate that the optimization of phase components through incorporating an appropriate amount of GNS and the construction of highly conductive networks can expand Na+ migration channels, facilitate ion and electron transport, and enhance electronic conductivity, thereby improving the durability under high-rate conditions. Conclusions In this study, the incorporation of GNS could effectively mitigate the fluorine loss during the synthesis process, thereby facilitating the increase of NVPF phase and improvement of the ionic conductivity of Bis-VPC material. The introduction of GNS constructed an interconnected three-dimensional porous architecture within the matrix of Bis-VPC particles, effectively mitigating the uncontrolled growth and agglomeration of particles and accelerating the transport of both ions and electrons. In addition, the generated porous structure also increased the interface between Bis-VPC particles and electrolyte. Consequently, the modified sample exhibited improved reversible capacity, exceptional rate performance and ultra-long cycle life. This novel modification strategy, which enhanced ion and electronic conductivity through graphene like nanosheets, could accelerate the application of high-energy-density vanadium-based phosphate cathode materials in large-scale energy storage for SIBs.