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.

A certain amount of sodium citrate (Na₃C₆H₅O₇) 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 (V₂O₅, 1 mmol) and oxalic acid dihydrate (H₂C₂O₄·2H₂O, 2 mmol) were dissolved in 30 mL 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 (NH₄H₂PO₄, 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.

A novel biphasic-VPC composite (Bis-VPC), composed of high-capacity Na₃V₂(PO₄)₂F₃ (NVPF) and high-stability Na₃V₂(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.

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.