The uneven electric field in cable accessory insulation can be optimized by field grading composite (FGC). We explored graphitic carbon nitride (g-C₃N₄) as a filler in liquid silicone rubber (LSR) matrices. Oxygen-doped g-C₃N₄ (O-g-C₃N₄) was prepared via calcination of g-C₃N₄ with ascorbic acid. Composites of g-C₃N₄/LSR and O-g-C₃N₄/LSR with different filler contents were fabricated. Microstructural and optical characterizations demonstrate that O-g-C₃N₄ retains the crystal structure of pristine g-C₃N₄ but exhibits thinner layers, modified elemental composition, and a 27.8% reduction in band gap; fillers are uniformly dispersed in the LSR matrix. Experimental measurements reveal that both composites exhibit nonlinear conductivity, while O-g-C₃N₄/LSR shows more pronounced nonlinearity at lower filler contents, accompanied by a faster decline in dielectric breakdown strength. There is little difference in thermal conductivity between g-C₃N₄/LSR and O-g-C₃N₄/LSR composites with the same filler content, which indicates that the change in band gap width has no significant influence on thermal conductivity. The low-cost synthesis and simple bandgap tuning method of g-C₃N₄ provide certain advantages for its use as a nonlinear filler in the preparation of FGC, broadening the application fields of g-C₃N₄, and verifying the possibility of reducing FGC filler usage through bandgap tuning.

With the rapid construction of new power systems characterized by high renewable energy penetration, high power electronics integration, and high voltage levels, the insulation reliability of critical power equipment—including cable accessories, gas-insulated switchgear (GIS), and power electronic modules—faces unprecedented challenges. Field grading materials (FGM), as core functional media for adaptive electric field homogenization and insulation failure prevention, have emerged as a research hotspot spanning materials science, electrical engineering, and polymer engineering. Starting from the current research status of FGM, this review systematically summarizes filler optimization strategies, covering single fillers, hybrid fillers, trace co-fillers, and structural modification approaches. The applications of FGM in transmission cables, GIS, high-voltage electrical machines, and wide-bandgap power electronic modules are then elaborated in detail. Emphasis is placed on performance enhancement routes of FGM, particularly thermal conductivity improvement via constructing three-dimensional thermally conductive networks and intelligent early warning based on thermochromic materials. Finally, the existing bottlenecks of FGM are analyzed in terms of material stability, multi-physical field coupling adaptation, and engineering industrialization. Future development trends are prospected toward high-performance, multifunctional, intelligent, and engineering-oriented FGM. This review aims to provide theoretical references and technical support for the design and application of advanced FGM in new power systems.