Dual-element synergy driven breakthrough in sodium storage performance of phenolic resin-based hard carbon

The inherent kinetic limitations of Na⁺ storage in hard carbon anodes plague the advancements in energy density for sodium-ion batteries. Conventional mono-heteroatom doping approaches, constrained by unilateral electronic modulation, fail to synergistically address the dual challenges of creating adsorption-active sites and enhancing ionic diffusion kinetics. Herein, the boron-phosphorus co-doped hard carbon microspheres (BPHCS) were synthesized through a copolymerization and crosslinking reaction. The combined effects of P–O and P–C bonds, along with boron compounds (BC₃, BC₂O, BCO₂) alter the microcrystalline structure of hard carbon, introduce additional Na⁺ storage sites, and establish the n/p-type heteroatom synergy, creating complementary electron-hole transport pathways in hard carbon. The BPHCS exhibited a reversible capacity of 344 mAh g⁻¹ at 0.05 A g⁻¹ and maintained 174 mAh g⁻¹ after 5000 cycles at 5 A g⁻¹, demonstrating superior rate performance and cycling stability. Furthermore, boron doping promotes the formation of graphite-like regions, enhancing the intercalation capability of Na⁺. GITT, ex-situ Raman, and XRD confirm that the sodium storage mechanism in hard carbon follows a "surface adsorption, interlayer intercalation, and nanopore filling" model.