Recently, a lateral heterostructure (LHS) combining black phosphorene and graphene edges was developed, addressing volume change issues and enhancing capacity retention. However, unresolved geometric concerns require further investigation, particularly regarding its suitability as a sodium-ion battery (SIB) anode. We present three LHSs models, labeled (LHS PC)1, (LHS PC)2, and P5C6, all featuring edge contact symmetry (armchair direction) but with different interface defects. Through first-principles calculations, we compare their structural stability, electronic properties, and charge transfer mechanisms. P5C6 exhibits superior energetic stability and dynamic stability, with type II band alignment and a 0.73 eV direct band gap, indicating optimal charge diffusion. Electrochemical assessments reveal a low sodium migration barrier (0.019 eV), facilitating rapid charge-discharge rates, with a storage capacity of 590.5 mAh/g and intercalation at a low average voltage (0.27 V). Additionally, ab initio molecular dynamics (AIMD) confirm its thermal stability during sodium storage, underscoring P5C6's potential as a promising SIB anode material. Display omitted •Inventive heterostructure P5C6 mixes BP and graphene synergy perks capably.•Na intercalation into P5C6 yields a theoretical capacity of 590.5 mAh/g.•P5C6 presents fast sodium diffusion, with a 0.019 eV low energy barrier.•Insertion of Na into P5C6 can enhance electronic conductivity.•P5C6 serves as a superb anode for SIBs: excellent performance.