A hard carbon material with free-standing porous structure and high contents of heteroatom functional groups is considered to be a potential anode for potassium-ion batteries (PIBs). Herein, a free-standing phosphorus/nitrogen cofunctionalized porous carbon monolith (denoted as PN-PCM) anode for PIBs is successfully fabricated via a supercritical CO2 foaming technology, followed by amidoximation, phosphorylation, and thermal treatment. Thanks to the synergistic effect of a three-dimensional macroporous open structure and high P/N contents of 6.19/5.74 at%, the PN-PCM anode delivers an excellent reversible specific capacity (396 mA h g-1 at 0.1 A g-1 after 300 cycles) with high initial Coulombic efficiency (63.6%), a great rate performance (168 mA h g-1 at 5 A g-1), and an ultralong cycling stability (218 mA h g-1 at 1 A g-1 after 3000 cycles). Theoretical calculations clarify that in a P/N cofunctionalized carbon, the P-C bonds devote more to enhancing the potassium storage via adsorption and improving electronic conductivity of carbon, while P-O bonds contribute more to enlarging the interlayer distance of carbon and reducing the ion diffusion barrier.
Keywords: hard carbon anodes; high initial Coulombic efficiency; phosphorus/nitrogen cofunctionalized; phosphorylation; porous carbon monoliths; potassium-ion batteries.