Controlled synthesis of multi-doped highly-disordered porous biomass carbon microsphere for ultra-stable and fast sodium storage

Biomass-derived hard carbon has been considered as one of the most promising anodes for sodium-ion batteries (SIBs), owing to its natural abundance, sustainability, and in-situ multi-heteroatom doping property. In this work, highly-disordered porous carbon microspheres with in-situ ternary N, P, O-doping (NOP-PCM) are synthesized through a hydrothermal and high-temperature activation process, using porphyra as precursor. Through seriously regulating the calcination temperature and KOH amount, the optimal NOP-PCM-800 can well preserve the microsphere structure, producing abundant pores and defects. Attributed to the porous structure and in-situ heteroatom doping, NOP-PCM-800 can shorten the Na+ transport distance, increase the active sites for the Na+ adsorption and enhance the electron conductivity, leading to enhanced high-rate sodium storage capability. As anode material for sodium ion batteries (SIBs), NOP-PCM-800 can deliver stable capacities of 56.1 mAh g−1 and high capacity retention of 94.4 % at 5.0 A g−1 over 8000 cycles, which is promoted by the enhanced pseudo-capacitive contribution. Based on the ex-situ Raman analysis, the adsorption-intercalation mechanism is responsible for the sodium storage. And in the full cell coupled with Na3V2(PO4)3@C, it can also keep a capacity of 76.9 mAh g−1 at 0.1 A g−1 over 100 cycles, demonstrating its potential practical application.