Ionic liquid-induced graphitization of biochar: N/P dual-doped carbon nanosheets for high-performance lithium/sodium storage

Biomass-derived graphitic carbon is becoming an attractive material for anodes in lithium-ion and sodium-ion batteries (LIBs and SIBs) owing to its sustainability. The graphitization of biochar by heating above 2600 °C not only requires high energy consumption but also removes heteroatoms that are conducive to electrochemical energy storage. In this study, graphitic carbon nanosheets with N/P-dual doping are facilely synthesized by one-step carbonization of pine sawdust at 800/1000/1200 °C that is priorly dissolved in 1-butyl-3-methyl-imidazolium ([Bmim]H₂PO₄) and used as anodes of LIBs/SIBs, respectively. [Bmim]H₂PO₄ simultaneously promotes the graphitization and porosities of the biochar as carbonization temperature increases in addition to providing N/P-dual doping. Used as anodes of LIBs, IWC-1200 demonstrates excellent rate performance and cyclic stability, delivering 385 mAh g⁻¹ at 1 Ag⁻¹ throughout 1000 cycles. For sodium storage, IWC-1000 exhibits stable capacities of 217 mAh g⁻¹ at 0.1 A g⁻¹ and 101 mAh g⁻¹ at 1 Ag⁻¹. The electrochemical performances benefit from the graphitized structure with N/P-dual doping, leading to redox pseudocapacitance for lithium/sodium storage. DFT calculations suggest that pyridinic N strongly attracts both Li and Na while P atoms inside the graphitic structure significantly increase the interlayer spacing.