Design advanced nitrogen/oxygen co-doped hard carbon microspheres from phenolic resin with boosted Na-storage performance

Although hard carbons are considered as promising anode candidates for commercial sodium-ion batteries (SIBs), the trade-off between high Na-storage capacity and initial coulombic efficiency (ICE) remains a critical challenge for their practical application. Herein, nitrogen/oxygen co-doped hard carbon microspheres (N/O–HCs) for SIBs are prepared by a simple solvothermal treatment to allow the efficient cross-linking of phenolic resin with melamine and followed high-temperature pyrolysis process. The heteroatom doping promotes structural disordering of hard carbons and generates more active sites for Na ion storage, whereas the cross-linking of melamine with phenol resin efficiently restricts the formation of surface defects. Therefore, the obtained N/O-HC-1200 electrode exhibits both high reversible Na-storage capacity of 319.7 mAh g−1 and much improved ICE of 81.6%. Moreover, the electrode demonstrates excellent cycle and rate performance with the capacity stabilized at 253.9 mAh g−1 at 1C over 1000 cycles. When coupled with NaFe1/3Ni1/3Mn1/3O2 cathode, the constructed full cell delivers a high energy density of 247.6 Wh kg−1 and outstanding rate capability with a retained capacity of 70 mAh g−1 at an ultrahigh current rate of 50C. The proposed work offers a new perspective on the surface chemistry and structural design of high-performance hard carbon anodes for SIBs.