Macro-micro structure engineering of bio-spore-derived hard carbon as an efficient anode in sodium ion batteries

Elucidating the macro–micro structures of hard carbon is critical to address the issues of deficient Na+ storage capacity at the anode in Na+ ion batteries. Herein, hard carbon materials with various macro–micro structures were synthesized by pyrolyzing the spores of Calvatia Gigantea (SCG) as precursors treated by a coupling strategy with a dehydration process via concentrated H2SO4 treatment and the removal of inherent Si species by NaOH. Research results demonstrate that the process of concentrated H2SO4 pretreatment can induce the collapse of original hollow spheres of SCG to form specific hard carbon hollow hemispheres, while the NaOH leaching treatment can further modulate the microstructures of hard carbon. Attributed to the unique characteristics of specific hollow hemi-spherical morphology, ultra-low surface area, rich closed pore and appropriate layer spacing, the hard carbon not only exhibits outstanding specific capacity up to 500 mA h g−1 with a high initial Coulombic efficiency of 90.23 % for Na+ storage, but also presents excellent high-rate capability with 254.88 mA h g−1 at 5 A/g and cycling stability with a capacity retention rate of ≥80 % after 4102 cycles. The outstanding performance exceeds the vast majority of reported hard carbons for Na+ storage. Theoretical analysis further reveals that increasing closed pores constructed by distorted graphite-like layers together with proper lattice distance and curvature in hard carbon is essential to improve Na+ storage capacity. The coupling strategy of tuning the morphologies and microstructures of spore-derived hard carbon in the work provides new insights for developing advanced hard carbon with efficient Na+ storage performance.