Orbital hybridization states of carbon functionalize the alkali-ion storage capability of hard carbons

Experimentally, hard carbons (HCs) synthesized under different conditions always show various alkali-ion (e.g., Li+, Na+, and K+) storage capabilities. However, the diversity of the precursor and the uncertain amorphous microstructure make it difficult to fully understand the origination of the electrochemical variety of HCs. Herein, fullerene (C60), a heteroatom-free and structure-confirmed precursor, is chosen to build “pure” carbon models (C60-Ts) for exploring the correlations between inherent characteristics and alkali-ion storage behaviors in HCs. The electrochemical results indicate that the C60-800 sample exhibits the highest specific capacity and best rate capability for Li+, Na+, and K+ storage. Various spectrometric characterizations and theoretical simulations demonstrate that the extra capacity of C60-800 mainly originates from the higher ratio of sp3 and sp2-hybridized carbon atoms (sp3/sp2-C). The existence of sp3-C could affect the local electronic distribution around sp2-C and even lower the absorption energy of alkali-ions. This work presents a novel orbital hybridization state-related strategy for designing high-capacity electrode materials of alkali-ion batteries.