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

Abstract

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-T_s) 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.