Mechanistic Mapping of Alkali-Ion Storage in Micro-Spherical Closed-Pores Hard Carbon: Electrochemical, Ex-Situ, and DFT Approaches

Abstract

Investigating porous closed-pore hard carbon (HC) anodes is crucial for advancing alkali-ion batteries. In this comprehensive study, electrochemical evaluations revealed that HC anodes demonstrated notable reversible capacities of 422 mAh g-1 at 0.1 C for SIBs, with 57% of this capacity originating from low-potential plateau regions, thus establishing a benchmark for undoped HCs. Similar performance was observed for LIBs (444 mAh g-1, ~25% more than graphite) and PIBs (235 mAh g-1), accompanied by excellent cyclic stability. To elucidate the ion storage mechanisms, we combined electrochemical analyses, differential capacity plots, galvanostatic intermittent titration technique (GITT), with ex situ characterizations (Raman, EPR, XPS), and density functional theory (DFT) simulations. The sloping capacity region arises from defect-assisted adsorption (AC) and intercalation (IC) facilitated by edge defects and expanded graphitic layers. In contrast, the plateau region originated from insertion followed by pore filling, leading to pseudo-metallic cluster formation. EPR confirmed metallic clusters at 0 V for Na and Li, supporting the pore-filling (FC) mechanism, while DFT calculations revealed that alkali-ion binding energetics depend strongly on interlayer spacing and micropore diameter, favouring Na-ion and K-ion storage in expanded graphitic layers and smaller micropores. Mechanistic analysis established the capacity contribution order as: LIBs: FC < AC < IC; SIBs: IC < AC < FC; and PIBs: FC < IC < AC. These insights bridge experimental and theoretical understanding, providing a framework for designing next-generation alkali-ion battery anodes.