Outer shell structural engineering for tunable sodium storage in pitch-derived carbon anodes

Abstract

Hard carbon (HC) is a promising anode for sodium-ion batteries (SIBs) due to its high theoretical capacity and low working potential. Previous studies have extensively correlated Na-ion storage behavior with bulk interlayer spacing, closed pore volume, and graphitization degree, but have failed to resolve the zonal characteristics of reversible capacity and rate capability in HC electrodes. Herein, we move beyond the conventional “bulk structure” perspective and demonstrate that the electrochemical behavior of pitch-derived carbon anodes is predominantly governed by the microstructure of the outer shell. We reveal that the outer shell serves as the primary electrochemically active domain, where large interlayer spacing microdomains and closed pores are directly accessible to the electrolyte and effectively contribute to low-potential Na-ion storage, while a sufficiently graphitized shell framework dictates the fast charge-transfer kinetics. Based on this spatially resolved understanding, we propose a new design paradigm for carbon-based anodes, selectively tailoring the large interlayer spacing microdomain, closed pore, and graphitization degree in the outer shell region. The pitch-derived carbon with a hard carbon shell (PCHC) delivers a higher reversible capacity at a low current density (310.2 vs. 294.2 mAh g⁻¹ at 25 mA g⁻¹), whereas that with a soft carbon shell (PCSC) exhibits a superior high-rate capability (193.1 vs. 133.1 mAh g⁻¹ at 500 mA g⁻¹). This work clarifies that the long-standing capacity-rate dilemma in HC anodes originates from a structural spatial mismatch rather than compositional limitations. We establish outer shell engineering as a generalizable strategy for reconciling high energy density and high power density in next-generation SIBs.