Design principles for fluoroethylene carbonate additive–electrode compatibility in nanoporous sugarcane bagasse-based hard carbon sodium ion anodes

Abstract

Nanopore sodium storage underpins the characteristic low-voltage plateau in hard carbon (HC) anodes. Yet, the interfacial impact of electrolyte additives on this confined process remains largely elusive. Herein, we unveil a pore-blocking mechanism in sugarcane bagasse-based hard carbon (SuHC) induced by fluoroethylene carbonate (FEC), a widely used solid electrolyte interphase (SEI) forming additive. Using a combination of electrochemical analysis, spectroscopy, electron microscopy, and density functional theory, we demonstrate that FEC decomposition induces NaF-rich deposits in near-surface pore domains and pore-entrance regions, which impede Na⁺ intercalation and suppress the low-voltage plateau. High-resolution transmission electron microscopy (HRTEM) reveals NaF-matching crystallites in subsurface regions close to particle edges. This interfacial chemistry drives a mechanistic shift from diffusion-dominated intercalation to surface-limited capacitive storage. Our findings highlight the critical importance of additive–structure compatibility and offer design principles for tailoring electrolyte formulations for nanoporous carbon anodes in next-generation SIBs.