Method-dependent Na-ion diffusion in nanocarbon materials: molten-salt-exfoliated graphene and C45 carbon nanoparticles

Abstract

Na-ion diffusion in nanocarbon materials plays a crucial role in regulating Na-ion cell performance; however, Na-ion diffusion coefficient (D_Na⁺) values reported across the literature vary widely. In this study, Na-ion diffusion in molten-salt-exfoliated graphene and C45 carbon nanoparticles was investigated using a diglyme-based electrolyte and multiple electrochemical techniques. The two nanocarbon materials exhibited substantially different apparent D_Na⁺ values based on the measurement approach employed. Notably, the determined D_Na⁺ values spanned many orders of magnitude across different techniques, underscoring the high sensitivity of apparent diffusion parameters to the electrochemical method used. These variations are discussed in terms of differences in Na-ion storage mechanisms, structural and morphological characteristics, and electronic conductivity. By comparing D_Na⁺ values obtained from cyclic voltammetry, the galvanostatic intermittent titration technique, and electrochemical impedance spectroscopy, this study highlights how each method examines distinct Na-ion transport regimes, ranging from surface- and interface-controlled processes to long-range ion transport. The results emphasize the importance of material properties and methodological context in interpreting Na-ion diffusion data, while providing insights to support the more informed evaluation and design of nanocarbon anodes for Na-ion batteries.