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

Abstract

Na-ion diffusion in nanocarbon materials plays an important role in governing sodium-ion cell performance; however, reported diffusion coefficients vary widely across the literature. In this work, Na-ion diffusion in molten-salt-exfoliated graphene and C45 carbon nanoparticles is examined using a diglyme-based electrolyte and multiple electrochemical techniques. The two nanocarbons exhibit substantially different apparent Na-ion diffusion coefficients depending on the measurement approach employed. These variations are discussed in relation to differences in Na-ion storage mechanisms, structural and morphological characteristics, and electronic conductivity. By comparing diffusion coefficients obtained from cyclic voltammetry, galvanostatic intermittent titration technique and electrochemical impedance spectroscopy, this study highlights how each method probes distinct Na-ion transport regimes, ranging from surface- and interface-controlled processes to longer-range ion transport. The results emphasise the importance of considering both material properties and methodological context when interpreting Na-ion diffusion data, and provide insights to support more informed evaluation and design of nanocarbon anodes for sodium-ion batteries.