Reversible sodium-ion storage in π-conjugated aromatic hydrocarbon molecules

Sodium-ion batteries (SIBs) are promising alternatives to lithium-ion systems, yet the development of suitable anode materials remains a major challenge due to the large ionic radius of Na⁺ and its poor intercalation in graphite. Here, we conduct the first systematic investigation of six unsubstituted π-conjugated arenes as molecular model systems to probe Na⁺-π interactions and their implications for Na⁺ storage. Through comprehensive electrochemical measurements, ultraviolet-visible spectroscopy, X-ray photoelectron spectroscopy depth profiling, and density functional theory (DFT) simulations, we explore how structural features influence reversible Na⁺ storage and dissolution resistance. Among the tested arenes, naphthalene, biphenyl, and 9,9-dimethylfluorene exhibit high reversible capacities, moderate initial Coulombic efficiencies, and outstanding cycling stability, attributed to their rigid, insoluble structures and stable SEI formation. In contrast, phenanthrene, p-terphenyl, and pyrene suffer from severe dissolution and capacity fading. DFT calculations further correlate favorable Na adsorption energies and minimal volume expansion with the experimentally observed stability of specific arenes. A direct-contact presodiation approach significantly improves the initial Coulombic efficiency and long-term performance of DiMF-based electrodes. This study provides comparative dataset of unsubstituted arenes in SIBs and suggests preliminary structure–function relationships that may inform future molecular design, while emphasizing the multifactorial nature of Na⁺-π interactions.