Elucidating the sodium insertion mechanism of an organic electrode material for sodium-ion batteries

Organic anode materials for sodium-ion batteries are attracting a great deal of interest due to their sustainability and design flexibility. However, the Na+ insertion mechanism is poorly understood, especially for disordered organic anode materials. A lack of understanding restricts optimization efforts and potential commercialization. Herein, we apply a range of characterization techniques, such as three-dimensional electron diffraction, powder X-ray diffraction, Raman spectroscopy, electron paramagnetic resonance spectroscopy, and pair distribution function (PDF) analysis to a model system, sodium naphthalene-2,6-dicarboxylate, to elucidate the Na+ storage mechanism. A combined ab initio random structure search and PDF study was conducted to postulate a structure of sodiated Na2+xNDC (s-NDC). Our work reveals an expansion in the Na+–O storage layer to allow for the accommodation of inserted Na+. Meanwhile, the naphthalene units exist as radical species, promoting a reorientation to accommodate the inserted Na+, as well as facilitating a stabilizing π interaction. Ultimately, our results illustrate the efficacy of using a multi-technique approach to study the sodiation mechanism of an organic anode material and offer insight into the sodiated structure. This approach can inform the strategic molecular design of future organic anode materials.