Electrolyte additives for tunnel-type Na₀.₄₄MnO₂ are often selected empirically. Our new research paper in Journal of Energy Storage asks a more specific question: what do FEC and nominally added trace NaF actually change in Na⁺ solvation, transport, and interfacial behaviour?
What the evidence shows
Raman spectroscopy and molecular-dynamics simulations identify FEC as the principal additive. It decreases Na⁺–EC coordination and, in the dual-additive electrolyte, promotes relatively greater ClO₄⁻ participation in the first solvation shell. Under the conditions examined, trace NaF has only a modest effect on homogeneous bulk solvation and is treated as a secondary co-additive.
The dual-additive formulation shows limited impedance growth, relatively balanced apparent Na⁺ transport during insertion and extraction, and largely reversible evolution of the tunnel framework by in situ synchrotron XRD.
Na‖NMO cells retain 95.6% of their cycle-2 capacity after 150 cycles at 1C. This is essentially the same as the FEC-only formulation (95.9%), but higher than NaF-only (74.3%) and baseline (66.6%) electrolytes. An NMO‖hard-carbon full cell retains 88.2% after 130 cycles at 1C and reaches an average Coulombic efficiency of 99.84% over cycles 2–187.
The interpretation remains deliberately bounded: the dissolved state of nominally added NaF was not quantified, direct Na–F coordination was not established, and the measurements do not determine a unique molecular interphase mechanism. The strongest conclusion is therefore formulation-level—FEC drives the measurable solvation change, while trace NaF provides a modest complementary contribution.
Read the paper
Weldegebrieal G. K., Kao-ian W., Gopalakrishnan M., Limphirat W., Buangam P., Zhang R., Liu W.-R., Kheawhom S. Anion-enriched Na⁺ solvation enabled by FEC and trace NaF for stable cycling of tunnel-type Na₀.₄₄MnO₂ cathodes. Journal of Energy Storage 181 (2026) 124418. DOI 10.1016/j.est.2026.124418