Single-Molecule Additive Integrating Na-Ion Reservoir, Cosolvent, and Diluent Functions for Low-Temperature Na-Ion Batteries.

Na-ion batteries hold great promise for low-temperature energy storage, but their performance is severely constrained by Na-ion loss, electrolyte freezing, and elevated viscosity. While each issue demands carefully tailored additive molecules, their combined use often leads to performance interference and complicates the manufacturing process. To overcome these concurrent challenges, we report an organic molecule, NaB(C2H5)4, designed via machine learning, which incorporates three functional components in a single entity: Na+ to replenish sodium inventory, B(C2H5)3 as a low-freezing-point cosolvent, and C4H10 as a viscosity-reducing diluent. NaB(C2H5)4 undergoes complete decomposition below 4.0 V via a free-radical cleavage pathway, as confirmed by NMR and mass spectrometry. The incorporation of this molecule reduces the freezing point of the ether electrolyte to -85.0°C, and when formulated in an ether-based electrolyte at -60°C, it achieves an ionic conductivity of 0.61 mS cm-1 and a viscosity of 49.7 mPa s. As a result, the hard carbon||P2-Na2/3Ni1/3Fe1/3Mn1/3O2 pouch cell exhibits an improved initial Coulombic efficiency from 64.8% to 83.7%, along with 90.2% capacity retention over 200 cycles at -60°C.