Aggregate-Dominated Dilute Electrolytes with Low-Temperature-Resistant Ion-Conducting Channels for Highly Reversible Na Plating/Stripping

Developing rechargeable batteries with high power delivery at low temperatures (LT) below 0 °C is significant for cold-climate applications. Initial anode-free sodium metal batteries (AFSMBs) promise high LT performances because of the low de-solvation energy and smaller Stokes radius of Na⁺, nondiffusion-limited plating/stripping electrochemistry, and maximized energy density. However, the severe reduction in electrolyte ionic conductivity and formation of unstable solid electrolyte interphase (SEI) hinder their practical applications at LT. In this study, a 2-methyltetrahydrofuran-based dilute electrolyte is designed to concurrently achieve an anion-coordinated solvation structure and impressive ionic conductivity of 3.58 mS cm⁻¹ at −40 °C. The dominant aggregate solvates enable the formation of highly efficient and LT-resistant Na⁺ hopping channels in the electrolyte. Moreover, the methyl-regulated electronic structure in 2-methyltetrahydrofuran induces gradient decomposition toward an inorganic-organic bilayer SEI with high Na⁺ mobility, composition homogeneity, and mechanical robustness. As such, a record-high Coulombic efficiency beyond 99.9% is achieved even at −40 °C. The as-constructed AFSMBs sustain 300 cycles with 80% capacity maintained, and a 0.5-Ah level pouch cell delivers 85% capacity over 180 cycles at −25 °C. This study affords new insights into electrolyte formulation for fast ionic conduction and superior Na reversibility at ultralow temperatures.