Dual-Sulfite Electrolytes for Stable Sodium Metal Batteries with a Low N/P Ratio

Grasping the intricacies of the solid electrolyte interphase (SEI) on sodium (Na) metal anodes is critical for developing sodium metal batteries (SMBs). Conventional carbonate electrolytes suffer from poor Na compatibility, forming organic-rich SEIs that degrade cycling stability. Here, A dual-sulfite electrolyte (DSE) is presented, where the synergistic effect of sp³ hybridization in -SO₃ and p²-pd hybridization in S═O enables a higher solvent coordination number, promoting the formation of a favored solvent-derived, inorganic-dominant SEI. The designed DSE accelerates the passivation process of Na anodes, yielding sulfide complexes in the SEI layer. As a result, paired with a high-loading layered oxide cathode (NaNi_1/3Fe_1/3Mn_1/3O₂, 2.6 mAh cm^-2) and thin Na (100 µm), the DSE enables SMBs to achieve an 85% capacity retention after 450 cycles at an N/P ratio of 4.3. Even with thinner Na (50 µm, N/P = 2.2), the cell retains 86% capacity after 200 cycles.