Molecular-level identification of organic species of ether-based solid-electrolyte interphase in sodium-ion batteries

The lack of precise chemical information of solid-electrolyte interphase (SEI) is a huge obstacle for battery technology breakthroughs. Herein we identify the organic component of diglyme-based SEI by artificially synthesizing bulk crystals and reveal its intrinsic properties. The organic component of SEI shows a layered structure and it actually is formed by the co-solvation of Na⁺, triflate ion and diglyme (Na₃T₃D), rather than by the electrochemical reduction of diglyme. Low-current-beam transmission electron microscopy revealed that the organic component is crystalline and soluble in most solvents, which upend humanity's understanding of its thin and amorphous features. Further, results disclosed that Na₃T₃D is the only organic SEI component and it is a super ionic conductor with rational migration pathways and facile energy barrier for Na⁺ migrating. Our work has overturned some existing misconceptions about the diglyme-based SEI and disclosed its essential characteristics.