Enhanced Sodium-Ion Transport across Solid Electrolyte Interphase via Electric-Field Modulation

The ion transport properties of the solid electrolyte interphase (SEI) critically govern the kinetics and cycling stability of rechargeable batteries. However, a mechanistic understanding of ion transport within this dynamic, multicomponent interphase remains limited. In this study, we demonstrate that spatial variations in the internal electric field across the SEI dictates a fundamental transition in Na+ transport mechanisms. Within the thin SEI region adjacent to the electrode, a strong electric field dominates, enabling ballistic Na+ transport with minimal scattering and thus facilitating rapid ion migration. In contrast, in thicker SEI regions where the electric field is attenuated, frequent ion collisions dominate, resulting in diffusive transport and reduced ion mobility. To validate this mechanism, we combined molecular dynamics and density functional theory simulations to evaluate electrolyte reducibility based on salt-solvent interactions. These computational insights are complemented by in situ, nondestructive potentiostatic chronocoulometry technique to quantitatively determine SEI formation charge. A systematic evaluation of 23 representative electrolytes reveals a strong correlation between SEI formation charge and SEI resistance (RSEI), consistent with the electric-field-dependent ion transport model (R2 = 0.993). Notably, eight electrolytes exhibit SEI formation charges below 50% and low RSEI, proving suppressed SEI formation promotes ballistic Na+ migration. The optimal electrolyte, 1 M NaPF6 in tetraethylene glycol dimethyl ether (TEGDME), forms a ∼21 nm SEI with merely 31.7% charge loss. Na||hard carbon cells using this electrolyte achieve 24.48 Ω cm2 interfacial impedance and retain 99.4% capacity after 1200 cycles. This work offers a mechanistic framework for designing high-performance interphases through electrolyte engineering.