Modulating Electric Double Layers via Polymer Electric Dipole Effects to Enhance Stability in Quasi-Solid-State Sodium Metal Batteries.

The interfacial instability of Na metal anodes poses a significant barrier to the practical applications of sodium metal batteries. According to electric double layer (EDL) theory, the potential difference in the Helmholtz layer critically affects electrochemical reactions at the electrode/electrolyte interfaces, which governs the solid electrolyte interphase (SEI) composition and the Na deposition process. Herein, we leverage the electric dipole effect of polymers, formed via in situ polymerization of butyl acrylate (BA), which preferentially adsorbs on the Na metal surface, to modulate the conformation of the EDL. Molecular dynamics simulations reveal that poly-BA facilitates the formation of a more compressed diffuse layer and reduces the potential difference in the Helmholtz layer. This compressed EDL with the change of species derives homogeneous SEI, enabling reversible Na plating/stripping. As a result, the poly-BA quasi-solid electrolyte extends the lifespan of the Na||Na cell to 3500 h at 0.1 mA cm⁻. Quasi-solid-state Na||NaV(PO) cells maintain stable cycling over 1500 cycles, with a capacity retention of 87.6% at 5 C. Our findings indicate that modulating the EDL structure via the electric dipole effect of polymers enables uniform Na deposition, offering a promising strategy for designing electrolytes for practical quasi-solid-state sodium metal batteries.