In Situ SEI Engineering with Sodiophilic Zn Sites and Fluoride‑Rich Interphase by Perovskite KZnF3 for Stable Sodium Metal Batteries.

Sodium metal batteries (SMBs) hold great promise for high energy storage system but are limited by dendrite growth and unstable solid electrolyte interphase (SEI). Herein, we report an in situ SEI engineering approach based on a KZnF3 (KZF) perovskite coating on Cu current collectors, which can spontaneously generate a fluoride-rich interfacial SEI during Na deposition. Experimental characterizations reveal that KZF undergoes sequential intercalation, conversion, and alloying reactions during electrochemical activation. The resulting KZF derived interface, composed of sodiophilic Zn/NaZn13 species and a robust NaF/KF rich SEI, promotes uniform electric field and Na+ flux distributions, effectively inhibiting dendrite formation and ensuring stable sodium deposition/dissolution. Consequently, the KZnF3 asymmetric cell delivers highly reversibility with an average CE of 99.6% over 500 cycles (1 mA cm-2, 1 mAh cm-2) and the KZF@Na symmetric cell demonstrates stable cycle for over 2500 h with a low overpotential of 27.2 mV at 1 mA cm-2 and 1 mAh cm-2. Moreover, the KZF@Na||NVP full cell achieves an excellent rate capability (97.4 mAh g-1 at 3 C) and cycle stability (94 mAh g-1 after 350 cycles with 90.9% retention under N/p = 2). This work provides a practical pathway for interfacial engineering toward the development of advanced SMBs.