A rooted interphase on sodium via in situ pre-implantation of fluorine atoms for high-performance sodium metal batteries

Abstract

Sodium plating/stripping with high reversibility is very challenging for sodium-based batteries. Building a robust solid-electrolyte interphase (SEI) film on the surface of a sodium electrode is a pragmatic and effective approach. The existing various SEI layers, however, are basically simply covered on a metal surface, thus the combination between them is not firm; this is the fundamental reason why these SEI layers are unstable during battery operation. Here, we propose and prepare a unique SEI that can be rooted in metal via in situ pre-implantation of atoms, called a rooted SEI (R-SEI), thus creating an integrated protective layer for sodium or other metals, thereby establishing a new concept. By utilizing the strong induction effect and electrostatic repulsion of functional molecules, the fluoroethylene carbonate (FEC) decomposed atoms are driven to penetrate deeply into the sodium from outside to inside, thereby achieving the in situ pre-implantation of the fluorine atoms with a certain depth, which will be the desired base for R-SEI formation. The pre-implanted F atoms will promote the reactions of SEI formation sequentially and become a part of them, so that the SEI is naturally rooted in sodium. A multilayer, inorganic-rich concentration gradient R-SEI is ultimately formed, enabling the carbonate electrolyte to achieve a high coulombic efficiency (CE) of 97.3% and an extended Na plating/stripping lifetime (1700 h) at 1 mA cm⁻². Moreover, the assembled 4.5 V Na@Cu‖Prussian Blue (PB) cell achieves impressive cycling stability with 86% capacity retention after 200 cycles with limited Na.