Electron localized separator design for stable Li-metal batteries.

Polyethylene (PE) separators are widely applied in Li-metal batteries (LMBs) for their chemical stability. However, their interactions with electrolyte components remain poorly understood. Here, we propose that the electron-deficient H atoms in CH bonds of PE may interact with anion lone pairs, thereby influencing local anion distribution and solid electrolyte interphase (SEI) formation. To address this, we propose an electron localization strategy by substituting H with F. In CF bond, the strong electron-withdrawing nature of F induces high electron localization, which makes the F atom less prone to gaining or losing electrons. This suppresses electron exchange and minimizes orbital overlap with anions, weakening the separator-anion interaction. Consequently, a C-F-rich polytetrafluoroethylene (PTFE) separator PTFE weakens separator-anion interactions, facilitating anion transport toward the Li interface and favoring anion-derived decomposition, contributing to the formation of an inorganic-rich SEI. As a result, Li||Li symmetric cells employing PTFE exhibit stable Li plating/stripping over 2800 h with low overpotential (< 35 mV) at 0.1 mA cm-2. Moreover, 4.4 V Li||LiNi0.8Co0.1Mn0.1O2 cells with PTFE retain over 80% of their initial capacity after 470 cycles at 1C, exceeding the 270-cycle lifespan of PE. This work proposes a rational separator design strategy for engineering stable interfaces in safe, long-lasting LMBs.