Beyond Polymerization: In Situ Coupled Fluorination Enables More Stable Interfaces for Solid-State Lithium Batteries

In situ polymerization strategies hold great promise for enhancing the physical interfacial stability in solid-state batteries, yet (electro)chemical degradation of polymerized interfaces, especially at high voltages, remains a critical challenge. Herein, we find interphase engineering is crucial for the polymerization process and polymer stability and pioneer an in situ polymerization-fluorination (Poly-FR) strategy to create durable interfaces with excellent physical and (electro)chemical stabilities, achieved by designing a bifunctional initiator for both polymerization and on-surface lithium donor reactions. The integrated in situ fluorination converts Li₂CO₃ impurities on LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) surfaces into LiF-rich interphases, effectively inhibiting the aggressive (de)lithiation intermediates and protecting the interface from underlying chemical degradation, thereby surpassing the stability limitations of polymerization alone. Furthermore, the Poly-FR mediated symmetric Li|Li cells achieve an impressive cycling stability of up to 12,000 h. Solid-state cells with NCM811 cathodes and Li metal anodes realize an ultrastable cycling performance of 400 cycles with 83.4% retention at a high voltage of 4.5 V. This work points toward advanced in situ polymerization and beyond.