Molecular Engineering of Highly Fluorinated Carbon Dots: Tailoring Li+ Dynamics and Interfacial Fluorination for Stable Solid Lithium Batteries

Fluorinated carbon dots (FCDs) have garnered interest owing to their distinct physicochemical properties. Nevertheless, intricate synthesis procedures and quite low fluorine doping levels limit its development and application. Herein, we propose a facile approach based on the Claisen-Schmidt reaction to realize gram-scale synthesis of highly fluorinated carbon dots (up to 20.79 at. %) at room temperature and atmospheric pressure, and a comprehensive exploration of the specific reaction mechanism is conducted. Furthermore, in consideration of the high fluorine content, good dispersibility, and compatibility with polymer electrolyte, the synthesized FCDs are utilized as an additive for PEO-based solid electrolytes of a Li battery to improve its ionic conductivity, interface stability, and mechanical properties. The introduction of FCDs can not only reduce the crystallinity of PEO and enhance the interaction of polymer chains, but also facilitate the establishment of uninterrupted pathways and in situ fluorination at the interface, which is substantiated by both theoretical calculations and experimental findings. As a result, the lithium symmetrical battery can operate stably for 1000 h at a current density of 0.4 mA cm(-2). Simultaneously, the LiFePO4/Li battery utilizing the composite electrolyte exhibits a capacity of 130.3 mAh g(-1) over 300 cycles while maintaining a capacity retention rate of 95.10%. This study develops a strategy for synthesizing highly fluorinated carbon dots, which demonstrate a useful influence on PEO electrolytes, thus boosting the advancement of FCDs and solid-state batteries.

Automated summary

Fluorinated carbon dots (FCDs) with high fluorine content were synthesized on a gram scale using a simple method based on the Claisen-Schmidt reaction. The synthesized FCDs, which have good dispersibility and compatibility with polymer electrolyte, were used as an additive in PEO-based solid electrolytes of a Li battery. The introduction of FCDs improved the ionic conductivity, interface stability, and mechanical properties of the electrolyte, leading to stable operation of the lithium symmetrical battery for 1000 hours and a capacity retention rate of 95.10% over 300 cycles for the LiFePO4/Li battery.