NaF-Rich Solid Electrolyte Interphase for Dendrite-Free Sodium Metal Batteries

Efficient energy storage systems are essential to the widespread application of renewable energy, as well as fulfilling the rapidly growing demand of mobile devices and electric vehicles. Sodium metal batteries (SMBs) have attracted much attention as a sustainable technology owing to the high theoretical energy density and abundance of Na. However, the practical realization is hindered by several critical challenges arising from high reactivity of metallic Na with organic electrolyte and its uneven deposition. A NaF-rich SEI layer on Na metal surface has been explored to demonstrate its beneficial effect on electrochemical performance of SMBs. Unlike previous strategies based on expensive and unstable fluorinated electrolytes to form NaF-rich SEI layers, this work employs PTFE micro powders that react with molten Na, whose process is much simpler at a lower cost. The spontaneous reaction with Na metal converts C-F to NaF which suppresses the formation of dendrites as proven by combined depth-profiling X-ray photoelectron spectroscopy (XPS) and time-of flight secondary ion mass spectrometry (ToF-SIMS) and theoretical DFT calculations. The full battery prepared with a Na₃V₂(PO₄)₃ cathode delivers 99% retention of Coulombic efficiency after 400 and 600 cycles at 1C in ether- and carbonate-based electrolytes, respectively. The SEI layer design strategy presented here can shed new insights into the development of high-performance dendrite-free Na metal and solid-state batteries.<br/>The authors acknowledge the supports by the Research Grant Council of Hong Kong SAR (Project numbers 16208718, 16304518), NSFC-RGC Joint Research Scheme (N_HKUST607/17), and the IER foundation (HT-JD-CXY-201907), “International science and technology cooperation projects” of Science and Technological Bureau of Guangzhou Huangpu District (2019GH06), Guangdong Science and Technology Department (2020A0505090003), Research Fund of Guangdong-Hong Kong-Macao Joint Laboratory for Intelligent Micro-Nano Optoelectronic Technology (2020B1212030010). Technical assistance from the Advanced Engineering Materials Facilities and the Materials Characterization and Preparation Facilities at HKUST is greatly appreciated.