Probing the heterogeneous nature of LiF in solid–electrolyte interphases

The electrolyte–electrode interface serves as the foundation for a myriad of chemical and physical processes. In battery chemistry, the formation of a well-known solid–electrolyte interphase (SEI) plays a pivotal role in ensuring the reversible operations of rechargeable lithium-ion batteries (LIBs) 1 , 2 . However, characterizing the precise chemical composition of the low crystallinity and highly sensitive SEI presents a formidable challenge 3 . Here, taking lithium fluoride (LiF)—a widely studied and considered crucial SEI component 4 , 5 , 6 , 7 —as an example, we use 19 F solid-state nuclear magnetic resonance (NMR) and identify that LiF formed in SEI (LiF SEI ) has fruitful spectroscopy features that originated from the formation of limited LiF–LiH solid solutions: H-rich phase (LiH 1 − y F y ) and F-rich phase (LiF 1 − x H x ), which is further validated by 6 Li isotope NMR, synchrotron X-ray diffraction and cryo-electron microscopy (cryo-EM). By characterizing SEI formed in various electrolytes, we confirm the dominance of LiH 1 − y F y in high-coulombic-efficiency electrolyte, which can be rationalized by the fact that LiF–LiH solid solution shows improved ionic conductivity over LiF. As a proof of concept, we demonstrate that LiH 1− y F y -rich coating layer presents obvious advantages compared with LiF-rich coating layer in lithium-metal batteries. This revised understanding of the heterogeneous nature of SEI components would provide new insights for electrode–electrolyte interface design.