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 challenge³. Here, taking lithium fluoride (LiF)-a widely studied and considered crucial SEI component^4-7-as an example, we use ¹⁹F 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-yF_y) and F-rich phase (LiF_1-xH_x), which is further validated by ⁶Li 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-yF_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-yF_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.