Deciphering the Role of Fluorination in Dual‐Halogen Electrolytes for All‐Solid‐State Batteries: A Case Study of New Li₂HfCl6−xFx Solid Electrolytes

Lithium metal chlorides are promising superionic conductors for all‐solid‐state batteries (SSBs) due to their favorable mechanical properties, high ionic conductivity, and good oxidative stability (up to >4.2 V versus Li/Li + ). Nonetheless, chloride solid electrolytes (SEs) still undergo electrochemical degradation when paired with high‐voltage cathodes such as LiNi 0.85 Co 0.1 Mn 0.05 O 2 . A viable strategy to enhance the intrinsic electrochemical stability of chloride electrolytes is to partially substitute Cl with F. By leveraging complementary insights from neutron and X‐ray diffraction, X‐ray absorption spectroscopy, X‐ray photoelectron spectroscopy (XPS), time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS), and electrochemical studies, we investigate the interplay between ionic and electronic conductivity, voltage stability, and overall battery performance of a family of new dual‐halogen SEs—Li 2 HfCl 6−x F x . All‐solid‐state cells utilizing Li 2 HfCl 5.5 F 0.5 as the electrolyte demonstrate much‐enhanced battery performance compared to Li 2 HfCl 6 . This improvement is mainly attributed to the formation of a kinetically stable LiF‐rich cathode electrolyte interphase (CEI), which inhibits detrimental reactions between the cathode and the SE, as revealed by ToF‐SIMS studies. The findings from this study are applicable to other dual‐halogen solid ionic conductors, offering valuable insights into the relationship between intrinsic electrochemical window (IEW), electronic and ionic conductivity, and battery performance in dual‐halogen solid‐state electrolytes.