Deciphering the Role of Fluorination in Dual-Halogen Electrolytes for All-Solid-State Batteries: A Case Study of New...

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_0.85Co_0.1Mn_0.05O₂. 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-Li₂HfCl_6-xF_x. All-solid-state cells utilizing Li₂HfCl_5.5F_0.5 as the electrolyte demonstrate much-enhanced battery performance compared to Li₂HfCl₆. 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.