Essential Factors in Lithium Ternary Fluorides for High‐Voltage Cathode Coating Materials

ABSTRACT Elevating the operation voltage is a key strategy to boost the energy density of lithium‐ion batteries. For stable high‐voltage lithium‐ion battery operation, it is effective to utilize fluorides, which have high oxidative stability, as a coating material for oxide cathode materials. However, the fluoride/oxide heterointerface can cause undesired insulating layer formation; therefore, it is necessary to clarify a key factor for fluorides as coating materials. Herein, a systematic investigation of lithium ternary fluorides (Li x M F 6 ; M = Al, Ti, Si, Sn, Zr, and Ge) is conducted, which have sufficient ionic conductivity and oxidative stability. These fluorides are simply synthesized by cation exchange method, and coated on a standard cathode material LiNi 1/3 Mn 1/3 Co 1/3 O 2 (NMC). Among them, Li 3 AlF 6 exhibits superior chemical compatibility with NMC, forming a stable interfacial layer that minimizes cycle degradation under high‐voltage operation (up to 4.6 V). These findings identify interfacial chemical stability as the most critical factor for effective cathode coatings, and offer a practical guideline for the rational design of advanced protective layers for high‐voltage cathode materials.