Synergistic Cation–Anion Engineering Enables Yttrium Chlorobromide Superionic Conductors for High‐Performance All‐Solid‐State Lithium Metal Batteries

Halide solid‐state electrolytes (HSSEs) deliver greater chemical stability, a wider electrochemical stability window, but lower room‐temperature ionic conductivities than existing sulfide solid‐state electrolytes. Herein, we report a novel HSSE, Li 2.5 Y 0.75 W 0.25 Cl 3.5 Br 2.5 , featuring synergetic anion–cation chemistry that enhances ionic conductivity and interfacial stability. The substitution of Br − at halide sites within Li 3 YCl 6 framework induces a space group transformation that expands the Li‐ion transport channels. Concurrently, the incorporation of W 5+ effectively generates cation vacancies and induces a cationic polarization effect, thereby reducing the energy barrier for Li‐ion migration. The resulting Li 2.5 Y 0.75 W 0.25 Cl 3.5 Br 2.5 achieves a high room‐temperature ionic conductivity of 6.38 mS cm −1 with a low activation energy of 0.22 eV. More importantly, the reaction products of WO 3 at the LiNi 0.8 Mn 0.1 Co 0.1 O 2 /Li 2.5 Y 0.75 W 0.25 Cl 3.5 Br 2.5 interface not only reduce the total interface energy but also significantly suppress oxygen release and enhance interfacial stability during cycling. Consequently, all‐solid‐state full cells paired with a LiNi 0.8 Mn 0.1 Co 0.1 O 2 cathode demonstrate long‐term cycling stability, with a high initial discharge capacity of 169.40 mAh g −1 at 0.3 C and a retained capacity of 100.14 mAh g −1 after 500 cycles. This work provides new insights into overcoming the performance limitations of HSSEs and holds significant importance for developing highly stable all solid‐state Li metal batteries (ASSLMBs).