Ultralow Young's Modulus Phosphosulfate Solid Electrolytes for High‐Voltage All‐Solid‐State Batteries

ABSTRACT Intimate solid‐solid interfacial contact is critical for high‐performance all‐solid‐state lithium batteries (ASSLBs), which require solid‐state electrolytes (SSEs) with exceptional mechanical softness to eliminate grain boundaries. Oxide, sulfide, and halide solid electrolytes all fail to balance Young's modulus, ionic conductivity, and high‐voltage oxidation stability, with each having a respective drawback in one of the three properties. Herein, we develop nanocrystalline/amorphous composite oxyanion‐halide SSEs through dual‐anion engineering by incorporating SO 4 2− or PO 4 3− into a zirconium chloride matrix. The optimized sulfate‐based and phosphate‐based electrolytes exhibit local Young's modulus of ∼0.4 and ∼0.9 GPa, with room‐temperature ionic conductivities of 2.1 and 2.2 mS cm − 1 , respectively. These SSEs also show high oxidative stability and cost‐effectiveness ($79.7 US kg −1 ). When paired with 4.6 V high‐voltage cathodes (LiCoO 2 , NCM88), ASSLBs deliver remarkable cycling performance: >90% capacity retention after 1000 cycles and >70% after 1800 cycles at 1 C. This design resolves the core interfacial challenge for ASSLBs, offering a practical, low‐cost electrolyte solution for next‐generation high‐energy‐density batteries.