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

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 SO₄ ^2- or PO₄ ^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 (LiCoO₂, 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.