Cyclic P₃O₉ ^3- Trimer: A Network Former for Amorphous Superionic Conductors in Sodium Solid-State Batteries

Achieving solid-state electrolytes (SSEs) that combine fast Na⁺ conduction, high-voltage stability, and deformability remains a formidable challenge for all-solid-state sodium-ion batteries (ASSNIBs). Here, we introduce a cyclic trimetaphosphate anion (P₃O₉ ^3-) as a transformative network-forming unit to construct a new family of amorphous oxyhalide SSEs via facile mechanochemical synthesis. The unique nine-oxygen-donor architecture of P₃O₉ ^3- enables robust, three-dimensional coordination with metal chlorides (MCl_n, M = Zr, Ta, Hf, Nb, Al), forming a rigid yet disordered framework where isolated Cl^- anions are strategically liberated. This distinctive structure enables a dynamic anion‑assisted transport mechanism: the P₃O₉ ^3-‑bridged network provides stable conduction channels, while the mobile Cl^- anions dynamically assist Na⁺ hopping by mitigating steric and electrostatic barriers, collectively achieving an ultralow activation energy of 0.33 eV. The optimized electrolyte exhibits a high room-temperature ionic conductivity of 0.80 mS·cm^-1 and a wide electrochemical window of 1.4-4.2 V. ASSNIBs assembled with a NaNi_0.33Fe_0.33Mn_0.33O₂ cathode demonstrate stable cycling at 4.2 V, retaining 92% capacity after 300 cycles at 0.5C. This work pioneers the use of macrocyclic polyphosphates in SSEs, establishing a new design paradigm that simultaneously addresses ionic conductivity, stability, and interfacial compatibility.