Cyclic P ₃ O ₉ 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 (P 3 O 9 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 P 3 O 9 3– enables robust, three‐dimensional coordination with metal chlorides ( M Cl 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 P 3 O 9 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 0.33 Fe 0.33 Mn 0.33 O 2 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.