Saturating the Matrix: Nanocomposite Solution-Processed Sodium Aluminophosphate Solid Electrolytes

Amorphous lithium solid electrolytes (SE) have enabled high performance lithium metal batteries, but sodium analogues are underexplored. Here, we report sodium aluminophosphate (NAPO) SEs synthesized via spin coating from aqueous solutions. Continuous, smooth, films with submicron thickness are produced after a mild annealing step. Exploration of the Na-Al-P-O phase space reveals nanocomposite materials comprising of an amorphous NAPO with crystalline NaNO₃ domains, suggesting a Na⁺ saturation limit within the Al-P-O matrix. A maximum ionic conductivity of ≈10^-8 S cm^-1 is achieved, with the presence of the insulating NaNO₃ precursor necessary for high ionic conductivity. Electron microscopy, time-of-flight secondary ion mass spectrometry and optical measurements reveal that at low concentrations the NaNO₃ phase is initially present as diffuse nanoparticle domains and at higher concentrations it forms isolated micron-sized particles. The optimal NAPO SE has an activation energy of 0.80(1) eV, a moderate reduced Young's modulus ≈30 GPa and low electronic conductivity (≈10^-14 S cm^-1), making these materials promising candidates for artificial solid electrolyte interphases or as solid electrolytes in sodium metal batteries.