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

## TL;DR

Researchers developed a sodium aluminophosphate solid electrolyte using a solution-based method, achieving promising ionic conductivity for sodium metal batteries.

## Contribution

The study introduces a novel nanocomposite sodium aluminophosphate solid electrolyte with high ionic conductivity and a unique NaNO3 domain structure.

## Key findings

- NAPO solid electrolytes with ionic conductivity of ≈10–8 S cm–1 were synthesized via spin coating.
- The presence of crystalline NaNO3 domains is essential for achieving high ionic conductivity.
- The material exhibits an activation energy of 0.80(1) eV and low electronic conductivity.

## Abstract

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.