# Reactive Carbide‐Based Synthesis and Microstructure of NASICON Sodium Metal All Solid‐State Electrolyte

## TL;DR

A new method for making a solid-state battery electrolyte using carbide precursors results in a denser, purer material that improves battery performance and safety.

## Contribution

A novel carbide-based synthesis method for NASICON electrolytes that enhances microstructure and dendrite suppression.

## Key findings

- Carbide-derived electrolytes achieved 98% density with minimal secondary phase impurities.
- Carbide-based electrolytes showed a 3.1 mA cm⁻² critical current density, significantly higher than the oxide-based baseline.
- Phase field simulations showed that zirconia particles deflect dendrites, while brittle glassy phases accelerate their growth.

## Abstract

Reactive carbide precursor‐based synthesis of NASICON‐type NZSP (Na1+xZr2SixP3‐xO12) solid‐state electrolyte (SSE) is demonstrated, in contrast to the established oxide‐based approach. Exothermic decomposition of ZrC and SiC in air homogenizes microstructure, yielding 98% compact density after conventional sintering at 1200 °C. Quantitative stereology demonstrates that significant microstructural differences are present. Compacts of carbide‐derived Carb‐NZSP are 98% dense with a secondary zirconium oxide (ZrO2) volume fraction of 0.2% ± 0.3%, versus 93% dense and 3% ± 1% for oxide‐derived baseline. For Carb‐NZSP, the secondary glassy phosphate phase is agglomerated, while for baseline, it is dispersed and percolated. Electrochemical testing combined with post‐mortem analysis demonstrates how microstructural control of secondary phases is critical for dendrite suppression: Carb‐NZSP critical current density (CCD) is 3.1 ± 0.8 mA cm−
2 at 0.1 mAh cm−
2, versus 1.0 ± 0.7 mA cm−2 at 0.1 mAh cm−2. Cryogenic focused ion beam (cryo‐FIB) analysis demonstrates that in both materials, the porous 2D sheet‐like sodium metal dendrites propagate around and subsume NZSP grains, likely following a path enriched with glassy phase and with porosity. Dendrites also flow around isolated zirconia particles. Phase field simulation reveals deflection of dendrites by mechanically tough zirconia, while brittle glassy phase accelerates dendrite growth, especially when finely distributed.

Sodium Metal All‐Solid State Batteries (Na‐ASSBs) are enabled by the synthesis of the solid state electrolyte, NASICON (Na1+xZr2SixP3‐xO12), using carbide‐based precursor compounds (ZrC and SiC); resulting in dense, pure, and mechanically improved microstructure. Heat released during solid state synthesis creates a uniform microstructure with a favorable distribution of secondary impurities – enabling faster current capabilities without dendrite driven short circuit.