Enhanced room-temperature Na+ ionic conductivity in Na4.92Y0.92Zr0.08Si4O12

Developing cost-effective and reliable solid-state sodium batteries with superior performance is crucial for stationary energy storage. A key component in facilitating their application is a solid-state electrolyte with high conductivity and stability. Herein, we employed aliovalent cation substitution to enhance ionic conductivity while preserving the crystal structure. Optimized substitution of Y³⁺ with Zr⁴⁺ in Na₅YSi₄O₁₂ introduced Na⁺ ​ion vacancies, resulting in high bulk and total conductivities of up to 6.5 and 3.3 ​mS ​cm⁻¹, respectively, at room temperature with the composition Na_4.92Y_0.92Zr_0.08Si₄O₁₂ (NYZS). NYZS shows exceptional electrochemical stability (up to 10 ​V vs. Na⁺/Na), favorable interfacial compatibility with Na, and an excellent critical current density of 2.4 ​mA ​cm⁻². The enhanced conductivity of Na⁺ ​ions in NYZS was elucidated using solid-state nuclear magnetic resonance techniques and theoretical simulations, revealing two migration routes facilitated by the synergistic effect of increased Na⁺ ​ion vacancies and improved chemical environment due to Zr⁴⁺ substitution. NYZS extends the list of suitable solid-state electrolytes and enables the facile synthesis of stable, low-cost Na⁺ ion silicate electrolytes.