Enhanced room-temperature Na⁺ ionic conductivity in Na_4.92Y_0.92Zr_0.08Si₄O₁₂

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 Na4.92Y0.92Zr0.08Si4O12 (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 Nathorn ion silicate electrolytes.