Homogeneous Na⁺ transfer dynamic at Na/Na₃Zr₂Si₂PO₁₂ interface for all solid-state sodium metal batteries

The instability and high resistance of metallic Na/solid-state electrolyte (especially oxide-based electrolyte) interface are still challenges for all-solid-state sodium batteries. Herein, we propose a grain-boundary engineering strategy to stabilize the Na/Na₃Zr₂Si₂PO₁₂ interface and improve the capability of sodium ion transfer at the interface. The chemical composition at the grain boundary of Na₃Zr₂Si₂PO₁₂ is mediated via the addition of sintering additive Na₂B₄O₇ to facilitate the densification sintering at relatively low temperature and boost sodium ion migration across the grain boundary. Na₃Zr₂Si₂PO₁₂-10 wt% Na₂B₄O₇ demonstrates an optimized conductivity of 1.72 mS cm⁻¹ at room temperature and the corresponding symmetric sodium cells exhibit ultra-stable sodium plating/stripping cycling under a current density of 0.3 mA cm⁻² for over 2500 h at room temperature. Analysis reveals that a kinetically stable interphase forms between electrolyte and metallic Na, reducing the interfacial resistance from 90 Ω cm² for Na₃Zr₂Si₂PO₁₂ to 36 Ω cm² for Na₃Zr₂Si₂PO₁₂-10 wt% Na₂B₄O₇. Cycling at stepwise changing temperature reconfirms the super stability of Na/Na₃Zr₂Si₂PO₁₂-10 wt% Na₂B₄O₇ interface. All solid-state batteries based on the Na₃Zr₂Si₂PO₁₂-10 wt% Na₂B₄O₇ demonstrates excellent cycling performance for over 200 cycles with limited capacity degradation. Our findings here open up a fertile avenue of exploration for all-solid-state sodium batteries utilizing NASICON-type electrolytes.