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.