Chemical Tuning of NaSICON Surfaces for Fast-Charging Na Metal Solid-State Batteries

Solid-state batteries (SSBs) with alkali metal anodes hold great promise as energetically dense and safe alternatives to conventional Li-ion cells. Whilst, in principle, SSBs have the additional advantage of offering virtually unlimited plating current densities, fast charges have so far only been achieved through sophisticated interface engineering strategies. Here, we reveal that such interface engineering can be easily achieved by tuning the chemistry of NaSICON solid electrolytes (Na_3.4Zr₂Si_2.4P_0.6O₁₂) and taking advantage of the thermodynamic stabilization of a Na₃PO₄ layer on their surface upon thermal activation. The optimized planar Na|NZSP interfaces are characterized by their exceptionally low interface resistances (down to 0.1 Ω cm² at room temperature) and, more importantly, by their tolerance to large plating current densities (up to 10 mA cm^-2) even for extended cycling periods of 30 minutes (corresponding to an areal capacity 5 mAh cm^-2).