The role of NaSICON surface chemistry in stabilizing 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. With a combination of surface sensitive analysis, we reveal that such sophisticated engineering is not necessary in NaSICON solid electrolytes (Na 3.4 Zr 2 Si 2.4 P 0.6 O 12 ) since optimised performances can be achieved by simple thermal treatments that allow the thermodynamic stabilization of a nanometric Na 3 PO 4 protective surface layer. The optimized surface chemistry leads to stabilized Na|NZSP interfaces with exceptionally low interface resistances (down to 0.1 Ω cm 2 at room temperature) and high tolerance to large plating current densities (up to 10 mA cm −2 ) even for extended cycling periods of 30 min (corresponding to an areal capacity 5 mAh cm −2 ). The created Na|NZSP interfaces show great stability with increment of only up to 5 Ω cm 2 after four months of cell assembly.