Regulating Na/NASCION electrolyte interface chemistry for stable solid-state Na metal batteries at room temperature

It is critical for solid-state alkaline metal batteries to solve the issues of large interfacial resistance and poor interfacial stability between the alkaline metal anode and the solid electrolyte. We tune the interface chemistry between Na metal and a NASICON-type solid electrolyte by a facile Cu²⁺-ion doping method. It is demonstrated that the Cu²⁺-ion incorporation into Na₃Zr₂Si₂PO₁₂ (NZSP) significantly reduces the interfacial resistance (R_inter) and boosts the interfacial stability during charge/discharge cyles at 25 °C. The optimal Na_3.4Zr_1.8Cu_0.2Si₂PO₁₂ (NZSP-0.2Cu) realizes an increased critical current density of 0.5 mA cm⁻² and an impressively low R_inter value of 19 Ω cm² at 25 °C, which is only 1/35 of the undoped one (665 Ω cm²). Moreover, the symmetrical Na|NZSP-0.2Cu|Na cell maintains steady Na plating/stripping cycles for as long as 1450 h under 0.2 mA cm⁻², clearly indicating the desired chemical compatibility at the Na metal anode interface. A self-formed dynamically stable Cu₃PO₄-dominant interphase layer at the Na metal/NZSP-0.2Cu interface is verified to explain for the outstanding interfacial performance. Furthermore, a room-temperature solid-state NaCrO₂|NZSP-0.2Cu|Na metal battery is assembled, exhibiting excellent cycling performance at 5 C for 660 cycles with a capacity retention of 86.5% and an overall Coulombic efficiency of 99.5%.