Aliovalent cation substitution in Na₃Zr₂Si₂PO₁₂ for practical solid-state sodium metal batteries

High-performance solid electrolytes with high conductivity and good electrode compatibility are critical for the operable solid-state sodium metal batteries. With the Na⁺ superionic conductor-typed Na₃Zr₂Si₂PO₁₂ as a matrix, an aliovalent cation substitution strategy using Ni²⁺, Mn³⁺, Nb⁵⁺, Mo⁶⁺ substituting Zr⁴⁺ is investigated on the ionic conductivity and interfacial performance of solid-state sodium metal batteries. The low-valence Ni²⁺ and Mn³⁺ show notable effect on both enlarging the bottlenecks in the grain lattices and reducing the barriers across the grain boundaries for Na⁺ migration, while the high-valence Nb⁵⁺ and Mo⁶⁺ mainly facilitate Na⁺ migration across the grain boundaries. By tuning the doping ratios, the Ni²⁺ doped Na_3.4Zr_1.8Ni_0.2Si₂PO₁₂ achieves the optimal total conductivity of 2.284 mS cm^-1 at 30 °C which is 6 times higher than the undoped Na₃Zr₂Si₂PO₁₂. Moreover, the aliovalent cation substitution essentially improves the interface compatibility with the sodium metal, achieving reduced interfacial resistances as low as 7.80 ohm cm² and enlarged critical current densities as high as 1.0 mA cm^-2. Besides, stable charge/discharge cycles at high rates for both the symmetric Na||Na cells over 3400 h and the full cells over 2400 cycles are achieved to signify the practical merits of the neat aliovalent cation substitution strategy.