Ruthenium doping of NASICON electrolyte augments the performance of solid-state sodium-ion batteries

Recently remarkably high ionic conductivities (up to 5.5 x 10-3 Ω-1cm−1) at 25 °C have been achieved in sodium superionic conducting (NASICON) solid-state electrolytes via aliovalent cation doping. Doping in NASICON leverages the synergic effects of bottleneck opening (lattice expansion) and correlated migration (higher carrier concentration). However, there is no report published that isolates the effect of correlated migration induced by heterovalent doping without the influence of structural/bottleneck changes. Therefore, here we report for the first time the substitution of trivalent Ru3+ cation into the Zr4+ site to enhance the ionic conductivity of the NASICON electrolyte. Even though Ru3+ has a smaller ionic radius than Zr4+ and leads to bottleneck shrinkage, high ionic conductivity is achieved on doping Ru as compared to undoped NZSP. Using first-principles calculations, we show that even though the bottleneck for Na+ migration shrinks, the diffusion barriers are nearly the same owing to the weaker Na-framework interaction. The doped electrolyte showed an ionic conductivity of 2.1 x 10-3 Ω-1cm−1, an ionic transference number of ∼ 98.9 %, and exhibited stable sodium plating/stripping at 0.5 mA-cm−2 for 100 h with an overpotential of 20 mV owing to the fused microstructure. The assembled all-solid-state battery with Ru-doped electrolyte exhibited an outstanding initial capacity of 87 mAh-g−1 at 0.3C as compared to 70 mAh-g−1 with undoped electrolyte. Compared to the pristine sample, which had capacity retention of 60 % after 100 cycles, the cell containing Ru-doped electrolyte demonstrated stable cycling with an 87 % capacity retention without external pressure.