Highly conductive Na2.804Sb0.879W0.046S3.7F0.075 with moisture tolerance enables stable all-solid-state sodium batteries

Abstract

The low ionic conductivity of sodium solid electrolytes and the instability with moisture and sodium metal are the main challenges for the development of all-solid-state sodium batteries. In this work, a Na₃SbS₄ solid electrolyte with coexisting W and F dopants (Na_2.804Sb_0.879W_0.046S_3.7F_0.075) is synthesized and achieves a high ionic conductivity of 11.13 mS cm⁻¹. Density-functional-theory calculations indicate that both the reduced Na-vacancy formation energy and Na-ion diffusion barrier by such co-doping strategy contribute to the improved ionic conductivity. Furthermore, although the promoted H₂O dissociation by W doping results in the extremely low stability of Na_2.95Sb_0.95W_0.05S₄ in moisture, the added F dopant dramatically increases the energy costs for H₂O adsorption and dissociation, effectively inhibiting the hydrolysis of Na_2.804Sb_0.879W_0.046S_3.7F_0.075. Moreover, the NaF layer formed at the interfaces of Na/Na_2.804Sb_0.879W_0.046S_3.7F_0.075/Na symmetric cell significantly ameliorates the interfacial instability caused by W, realizing four times longer stable cycle time to 800 h. As a result, the TiS₂/Na_2.804Sb_0.879W_0.046S_3.7F_0.075/Na battery shows an initial reversible capacity of 173.3 mAh g⁻¹, with a capacity retention of 85 % after 100 cycles at 0.1 C.