To advance the development of all-solid-state Na-ion batteries (ASSNIBs), optimal Na solid-state electrolyte (SSE) materials must meet critical requirements, including high ionic conductivity (>10−3 ​S ​cm−1), low electronic conductivity (<10−10 ​S ​cm−1), and cost-effectiveness (<$50 ​kg−1). In this study, we present a mixed-anion strategy for designing SSEs containing earth-abundant elements only. Density functional theory (DFT) and bond valence site energy (BVSE) calculations show that Na2ZrO3 offers better electrochemical stability but poor Na ​+ ​conductivity compared to Na2ZrCl6. Mixed-anion SSE Na2ZrCl6-4xO2x has the potential for combining the strengths of high electrochemical stability of the oxide and high ionic conductivity of the halide. The optimal composition Na2ZrCl3O1.5 synthesized by a mechanochemical method exhibits a high ionic conductivity of 5.17 ​× ​10−5 ​S ​cm−1 at room temperature, nearly an order of magnitude improvement over Na2ZrCl6 and orders of magnitude higher than that of Na2ZrO3. This enhancement is attributed to the more disordered phase within Na2ZrCl3O1.5. Cost analysis reveals that Na2ZrCl6-4xO2x can be produced at a large scale and low cost (≤£25.53/kg). These findings pave the way for the mixed-anion strategy for developing high-performing SSE materials for ASSNIBs.