Scandium-Substituted Na3Zr2(SiO4)2(PO4) Prepared by a Solution-Assisted Solid-State Reaction Method as Sodium-Ion Conductors

As possible electrolyte materials for all-solid-state Na-ion batteries (NIBs), scandium-substituted Na3Zr2(SiO4)2(PO4) in the structure of NASICONs (Na superionic conductors) has received hardly any attention so far, although among all the trivalent cations, Sc3+ might be the most suitable substitution ion for Na3Zr2(SiO4)2(PO4) because the ionic radius of Sc3+ (74.5 pm) is the closest to that of Zr4+ (72.0 pm). In this study, a solution-assisted solid-state reaction (SASSR) method is described, and a series of scandium-substituted Na3Zr2(SiO4)2(PO4) with the formula of Na3+xScxZr2-x(SiO4)2(PO4) (NSZSPx, 0 ≤ x ≤ 0.6) have been prepared. This synthesis route can be applied for powder preparation on a large scale and at low cost. With increasing degrees of scandium substitution, the total conductivity of the samples also increases. An optimum total Na-ion conductivity of 4.0 × 10–3 S cm–1 at 25 °C is achieved by Na3.4Sc0.4Zr1.6(SiO4)2(PO4) (NSZSP0.4), which is the best value of all reported polycrystalline Na-ion conductors. The possible reasons for such high conductivity are discussed.