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

Abstract

As possible electrolyte materials for all-solid-state Na-ion batteries (NIBs), scandium-substituted Na 3 Zr 2 (SiO 4 ) 2 (PO 4 ) in the structure of NASICONs (Na super-ionic conductors) have received hardly any attention so far, although among all the trivalent cations, Sc 3+ might be the most suitable substitution ion for Na 3 Zr 2 (SiO 4 ) 2 (PO 4 ) because the ionic radius of Sc 3+ (74.5 pm) is the closest to that of Zr 4+ (72.0 pm). In this study, a solution-assisted solid-state reaction (SASSR) method is described and a series of scandium-substituted Na 3 Zr 2 (SiO 4 ) 2 (PO 4 ) with the formula of Na 3+x Sc x Zr 2-x (SiO 4 ) 2 (PO 4 ) (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 Na 3.4 Sc 0.4 Zr 1.6 (SiO 4 ) 2 (PO 4 ) (NSZSP0.4), which is the best value of all reported polycrystalline Na-ion conductors. The possible reasons for such high conductivity are discussed.

Fast Na ion transport triggered by rapid ion exchange on local length scales

“…In the monoclinic form of NZSP the Na + ions occupy three crystallographically inequivalent sites 4 d , 4 e , and 8  f , which are partially occupied. The large fraction of vacant Na sites and the favourable connection of their polyhedra ensure rapid exchange of the Na + ions as discussed in literature 22,40 . The preparation and characterisation of the samples studied here (NZSP and Sc-NZSP) has already been described elsewhere 18,22 .…”

“…The large fraction of vacant Na sites and the favourable connection of their polyhedra ensure rapid exchange of the Na + ions as discussed in literature 22,40 . The preparation and characterisation of the samples studied here (NZSP and Sc-NZSP) has already been described elsewhere 18,22 . The characterization also includes analyses by X-ray powder diffraction and differential scanning measurements.

Figure 1Crystal structure and conductivity.

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“…Crosses show conductivities taken from ref. 22 . ( c ) 23 Na spin-lattice relaxation rates ( R 1 , R 1ρ ) of Sc-NZSP recorded in both the laboratory frame of reference (79 MHz, R 1 ) and the rotating-frame of reference (20 kHz, R 1ρ ).…”

“…e ., air-insensitive, Na + electrolytes with extremely high ion conductivities exceeding 1 mS/cm at room temperature have, apart from the well-known Na- β ′′-alumina 12 , only very rarely been characterized in detail 16,18,22 . Here we present an in-depth study to explore the roots of extremely rapid 3D ion dynamics in Na 3.4 Sc 0.4 Zr 1.6 (SiO 4 ) 2 PO 4 (Sc-NZSP), which was prepared via a wet-chemical route 18 .…”

“…The parent compound Na 3 Zr 2 (SiO 4 ) 2 PO 4 (NZSP), following the pioneering work of Hong and Goodenough 23,24 , served as benchmark. It has been shown recently that substitution of Zr by Sc in NZSP leads to an increase of ionic conductivity 22 . For the reason of charge compensation, the replacement of parts of Zr ions with an equimolar amount of Sc ions increases the number density of (mobile) Na ions.…”