Elsevier

Solid State Ionics

Volume 293, 1 October 2016, Pages 18-26
Solid State Ionics

New promising NASICON material as solid electrolyte for sodium-ion batteries: Correlation between composition, crystal structure and ionic conductivity of Na3 + xSc2SixP3  xO12

https://doi.org/10.1016/j.ssi.2016.06.005Get rights and content

Highlights

  • Systematic study of the introduction of Si in Na3Sc2(PO4)3

  • Very high conductivity: 8.3 × 10 4 S cm 1 at 30 °C for x = 0.4

  • Correlation of structural parameters and conductivity values

  • Validation of the influence of the bottleneck for the Na+ conduction

Abstract

In the search for novel sodium-ion conductors to be used in batteries for grid application, the thoroughly studied class of NASICON materials is of great interest due to compositional diversity and high ionic conductivity. The solid solution Na3 + xSc2(SiO4)x(PO4)3  x with 0.05  x  0.8 was investigated for the first time. The various compositions were synthesized by solid state reaction and their crystallographic and electrical properties were measured. As a result, one of the best sodium-conductive NASICON materials to date was obtained for x = 0.4 (σNa,Total = 6.9 × 10 4 S cm 1 at 25 °C). Furthermore, the importance of the sodium concentration and size of lattice parameters on the ionic conductivity were investigated. The bulk ionic conductivity was correlated with the structural parameters along the conduction pathway of the sodium ions and confirm the key influence of the interatomic Na–O distances on sodium ion transport.

Introduction

In 1976, Hong et al. introduced the term NASICON [1], [2] to describe the very high ionic conductivity in Na3Zr2Si2PO12. This term is now used for all ceramic materials with the same crystal structure and the general composition A1 + 2x + y + zM(II)xM(III)yM(IV)2  x  ySizP3  zO12 where A is usually a mono- or divalent cation (here, A = Na) and M are divalent, trivalent or tetravalent cations; P can also be substituted with Si or As. The materials belonging to the NASICON family are very attractive because of their compositional diversity leading to many possible applications, such as electrode materials or solid electrolytes in batteries as tested in an all-NASICON battery by Lalère et al. [3], Cl2 and CO2 sensors [4], [5], or photoluminescent devices [6]. The conductivity of the NASICON materials is strongly related to their Na concentration and their crystallographic structure, which is influenced by the size of the M cations. It has been concluded from a literature survey [7] that the average ionic radius of M cations should be close to the ionic radius of Zr, i.e. rZr = 0.72 Å [8], in order to obtain highly conductive materials comparable to β- and β″-alumina [9], [10]. In addition, the NASICON materials with the highest sodium ion conductivity contain 3–3.5 mol Na per formula unit and show a monoclinic distortion of the crystallographic lattice [7].

Following these guidelines for designing sodium ion conductors, the new solid solution Na3 + xSc2SixP3  xO12 (abbreviated hereafter as NSSiPx) was investigated. The ionic radius of Sc of 0.745 Å [8] is close to the ionic radius of Zr and the presence of the trivalent Sc leads to a high amount of Na per formula unit. The introduction of Si in the highly conductive Na3Sc2(PO4)3 (2.3 × 10 5 S cm 1 at room temperature) [11] was inspired by the work of Hong on Na1 + xZr2SixP3  xO12 [1] and of Vogel et al. on Na1 + xHf2SixP3  xO12 [12]. In both cases, the substitution with Si in the structure led to a significant increase in conductivity. Different structural parameters were correlated with the measured ionic conductivity in order to better understand the impact of Si substitution on the Na+ transport in the scandium-based NASICON materials.

Section snippets

Experimental

All compositions were synthesized via conventional solid state reaction. A stoichiometric homogenized mixture of NH4H2PO4 (Merck KGaA, 99%), Sc2O3 (Projector GmbH, 99.5%), Na2CO3 (Alfa Aesar GmbH & Co KG, 99.5%), and SiO2 (Alfa Aesar GmbH & Co KG, 99.8%) was heated with 300 K h 1 to 900 °C for 4 h. After grinding, the powder was again annealed for 20 h between 1280 °C and 1350 °C depending on the composition [11], [13], [14], [15]. The obtained powder was milled and pressed into pellets (13 mm in

Crystal structure

NSSiPx was synthesized in the compositional range of 0.05  x  0.8. For x  0.1, NSSiPx crystallizes with rhombohedral structure at room temperature (Fig. 1a). NSSiP0.05 is a mixture of rhombohedral and monoclinic phases as shown in Fig. 1b, in which the XRD patterns of the samples NSSiP0.05 and NSSiP0.1 are enlarged for the diffraction angles between 18° and 30°. The appearance of three reflections at 2θ = 19.5° and two reflections at 2θ = 28° are characteristic for the presence of the monoclinic phase

Conclusion

Substituting P with Si in the ion-conductive Na3Sc2(PO4)3 [11] leads to Na3.4Sc2(SiO4)0.4(PO4)2.6 with very high sodium ion conductivity of σNa,Total = 6.9 × 10 4 S cm 1 at 25 °C. The systematic study of the solid solution Na3 + xSc2(SiO4)x(PO4)3  x with 0.05  x  0.8 led to a better understanding of the influence of the substitution of P with Si in the NASICON materials. The increase in charge carriers led to an increase in conductivity until a maximum was reached, after which the ratio of the amount of

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