The Influence of Synthesis Procedure on the Structure and Ionic Transport Property of Substituted Sodium Metal Halide

Sodium all-solid-state batteries have attracted growing interest in recent years because of their potentially high energy densities and superior safety as well as the abundance of sodium. Sodium metal halides are viewed as promising candidates for catholytes in sodium all-solid-state batteries since they have wide electrochemical windows and excellent electrochemical compatibility toward oxide cathode materials, as well as good mechanical deformability and scale-up capability. However, the low ionic conductivity hinders their practical application. Substitution is an effective way to improve the ionic transport properties of ionic conductors.<br/> <br/>Herein, we introduced aliovalent cation In³⁺ in Na₂ZrCl₆ and the X-ray diffraction analyses show a full indium solubility in the 200°C annealed Na_2+<i>x</i>Zr_1-<i>x</i>In<i>_x</i>Cl₆ crystalized in a <i>P</i>2₁/<i>n</i> phase while that exsolves at higher heat treatment temperatures because the indium-rich Na_2+<i>x</i>Zr_1-<i>x</i>In<i>_x</i>Cl₆ compound tends to partially transform to a <i>P</i>1<i>c</i> phase. By assessing the ionic conductivity of the differently synthesized Na_2+<i>x</i>Zr_1-<i>x</i>In<i>_x</i>Cl₆ series, we can show the synergistic effect of the Na⁺/vacancy ratio and crystallinity on sodium ion transport in this class of materials which can enhance the ionic conductivity by three orders of magnitude.¹ Besides the cation doping, anion-mixed sodium halide (Na₃InCl₆_-<i>_x</i>Br<i>_x</i>) is also explored. By milling, the Na₃InCl₆_-<i>_x</i>Br<i>_x</i> (0 ≤ <i>x </i>≤ 1.5) solid solution series crystallizes in a monoclinic <i>P</i>2₁/<i>n </i>phase, while the subsequently annealed Na₃InCl₆_-<i>_x</i>Br<i>_x</i> (0 ≤ <i>x </i>≤ 2) series transforms into a trigonal <i>P</i>1<i>c</i> phase. A greater anion solubility can be achieved by annealing and changing the structure type. Unlike the cation substituted Na_2+<i>x</i>Zr_1-<i>x</i>In<i>_x</i>Cl₆ series, in Na₃InCl₆_-<i>_x</i>Br<i>_x</i> only slight improvements to the ionic conductivity are observed possibly due to an unfavorable trade-off between high pre-factor and low activation energy resulting from an enthalpy-entropy compensation behavior fulfilling the Meyer-Neldel rule.²<br/> <br/>These works further highlight the strong dependence of the structure and solubility of sodium metal halides on the synthesis protocol and shed light on the design principle and synthesis strategy of this class of materials as solid electrolytes.<br/> <br/><b>Reference</b><br/>1. Zhao, T.; Sobolev, A. N.; Schlem, R.; Helm, B.; Kraft, M. A.; Zeier, W. G., Synthesis-Controlled Cation Solubility in Solid Sodium Ion Conductors Na_2+<i>x</i>Zr_1–<i>x</i>In_xCl₆. <i>ACS Applied Energy Materials </i><b>2023,</b> <i>6</i> (8), 4334-4341.<br/>2. Zhao, T.; Kraft, M. A.; Zeier, W. G., Synthesis-Controlled Polymorphism and Anion Solubility in the Sodium ion-conductor Na₃InCl_6-<i>x</i>Br<i>x</i> (0 ≤ <i>x</i> ≤ 2). <i>Inorganic Chemistry</i>. Under review.