Enhancement of Superionic Conductivity by Halide Substitution in Strongly Stacking Faulted Li₃HoBr_6–xI_x Phases

Abstract

Ternary lithium halide-based solid electrolytes have attracted broad scientific interest due to their high ionic conductivities in conjunction with good electrochemical stabilities against high-voltage cathode materials. Here, we analyze the structure of the rare earth halide solid solution series Li3HoBr6–x I x and quantify structural defects such as intralayer cation disorder and stacking faults. Almost all members of the solid solution series show strong stacking fault disorder, whereas the intralayer cation disorder systematically increases with increasing iodide content. The substitution of bromide by iodide in Li3HoBr6–x I x leads to an increasing lattice softness and therefore to a higher ionic conductivity and to a lower activation energy. This is counteracted by the increased cation disorder, which also has a strong influence on the ionic conductivity of the solid solution series. Thus, a decrease in ionic conductivity by one order of magnitude is observed when the iodine content exceeds an optimum value. Stacking faults on the other hand do not have a significant impact on the ionic conductivity as the connectivity between neighboring lithium halide octahedra and hence the energy landscape of their migration pathways is not affected for equivalent stacking vectors. The competing factors above give rise to an optimum ionic conductivity at x ≈ 3 with 2.7 × 10–3 S cm–1 at 20 °C and an activation energy of 0.21 eV. While thermal annealing reduces the structural disorder of Li3HoBr6–x I x , the self-healing properties are significantly impeded by mechanical sample treatment, demonstrating the complex interplay between thermal and mechanical effects on the microstructure and, hence, ionic conductivity of layered superionic conductors.

Synthetic control of structure and conduction properties in Na–Y–Zr–Cl solid electrolytes

“…The compositional exibility of this materials class has led to the synthesis of a wide range of A 3 MX 6 chemistries (A ¼ Li, Na, M ¼ Sc, In, Y, Yb, Er, Hf, Ho, Dy, Al, Zr, and X ¼ F, Cl, Br, I) and remarkably tunable properties. [11][12][13][14][15][16][17][18][19][20][21][22] Many of the Li analogues exhibit conductivities on the order of 10 −3 S cm −1 thanks to a large number of intrinsic cation vacancies and low Li-ion migration barriers due to the weak Coulombic interactions between the Li-ions and the monovalent anion framework. 13,16,18,23 These conductivities can be further improved through iso-and aliovalent substitution on the halide or metal site.…”

“…13,16,18,23 These conductivities can be further improved through iso-and aliovalent substitution on the halide or metal site. [6][7][8]15,18,19,21,[24][25][26][27] While none of the halides have been shown to be stable against a metal anode, chlorides and uorides are stable at potentials >4 V as their highly electronegative anions resist oxidation, enabling stable long-term cycling even when tested against (uncoated) oxide cathodes. 11 Finally, scalable solution-based synthesis methods that can yield >100 g of material per batch have been reported for many A 3 MX 6 chemistries.…”

Investigation of Structure, Ionic Conductivity and Electrochemical Stability of Halogen-substitution in solid-state ion conductor Li3YBrxCl6-x

“…This suggests that the anisotropic, platelet-like morphology, which can show as preferred orientation due to non-random orientation of the powder when the x-ray diffraction data is measured in Bragg-Brentano geometry, could be at least a source of the discrepancy. However, recent literature has reported the occurrence of stacking faults in Li3YCl6 17 and Li3HoBrxI6-x 4 , presenting an alternative origin for the discrepancy (SI Text 1). As described in ref.…”

“…The Rietveld refinement shows that the Cl and Br atoms distribute statically across the halogen sites (as consistent with literature, see ref. [4][5][6] ) and small differences in the Y and Li occupancies (SI tables S4-S5). The monoclinic phase with the compositions Li3YBrxCl6-x at x = 3, 4.5 and 6 on the other hand, shows cubic close packing of the halogen atoms.…”

“…A strategy that has been successfully employed to tune the ionic conductivity of Li3M(III)X6 materials is halogen substitution [4][5][6][7][8][9] . Specifically for the compositional tie-line between Li3YCl6 and Li3YBr6, the Li3YBr3Cl3 composition was shown to have a 7.2 10 -3 S/cm ionic conductivity at room temperature (compared to the ~1 10 -3 S/cm of the full Li3YCl6/Li3YBr6 2 ).…”