Investigation of Structure, Ionic Conductivity, and Electrochemical Stability of Halogen Substitution in Solid-State Ion Conductor Li₃YBr_xCl_6–x

Abstract

Li 3 YX 6 (X = Cl, Br) materials are Li-ion conductors that can be used as solid electrolytes in all solid-state batteries. Solid electrolytes ideally have high ionic conductivity and (electro)chemical compatibility with the electrodes. It was proven that introducing Br to Li 3 YCl 6 increases ionic conductivity but, according to thermodynamic calculations, should also reduce oxidative stability. In this paper, the trade-off between ionic conductivity and electrochemical stability in Li 3 YBr x Cl 6−x halogensubstituted compounds is investigated. The compositions of Li 3 YBr 1.5 Cl 4.5 and Li 3 YBr 4.5 Cl 1.5 are reported for the first time, along with a consistent analysis of the whole Li 3 YBr x Cl 6−x (x = 0− 6) tie-line. The results show that, while Br-rich materials are more conductive (5.36 × 10 −3 S/cm at 30 °C for x = 4.5), the oxidative stability is lower (∼3 V compared to ∼3.5 V). Small Br content (x = 1.5) does not affect oxidative stability but substantially increases ionic conductivity compared to pristine Li 3 YCl 6 (2.1 compared to 0.049 × 10 −3 S/cm at 30 °C). This work highlights that optimization of substitutions in the anion framework provide prolific and rational avenues for tailoring the properties of solid electrolytes.

Re-investigating the structure–property relationship of the solid electrolytes Li 3−xIn1−xZrxCl6 and the impact of In–Zr(iv) substitution

“…This paper shows that aliovalent doping does not only affect the charge carrier concentration, but can also lead to changes in the distribution of cations in the cell, affecting diffusivity on different timescales. For future studies, it would be interesting to combine this strategy with other successful strategies such as for example halogen alloying, 66 which can affect ionic conductivity due to a change in lattice parameter (bottleneck size), lattice polarizability as well as potentially the configurational entropy.…”

Synthesis-Controlled Polymorphism and Anion Solubility in the Sodium-Ion Conductor Na3InCl6–xBrx(0 ≤x≤ 2)

“…), all of which can also have a significant impact on the ionic conductivity. 50 Moreover, unlike the case of bromine-substituted doped Li 3 InCl 6 , 36 bromine substitution can only slightly improve the ionic conductivity within the Na 3 InCl 6−x Br x series. Previous calculation results suggest that Li 3 InCl 3 Br 3 separates into bromine-and chlorine-rich regions, and this kind of separation increases lithium-ion diffusivity at the interface.…”

“…35 Additionally, for a series of compounds Li 3 YBr x Cl 6−x synthesized by comelting of the precursors, the composition Li 3 YCl 4.5 Br 1.5 (x = 1.5) crystallizes in the trigonal P3̅ m1 phase (like Li 3 YCl 6 ), while the compositions Li 3 YCl 6−x Br x with x = 3 and 4.5 crystallize in the monoclinic C2/m phase (like Li 3 YBr 6 ), and halogen mixing allows higher conductivities (a maximum of 5.4 × 10 −3 S•cm 1 at 30 °C for Li 3 YBr 4.5 Cl 1.5 ) especially compared to that of Li 3 YCl 6 (4.9 × 10 −5 S•cm −1 at 30 °C). 36 Similarly, the Li 3 InBr x Cl 6−x series of compounds synthesized by ball milling and subsequent annealing also show various ionic conductivities, which reach a maximum of 1.2 × 10 −4 S•cm 1 at x = 3. 37 A similar complex relationship between the composition and structure was also observed in a halogenmixed sodium solid solution Na 3 GdBr x I 6−x system.…”

“…Here, Na 2.25 Zr 0.75 Y 0.25 Cl 6 and Na 2.4 Zr 0.6 Er 0.4 Cl 6 show ionic conductivities of 6.6 × 10 –5 and 3.5 × 10 –5 S·cm –1 , about 3 and 4 orders of magnitude higher than those of Na 3 YCl 6 and Na 3 ErCl 6 , reepectively. , Besides cation doping, anion substitution is also commonly explored to develop more conductive solid electrolytes, where especially halogen mixing seems beneficial for conductivity. , For example, the incorporation of chlorine in Li 2 ZrF 6 enhances its room temperature ionic conductivity by about 5 orders of magnitude up to 5.5 × 10 –7 S·cm –1 in Li 2 ZrF 5 Cl because of weaker Li–Cl bonds enabling more facile lithium-ion transfer . Additionally, for a series of compounds Li 3 YBr x Cl 6– x synthesized by comelting of the precursors, the composition Li 3 YCl 4.5 Br 1.5 ( x = 1.5) crystallizes in the trigonal P 3̅ m 1 phase (like Li 3 YCl 6 ), while the compositions Li 3 YCl 6– x Br x with x = 3 and 4.5 crystallize in the monoclinic C 2/ m phase (like Li 3 YBr 6 ), and halogen mixing allows higher conductivities (a maximum of 5.4 × 10 –3 S·cm 1 at 30 °C for Li 3 YBr 4.5 Cl 1.5 ) especially compared to that of Li 3 YCl 6 (4.9 × 10 –5 S·cm –1 at 30 °C) . Similarly, the Li 3 InBr x Cl 6– x series of compounds synthesized by ball milling and subsequent annealing also show various ionic conductivities, which reach a maximum of 1.2 × 10 –4 S·cm 1 at x = 3 .…”

Influence of synthesis and substitution on the structure and ionic transport properties of lithium rare earth metal halides

“…32 Similar effects were also observed for Na 3− x Y 1− x Zr x Cl 6 , 33 Li 3− x Er 1− x Zr x Cl 6 34 or Li 3− x Yb 1− x M x Cl 6 with M = Zr 4+ , Hf 4+ . 35,36 In Na 3 GdBr 6− x I x , 27 Li 3 YCl 6− x Br x 37 and Li 3 HoBr 6− x I x 19 it was shown that isovalent anion substitution can be used to tune the ionic conductivity and its activation energy. While in Li 3 HoBr 6− x I x an increase in iodine content led to a decrease in activation energy due to softening of the anion lattice, this is also accompanied by an increase in Li/rare earth (RE) site disorder, which led to a reduction of the ionic conductivity due to an increased electrostatic repulsion along the lithium conduction pathways.…”