State-of-the-art
oxides and sulfides with high Li-ion conductivity
and good electrochemical stability are among the most promising candidates
for solid-state electrolytes in secondary batteries. Yet emerging
halides offer promising alternatives because of their intrinsic low
Li
+
migration energy barriers, high electrochemical oxidative
stability, and beneficial mechanical properties. Mechanochemical synthesis
has enabled the characterization of LiAlX
4
compounds to
be extended and the iodide, LiAlI
4
, to be synthesized for
the first time (monoclinic
P
2
1
/
c
,
Z
= 4;
a
= 8.0846(1)
Å;
b
= 7.4369(1) Å;
c
=
14.8890(2) Å; β = 93.0457(8)°). Of the tetrahaloaluminates,
LiAlBr
4
exhibited the highest ionic conductivity at room
temperature (0.033 mS cm
–1
), while LiAlCl
4
showed a conductivity of 0.17 mS cm
–1
at 333 K,
coupled with the highest thermal and oxidative stability. Modeling
of the diffusion pathways suggests that the Li-ion transport mechanism
in each tetrahaloaluminate is closely related and mediated by both
halide polarizability and concerted complex anion motions.
Establishing links between the structure and physical properties of solid-state ionic conductors contributes not only to a rationale of their fundamental nature, but also provides design principles to accelerate the...
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