β-SnSb
is known to be a highly stable anode for sodium ion
batteries during cycling, but its sodiation–desodiation alloying
reactions are poorly understood. Combining in situ TEM with electroanalytical
methods, we demonstrate that β-SnSb forms Na3Sb and
Na15Sn4 in sequence upon sodiation and re-forms
as β-SnSb upon desodiation. The negative enthalpy of mixing
for Sn and Sb is sufficient to cause sequentially deposited bilayers
of Sn/Sb to transform into β-SnSb, resulting in comparable cycling
stability. The good cycling stability of β-SnSb results from
the complex two-phase amorphous–nanocrystalline microstructure
in the partially charged–discharged states, as well as the
intrinsic mechanical toughness of the β phase. Per the in situ
TEM results, the sequential phase transformation shows minimal fracturing
of the β-SnSb, indicating facile buffering of stresses. Extensively
cycled specimens eventually show crystalline Sn phase segregation,
which may be the source of the ultimate capacity fade in the alloy
and bilayers.
The cycling stability of Sb-based anodes for sodium-ion batteries can be greatly improved by adding Sn and Bi atoms in a substitutional solid solution with the host Sb lattice.
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