Abstract
Trace additives improve battery durability, but the atomic‐scale reasons for their effectiveness are often unclear. In lead–acid batteries, antimony suppresses softening and shedding of positive active material, yet its impact depends strongly on its chemical form. Here, we systematically elucidate how antimony chemical speciation governs structural evolution and electrochemical durability in battery cathodes, using the lead–acid system as an industrially relevant model. Quantitative multicomponent synchrotron X‐ray diffraction combined with cryogenic extended X‐ray absorption fine structure spectroscopy directly compares the behavior of Sb
2
O
3
, Sb
2
(SO
4
)
3
, and Sb
2
O
5
. For Sb
2
O
3
and Sb
2
(SO
4
)
3
, which enhance cycle life, Sb
5+
substitutionally incorporates into Pb sites in the PbO
2
lattice. In contrast, Sb
2
O
5
—ineffective for durability—shows no such lattice incorporation. These results demonstrate that reversible incorporation of Sb into the host lattice is the key factor governing electrode stability and suppressing active‐material degradation. Broadly, these results establish a general design principle for trace additives in electrochemical materials: durability improvement requires chemical species capable of specific, reversible atomic‐scale interactions with the host lattice. By explicitly linking chemical speciation, lattice incorporation, and degradation suppression, this work demonstrates how advanced structural characterization can transform mature battery systems into platforms for extracting transferable materials‐design rules.