Lithium extraction from brines is essential to onshoring
battery
supply chains. Yet, cation-exchange sorbents often exhibit poor selectivity
and require pH swings, contributing to increased capital and operating
expenditures. Here, we develop lithium extraction sorbents that obviate
pH swings by reversibly binding Li+ within aqueous brine-compatible
polymers of intrinsic microporosity (AquaPIMs) bearing metal-complexing
aza-crown-ether pendants. We conduct selectivity screens with synthetic
and natural brines, the latter obtained from oilfield and salt lake
sites across North America. Sorption selectivities for Li+/Na+, Li+/K+, Li+/Mg2+, and Li+/Ca2+change with respect to
sorbent and brine composition due to pH, mixed-ion effects, and ion–polymer
interactions; this behavior was pronounced for natural brines. Using
an AquaPIM sorbent embodying aza-12-crown-4 pendants, we demonstrated
a lithium extraction process for Smackover Formation brine (Arkansas,
USA), enriching Li by a factor of 3.7 when desorbing it with pure
water. Our work underscores the importance of designing sorbents based
on brine composition and Li+/M
n+ separation priorities.