Lithium–aluminum layered double hydroxides (LDHs) selectively sorb lithium from brines, concentrating and purifying this critical element for subsequent conversion to active battery components. Lithium ion partitioning into lattice vacancies within the LDH structure is selectively enhanced with iron doping. However, this process leads to a highly coupled set of intercalation interactions whose mechanisms are challenging to assess in situ. Here, we show that iron modulates the size- and shape-dependent composition of LDHs and imposes a powerful control on lithium sorption processes in complex fluids. We observe fundamental units of LDH layers and aluminum ferrihydrite nanoclusters that (dis)assemble to form at least five distinct particle types that influence LDH lithium capacity and cyclability. Importantly, lithium sorption is controlled by feedbacks arising from the dynamic interconversion of planar stacks and scrolls of LDH layers, which exchange lithium, water, and other species in the process of (un)rolling due to similar energy scales of hydration, sorption, and deformation. Under appropriate iron redox conditions, the cycling efficiency and stability of lithium sorption can be optimized for the range of lithium concentrations found in many natural brines.