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An energy storage paradigm orthogonal to Li-ion battery chemistries can be conceptualized by employing anions as the primary charge carriers. F-ion conversion chemistries show promise but have limited cyclability as a result of the significant change in volume of active electrodes upon metal–metal fluoride interconversion. In contrast, the exploration of insertion chemistries has been stymied by the lack of hosts amenable to reversible F-ion insertion at room temperature. Here we show the reversible and homogeneous topochemical insertion/deinsertion and bulk diffusion of F ions within the one-dimensional tunnels of submicrometer-sized FeSb₂O₄ particles at room temperature. The insertion of F ions is evidenced by formal oxidation of the iron centers from Fe²⁺ to Fe³⁺ with a lattice volume contraction of <1% at a capacity of one F ion per iron center. The topochemical insertion of F ions is observed to be …