  Electrochemical ion insertion involves coupled ion-electron transfer
reactions, transport of guest species, and redox of the host. The hosts are
typically anisotropic solids with two-dimensional conduction planes, but can
also be materials with one-dimensional or isotropic transport pathways. These
insertion compounds have traditionally been studied in the context of energy
storage, but also find extensive applications in electrocatalysis,
optoelectronics, and computing. Recent developments in operando, ultrafast, and
high-resolution characterization methods, as well as accurate theoretical
simulation methods, have led to a renaissance in the understanding of
ion-insertion compounds. In this Review, we present a unified framework for
understanding insertion compounds across time and length scales ranging from
atomic to device levels. Using graphite, transition metal dichalcogenides,
layered oxides, oxyhydroxides, and olivines as examples, we explore
commonalities in these materials in terms of point defects, interfacial
reactions, and phase transformations. We illustrate similarities in the
operating principles of various ion-insertion devices ranging from batteries
and electrocatalysts to electrochromics and thermal transistors, with the goal
of unifying research across disciplinary boundaries.
