Every time a battery is cycled between charged and discharged states, ions and electrons are shuttled between electrodes. In this process, beyond storing charge, the addition and removal of ions in electrodes provides a reversible and tunable handle over the electronic structure and magnetic order. This Research Update paper connects the chemistry of battery electrode materials with the physics of emergent phenomena, including insulator–metal transitions, geometric magnetic frustration, charge and vacancy ordering, and cooperative Jahn–Teller effects. Ion insertion is suggested as being a potentially overlooked handle for the design of functional materials, and open experimental and theoretical challenges are highlighted.