AbstractBattery separators are a critical component that greatly determine cell calendar life and safety. Generally, these separators are passive with no ability to reversibly change their properties in order to optimize battery performance. Here, an iongate separator is demonstrated, which allows ion transport while in the oxidized “on” state but limits ion transport when switched to the reduced “off” state. This is achieved by depositing a dense 300 nm thin film of polypyrrole:polydopamine (PPy:PDA) on a conventional polyolefin separator. By using this iongate separator as a third electrode, a rapid and reversible order of magnitude increase of iongate resistance is achievable. The iongate battery shows similar cycling performance to a normal battery while in the “on” state, but cycling can be reversibly shut‐off when the iongate separator is reduced to the “off” state. During elevated temperature storage with the iongate separator in the “off” state, battery capacity loss is decreased by 37% and transition metal crossover is greatly suppressed when compared to a normal battery without the iongate. Additionally, rapid shut‐off during discharge is demonstrated by directly shorting the iongate separator to the anode.