Electrical energy storage and management is an urgent issue due to climate change and energy shortage. Dielectric and double-layer capacitors are the two basic energy-storage devices currently available. However, the application of the dielectric capacitor is very limited due to its low capacitance, while the double-layer capacitor suffers from difficult scaling-up and high fabrication cost, even though its capacitance is high. In this work, we present the first energy-storage membrane which stores charge when simply sandwiched between two metal plates. With an ionic conductivity of 2.8 × 10⁻⁴ S cm⁻¹, the membrane is highly polarizable, and its energy-storage mechanism is based on the condensation–decondensation of mobile cations in the membrane negative matrix. The capacitance of a 1.0 cm² area membrane is 0.2 F cm⁻², and it can be readily scaled up simply by using larger membrane pieces. In view of its extreme simplicity, excellent scalability and practical viability, the novel game-changing membrane reported here may provide a sustainable solution to energy storage.