Piezoionic Interface Engineering Enabled High Energy Density and Suppressed Self‐Discharge in Flexible Supercapacitors

Self‐discharge remains a persistent challenge in electrochemical energy storage, especially in interface‐dominated flexible supercapacitors, where suppressing charge loss often compromises ion transport and energy density. Here, we report a piezoionic interface engineering strategy that uses mechanical pressure as an active variable to regulate interfacial ion transport and charge relaxation. Guided by the Hofmeister effect, a PVA‐based hydrogel electrolyte with tunable piezoionic behavior is developed to generate pressure‐induced ionic polarization. Upon compression, improved interfacial conformality drives ions into deeper micropores and promotes the formation of a more compact electric double layer, thereby enhancing capacitance. Meanwhile, the induced polarization field stabilizes interfacial charge distribution and suppresses ion desorption, leading to slower self‐discharge. As a result, the flexible supercapacitor achieves an energy density of 112.4 µWh cm −2 at 100 µW cm −2 under 100 kPa and retains 90.1% of its stored energy after 1 h of open‐circuit rest, representing a 3.2‐fold improvement over the unpressurized state. As a proof of concept, the flexible supercapacitor powers a wireless wearable insole system. This work provides a pressure‐regulated interfacial design paradigm for simultaneously enhancing energy density and charge retention in flexible supercapacitors.