Separator with high ionic conductivity enables electrochemical capacitors to line-filter at high power

Line-filtering electrochemical capacitors (LFECs) are demonstrating advantages in line filtering over traditional electrolytic capacitors. However, they can only function at no-load or low-power conditions due to the limited high-frequency capacitance resulting from the excessive ionic resistance, despite much progress in electrode materials. Here, we show separators dominate both ion migration and capacitance in LFECs. A 3 μm-thick thread-anchor structured separator is developed, featuring both accelerated ionic transport and reliability, leading to a low ionic resistance of 25 mΩ cm². With a phase angle of -80° at 120 Hz, the assembled device has an areal capacitance of 6.6 mF cm^-2. Furthermore, stack integration in parallel breaks the trade-off between capacitance and frequency response, boosting the areal capacitance by two orders of magnitude without decay of frequency characteristics. The On-board field test demonstrates that voltage ripples are steadily suppressed below 5% even for practical high-power line filtering with a load power density of 2.5 W cm^-2, three orders of magnitude higher than previous instances. This work opens up a perspective of separator engineering for the development of high-performance line-filtering electrochemical capacitors and promotes their applications in practical high-power scenarios.