Gradient architecture design of porous aramid nanofiber separators for robust and safe lithium-ion batteries

Separator failure remains a critical safety challenge for the application of lithium-ion batteries (LIBs). Conventional polyolefin separators lack thermal stability that limits high-temperature operations, and the nonuniform, uncontrolled pores tend to induce lithium dendrite growth that compromises safety and performance. Herein, we develop a proton donor-regulated assembly strategy to incorporate a gradient nanopore architecture to aramid nanofibers (GANFM), which serves as a thermally stable and electrochemically superior separator for LIBs. Comparative experiments and simulations involving gradient separators in opposite orientations and a polyethylene (PP) separator reveal the mechanism of GANFM design. The larger nanopores near the cathode function as ion guides that facilitate Li-ion transport, while the smaller nanopores near the anode act as ion regulators smoothing the ion distribution. As a result, the GANFM achieves superior ionic conductivity and significantly reduces Li-ion concentration fluctuations (standard deviation is 0.39 times lower than that of PP). Both symmetric and full cells incorporating GANFM exhibit excellent reversible capacity and C-rate performance. The LiFePO₄//Li cells retain 85.3% capacity after 300 cycles with a high current density (5C) at room temperature. Even at 55 °C, capacity retention remains at 86.7% after 250 cycles. Our work deepens the understanding of pore structure-related electrochemistry and provides valuable insights into the design of high-safety separators for LIBs.