Self-extinguishing polyimide sandwiched separators for high-safety and fast-charging lithium metal batteries

Developing fast-charging lithium metal battery (LMB) with high specific capacity arouses enormous attentions. Unfortunately, during the operation of fast-charging LMB, substantial amount of Joule heat generated within battery often induces inhomogeneous heat accumulation that concomitantly deteriorates the lithium dendrite growth issues, resulting in rapid capacity fading and potentially fatal thermal runaway. It is critically essential to develop advanced separator that possesses high ionic conductivity, effective dendrite inhibition and self-extinguishing property for fast-charging LMB with high-safety. Here, we report a facile approach to fabricate polyimide composite separator (denoted as PIFAP), where the polyimide fiber (PIF) membrane is sandwiched by aerogel layers of polyimide/ammonium polyphosphate composite. Notably, compared with PIF, the PIFAP features significant enhancement in flame retardance, electrolyte affinity, electrolyte adsorption and ionic conductivity. Moreover, beneficiating from the aerogel layers with uniform nanopores of ∼100 nm, the optimized PIFAP separator can enable homogeneous lithium deposition, achieving a dendrite-free lithium deposition on the surface of anode for over 1600 h, superior to the PIF and Celgard separators. The resulting "LiFePO4/PIFAP/Li" cell can exhibit a remarkably high cycling stability with 99.1 % of capacity retention after long-term cycling at a current density of 10 C.