An improved 9 micron thick separator for a 350 Wh/kg lithium metal rechargeable pouch cell

The use of separators that are thinner than conventional separators (> 20 µm) would improve the energy densities and specific energies of lithium batteries. However, thinner separators increase the risk of internal short circuits from lithium dendrites formed in both lithium-ion and lithium metal batteries. Herein, we grow metal-organic frameworks (MOFs) inside the channels of a polypropylene separator (8 µm thick) using current-driven electrosynthesis, which aggregates the electrolyte in the MOF channels. Compared to unmodified polypropylene separators, the MOF-modified separator (9 µm thick) vastly improves the cycling stability and dendrite resistance of cells assembled with Li metal anodes and carbonate-based electrolytes. As a demonstration, a 354 Wh kg^-1 pouch cell with a lithium metal anode and LiNi_0.8Co_0.15Al_0.05O₂ (NCA)-based cathode (N/P = 3.96) is assembled with 9 µm layer of the MOF-modified separator and retains 80% of its capacity after 200 cycles (charged at 75 mA g^-1, discharged at 100 mA g^-1) at 25 °C.