Carbon Nanofiber Sheet Cathodes Designed for 500 Wh kg ^–1 Class Lithium–Oxygen Batteries

Enhancing energy density is essential for the practical application of lithium–oxygen batteries. Here, we report a self-standing cathode composed of ultralong carbon nanofibers with lengths of at least several tens of micrometers and a typical diameter of 80 nm, designed to operate under lean electrolyte conditions with minimal cathode weight. The sheet, with a thickness of 0.136–0.180 mm and a weight of 2.5–2.6 mg cm–2, consistently achieved discharge capacities exceeding 4 mAh cm–2 at 0.1 mA cm–2, even when the electrolyte filled only 36 vol % of its void volume. This performance corresponds to an energy density of ∼500 Wh kg–1. Analysis of the discharge capacity and electrolyte utilization indicates that discharge proceeds via electrolyte migration within the sheet. Comparative studies with fibers of different diameters suggest that large voids naturally formed between carbon nanofibers with moderate-sized diameters play a key role in facilitating electrolyte migration.