Ultra‐Thick Carbon Microhoneycomb Cathode for High‐Capacity Lithium‐Oxygen Batteries

The demand for high‐energy‐density batteries triggers interest in designing thick electrodes for lithium oxygen batteries (LOBs). Although thick electrodes are essential for achieving high discharge capacities, their performance is often limited by insufficient mass transport. Here, we report an ultra‐thick, biochar‐derived carbon microhoneycomb cathode (CMC) electrode produced from cellulose nanofibers (CNFs)‐based precursors via an ice‐templating process. This template‐directed method converts CNFs into structurally ordered biochar with aligned channels and a thickness of ca . 800 µm. We investigate their discharge behavior in LOB under varying electrolyte conditions. The optimized CMC exhibits a high areal capacity of 38.2 mAh cm –2 , owing to its highly penetrated channels that ensure efficient Li + and O 2 transport and sufficient accommodation space for the main discharge product, lithium peroxide (Li 2 O 2 ). Comprehensive characterizations reveal that the discharge capacity strongly depends on the electrolyte amount and its spatial distribution in ultra‐thick cathodes. Insufficient electrolyte (100 µL) limits Li + transport, while excess electrolyte (400 µL) impedes O 2 transport and suppresses the oxygen reduction reaction. We also reveal that asymmetric Li 2 O 2 deposition occurs across the electrode thickness, with a larger amount near the glass fiber separator, due to localized Li + enrichment. The finite element modeling simulates the change of discharge performance and Li 2 O 2 distribution with various amounts of electrolyte, suggesting the importance of optimizing electrolyte distribution. This work provides new insights into the design of biomass‐derived ultra‐thick carbon electrodes for high‐capacity LOBs and clarifies the critical role of electrolyte‐structure interactions in thick‐electrode operation.