Hollow carbon fibers with diameters on the order of 30 nm were grown on a ceramic porous substrate, which was used as the oxygen electrode in lithium-oxygen (Li–O₂) batteries. These all-carbon-fiber (binder-free) electrodes were found to yield high gravimetric energies (up to 2500 W h kg_discharged⁻¹) in Li–O₂ cells, translating to an energy enhancement 4 times greater than the state-of-the-art lithium intercalation compounds such as LiCoO₂ (600 W h kg_electrode⁻¹). The high gravimetric energy achieved in this study can be attributed to low carbon packing in the grown carbon-fiber electrodes and highly efficient utilization of the available carbon mass and void volume for Li₂O₂ formation. The nanofiber structure allowed for the clear visualization of Li₂O₂ formation and morphological evolution during discharge and its disappearance upon charge, where Li₂O₂ particles grown on the sidewalls of the aligned carbon fibers were found to be toroids, having particle sizes increasing (up to 1 μm) with increasing depth-of-discharge. The visualization of Li₂O₂ morphologies upon discharge and disappearance upon charge represents a critical step toward understanding key processes that limit the rate capability and low round-trip efficiencies of Li–O₂ batteries, which are not currently understood within the field.