A kinetic window framework governing depth of discharge-dependent charging in Li–CO₂ batteries

The sluggish decomposition kinetics of insulating discharge products impose large charging overpotentials and limit the practical application of Li–CO₂ batteries. While constant-voltage charging has shown promise in overcoming kinetic barriers, the optimal charging voltage and its evolution with depth of discharge remain unclear. Here we establish, for the first time, a kinetic-window framework that identifies the depth of discharge (DOD) as the master variable governing the feasible voltage range for reversible operation in a model Ru@rGO cathode with 1 M LiTFSI/TEGDME electrolyte. The lower bound of this window rises progressively with increasing depth of discharge, driven by continuous accumulation of crystalline Li₂CO₃ and carbon that thickens the passivation layer, whereas Li₂C₂O₄ persists at a quasi-steady level through a dynamic cycle of formation, disproportionation, and regeneration. Guided by this framework, we demonstrate that the shallow-optimized 4.0 V protocol fails at deeper discharge, while 4.2 V supports 87 stable cycles at 500 μAh cm^–2. At 1000 μAh cm^–2, even 4.4 V fails to sustain long-term operation, as the required driving force exceeds the electrolyte stability limit. This work establishes DOD as the master variable for protocol design and offers a universal, boundary-governed principle that can be extended to other metal–gas batteries facing similar polarization-accumulation challenges.