Insights into gas generation and self-absorption in lithium-ion batteries after fast charge aging: Mechanism and implications

• Discovered lithium deposition causes gas generation during storing in fully discharged batteries. • Identified gas self-absorption by graphite and cathode during charging and its performance benefits. • Found gas self-absorption reduces impedance and restores capacity in aged batteries. • Revealed gas generation reduces thermal stability in fast-charging aged batteries. • Recommended storing fast-charging aged batteries at high SOC to prevent swelling. Lithium-ion batteries (LIBs) are susceptible to accelerated degradation, swelling, and safety concerns under fast-charging conditions, largely due to gas generation and related side reactions. This work investigates the electrochemical degradation mechanisms and coupled gas generation/self-absorption behaviors of pouch cells aged by fast charging. Through a combination of electrochemical impedance spectroscopy (EIS) and distribution of relaxation times (DRT) analysis, we identify a pronounced impedance rise and capacity fade linked to lithium plating on the anode and interfacial layer thickening on both electrodes. The gas emitted caused by lithium plating during storing in fully discharged batteries is primarily H 2 (∼70%), with CO 2 and hydrocarbon gases constituting the remainder. Significantly, during constant-voltage charging at high SOC, the swollen cells exhibit a remarkable shrinkage phenomenon, driven by in situ absorption of the internal gas. Extensive surface and structural characterizations indicate that both the fully lithiated graphite anode and high-voltage cathode actively contribute to H 2 absorption and reactive consumption of CO 2 , leading to partial recovery of cell capacity and a temporary reduction in internal resistance. Nevertheless, this process accelerates interfacial growth (SEI/CEI) and triggers additional side reactions, ultimately lowering thermal stability as confirmed by Accelerating Rate Calorimetry. These findings offer critical insights into the synergistic degradation pathways of fast-charged LIBs and highlight the potential of leveraging gas self-absorption for mitigating swelling. More broadly, this study underscores the importance of managing gas behavior to balance performance, lifetime, and safety in next-generation LIBs systems.