1. SEI Film Reconstruction and Active Lithium Consumption

Although high‑temperature resting (45 °C) after formation can promote the complete intercalation of residual lithium and reduce unreacted metallic lithium on the anode surface, this process also consumes active lithium.

During high‑temperature resting, the SEI film undergoes reconstruction and stabilization. The initially formed SEI may contain unstable organic components (such as ROCO₂Li), which decompose at elevated temperatures and transform into more stable inorganic components (such as Li₂CO₃). At the same time, gases (e.g., CO₂) may be released, causing structural defects in the film. This process consumes active lithium ions and reduces the amount of reversible lithium, thereby directly lowering the initial coulombic efficiency.

2. Intensified Electrolyte Side Reactions

High temperatures accelerate electrolyte decomposition, especially at the interfaces of the cathode and anode. Solvent molecules (such as EC and DEC) may co‑intercalate into graphite layers with lithium ions, leading to graphite exfoliation and the formation of new SEI films, which further consume active lithium.

Studies have shown that when the resting time exceeds 24 hours, electrolyte decomposition side reactions increase significantly. This not only reduces the amount of free electrolyte but also generates irreversible by‑products, resulting in a lower initial efficiency.

3. Lithium Deposition and Intercalation Kinetics

During the high‑temperature resting stage after formation, lithium ions that have not fully intercalated into graphite continue to migrate and embed into the graphite structure. However, part of the lithium may deposit in irreversible forms (such as lithium dendrites or dead lithium), reducing the amount of reversible lithium.

Although extending the high‑temperature resting time can promote lithium intercalation, excessive lithium replenishment or overly long resting periods can lead to the loss of active lithium.

4. Internal Resistance and Polarization Effects

Excessively long high‑temperature resting periods may increase the internal resistance of the battery. While extended resting can improve electrolyte wetting, LiPF₆ decomposition at high temperatures produces LiF, which can deposit on the electrode surface and increase interfacial impedance.

The increase in internal resistance intensifies charge–discharge polarization, resulting in a reduction in the actual releasable capacity and a decline in initial efficiency.

5. Conclusion

High‑temperature resting time mainly reduces the initial efficiency through three pathways: lithium consumption caused by SEI film reconstruction, intensified electrolyte side reactions, and irreversible lithium deposition.