1. SEI Film Reformation and Active Lithium Consumption
Although high-temperature aging (45 °C) after formation facilitates full intercalation of residual lithium and reduces unreacted metallic lithium on the anode surface, active lithium is inevitably consumed during this process.
During high-temperature aging, reformation and stabilization of the solid electrolyte interphase (SEI) film take place. The initially formed SEI contains unstable organic components (e.g., ROCOLi), which decompose at elevated temperatures and convert into more stable inorganic species (e.g., Li₂CO₃), meanwhile releasing gases such as CO and creating structural defects in the film. This reaction consumes active lithium ions and reduces the inventory of reversible lithium, directly lowering the first Coulombic efficiency.
2. Aggravated Electrolyte Side Reactions
High temperatures accelerate electrolyte decomposition, especially at the interfaces between cathode and anode. Solvent molecules (e.g., EC, DEC) may co-intercalate into graphite interlayers along with lithium ions, triggering graphite exfoliation and generating new SEI films, which further deplete active lithium.
Literature indicates that electrolyte decomposition side reactions rise significantly when aging exceeds 24 hours. This reduces free liquid electrolyte while generating irreversible byproducts, resulting in lower first Coulombic efficiency.
3. Lithium Deposition and Intercalation Kinetics
During high-temperature aging post formation, lithium ions that fail to fully intercalate into graphite continue migrating and embedding into the anode. Nevertheless, part of the lithium may deposit in irreversible forms such as lithium dendrites or dead lithium, cutting down the quantity of reversible lithium. Extended high-temperature aging promotes lithium intercalation, yet excessive supplementary lithium storage or overlong aging periods lead to net loss of active lithium.
4. Internal Resistance and Polarization Effects
Excessively prolonged high-temperature aging may increase battery internal resistance. While extended aging improves electrolyte wetting, LiPF₆ decomposes under heat to produce LiF that precipitates on electrode surfaces and elevates interfacial impedance. Higher internal resistance intensifies polarization during charge and discharge, reducing the practically deliverable capacity and degrading first Coulombic efficiency.
5. Conclusion
High-temperature aging duration reduces the first Coulombic efficiency mainly through three pathways: lithium consumption induced by SEI reformation, aggravated electrolyte side reactions, and irreversible lithium deposition.
