The initial coulombic efficiency (ICE) of a lithium battery is a key indicator of its electrochemical performance. However, prolonged resting time under high temperature can significantly reduce ICE by influencing electrolyte behavior, electrode stability, binder performance, and SEI layer formation.
1. Electrolyte Decomposition
Extended exposure to heat may trigger decomposition of organic solvents in the electrolyte, producing gases such as CO, CO₂, and CH₄. This not only increases internal pressure but also disrupts the internal structure of the cell, ultimately reducing its initial efficiency.
2. Electrode Material Degradation
High temperatures can cause structural collapse or phase transitions in cathode materials such as NCM (nickel-cobalt-manganese oxides) or lithium cobalt oxide. Once the lithium-ion pathways are obstructed, the effective capacity decreases, leading to lower ICE.
3. Binder Performance Loss
Polyvinylidene fluoride (PVDF) and other binders may undergo chain scission or swelling under heat. This weakens the structural integrity of the electrode, causing active material to detach and lowering the battery’s electrochemical performance during the first cycle.
4. Poor SEI Layer Formation
Excessive high-temperature resting accelerates side reactions between the anode and electrolyte, consuming more lithium ions. This results in a low-quality solid electrolyte interphase (SEI) with higher resistance to lithium-ion transport, further reducing ICE.
Conclusion
In short, excessive high-temperature resting time negatively impacts a lithium battery’s initial coulombic efficiency by accelerating degradation processes at multiple levels. To optimize performance and ensure long-term reliability, it is essential to carefully control storage conditions and avoid unnecessary heat exposure prior to cycling.
Recommended Storage Conditions
Polymer lithium batteries should be stored in a cool, dry, and ventilated environment, ideally at 15–25 °C (59–77 °F) with relative humidity below 65%. They should be kept away from direct sunlight, heat sources, and flammable materials. For long-term storage, maintaining the battery at 40–60% state of charge (SOC) helps minimize degradation.