After the lithium battery is fully charged, during the first charge and discharge cycle, electrochemical reactions occur at the solid-liquid interface between the electrode material and the electrolyte, leading to the formation of a solid electrolyte interphase (SEI) film that covers the surface of the electrode. The quality of this SEI film plays a crucial role in determining the battery's cycle life. The SEI is composed of various compounds such as Li₂O, LiF, LiCl, Li₂CO₃, LiCO₂-R, alkoxides, and non-conductive polymers. It has a multi-layered structure, with a porous side facing the electrolyte and a denser side in contact with the electrode.
On one hand, the formation of the SEI film consumes some lithium ions, increasing the irreversible capacity during the initial charge and discharge cycles, which reduces the overall charge and discharge efficiency of the electrode material. On the other hand, the SEI film is insoluble in organic solvents and can remain stable in the electrolyte solution, thereby enhancing the battery’s cycle life.
The chemical conversion process significantly influences the formation of the SEI film and directly affects the battery’s performance. Typically, the formation potential for the SEI film lies between 0.6 V and 0.8 V. Therefore, during the initial formation phase, it is common to use a very small current to ensure the SEI film forms more densely, which benefits the long-term cycle performance of the battery.
Although traditional small-current pre-charging methods help form a stable SEI film, prolonged low-current charging can increase the impedance of the SEI, affecting both the cycle life and rate performance of the battery. Additionally, gas generation during the charging process can accumulate between the battery’s separators if the gas production rate exceeds the injection rate, potentially interfering with the SEI film formation on the negative electrode. Thus, selecting an appropriate current and formation time is essential.
Currently, the chemical conversion process is mainly divided into two types: a multi-step stepwise process and a constant current process. Which method is more suitable?
Group A uses a multi-step formation process:
Charging (0.05CC/4h → 0.1CC/2h → 0.2CC/1h → 0.4CC CV/4.2V → 0.02C cut) → Standby 0.5h → Discharging (0.5C to cutoff voltage) → Standby 0.5h, repeating three times, then 0.2C/2h to 4.0V.
Group B uses a constant current formation process:
Charging (0.2C/2.5h) → Standby 12h → Discharging (0.2C to cutoff voltage) → Standby 0.5h → Charging (0.2C/4.2V, 0.02C cut) → Standby 0.5h → Discharging (0.2C to cutoff voltage) → Standby 0.5h → Charging (0.2C/4.0V).
As you can see, Group A employs a slow, incremental charging approach, gradually increasing the current while reducing the charging time. Group B, on the other hand, uses a consistent 0.2C current without many parameter changes. What are the differences between these two formation processes in terms of SEI film and electrochemical performance?
First, the SEI film:
SEM analysis of the negative electrode surfaces showed that both groups had an SEI layer covering the electrode surface. However, no significant difference was observed in the thickness or coverage area between the two groups.
Second, electrochemical performance:
Analyzing the basic electrochemical performance of the two groups, it was found that the positive electrode material capacity of the batteries using the stepwise formation process was 3 mAh/g higher than that of the constant current process. Additionally, the overall charge and discharge efficiency was better. After 50 weeks of cycling, the constant current process showed a slower capacity fade compared to the stepwise process.
In the first cycle, the constant current type performed lower than the step type, but by the second cycle, it outperformed the stepwise method. This suggests that the reversible reaction degree of the battery formed using the constant current method is higher, resulting in less irreversible capacity loss.
Third, SEI component analysis:
By analyzing the SEI of both groups, the following conclusions were drawn:
1. After the stepwise formation, the lithium content on the surface of the negative electrode was higher than that in the constant current group. This may be due to the formation of various lithium-containing compounds under different current densities, leading to excess lithium.
2. XPS results confirmed the presence of Li₂CO₃ or LiCO₂-R in the SEI films of both groups.
3. The SEI thickness formed by both processes exceeded 3 nm.
Through the analysis of these two different formation processes, it becomes clear that varying current magnitudes and durations have distinct impacts on battery performance. The composition and properties of the SEI film also differ, which ultimately affects the battery’s performance.
In real-world production, it is necessary to adjust the current and duration based on the specific battery system and capacity. It is not reasonable to label one method as entirely unsuitable. Regardless of the method used, both approaches contribute to the formation and stabilization of the SEI film. This article aims to provide insights and encourage further discussion. Feel free to leave a comment below!
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