Knowledge about Passivation in Lithium Thionyl Chloride Batteries

March 6, 2025

 Knowledge about Passivation in Lithium Thionyl Chloride Batteries
 
 1. Definition of Passivation
Passivation in lithium thionyl chloride (Li/SOCl₂) batteries refers to the formation of an insulating film on the surface of the lithium anode. This film, primarily composed of lithium chloride (LiCl), is a product of the reaction between thionyl chloride (SOCl₂) and lithium. Passivation is an inherent characteristic of Li/SOCl₂ batteries, which helps extend their storage life by limiting internal chemical reactions.
 2. Mechanism of Passivation Layer Formation
The passivation layer forms due to the chemical reaction between the thionyl chloride electrolyte and the lithium anode. When SOCl₂ comes into contact with lithium, a dense passivation film immediately forms on the lithium surface. Although this film allows lithium ions to pass through, its ion migration rate is low, which hampers the normal discharge of the battery. The thickness of the passivation layer increases with storage time but grows more slowly as the layer itself acts as a barrier to further reactions.
 
 3. Impact of Passivation on Battery Performance
The presence of the passivation layer has both positive and negative effects on battery performance:
- **Positive Impact**: The passivation layer significantly reduces the self-discharge rate of the battery, allowing Li/SOCl₂ batteries to maintain high capacity during long-term storage. This makes them suitable for applications requiring long standby times.
- **Negative Impact**: The passivation layer increases the internal resistance of the battery, leading to initial voltage drops during discharge (voltage lag) and potentially reducing overall battery capacity. In applications requiring high current pulses, the passivation layer may limit battery performance.
 
 4. How to Mitigate the Effects of Passivation
To reduce the impact of passivation on battery performance, the following methods can be employed:
1. **Low-Current Discharge Activation**: Discharging the battery with a low current (e.g., 10 mA) or using an external resistor can gradually remove the passivation layer and restore battery performance.
2. **Pulsed Current Activation**: Using pulsed current to activate the battery can more effectively break down the passivation layer.
3. **Controlled Storage Conditions**: Storing the battery in a low-temperature, dry environment can slow down the formation of the passivation layer.
4. **Chemical Additives**: Some battery manufacturers add chemicals to the electrolyte to limit the growth of the passivation layer while maintaining the battery's safety and storage life.
 
 5. Applications and Limitations of Passivation
The presence of the passivation layer allows lithium thionyl chloride batteries to exhibit extremely low self-discharge rates (less than 0.5% per year), making them ideal for long-term storage. However, it also limits their performance in high-current pulse applications. Modern Li/SOCl₂ batteries often optimize the thickness of the passivation layer to balance self-discharge rates and discharge performance.
 
 6. Conclusion
Passivation is an inherent characteristic of lithium thionyl chloride batteries. It plays a crucial role in extending their storage life and reducing self-discharge rates but also imposes certain limitations on discharge performance. By optimizing storage conditions, employing activation methods, or using chemical additives, the negative impacts of passivation on battery performance can be effectively mitigated, thereby enhancing the battery's performance in practical applications.