Lithium-ion batteries have revolutionized the way we power portable electronic devices and electric vehicles. However, as these batteries age, they often experience a gradual decline in capacity, impacting their performance over time.
Understanding and identifying the reasons behind this capacity attenuation are crucial for improving battery technology. One significant factor contributing to capacity loss is the formation of the Solid Electrolyte Interface (SEI) membrane due to the accumulation of lithium metal on the graphite particles within the battery.
In this article, we explore the methods used to detect and analyze lithium in lithium-ion batteries, shedding light on capacity attenuation and cell aging.
Small Current Discharge Method
The small current discharge method offers insights into the live lithium stripping reaction that occurs within the battery. During small current discharge, a voltage platform is produced as a result of this live lithium stripping reaction. Researchers utilize differential analysis of the voltage platform to quantitatively evaluate this reaction. This analysis employs two key approaches:
Differential Voltage (DV): DV analysis is a widely used method that can effectively explain electrode reaction processes. It identifies the peak in DV when phase changes occur, allowing for the determination of stripped power (Qstripping).
Differential Capacity (DC): DC analysis, which examines the relationship between dQ/dV and V, provides information on the reversible component of lithium precipitation. However, it’s essential to note that this reversible component does not contribute to capacity loss, and it’s the irreversible component that plays a significant role in capacity reduction.
Researchers have observed that the reversible component of lithium precipitation increases linearly when the State of Charge (SOC) is below 80%. However, beyond 80% SOC, this component tends to saturate. Interestingly, SOC levels exceeding 90% mark an important stage in the transformation of “live lithium” into “dead lithium,” with a sudden shift from the reversible to the irreversible component.
Voltage Relaxation Method
The voltage relaxation method offers an alternative approach to analyzing lithium in lithium-ion batteries. After charging the battery, it is allowed to sit for a few hours, and then the relaxation voltage curve is analyzed using differential voltage or differential time methods.
Here’s how this method works:
Differential Analysis: Unlike the small current discharge method, there is no net current flow during voltage relaxation. Consequently, traditional methods like dV/dQ or dQ/dV cannot be employed. Instead, researchers use dV/dt and dt/dV to extract differential information.
Observations from this method have led to some key findings:
- An inflection point in the differential voltage signal is detected when the end State of Charge (SOC) is greater than or equal to 70%, indicating the presence of lithium precipitation.
- Charging from different initial SOC states shows lithium precipitation occurring when the initial SOC is less than or equal to 60%.
Overall, the occurrence of lithium precipitation depends not only on SOC states but also on the corresponding charging currents in the SOC state.
Lithium behavior is key to understanding battery aging and improving lifespan. Techniques like small current discharge and voltage relaxation reveal insights into how lithium reacts in different conditions and charge states.
By studying lithium behavior, researchers can develop better, longer-lasting batteries, boosting performance and reliability in everything from laptops to electric vehicles.
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