"The Secret Life of Lithium-Ion Batteries: Understanding the Longevity and Performance Factors"

The performance of lithium-ion batteries experiences varying degrees of degradation over time, marking a slow and irreversible process. The lifespan, a crucial performance parameter, is subject to current industry standards that specify it in terms of the number of full cycles (80% capacity retention) in standard charge-discharge cycle tests.

 

The complexity of real-world operating conditions significantly diverges from standard tests, with fluctuations in environmental conditions, operating rates, depth of charge-discharge, and more. This divergence renders direct measurement of the lifespan of lithium-ion batteries in real-world operation by cycle count alone unfeasible.

 

Consider a mobile phone battery with 100% charge. Using it until it reaches 0% and then charging it back to 100% constitutes one charge-discharge cycle. However, the battery cycle count is 1, and the charge count is also 1. Now, if the battery is used until it reaches 40% in the first instance and then charged back to 100%, and in the second instance, used until it reaches 60% and then charged back to 100%, these two uses together still constitute one complete charge-discharge cycle. In this case, the battery cycle count is 1, but the charge count is 2.

 

The equivalent full cycle count for lithium-ion batteries is defined as the count obtained by accumulating the capacity under various operating conditions divided by the battery's nominal capacity. A complete 100% discharge/charge cycle can involve just one charge or two, three, or more charges. What ultimately determines the lifespan of a lithium-ion battery is the equivalent full cycle count at 80% initial capacity, not the number of charges.

 

The lifespan study of various batteries under different temperatures, charge-discharge depths, and rates shows that LFP batteries exhibit longer cycle lifespans, ranging from 2500 to 9000 EFC (Equivalent Full Cycles), with most LFP batteries not yet reaching 80% capacity. NCA batteries have a poorer lifespan, ranging from 250 to 1500 EFC, and NMC batteries range from 200 to 2500 EFC. All batteries exhibit linear degradation behavior, with slightly faster degradation rates at the beginning and end of the cycles.

 

The capacity degradation rate of LFP batteries increases with higher temperatures, whereas the capacity degradation rate of NMC batteries decreases with higher temperatures. NCA batteries do not show strong temperature dependence within the experimental range. The capacity degradation rate of all batteries increases with higher discharge depths.

 

Due to the increased pressure on the electrode from rapid volume changes, higher discharge rates are expected to accelerate capacity degradation. NMC and LFP batteries have a lower dependence on discharge rates, but the capacity degradation of NCA batteries decreases with higher discharge rates.

 

The trends in battery dependence on temperature, discharge depth, and discharge rate vary in different chemical systems. The observed dependence in one chemical system cannot be broadly extrapolated to all lithium-ion batteries. In the temperature range of 15°C to 35°C, the capacity degradation rate of LFP batteries increases with higher temperatures, while NMC batteries show a decrease, indicating the existence of different dominant degradation mechanisms. NMC and NCA batteries have a stronger dependence on discharge depth and are more sensitive to full SOC (State of Charge) range cycling than LFP batteries.

 

In conclusion, it's important to emphasize that one charge-discharge cycle refers to one complete charge-discharge process of a lithium-ion battery. This means that when the battery's state of charge reaches 100%, it has completed one charging cycle, but not necessarily through one single charge.