Study on the mechanism of cycle life decline of iron phosphate lithium ion batte
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2022/08/03 08:52:34
Acid lithium iron batteries because of the excellent safety performance, making it one of the best electric bus, with many huge iron phosphate lithium-ion batteries will face a variety of using environment and conditions of use of the test, let us know, such as temperature and SoC, the discharge rate will impact on their lives there, so in order to ensure the iron phosphate lithium ion battery can satisfy the use in different conditions It is required that we have an understanding of the failure characteristics and mechanism of iron phosphate lithium ion batteries in some use environments.
In this article, we will take you to the MeinertLewerenzetalLFP/ graphite batteries in different ratios, MOD and temperature fault characteristics are discussed, and a theoretical understanding of the new capacity of lithium ion batteries after storage for a period of time is proposed Human research results.
Under different temperatures and states of charge, the curve of battery capacity decline and internal resistance change shows that at 25℃, the internal resistance of the battery decreases by about 10% and the battery capacity decreases by 1-2%. However, during the initial 200-400 days of storage, the capacity increased by 0.7-1.3%. In fact, according to MeinertLewerenz, the LFP battery declines very slowly at 25 ° C and loses capacity during storage due to regular capacity testing itself.
For example, the capacity of 20%SoC battery increases significantly in the first 200 days of storage, and the capacity of 50%SoC battery increases somewhat. The capacity loss after 2 years of storage is 10-15%, which is much higher than the capacity loss of batteries at 25℃. At 60℃, the capacity decline of the battery is significantly higher than that at 25℃ and 45℃, reaching 20-25%.
At the beginning of storage, we found a small increase in battery capacity, which MeinertLewerenz believes is important because there are more positive terminals at the negative edge, as shown below. The proportion of negative electrode more than positive electrode is about 5.7%, and the proportion of positive electrode relative to negative electrode has higher Li content at the beginning of charging. However, driven by the voltage difference, Li will disperse to the periphery, shortening this distance and eventually making the negative electrode reach 80% SOC. Therefore, if the SoC state of the battery is stored in a low, the Lee concentration around the negative electrode is higher than the middle, then the Lee dispersion is promoted from the surrounding to the center, and then the positive polarity part of the negative electrode of the Lee concentration is added, thus improving the capacity of the battery after storage. We know that the dispersion of Li is caused by the voltage difference between the different parts of the negative terminal. In other words, when SoC is greater than 80%, the voltage curve of the graphite negative electrode is relatively flat, so the voltage difference between different parts of the negative electrode becomes very small. Therefore, the diffusion rate of Li is slow, and the increasing trend of capacity is not obvious. However, at low charged state, the negative potential difference is large, which can effectively promote the dispersion of lithium in the negative electrode, and then increase the battery capacity. The following table shows the voltage difference of the negative electrode at different temperatures and SoC. The voltage difference in this part will push the lithium to disperse inside the negative electrode, rebalancing it. The test results show that, under 20% SOC, the rear storage capacity is increased by 1.5-3.4%, 0.6-1.7%, 0.5-0.7% and 0.7%, respectively, under 50% SOC.
This theory is very simple, but does not think the cathode inhomogeneity, closer to the edge of the anode part may be elements, such as faster reaction rate, also cannot well explain ability in high SoC failure phenomenon, but the SoC theory can better explain the low, based on the storage capacity of graphite anode after rising phenomenon, is worth further discussion.
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