Research on the Decline Mechanism of Cycle Life of Lithium Iron Phosphate Battery

Research on the mechanism of the cycle life reduction of lithium iron phosphate batteries. Because of its excellent safety performance, the lithium iron phosphate battery makes it the best electric bus. The huge use makes the lithium iron phosphate battery face a variety of use environments and uses Conditional testing, let us know, such as temperature, SoC, and the discharge rate will have an impact on their lives, so in order to ensure that the lithium iron phosphate battery can meet the requirements under different conditions, we must treat phosphoric acid in some use environments. The failure characteristics and mechanism of iron-lithium batteries are known. The MeinertLewerenzetalLFP/graphite battery at RWTH Aachen University in Germany has a discussion on the characteristics of different ratios, Ministry of Defense and temperature failures. After a period of storage, the new capacity of lithium batteries is proposed. Will take you to know the results of Meinert Lewerenz and others' research. Under different temperature and state of charge, the battery capacity decrease and internal resistance change curve can be seen that at 25°C, the battery internal resistance decreases by about 10%, and the battery capacity decreases by 1-2%. However, during the initial 200-400 days of storage, capacity increased by 0.7-1.3% instead. MeinertLewerenz believes that, in fact, the LFP battery drops very slowly at 25°C, and the battery capacity is lost due to the periodic capacity test itself during storage. For example, the capacity of a 20% SoC battery has a significant increase in the first 200 days of storage, and the capacity of a 50% SoC battery has a certain increase. The capacity loss of storage for 2 years is 10-15%, which is much higher than the capacity loss of the battery at 25°C. At 60°C, the decrease in battery capacity is significantly higher than that at 25°C and 45°C, reaching 20-25%. At the beginning of storage, we found a small increase in battery capacity. MeinertLewerenz believes that this is important because there are more positive electrodes on the edge of the negative electrode, as shown in the figure below. The part of the negative electrode more than the positive electrode is about 5.7%, and the part of the positive electrode relative to the negative electrode has a higher Li content at the beginning of charging. However, under the drive of the voltage difference, Li will be dispersed to the periphery, shortening this distance, and finally make the negative electrode reach 80% state of charge. Therefore, if the battery SoC state is stored at a low level, the Li concentration around the negative electrode is higher than the middle part, and then the Li dispersion from the periphery to the center is promoted, and then the positive part of the negative electrode of the Li concentration is added, thereby increasing the battery capacity After storage. We know that the dispersion of Li is mainly caused by the voltage difference of different parts of the negative electrode. In other words, when the 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 speed of Li is relatively slow, and the capacity increase trend is not obvious. In the low-charge state, the negative electrode potential difference is large, which can effectively promote the dispersion of lithium in the negative electrode, thereby increasing the battery capacity. The following table shows the voltage difference of the negative electrode under different temperatures and SoCs. This part of the voltage difference will promote the dispersion of lithium in the negative electrode and rebalance. The test results show that under 20% state of charge, the storage capacity increases by 1.5-3.4%, under 50% state of charge, an increase of 0.6-1.7%, and under 100% state of charge, an increase of 0.5-0.7%, 80% Reduced by 0.7% when charged. This theory is very simple, but it does not consider the inhomogeneity of the cathode. The edge part closer to the anode may be an element, such as a faster reaction rate, and it cannot explain the failure phenomenon in high SoC well, but the SoC theory can be more A good explanation is that the battery capacity increase phenomenon based on the graphite anode is worthy of our follow-up discussion. Disclaimer: Some pictures and content of articles published on this site are from the Internet. If there is any infringement, please contact to delete. Previous post: What are the types of dry batteries?