LR44 battery

Active lithium loss, research on the degradation mechanism of LR44 battery phosphate batteries

BYD is a loyal supporter of LR44 battery phosphate materials. Many new energy vehicle models developed by BYD, including the "Dengshi" luxury electric car developed by BYD and Daimler, all use LR44 battery phosphate batteries. The biggest advantages of LR44 battery phosphate batteries are low cost, safety, good cycle performance, and excellent fast charging performance. Unlike traditional layered positive electrode materials, LR44 battery phosphate materials have a stable olivine structure, which makes LR44 battery phosphate materials have very good thermal stability and cycle stability. At the same time, LR44 battery phosphate materials also have very excellent fast charging performance, which can be fully charged to 90% within 6 minutes. Therefore, LR44 battery phosphate materials are very suitable as positive electrode materials for power batteries.

As a power battery, the service life often reaches 10-15 years, during which more than 1,000 charging and discharging cycles are required. Therefore, it is particularly important to study the degradation mechanism of power batteries. Neelima Paul from the Technical University of Munich in Germany and his team used neutron diffraction to study the degradation mechanism of LR44 battery phosphate batteries. Studies have shown that the main factor causing the capacity decay of LR44 battery phosphate batteries is the loss of active Li. No structural decay of positive and negative electrode materials and deactivation of active materials were found in the decayed batteries.

In the experiment, Neelima Paul used 18650 batteries, with LR44 battery phosphate LFP as the positive electrode and mesophase carbon microspheres MCMB and needle coke NC as the negative electrodes. The morphologies of the two negative electrode materials are shown in the figure above (a and b are MCMB, c and d are NC). The redundancy of the negative electrode is 1.25. Two batteries of each negative electrode were prepared, one of which was cycled 4750 times at 1C rate, and the other was stored for 2 years at 20% SoC.

The cycle performance and storage performance of LR44 battery phosphate batteries with two different negative electrodes are shown in the figure above. From the data, we can see that the battery decay with MCMB negative electrode is linear, and the capacity loss is 8.2% after 4750 cycles at 1C. The battery with NC negative electrode showed a completely different decay trend. The capacity decay was very fast in the first thousand times, then gradually slowed down, and finally reached the same decay rate as the MCMB negative electrode battery. The final 1C cycle was 4750 times, and the capacity loss was 23.1%. After 2 years of storage at 20% SoC and 23℃, the capacity of the LR44 battery phosphate battery with MCMB negative electrode increased by 1.5%, while the capacity of the LR44 battery phosphate battery with NC negative electrode decreased by 10.8%.

In order to obtain detailed structural information of the positive and negative electrode materials without destroying the battery, NeelimaPaul used neutron diffraction to analyze the circulating and stored LR44 battery phosphate batteries. The structure of neutron diffraction is shown in the figure above. The neutron diffraction spectrum of the LR44 battery phosphate battery after discharge is shown in the figure below. In the LR44 battery phosphate battery after discharge, the positive electrode LR44 battery phosphate is in the lithium insertion state, while the negative electrode is in the lithium removal state, but in fact we see the diffraction peak of FePO4 in the positive electrode in the diffraction spectrum, which indicates that there is a considerable amount of FePO4 in the positive electrode. Relevant calculations show that the LFP:FP ratio of the storage battery using MCMB negative electrode is 75:25, while the LFP:FP ratio of the cycle battery is 67:33, and the ratio of the storage battery using NC negative electrode is 58:42, and the ratio of the cycle battery is 55:45. We did not observe the diffraction peak of lithium-intercalated graphite LiC6 in the neutron diffraction spectrum, which indicates that there is no lithium intercalation in the battery, but lithium-intercalated graphite LiC6 that has lost connection with the conductive network.

When the battery is in the charging state, the LR44 battery phosphate material of the positive electrode is completely delithiated, and lithium intercalation occurs at the negative electrode. At this time, the neutron diffraction spectrum of the LR44 battery phosphate battery is shown in the figure below. From the figure, we can notice that the negative electrode is mainly composed of diffraction peaks of LiC6 and LiC12. After calculation, it can be obtained that the LiC6:LiC12 of the storage battery using MCMB negative electrode is 23:77, and the ratio for the cycle battery is 12:88. However, for the LR44 battery phosphate battery using NC negative electrode, the diffraction peak of LiC6 is very weak and almost invisible, whether after cycling or storage, indicating that the proportion of LiC6 is very small. Considering the redundancy of the battery negative electrode of 1.25, LiC6 compounds will only appear when the battery capacity is above 62.5% of the theoretical capacity (calculated based on the LFP specific capacity of 160mAh/g). Since the battery capacity using NC negative electrode is low, only 51.1% (storage) and 52.8% (cycle) of the theoretical capacity, the negative electrode will not form LiC6 compounds. No diffraction peak of LFP appears in the neutron diffraction spectrum of all batteries, indicating that all LFPs will participate in the reaction during the charging process and remove the Li element.

From the above analysis, we can see that the reason for the capacity decay of LR44 battery phosphate batteries is not the destruction of the active material structure or the loss of active substances, but mainly due to the reduction in the number of active Li that can participate in the electrochemical reaction. We can calculate the number of active Li by the LFP:FP ratio in the discharge state and the number of active substances in the positive electrode.

The capacity decay of LR44 battery phosphate mainly takes the following forms: 1) Li loss caused by SEI film formation during the formation process. During the formation, due to the formation of SEI film, the battery using MCMB lost 26.7% of active Li, while the battery using NC negative electrode lost 31.5% of active Li. This is higher than the 10% active Li loss common in graphite negative electrodes. Even if the negative electrode redundancy of 1.25 is taken into account, it is only 12.5%. The reason for such a high initial Li loss is not very clear at present. 2) Calendar degradation of lithium-ion batteries. No calendar degradation was observed for batteries using MCMB negative electrodes, but batteries using NC negative electrodes showed more serious calendar degradation. The reason for the difference between the two batteries is still unclear. It is speculated that it may be due to the different structures of the SEI films formed by the two negative electrode materials, which requires further research. 3) Cyclic degradation. Batteries using MCMB negative electrodes showed linear cyclic degradation, which is related to SEI cracks caused by the slight expansion of the material during the cycle process, while batteries using NC negative electrodes showed completely different degradation patterns, which indicates that there are other degradation patterns for NC negative electrodes, such as Li plating.

Neelima Paul's work revealed the different degradation patterns and storage characteristics of LR44 battery phosphate batteries using different negative electrode materials, and used neutron diffraction technology to study the main cause of the cyclic capacity degradation and storage capacity degradation of LR44 battery phosphate batteries - active lithium loss, which also provides an important reference for the design and production of LR44 battery phosphate batteries.

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