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Lithium iron phosphate material has become an ideal positive electrode material choice for power lithium batteries due to its low cost, good thermal stability, and cycling stability. Especially this year, with the decline of subsidies, the value of lithium iron phosphate has become more prominent.

The residual moisture inside the battery can cause the decomposition of the electrolyte and deteriorate battery performance. Recently, E R. Logan (first author) and J R. Dahn et al. analyzed the effect of residual moisture inside the electrolyte on the electrical performance of LFP/graphite system batteries.

The dissolution of Fe and its deposition on the negative electrode are considered important reasons for the degradation of cycling performance in lithium iron phosphate batteries. It is generally believed that the decomposition of LiPF6 in trace amounts of water, resulting in HF, is a significant cause of Fe dissolution. Electrolyte additives are an important method to reduce Fe leaching, for example, studies have shown that VC additives can improve the capacity retention rate of LFP/MCMB system batteries after high-temperature cycling.

The basic electrolyte used in the experiment was a mixture of EC: DMC=3:7, and important additives including VC, FEC, LiPO2F2 (LFO), and DTD were used. The addition of additives was concentrated in the following ways: 2% VC (2VC), 2% FEC (2FEC), 1% LFO (1LFO), 2% VC+1% DTD (2VC+1DTD), and 2% FEC+1% LFO (2FEC+1LFO).

The battery used in the experiment is a 402035 type battery, with a positive electrode of LFP and a negative electrode of artificial graphite. The following figure shows the moisture content of LFP electrodes after drying at different temperatures. The electrode that has not been dried corresponds to 25 ℃. From the figure, we can notice that the moisture content of the electrode that has not been dried is very high, reaching about 1000ppm. High temperature drying can significantly reduce the moisture content of LFP electrodes. After drying at 100 ℃ for 14 hours, the moisture content of the electrode decreases to 500ppm. Further increasing the drying temperature to 120 ℃ and 140 ℃ can reduce the moisture content inside the LFP electrode to 100ppm. However, 140 ℃ may cause membrane closure. Therefore, the subsequent experimental authors chose 120 ℃ as the drying temperature.

Previous studies have shown that high-temperature drying can cause damage to the binder, leading to a decrease in the mechanical strength of the electrode. Therefore, the author also focuses on the changes in the mechanical strength of the electrode after high-temperature drying. Bending tests show that electrodes dried at 100 ℃ and 120 ℃ did not break or fall off in various radius bending tests, indicating that drying at temperatures below 120 ℃ does not affect the mechanical strength of the electrode. However, the capacity retention rate of electrodes dried at high temperatures in long-term cycling is slightly lower, especially at higher cycling rates, where this phenomenon is evident.

The following figure shows the gas production and electrode interface charge exchange impedance during the formation process of batteries dried at 100 ℃ and 120 ℃, with different electrolyte additives. From the figure below, it can be seen that increasing the drying temperature in the control group electrolyte can reduce the gas production and interface charge exchange impedance of the battery. However, in electrolytes containing various additives, the effect of drying temperature on gas production and charge exchange impedance is relatively small.

The following figure shows the UHPC test results, and Figure a shows the cyclic voltage curves of the battery after drying at 100 ℃ (black) and 120 ℃ (red) using the control group electrolyte. From the graph, it can be seen that there is a significant deviation in the voltage curve during the cycling process of the battery dried at 100 ℃. This is generally caused by the oxidation of the electrolyte at the positive electrode or the dissolution of transition metal elements at the positive electrode. However, LFP materials have low operating voltage, good stability, and will not undergo such serious decomposition phenomena. Therefore, the author believes that this may be due to the reaction of the electrolyte’s decomposition products at the negative electrode migrating to the surface of the positive electrode. When we increase the drying temperature to 120 ℃, most of the moisture in the battery is removed, which can effectively reduce this side reaction and significantly reduce the deviation of the voltage curve.

If we add 2% VC to the electrolyte, the drying temperature of the battery will not have a significant impact on the deviation of the voltage curve, indicating that VC can significantly suppress the occurrence of negative electrode side reactions.

From the above analysis, it can be seen that electrolyte additives can effectively suppress the negative impact of moisture on the performance of LFP batteries. Therefore, the author tested several electrolyte additives applied in NCM battery systems. The following figure shows the Coulombic efficiency of LFP dried at 100 ℃ and 120 ℃ with different electrolyte additives as a function of the number of cycles. The Coulombic efficiency of batteries that can use the control group electrolyte is relatively low, especially for batteries that are dried at 100 ℃. After 5 cycles, the Coulombic efficiency is only 0.95, while batteries dried at 120 ℃ have a significant improvement in Coulombic efficiency, reaching 0.99 or above due to their lower moisture content. However, compared to batteries that use electrolyte additives, the Coulombic efficiency still appears to be lower. After adding various additives to the electrolyte, the effect of drying temperature (moisture content in the electrode) on the Coulombic efficiency of the battery becomes smaller.

The following figure shows the 1C/1C cycle performance curves of batteries with different electrolyte additives at 20 ℃. At the same time, the author tests the battery’s capacities of 0.2C, 2C, and 3C every 100 cycles to analyze the changes in battery rate performance during the cycling process. In Figure i, the author summarizes the capacity degradation of batteries with different electrolyte systems after 1500 cycles. It can be seen that the choice of electrolyte has a significant impact on the cycling performance of the battery. The electrolyte with 2% FEC or 1% LFO added has the best cycling performance, and the capacity retention rate can basically reach 100% or above after 1500 cycles. The drying temperature (electrode moisture content) in the control group electrolyte also has a significant impact on the cyclic degradation of the battery. After 1500 cycles, the capacity loss of the battery after drying at 120 ℃ is about 2%, while the capacity loss rate of the battery after drying at 100 ℃ reaches over 8%. However, in electrolytes containing additives, the impact of different drying temperatures (electrode moisture content) on the cycling performance of the battery is very small. This is important because at lower temperatures, the LFP electrode is more stable and there are very few interface side reactions. Therefore, the influence of moisture content at low temperatures on the cycling performance of LFP batteries is relatively small.

At high temperatures, as the interface side reactions intensify, the moisture content will have a significant impact on the performance of LFP batteries. In the figure below, the author compared the cycling performance of different electrolyte additives at a C/3 ratio of 40 ℃. Similarly, we found that low moisture content (drying at 120 ℃) in batteries using control group electrolytes would result in less capacity loss, while in batteries containing various types of additives, the influence of moisture content on battery performance was relatively small.

The following figure shows the cycling performance of different batteries at a C/3 ratio at 55 ℃. It can be seen that the moisture content has little effect on the cycling performance of the battery at this temperature. This indicates that there is a significant difference in the degradation mode of the battery at 55 ℃ compared to 40 ℃ and 20 ℃. It may be that moisture has a significant impact on the battery performance at high temperatures of 55 ℃. Therefore, although a high drying temperature reduces the moisture content of the electrode, the residual small amount of moisture in the electrode is sufficient to have a significant impact on the LFP battery.

The following figure shows the variation of open circuit voltage during storage of batteries with different electrolyte additives at 60 ℃. It can be seen from the figure that the storage performance of the control group electrolyte is the worst. Batteries with higher moisture content (dried at 100 ℃) have a voltage drop of 2.5V during storage, while batteries with lower moisture content (dried at 120 ℃) have slightly better high-temperature storage performance, but still significantly worse than other groups of electrolytes. During the storage process of batteries containing electrolyte additives, the open circuit voltage of the battery is higher than 3.35V. Under the condition that the electrolyte contains additives, the influence of electrode moisture content on the storage performance of the battery is weak. Only in batteries using 2% VC additives, the capacity loss during storage is more severe in batteries dried at 120 ℃.

In the figure below, the author compared the cycling and storage performance of batteries using CTRL, 2VC, 1LFO, and 2VC+1DTD electrolytes. It can be seen from the figure that the influence of moisture content in the control group electrolyte is the greatest, especially at a lower temperature of 20 ℃, the capacity loss of electrodes with lower moisture content after 1500 cycles of high-temperature drying is only 2%, while the capacity loss of batteries with higher moisture content after drying at 100 ℃ reaches 8%. However, at higher temperatures, such as 55 ℃ and 60 ℃, the influence of moisture content on battery cycling and storage performance is relatively weak. In batteries containing electrolyte additives, the influence of moisture content on battery cycling and storage performance is relatively small.

The most important decay mode of LFP materials is the dissolution of Fe element, which is usually believed to be caused by HF erosion of the positive electrode caused by the decomposition of LiPF6. The author adoptsμ The XRF tool tests the disassembled graphite negative electrode to analyze the content of Fe element. From the figure below, it can be seen that at all temperatures, even at 20 ℃, the dissolution of Fe element in the battery using the control group electrolyte is significantly higher than that of other electrolytes. At the same time, the moisture content also has a significant impact on the dissolution of Fe elements. For example, at 40 ℃, when the moisture content is high (dried at 100 ℃), the Fe element content on the negative electrode surface is 5.5μ G/cm2, and when the moisture content is low (dried at 120 ℃), the Fe element content on the negative electrode surface decreases to 0.2μ G/cm2. However, at 55 ℃, the influence of moisture content is relatively small. This indicates that high moisture content will exacerbate the dissolution of iron elements in the LFP positive electrode, leading to a decline in the cycling performance of the battery. However, for batteries with additives, both the positive and negative electrodes are well passivated, so the influence of moisture content on battery performance is relatively small.

E. R. Logan’s research shows that the drying temperature of LFP electrodes has a significant impact on the moisture content of the electrodes. Drying at 120 ℃ can effectively remove moisture from the electrodes. At the same time, excessive water content in additive free electrolytes can lead to battery performance degradation. This is important because higher water content intensifies the dissolution of Fe elements in the positive electrode. Adding some additives such as VC, FEC, LFO, etc. to the electrolyte can effectively passivate the interface between the positive and negative electrodes, thereby reducing the impact of moisture on LFP battery performance.

This article is an important reference to the following literature. It is only intended for the introduction and commentary of relevant scientific works, as well as classroom teaching and scientific research, and cannot be used for commercial purposes. If you have any copyright issues, please feel free to contact us at any time.

Performance and Degradation of LiFePO4/GraphiteCells: TheImpact of WaterContamination and anEvaluation of CommonElectrolyteAdditivities, Journal of TheElectrochemical Society, 2020167130543, E.R. Logan, Helena Hebecker, A. Eldesoky, AidanLuscombe, MichelB Johnson, 1andJ R. Dahn