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With the continuous development of electronic technology, lithium batteries have gradually entered our daily lives. Whether it is smartphones, tablets, etc., lithium batteries can be seen. Generally speaking, the update and replacement speed of consumer electronic products is very fast. Therefore, the lifespan of lithium batteries is generally designed to be more than 500 times, which basically meets the demand. However, in some fields that require long-term use, such as electric vehicles, the design lifespan is generally about ten years. To meet such a long service life demand, the lifespan of lithium batteries is generally designed to be more than 1000 times, even 3000 times. This requires us to have a deep understanding of the mechanism of lithium battery aging and degradation process.

There are many factors that affect the lifespan of lithium batteries, such as the composition and structure of electrodes, the selection of electrolytes, and the conditions of use.

The electrolyte of lithium batteries generally consists of solvent salts (commonly LiPF6) and linear carbonates such as DMC, EMC, and DEC, as well as cyclic carbonates such as EC and PC. Due to the high electrochemical potential of the lithium battery system, the positive electrode generally exceeds 4V and the negative electrode can reach about 0.1V. Therefore, the electrolyte faces a dual test inside the lithium battery, which cannot be oxidized by the positive electrode or reduced by the negative electrode. In order to improve the electrochemical stability of the electrolyte, some additives such as FEC and VC need to be added. During the initial charging process of the lithium battery, these additives will react with the negative electrode and be reduced, forming a protective layer on the surface of the negative electrode to prevent further reaction between the solvent and the negative electrode. The sentence is:.

However, it is difficult to prevent the decomposition and oxidation of the electrolyte during the cycling process, resulting in the loss of some active Li. In order to study the changes in the electrolyte during battery aging, Xaver Monnighoff et al. from the University of M ü nster in Germany used methods such as supercritical carbon dioxide extraction and gas chromatography to analyze the composition of the electrolyte in aged batteries. They found 17 unstable aging products in the electrolyte, of which 7 had never been reported in previous literature.

In the experiment, Xaver Monnighoff used an 18650 battery structure (NMC532/C) and conducted cyclic tests at 20 ° C and 45 ° C according to a 1C/1C system (2.75V-4.2V). The end-of-life EOL was located at 70% of the initial capacity. The tested battery was disassembled in a glove box, and the extracted cells were extracted using a supercritical carbon dioxide extraction device. The separated electrolyte was then analyzed for composition using a gas chromatograph.

The following figure shows the gas chromatography analysis results of the electrolyte extracted from a brand new battery, from which common solvents and additives in the electrolyte can be seen.

The cycling performance curves of the battery at 20 ℃ and 45 ℃ are shown in the following figure. From the results, it can be seen that temperature has a significant impact on the cycling performance of the battery. The battery cycled at 45 ℃ has better cycling performance, with around 1500 cycles at the end of its life. However, the cycling performance of the battery at 20 ℃ is very poor, with only about 300 cycles reaching the end of its life. Analysis suggests that the main reason for the poor cycling performance of the battery at 20 ℃ is the co embedding of PC solvent and the peeling of graphite layers.

The following figure shows the gas chromatography analysis results of the electrolyte obtained from new batteries, 20 ℃ and 45 ℃ cycling batteries. In order to facilitate analysis, Xaver Monnighoff divided the analysis results into three parts, namely 3-7 minutes, 7-10 minutes and 10-13 minutes. In Region 1, three peaks were detected in the electrolyte of the new battery, corresponding to EMC and monofluorophosphate EMFP (possibly due to VC decomposition during battery formation and SEI film formation), as well as VC. Only EMC and EMFP were found in the electrolyte of the 45 ℃ cycling battery, indicating that VC has been completely consumed during the film formation process. In the battery cycled at 20 ℃, multiple decomposition products were found, including EMC (peak 1), DMFP (peak 2), and EMFP (peak 5), as well as three other products containing propylene chains (peaks 3, 4, and 6), namely methyl isopropyl carbonate (peak 3 MiPrC), methyl propyl carbonate (peak 4 MPrC), and 1,2-diethoxypropane (peak 6). No VC was detected.

The formation mechanism of the products corresponding to peaks 1, 2, and 5 has been reported, while the products corresponding to peaks 3, 4, and 6 have not been reported yet. After analysis, Xaver Monnighoff believes that the mechanism of the appearance of products 3 and 4 is shown in the following formula. The formation mechanism of the product corresponding to peak 6 may be the ring opening reaction of PC solvent,

Within the range of 7-10 minutes, FEC and Diethyl Fluorophosphate DEFP (peak 7) were detected in the electrolyte of the new battery. FEC was detected in the battery cycled at 45 ℃, but not in the battery cycled at 20 ℃, indicating that all FEC had been consumed. Several other decomposition products were also detected, with peak 8 corresponding to 2,2-dimethoxyacetic acid methyl ester, peak 9 to 2-methoxyethyl methyl carbonate, and peak 10 to TMP, which have been reported in previous articles.

Within the range of 10-13 minutes, PS and methoxy EC (peak 11) were detected in the electrolyte extracted from the new battery. Methoxy EC is a product of the reaction between VC and lithium methoxide LiOMe during the formation process. Due to the capture effect of VC on LiOMe, VC can inhibit the formation of alkyl carbon esters during the formation and cycling process (such as DMDOHC corresponding to peak 12 and EMDOHC corresponding to peak 15). The battery cycled at 45 ℃ can detect products corresponding to peaks 11 and 12, as well as a decomposition product whose structure cannot be determined. And for the battery cycled at 20 ℃, in addition to detecting the products corresponding to peaks 11 and 12, six other decomposition products were also detected. The following figure shows the formation mechanism of the decomposition products corresponding to peak 13, and the product corresponding to peak 15 is EMDOHC, which may be the reaction product of EC and LiOMe or EMC and DMC. Detailed analysis of peak 16 revealed the presence of two methoxy side chains in the structure of the decomposition product, but more detailed structural information is currently unavailable. Analysis of peak 17 revealed that the product corresponding to this peak contains side chains of methoxide and propoxide, while the product corresponding to peak 18 contains side chains of two methoxide salts.

From the above analysis results, it can be seen that there are significant differences in the decomposition products of the electrolyte between 20 ℃ and 45 ℃ cycling batteries. At 20 ℃, due to insufficient SEI film protection, many linear and cyclic carbonates in the electrolyte decompose, resulting in a rapid decline in battery performance at 20 ℃.

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