Effect of Cyclic Aging on Lithium and Electrolyte Distribution in CR2016 battery

Continuous interfacial side reactions during the cycling process of CR2016 battery will cause electrolyte consumption and loss of active Li. At the same time, due to the inhomogeneity of temperature, current, pressure and other factors inside the lithium battery, there will also be obvious inhomogeneity in the side reactions inside the lithium battery.

Recently, M.J.Mühlbauer (first author) and A. Senyshyn (corresponding author) of the Technical University of Munich in Germany analyzed the distribution of Li and electrolyte inside the lithium battery at the end of its life through neutron diffraction. The research showed that The distribution of Li and electrolyte in CR2016 battery at the end of their life is significantly uneven.

Using the neutron diffraction method to study electrolytes requires a full understanding of the structure and characteristics of the electrolyte. In fact, EC and DEC have been fully studied in relevant literature, but there is still relatively little research on DMC. The conductivity of the electrolyte will significantly decrease at low temperatures, and may even solidify. Neutron diffraction can also be used to analyze the quantity and spatial distribution of the solidified electrolyte inside the lithium battery.

The research object used by the author in the experiment is a 3.4Ah 18650 battery from Panasonic Corporation of Japan. The positive electrode is NCA and the negative electrode is graphite. Figure a below is the internal structure diagram of the lithium battery obtained by the author through CT. The cycle test in the experiment was conducted at room temperature using Neware's BTS3000 test system. The charging system was CC-CV, the charging current was 1.675A, the discharge system was 6A constant current discharge, and the battery was cycled 60, 120, 210, 400, 600, 800, 1000, 1112 and 1392 times respectively. .

The author uses the change in battery voltage caused by the current change at the beginning and end of discharge to calculate the internal resistance of the battery at 0% SoC and 100% SOC. Figure b below shows the internal resistance of the battery at 0% and 100% SoC. As the cycle number changes curve, we can notice that the internal resistance of the fully charged battery increases rapidly at 810-850 times. This is important because the battery is very polarized at this time, and the battery voltage changes from 4.2 in the early stages of discharge. V dropped to 2.5V.

The author used pulse charging and discharging (400mA) to measure the internal resistance information of the battery in different SoC states. The figure d below shows the internal resistance during the charging process, and the figure e below shows the internal resistance during the discharging process.

Figure a below shows the distribution of Li concentration in the negative electrode of a new battery and a cycled 18650 battery. It can be seen that the Li concentration of the cycled battery is significantly lower than that of a fresh battery. This is mainly due to the consumption of Li by side reactions at the interface during the cycle. More active Li. At the same time, there is also a significant uneven Li concentration in the height direction of the battery. The Li concentration in the upper and lower parts of the battery is lower. This gap may be due to uneven pressure and temperature during the operation of the battery, as well as the electrolyte. Caused by factors such as uneven infiltration and gas accumulation.

Among these factors, the electrolyte factor is relatively difficult to study. Here, the author used neutron diffraction tools to study the distribution of low-temperature solidified electrolyte inside the lithium battery (as shown in Figure b below). From the figure, it can be seen that in the fresh There is significant inhomogeneity in the electrolyte in the diameter direction of the battery. There is significantly more electrolyte outside the cell than inside the battery, but the electrolyte distribution in the height direction of the battery is almost the same. However, after the cycle, first of all, the overall concentration of the electrolyte dropped significantly, and the distribution of the electrolyte still had a significant phenomenon of more outside and less inside. At the same time, aging also led to significant differences in the electrolyte in the height direction of the battery. The electrolyte at the bottom of the battery is significantly higher than at the top of the battery.

This non-uniform distribution of Li and electrolyte inside the cycled lithium battery indicates that there is a relatively complex mechanism for the degradation of CR2016 battery during cycling. In order to analyze the relationship between electrolyte and lithium loss, the author analyzed batteries with different cycle times. From the insert in Figure a below, we can see that the ratio of (001)/(002) LiC6/LiC12 changes with the number of cycles. The lithium content in the graphite anode continues to decrease, and the intensity of the (002) peak of the reaction electrolyte also decreases. Based on the above diffraction data, the author calculated the electrolyte and Li content diagram in the battery (as shown in Figure b below). From the figure, it can be seen that as the number of cycles increases, the contents of the electrolyte and active Li also appear. There is an obvious decrease, but the decline process is obviously divided into two processes. In Figure c below, the author created a relationship diagram between the relative values of active Li and electrolyte. From the figure, we can see two areas more clearly. In the 1-2 area, the loss of Li and the loss of electrolyte are linear. But after 600 cycles, the loss of active Li in the negative electrode basically stopped, but the electrolyte began to decompose massively.

Inhomogeneous distribution of lithium and electrolyteinaged Li-ion cylindrical cells, Journal of Power Sources 475 (2020) 228690, M.J.Mühlbauer, D.Petz, V.Baran, O.Dolotko, M.Hofmann, R.Kostecki, A.Senyshyn

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