Li concentration distribution in the process of lithium insertion in the negative electrode of power battery

by:CTECHi     2021-08-14

Lithium-ion batteries are mainly composed of positive electrode, negative electrode and electrolyte. During charging, Li+ is released from the positive electrode, diffuses through the electrolyte, migrates to the surface of the negative electrode, and is embedded in graphite. As an intercalated negative electrode material, graphite mainly produces two products in the process of lithium insertion: LiC12 and LiC6. LiC12 is an intermediate product. In the end, the graphite material completely intercalated with lithium will generate LiC6 product, but in the actual process Since the diffusion coefficient of Li+ in graphite is small, the final product LiC6 will be generated locally first, while the other parts are still in a lithium-poor state, resulting in a concentration gradient inside the graphite anode. Many models have predicted this Li concentration The existence of gradient, but accurate measurement of this phenomenon is still a big challenge. We have previously reported that KoffiP.C.Yao of the Argonne National Laboratory in the United States used in-situ energy dispersive X-ray diffraction (EDXRD) technology to study the lithium concentration gradient phenomenon of graphite anodes. Recently, WeiYang of Tianjin University (first Author), WeiQiu (corresponding author), QianZhang (corresponding author) and others have realized in-situ analysis of the distribution of Li concentration in the process of lithium insertion in graphite negative electrodes and the uneven stress distribution caused by the optical imaging system. In the experiment, WeiYang mixed artificial graphite (93%), conductive agent SP (3%) and binder PVDF (4%) with NMP, then coated on the surface of copper foil, dried and rolled, and cut into discs . In order to be able to observe the changes in the process of lithium insertion in the negative electrode, the author designed the battery structure shown in the figure below, in which an observation window is set on the upper cover of the battery to realize real-time observation. The disc-shaped graphite electrode is placed in the center, and the metal lithium negative electrode is made into a ring and placed around the graphite negative electrode. Therefore, during the charging and discharging process, Li is embedded from the edge of the graphite electrode sheet and gradually diffuses to the central position, so that we can observe The window directly observes the concentration distribution in the graphite electrode sheet.

In the process of lithium insertion, graphite will produce different products with the change of Li concentration, and different products have different absorption frequencies of visible light, so different lithium insertion The state will cause graphite to show different colors (as shown in the table below). As the concentration of lithium intercalation goes from low to high, the color of graphite will also change from gray to blue, then to red, and finally to gold, so We can use this characteristic of graphite to judge the lithium intercalation concentration of graphite.

The following figure a shows the change of graphite potential during the lithium insertion process. From the figure, it can be seen that there is an obvious voltage platform at 0.2V during the graphite lithium insertion process, the following figure b It shows the color change of the graphite electrode sheet after lithium insertion for different time. From the figure, it can be noticed that the graphite electrode appears gray when the lithium insertion starts. As the amount of lithium insertion increases, the graphite electrode starts at 4.2h Turns to blue. At this time, the distribution of Li in the graphite anode is relatively uniform. After that, as the degree of lithium intercalation further increases, the color change of graphite also begins to show a certain degree of unevenness, from the edge of the graphite sheet It starts to gradually change from blue to red from the outside to the inside, and finally to gold, indicating that Li concentration gradient in the graphite electrode begins during this process. The Li concentration is higher near the edge of the metal Li negative electrode, and the Li concentration is relatively higher in the middle position. It is lower, but the whole graphite electrode turns to gold in the final stage of lithium insertion, indicating that the final Li concentration of the graphite negative electrode becomes uniform again.

Because there is a close relationship between the color of graphite and the amount of lithium intercalation, we can analyze the concentration distribution of Li in the graphite electrode through the color change of the graphite electrode. The picture shows the change of the Li concentration in the graphite electrode in the diameter direction at different times according to the color change of the graphite electrode. It can be noticed from the picture that since Li diffuses from the edge to the center of the graphite electrode sheet, Li is in the graphite at any time The concentration of is not uniform, the concentration gradient is small at the beginning, the concentration gradient rises rapidly in the middle and late period, and the concentration gradient decreases again at the end of lithium insertion.

As the degree of lithium intercalation increases, the volume of graphite will also change accordingly, so a certain stress will be generated inside the graphite. The picture below shows the use of grayscale imaging. The obtained graphs of the stress distribution of graphite at different times of lithium insertion, it can be seen from the figure that as the degree of lithium insertion increases, the strain of graphite is gradually increasing, and the stress distribution also presents an approximately circular distribution around the center, close to The edge position of the Li negative electrode has a higher Li concentration, so the strain is also greater, and the center position has a lower Li concentration, so the strain is relatively small.

It is a well-known phenomenon that the color of graphite intercalation varies with the degree of lithium intercalation, but few people try to infer the degree of lithium intercalation of graphite anode through the color change of graphite anode. Wei Yang et al. used this phenomenon and combined with a CMOS sensor to achieve rapid in-situ detection of the Li concentration gradient of the graphite negative electrode through a refined analysis of the color change of the graphite negative electrode, which is of groundbreaking significance.

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