What are the differences in the expansion behavior of the three common cathode systems?
Author：INITIAL ENERGY SCIENCE&TECHNOLOGY Co.,Ltd（IEST）
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Time：2022-09-15 15:02:08
As one of the most common energy carrier, lithium battery has been widely used in all aspects of life. When the battery is in use, the internal chemical and electrochemical reactions occur all the time, resulting in the shape of the lithium battery will also change to a certain extent with these reactions. The expansion of batteries is usually divided into two kinds: one is the hard expansion caused by the change of the embedded lithium embedded structure of the positive and negative electrode material, and the other is the soft expansion caused by the gas production reaction inside the lithium battery. Hard expansion has been accompanied by the cycle charge and discharge process of lithium battery, and with the increase of the cycle number, there will also be some irreversible expansion. Current lithium battery system, the negative most commonly used for graphite material, and the most common positive lithium cobalt oxide LCO, ternary NCM and lithium iron phosphate LFP, different positive and negative material in charge and discharge structure is different, so will show different expansion behavior in the cell end, understand the expansion of these different systems, can help the researchers according to the actual application requirements to choose the appropriate material system design. In this paper, the in-situ expansion test system is used to monitor the expansion force and expansion thickness curve in the charging and discharge process of the three systems, and analyze the expansion difference in combination with the electric performance.
Figure 1. Comparison of the three cathode structures1¹
1.Test equipment: in-situ expansion test system SWE2110, the appearance is shown in the figure below.
Figure 2. SWE2110 Equipment appearance diagram
2.Test parameter:：2.1 The cell information is shown in Table.
Table 1. Cell information
2.2 Test process: Put the cell in the test cavity of in situ expansion analyzer (SWE2110), and the constant pressure test mode and constant gap test mode are set respectively, adjust the initial pressure is 0.2MP a, the charge and discharge is opened, and the expansion curve of each cell system in the charge and discharge process is monitored in situ.
In the process of charge and discharge of lithium battery, lithium ions will shuttle back and forth between the negative electrode, electrolyte, positive electrode, etc., so that the battery has a certain degree of expansion deformation, which is mainly the result of the unembedded lithium phase change of the positive and negative electrode materials. Usually, the cell volume change of the cathode material is smaller than that of the cathode material, such as during charging2Crystal cell volume shrinkage (delithium removal 0) is about 1.9%, Li (Ni, Co, Mn) O.52Crystal cell volume changes by about 2%, with the olivine structure of Li FePO₄The volume of crystal cell changes by about 7%, while the volume expansion of graphite after lithium embedding is about 10%, so the expansion trend in the process of charging and discharge is basically based on cathode materials. Therefore, whether it is LCO / graphite system, NCM / graphite system, or LFP / graphite system cell, the expansion thickness change curve and expansion force change curve we measured all show the phenomenon of charging expansion and discharge contraction (as shown in Figure 3 (a)). From the actual cell expansion thickness percentage, the actual cell expansion thickness and theoretical cell expansion difference, this is mainly because in the cell expansion percentage calculation, is using the total expansion thickness divided by the total thickness, the denominator part is the sum of the positive and negative pole sheet, and the molecular part is mainly negative expansion, thus will lead to the calculated percentage is less than the proportion of graphite cell expansion alone.
Figure 3. In-situ test of expansion thickness and expansion force curve and differential capacity change curve of the cell
(Green constant gap mode, red constant pressure mode)
Although the expansion curve trend of the conventional system cell is consistent with the charge and discharge process of the cathode material, the expansion behavior of the cathode material will also affect the trend change of the whole process. For example, the expansion curve of the LFP system cell in Figure 3 will have the phenomenon of "hump", which may be due to LiFePO₄ For the olivine structure, the LiFePO during the charging process4The delithium degradation product is iron phosphate (FePO)₄), The charge and discharge process is in the FePO₄/LiFePO₄ The state of two-phase coexistence, FePO₄ take part in LiFePO₄ The structure of is similar, in volume, FePO₄ The volume is 6.81%² smaller And the battery is LiFePO in the early stage of the charging period4The contraction is not obvious, the graphite expands, so the pressure rises, charging to the middle FePO₄ Shtraction, which will offset part of graphite expansion, expansion force drops, later LFP no longer contraction, but graphite continues to expand, so the expansion force increases, and the discharge process is the opposite3. NCM / graphite system in figure 3, in the charging constant pressure stage, we can find that the expansion thickness and expansion force are slightly reduced trend, this is mainly due to the cathode material transition metal in a high valence state, Li content is low, Li layer in unstable expansion state, when the material further Li, material structure collapse, c axis reduction, and the cell expansion decreases.
Figure 4. Expansion amount of lithium-embedded structure of graphite crystal³
Further differential capacity analysis of the three systems cells. As shown in Figure 3 (b), 2~3 obvious off-embedded lithium peaks appeared in the charge and discharge process, which may mainly represent the lithium-embedded Li Cx phase transition of graphite, and the peak position and peak strength have a certain correlation with the polarization and reaction capacity of the cell. From the comparison of the two different expansion measurement modes, the two different boundary conditions, constant pressure or constant gap, have little impact on the electrical performance in the short-term cycle, but if we consider the effect on the long-term cycle in detail.
By using in situ expansion analysis system (SWE2110) characterize the three different system cell expansion force and expansion thickness curve, and combined with the positive and negative electrode material embedded lithium phase change analysis the difference of the expansion curve, the LFP system cell in the process of charge and discharge "hump" phenomenon, and LCO and NCM system is no phenomenon, and NCM cells in the charging constant pressure stage will appear slightly reduced expansion trend. The difference comparison of these expansion phenomena, on the one hand, can provide lithium battery researchers with a way to describe the cell expansion performance in situ, and on the other hand, it can also provide a data mechanism reference for studying the expansion performance of specific system cells.
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