In order to improve battery capacity and performance, domestic and foreign scholars have fully studied the mechanism of lithium battery capacity loss. At present, it is known that the main factors that cause the capacity decay of lithium-ion batteries include the formation of CEI/SEI passivation films on the surfaces of positive and negative electrodes, the deposition of metallic lithium, the dissolution of electrode active materials, the occurrence of cathodic and anodic redox reactions or side reactions, structural changes and phase changes. Change and so on^1~3. At present, the capacity fading changes of lithium-ion batteries and their causes are still in the process of continuous research. In this paper, by studying the stress change and electrochemical behavior of NCM/graphite cells during cycling, the reasons for the cell cycle capacity decay were analyzed.

1. Test equipment: In-situ expansion analyzer, model SWE2110 (IEST Yuanneng Technology), the applied pressure range is 50~10000N.

Figure 3. Schematic diagram of the in situ expansion analyzer

As the number of cycles increases, the capacity of the cell decreases and the thickness of the cell increases. As shown in Figure 3, the initial charge thickness expands by 3.4%, the discharge volume shrinks by 3.1%, and the irreversible expansion is about 0.3%. When the capacity retention rate remains at 80%, the maximum expansion thickness of the cell reaches about 20%, and the expansion curve also increases sharply with the sharp attenuation of the capacity.

Figure 3. a) Charge-discharge voltage and expansion thickness curve

b) Charge capacity and charge maximum thickness curve

For NCM/graphite cells, lithium ions intercalate into graphite to gradually form Li-C compounds including LiC72, LiC36, LiC24, LiC12, and LiC6, resulting in graphite lattice expansion, and the microscopic stress generated by lattice expansion is the main driving force for electrode expansion. The electrodes are composed of active particles, binders, conductive additives, and pores formed between them. The lattice expansion caused by lithium intercalation is accompanied by the structural evolution of the binder and the changes in the porous structure in the electrodes. The evolution of the pore structure can be changed. Lithium ion transport and diffusion processes and associated stresses in electrode films during charging and discharging. The thickness evolution of the cell can be divided into electrochemical expansion caused by the process of lithium extraction and intercalation; physical expansion caused by the volume evolution of polymers (such as binders and dispersants) and the mechanical and structural changes of electrodes. From the lattice expansion curve, the lithiation and delithiation processes are reversible. However, the continuous accumulation of stress due to the microscopic lattice expansion of graphite may cause defects such as material structural damage and electrode mechanical cracks, while the mechanical or structural changes related to the electrode film are irreversible. Therefore, there may be several reasons for the increasing irreversible thickness of the cell: structural damage of electrodes and materials, side reactions, lithium precipitation, etc.

In order to further analyze the reasons for the expansion, we select the charge thickness variation curves corresponding to different cycle numbers, and analyze the differences after merging. 4 shown. As the number of cycles increases, it can be seen that the charging capacity of the battery cell decreases continuously, and after 110 cycles, the thickness expansion curve is significantly different from the previous expansion curve. Especially in the later stage of charging, the slope of the expansion curve increases significantly. Refer to Before this official account, there is research on lithium precipitation (Lithium-ion soft-pack battery non-destruction lithium analysis - temperature window, Li-ion soft-pack battery non-destruction lithium analysis - rate window), it can be speculated that the battery will continue to accumulate stress in the charge-discharge cycle after 110 cycles , irreversible mechanical damage, lithium precipitation and other side reactions have occurred, so the expansion rate of the cell will be larger than the initial expansion rate.

Figure 4. The change curve of the expansion force of the battery in each cycle of charging

In addition, the differential capacity curves corresponding to different cycle numbers are analyzed, as shown in Figure 5. During the charging process, there are three characteristic peaks of charging phase transition, and as the cycle increases, the voltage corresponding to each peak (as shown in Table 2) first decreases and then increases, that is, the cell polarization first decreases and then increases. It shows that applying a certain external pressure to the cell can reduce the polarization of the cell during the charging and discharging process at the beginning of the cycle, but with the accumulation of subsequent side reactions and lithium precipitation, it will increase the polarization of the cell. 

Figure 5. Differential voltage curve of cell charging expansion force

The intensity changes of each peak are shown in Table 2 and Figure 6. The ratios of the intensity changes of the three characteristic peaks are inconsistent, indicating that the reason for the cycle decay of this cell is not due to the structural damage of the active material, but mainly due to the mechanical damage of the electrode, lithium precipitation and other side effects caused by 3.

Table 2. Differential voltages for cell charging 

Figure 6. Variation trend of the peak intensity of each phase transition

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