Research on Abnormal Expansion and Improvement of Soft Pack Battery Cycle

Cycle testing is a typical method for evaluating soft pack polymer batteries. This article analyzes a cyclic failure model of commercial polymer lithium-ion batteries and identifies the main factors affecting battery cyclic performance, especially cyclic expansion; By conducting case studies under different conditions, analyzing failure models, and proposing suggestions for improving the cycling of flexible packaging lithium batteries.
1 Experimental and testing methods
To study the cyclic expansion of lithium-ion batteries, a commercial soft pack polymer battery with a nominal thickness of 5.0mm manufactured using winding technology was tested. The positive electrode active material of the battery is lithium cobalt oxide, and the negative electrode active material is artificial graphite. The reference electrolyte composition is 1 mol/L LiPF6 (EC+PC+DEC+EMC) (volume ratio of 40:30:20:10), 4% FEC/3% PS/3% ADN, and the separator is made of 7um ceramic coated separator.
The cyclic testing method involves using two different charging processes in an environment of 25 ℃, with the same discharge current, to cycle the A group (two cells) and B group (two cells) batteries: Group A uses the 1.0C stepwise charging defined in the specification book, with the specific charging process being 1.0C CC to 4.2V, 4.2V CV to 0.6C, 0.6C CC to 4.4V, 4.4V CV to 0.05C; Group B adopts 1.2C stepwise charging, with the specific charging process being 1.2C CC to 4.2V, 4.2V CV to 0.6C, 0.6C CC to 4.4V, 4.4V CV to 0.05C. After two groups are fully charged, let them stand for 10 minutes, then discharge at 0.5C and continue to stand for another 10 minutes, repeating the full charge current cycle to 800 cycles.
An analysis was conducted on the causes of the failure of the battery cells with abnormal cycles mentioned above. Firstly, the tested cells were subjected to EIS testing, followed by non-destructive testing characterization such as laser thickness scanning and CT testing to confirm the type of failure and locate the thickness failure area; Secondly, fully charge the abnormally swollen battery, dissect it in a glove box, observe the appearance of the electrode pieces, test SEM, observe the surface morphology of the particles, and test the metal dissolution by EDS; Next, based on the characterization and analysis data, establish a failure model; Finally, based on this failure model, experimental design and verification are carried out to confirm the matching degree between the verification results and the failure model, and to complete the analysis and verification.

Analysis of 2 failed batteries and improvement measures for cycling
2.1 Analysis of failed batteries
2.1.1 Loop Data Analysis
Figure 1 shows the capacity decay curve of the battery cell after 800 cycles under different charging processes, and Figure 2 shows the corresponding thickness variation curve. As shown in Figure 1, the cycling trend of the two cells charged at 1.2C is poor. After 400 cycles, the capacity of the two cells bifurcates. One of the cells just reached 70% after 800 cycles.

Upon further observation of the thickness variation curve of the battery cell, it can be seen that the 1.2C charged battery cell showed a significant increase of 3.5% in thickness after 200 cycles compared to 1.0C cycles, with a difference of about 5.0% after 800 cycles, further increasing by 1.5%. Therefore, it is crucial to study the significant increase in thickness after 200 cycles.

In order to investigate the failure modes of battery cells, the EIS of the battery cells was first analyzed. The Nyquist plot and equivalent circuit are shown in Figure 3, and the fitting results of relevant parameters are shown in Table 1. Due to the limited number of points taken in the low-frequency region, Weber impedance was not fitted here. Csei is the SEI film capacitor, and Cdl is the double-layer capacitor.

                           
From Table 1, it can be seen that the 1.0C cycle cells Rs, Rsei, and Rct are all the smallest; The 1.2C cycle group is exactly the opposite, with Rs, Rsei, and Rct being the highest. However, the trend of Rsei and Csei changes is different between the 1.2C cycle and the 1.0C cycle, with the former showing a larger Rsei but a smaller Csei. According to literature, if Rsei is large and Csei is small, it indicates an increase in the thickness of the SEI film of the battery cell, that is, 1.2C high rate charging is more likely to cause the SEI film of the battery cell to become thicker.

Randomly select one failed cell and one cycled cell for laser thickness scanning in the length direction, as shown in Figure 4. It can be seen that the 1.0C cycle of the battery cell has the highest thickness near the head in the length direction, which is the ear position of the battery cell, consistent with the battery cell design.

For a 1.2C rechargeable battery cell, the areas with the highest thickness are seen at the head and tail in the length direction. From the perspective of laser scanning, the thickness of the head and tail of the battery cell is relatively the largest compared to the normal area, and the thickness of the tail of the battery cell is the largest. The difference between the thickest area and the ear position area of the battery cell reaches 0.22mm, a difference of 4.4%; For a battery cell with a 13% expansion, if the abnormal areas at the head and tail are removed, the thickness expansion is 8.6%. For the 1.0C cycle group, the battery cell expansion is 8.3%, which is within the range of 1%.
In order to further characterize and analyze the battery cell, combined with the change trend of the battery cell at 200 cycles mentioned above, CT scans were performed on the two groups of battery cells after 200 cycles, as shown in Figure 5. From the figure, it can be clearly seen that the outer packaging aluminum foil of the battery cell is tightly adhered to the main body area of the battery cell without obvious gaps. It can be ruled out that the lifting of the head and tail of the battery cell is caused by gas production, indicating that the thickness increase of the battery cell is due to physical structure. Further enlarging the head and tail regions of the battery cell, it was found that the two regions with the highest thickness of the 1.2C cycle battery cell had a certain gap in the innermost winding structure, and the thickness of the battery cell in the region with the gap had the maximum value. However, for cells with a 1.0C cycle, CT did not observe any significant internal gaps.

2.1.4 Disassembly and analysis of battery cells
Disassemble the fully charged battery cells after 200 cycles of CT completion, as shown in Figure 6. From the figure, it can be seen that there is a significant color difference between the edge area of the 1.2C cycle cell anode plate in the width direction, namely the eaves area, and the main area. The main area is golden yellow, and the eaves area of the electrode plate is dark gray

To analyze the region, the anode eaves area and main electrode area of the sample were disassembled after a 1.2C cycle and full charge, and SEM and EDS were performed, as shown in Figure 7. From Figure 7, it can be seen that there are many amorphous gray substances on the anode surface of the eaves area; There are no obvious other products visible in the anode particles in the main area.

From Figure 8 and Table 2, it can be seen that in the anode eaves area, oxygen and fluorine elements are significantly higher, while carbon elements are significantly lower, and sulfur and phosphorus elements are also higher.

Shengshui Zhang believes that the formation of SEI film in the solvent system composed of ethylene carbonate (EC) mainly involves two steps: one is the generation of gas-phase ethylene and unstable soluble lithium carbonate salt; The second is that EC reacts with ethylene to generate a stable inert organic salt C4H4O6Li2 containing six oxygen elements, which is manifested as an increase in oxygen elements and a decrease in carbon elements in the anode electrode. At the same time, due to the consumption of electrolyte, the ratio of fluorine and phosphorus elements in the anode electrode will also increase. Combined with EIS, the larger Rsei and smaller Csei further confirmed that the main product in the anode edge region is the grown SEI film.

Qianqian Liu et al. believe that an increase in SEI film can lead to an increase in cell impedance, which in turn increases anode polarization, resulting in a lower anode potential and easier lithium deposition during the charging process. Once lithium metal deposits on the anode, it will spontaneously react with the electrolyte to form insoluble salts that make up the SEI film, further increasing the thickness of the SEI film. Therefore, Qianqian Liu et al. believe that lithium evolution reaction is a self accelerating reaction.
In order to further investigate whether the anode eaves area or the edge area of the anode electrode plate contains lithium metal deposits, another 200 times of discharge were taken from the bulging head and tail of the battery cells, and discharged to 0% SOC and 60% SOC. The cells were disassembled in a glove box, as shown in Figure 9. From Figure 9, the anode film area can be observed, and it can be seen that the anode main area of the 0% SOC cell is gray, the edge area is brown, and there is a white line in the middle transition area, located at the junction of the positive and negative electrodes, showing a trend of growing towards the interior of the cell; The anode area of the 60% SOC cell is brown, the edge area is golden yellow, and there is also a white line in the middle transition area.

By comparing the colors of the anode membrane, it can be seen that the lithium insertion state in the edge region is nearly 0% to 60% higher than that in the main region, indicating that the anode edge region is more prone to lithium metal precipitation. This white line may be a mixture of lithium metal deposition and SEI film. Through the above analysis, it can be concluded that the reason for the abnormal lifting of the head and tail of the battery is the occurrence of lithium deposition in the edge area of the battery cell tail, which leads to the thickening of the SEI film. As lithium deposition is a self accelerating reaction, the thickness of the SEI film and lithium deposition in this tail edge area are increasing, resulting in capacity decay and abnormal thickness expansion of the battery cell.
3 Failure mechanisms
3.1 Failure Process
Through the above analysis, it is known that lithium deposition and an increase in SEI film thickness are the main reasons for the curling of the head and tail of the battery cell. But why is the anode edge area of the battery cell higher and more prone to lithium deposition? The specific process and failure mechanism still need further in-depth analysis and explanation.
Lithium ion batteries require a larger anode capacity than cathode capacity based on safety design requirements. From the perspective of battery cell capacity, N/P>1; Based on size considerations, the anode area needs to completely cover the cathode in both the length direction (electrode width direction) and width direction (electrode length direction) of the battery cell. Analyzing the length direction of a single-layer battery cell, the anode region of the cell is longer than the cathode region, similar to the protruding eaves outside the main area of a house. The specific process is described in the following text:
(1) Uneven interface and special lithium ion movement. The existence of the eaves area leads to local unevenness that is different from the main area. Specifically, the main area of the battery cell is subjected to high pressure, with good adhesion between the positive and negative electrodes and small gaps between the electrode plates; The stress distribution between the eaves area and the transition area of the balcony is uneven, and the anode plates are relatively loose, with weak adhesion between the plates. During the heating and pressurization process, the force is relatively small here, and there are differences between the SEI film formed by the anode and the main area. Moreover, due to the absence of a corresponding cathode region in this eaves area, lithium ions can only migrate from the adjacent anode region to the eaves area during the charging and discharging process. During the discharging process, lithium ions can only diffuse from the eaves area to the adjacent anode region, with solid-phase concentration diffusion playing a dominant role. Myounggu Park et al. believe that the concentration diffusion coefficient of lithium ions in the solid phase is strongly correlated with temperature. As the temperature increases, Ds increases, and as the temperature decreases, Ds decreases significantly;
(2) Acceleration factor. (a) If the charging current of the battery cell is high, the anode expansion of the battery cell will be relatively larger, and the gap will easily become larger at this time; Moreover, if the current increases, the diffusion rate of lithium ions from the anode region to the adjacent region cannot catch up with the rate of lithium ion deintercalation from the anode and cathode, resulting in the accumulation of lithium ions in the eaves region and an excessively high concentration of lithium ions; (b) If the temperature is low, the diffusion of lithium ions in the electrode will slow down, and the relative speed of lithium ion deintercalation will be slower, which will also be manifested as the accumulation of lithium ions in the eaves area. Overall, it will be manifested as a higher concentration of lithium ions in the eaves area;

(3) Qualitative change, self acceleration, and increased thickness of the head and tail. When the gap at the edge of the battery cell increases, the impedance of the battery cell will also increase. During the charging process, in the adjacent edge region, lithium ions are more difficult to embed into the anode region after being released from the cathode, making it easier for them to accumulate at the anode interface, resulting in lithium deposition in this anode region. When lithium deposition occurs, the anode membrane in this area becomes thicker, which increases the edge gap and leads to a self accelerating lithium deposition reaction. And when the edge gap is not considered, there is an independent high eave area in the anode edge region. At the end of charging, when the main area reaches full charging voltage, this area has already been overcharged, which is also prone to lithium deposition. When the two effects are combined, more and more lithium metal precipitates, which will react negatively with the electrolyte, consume the electrolyte, and cause the SEI film to continue to grow, manifested as a significant increase in the thickness of the head and tail regions of the battery cell.
3.2 Design Improvement
Based on the above cyclic thickness failure mechanism, the cyclic performance of soft pack lithium batteries can be improved in the following aspects: improving the interface of the battery cell, reducing the charging rate of the battery cell, and increasing the diffusion coefficient of the battery cell. To improve the battery cell interface, the battery cell can be placed in a fixed fixture to fix the interface of the battery cell, avoiding gap growth and causing the head and tail of the battery cell to rise. In this validation experiment, two groups were considered: the first group, fresh battery cells were placed in two glass plates under a pressure of 0.1 MPa and subjected to the same charging cycle at 25 ℃ and 1.2 ℃; In the second group, place the battery cells that have bulged after 200 cycles in the 0.1MPa flat fixture of the first group and continue to cycle in the same process.
From Figure 10, it can be seen that both groups of cells showed no significant improvement compared to the cells before improvement, and both experienced rapid capacity decay within 800 cycles. At the same time, it can be seen that for the cells on the upper flat fixture after 200 cycles of cyclic bulging, a plateau area with a 50 cycle curve appears. Within the plateau area, the capacity change of the cells is relatively small; Subsequently, the battery cell experienced a rapid decrease in cycling capacity. After the fresh battery cells were mounted on the flat fixture, a similar phenomenon also occurred. The occurrence of this phenomenon may be related to the shaping of the flat fixture.

When the swollen cell is shaped by a flat plate, the interface between the head and tail electrodes significantly improves, and the impedance of the cell decreases, resulting in a decrease in cell attenuation and even a certain increase. However, due to the self accelerating nature of lithium evolution, as the side reactions of lithium evolution continue to increase and the electrode interface is restricted, lithium dendrites can puncture the separator, leading to a deterioration in cycling performance. For the flat fixture on fresh battery cells, only improving the gap between the cells cannot eliminate the phenomenon of overcharging and lithium deposition in the anode eaves area of the cells, which can also lead to the same plateau area and rapid decay area in the later stage of cycling. Therefore, the method of improving the battery cell interface has little effect.

Reducing the charging rate of the battery cell can effectively avoid the phenomenon of high voltage in the eaves area of the battery cell, prevent local overcharging of the anode, and prevent the gap between the electrode pieces in the eaves area from becoming larger due to current impact. As shown in Figures 11 (a) and (b), after a 0.5C charging cycle, the cell capacity retention rate reached 90%, and the thickness expansion rate was less than 5%.
Improving the lithium ion diffusion coefficient of the anode can also effectively avoid the phenomenon of high diffusion in the eaves area of the battery cell. The main method is to coat the surface of the anode graphite particles with amorphous carbon, optimizing the diffusion path of lithium ions from the graphite layer to be able to move from multiple paths. As shown in Figures 11 (a) and (b), the carbon coated graphite increases the capacity retention rate of the battery cell to 85%, and the thickness expansion rate is less than 8%.
4 Conclusion
This study used high-voltage lithium cobalt oxide as the positive electrode material and artificial graphite as the negative electrode material to fabricate a 4.4V system soft pack lithium-ion battery. It was found that when the charging rate of the battery cell is increased, there will be abnormal expansion caused by the lifting of the head and tail of the battery cell. A thorough study was conducted on the abnormal expansion, and it was found that the curling of the head and tail of the battery cell was caused by the special weak interface and lithium ion transfer path in the eaves area inside the battery cell, which led to the failure mode of lithium deposition and increased SEI film.
Research has shown that selecting anode materials with good dynamic performance and reducing the charging rate of the battery can suppress abnormal expansion of the battery, improve cycling performance and thickness expansion. At the same time, research has also shown that applying pressure to the battery cell can improve the interface contact of the battery cell, but it cannot change the transfer of lithium ions in the eaves area. It can still lead to high SOC in the eaves area, resulting in overcharging and internal lithium deposition. Based on the current safety design requirements for lithium-ion batteries, it is difficult to avoid the eaves area. Further optimization design is needed to fundamentally reduce or alleviate abnormal expansion.