The impact of electrolyte dosage on battery performance!

The amount of electrolyte injected into lithium-ion batteries directly affects the performance of the battery. When the amount of electrolyte injected into the battery is too high, it not only increases the cost of battery preparation, but also decomposes the excess electrolyte during charging and discharging, generating gas, resulting in poor contact between the positive and negative electrodes of the battery, deterioration of cycling performance, and a series of safety issues; When the electrolyte injection amount is too low, the conduction of lithium ions between the positive and negative electrodes is limited, which can cause an increase in internal resistance and a decrease in cycling stability of the battery during long-term cycling.
This article investigates the relationship between electrolyte injection amount and electrolyte retention amount in flexible packaging lithium-ion batteries under different formation pressures, investigates the relationship between electrolyte retention amount and battery cycling performance, and studies how to further reduce battery manufacturing costs by controlling formation pressure and reducing electrolyte injection amount while ensuring battery performance in the preparation process of digital lithium-ion batteries.
1 Experiment
Two batteries with high volumetric energy density were selected for this experiment, namely 406072-2920mAh (650Wh/L) and 426168-3020mAh (667Wh/L) batteries.
1.1 Preparation of Lithium ion Batteries
Dissolve the binder polyvinylidene fluoride (PVDF) in N-methylpyrrolidone (NMP) to make a gel solution, then mix it thoroughly with lithium cobalt oxide sample (LiCoO2), conductive carbon black (SP), and carbon nanotubes (CNT). Use NMP to adjust the slurry to make a slurry with appropriate solid content and viscosity. Uniformly coat the slurry on a 12 μ m thick aluminum foil, vacuum dry it at 110 ℃ for 6 hours, and then roll it with a pressure of 200MPa to a thickness of 98 μ m (compaction density of 3.95g/cm3) to make a positive electrode sheet with an active material content of 98.0%.
Dissolve the thickener CMC (battery grade) in deionized water (homemade) to make a gel solution, then mix it with artificial graphite, conductive carbon black (SP), and binder SBR. Use deionized water to adjust the slurry to make a slurry with appropriate solid content and viscosity. Coat it on an 8 μ m thick copper foil, vacuum dry it at 90 ℃ for 6 hours, and then roll it with a pressure of 100MPa to make it 122 μ m thick (compaction density 1.60 g/cm3) to make a negative electrode sheet with an active material content of 96.0%.
After vacuum drying the prepared electrode sheets at 85 ℃ for 12 hours, the positive electrode sheet, (9+3) μ m ceramic separator, and negative electrode sheet were wound into a core structure, encapsulated with aluminum-plastic film, and vacuum baked at 85 ℃ for 24 hours. After the moisture content was qualified, the electrolyte was injected into the glove box and the battery quality was recorded. The battery was then left to stand at high temperature for 24 hours at 45 ℃.

1.2 Lithium ion battery formation process and electrochemical performance testing
The formation process of lithium-ion batteries is as follows: NP-5AFF (5V5A-128CH) high-temperature pressurized formation machine is used to charge at a constant current of 0.1C for 45 minutes, with a controlled voltage of 3.5V; then charged at a constant current of 0.2C for 30 minutes, with a controlled voltage of 3.8V; and finally charged at a constant current of 0.5C for 90 minutes, with a controlled voltage of 4.2V. According to experimental requirements, the formation pressure is controlled at 1.0-3.0MPa, and the formation temperature is controlled at 75 ℃. After the battery formation is completed, it should be left at room temperature for more than 4 hours, subjected to secondary sealing and air extraction molding, and the battery quality should be recorded.
The battery capacity division steps are as follows: at (25 ± 2) ℃, charge at a constant current of 0.5C to 4.4V, then charge at a constant voltage of 0.02C, and then discharge at a constant current of 0.5C to 3.0V, cycle twice, divide the battery, record the charge and discharge capacity of the battery, and the rated capacity of the battery is between 3000-4000mAh.
The high-precision battery performance testing system CT-4008-5V6A is used for battery charge and discharge testing. The cyclic testing method is as follows: the battery is charged at a constant current and voltage of 0.7C to 4.40V at room temperature, with a cut-off current of 0.02C, and discharged at a constant current of 0.7C to 3.0V. The charging and discharging capacity and capacity retention rate of the battery are recorded at different charging and discharging times.
2 Results and Discussion
2.1 Relationship between electrolyte injection amount under low formation pressure and electrolyte retention amount after formation
The amount of electrolyte in lithium-ion batteries usually affects the cycling stability of the battery. In the production process, it is crucial to control the injection amount of electrolyte within a reasonable range. After the formation of lithium-ion batteries, the retention amount of electrolyte is closely related to the injection amount of electrolyte, the formation process, and the characteristics of the positive and negative electrode materials of the battery. Under the condition of controlling the formation pressure ≤ 1.0MPa and maintaining consistency between the formation process and the positive and negative electrode materials of the battery, a lithium-ion battery with a capacity of about 3000mAh was selected, and the electrolyte injection amounts were 1.60, 1.75, and 1.90g/Ah, respectively. The basic relationship between the electrolyte injection amount and the retention amount of the electrolyte after formation was studied, as shown in Table 1. The selection of the minimum retention value mentioned above is based on actual production data. For lithium cobalt oxide batteries, when the formation pressure is 1.0 MPa, injection below the minimum value will result in poor wetting of the positive and negative electrodes.

The use of a formation pressure of ≤ 1.0MPa ensures that there is appropriate adhesion between the positive and negative electrode plates and the separator during the formation of lithium-ion batteries, while also ensuring that the lithium-ion battery is not excessively compressed and that there is sufficient electrolyte reserve. According to the recording and calculation of the injection amount and the retention amount of electrolyte after formation, it can be found that when the formation pressure is small (<1.0MPa), a battery cell with a capacity of about 3000mAh and an electrolyte injection amount ≥ 1.60g/Ah can fully meet the consumption of electrolyte by SEI film generation during the formation process. As the injection amount of electrolyte increases, the retention amount of electrolyte after formation also gradually increases, and the injection amount is positively correlated with the retention amount of electrolyte.
In order to further clarify the relationship between the retention amount and injection amount of electrolyte in the actual production process of lithium-ion batteries, data was collected from multiple sets of samples in the 1 #~2 # series experiments, and box plots were made to observe the dispersion of data. As shown in Figure 1, when the injection amount of electrolyte is 1.60g/Ah, the numerical distribution of electrolyte retention amount after lithium-ion battery formation is relatively narrow. With the increase of electrolyte injection amount, the distribution of electrolyte retention amount gradually widens, indicating that when the electrolyte of the battery is sufficient, reducing the injection amount of electrolyte will gradually narrow the distribution of electrolyte retention amount.

Figure 1 Box plot of electrolyte retention after lithium-ion battery formation under different electrolyte injection amounts
2.2 Impact of Formation Pressure on Cycle Performance
In actual production, in order to control costs while ensuring battery performance, it is better to inject as little electrolyte as possible. The above research confirms that when the formation pressure is ≤ 1.0MPa and the electrolyte injection rate of lithium-ion batteries is 1.60g/Ah, it can ensure that the formed lithium-ion batteries have sufficient electrolyte. On this basis, investigate the cyclic performance under different formation pressures. When the formation pressure increases, the contact between the positive and negative electrode plates and the separator becomes tighter, resulting in a decrease in the space available for absorbing electrolyte inside the lithium-ion battery. Therefore, an electrolyte injection amount of 1.60g/Ah was selected, and the formation pressure was controlled at 2.2, 1.6, and 1.0MPa, respectively. A series of batteries were prepared and their cycling performance was tested, as shown in Figure 2.

Figure 2: Long term cycling performance of batteries under different formation pressures
As shown in Figure 2, when the formation pressure gradually decreases, the long-term cycling performance of the battery deteriorates. When the formation pressure is 2.2MPa, the capacity retention rate of the battery is still greater than 80% after 1150 cycles. At this point, after calculation, the electrolyte retention of the battery is greater than 1.56g/Ah, which means that using a lower electrolyte injection amount can meet the capacity retention rate of the battery after 1000 cycles, which is greater than 80%. As the electrolyte injection amount of the battery increases, the probability of long-term cycling failure of the battery will also decrease.
2.3 The influence of electrolyte retention on cycling performance
The retention amount of electrolyte has a significant impact on cycling performance, and sufficient electrolyte is a necessary condition for maintaining battery cycling performance. The main reasons for insufficient electrolyte are: firstly, insufficient injection of electrolyte; The second issue is that the electrolyte injection is sufficient, but due to insufficient immersion, the injected electrolyte is extracted, resulting in insufficient retention of the electrolyte; The third is that the electrolyte inside the battery is completely consumed during the cycling process. During the formation stage, the film-forming additives in the electrolyte will decompose, and the decomposition products will precipitate on the negative electrode surface to form an SEI film. The structure of the SEI film affects the cycling performance of the negative electrode material.
The unstable SEI film during the cycling process will repeatedly decompose and regenerate, consuming reversible lithium sources and electrolytes, resulting in cycling failure. Increasing the retention of electrolyte can improve cycling performance while ensuring sufficient electrolyte injection and infiltration. In order to further investigate the effect of electrolyte retention on cycling performance, 1 #~5 # lithium-ion batteries were selected and injected with different amounts of electrolyte, and the retention amount of electrolyte after formation was recorded, as shown in Table 2. When the electrolyte injection amount was 1.58-1.68g/Ah and the formation pressure was 2.2MPa, the retention amount of electrolyte was not less than 1.54g/Ah. According to the statistical results of the difference between the electrolyte injection amount and the retention amount, it can be concluded that as the electrolyte injection amount increases, the retention amount of the electrolyte also gradually increases.

Long term cycling performance tests were conducted on cells with different electrolyte retention amounts in Table 2, with a charge discharge current of 0.7C and a charge discharge voltage range of 3.0-4.4V. The specific test results are shown in Figure 3.

Figure 3: Long term cycling performance of batteries with different electrolyte injection and retention amounts
As shown in Figure 3, the cycling performance of Battery 1 is relatively poor. After 498 cycles, the capacity retention rate of the battery is lower than 80% of its initial capacity; The cycling performance of batteries 2 #, 4 #, and 5 # is relatively consistent, with a capacity retention rate of 80% even after 650 cycles; The cycling performance of battery # 3 is optimal, with 770 charge and discharge cycles achieved when the battery capacity retention rate is 80%. When the electrolyte injection amount is sufficient and the electrolyte retention rate is above 1.56g/Ah, the capacity retention rate of the battery after 500 cycles at 0.7C can be greater than 80%. With the increase of injection amount, the capacity retention rate increases and the probability of cycle failure decreases; When the difference between the electrolyte injection amount and the retention amount is 0.06g/Ah, the battery has good cycling performance. After the battery formation step is completed, the optimal combination of battery performance and cost can be achieved by controlling the loss of electrolyte after formation to around 0.06g/Ah.
3 Conclusion
This article investigates the relationship between electrolyte injection rate and battery performance in flexible packaging lithium-ion batteries prepared with lithium cobalt oxide and artificial graphite as positive and negative electrodes. For digital soft pack lithium-ion batteries with a capacity of approximately 3000mAh, the retention amount of electrolyte is directly proportional to the injection amount of electrolyte. When the injection amount of electrolyte is sufficient, reducing the injection amount of battery electrolyte will gradually narrow the distribution of electrolyte retention. When the pressure used for battery formation is 2.2MPa, the electrolyte retention of the battery is greater than 1.56g/Ah, which can achieve a capacity retention rate of over 80% after 1000 cycles. In order to further reduce battery costs and ensure battery performance, the optimal combination of battery performance and cost can be achieved by controlling the loss of electrolyte after formation to around 0.06g/Ah.