The high-temperature cycle life of high-nickel ternary cathode material lithium-ion batteries is difficult to meet the requirements of large-scale application in the field of power batteries, and it is of great significance to study the high-temperature cycle failure mechanism for the development of high-temperature and long-life high-nickel ternary cathode lithium-ion batteries.
In view of the above objectives, the authors of this paper divide the capacity attenuation into polarization loss, active Li+ loss, structural phase transformation loss and metal ion dissolution loss, etc., hoping to clarify the capacity loss of each part and find the main factors of high-temperature cycle failure of lithium-ion batteries, which are high-nickel ternary cathode materials, so as to improve the cycle performance of the battery.
1 experiment
1.1 Battery preparation
The raw materials used in the experiment are the materials used in the preparation of the company's commercial lithium-ion batteries. NCM811, PVDF and conductive carbon black were mixed according to the mass ratio of 97.1:1.4:1.5, NMP was added, stirred evenly to make a slurry, and coated on 15um thick aluminum foil on both sides, and the positive electrode piece was obtained according to the company's production process.
Artificial graphite, conductive carbon black, CMC and SBR are made into a slurry according to the mass ratio of 97.1:0.5:1.0:1.4 with water as the solvent, and the negative electrode is coated on the copper foil with a thickness of 8um on both sides, and the negative electrode piece is obtained according to the company's production process.
The positive and negative electrode sheets and ceramic separators are made into flexible packaging batteries with a rated capacity of 3.58Ah according to the company's production process. The electrolyte is a LiPF6-based electrolyte for production.
1.2 Performance Testing
1.2.1 Battery Testing
Perform a 45°C charge-discharge cycle with a battery tester. The charging process is as follows: charge to 4.200V with 1.00C constant current, transfer to 0.05C with constant voltage, and discharge to 2.800V with 1.00C constant current. Between charging and discharging, let stand for 30min.
The DC internal resistance (DCIR) of the battery was tested with a battery tester, and the battery SOC was adjusted to 70% at a current of 0.10C, and discharged at 1.00C for 1s. The alternating current internal resistance (ACIR) of a battery is determined with a battery internal resistance tester. Measure the thickness of the battery with a digital thickness gauge.
Electrochemical impedance spectroscopy (EIS) testing is performed with an electrochemical workstation using the positive working electrode, the negative electrode, and the reference electrode of the flexible packaging battery.
1.2.2 Pole piece test
XRD analysis was performed with an X-ray diffractometer with a test step size of 0.02°. The positive electrode piece NCM811 particles were cut by focused ion beam (FIB) technology, and the profile was observed by scanning electron microscope. The composition of the anode element was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES).
The positive and negative electrodes in the empty state are punched into a disc with a diameter of 13mm in the glove box, and the separator, lithium metal sheet and electrolyte are formed into a button battery, and the positive electrode capacity test is carried out after standing for 24 hours (0.10C is charged to 4.250V, transferred to constant voltage to 0.05C; static for 30min; 0.10C discharged to 2.800V, and left for 30min) and negative electrode capacity test (0.10C discharged to 0.005V; static for 30min; charged to 2.000V at 0.10C). Let stand for 30min).
2 Results and Discussion
2.1 45°C cycling performance
The cycling performance of the battery at 45°C is shown in Figure 1. As can be seen from Figure 1, the capacity retention rate of the battery for 523 cycles is 76.05%.
2.2 Failure battery state and impedance
2.2.1 Battery Status
Failed batteries experience some degree of bulging and an increase in thickness, but no gas production. The battery was discharged to 2.800V at 0.05C for dissection, and it was found that the fresh battery had a large amount of electrolyte, while the failed battery had less electrolyte, indicating that a large amount of electrolyte was consumed during the 45°C cycle process, and the side reactions were serious.
The fresh battery and the failed battery were dissected, and the state of the positive and negative electrodes was shown in the figure. 2. The thickness of the battery and pole piece of fresh battery and failed battery is listed in Table 1.
As can be seen from Figure 2, the negative electrode piece of the failed battery has a dull gloss, powder loss, wrinkles, lithium precipitation at the folds, and wrinkles at the corresponding positive electrode pieces. As can be seen from Table 1, the thickness of the failed battery increases by 35.3%, and the thickness of the positive electrode piece increases by up to 8.5% and the thickness of the negative electrode piece increases by up to 12.5% due to the influence of deformation folds. This indicates that in addition to the expansion and deformation of the positive and negative electrodes, the decomposition of the electrolyte and the accumulation of by-products also lead to an increase in the thickness of the battery.
2.2.2 DC internal resistance
DCIR is divided into ACIR and polarized internal resistance, and the DCIR of fresh and failed battery impedances is listed in Table 2.
As can be seen from Table 2, compared with fresh batteries, the DCIR of the failed battery increased by 31.6 mΩ, of which the ACIR increased by 22.1 mΩ and the polarization internal resistance increased by 9.5 mΩ, indicating that the increase in DCIR was mainly due to the change of ACIR.
The results of EIS and the corresponding fitting calculation of the equivalent circuit are shown in Figure 3. The ohmic impedance Rs of the battery, the solid electrolyte phase interface film impedance Rsei charge transfer impedance Rct and the Warburg diffusion impedance Rw are calculated from the fitting circuit, as shown in Table 3.
As can be seen from Table 3, the Rct and Rw of failed batteries increased less, while the Rs and Rsei increased significantly. This is consistent with the results of DCIR and ACIR tests, that is, the increase in ACIR is the main reason for the increase in the impedance of the failed battery, and the increase in ACIR is mainly due to the gradual decomposition and consumption of the electrolyte in the cycle.
2.3 Loss of polarization capacity
The relationship between the constant current charge ratio and the number of cycles is shown in Figure 4. As can be seen from Figure 4, with the increase of the number of cycles, the constant current charge ratio decreases, indicating that the polarization of the battery continues to increase, which leads to the loss of part of the capacity.
In general, the capacity measured at a lower charge-discharge rate can basically ignore the capacity change caused by polarization, so the battery is charged and discharged at 1.00 C and 0.05C currents to determine the polarization capacity loss. The experimental results show that the difference in discharge capacity between fresh and failed batteries is 0.549Ah at 0.05C and 0.735Ah at 1.00C. Cyclication-induced polarization resulted in a capacity loss of 0.186 Ah, or 5.25% of the initial capacity of 3.543 Ah.
2.4 Loss of cathode capacity
2.4.1 Structure and morphology
The unit cell parameter c/a is an important factor to judge the layered structure, and the ratio of (003) diffraction peak intensity [I(003)] to (104) diffraction peak intensity [I(104)] in the XRD diagram of layered materials can be used to measure the degree of cation mixing. The unit cell parameters of the cathode of fresh and failed cells are listed in Table 4.
It can be seen from Table 4 that the c/a of the cathode material of fresh and failed batteries is greater than 4.9, indicating that the layered structure of the material remains intact. In addition, the unit cell parameters of the cathode material of the failed battery decrease and c decrease, while Li+ is detached from the cathode, the transition metals Ni and Co are oxidized, and the repulsion between adjacent oxygen layers increases. This suggests that the state of charge of the positive electrode of a failed battery may be slightly higher than that of a fresh battery, and it is assumed that the voltage of a button half-cell assembled with a lithium sheet will be higher, and this is indeed the case. The I(003)/I(104) of the cathode material of the failed battery did not decrease, indicating that the Li/Ni mixed discharge was not aggravated.
SEM images of the NCM811 particles on the cathode piece of a fresh and failed battery after FIB cutting are shown in Figure 5. It can be seen from Figure 5 that the inside of the NCM material particles of fresh batteries is relatively compact and there are no significant cracks, while there are more network microcracks inside the NCM particles of failed batteries. This indicates that serious microcracks occur in the cathode material during the cycling process, and a large number of new crystal interfaces are generated, which accelerates the side reaction between NCM and the electrolyte, resulting in electrolyte consumption and increased ohmic internal resistance of the battery.
2.4.2 Analysis of cathode/Li button batteries
The capacities of fresh and failed batteries for cathode/Li button batteries are shown in Table 5. It can be seen from Table 5 that the average capacity loss of the cathode of the failed battery is about 0.740mAh, accounting for 17.77% of the charging capacity of the fresh battery, and the average capacity loss of the cathode is about 0.279mAh, accounting for 6.55% of the discharge capacity of the fresh battery. This indicates that the capacity loss caused by the lack of cathode active lithium accounts for about 11.22% of the initial capacity of the battery, while the capacity loss caused by the change of the cathode material structure accounts for about 6.55%.
2.5 Loss of negative electrode capacity
2.5.1 ICP-AES分析
The contents of transition metals Ni, Co and Mn in the graphite of the anode of fresh and failed batteries and the consumption of Li+ in the anode graphite of fresh and failed batteries were quantitatively analyzed by ICP-AES, and the results are shown in Table 6.
It can be seen from Table 6 that after 523 cycles at 45°C, the transition metal elements Ni, Co and Mn in NCM materials were dissolved to varying degrees, and accumulated in the negative electrode graphite, among which Ni had the largest dissolution amount, followed by Co and Mn the least. According to Faraday's law, the capacity loss caused by Li+ consumed by the negative electrode and the dissolved metals Ni and Co from the positive electrode can be quantitatively calculated, and the calculation formula is shown in Eq. (1).
In Eq. (1), Q loss is the capacity loss caused by the consumption of metal elements (Li, Ni and Co) in the cycle, m loss is the mass of the metal element consumed in the cycle, F is the Faraday constant, and M is the molar mass of the corresponding metal element.
The calculation shows that the capacity loss caused by the accumulation of Li, Co and Li metal in the negative electrode is 1.428mAh, 0.097mAh and 2.365mAh, respectively, and the sum accounts for 0.11% of the initial capacity of the battery. It can be seen that the capacity loss caused by the consumption of accumulated metal elements in the anode is negligible.
2.5.2 Analysis of anode/Li button battery
The capacities of fresh and failed batteries/Li button batteries are shown in Table 7, and the XRD plots of fresh batteries and failed batteries are shown in Figure 6.
As can be seen from Table 7, the negative electrode capacity of the failed battery is almost the same as that of the fresh battery, and from Figure 6, it can be seen that the negative electrode structure does not change significantly after cycling. It can be seen that the negative graphite structure almost does not lead to capacity loss.
3 Conclusion
The thickness and impedance of the battery increase significantly due to the electrolyte consumption, and the thickness increase is mainly due to the accumulation of by-products between the separator and the electrode piece and the deformation of the battery, and the impedance increase is mainly due to the increase of AC internal resistance. Polarization causes a capacity loss of about 5.25%, a change in the structure of the cathode material causes a capacity loss of about 6.55%, a loss of cathode activity Li+ causes a capacity loss of about 11.22%, and the accumulation of lithium metal in the anode during the cycle causes a capacity loss of about 0.11%.
The sum of the above four factors was 23.13%, which was very close to the capacity retention rate of 76.05% (i.e., 23.95%) for 523 cycles. The experimental results show that the factors leading to the attenuation of circulating capacity at 45°C are as follows: loss of active Li+ of cathode material> change of cathode material structure, system polarization> accumulation of anode metal.
Literature reference: Liu Bozheng, Xu Xiaoming, Zeng Tao, Wu Shaozhong.45°C capacity attenuation of lithium-ion battery with high nickel ternary cathode material[J].Battery,2020,50(5):458-461
Source: Battery Technology TOP+
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