Recently, ChengyuMao (first author) and ZhijiaDu (corresponding author) of Oak Ridge National Laboratory in the United States analyzed the influence of 6 mainstream artificial and natural graphite materials on the cycle performance of NCM811 batteries, and the analysis showed that the specific surface area is smaller. The material was able to achieve higher first Coulomb efficiencies and also performed better in long-term cycling.
In the experiment, ChengyuMao used NCM811 material from Targray, Canada as the positive electrode, and the six graphite negative electrodes were A12 from ConcoPhillips, APS19 from GrafTech, SCMG-BH from Showa Denko, MAGE and MAGE3 from Hitachi Chemical, and SLC1520T from Superior.
The figure below shows the morphologies of several graphite materials. From the figure, it can be seen that SCMG-BH, MAGE and MAGE3 materials are basically "potato" shaped, A12 and APS19 have a sheet-like structure, and SLC1520T material is closer to a spherical structure. The surface is relatively smooth, so SLC1520T also obtains the smallest specific surface area (as shown in the table above).
The graphite crystal size can be obtained from the cross-sectional view of the particles. It can be seen from the figure below that the graphite crystal size of the SCMG-BH material is the smallest. Since the electrolyte is easier to decompose at the edge of the graphite crystal sheet, the crystal particles are smaller. SCMG-BH The material will cause more electrolyte to decompose, resulting in low coulombic efficiency of the battery and affecting the cycle life of lithium batteries.
The figure below shows the reversible capacity of 6 kinds of negative electrode materials discharged at C/3 rate in the coin cell. It can be seen from the figure that the reversible capacity of most graphites can reach more than 350mAh/g, only the SCMG-BH material of Showa Denko The reversible capacity of 322mAh/g.
During the initial lithium insertion process, the electrolyte will decompose on the surface of the graphite negative electrode as the potential decreases. Therefore, the specific surface area of graphite will have a significant impact on the coulombic efficiency of the first charge and discharge of the battery. The following figure shows the NCM811 material and different The charge-discharge curve of the full battery (soft pack) composed of a graphite negative electrode during the formation process, the capacity of NCM811 material in the first charge and discharge is shown in the following table, among which the MAGE material of Hitachi Chemical with a smaller specific surface area has the highest first efficiency, reaching The MAGE3 material has the lowest coulombic efficiency of 82.2%, which is also related to its large specific surface area of 4.97m2/g.
The figure below shows the cycle curves of batteries using several different graphite negative electrodes (3.0-4.2V, C/3 charge and C/3 discharge). In order to speed up the decay of the battery, the author also added a 3-hour charge to each charge. It can be seen from the figure that the addition of the 3-hour constant pressure process greatly accelerates the decay rate of the lithium battery. After 300 cycles, only the battery capacity retention rate of MAGE and SLC1520T materials exceeds 80%. , MAGE3 and SCMG-BH materials have the worst battery cycle performance, reaching end-of-life first. Batteries using different anodes also showed different decay characteristics. For example, batteries using A12 and APS19 materials began to accelerate their decay rate after 200 cycles, while MAGE3 and SCMG-BH materials showed faster in the early stage. At the same time, we can also notice from the table below that the initial capacity of batteries using MAGE3 and SCMG-BH is also lower than that of batteries with other materials, which is also important because of the relatively low Coulombic efficiencies of these two materials. caused.
In order to analyze the decay mechanism of several lithium batteries with different negative electrodes during cycling, the author dissected the batteries after cycling, and made button batteries with positive and negative electrodes. The capacity has decreased significantly, and the rate performance has also decreased significantly. In contrast, the negative electrode (Figure d below) has only a slight decrease in reversible capacity (less than 3%) after cycling, but the rate performance has decreased. , the cycled SCMG-BH, A12 and MAGE3 materials have relatively low capacities at high rates.
In order to analyze the reasons for the decline in rate performance after the aging of positive and negative materials, the author also used the method of AC impedance to analyze the button battery. In the figure, we can notice that in addition to a semicircle in the high frequency region, the cycled NCM811 material also has a new semicircle in the intermediate frequency region, which may be a relatively slow charge exchange process in the cycled NCM811 material, such as It is possible that a new phase appeared on the surface of the NCM811 particles, leading to an increase in charge exchange resistance.
The figure below shows the AC impedance spectrum of the coin-type half-cell made of the negative electrode after formation and the negative electrode after cycling. Compared with the positive electrode, the impedance of the negative electrode is relatively small, indicating that the increase in the internal resistance of the full battery after cycling is more due to the positive electrode. Added impedance. And the AC impedance changes of different anodes after cycling are also different. The impedance of A12, SCMG-BH and MAGE3 materials almost doubles after cycling. It leads to a large amount of active Li consumption, which leads to the poor cycle performance of the full battery of these materials, while the MAGE and SLC1520T materials with better cycle performance also have relatively little increase in impedance after cycling, which is mainly due to the fact that The smaller specific surface area of these two materials reduces the decomposition of the electrolyte.
The figure below shows the relationship between the specific surface area, primary efficiency and negative particle size of several anode materials and the capacity retention rate of the full cell. From the figure, it can be seen that the MAGE and SLC1520T materials with the smallest specific surface area not only have the highest coulombic efficiency for the first time, but also The highest capacity retention was also exhibited during long-term cycling, while MAGE3 and APS19 materials with larger specific surface areas exhibited lower first coulombic efficiencies and poor cycling performance.
In general, the specific surface of the graphite anode has a crucial impact on its coulombic efficiency and long-term cycling stability. Materials with smaller specific surface can reduce the decomposition of the electrolyte, thereby improving the first coulombic efficiency and long-term cycling stability of the battery. Therefore, for lithium batteries that require higher life characteristics, graphite materials with smaller specific surface areas should be selected.