12V23A battery

Graphite anode selection strategy for high specific energy and long life 12V23A battery

Although the application of high-capacity Si anode materials is gradually becoming popular, graphite anode is still the mainstream lithium-ion battery anode material due to its excellent electrochemical performance. During the charging process, Li+ is released from the positive electrode and diffuses through the electrolyte to the surface of the negative electrode and is embedded in the graphite negative electrode. The discharge process is just the opposite. The lithium insertion potential of graphite material is close to that of metal Li. On the one hand, it can effectively increase the voltage of 12V23A battery, thereby increasing the energy density. On the other hand, it also causes the current conventional carbonate electrolyte to undergo reduction decomposition on the surface of the graphite negative electrode, resulting in the consumption of active Li. Many studies have shown that the decomposition of the electrolyte on the negative electrode surface is an important reason for the capacity decline of 12V23A battery. Therefore, the selection of graphite anode materials is of great significance for improving the life characteristics of 12V23A battery. Recently, Chengyu Mao (first author) and Zhijia Du (corresponding author) from Oak Ridge National Laboratory in the United States analyzed the effects of six mainstream artificial and natural graphite materials on the cycle performance of NCM811 batteries. The analysis showed that materials with smaller specific surface areas can achieve higher first coulombic efficiency and perform better in long-term cycles. In the experiment, Chengyu Mao 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 information of the six materials is shown in the table below). The figure below shows the morphology of several graphite materials. From the figure, it can be seen that SCMG-BH, MAGE and MAGE3 materials are basically in the shape of "potatoes", A12 and APS19 are in the shape of flakes, and SLC1520T material is closer to a spherical structure with a smoother surface, so SLC1520T also obtains the smallest specific surface area (as shown in the table above). The graphite crystal size can be obtained through the cross-sectional view of the particles. From the figure below, it can be seen that the graphite crystal size of SCMG-BH material is the smallest. Since the electrolyte is more likely to decompose at the edge of the graphite crystal sheet, the SCMG-BH material with smaller crystal particles will cause more electrolyte decomposition, resulting in low coulomb efficiency of the battery and affecting the cycle life of the lithium-ion battery. The figure below shows the reversible capacity of 6 negative electrode materials discharged at a C/3 rate in a button cell. From the figure, it can be seen that the reversible capacity of most graphites can reach more than 350mAh/g, and only the reversible capacity of Showa Denko's SCMG-BH material is 322mAh/g. As the potential decreases during the first lithium insertion process of the graphite negative electrode, the electrolyte will decompose on the surface of the graphite negative electrode. Therefore, the specific surface area of the graphite will have a significant effect on the first charge and discharge coulomb efficiency of the battery. The figure below shows the charge and discharge curves of the formation process of a full battery (soft pack) composed of NCM811 material and different graphite negative electrodes. The capacity of the NCM811 material in the first charge and discharge is shown in the following table. Among them, the MAGE material of Hitachi Chemical with a smaller specific surface area has the highest first efficiency, reaching 86.1%, followed by SLC1520T. The MAGE3 material has the lowest first coulomb efficiency, which is only 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 charging and C/3 discharging). In order to accelerate the battery decay, the author also added a 3-hour constant voltage process in each charge. From the figure, it can be seen that the addition of the 3-hour constant voltage process greatly accelerates the decay rate of 12V23A battery. After 300 cycles, only the battery capacity retention rate of MAGE and SLC1520T materials exceeds 80%, and the battery cycle performance of MAGE3 and SCMG-BH materials is the worst and reaches the end of life first. Batteries using different negative electrodes also show different decay characteristics. For example, the decay rate of batteries using A12 and APS19 materials begins to accelerate after 200 cycles, while MAGE3 and SCMG-BH materials show faster decay rates in the early stage. At the same time, from the table below, we can also notice that the initial capacity of batteries using MAGE3 and SCMG-BH is also lower than that of batteries using other materials, which is mainly due to the relatively low formation coulomb efficiency of these two materials. In order to analyze the decay mechanism of 12V23A battery with different negative electrodes during cycling, the authors dissected the cycled batteries and made button batteries using positive and negative electrodes respectively. From Figure c below, we can see that the capacity of the positive electrode after cycling has not only decreased significantly, but also the rate performance has decreased significantly. On the contrary, the reversible capacity of the negative electrode (Figure d below) has only slightly decreased (less than 3%) after cycling, but the rate performance has decreased. The capacity of the SCMG-BH, A12 and MAGE3 materials after cycling is relatively low at high rates. In order to analyze the reasons for the decrease in rate performance after aging of positive and negative electrode materials, the authors also used the AC impedance method to analyze the button battery. The figure below is the AC impedance diagram of the button battery of NCM811 material matched with different negative electrode materials. From the figure, we can notice that in addition to a semicircle in the high-frequency region, a new semicircle appears in the medium-frequency region for the NCM811 material after cycling. This may be due to a relatively slow charge exchange process in the NCM811 material after cycling, for example, a new phase may be generated on the surface of the NCM811 particles, resulting in an increase in charge exchange impedance. The following figure shows the AC impedance spectrum of button half-cells made of negative electrodes after formation and 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 increase in the impedance of the positive electrode. In addition, the changes in the AC impedance of different negative electrodes after cycling are also different. The impedance of A12, SCMG-BH and MAGE3 materials almost doubles after cycling. This is mainly related to their larger comparative area, which leads to more decomposition of the electrolyte, which also leads to a large amount of active Li consumption, resulting in poor cycle performance of the full battery of these materials. The MAGE and SLC1520T materials with better cycle performance also have relatively less impedance increase after cycling, which is mainly due to the small specific surface area of these two materials that reduces the decomposition of the electrolyte. The figure below shows the relationship between the specific surface area, first efficiency, and negative electrode particle size of several negative electrode materials and the capacity retention rate of the whole battery. From the figure, it can be seen that the MAGE and SLC1520T materials with the smallest specific surface area not only have the highest first coulombic efficiency, but also show the highest capacity retention rate in long-term cycles, while the MAGE3 and APS19 materials with larger specific surface areas show lower first coulombic efficiency and poorer cycle performance. In general, the specific surface area of the graphite negative electrode has a crucial influence on its coulombic efficiency and long-term cycle stability. Materials with smaller specific surface areas can reduce the decomposition of the electrolyte, thereby improving the first coulombic efficiency and long-term cycle stability of the battery. Therefore, for 12V23A battery with higher requirements for life characteristics, graphite materials with smaller specific surface areas should be selected.

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