Synthesis of Alumina-Coated Natural Graphite for High Cycling Stability and Safety of Li-ion Batteries
Research Background：
Al 2 O 3 in lithium-ion batteries has a long history, and the most famous one is that the separator surface is coated with Al 2 O 3 to improve the thermal stability of the separator, and the aluminum oxide coating on the negative electrode of Samsung SDI is proposed by Dr. Zhang Zhengming of Celgard. Overlay improves battery performance and safety. However, there are more and more studies on the application of Al 2 O 3 in lithium batteries recently. A little further, the research of the research group of Liu Zhaoping from the Ningbo Institute of Materials, Chinese Academy of Sciences shows that adding silane-Al 2 O 3 to the conventional electrolyte can not only improve the electrochemical performance of the battery such as cycle and rate, but also effectively improve the safety of the battery. Then, the research of Dae Sik Kim et al. of Korea showed that the graphite surface coated with amorphous Al 2 O 3 can improve the fast charging performance of the battery. Recently, Jeff Dahn put forward a new insight that Al 2 O 3 coated on the surface of NCM can react with LiPF 6 to form LiPO 2 F 2 , which can improve battery performance.
Due to the trend of various factors, the capacity and energy density of power batteries continue to increase, and the safety of batteries is increasingly concerned. The safety issues of batteries are complex, but one of the root causes is the chemical system. Only by fundamentally improving the safety of materials can it be possible to improve the overall safety characteristics of batteries. Headquartered in Shenzhen, Battery has a good reputation in the field of anode materials, and its R&D strength is second to none among domestic material factories. Recently, Dr. Xu Tao, who specializes in improving the safety of negative electrodes, proposed that Al 2 O 3 can be coated on the surface of natural graphite by the sol-gel method, which can not only improve the cycle stability of the battery, but also improve the safety. Alumina-Coated Natural Graphite for Highly Cycling Stability and Safety of Li-Ion Batteries was published in Chinese Journal of Chemistry.
Analysis of pictures and texts：
Figure 1. (ac) Natural graphite (NG), 1 wt% Al2O3-coated natural graphite ( denoted as AN-1), and 3 wt% Al2O3-coated natural graphite (denoted as AN-1 ).
Figure 2. (a) and (e) are the SEM images of AN-1 and AN-3, respectively; (bd) and (fh) are the elemental distribution images of AN-1 and AN-3.
First, the authors coated Al 2 O 3 on the surface of natural graphite (NG) by sol-gel method . The preparation method of Sol-gel is very simple. First , add deionized water to Al(NO 3 ) 3 ·9H 2 O, then add natural graphite under stirring conditions, and finally dry to obtain natural graphite AN coated with Al 2 O 3 on the surface. -1 (alumina coating amount of 1 wt%) and AN-3 (alumina coating amount of 3 wt%). The SEM images of NG, AN-1 and AN-3 are shown in Figure 1, and the particle sizes of the three are about 11.55 μm, 11.58 μm and 11.67 μm, respectively. It can be seen from Figure 1 and Figure 2 that the Al 2 O 3 coated on the surface of natural graphite by the sol-gel method is relatively uniform, but the natural graphite particles do not achieve complete coating, but there is a considerable part of the exposed area .
Figure 3. XRD patterns and TGA patterns of NG, AN-1 and AN-3.
The XRD patterns of AN-1 and AN-3 are almost the same as those of NG, and do not show the existence of Al 2 O 3 , which is mainly caused by too little coating of Al 2 O 3 . But the TGA curve can clearly see that NG burns completely at 900 ℃, while AN-1 and AN-3 still have quality, which is consistent with the amount of Al 2 O 3 in them.
Figure 4. Charge-discharge curves of NG, AN-1 and AN-3 at 0.005-3 V and 0.1 C.
Figure 5. Charge-discharge curves of NG, AN-1 and AN-3 at 3.0-4.35 V and 1 C.
Subsequently, the authors conducted a comparative analysis of the electrochemical properties of NG, AN-1 and AN-3. First, in terms of discharge capacity, the discharge capacities of NG, AN-1, and AN-3 are 364.0, 359.8, and 350.4 mA/g, respectively, and the Coulombic efficiencies are 93%, 93.4%, and 93.5%, respectively. For the reduction of discharge capacity by Al 2 O 3 coating, the author believes that the coating leads to the reduction of lithium ion diffusion channels; while the increase in Coulomb efficiency is caused by the reduction of side reactions by coating ( Note: the most conventional explanation is not good. Can’t say it’s bad either! )
Figure 6. (a)-(c) are the results of acupuncture experiments for 2 Ah LCO pouch cells using NG, AN-1 and AN-3 graphites, respectively.
The highlight is here. Finally, the author conducted acupuncture experiments on soft-pack batteries using NG, AN-1 and AN-3 three kinds of graphite, and compared the modification of battery safety by Al 2 O 3 coating. The needle diameter is 3 mm, the taper angle is 60 degrees, and the needling speed is 80 mm/s. As shown in Figure 7, thermal runaway occurred rapidly during the needling process of pouch battery using NG, and the maximum temperature of thermal runaway exceeded 600 °C; while the pouch battery using AN-1 and AN-3 did not experience thermal runaway during the needling test. The maximum temperature of the battery surface does not exceed 100 ℃, and the maximum temperature reached by AN-3 is lower than that of AN-1. The above acupuncture comparison experiments show that the surface Al 2 O 3 coating of natural graphite can definitely improve the safety of the battery to a certain extent.
summary:
In this paper, the surface of natural graphite is coated with Al 2 O 3 by sol-gel method. The coating can not only inhibit the side reaction, improve the cycle stability of the battery, but also improve the safety. The coating amount of Al 2 O 3 is 1 wt% of natural graphite, and thermal runaway does not occur under the acupuncture experiment. Although the impact factor of this article is not high, it is still commendable to publish papers in enterprises, and the final battery safety improvement results are also very convincing.