A powerful tool to gain insight into the reaction mechanism of 18650 battery 1800mah—in-situ transmission electron microscopy technology

　　Since their invention, 18650 battery 1800mah have entered all areas of our lives and have had a profound impact on our modern lives. 18650 battery 1800mah are mainly composed of key parts such as positive electrode, negative electrode and electrolyte. During the charging process, Li is released from the positive electrode, diffuses to the surface of the negative electrode through the electrolyte, and is embedded in the crystal structure of the negative electrode. The discharge process is exactly the opposite. Although 18650 battery 1800mah have been developed for many years, they still face many challenges, such as the problem of lithium dendrites in the negative electrode and the thermal stability of the positive electrode, including the new all-solid-state metal lithium batteries and thin-film 18650 battery 1800mah that have appeared in recent years. We are faced with various problems in the process, but because 18650 battery 1800mah are closed systems, we lack powerful research tools on the mechanisms of these problems inside 18650 battery 1800mah, so we have more understanding of these problems. It remains on the basis of theoretical research. The in-situ detection technology that has emerged in recent years has opened a new window for us to study the mechanisms of these problems inside 18650 battery 1800mah.

　　In-situ transmission electron microscopy technology is a powerful tool that has emerged in recent years to study the internal reaction mechanism of 18650 battery 1800mah. It has unique advantages in studying the structure, thermal stability of positive and negative electrode materials, and the stability of the electrode and electrolyte interface. .

　　Generally speaking, the storage mechanism of Li+ in 18650 battery 1800mah is mainly divided into three categories: 1) intercalation; 2) alloying; 3) conversion reaction. The intercalation reaction mainly refers to the embedding of Li+ into the active material without causing significant structural changes. Alloying mainly refers to the direct reaction between Li and metal element A (such as Si, Ge, Sn, etc.) to form a Li-A alloy. The conversion reaction refers to the reaction between Li and the binary compound MX (M mainly refers to the transition metal elements Fe, Co, Cu, etc., and X mainly refers to elements such as S, O, F, etc.) to generate MO and LiX. The three reaction mechanisms are different, so the active materials involved are also very different. In-situ transmission electron microscopy technology can help us better understand the mechanisms of the above three reactions.

　　For the intercalation reaction, ideally, when Li+ is intercalated into the active material, the structure of the material should not change significantly. However, in actual situations, the positive electrode material often causes localized lithium intercalation during the intercalation process. Structural instability causes structural collapse, leading to rapid capacity decline. Taking MnO2 as an example, MnO2 has a one-dimensional Li+ diffusion channel inside. Due to the existence of the John-Teller effect, uneven volume expansion will occur during the lithium insertion process, causing structural instability. This uneven Expansion will reduce the capacity of MnO2 during actual use.

　　Our common LiFePO4 materials usually have uneven reactions during the lithium insertion process. In-situ transmission electron microscopy studies have found that there are many dissolution zones or two-phase zone (LFP/FP) boundaries in some lithium-embedded LFP materials. This will lead to poor cycle performance of LFP, which is also a problem that needs to be paid attention to when improving the performance of LFP materials.

　　The most common alloying reaction is Si anode. Through the alloying reaction of Li and Si, a capacity of 4200mAh/g can be obtained. However, silicon materials also face problems such as powdering and capacity fading. In-situ transmission electron microscopy technology has well revealed the failure mechanism of Si materials for us. The mechanisms leading to the failure of Si materials include amorphization caused by lithium insertion, material fragmentation, self-limiting lithium insertion, uneven lithium insertion and interface insertion. Lithium delamination and exfoliation, etc. These findings provide new ideas for the design of highly stable alloyed anodes. Through surface modification and nanotechnology, the structural stability and capacity retention rate of Si alloy anodes have been greatly improved. promote.

　　4.4Li++4.4e-+Si=Li4.4Si

　　Transition metal oxides and sulfide materials MX (such as MoS2, etc.) are also emerging high-capacity anode materials in recent years. However, due to the high reactivity of LiX generated by the lithium insertion reaction, it will lead to multi-phase reactions, resulting in low Coulombic efficiency. Problems such as large polarization voltage and fast capacity fading hinder its widespread application. In-situ transmission electron microscopy studies on the lithium insertion mechanism of MX materials show that during the lithium insertion process, nanoparticles of the transition metal element M will be formed. However, during the reverse reaction process, the M nanoparticles often cannot be oxidized to the initial valence state. This It also causes the first Coulomb efficiency of the MX material to be very low. At the same time, repeated lithium insertion and delithiation processes will cause the material to generate fragments, causing the material's capacity to continue to decline.

　　Compounding and structural modification of materials are currently effective methods to improve the performance of electrode materials, but we have not studied the mechanism in depth enough. In-situ transmission electron microscopy technology provides us with a very powerful way to study the mechanism of these modification methods. Tool of.

　　For example, Si-C composite material technology is currently a common technology for preparing high-performance Si materials. Research using in-situ transmission electron microscopy shows that the mechanism of this method mainly consists of two: First, composite materials can well suppress the volume of Si materials. expansion to maintain the stability of the SEI film. Secondly, the composite material also provides a better diffusion channel for e-/Li+ diffusion.

　　As the energy density of 18650 battery 1800mah continues to increase, thermal stability has become the focus of our attention, especially for power batteries. Thermal stability has an intuitive and important impact on the safety of the battery. Therefore, the thermal failure mechanism of electrode materials research is particularly important.

　　Taking the currently common NCA (LiNi0.8Co0.15Al0.05O2) as an example, high-temperature in-situ transmission electron microscopy technology was used to find that when heated to 450°C, the crystal structure inside the material changed from layered to salt rock structure, and the surface of the particles appeared Micropores and the release of active O. Further studies show that the release of active O is mainly due to the reduction of Ni, which further leads to the reduction of the stability of Mn and Co elements.

　　Interface issues are important issues that limit the electrochemical performance and safety performance of 18650 battery 1800mah. In-situ transmission electron microscopy technology can be used to study the growth issues of SEI films, Li coatings, and dendrites. For example, research on SEI films shows that the SEI film is not formed uniformly in the electrolyte. Instead, dendrite-like products are first generated and continue to grow along with the Li plating. The growth of the SEI film is affected by the decomposition of electrolyte components and electrodes. product impact. These studies have improved important information for our understanding of Li deposition and SEI film growth, and provided new ideas for solving related problems and improving the life of 18650 battery 1800mah.

　　The development of in-situ transmission electron microscopy technology provides us with a powerful tool for understanding the mechanisms of various reactions in 18650 battery 1800mah. Understanding the reaction mechanisms can in turn help us continue to improve the performance of electrode materials.

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