CR2032 button cell.Si anode faces strong enemy again: Peking University develops rare earth reinforced graphite material

　　In recent years, with the continuous improvement of the energy density of lithium-ion batteries, traditional lithium-ion battery materials are gradually declining and being gradually replaced by rising stars. The positive electrode is NCA and NCM materials, while the negative electrode is high-capacity Si material. Compared with the theoretical capacity of graphite material of 372mAh/g, the theoretical capacity of Si material can reach 4200mAh/g (Li4.4Si). In practice, it can also reach about 3000mAh/g, which is much higher than graphite materials. However, there is a problem with Si material The obvious shortcoming is huge volume expansion. The volume expansion of the Si material can reach 300% in the fully lithium-embedded state. This will not only destroy the SEI film on the surface of the Si particles, cause interface instability, and lead to the loss of Li, but also Destroying the conductive network structure of the electrode, resulting in the loss of active materials, these factors have led to the cycle performance of Si materials being significantly lower than graphite materials. Although many measures have been taken to suppress the volume expansion of Si materials, such as nanotechnology, nano-Si- Carbon composite and synthetic SiOX, etc., but the effect is not ideal. At present, only SiOX materials have achieved certain results in practice.

　　For this reason, while people continue to promote the research of SiOX materials, they have not given up the development of other high-capacity anode materials. Today we introduce to you the rare earth element reinforced graphite material developed by Xinyao Zheng and others from Peking University. This material has excellent The electrochemical performance of the product still maintains a high capacity of 720mAh/g after 250 cycles, which is much higher than ordinary graphite materials and higher than currently commonly used SiOX/graphite composite materials, and has good application prospects.

　　The theoretical capacity of graphite material is only 372mAh/g. In order to solve the problem of low capacity of graphite material, Xinyao Zheng used rare earth elements to synthesize YH3-graphite composite material (Y represents rare earth element). The study found that one H atom in YH3 has electrical Chemical activity can serve as a negative charge center to improve the ability of graphite to embed Li. On average, one active H atom can fix 3.1-3.4 Li atoms. Therefore, by synthesizing YH3-graphite composite materials, the lithium embedding in the negative electrode changes from forming LiC6 to The formation of Li5C16H greatly enhances its ability to embed Li.

　　The synthesis of YH3-graphite composite material is obtained by ball milling YH3 powder and graphite material in a H2 atmosphere at 0.4MPa. EDS analysis can find that both C and Y elements are very uniformly distributed in the material (a), above. c shows the first charge and discharge curves of the sample of YH3/graphite=0.5:1 and the sample of YH2/graphite=0.5:1. It can be seen from the figure that the first lithium insertion capacity and delithiation capacity of the YH3/graphite sample are 1430mAh respectively. /g and 800mAh/g, it can be noticed from the XRD diffraction patterns in different lithium-intercalation states that after lithium-intercalation occurs in the YH3/graphite sample, the diffraction peak of YH2 begins to appear in the material, and the sample in the completely lithium-intercalation state is mainly composed of It is composed of YH2 and graphite. After delithiation, YH2 is converted into YH3, which shows that only one H atom in YH3 is active during the charge and discharge process.

　　The addition of YH3 not only increases the specific capacity of the composite material to 800mAh/g, but the material also has very good cycle performance. After 250 cycles at 50mA, the capacity can still reach more than 720mAh/g (as shown in Figure a above). In contrast, the rate performance of this material is relatively poor. When the current is increased from 50mA to 2500mA, the remaining capacity of the material is only 170mAh/g (as shown in Figure b above).

　　In order to study the mechanism of YH3 increasing the capacity of graphite materials, Xinyao Zheng conducted cyclic voltammetry tests on YH3, YH3/graphite and YH2/graphite materials. The results are shown in Figure e above. From the figure, you can notice the curve of YH2/graphite. Very similar to pure graphite materials, there is a current peak at 0.2V, but the current peak of YH3/graphite material is obviously different. There is a one-degree current peak around 0.18V, which is different between pure YH3 and YH2/graphite. None have been observed in the material, which indicates that the addition of YH3 creates a new lithium insertion mechanism in the graphite material. Based on the above research, Xinyao Zheng believes that an H atom in YH3 is electrochemically active and can significantly enhance the lithium insertion ability of graphite materials. The reaction is as shown in the following formula.

　　In order to deeply understand the mechanism of YH3 in graphite materials, Xinyao Zheng used density function theory to calculate and analyze the lithium insertion process of YH3/graphite composite materials. The calculation results show that the role of H atoms in the material is to provide an electronegativity In the center of the graphite material, H atoms occupy the center of a carbon hexagon, and Li is embedded in the surrounding carbon hexagons, thereby greatly improving the Li storage capacity of the graphite material.

　　The YH3/graphite material developed by Xinyao Zheng et al. enhances the lithium intercalation ability of the graphite material through the addition of rare earth element hydride. Different from ordinary material mixing, the addition of YH3 forms a lithium-intercalation layer with H atoms in the carbon atomic layer of the graphite material. The electronegativity center in the center can effectively increase the Li-embedding ability of graphite materials and increase the capacity of the material. More importantly, the material not only has high capacity, but also has very excellent cycle performance. After 250 cycles of 50mA, the capacity is almost No fading. However, this material still has problems such as low first efficiency and poor rate performance, which need to be further solved.

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