In recent years, with the continuous improvement of the energy density of lithium-ion batteries, the traditional lithium-ion battery materials are being gradually miniaturized and gradually replaced by the rising star, NCA and NCM materials on the positive side and Si materials on the negative side with high capacity. The theoretical capacity of graphite material 372mAh / g, the theoretical capacity of Si material up to 4200mAh / g (Li4.4Si), in practice it can reach about 3000mAh / g, much higher than the graphite material, but there is a very obvious Si material Disadvantages - huge volume expansion, in the complete state of lithium embedded Si material volume expansion up to 300%, which will not only damage the surface of the Si particles SEI film, causing instability of the interface, leading to the loss of Li, but also damage the electrode Conductive network structure, resulting in the loss of active materials, these factors have led to the Si material significantly lower than the cycle performance of graphite materials, although many measures taken to inhibit the volume expansion of Si materials, such as nano-technology, nano-Si- Synthetic SiOX, but the effect is not satisfactory, so far only the SiOX material has achieved some success in practice.
To this end, people continue to promote the research of SiOX materials, without abandoning the development of other high-capacity anode materials. Today we introduce the rare earth element-reinforced graphite materials developed by Xinyao Zheng et al. After 250 cycles, the high capacity of 720mAh / g is maintained, which is much higher than that of common graphite materials and also higher than the currently used SiOX / graphite composite materials, and has good application prospect.
The theoretical capacity of graphite material is only 372mAh / g, in order to solve the problem of low capacity graphite material, Xinyao Zheng YH3-graphite composite material synthesized by rare earth elements (Y on behalf of rare earth elements), the study found that one H atom YH3 has electricity Chemical activity, can serve as a negative charge center, with the ability to enhance the Li embedded in graphite, an average of an active H atoms can be fixed 3.1-3.4 Li atoms, so by synthesizing YH3-graphite composite, making the negative Li by the formation of LiC6, into The formation of Li5C16H, greatly strengthened its ability to embed Li.
YH3-graphite composites were synthesized by ball milling the YH3 powder and graphite in H2 atmosphere at 0.4MPa. EDS analysis showed that the C and Y elements were uniformly distributed in the material (top a) c shows the first impulse curves for samples with YH3 / graphite = 0.5: 1 and samples with YH2 / graphite = 0.5: 1. It can be seen from the figure that for the first time the lithium insertion capacity and delithiation capacity of YH3 / graphite samples are respectively 1430 mAh / g and 800mAh / g. It can be noticed from the XRD diffraction patterns of different intercalation states that diffraction peaks of YH2 begin to appear in the YH3 / graphite sample after lithium intercalation, while those in the fully intercalated lithium state mainly consist of YH2 and graphite, YH2 is converted back to YH3 after the delithiation, indicating that only one H atom in YH3 is active during charging and discharging.
The addition of YH3 not only makes the specific capacity of the composite reach 800mAh / g, but also the material has very good cycling performance, and the capacity can still reach more than 720mAh / g after 250 cycles at 50mA (as shown in the above picture a) The material under the rate of performance is relatively poor, when the current increased from 50mA to 2500mA, the material remaining capacity of only 170mAh / g (Figure b).
In order to investigate the mechanism of YH3 on the capacity of graphite materials, Xinyao Zheng conducted cyclic voltammetry tests on YH3, YH3 / graphite and YH2 / graphite materials, respectively. The results are shown in the above figure e, from which YH2 / graphite curve Similar to the pure graphite material, a current peak appears at 0.2V, but the current peak of YH3 / graphite material shows obvious difference. There is a current peak at about 0.18V, which shows that in pure YH3 and YH2 / graphite The results showed that the addition of YH3 resulted in a new mechanism of lithium intercalation in graphite.According to the above research, Xinyao Zheng thought that one H atom in YH3 is electrochemically active and can significantly enhance the Lithium insertion capacity, the reaction shown below.
In order to further understand the mechanism of YH3 in graphite materials, Xinyao Zheng calculated the lithium insertion process of YH3 / graphite composites by using the density function theory. The calculation results show that the role of H atoms in the material is to provide a negative charge Sex centers, H atoms occupy the center of a hexagonal hexagonal carbon hexagonal will be embedded within the Li, which greatly improved graphite material storage Li ability.
The YH3 / graphite material developed by Xinyao Zheng et al. Enhances the lithium intercalation ability of graphite materials by adding rare earth element hydride. Unlike the ordinary material mixing, the addition of YH3 forms a carbon atom layer with H atoms as The electronegativity center of the center can effectively increase the embedded Li ability of graphite materials and improve the material capacity. More importantly, the material not only has high capacity, but also has very excellent cycling performance. The capacity of the battery is almost 250mA at 250 cycles There is no decline, but the material for the first time there are still low efficiency and rate of poor performance and other issues remain to be further resolved.
