Enhancing the specific energy of Li-ion batteries is inseparable from the continuous progress of materials technology. The theoretical specific capacity of traditional graphite anode materials is only about 372 mAh/g. At present, some modified artificial graphite materials have reached about 360 mAh/g, and the space for continuous improvement is limited. For high-capacity negative electrode materials, Si-based materials are widely expected in the market today, including SiOx and Si-C composite materials. With the improvement of Si-C composite technology, Si-C materials have a great deal of control. Trends. Under the background of Si-based materials, the majority of scholars have not abandoned the research and development of other high-capacity materials, such as metal sulfides, such as MoS2, metal oxides, such as SnO, carbon-nitrogen compounds, and Ge that we will introduce today. Base alloy anode.
Speaking of Ge and Si, there is still a legendary story. It is said that when the semiconductor industry was developed, it was also faced with competition between Si and Ge. It was only that the final Si material won. Otherwise, the chip we use today is probably made of Ge material. Ge has lost. In the field of semiconductors, Ge's status in energy storage is also faltering. Ge also faces the same problem of volume expansion as Si in the process of intercalation of lithium. Therefore, life expectancy declines very quickly. Recently, the Wei of Henan Shangqiu Teachers College. Wei solves the problem of large volume expansion of Ge anodes by compounding Ge quantum dots with nano-carbon fibers, achieving a reversible capacity of 1,204 mAh/g (current density, 200 mA/g), and a capacity retention rate of 87.1% for 100 cycles. , The results have been published in the magazine Nanoscale.
After the Si anode is completely intercalated, the volume expansion reaches up to 300%. In order to reduce the particle powdering caused by the expansion of the Si anode, nanocrystallization and Si-C compounding are common methods. Dr. Wei Wei also used this experience to solve the negative electrode of Ge. Using electrospinning technology, Ge quantum dots with sizes of 4-7nm are mixed into the micropores of nano-carbon fibers (pore diameter 10-150nm), which is a good solution to the problem of the volume expansion of Ge during electrode insertion. Destruction, improve the cycle performance of Ge anode.
The following figure shows SEM images of carbon nanofibers CNFs (Fig. a, b) and Ge/carbon nanofibers (Fig. c, d). From the figure, it can be seen that the diameter of the nanocarbon fibers is 400-600 nm, and the diameter of the micropores in the carbon fibers. Around 10-150 nm, Ge quantum dots are dispersed among these micropores. Through the distribution analysis of EDS elements, it can be found that the distribution of Ge elements in nanocarbon fibers is very uniform. The unique structure of Ge/CNFs ensures good electrochemical properties of the material. .
To further analyze the chemical composition of Ge/CNFs, Wei Wei used XPS to perform elemental valence analysis on the above materials (as shown in the figure below). From figure b below, it can be seen that Ge 3d has a distinct peak at 29.1 eV. This indicates that the Ge in the material is present in a metallic state. The weak peak at the 30.8 eV cathode indicates that some Ge and N elements form Ge-N bonds. The strong peak at 284.6 eV in the lower panel b is the C 1s peak. The peak at 286.9 eV indicates the existence of the CN bond. The existence of the N element brings two benefits: 1) The preferred N element can reduce the electronic impedance and the ion impedance of the material; 2) Secondly, the N element can effectively reduce the material Polarization. The content of Ge in the material can be obtained by a thermogravimetric reaction (as shown in Figure f below). The test shows that the content of Ge in Ge/CNFs is about 40.6%.
Figure a shows the cyclic voltammetry results of Ge/CNFs materials. The broad current peaks appearing in the first intercalation of lithium between 0.5 and 0.0V are mainly Li-Ge alloying, and a 0.67V appearance during delithiation. The current peak corresponds to the dealloying reaction. Figure b below shows the charge/discharge curve of Ge/CNFs. It can be seen that the reversible capacity of the material reaches 1204mAh/g, which is much higher than that of graphite, but we also noticed that the material For the first time, the irreversible capacity reached 577mAh/g, and the first Coulomb efficiency was only 67.6%. The following figure c shows the results of cyclic performance tests of several different materials (current density 200mA/g, cycle 100 times). From the figure we see that Ge The /CNFs exhibited excellent cycle performance with a capacity retention of 87.2% over 100 cycles.
The following figure d shows the rate performance test results of different materials. From the figure, we can see that when the current density is increased to 200, 1000, 2000 and 3000mA/g, the material capacity can reach 1150, 1050, 920 and 760mAh/g respectively. Shows very excellent rate performance.
The application of Ge-based negative electrodes faces the same problem of large volume expansion as that of Si-based negative electrodes. Dr. Wei Wei achieves uniform nanocomposite of nano-Ge and nano-carbon fibers through electrospinning technology, which can well suppress the volume expansion of Ge negative electrode during lithium insertion. For the destruction of the active material structure, the porous structure of carbon fiber also ensures the diffusion rate of Li+ and improves the rate performance of the material. At the same time, the results can also provide a good reference for solving the negative volume expansion of Si.
