With the rapid development of modern electronic and electrical materials towards miniaturization and integration, the requirements for energy storage materials are compact and light. Thick electrode materials with high area capacity have excellent prospects in improving the energy density of batteries. However, the development of thick electrode materials is limited by factors such as poor electrochemical performance, poor mechanical properties, and complicated manufacturing processes. Therefore, a method for preparing thick electrodes with low cost, simple process and continuous production is urgently needed to be developed. Recently, the Fu Kun team and collaborators of the University of Delaware have ingeniously developed a vertically oriented thick positive electrode (FAT) with high capacity, low tortuosity, high electrical conductivity, high thermal conductivity, and excellent mechanical properties. The performance provides an effective channel for high-speed electron/ion transmission. The volume energy density and mass energy density of the assembled battery reached 431.2 Wh/L and 164.8 Wh/kg, respectively, far superior to the battery prepared by the traditional coating method. The research was published in "Nano letters" with the title "Low Tortuous, Highly Conductive, and High-Areal-Capacity Battery Electrodes Enabled by Through-thickness Aligned Carbon Fiber Framework".
The prepared FAT electrode has a highly oriented structure, which can provide an effective channel for the transmission of electrons and Li ions. Their preparation method is as follows: coating the positive electrode slurry on the oriented carbon fiber film, and then curling it into a column shape and cutting it perpendicular to the length direction, a FAT electrode with a thickness of about 1 mm can be obtained. The method is simple in process, low in cost, and applicable to various types of positive electrode materials, and the winding-cutting method is very suitable for large-scale production. Compared with the electrode prepared by direct coating method, FAT electrode has better compressive strength and compressive modulus.
Microstructure of FAT electrode
Subsequently, the author used X-ray three-dimensional imaging and scanning electron microscopy to analyze the microstructure of the FAT electrode, as shown in Figure 2. It can be seen that the cathode material (lithium iron phosphate, LFP) is uniformly dispersed around the oriented carbon fibers. Compared with the electrode prepared by the direct coating method, the FAT electrode has higher vertical conductivity and better electrolyte wetting, which also indicates that the vertically oriented carbon fibers inside the FAT electrode play a role in improving the conductivity and improving electrolyte wetting. Sex.
Thermal conductivity of FAT electrode
Since the battery will generate heat during the charge and discharge process, especially during the high-rate cycle, if the heat cannot be conducted out in time, the battery may run out of control or even explode. Therefore, the author characterized the thermal conductivity of the FAT electrode, as shown in Figure 3. The thermal conductivity of the FAT electrode in the vertical direction reached 1.12 W/m·K, which is four times that of the traditional electrode, and the improvement of thermal conductivity mainly comes from the highly oriented carbon fiber providing a perfect heat conduction channel.
Battery performance of FAT electrode
Subsequently, the author assembled the FAT electrode into a full battery and tested its electrochemical performance. It can be seen that the peak intensity in the cyclic voltammetry curve of the FAT electrode is higher and the width is narrower than that of the traditional electrode. It shows that the kinetics of electrochemical reaction of FAT is better. At the same time, the transfer resistance of FAT is also significantly lower than traditional electrodes. In addition, under the same current density, the capacity of the FAT electrode is also significantly higher than that of the traditional electrode. For example, at a current density of 1 mA/cm2, the capacity of the FAT electrode is 150 mAh/g, which is 2.3 times that of the traditional electrode (65 mAh/g). In addition, under the same current density, the over-potential of the FAT electrode is also lower than that of the traditional electrode. After 150 cycles of FAT electrode assembled batteries, the copper beam retention rate was 68%, which was significantly better than traditional electrode assembled batteries (45%).
Comparison of battery performance of FAT electrode
In order to further demonstrate the advantages of this work, the author also compared with the results reported in the previous literature. It can be seen that the area capacity, volume capacity and specific capacity of the battery assembled by the FAT electrode are better than the results reported in the previous literature. This is mainly due to the higher proportion of inactive materials (current collectors and separators) in the total battery (the mass and volume accounts for 21.2% and 22.2%, respectively). The proportion of inactive substances in FAT is only 3% and 2.6%. Therefore, the volume energy density and mass energy density of the battery assembled with FAT electrodes reached 431.2 Wh/L and 164.8 Wh/kg, respectively, which was 1376.% and 135% of the conventional battery.
to sum up
The author ingeniously used the winding-cutting method with simple process and large-scale production to prepare the highly oriented carbon fiber frame electrode in vertical direction. The electrode also has the characteristics of high electrical conductivity, high thermal conductivity, excellent mechanical properties, high capacity, high capacity and excellent rate performance, which provides new ideas for the design and preparation of high energy density batteries.