Scientists at Lund University in Sweden and University College London in the United Kingdom used 4D computer-assisted X-ray tomography (XCT) imaging to visualize the first lithification of silicon-based electrodes. The volume of the silicon-based electrode will change dramatically during the lithium process, even more than 300%.

Lithium-ion battery is a kind of high-capacity, long-life, environment-friendly battery, which has many advantages and is widely used in energy storage, electric vehicles, portable electronic products and other fields. With the development of society, various application fields, especially the development of electric vehicles, have put forward higher requirements for the specific energy, life, safety and price of lithium ion batteries. Therefore, a deeper understanding of the highly complex electrochemical transport mechanisms in batteries is necessary. The microstructures of anode and cathode plates are closely related to the electrochemical performance of batteries. Many scientists devote themselves to studying electrode materials and charging and discharging mechanism. If we can visualize the evolution of the microstructure during charge and discharge, we can better understand the mechanism of the battery, which can provide a useful basis for battery design optimization and even the development of the next generation battery.

Computer-aided X-ray tomography (XCT) imaging is a high-resolution, non-destructive, non-destructive imaging technique that allows qualitative and quantitative analysis of the structure and properties of materials. XCT has been shown to visualize the microstructural evolution of various components of a battery at multiple scales and to be an effective tool for diagnosing the mechanism of battery failure. XCT is also used to study the microstructure of electrode materials for lithium-ion batteries. In addition, continuous three-dimensional images can be used for 4D(3D+ time) analysis, including possible in-situ and online detection (such as XCT imaging in electrochemical tests).

Scientists at Lund University in Sweden and University College London in the United Kingdom used 4D computer-assisted X-ray tomography (XCT) imaging to visualize the first lithification of silicon-based electrodes. The volume of the silicon-based electrode will change dramatically during the lithium process, even more than 300%. This will lead to significant mechanical deformation of the battery components, or even failure. The authors hope to visualize the lithium process and understand the mechanism of volume change.

The battery assembly

The silicon electrode is assembled into a Swagelok type half battery with lithium metal. The thin shell of the battery is made of X-ray penetrable PFA plastic. Micron Si powder: conductive graphite: PVDF= 80:10:10 (weight ratio). The borosilicate glass fiber is used as the diaphragm.

Electrochemical and XCT tests

The theoretical capacity of the assembled cell is about 7.45mAh, and a potentiostat is used to continuously drain the battery. The XCT was tested using the XRadia Versa MicroxCT-520 sectional imager. The battery was discharged at A constant current of 25μA for A certain time. After each discharge, the XCT imaging was performed. The battery was discharged for 10 times in this process. In the first step, the discharge lasted for 10 hours, and 3.36% of the electrode was lithified. Subsequent steps lasted 20 h for each discharge, and 6.72% of each electrode was lithified. Figure 2 shows the 10 steps of partial lithium on the silicon electrode and the 11 XCT test moments.

The X-ray source and detector were placed 15 mm from the center of the sample, respectively, in front of and behind the sample, using a quadruple eyepiece, and the pixel size of the image was 1.7μm. The light source tube voltage of the scanner is 45kV, the exposure time of each projection is 30s, and 2001 photos are obtained for each scan. The reconstructed volume image is 16-bit grayscale, 2000x2000 pixels.

The analysis of the DVC

The study used Digital Volumetric Correlation (DVC) algorithm to quantify the mechanical deformation of the battery pole and membrane during lithification. DVC technology is to obtain the calculation method of displacement field and strain field in the process of object deformation by analyzing two groups of three-dimensional images with correlation. This method can measure the displacement and strain of the sampling point at any position before and after 3D image deformation.