Preface

X-ray diffraction (XRD) is an important tool for studying the crystal structure and properties of electrode materials. In addition, it can also be used to study the principles of chemical reactions. In electrochemical systems, XRD is used to study novel electrode materials for rechargeable lithium-ion batteries. During the charging and discharging process, in-situ XRD testing is performed on the material, and the changes in diffraction peak positions can be used to infer the electrochemical reaction mechanism.

X-ray diffractometer qualitatively analyzes the crystal type, crystal defects, and content of different structural phases of materials based on the diffraction phenomenon of X-rays in the sample, using the position and intensity of diffraction peaks. During the charging and discharging process of the battery, the crystal type and cell parameters of the electrode material will change. In order to determine the specific changes that occur in the electrode material during the battery charging and discharging process, we can use in-situ XRD to observe the electrode material in real time, thereby inferring the intermediate products generated during the electrochemical reaction process. Through these intermediate products, the reaction mechanism can be revealed.

Figure 1. (a) Schematic diagram of in-situ battery design; (b) Schematic diagram of in-situ XRD patterns of four typical battery reaction mechanisms:

Single phase reaction, phase transition reaction, alloying reaction, transformation reaction. [1]M. T. Xia, T. T. Liu, N. Peng, R. T. Zheng, X. Cheng, Small Methods 2019, 1900119, 3.

Advantages of in-situ XRD

Compared to non in situ XRD, the advantages of in situ XRD are as follows:

1. In situ XRD provides real-time structural change information during the material reaction process, which can deepen the understanding of the reactions that occur in the material during charge and discharge, and has great guiding significance for how to improve the material.

Non in situ XRD testing often cannot accurately reproduce the true condition. Additionally, if the material being tested is sensitive to air, it must be tested in an isolated device to reflect its true state.

2. In situ XRD testing can obtain a large amount of comparable information in a short period of time. Since the entire process of in-situ testing is testing the same material at the same location, the information obtained (whether it is unit cell parameters, peak intensity, or other parameters) is relatively comparable.

3. The information obtained from non in situ XRD has relatively poor comparability and requires high operational requirements during the testing process. For example, if the electrode is disassembled and washed, and the electrode is in a wrinkled state, it will cause changes in the height of the material testing surface, resulting in a shift in the peak of the measured XRD and corresponding changes in the refined unit cell parameters; However, the quality and distribution of active materials in different electrode plates are inevitably different, which inevitably leads to poor comparability of peak intensities under different charge and discharge states.

Application Cases

(1) Sample/Preparation

This experiment used the FRINGE desktop X-ray diffractometer from Suzhou Langsheng Scientific Instrument Co., Ltd. to perform in-situ XRD testing on the assembled electrode materials provided by a research group at a certain school.

Figure 2a~c) In situ battery assembly diagram; d) Installation diagram for in-situ battery testing

Figure 4. Field case of in-situ XRD testing using FRINGE

(2) Testing and Analysis

A certain research group is studying graphite interlayer compounds as cathode materials for lithium-ion batteries, and in situ XRD is needed to reveal the electrochemical reaction mechanism.

For graphite interlayer compounds, their structural information can be determined through X-ray diffraction experiments. As shown in Figure 5, the interlayer of graphite in the initial graphite is subjected to van der Waals forces, with a spacing d0 of about 0.335nm. As the intercalated species enter the graphite interlayer, the spatial arrangement in the vertical basal plane direction changes. Generally, the ratio of the number of carbon atomic layers to the number of intercalated species layers is defined as the order n of graphite interlayer compounds, which measures the depth of intercalation reactions. On this basis, there exists a quantitative relationship:

Ic=di+(n-1)d0

Figure 5. Schematic diagram of the structure of graphite interlayer compounds

Figure 6. PF6-The first cycle constant current charge discharge curve of anionic electrochemically intercalated graphite positive electrode

Figure 7. Graphite intercalation compound GIC formed by PF6 anion electrochemical intercalation of graphite positive electrode during charging process,

The left and right sides of the initial graphite peak are the (00n+1) and (00n+2) diffraction peaks that form GIC

Figure 8. In situ XRD offset overlay pattern displayed by CrystalX software during the charging process of PF6 anionic electrochemically intercalated graphite positive electrode

Figure 9. First cycle constant current charge discharge curve of BF6 anionic electrochemically intercalated graphite positive electrode

Figure 10. In situ XRD offset overlay spectra displayed by CrystalX software during the charge and discharge process of BF4 anion intercalated graphite positive electrode solvated by sulfolane SL molecule

(3) Results and Discussion

1. The constant current charge discharge test of Li/graphite half electrode is often used for the preliminary electrochemical performance characterization of positive electrode materials, as shown in Figure 6. The first cycle constant current charge discharge curve of PF6 anionic electrochemically intercalated graphite positive electrode has a charge specific capacity of 143mAh/g and a discharge specific capacity of 92mAh/g. In addition, the charge discharge curve shows multiple platforms, indicating that PF6-The process of entering the graphite interlayer is divided into "stages".

2. By conducting X-ray diffraction tests on graphite electrodes, we can obtain the intensity and position of several (001) diffraction peaks of graphite interlayer compounds.

PF6 during charging process-The anionic electrochemical intercalation graphite positive electrode forms a graphite intercalation compound GIC as shown in Figure 7. The (00n+1) and (00n+2) diffraction peaks that form GIC are located on the left and right sides of the initial graphite peak.

During the charge and discharge process, the tetrafluoroborate BF4 anion intercalated graphite positive electrode solvated by sulfolane SL molecules forms a graphite intercalation compound GIC as shown in Figure 10. The (00n+1) and (00n+2) diffraction peaks that form GIC are located on the left and right sides of the initial graphite peak. During the discharge process, the (00n+1) and (00n+2) diffraction peaks that form GIC on the left and right sides gradually disappear, and the diffraction peak position returns to the initial graphite position. The diffraction intensity of the (002) crystal plane is greatly reduced.

3. Figures 8 and 10 show the offset superposition pattern function in the CrystalX control analysis software that is compatible with the Suzhou Langsheng FRINGE X-ray diffractometer. This feature can help users compare multiple historical data, especially for graphs that have undergone phase transitions, which can intuitively reflect the differences in the graphs and be clear and concise.