Comparative studies between in situ XRD and operando XRD

Advantages of In situ XRD over Non-In situ XRD:

1. In situ XRD provides real-time structural information during material reaction processes, allowing for a deeper understanding of the reactions occurring during charge and discharge processes. This is of great significance for improving materials. Non-in situ XRD testing often fails to accurately reflect the true conditions due to the influence of processes such as battery disassembly (due to the presence of polarization, a simple battery standby can generate a certain voltage drop, which can be more severe under high current), electrode washing, etc. Additionally, if the tested material is sensitive to air, it must be tested in a device isolated from air to reflect the true state of the material.

2. In situ XRD testing can obtain a large amount of comparable information in a short period. Because the entire process of in situ testing involves testing the same material at the same position, the obtained information (whether it is lattice parameters, peak intensity, or other parameters) is relatively comparable. In contrast, the comparability of information obtained from non-in situ XRD is poor, and there are higher requirements for operations during the testing process. For example, if several electrodes are disassembled and washed, and the electrode is wrinkled, the material test will produce variations in intensity, resulting in peak shifts in the XRD and corresponding changes in the refined lattice parameters. Additionally, different electrode active material qualities and distributions inevitably lead to poor comparability of peak intensities under different charge and discharge states.

Modes of In situ XRD:

Transmission Mode vs Reflection Mode

The fundamental difference between these two modes is based on the X-ray diffraction light source provided. Since the power of X-ray diffraction sources provided by laboratory X-ray diffractometers is generally low, only the reflection mode can be adopted. However, if the light source generated by a synchrotron accelerator has higher energy, the transmission mode can be used. Of course, different light sources also determine the differences and difficulties in the design of in situ XRD for electrolysis cells.

1. Transmission Mode

In transmission mode, X-rays enter from one end of the electrolysis cell, diffracted X-rays exit from the other end of the electrolysis cell, and the detector receives signals to obtain data. However, due to the special requirements for X-ray source intensity, currently, only research groups or laboratories with advantages in synchrotron accelerator-generated light sources have the prerequisites and advantages to develop such technology.

Figure 1:J-M. Tarascon published a paper using transmission mode in situ XRD to study the structural changes of LiCoO2 during charge and discharge processes.

2. Reflection Mode

In reflection mode, X-rays enter the electrolysis cell through a window, pass through the window to reach the material, and diffracted X-rays exit from the same window. The detector receives signals to obtain data. The earliest in situ XRD electrolysis cells in the world were designed using the reflection mode.

Figure 2: Studying the charge and discharging processes of Li/TiS2.

With the development of in situ techniques, many research groups are now attempting to use pouch-type batteries for reflection-mode in situ XRD testing.

Figure 3: K. Zaghib utilized pouch-type batteries to investigate the structural changes of spinel lithium nickel manganese oxide before and after chromium doping during the charge and discharge processes.

Currently, research groups that focus on in situ XRD testing (for positive electrode materials, especially high-voltage positive electrode materials, achieving almost identical in situ charge-discharge curves and actual charge-discharge curves is still very rare. Below are just a few examples, and feedback and additions from experts are welcome!)

• In situ XRD pioneer B. M. L. Rao, Exxon Research and Engineering Company, Linden, New Jersey, first published in situ XRD testing on battery material charge and discharge in 1978.

• Jeff Dahn, Department of Physics and Department of Chemistry, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada, conducted the earliest in situ characterization of LixTiS2 in 1981. In 2002, he published an article titled "Understanding the Anomalous Capacity of Li/LiyNixLi(1/3-2x/3)Mn(2/3-2x/3)O2 Cells Using In Situ X-Ray Diffraction and Electrochemical Studies" in the Journal of the Electrochemical Society. This was the earliest article to use in situ XRD to study lithium-rich materials and obtained relevant patents.

• Claus Daniel, Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6083, USA; Energy and Transportation Science Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6083, USA; Bredesen Center for Interdisciplinary Research and Graduate Education, University of Tennessee, Knoxville, TN 37996, USA. In 2013, he published an article titled "Structural transformation of a lithium-rich Li1.2Co0.1Mn0.55Ni0.15O2 cathode during high voltage cycling resolved by in situ X-ray diffraction," achieving in situ XRD studies of high-voltage lithium-rich positive electrode materials under normal charge and discharge conditions for different cycle numbers, illustrating the possible causes of voltage decay in lithium-rich batteries.

• J-M. Tarascon. Almost all articles published in recent and past issues of Nature Materials have used in situ XRD techniques to characterize changes in material structure.

In addition to in situ XRD during charge and discharge processes, there is another type of in situ technique called high-temperature in situ XRD. This in situ XRD technology can be used to observe the structural changes of materials during the synthesis process, allowing for an understanding of the material synthesis conditions. It can also be used to detect the corresponding structural changes of materials as the temperature changes during charge and discharge to a certain potential. This is an important means for investigating one of the reasons for the safety issues of actual batteries, namely, the safety problems caused by structural changes in materials.