Pouch lithium-ion batteries are one of the primary assembly methods for finished battery products. These batteries consist of a liquid lithium-ion battery encased in a polymer shell, packaged with aluminum-plastic film. Pouch batteries offer advantages including better safety, lightweight, high capacity, low internal resistance, and flexible design.

Figure 1. Pouch Cell Battery

As a derivative testing method of XRD, in-situ XRD characterization can not only meet the phase analysis requirements of crystalline materials for ex-situ XRD but also enable the testing and analysis of the crystal structure of crystalline materials and secondary battery components under conditions such as in-situ high/low temperatures, charge-discharge, and specific atmospheres. The performance of lithium-ion battery electrode materials primarily depends on their composition and structure. Systematically studying the relationship between material composition, structure, and performance, and exploring structural evolution, ion, and charge transfer during charge-discharge processes are crucial for deeply understanding the lithium storage mechanism of electrode materials and optimizing their chemical composition, crystal structure, and morphology.

In-situ X-ray diffraction (In-situ XRD) technology for batteries refers to performing XRD scans during the battery's charge-discharge process. It is primarily used to observe the structural and phase transformations occurring in the electrode materials during cycling, precisely revealing the battery's reaction mechanism. Using a short-wavelength (Mo or Ag) X-ray source and transmission diffraction geometry allows direct measurement of in-situ XRD on pouch cells. The benefit is the simultaneous observation of structural and/or phase changes in both cathode and anode materials during charge-discharge.

Figure 2. In-Situ X-ray Diffraction (In-Situ XRD) Technology

The in-situ battery device is crucial for in-situ XRD testing. For transmission-mode in-situ XRD characterization, the device typically consists of three main parts: a stainless steel battery housing, an X-ray transmission window (made of Kapton film), and the lithium-ion battery materials. The combination of all components ensures tight contact of the battery materials, resistance to electrolyte corrosion, and the seal integrity of the entire assembly. For transmission-mode in-situ XRD characterization, researchers can use pouch cells for testing. The incident X-rays can penetrate the pouch cell, simultaneously acquiring information on the crystal structure evolution of both cathode and anode materials during cycling.

Figure 3. In-Situ Variable Temperature Charge-Discharge Device for XRD Transmission Pouch Cells

Figure 4. Operational Diagram of the In-Situ Variable Temperature Charge-Discharge Device for XRD Pouch Cells (Left - Full View, Right - Internal Detail View)

The following is an example of an in-situ XRD experiment using an NCM811/Graphite pouch cell. The figure shows the experimental setup for the in-situ XRD test on the pouch cell. In this case, the pouch cell is placed in a device with a Kapton film window to enable environmental temperature control. The figure also displays the changes in the crystal structure of the NCM811 material and the graphite material during the charge-discharge cycles. The (003) peak of NCM811 initially shifts to lower angles, indicating a phase transition from H1 to H2 in the cathode material, accompanied by expansion of the cathode material grains. Upon further charging to the upper cutoff voltage, the (003) peak shifts to the right towards higher angles, indicating a transition from H2 to H3, where the cathode material grains contract. As shown in Figure 6, the peak shifts of the graphite material represent the reversible changes between graphite and lithium-carbon intercalation compounds. The example presents two sets of in-situ XRD data during the charge-discharge cycles, showing the reversible changes in the cathode material during lithium de/intercalation. This method can also be applied to study the evolution of the crystal structure of active materials in lithium-ion batteries over long-term cycling.

Figure 5. In-Situ Variable Temperature Charge-Discharge X-Ray Diffraction Patterns of Pouch Cell

Figure 6. In-Situ Variable Temperature Charge-Discharge X-Ray Diffraction Patterns of Pouch Cell