X-ray diffraction (XRD) is a critical analytical technique for characterizing layered oxide cathode materials such as lithium nickel manganese cobalt oxide (NMC) and lithium nickel cobalt aluminum oxide (NCA). These materials are widely used in lithium-ion batteries due to their high energy density and structural stability. XRD provides detailed insights into crystallographic parameters, phase purity, and structural defects, which directly influence electrochemical performance. This article explores the application of XRD in analyzing layered oxide cathodes, focusing on peak indexing, Rietveld refinement, and the detection of cation mixing or structural defects. The correlation between XRD-derived structural parameters and electrochemical behavior is also discussed.

Layered oxide cathodes typically adopt a hexagonal crystal structure with an R-3m space group, where lithium and transition metal (TM) ions occupy octahedral sites in alternating layers. The XRD pattern of these materials exhibits characteristic peaks corresponding to the (003), (101), (104), (015), (107), (108), and (110) planes. The (003) peak is particularly sensitive to the interlayer spacing, while the (104) and (110) peaks provide information about cation ordering and TM layer integrity. Peak indexing involves assigning Miller indices to each diffraction peak based on the expected crystal structure. Accurate indexing is essential for identifying secondary phases or structural deviations, such as the presence of rock-salt impurities or spinel-like domains.

Rietveld refinement is a powerful method for quantifying structural parameters from XRD data. This technique fits a theoretical diffraction pattern to the experimental data by adjusting variables such as lattice constants, atomic positions, and site occupancies. For layered oxide cathodes, Rietveld refinement can determine the lattice parameters a and c, which reflect the in-plane and out-of-plane dimensions of the unit cell. The c/a ratio is a key indicator of structural stability, with higher values suggesting better lithium-ion diffusion pathways. Additionally, refinement can reveal the degree of lithium/Nickel mixing, where nickel ions migrate into lithium layers, disrupting the ideal layered structure. Cation mixing is quantified by refining the occupancy of lithium and nickel sites, with values above 5% often leading to reduced capacity and rate capability.

Cation mixing is a common defect in layered oxide cathodes, particularly in high-nickel compositions like NMC811 or NCA. XRD can detect this phenomenon through changes in peak intensities and positions. For example, the intensity ratio of the (003) and (104) peaks (I003/I104) is a widely used metric for assessing cation disorder. A lower I003/I104 ratio indicates higher nickel occupancy in the lithium layer, which hinders lithium-ion mobility. Similarly, the splitting of the (006)/(012) and (018)/(110) doublets is sensitive to structural distortions caused by cation mixing. Advanced refinement techniques, such as the use of anisotropic displacement parameters, can further improve the accuracy of cation mixing quantification.

Structural defects, such as stacking faults or oxygen vacancies, can also be identified through XRD analysis. Stacking faults manifest as peak broadening or asymmetry, particularly in the (003) and (104) reflections. Williamson-Hall analysis or whole-pattern fitting methods can separate the contributions of crystallite size and microstrain to peak broadening, providing insights into defect density. Oxygen vacancies, which are common in high-voltage cycling, may cause shifts in peak positions due to changes in the metal-oxygen bond lengths. Pair distribution function (PDF) analysis of high-energy XRD data can offer additional resolution for detecting local structural deviations that are not visible in conventional XRD patterns.

The electrochemical performance of layered oxide cathodes is closely tied to their structural characteristics, as revealed by XRD. For instance, materials with low cation mixing exhibit higher reversible capacity and better cycling stability due to unimpeded lithium-ion diffusion. The lattice parameter c, which reflects interlayer spacing, correlates with rate performance; larger c values facilitate faster lithium-ion transport. Conversely, excessive cation disorder or defect density leads to increased polarization and capacity fade. In high-nickel cathodes, XRD can predict the onset of phase transitions during cycling, such as the formation of inactive rock-salt phases, which degrade performance.

XRD analysis also plays a crucial role in optimizing synthesis conditions for layered oxide cathodes. For example, high-temperature calcination can reduce cation mixing but may also promote oxygen loss, as detected by changes in lattice parameters. Post-synthesis treatments, such as coatings or doping, can be evaluated using XRD to confirm the preservation of the layered structure and the absence of secondary phases. In-situ XRD studies during battery operation provide real-time insights into structural evolution, such as phase transitions or lattice expansion during lithium extraction.

In summary, XRD is an indispensable tool for characterizing layered oxide cathode materials. Through peak indexing and Rietveld refinement, it provides quantitative data on lattice parameters, cation mixing, and defect density. These structural parameters are directly linked to electrochemical performance, guiding the development of high-performance cathodes. By correlating XRD findings with battery metrics, researchers can design materials with enhanced stability, capacity, and rate capability. The continued advancement of XRD techniques, including in-situ and high-resolution methods, will further deepen the understanding of structure-property relationships in layered oxide cathodes.