X-ray diffraction (XRD) plays a critical role in the development of sodium-ion (Na-ion) and potassium-ion (K-ion) battery materials by providing essential insights into their crystal structures, phase transitions, and stability mechanisms. Unlike lithium-ion (Li-ion) batteries, Na-ion and K-ion systems face unique challenges due to the larger ionic radii of Na+ (1.02 Å) and K+ (1.38 Å) compared to Li+ (0.76 Å). These differences significantly influence the structural stability, electrochemical performance, and material design strategies for cathodes and anodes. XRD serves as a fundamental tool for characterizing these materials, enabling researchers to identify suitable host structures, monitor phase evolution, and optimize synthesis conditions.

The structural differences between Na-ion/K-ion and Li-ion materials arise primarily from the size mismatch between alkali ions. While layered oxides (e.g., NaxMO2, where M is a transition metal) are common cathode materials for Na-ion batteries, their structures often deviate from the well-known LiCoO2-type arrangement due to Na+’s tendency to occupy prismatic or octahedral sites rather than tetrahedral ones. XRD analysis reveals distinct stacking sequences, such as P2 or O3 phases in Na-ion layered oxides, which directly impact Na+ mobility and cycling stability. Similarly, K-ion cathodes like KxMO2 exhibit even more pronounced structural distortions due to K+’s larger size, leading to challenges in maintaining reversible intercalation. XRD helps identify these structural nuances, guiding the development of stable frameworks.

Phase stability is a major challenge for Na-ion and K-ion materials, particularly during charge-discharge cycles. Unlike Li-ion systems, where phase transitions are often smooth, Na-ion and K-ion electrodes frequently undergo complex multi-phase reactions or irreversible structural collapses. For example, Prussian blue analogs (PBAs), a promising class of K-ion cathode materials, can suffer from phase segregation or vacancy ordering, which XRD detects through peak splitting or disappearance. In anode materials like hard carbon or alloy-based systems (e.g., Sn, Sb), XRD provides real-time monitoring of phase transformations, such as the conversion of crystalline Sn to amorphous NaxSn alloys, which are critical for understanding capacity fade mechanisms.

Emerging cathode systems for Na-ion batteries include polyanionic compounds such as Na3V2(PO4)3 (NVP) and NaFePO4, which offer robust frameworks due to their open 3D structures. XRD confirms the stability of these materials by tracking lattice parameter changes during cycling, ensuring minimal volume expansion. For K-ion batteries, layered transition metal oxides like K0.5MnO2 and polyanionics like KVPO4F are under investigation. XRD reveals how K+ insertion affects the host lattice, particularly in mitigating Jahn-Teller distortions in Mn-based oxides. These insights are vital for designing cathodes with high energy density and long cycle life.

Anode materials for Na-ion and K-ion batteries also benefit from XRD analysis. Hard carbon, a leading Na-ion anode, exhibits a turbostratic structure that XRD characterizes through broad (002) peaks, correlating with Na+ storage capacity. Alloy-based anodes like Sb and P undergo significant volume changes, and XRD helps identify intermediate phases (e.g., Na3Sb or K3P) that form during cycling. Additionally, conversion-type anodes such as metal sulfides (e.g., MoS2) rely on XRD to confirm phase reversibility after repeated sodiation or potassiation. These findings inform strategies to mitigate mechanical degradation.

XRD also aids in exploring novel materials like organic electrodes or dual-ion systems for Na-ion and K-ion batteries. Organic cathodes, such as quinone-based polymers, exhibit amorphous or semi-crystalline structures that XRD analyzes to correlate molecular packing with electrochemical performance. Dual-ion systems, where both cations and anions participate in charge storage, require XRD to verify the coexistence of multiple phases during operation. This is crucial for optimizing electrolyte composition and electrode design.

In summary, XRD is indispensable for advancing Na-ion and K-ion battery technologies by elucidating structural properties, phase stability, and degradation mechanisms. Its ability to provide precise crystallographic data enables researchers to overcome the inherent challenges posed by larger alkali ions, paving the way for next-generation energy storage systems. By focusing on emerging cathode and anode materials, XRD continues to drive innovation in this rapidly evolving field.