X-ray diffraction (XRD) is a powerful analytical technique for identifying crystalline phases in battery materials, particularly for detecting degradation products that form during aging. As lithium-ion batteries cycle, especially under high-voltage conditions, electrode materials undergo structural changes that lead to the formation of secondary phases such as lithium fluoride (LiF) and transition metal oxides or hydroxides. These degradation products contribute to capacity fade and impedance rise, making their identification critical for understanding failure mechanisms.

One of the primary degradation products observed in aged cathodes is LiF, which forms due to the decomposition of lithium salts in the electrolyte, particularly at elevated voltages. XRD analysis reveals LiF as a crystalline phase with distinct diffraction peaks, typically appearing at 2θ angles around 38.7° and 45.1° (Cu Kα radiation). The intensity of these peaks correlates with the extent of electrolyte decomposition, which accelerates in high-voltage systems operating above 4.3V. Studies on layered oxide cathodes, such as NMC (LiNi_xMn_yCo_zO₂), show that prolonged cycling leads to increased LiF formation, coinciding with capacity loss.

Transition metal dissolution is another critical degradation mechanism detectable via XRD. At high voltages, transition metals (Ni, Mn, Co) leach from the cathode and migrate to the anode, where they deposit as oxides or hydroxides. XRD patterns of aged anodes often show additional peaks corresponding to phases like MnO₂ or Ni(OH)₂. For instance, in NMC-based cells cycled to 4.5V, Mn dissolution leads to the appearance of Mn₃O₄ peaks in the anode XRD spectrum. This phenomenon is particularly severe in Mn-rich cathodes, where structural instability at high potentials exacerbates metal ion migration.

Phase evolution in cathodes is also a key factor in capacity fade. Layered oxide cathodes experience gradual phase transitions from a well-ordered R-3m structure to disordered spinel (Fd-3m) or rock-salt (Fm-3m) phases. XRD analysis tracks these transitions through peak broadening, shifts in lattice parameters, and the emergence of new reflections. For example, NMC811 cathodes cycled at 4.4V exhibit a progressive shift in the (003) peak toward lower angles, indicating layer spacing expansion due to oxygen loss and cation mixing. Over time, new peaks corresponding to spinel phases appear, confirming structural degradation.

High-voltage degradation case studies highlight the role of XRD in diagnosing failure modes. In one study, LiCoO₂ cells cycled at 4.5V showed a pronounced increase in LiF content after 500 cycles, accompanied by the appearance of Co₃O₄ peaks. The XRD data correlated with a 25% capacity loss, confirming that electrolyte decomposition and cobalt dissolution were primary degradation pathways. Another study on NMC532 cathodes revealed that cycling at 4.6V induced a phase transition to a disordered rock-salt structure, with XRD quantification showing a 30% reduction in the original layered phase after 300 cycles.

XRD also identifies anode-side degradation products, particularly in silicon or graphite-based systems. In silicon anodes, cycling leads to the formation of crystalline Li₁₅Si₄, detectable via XRD, while prolonged aging results in amorphous silicon due to pulverization. Graphite anodes exposed to dissolved transition metals exhibit peaks corresponding to metal fluorides or carbides, which increase charge transfer resistance.

Quantitative XRD analysis, such as Rietveld refinement, enables precise determination of phase fractions in degraded electrodes. For instance, a study on high-nickel NCA cathodes (LiNi₀.₈Co₀.₁₅Al₀.₀₅O₂) revealed that after 1000 cycles at 4.3V, the spinel phase fraction reached 12%, while the original layered phase decreased by 18%. These phase changes directly correlated with a 20% capacity fade, demonstrating the utility of XRD in linking structural evolution to performance loss.

In summary, XRD provides critical insights into the phase transformations and degradation products that accumulate in lithium-ion batteries, particularly under high-voltage operation. By identifying crystalline byproducts such as LiF and transition metal compounds, as well as tracking cathode phase transitions, XRD helps establish clear correlations between structural degradation and capacity fade. Case studies on high-voltage systems underscore its value in diagnosing failure mechanisms and guiding material improvements for enhanced battery longevity.