Small Crystals, Big Insights: How Electron Diffraction is Transforming Materials, Life Science, and Chemistry Research

Battery charging profiles, which plot voltage against specific capacity, reveal insights into the electrochemical processes occurring within the battery. A "solid solution" type charging profile, typically seen in lithium-ion batteries like NMC (nickel manganese cobalt) cathodes, appears relatively smooth and continuously increasing. This indicates that ions (e.g., lithium) are randomly removed from the material, largely staying within the same crystal phase, though lattice parameters may change, causing strains.

In contrast, "phase transition" or "steppy" profiles, common in sodium-ion batteries (e.g., sodium cobalt oxide), show distinct steps and plateaus. Each step represents a single stable phase, and the plateaus between them indicate a two-phase reaction. These transitions occur because the larger sodium ions have a more significant impact on the cathode structure when removed, causing the material to jump between different stable phases. While thermodynamically interesting, undergoing multiple phase transitions during thousands of charge/discharge cycles can be a major cause of degradation, leading to hysteresis, cracking, volume changes, and the formation of "dead areas," thus negatively affecting the battery's long-term reversibility and efficiency. Understanding these phase transitions is critical for designing more durable batteries.

MicroED (Micro-Electron Diffraction) addresses the challenges of structural characterization by enabling the collection of full 3D single-crystal diffraction data from micro- or nanocrystalline battery materials, which are the realistic forms found in batteries. Unlike traditional single-crystal X-ray diffraction that requires large single crystals, MicroED can be performed on tiny portions of individual crystallites (typically 200-300 nanometers thick) within a powder sample, as long as they are electron-transparent.

Its main advantages include:

• Single-crystal data from realistic samples: It allows for the study of the actual materials used in batteries, which are typically very small crystallites.
• Fast collection times: Data collection takes only a few minutes (2-5 minutes per sample) compared to hours for other techniques. This speed allows researchers to analyze multiple particles in one session and quickly survey the diversity of phases present in a sample.
• Targeted analysis: Using a TEM, researchers can precisely target and analyze individual micro- or nanophases, including side products or distinct stacking sequences, that would be difficult to isolate otherwise.
• High resolution: It can achieve resolutions down to 0.3-0.5 angstroms, providing detailed structural information.

Several practical considerations and challenges arise when performing MicroED on battery materials:

• Sample preparation: Battery materials, especially those from cycled electrodes, require careful preparation. This involves grinding the powder finely, placing it on a TEM grid, and often manipulating it in an air-free, inert environment (like a glovebox) to prevent degradation from air exposure, especially for delithiated or desodiated samples. Cryostages can be used for sample loading under liquid nitrogen to maintain an inert and cold environment.
• Incomplete Ewald sphere: The data collected in MicroED often results in an incomplete Ewald sphere in reciprocal space, appearing as a "wedge." This is due to limitations in the sample's rotation range and can lead to ambiguity in space group determination.
• Dynamical diffraction: Electrons interact more strongly with atoms than X-rays, leading to dynamical diffraction effects (e.g., double diffraction). This can cause intensity in positions that should be systematically absent based on the space group, making structure solution more complex. While often manageable by refining an extinction coefficient, it's a significant difference from X-ray crystallography.
• Electron scattering factors: The scattering factors for electrons are less straightforward than X-ray scattering factors and can vary significantly between neutral atoms and ions, which is crucial for battery materials where ion states change. Using tabulated factors for ions is recommended.
• Verification: Because MicroED analyzes individual tiny particles, it's essential to verify that the derived structure is representative of the bulk material. This is typically done by comparing the MicroED-derived structure with bulk powder diffraction data, ensuring consistency even if the powder data alone couldn't resolve all the subtle features.
• R-values: R-values (a measure of fit quality) from MicroED refinements are typically higher than those in X-ray crystallography (e.g., 15% is considered good, whereas 10% is excellent), reflecting the complexities of electron diffraction and the approximate handling of dynamical effects.

The detailed structural information obtained from MicroED is crucial for understanding and improving battery performance. By solving the 3D crystal structures of battery materials at different states of charge, researchers can:

• Identify phase transitions: Precisely determine the structures of various phases that occur during charging and discharging, especially the subtle "sodium vacancy orderings" or stacking sequence transitions in sodium-ion batteries, which are often the root cause of degradation.
• Inform simulations: Provide fundamental structural data (e.g., atomic positions, lattice parameters) that can serve as the basis for various computational simulations, such as density functional theory (DFT) calculations, to predict material behavior and optimize designs.
• Map ion conduction pathways: Analyze the crystal structure to identify pathways through which ions (e.g., sodium) can move. Techniques like bond valence sum mapping can highlight areas suitable for ion occupation and connectivity, providing insights into ion mobility and overall battery performance.
• Rationalize material design: With a clear understanding of structure-property relationships, researchers can design new battery materials with improved characteristics, such as higher energy density, longer lifetime, better safety, and lower cost, by avoiding detrimental phase transitions or optimizing ion transport.