Such structural defects (e.g. stacking faults, twinnings, irregular structural intergrowths) are found in many examples of battery materials. Recent results obtained with the program FAULTS have shown that the presence of these structural faults can have a positive impact on the electrochemical properties of electrode materials.

Understanding the correlation between materials’ microstructure and their electrochemical performance enables to pave new ways to design better electrode materials for Li-ion and Na-ion batteries. At CIC energiGUNE, we count with experts in understanding reaction mechanisms through the use of in situ and operando experiments and, in particular, with a unique expertise in characterizing structural disorder and defects.

Imperfection is a universal feature in crystalline materials

“Crystals are like people: it is the defects in them which tend to make them interesting!” This citation from Prof. Collin Humphreys, physicist specialist of semiconductors (University of Cambridge), also holds true for battery materials. However, the presence of faults in their crystalline structures are often disregarded or oversimplified, for lack of means to properly characterize them.

Crystals are generally defined as solids in which the constituent elements are ordered in a specific pattern that is periodically repeated in the three dimensions of the space. Such a definition may suggest that crystalline materials (including most of the electroactive materials for Li-ion and Na-ion batteries) are perfect arrangements of atoms, but in reality, imperfection is a universal feature in crystalline materials.

Crystalline defects generally have bad reputation. By themselves, the names “defect” and “fault” carry a distinctly negative connotation. As a result, they are regarded a priori as detrimental features to functional materials’ properties and performance, and in turn, considerable efforts can be made to either minimize their concentration or to counterbalance their detrimental impact.

However, defects can indeed generate value! The defects and impurities that cause highly desirable color in gem-quality diamonds are in fact a manifest illustration of the opportunity offered by structural imperfections. When renamed “dopants”, defects start to be more attractive. The entire semiconductor industry is indeed built on methods for preparing Si, Ge or GaAs materials doped with precise amounts of desired impurities, which enable to finely tune their electrical properties.

Therefore, in Materials Science, instead of avoiding defects, the intentional and purposely introduction of defects (with control over type, concentration and location) offers novel opportunities for materials engineering. The material scientist can then use them as a tool for tuning and enhancing properties and even enabling new functionalities.

In the field of battery materials, disorder and structural faults are also omnipresent. The challenge resides in accurately characterize the microstructure of the electroactive compounds, understanding the correlations between the microstructural features of the samples and their performance, and eventually control their location and amount during the materials’ synthesis and/or processing, to turn them into advantage and enhance their properties. This is the job of the Crystal Chemist and CIC energiGUNE counts some internationally recognized experts in this field.

How to characterize structural defects using diffraction techniques

X-ray Diffraction and Neutron Powder diffraction (usually abbreviated as XRD and NPD or ND) are two techniques of choice to characterize crystalline materials. Diffraction patterns include information about the average crystal structure of the phases present in the sample, which can be extracted from the peak positions and their relative intensities. From a more thorough analysis of the diffraction patterns, one can also extract precise information about the microstructure of the materials, in particular about the crystallite size, strains (variations of cell parameters), but also about defects.

The conventional method to extract that information from a powder diffraction pattern consists in performing Rietveld refinements. This type of refinements consists in calculating the diffraction pattern of an average model of a 3D structure and in adjusting the relevant structural parameters to fit the experimental pattern. Most of the software dedicated to such Rietveld refinement (e.g. FullProf Suite, GSAS II, Jana2006, Rietan-PF) include suitable microstructural models to take into account size and strain broadening, point defects or anti-phase boundaries. To learn more about Rietveld refinements, CIC energiGUNE organizes every year a training on structural analysis from powder diffraction data using the FullProf Suite of programs.

However, Rietveld refinements cannot take into account defects that cause the break in the 3D periodicity, and hence this method fails in modelling planar defects, stacking faults and structural intergrowths. Yet, this kind of structural defects appears in many battery materials of interest (e.g. Li-rich and Na-rich layered oxides, β-Ni(OH)₂, EMD-MnO₂, graphite and soft carbons), and their careful characterization is essential for a comprehensive understanding, control and optimization of electrode materials’ properties.

Working with such non-periodic structures implies forgetting about the conventional way of thinking structural models, that is leaving the crystallographic concepts of unit cell and space group out. Instead, the structure has to be described as building blocks or layers that are stacked on top of other, in the same way as building a LEGO®. Describing structures in such way provides a great versatility to introduce stacking faults, crystal twinning or structural intergrowths in a structural model.

Since its release in the early 1990’s, the open software DiFFAX, which enables to simulate the diffracted intensities (XRD, NPD and electron diffraction patterns) from crystals containing planar faults, has been used by many groups to carry out qualitative studies of such defective battery materials. Yet, quantitative analyses were desirable.

This is now possible thanks to the program FAULTS. This program is developed through a collaboration between the Institut Laue Langevin (Dr. Juan Rodríguez Carvajal) and CIC energiGUNE (Dr. Montse Casas-Cabanas and Dr. Marine Reynaud) and is distributed since 2015 within the FullProf Suite or as an individual program on CIC energiGUNE’s website. In addition to the simulation of X-ray and Neutron powder diffraction and selected area electron diffraction (SAED) patterns of structures with extended defects, the program FAULTS enables to perform real refinements of the XRD and NPD data of these defective structures. Then, thanks to this full pattern treatment, the FAULTS software makes possible to decouple the size and faults contributions to the peak broadening in powder diffraction patterns and to accurately quantify the amount of planar defects in a sample. These specific capabilities have already contributed to numerous works on battery materials and on materials for others applications.

Understanding the role of defects in battery materials

In a recent work, combined with operando XRD experiments, we have revealed that the structural faults observed in the Na-rich layered material Na₂RuO₃ reorganize towards a defect-free structure during charge and reversibly reappear during discharge. The FAULTS analyses of synchrotron data enabled to precisely quantify the amount of stacking faults within the material at different states-of-charge, and showed that this structural self-ordering prevents the degradation of the electrode material upon extended cycling. The results of this international collaboration, published in the journal Nature Communications, pave a new way to design new electrode materials for lithium and sodium batteries.

Note/acknowledgements: Related research works developed at CIC energiGUNE have received funding from the Spanish Ministerio de Economia y Competitividad and Ministerio de Ciencia e Innovación y Universidades through the projects ION-STORE (ref. ENE2013-44330-R) and ION-SELF (ref. PID2019-106519RB-I00), the postdoctoral fellowship Juan de la Cierva-Formación (ref. FJCI-2014-19990), and the mobility grant José Castillejos (CAS15/00354).