54 55 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 2 / 2 3 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 2 / 2 3 Local cation ordering in ‘disordered’ Li-ion cathodes Energy Storage - Energy - Physical Chemistry - Energy Materials - Chemistry - Materials Science The modern world relies on high-performance lithium-ion (Li-ion) batteries to power mobile devices and electric-powered vehicles, and for the storage systems needed to ensure continuous supplies of low-carbon energy. Demand for these batteries is increasing, but current cathode materials limit the energy density and dominate the cost. In recent years, there has been a surge of interest in lithium- rich cathodes with a cation Disordered Rock Salt (DRS) structure. DRS cathodes are relatively low cost, and their high first charge capacities offer tantalising promise for high-energy-density Li-ionbatterieswith capacities vs Li/Li + beyond 300 mAh g -1 . However, DRS cathodes suffer from a large irreversible capacity loss on the first charge-discharge cycle. The charge compensation mechanism responsible for the large first charge capacity and the origin of the capacity loss in DRS are not well understood. Understanding these phenomena requires operando investigations of structural and charge effects. Researchers therefore used operando techniques to investigate the evolution of the average structure, short-range ordering and charge during the electrochemical cycling of a DRS, using both spectroscopic and structural probes. To understand the structural evolution of nanostructured DRS cathodes operando , they acquired pair distribution function (PDF) data at Diamond’s I15-1 beamline using Diamond Radial In Situ X-ray ( DRIX) electrochemical cells. Performing operando HighEnergyResolution Fluorescence Detected (HERFD-)XANES measurements at the I20 beamline provided a better understanding of the charge compensation mechanism. The combined use of advanced PDF refinement methods and Bond Valence Sum (BVS) mismatch mapping revealed the local cation ordering of Li-ions with battery cycling can perturb the percolating Li-diffusion network in DRS. Total scattering and X-ray Emission Spectroscopy (XES) show that the cation and lithium vacancies in the layered domain that form during cycling become less accessible in subsequent charge cycles. The trapping of Li-ions in short-range- ordered domains could be associated with the capacity fade of DRS and could be a significant source of capacity fade alongside contributions from oxygen redox irreversibility. The key to reducing capacity losses of DRS could be in preventing the formation of layered domains during cycling or controlling their size. Potentially, this could be achieved via nanostructuration or by introducing electrochemically inactive dopants. Future research should investigate the formation of short- range ordering in DRS cathodes, whether it forms during ball milling used to prepare the composite or through evolution of these domains with electrochemical cycling. Although the growth of layered domains can act as a trap for lithium, layered cathode materials have an inherently high capacity and rate capability. Further investigation into the interplay between the layered and DRS sublattices is needed, and the impact of short-range ordering should be optimised. This work provides insight into the design of better DRS cathodes and highlights the importance of local structures in the cyclability of battery materials. Furthermore, successful control of the coexistence of layered and DRS sublattices offers a novel route to electrode design, opening a new path to developing high-performance cathode materials. This research is an example of how multimodal, operando experiments across complementary techniques including HERFD-XANES, XES (Kβ main line, V2C), and X-ray Total Scattering (Bragg, XPDF), can aid the complete understanding of complex electrochemical processes in new battery materials. Related publication: Diaz-Lopez, M. et al. Li trapping in nanolayers of cation ‘disordered’ rock salt cathodes. Journal of Materials Chemistry A 10 , 17415-17423 (2022). DOI: 10.1039/D2TA04262B Corresponding authors: Maria Diaz-Lopez, Institute Néel, CNRS,
[email protected] Local structure and Li diffusion of LMTO DRS from PDF refinements with electrostatic potential constraints. Left: Magnified view of the short r-range of the PDF refinements. Right: refined average structural models (top) and supercells of atomistic disorder (bottom). Green, purple, blue and red spheres denote Li, Mn, Ti and O, respectively. The yellow isosurfaces represent the regions with an energy threshold suitable for Li-diffusion. This figure is taken from our published work (https://doi.org/10.1039/d2ta04262b) . Crystallography Group Beamline I15-1 Electronmovement between components ofmechanically interlockedmolecules Physical Chemistry - Chemistry Controlling photoinduced charge transfer within molecules is a significant challenge. An international team of researchers previously demonstrated that a macrocycle (wheel-shaped molecule) could reversibly lose an electron. They wanted to see if threading an electron-accepting molecule through this macrocycle could make a mechanically interlocked system, known as a rotaxane, where the electron could be stimulated to hop from the wheel to the thread. Theymade two rotaxanes with different numbers, types and positions of macrocycles to compare electron movement in these different systems. In order to track the location of electrons before and after excitation, they needed to investigate the optical properties of the rotaxanes in different oxidation states and then perform time-resolved spectroscopic studies after photoexcitation of the molecules to determine the pathways of electron transfer in the molecule. To appreciate all these factors, it was vital to prove the orientation and arrangement of the complex structure of the threaded molecule. Although the structure could be evidenced by NMR spectroscopy, obtaining a crystal structure provided a more elegant and accurate determination of the molecular arrangement. They obtained the single crystal structure of the most complex compound, a hetero[4]rotaxane, using data from Diamond’s I19 beamline. A [4]rotaxane is a molecular thread passing through three wheel-shaped molecules. Crystal structures of such intricate molecular systems are incredibly rare, so it was valuable for this work to obtain this structure that showed the position and arrangement of all the interlocked pieces. Crystals of this compound are both small and very weakly diffracting. Much of the team’s research focuses on crystal engineering – fine-tuning properties and positions of active groups within a crystal. For this, good- quality crystallography data is essential. Diamond, particularly beamline I19, can provide high-energy X-rays for crystallography studies, allowing scientists to investigate crystals smaller than those they can study in their labs. This is particularly pertinent for challenging molecules, such as mechanically interlocked molecules, which often form small or weakly diffracting crystals. The intensity of the X-ray source at Diamond enables the collection of more accurate information about the arrangement of atoms within the target molecules. In this study, combining the data from the synchrotron X-ray source with electron diffraction studies allowed the complete determination of the structure of this fascinating molecule. The movement of the electron can be stimulated with photons (laser light), which causes an electron to rapidly hop from the wheel to the thread. This event occurs at different rates in the different systems, which led the research team to conclude that a distinct wheel molecule was supplying the electron in each system. Understanding how to control the movement of electrons in complex molecular systems is useful in the construction of organic electronic devices, such as OLEDs and solar cells. This study demonstrates the use of a relatively compact electron-donatingmolecule that can easily be incorporated intomore complex molecules. Exploring a mechanically interlocked system, specifically a rotaxane, demonstrates how we might control the movement of electrons in a molecular machine. Related publication: Pearce, N. et al. Selective photoinduced charge separation in perylenediimide-pillar[5]arene rotaxanes. Nature Communications 13 , 415 (2022). DOI: 10.1038/s41467-022-28022-3 Funding acknowledgement: EPSRC (EP/S002995/1) Corresponding authors: Neil R. Champness, University of Birmingham,
[email protected] X-ray crystal structure of a [4]rotaxane obtained using synchrotron radiation at Diamond. (Top) coloured by atom type, red=oxygen, blue=nitrogen, grey=carbon; (bottom) coloured to show the distinct molecular components, red = thread, purple =wheels 1&3, blue =wheel 2. Crystallography Group Beamline I19