X-rays, the radiation used to detect broken bones in our body, have been invaluable in investigating structures of materials at multiple length scales. X-rays generated from a synchrotron light source have advantages of greater intensity, higher focus, and energy tunability over X-rays from a laboratory source. A synchrotron is a giant circular machine with a size close to that of a football field. It works on the principle that energy is emitted when a moving electron changes its direction. Energy is emitted as X-rays when the electrons are accelerated to a very high speed, near the speed of light, in a circular loop inside a synchrotron. Synchrotron X-rays have advanced research and development in several areas including condensed matter physics, structural biology, energy, and environmental science.
Several Department of Energy (DOE) Energy Frontier Research Center’s (EFRCs) take advantage of the unique capabilities of synchrotron techniques to further energy research. X-ray diffraction (XRD) or scattering is the most widely used X-ray-based technique for materials characterization in which the sample is irradiated with X-rays and the intensities of X-rays scattered from the sample are measured to obtain structural information. The Center for Lignocellulose Structure and Formation (CLSF), a DOE EFRC, has used advanced forms of synchrotron based XRD techniques such as Grazing incidence wide angle X-ray scattering (GIWAXS) [1] and resonant soft X-ray scattering (RSoXS) [2], to study plant cell walls, a naturally abundant energy-rich material. GIWAXS is a form of XRD in which X-rays are incident on the sample at a very shallow angle and the X-rays scattered from the sample provide structural information in directions both parallel and perpendicular to the sample plane. Using GIWAXS, researchers at CLSF revealed that crystals of cellulose, a cell wall component, have preferential alignment with respect to the plane of cell wall [1]. Figure 1(a) shows a GIWAXS 2D data of a primary cell wall in which two reflections (cellulose (200) and cellulose (110/1-10)) are seen in the vertical direction. They represent characteristic molecular packing in cellulose, and their presence as arcs indicates preferential alignment of cellulose crystals. This finding contradicts the previously predominant notion of cellulose twisting in cell walls of growing cells, which could have important implications in cell wall synthesis and mechanics. Researchers at CLSF also studied plant cell walls using RSoXS, another specialized XRD in which the intensities of X-rays scattered from a sample are sensitive to the chemical composition. Distances between cellulose fibrils in cell walls were determined based on the enhanced intensities of scattered X-rays originating from the distribution of calcium in the cell walls [2]. Since cellulose is the major load bearing component of cell walls, the detailed structure and organization of cellulose in cell walls obtained from GIWAXS and RSoXS has important implications in plant growth and mechanics. Advanced characterization of plant cell walls using synchrotron radiation will also further the advancement of this energy rich material as a renewable and sustainable energy source.
In the field of energy storage, in-situ and in-operando synchrotron techniques have been particularly critical for revealing how materials transform and degrade within battery systems during charging and discharging. These experiments probe reaction processes and dynamics as they occur in their real operating conditions and thus, provide more precise and reliable data. Additionally, real-time monitoring of reactions at the site of occurrence (in-situ) in these measurements enables the investigation of short-lived intermediate reaction states or species, eliminating the risk of contamination during transfer from ex-situ or off-site measurements. Therefore, in-situ experiments leveraging high intensity synchrotron X-rays enable observation of the structural evolution of pristine battery materials with high spatial and temporal resolution. Several EFRCs have taken advantage of the in situ and in operando capabilities of synchrotron techniques to monitor what happens inside a battery. Understanding the innerworkings of a battery can help us to improve its performance, but the problem is that they are not transparent to X-rays and other radiation. Researchers in the NorthEast Center for Chemical Energy Storage (NECCES) EFRC built an electrochemical cell called AMPIX for in situ and in operando synchrotron X-ray studies during battery charging and discharging [3]. AMPIX has X-ray transparent windows and is compatible with multiple X-ray characterization techniques, thus eliminating cell variability during comparison between complementary techniques.
More recently the next Generation Synthesis center (GENESIS) EFRC has used in situ XRD to probe lithium-ion exchange processes [4]. Elucidating the rate of Li-ion exchange is important for development of high-performance rechargeable lithium batteries. Researchers at GENESIS used in situ XRD to reveal the strong effects of the choice and concentration of salt and its effect on ion exchange rates. XRD probes lattice parameters to identify the molecular arrangement in a compound. Availability of high energy X-rays and advanced detectors at the synchrotrons enable real time monitoring of change of lattice parameters (Figure 1(b)) with high time resolution and good data quality. The synchrotrons also enabled understanding the ion exchange process which was previously not possible due to insufficient time resolution.
Synchrotron X-rays are also used for direct visualization of nanoscale structures at high resolution through X-ray microscopy. Researchers in Center for Mesoscale Transport Properties (m2M/t) used two different X-ray microscopy techniques to investigate the stability of aqueous battery systems [5]. Transmission X-ray microscopy (TXM) was used in operando mode to visualize the stability of electrolyte, a material which promotes the movement of ions during charging and discharging of a battery. The researchers studied a new type of electrolyte called a water-in-salt (WIS) electrolyte, consisting of a high salt concentration. Figure 1(c) shows a stack of TXM images collected in operando. The study found that WIS electrolytes have higher stability as compared to conventional aqueous electrolytes under high voltage. Another X-ray imaging technique called X-ray absorption near edge structure (XANES) microscopy, a chemically sensitive imaging technique, was also used to probe the chemical heterogeneity of the electrolyte based on the distribution of metal species within the sample. Understanding the stability of WIS electrolytes under different voltage conditions will push their development as a safer and more sustainable alternative to current Li-ion batteries.
Developments in synchrotron X-ray optics and instrumentations have enabled the study of structure at the nanoscale, which in turn helps explain the macroscale properties of materials. Consequently, high intensity, well-focused, and tunable synchrotron X-rays have immensely contributed to advances in energy science by helping to understand energy-relevant materials and processes at multiple length scales. Such knowledge of structure–property relationships will help in the design of new materials and improve the performance of existing materials for energy generation, storage, and conversion.
