Cold Microscopy Solves Hot Problems in Energy Research

The recent advances made in cryogenic electron microscopy (Cryo-EM) for structural biology applications are deservedly celebrated. The ability to perform structural characterization at resolutions in the single angstrom range, which is now routine, was completely unheard of just ten years ago. Due to this rapid advance, Cryo-EM was chosen as Nature’s Method of the Year in 2015¹ and its developers were subsequently awarded the 2017 Nobel Prize in Chemistry.² Massive investments in scientific infrastructure to offer Cryo-EM in centralized user facilities have followed,³ including the Electron BioImaging Centre (eBIC) in the United Kingdom, the Netherlands Center for Electron Nanoscopy (NeCEN), the Janelia Research Campus of the Howard Hughes Medical Institute, the National Cancer Institute, three national centers overseen by the National Institutes of Health, and Department of Energy centers at Brookhaven National Laboratory and the SLAC National Accelerator Laboratory to name but a few in a growing list. These new investments and new capabilities offer tantalizing opportunities for materials characterization outside of structural biology, but few examples have yet emerged to demonstrate the vast potential.

In a pair of recently published papers, novel research using Cryo-EM methods to solve difficult materials characterization problems show that this may be changing. First, a Stanford University-based team led by materials scientist Yi Cui has revealed, in remarkable detail, the degradation process in hybrid perovskite solar cells (PSC).⁴ PSC’s show tremendous promise for solar energy conversion, but their commercial development has been hindered due to poor chemical and structural stability during use. Since this degradation is driven by nanoscale phenomena, TEM imaging and spectroscopy would be the characterization tool of choice to monitor the controlling factors. However, the materials involved undergo rapid alteration upon exposure to the electron beam. That’s where Cryo-EM comes in. Since modern instruments were developed especially for analyzing materials that exhibit the ultimate in beam sensitivity, they are equipped with high-speed, ultra-sensitive detectors and employ automated data acquisition methods to collect the required information with extremely low levels of electron exposure. Indeed, the Stanford researchers have measured the critical dose threshold for a common PSC material and found it to be just 12 electrons per square Angstrom. By staying below this threshold, the team was able to reveal the true nanoscale structure and chemistry of these materials, and several fascinating observations were made. Perhaps the most interesting and technologically relevant is that the surface of the PSCs were found to undergo structural changes after even short exposures to UV light. These changes have gone undetected by other methods such as XRD or UV-Vis spectroscopy, which are less sensitive to such localized variations. The presence of this unexpected phase on the PSC surface will alter their optoelectronic behavior and may explain the large variation in performance observed between production batches.

Analysis of these materials is also complicated by their sensitivity to air exposure, as they can be chemically altered even during the short time required to insert the specimen into a conventional TEM. By contrast, in Cryo-EM the sample is rapidly frozen from solution and transferred to the microscope without exposure to air. By eliminating this uncontrolled environmental factor, the authors were able to study the effects of moisture on these materials and found that the PSC surface structure is even more sensitive than previously thought as changes to the surface structure were observed almost immediately upon exposure. Again, these changes had gone undetected via optical or X-ray characterization and suggest that environmental barriers to the PSC surface will be very important for preventing degradation. Taken as a whole, these results should drive an entirely new engineering effort for PSC solar cell design and optimization.

In the second paper, a different team also led by Professor Cui has employed Cryo-EM to study the behavior of vitally important solid-state lithium-ion battery materials.⁵ Specifically, the work focuses on the characterization of silicon anodes, which can potentially achieve energy storage capacities that are ten times greater than the more widely used graphite anode. As in the case of the PSC’s already discussed, the use of silicon anodes has been limited due to poorly understood degradation behavior that occurs in service. Silicon undergoes a large volume expansion upon lithium uptake that causes continuous fracture and reformation of the solid electrolyte interface (SEI) layer; a crucial component of the anode that kinetically stabilizes the electrolyte and mediates lithium ion transport. The changes undergone by this nanoscale SEI layer during cycling are the principle source of capacity loss in batteries with silicon anodes. TEM imaging could be used to probe these changes but the SEI layer is highly reactive in air and is very sensitive to the electron beam. By overcoming these limitations through Cryo-EM, the group was able to directly reveal the structure and chemical dynamics of the SEI layer during cycling for the first time. To do this, they fabricated a model battery system by growing silicon nanowires directly on a TEM-compatible support grid and integrating it with a Li-containing organic electrolyte and a Li-metal electrode. The anode was then lithiated or delithiated and then plunge frozen before inserting it in the microscope for examination.

This novel approach allowed the team to lock-in the state of the anode at various points in the lithiation process and reveal the dynamic changes that occur during cycling. They were able to show that the lithiation process proceeds radially from the outside inward resulting in an amorphous Li_xSi shell which surrounded the residual crystalline Si core. By leveraging the increased stability of the structure afforded by the cryogenic sample cooling, electron energy-loss spectroscopy could be used to examine the elemental distribution within. It was found that the amorphous shell actually consisted of two distinct regions: a Li-rich inner shell and an outer shell consisting of Li, C, and O. Upon reaching a fully lithiated state, the formerly amorphous inner shell Li_xSi was found to crystallize to form Li₁₅Si₄, the most stable room temperature phase, while the outer shell remained amorphous. Perhaps most interesting is the behavior observed after delithiation, where the crystallized Li₁₅Si₄ inner shell layer reverted to an amorphous state, demonstrating that the lithiation process is completely reversible. Previously, it was thought that the SEI monotonically grew with increased cycling, but these dynamic Cryo-EM observations show directly that the SEI changes reversibly with the charge state. These repeated changes give rise to the poor stability and rapid failure of the anode. Finally, the authors used the same approach to study the addition of fluorinated ethylene carbonate (FEC) to the electrolyte which is known to dramatically increase the anode lifetime.⁶ The Cryo-EM analysis provides direct evidence that the FEC stabilizes the SEI structure through the formation of a polymerized outer carbonate layer upon lithiation. This layer was retained after delithiation and prevented further changes to the SEI layer. The result was a more robust anode which could be more extensively cycled without a fall-off in performance.

These two reports using state of the art Cryo-EM methods to analyze materials which are unstable under exposure to both the electron beam and the environment offer just two examples of the capabilities now afforded by modern instrumentation. In each case, model nanowire structures were studied in order to lessen the experimental difficulties which were already daunting. Going forward, ideally this type of analysis could be done using more realistic PSC and Li-ion battery architectures and the results could be quantified so they could be fit into a theoretical framework. More generally, materials scientists should endeavor to harness the tools that have resulted from the decades-long work of our colleagues in the biological fields. These capabilities could be instrumental in solving long-standing problems which have required nano- or atomic-scale imaging but where such analysis was not feasible due to the sensitivity of the materials. Polymers are an obvious starting point for this type of work, as they have a strong overlap with biological materials in terms of their composition and sensitivity to electron beam exposure. The ability to carefully control the electron dose imparted to a polymeric specimen and to collect the images and diffraction patterns required with very high speed and sensitivity could help to solve age old problems in polymer science, shine new light on our existing theories, or even open entire new fields of study. True, these experiments will be very challenging, but the work presented in these two papers would have been dismissed as impossible just a few short years ago.