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electrochemical stabilities of electrode-electrolyte interfaces (although sub-10 meV resolution has recently been demonstrated).29,30 Because very different electron doses are required for imaging hard ceramics and soft matter, hard-soft matter interfaces, which play a significant role in future lithium metal batteries, cannot yet be routinely characterized at the atomic scale. Furthermore, it remains challenging to link local nanoscale features to both the bulk properties and the overall function of a material. Integrated characterization techniques, which measure various key parameters (not only structure and chemistry, but also function), and which cover different length scales simultaneously, will be critical to providing a holistic characterization of how batteries function. Overcoming these challenges will not only require significant advances in instrumentation, such as highly stable control electronics and power supplies, but also the integration of new computing algorithms and data science. Advances in Mechanical Property Measurement: There is a strong connection between the ion concentration and mechanical properties of materials such as material volume, mechanical stresses, and stiffness. This electrochemical-mechanical coupling can follow from a change in the unit cell31,32 or a change in solid-liquid interaction under confinement in pores or between layers of quasi-2D materials.33-36 The solid-liquid interaction will be strongly determined by the ion size, as well as solvation and surface chemistry. In turn, the mechanical properties and dimensional constraints can limit ionic transport, an important but still unexplored area. The electrochemical-mechanical coupling can be considered in multiple ways. The volume change caused by electrochemically induced stoichiometry changes can lead to local stress and mechanical degradation. On the other hand, electro-chemo-mechanical coupling may also provide new opportunities to characterize materials by measuring the stress/strain generated by the stoichiometry change. There is a great opportunity to design new in situ detection and imaging strategies on mesoscopic length scales, which are largely missing, and to implement testing strategies for full battery cells. In the first case, contact resonance scanning probe microscopy can be used to image the ion insertion pathways through changes in Young’s modulus when Li+ and K+ are electrochemically driven into layered materials such as MXene (see Figure 2.3.9 in Chapter 2).37 This approach reveals where the ions cross the solid-liquid interface and where they are incorporated into the electrode. In the latter example, electrochemical-acoustic time-of-flight analysis was used to study changes in elasticity of full commercial cells.38 Approaches like this have the potential to image the state of charge of batteries and track the development of interphase formations during electrochemical operation over a long cycle life. Advances in X-ray Imaging: Much progress has been made over the past five years in using photon-based imaging techniques applied to energy storage materials, as highlighted in recent publications.39,40 These have proven useful both operando and ex situ and include transmission X-ray microscopy, micro-computed tomography, and scanning transmission X-ray microscopy. The two imaging modalities, morphological (including 3D morphology) and chemical using spectro-microscopies, can be used separately or in combination. The latter is especially powerful as this allows mapping phase changes in both primary and secondary particles. Full field transmission X-ray microscopy and micro-computed tomography of most energy storage materials employ hard X-rays, while scanning transmission X-ray microscopy of such materials primarily uses soft X-rays. Soft X-rays are more chemically sensitive and so provide greater fidelity. In contrast, operando and in situ investigations are much easier with transmission X-ray microscopy and tomography using hard X-rays, although there has been recent development of in situ scanning transmission X-ray microscopy with soft X-rays but at some sacrifice in resolution.41 While most imaging is 2D, the 3D imaging that is inherent to micro-computed tomography and achievable in transmission X-ray microscopy can be much more informative in some cases.42 Advances in Use of Model Systems: Over the past decade, researchers have adopted a reductionist approach by using model, but still relevant, systems to obtain a detailed understanding of energy storage phenomena, especially related to interfaces and interphases. Typically, this involves in situ characterization using single crystals such as Si thin films,43 or open cell geometries for nanostructured electrodes as used for transmission electron microscopy (see above). This methodology uses techniques such as surface X-ray and neutron scattering and transmission electron microscopy to provide exquisite details on interface structure and chemistry at length scales less than 1 nm. Due to the simplicity of the experiment, direct comparison to theory is often more straightforward than for more complex, but more realistic (closed) half-cells. Often, highly idealized geometries and in situ cells are used, with the knowledge gained being transferable to real cells. For example, X-ray and neutron reflectivity have provided insight into phase transformations and the formation and “breathing” of the solid-electrolyte interface on silicon.44 NEXT GENERATION ELECTRICAL ENERGY STORAGE PANEL 6 REPORT 149PDF Image | Next Generation Electrical Energy Storage
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