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REPORT OF THE BASIC RESEARCH NEEDS WORKSHOP to unravel structure-function relationships in both explicit and buried interfaces. Each of these approaches brings obvious drawbacks. Operando investigations, for example, require more intensive, longer-term commitment to instrument development and large-scale collaborative efforts between theory, computation, and experiment for success. Thus, a multifaceted approach is needed, and studies of the same model systems by multiple teams can be very beneficial. Bridging Theory and Experiment to Understand Underlying Physical Phenomena: Equilibrium properties— such as the potential of mean force of an ion moving from a bulk phase (e.g., the electrolyte) through an interphase or interface to another bulk phase (e.g., the electrode)—directly impact the free-energy landscape of ion transport. To this end, methods must be further developed to allow for the accurate determination of concentration profiles in the vicinity of a solid-electrolyte interface. Grand canonical methods using classical force fields between a bulk reservoir of electrolyte and an interfacial region have been previously used.63 The aforementioned methods may suffer from inaccurately representing the response of the electrolyte to a dielectric interface. Extension of grand canonical methods to couple accurate descriptions of electrostatics of the dielectric as described by quantum methods (e.g., quantum density functional theory) to reduced descriptions of the electrolyte solution by using classical liquid theories will be vital to advance understanding of the electrode-electrolyte interface down to the molecular level. The deep understanding of equilibrium properties must also go hand in hand with the study of non-equilibrium properties necessary to understand phenomena at the device level.64 The coupling of electronic structure methods with reduced theoretical frameworks will enable the connection of molecular detail to emergent phenomena. To model a true electrochemical system, it will be necessary to include a driving force such as bias potential. Neither interfacial systems nor interphases present metrics that can be directly compared with new experiments. Therefore, leveraging models to understand distinguishing features of the interface will rely heavily on extending computational models to include simulation of spectroscopic or microscopic observables.65 Operando X-ray Methods: In addition to the progress made with operando Raman spectroscopy, there has been much progress with analytical tools utilizing X-ray radiation. X-ray scattering (e.g., crystal truncation rod and X-ray reflectivity) probes the structure of interfaces, while X-ray spectroscopies such as X-ray photoelectron spectroscopy66 and X-ray absorption spectroscopy67 probe element-specific chemistries and electronic structures of surfaces and interfaces. The use and application of hard X-ray (> 8 keV) tools benefit from a number of favorable characteristics, including their ready penetration through matter (e.g., through millimeters to centimeters of material), their ability to resolve structures with angstrom-scale resolution (wavelengths that are comparable to atomic dimensions), and the development of high-brilliance synchrotron sources that provide opportunity for high dynamic range and real-time studies. The energy tunability of synchrotron sources enables element-specific probes that couple with the excitation of core electrons (e.g., X-ray absorption spectroscopy). The state of the art has demonstrated robust interfacial sensitivity, such as the formation of interfacial hydration layers, adsorbed species, and solid surface reconstructions, especially for well-defined (i.e., flat) interfaces. Soft and tender X-ray (< 2 keV and 2 keV to 8 keV, respectively) spectroscopies and microscopies have made significant advances in the last few years—bringing atomic concentration, chemical structure, and electronic structure insight, from traditional ultra-high-vacuum surface science, to functioning electrochemical interfaces (Figure 2.3.12).67 With the emergence of X-ray transparent windows, differentially pumped analyzers, and tunable synchrotron X-ray facilities, solid/gas,68,69 solid/liquid,65,69,70-74 and solid/solid75-77 interfaces are now at the forefront of characterization. The anticipated availability of coherent hard X-ray sources will enable the extension of the molecular-scale understanding of model interfaces to obtain a robust understanding of materials having complex morphologies. Applied to crystalline grains within a binder, proof-of-principle studies have demonstrated the ability to image the structure changes, in particular, the evolution of lattice strain as a function of state of charge. These capabilities will become widely available with next generation synchrotron facilities (such as the planned upgrades of the Advanced Photon Source and Advanced Light Source, as well as the Linac Coherent Light Source). In addition to the chemical identity of species at the solid-liquid interface, mechanical stress can have a large impact on ion transport. To study these mechanical effects requires development of in situ microscopic techniques, including coherent diffraction imaging. 48 PRIORITY RESEARCH DIRECTION – 3PDF Image | Next Generation Electrical Energy Storage
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