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REPORT OF THE BASIC RESEARCH NEEDS WORKSHOP experimental measurements are made with electrodes at a constant potential, requiring referencing the Fermi level to an external potential. For electrochemical capacitor applications, this can be achieved with newly developed methods and boundary conditions that combine DFT and implicit solvent-like methods, such as the effective medium approach. When direct comparisons between atomistic simulations and experiments are not possible or are ambiguous, meaningful connections can still be made via continuum modeling, which can capture both the experimental time and length scales. For example, just comparing energy barriers may not be adequate to distinguish different diffusion mechanisms. However, a one-dimensional diffusion model that builds upon the mechanisms, mimics the testing conditions, and takes key input from DFT simulations can be constructed to predict measurable concentration profiles that can be compared to experiment.32 While there is a need for more complete measurements of the fundamental properties of electrodes, such as the Li wetting of materials and rate constants for various dynamic processes, new characterization tools and methodologies are needed for more fully linking electronic, chemical, structural, and mechanical phenomena. How do we accurately measure mechanical and electronic properties at the same spatial location on an electrode, for example, by developing a combined microscope for scanning probes (mechanical), scanning electrochemical probes (redox), and scanning X-ray probes (chemical, electronic)? Can we more accurately relate the electrochemistry (measured in situ in the characterization cell) to the structure and electronic properties of the electrode and its interface with the electrolyte? Can the electrode state of charge be quantitatively connected to the spatial distribution of Li, while directly addressing the question “Where does the Li go, and how does it interact with its local environment?” Connecting experiments and simulations to understand complex, coupled phenomema can be facilitated by careful selection of the system to be studied. Simple model systems have significantly advanced our understanding of energy storage, as they reduce complexity and focus on a few variables and outcomes that often reveal mechanistic causes and relationships. Researchers have adopted model systems to understand energy storage mechanisms by using these idealized geometries to find and test specific hypotheses that otherwise may be lost in unrelated complexities. This approach is well expressed in the catalysis community where, for example, single crystals are used as model substrates rather than nanoparticles.35 Models may involve in situ characterization using single crystals,36,37 thin films,38 or open cell geometries39,40 for nanostructured electrodes to explore specific features and relationships. One example is the coupled mechanical and transport properties of multi-component interphase layers that are generally too complex for modeling studies to capture. These interphases are strongly affected by the composition of the electrolyte, including uncontrolled impurities, the charging and discharging rates, voltage variation across the charging and discharging profile, and repeated cycling. Instead of trying to treat all of these variables, a simpler experimental model carried out with pure electrolytes with systematically controlled voltages and charging rates allows specific phenomena and their causes to be isolated and controlled. Such a model experimental system can be simulated, and when the simulation is refined by experimental validation and feedback, predictive capability outside the range of measured experience can often be achieved. An example of coordinated experiment and simulation of a model SEI comes from the study of the lithiation of Si on a single-crystal surface in the presence of an electrolyte composed of ethylene carbonate and dimethyl carbonate solvent and LiPF6 salt.37 The surface and subsurface layers were examined with X-ray, neutron, and electron scattering, and the surface with atomic force microscopy (Figure 2.2.4). These in situ studies revealed not only a structure of sequential layers produced by the lithiation process, but also the evolution and breathing of the layers with charge and discharge cycling. 32 PRIORITY RESEARCH DIRECTION – 2 • X-rays • Neutrons • Electrons • Laser Light Li+ Li+ Atomic Force Microscopy Organic SEI Inorganic SEI LixSi Si Figure 2.2.4. A model Si-SEI lithiation system composed of a single Si crystal substrate in ethylene carbonate-dimethyl carbonate solvent with LiPF6 salt. Based on Refs. 7, 36, 37.PDF Image | Next Generation Electrical Energy Storage
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