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3.2 Panel 2 Report — Structure, Interphases, and Charge Transfer at Electrochemical Interfaces The structure, composition, and evolution of interfaces are critical determinants of the performance of electrical energy storage systems. In all electrochemical devices, charge transfer and, in some cases, charge transport occur across (or along) hetero-material interfaces and interphases. The rates of charge and discharge (power density) achievable in electrochemical cells are, as a consequence, sensitive functions of the morphology and topology of interfaces and the efficiency with which ions, charge, and mass are transported across/along the many interfaces. These interfaces can be either abrupt or evolving. An abrupt interface refers to boundaries between functionally or characteristically different components of an electrochemical cell, such as between the solid electrode and liquid electrolyte or even solid electrolyte (Figure 3.2.1).1 An evolving interface may arise within cell components dynamically, while they function, e.g., between chemically distinct components in a composite electrode, across or along grain boundaries in polycrystalline electrodes, or between phases under heterogeneous loading (see Figure 2.3.1 in Chapter 2). Chemistry, electrodynamics, mechanics, and their various couplings at interfaces are likewise known to affect the kinetics and stability of electrochemical processes that determine active material utilization (energy density) and time-dependent performance degradation. There is a critical need for analytical and predictive methods that advance understanding and control of electrochemical processes at interfaces. From an understanding of the processes responsible for SEI formation and function, to those that lead to fast transport of ions in solids, and to morphological and chemical instabilities in electrodes based on high-energy reactive metals such as lithium (Figure 3.2.2),2 new approaches that enable rational design and synthesis of interphases are required for advances in the field. Many interfaces in electrochemical energy storage are multiphase junctions consisting of, but not limited to, the active electrodes, separator membrane, and electrolyte, whether the latter is a liquid, solid, polymer, or gel. The presence of binders, conductivity aids, salt, functional additives, and finite amounts of contaminants (sometimes present at low parts per million) in each of these components means that these interfaces are as a rule complex. In some cases, under the action of typical potentials utilized in energy storage devices, these entities may undergo chemical, physical, and/or electrochemical transformation to create self-limiting interphases that couple the bulk electrode subsurface and electrolyte. This SEI 50 nm Li ion Graphite anode SEI can evolve as the potentials of the positive and negative electrodes are frequently operated beyond the electrochemical stability of one or more of the components in the multiphase junction of the surface. In most cases of practical interest, an effective interphase forms through a mechanism that involves contributions of a number of components and cannot be fully identified by isolating the reaction of single components. Electrolytes LEDC LiF NEXT GENERATION ELECTRICAL ENERGY STORAGE Figure 3.2.1. Illustration of functional electrode-electrolyte interface in electrical energy storage. A binary electrolyte (LiPF6 in ethylene carbonate) generates an initial smooth SEI composed of lithium ethylene dicarbonate (LEDC) and LiF on graphite electrode. From Ref. 1. See also Figure 2.3.11 in Section 2. PANEL 2 REPORT 93PDF Image | Next Generation Electrical Energy Storage
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