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REPORT OF THE BASIC RESEARCH NEEDS WORKSHOP This concept has been generalized to Na-based batteries4,5 and Na-O2 batteries with superconcentrated electrolyte of sodium bis(trifluoromethanesulfonyl)imide (TFSI) in dimethyl sulfoxide (DMSO) (Figure 3.4.1).6 A unique solvated structure of loosely crosslinked (DMSO)3-Na+-TFSI basic units binds a significant proportion of the DMSO solvent molecules, leaving only a few available for parasitic reactions. This implies that to minimize solvent decomposition from volatile side reactions yet maintain full utilization of the electrolyte, one can reduce the fraction of “free solvent” (i.e., non-fully solvated solvent molecules) in dilute electrolytes by increasing the salt concentration. However, to generalize this novel concept and apply it to diverse solvent and salt systems, better fundamental understanding about the nature of superconcentrated electrolytes is essential. The electrolytes used in electrochemical energy storage are either liquids, as in the case of lithium-ion batteries, or solids, as in solid-state batteries. They are very rarely a combination of the two. This is unfortunate because an ideal electrolyte is one that has the conductivity of a liquid but the thermal, chemical, and dimensional stability of a solid. Recently, researchers have used the sol-gel process to prepare materials by using an ionic liquid as the solvent phase.7 The final material consists of an interconnected porous inorganic network that effectively holds the ionic liquid phase by capillary forces, as the pore size is on the order of 10 nm. This material, termed an “ionogel,” has been adapted so that the ionic liquid is ionically conducting through the addition of a lithium-based salt. Moreover, because the ionic liquid has extremely low vapor pressure, the liquid phase is completely stable in ambient conditions. The resulting composite material is solid-like in its appearance but has much greater ionic conductivity because of the liquid-phase character of the ionic liquid domains. The ionogel approach is just at the proof-of-concept stage but already shows great promise. Because of the continuous liquid phase, ionogel conductivities are in the range of 10-3 S/cm at room temperature, only 2 to 5 times less conductive than the corresponding ionic liquid but many orders of magnitude greater than solid electrolytes. However, a key question that has yet to be fully answered is whether the electrolyte/electrode interface will have much lower impedance than that of solid electrolytes. If the presence of the liquid phase produces a low interfacial resistance, as occurs in liquid electrolyte systems, this will overcome one of the limitations of solid- state battery technology. Interfaces: Where All the Action Is...The various battery components interact through physical interfaces. Lately, new experimental methodologies with ultrafast time resolution have become available for investigating the molecular structure of bulk electrolytes, interfaces, and solids: in particular, those exhibiting fast dynamic behavior. These new ultrafast techniques, coupled to computational analyses, are starting to answer unresolved questions in electrochemical energy storage. For example, vibrational sum frequency generation has been combined with cyclic voltammetry to investigate the formation of the solid-electrolyte interphase on anode surfaces.8 Other femtosecond time-resolved spectroscopies have been used to investigate the structure and dynamics of bulk electrolyte solutions.9,10 Interestingly, the experimental findings provided by these techniques can be used not only to bridge the gap between other structural methodologies with lower time resolution such as nuclear magnetic resonance, but also to generate databases of experimental observables required for methodologies such as machine learning. Other electrochemical research communities have helped in the advancement of the understanding of solid-liquid interfaces. For example, analysis of the interfacial water on gold electrodes studied by X-ray absorption spectroscopy and first-principles calculations revealed that the interfacial water molecules have a different structure from those in the bulk.11 Recently, studies have also started unravelling the electrochemical double layer by direct probing of the solid-liquid interface (see Figure 2.3.12 in Chapter 2) by means of ambient pressure X-ray photoelectron spectroscopy performed under polarization conditions.12 These newly emerging experimental techniques are opening up many possibilities and enabling comparison, validation, and benchmarking of theory and computational approaches. Charge Transport and Storage: The Pulse of the Battery: Our current understanding of ionic diffusion in the solid state is that the basic process occurs through migration of ions between empty interstices or vacancies in the lattice.13 This motion of ions is strongly influenced by the nature of the bonding in the host framework because of electrostatic interactions that occur between the positively charged alkali ions and the negatively charged anions.14-16 In oxides, the distortions that occur as Li ions are inserted or removed can induce severe changes in the structural framework. In the classic LiCoO2 material, as Li is removed from the parent structure, the electrostatic repulsion between the neighboring layers of CoO6 results in an elongation along the c-axis of the unit cell.16,17 This expansion continues up to the point where all the Li is removed (i.e., CoO2 remains) and the compound 116 PANEL 4 REPORTPDF Image | Next Generation Electrical Energy Storage
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