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significantly reduce crossover of redox species due to their high ion selectivity. To realize wide application of solid electrolytes in flow systems, fundamental research is needed to understand the physical and chemical processes that occur at the interfaces between the solid electrolyte and the anolyte/catholyte.55 Semi-solid batteries: Although conventional flow batteries have liquid electrodes, solid electrode materials can be made to flow in suspension. Duduta et al.61 have explored the semi-solid redox-flow battery architecture using thick suspensions made of Li-ion battery electrode materials in liquid electrolyte.This approach enabled exceptionally thick electrodes ranging from 100 μm to 700 μm, compared to less than 100 μm in traditional Li-ion batteries with solid electrodes. The flowing suspension electrode is also “self-healing” in the sense that it cannot fail under shear stress like a solid. Yet-to-emerge concepts in this vein may yield further scientific breakthroughs in large-scale energy storage. Molecular-Level Tailoring of Electrolytes and Interphases: The electrolyte is the “blood” of all electrochemical cells and is responsible for ion transport between the two electrodes; however, it is arguably the least understood component in a battery. It is reasonable to say that the overall behavior of advanced batteries is largely dictated by the properties of electrolytes and electrolyte/electrode interphases. In the widely used Li-ion battery system, the power of the cell is often determined by how fast the Li ions are transported across the solid-electrolyte interphase (SEI), as well as through electrode materials. The formation and characteristics of the SEI are key factors that determine the energy, power, and stability of today’s Li-ion batteries. To enable simultaneous high energy and power capabilities, researchers face the following scientific challenges: (1) how can the electrochemical window of electrolytes be widened while either controlling or preventing the formation of harmful SEI layers? (2) How can compatible electrolytes be designed with tailored attributes to accelerate ion transport and enhance the compatibility between the electrolyte and electrodes? To answer these scientific questions, it is critical to fundamentally understand the behavior of solute and solvent molecules in the bulk electrolyte and in the vicinity of the electrode/electrolyte interface. The synergistic action of salt and solvent molecules affects the quality of the SEI derived from an electrolyte. Considering the solvation structure, a Li+ ion is normally coordinated with three to four solvent molecules in the conventional 1 molar electrolyte solution, which primarily consists of solvent-separated ion pairs and free solvent molecules.66 In the vicinity of the electrode, the free solvent molecules dominate the inner Helmholtz layer, which dictates the side reactions on the electrode once the electric field is applied. Therefore, the SEI layer formed in regular electrolytes is mainly derived from the decomposition of electrolyte solvents.67 Due to the uneven distribution of the electric field on the electrodes, the morphology and composition of SEI layers formed in a functioning electrochemical cell are not uniform. In concentrated electrolytes, however, the overwhelming population of Li salt increases the association among Li+ ions, anions, and the solvent molecules, which reduces the presence of the free solvent molecules as well as their activity towards the electrode materials. Therefore, when the electrode is polarized, anions have a higher chance of being decomposed, strengthening the anion contribution to the SEI layer.15 Such changes in the inner Helmholtz layer greatly alter the formation process of the SEI. The SEI layer formed in concentrated electrolyte tends to be thinner and more compact, and it consists of simpler chemical compositions, effectively suppressing further reactions between the active electrode and the electrolyte. A few interesting results featuring concentrated electrolytes have recently been discovered, such as corrosion prevention of aluminum current collectors at high voltages,68-70 lithium dendrite suppression,71 and improved cycling stability of Li-S72 and Li-O2 batteries.73 Even in aqueous systems, concentrated electrolytes are found to significantly expand the electrochemical stability window to 3 V and beyond, which could lead to new aqueous systems with high energy density.16 Although concentrated electrolytes provide new insights on how to stabilize electrolytes by controlling the solvation properties of the organic molecules and preventing SEI formation under high charge/discharge rates, high salt concentrations also increase the viscosity and reduce ion conductivity in electrolytes. These results have a negative effect on the power density of cells. The fundamental question, therefore, is how to understand the interplay between ions and solvent molecules at different length scales, with the goal of increasing the stability of the electrolyte and maintaining high ion mobility (Figure 3.1.6).74 State-of-the-art computational tools should be used to predict the characteristics and properties of electrolytes from the molecular scale to the mesoscale. Guided by computational studies, rapid synthesis techniques and high-throughput electrochemical measurements should be developed to facilitate the synthesis, selection, and investigation of electrolytes with NEXT GENERATION ELECTRICAL ENERGY STORAGE PANEL 1 REPORT 89PDF Image | Next Generation Electrical Energy Storage
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