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REPORT OF THE BASIC RESEARCH NEEDS WORKSHOP a 1 0 Initial Stage Parent γ-MnO2 Early Stage Zn2MnO4 Spinel Intermediate Stage Final b Stage c 2 Layered Zn0.25 V2O3-γH2O +Zn2+/2e– -H2O H2O Layered Zn0.25+? V2O3+2H2O 0 200 Specific Capacity (mAh/g) MnO2 Zn Zn2+ Zn2+ 100 Zn2+ Zn2+ 18 PRIORITY RESEARCH DIRECTION – 1 γ=Zn2MnO2 Tunnel-type Zn2MnO4 Spinel 300 Zn (s) L=Zn2MnO2 Layered type OH Zn2+ Zn2+ Figure 2.1.5. Schematic illustrations of different mechanisms for Zn ion storage. (a) Zn-ion intercalation in a γ-MnO2 cathode (from Ref. 68). (b) Chemical conversion reaction for reversible Zn-ion storage. (c) Water molecule-assisted Zn-ion interaction in Zn0.25V2O5. Reprinted with permission from Ref. 61. Copyright (2015) American Chemical Society. The example of Zn-based systems illustrates the complexity encountered in alternative materials and battery chemistries. In aqueous systems, in particular, the interaction of the electrode materials with water, as well as the solvation of the cations, must be carefully studied to identify and overcome barriers to achieve high capacity and fast charge transport.62 Aqueous Battery Systems: Aqueous electrolytes are particularly attractive due to their low cost, safety, and abundance, but they suffer from a relatively small usable voltage window of ~1.7 V. Above this voltage, water decomposition and gas evolution occur at the electrodes, leading to coulombic losses. Aqueous systems are particularly desirable if they could be redesigned to enable higher voltage operation, and indeed recent findings demonstrate that voltage windows approaching or even exceeding 2 V are possible between electrode pairs immersed in an aqueous electrolyte.71,72 These results suggest a promising route to high-performance energy storage systems that can be produced at scale. Attaining this goal may well depend on fundamental research on novel high-solubility aqueous salts, electrode structures for cultivating favorable pH environments, the fundamentals of electrochemical stability in aqueous electrolytes, and ionic transport in electrolytes with different degrees of solvated species, effects that are largely undocumented and poorly understood. Diverse Chemistries with New Architectures: Mediator-ion solid electrolytes73-75 provide the basis for an architecture that enables novel combinations of aqueous and non-aqueous chemistries and the use of abundant and environmentally benign elements like iron, zinc, oxygen, sulfur, etc., in aqueous media. While conventional polymeric porous separators are inadequate to prevent dendrite penetration or chemical crossover between the positive and negative electrodes, a solid electrolyte permeable to divalent ions like Zn2+ and Fe2+ would be attractive, but it is exceedingly difficult to realize, considering the challenges already observed when using solid electrolytes to transport even lighter monovalent ions like Li+ and Na+. This problem could be overcome by adopting new strategies, such as the concept of a mediator-ion solid electrolyte as depicted in Figure 2.1.6. With solid electrolytes that act as ion mediators and also prevent chemical crossover, it may be possible to achieve low-cost, safe, aqueous battery systems, including a metallic Zn or Fe anode, an aqueous electrolyte, a lithium-ion or a sodium-ion solid electrolyte, and a variety of redox systems (oxygen, sulfur, bromine, permanganate, ferrocyanide, quinones, etc.). Importantly, a solid electrolyte separator with liquid anolytes and catholytes eliminates the commonly encountered problems of huge charge-transfer resistance between solid electrolytes and solid electrodes (as in all-solid-state batteries). The strategy also offers the flexibility to use, for example, an alkaline medium at one electrode and acidic medium at the other electrode, or a non-aqueous medium at one electrode and aqueous medium at the other electrode (Figure 2.1.6), in contrast to conventional battery systems ( e.g., in Li-ion batteries, transition-metal ions can crossover from cathode to anode). Such hybrid battery configurations, featuring solid electrolytes as ion mediators, are in their infancy in exploiting new architectures to enable diverse chemistries, but research opportunities are plentiful, including issues such as design and development of mediator-ion solid electrolytes for Li or Na ion transport while allowing different electrolytes on either side; solid electrolyte surface protection with ion-transporting surface coatings or compositions, which may also drop voltages at the liquid electrolyte interface; compatible catholyte/anolyte systems for a given mediator-ion solid electrolyte; and long-term stability, kinetics, and reaction mechanism. H+ Voltagge (V vs. Zn2-/Zn) Zn2+PDF Image | Next Generation Electrical Energy Storage
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