Basic Research Needs for Electrical Energy Storage: Report of the ...

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Correlation Between Pore Size, Ion Size, and Specific Capacitance The electrolyte environment is very complex at the nanoscale. Surrounding each electrolyte ion is a layer of solvent molecules that is attracted to the charged ions. It was long thought that this so-called solvation layer was tightly bound to its ion nucleus, and therefore pores would need to be larger than the diameter of both the solvent layer and the ion. Therefore, scientists were interested in increasing the pore size as much as possible without significantly compromising the surface area. Recent work on carbon derived from metal carbide precursors has elucidated the mechanism governing this capacitance increase with increasing pore size. However, these studies have also shown that the capacitance increases anomalously if the pore size is decreased below the size of the solvation shell. This approach leads to an increase in capacitance of almost 50% compared with the best-performing commercially available activated carbons. Additional studies are needed to fully understand these complex relationships. Multifunctional materials for pseudocapacitors and hybrid devices. There are extraordinary opportunities for taking advantage of materials that exhibit pseudocapacitance to produce high-performance ECs. Improved understanding of charge-transfer processes in pseudocapacitance is a critical step that will lead to the design of new materials and multifunctional architectures offering substantially higher levels of energy and power density. Electrolytes for capacitive energy storage. At present, there is a lack of understanding of the influence of electrolyte components on molecular interactions, physical properties, and EC performance. Fundamental understanding of molecular interactions in the bulk solution and at interfaces will be a key consideration in identifying the next generation of electrolyte 20 Dependence of material capacitance normalized by surface area on pore size. The results show that increasing the pore size above ~1 nm marginally increases the normalized capacitance. Decreasing the pore size below ~1 nm sharply increases the normalized capacitance due to constriction of the ion solvation shell (inset). Phenomena of this kind must be understood, described theoretically, and used in designing new material/solvent systems for capacitive energy storage.
systems that offer high conductivity and wider electrochemical windows, allowing higher operating voltages to be realized. As was noted in the Chemical Energy Storage panel report, theory, modeling, and simulation tools are also needed for ECs. These needs are described in PRD 6, “Rational Materials Design through Theory and Modeling.” New computational tools can guide experiments that test our fundamental understanding of the various processes involved in capacitive storage science. To date, relatively few models have been applied to ECs, although various modeling approaches are available and seem appropriate. Numerous topics of central importance to capacitive energy storage processes in such areas as dynamics, charge transfer, and interfacial properties need to be explored to identify new directions in both materials and electrode architectures. The potential impact of developing new generations of ECs is significant. 7 There are many opportunities for storing energy and recovering waste energy for which batteries are either not appropriate or are far from optimal because of their limited power capability in charge or discharge modes and/or their limited cycle life for deep discharge. These applications include energy storage and load leveling in solar, wind, and other energy sources and for power quality regulation on the electrical grid. ECs have particular promise for energy recovery from regenerative braking in vehicles and/or stop-start in industrial equipment, reducing energy requirements. Further applications in which energy can be recovered from intermittent light, vibration, and motion sources (e.g., walking) may also be possible with new EC technologies. The current lack of fundamental knowledge of how these devices work at the atomic and molecular level is a major impediment to needed advances in this field. Basic research is needed to obtain this knowledge and provide breakthroughs that are needed in the design of electrodes and electrolytes for future EC devices. References 1. R. Kotz and M. Carlen, “Principles and Applications of Electrochemical Capacitors,” Electrochimica Acta 45, 2483–2498 (2000). 2. M. S. Halper and J. C. Ellenbogen, MITRE Nanosystems Group, 2006. 3. F. Beguin and E. Frackowiak, “Nanotextured Carbons for Electrochemical Energy Storage,” Nanomaterials Handbook, CRC Press/Taylor and Francis, pp. 713–737, Ed. Y. Gogotsi, 2006. 4. A. G. Pandolfo and A. F. Hollenkamp, “Carbon Properties and Their Role in Supercapacitors,” J. of Power Sources 157, 11–27 (2006). 5. J. Chmiola and Y. Gogotsi, “Supercapacitors as Advanced Energy Storage Devices,” Nanotechnology Law and Business 4(1), 577–584 (2007). 6. W. G. Pell and B. E. Conway, “Peculiarities and requirements of asymmetric capacitor devices based on combination of capacitor and battery-type electrodes,” J. of Power Sources 136, 334–345 (2004). 7. R. F. Service, “New ‘Supercapacitor’ Promises to Pack More Electrical Punch,” Science 313, 902–905 (2006). 21
Page 2 and 3: On the cover: The cover depicts the
Page 5: CONTENTS iii Page Figures..........
Page 8 and 9: 14. Calculated and scaled Raman spe
Page 10 and 11: SEI solid electrolyte interphase Sh
Page 12 and 13: transportation and electricity dist
Page 14 and 15: time, during charge transport and t
Page 17 and 18: INTRODUCTION A modern economy depen
Page 19: developments have stimulated interd
Page 23 and 24: CHEMICAL ENERGY STORAGE Abstract Ba
Page 25 and 26: What Is a Battery? A battery contai
Page 27 and 28: Further, an understanding is needed
Page 29 and 30: computation and analysis will be ne
Page 31 and 32: charge-discharge cycles each can un
Page 33: dimensional architectures. By desig
Page 39 and 40: NOVEL DESIGNS AND STRATEGIES FOR CH
Page 41 and 42: nanostructured, high-surface-area e
Page 43 and 44: and electronic conductors. For exam
Page 45 and 46: SOLID-ELECTROLYTE INTERFACES AND IN
Page 47 and 48: Research Directions Strategies for
Page 49 and 50: Many opportunities exist for design
Page 51 and 52: CAPACITIVE ENERGY STORAGE MATERIALS
Page 53 and 54: understood at present, nor are the
Page 55 and 56: ELECTROLYTE INTERACTIONS IN CAPACIT
Page 57 and 58: Figure 12. Ions in the ionic liquid
Page 59 and 60: species dynamically at the interfac
Page 61 and 62: MULTIFUNCTIONAL MATERIALS FOR PSEUD
Page 63 and 64: the functionalities described. New
Page 65 and 66: conductive architectures. 7 Templat
Page 67 and 68: RATIONAL MATERIALS DESIGN THROUGH T
Page 69 and 70: Figure 16. (Left) Sketch of a singl
Page 71 and 72: changes in performance through the
Page 73 and 74: is often more relevant for the prac
Page 75 and 76: validated against quantum-mechanica
Page 77: References 1. M. Bervas et al., “
Page 81 and 82: CROSS-CUTTING SCIENCE FOR ELECTRICA
Page 83 and 84: with theory, modeling, and simulati
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path for the preparation of metal o
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studying double layer capacity and
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An even more demanding goal is to u
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CONCLUSION
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This Report outlines substantial te
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demanding goal for theory is the de
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TECHNOLOGY AND APPLIED R&D NEEDS FO
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Electrical Energy Storage Factual D
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2.1 TRANSPORTATION Electrical Energ
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designed with electronically contro
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Electric Energy per cycle in Wh Fig
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Capacitor range and self-discharge.
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Figure 4 shows the effects of and o
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The following applications are ways
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oversized, and load fluctuations⎯
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atteries with higher discharge rate
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3. RECHARGEABLE BATTERIES FOR ENERG
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Such failure mechanisms can be held
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The open circuit voltage (OCV) and
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Figure 10. Charge and overcharge pr
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3.4 HIGH-TEMPERATURE SODIUM BETA BA
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Batteries for Load Leveling Beta ba
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that protects from further reaction
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LiFePO4 (olivine) have been used as
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In similar metal-water batteries, t
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Lithium-air Batteries There have be
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Fig. 13. A stack of four redox flow
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the redox flow cells can be dispatc
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laboratory studies and observations
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46 Fig. 14. Equivalent circuit diag
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densities, and excellent cycle life
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system operating voltage is relativ
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52 Fig. 17. Cutaway drawing of a bi
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54 Fig. 19. Commercial large-format
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typically in 1 minute or less, whic
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58 removing impurities from the car
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PROBING ELECTRICAL ENERGY STORAGE C
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electrolytes, as demonstrated for 1
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Figure B.2. Imaging lithium in a bl
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Since most chemical reactions invol
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Workshop on Basic Research Needs El
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Breakout Sessions—Chemical Storag
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John Ferraris, University of Texas
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DISCLAIMER This report was prepared
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