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

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To achieve the goals of using electrical energy in transportation and electricity grid applications, a major step change in battery technology is needed—a step that can be taken only by obtaining a fundamental understanding of the physical and chemical processes that occur in these complex systems to enable breakthroughs in batteries, including electrode materials and electrolyte chemistries. Fundamental Challenges Batteries are inherently complex and virtually living systems—their electrochemistry, phase transformations, and transport processes vary not only during cycling but often also throughout their lifetime. Although they are often viewed as simple for consumers to use, their successful operation relies on a series of complex, interrelated mechanisms involving thermodynamic instability in many parts of the charge-discharge cycle and the formation of metastable phases. The requirements for long-term stability are extremely stringent and necessitate control of the chemical and physical processes over a wide variety of temporal and structural length scales. A battery system involves interactions among various states of matter—crystalline and amorphous solids, polymers, and organic liquids, among others (see sidebar “What Is a Battery?”). Some components, such as the electrodes and electrolytes, are considered electrochemically active; others, such as the conductive additives, binders, current collectors and separators, are used mainly to maintain the electrode’s electronic and mechanical integrity. Yet all of these components contribute to battery function and interact with one another, contributing to a convoluted system of interrelated reactions and physico-chemical processes that can manifest themselves indirectly via a large variety of symptoms and phenomena. To provide the major breakthroughs needed to address future technology requirements, a fundamental understanding of the chemical and physical processes that occur in these complex systems must be obtained. New analytical and computational methods and experimental strategies are required to study the properties of the individual components and their interfaces. An interdisciplinary effort is required that brings together chemists, materials scientists, and physicists. This is particularly important for a fundamental understanding of processes at the electrode-electrolyte interface. The largest and most critical knowledge gaps exist in the basic understanding of the mechanisms and kinetics of the elementary steps that occur during battery operation. These processes—which include charge transfer phenomena, charge carrier and mass transport in the bulk of the materials and across interfaces, and structural changes and phase transitions— determine the main parameters of the entire EES system: energy density, charge-discharge rate, lifetime, and safety. For example, understanding structure and reactivity at hidden or buried interfaces is particularly important for understanding battery performance and failure modes. These interfaces may include a reaction front moving through a particle in a twophase reaction (Figure 1, ii); an interface between the conducting matrix (e.g., carbon), the binder, or the solid electrolyte interphase (SEI) (see PRD “Rational Design of Interfaces and Interphases”) and the electrode material (Figure 1, i and iv); or a dislocation originally present in the material or caused by electrochemical cycling (Figure 2). New analytical tools 10
What Is a Battery? A battery contains one or more electrochemical cells; these may be connected in series or parallel to provide the desired voltage and power. The anode is the electropositive electrode from which electrons are generated to do external work. In a lithium cell, the anode contains lithium, commonly held within graphite in the well-known lithium-ion batteries. The cathode is the electronegative electrode to which positive ions migrate inside the cell and electrons migrate through the external electrical circuit. The electrolyte allows the flow of positive ions, for example lithium ions, from one electrode to another. It allows the flow only of ions and not of electrons. The electrolyte is commonly a liquid solution containing a salt dissolved in a solvent. The electrolyte must be stable in the presence of both electrodes. The current collectors allow the transport of electrons to and from the electrodes. They are typically metals and must not react with the electrode materials. Typically, copper is used for the anode and aluminum for the cathode (the lighterweight aluminum reacts with lithium and therefore cannot be used for lithium-based anodes). The cell voltage is determined by the energy of the chemical reaction occurring in the cell. The anode and cathode are, in practice, complex composites. They contain, besides the active material, polymeric binders to hold the powder structure together and conductive diluents such as carbon black to give the whole structure electronic conductivity so that electrons can be transported to the active material. In addition these components are combined so as to leave sufficient porosity to allow the liquid electrolyte to penetrate the powder structure and the ions to reach the reacting sites. are needed to allow monitoring of a reaction front moving through a particle in a two-phase reaction (Figure 1, ii) in real time, and to image concentration gradients and heterogeneity in these complex systems. A detailed, molecular-level understanding is needed of the mechanism by which an ion intercalates or reacts at the liquid-solid interface (Figure 1, iii) or at the gas-solid interface, depending on the type of battery being studied. 11
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: CHEMICAL ENERGY STORAGE Abstract Ba
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 and 34: dimensional architectures. By desig
Page 35: systems that offer high conductivit
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
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validated against quantum-mechanica
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References 1. M. Bervas et al., “
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CROSS-CUTTING SCIENCE FOR ELECTRICA
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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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