What this Department of Energy document shows is that we can’t make the necessary REVOLUTIONARY breakthroughs to electrify cars until we understand the physics of batteries, and points out that “battery technology has not changed substantially in nearly 200 years.” page 3.
It’s how scientists like to say “don’t hold your breath” in as understated a way as possible. Laws of physics? That should have exclamation points. And it sounds very expensive…
These are just a few of the challenges batteries and other kinds of electrical energy storage (EES) face. I ran out of steam extracting them by page 35.
“Basic Research Needs for Electrical Energy Storage”. Report of the Basic Energy Sciences Workshop on Electrical Energy Storage April 2-4, 2007. Office of Science, U.S. Department Of Energy. http://www.sc.doe.gov/BES/reports/files/EES_rpt.pdf
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 electro-positive 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.
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; 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; or a dislocation originally present in the material or caused by electrochemical cycling (Figure 2). New analytical tools 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 or at the gas-solid interface, depending on the type of battery being studied.
Further, an understanding is needed of how these mechanisms vary with surface and bulk structure, particle morphology, and electronic properties of the solid for both intercalation and conversion reactions. Also important is the ability to correlate the structure of the interface with its reactivity, to bridge the gap between localized ultrafast phenomena that occur at the Å–micron length scale and the macroscopic long-term behavior of the battery system. Gaining insight into the nature of these processes is key to designing novel materials and chemistries for the next generation of chemical EES devices. Recent advances in nanoscience, analytical techniques, and computational modeling present unprecedented opportunities to solve technical bottlenecks. New synthetic approaches can allow the design of materials with exquisite control of chemical and physical processes at the atomic and molecular levels. Development of in situ methods and even multi-technique probes that push the limits of both spatial and temporal resolution can provide detailed insight into these processes and relate them to electrode structure. New computational tools, which can be employed to model complex battery systems and can couple with experimental techniques both to feed data into modeling and to use modeling/theory to help interpret experimental data, are critically important.
The Potential of nano-science
The lack of a fundamental understanding of how thermodynamic properties, such as phase co-existence, change at the nanoscale is in stark contrast to the wealth of information available on the novel electronic, optical, and magnetic properties of nanomaterials. While the latter properties typically arise from the interaction of the electronic structure with the boundary conditions (e.g., electron confinement and/or localization), purely energetic properties and thermodynamic behavior change in a less transparent way at the nanoscale.
Many fundamental questions remain to be answered. For example, are the differences in the electrochemical properties of bulk and nanosize electrode materials simply due to the higher concentrations of different surfaces available for intercalation, or are the electronic properties of the nanomaterials significantly different? Are surface structures at the nanoscale significantly different from those in the bulk or are the improved properties simply a transport effect? At the nanoscale, can we conceptually separate pseudocapacitive from storage reactions? Can we develop general rules and, if so, how widely do we expect them to apply? How are ionic and electronic transport processes coupled in complex heterogeneous nanostructured materials? The ability to modify the properties of materials by treating size and shape as new variables presents great opportunities for designing new classes of materials for EES.
It is imperative to explore how the different properties of nanoparticles and their composites can be used to increase the power and energy efficiency of battery systems. A tremendous opportunity exists to exploit nanoscale phenomena to design new chemistries and even whole new electrode and electrolyte architectures—from nanoporous mesoscopic structures to three-dimensional electrodes with active and passive multifunctional components interconnected within architectures that offer superior energy storage capacity, fast kinetics and enhanced mass transport, and mechanical integrity. To do so, we need to be able to control chemistries and assembly processes. Furthermore, low-cost, high-volume synthesis and fabrication techniques and nanocomposites with improved safety characteristics must be designed, to satisfy requirements for large-scale manufacturing of nanostructure materials and for their use in practical battery systems.
New Capabilities in Computation and Analysis
Although clever engineering can address some inherent problems with a particular battery chemistry, dramatic improvements in performance will ultimately come from the development of different electrode and electrolyte materials. New computational and analysis tools are needed to realize significant breakthroughs in these areas. For example, new analytical tools will provide an understanding of how the phase behavior and electrochemical properties of materials are modified at the atomic level. With this information, computational tools will expedite the design of materials with structures and architectures tailored for specific performance characteristics. It is now possible to predict many properties of materials before attempting to synthesize and test them (see Appendix B, “Probing Electrical Energy Storage Chemistry And Physics Over Broad Time And Length Scales,” for further details), and expanded computational capabilities specific to chemical energy storage are a critical need. New capabilities in modeling and simulation could help unravel the complex processes involved in charge transport across the electrode-electrolyte interface and identify underlying reactions that cause capacity degradation.
Tremendous opportunities exist to develop and apply novel experimental methodologies with increased spatial, energy, and temporal resolution. These could answer a wide range of fundamental questions in chemical electrical storage, identifying and providing ways to overcome some of the barriers in this field. In particular, techniques that combine higher resolution imaging, fast spectroscopic tools, and improved electrochemical probes will enable researchers to unravel the complex processes that occur at electrodes, electrolytes, and interfaces.
CAPACITIVE ENERGY STORAGE
Abstract
To realize the full potential of electrochemical capacitors (ECs) as electrical energy storage (EES) devices, new materials and chemical processes are needed to improve their charge storage capabilities by increasing both their energy and their power densities. Incremental changes in existing technologies will not produce the breakthroughs needed to realize these improvements. Rather, a fundamental understanding of the physical and chemical processes that take place in the EC—including the electrodes, the electrolytes, and especially their interfaces—is needed to design revolutionary concepts. For example, new strategies in which EC materials simultaneously exploit multiple charge storage mechanisms need to be identified. Charge storage mechanisms need to be understood to enable the design of new materials for pseudocapacitors and hybrid devices. There is a need for new electrolytes that have high ionic conductivity in combination with wide electrochemical, chemical, and thermal stability; are non-toxic, biodegradable, and/or renewable; can be immobilized; and can be produced from sustainable sources. New continuum, atomistic, and quantum mechanical models are needed to understand solvents and ions in pores, predict new material chemistries and architectures, and discover new physical phenomena at the electrochemical interfaces. From fundamental science, novel energy storage mechanisms can be designed into new materials. With these breakthroughs, ECs have the potential to emerge as an important energy storage technology in the future.
FUNDAMENTAL CHALLENGES
Little is known about the physico-chemical consequences of nanoscale dimensions (see sidebar “Correlation Between Pore Size, Ion Size, and Specific Capacitance”). Further, it is necessary to understand how various factors—such as pore size, surface area, and surface chemistry— affect the performance of ECs. This knowledge can be used to design nanostructured materials with optimized architectures that could yield dramatic improvements in current capabilities in energy and power. Novel electrolyte systems that operate at higher voltages and have higher room-temperature conductivity are critically needed for the next generation of ECs. Fundamentals of solvation dynamics, molecular interactions at interfaces, and ion transport must be better understood to tailor electrolytes for optimal performance. Exciting opportunities exist for creating multifunctional electrolytes that scavenge impurities and exhibit self-healing. A potential bridge between ECs and batteries is combining a batterytype electrode with a capacitor-type electrode in so-called hybrid or asymmetric ECs.6 This approach needs to be better understood at the fundamental level so that it enables the tailoring of energy density without compromising power density. In situ characterization of the electrolyte/electrode interface during charging/discharging at molecular and atomic levels is critical to understanding the fundamental processes in capacitive energy storage. This will require the development of new experimental techniques that combine measurement and imaging, including so-called chemical imaging, where chemical information can be obtained at high spatial resolution. In addition, new computational capabilities can allow modeling of active materials, electrolytes, and electrochemical processes at the nanoscale and across broad length and time scales. These models will assist in the discovery of new materials and the performance evaluation of new system designs.
Background and Motivation
A chemical energy storage system (battery) is inherently complex, consisting of a cathode, an
electrolyte, and an anode (see sidebar “What is a Battery?” on page 11). Any future system
must be designed to include a number of essential characteristics, including
• high energy density;
• sufficient power achieved through holistic design of the storage materials, supporting
components, and device construction;
• electrochemical and materials stability to ensure long lifetimes;
• practical materials synthesis and device fabrication approaches;
• reasonable cost; and
• optimized safe operation and manageable toxicity and environmental effects.
Future chemical energy storage applications, ranging from portable consumer products to
hybrid and plug-in electric vehicles to electrical distribution load-leveling, require years to
decades of deep discharge with subsequent recharging (charge-discharge cycles). This level
of use must occur with minimal loss of performance so that the same capacity is available on
every discharge (i.e., with minimal capacity fade). The necessity of ensuring stable cycle-life
response has restricted the number of electrons that can be transferred in any given discharge
or charge reaction, thereby limiting the utilization of the electrodes and the amount of energy
that could be available from the batteries.
This restriction in battery operation is driven by the fact that deep, but thermodynamically
allowable, discharge reactions usually drive the electrodes toward physical and chemical
conditions that cannot be fully reversed upon charging. The extent to which the physical and
chemical properties of electrode materials change during electrochemical cycling is
dependent on the battery’s chemistry. For example, during charge-discharge, the electrode
materials can undergo damaging structural changes. They can fracture, resulting in the loss of
electronic contact, and they can dissolve in the electrolyte, thereby lowering the cycling
efficiency and delivered energy of the batteries.
I’m amazed you got this far. This is just page 35 of 186 pages, go read the rest online if your eyes haven’t glazed over yet!