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3.1 Panel 1 Report — Pathways to Simultaneous High Energy and Power Both the amount of energy stored, typically defined in terms of the volumetric or gravimetric energy density, and the rate at which that energy is delivered (power density) are critical aspects of electrochemical energy storage systems. The energy density determines how long the system can last between charges, while power density relates to how fast the energy can be extracted from or introduced into the system. Simultaneous high energy and power are required for many applications. For instance, batteries for electric vehicles must contain sufficient energy to enable a long driving range (>300 miles), but they also must feature high power capabilities for acceleration and fast charging (less than 15 minutes). For mobile power sources, consumers also demand extended operation between charging (high energy density) as well as fast charging capabilities. For grid-based energy storage, the energy and power demands vary greatly depending on the specific point of integration within the grid. Fast charging is needed to respond to and accommodate the unpredictable variation of the output from renewable energy sources. Deep charge and discharge cycling is required for load leveling and cyclical day-night storage. Regardless of the application, an exceptionally long cycle life is required to reduce the cost for energy storage. To achieve the goal of simultaneous high energy and power, improved understanding of how energy and power are determined by materials and component behavior across length scales within battery cells is urgently needed. 3.1.1 CURRENT STATUS AND RECENT ADVANCES Simultaneous high energy and power require deep understanding of fundamental scientific issues. The near- universal relationship between power and energy across a variety of energy storage systems is illustrated in the Ragone plot in Figure 3.1.1.1 The energy density of a given electrochemical energy storage system is intrinsically governed by the quantity of ions that participate in the electrochemical reactions, the molecular weight or volume of the active materials, and the electrochemical potential difference between different electrodes. The power capability is limited by how quickly ions and electrons can be transported in the bulk and across different interfaces within the system. Ion transport is usually much slower than electron transport and is dependent on a number of factors, including the kinetics of phase transformations within active materials, impedance at interfaces, tortuosity and arrangement of particles within electrodes, and the “transference number” (the fraction of the total current carried either by the anion or the cation) of ions in electrolytes. Conventional high- power batteries are realized by using thin electrodes that include significant volume fractions of electrically and ionically conductive materials, including void spaces to accommodate electrolytes. This formulation balances the diffusion length of the electrons and ions but also decreases the volume available for the materials that are storing energy, thus resulting in lower energy density. High energy systems are produced in a converse fashion, which normally requires increasing the amount of active materials and increasing the electrode thickness, along with reducing the amount of conductive additives and void space. The resulting high internal resistances reduce the available power and increase the possibility of electrode failure. Although a large body of literature has been focused on developing high energy electrode materials, less attention has been paid to the electron and ion transport problems for high energy structures in which the utilization rate of active materials needs to be high.2-4 There is thus a significant need to advance the synthesis and understanding of stable architectures to simultaneously provide energy and power. NEXT GENERATION ELECTRICAL ENERGY STORAGE PANEL 1 REPORT 79PDF Image | Next Generation Electrical Energy Storage
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