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REPORT OF THE BASIC RESEARCH NEEDS WORKSHOP 2.2.1 SCIENTIFIC CHALLENGES This PRD focuses on understanding the interplay of electronic, chemical, structural, and mechanical phenomena that underlie the capacity, kinetics, reversibility, irreversibility, and ultimately degradation and failure of electrochemical energy storage systems. The ultimate goal is to understand the spatial and temporal evolution of operating electrochemical systems, allowing rational design of energy storage devices from atomic and molecular to mesoscopic and system levels. Approaches are needed to link experiments and simulations across spatial scales from atomic to particle level (100s of nanometers to microns) to the electrode level (millimeter-centimeter) and time scales from picoseconds to seconds to years. The challenge is to develop and apply methodologies and tools that can seamlessly link the spatial and temporal scales of coupled electronic, chemical, structural, and mechanical phenomena. This large range of spatial and time scales is illustrated in Figure 2.2.1 for Li-ion battery electrodes.8-14 Similar spatial and temporal ranges apply for interphases (see PRD 4), supercapacitors, membranes, and electrolytes. At the smallest scale is atomic lithium hopping through the lattice by diffusion and entering or leaving the electrode by intercalation, alloying, or surface redox chemistry. Associated with these atomic motions are local, coupled, structural and electronic changes that directly impact the redox processes and storage capacity. As ions continue to enter the electrode, its volume expands and often there is a phase transition, especially for alloying or conversion electrodes such as Li alloying with Si to form Li15Si4-like phases or Li reducing FeO to form Li2O and Fe0. As the volume change propagates throughout the electrode, stress builds up in ~100-nm size particles through chemo-mechanical coupling and mechanical incompatibilities,15 which can cause particle cracking. Alternatively, mechanical constraints can limit volume expansion, which can close internal pores and limit ion transport. If the particle state of charge is spatially heterogeneous, differential expansion can build up within and between particles and become another source of cracking. Such mechanical failure can cause fragments to disconnect from the conduction pathway to the current collector, and SEI formation on the fractured surface can consume Li, leading to degradation in capacity. These detrimental effects accumulate during cycling and can cause catastrophic failure, with implications for battery safety. Capturing the Coupling of Electronic, Structural, Chemical, and Mechanical Phenomena over a Wide Range of Space and Time: This is especially important for rare events in space and time that trigger coupled phenomena that grow uncontrollably and ultimately lead to catastrophic failure. By their nature rare events are infrequent and usually irreversible, but the inhomogeneous structures and coupled phenomena of energy storage devices can promote the occurrence of rare events. While the consequences of the event can span nanoseconds to days and can be local or extended, the event itself initiates at a small length and short time scale. At present, the occurrence of rare events cannot be predicted, nor can the sequence of coupled electronic, structural, chemical, and mechanical events that they may trigger leading to macroscopic failure. An example is dendrite nucleation (see Figure 2.2.2),16 which initiates locally in nanoseconds at a nucleation site of nanometer dimensions, but grows to micrometer size during cycling and eventually creates a short circuit between the anode and cathode, which often leads to thermal runaway. Correlation and Analysis of Coupled Phenomena and Processes: This is a key challenge across all aspects of understanding and improving electrochemical energy storage. Many techniques and approaches are available to address a limited range of individual phenomena at specific space and time scales, such as the change in microstructure of electrodes on lithiation.2,17,18 By linking the necessary length and time scales across diverse phenomena into a global, 3D representation of an electrochemical energy storage device, one could selectively zoom in or out of spatial regions of interest (e.g., a primary particle or the surface region of a particle) over a desired time scale and select the properties to display (e.g., elastic modulus or local state of charge). Understanding the interaction of coupled phenomena is critical for many energy storage processes, including interfacial evolution, electron and ion transport, charge transfer at interfaces, and degradation with charge- discharge cycling. Interface Evolution: This includes SEI formation and evolution, general passivation, and interface (surface) segregation. The length scales relevant for this phenomenon are nanometers to micrometers and the time scales are nanoseconds to years, as the interface continually evolves. For example, while the initial SEI formation is a one-time event occurring over milliseconds to hours, the SEI evolution is a continuous process effectively spanning the lifetime of the battery. For interphases, traditional electrochemical modeling solves coupled diffusion equations with predefined boundaries and boundary conditions. As materials change their size, interphases form and grow over time, and it is important to model and track the moving boundaries to 24 PRIORITY RESEARCH DIRECTION – 2PDF Image | Next Generation Electrical Energy Storage
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