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describe the interface evolution. Approaches such as the phase-field method, which has been successfully applied to modeling temporal and spatial microstructure evolution in materials undergoing phase transformation, deformation, and particle coarsening, could be extended to include electrochemical processes. Hour, Day, Year Min. Sec. Micro- Milli Sec. Rare, but Instantaneous Particle Expansion Electrode: 20 10 0 Si Li+ Pico- Nano Sec. Dendrite Nucleation Dendrites Cathode Electrolyte and Separator Li Anode Mechanical Stress and Strain 2 Strain Unit Cell and Volume Expansion Ion Hopping 4 Strain 0.5 1 0 Delamination, Cracking, Loss of Active Material, Dendrites 1.0 3 Primary and Secondary Particle: 0 Å nm μm mm cm Length Figure 2.2.1. Plot of approximate time and length scales for phenomena occurring in the electrochemical energy storage materials during charging and discharging using Li-ion batteries as the example. The insertion and removal of Li ions in the electrode (lower left showing Li diffusion and alloying) cause volume changes and defect formation in particles (middle images of expansion and spatially resolved strain), which induce stress and strain on larger length scales, leading to degradation and, possibly, battery failure (upper right images showing local elastic modulus and dendrites). The upper left corner indicates rare events that might occur over short time periods and infrequently in the space of a few cubic nanometers, but are a root cause for an effect that evolves over days to seriously impact the entire battery. Images in figure drawn from Refs. 8-10, 12. Electron and Ion Transport: This is critical to the operation of a battery, whether driven by diffusion, migration, or convection. The length scales of interest span from the Ångstroms associated with ion hops to the full cell dimensions of millimeters to centimeters, and the time scale ranges from picoseconds to hours, largely determined by diffusivities. While much progress has been made in understanding phenomena at slower time scales of seconds and longer, there is a critical challenge, especially with experiments, to understand faster dynamics associated, for example, with ion motion (hopping) or transport across interphase regions. Electrochemical impedance spectroscopy can reveal dominant transport processes both in electrodes and NEXT GENERATION ELECTRICAL ENERGY STORAGE Battery Failure 700 nm -0.5 -1.0 400 nm PRIORITY RESEARCH DIRECTION – 2 25 Time E⊥ (GPa)PDF Image | Next Generation Electrical Energy Storage
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