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The control of experimental parameters and the relatively simple structure of this model system were amenable to simulation, and the combined results were consistent with a three-stage lithiation mechanism with a reaction- limited, layer-by-layer lithiation of the Si substrate. NEXT GENERATION ELECTRICAL ENERGY STORAGE ION DIFFUSION AT ATOMIC LENGTH AND PICOSECOND TIME SCALES: CONNECTING LENGTH SCALES The diffusion dynamics of the ions (e.g., Li+, Na+, or multivalent ions) through the crystal lattice of an electrode have a tremendous impact on the energy storage capacity, material lifetime, and especially, rate capability. Often, slow diffusion is the limiting factor in charge and discharge rates in electrodes, and, therefore, inhibits faster charging of electric vehicles. While first-principles calculations create a detailed picture of electroactive ions diffusing through a lattice,6 little is known experimentally about the atomic- scale processes involved in ion diffusion. Time scales for individual ion hopping events between adjacent interstitial sites approach ~100 fs to ps. These “jumps” Lattice Distortions Near Ions Electrolytes Ions 3D Metal-oxide or Nanostructured Electrode Materials are associated with significant transient changes in the crystal strain field, which, in turn, can influence the dynamics of neighboring ions at high concentrations. Such statistical ultrafast local events ultimately link to longer range dynamics spanning many orders of magnitude in time, because these slower coupled diffusional motions occur as a result of many local hops. Our ability to characterize such phenomena over the length and time scales required for comparison with theory has been limited by available experimental tools, and this has restricted our ability to develop validated design guidelines to improve ion diffusion in functional materials. The capabilities of free electron X-ray lasers will enable much more detailed insight into ion and atom migration in complex materials under operating conditions. Dynamic scattering with coherent X-rays (X-ray photon correlation spectroscopy) has already been shown to be a powerful probe of atomic diffusion,33 but is limited to relatively slow timescales (>milliseconds) by present X-ray sources. Advanced high-flux X-ray photon correlation spectroscopy with high-repetition-rate free electron lasers34 will enable operando studies of the local aspects of ion diffusion at high spatial resolution. Systematic studies will reveal how these processes depend on electrode nanostructure, crystal structure, diffusion direction, and ion concentration. Coherent X-ray scattering from the electrode material will directly probe transient distortions of the lattice and associated longer range strain fields that arise from stochastic ion hopping events. By using a megahertz repetition rate and split-delay-line X-ray photon correlation spectroscopy, times scales between picoseconds and milliseconds can be bridged. On the hopping time scale, <1 ps, this approach can determine the local distortion during the hop, which can be related to the energy barrier and speed of the hop, key features of simulations. Image courtesy of Hans-Georg Steinruck, SLAC National Accelerator Laboratory. PRIORITY RESEARCH DIRECTION – 2 33PDF Image | Next Generation Electrical Energy Storage
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