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An alternative strategy is to identify totally new solid electrolytes that do not exhibit these flaws. Unfortunately, the prediction of stable crystal structures based on a specified composition or property remains an unmet challenge. Nevertheless, computational techniques based on atomic-scale simulations can definitely aid in this search. A more practical, but still challenging strategy would involve screening of existing databases of known (or hypothetical) materials, such as the Inorganic Crystal Structure Database.33 Given a composition and crystal structure, arguably the most straightforward way to determine whether the corresponding compound exhibits high ionic conductivity is through molecular dynamics simulation. A drawback to this approach, however, is its limited throughput. Ab initio molecular dynamics although highly accurate, can typically access only a few picoseconds of simulation time due to its extreme expense, resulting in limited transport statistics. In contrast, classical molecular dynamics, which uses a prescribed interatomic potential, can access much longer simulation times, typically on the order of nanoseconds. Nevertheless, classical molecular dynamics remains moderately expensive, and the determination of accurate interatomic potentials that are transferrable across materials classes can pose a challenge. In summary, while molecular dynamics is well-suited for careful characterization of the transport properties of a small number of compounds, it is not amenable to high-throughput screening. Other methods for screening candidate solid electrolytes are needed. Rapid screening becomes possible when design rules or “descriptors” for ionic conductivity (and other key properties) are identified. In principle, these descriptors can then be evaluated with low-to-moderate computational expense for many candidate materials.34 In the case of ionic conductivity, correlations between ion mobility and several other structural or chemical features have been discussed.35,36 These include the topology of the migration channel, ionic radius and volume effects, ionic polarizability, bond valence, and phonon modes.35 To cite one example related to crystal topology, Figure 3.5.3 illustrates energy barriers associated with Li-ion migration within prototypical crystal lattices based on three anionic crystal structures: hexagonal close packed (hcp), body-centered cubic (bcc), and face-centered cubic (fcc).36 At present, the importance and generality of any single descriptor from the preceding list remain a matter of debate, as opportunities for validation are limited by the dearth of experimental data. Thus, the speed at which computationally guided discovery can proceed will be influenced by the availability of high-quality measurements drawn from diverse materials. In addition to high ionic conductivity, a viable solid electrolyte should also exhibit chemical and electrochemical stability with respect to the electrodes,37-39 have a wide electrochemical window,37 and display suitable mechanical properties40-42 and microstructural features. Not all of these requirements are well-defined (e.g., mechanical and microstructural); moreover, the sheer number of criteria complicates the discovery process. Although many of these properties can be calculated, doing so is expensive in some cases. At present, little or no effort has been devoted to identifying descriptors for these properties. A comprehensive strategy for the discovery of viable solid electrolytes must take these properties into account, and not focus solely on ionic conductivity. Key scientific challenges for discovery of new solid electrolytes include the following: identify what chemical and structural features provide for efficient ion transport properties, stability with electrodes, and dendrite suppression; predict electronic and ionic transport processes from computational methods, in concert with ab initio crystal structure prediction algorithms; develop and apply new in situ/operando experimental probes of interfacial stability, reaction, and interphase formation in model materials; and accelerate materials discovery through rapid synthesis and characterization protocols. While the mainline approach to enhanced solid electrolytes places a premium on high ionic conductivity materials, another route to high performance solid electrolytes is through much thinner (<100 nm) solid electrolytes. Basic principles of diffusion dictate that in such cases the charge/discharge rates are determined by diffusivity in the electrodes, so that electrolytes with inherently lower (10-100X) ionic conductivity can be tolerated in such thin layers. This route poses a different kind of synthesis challenge, based on vapor phase chemistry for materials growth under self-limiting surface reaction conditions employed in atomic layer deposition. Syntheses of LiPON43,44 and LLZO45 solid electrolytes by atomic layer deposition have been reported, with LiPON having been employed as an ultrathin (~40-100 nm) electrolyte in solid-state batteries.44 NEXT GENERATION ELECTRICAL ENERGY STORAGE PANEL 5 REPORT 133PDF Image | Next Generation Electrical Energy Storage
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