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To facilitate material discovery, in situ monitoring of the synthesis process may help chart chemical phase evolution pathways and identify metastable phases far from thermodynamic equilibrium. For example, in situ electrochemical impedance spectroscopy integrated with solid-state synthesis of Li7S3P11 has been used to identify degradation processes at high temperatures, leading to the formation of undesired phases and reduced ionic conductivities (Figure 3.5.4). With this information, the authors were able to identify optimal synthesis conditions.46 Moreover, simultaneous determination of structure and properties of target and intermediate compounds will allow fast experimental screening of materials. In situ techniques, including X-ray diffraction,46 Raman spectroscopy,47 X-ray photoelectron spectroscopy,48 electron microscopy,49 and solid-state nuclear magnetic resonance,50 have been recently employed to create “panoramic” reaction maps and accelerate the discovery of efficient synthesis routes for many classes of materials. Al heat sink Al2O3 galvanic decoupling thermocouple Al2O3 galvanic decoupling electronic pressure gauge 1 100 crystallization Li7P3S11 50 1.25 NEXT GENERATION ELECTRICAL ENERGY STORAGE hydraulic cylinder hardened steel frame heatable die/EIS cell splinter protection coaxial heating coil Al heat sink 1000 300 Tc 250 100 1.75 Tg 200 10 150 1.50 0.1 cooldown 0 1.00 01234567 time/h Figure 3.5.4. (Left) Schematic of hot press apparatus designed for real-time acquisition of process parameters (pressure and temperature) combined with electrochemical impedance spectroscopy. (Right) Synthesis diagram showing the evolution of Li7S3P11 resistivity during thermal treatment. Sample temperature (orange), pressure (green), and resistance (purple). From Ref. 46. Reprinted with permission of American Chemical Society. Sintering to consolidate loose electrolyte powders into dense layers is another challenge and is traditionally accomplished by solid-state diffusion at high temperatures. The process is controlled by the slowest moving species and can thus require several hours at temperatures above 1000°C to achieve densification. Ironically, the high diffusivity of Li ions at room temperature, which is desired in solid-state electrolytes, causes loss of Li during sintering. Thus, from a practical perspective, sintering is often a bottleneck in ceramic processing that must be overcome to dramatically improve manufacturing throughput and lower costs. Recent advances in ceramic processing science have the potential to address these challenges. Spark plasma sintering is one example of field-assisted sintering techniques that hold promise in consolidating ceramic electrolytes to full density without a significant change in composition and phase.51 This method has been used in the preparation of NASICON,52 garnet,53 and Li7P3S1154 electrolytes, in addition to the assembly of complete solid-state electrochemical cells.52 Other advanced material processing capabilities, such as rapid thermal annealing, laser processing, and rapid quench processing, also offer opportunities to control the morphology of solid-state materials. For example, glassy phases can be kinetically trapped as the thermal quench outpaces crystallization. These processing techniques will provide a rich foundation for a full range of basic research on synthesis and should enable revolutionary advances in structures and properties. As promising solid electrolytes with high ionic conductivities and enhanced electrochemical stabilities have been developed over the past decade, efforts to demonstrate solid-state energy storage in practical devices have highlighted fundamental challenges in fabrication. Literature demonstrations of solid-state cells typically show non-optimized designs where the solid electrolyte is several hundreds of micrometers thick. However, the solid electrolyte should be as thin as possible to minimize its volume and mass contribution to the cell. Thinness is a critical design parameter in terms of transport as well. For solid-state cells to have rate performance comparable to that of conventional lithium-ion cells, the area-specific resistance of the solid electrolyte should be on the order of 10 W·cm2. Therefore, solid lithium conductors with relatively high conductivities of 10-4 and 10-3 S·cm-1 need to be fabricated with thicknesses on the order of 10 and 100 mm, respectively. For materials with inferior lithium conductivities, the solid electrolyte layer thickness must be further minimized to accommodate the area specific resistance requirement. Advanced film or coating fabrication technologies, such as various solution casting approaches and physical or chemical vapor deposition, can provide these capabilities. These transport requirements also highlight fundamental challenges in integrating these thin solid electrolyte layers with energy- 2.00 PANEL 5 REPORT 135 P (ton) T (°C) R (Ω) nucleation softeningPDF Image | Next Generation Electrical Energy Storage
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