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A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A Y / J U N E 2 0 2 1 2 4 the anode side (orders of magnitude). During stripping, lithium voids are of- ten formed resulting in incomplete cy- cling or orphaning of the lithium metal. In experimental cells this loss of lith- ium is compensated for through the addition of stack pressure upwards of >1000 psi [11] . These pressures are clearly untenable for a cell where the hardware to achieve these pressures would neg- atively affect battery pack energy den- sity. These challenges require unique packaging engineering and cell designs or new ways to control the nucleation and growth of electrodeposited metals from solid electrolytes. The fourth major challenge in- volves interface control with time and potential [12] . Interfaces mediate ion transport but in a solid-state battery, in- terfaces are subject to the same poten- tials that form the CEI and SEI. At the cathode-SE interface, oxidation reac- tions dominate leading to oxygen losses or oxidation of the solid electrolyte (e.g., Li-S toLi-S-O). At theanode-SE interface, the extreme reductive ability of lithium reduces the solid electrolyte to simple species like Li 3 P, Li 2 O, and Li 2 S, while si- multaneously reducing transition met- als like titanium in Li 0.54 La 0.33 TiO 3 . These types of reactions introduce new inter- faces which increase cell resistance or electrically short the cell through prop- agation of the reduced transition metal species. A second form of interface evo- lution is the diffusion of transition met- als from the cathode into the SE during sintering to form a “good” interface. This diffusion results in ionically insu- lating interfaces, loss of electroactive materials, and electrical pathways for cell shorting. These challenges require new ways to predictively assemble materials to prevent interdiffusion/re- actions or the introduction of barrier layers, like the common LiNbO 3 , to form stable interfaces. CONCLUSION The challenges associated with the next generation of energy storage revolve around our limited ability to synthesize and process dissimilar ma- terials while maintaining high lithium transport. The interface-driven phe- nomenon inherent in solid-state bat- teries bridge all aspects of materials science and will require new entrants to the field with unique skills developed on analogous problems. Successfully addressing these challenges will revolu- tionize energy storage and safety while addressing the critical challenge of cell lifetime and energy density. ~AM&P For more information : Gabriel M. Veith, senior staff scientist, Oak Ridge Nation- al Laboratory, Oak Ridge, TN 37831, veithgm@ornl.gov . Acknowledgment This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Science and Engineering. References 1. A. Jain, et al., Commentary: The Materials Project: A Materials Genome Approach to Accelerating Materials Innovation, APL Materials, 1 (1), 011002, 2013. 2. K. Xu, Electrolytes and Interphases in Li-Ion Batteries and Beyond, Chem. Rev., 114 (23), p 11503-11618, 2014. 3. Z. Zou, et al., Mobile Ions in Composite Solids, Chem. Rev., 120 (9), p 4169-4221, 2020. 4. N.J. Dudney, Solid-state Thin-film Rechargeable Batteries, Mater. Sci. Eng., R Rep., 116 (3), p 245-249, 2005. 5. A. Hayashi, et al., Superionic Glass-ceramic Electrolytes for Room- temperature Rechargeable Sodium Bat- teries, Nat Commun, 3 , 856, 2012. 6. V. Bocharova, A.P. Sokolov, Per- spectives for Polymer Electrolytes: A View from Fundamentals of Ionic Conductivity, Macromolecules, 53 (11), p 4141-4157, 2020. 7. T. Thompson, et al., Electrochemical Window of the Li-Ion Solid Electrolyte Li7La3Zr2O12, ACS Energy Letters, 2 (2), p 462-468, 2017. 8. X. Liu, et al., Elucidating theMobility of H+ and Li+ Ions in (Li6.25−xHxAl0.25) La3Zr2O12 via Correlative Neutron and Electron Spectroscopy, Energy & Envir- onmental Science, 12 (3), p 945-951, 2019. 9. K. Xiao, et al., Bioinspired Ionic Sen- sory Systems: The Successor of Elec- tronics, Advanced Materials, 32 (31), 2000218, 2020. 10. P. Albertus, et al., Status and Challenges in Enabling the Lithium Metal Electrode for High-energy and Low-cost Rechargeable Batteries, Nature Energy, 3 (1), p 16-21, 2018. 11. J.-M. Doux, Stack Pressure Considerations for Room-Temperature All-Solid-State Lithium Metal Batter- ies, Advanced Energy Materials, 10 (1), 1903253, 2020. 12. Y. Xiao, Understanding Interface Stability in Solid-state Batteries, Nature Reviews Materials, 5 (2), p 105-126, 2020.