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In this Perspective, we highlight recent progress and challenges related to the integration of lithium metal anodes in solid-state batteries. While prior reports have suggested that solid electrolytes may be impermeable to lithium metal, this hypothesis has been disproven under a variety of electrolyte compositions and cycling conditions. Herein, we describe the mechanistic origins and importance of lithium filament growth and interphase formation in inorganic and organic solid electrolytes. Multimodal techniques that combine real and reciprocal space imaging and modeling will be necessary to fully understand nonequilibrium dynamics at these buried interfaces. Currently, most studies on lithium electrode kinetics at solid electrolyte interfaces are completed in symmetric Li–Li configurations. To fully understand the challenges and opportunities afforded by Li-metal anodes, full-cell experiments are necessary. Finally, the impacts of operating conditions on solid-state batteries are largely unknown with respect to pressure, geometry, and break-in protocols. Given the rapid growth of this community and the diverse portfolio of solid electrolytes, we highlight the need for detailed reporting of experimental conditions and standardization of protocols across the community.

Realizing solid‐state lithium batteries with higher energy density and enhanced safety compared to the conventional liquid lithium‐ion batteries is one of the primary research and development goals set for next‐generation batteries in this decade. In this regard, polymer electrolytes have been widely researched as solid electrolytes due to their excellent processability, flexibility, and low weight. With high cationic transference numbers (tLi+ close to 1), single‐ion conducting polymer electrolytes (SICPEs) have tremendous advantages compared to polymer electrolyte systems (tLi+ < 0.4) because of their potential to reduce the buildup of ion concentration gradients and suppress growth of lithium dendrites. The current review covers the fundamentals of SICPEs, including anionic unit synthesis, polymer structure design, and film fabrication, along with simulation and experimental results in solid‐state lithium–metal battery applications. A perspective on current challenges, possible solutions, and potential research directions of SICPEs is also discussed to provide the research community with the critical technical aspects that may advance SICPEs as solid electrolytes in next‐generation energy storage systems.

Solid-state batteries utilizing Li metal anodes have the potential to enable improved performance (specific energy >500 Wh/kg, energy density >1,500 Wh/L), safety, recyclability, and potentially lower cost (< $100/kWh) compared to advanced Li-ion systems. 1,2 These improvements are critical for the widespread adoption of electric vehicles and trucks and could create a short haul electric aviation industry. [1][2][3] Expectations for solid-state batteries are high, but there are significant materials and processing challenges to overcome.On May 15 th , 2020, Oak Ridge National Laboratory (ORNL) hosted a 6-hour, national online workshop to discuss recent advances and prominent obstacles to realizing solid-state Li metal batteries. The workshop included more than 30 experts from national laboratories, universities, and companies, all of whom have worked on solid-state batteries for multiple years. The participants' consensus is that, although recent progress on solid-state batteries is exciting, much has yet to be researched, discovered, scaled, and developed. Our goal was to examine the issues and identify the most pressing needs and most significant opportunities. The organizers asked workshop participants to present their views by articulating fundamental knowledge gaps for materials and processing science, mechanical behavior and battery architectures critical to advancing solid-state battery technology. The organizers used this input to set the workshop agenda. The group also considered what would incentivize the adoption of US manufacturing and how to accelerate and focus research attention for the benefit of the US energy, climate, and economic interests. The participants identified pros and cons for sulfide, oxide, and polymerbased solid-state batteries and identified common science gaps among the different chemistries. Addressing these common science gaps may reveal the most promising systems to pursue in the future.

Lithium phosphorus oxynitride, also known as Lipon, solid-state electrolytes are at the center of the search for solid-state Li metal batteries. Key to the performance of Lipon is a combination of high Li content, amorphous character, and the incorporation of N into the structure. Despite the material's importance, our work presents the first study to fully resolve the structure of Lipon using a combination of  ab initio molecular dynamics, density functional theory, neutron scattering, and infrared spectroscopy. The modeled and experimental results have exceptional agreement in both neutron pair distribution function and infrared spectroscopy. Building on this synergy, the structural models show that N forms both bridges between two phosphate units and nonbridging apical N. We further show that as the Li content is increased the ratio of bridging to apical N shifts from being predominantly bridging at Li contents around 2.5:1 Li:P to only apical N at higher Li contents of 3.38:1 Li:P. This crossover from bridging to apical N appears to directly correlate with and explain both the increase in ionic conductivity with the incorporation of N and the ionic conductivity trends found in the literature.