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.

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Engineering a stable solid electrolyte interphase (SEI) is critical for suppression of lithium dendrites. However, the formation of a desired SEI by formulating electrolyte composition is very difficult due to complex electrochemical reduction reactions. Here, instead of trial-anderror of electrolyte composition, we design a Li-11 wt % Sr alloy anode to form a SrF 2 -rich SEI in fluorinated electrolytes. Density functional theory (DFT) calculation and experimental characterization demonstrate that a SrF 2 -rich SEI has a large interfacial energy with Li metal and a high mechanical strength, which can effectively suppress the Li dendrite growth by simultaneously promoting the lateral growth of deposited Li metal and the SEI stability. The Li−Sr/Cu cells in 2 M LiFSI-DME show an outstanding Li plating/stripping Coulombic efficiency of 99.42% at 1 mA cm −2 with a capacity of 1 mAh cm −2 and 98.95% at 3 mA cm −2 with a capacity of 2 mAh cm −2 , respectively. The symmetric Li−Sr/Li−Sr cells also achieve a stable electrochemical performance of 180 cycles at an extremely high current density of 30 mA cm −2 with a capacity of 1 mAh cm −2 . When paired with LiFePO 4 (LFP) and LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811) cathodes, Li−Sr/LFP cells in 2 M LiFSI-DME electrolytes and Li−Sr/NMC811 cells in 1 M LiPF 6 in FEC:FEMC:HFE electrolytes also maintain excellent capacity retention. Designing SEIs by regulating Li-metal anode composition opens up a new and rational avenue to suppress Li dendrites.

P2‐type layered oxides suffer from an ordered Na+/vacancy arrangement and P2→O2/OP4 phase transitions, leading them to exhibit multiple voltage plateaus upon Na+ extraction/insertion. The deficient sodium in the P2‐type cathode easily induces the bad structural stability at deep desodiation states and limited reversible capacity during Na+ de/insertion. These drawbacks cause poor rate capability and fast capacity decay in most P2‐type layered oxides. To address these challenges, a novel high sodium content (0.85) and plateau‐free P2‐type cathode‐Na0.85Li0.12Ni0.22Mn0.66O2 (P2‐NLNMO) was developed. The complete solid‐solution reaction over a wide voltage range ensures both fast Na+ mobility (10−11 to 10−10 cm2 s−1) and small volume variation (1.7 %). The high sodium content P2‐NLNMO exhibits a higher reversible capacity of 123.4 mA h g−1, superior rate capability of 79.3 mA h g−1 at 20 C, and 85.4 % capacity retention after 500 cycles at 5 C. The sufficient Na and complete solid‐solution reaction are critical to realizing high‐performance P2‐type cathodes for sodium‐ion batteries.

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