Nanostructured Li-Rich Fluoride Coated by Ionic Liquid as High Ion-Conductivity Solid Electrolyte Additive to Suppress Dendrite Growth at Li Metal Anode.

Blending additive with electrolyte is a facile and effective method to suppress anode dendrite growth in Li metal batteries (LMBs), especially when a LiF-rich solid electrolyte interface (SEI) is formed as a consequence of additive decomposition or deposition. However LiF still suffers from poor bulk ion conductivity as well as the difficult access to tailored nanostructure. Exploring new Li fluoride of high Li-ion conductivity as SEI component is still a big challenge in view of the lacking of desired structure prototype or mineral phase. Here, we propose a Li-rich Li3AlF6 derivative from cryolite phase as solid electrolyte additive, which is characterized by textured nanoporous morphology and ionic liquid coating. Its room temperature ion conductivity is as high as ∼10-5 S/cm with a low activation energy of 0.29 eV, the best level among fluoride-based solid electrolytes. These features guarantee a homogenization of Li+ fluxing through bulk and grain boundary of Li3AlF6-rich SEI and reinforce the effect on Li dendrite suppression. Li3AlF6 additive enables a stable cyclability of Li∥Li symmetric cells for at least 100 cycles even under a high areal capacity of 3 mA h/cm2 and a significant improvement on capacity retention for various LMBs based on LiFePO4, FeS2, and S cathodes.