Liquid phase therapy with localized high-concentration electrolytes for solid-state Li metal pouch cells

Solid-state Li metal batteries are considered promising next-generation energy storage systems due to its exceptional advantages in terms of safety and high energy density. The continuous process on the development of solid-state fast ionic electrolytes enables the solid-state battery to operate at room temperature. Among these, sulfide-based solid electrolytes have attracted significant attentions due to their extremely high ionic conductivity, excellent deformability, and mild low-temperature processability. However, the full demonstration of practical batteries remains challenging due to the slow lithium-ion transport kinetics at working solid-solid interfaces. The sluggish interfacial transport kinetics mainly result from the poor solid-solid contacts, resulting in poor battery performance. Especially for solid-state pouch cells, the high local current due to the poor contact is amplified by the high working current, leading to rapid failure. Constructing fast ion transport paths between the Li metal anode and solid electrolyte interface is key for the practical application of solid-state batteries. Here a simple protocol was developed to realize fast ionic transportation by wetting the solid electrolyte/Li metal anode interface with localized high salt concentration liquid electrolyte. First, 3.5 mmol lithium trifluoroalfonylimide (LiTFSI) was added into 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (HFE) and dimethoxyethane (DME) mixed solvent, and stirred to obtain uniformly dispersed localized high-concentration liquid electrolyte, denoted as HFE-DME LiTFSI. The fluidity of liquid electrolyte ensures sufficiently conformal contacts between lithium anode and liquid electrolyte, as well as solid-state electrolyte and liquid electrolyte. Thus, fast ion transportation channels were constructed between the solid electrolyte and Li metal anode by wetting HFE-DME LiTFSI at a concentration of 3.0 μL·cm⁻². After liquid phase therapy, the interfacial resistance of solid-state Li|Li4Ti5O12 (LTO) pouch cell rapidly reduced from 4366 to 64 Ω·cm⁻² and even lower than the cell that was pressed at 3 MPa in the assemble process (340 Ω·cm⁻²). This suggests that the ion transport kinetics are significantly improved by liquid phase therapy. Therefore, the solid-state Li metal pouch cell with dimensions of 30 mm × 30 mm showed excellent cycling performances with specific capacities of 107 and 96 mAh·g⁻¹ at 0.1C and 0.5C, respectively. Furthermore, the solid-state Li-S pouch cell delivered capacities of 1100 and 932 mAh·g⁻¹ at 0.01C and 0.02C, respectively. This study demonstrates the effectiveness of the novel liquid phase therapy to construct fast ionic transportation channels, which providing an effective strategy for the practical application of solid-state Li metal pouch cells.