Bilayer solid electrolyte enabling quasi-solid-state lithium-metal batteries

NASICON-type electrolytes such as Li1.5Al0.5Ge1.5(PO4)3 (LAGP) potentially enable high safety and high energy for solid-state batteries. However, the poor interfacial stability with lithium metal remains a main issue. To overcome this challenge, we proposed a bilayer solid electrolyte architecture implementing a novel ultrathin solid polymer electrolyte (SPE) film in combination with LAGP to improve the interface with lithium metal. The SPE film is composed of a bis(fluorosulfonyl)imide-based ionic liquid and polyethylene oxide, which shows a remarkable ionic conductivity of 1.25 × 10−3 S cm−1 at room temperature. The application of this thin interlayer leads to an outstanding interface stability, allowing >2000 h of continuous Li stripping/plating in symmetric Li|SPE/LAGP/SPE|Li cells without any increase in polarization or indication of a short circuit. As a result, this approach enables Li|SPE/LAGP|NCM811 cells with a discharge capacity of ∼200 mAh g−1 at 0.1C and stable cycling for >400 cycles at 0.2C with a capacity retention of 83%. Additionally, the cell shows an extremely high average Coulombic efficiency of 99.96% demonstrating that the approach enables to achieve high-energy and long-term stable solid-state lithium-metal batteries.