Constructing Li‐Rich Artificial SEI Layer in Alloy–Polymer Composite Electrolyte to Achieve High Ionic Conductivity for All‐Solid‐State Lithium Metal Batteries

Abstract To achieve high ionic conductivity for solid electrolyte, an artificial Li‐rich interface layer of about 60 nm thick has been constructed in polymer‐based poly(ethylene oxide)‐lithium bis(trifluoromethanesulfonyl)imide composite solid electrolyte (briefly noted as PEO m ) by adding Li‐based alloys. As revealed by high‐resolution transmission electron microscopy and electron energy loss spectroscopy, an artificial interface layer of amorphous feature is created around the Li‐based alloy particles with the gradient distribution of Li across it. Electrochemical analysis and theoretical modeling demonstrate that the interface layer provides fast ion transport path and plays a key role in achieving high and stable ionic conductivity for PEO m ‐Li 21 Si 5 composite solid electrolyte. The PEO m ‐5%Li 21 Si 5 composite electrolyte exhibits an ionic conductivity of 3.9 × 10 –5 S cm −1 at 30 ° C and 5.6 × 10 −4 S cm −1 at 45 ° C. The LiFePO 4 | PEO m ‐5%Li 21 Si 5 | Li all‐solid‐state batteries could maintain a stable capacity of 129.2 mA h g −1 at 0.2 C and 30 ° C after 100 cycles, and 111.3 mA h g −1 after 200 cycles at 0.5 C and 45 ° C, demonstrating excellent cycling stability and high‐rate capability.