Synergistic Enhancement of Mechanical and Electrochemical Properties in Grafted Polymer/Oxide Hybrid Electrolytes

Abstract Lithium metal batteries operated with high voltage cathodes are predestined for the realization of high energy storage systems, where solid polymer electrolytes offer a possibility to improve battery safety. Al 2 O 3 PCL is introduced as promising hybrid electrolyte made from polycaprolactone (PCL) and Al 2 O 3 nanoparticles that can be prepared in a one‐pot synthesis as a random mixture of linear PCL and PCL‐grafted Al 2 O 3. Upon grafting, synergistic effects of mechanical stability and ionic conductivity are achieved. Due to the mechanical stability, manufacture of PCL‐based membranes with a thickness of 50 µm is feasible, yielding an ionic conductivity of 5·10 −5 S cm −1 at 60 °C. The membrane exhibits an impressive performance of Li deposition in symmetric Li||Li cells, operating for 1200 h at a constant and low overvoltage of 54 mV and a current density of 0. 2 mA cm −2. NMC 622 | Al 2 O 3 PCL | Li cells are cycled at rates of up to 1 C, achieving 140 cycles at >80% state of health. The straightforward synthesis and opportunity of upscaling as well as solvent‐free polymerization render the Al 2 O 3 PCL hybrid material as rather safe, potentially sustainable and affordable alternative to conventional polymer‐based electrolytes.