Grafted polyrotaxanes as highly conductive electrolytes for lithium metal batteries

Abstract

Hyperbranched polymers comprised of polyrotaxanes as mechanically stable backbone and grafted polycaprolactone (PCL) side chains are utilized as solid polymer electrolyte for application in lithium metal (LMBs) and lithium ion batteries (LIBs). The polyrotaxanes were obtained from self-assembly of Cyclodextrin (CD) host molecules threading onto polyethylenoxide (PEO) chains. In particular, CD serves as initiator for a ring-opening-polymerization of PCL affording pendant side chains with merely a few monomer unit lengths that foster enhanced lithium ion transport, as mediated by well-defined lamellar morphology of the PCL side chains. An impressive ionic conductivity of 1 mS cm−1 of the solid polymer electrolyte at 60 °C and more than 0.1 mS cm-1 at room temperature in addition to a superior oxidative electrochemical stability of up to 4.7 V vs. Li/Li+ allows for robust galvanostatic cycling in LiFePO4|Li cells, even at reduced temperatures not accessible by commonly utilized PEO-based electrolytes. The hyperbranched polymers can be readily up-scaled and further modified, thereby demonstrating the versatility of the introduced class of solid-state polymer electrolytes, as reflected by its interfacial stability against the high-capacity Lithium metal anode.