Polymalonamide-Modified Poly(Ethylene Oxide) Solid Polymer Electrolytes with Tunable Diamine Structures for Lithium Batteries

Abstract

A series of polymalonamide (PM)-modified poly(ethylene oxide) (PEO) solid polymer electrolytes (SPEs) were developed to enhance the electrochemical performance of all-solid-state lithium–ion batteries. The PMs were synthesized from reacting a reactive intermediate bis(azetidine-2,4-dione), derived from methylene diphenyl diisocyanate, with either the short-chain hexamethylenediamine (PM-HDA), or the polyetheramine, JEFFAMINE XTJ-500 (PM-XTJ). The β-dicarbonyl and amide functional groups in the PMs act cooperatively to enhance lithium salt dissociation while restricting anion mobility, thereby improving both ionic conductivity and lithium–ion transference number. The short-chain PM-HDA-based SPE (SPMH-3, 20 wt %) achieved a high Li+ transference number of 0.287 and supported faster charge–discharge kinetics, demonstrating excellent midterm stability with 93.4% capacity retention after 100 cycles. In comparison, the sample containing 10 wt % PM-XTJ (namely SPMX-2) delivered the long-term performance, exhibiting an initial discharge capacity of 166 mAh g–1 and retaining 87.6% of its capacity after 300 cycles at 0.2 C in LiFePO4 cells. These findings indicate that careful selection of diamine monomers allows fine-tuning of PEO-based SPE properties and provides a molecular-level design strategy for developing safe, high-performance solid-state electrolytes.