Porosity Engineering in Thermally Robust Polymer Nanocomposite Electrolytes for Sustainable Energy Storage: Salt Speciation, Trapping, and Collective Ion Transport

Porosity is increasingly used to engineer solvent-free solid and quasi-solid polymer nanocomposite electrolytes for Li-, Na-, and Zn-based batteries, but its transport role is often obscured by retained liquids, polymer plasticization, interfacial chemistry, and thermally induced structural change. This review organizes the literature by physical state, pore location, and dominant conducting phase, then examines how confinement modifies salt speciation, effective carrier size, ion-wall coordination, connectivity, occupancy, trapping, and interfacial flux. A physical-state audit separates intrinsic solid transport from liquid-assisted conduction, while comparative data show that pore size or total porosity alone does not predict conductivity or cation selectivity. Across chemistries, useful transport depends on connected ion-accessible pathways, reversible ion-specific interactions, and retention of the conducting architecture during thermal and electrochemical operation. The resulting thermal–porosity–transport framework distinguishes geometric percolation from dynamic ion coupling and treats trapping through species-specific residence times. It is demonstrated on selected literature series as a diagnostic rather than a universal constitutive model. The review concludes with falsifiable tests, minimum reporting requirements, and sustainability considerations for porous polymer nanocomposite electrolytes.