Synthesis of porous magnesia fibers with enhanced performance as a binder for molten electrolyte

•Porous MgO fibers with controlled pore size were prepared by hydrothermal method.•The effect of the PEG addition on the pore size of MgO fibers was studied.•Porous MgO fibers improve the immobilizing efficiency of molten electrolyte.•Discharge capacity and ionic conductivity of electrolyte pellets filled with porous MgO fibers were enhanced especially.