Synergistic polymer-ceramic separator design for high lithium transference and stable cycling of lithium-ion...

A polymer-ceramic composite separator was developed by coating a commercial polypropylene (PP) membrane with a magnesium oxide (MgO)-filled dual-polymer adhesive layer composed of poly(acrylic acid) (PAA) and poly(vinylidene fluoride) (PVDF). By systematically tuning the PAA:PVDF ratio at a fixed MgO content, the effects of composition on coating morphology, thermal tolerance, electrolyte affinity, and ion-transport behavior were clarified. The optimized PAA:PVDF:MgO separator with a ratio of 15:5:80, hereafter referred to as PPM-15:5, exhibits near-zero electrolyte contact angle, high electrolyte uptake, and an enhanced ionic conductivity of 1.46 mS cm^-1 at room temperature, together with a high lithium-ion transference number of 0.71. Electrochemical measurements further indicate improved oxidative stability and accelerated interfacial charge-transfer kinetics relative to PP. In lithium||lithium iron phosphate (Li||LiFePO₄) half cells, the PPM-15:5 separator delivers superior rate capability and stable long-term cycling, retaining 92% capacity after 200 cycles at 0.5C and delivering a high discharge capacity of 123.8 mAh g^-1 at high c-rate of 5C. Accelerated electrolyte-aging tests, ¹⁹F NMR, and recovered-MgO XRD provide complementary evidence for MgO-assisted electrolyte stabilization under LiPF₆/H₂O aging conditions. Overall, this work establishes a clear composition-structure-transport relationship for MgO-reinforced PAA/PVDF coatings and provides a practical separator design strategy for high-rate, long-life lithium-ion batteries.