Structure and performance study of iPP microporous membrane derived from β-transcrystalline precursor for lithium-ion battery separators

A β-transcrystalline (β-TC) polypropylene microporous membrane fabricated by synchronous biaxial stretching of a precursor film with continuous and ordered β-transcrystallinity was evaluated as a lithium-ion battery separator, with a conventional β-spherocrystalline (β-SC) membrane as the comparative sample. The β-TC separator exhibited greater elongation at break than its counterpart and favorable elastic recovery under compressive stress, and its maximum puncture force was approximately twice that of its counterpart. It also maintained structural integrity and microporosity to higher temperatures, with a considerably lower degree of thermal shrinkage. EIS measurements revealed that the β-TC separator possessed higher ionic conductivity and lower bulk resistance than its counterpart. In LiFePO₄/Li half-cells, the β-TC separator delivered superior rate capability, with the capacity gap widening at elevated C-rates, and its capacity retention over 200 cycles was 78.6%, compared with 54.0% for the β-SC separator. These performance advantages are attributed to the uniformly fine fibrillar network and narrow pore size distribution of the β-TC membrane, which provide more efficient ion transport pathways and consequently enable superior rate capability and cycling stability in the cells.