Polymer/ceramic co-continuous nanofiber membranes via room-curable organopolysilazane for improved lithium-ion battery performance

Polyacrylonitrile (PAN) and ambient temperature-curable organopolysilazane were combined to successfully fabricate PAN/polymer-derived ceramic (PDC) hybrid nanofiber separator using a single-step electrospinning process. The amount of added precursor was varied from 10 to 30% to characterize the effects of various loadings on the mechanical, thermal properties, and electrochemical performance on the hybrid membranes. TEM images reveal that all composite fibers have a thin (~5 nm) ceramic-rich sheath layer surrounding each fiber. In addition, there is also ceramic present inside the fiber with the ceramic forming continuous network within the nanofiber at high concentrations. The interconnected ceramic network within the fiber disrupts the polymers ability to properly crystallize, leading to an increase in amorphous regions with an increase in ceramic inclusion. The presence of the ceramic on the surface of the membrane in addition to the increased amorphous regions leads to excellent ionic conductivity and cycling performance. The 30 wt% PDC sample has an ionic conductivity of 1.05 mS cm−1 compared to 0.29 mS cm−1 of pristine PAN separator. All separators with additional PDC content showed increased initial capacity and capacity retention at 0.2C charging and discharging rate, with the 90:10, 80:20, and 70:30 wt% of PAN:PDC showing 89, 90 and 93% capacity retention of graphite/LiCoO2 full cells over 100 cycles, respectively. In rate capability testing, PAN/PDC fibers demonstrated increased capacity retention even at increased charge rates. The results suggest that the increased ionic conductivity and wetting behavior by both the ceramics within the membrane and on the surface of the membrane are more correlated to an increase in capacity retention and rate capability than the porosity.