Enhancement of physical and electrochemical properties of electrospun P (VdF-co-HFP) separators by incorporating magnesium borate for advanced lithium-ion batteries

In this study, fibrous membrane composites containing poly (vinylidene fluoride-co-hexafluoropropylene) P(VdFco-HFP) copolymer and various amounts of magnesium borate Mg2B2O5 were prepared via electrospinning method. The crystallization behavior, thermal stability, microstructure, porosity, liquid electrolyte uptake and electrochemical performance of electrospun membranes were investigated in detail. X-ray diffraction (XRD) and differential scanning calorimetry (DSC) show that, the crystallinity of P(VdF-co-HFP) tends to decrease with increasing amount of Mg2B2O5 particles. The electrospun membrane containing P(VdF-co-HFP) and 2.5 wt% of Mg2B2O5 demonstrates stronger anti-shrinkage properties compared to those of commercial polypropylene (PP) or pure P(VdF-co-HFP) separators at 140 degrees C. The fibrous membrane consisting of 2.5 wt% Mg2B2O5 exhibits high electrolyte uptake (381 %), low interfacial resistance and good porosity at room temperature (81 %), even after heating at 140 degrees C the porosity becomes 78 %. Moreover, Li||LiFePO4 cell using P(VdF-co-HFP) separator with 2.5 wt% of Mg2B2O5 shows better discharge capacities of 167.5 and 146.8 mAh g- 1 at 0.2 C and 5 C, respectively, compared to those of commercial PP separator which delivers only 158 and 127 mAh g- 1 at 0.2 C and 5 C, respectively. [P(VdF-co-HFP) + 2.5 wt% Mg2B2O5] separator also provides discharge capacity retention of 99.9 % after 100 cycles at 9 C, in comparison with only 87 % for polypropylene separator. Such results reveal that P (VdF-co-HFP) membrane containing 2.5 wt% Mg2B2O5 is a highly promising separator with good safety which can be used in high-performance lithium-ion batteries.