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

Abstract

In this study, fibrous membrane composites containing poly (vinylidene fluoride-co-hexafluoropropylene) P(VdF-co-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 °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 °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.