Poly(vinylidene Fluoride)/poly(acrylonitrile) Blend Fibers Based Separators For Lithium-ion Batteries

The demand for high performance lithium-ion batteries(LIBs) is increasing rapidly, while the separator is an obstacle to the development of high performance LIBs. Currently, polyolefin based separators are not suitable for high performance LIBs due to their instinct characteristics such as low porosity, poor affinity with electrolyte and inferior thermal stability. Therefore, it is urging to develop new separators with better properties for high performance LIBs.Fibrous membranes are very attractive as separators due to high porosity and superior electrolyte uptake. In this dissertation, poly(vinylidene fluoride)/poly(acrylonitrile)(PVDF/PAN) fibrous membranes were fabricated for high performance LIBs, and the results are as following.(1) PVDF/PAN fibrous membranes with different weight ratio were prepared by electrospinning. By doped a PVDF solution, mechanical strength of 5/5 blend membranes was further enhanced, i.e. achieved 20.4 MPa. Besides, the electrolyte uptake of 5/5 blend membrane was about 320% and the ionic conductivity was 1.45 mS/cm. PAN nanofibrils were observed in an individual blend nanofiber. The presence of continuous PAN component in the blend nanofibers can serve as skeletons to sustain dimensional stability of membranes at elevated temperature. Besides, the blend membranes have an excellent cycling performance and C-rate performance. Compared with commercial separators, the doped blend membranes show better comprehensive performance.(2) Electrospinning is a popular approach to produce fibrous membranes, but its production rate is very low. As a comparison, mass production of fibrous membranes can be achieved by centrifugal spinning. In this section, PVDF, PVDF/PAN and PAN fibrous membranes were mass produced by centrifugal spinning. It was found that the centrifugal spun membranes exhibited higher porosity, higher ionic conductivity, better thermal stability and superior electrochemical performance than commercial Celgard 2400 separators. Besides, the mechanical strength of PVDF/PAN with weight ratio of 5/5 was 18.6 MPa. The initial discharge capacity at 0.2C-rate was 147.7 mAh/g. It demonstrated that the blend fibrous membranes, massively produced by centrifugal spinning, have great potentials in the application as separators of high performance lithium-ion battery separators.(3) Hybrid silica membranes with a PVDF/PAN nanofiber network with weight ratio of 5/5 were prepared via penetrated by a silica nanoparticle suspension under vacuum. It was found that the hybrid silica membranes showed excellent thermal stability. Meanwhile, the mechanical property and electrolyte uptake capability were good too. Moreover, the cycling performance and C-rate performance of the coin cells with hybrid silica membranes as separators were better than commercial Celgard 2400 separators. Initial discharge capacity was 150.6 mAh/g for hybrid silica membranes as separators at 0.2C-rate, and high coulombic efficiency of about 99 % was observed for coin cells with hybrid silica membranes as separators.