Study On The High-performance Composite Meltblown Li-ion Battery Separator

Because Li-ion battery has many advantages over other conventional batteries, including longer service life, higher operational voltage, lower self-discharge rate and higher energy density, it has been the major power source for various electronic devices like mobile phones, MP3 player, digital cameras, laptops, tablet computers and so on. With the deterioration of environment and the enhancement of people’s environmental awareness, as a clean energy source, lithium ion battery has been applied to electric cars and large energy storage systems, which puts forward some new requirements on lithium ion batteries, such as rate and safety performance. As one of the core components of lithium-ion batteries, lithium ion battery separator plays a crucial role in Li-ion batteries. In order to develop a composite separator with a satisfying performance, the effects of heat treatment and drawing on the structure and mechanical properties of PP meltblown nonwovens, the fabrication process of microporous PVDF-HFP membrane and its properties, and physical and electrochemical properties of the PVDF-HFP/Si O2 composite meltblown separators were studied. PE commercial separator was studied for comparison. The following conclusions were reached:(1)In order to obtain PP meltblown fabric with good mechanical properties, changes of the properties of PP meltblown materials were studied after heat treatment under certain draft condition, and it was found that more fibers oriented along the MD(machine direction) of the nonwoven and improved crystallinity of the fibers was observed, leading to enhanced mechanical strength of PP meltblown nonwoven and decreased elongation at break in MD. The heat treatment temperature at 130℃ showed the best result, and the breaking strength of PP meltblown materials in machine direction is increased by 45.2% after heat treatment at this temperature.(2) Ideal PVDF-HFP film should possess a submicron average pore size, a small maximum pore diameter, as well as high electrolyte uptake and porosity. The effects of different concentrations of PVDF-HFP and deionized water on the film properties were studied. It was found that with the increase of PVDF-HFP concentration in the casting solution, porosity, electrolyte uptake, average pore size and its distribution decreased. The results also indicated thatas the deionized water concentration increased, the average pore size, maximum pore diameter, porosity and electrolyte uptake were improved.(3)To further optimize the performance of the film, especially reducing the maximum pore size, novel PVDF-HFP films were prepared by coating a thin layer of nano-Si O2 particles on the surface of glass before casting the PVDF-HFP solution. After evaporation of acetone, Si O2 particles were adhered to the surface of PVDF-HFP membrane. For example, the maximum pore size of 5% and 7% PVDF-HFP films after coating Si O2 were reduced from 23.1 and 16.5μm to 2.23 and 1.95μm respectively. The maximum pore size of PVDF-HFP films were controlled around 2μm. In addition, it was found that after coating Si O2 particles, the porosity and average pore size of the PVDF-HFP films changed a little.(3)To investigate the influence of maximum pore size on the battery performance, 5%PVDF-HFP and 5%PVDF-HFP/Si O2 composite meltblown separators were assembled into batteries respectively. At the initial cycle, these two cells showed similar discharge capacity. After 52 cycles, the discharge capacity of the cell using 5% PVDF-HFP composite meltblown separator, which possess a large maximum pore size, exhibited a discharge capacity retention rate as low as 26.5%. While for the cell using the 5% PVDF-HFP/Si O2 composite meltblown separator, capacity retention rate reached 84.3%, which illustrates that Li-ion battery separators with a maximum pore size less that 3μm could allow for normal cycling of batteries.(4) A novel sandwich-like type of composite nonwoven separators was prepared by combining Si O2/PVDF-HFP membranes with PVDF-HFP dipped polypropylene nonwoven fabric. The physical and electrochemical properties of the composite meltblown separators were investigated. The mean pore size of the composite meltblown separators was in the range of 100 to 300 nm with maximum pore size around 2um. The composite nonwoven separators possess higher electrolyte uptake, stronger thermal stability, better wettability and lower interfacial resistance than a kind of commercial PE separator. The batteries assembled with composite nonwoven separators showed stable cycling performance and improved rate performance.To sum up, the composite meltblown separators are expected to be a promising alternative to commercial PE separator.