Fabrication and characterization of novel membranes for battery separator applications

Abstract

With the prospering industry of electrical mobility, the need for high-performance batteries and energy sources is on the rise. The demand on efficient and safe batteries has exponentially increased. In general, batteries consist of three main critical components; anode, cathode and a separator. While anode and cathode are the active parts in charge and power generation, battery separators are the non-active insulative part. They are a porous structure that allows the passage of ions back and forth while they are made of inherently electrically insulative materials that prohibits the contact of the two poles preventing any short circuit possibility. Battery separators can be made of several materials and substances and carry several key properties. They can be made of any porous insulative material with defined pore size and porosity for their intended application. Along their porous structure, they should carry high chemical stability, strong integrity, and stability under elevating temperatures. Since there are various reactions occurring inside a battery which may lead to an increase in the cell temperature, several approaches are used to fabricate battery separators with high thermal stability and flame retardancy. Among these approaches, coating the separator with inherently flame-retardant materials is a common method. Sodium alginate, as a natural polysaccharide, that shows significant flame retardancy performance when crosslinked with calcium. Cross-linked calcium alginate is reported to exhibit a limiting oxygen index (LOI) of 34. Several polymers are used to fabricate the battery separator. Ultra-high molecular weight polyethylene (UHMWPE) is widely used in battery separator applications due to its high mechanical strength and chemical stability. Film casting is a process widely used to fabricate battery separators. In the wet process film casting, UHMWPE is melt-mixed with a low molecular weight diluent (also called porogen) and casted through a film die before its conveyed into stretching and extraction steps to obtain final porous membrane. Another type of common separator is the nanofiber-based. They can be fabricated via several methods including centrifugal spinning, where a rotor is ejecting the polymeric solution into fibers while rotating at very high speeds. The aim of this thesis is to fabricate lithium and sodium ion battery separators with different fabrication methods and enhance their thermal performance by coating them with calcium alginate. Film casting and centrifugal spinning processes were used to obtain two different membrane structures from two different polymers. In the film casting process, UHMWPE was mixed with paraffin oil (PO) then melted and extruded through a twin-screw extruder (TSE). 30% UHMWPE, 70% PO, and 1% antioxidant mixture was prepared. The temperature of the 6 heating zones and die was held at 130, 140, 150, 160, 170, 180, and 180℃ respectively. The screw rotating speed was held constant at 35rpm. The melt blend was cast through a stripe die with a thickness of 3 mm. The obtained sheets were hot pressed at 180℃ and 8 tons of load for 8 minutes then cooled down to room temperature. The final film thickness obtained was in the range of 200-400μm. Optimization experiments were conducted on the samples to study the effect of different stretching ratios, the importance of extraction step order, the effect of the constrained and nonconstrained uniaxial stretching, and the effect of heating distance between the sample and the stretching machine. It was found that the extraction of the oil after stretching led to a smaller and more uniform pore size in comparison to bigger pores formed when the oil was extracted before stretching. Heating of the samples in the stretching machine was done with the open system using thermal irradiators. Distances of 13.5, 16.5, and 19.5cm were studied. Heaters caused the film to melt fastly and close the pores formed at close distances while at 19.5cm the heating wasn't enough to initiate any pores. The films are then stretched with a custom-made uniaxial stretching machine, where pore formation is initiated. Two stretching ratios were applied; 2×1.5 and 4×1.5. Higher stretching ratios showed the closure of pores. The optimum parameters were found to be a 1.5×2 constrained stretching ratio with a 16.5 cm heating distance and 110℃ heating temperature. After that three Samples (S1, S2, and S3) were fabricated and hot pressed into three thicknesses of 280, 200, and 280μm. These samples are then stretched and annealed to obtain samples of three final thicknesses of 40, 60, and 80μm. Finally, these samples are immersed in n-hexane to extract the oil and obtain the desired porous structure and the final membrane. These membranes are then put in a coin cell configuration and tested for sodium ion (Na-ion) battery separator performance. The ionic conductivities of S1, S2, and S3 samples are 0.09, 0.48, and 0.04mS/cm2 respectively. The S2 sample showed better performance in comparison to other samples due to its higher porosity and uniform pore distribution. Nanofibrous membrane on the other hand was fabricated from thermoplastic polyurethane (TPU) by centrifugal spinning process. TPU was dissolved in dimethylformamide (DMF)/acetone mixture and spun into nanofibers. Optimization of the process was done by altering the polymer concentration, solvent ratios, needle diameter (gauge), and rotating speeds. The optimum nanofibers were obtained at 10% TPU dissolved in a 2:1 DMF/acetone ratio with 30G needle diameter and 13,000 rpm rotating speed. Obtained nanofibers were treated with sodium alginate which was then crosslinked with calcium chloride (CaCl2). After Calcium Alginate (Ca-alg, CA) treatment, the samples were hot pressed at 120℃ and 8 tonnes of load for 2 minutes. Treatment with CA enhances the thermal performance of the separators. The obtained coated nanofibrous membranes were then put into coin cell configuration with 1M LiPF6 and 1M NaClO4 electrolytes and its performance was measured. These membranes were tested for sodium ion (Na-ion) and lithium-ion (Li-ion) batteries. The nanofibrous membranes exhibited significantly better performance when compared to the commercial Celgard 2500 polypropylene (PP) separator. The ionic conductivities of the neat TPU, pressed TPU, 0.5 TPU, and 2 TPU were 0.23, 0.67, 0.17, 0.16, and 0.05 mS/cm2 respectively. The coated samples exhibited significantly larger ionic conductivity numbers and lower resistance values in comparison to the commercial PP separator. In full cell configuration, where anode and cathode are used, the samples exhibited the same trend with neat TPU performing better than other samples. The ionic conductivities of the full-cell samples were 1.53, 0.19, 0.05, and 0.03 mS/cm2 respectively. The drop in the ionic conductivities of the samples can be referred to as the effect of coating the samples which leads to pore closure. This led to the domination of the diffusion phenomena in the samples which was demonstrated by the straight line in the Nyquist plot. In terms of thermal performance, significant enhancement was achieved. On the other hand, when these samples were examined for Li-ion separator tests, the ionic conductivities of Neat TPU, 0.5 TPU, 2 TPU, and PP separators were 0.10, 0.21, 0.09, and 0.07mS/cm2 respectively. The 0.5 TPU sample showed the best performance and proved that calcium alginate can enhance the chemical performance of the separator. The coated samples showed 0% shrinkage in comparison to 63% and 100% shrinkage of the neat TPU and commercial samples. The samples also showed significant fire retardation when exposed to flame source. A comparison between the condition of the samples after 1 and 10 seconds was recorded. Neat and pressed samples melted directly within the first seconds, while the treated 0.5 TPU and 2 TPU samples kept their integrity. 0.5 TPU sample showed marks of burning and its color started turning into brownish while the 2 TPU sample kept its initial state even after 10 seconds. The obtained results showed the high potential calcium alginate carries in battery applications. Studies to enhance the ionic conductivities of the calcium alginate can be conducted and better integrating methods can be proposed.