Polyolefin polymers and polyester polymer materials commonly used in diaphragms

Polyolefin polymers and polyester polymer materials commonly used in diaphragms

Polyolefin polymers

Polyolefin polymers, mainly polypropylene and polyethylene, are widely used as the main polymer material of the diaphragm due to their good mechanical strength, excellent chemical stability (acid and alkali corrosion resistance, organic solvent resistance) and high electrical insulation properties. The overall comparison between polypropylene and polyethylene: ①Polypropylene is relatively more resistant to high temperatures, and polyethylene is relatively more resistant to low temperatures; ②Polypropylene has a lower density than polyethylene; ③Polypropylene has a higher melting point and self-closing cell temperature than polyethylene; ④Polypropylene products are more brittle than polyethylene; ⑤Polyethylene is more sensitive to environmental stress.

Polyolefin separators have shortcomings such as low heat-resistant temperature (generally not more than 150°C), easy shrinkage when heated, and flammability. Potential safety hazards are becoming prominent, but they are the lithium-ion battery separators with the best overall performance at this stage. Polyethylene diaphragm products are mainly made by wet process, and polypropylene diaphragm products are mainly made by dry process.

At present, China’s special PP resin for lithium-ion battery separator production mainly relies on imports. Representative producers in other countries include South Korea’s Daelim Corporation and Japan’s Asahi Kasei Corporation. Through analysis and research, it can be seen that the special PP resin for lithium-ion battery separator production should have the characteristics of high orientation, low ash content, and broad molecular weight distribution, as well as strong resistance to thermal oxidation degradation. In 2015, Sinopec Corporation initially realized the small-batch production of PP resin for domestic diaphragms.

Ultra-high molecular weight polyethylene (UHMWPE) resin is an unbranched linear polyethylene with a molecular weight of 1.5 million or more. It has attracted much attention in the field of battery separators in recent years. The ultra-high molecular weight polyethylene diaphragm has strong resistance to external force puncture (can effectively reduce the short-circuit rate of the battery), better heat resistance (improved closed cell temperature, film breaking temperature, low thermal shrinkage) and excellent corrosion resistance. In the past, the high-end UHMWPE raw materials (special grade for diaphragms) used by China’s wet-process diaphragm companies mainly relied on imports. The import volume reached 10,000 tons/year (showing a trend of rapid growth), resulting in the price of its raw materials as high as 20,000 yuan/ton or more. Representative producers in other countries include Ticona’s GUR series of ultra-high molecular weight polyethylene and Mitsui Chemicals’ Hi-Zex Million series of ultra-high molecular weight polyethylene. In 2017, Yangzi Petrochemical Research Institute successfully trial-produced YEV-4500 (hundred-ton level), a new product of ultra-high molecular weight polyethylene for lithium-ion battery separators for the first time, filling the gap in China. This is a major breakthrough in the field of high-end polyolefin materials in China.

Polyester polymers

When polyester materials are used in battery separators or electrolytes, they mainly refer to polyethylene terephthalate (PET) and polymethyl methacrylate (PMMA) (Figure 1). PET is often made into a non-woven fabric with a three-dimensional structure through meltblown or spunbond methods, and has been widely used in lead-acid batteries, nickel-hydrogen batteries and other fields. The melting point of PET is as high as 220°C, and its heat resistance far exceeds that of polyolefin separators. It also has higher puncture resistance, porosity and electrolyte wettability.

Polyolefin polymers and polyester polymer materials commonly used in diaphragms
Figure 1 – The molecular structure of PET and PMMA

However, when applied to lithium-ion batteries, meltblown or spunbond non-woven fabric separators have a large pore size, which easily leads to a large self-discharge effect of the battery, and the non-woven fabric has a relatively high water content, which affects the safety of the battery. Therefore, meltblown or spunbond non-woven fabrics are usually used as the support matrix of lithium-ion battery separators, supplemented by other polymers or inorganic materials to prepare non-woven composite separators, which not only maintains the excellent performance of the non-woven fabric, but also greatly improves the safety of the separator.

PMMA is commonly known as plexiglass. The macromolecular functional group (O=C-O-CH3) in the monomer makes the branch chain flexibility of PMMA high. The electron-donating functional group carbonyl (-C=O) easily forms complexes with alkaline earth metals, and the density of PMMA is low. As a separator, it can further increase the energy density of the battery. The amorphous structure is suitable as a matrix material for the electrolyte of lithium-ion batteries.