Research progress and market status of lithium-ion PP OPP PET separators

Lithium battery separatorThe following performance is required. ① High stability. To prevent battery short circuits, the separator needs to have good insulation properties to achieve the function of isolating the positive and negative poles; Due to the fact that the solvents in the electrolyte are mostly strongly polar organic compounds, it is necessary for the separator to have sufficient chemical stability to avoid corrosion by the battery electrolyte and ensure battery life; Adequate tensile strength and small shrinkage rate can effectively prevent diaphragm deformation Good consistency. The diaphragm should be as thin as possible while ensuring a certain mechanical strength, and the porosity should be as large as possible while ensuring a certain pore size. It should have good permeability and good infiltration ability into the electrolyte to ensure low resistance and high ion conductivity of the diaphragm, thereby improving the energy density and charge discharge performance of the battery High security. The diaphragm should have sufficient puncture strength, as well as the highest possible melting temperature and appropriate closed cell temperature, which can prevent lithium dendrites from piercing the diaphragm or causing the diaphragm to melt, resulting in battery short circuit, and also prevent battery overheating.

1. Diaphragm production process

The core process for preparing lithium-ion battery separators is microporous preparation technology, which is mainly divided into dry process, wet process, and spinning process according to different processes.

1.1 Dry process technology

The difference between dry and wet methods mainly lies in whether solvents are required during the production process. The dry process for producing membranes mainly consists of four steps: heating and melting, extrusion, annealing, and stretching. The dry process flow is shown in Figure 1. Firstly, the polymer raw material is heated to its melting point, and the molten polymer is extruded intoNon porous polymer filmHeat annealing treatment is performed on non porous polymer films to promote the generation of microcrystals (highly oriented layered crystals). The high crystallinity pore free polymer film generated after annealing generates pores through stretching (low-temperature stretching, high-temperature stretching, and relaxation). According to the difference in uniaxial or biaxial direction during the stretching process, dry process can be divided into dry uniaxial stretching and dry biaxial stretching. The former only stretches in the longitudinal direction (the diaphragm has anisotropy), while the latter generally undergoes longitudinal stretching at lower temperatures and then transverse stretching at higher temperatures.

The dry uniaxial stretching process mainly uses PP as the raw material. The dry uniaxial stretching technology in foreign countries was originally controlled by Celgard company, and later Japan's Ube company purchased this technology from Celgard. The domestic dry uniaxial stretching technology was successfully breakthrough by Xingyuan Material in 2008. After years of development, the domestic dry uniaxial stretching technology has become very mature, with a high domestication rate and low cost. However, the transverse strength of dry uniaxial stretching membranes is relatively low, the pore structure is narrow, the pore size distribution and porosity are difficult to control, and the membrane thickness is large, all of which can affect the exchange of lithium ions [11-12]. Unidirectional stretching membranes are prone to shrinkage or even melting at high temperatures, posing a threat to battery safety. Dry biaxial stretching is an improved process of uniaxial stretching, developed by the Institute of Chemistry of the Chinese Academy of Sciences. The separators produced by this method usually have a wide pore size distribution, which leads to uneven current transmission during battery use, and is prone to problems such as poor cycling performance and micro short circuits. Moreover, the improvement space of this process is limited. At present, manufacturers such as Celgard in the United States, UBE in Japan, and Xingyuan Materials all use dry uniaxial stretching technology to produce diaphragm products.

1.2 Wet process technology

The wet process, also known as thermally induced phase separation (TIPS), mainly utilizes the principle of thermally induced phase separation to produce single-layer PE membranes. As shown in Figure 2, the basic process of the wet process is to dissolve the polymer in a high boiling point, low volatility solvent at high temperature to form a homogeneous liquid, then cool it down to form liquid-solid or liquid-liquid phase separation, and extract the high boiling point solvent. After drying, heat treatment and other processes, a polymer microporous membrane with a certain structural shape is obtained. The unidirectional or bidirectional stretching of microporous membrane manufacturing can be carried out before or after solvent extraction. Compared with the dry process, the wet process requires both horizontal and vertical stretching, resulting in higher production difficulty, longer process flow, and higher precision requirements for equipment. At the same time, the product aperture is more uniform and the performance is better. The wet process prepared separator is suitable for high-power batteries and has a high permeability in power batteries. Currently, AsahiKasai, Japan Toray、 South Korean manufacturers such as SK, Enjie Co., Ltd., Sinoma Technology, and Cangzhou Mingzhu all adopt wet process routes.

1.3 Spinning process

The membrane spinning process mainly includes electrospinning process and melt blown spinning process. Electrospinning technology is used to prepare nanofibers and non-woven membranes, resulting in membranes with high porosity and good heat resistance, but there is a problem of poor strength. WU et al. prepared composite nanofiber membranes using electrospinning method, which significantly improved the safety performance of batteries using this separator. BAO et al. prepared nanofiber films doped with graphene using electrospinning method and found that the addition of graphene effectively enhanced the mechanical properties of the films. The melt blown spinning process uses a single polymer or a blend of multiple polymers as raw materials, and after melt blown drawing, it is thermally bonded and solidified into a network. The porosity and safety performance of the membrane products are greatly improved, but there is a defect of poor heat resistance. Gaohuipu utilized melt spinning technology to prepare polyvinylidene fluoride (PVDF) - hexafluoropropylene (HFP)/SiO2 composite membranes, which exhibited better porosity compared to commercial PE membranes. The batteries assembled from this membrane showed higher capacity retention.

2. Membrane modification

Although currently commercial lithium-ion battery separators are mainly made of polyolefin materials, there are also many defects. The melting points of PP and PE are relatively low, at 130 and 110 ℃ respectively, which limits the use of membranes under certain conditions. In addition, the tensile strength of the diaphragm will decrease when exposed to different temperatures. Meanwhile, the low porosity of the separator and its poor affinity for the electrolyte seriously affect the performance of the battery.

Surface coating on the existing polyolefin membrane can effectively improve its high temperature resistance and electrochemical performance. Coating modification mainly includes inorganic organic modification and organic organic modification. Inorganic organic modified coating materials, represented by Al2O3, also include inorganic particles such as SiO2, TiO2, ZrO2, MgO, etc. PVDF and polymethyl methacrylate (PMMA) are usually used as adhesives. For example, Leijing et al. coated Al2O3 on the surface of the diaphragm, and the internal resistance and polarization voltage of the battery cells modified by this coating were significantly reduced. Organic organic modified coating materials, represented by PVDF, also include polyacrylonitrile (PAN), PMMA, and mixtures of various materials. Zhou Xingping's research group alternately coated the surface of PE membranes with molybdenum disulfide (MoS2) and polyacrylic acid (PAA) coatings, enhancing the mechanical properties of the membranes and effectively inhibiting the growth of lithium dendrites. At present, battery or separator manufacturers such as Asahi Kasei, Sumitomo, SK, LG, CATL, and Enjie have their own separator coating production lines.

3. Current situation of domestic and international markets

Under the influence of policy guidance from various countries, the trend of electrification in the global automotive market is clear, and the sales of electric vehicles are increasing day by day. In 2020, global sales of electric vehicles exceeded 3.24 million units, with 1.367 million units sold in China and 1.41 million units sold in Europe, and 253000 units sold in the United States. With the intensification of policies and the promotion of subsidies, the sales of electric vehicles have further increased in 2021. In addition, the booming 3C digital field and the growing energy storage industry have further promoted the vigorous development of the diaphragm industry. In 2020, the global demand for lithium battery separators was 5.42 billion square meters, of which Enjie Co., Ltd., Xingyuan Materials, Japan's Toray, Asahi Kasei, South Korea's SK and other major manufacturers accounted for nearly 60% of the global market share of wet process separators; Xingyuan Material, Asahi Kasei, and Ube Dry Diaphragm from Japan account for approximately 37.7% of the global market share. According to the 2020 lithium battery consumption of 16.67 million m2/GWh and combined with the expansion plans of major battery manufacturers worldwide, it is expected that the global demand for separators will reach 37.42 billion m2 by 2025.

In 2020, the total production capacity of lithium-ion battery separators in China was about 5.5 billion square meters, and the shipment volume of separators (base films) was 3.87 billion square meters, a year-on-year increase of 29.9%. Among them, the shipment volume of wet process membranes was 2.72 billion m2, a year-on-year increase of 36%, accounting for 70% of the total shipment volume of membranes; The shipment volume of dry process membranes was 1.15 billion m2, a year-on-year increase of 17%, accounting for 30% of the total shipment volume of membranes. The overall market share of Chinese diaphragm enterprises in 2020 is shown in Figure 3. The market share of wet and dry diaphragm enterprises in China in 2020 is shown in Figure 4.

Wet process diaphragmWith its superior performance and continuously optimized cost-effectiveness, it has become the mainstream in the market, and its market share will further increase in the future. In addition, coating membranes is also one of the future development trends. With the continuous expansion of coating materials and the continuous improvement of technical formulas, the price difference between coating membranes and base membranes is gradually narrowing. Currently, almost all wet membrane projects under construction or planned have corresponding membrane coating production lines, and LG Chem's membrane coating rate for power batteries is close to 100%.

4. Summary and Prospect

The rapid development of new energy electric vehicles and the acceleration of digital electronic product updates have led to remarkable progress in the separator, which is an important component of batteries. In recent years, with the development and mass production of separators with high porosity, excellent mechanical properties, stability and electrochemical performance, and low thickness, the battery industry has been further promoted to better serve people's daily lives. After years of development, China's diaphragm enterprises have also achieved gratifying results, with market share and production capacity ranking among the top in the world. However, China's diaphragm industry started late, and most of the core equipment for producing wet process diaphragms needs to be imported. The high-end diaphragm market is still monopolized by foreign companies, and the patent barriers faced by coating materials urgently need to be solved. With strong support from the government in the fields of new energy and new materials, major enterprises and research institutes have increased their research and investment in membrane materials, preparation processes, equipment, and coating technologies. In the future, China's battery membrane industry will make significant progress, and battery membrane products with higher performance will enter the market.