As a major producer and consumer of lithium-ion batteries, my country has basically formed a complete industrial chain, from mineral resources, battery materials and components to lithium-ion batteries and end-use products. In recent years, my country's lithium-ion battery market has maintained rapid growth, increasing from 27.7 billion yuan in 2011 to 85 billion yuan in 2015, with an average annual compound growth rate of 32.4%. Although my country's lithium-ion battery market is booming, my country is not yet a leading power in the field. The following discussion will focus on our shortcomings in two aspects: lithium-ion battery separators and aluminum-plastic films.
diaphragm
1. Applications of lithium-ion battery separators
The separator is a crucial component of lithium-ion batteries. Located between the positive and negative electrodes inside the battery, it allows lithium ions to pass through while hindering electron transport. The performance of the separator determines the battery's interface structure, internal resistance, and other characteristics, directly affecting the battery's capacity, cycle life, and safety performance. A high-performance separator is essential for improving the overall performance of the battery.
2. Requirements for separators in lithium-ion batteries
The requirements for separators in lithium-ion batteries include:
(1) It has electronic insulation properties, ensuring mechanical isolation between the positive and negative electrodes;
(2) It has a certain pore size and porosity, ensuring low resistance and high ionic conductivity, and good permeability to lithium ions; (3) It is resistant to electrolyte corrosion and has sufficient chemical and electrochemical stability, which is due to the fact that the solvent of the electrolyte is a strongly polar organic compound;
(4) It has good wettability in electrolytes and strong liquid absorption and moisture retention capabilities;
(5) High mechanical stability, including puncture strength and tensile strength, but with the thickness as small as possible;
(6) Good spatial stability and flatness;
(7) It has good thermal stability and automatic shutdown protection performance;
(8) The thermal shrinkage rate should be small, otherwise it will cause a short circuit and lead to battery thermal runaway. In addition, power lithium-ion batteries usually use composite membranes, which have higher requirements for the separator.
3. Classification of Lithium-ion Battery Separators
Based on differences in physical and chemical properties, lithium-ion battery separators can be classified into several categories, including woven membranes, non-woven membranes (non-woven fabrics), microporous membranes, composite membranes, separator paper, and rolled membranes. Despite the variety of types, commercially available lithium-ion battery separator materials primarily utilize polyethylene and polypropylene microporous membranes.
4. Lithium-ion battery separator process
Currently, the main methods for preparing lithium-ion battery separators are wet and dry processes. The wet process, also known as the phase separation method or thermally induced phase separation method, involves mixing liquid hydrocarbons or small molecules with polyolefin resin, heating and melting them to form a homogeneous mixture, then cooling to allow phase separation, pressing to obtain a membrane, and then heating the membrane to near its melting point for biaxial stretching to orient the molecular chains. Finally, it is held at this temperature for a certain period, and residual solvent is washed away with volatile substances to prepare an interconnected microporous membrane. The dry process involves melting, extruding, and blowing polyolefin resin to form a crystalline polymer film. After crystallization and annealing, a highly oriented multilayer structure is obtained. Further stretching at high temperatures peels off the crystalline surfaces, forming a porous structure, which can increase the pore size of the film.
Wet and dry processes each have their advantages and disadvantages. Wet processes produce thinner films with smaller and more uniform pore sizes, but require larger investments, involve more complex processes, and cause greater environmental pollution. Dry processes, on the other hand, are relatively simple, have higher added value, and are more environmentally friendly, but the pore size and porosity are difficult to control, making it difficult to produce thinner products.
5. Two core technologies for lithium-ion battery separator processes
For wet-process resin mixing and extrusion, the stretching process is the two core issues. The extrusion process requires good material mixing, strong plasticizing ability, and stable extrusion, while the stretching process determines the orientation of molecular chains and the uniformity of pore-forming agent distribution. For dry-process resin mixing, in addition to the extrusion mixing process, melt stretching ratio and thermal desorption are also core processes.
Currently, global manufacturers of separators mainly use the wet process, which makes separators more expensive. In the future, wet-process separators will continue to dominate the high-end market for power lithium-ion batteries, while mid-to-low-end power lithium-ion batteries will still be dominated by the dry process.
6 Global Lithium-ion Battery Separator Companies
The global market demand for lithium-ion battery separators has been increasing year by year, with separator shipments rising from 240 million square meters in 2009 to 1.185 billion square meters in 2014. Asahi Kasei, Tonen Chemical, and Celgard (acquired by Asahi Kasei in February 2015, a leading company in wet process technology, with its dry process line discontinued and a new wet process line established) were the three giants in the separator market, once holding as much as 77% of the global market share. However, with the rise of South Korean and Chinese companies, the market share of these three giants has been declining rapidly, reaching approximately 56% in 2014.
7. Gap in Lithium-ion Battery Separators in my country
The separator in lithium-ion batteries has the highest technological barrier among the four major materials. Its cost accounts for about 10% to 14%, second only to the cathode material. In some high-end batteries, the cost of the separator can even reach 20%.
my country has made significant breakthroughs in dry-process lithium-ion battery separators, achieving world-class manufacturing standards. However, in the wet-process separator sector, domestic companies are limited by factors such as process and technology, resulting in lower product quality and a heavy reliance on imported processing equipment. Chinese separator products lag significantly behind foreign products in terms of thickness, strength, and porosity consistency, and batch-to-batch consistency also needs improvement.
Aluminum-plastic film
1. Applications of aluminum-plastic film for lithium-ion batteries
Aluminum-plastic film is one of the five key materials for lithium-ion batteries, and it's used as a packaging material for soft-pack lithium-ion batteries. Aluminum-plastic film consists of five layers: an outer nylon layer, an adhesive layer, an intermediate aluminum foil layer, another adhesive layer, and an inner heat-sealing layer. Each layer has relatively high functional requirements. A typical aluminum-plastic film structure is shown in the figure below:
2. Requirements of aluminum-plastic film for lithium-ion batteries
The barrier properties, puncture resistance, electrolyte stability, high-temperature resistance, and insulation of the aluminum-plastic film affect the performance of lithium-ion batteries. Deficiencies in any of these aspects can lead to performance degradation and even render the battery unusable. The aluminum-plastic film is processed using precision coating technology, and currently, Japanese companies possess the world's most advanced precision coating technology.
3. Lithium-ion battery aluminum-plastic film process
Dry and hot processes are the main processing techniques used for aluminum-plastic film. The dry process involves bonding aluminum and polypropylene with an adhesive and then pressing them vertically together. The hot process involves bonding aluminum and polypropylene with MPP and then hot-pressing them together under slowly increasing temperature and pressure.
Dry-processed aluminum-plastic films are thin, have a good appearance, and possess excellent deep-drawing and short-circuit protection properties. The process is simple and low-cost. However, compared with the hot-process, they have poorer resistance to electrolytes and water. The advantage of the hot-process is its good resistance to electrolytes and water, but its deep-drawing and short-circuit protection properties are not as good as the dry-process, and its appearance and cutability are poor.
4 Global Lithium-ion Battery Aluminum-Plastic Film Companies
Aluminum-plastic film plays a crucial role in pouch lithium-ion batteries, typically accounting for 15-20% of the cell cost. However, due to insufficient technology, the domestic market share of aluminum-plastic film is very small, less than 5%. Currently, Japanese manufacturers monopolize 90% of the domestic aluminum-plastic film market, primarily DNP (Dai Nippon Printing), Showa Denko, and TT.
As a lithium battery material that has not yet been domestically produced, aluminum-plastic film boasts a gross profit margin as high as 60-80%. It is estimated that the current global market size for aluminum-plastic film is only several billion yuan, but with the increase in downstream demand, the industry growth rate is expected to exceed 40%, and the potential market size will reach tens of billions of yuan.
5. Gap between aluminum-plastic film and lithium-ion battery production in my country
As a core material for pouch batteries, aluminum-plastic film's processing technology is far more challenging than that of separators, positive electrodes, negative electrodes, and electrolytes, making it one of the three high-tech areas in the lithium-ion battery industry. In terms of product performance, Chinese aluminum-plastic film products lag significantly behind their foreign counterparts, primarily due to: outdated and polluting aluminum foil surface treatment processes; hydrogen embrittlement in aluminum foil water treatment leading to poor film durability; insufficient aluminum foil surface stiffness resulting in low yield; easy curling and layered crystallization when polypropylene is laminated with highly thermally conductive aluminum foil; and poor domestic adhesive formulation processes, leading to delamination and peeling issues.
Due to deficiencies in these processing technologies, the maximum drawing depth of aluminum-plastic film products processed in my country is only around 5mm, which has consistently failed to meet good performance requirements. In contrast, foreign products can reach 8mm, and some even reach 12mm, indicating a significant gap compared to foreign products. Regarding thickness, the thinnest aluminum-plastic film produced domestically is only 70μm, with mass-produced thicknesses of 112, 88, and 72μm. In contrast, Japanese aluminum-plastic film can be made as thin as 40μm, with 65μm and 48μm films also in mass production.
The reason why the manufacturing technology of aluminum-plastic film is difficult to break through is mainly due to deficiencies in materials, equipment, and processes. The technical difficulty lies in the control of the process—the precise control of reaction conditions.
