Comprehensive analysis of lithium battery diaphragm and aluminum plastic film technology

 

 

【China Aluminum Network] As a large producer and consumer of lithium-ion batteries, China has basically formed a complete industrial chain from mineral resources, battery materials and accessories to lithium-ion batteries and terminal application products. In recent years, China's lithium-ion battery market has maintained rapid growth in the form of China's lithium-ion battery market size from 27.7 billion yuan in 2011 to 85 billion yuan in 2015, the average annual compound growth rate of up to 32.4. The following introduces the lithium ion battery separator and aluminum plastic film technology.

Diaphragm

1 the role of lithium ion battery separator

The separator is an important part of the lithium-ion battery. It is located between the positive and negative electrodes inside the battery to ensure the passage of lithium ions while hindering the transmission of electrons. The performance of the separator determines the interface structure and internal resistance of the battery, and directly affects the capacity, cycle and safety performance of the battery. The excellent separator plays an important role in improving the overall performance of the battery.

2 Lithium-ion battery diaphragm requirements

Requirements for separators in lithium-ion batteries include:

(1) It has electronic insulation to ensure the mechanical isolation of the positive and negative electrodes;

(2) has a certain pore size and porosity, to ensure low resistance and high ionic conductivity, lithium ion has a good permeability;(3) resistance to electrolyte corrosion, there is enough chemical and electrochemical stability, this is due to the electrolyte solvent for strong polar organic compounds;

(4) It has good electrolyte wettability and strong liquid absorption and moisturizing ability;

(5) High mechanical stability, including puncture strength, tensile strength, etc., but the thickness is as small as possible;

(6) Good space stability and smoothness;

(7) Good thermal stability and automatic shutdown protection performance;

(8) The thermal shrinkage rate is small, otherwise it will cause a short circuit and cause thermal runaway of the battery. In addition, power batteries usually use composite membranes, which have higher requirements for diaphragms.

3 Li-ion battery separator classification

According to the differences in physical and chemical properties, lithium battery separators can be divided into: woven membranes, non-woven membranes (non-woven fabrics), microporous membranes, composite membranes, separator paper, rolled membranes and other categories. Although there are many types, so far commercialized lithium battery separator materials mainly use polyethylene and polypropylene microporous membranes.

4 Lithium-ion battery separator process

At present, the preparation method of lithium ion battery separator mainly includes wet method and dry method. Wet method is also called phase separation method or thermally induced phase separation method. Liquid hydrocarbon or small molecular substances are mixed with polyolefin resin, heated and melted to form a uniform mixture, then cooled to carry out phase separation, pressed to obtain a diaphragm, then the diaphragm is heated to a temperature close to the melting point, biaxial stretching is carried out to orient the molecular chain, and then the residual solvent is eluted with volatile substances to prepare a microporous membrane that penetrates each other. The dry method is to melt, extrude and blow the polyolefin resin into a crystalline polymer film. After crystallization treatment and annealing, a highly oriented multilayer structure is obtained. Further stretching at high temperature, the crystal surface is peeled off to form a porous structure, which can increase the pore size of the film.

Wet and dry methods have their own advantages and disadvantages, among them, the wet process film pore size is small and uniform, the film is thinner, but the investment is large, the process is complex, environmental pollution is large, while the dry process is relatively simple, high value-added, environmentally friendly, but the pore size and porosity is difficult to control, the product is difficult to do thin.

5 Two core technologies for lithium-ion battery separator processes

For the wet process, the resin andAdditivesThe extrusion mixing process and the stretching process are the two core issues of the process. The extrusion process requires good material mixing effect, strong plasticizing ability, stable extrusion process, and the stretching process determines the orientation of the molecular chain and whether the pore-forming agent is evenly distributed. For the dry process, in addition to the extrusion mixing process, the melt draw ratio andHeat treatmentAll are core processes.

At present, the global manufacturers of diaphragm to wet-based, wet diaphragm more expensive, the future of wet diaphragm in the power battery will still take the high-end market route, while the low-end power battery will still be dry-based.

6 Global Lithium-ion Battery Separator Enterprises

The global market demand for lithium-ion battery separators is increasing year by year, with separator shipments increasing from 0.24 billion square meters in 2009 to 1.185 billion square meters in 2014. Japan's Asahi Kasei, Japan's East Gas Chemical and U.S. Celgard(Celgard acquired in February 2015 by wet technology representative company Asahi Kasei, dry production line discontinued and a new wet production line) are the diaphragm giants, accounting for 77% of the global market share. But with the rise of South Korean and Chinese companies, the share of the Big Three is declining rapidly, accounting for about 56% in 2014.

7 China's lithium-ion battery separator gap

Lithium battery separator is a part of the four major materials with high technical barriers, and its cost ratio is second only to cathode materials, about 10%-14%. In some high-end batteries, the cost of separator even reaches 20%.

China's lithium-ion battery separator has made a major breakthrough in the dry process, and now has a world-class manufacturing level. However, in the field of wet diaphragm, domestic diaphragm enterprises are limited by many factors such as process and technology, the product level is still low, and the production equipment mainly depends on imports. China's diaphragm products in the thickness, strength, porosity consistency and foreign products have a large gap, product batch consistency also needs to be improved.

aluminum plastic film

1 the role of lithium ion battery aluminum plastic film

Aluminum-plastic film is one of the five major materials for lithium-ion batteries and is a soft-pack lithium battery packaging material. Aluminum plastic film made of outer nylon layer/adhesive/middle layerAluminum Foil/adhesive/inner heat sealing layer, a total of five layers, each layer of functional requirements are relatively high. A typical aluminum-plastic membrane structure is shown in the following figure:

2 lithium ion battery requirements for aluminum plastic film

The barrier ability, puncture resistance, electrolyte stability, high temperature resistance and insulation of aluminum-plastic film affect the performance of lithium-ion batteries. Any lack of any aspect may lead to a decrease in battery performance and direct scrap. Aluminum plastic film is produced by precision coating technology. At present, Japanese enterprises have the most advanced precision coating technology in the world.

3 Lithium ion battery aluminum plastic film process

Dry and thermal processes are the main production processes used in aluminum-plastic films. The dry process is formed by directly pressing aluminum and polypropylene after bonding with an adhesive, and the thermal process is formed by hot pressing aluminum and polypropylene with MPP followed by slow heating and boosting.

The aluminum-plastic film produced by dry method is thin, good in appearance, excellent in deep drawing performance and short circuit resistance, simple in process and low in cost. However, compared with heat method, it has poor electrolyte resistance and water resistance. The advantages of heat method are good in electrolyte resistance and water resistance, but the deep drawing forming performance and short circuit prevention performance are not as good as dry method, and the appearance and cutting performance are poor.

4 Global Lithium-ion Battery Aluminum Plastic Film Enterprise

Aluminum-plastic film plays a key role in soft-packed lithium batteries, generally accounting for about 15-20% of the cost of batteries. However, due to the lack of technology in China, the proportion of aluminum plastic film market is very small, accounting for less than 5%. At present, 90% of the domestic aluminum-plastic film market is monopolized by Japanese manufacturers, mainly DNP (Great Japan Printing), Showa Electric and T & T.

As a lithium-electric material that has not yet been localized, aluminum-plastic film has a gross profit margin of 60-80%. It is estimated that the current global market space of aluminum-plastic film is only a few billion yuan. With the downstream demand volume, the industry growth rate is expected to exceed 40%, and the potential market size will reach tens of billions.

5 China's lithium-ion battery aluminum-plastic film gap

As the core material of soft-pack batteries, the production technology of aluminum-plastic film is much more difficult than that of diaphragm, positive electrode, negative electrode and electrolyte. It is one of the three high technologies in the lithium battery industry. From the perspective of product performance, there is a big gap between China's aluminum-plastic film products and foreign products, mainly manifested in: backward surface treatment process of aluminum foil and large pollution; Water treatment of aluminum foil will produce "hydrogen embrittlement", resulting in poor disassembly resistance of aluminum-plastic film. The surface stiffness of aluminum foil is not enough and the yield is poor. Polypropylene is easy to curl when compounded with high thermal conductivity aluminum foil surface, resulting in layered crystallization; The domestic adhesive formulation process is poor, the problem of delamination and peeling is easy to occur.

Due to the lack of these production technology, China's production of aluminum-plastic film products in the larger depth of about 5mm, has been unable to achieve good performance requirements. However, foreign countries can reach 8mm, some even reach 12mm, which is still a certain gap with foreign products. In terms of thickness, the domestic aluminum-plastic film can only reach 70μm when it is thinner, and the mass production is 112, 88 and 72μm, while the Japanese aluminum-plastic film can reach 40μm when it is thinner, and the mass production is also realized when it is 65 and 48μm.

Why is it difficult to break through the manufacturing technology of aluminum-plastic film? Generally speaking, there are mainly deficiencies in materials, equipment and technology, and its technical difficulty mainly lies in the control of the process-the accurate control of the reaction conditions.