1 Introduction
Lithium battery separators have relatively strict technical indicators and high production process requirements, and the core manufacturing technology is monopolized by a few . The development of CHINA's lithium battery industry is relatively late. In the early stage, almost all lithium battery separators were imported from abroad, especially the demand for high-end separators. Therefore, the development of CHINA's lithium battery industry was greatly restricted.
At present, the separator products are mostly low-end polyolefin microporous membranes, which are generally difficult to meet the high-current and high-rate discharge of lithium batteries used in the high-end market, high heat resistance and dimensional stability, and good adsorption electrolyte and Requirements such as the ability to maintain the electrolyte. In the high-end market application field, especially the power battery application field, there will be safety hazards such as fire and explosion . Therefore, the development of high-performance separators that can meet the high-end market applications has become an urgent need for the lithium battery industry. Coating-modified diaphragms have become a hot research topic in the lithium battery diaphragm market due to their advantages of high safety, high stability and adaptability to high-power discharge. This paper reviews the research status of coating modified separators at home and abroad, and looks forward to the development direction of coating modified lithium battery separators.
2 Working principle of lithium battery
How Lithium-Ion Batteries Work
[1,2], taking graphite as the negative electrode and LiMOx as the positive electrode (where M is Co, Ni, Mn, Fe, etc.) as an example, as shown in Figure 1. When charging, Li+ is deintercalated from the positive electrode and inserted into the negative electrode after passing through the electrolyte and separator ; during discharge, Li+ is deintercalated from the negative electrode and inserted into the positive electrode after passing through the electrolyte and separator. As an important has two main functions: first, it separates the positive and negative electrodes of the lithium battery to prevent direct contact between the positive and negative electrodes and cause a battery short circuit, while allowing Li+ to pass freely to make the lithium battery Form a closed circuit; second, when the temperature is too high, the diaphragm can cut off the circuit by closing the pores in time to avoid rupture of the membrane and prevent the battery from exploding due to continuous temperature rise.
3 Brief introduction of lithium-ion battery separator
3.1 Definition of lithium-ion battery separator
Lithium-ion battery separator, as one of the important components of lithium-ion batteries, has been receiving much attention. The performance of lithium-ion battery separators directly affects key performance indicators such as current, capacity, and cycle life of lithium batteries [3]. The main function is to separate the positive and negative electrodes of the lithium battery to avoid short circuit, and at the same time provide a channel for lithium ions to realize a closed circuit. Since the electrolyte of the battery is an organic solvent system, the lithium-ion battery separator is a polymer functional membrane material with nano-scale pores [4], which is required not to react with other related materials such as the electrolyte in the lithium battery system .
3.2 The role of lithium-ion battery separator
Lithium-ion batteries are widely used because of their high energy density, no memory effect, long cycle life, and environmental friendliness. As one of the core materials of lithium-ion batteries, the separator directly affects the energy density, power density, cycle life and safety of the battery. Lithium-ion battery separator is one of the four key raw materials for lithium batteries. Its performance plays a decisive role in the ability of the battery electrolyte to absorb and retain liquid, and thus directly determines the internal resistance, current, charge and discharge of the battery after assembly. Key characteristics such as cycle life. A separator with excellent performance plays an important role in improving the overall performance of the battery [5]. The four key materials of lithium-ion batteries are positive, negative, diaphragm and electrolyte. Among them, the diaphragm is a material . It plays two main roles in the charging and discharging process of the battery: to separate the positive and negative electrodes of the lithium battery to avoid internal short circuits, and to provide free channels for lithium ions to achieve a closed circuit [6]. The separator material itself is insulating , and its physical and chemical properties have a direct impact on the overall performance of lithium batteries [7]. At present, the materials used for battery separators in the industry are mostly polyolefin microporous films, non-woven mats or multi-layer porous films [8,9]. The separators used in the direction of power batteries require large discharge power And higher safety, coating modified diaphragm becomes the preferred choice.
3.3 Development Status of Lithium-ion Battery Separator
3.3.1 Types of lithium-ion battery separators
Different types of separators are used in batteries . [10] At present, in the lithium-ion battery system, the electrolyte is mostly an organic solvent system. Therefore, the lithium battery diaphragm material is required to be chemically inert and not react with organic solvents or be corroded. Generally, high -strength polyolefin microporous membranes are used. In order to meet the requirements of power batteries with high energy density and high discharge rate, research on modified lithium-ion battery separators is increasing.
At present, the separator material products on the market mainly include single-layer PP, single-layer PE, double-layer PP/PE, double-layer PP/PP and three-layer PP/PE/PP , among which the first two types of products are mainly used for 3C small batteries The latter types of products are mainly used in the field of power lithium batteries. [11] Among the separator materials for power lithium batteries, the double-layer PP/PP separator materials are mainly localized and mainly used in the Chinese mainland. This is mainly because Chinese companies have not yet made breakthroughs in making PP and PE double-layer materials. Technical barriers of composite membranes. The separators used in global automotive power lithium batteries are mainly three-layer PP/PE/PP, double-layer PP/PE and other separator materials.
At the same time, new modified diaphragm material products are also emerging and starting to be applied. However, due to the small quantity and high price, they are mainly used in the field of power lithium battery manufacturing. These products mainly include: PP+ ceramic single and double-sided coating, PE+ ceramic single and double-sided coating, coated polyester film (PET, Polyethylene Terephthalate), separator coated with heat-resistant polymer, etc. The advantages of these coating-modified separators are good heat-resistant dimensional stability, stable charging and discharging, long cycle life, and moderate mechanical strength. In general, lithium battery diaphragm material products show an obvious development trend of high safety.
3.3.2 Performance requirements of lithium battery separator
With the rapid development of lithium-ion battery applications, the market has put forward more stringent requirements for the performance of lithium-ion batteries, and battery manufacturers have higher and higher requirements for battery separators. At present, qualified lithium battery separator materials need to have the following characteristics [9]:
(1) The thickness is uniform and moderate. The thicker the diaphragm, the higher the mechanical strength and the higher the safety performance, but the greater the internal resistance of the battery, and vice versa; the better the uniformity of the diaphragm thickness, the greater the energy density, and vice versa;
(2) Good permeability and micropore uniformity. The greater the permeability, the more uniform the micropore distribution, the better the ion permeability, the higher the discharge rate, and the better the battery cycle performance and life;
(3) Strong ability to absorb and retain liquid. Strong liquid absorption and liquid retention ability is conducive to the rapid and complete infiltration of the diaphragm by the electrolyte and keeps it wet , which helps to ensure that the battery maintains good electrical performance during repeated charging and discharging;
(4) Good chemical stability and electrochemical stability. It is required that the diaphragm can be soaked in the organic electrolyte for a long time and stably without reacting with the electrode material and the electrolyte to cause failure or cause safety hazards ;
(5) Good dimensional stability and mechanical strength. The separator cannot shrink in the electrolyte , and should have a certain mechanical strength to ensure that it will not be pierced by burrs on the surface of the electrode material and cause a short circuit, and can meet the stretching requirements during battery assembly;
(6) Good thermal stability [7]. GB/T12027-2004 stipulates that the lithium battery separator needs to meet the requirements that, when placed at 110°C for 60min, the transverse and longitudinal shrinkage of the separator should be less than 0.5% and 1.5% respectively;
(7) High safety performance and good thermal self-closing effect. Lithium-ion batteries have relatively high safety performance requirements, especially for high-power lithium-ion batteries used in the direction of power batteries. When the battery is short-circuited or the temperature rises, the diaphragm needs to be able to melt and close the cells quickly to achieve Open circuit to avoid safety hazards [12,13];
(8) Low cost. At present, the lithium battery separator accounts for about 20% of the total cost of lithium batteries, and the main cost of the separator is generated by the manufacture of the separator. Therefore, reducing the cost of diaphragm manufacturing technology has become the key to reducing the cost of lithium battery diaphragms.
The performance requirements of the separator are not independent, and have certain constraints or gain relationships with each other. For example, reducing can increase the energy density of the battery, but at the same time reduce its mechanical properties. Therefore, in the actual development and application process, the performance requirements of various diaphragm materials should be considered comprehensively and a reasonable selection should be made.
3.3.3 Development Status of Lithium Battery Separators in CHINA
From the perspective of the overall domestic lithium battery separator market, the main separator market is still monopolized by foreign brands. The main foreign brands include SK from Korea, Asahi Kasei from Japan, Teijin, and cellguard from the United States. However, the development of domestic diaphragm technology has not stopped because of this. On the contrary, domestic diaphragm technology has emerged
There are a number of diaphragm companies that are gradually scaled up, such as Shenzhen Xingyuan Material Technology Co., Ltd., Foshan Jinhui Hi-Tech Optoelectronic Materials Co., Ltd., Xinxiang Green New Energy Materials Co., Ltd., Nantong Materials Co., Ltd., etc., the performance of its diaphragm products is basically similar or equivalent to that of imported diaphragms, and at the same time, the price has dropped significantly compared with imported diaphragms. In addition, the localization of diaphragms has a unique geographical advantage for domestic battery manufacturers with a huge demand for diaphragms , which can greatly shorten the supply cycle, timely track and efficiently realize after-sales service. At present, domestic diaphragm products have been basically recognized in the domestic middle and low-end market applications , and some markets have been replaced by imported products with domestic products. At the same time, a small number of domestic diaphragm products have entered the high-end market. The price has been greatly reduced due to the increase in quantity.
At present, more than 40 companies across the country have invested in lithium battery diaphragm production lines. Due to the relatively late rise of the domestic lithium battery diaphragm industry, most companies do not have a thorough grasp of the diaphragm production process, resulting in many diaphragm manufacturers failing to start normal production as scheduled. Among the more than 40 diaphragm manufacturers, there are only more than 20 production lines that can truly achieve industrialization. Although the lithium battery separator industry is in the early stage of new industry development in CHINA, the national policy of vigorously developing green new energy also provides a good development opportunity for it, but it needs to be clear that the domestic pre-investment diaphragm production line project will be completed and put into operation next year, when the production capacity will reach 7~ 800 million square meters, the scale expansion of foreign manufacturers will also increase the production capacity of about 5~ 600 million square meters, and the production capacity of low-end diaphragm products may far exceed the market demand.
3.3.4 problems facing the domestic diaphragm industry
Lithium-ion battery separators are one of the four key raw materials in the lithium battery structure. In recent years, with the rise of the lithium battery industry, domestic research on lithium battery separators has gradually heated up, but there are relatively few companies that have actually mass-produced them. The main reason is that CHINA has not fully broken through its production technology, but its high gross profit has made domestic separator manufacturers more enthusiastic about its research. Domestic companies are trying to break through foreign technical barriers and develop lithium battery separators, but the difficulty is greater. Mainly reflected in the following aspects [14]:
(1) The intellectual property rights of traditional diaphragm preparation processes are basically monopolized by a few countries such as the United States, South Korea, Germany, and Japan. CHINA's R & D and production started relatively late, and independent intellectual property rights are scarce.
(2) The research is not deep enough, the formula research is not systematic, the consistency and uniformity of the diaphragms produced are relatively poor, the pass rate is low, and mass production is difficult to achieve ;
(3) The manufacturing process of the diaphragm has strict requirements on the equipment. Poor equipment operation and control accuracy will directly affect the performance Generally, domestic equipment is difficult to meet the demand, but the price of imported equipment is relatively high. Industrialization has received certain equipment restrictions.
3.3.4 Analysis of Patent Technology of Coating Modified Lithium Battery Separator
3.3.4.1 General Overview of Patent Search
A total of 2,666 Chinese patents announced/published before March 3, 2015 were searched in the Chinese patent database , involving 770 lithium- ion battery separators and 325 coating-modified separators. Figure 2 is a diagram of the patent area composition. It can be seen from the figure that the coating modified diaphragm is mainly in CHINA, Japan, and South Korea, accounting for about 97% of the total patents of the coating modified diaphragm.
A detailed analysis of 325 coating modified diaphragm patents found that although the number of patents in CHINA is the largest, the patent applicants are relatively scattered, and there are many patents applied by research institutions and schools, and most of them are conceptual patents, and no product launches. In Japan and South Korea, the same applicant will have a series of coating modified diaphragm patents (Figure 3), with a large number of patents, wide coverage, serialization, and corresponding product launches.
