Lithium-ion battery requirements for diaphragm materials.

Time: 2019-06-26
Summary: The diaphragm is an important material for lithium-ion batteries. It embodies two important functions: one is to ensure battery safety; the other is to enable the battery to be charged and discharged.

The diaphragm is an important material for lithium-ion batteries. It embodies two important functions: one is to ensure battery safety; the other is to enable the battery to be charged and discharged. The increase of battery energy density is mainly based on the development and optimization of electrode material system; and the important characteristics of battery capacity, rate performance, cycle life, charge potential, first coulombic efficiency, self-discharge, high and low temperature characteristics, internal short circuit and lithium deposition. Both are related to the properties and quality of the diaphragm material.

 

The basic function of the separator is to isolate the positive and negative electrodes to avoid short circuits. At the same time, based on the working mechanism of lithium-ion battery: lithium ions are removed from the positive electrode material during charging, and migrated through the membrane to be inserted into the layered structure of the negative electrode material; lithium ions are removed from the negative electrode material during discharge and re-migrated through the diaphragm. Embedded in the cathode material (Figure 1).


Figure 1:Schematic diagram of the working process of lithium ion battery

 

Therefore, there is a need for through-holes for lithium ion migration on the separator; the most important property for the separator is the microporous structure. The technical parameters of the microporous structure of lithium battery diaphragm include: pore size, pore size distribution, porosity; structural features that are difficult to quantify, the degree of fiber formation during stretching, the uniformity of pore formation, the ratio of blind and closed pores. Wait. The microporous structure of a membrane can clearly reflect the characteristics of its production process and the level of equipment. The most convenient method for antimicroporous structure is scanning electron microscopy (SEM). The use of SEM can visually reflect the difficulties encountered by manufacturers in improving the quality of the diaphragm. The uniformity of the membrane, the shape, size and approximate distribution of the surface pores are characterized by characterizing the surface of a membrane. More accurate information needs to be obtained by a mercury intrusion meter and a capillary flow analyzer.

 

At present, there are two main types of commercial lithium battery separators: one is a polyolefin separator (PP, PE, PP/PE/PP) manufactured by mechanical stretching and hole making process; the other is a non-woven/ceramic particle composite diaphragm. . Ceramic coatings (or other types of coatings) developed to enhance the safety of polyolefin separators complement the performance of the first type of membranes. The research and industrialization of high-power, high-power power batteries have placed high demands on the development of diaphragms. High-quality, high-voltage and high-voltage diaphragms have always been a hot spot in the field of lithium battery research. However, there is currently no new diaphragm that has actually entered the industrial manufacturing stage.


Figure 2:SEM image of ultra-high molecular weight PE separator of Sinoma Lithium Film Co., Ltd.

 

Globally, the main markets for lithium-ion diaphragms are concentrated in Japan, China, South Korea and the United States; and leading production technologies are basically in the hands of a few companies in Japan and the United States. But with the acquisition of Celgard by Asahi Kasei in 2015, Japan became the leader in lithium-ion diaphragm manufacturing. In 2016, China's diaphragm demand is about 2 billion m2, the output is about 1.2 billion m2, and the import ratio is about 40%. The high-end power battery separator has a higher import rate of 70%. In 2017, the global diaphragm production is estimated to be around 2.38 billion m2, a year-on-year increase of 25.3%, of which dry membranes account for 42%, wet membranes account for 58%, and global lithium battery separators grow faster. Due to the increase in downstream demand, it is expected The global market will maintain a growth rate of more than 20%.

 

In April 2017, the Ministry of Industry and Information and other three ministries and commissions issued the "Notice on Printing and Distributing the Medium- and Long-Term Development Plan for the Automobile Industry", stating: "In 2020, the annual production and sales of new energy vehicles will reach 2 million units... By 2025, new energy sources Automobiles account for more than 20% of automobile production and sales.” China EV100 “Lithium and Battery Enterprise Sustainability Research Report 2017” quotes data from China's power lithium battery market demand forecast for 2020-2025, 2 million in 2020 and 15.2 million in 2030. A new energy vehicle requires 130 GW·h and 500 GW·h of power batteries. Accordingly, the demand for high-quality power battery separators can be calculated to be roughly 2 billion m2 in 2020 and 9 billion m2/year in 2030. If the demand for diaphragms for energy storage batteries and consumer electronics batteries is added, the diaphragm demand calculated above will roughly be 3 billion m2 in 2020 and 10 billion m2 in 2030.

 

Status of domestic lithium battery separator related standards.

 

The production process and equipment of different membrane companies are different, and the characteristics of the membrane products are highly correlated with the process and equipment. Therefore, the standard for the development of diaphragm products in the industry is not significant. What makes sense is that companies develop their own product standards, standardize production processes, and ensure product quality consistency. The main standards that can be retrieved are the test standards for some diaphragms, as shown in Table 1. China has promulgated relevant standards for diaphragm materials in the past ten years, including 14 national standards, 1 national quality testing standard, and 1 industry standard. From the category point of view, there are 1 product standard, 15 test and analysis methods. Most of them are drafted, reviewed and released by the China National Standardization Administration. Relative to manufacturing and product standards, diaphragm testing standards are relatively easy to develop and accepted by the industry, and have been gradually referenced or applied in actual testing work.

 

Lithium-ion battery requirements for diaphragm materials

 

The performance of the separator affects the interface between the battery pole piece and the diaphragm, the maintenance of the electrolyte, the conduction of lithium ions, the internal resistance of the battery, the charge and discharge performance, the cycle life, and the safety performance. Generally, the performance of the separator is as follows.

1 uniformity, including uniform thickness, uniform tension, uniformity of micropore distribution, etc.;

2 Consistency refers to batch consistency of product quality;

3 Under the premise of ensuring safety, reduce the thickness of the diaphragm and increase the energy density/power density of the battery;

4 increase the porosity to a reasonable range and improve the rate performance of the battery;

5 optimized pore size distribution;

6 Improve the heat resistance and ensure that the diaphragm does not shrink at a large rate when the battery is abnormally heated;

7 has sufficient mechanical strength and tensile elongation ratio to ensure the reliability when external force is applied;

8 Improve oxidation resistance, ensure battery life and suitability for high voltage conditions;

9 provides a closed cell temperature that can effectively protect the battery;

10 chemical / electrochemical stability; high breakdown voltage; low water content (to reduce electrolyte decomposition, improve battery yield).

 

In the various aspects of raw material selection, production, transportation and sales of the diaphragm, fluctuations occur due to changes in human, machine, material, law, ring, measurement and other conditions. Therefore, from the screening of raw materials to the final sale, it is required to operate according to standards and be tested according to standards to ensure the practicability, consistency and reliability of the products. The user cares about the function, consistency and reliability of the product. The control of the product quality mainly detects the following technical indicators.

 

Diaphragm thickness

 

The thickness is determined by considering the overall performance and safety of the battery. Thickness uniformity is one of the quality indicators that need to be strictly controlled in the production process. The thickness deviation is determined by the current level of the diaphragm industry and the degree to which the battery assembly can accept. The thinner the separator, the smaller the resistance encountered when solvating lithium ions crosses, the better the ion conductivity, the lower the impedance, but when the diaphragm is too thin, its liquid retention capacity and electronic insulation are reduced, which also brings battery performance. negative effect. For consumer lithium-ion batteries (cells, laptops, batteries used in digital cameras), thinner diaphragms, such as 7μm, or even thinner diaphragms, are beginning to be used in a wide range of applications as they are used. The development of polyolefin membranes in a thinner direction to meet the performance requirements of 3C lithium-ion batteries is a key entry point for improving battery performance in the future. For power batteries, thicker diaphragms are often required due to the mechanical requirements of the assembly process. Of course, for large power batteries, safety is also very important, while thicker diaphragms often mean better safety. EV /HEV uses a diaphragm with a total thickness of 16 to 25 μm. Generally speaking, the thicker the thickness of the diaphragm, the higher the mechanical strength, which can ensure the safety of the battery to a certain extent, but the effect on the puncture damage, battery structure damage and ultra-high temperature impact is small, and the final safety of the battery. There is also a need to improve the stability of the positive and negative materials and the electrolyte to ensure. At present, diaphragm manufacturers generally refer to GB/T 6672-2001 Plastic Film and Sheet Thickness Measurement Mechanical Measurement Method or ISO4591:1992 "Plastic-film and sheet-sample average thickness measurement, coil average thickness and yield measurement. - Weight measurement method (weight analysis thickness), the method mainly specifies the sampling method, instrument test accuracy, measurement pressure, measurement area, etc., but there is currently no standard for lithium battery separators. During the actual test process, each The test conditions are different from the experimental parameters, which makes the test results different.

 

The weight of the diaphragm

 

The grammage of the lithium battery separator material, ie the areal density, indirectly reflects the porosity of the separator material of the same thickness and raw material specification, mainly related to the density of the separator material and the thickness specification of the separator material. The weight of the lithium battery separator material also affects the internal resistance, rate, cycle performance and safety performance of the lithium battery. Table 1 lists some of the standards in the corporate standards. Most of the methods for detecting the grammage of different types of lithium battery separators are to weigh the masses of several fixed length and width sizes by an electronic balance, and calculate the corresponding gram weights. The gram weight calculation method of the sample with a fixed size is usually set on the electronic balance, and the gram weight reading corresponding to the sample is directly displayed after weighing, and the calculation formula is shown in formula (1).

(1)

ρ:gram weight (area density), g/m2; m: the mass of the sample, g;

L: the length of the sample, m;b:the width of the sample, m.

Microporous structure characteristics of the diaphragm

The microporous structure of the diaphragm is mainly reflected in the two aspects of porosity and pore size distribution. Relatively speaking, the pore size distribution is a more important indicator for evaluating the quality of the diaphragm. The porosity (p) is the ratio of the volume of the pores to the total volume of the membrane, that is, the percentage of the volume of the pores per unit membrane, which is related to the density of the raw material resin and the final product. The pores of the membrane material, including through-holes, blind vias, and closed-cells, are only useful for through-holes for lithium-ion battery separators. To determine the porosity of a diaphragm sample, the following methods can be used: electron microscopy combined with software processing, weighing calculation, liquid infiltration, and instrumental measurement (mainly mercury intrusion method, capillary flow method). Scanning electron microscopy can be used to visually observe the pore formation state of the membrane surface, and then image processing software is used to mark the pore size and calculate the pore size distribution to estimate the porosity. The porosity obtained by this method is relatively rough and does not necessarily reflect the pore formation inside the membrane. The weighing method is based on the density of the film material and the apparent density of the film. formula (2)

(2)

In the formula, ρf is the apparent density of the film, which is calculated by the weighing method, and the film of a certain area is cut, and the thickness of the film is measured by a spiral micrometer to obtain the volume of the film, and the density of the film is calculated after weighing. P is the density of the film raw material. This method requires first measuring the apparent density of the film, there is a measurement error, and it is necessary to know the density of the film raw material. The liquid infiltration method is to take a certain size of the film after drying and weigh it into the n-butanol solution for a certain period of time, and then remove the n-butanol on the surface of the film by using a filter paper, weigh the weight, and then according to the formula (3) Calculate porosity.

(3)

W1 is the mass of the film after infiltrating n-butanol, W2 is the mass of the dry film, V is the apparent volume of the film, and d-n-butanol is the density of n-butanol. The weighing method is to compare the mass of the sample with the mass of the same material without pores. The main error comes from the calculation of the sample volume, and the calculation results include the contribution of closed cells. The n-butanol is absorbed by the weighing diaphragm. The difference in weight before and after the solvent such as alkane has a large random error in the calculated porosity, which can roughly characterize the ratio of the through hole and the blind hole of the diaphragm. Porosity was measured by capillary flow analyzer and mercury intrusion meter, and the results obtained by software based on pore size distribution were obtained. The results measured by different instruments were slightly different. Most lithium-ion battery separators have a porosity of 35% to 50%. Some commercial separators (such as surfactants on the surface) have a porosity of less than 30%, and some have a high porosity of up to 60%. . In principle, for certain electrolytes, membranes with high porosity can reduce the impedance of the battery, but the porosity is not as high as possible. The higher the porosity, the worse the mechanical properties and the anti-opening properties. Even if the porosity and thickness are the same, the impedance may be different, which is due to the difference in the penetration of the holes. The calculation of porosity (p) is given by equations (4) and (5). The theoretical density of polyethylene diaphragm is generally 0.95 g/cm3.

(4)、(5)

 

ρ1 is the areal density of the sample, g/m2; m is the mass of the sample, g; L is the length of the sample, m; b is the width of the sample, m; p is the porosity of the sample, Indicated by %; d is the thickness μm of the sample; ρ0 is the density of the raw material, g/cm3. The pore size distribution can be measured by a capillary flow analyzer, a mercury intrusion meter or the like. The capillary flow analyzer measures the pore size parameters by the bubble point method. The standard can be referred to ASTM F316-03. The mercury intrusion test can refer to the standards listed in Table 1. There are differences in the results of the two tests. The capillary flow analyzer test reflects the condition of the through hole, and the data of the mercury intrusion meter contains the contribution of the blind hole.

 

Mechanical properties of the diaphragm

 

The mechanical properties of a lithium-ion battery separator are a sign of whether the diaphragm is easily broken. The diaphragm rupture will cause a short circuit in the battery, reduce the yield, and the safety performance of the battery will also be affected. The mechanical properties of the diaphragm are mainly represented by the mechanical strength of the diaphragm. The certain mechanical strength can ensure that the diaphragm does not break under the condition of battery deformation, reduce the risk of short circuit of the battery, improve the yield, improve the safety performance of the battery, and prolong the service life of the battery. The mechanical strength has two parameters, one is the tensile strength and elongation at break of the separator in the longitudinal direction and the vertical direction; the other is the puncture strength in the thickness direction. The tensile strength of the separator refers to the tensile force of the separator of a unit cross section at the time of tensile fracture, indicating the ability of the separator to resist stretching, and sufficient tensile strength can prevent deformation of the separator. Associated with the process of film making. With uniaxial stretching, the film is different in strength from the vertical direction in the stretching direction; and in the case of biaxial stretching, the film has similar consistency in both directions. Generally, the tensile strength mainly means that the longitudinal strength is more than 100 MPa. In actual battery manufacturing, it is required that the tensile strength in the longitudinal direction is sufficiently high, and the lateral strength is not too large, and excessively, the lateral shrinkage rate is increased, and this shrinkage increases the probability of contact between the positive and negative electrodes of the lithium battery. Elongation at break mainly refers to the percentage increase of the length of the diaphragm sample when the diaphragm is subjected to the tensile test. This value is mainly used to measure the elongation of the diaphragm when it is not broken. The test for tensile strength and elongation at break of the separator is GB/T 1040 3—2006 Determination of tensile properties of plastics. Part 3: Test conditions for films and sheets. The experimental parameters involved are mainly clamp distances. , tensile rate, sample size, etc., the current test speed is 50 (200, 250) mm / min, the clamp spacing is 100 mm, the sample size is mostly long strip type and dumbbell type, the test sample edge is required to be smooth and no gap; The general test parameters will not affect the results. In the case of ensuring the quality of the sample preparation, the results obtained are more parallel and have higher accuracy. The puncture strength is related to the roughness of the surface of the electrode plate. Since the separator is sandwiched between the uneven positive and negative plates, it is required to withstand a large pressure. In order to prevent short circuits, the diaphragm must have a certain penetration strength. The materials used for the electrodes are different, and the puncture strength of the diaphragm is also required to be different. If the carbon material particles are fine and have no edges and corners, the puncture strength value of the diaphragm is required to be relatively low. On the contrary, if the particles are thick and large, and the edges are sharp, the required diaphragm puncture strength is high. Sufficient puncture strength can prevent lithium dendrites and pole pieces from piercing the diaphragm and cause short circuit. The puncture strength is generally between 300 and 500 gf. However, the method used in the test is quite different from the actual battery. It is not particularly reasonable to directly compare the puncture strength of the two diaphragms. At present, the diaphragm puncture strength is mostly tested by ASTM D4833-2007 or GB/T 10004-2008. The test results of the puncture strength are related to the specification of the puncture needle, the puncture rate, the size of the lower clamp, etc. The current diameter of the puncture needle is 1.0 mm. The radius of the sphere is 0.5mm, etc., and the puncture rate is mostly based on the national standard.

Heat shrinkage and closed cell of the diaphragm.

The heat shrinkage of polyolefin separators is closely related to the safety performance of lithium batteries. In general, the lateral direction (TD direction) of the diaphragm corresponds to the height direction of the battery. The positive pole piece width + (1 ~ 2) mm = negative pole piece width, negative pole piece width + (1 ~ 2) mm = diaphragm width. During the baking process, the shrinkage amount in the TD direction cannot make the positive and negative pole pieces and the aluminum shell contact, that is, it cannot exceed 1 to 2 mm, and if it exceeds, the positive and negative electrodes are short-circuited. The longitudinal direction (MD direction) of the diaphragm corresponds to the width direction of the battery, that is, the winding or lamination direction of the preparation process. During the winding or lamination process, the diaphragm is tightened and the tape is bonded. Longitudinal heat shrinkage will strain the diaphragm, and the lateral burrs of the pole piece will more easily pierce the diaphragm to short the internals of the battery. Lithium-ion manufacturers generally require vertical and lateral heat shrinkage at different temperatures and times for different types and thicknesses of separators. The test method generally adopts "GB/T 12027-2004 Plastic, film and sheet heating dimensional change rate experimental method". However, for the lithium ion battery separator, considering the condition of the diaphragm between the battery pole pieces, it is recommended to refer to UL2591-2009, and clamp the diaphragm between the two steel plates to add a fixed pressure test.

 

The concept and meaning of the "closed hole" of lithium-ion diaphragms is often abused by some product propaganda and confused with the melting temperature. At what abnormal temperature does a lithium-ion battery need to be closed by a diaphragm to stop the battery? There is currently no consensus in the industry. The closed-cell test method uses a simulated battery to determine the point of change in the resistance of the two sides of the membrane immersed in the electrolyte during the temperature rise, with reference to the standard UL2591-2009.

Electrochemical stability and oxidation resistance of the separator

The parameter of the electrochemical characteristics of the separator is an electrochemical stabilization window, which mainly reflects the upper limit of the voltage at which the separator can be normally charged and discharged. The electrochemical window can be tested for cyclic voltammetry using an analog battery. However, the results of this test will be affected by the type of electrolyte and additives. The oxidation resistance of the separator is mainly determined by the basic properties of the material and does not vary depending on the preparation method. The difference in oxidation resistance of different membranes will gradually appear after the battery has been in operation for a while, and there is currently no good test recommendation method.

Ceramic coating on the surface of the diaphragm

As the thickness of the separator becomes thinner, ceramic coating has become a mandatory additional process. Single-sided or double-sided coating, symmetric coating or asymmetric coating, coating thickness, areal density and particle diameter of the coating layer, binder and other additive components are all variables that affect the performance of the coated membrane. The various factors involved in the coating will have a very complex effect on the performance of the membrane. However, there is currently no information on relevant norms or standards in this area.

Water content of the diaphragm

The moisture content of the diaphragm material is closely related to the characteristics of the separator material and the moisture content, and the moisture control of the production site. There is no uniform industry standard for the current standard requirements for moisture content in the lithium-ion battery separator material industry, but with the rapid development of the electric vehicle industry, especially the quality requirements of power lithium-ion batteries are significantly improved, lithium-ion battery separator materials The moisture content will also be one of the important indicators for measuring the quality of diaphragm materials. The interior of a lithium battery is a relatively complex chemical system. The reaction results and processes of these chemical systems are closely related to moisture. The loss of control or coarsening of water leads to the presence of excessive moisture in the battery, which not only leads to the decomposition of the electrolyte lithium salt, but also has a bad influence on the film formation and stability of the positive and negative materials, resulting in the electrochemical properties of the lithium ion battery. Such as capacity, internal resistance, product characteristics will produce more obvious deterioration. The moisture content control of the diaphragm material is also an important part of controlling the water content inside the battery.

Because the moisture content is important, it needs to be tested by the highly sensitive Karl Fischer method. It is usually tested with a Coulomb moisture meter: I2, SO2, pyridine, and anhydrous CH3OH are used as reagents to react with water in the sample to calculate The water content of the sample. The test principle is shown in equations (6) and (7).

(6)、(7)

Diaphragm product outlook

The diaphragm has multiple material systems. Even the same type of diaphragm will form different series, specifications and quality differences due to factors such as manufacturer, process, thickness variation and surface coating. Membrane companies are concerned about the physical properties of the diaphragm, and the battery manufacturer as the user is concerned with the application of the diaphragm in the battery. Therefore, the starting point and requirements for considering the diaphragm quality standards are different. We need to be fully aware of the complexity of the battery manufacturing process and the complexity of the factors that affect battery performance. For the battery of the characteristic model, the selection of the diaphragm material is very important. Choosing the right materials and specifications is a necessary condition for making high quality batteries. With the rapid development of the lithium battery industry, the demand for diaphragms in the industry is also increasing. Diaphragm enterprises should be reasonably guided to develop in the direction of improving product quality, standardizing production, and enhancing the international competitiveness of enterprises. 

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