Introduction to Lithium Ion Battery Isolation Film What are the effects of raw materials and manufacturing processes on the performance of the isolation film?

1. Introduction to Isolation Film

1.1 The impact of raw materials and manufacturing processes on the performance of the isolation membrane

1.1.1 Definition and function of isolation membrane

Separator is an important part of lithium-ion batteries, a microporous membrane used to separate the positive and negative pole pieces, and a polymer functional material with nano-scale microporous structure. Its main function is to prevent short circuit due to contact between the two poles while allowing electrolyte ions to pass through. Its performance determines the battery's interface structure, internal resistance, etc., and directly affects the battery's capacity, cycle, and battery safety performance.

1.1.2 Raw materials of isolation membrane

At present, most of the commercial lithium-ion battery separator products are microporous membranes made of polyolefin materials. The main raw materials are high-molecular-weight polyethylene and polypropylene. The products include polyethylene PE monolayer film, polypropylene PP monolayer film and PP/PE/PP multilayer microporous film composed of PP and PE. Polyolefin materials have the advantages of high strength, good acid and alkali corrosion resistance, water resistance, chemical resistance, good biocompatibility, non-toxicity, etc., and their industrial preparation is relatively mature. There are also PET/cellulose non-woven fabrics, polyvinylidene fluoride (PVDF) porous membranes, polyimide (PI) electrospun porous membranes, and other lithium-ion battery separators that are in the research stage or have not yet been applied on a large scale. Kinds of PE, PVDF, PP, PI modified membranes, etc.

PE film requirements for HDPE raw materials:

1) Excellent miscibility, HDPE has good solubility, the melting temperature is greater than 135℃, and the density is 95%-99%. It can be dissolved with organic alkanes to form a uniform solution, which is the guarantee of the consistency of the diaphragm.

2) Appropriate molecular weight and molecular weight distribution, the molecular weight is greater than 300,000, and the distribution is narrow, PDI=Mw/Mn=6-8, ensuring the processing performance and mechanical properties of the diaphragm.

3) Low gel and impurity content, only one main degradation peak in the DSC curve, single raw material composition, low inorganic impurities, ensuring the quality of the diaphragm.

4) Plasticizer and extractant, liquid paraffin (C16-C20 n-alkane) as plasticizer, and dichloromethane as extractant to ensure the uniformity of pore formation.

Requirements of PP film to PP raw materials:

5) It has a high isotactic index, the gauge composition must be greater than 95%, and the melting temperature is greater than 163℃ to ensure good crystallization and pore formation

6) Appropriate molecular weight and molecular weight distribution, the molecular weight is greater than 400,000, the distribution is relatively, PDI=Mw/Mn=6-8, to ensure the processing performance and mechanical properties of the diaphragm

7) Low gel and impurity content, only one main degradation peak in the DSC curve, single raw material composition, low inorganic impurities, ensuring the quality of the diaphragm.

8) β crystal modifier, the dry biaxial stretching process also needs to add β crystal modifier, even mixing is an important factor for the uniformity of biaxial stretching.

1.1.3 Process of isolation film

The material of lithium ion battery separator is mainly porous polyolefin, and its preparation methods mainly include wet method and dry method. The wet method is also called phase separation method or thermally induced phase separation (TIPS); dry method, that is, stretching The pore formation method is also called melt stretching (MSCS). Both are aimed at improving the porosity and strength of the separator. The classification and process of the diaphragm, the characteristics are shown in the following table. In addition, PET/cellulose non-woven fabrics use a non-woven technology process, polyvinylidene fluoride (PVDF) porous membranes also use phase separation methods, and polyimide (PI) and polyamide (PAI) use electrospinning. And casting phase separation process.

1.1.3.1 Process production process of dry diaphragm

The dry method is to melt, extrude, and blow a polyolefin resin into a crystalline polymer film. After crystallization heat treatment and annealing, a highly oriented multilayer structure is obtained. It is further stretched at a high temperature to peel off the crystal interface to form Porous structure can increase the pore size of the diaphragm. The porous structure is related to the crystallinity and orientation of the polymer. The key technology of the dry method is that the polymer must be stretched about 300 times under the viscous flow state of the polymer to form a hard elastomer material when the polymer is melted and extruded. The process flow of the multi-layer PP and PE composite film is as follows: ①The PE and PP are separately melted and extruded, and the cast sheet is stretched about 300 times to form a 12μm film; ②The PE and PP films are thermally composited, heat treated, and longitudinally Stretching and heat setting. The process flow of dry diaphragm is as follows:

Polyolefin membrane dry process flow chart

1) Melt extrusion/ stretching/ heat setting method (uniaxial stretching method)

The preparation principle of the melt extrusion/stretching/heat setting method is that the polymer melt is crystallized under a high stress field to form a lamella structure perpendicular to the extrusion direction but arranged in parallel, and then heat treated to obtain an elastic material. After stretched, the polymer film with rigid elasticity separates between the lamellae, and a large number of microfibers appear, thereby forming a large number of microporous structures, and then after heat setting, the microporous film is prepared. Relevant patents introduce this preparation process of polyolefin microporous film. The stretching temperature is higher than the glass transition temperature of the polymer and lower than the crystallization temperature of the polymer. For example, the blown and extruded polypropylene film is heat treated to obtain hard elasticity. The film is first cold drawn by 6% to 30 %, then heat stretched at 120 to 150 ℃ for 80% to 150 %, and then heat-set to obtain a highly stable microporous film. The process of melt extrusion/stretching/heat setting method is relatively simple and pollution-free. It is a common method for preparing lithium-ion battery separators. However, this method has disadvantages such as difficult to control pore size and pores.

Figure 11: Before and after dry stretching of polyolefin membrane

The PP and PP/PE/PP membranes prepared by the uniaxial stretching dry process have slender pores with a length of about 0.1～0.5μm and a width of about 0.01～0.05μm. The pore structure is straight through Pores, the pore size of the prepared membrane ranges from 0.1 to 3 μm, and the crack pore diameter of the membrane is the longest 0.4 μm and the widest 0.04 μm. Because the uniaxially stretched dry film is not stretched in the TD direction, its strength in the TD direction is poor, only about 10MPa (about 1/10 of the wet film), and it is easy to tear in the TD direction, but It is also because there is no stretching in the TD direction, there is almost no thermal shrinkage in the TD direction. In addition, PP polypropylene has poor ductility and low surface energy. It is a difficult-to-stick plastic and is not conducive to bonding with the positive and negative plates. The interface between the separator and the electrode is not tight, which affects the performance of the battery.

2) Co-extrusion / stretching / heat fixing method with nucleating agent added (biaxial stretching method)

Co-extrusion with nucleating agent is added to form a film containing solid additives. The solid additives are uniformly distributed in the polymer phase with sub-micron particle size. Due to the phase separation caused by the stress concentration during stretching, a microporous film is formed. The microporous membrane is prepared by biaxially stretching a polypropylene membrane containing a large amount of β crystal form, and then heat-fixing it. The pore size is 0.02～0.08μm and the porosity is 30%～40%. The strength of the direction is consistent, about 60 ~ 70 MPa. Because the β-crystal polypropylene morphology is composed of lamellae grown in bundles, the density of spherulites is low, so the amorphous regions between the wafer bundles are easily pulled apart to form microcracks or micropores. After adding the nucleating agent, because the crystal structure becomes loose, it is easy to form pores during stretching, and there is no pollution. This method was first developed by the Chinese Academy of Sciences, and domestically, Xinxiang Green, and New Times Technology used this method to produce biaxially stretched single-layer PP diaphragms.

The PP film prepared by the biaxial stretching dry process is stretched in both MD and TD directions, and its strength in the TD direction is about 6 times greater than that of the uniaxial stretching dry process, so its TD direction will not Easy to tear. The pore structure is similar to that of the wet method, which is a dendritic non-straight pore. Because it needs to add a solid nucleating agent, the degree of dispersion of the nucleating agent in the PP melt directly affects the uniformity of its pore formation, but the degree of dispersion in the solid melt is more difficult to control, so the uniformity of pore formation It is the biggest disadvantage of biaxial dry stretching.

1.1.3.2 Process production process of wet diaphragm

The thermally induced phase separation method is a method of preparing microporous membranes developed in recent years. It uses polymers and some small molecular compounds with high boiling points at a higher temperature (generally higher than the melting temperature Tm of the polymer) When the temperature is reduced, a homogeneous solution is formed, and solid-liquid or liquid-liquid separation occurs again when the temperature is lowered. In this way, in the high polymer phase, the low-molecular substances can be removed after stretching to form an interpenetrating microporous membrane material. The wet extrusion casting uses thermally induced phase separation. The wet method mixes liquid hydrocarbons or some small molecular substances with polyolefin resins, and after heating and melting, a uniform mixture is formed, the solvent is volatilized, the phase separation is carried out, and then pressed Obtain the membrane; heat the membrane to close to the crystalline melting point, keep it for a certain period of time, use volatile substances to elute the residual solvent, add inorganic plasticizer powder to form a film, and further use the solvent to elute the inorganic plasticizer, and finally It is extruded into a sheet. Polymers such as PE, PP and small molecular compounds with high boiling points such as paraffin and DOP form a homogeneous solution at elevated temperature (higher than the melting point of polymers such as PE), and phase separation occurs when the temperature is reduced, and undergoes biaxial stretching. Then, the small molecule compounds such as paraffin wax can be eluted with solvent to become microporous material.

The process flow is as shown in the figure: twin-screw extruder extrusion, cast sheet molding, synchronous/asynchronous two-way stretching, solvent extraction, blow-drying, horizontal stretching, online thickness measurement, winding, aging treatment, slitting, etc. . The membrane prepared by this method can change its performance and structure by controlling the composition of the solution and the volatilization of the solvent during the gel curing process.

Figure wet process polyolefin diaphragm production process

The wet process of biaxial stretching is also divided into synchronous stretching and asynchronous stretching. Simultaneously stretched MD and TD directions are stretched at the same time. The PE membrane prepared by this method has better uniformity, higher pass rate, and smaller strength difference between TD and MD directions. Asynchronous stretching is to first stretch in the MD direction, and then stretch in the TD direction. The stretching ratio of the two directions is controllable and adjustable, with higher flexibility and greater strength than synchronous stretching. The disadvantage is The uniformity in the TD direction is not as good as simultaneous stretching. Generally speaking, the TD-direction strength of the membrane prepared by the wet process is higher than that of the dry process, the pore size is uniform, the tortuosity of the pores is high, the porosity is high, and the air permeability is good.

1.1.3.3 Process production process of non-woven diaphragm

Non-woven fabric is a kind of fabric that does not need to be spun and weaves, but only oriented or randomly arranged woven short fibers or filaments to form a web structure, which is then reinforced by mechanical, thermal bonding or chemical methods. It directly uses polymer chips, short fibers or filaments to form new fiber products with soft, air-permeable and planar structure through various web forming methods and consolidation technologies. Due to its porous structure and low price, non-woven membranes are widely used in nickel-metal hydride and nickel-cadmium batteries. At present, more and more researchers are applying non-woven membranes to lithium-ion batteries, but they are in their infancy. Non-woven fabric separators for lithium-ion batteries are classified according to materials, mainly polypropylene non-woven fabric separators, polyester (PET) non-woven fabric separators, and cellulose separators.

The main processes of non-woven fabrics are as follows:

1) Spunlace non-woven fabric: The spunlace process sprays high-pressure fine water onto one or more layers of fiber webs to entangle the fibers with each other, so that the webs can be reinforced and have a certain strength.

2) Thermally bonded non-woven fabric: Thermally bonded non-woven fabric refers to the addition of fibrous or powdery hot-melt adhesive reinforcement materials to the fiber web, and the fiber web is heated, melted and cooled to reinforce the fabric.

3) Pulp air-laid non-woven fabric: Air-laid non-woven fabric can also be called dust-free paper and dry paper-making non-woven fabric. It uses air-laid technology to open the wood pulp fiberboard into a single fiber state, and then uses air-flow method to agglomerate the fibers on the web curtain, and then reinforce the fiber web into a cloth.

4) Wet-laid non-woven fabrics: Wet-laid non-woven fabrics are used to open fiber raw materials placed in an aqueous medium into single fibers, and at the same time mix different fiber raw materials to form a fiber suspension slurry, which is transported to a web forming mechanism. The fibers are formed into a net in a wet state and then reinforced into a cloth.

5) Spunbond non-woven fabric: Spunbond non-woven fabric is formed after the polymer has been extruded and stretched to form continuous filaments, the filaments are laid into a web, and the web is then bonded by itself, thermally bonded, and chemically. Bonding or mechanical reinforcement method, the fiber web becomes non-woven.

6) Melt-blown non-woven fabric: The process of melt-blown non-woven fabric: polymer feeding-melt extrusion-fiber formation-fiber cooling-net formation-reinforcement into cloth.

The pore structure of this kind of non-woven fabric is interwoven with fibers, so it has the advantages of large pore size and high pore size, but its shortcomings are also obvious: easy to absorb moisture, low strength, wide pore size distribution, and difficult to thin ( >16um)

1.1.3.4 Process production process of electrospinning diaphragm

Electrospinning is the most important basic method to obtain nanofibers. The main principle is to make the charged polymer solution or melt flow and deform in the electrostatic field, form a Taylor cone at the tip of the spinneret to produce nanowires and spray them, and then solidify by solvent evaporation or melt cooling to obtain fiberization substance. Therefore, this process is also called electrospinning. The meaning of nanofiber refers to the diameter of the fiber. Generally, fibers with a diameter in the range of 1-100nm are called nanofibers. Of course, this definition of upper and lower limits is not absolute. The diameter of the fiber produced by electrospinning varies with spinning conditions, and the typical data varies from 40 to 2000 nm. That includes the micron, sub-micron and nanometer ranges. The basic principle of electrospinning is shown in the figure:

Electrospinning system mainly includes four parts: spinneret, infusion system, high-voltage generator and threading system. Electrospinning process (referred to as electrospinning process) is that polymer solution or melt passes through a charged spinneret. Under the action of the high-voltage electrostatic field formed by the spinneret and the spinning system, the liquid stream is divided into multiple streams. The solvent continuously volatilizes and the polymer solidifies, forming a non-woven fiber membrane on the threading system. To be precise, during the electrospinning process of the polymer solution, the liquid stream is split due to the mutual repulsion of the electric charges, and the electric field causes the split liquid beam to move to the wire connection system and land on the wire connection system. In the whole process, it is electric field force that plays a fundamental role.

Electrospinning separator has the characteristics of high porosity, high magnification, high resistance, etc. If polyimide is used as the spinning material, its heat resistance can be as high as 500 degrees, which can improve the safety performance of the battery. However, due to the spinning process, its mechanical strength is poor, only 1/10 of wet PE film.

1.2 Characterization method of isolation membrane performance parameters

1.2.1 Technical requirements for lithium ion battery diaphragm

The performance of the lithium-ion battery separator determines the interface structure and internal resistance of the battery, and directly affects the battery's capacity, cycle and battery safety performance. Therefore, the technical requirements for lithium-ion battery separators are:

1) Insulation performance, is an electronically conductive insulator

2) The least repellent to electrolyte, with good electrolyte wettability

3) The ion conductivity is high, that is, the resistance to the movement of dielectric ions is small

4) It can effectively prevent the migration of particles, colloids or other soluble substances between the positive and negative electrodes

5) The mechanical strength should be high to ensure that it will not tear or deform during processing.

6) Dimensional stability, small dimensional changes at temperatures below the melting point, will not cause short circuit of the positive and negative electrodes

7) Chemical stability and electrochemical inertness. It must be stable enough for electrolyte, possible impurities, electrode reactants and electrode reaction products, and will not dissolve or degrade.

8) The uniformity of thickness and hole diameter should be high

Different lithium-ion battery systems and application areas have different requirements for separators.

1.2.2 Characterization of performance parameters of isolation membrane

The characterization of the performance parameters of lithium-ion battery separators can be divided into three aspects: structural properties, mechanical properties and physical and chemical properties.

1.2.2.1 The structural characteristics of the diaphragm:

It mainly includes parameters such as thickness, pore size and distribution, porosity, permeability, and microscopic morphology.

1) Thickness: The thickness of the lithium ion battery separator is generally <25μm. Under the premise of ensuring certain mechanical strength, the thinner the thickness of the diaphragm, the better. At present, consumer electronics batteries use wet-process PE thin membranes due to their high energy density requirements, which have reached the level of applying 9um membranes. A company has mass-produced 7um substrates. Electric vehicles (EV) and hybrid electric vehicles (HEV) mostly use dry-process diaphragms with a film thickness of 20 μm or 16 μm, mainly considering price issues. Its thickness uniformity is also an important indicator of battery consistency.

2) Pore size and distribution: As a lithium-ion battery separator material, it has a microporous structure that allows the electrolyte to be absorbed; in order to ensure consistent electrode/electrolyte interface properties and uniform current density in the battery, the micropores in the entire separator material The distribution should be uniform. The size and uniformity of the pore size have a direct impact on the battery performance: too large pore size, it is easy to make the positive and negative electrodes directly contact or easy to be pierced by lithium dendrites and cause short circuit; too small pore size will increase the resistance. The uneven distribution of micropores will cause excessive local current during operation, which will affect the performance of the battery. Using a capillary flow pore meter (CFP), using a non-volatile fluorine-containing organic liquid as the medium, the relationship between pressure and gas flow rate was measured for different commercial lithium-ion battery diaphragms. The results show that (Table 1 and Figure 1): The pore size of commercial membranes is generally 0.03-0.05μm or 0.09-0.12μm, and it is believed that the difference between the maximum pore size and the average pore size distribution of most commercial membranes is less than 0.01μm

Table 1 Diaphragms of different thicknesses used for testing

Figure 1 Diaphragms of different thicknesses used for testing

The pore diameter of the diaphragm can be obtained by formula (1), T represents the surface tension of the test liquid, C is the capillary constant, p is the gas pressure, and d is the pore diameter. At the same time, this method can combine the wet line and the dry line to get the pore size distribution.

Figure 2 The pore size and distribution of common diaphragm and *** diaphragm of a company

As shown in Figure 2, a certain company often has diaphragm test results: the results show that the average pore diameter of diaphragms 1 and 2 is 0.032μm and 0.046μm. It is consistent with the results of the literature.

3) Porosity: Porosity is very important to membrane permeability and electrolyte capacity. It can be defined as: the ratio of the volume of pores to the volume occupied by the diaphragm, that is, the volume percentage of pores per unit volume of the membrane, which is related to the density of the raw resin and the product. There are three commonly used methods to test the porosity, one is to use the weighing method, that is, to test the volume of the diaphragm, and calculate the volume of the pores in the diaphragm by the true density of the diaphragm material is the porosity:

The second method is the liquid absorption method to measure, weigh the diaphragm sample, then immerse it in analytically pure hexadecane for 1 hour, take it out and wipe off the remaining liquid on the surface with filter paper, and calculate the porosity by the following formula:

There is also a mercury intrusion method to test the volume of mercury that the diaphragm can hold, which is the porosity. A company often uses mercury intrusion method and weighing method to test the porosity of the diaphragm. The commonly used diaphragm test results are as follows:

Figure 3 A company’s commonly used diaphragm mercury porosimeter to test the pore size and its distribution

Table 2 A company commonly used diaphragm mercury porosimeter test and weighing method to test porosity

The test results of mercury intrusion method and weighing method have a certain deviation, which comes from the deviation of the thickness test and the deviation of the porosity uniformity of the diaphragm itself. However, the porosity of most commercial lithium ion battery separators is between 30% and 50%. In principle, for a certain electrolyte, a separator with a high porosity can reduce the impedance of the battery, but it is not as high as possible. Too high porosity will make the material's mechanical strength and self-discharge worse.

4) Permeability: Permeability can be characterized by the amount of gas passing through the diaphragm under a certain time and pressure, which mainly reflects the patency of lithium ions through the diaphragm. The permeability of the diaphragm is the result of the comprehensive factors affecting the internal pore structure of the diaphragm, such as the porosity, pore diameter, pore shape and pore tortuosity of the diaphragm. Among them, pore tortuosity has the greatest impact on permeability, and an increase in pore tortuosity will cause permeability to decrease in a square order. Hole tortuosity is defined as the ratio of the distance the gas or liquid actually passes through the diaphragm to the thickness of the diaphragm:

In the formula: T—the tortuosity of the hole, Ls—the length of the actual passage of gas or liquid, and d—the thickness of the diaphragm. A pressure drop meter can be used to measure the air permeability of the battery diaphragm. The faster the pressure drop decreases over time, the higher the air permeability of the diaphragm is, and vice versa. Generally speaking, the lower the porosity, the slower the pressure drop and the lower the air permeability. The air permeability can also be characterized by the Gurley value [4], which refers to the time required for a specific amount of air to pass through a specific area of ​​the diaphragm under a specific pressure (standard Gruley: 100mL of gas passes through 1 square inch under a pressure of 4.88 inches of water column Diaphragm time). It is related to porosity, pore size, thickness and pore tortuosity, and is a measure of the permeability of the membrane.

In the formula: 5.18*10-3 is the empirical constant of Celgard dry process diaphragm, tGur-Gurley value; T-hole tortuosity; L-film thickness (cm); ω-porosity; d-pore size. The Gurley value is used to characterize the film because the value is easy to measure and more accurate, and its deviation from a certain characteristic value can reflect the problems of the film. If it is higher than a certain standard value, it indicates that the membrane surface is damaged, or the pores shrink by heat, and if it is lower than the standard value, it indicates that the diaphragm may have pinholes. Moreover, for the same diaphragm sample, the Gurley value is proportional to the resistance of the diaphragm.

Since the pore diameter, pore curvature, and porosity are directly related to permeability, the permeability constant can also be tested and the pore diameter and pore curvature parameters can be inversely calculated using the empirical equation of fluid mechanics. Assuming that the air permeability meets the Knudsen fluid equation and the liquid permeability meets the Hagen-Poiseuill fluid equation, the details are as follows:

①

Knudsen: Qgas=2/3×π×r3×(8RT/πM)1/2×⊿P/τd×1/Ps--------Formula 5

②

Hagen-Poiseuill: Qliq=πr4/8η×⊿P/τd--------Formula 6

Combining the above two equations (Equations 5 and 6), only need to test the Rgas-air permeability constant (m3/(m2.s.Pa) and Rliq-liquid permeability constant (m3/(m2.s.) Pa), the pore diameter 2r and pore curvature τ can be calculated.

∵Rgas= Qgas × ε/πr2τ= 2/3×rε×(8RT/πM)1/2×⊿P/τ2d×1/Ps--------Formula 7

Rliq = Qliq × ε／πr2τ= r2ε／8η×⊿P／τ2d--------Formula 8

Simultaneous formulas 7 and 8 can get the hole diameter 2r and hole curvature τ:

∴2r=Rliq/Rgas×(32η×v)/(3×101325)

τ= (2/3rε.v. ⊿P/ (Rgas.d.Ps))1/2

In the above formula, 2r-pore size, R-gas constant, M-air molecular weight, ⊿P-pressure difference, η-liquid viscosity, T-temperature, ε-porosity, d-diaphragm thickness, τd-pore length, v-molecule Average speed of movement.

Table 3 below is the pore diameter and pore curvature data calculated from the above equation:

Table 3 Calculated pore diameter and pore curvature of a company’s common diaphragm

Wet diaphragms generally have a pore curvature between 2-3, and the pore diameter calculated by this method is larger than that measured by CFP.

5) Microscopic morphology: The surface morphology of the diaphragm can also be observed by scanning electron microscope (SEM) or atomic force microscope (AFM). The morphology of dry method and wet method is quite different, as shown in the figure below:

It can be clearly seen from Figure 4 that the surface morphology, pore size and distribution of the two are very different. The wet process can obtain complex three-dimensional fibrous pores with tensile structure, and the tortuosity of the pores is relatively high. The dry process creates pores, so the pores are narrow and long, the pore tortuosity is low, and the air permeability and strength are improved.

1.2.2.2 Mechanical properties of diaphragm

In the process of battery assembly and charge-discharge cycle use, the separator material itself needs to have a certain mechanical strength. The mechanical strength of the diaphragm can be measured by the tensile strength and the puncture resistance. In addition, the tension consistency is also an important performance parameter to evaluate. Since the diaphragm below 9um needs to be coated with a ceramic layer before it can be used, the tension in the TD direction of the diaphragm Consistency must meet certain requirements to meet the requirements of the coating process.

1) Tensile strength: The tensile strength of the diaphragm is related to the production process of the membrane. Generally speaking, if the porosity of the membrane is high and the pore size is large, although its impedance is low, the strength will decrease; and when uniaxial stretching is used, the strength of the membrane in the stretching direction is different from the vertical stretching direction. The strength of the separator prepared by biaxial stretching is basically the same in both directions. The wet method is basically biaxial stretching, so the tensile strength in the TD and MD directions are basically close, and can reach more than 100MPa. The dry method is mostly uniaxial stretching, so the tensile strength in the MD direction is higher. The tensile strength of the unstretched TD direction is very small, which can only reach about 10MPa. The tensile strength of two diaphragms with the same thickness is as follows:

Figure 5 MD and TD tensile curves of dry and wet diaphragms

2) Puncture resistance strength: The puncture resistance strength refers to the mass of a sample applied to a given needle to pierce a given diaphragm. It is used to characterize the tendency of short-circuiting during the assembly of the diaphragm. Since the electrode is composed of active material, conductive carbon black, and adhesive glue, even after rolling, the electrode surface is still a convex and concave surface composed of tiny particles of a mixture of active material and carbon black. The separator material sandwiched between the positive and negative plates also needs to withstand a lot of pressure during the shaping process. Therefore, in order to prevent short circuits, the diaphragm must have a certain puncture resistance. To a certain extent, the puncture resistance can also roughly characterize the quality of self-discharge. Empirically, the puncture strength of the lithium-ion battery separator is greater than 100gf, the dry PP film is generally greater than 100gf, and the wet PP film is generally greater than 200gf.

3) Tension consistency: It is mainly reflected in the flatness of the diaphragm material in the TD direction after unwinding. Due to the deviation of the thickness in the TD direction, the tension will be uneven. Once there is uneven tension, the diaphragm after unwinding In the TD direction, there will be intermediate waves and sagging edges, which will eventually cause the diaphragm to wrinkle and miss coating.

Figure 6 Uneven tension of diaphragm unwinding

1.2.2.3 The physical and chemical properties of the diaphragm:

Wetting ability and wetting speed, chemical stability, thermal stability, electrical conductivity or resistivity, self-closing performance of holes, etc.

1) Wetting ability and wetting speed: better wettability is conducive to the affinity between the diaphragm and the electrolyte, expanding the contact surface between the diaphragm and the electrolyte, thereby increasing the ion conductivity and improving the charge and discharge performance of the battery. capacity. Poor wettability of the separator will increase the resistance of the separator and the battery, and affect the cycle performance and charge and discharge efficiency of the battery. The wetting speed of the diaphragm refers to the speed at which the electrolyte enters the micropores of the diaphragm, and it is related to the surface energy, pore size, porosity, and tortuosity of the diaphragm. The wettability of the diaphragm to the electrolyte can be measured by measuring its liquid absorption rate and liquid holding rate. The dry sample is weighed and immersed in the electrolyte. After the absorption is balanced, the wet sample is taken out and weighed, and finally the percentage difference is calculated. However, this method causes large errors, so it is also useful to crawl the electrolyte on the diaphragm. Liquid height and speed are used to measure its wettability to electrolyte. In addition, the wettability can also be measured by the contact angle between the electrolyte and the diaphragm material. The dynamic contact angle meter is a more accurate instrument for testing the contact angle of the solid-liquid interface.

2) Chemical stability: The diaphragm should maintain long-term stability in the electrolyte. Under the conditions of strong oxidation and strong reduction, the chemical stability of the diaphragm is not related to the electrolyte and electrode materials by measuring the corrosion resistance of the electrolyte. And the rate of expansion and contraction. In the literature, the corrosion resistance of the electrolyte is that the electrolyte is heated to 50 ℃, the diaphragm is immersed for 4-6 hours, taken out, washed, dried, and finally compared with the original dry sample to observe whether the diaphragm is dissolved or changed in color. . Expansion and shrinkage rate is to detect the dimensional change after immersing the diaphragm in the electrolyte for 4-6 hours, and find the difference percentage. Commercial polyolefin diaphragms are made of PP or PE material, which is resistant to electrolyte corrosion and expansion and contraction. Both are better and can be used in lithium ion batteries.

3) Thermal stability: The battery will release heat during charging and discharging, especially when it is short-circuited or overcharged, a large amount of heat will be released. Therefore, when the temperature rises, the diaphragm should maintain the original integrity and certain mechanical strength, continue to play the role of isolating the positive and negative electrodes, and prevent the occurrence of short circuits. Thermomechanical analysis (TMA) can be used to characterize this characteristic, which can provide a repeatable measurement of the melt integrity of the diaphragm material. TMA measures the deformation of the diaphragm under load when the temperature rises linearly. Usually the diaphragm first shows shrinkage, then begins to stretch, and finally breaks. The following are the TMA test results of a company's commonly used diaphragm:

Figure 7 TMA test curve of KN9 and TN9 diaphragm

From the results in Fig. 7, the thermal shrinkage of TN9 diaphragm is larger than that of KN9 diaphragm in MD direction, and the film rupture temperature is close to 150 degrees. In TD direction, the thermal stability of TN9 diaphragm is better than that of KN9. difference.

4) The resistance of the diaphragm: The resistance of the diaphragm directly affects the performance of the battery, so the measurement of the diaphragm resistance is very important. The resistivity of the diaphragm is actually the resistivity of the electrolyte in the micropores, which is related to many factors, such as porosity, pore tortuosity, electrolyte conductivity, film thickness and the degree of electrolyte wetting to the diaphragm material, etc. . The more commonly used method to test the resistance of the diaphragm is the alternating current impedance method (EIS). The resistance of the diaphragm in the electrolyte is compared with the resistance of the electrolyte to obtain the Nm value, which is the MacMullini constant. A sinusoidal AC voltage signal is applied to the measuring device. By measuring the impedance values ​​of different frequencies within a certain range, and then analyzing the data with an equivalent circuit, the information of the diaphragm ionic resistance is obtained. Because the film is very thin, there are often defects that increase the error of the measurement results. Therefore, multilayer samples are often used and the average value of the measurement is taken. The current evaluation method of a company is shown in the figure below. The repeatability and reliability of the experiment are still Awaiting further research and development.

Figure 8 Fixture for testing the Nm value (ionic conductivity) of a company's diaphragm

5) Self-closing performance: When the temperature is above a certain temperature, the components in the battery will have an exothermic reaction and cause "self-heating". In addition, due to charger failure, safety current failure, etc., it will cause overcharging or external short-circuit of the battery. These conditions will generate a lot of heat. Due to the thermoplastic nature of polyolefin materials, when the temperature is close to the melting point of the polymer, the porous ion-conducting polymer film will become a non-porous insulating layer, and the pores will be closed to produce self-closing phenomenon, thereby blocking the continued transmission of ions. It forms an open circuit to protect the battery, so the polyolefin separator can provide additional protection for the battery.

Figure 9 Fixture for temperature test (ionic conductivity) of a certain company

1.2.3 The impact of isolation membrane performance parameters on battery performance

1) Uniformity of film thickness and its distribution

As a component that does not participate in the electrochemical reaction and does not provide energy, the thickness of the separator should be as thin as possible. Transferring the space to the positive electrode and the negative electrode can increase the energy density of the battery. At present, a company has mass-produced 7um base film, plus 3-4um coating, the total thickness is 10-11um.

The uniformity of the diaphragm thickness directly affects the uniformity of the battery thickness. The difference between domestic diaphragms and foreign diaphragms is not the difference in performance, but the difference in consistency.

Remark: L: left; M: middle; R: right (left, middle and right in the TD direction of the diaphragm)

As shown in the figure above, the thickness tolerance of the world-class diaphragm manufacturer is less than ±1um, and its CPK is greater than 1.67

2) Consistency of the processing strength and tension of the diaphragm

Factors such as the processing strength and uneven strength of the diaphragm will affect the coating and winding process of the diaphragm.

During the coating process, the diaphragm is prone to local stretching due to the cumulative effect of uneven thickness or poor winding tension control, resulting in poor flatness of the diaphragm and severe wavy edges, resulting in failure to coat wrinkles or miss coating. (As shown below).

During the winding process, the uneven tension of the diaphragm will also affect the overhance misalignment.

3) Dimensional stability (heat shrinkage performance)

In the battery manufacturing process, the separator needs to withstand high-temperature vacuum baking and high-temperature shaping and other thermal processes. Therefore, the diaphragm needs to be able to maintain dimensional stability when heated. If the thermal shrinkage in the MD direction is too large, it is easy to deform the battery (arching) during the vacuum baking process, and if the shrinkage in the TD direction is too large, it is easy to make the battery overhance smaller. The general requirement is the thermal shrinkage rate MD of the diaphragm in the free baking of 90 degrees/1 hour

4) Pore structure

The higher the porosity of the separator, the larger the pore size and the smaller its Gurley value, the stronger the ion conduction and electrolyte retention performance, but too large porosity and pore size will also affect the self-discharge performance of the battery.

As shown in the figure above, the self-discharge of different Gurley separators produced by the same supplier and the same process has a large degree of opposite relationship with Gurley's, which shows that high porosity and low Gruley cannot be blindly pursued.

5) Current blocking (shultdown & meltdown)

When the battery is abused by short circuit or overcharge, the battery temperature will increase between 100-130 degrees, the diaphragm can play a thermal closed cell effect, block the current, prevent thermal runaway, but ordinary PE diaphragm and three-layer PP/PE/ The thermal closed cell effect of the PP separator does not significantly improve the safety performance of large-capacity (>4Ah) batteries. It can be seen that the temperature difference between closed cells and membrane rupture needs to be increased to play a better role.

6) Electronic insulation and chemical stability

The polyolefin membrane material itself has better electronic insulation, the dielectric constant of PE material is 2.33, and the dielectric constant of PP material can reach 1.5. The polyolefin material has excellent solvent resistance. It is almost insoluble in any organic solvent at room temperature, and the electrolyte will not cause the membrane to dissolve or chemically react.

7) Mechanical strength

Mechanical strength includes tensile strength (ie, tensile strength) and puncture strength. Traditional polyolefin separators are stretched films, so their mechanical strength is relatively large, and the MD direction is basically greater than 100MPa (1000kgf/cm2). There is no problem with separator coating and winding. The puncture strength is related to the self-discharge of the battery. The greater the strength, the more difficult it is for the burrs and protruding particles on the pole piece to penetrate the separator (causing a short circuit), or the battery pierces the separator when lithium dendrites appear, but the puncture strength The test method does not reflect this well, and it cannot be concluded that the greater the current puncture intensity, the smaller the self-discharge. The hybrid puncture test is closer to the actual situation of the separator in the battery, but the current test method has yet to be developed.