Brief description of the separator of lithium ion battery

1. Introduction to the separator (1)

1.1 The influence of raw materials and manufacturing processes on the performance of the separator

1.1.1Isolation membrane definition and function

The separator is an important component of a lithium ion battery, and is a microporous membrane for separating positive and negative electrode sheets, and is a polymer functional material having a nanometer-scale microporous structure. Its main function is to prevent the two poles from contacting and short-circuiting while allowing electrolyte ions to pass. Its performance determines the interface structure and internal resistance of the battery, which directly affects the battery capacity, circulation and battery safety performance.

1.1.2 Raw materials for the separator

At present, the commercial lithium ion battery separator products are mostly microporous membranes prepared from polyolefin materials. The main raw materials are high molecular weight polyethylene and polypropylene. The products include polyethylene PE single layer film, polypropylene PP single layer film and PP/PE/PP multilayer microporous membrane compounded by PP and PE. Polyolefin materials have the advantages of high strength, good acid and alkali corrosion resistance, water resistance, chemical resistance, good biocompatibility, and no toxicity. The industrial preparation is mature. Lithium-ion battery separators that are in the research stage or have not yet been used on a large scale include PET/cellulose nonwoven fabrics, polyvinylidene fluoride (PVDF) porous membranes, polyimide (PI) electrospun porous membranes, and PE, PVDF, PP, PI modified film, and the like.

PE film requirements for HDPE raw materials:

1) Excellent miscibility, HDPE solubility is good, melting temperature is greater than 135 ° C, density 95% -99%, guaranteed to be co-dissolved with organic alkanes, forming a homogeneous solution, is the guarantee of the consistency of the membrane.

2) Appropriate molecular weight and molecular weight distribution, molecular weight greater than 300,000, narrow distribution, PDI = Mw / Mn = 6-8, to ensure the processing properties and mechanical properties of the membrane.

3) Low gel and impurity content, there is the only one main degradation peak in the DSC curve, the raw material component is single, and the inorganic impurities are low, which ensures the quality of the separator.

4) Plasticizer and extracting agent, liquid paraffin (C16-C20 normal paraffin) as plasticizer, dichloromethane as extracting agent, guarantee the uniformity of pore formation.

PP film requirements for PP raw materials:

5) It has a higher isotactic index, the gauge component must be greater than 95%, and the melting temperature is greater than 163 ° C to ensure good crystallization and hole formation.

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

7) Low gel and impurity content, there is the only one main degradation peak in the DSC curve, the raw material component is single, and the inorganic impurities are low, which ensures the quality of the separator.

8) β crystal modifier, the dry biaxial stretching process also needs to add β crystal modifier, and uniform mixing is an important factor for biaxial stretching into pore uniformity.

1.1.3 Process of separator film

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

Super detailed! Contains calculation formula - Introduction to lithium ion battery separator

1.1.3.1 Process flow of dry diaphragm

In the dry method, a polyolefin resin is melted, extruded, and blown into a crystalline polymer film, which is subjected to crystallization heat treatment and annealing to obtain a highly oriented multilayer structure, which is further stretched at a high temperature to peel off the crystal interface. The porous structure increases the pore size of the membrane. The porous structure is related to the porous structure and orientation of the polymer. The key technology of the dry process is that the polymer melts and extrudes the cast sheet by stretching about 300 times in the viscous flow state of the polymer to form a hard elastomer material. The process of multi-layer PP and PE composite membrane is as follows: (1) PE and PP are separately melt-extruded, and the film is cast into a film of 12 μm by stretching about 300 times; 2. The PE and PP films are thermally composited, heat-treated, and longitudinally. The process flow of the dry diaphragm is as follows:

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Polyolefin diaphragm dry process flow chart

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

The melt extrusion/stretching/heat setting method is prepared by crystallizing a polymer melt under a high stress field to form a platelet structure having a direction perpendicular to the extrusion direction and parallel alignment, and then heat-treating to obtain an elastic material. After the polymer film having hard elasticity is stretched, the platelets are separated, and a large amount of microfibers are formed, thereby forming a large number of microporous structures, and then heat-setting forms a microporous membrane.

The related patent describes the preparation process of polyolefin microporous membranes and the stretching temperature is higher than the glass transition temperature of the polymer and lower than the crystallization temperature of the polymer, such as the blow-molded polypropylene film is heat-treated to obtain hard elasticity. The film is firstly drawn by 6% to 30%, and then thermally stretched between 80% and 150% between 120 and 150 ° C. After heat setting, a microporous film having high stability is obtained. The process of melt extrusion/stretching/heat setting method is simple and non-polluting, and is a common method for preparing lithium ion battery separators, but the method has disadvantages such as difficulty in controlling pore size and pores.

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Figure 11: Polyolefin separator before and after dry stretching

The PP and PP/PE/PP separators prepared by the uniaxial stretching dry process have a slender shape, a length of about 0.1 to 0.5 μm, a width of about 0.01 to 0.05 μm, and a pore structure of a through-hole, and the film is obtained. The pore diameter ranges from 0.1 to 3 μm, and the pore diameter of the membrane is 0.4 μm at the longest and 0.04 μm at the widest.

The uniaxially stretched dry film is not stretched in the TD direction, so that its strength in the TD direction is poor, and the strength is only about 10 MPa (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, and there is almost no heat shrinkage in the TD direction. In addition, PP polypropylene has poor ductility and low surface energy, and is a difficult-to-bond plastic. It is not conducive to the bonding with positive and negative electrodes. The interface between the separator and the electrode is not tight, which affects the performance of the battery.

2) Adding nucleating agent coextrusion / stretching / heat setting method (biaxial stretching method)

The nucleating agent is co-extruded to form a film containing a solid additive, and the solid additive is uniformly distributed in the polymer phase with a submicron particle size, and a microporous film is formed due to phase separation of stress concentration during stretching, polypropylene the method for preparing a microporous membrane, biaxially stretching a polypropylene film containing a large amount of β crystal form, and then thermally fixing it, the pore diameter is 0.02-0.08 μm, the porosity is 30% to 40%, and the strength of the film in all directions Consistent, about 60 ~ 70MPa.

Since the polypropylene form of the β crystal form is composed of bundled crystals grown, the density of the spherulites is low, so that the amorphous regions between the wafer bundles are easily pulled apart to form micro-silver or micropores. After the addition of the nucleating agent, since 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. It is produced by Xinxiang Gryen and Xinshi Technology in this way to produce biaxially stretched single-layer PP separators.

The PP film prepared by the biaxial stretching dry process has tensile strength in both the MD and TD directions, and its strength in the TD direction is about 6 times larger than that of the uniaxial stretching dry process, so the TD direction does not Easy to tear. The pore structure is similar to the wet method and belongs to a dendritic non-straight hole. Due to the need to add a solid nucleating agent, the degree of dispersion of the nucleating agent in the PP melt directly affects the uniformity of 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 double-axis dry stretching.

1.1.3.2 Process production process of wet diaphragm

Thermally induced phase separation is a method for preparing microporous membranes developed in recent years. It utilizes high polymers and certain high boiling small molecule compounds at higher temperatures (generally higher than the melting temperature Tm of the polymer). At the same time, a homogeneous solution is formed, and the temperature is lowered to cause solid-liquid or liquid-liquid phase separation, so that in the high polymer phase, the low-molecular substance can be removed after stretching to form a microporous membrane material which penetrates each other.

The wet extruded sheet is separated by thermal phase separation. The wet method is to mix liquid hydrocarbon or some small molecular substances with polyolefin resin, heat and melt, form a homogeneous mixture, volatilize the solvent, perform phase separation, and then press. Obtaining a film; heating the film to a melting point close to the crystal, holding it for a certain period of time, eluting the residual solvent with a volatile substance, adding an inorganic plasticizer powder to form a film, further eluting the inorganic plasticizer with a solvent, and finally It is extruded into pieces. For example, polymers such as PE and PP, and high-boiling small-molecule compounds such as paraffin and DOP form a homogeneous solution at an elevated temperature (higher than the melting point of a polymer such as PE), and phase separation occurs when the temperature is lowered. Thereafter, a small molecule compound such as paraffin is eluted with a solvent to become a microporous material.

The process flow is as follows: twin-screw extruder extrusion, cast sheet forming, simultaneous/asynchronous biaxial stretching, solvent extraction, blow drying, cross-tensioning, on-line thickness measurement, winding, aging treatment, slitting, etc. . The separator prepared by this method can change its properties and structure by controlling the composition of the solution and the volatilization of the solvent during the gel curing process.

Super detailed! Contains calculation formula - Introduction to lithium ion battery separator

Super detailed! Contains calculation formula - Introduction to lithium ion battery separator

Figure wet polyolefin diaphragm production process

The wet process of biaxial stretching is also divided into two types: synchronous stretching and asynchronous stretching. The MD of the synchronous stretching is simultaneously stretched in the TD direction. The uniformity of the PE separator prepared by this method is better, the yield is higher, and the difference between the two directions of TD and MD is smaller. Asynchronous stretching is performed by stretching in the MD direction and then stretching in the TD direction. The stretching ratio in both directions is controllable and adjustable, the flexibility is higher, and the strength is larger than that of the synchronous stretching. The disadvantage is that Uniformity in the TD direction is less than synchronous stretching.

In general, the wet process has higher strength in the TD direction than the dry process, uniform pore size, high porosity of the pores, high porosity, and good gas permeability.

1.1.3.3 Process flow of non-woven diaphragm

The non-woven fabric is a fabric formed without spinning a woven fabric, and the woven short fibers or filaments are oriented or randomly arranged to form a web structure, and then reinforced by mechanical, thermal bonding or chemical methods. It directly utilizes high-polymer chips, staple fibers or filaments to form new fiber products with soft, permeable and planar structures formed by various web forming methods and consolidation techniques. Due to its porous structure and low price, non-woven membranes are widely used in nickel-hydrogen and nickel-cadmium batteries. At present, more and more researchers use non-woven membranes in lithium-ion batteries, but they are in their infancy.

Non-woven separators for lithium ion batteries are mainly classified into 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) Spunlaced nonwoven: The spunlace process is to spray a high-pressure fine water stream onto one or more layers of the fiber web to entangle the fibers with each other, so that the fiber web can be reinforced and has a must be strong.

2) Heat-bonded non-woven fabric: Thermally-bonded non-woven fabric refers to a fibrous or powdery hot-melt adhesive reinforcing material added to the fiber web, and the fiber web is further fused and cooled to form a cloth.

3) Pulp air-laid non-woven fabric: air-laid non-woven fabric can also be called dust-free paper, dry-process paper non-woven fabric. It uses the air-laid technology to open the wood pulp fiber board into a single fiber state, and then uses a gas flow method to agglomerate the fibers on the mesh curtain, and the fiber web is further reinforced into a cloth.

4) Wet-laid non-woven fabric: The wet-laid non-woven fabric is obtained by opening a fiber raw material placed in an aqueous medium into a single fiber, and simultaneously mixing different fiber raw materials to form fiber suspension slurry, and transporting the suspended pulp to a forming mechanism. The fibers are netted in a wet state and then reinforced into a cloth.

5) Spunbond non-woven fabric: Spunbond non-woven fabric is a filament laid in a web after the polymer has been extruded and stretched to form continuous filaments. The web is then bonded, thermally bonded, and chemically bonded. Bonding or mechanical reinforcement means that the web becomes a non-woven fabric.

6) Meltblown non-woven fabric: the process of melt-blown non-woven fabric: polymer feeding---melt extrusion---fiber formation---fiber cooling---forming into the net---reinforcing into cloth.

The pore structure of the non-woven fabric is made of fibers interlaced, so it has the advantages of large pore size and high pore size, but its disadvantages are also obvious: easy to absorb moisture, low strength, wide pore size distribution, and thinner thickness ( >16um)

1.1.3.4 Process flow of electrospinning diaphragm

Electrospinning is the most important basic method for obtaining 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 nanofibers refers to the diameter of the fibers, and fibers generally defined as having a diameter in the range of 1 to 100 nm are referred to as nanofibers. Of course, this upper and lower definition is not absolute. The fiber diameter obtained by electrospinning varies depending on the spinning conditions, and typical data varies from 40 to 2000 nm.

That includes the range of micrometers, submicrometers, and nanometers. The basic principle of electrospinning is shown in the figure:

Super detailed! Contains calculation formula - Introduction to lithium ion battery separator

Electrospinning system mainly includes: spinneret, infusion system, high voltage generator and wire system. The electrospinning process (referred to as electrospinning process) is a polymer solution or a melt passing through a charged spinneret. Under the action of a high-voltage electrostatic field formed by the spinneret and the wire system, the liquid stream is divided into a plurality of thin streams. The solvent is continuously volatilized and the polymer is solidified to form a non-woven fibrous film on the wire joining system.

Specifically, in the electrospinning process, the polymer solution splits the liquid stream due to mutual repelling of electric charges, and the electric field causes the split liquid beam to move toward the wire receiving system and falls on the wire receiving system. Throughout the process, the fundamental role is the electric field force.

The electrospinning membrane has the characteristics of high porosity, high magnification, high resistance and the like. If polyimide is used as the spinning material, the heat resistance can be as high as 500 degrees, and the battery safety performance is better improved. However, due to the spinning process, the mechanical strength is poor, only 1/10 of the wet PE film.

1.2 Characterization method of isolation membrane performance parameters

1.2.1 Lithium-ion battery diaphragm technical requirements

The performance of the lithium ion battery separator determines the interface structure and internal resistance of the battery, which directly affects the capacity, circulation and safety of the battery. Therefore, the technical requirements of lithium ion battery separator:

1) Insulation performance, is an electronically conductive insulator

2) Minimize rejection of electrolyte and have good wetting property of electrolyte

3) High ionic conductivity, that is, the resistance to dielectric ion motion is small

4) Effectively prevent migration of particles, colloids or other soluble substances between positive and negative electrodes

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

6) Dimensional stability, small dimensional change below the melting point temperature, does not cause short circuit between positive and negative

7) Chemical stability and electrochemical inertness, which are stable enough to dissolve, degrade or degrade the electrolyte, possible impurities, electrode reactants and electrode products.

8) The uniformity of thickness and aperture is high

Different lithium-ion battery systems and applications have different requirements for diaphragms.

1.2.2 Characterization of the performance parameters of the separator

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

1.2.2.1 Structural characteristics of the diaphragm:

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

1) Thickness: The thickness of the lithium ion battery separator is generally <25 μm. Under the premise of ensuring a certain mechanical strength, the thinner the separator is better. At present, consumer electronics batteries use wet PE thin diaphragms because of their high energy density requirements, and have reached the level of 9um diaphragm. A company has mass-produced 7um substrates. Most electric vehicles (EVs) and hybrid electric vehicles (HEVs) use dry 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 to allow absorption of electrolyte; in order to ensure consistent electrode/electrolyte interface properties and uniform current density in the battery micropores are in the entire diaphragm material. The distribution should be uniform. The uniformity of the size and distribution of the pore size has a direct effect on the performance of the battery: the pore size is too large, and the positive and negative electrodes are easily contacted or easily pierced by the lithium dendrites to cause a short circuit; if the pore size is too small, the resistance is increased. The micropore distribution is uneven, and local current is too large during operation, which affects the performance of the battery.

Using a capillary flow aperture analyzer (CFP), a non-volatile fluorinated organic liquid was used as a medium to measure the relationship between pressure and gas flow rate for different commercial lithium ion battery separators. The results show that (Table 1 and Figure) 1): The pore size of the commercial membrane is generally 0.03-0.05μm or 0.09-0.12μm, and it is considered 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 Diameters of different thicknesses for testing

Super detailed! Contains calculation formula - Introduction to lithium ion battery separator

Figure 1 is used to test different thickness of the diaphragm

Super detailed! Contains calculation formula - Introduction to lithium ion battery separator

The pore size of the membrane can be obtained from the 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 and dry lines to obtain the pore size distribution.

Super detailed! Contains calculation formula - Introduction to lithium ion battery separator

Figure 2 A company's commonly used diaphragm and *** diaphragm aperture and distribution

As shown in Figure 2, a company often has a diaphragm test result: the results show that the average pore size of the separators 1 and 2 is 0.032 μm and 0.046 μm.

3) Porosity: Porosity is very important for the permeability of the membrane and the capacity of the electrolyte. It can be defined as the ratio of the volume of the pore to the volume occupied by the membrane, that is, the volume fraction of the pores per unit volume, which is related to the density of the raw material resin and the product. It is more common to use three 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 pore in the diaphragm by the true density of the diaphragm material:

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The second method is a liquid absorption method for weighing, the diaphragm sample is weighed, and then immersed in analytically pure hexadecane for 1 h, and the surface residual liquid is removed by using a filter paper, and the porosity is calculated by the following formula:

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There is also a method of testing the volume of mercury that can be contained in the membrane by mercury intrusion, which is the porosity. A company uses the mercury intrusion method and the weighing method to test the porosity of the diaphragm. The commonly used diaphragm test knots are as follows:

Super detailed! Contains calculation formula - Introduction to lithium ion battery separator

Figure 3: A company's common diaphragm pressure mercury meter test aperture and its distribution

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

Super detailed! Contains calculation formula - Introduction to lithium ion battery separator

There is a certain deviation between the mercury intrusion method and the weighing method test result, which is derived from the deviation of the thickness test and the porosity uniformity deviation 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 lower the impedance of the battery, but the higher the better, the higher the porosity, the worse the mechanical strength of the material, and the worse the self-discharge.

4) Permeability: The permeability can be characterized by the amount of membrane gas that can be used under a certain time and pressure, mainly reflecting the patency of lithium ions through the membrane. The permeability of the membrane is the result of the comprehensive factors of the pore structure of the membrane, such as the porosity of the membrane, the pore diameter, the shape of the pores, and the tortuosity of the pores. Among them the holes tortuosity has the greatest influence on the permeability, and the increase in the holes tortuosity will cause the permeability to decrease in a square order. The holes tortuosity is defined as the ratio of the path actually traveled by a gas or liquid in the diaphragm to the thickness of the diaphragm:

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Where: T-hole tortuosity, Ls - the length of the path through which the gas or liquid actually passes, d - the thickness of the diaphragm. The pressure drop meter can be used to determine the gas permeability of the battery separator. The faster the pressure drop decreases with time, indicating that the membrane has higher gas permeability, and vice versa. In general, the lower the porosity, the slower the pressure drop and the lower the gas permeability is. The gas 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 membrane at a specific pressure (standard Gruley: 100 mL gas passes 1 square inch at 4.88 inches of water column pressure) the time of the diaphragm).

It is related to porosity, pore size, thickness and the tortuosity of the pores and is a measure of the permeability of the membrane.

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Where: 5.18*10-3 is the empirical constant of the celgard dry diaphragm, tGur-Gurley value; T-hole tortuosity; L-film thickness (cm); ω-porosity; d-aperture. The film is characterized by the Gurley value because the value is easy to measure and accurate, and its deviation from a certain eigenvalue reflects the problem of the film. If a value above a certain standard indicates damage to the surface of the membrane, or shrinkage of the heated pore, below the standard value indicates that there may be pinholes in the membrane. Moreover, for the same diaphragm sample, the magnitude of the Gurley value is proportional to the level of the diaphragm resistance.

Since the pore size, pore curvature, and porosity are directly related to permeability, the permeability constant can also be tested and the pore size and pore curvature parameters can be inversely calculated using the empirical equation of fluid mechanics. Assuming that the gas permeability conforms to the Knudsen fluid equation, the liquid permeability is in accordance with the Hagen-Poiseuill fluid equation, 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

In conjunction with the above two equations (Equations 5 and 6), it is only necessary to test the Rgas-air transmission velocity constant (m3/(m2.s.Pa) and Rliq-liquid permeation velocity constant (m3/(m2.s. Pa), that is, the aperture 2r and the aperture 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

The simultaneous equations 7 and 8 give the aperture 2r and the hole curvature τ:

∴2r=Rliq/Rgas×(32η×v)/(3×101325)

τ=(2/3rε.v.⊿P/(Rgas.d.Ps))1/2

2r-pore diameter, R-gas constant, M-air molecular weight, ⊿P-pressure difference, η-liquid viscosity, T-temperature, ε-porosity, d-membrane thickness, τd-hole length, v-molecule Average speed of movement.

Table 3 below shows the aperture and hole curvature data calculated by the above equation:

Table 3 calculated common diaphragm aperture and hole curvature of a company

Super detailed! Contains calculation formula - Introduction to lithium ion battery separator

The wet diaphragm generally has a pore curvature of between 2-3, and the calculated aperture is larger than that of the CFP test.

5) Micromorphology: The surface morphology of the separator can also be observed by scanning electron microscopy (SEM) or atomic force microscopy (AFM). There is a big difference between the dry and wet film morphology, as shown below:

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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 a complex three-dimensional fiber-like pore that is a tensile structure, and the tortuosity of the hole is relatively high. The dry process is formed into pores, so the pores are long and narrow, the pore tortuosity is low, and the gas permeability and strength are improved.

1.2.2.2 Mechanical properties of the diaphragm

During battery assembly and charge and discharge cycles, the diaphragm material itself is required to have a certain mechanical strength. The mechanical strength of the diaphragm can be measured by tensile strength and puncture strength. In addition, the consistency of tension is also an important evaluation parameter. Since the diaphragm below 9um needs to be coated with a ceramic layer, 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 separator is related to the manufacturing process of the film. In general, if the separator has a high porosity and a large pore size, although the impedance is low, the strength is lowered; and in the case of uniaxial stretching, the film has different strength in the stretching direction and the vertical stretching direction, and The separator prepared by biaxial stretching has substantially the same strength in both directions. The wet method is basically biaxially stretched, so the tensile strength in the TD and MD directions is substantially close to 100 MPa or more, and the dry method is mostly uniaxial stretching, so the tensile strength in the MD direction is higher. Large, can reach 150MPa or more, and the tensile strength of the unstretched TD direction is very small, only about 10MPa. The tensile strength of the two different thickness diaphragms is as follows:

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Figure 5 Dry and wet diaphragm MD and TD tensile curves

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

3) Tension consistency: mainly reflected in the flatness of the TD direction after the diaphragm coil is unwound. The deviation of the thickness in the TD direction will cause unevenness of tension. Once there is uneven tension, the diaphragm after unwinding In the TD direction, there will be intermediate waves, sagging edges, etc., which eventually cause the diaphragm to wrinkle and leak.

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Figure 6 diaphragm unwinding tension unevenness

1.2.2.3 Physicochemical properties of the membrane:

Wettability and wetting speed, chemical stability, thermal stability, electrical conductivity or electrical resistivity, self-closing properties of the pores, and the like.

1) Wettability and wetting speed: better wettability is beneficial to the affinity between the separator and the electrolyte, and enlarges the contact surface between the separator and the electrolyte, thereby increasing the ionic conductivity and improving the charge and discharge performance of the battery capacity. Poor wettability of the separator increases the resistance of the separator and the battery, affecting 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 pores of the membrane. It is related to the surface energy, pore size, porosity, and tortuosity of the separator. The wettability of the separator to the electrolyte can be measured by measuring its liquid absorption rate and liquid retention rate. The dry sample is weighed and immersed in the electrolyte. After the equilibrium is absorbed, the wet sample is taken out and weighed. Finally, the percentage difference is calculated. However, this method artificially causes a large error, so it is also useful to climb the electrolyte on the diaphragm. The liquid height and speed are used to measure its infiltration performance to the electrolyte.

In addition, the wettability can also be measured by the contact angle of the electrolyte with the separator material. The dynamic contact angle meter is an instrument for testing the contact angle between the solid and the liquid.

2) Chemical stability: The separator should maintain long-term stability in the electrolyte. Under the conditions of strong oxidation and strong reduction, the chemical stability of the separator without electrolyte and electrode material is determined by the ability to resist electrolyte corrosion. And the rate of expansion and contraction is evaluated. In the literature, the ability to resist electrolyte corrosion is to dilute the electrolyte to 50 ° C, then immerse the membrane for 4~6h, take it out, wash it, dry it, and finally compare it with the original sample to see if the diaphragm is dissolved or the color changes. . The expansion and contraction rate is the thickness change after immersing the separator in the electrolyte for 4~6h, and the percentage difference is obtained. The commercial polyolefin separator is made of PP or PE material, which resists electrolyte corrosion and expansion and contraction rate. Both are good and can be used in lithium ion batteries.

3) Thermal stability: The battery will release heat during charging and discharging, especially when short circuited or overcharged, a large amount of heat will be released. Therefore, when the temperature rises, the diaphragm should maintain its original integrity and a certain mechanical strength continue to play the role of isolation of the positive and negative electrodes to prevent the occurrence of short circuits. Thermomechanical analysis (TMA) can be used to characterize this property, which provides repeatable measurements of the melt integrity of the membrane material. TMA is a measure of the deformation of a diaphragm at a load when the temperature rises linearly. Usually, the diaphragm first exhibits shrinkage, then begins to elongate and eventually breaks. The following is the TMA test results of a company's commonly used diaphragm:

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Figure 7KN9 and TN9 diaphragm TMA test curve

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

4) 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 separator is actually the resistivity of the electrolyte in the micropores, which is related to many factors such as porosity, tortuosity of the pores, conductivity of the electrolyte, film thickness, and degree of wetting of the separator material by the electrolyte. . The most commonly used test resistor is the AC impedance method (EIS), which measures the resistance of the diaphragm in the electrolyte to the Nm value of the electrolyte, which is the MacMullini constant. A sinusoidal alternating voltage signal is applied to the measuring device, and the impedance value of the different frequency in a certain range is measured, and then the data is analyzed by the equivalent circuit to obtain the information of the diaphragm ionic resistance. Since the film is very thin, there are often defects and the error of the measurement result is increased. Therefore, a multi-layer sample is often used, and the average value of the measurement is taken. At present, the evaluation method of a company is as shown in the following figure, and the experiment repeatability and reliability are still Need to be further research and development.

Super detailed! Contains calculation formula - Introduction to lithium ion battery separator

Figure 8 A company's diaphragm Nm value test (ion conductivity) fixture

5) Self-closing performance: When the temperature is above a certain temperature, the components in the battery will undergo an exothermic reaction, resulting in “self-heating”. In addition, due to charger failure, safety current failure, etc., which will cause overcharging or external short circuit of the battery, these conditions will generate a lot of heat. Due to the thermoplastic nature of the polyolefin material, when the temperature is close to the melting point of the polymer, the porous ion-conducting polymer film becomes a non-porous insulating layer, and the micropores are closed to cause self-closing, thereby blocking the continued transport of ions. An open circuit is formed to protect the battery, so the polyolefin diaphragm provides additional protection for the battery.

Super detailed! Contains calculation formula - Introduction to lithium ion battery separator

Figure 9 A company's closed-cell temperature test (ion conductivity) fixture

1.2.3 Effect of barrier film performance parameters on battery performance

Super detailed! Contains calculation formula - Introduction to lithium ion battery separator

1) Uniformity of film thickness and its distribution

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

Super detailed! Contains calculation formula - Introduction to lithium ion battery separator

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

Super detailed! Contains calculation formula - Introduction to lithium ion battery separator

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

Super detailed! Contains calculation formula - Introduction to lithium ion battery separator

Super detailed! Contains calculation formula - Introduction to lithium ion battery separator

As shown above, world-class diaphragm manufacturers have a thickness tolerance of less than ±1um and a CPK greater than 1.67.

2) Processing strength and tension consistency of the diaphragm

Factors such as the processing strength and unevenness of the separator affect the application of the separator and the winding process.

During the coating process, the diaphragm may be prone to local stretching due to the cumulative effect of uneven thickness or poor control of the winding tension, so that the smoothness of the diaphragm flatness is severe, resulting in the inability to apply wrinkles or leak coating (As shown below).

Super detailed! Contains calculation formula - Introduction to lithium ion battery separator

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

3) Dimensional stability (heat shrinkage performance)

In the battery process, the diaphragm needs to withstand high temperature vacuum baking and high temperature shaping and other thermal processes. Therefore, the diaphragm needs to maintain dimensional stability under heat. If the heat shrinkage in the MD direction is too large, the battery is easily deformed (arched) during the vacuum baking process, and if the shrinkage in the TD direction is too large, the overhance of the battery is easily reduced. The general requirement is that the thermal shrinkage of the separator at 90 degrees / 1 hour of freebaking is MD < 5%, TD < 3%. Of course, the thermal shrinkage of the separator in the cell will be much smaller than in the free case.

4) Pore structure

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

Super detailed! Contains calculation formula - Introduction to lithium ion battery separator

As shown in the above figure, the different Gurley diaphragms produced by the same manufacturer from the same process, the self-discharge and Gurley's larger degree of inverse relationship, cannot blindly pursue high porosity and low Gruley.

5) Current blocking (shultdown & meltdown)

When the battery is abused by short circuit or overcharge, the battery temperature rises between 100-130 degrees, the diaphragm can play the role of thermal closed hole, block the current, prevent thermal runaway, but ordinary PE diaphragm and three layers 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 it is necessary to increase the temperature difference between closed cells and membranes to play a better role.

Super detailed! Contains calculation formula - Introduction to lithium ion battery separator

Super detailed! Contains calculation formula - Introduction to lithium ion battery separator

6) Electronic insulation and chemical stability

The polyolefin separator material itself has good electronic insulation, the dielectric constant of the PE material is 2.33, and the dielectric constant of the PP material can reach 1.5. The polyolefin material has excellent solvent resistance and is almost insoluble in any organic solvent at normal temperature, and the electrolyte does not cause dissolution or chemical reaction of the separator.

7) Mechanical strength

The mechanical strength includes tensile strength (tensile strength) and puncture strength. The conventional polyolefin separator has a relatively large mechanical strength due to the stretched film, and is substantially larger than 100 MPa (1000 kgf/cm 2) in the MD direction. There is no problem with diaphragm 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 pierce the diaphragm (causing a short circuit), or the piercing of the diaphragm when the lithium dendrite appears in the battery, but the puncture strength The test method does not reflect this well, and it cannot be concluded that the greater the current puncture strength, the smaller the self-discharge. The hybrid puncture test is closer to the actual diaphragm in the battery, but this test method is currently to be developed.

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