Lithium-ion batteries are representatives of modern high-performance batteries, consisting of four important parts of the positive, negative electrode, diaphragm and electrolyte.. Among them, the diaphragm is a film having a microporous structure, and is the key inner assembly of the most technical barriers in the lithium ion battery industry chain, which acts as the following two important uses in the lithium ion battery: 1) Separate the lithium ion battery.
Positive and negative, prevent positive and negative contact forming short circuit. 2) The micropores in the film can pass the lithium ion to form a charge and discharge circuit..
The basic properties of the diaphragm should be prevented from preventing physical contact to prevent short circuits. 2) Easy to wet, have good venting capacity. 3) Transmitability and low ion resistance of electrolyte ions.
4) Have chemical and electrochemical stability. 5) The diaphragm is as thin as possible. 6) The diaphragm is guaranteed to have a certain strength and has sufficient physical mechanical properties.
. 7) The diaphragm does not contain a particulate and metal that can dissolve the electrolytic solution and a substance that is harmful to the battery..
Diaphragm use 1) isolate the positive and negative of the battery to prevent short circuit. 2) Adsorption of electrolytic solution required for electrochemical reaction to ensure high ion conductivity. 3) Ensure that the battery is stopped to stop the battery when the battery is abnormal.
. Requirements of the diaphragm 1) have a certain mechanical strength to ensure that it does not rupture under battery deformation conditions..
2) Have good ion through ability to reduce battery internal resistance. 3) Excellent electron insulation to ensure effective isolation of electrodes. 4) Have the ability to resist chemistry and electrochemical corrosion, stability in the electrolyte.
5) Ability to absorb electrolyte. 6) Low cost, suitable for large-scale industrial production. 7) Less impurity content, uniform performance.
Structural characteristics of diaphragm 1) thickness. The thickness of the lithium-ion battery diaphragm is generally ≤25 μm. Under the premise of ensuring a certain mechanical strength, the thinner the thickness of the diaphragm is.
Now, the new type of high energy cell is mostly a single layer of film thickness of 20 μm or 16 μm; the separator of the battery used by the electric vehicle (EV) and the mixed electric vehicle (HEV) is about 40 μm, which is the battery large current discharge and high capacity. Moreover, the thicker the diaphragm, the better the mechanical strength, and it is not easy to short during the assembly battery..
2) Aperture and distribution. As the battery diaphragm material, itself has a microporous structure, allowing the absorbed electrolyte; in order to ensure the identity of the electrode / electrolyte interface properties and uniform current density in the battery, the distribution of micropores in the entire separator material should be uniform..
The size and distribution of the aperture have a direct impact on the battery performance: too much aperture, easy to contact the positive and negative contact or easy to be short-circuited by the lithium delegranes; the aperture is too small to increase the resistance.. The microporous distribution is uneven, and the local current is excessively large, affecting the performance of the battery.
. 3) Porosity. Transmissivity can be characterized by the amount of diaphragm gas by time and pressures, important reflecting the heat of lithium ion transmitting the diaphragm.
Porosity to the membrane permeability and electrolyte capacity. Most commercial lithium-ion battery diaphragm is between 40% and 50%. After the weighted microporous membrane (Wd) was soaked in n-butanol 2h, the liquid was gently discharged by filter paper, and the weighed (Ww) was again obtained, and the microporous membrane can be obtained.
Quality WB = WW-WD. WD-microporous membrane weight (g); WW- Soaked weight (G); Wb-n-butanol mass (g); ρB-n-butanol density (g / cm3); VP-dry film volume (cm3) 4) Transmitability. Under certain conditions (pressure, measurement area) a quantity of air through the separator, referred to as a gurly value.
The size of the diaphragm permeability is the result of the synthesis of the diaphragm porosity, the shape of the aperture, the shape of the aperture, and the internal hole structure of the diaphragm of the hole tomography.. 5) Surface morphological structure of the SEM diaphragm.
6) Base weight of the diaphragm. 1) Intercept three long 30cm diaphragm samples. 2) Store these three samples and fold together.
3) Weigh and record the quality of the sample (mg). BW (mg / cm2) = weight (mg) / [3 × 30 cm × width (cm)] Mechanical properties of the diaphragm 1) Top strength: the tensile strength of the diaphragm and the production process of the film. Tensile strength: Mdtension, TDTension East The tensile strength of the TDtension is less than Celgard, but the elongation is greater than Celgard.
When using uniaxial stretching, the film is different in the stretch direction and the vertical stretch direction, and the strength of the diaphragm prepared by biaxial stretch is basically consistent in both directions.. 2) Response intensity.
The anti-puncture strength means that the quality applied to a given needle is used to poke a sample of the given separator, which is used to characterize short circuit during the assembly process.. In related experience, the puncture strength of the lithium-ion battery diaphragm is at least 11.
38kg / mm.. Since the electrode is mixed by the active substance, carbon black, plasticizer and PVDF, it is evenly applied to the metal foil, and then manufactured after 120 ° C, it is formed, so the surface of the electrode is made of active substance and carbon black.
The tiny particles of the mixture constitute a convex surface. The diaphragm material being sandwiched between the positive and negative plates is to bear great pressure..
Physicochemical properties of diaphragm 1) Wetability and wetting speed. The diaphragm is not good, the resistance of the separator and the battery will be added, which affects the cycle performance and charge and discharge efficiency of the battery..
The wetting speed of the diaphragm means that the electrolytic solution enters the micropores of the separator, and it is related to the surface energy of the separator, aperture, porosity, and metrics.. 2) Sipioflow rate of diaphragm.
Due to the function of the battery diaphragm material, there must be the following conditions: sufficient suction rate to ensure unobstructed inexplicable ion channel, and in the battery system, there will be a large number of side effects, consume a large amount of electrolyte Therefore, there must be sufficient storage, otherwise it will increase the new interfacial resistance due to the lack of the electrolyte, and the consumption of the electrolyte will be accelerated, which will be a malignant cycle, so the suction rate is a very important diaphragm parameter.. Determination of membrane liquid absorbing: Determination by electrolytic solution.
Take a small piece of film, and the dry weighing weight M1 after extracting the plasticizer. The film is then soaked in the electrolytic solution for 30 min, and the film is fully absorbed and the electrolyte is removed..
Gently absorb the electrolyte of film surface with filter paper, weigh M2. 3) Chemical stability. The diaphragm should maintain long-term stability in the electrolytic solution, under the conditions of strong oxidation and strong reduction, no with the electrolyte and electrode substance.
The chemical stability of the diaphragm is evaluated by measuring the corrosion resistance and bulging ratio of the electrolyte resistance.. The electrolyte corrosion resistance is to warm the electrolyte to 50 ° C, impregnate the separator for 4 to 6 h, remove the wash, dry, and finally compared to the original dry sample.
. The spread rate is to detect the diaphragm to detect the dimensional change after 4 to 6 hours of the electrolyte, and seek a percentage of difference..
ExpansionTMA (MD) ExpansionTMA (TD) 4) Heat Stability. The battery will release calories during the charge and discharge process, especially when short circuit or overcharge, there will be a lot of heat. Therefore, when the temperature increases, the diaphragm should maintain the original integrity and a certain mechanical strength, continue to function as a positive and negative electrode, prevent short-circuit occurrence.
TMA (ThermalmechanicalAlanaLaysis) technology is a method of measuring high temperature diaphragm integrity, which can measure the diaphragm shape with temperature changes. TMA is a deformation of the diaphragm at the time of weight when the temperature line is rising, and the diaphragm is usually referred to, then start elongation, final breakage..
5) resistance of the diaphragm. The resistivity of the diaphragm is actually the resistivity of the electrolyte in the microporous, which is related to many factors, such as porosity, aperture of the aperture, the electrical conductivity of the electrolyte, the film thickness, and the degree of wetting of the membrane material, etc..
The test diaphragm resistor is more commonly used in the AC impedance method (EIS), applying a sinusoidal AC voltage signal on the measuring device, by measuring the impedance value of different frequencies within a certain range, and then analyzes data to obtain information about the separator and electrode interface.. Since the film is very thin, there is often a defect, and the error of the measurement results is increased, and therefore often uses a multi-layer specimen, and the average value of the measurement is again.
. 6) Automatic performance. At a certain temperature or more, the components within the battery will cause “self-heating”, and due to the failure of the charger, the safety current loss, etc.
will cause excessive charging or when the battery is short-circuited, these conditions will have a large amount. Heat. Due to the thermoplastic properties of the polyolefin material, when the temperature is close to the polymer melting point, the porous ion-conductive polymer film becomes a non-porous insulating layer, and the microporous closure occurs, there is a self-closing phenomenon, thereby blocking the continued transmission of ions.
Forming a discharging to protect the battery, so the polyolefin diaphragm can supply additional protection for the battery.. The cost of lithium ion batteries constitutes a complex lithium-ion battery diaphragm production process, and the technical barrier high-performance lithium-ion battery to have a thickness uniformity and excellent mechanical properties (including tensile strength and anti-puncture strength), gas permeability, physical and chemical properties.
Including wettability, chemical stability, thermal stability, safety). It is understood that the excellent and whether the diaphragm directly affects the capacity of the lithium-ion battery, the characteristics such as cycle capacity, and safety performance, and the performance excellent in performance is important to improve the overall performance of the battery..
Many characteristics of lithium-ion battery diaphragm and its performance indicators are difficult to determine the high barriers of production technology, and the development is difficult.. The diaphragm production process includes many processes such as raw material formulation and fast formulation adjustment, micropore preparation technology, complete design of their own equipment.
. Among them, microporous preparation techniques are the core of the preparation process of lithium ion battery diaphragm. According to the difference in microporous complex mechanism, the diaphragm process can be divided into two kinds of dry and wet.
. The dry diaphragm is divided into a monorant and double grade diaphragm diaphragm. The method is most commonly used in the preparation process of the diaphragm, which is a uniform melt in which a high molecule polymer, an additive is mixed, and when extruded The sheet is formed under the tensile stress, the heat treatment sheet structure obtains hard-elastic polymer film, and then stretches the slit micropores at a certain temperature, and the microporous membrane is obtained after heat setting.
. At present, the dry process is important including dry one-way stretching and two-way stretching..
The dry monohydrate monogram is a polyethylene (PE) or polypropylene (PP) polymer having a good fluidity and low molecular weight, and the manufacturing principle of hard elastic fibers is prepared, and the preceding is prepared from high-degree or low crystallization. After the olefin casting piece, low temperature stretching forms a micro-defect, the defect is opened by high temperature annealing, and the aperture is uniaxially oriented microporous film..
The dry trial process is: 1) Trolution: PE or PP and additives such as PE or PP and additives are delivered to the extrusion system after preprocessing the formulation.. 2) Returning: In the extrusion system, the preproced raw material is extruded from the die after melt, the melt is extruded, and the melt forms a base film of a particular crystallization structure.
. 3) Heat treatment: After the base film is heat-treated, the hard elastic film is obtained..
4) Stretch: Forming a hard-elastic film for cold stretching and heat stretching to form a nanoporous membrane. 5) Split: Cut the nanoporous membrane according to the customer’s specifications as a finished film. Dry Method Dry Method Double Dependence.
Since the beta crystal form of PP is a hexalectine system, single crystal nucleation, wafer arrangement loose, and has a single crystal structure that grows radially into a deduplicated beam shape, does not have a complete spherical structure, under thermal and stress use It will be transformed into a more dense and stable α crystal, which will appear in the material inside the material after absorbing a lot of impact.. The process uses PP differential inter-phase density in the PP, which occurs during the stretching process, forming a micropore during the stretching process.
. The dry double pulling process is: 1) Trim: PP and a pore agent and other raw materials are delivered to the extrusion system after formula pretreatment. 2) The casting: a PP casting piece having a high β crystal content and a homogeneity of β crystal form.
3) Longitudinal stretching: vertical stretching of the casting piece at a certain temperature, using the β-crystalline stretching force to easily integrate the characteristics of the hole. 4) Transverse stretch: The sample is laterally stretched to the sample at a higher temperature while increasing the uniformity of the pore size distribution..
5) Treatment of the reward: by heat treatment by heat treatment at high temperatures, reduce the thermal shrinkage ratio, improve dimensional stability. The wet diaphragm is divided into an asynchronous and synchronous wet process in accordance with the tensile orientation. Mixing, the phenomenon of solid-liquid phase or liquid-liquid phase separation occurs during the cooling process of the molten mixture, and the press is stretched to close to the melting point temperature, and the molecular chain is tvged, and the heat preservation is used to volatilize the solvent after time.
For example, dichloromethane and trichlorethylene) extract plasticizers from the film, and the mutually penetrated submicron-sized microporous membrane material is obtained.. Wet process is suitable for producing thinner single-layer PE diaphragms, is a preparation process with better uniformity, physical and mechanical properties and mechanical properties.
. Depending on whether or not the tensile is orientation, the wet process can also be divided into wet two-way asynchronous stretching process and two-way synchronous tensile processes..
Wet asynchronous stretching process is: 1) Trim: PE, a porous agent and other raw materials are pretreated to the extrusion system in accordance with the formulation. 2) Retall: After the pre-treated raw material is extruded from the die after melt in the twin screw extrusion system, the melt is extruded from the die, and the melt forms a cast film containing a pore agent..
3) Longitudinal stretch: vertical stretching. 4) Transverse stretching: transversely stretch the casting thick sheet after longitudinal stretching to obtain a base film containing a pore agent. 5) Extraction: After the base film is extracted after the solvent is extracted, the base film that does not contain a porous agent is formed.
. 6) Type: The base membrane without a pore agent is dried, and the nanoporous membrane is obtained..
7) Split: Cut the nano microporous membrane according to the customer’s specifications as a finished film. Wet asynchronous stretching process wet synchronous stretching technology process flow and asynchronous tensile technology are basically the same, but the process can be oriented simultaneously in the horizontal, vertical direction, except for the longitudinal stretching process, enhance the separator Thickness uniformity. However, the problem of synchronous stretching existence is the slow speed, and the second is a slightly poor adjustability.
Only the lateral stretch is adjustable, and the longitudinal stretching ratio is fixed.. Wet synchronous tensile process wet coating is the performance of the moisture diaphragm of lithium-ion battery diaphragm superior to the dry separator diaphragm product.
. Stability, consistency, safety of the diaphragm, decisive effect on the discharge ratio, energy density, cycle life, and safety of the lithium ion battery. Compared to dry diaphragm, wet separator is more than material in thickness uniformity, mechanical properties (tensile strength, anti-puncture strength), gas permeability, physical and chemical properties (wettability, chemical stability, safety).
For excellent, it is conducive to the suction fluid of the electrolyte and improves the charging and discharge and cycle capacity of the battery, suitable for high-capacity batteries. From the perspective of the product, stronger than dry diaphragm. The wet diaphragm also has a disadvantage.
Depending on the fact that is limited to the matrix material, the thermal stability is more non-product factors, such as a large amount of solvent, which is easy to cause environmental pollution; complicated with the dry process, large investment Long cycle, high cost, large energy consumption, large production difficult, low production efficiency, etc.. In the wet diaphragm, the bidirectional synchronous tensile technique can be oriented simultaneously in the horizontal, vertical direction, eliminating the process of carrying longitudinal stretching separately, enhancing the thickness of the diaphragm, high product transparency, no scratch, optical performance and Excellent surface performance, is the best diaphragm for comprehensive performance, accounting for an important position in the high-end market of the diaphragm, and is also the best lithium-ion battery diaphragm in the current market performance.
. Lithium-ion battery diaphragm dry and wet process compared to dry and wet process diaphragm properties Comparison from product performance, compared to dry diaphragms, wet separators in mechanical properties, gas permeability, and physical and chemical properties have a certain advantage, through the base film Coating ceramic alumina, PVDF, aramid, etc., can greatly improve the thermal stability of the diaphragm, reduce the high temperature shrinkage, prevent the exemption of the pole exemption caused by large shrinkage of the diaphragm, and compensate for the unique thermal stability short board, product Performance has been fully leading dry film.
Coating diaphragm and conventional diaphragm ceramic coating diaphragm ceramic particles under high temperature conditions, the film is applied to the base film, and the surface is coated with an inorganic ceramic particles of Al2O3, SiO2, Mg (OH) 2 or other heat resistance. After special processes, the matrix is closely bonded together, stabilize the flexibility of the organic matter and the thermal stability of the inorganic matter, improve the high temperature resistance of the diaphragm, heat resistant shrinkage and puncture strength, thereby improving the safety of the battery..
It is understood that on the one hand, the ceramic composite layer can solve the thermal displacement caused by the heat shrinkage of PP and PE diaphragm to cause safety problems of battery combustion, explosion; on the other hand, the ceramic composite diaphragm and electrolyte and positive and negative electrode materials have good infiltration and suction The ability to protect the liquid, greatly improve the life of the battery. In addition, the ceramic coated diaphragm can also neutralize a small amount of hydrofluoric acid in the electrolyte to prevent battery air..
PVDF coated diaphragm PVDF, ie, polyfial fluoride, is a white powder crystalline polymer, melting point 170 ° C, thermally decomposed temperature 316 ° C or higher, long-term use temperature -40 to 150 ° C, has excellent chemical resistance, High temperature resistance, oxidation resistance, wear resistance, flexibility, and high resistance strength and impact resistance. PVDF coated diaphragm has low internal resistance, high (thickness / void ratio), good mechanical properties, chemical and electrochemical stability. Due to the presence of nanofiber coatings, the novel diaphragm has better compatibility and adhesion than ordinary battery separators, which can greatly improve the high temperature resistance and safety of the battery.
. Further, the new diaphragm has good absorption resistance to the liquid electrolyte, the ability to infiltrate and suction fluid, extend the battery cycle life, new battery’s large rate discharge performance, and increase the output capacity of the battery by 20%, especially suitable High-end storage battery, automotive power lithium battery. Aramid coating diaphragm aramid fiber is used as a high performance fiber, which has high temperature resistance and excellent fire flame retardancy that can withstand high temperature of 400 ° C, which can effectively prevent the surface of thermal melt.
. Coating the coating obtained using high thermal antifungal resins, on the one hand, the heat resistance of the diaphragm can greatly increase, realistic of the overall combination of closed cell characteristics and heat resistance; on the other hand due to aramid resin pair The electrolyte has high affinity, which makes the diaphragm have good infiltration and suction oil, and this excellent high infiltration can extend the battery life..
Further, the aramid resin plus the filler, which can improve the antioxidant properties of the diaphragm, thereby achieving high potentialization, thereby increasing energy density.. Comparison of physical properties indicators of three important coated separators of conventional separators and coated separators.