Composite diaphragm, preparation method thereof and lithium ion battery

Abstract

The invention relates to the technical field of lithium ion batteries, and discloses a composite diaphragm, a preparation method thereof and a lithium ion battery. The composite diaphragm comprises a first surface layer, a second surface layer, a third surface layer and a fourth surface layer, the middle functional layer is formed by compounding aramid nanofibers and hydroxyapatite; the second surface layer is formed by polyacrylonitrile nanofibers; wherein the thickness of the first surface layer and the thickness of the second surface layer are 4-10 [mu] m, and the thickness of the middle functional layer is 2-6 [mu] m. The composite diaphragm can simultaneously combine the chemical stability and electrolyte affinity of PAN and the high-temperature mechanical strength of ANF, and also has the ion guiding and interface regulation and control capabilities of HAP.

Description

Technical Field

[0001] The present application relates to the technical field of lithium-ion batteries, and in particular to a composite diaphragm and a preparation method thereof, and a lithium-ion battery. Background Art

[0002] As new energy vehicles, energy storage power stations, and portable electronic devices continue to demand higher energy density, safety, and cycle life, the safety performance of lithium-ion batteries, as core energy units, has become a key factor restricting their further development. As a key component within the battery, the separator not only physically separates the positive and negative electrodes but also directly affects the battery's ion conduction efficiency, interfacial stability, and thermal safety.

[0003] During the implementation of the embodiments of the present disclosure, it was found that at least the following problems exist in the related art:

[0004] Traditional polyolefin separators (such as polyethylene and polypropylene) have average lithium ion conductivity and poor thermal stability, and are prone to shrinkage at high temperatures, causing safety accidents. In addition, in lithium metal anode or high-rate applications, traditional separators cannot effectively inhibit the formation and puncture of lithium dendrites, which can easily cause safety accidents such as thermal runaway. Therefore, how to design a new functional separator that combines electrolyte affinity, high-temperature stability and anti-dendrite ability becomes the key.

[0005] It should be noted that the information disclosed in the above background technology section is only used to enhance the understanding of the background of this application, and therefore may include information that does not constitute prior art known to ordinary technicians in this field. Summary of the Invention

[0006] In order to provide a basic understanding of some aspects of the disclosed embodiments, a brief summary is given below. The summary is not an extensive review, nor is it intended to identify key / critical elements or delineate the scope of protection of these embodiments, but rather serves as a prelude to the detailed description that follows.

[0007] The embodiments of the present disclosure provide a composite diaphragm, a preparation method thereof, and a lithium-ion battery, which significantly improve the electrolyte affinity, high-temperature stability, and anti-dendrite ability of the diaphragm while ensuring the overall flexibility and mechanical stability of the diaphragm.

[0008] In some embodiments, the composite membrane comprises:

[0009] a first surface layer composed of polyacrylonitrile nanofibers;

[0010] The middle functional layer is composed of aramid nanofiber and hydroxyapatite composite;

[0011] a second surface layer composed of polyacrylonitrile nanofibers;

[0012] Wherein, the thickness of the first surface layer and the thickness of the second surface layer are 4-10 μm, and the thickness of the intermediate functional layer is 2-6 μm.

[0013] Optionally, the mass ratio of the aramid nanofiber to the hydroxyapatite in the intermediate functional layer is 19 to 3:1, wherein the diameter of the aramid nanofiber is 5 to 50 nm, and the median particle size D of the hydroxyapatite is 100 nm. 50 It is 10 to 130 nm.

[0014] Optionally, the contact angle of the composite membrane in the liquid electrolyte is ≤15°, and the membrane rupture temperature of the composite membrane is ≥200°C.

[0015] Optionally, the composite separator has a dimensional shrinkage rate of less than 4.3% after being treated at 180° C. for 30 minutes.

[0016] Optionally, the ionic conductivity of the composite membrane is greater than 0.39 mS / cm, and the constant current density is 1 mA / cm 2 Under the condition of , the cycle life of the composite membrane exceeds 1000h.

[0017] Optionally, the composite membrane is a three-layer gradient structure, which is deposited in sequence through a continuous electrospinning process and formed by calendering.

[0018] In some embodiments, the method for preparing the composite membrane comprises:

[0019] Step 101: preparing a spinning solution, and depositing the spinning solution on a rotary drum current collector through an electrospinning process to form a first surface layer composed of polyacrylonitrile nanofibers;

[0020] Step 102: preparing a polymer mixed dispersion of aramid nanofibers and hydroxyapatite, and depositing the polymer mixed dispersion on the first surface layer through an electrospinning process to obtain an intermediate functional layer;

[0021] Step 103: using the spinning solution in step 101, depositing the spinning solution on the intermediate functional layer through an electrospinning process to form a second surface layer composed of polyacrylonitrile nanofibers, thereby obtaining a composite diaphragm with a three-layer gradient structure;

[0022] Step 104: The composite diaphragm is subjected to roller pressing treatment at a hot pressing temperature of 60 to 120° C. and a roller pressing pressure of 0.5 to 4.0 MPa, followed by vacuum drying at 60° C. for 12 to 48 hours to remove residual solvent.

[0023] Optionally, in step 101, the spinning solution is prepared by dissolving 5 to 15 wt % of polyacrylonitrile (PAN) in N,N-dimethylformamide (DMF) and stirring for 12 hours to form a uniform spinning solution; the voltage in the electrospinning process is 5 to 20 kV, the collection distance is 5 to 15 cm, and the flow rate is 0.2 to 1.5 mL / h; the thickness of the first surface layer composed of polyacrylonitrile nanofibers is 2 to 4 μm.

[0024] Optionally, in step 102, the method for preparing the aramid nanofiber-hydroxyapatite polymer mixed dispersion is as follows: dissolving the aramid fiber in a dimethyl sulfoxide / water system at a concentration of 2 to 15 wt%, adding potassium hydroxide and chloroform to obtain an aramid nanofiber dispersion, and then adding a hydroxyapatite nanofiber having a median particle size D 50 The electrospinning process comprises an electrospinning process of hydroxyapatite with a diameter of 10 to 130 nm and a uniform dispersion of the hydroxyapatite by ultrasonic treatment for 10 to 60 minutes. The volume ratio of dimethyl sulfoxide to water is 8:2, the diameter of the aramid nanofiber is 5 to 50 nm, and the mass ratio of the aramid nanofiber to the hydroxyapatite is 85:15. The voltage in the electrospinning process is 5 to 25 kV, the collection distance is 10 to 25 cm, and the flow rate is 0.2 to 1.0 mL / h. The thickness of the intermediate functional layer is 5 to 7 μm, and the mass percentage of the hydroxyapatite is 15%.

[0025] In some embodiments, the lithium-ion battery comprises a positive electrode sheet, a negative electrode sheet, and the composite separator as described in the present application, wherein the capacity retention rate of the lithium-ion battery is greater than 85% when discharged at a 10C rate.

[0026] The composite diaphragm and its preparation method, as well as the lithium-ion battery provided by the embodiments of the present disclosure can achieve the following technical effects:

[0027] By designing a composite membrane with a gradient interface structure, the first surface layer is composed of polyacrylonitrile (PAN) nanofibers, the middle functional layer is composed of an aramid nanofiber (ANF)-hydroxyapatite (HAP) composite, and the second surface layer is composed of polyacrylonitrile (PAN) nanofibers. This composite membrane combines the chemical stability and electrolyte affinity of PAN, the high-temperature mechanical strength of ANF, and the ion guidance and interface control capabilities of HAP. On this basis, the nanofiber gradient interface structure ensures the overall flexibility and mechanical stability of the composite membrane while further improving the wettability, thermal stability, and lithium dendrite suppression capabilities of the composite membrane.

[0028] The above general description and the following description are exemplary and explanatory only and are not intended to limit the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] One or more embodiments are exemplarily described by corresponding drawings. These exemplary descriptions and drawings do not limit the embodiments. Elements with the same reference numerals in the drawings are shown as similar elements. The drawings do not constitute a scale limitation. In addition,

[0030] Figure 1 is a schematic structural diagram of a lithium-ion battery provided by an embodiment of the present disclosure;

[0031] Figure 2 1 is a schematic structural diagram of a battery roll core provided by an embodiment of the present disclosure;

[0032] Figure 3 1 is a schematic diagram of an unfolded battery roll core provided by an embodiment of the present disclosure;

[0033] Figure 4 This is a flow chart of a method for preparing a lithium-ion battery provided in an embodiment of the present disclosure.

[0034] Reference numerals:

[0035] 1-positive terminal; 10-battery core; 11-positive column; 12-negative terminal; 2-shell; 3-negative electrode sheet; 4-composite diaphragm; 5-positive electrode sheet. DETAILED DESCRIPTION

[0036] In order to be able to understand the features and technical content of the embodiments of the present disclosure in more detail, the implementation of the embodiments of the present disclosure is described in detail below in conjunction with the accompanying drawings. The accompanying drawings are for reference only and are not used to limit the embodiments of the present disclosure. In the following technical description, for the sake of convenience of explanation, a full understanding of the disclosed embodiments is provided through multiple details. However, one or more embodiments can still be implemented without these details. In other cases, to simplify the drawings, well-known structures and devices can be simplified for display.

[0037] In the description and claims of the embodiments of the present disclosure, as well as in the accompanying drawings, the terms "first," "second," and the like are used to distinguish similar items and are not necessarily used to describe a particular order or precedence. It should be understood that the terms used in this manner are interchangeable where appropriate to describe the embodiments of the present disclosure herein. In addition, the terms "including," "having," and any variations thereof are intended to cover non-exclusive inclusions.

[0038] In the embodiments of the present disclosure, the terms "upper", "lower", "inside", "middle", "outside", "front", "back" and the like indicate directions or positional relationships based on the directions or positional relationships shown in the accompanying drawings. These terms are mainly intended to better describe the embodiments of the present disclosure and their embodiments, and are not intended to limit the indicated devices, elements or components to having a specific direction, or to be constructed and operated in a specific direction. Moreover, in addition to being used to indicate directions or positional relationships, some of the above terms may also be used to indicate other meanings. For example, the term "upper" may also be used to indicate a certain dependency or connection relationship in certain circumstances. For those of ordinary skill in the art, the specific meanings of these terms in the embodiments of the present disclosure can be understood according to specific circumstances.

[0039] Furthermore, the terms "disposed," "connected," and "fixed" should be interpreted broadly. For example, "connected" can mean a fixed connection, a removable connection, or an integral structure; it can be a mechanical connection or an electrical connection; it can be a direct connection, an indirect connection through an intermediary, or an internal connection between two devices, elements, or components. Those skilled in the art will understand the specific meanings of these terms in the embodiments of this disclosure based on the specific circumstances.

[0040] Unless otherwise stated, the term "plurality" means two or more.

[0041] In the embodiment of the present disclosure, the character " / " indicates that the preceding and following objects are in an "or" relationship. For example, A / B means: A or B.

[0042] The term "and / or" describes an association between objects, indicating that three relationships can exist. For example, A and / or B means: A or B, or A and B.

[0043] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present disclosure can be combined with each other.

[0044] Combine Figure 1 As shown, the embodiment of the present disclosure provides a lithium-ion battery, including a cylindrical shell 2, the interior of which is used to accommodate a battery coil core, a positive terminal 1 at the top, a negative terminal 12 at the bottom, and a positive column 11 provided on the positive terminal 1. Specifically, Figure 2 It shows the structural diagram of the battery roll core in this application, Figure 3 The diagram of the unfolded battery roll core in this application is shown. Among them, the positive electrode sheet 5, the negative electrode sheet 3 and the composite separator 4 are as shown in FIG. Figure 3 As shown in the stacking, and then winding to form Figure 2The cylindrical battery core 10 shown has the electrode sheet end at the cylindrical axis at the beginning of winding and the electrode sheet end at the cylindrical outer surface at the end of winding. The positive electrode sheet 5 comprises a strip of positive electrode foil, a positive electrode coating applied to the surface of the positive electrode foil strip, and a first hollow foil region. The negative electrode sheet 3 comprises a strip of negative electrode foil, a negative electrode coating applied to the surface of the negative electrode foil strip, and a second hollow foil region. The first and second hollow foil regions are perpendicular to the winding direction and are formed by flattening or cutting and stacking to form the top and bottom end faces of the lithium-ion battery.

[0045] In this regard, in order to ensure the overall flexibility and mechanical stability of the diaphragm while significantly improving the electrolyte affinity, high temperature stability and anti-dendrite ability of the diaphragm, the present application provides a composite diaphragm, including a first surface layer, an intermediate functional layer and a second surface layer, wherein the first surface layer is composed of polyacrylonitrile PAN nanofibers, the intermediate functional layer is composed of a composite of aramid nanofibers ANF and hydroxyapatite HAP, and the second surface layer is composed of polyacrylonitrile PAN nanofibers, and the thickness of the first surface layer and the thickness of the second surface layer is 4 to 10 μm, and the thickness of the intermediate functional layer is 2 to 6 μm.

[0046] The composite diaphragm of the lithium-ion battery provided by the embodiment of the present disclosure is designed with a gradient interface structure, wherein the first surface layer is composed of polyacrylonitrile PAN nanofibers, the middle functional layer is composed of aramid nanofiber ANF-hydroxyapatite HAP composite, and the second surface layer is composed of polyacrylonitrile PAN nanofibers. In this way, the composite diaphragm can simultaneously combine the chemical stability and electrolyte affinity of PAN, the high-temperature mechanical strength of ANF, and the ion guidance and interface regulation capabilities of HAP. On this basis, through the nanofiber gradient interface structure, it is possible to ensure that the composite diaphragm has overall flexibility and mechanical stability while further improving the wettability, thermal stability and lithium dendrite suppression ability of the composite diaphragm.

[0047] Before explaining the various performances of the composite diaphragm of this application, it is first necessary to remove the composite diaphragm from the lithium-ion battery. The specific processing method is: discharge the lithium-ion battery to 2.5V at a constant current to ensure that it is in a safe state to reduce the risk of short circuit or thermal runaway during the disassembly process. In a glove box (argon or other inert atmosphere protection), disassemble the battery and take out the composite diaphragm in the cylindrical battery cell. Soak the removed composite diaphragm in anhydrous dimethyl carbonate DMC solution for 30 minutes to dissolve and remove residual electrolyte and possible by-products. Subsequently, gently wipe the surface with dust-free wipe paper, then replace with fresh DMC solution, and repeat the soaking-wiping process three times to ensure that there are no residual contaminants on the surface of the composite diaphragm. Finally, rinse the composite diaphragm with anhydrous ethanol, wipe it again, and place it in a glove box for 48 hours to ensure that the composite diaphragm is completely dry to prevent subsequent tests from being interfered with by solvent residues.

[0048] Optionally, the mass ratio of the aramid nanofiber to the hydroxyapatite in the intermediate functional layer of the present application is 19 to 3:1 (preferably 17:3), and the diameter of the aramid nanofiber is 5 to 50 nm (preferably 20 nm), and the median particle size D of the hydroxyapatite is 100 nm. 50 10 to 130 nm (preferably 32 nm).

[0049] Specifically, the hydroxyapatite (HAP) content in the intermediate functional layer is determined by gently peeling the upper, middle, and lower layers of the dried composite separator using a plastic scraper or blade (or by selective solvent stripping). The intermediate functional layer is then placed in a thermogravimetric analyzer. The sample is heated to 600°C in a nitrogen atmosphere at a constant heating rate (e.g., 5°C / min) to completely decompose the organic matter (residue being HAP). The HAP content is calculated by the ratio of the initial mass to the residual mass after constant weight at high temperature. The HAP mass percentage is calculated as: post-calcination mass / original sample mass × 100%.

[0050] And, D of HAP in the middle functional layer 50 The ANF diameter is determined as follows: the intermediate functional layer is chopped into small pieces, placed in a test tube, and sonicated with ethanol for 30 minutes. The intermediate functional layer is then removed, the remaining solution is centrifuged (at 10,000 rpm for 10 minutes), the supernatant is discarded, and the powder is redispersed with anhydrous ethanol. The powder is sonicated again for 10 minutes, and then centrifuged again. This process is repeated three times to ensure the purity of the powder sample. High-resolution images of HAP particles (round or quasi-spherical) are then acquired using transmission electron microscopy (TEM). Image analysis software (such as ImageJ) is used to count the two-dimensional particle size (diameter, count ≥ 100 particles). Based on the equivalent circle assumption, the projected area of ​​non-spherical particles is equated to the area of ​​a circle, and their equivalent diameter is calculated. Finally, the D50 value of the HAP is calculated. Similarly, high-resolution images of ANFs (elongated nanowires) are acquired using TEM, and the diameters of elongated nanowires (count ≥ 100 particles) are counted using image analysis software. The average ANF diameter is then calculated.

[0051] In this way, the present application controls the particle size of HAP particles to 10 to 130 nm, which helps to uniformly disperse them in the diaphragm and build a stable inorganic skeleton structure. The smaller particle size increases the specific surface area, which is beneficial to the electrolyte wettability and ion conductivity; the medium particle size can enhance the thermal stability and interface structure integrity of the intermediate functional layer; the larger particle size has good mechanical support and lithium dendrite blocking ability, thereby achieving a synergistic improvement in ion conductivity, thermal safety and mechanical strength.

[0052] At the same time, this application helps ANF form a dense nanofiber network structure in the diaphragm by controlling the diameter of ANF to 5-50nm. Fibers with smaller diameters can penetrate into the gaps in the PAN matrix to enhance hydrogen bond cross-linking, thereby improving the flexibility and fracture strength of the diaphragm; medium to larger diameters can further construct a supporting skeleton, enhance the mechanical modulus and thermal dimensional stability of the intermediate functional layer, and achieve a composite reinforcement effect with high strength and high thermal stability.

[0053] Optionally, the contact angle of the composite membrane of the present application in the liquid electrolyte is ≤15°, and the membrane rupture temperature of the composite membrane of the present application is ≥200°C.

[0054] Specifically, the contact angle measurement method of the composite diaphragm is as follows: 2 to 5 μL of trace electrolyte (EC:DMC=1:1) is added to the treated clean diaphragm surface using a micro syringe, a side view image of the droplet is taken, the droplet profile is fitted using software (such as the Young-Laplace equation or the ellipse fitting method), and the contact angle is calculated after the baseline is automatically detected.

[0055] The hot box failure temperature (i.e., membrane rupture temperature) is determined by placing the lithium-ion battery in a 25°C constant temperature oven for 4 hours and charging it to 4.2V at a constant current and voltage of 1C with a cutoff current of 0.01C. The battery is then left to stand for 10 minutes. The temperature is then increased at a rate of 5°C / min, with each 5°C increase followed by a 10-minute hold. The surface temperature of the lithium battery is monitored during the heating process. When the temperature begins to rise sharply, the corresponding oven temperature is the hot box failure temperature of the secondary battery.

[0056] In this way, the contact angle of the composite diaphragm of the present application in the liquid electrolyte is ≤15°, which significantly improves the infiltration speed and liquid absorption capacity of the electrolyte, helps to reduce the interfacial impedance and improve the battery rate performance; at the same time, its membrane rupture temperature is ≥200°C, indicating that the diaphragm can still maintain structural integrity in a high temperature environment, has good hot melt closed hole delay ability and safety performance, can effectively cope with thermal runaway conditions, and ensure the high safety operation of lithium-ion batteries.

[0057] Optionally, the dimensional shrinkage of the composite membrane of the present application after thermal shrinkage measurement is less than 4.3%.

[0058] Specifically, the method for determining the thermal shrinkage rate of the composite diaphragm is as follows: cut the composite diaphragm into a rectangular specimen of 10mm×100mm; spread the sample flat between A4 papers to prevent curling at high temperature, place it in an oven with a temperature control accuracy of ±1°C, set the temperature to 180°C, heat for 1 hour without external force, and then cool to room temperature; use a vernier caliper (accuracy 0.01mm) or a laser rangefinder to measure the initial length L0 (length direction) and the length L1 after heating, respectively, and calculate the shrinkage rate according to the formula (L0-L1) / L0×100%. Three samples need to be tested in each direction to take the average value, and retesting is required if the deviation exceeds 5%.

[0059] In this way, the composite diaphragm of the present application has a dimensional shrinkage rate of less than 4.3% after being treated at 180°C for 30 minutes, reflecting its excellent high-temperature dimensional stability. This is mainly attributed to the nanostructured reinforcing materials (such as ANF and HAP) introduced in the intermediate functional layer to construct a dense three-dimensional support skeleton, which effectively inhibits the shrinkage behavior of the polymer chain segments at high temperatures; at the same time, the upper and lower layers of PAN matrix show a certain oxidative cross-linking trend during the heat treatment process, further enhancing the thermal stability of the membrane. This performance is significantly better than that of traditional polyolefin diaphragms (such as PP / PE), which helps to maintain the integrity of the diaphragm structure in the early stage of thermal runaway of the battery and prevent short circuits between the positive and negative electrodes, thereby significantly improving the thermal safety performance and reliability of the battery. In addition, the low shrinkage rate is conducive to maintaining the stability of the internal structural dimensions of the battery cell under high-temperature conditions, and is suitable for harsh application environments such as fast charging and high rate.

[0060] Optionally, the ionic conductivity of the composite membrane of the present application is greater than 0.39 mS / cm, and the ionic conductivity of the composite membrane of the present application is greater than 0.39 mS / cm at a constant current density of 1 mA / cm 2 Under the condition of , the cycle life of the composite diaphragm of the present application exceeds 1000h.

[0061] Specifically, the ionic conductivity is determined as follows:

[0062] (1) Sample preparation: The prepared three-layer composite membrane was soaked in an electrolyte (e.g., 1 M LiPF6, EC / DEC = 1:1, v / v) for at least 12 h to ensure adequate wetting.

[0063] (2) Assembling a symmetrical cell: In an inert atmosphere (such as a glove box), two stainless steel electrodes (SS) and a diaphragm soaked in electrolyte are clamped together to form a symmetrical structure of SS|diaphragm|SS, i.e., an impedance test device with “inactive electrodes”.

[0064] (3) Test equipment and parameters: An electrochemical workstation was used to perform the AC impedance spectroscopy (EIS) test. The test conditions were as follows: frequency range: 1 MHz to 0.1 Hz, AC disturbance voltage: 5 to 10 mV, and test temperature: 25°C (the temperature can be controlled in a constant temperature box).

[0065] Conductivity calculation formula: σ = L / (R × A);

[0066] Where, σ is the ionic conductivity (S / cm), L is the thickness of the diaphragm (cm), R is the high-frequency semicircle intercept in the Nyquist plot (Ω), and A is the effective area of ​​the peripheral electrode (cm 2 ).

[0067] And the Li||Li symmetric battery cycle test method is: the lithium ion battery separator anti-dendritic puncture ability can be tested by Li||Li symmetric battery at 1mA / cm 2 Long-term cycling tests were conducted at a constant current density to evaluate the performance. Inside a glove box, the separator to be tested was sandwiched between two sheets of lithium metal to form a 2032-type button cell. The electrolyte was a mixture of 1M LiPF6 in EC and DMC (EC / DEC = 1:1, v / v). Subsequently, repeated lithium deposition / stripping cycles were performed in a 25°C constant temperature oven. If the voltage plateau remained stable and the cycle life exceeded 1000 hours without short circuiting, the separator effectively suppressed lithium dendrite penetration and short circuiting. If the voltage suddenly dropped or rapidly dropped to zero, it indicated that dendrites had penetrated the separator, causing a short circuit.

[0068] Optionally, the lithium-ion battery of the present application has a capacity retention rate greater than 85% when discharged at a 10C rate.

[0069] Specifically, the 10C discharge capacity retention rate test method is:

[0070] Place the battery in a 25°C constant temperature box for 4 hours and test it according to the following steps:

[0071] (1) Charge to 4.2V at 0.2C constant current and constant voltage, with a cut-off current of 0.1C, and let it rest for 30 minutes;

[0072] (2) Discharge at a constant current of 1C until the cell is cut off at 2.5V, with the capacity measured as Q1, and allow to stand for 30 minutes;

[0073] (3) Charge the battery to 4.2V at a constant current and constant voltage of 0.2C, with a cut-off current of 0.1C, and let it rest for 30 minutes.

[0074] (4) Discharge at a constant current of 10C until the cell is cut off at 2.5V, with the capacity measured as Q2, and allow to stand for 30 minutes;

[0075] The calculation method of 10C capacity retention rate is: Q2 / Q1×100.

[0076] The lithium-ion battery of the present invention exhibits a capacity retention greater than 85% at a 10C discharge rate, demonstrating that the hydroxyapatite (HAP) incorporated into the composite separator exhibits a highly ordered crystalline channel structure at the nanoscale, contributing to the establishment of a continuous lithium ion migration path and significantly improving the separator's overall ion conductivity. Furthermore, the abundant lithiophilic functional groups on the HAP surface synergize with the polar structure of the PAN matrix to enhance electrolyte adsorption and retention, further reducing interfacial resistance. Furthermore, the aromatic nanofibers (ANF) in the intermediate functional layer form a dense nanofiber network structure, which not only enhances the separator's mechanical strength and dimensional stability but also helps inhibit lithium dendrite penetration and growth, effectively extending the stable operation time of the lithium metal symmetric battery under constant current cycling conditions.

[0077] Optionally, the composite diaphragm of the present application is a three-layer gradient structure, which is sequentially deposited by a continuous electrospinning process and formed by calendering.

[0078] In this way, the composite membrane of the present application adopts a three-layer gradient structure design, and the upper layer PAN, the core functional layer ANF-HAP composite layer and the lower layer PAN are sequentially deposited through a continuous electrospinning process, and then integrated into a calendering process, taking into account both structural integrity and multifunctional performance improvement. The gradient structure provides good flexibility, density and interface affinity through the upper and lower layers of PAN nanofibers, while the dense network skeleton of the middle functional layer, which is synergistically constructed by nano-aramid fiber ANF and hydroxyapatite HAP, gives the membrane excellent thermal stability, mechanical strength and ion conductivity. The continuous electrospinning process ensures a natural transition between the three layers of materials without obvious interface defects, avoids the interlayer peeling problem that is prone to occur in traditional multi-step preparation processes, and improves the overall mechanical and thermal stability of the membrane. At the same time, the calendering process further optimizes the fiber arrangement and thickness uniformity, enhances the fit between the membrane and the battery electrode, reduces the interface resistance, and significantly improves the dimensional stability and thermal shrinkage suppression ability. Overall, this structure-process integration strategy gives the composite membrane excellent comprehensive performance.

[0079] Combine Figure 4 As shown, the embodiment of the present disclosure also provides a method for preparing a composite diaphragm, comprising:

[0080] Step 101: preparing a spinning solution, and depositing the spinning solution on a rotary drum current collector through an electrospinning process to form a first surface layer composed of polyacrylonitrile nanofibers.

[0081] In the embodiment of the present application, the spinning solution is prepared by dissolving 5 to 15 wt % (preferably 8 wt %) of polyacrylonitrile (PAN) in N,N-dimethylformamide (DMF), stirring and dissolving for 12 hours to form a uniform spinning solution.

[0082] Optionally, the voltage in the electrospinning process is 5-20 kV (preferably 15 kV), the collection distance is 5-15 cm (preferably 15 cm), and the flow rate is 0.2-1.5 mL / h (preferably 0.5 mL / h).

[0083] Optionally, the thickness of the first surface layer composed of polyacrylonitrile nanofibers is 2-4 μm.

[0084] Step 102: preparing a polymer mixed dispersion of aramid nanofibers and hydroxyapatite, and depositing the polymer mixed dispersion on the first surface layer through an electrospinning process to obtain an intermediate functional layer.

[0085] In the embodiment of the present application, the method for preparing the aramid nanofiber-hydroxyapatite polymer mixed dispersion is as follows: aramid fibers are dissolved in a dimethyl sulfoxide / water system at a concentration of 2 to 15 wt% (preferably 5 wt%), potassium hydroxide and chloroform are added to obtain a dispersion of aramid nanofibers, and then a particle size of median diameter D is added. 50 The hydroxyapatite is 10 to 130 nm (preferably 32.1 nm), and is finally uniformly dispersed by ultrasonication for 10 to 60 minutes (preferably 30 minutes).

[0086] The volume ratio of dimethyl sulfoxide to water is 8:2, the diameter of the aramid nanofiber is 5 to 50 nm (preferably 20.3 nm), and the mass ratio of the aramid nanofiber to hydroxyapatite is 85:15.

[0087] The voltage in the electrospinning process is 5 to 25 kV (preferably 18 kV), the collection distance is 10 to 25 cm (preferably 18 cm), and the flow rate is 0.2 to 1.0 mL / h (preferably 0.4 mL / h); the thickness of the intermediate functional layer is 5 to 7 μm, wherein the mass percentage of hydroxyapatite is 15%.

[0088] Step 103: Using the spinning solution in step 101, the spinning solution is deposited on the middle functional layer through an electrospinning process to form a second surface layer composed of polyacrylonitrile nanofibers, thereby obtaining a composite diaphragm with a three-layer gradient structure.

[0089] Step 104: The composite membrane is subjected to roller pressing treatment at a hot pressing temperature of 100° C. and a roller pressing pressure of 2 MPa, followed by vacuum drying at 60° C. for 24 hours to remove residual solvent.

[0090] The present invention is further explained below with reference to the following examples.

[0091] Example 1

[0092] Example 1 provides a lithium ion battery, which is prepared by the following method:

[0093] 1. Positive electrode production method:

[0094] Take the positive electrode active material (LiNi 0.8 Co 0.1 Mn 0.1 O2, NMC811), conductive carbon black, carbon nanotubes, and polyvinylidene fluoride (PVDF) were thoroughly mixed in an N-methylpyrrolidone solvent system at a mass ratio of 96:1:1:2. The positive electrode coating material was then evenly coated onto 12.0μm-thick aluminum foil. The electrode sheet was dried, cold-pressed, slit, and cut to obtain a compacted density of 2.45g / cm3.

[0095] 2. Negative electrode production method:

[0096] The negative electrode coating material is prepared by mixing graphite, carbon nanotubes, sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) in a mass ratio of 96:1.5:1.0:1.5 in deionized water. The negative electrode coating material is then applied to a 15μm-thick copper foil. After drying, cold pressing, slitting, and cutting, the negative electrode sheet is obtained, with a compaction density of 1.5g / cm³.

[0097] 3. Preparation of electrolyte:

[0098] The electrolyte was obtained by mixing lithium salt lithium hexafluorophosphate (LiPF6), organic solvent ethylene carbonate (EC), dimethyl carbonate (DMC), fluoroethylene carbonate (FEC), vinyl sulfate (DTD) and vinylene carbonate (VC) in a mass percentage ratio of 10.0:20.0:55.0:2.0:8.0:5.0.

[0099] 4. Preparation of diaphragm:

[0100] (1) Preparation of PAN layer (i.e., first surface layer)

[0101] 8 wt% polyacrylonitrile (PAN) was dissolved in N,N-dimethylformamide (DMF) and stirred for 12 hours to form a uniform spinning solution. The first layer of PAN fiber membrane was deposited onto a rotating drum current collector using electrospinning (voltage: 15 kV, collection distance: 15 cm, flow rate: 0.5 mL / h), with a controlled thickness of approximately 3 μm.

[0102] (2) Preparation of ANF-HAP intermediate functional layer

[0103] Aramid fibers were dissolved in DMSO / water (v / v=8:2) at a concentration of 5 wt%, and potassium hydroxide and chloroform were added to obtain an ANF dispersion. The diameter of the ANF was 20.3 nm. 50HAP (32.1 nm in diameter) was added to the PAN layer at a mass ratio of 85:15. Ultrasonic dispersion was achieved for 30 minutes. Electrospinning was then performed on the PAN layer (voltage: 18 kV, distance: 18 cm, flow rate: 0.4 mL / h) to produce a functional layer approximately 6 μm thick (HAP content: 15% by mass).

[0104] (3) Preparation of PAN layer (i.e., second surface layer)

[0105] Repeat step 1 to continue electrospinning a PAN layer on the middle functional layer with a thickness of about 3 μm to form the final gradient three-layer composite membrane.

[0106] Post-treatment: The composite diaphragm was subjected to roller pressing treatment at a hot pressing temperature of 100°C and a roller pressing pressure of 2 MPa, and then vacuum dried at 60°C for 24 hours to remove the residual solvent.

[0107] 5. Assembly of lithium-ion batteries:

[0108] The positive and negative electrode sheets are rolled and slit, respectively, and then wound together with the separator to obtain a 21700 cylindrical battery core. The battery core is then welded to the connecting sheet and loaded into the battery casing. After completing the injection, sealing, and formation processes, the lithium-ion battery of Example 1 is obtained. The casing of the lithium-ion battery is cylindrical, and its dimensional parameters are: diameter: 21.0 mm, length: 70.0 mm.

[0109] Example 2

[0110] The difference between this embodiment and embodiment 1 is that the mass percentage of HAP in the intermediate functional layer is 5%, and the rest is the same as embodiment 1.

[0111] Example 3

[0112] The difference between this embodiment and embodiment 1 is that the mass percentage of HAP in the intermediate functional layer is 10%, and the rest is the same as embodiment 1.

[0113] Example 4

[0114] The difference between this embodiment and embodiment 1 is that the mass percentage of HAP in the intermediate functional layer is 20%, and the rest is the same as embodiment 1.

[0115] Example 5

[0116] The difference between this embodiment and embodiment 1 is that the mass percentage of HAP in the intermediate functional layer is 30%, and the rest is the same as embodiment 1.

[0117] Example 6

[0118] The difference between this embodiment and embodiment 1 is that the D 50 is 10.5 nm, and the rest are the same as in Example 1.

[0119] Example 7

[0120] The difference between this embodiment and embodiment 1 is that the D 50 The thickness is 61.6 nm, and the others are the same as those in Example 1.

[0121] Example 8

[0122] The difference between this embodiment and embodiment 1 is that the D 50 The thickness is 92.5 nm, and the others are the same as those in Example 1.

[0123] Example 9

[0124] The difference between this embodiment and embodiment 1 is that the D 50 is 123.2 nm, and the rest are the same as in Example 1.

[0125] Example 10

[0126] The difference between this embodiment and embodiment 1 is that the diameter of the ANF added to the intermediate functional layer is 5.2 nm, and the rest is the same as embodiment 1.

[0127] Example 11

[0128] The difference between this embodiment and embodiment 1 is that the diameter of the ANF added to the intermediate functional layer is 9.8 nm, and the rest is the same as embodiment 1.

[0129] Example 12

[0130] The difference between this embodiment and embodiment 1 is that the diameter of the ANF added to the intermediate functional layer is 35.3 nm, and the rest is the same as embodiment 1.

[0131] Example 13

[0132] The difference between this embodiment and embodiment 1 is that the diameter of the ANF added to the intermediate functional layer is 49.7 nm, and the rest is the same as embodiment 1.

[0133] Example 14

[0134] The difference between this embodiment and embodiment 1 is that the hot rolling temperature of the composite diaphragm is 60° C., and the rest is the same as embodiment 1.

[0135] Example 15

[0136] The difference between this embodiment and embodiment 1 is that the hot rolling temperature of the composite diaphragm is 80° C., and the other aspects are the same as those of embodiment 1.

[0137] Example 16

[0138] The difference between this embodiment and embodiment 1 is that the hot rolling temperature of the composite diaphragm is 120° C., and the rest is the same as embodiment 1.

[0139] Example 17

[0140] The difference between this embodiment and embodiment 1 is that the thickness of the first surface layer / the intermediate functional layer / the second surface layer is 2 μm / 8 μm / 2 μm, and the rest is the same as embodiment 1.

[0141] Example 18

[0142] The difference between this embodiment and embodiment 1 is that the thickness of the first surface layer / the intermediate functional layer / the second surface layer is 4 μm / 4 μm / 4 μm, and the rest is the same as embodiment 1.

[0143] Comparative Example 1

[0144] The difference between this embodiment and embodiment 1 is that HAP nanoparticles are not added to the intermediate functional layer. Other aspects are the same as those of embodiment 1.

[0145] Comparative Example 2

[0146] The difference between this embodiment and embodiment 1 is that the D 50 The thickness is 1.2 μm, and the others are the same as those in Example 1.

[0147] The specific results are as follows:

[0148] Examples 1 to 5 are investigations of the mass percentage of HFP in the intermediate functional layer of the present application. See Table 1 for specific results.

[0149] Table 1

[0150]

[0151] As shown in Table 1, comparing Examples 1 to 5, as the HAP content increases from 5% to 15%, the contact angle decreases significantly, while the ionic conductivity, discharge capacity retention, and thermal stability increase significantly. Optimal performance is achieved at 15% HAP by mass. As the HAP content continues to increase, all separator properties decline. Furthermore, the separators exhibited no short-circuiting after 1000 hours of cycling. This phenomenon can be explained by the fact that an appropriate amount of HAP strengthens the separator's polar surface and forms a stable three-dimensional network with the ANF, effectively enhancing lyophilicity and providing ion migration pathways, thereby improving the kinetic performance of lithium batteries. Furthermore, an appropriate amount of ANF enhances the separator's thermal stability. However, excessive HAP can lead to particle agglomeration, resulting in decreased porosity, reduced electrolyte retention, and network disruption, leading to reduced wettability, ionic conductivity, and thermal stability.

[0152] Examples 1, 6 to 9 are the HFP size (D 50 ), and the specific results are shown in Table 2.

[0153] Table 2

[0154]

[0155] As shown in Table 2, by comparing Examples 1, 6 to 9, it can be seen that when the HAP particle size increases from ~10nm to ~32nm, the overall performance of the separator improves. However, when the particle size further increases to 123nm, the performance decreases significantly, which is manifested by uneven pore structure, poor electrolyte wettability, decreased ionic conductivity, lower membrane rupture temperature, and decreased lithium battery kinetic performance. In addition, the above separator is 2 At constant current density, no short circuiting occurred after 1000 hours of cycling. This phenomenon can be explained by the fact that small-particle HAP has a larger specific surface area, allowing it to be evenly dispersed in the ANF, optimizing the microporous structure and forming a continuous ion migration path. It also creates a denser and more uniform inorganic skeleton reinforcement point, which helps support the fiber network, inhibit thermal shrinkage, and effectively delay the collapse of the separator structure under high temperature environments. However, large-particle HAP has difficulty embedding into the network and tends to agglomerate and block pores, forming "structural islands." This leads to interrupted conductive paths, interfacial stress concentration, and performance degradation. Furthermore, it is more likely to cause local delamination under thermal stress, reducing the overall resistance to thermal deformation, resulting in increased shrinkage and lower membrane rupture temperature at high temperatures.

[0156] Examples 1, 10 to 13 are investigations of the ANF diameter in the intermediate functional layer in this application. See Table 3 for specific results.

[0157] Table 3

[0158]

[0159] As shown in Table 3, by comparing Examples 1, 10 to 13, it can be seen that as the ANF diameter increases from 5 nm to 20 nm, the pore structure of the membrane becomes more uniform, the mechanical support is better, and the liquid absorption, ion conductivity and thermal stability are simultaneously improved; when the diameter exceeds 35 nm, the pore size is uneven due to the excessive fiber thickness, the liquid absorption rate and cycle performance decrease, the shrinkage rate increases at 180 ° C, and the membrane rupture temperature decreases. In addition, the above-mentioned membrane is at 1 mA / cm 2 Under constant current density, no short circuit phenomenon occurred after 1000h of cycling, indicating that the above-mentioned diaphragm can effectively inhibit dendrite growth. This phenomenon can be explained by the fact that fine-diameter ANF helps to build a densely connected nanofiber network, providing uniform pores to improve the membrane wettability and lithium ion conductivity. There are more physical entanglements and hydrogen bonds between fine fibers, which can also share stress and resist structural collapse at high temperatures, thereby effectively reducing thermal shrinkage and increasing membrane rupture temperature; and when the fiber diameter increases, the rigidity is enhanced, the interface bonding area is reduced, the network is loose, the pores are coarse, the electrolyte is not easy to penetrate evenly, and local stress is concentrated, and the dynamic performance decreases accordingly. In addition, the connection between large-diameter fibers is loose, the mechanical support is weakened, and it is easier to produce pore closure or local melting at high temperatures, which reduces the overall thermal stability of the diaphragm.

[0160] Examples 1, 14 to 16 are investigations of the hot pressing temperature in the intermediate functional layer in this application. For specific results, see Table 4.

[0161] Table 4

[0162]

[0163] As shown in Table 4, by comparing Examples 1, 14 to 16, it can be seen that increasing the hot pressing temperature can promote the adhesion of the interface between the functional layer and the intermediate functional layer, improve the overall stability, liquid absorption performance and capacity retention rate, and achieve the best performance at 100°C; after continuing to rise to 120°C, the overall kinetic and thermodynamic properties decrease slightly. This phenomenon is attributed to the fact that the appropriate hot pressing temperature can promote the adhesion of the nanofiber layer and the intermediate functional layer, forming a tight interface structure, which is beneficial to improving the overall structural stability of the diaphragm and the integrity of the ion channel. The hot pressing effect is optimal at 100°C and will not significantly damage the nanopore structure. Above this temperature, some fibers will adhere or fuse, resulting in pore collapse, restricted electrolyte pathways, and a decrease in liquid absorption rate. At the same time, the pore structure becomes smaller, resulting in poor electrolyte wetting and a slight increase in the contact angle; overheating may also cause uneven surface stress, resulting in increased thermal shrinkage and a slight decrease in structural stability.

[0164] Examples 1, 17 to 18 are investigations of the thickness of the first surface layer, the intermediate functional layer and the second surface layer in this application. For specific results, see Table 5.

[0165] Table 5

[0166]

[0167] As shown in Table 5, comparing Examples 1, 17, and 18, it can be seen that when the thickness of the middle functional layer is moderate (3:6:3 μm), the membrane performance is optimal, as reflected in high wettability, low thermal shrinkage, high ionic conductivity, and thermal stability. If the middle layer is too thick or too thin (such as 2:8:2 μm or 4:4:4 μm), the overall performance of the membrane is significantly reduced. This phenomenon can be explained by the fact that the middle HAP-ANF composite layer of appropriate thickness can form an effective ion screening channel and physical barrier. In addition, the dense and uniform inorganic skeleton reinforcement points formed help support the fiber network and inhibit thermal shrinkage in high temperature environments. If the middle layer is too thin, the entire membrane loses its rigid support and thermal barrier capabilities. If it is too thick, it will increase the resistance to ion migration and cause an uneven interface, thus affecting lithium ion dynamics and safety.

[0168] Table 6

[0169]

[0170] As shown in Table 6, by comparing Example 1 with Comparative Examples 1 to 2, it can be seen that when no HAP is added to the intermediate functional layer or when the HAP is added in micron-sized form, the contact angle of the separator increases significantly, the ionic conductivity decreases significantly, and thus the battery kinetic performance decreases significantly. In addition, the thermal stability of the separator also decreases significantly, even at 1 mA / cm 2 At constant current density, a short circuit occurred after 1000 hours of cycling. This phenomenon was attributed to the fact that, in the absence of nano-HAP, the PAN / ANF matrix struggled to form a dense and continuous composite network structure, resulting in unstable pore structure and poor lyophilicity, which reduced electrolyte wetting and retention. Furthermore, the poor dispersion of micron-sized HAP particles made it easy for them to agglomerate within the membrane, causing pore blockage, interruption of charge transfer paths, and localized stress concentration. This accelerated dendrite penetration and separator thermal deformation during cycling, ultimately leading to battery short-circuit failure.

[0171] In summary, this application can effectively adjust the lyophilicity, thermal stability, and ion transport performance of the diaphragm through systematic regulation of the HAP mass percentage, HAP particle size, ANF diameter, hot pressing temperature, and the thickness ratio of the three-layer gradient structure. This provides a clear structural design path and process parameter guidance for constructing a gradient functional diaphragm with high safety, high ion conductivity, and excellent dendrite suppression ability.

[0172] The above description and the accompanying drawings sufficiently illustrate the embodiments of the present disclosure to enable those skilled in the art to practice them. Other embodiments may include structural and other changes. The embodiments represent only possible variations. Unless expressly required, individual components and functions are optional, and the order of operations may vary. Portions and features of some embodiments may be included in or replace portions and features of other embodiments. The embodiments of the present disclosure are not limited to the structures described above and shown in the accompanying drawings, and various modifications and changes may be made without departing from the scope thereof. The scope of the present disclosure is limited only by the appended claims.

Claims

1. A composite diaphragm, characterized in that: include: a first surface layer composed of polyacrylonitrile nanofibers; The middle functional layer is composed of aramid nanofiber and hydroxyapatite composite; a second surface layer composed of polyacrylonitrile nanofibers; Wherein, the thickness of the first surface layer and the thickness of the second surface layer are 4-10 μm, and the thickness of the intermediate functional layer is 2-6 μm.

2. The composite diaphragm according to claim 1, characterized in that The mass ratio of the aramid nanofiber to the hydroxyapatite in the intermediate functional layer is 19 to 3:1, wherein the diameter of the aramid nanofiber is 5 to 50 nm, and the median particle size D of the hydroxyapatite is 100 nm. 50 It is 10 to 130 nm.

3. The composite diaphragm according to any one of claims 1, characterized in that The contact angle of the composite membrane in the liquid electrolyte is ≤15°, and the membrane rupture temperature of the composite membrane is ≥200°C.

4. The composite diaphragm according to claim 1, characterized in that The dimensional shrinkage of the composite diaphragm after thermal shrinkage measurement is less than 4.3%.

5. The composite diaphragm according to claim 1, characterized in that The ionic conductivity of the composite membrane is greater than 0.39 mS / cm, and the ionic conductivity is greater than 0.39 mS / cm at a constant current density of 1 mA / cm 2 Under the condition of , the cycle life of the composite membrane exceeds 1000h.

6. The composite diaphragm according to any one of claims 1 to 5, characterized in that: The composite diaphragm is a three-layer gradient structure, which is deposited in sequence through a continuous electrostatic spinning process and formed through a calendering process.

7. A method for preparing a composite diaphragm according to any one of claims 1 to 6, characterized in that: include: Step 101: preparing a spinning solution, and depositing the spinning solution on a rotary drum current collector through an electrospinning process to form a first surface layer composed of polyacrylonitrile nanofibers; Step 102: preparing a polymer mixed dispersion of aramid nanofibers and hydroxyapatite, and depositing the polymer mixed dispersion on the first surface layer through an electrospinning process to obtain an intermediate functional layer; Step 103: using the spinning solution in step 101, depositing the spinning solution on the intermediate functional layer through an electrospinning process to form a second surface layer composed of polyacrylonitrile nanofibers, thereby obtaining a composite diaphragm with a three-layer gradient structure; Step 104: The composite diaphragm is subjected to roller pressing treatment at a hot pressing temperature of 60 to 120° C. and a roller pressing pressure of 0.5 to 4.0 MPa, followed by vacuum drying at 60° C. for 12 to 48 hours to remove residual solvent.

8. The preparation method according to claim 7, characterized in that In step 101, a spinning solution is prepared by dissolving 5 to 15 wt% of polyacrylonitrile (PAN) in N,N-dimethylformamide (DMF) and stirring for 12 hours to form a uniform spinning solution. The voltage in the electrospinning process was 5–20 kV, the collection distance was 5–15 cm, and the flow rate was 0.2–1.5 mL / h; The thickness of the first surface layer composed of polyacrylonitrile nanofibers is 2 to 4 μm.

9. The preparation method according to claim 7, characterized in that In step 102, the method of preparing the aramid nanofiber-hydroxyapatite polymer mixed dispersion is as follows: dissolving the aramid fiber in a dimethyl sulfoxide / water system at a concentration of 2 to 15 wt%, adding potassium hydroxide and chloroform to obtain the aramid nanofiber dispersion, and then adding the median particle size D 50 The hydroxyapatite is 10 to 130 nm in diameter and is finally uniformly dispersed by ultrasonication for 10 to 60 minutes. The volume ratio of dimethyl sulfoxide to water is 8:2, the diameter of the aramid nanofiber is 5 to 50 nm, and the mass ratio of the aramid nanofiber to the hydroxyapatite is 85:

15. The voltage in the electrospinning process was 5–25 kV, the collection distance was 10–25 cm, and the flow rate was 0.2–1.0 mL / h; The thickness of the intermediate functional layer is 5-7 μm, wherein the mass percentage of hydroxyapatite is 15%.

10. A lithium ion battery, characterized in that: The invention comprises a positive electrode sheet, a negative electrode sheet and the composite separator according to any one of claims 1 to 6, wherein the capacity retention rate of the lithium-ion battery is greater than 85% when discharged at a 10C rate.

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