TWI640434B - Composite separator and preparation method thereof - Google Patents

Abstract

A composite separator comprising a layer of nanofibers and two layers of porous polymer electrolyte membrane layers. The nanofiber layer comprises a plurality of organic nanofibers and a plurality of nanoparticles dispersed on the organic nanofibers, the nanoparticles being inorganic oxide nanocrystals functionalized by a nitrogen-based quaternary phosphonium salt Particles. The two layers of porous polymer electrolyte membrane layers are respectively formed on opposite sides of the nanofiber layer, and the porous polymer electrolyte membrane layers comprise a difluoroethylene-hexafluoropropylene copolymer. The invention also provides a preparation method of a composite separator. The lithium ion battery made of the composite isolation membrane has better discharge capacity and charge and discharge stability in high power and high temperature environments, and is favorable for storage and power supply equipment in high power and high temperature operation environments.

Description

Composite separator and preparation method thereof

The invention relates to a composite separator, in particular to a composite separator comprising a nanofiber layer and a porous polymer electrolyte membrane layer and a preparation method thereof.

Lithium-ion batteries have high open circuit voltage, high energy density, fast charge/discharge rate, long charge/discharge cycle life, and self-discharge (self- Discharging) Low and light weight characteristics, so it is often used as a storage and power supply system for consumer electronics and transportation equipment.

The structure of a conventional lithium ion battery includes: a positive electrode made of a lithium transition metal oxide, a negative electrode made of lithium metal or carbon, and a separator which absorbs a lithium salt electrolyte solution. The ideal separator should have low ionic resistance, good mechanical and thermal stability, and chemical resistance. Generally, commercially available microporous polyolefin films such as polyethylene (PE), polypropylene (PP) or PP/PE/PP three-layer composite film and the like have been widely used. However, these commercially available separators have poor hydrophilicity and thermal stability, and are also challenging in electrical performance and stability in high power (high current density) and high temperature environments.

Accordingly, it is an object of the present invention to provide a composite separator that overcomes the above-discussed shortcomings of the prior art.

Thus, the composite separator of the present invention comprises a layer of nanofiber layer and two layers of porous polymer electrolyte membrane layers. The nanofiber layer comprises a plurality of organic nanofibers and a plurality of nanoparticles dispersed in the organic nanofibers, the nano particles being nitrogen-based quaternary phosphonium salts (nitrogen) Family quaternary onium salt) functionalized inorganic oxide nanoparticles. The two layers of porous polymer electrolyte membrane layers are respectively formed on opposite sides of the nanofiber layer, and the porous polymer electrolyte membrane layers include a difluoroethylene-hexafluoropropylene copolymer (PVDF-HFP).

Therefore, another object of the present invention is to provide a method for preparing a composite separator comprising the following steps:

(a) dispersing a mixture in a first solvent to obtain an electrospinning solution, the mixture comprising an organic polymer and a plurality of inorganic oxide nanoparticle functionalized by a nitrogen-based quaternary phosphonium salt;

(b) electrospinning the electrospinning solution to form a plurality of organic nanofibers dispersed with the inorganic oxide nanoparticles;

(c) drying the organic nanofibers to obtain a layer of nanofibers;

(d) dissolving the difluoroethylene-hexafluoropropylene copolymer in a second solvent to obtain a slurry;

(e) coating the slurry on a substrate, and dipping the nanofiber layer into the slurry on the substrate to form a polymer electrolyte film pre-forming layer on each of the two sides of the nanofiber layer ;and

(f) drying the polymer electrolyte membrane pre-formed layer to form a porous polymer electrolyte membrane layer, and obtaining the composite separator.

The effect of the invention is that the lithium ion battery made of the composite separator has better discharge capacity and charge and discharge stability in a high power and high temperature environment.

The contents of the present invention will be described in detail below:

Preferably, the nitrogen quaternary phosphonium salt is a chloride having a 3-(trimethylammonium)propyl moiety, a chloride having a tetraphenylphosphonium moiety or having Iodide of the tetramethylguanidine moiety.

Preferably, the inorganic oxide nanoparticles are selected from the group consisting of cerium oxide nanoparticles, alumina nanoparticles, graphene oxide nanoparticles or a combination thereof.

Preferably, the nanoparticles have a particle size ranging from 5 to 500 nm. More preferably, the nanoparticles have a particle size ranging from 20 to 100 nm.

Preferably, the nanofiber layer is a nanofiber nonwoven fabric layer, and the material of the organic nanofiber is selected from the group consisting of polyethylene terephthalate (PET) and poly Fluoroethylene (PVDF), polyvinyl alcohol (PVA), polyacrylonitrile (PAN), polyimine (PI), polyvinyl chloride (PVC), polyethylene (PE), polypropylene (PP) or combinations thereof . In a specific embodiment of the invention, the material of the organic nanofiber is polyethylene terephthalate.

Preferably, the nanofiber layer has a thickness ranging from 10 to 100 μm. More preferably, the nanofiber layer has a thickness ranging from 10 to 60 μm. In a particular embodiment of the invention, the nanofiber layer has a thickness of about 60 μm.

Preferably, each layer of the porous polymer electrolyte membrane layer has a thickness ranging from 10 to 300 μm. More preferably, each layer of the porous polymer electrolyte membrane layer has a thickness ranging from 10 to 50 μm. In a specific embodiment of the invention, each layer of the porous polymer electrolyte membrane layer has a thickness of about 20 μm.

Preferably, the surface of each layer of the porous polymer electrolyte membrane layer is formed with a plurality of pores having a pore diameter ranging from 0.5 to 1.8 μm.

Preferably, in the nanofiber layer, the weight ratio of the nano particles to the organic nano fibers ranges from 5:100 to 10:100. In a particular embodiment of the invention, the weight ratio of the nanoparticles to the organic nanofibers is 10:100.

Preferably, the organic nanofibers have a diameter ranging from 500 to 900 nm.

Preferably, in the step (b), the voltage of the electrospinning is 5.0 to 30.0 kV. In a specific embodiment of the invention, the electrospinning voltage is 9.0 kV.

Preferably, in the step (b), the electrospinning solution is subjected to electrospinning at a flow rate ranging from 0.045 to 0.065 mL/min. In a specific embodiment of the invention, the electrospinning solution is subjected to electrospinning at a flow rate of 0.06 mL/min.

Preferably, in the step (b), the spinning distance of the electrospinning solution for electrospinning ranges from 10 to 20 cm. In a specific embodiment of the invention, the electrospinning solution is electrospun at a spinning distance of 15 cm.

Preferably, in the step (e), the coating thickness of the slurry ranges from 50 to 300 μm. More preferably, the slurry has a coating thickness in the range of 100 to 200 μm. In a particular embodiment of the invention, the slurry has a coating thickness of 200 μm.

Before the present invention is described in detail, it should be noted that in the following description, similar elements are denoted by the same reference numerals.

The present invention will be further illustrated by the following examples, but it should be understood that this embodiment is intended to be illustrative only and not to be construed as limiting.

[ Example] Composite insulation membrane S _E

Referring to Figure 1, an embodiment of the composite separator of the present invention comprises a layer of polyethylene terephthalate (PET) nanofiber layer 1 and two layers of porous difluoroethylene-hexafluoropropylene copolymer (PVDF-HFP). ) Electrolyte film layer 2.

The PET nanofiber layer 1 includes a plurality of PET nanofibers 11 and a plurality of nanoparticles 12 dispersed in the PET nanofibers 11. In this embodiment, the nanoparticles 12 are chloride-functionalized cerium oxide nanoparticles granulated with 3-(trimethylammonium)propyl.

The two-layer porous PVDF-HFP electrolyte membrane layer 2 is formed on the opposite faces of the PET nanofiber layer 1, respectively.

Referring to Figure 2, this embodiment can be made by a method comprising the following steps:

(a) 18 g of PET was dissolved in a mixed solvent of 82 g of trifluoroacetic acid (TFA) and dichloromethane (DCM) (weight ratio of 3:2), and then added 1.8 g of 3-(trimethylammonium) a propyl chloride-functionalized cerium oxide nanoparticle (Q-SiO ₂ , purchased from Sigma-Aldrich, having an average particle size of 40 nm), stirred at room temperature until uniformly dispersed to obtain an electrospinning Solution.

(b) The electrospinning solution was filled into a syringe, and a needle of an electrospinning machine (purchased from MECC, Japan, model SNAN series) was injected at a flow rate of 0.06 mL/min using a syringe pump (inner diameter of 0.26). Mm), for electrospinning (DC voltage 9.0 kV, 15 cm from the collection), the PET fibers were collected on stainless steel with a collection area of approximately 675 cm ² (collection time approx. 80 min).

(c) drying the collected PET fibers at room temperature, to obtain a PET nonwoven fabric layer nanofiber L _E.

(d) Add 18 g of PVDF-HFP (HFP content of 6 mol%) to 87.4 g of dimethylformamide (DMF)/acetone mixture (weight ratio 1:3) and 4.6 g of N -methylpyrrolidine The ketone (NMP) was stirred in a water bath at 80 ° C for 2 hours to dissolve to obtain a slurry.

(e) The slurry was cooled at room temperature while stirring slowly to remove bubbles in the slurry. The cooled slurry is coated on a glass substrate (coating thickness: 200 μm) by a reciprocating coater, and the PET nanofiber non-woven layer L _{E is} immersed in the slurry on the glass substrate to A PVDF-HFP electrolyte membrane pre-formed layer was formed on each of the sides of the PET nanofiber nonwoven fabric layer L _E.

(f) drying the PVDF-HFP electrolyte membrane pre-formed layer at room temperature to form a porous PVDF-HFP electrolyte membrane layer, and rolling to obtain a sandwich structure PVDF-HFP/PET/PVDF-HFP composite separator S _E (thickness of about 100 μm).

[ Application example] Lithium-ion battery LIB _E

Positive electrode tab - LiH ₂ PO ₄ , FeC ₂ O ₄ · 2H ₂ O, glucose, tartaric acid are mixed in a weight ratio of 45:45:5:5, and then acetone is added for ball milling, followed by sintering in the presence of hydrogen to obtain LFP ( LiFePO ₄ ) sample. 100 wt% of LFP sample, additional 1 wt% carbon black (water as dispersion, purchased from Cabot, USA, average particle size 10 nm, surface area 1500 m ² /g) and 5 wt% glucose (aqueous solution) Spray drying was carried out, followed by sintering at 600 ° C for 5 h under an argon atmosphere to obtain an LFP/C composite powder. 0.4 g of conductive carbon black Super P ^® (purchased from TIMCAL, Switzerland, average particle size 30 nm, surface area 50 m ² /g), 5.4 g PVDF (7 wt% NMP solution), 3 g LFP/C composite powder (equivalent to conductive carbon black Super P ^® , PVDF, LFP/C composite powder weight ratio of about 10:10:80), added to 4 g of NMP, stirred until evenly mixed, after removing bubbles, It was coated on an aluminum foil having a thickness of 20 μm by a coater (wet film thickness of 200 μm), and dried in an oven at 60 ° C to remove the solvent, followed by baking in an oven at 120 ° C for 2 hours. The residual solvent and moisture are removed, and then rolled and flattened by a roller to a thickness of about 40 to 45 μm (the compaction density is about 1.8 to 2.1 g/cm ³ ), and finally cut into a diameter. 13 mm round pole piece.

Negative electrode tab - a lithium foil with a diameter of 16 mm and a thickness of 200 μm.

Isolation film and electrolyte solution—The composite separator S _E obtained in the above embodiment was cut into a circle having a diameter of 18 mm, and immersed in 1 M LiPF ₆ of ethylene carbonate (EC) and carbonic acid. A solution of diethyl carbonate (DEC) (volume ratio of EC and DEC is 1:1).

In the argon operating environment, the positive electrode tab, the negative electrode tab, the separator coated with the electrolyte solution, and the upper cover, the lower cover, the reed, and the gasket are packaged into a button type lithium ion battery LIB _E .

[ Comparative application example] Lithium-ion battery LIB _CE

The process of the comparative application example is similar to the above-mentioned application example, except that a polyethylene (PE) separator S _CE (available from Asahi Kasei Co., Ltd., Japan, thickness: 16 μm) is used instead of the composite separator obtained in the above embodiment. S _E , packaged into a button type lithium ion battery LIB _CE .

[ Observation of the structure of the isolation membrane]

The PET nanofiber non-woven fabric layer L _E and the composite separator S _E of the above embodiment and the PE separator S _{CE of the} above comparative application were respectively observed by a scanning electron microscope, and the results are shown in Fig. 3(A) to 3, respectively. B), Figure 4 (A) ~ 4 (B) and Figure 5.

As can be seen from Figures 3(A) to 3(B), the PET nanofiber nonwoven fabric layer L _{E of the} embodiment comprises a plurality of PET nanofibers having a diameter of about 500 to 900 nm, and on the PET nanofibers. A plurality of chloride-functionalized cerium oxide nanoparticles having a 3-(trimethylammonium)propyl group are dispersed [as indicated by the arrow in Fig. 3(B)].

As can be seen from Fig. 4(A), the surface of the composite separator S _{E of the} embodiment is formed with a plurality of pores having an average pore diameter of about 1.1 μm. As can be seen in FIG. 4 (B), compound of formula S _E cross-section an embodiment of the insulating film which is a sandwich structure of the display, and a thickness of approximately 100 μm; wherein a thickness of the PET non-woven layer L _E about nanofiber At 60 μm, each porous PVDF-HFP electrolyte membrane layer has a thickness of about 20 μm.

As can be seen from Fig. 5, the surface of the PE separator S _{CE of the} comparative application example is formed with a plurality of pores having an average pore diameter of about 80 nm.

[ Contact angle measurement]

The PE separator S _CE in the above comparative application, the PET nanofiber non-fabric layer L _{PET in} which the nanoparticles are not dispersed, and the PET nanofiber in which the nanoparticles are dispersed in the above embodiment are respectively measured by deionized water droplets. The contact angle of the deionized water droplets on the surface of the fabric layer L _E changes with time, and the result is shown in FIG.

As can be seen from Figure 6, at the time of 1 s (initial stage), L droplets on the surface of the _PET PE and PET separator S _CE nanofiber nonwoven layers are not dispersed nano-particles of Comparative Application Example contacting The angles are about 123° and 131°, respectively, and the droplet contact angle on the surface of the PET nanofiber nonwoven layer L _{E in which} the nanoparticles are dispersed is about 120°; after 10 minutes, in the comparative application. The contact angle of the droplets on the surface of the PE separator S _CE and the PET nanofiber non-fabric layer L _PET on which the nanoparticles are not dispersed is about 114°, and in the embodiment, the PET nanoparticles in which the nanoparticles are dispersed are used. The droplet contact angle on the surface of the fibrous nonwoven fabric layer L _E was about 110°, indicating that the surface of the PET nanofiber nonwoven fabric layer L _{E in which} the nanoparticles were dispersed was relatively hydrophilic.

Further, the droplet contact angle of the PE separator S _{CE of the} above comparative application example and the composite separator S _E of the above-described embodiment was measured with time by the DMSO electrolyte droplets containing 1 M LiClO ₄ .

The results showed that at 1 s (initial stage), the droplet contact angle on the surface of the PE separator S _CE of the comparative application example was 78.5°, and the liquid on the surface of the composite separator S _E of the example The droplet contact angle was 49.5°; after 10 minutes, the droplet contact angle on the surface of the PE separator S _CE of the comparative application example was 76.2°, and the liquid on the surface of the composite separator S _E of the example The dropping contact angle was 2.9°, which showed that the hydrophilicity of the surface of the composite separator S _E of the example was remarkably high, and the electrolyte droplets could infiltrate into the composite separator S _{E of the} example over time.

[ Thermal stability measurement]

The PE separator S _{CE of the} above comparative application, the PET nanofiber non-fabric layer L _PET not dispersed with the nanoparticles, and the nanoparticle dispersed in the above embodiment were respectively measured by a thermogravimetric analysis method. The weight of the PET nanofiber non-woven fabric layer L _E changes with temperature, and the result is shown in FIG.

It can be seen from Fig. 7 that in the process of heating from 200 ° C to 600 ° C, the weight loss rates of the PE separator S _{CE of the} comparative application example and the PET nanofiber non-woven layer L _PET of the non-dispersed nanoparticles are respectively about The weight loss rate of the PET nanofiber non-woven fabric layer L _E of the example was only about 72.7%, which shows the heat of the PET nanofiber non-woven layer L _E of the embodiment in which the nanoparticle was dispersed. The stability is relatively good.

Further, the PE separator S _CE of the above comparative application of the same size and the composite separator S _{E of the} above-described embodiment were heated at 150 ° C for 30 min, and the area shrinkage after heating was measured.

The results show that the area shrinkage of the PE separator S _{CE of the} comparative application is 92% after heating, and it changes from white to transparent, and the area shrinkage of the composite separator S _{E of the} embodiment after heating is only 8%. And maintaining white opacity, it is shown that the thermal stability of the composite separator S _E of the embodiment is remarkably better.

[ Lithium-ion battery electrical measurement]

The battery test equipment (purchased from Jiayou Technology, model BAT-750B) was used to measure the discharge specific capacity of the lithium ion battery LIB _{CE of the} above comparative application and the lithium ion battery LIB _E of the above embodiment. , Q _sp ), test conditions: ambient temperature is 55 ° C, 1 C = 170 mAh / g, and the discharge capacity retention (CR) is calculated by the following mathematical formula 1, charge and discharge conditions and results are shown in Table 1 below .

The resistance (R _b and R _ct ) were measured by AC impedance spectroscopy, and the total resistance difference (ΔR _t ) was calculated by the following mathematical formula 2. The charge and discharge conditions and results are shown in Table 1 below. [Math 1] CR = [(Q _sp ) _cycle / _before (Q _sp ) _cycle ] × 100% [Math 2] ΔR _t = (R _b + R _ct ) _{after the 200th cycle} - (R _b + R _ct ) After _{the 100th cycle} [Table 1] <TABLE border="1"borderColor="#000000"width="85%"><TBODY><tr><td></td><td> Charging 0.2 C discharge 10C cycle 3 times</td><td> charge 0.1C discharge 0.1C cycle 60 times</td><td> charge 1C discharge 1C cycle 200 times</td><td> charge 1C discharge 1C cycle 200 times </td></tr><tr><td>Q_sp (mAh/g) </td><td> CR </td><td> CR </td><td >ΔR_t (Ω) </td></tr><tr><td>LIB_CE</td><td> 115±0.8 </td><td> 92.0% </td><td> 64.2% </td><td> +6 </td></tr><tr><td>LIB_E</td><td> 132±0.3 </td><td> 96.3% </td><td> 84.6% </td><td> -18 </td></tr></TBODY></TABLE>

As can be seen from Table 1, in the environment of 55 ° C, compared to the lithium ion battery LIB _{CE of the} comparative application:

(1) When the lithium ion battery LIB _{E of the} embodiment is discharged at a high speed (high current density) (10C), its discharge gram capacity is about 15% higher and is more stable.

(2) whether a low speed (0.1C) or a higher speed charge and discharge (1C) charge and discharge, after repeated charge and discharge cycles, the discharge capacity retention embodiments the lithium ion battery according to the LIB _E are high.

(3) after 200 charge-discharge cycles, the lithium ion battery LIB _E embodiment embodiment can still effectively suppress an increase in resistance of the battery.

In summary, the composite separator of the present invention can improve the hydrophilicity and thermal stability of the nanoparticle in the nanofiber layer, and can provide the lithium ion battery thereof at high speed (10C). The high discharge (55 ° C) environment in the preferred discharge capacity and charge and discharge stability (preferred discharge capacity maintenance rate), is conducive to the application of high-power and high-temperature operating environment in the storage and power supply equipment, so it is true The object of the invention can be achieved.

However, the above is only the embodiment of the present invention, and the scope of the invention is not limited thereto, and the simple equivalent change and functionalization according to the scope of the patent application and the patent specification of the present invention are It is still within the scope of the invention patent.

1‧‧‧PET nanofiber layer
11‧‧‧PET nanofiber
12‧‧‧Nano granules
2‧‧‧PVDF-HFP electrolyte membrane

Other features and effects of the present invention will be apparent from the following description of the drawings, wherein: FIG. 1 is a perspective, cross-sectional view showing an embodiment of the composite separator of the present invention; [FIG. 2] is A flow chart of an embodiment of a method for preparing a composite separator; [Fig. 3] is a (A) PET nanofiber nonwoven layer and (B) dispersed with 3-(3) of the composite separator of the present invention; Scanning electron micrograph of PET nanofiber of chloride-functionalized cerium oxide nanoparticle of trimethylammonium) propyl; [Fig. 4] (A) of the embodiment of the composite separator of the present invention Top view and (B) cross-sectional scanning electron micrograph; [Fig. 5] is a scanning electron micrograph of a PE separator of a comparative application example; [Fig. 6] is a PE separator in the comparative application example, Contact angle of the deionized water droplets on the surface of the PET nanofiber non-woven layer in which the nanoparticles are dispersed and the PET nanofiber non-woven layer in which the nanoparticles are dispersed; and [Fig. 7] The PE separator of the comparative application example, P which is not dispersed with nanoparticles Thermogravimetric graph of the ET nanofiber nonwoven layer and the PET nanofiber nonwoven layer in which the nanoparticles are dispersed in this example.

Claims (10)

A composite separator comprising: a layer of nanofibers comprising a plurality of organic nanofibers and a plurality of nano particles dispersed in the organic nanofibers, the nano particles being nitrogen-based quaternary phosphonium salts a functionalized inorganic oxide nanoparticle; a two-layer porous polymer electrolyte membrane layer formed on two opposite faces of the nanofiber layer, respectively, the porous polymer electrolyte membrane layer comprising difluoroethylene-hexafluorocarbon Propylene copolymer. The composite separator according to claim 1, wherein the nitrogen quaternary phosphonium salt is a chloride having a 3-(trimethylammonium)propyl moiety, a chloride having a tetraphenylphosphonium moiety or having a tetra The iodide of the methyl hydrazine moiety. The composite separator according to claim 1, wherein the inorganic oxide nanoparticles are selected from the group consisting of cerium oxide nanoparticles, alumina nanoparticles, graphene oxide nanoparticles, or a combination thereof. The composite separator according to claim 1, wherein the nano particles have a particle size ranging from 5 to 500 nm. The composite separator according to claim 1, wherein the nanofiber layer is a nanofiber non-woven layer, and the material of the organic nanofiber is selected from the group consisting of polyethylene terephthalate and poly Difluoroethylene, polyvinyl alcohol, polyacrylonitrile, polyimine, polyvinyl chloride, polyethylene, polypropylene or a combination thereof. The composite separator according to claim 1, wherein the nanofiber layer has a thickness ranging from 10 to 100 μm. The composite separator according to claim 1, wherein each of the porous polymer electrolyte membrane layers has a thickness ranging from 10 to 300 μm. A method for preparing a composite separator, comprising the steps of: (a) dispersing a mixture in a first solvent to obtain an electrospinning solution, the mixture comprising an organic polymer and a plurality of nitrogen-based quaternary phosphonium salt functional groups (i) electrospinning the electrospun solution to form a plurality of organic nanofibers dispersed with the inorganic oxide nanoparticles; (c) drying the organic a nanofiber to obtain a nanofiber layer; (d) dissolving the difluoroethylene-hexafluoropropylene copolymer in a second solvent to obtain a slurry; (e) coating the slurry on a substrate And immersing the nanofiber layer in the slurry on the substrate to form a polymer electrolyte film pre-forming layer on each of the two sides of the nanofiber layer; and (f) drying the polymer electrolyte film to form The layer is formed to form a porous polymer electrolyte membrane layer, and the composite separator is obtained. The production method according to claim 8, wherein the nitrogen quaternary phosphonium salt is a chloride having a 3-(trimethylammonium)propyl moiety, a chloride having a tetraphenylphosphonium moiety or having a tetramethyl group. Part of the iodide. The production method according to claim 8, wherein the inorganic oxide nanoparticles are selected from the group consisting of cerium oxide nanoparticles, alumina nanoparticles, graphene oxide nanoparticles or a combination thereof.