WO2016165633A1

WO2016165633A1 - Polymer composite membrane and preparation method thereof, gel electrolyte and lithium ion battery having the same

Info

Publication number: WO2016165633A1
Application number: PCT/CN2016/079326
Authority: WO
Inventors: Lina JIN; Jialing HU; Jun Shan; Long He
Original assignee: Byd Company Limited
Priority date: 2015-04-14
Filing date: 2016-04-14
Publication date: 2016-10-20
Also published as: CN106159173A; CN106159173B

Abstract

A method of preparing a polymer composite membrane is provided, and includes: providing a separator, providing a spinning solution comprising a mixed solvent and a spinning polymer dissolved in the mixed solvent, the mixed solvent comprising a first solvent having a boiling point ranging from about 150 ℃ to about 170 ℃, and a second solvent having a boiling point ranging from about 175 ℃ to about 220 ℃, a mass ratio of the first solvent and the second solvent ranging from about 7: 3 to about 3: 7; and electrostatic spinning on the separator with the spinning solution to obtain the polymer composite membrane having a separator and a fiber layer coated on the separator. A polymer composite membrane obtained from the method, a gel electrolyte including the polymer composite membrane and a lithium battery including the gel electrolyte are also provided.

Description

POLYMER COMPOSITE MEMBRANE AND PREPARATION METHOD THEREOF, GEL ELECTROLYTE AND LITHIUM ION BATTERY HAVING THE SAME

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to and benefits of Chinese Patent Application No. 201510175539.5, filed with the State Intellectual Property Office (SIPO) of the People's Republic of China on April 14, 2015, the entire content of which is hereby incorporated by reference.

FIELD

Embodiments of the present disclosure generally relate to the field of lithium ion battery, and more particularly to a method of preparing a polymer composite membrane, a polymer composite membrane obtained from the method, a gel electrolyte including the polymer composite membrane, and a lithium battery including the gel electrolyte.

BACKGROUND

With the development of portable electronic devices and an application of a lithium ion battery in an electric vehicle, requirements for performances of the lithium ion battery are increasingly high in the related art, such as energy density, power density and etc. In addition, a requirement for a safety performance of the lithium ion battery is also more rigorous.

However, a polymer composite membrane obtained from a method according to prior arts has a fiber layer with poor adhesiveness, which is inconvenient to operations. And the fiber layer cannot be effectively adhered to an electrode sheet and polyethylene, which may result in a poor safety performance of the lithium ion battery. Meanwhile, the method in the prior arts generally uses acetone as a solvent, which may cause a poor construction safety.

SUMMARY

In viewing thereof, the present disclosure is aimed to provide a polymer composite membrane, thus to solve at least one problem existing in the art, for example, to improve the adhesiveness of the fiber layer of the polymer composite membrane in the related art.

According to embodiments of a first aspect of the present disclosure, a method of preparing a polymer composite membrane is provided. The method for preparing a polymer composite membrane includes:

providing a separator,

providing a spinning solution including a mixed solvent and a spinning polymer dissolved in the mixed solvent, the mixed solvent including a first solvent having a boiling point ranging from about 150 ℃ to about 170 ℃, a second solvent having a boiling point ranging from about 175 ℃ to about 220 ℃, and a mass ratio of the first solvent and the second solvent ranging from about 7: 3 to about 3: 7； and

electrostatic spinning on the separator with the spinning solution to obtain the polymer composite membrane having the separator and a fiber layer coated on the separator.

In some embodiments, the spinning polymer is at least one selected from a group consisting of polyvinylidene fluoride, and poly (vinylidene fluoride-hexafluoropropylene) ；

the first solvent is at least one selected from a group consisting of N, N-dimethylformamide, and N, N-dimethylacetamide； and

the second solvent is at least one selected from a group consisting of N-methylpyrrolidone, dimethylsulfoxide, tetramethylurea, and triethyl phosphate.

In some embodiments, the spinning polymer is poly (vinylidene fluoride-hexafluoropropylene) with a hexafluoropropylene copolymerized unit ranging from about 5 wt％to about 15 wt％.

In some embodiments, the spinning polymer in the spinning solution has a concentration ranging from about 5 wt％to about 30 wt％.

In some embodiments, the first solvent has a flash point ranging from about 58 ℃ to about 70 ℃, and the second solvent has a flash point ranging from about 65 ℃ to about 117 ℃.

In some embodiments, the separator is a polyolefin separator.

In some embodiments, the electrostatic spinning is performed in a manner of needle spinning under a condition including: a flow velocity ranging from about 0.3 mL/h to about 5 mL/h, a spinning temperature ranging from about 25 ℃ to about 70 ℃, a spinning humidity ranging from about 10％to about 60％, and a voltage ranging from about 5 kV to about 20 kV.

In some embodiments, the electrostatic spinning is performed under the condition further including: a receiving distance ranging from about 6 cm to about 15 cm, a moving velocity of a needle ranging from about 6 mm/sec to about 7 mm/sec, an inner diameter of the needle of about 0.46 mm, the voltage ranging from about 7 kV to about 15 kV, and a rotating speed of a metal drum ranging from about 100 rpm to about 300 rpm.

In some embodiments, the electrostatic spinning is performed in a manner of needle-free spinning under a condition including: a spinning temperature ranging from about 25 ℃ to about 70 ℃, a spinning humidity ranging from about 10％to about 60％, a moving velocity of a liquid pool ranging from 0 to about 2000 mm/sec, a moving velocity of a substrate ranging from 0 to about 20000 mm/min, an anode voltage ranging from about 0 to about 150 kV, a cathode voltage ranging from about -50 kV to about 0 kV, and a voltage difference ranging from about 20 kV to about 100 kV.

In some embodiments, both surfaces of the separator are provided with the fiber layer with the electrostatic spinning.

In some embodiments, the fiber layer has a porosity ranging from about 70％to about 98％, and a peel strength ranging from about 0.03 kN/m to about 0.50 kN/m.

In some embodiments, the fiber layer has a liquid absorbency ranging from about 200％to about 1300％.

In some embodiments, the fiber layer has an areal density ranging from about 0.2 g/m² to about 3 g/m².

In some embodiments, the fiber layer has a fiber diameter ranging from about 100 nm to about 1500 nm, and a thickness ranging from about 0.5 μm to about 5 μm.

In some embodiments, the method further includes:

providing a slurry including an inorganic particle, a third solvent and a binder；

coating the slurry on a surface of the separator to obtain an inorganic particle layer on the surface of the separator； and

electrostatic spinning on the separator coated with the inorganic particle layer with the spinning solution.

In some embodiments, a mass ratio of the inorganic particle, the third solvent and the binder is about 9-24: 67-500: 1.

In some embodiments, the inorganic particle is at least one selected from a group consisting of Al₂O₃, SiO₂, BaSO₄, TiO₂, CuO, MgO, LiAlO₂, ZrO₂, carbon nanotube, BN, SiC, Si₃N₄, WC, BC, AlN, Fe₂O₃, BaTiO₃, MoS₂, α–V₂O₅, PbTiO₃, TiB₂, CaSiO₃, molecular sieve, clay, and kaolin；

the binder is at least one selected from a group consisting of polyvinylidene fluoride, poly (vinylidene fluoride -hexafluoropropylene) , polymethyl methacrylate, polyacrylonitrile, polyimide, polyvinyl pyrrolidone, polyethylene oxide, polyvinyl alcohol, sodium carboxymethyl cellulose, and styrene-butadiene rubber； and

the third solvent is at least one selected from a group consisting of N-methylpyrrolidone, N, N-dimethylformamide, N, N-dimethylacetamide, toluene, methylene chloride, chloroform, deionized water, and ethanol.

In some embodiments, the inorganic particle has a diameter ranging from about 50 nm to about 3 μm.

In some embodiments, the inorganic particle layer has a thickness ranging from about 0.5 μm to about 3 μm.

In some embodiments, both surfaces of the separator are provided with the inorganic particle layer.

In some embodiments, the electrostatic spinning is performed on both surfaces of the separator coated with the inorganic particle layer respectively.

According to embodiments of a second aspect of the present disclosure, a polymer composite membrane obtained by a method according to the present disclosure is provided. The polymer composite membrane includes a separator, and a fiber layer on the separator, having a peel strength more than 0.03 kN/m.

In some embodiments, the fiber layer has a porosity more than 70％.

In some embodiments, the fiber layer has a porosity ranging from about 70％to about 98％, and the peel strength ranging from about 0.03 kN/m to about 0.50 kN/m.

In some embodiments, the fiber layer has a liquid absorbency ranging about 200％to about 1300％.

In some embodiments, the fiber layer is made of polyvinylidene fluoride, poly (vinylidene fluoride-hexafluoropropylene) or a combination thereof.

In some embodiments, the fiber layer is made of poly (vinylidene fluoride-hexafluoropropylene) with a hexafluoropropylene copolymerized unit ranging from about 5 wt％to about 15 wt％.

In some embodiments, both surfaces of the separator are provided with the fiber layer.

In some embodiments, the separator is a polyolefin separator.

In some embodiments, the polymer composite membrane further includes an inorganic particle layer between the separator and the fiber layer； in which the inorganic particle layer includes:

an inorganic particle selected from a group consisting of Al₂O₃, SiO₂, BaSO₄, TiO₂, CuO, MgO, LiAlO₂, ZrO₂, carbon nanotube, BN, SiC, Si₃N₄, WC, BC, AlN, Fe₂O₃, BaTiO₃, MoS₂, α–V₂O₅, PbTiO₃, TiB₂, CaSiO₃, molecular sieve, clay, kaolin and combinations thereof； and

a binder selected from a group consisting of polyvinylidene fluoride, poly (vinylidene fluoride -hexafluoropropylene) , polymethyl methacrylate, polyacrylonitrile, polyimide, polyvinyl pyrrolidone, polyethylene oxide, polyvinyl alcohol, sodium carboxymethyl cellulose, styrene-butadiene rubber, and combinations thereof.

In some embodiments, a mass ratio of the inorganic particle and the binder is about 9-24: 1.

According to embodiments of a third aspect of the present disclosure, a gel electrolyte is provided. The gel electrolyte includes a polymer composite membrane according to the present disclosure or obtainable by the method according to the present disclosure, and an electrolyte adsorbed to the polymer composite membrane.

In some embodiments, the electrolyte includes:

a lithium salt being any one selected from a group consisting of lithium hexafluorophosphate, lithium perchlorate, and lithium tetrafluoroborate, having a concentration ranging from about 0.8 mol/L to about 1.5 mol/L； and

a fourth solvent selected from a group consisting of ethylene carbonate, propylene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, vinylene carbonate, and combinations thereof.

According to embodiments of a fourth aspect of the present disclosure, a lithium ion battery is provided. The lithium ion battery includes an anode, a cathode, and a gel electrolyte according to the present disclosure, disposed between the anode and the cathode.

Through a large number of experiments by the inventor of the present disclosure, it was found that, at least one selected from a group consisting of acetone, N, N-dimethyformamide (DMF) , and N, N-dimethylacetamide (DMAc) is usually used as the solvent according to methods referring to electrostatic spinning in the prior arts. The fiber layer obtained by above methods in the prior arts shows a poor adhesiveness, and thus is very hard to be adhered. Filaments inside the fiber layer also show the poor adhesiveness, and are very easy to fall off, thus cannot ensure a good adhesion of the whole fiber layer. Especially while used in the lithium ion battery, the fiber layer may crack easily, and it is very difficult to effectively bond an electrode material with the separator. Only using N-methylpyrrolidone with a high boiling point as the solvent will result in a fiber layer with a far low porosity, thereby a polymer composite membrane thus obtained may show a greatly reduced ionic conductivity, thus affecting electrical properties of the lithium ion battery.

But in the method according to the present disclosure, by using a lower boiling point solvent (the second solvent) in conjunction with a higher boiling point solvent (the first solvent) , a fiber layer in the polymer composite membrane obtained shows a good adhesiveness, especially between filaments inside the fiber layer, thus effectively bonding the electrodes with the separator to form an integrated body. Moreover, the fiber layer of the polymer composite membrane of the present disclosure has a very high porosity to effectively ensure a good ionic conductivity thereof. The lithium ion battery including the polymer composite membrane according to the present disclosure shows greatly improved rate discharge ability, cycling performance and safety performance, and a long using life.

Additional aspects and advantages of the embodiments of the present disclosure will be given in part in the following descriptions, become apparent in part from the following descriptions, or be learned from the practice of the embodiments of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects and advantages of the disclosure will become apparent and more readily appreciated from the following descriptions taken in conjunction with the drawings, in which:

Fig. 1 is a schematic view of a polymer composite membrane according to EMBODIMENT 1 of the present disclosure；

Fig. 2 is a scanning electron microscope (SEM) image of a polymer composite membrane according to EMBODIMENT 2 of the present disclosure with a magnification of 5000 times；

Fig. 3 is a scanning electron microscope (SEM) image of a polymer composite membrane according to EMBODIMENT 3 of the present disclosure with a magnification of 5000 times；

Fig. 4 is a scanning electron microscope (SEM) image of a polymer composite membrane according to EMBODIMENT 6 of the present disclosure with a magnification of 5000 times；

Fig. 5 is a scanning electron microscope (SEM) image of a polymer composite membrane according to EMBODIMENT 7 of the present disclosure with a magnification of 5000 times；

Fig. 6 is a scanning electron microscope (SEM) image of a polymer composite membrane according to COMPARATIVE EMBODIMENT 1 of the present disclosure with a magnification of 5000 times；

Fig. 7 is a scanning electron microscope (SEM) image of a polymer composite membrane according to COMPARATIVE EMBODIMENT 2 of the present disclosure with a magnification of 5000 times； and

Fig. 8 is a scanning electron microscope (SEM) image of a polymer composite membrane according to COMPARATIVE EMBODIMENT 3 of the present disclosure with a magnification of 5000 times.

DETAILED DESCRIPTION

Reference will be made in detail to embodiments of the present disclosure. The embodiments described herein with reference to the accompany drawings are explanatory and illustrative, which are used to generally understand the present disclosure. The embodiments shall not be construed to limit the present disclosure.

For the purpose of the present description and of the following claims, the definitions of the numerical ranges always include the extremes unless otherwise specified.

In addition, terms such as “first” and “second” are used herein for purposes of description and are not intended to indicate or imply relative importance or significance.

A polymer composite membrane according to the present disclosure includes a separator, and a fiber layer on the separator, and the fiber layer has a peel strength more than 0.03 kN/m. In some embodiments, the fiber layer has a peel strength ranging from about 0.03 kN/m to about 0.50 kN/m.

The separator may be a polyolefin separator known to those skilled in the related art. The polyolefin separator is a general separator for a lithium ion battery, and includes a polypropylene (PP) separator, a polyethylene (PE) separator, a three-layer separator with a structure of PE/PP/PE and etc.

According to the present disclosure, the fiber layer may be made of any conventional material. In some embodiments, the fiber layer may be made of polyvinylidene fluoride (PVDF) , poly (vinylidene fluoride-hexafluoropropylene) (P (VDF-HFP) ) or a combination thereof. In one embodiment, the fiber layer is made of poly (vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP) ) with a hexafluoropropylene (HFP) copolymerized unit ranging from about 5 wt％to about 15 wt％. In this embodiment, the fiber layer has a better adhesive strength, and may be benefit to further improve an adhesive property to electrodes while applied in a battery.

According to the present disclosure, in some embodiments, the fiber layer has a porosity more than 70％. In one embodiment, the fiber layer has a porosity ranging from about 70％to about 98％. In this embodiment, the fiber layer is of a good adhesiveness, thus may effectively bond the electrodes with the separator to form an integrated body. Moreover, the fiber layer has a very high porosity, thus may effectively ensure a good ionic conductivity thereof.

According to the present disclosure, in one embodiment, the fiber layer has a liquid absorbency ranging about 200％to about 1300％, which is benefit to further improve electrical properties of the lithium ion battery.

In some embodiments, the fiber layer has an areal density ranging from about 0.2 g/m² to about 3 g/m². The areal density refers to a weight of material coated on a separator substrate per unit area, which indicates a coating amount on the separator substrate. In addition, the porosity of the fiber layer may be calculated by the areal density of the fiber layer, a thickness of the fiber layer, and a density of a polymer itself of the fiber layer. In the present disclosure, the fiber layer having above areal density may effectively ensure a good electrical conductivity, not affect migration of lithium ions, shows a better adhesiveness, and is benefit to improve the safety performance of the lithium ion battery.

According to the present disclosure, there is no special limit to a fiber diameter of the fiber layer. In some embodiments, the fiber layer has a fiber diameter ranging from about 100 nm to about 1500 nm.In some embodiments, the fiber layer has a thickness ranging from about 0.5 μm to about 5 μm. According to the present disclosure, the fiber layer having above thickness may effectively bond the electrodes with the separator, thus to improve the cycling performance of the lithium ion battery.

According to the present disclosure, the fiber layer may be provided on just one surface of the separator, and also may be provided on both surfaces of the separator. In some embodiments, both surfaces of the separator are provided with the fiber layer.

In some embodiments, the polymer composite membrane further includes an inorganic particle layer between the separator and the fiber layer. In some embodiments, the inorganic particle layer includes an inorganic particle and a binder. The inorganic particle may be at least one selected from a group consisting of Al₂O₃ (including α-, β-, γ-type) , SiO₂, BaSO₄, TiO₂ (rutile or anatase) , CuO, MgO, LiAlO₂, ZrO₂, carbon nanotube (CNT) , BN, SiC, Si₃N₄, WC, BC, AlN, Fe₂O₃, BaTiO₃, MoS₂, α–V₂O₅, PbTiO₃, TiB₂, CaSiO₃, ZSM-5 molecular sieve, clay, and kaolin. In some embodiments, the inorganic particle is Al₂O₃, SiO₂, or BaSO₄.

In some embodiments of the present disclosure, when using Al₂O₃, especially α-Al₂O₃ as the inorganic particle, the inorganic particle layer shows an excellent heat isolation and electrochemical stability, which may improve heat stabilities of the separator and the lithium ion battery, and be benefit to improve the safety performance of the lithium ion battery. In some embodiments, when using SiO₂ or BaSO₄ as the inorganic particle , the heat stability of the lithium ion battery may be improved.

Through a large number of experiments by the inventor of the present disclosure, it was found that, with the inorganic particle layer provided on the surface of the separator, an affinity between the fiber layer and the inorganic particle layer is more than that between the fiber layer and the separator； and a surface of the inorganic particle layer is not even and configured with a plurality of particle projections, which may provide more fiber attaching points to the fiber layer and be benefit for improving the adhesive strength of the fiber layer on the inorganic particle layer. The fiber layer may better bond the electrodes with the separator to form an integrated body. Meanwhile, the inorganic particle layer may provide an overall dimensional stability and a high anti-heat-shrink performance to the polymer composite membrane. In case that the fiber layer has an excellent adhesiveness as mentioned above, and is cooperated with the inorganic particle layer, a lithium ion battery thus obtained may has an improved rigidity, a not easily deformed electrode sheet during a cycling process, and a high security. Besides, the inorganic particle layer may improve the strength of the separator and the affinity with the electrolyte.

In some embodiments, the inorganic particle has a diameter ranging from about 50 nm to about 3 μm. It was found by the inventor that, the inorganic particle having above diameter, may not only avoid a porous polyolefin separator being clogged by the inorganic particle, but also is benefit for adjusting the thickness of the separator, thus may effectively improve a mechanical strength and a thermal stability of the separator, , and the safety performance of the lithium ion battery.

The binder is used for adhering the inorganic particle, and is adhered to the separator. In the present disclosure, the binder may be at least one selected from a group consisting of polyvinylidene fluoride (PVDF) , poly (vinylidene fluoride-hexafluoropropylene) (P (VDF-HFP) ) , polymethyl methacrylate (PMMA) , polyacrylonitrile (PAN) , polyimide (PI) , polyvinyl pyrrolidone (PVP) , polyethylene oxide (PEO) , polyvinyl alcohol (PVA) , sodium carboxymethyl cellulose (Na-CMC) , and styrene-butadiene rubber (SBR) , which can be dissolved in an organic solvent or deionized water.

A thickness of the inorganic particle layer may be varied in a large range and may be adjusted according to actual requirements by those skilled in the related art. In some embodiments, the inorganic particle layer has a thickness ranging from about 0.5 μm to about 3 μm. With the inorganic particle layer having above thickness, the mechanic strength and the anti-heat-shrink of the separator may be effectively improved on one hand； on the other hand, a migration speed of the lithium ion in the polymer composite membrane may be ensured, which is benefit to ensure the electrical performances of the lithium ion battery.

As mentioned above, in some embodiments, both surfaces of the separator are provided with the fiber layer. Similarly, both surfaces of the separator are provided with the inorganic particle layer. Hereon, each fiber layer is disposed on each inorganic particle layer respectively.

A method of preparing the polymer composite membrane of the present disclosure is also provided, and the method includes:

providing a separator,

providing a spinning solution including a mixed solvent and a spinning polymer dissolved in the mixed solvent, the mixed solvent including a first solvent having a boiling point ranging from about 150 ℃ to about 170 ℃, and a second solvent having a boiling point ranging from about 175 ℃ to about 220 ℃, a mass ratio of the first solvent and the second solvent ranging from about 7: 3 to about 3: 7； and

electrostatic spinning on the separator with the spinning solution to obtain the polymer composite membrane having a separator and a fiber layer coated on the separator.

According to the present disclosure, the separator is provided at first as a substrate, then subsequential operations may be performed on a surface thereof. As mentioned above, the separator may be a polyolefin separator known to those skilled in the related art.

According to the present disclosure, a spinning solution is then provided. The spinning solution includes a mixed solvent and a spinning polymer dissolved in the mixed solvent. The mixed solvent includes a first solvent having a boiling point ranging from about 150 ℃ to about 170 ℃, and a second solvent having a boiling point ranging from about 175 ℃ to about 220 ℃, and a mass ratio of the first solvent and the second solvent ranges from about 7: 3 to about 3: 7.

In specific, any of the first and second solvents fulfilling above mentioned conditions may be mixed together according to the above mentioned mass ratio to obtain the mixed solvent as required. Then, the spinning polymer may be sufficiently dissolved into the mixed solvent with magnetic stirring in a water bath to obtain the spinning solution. Usually, the step of dissolving the spinning polymer into the mixed solvent may be performed at a temperature ranging from about 25 ℃ to about 60 ℃.

According to the present disclosure, in some embodiments, the spinning polymer is at least one selected from a group consisting of polyvinylidene fluoride (PVDF) , and poly (vinylidene fluoride-hexafluoropropylene) (P (VDF-HFP) ) .

In some embodiments, the spinning polymer is poly (vinylidene fluoride-hexafluoropropylene) (P (VDF-HFP) ) with a hexafluoropropylene (HFP) copolymerized unit ranging from about 5 wt％to about 15 wt％. It was found by the inventor that, on the basis that poly (vinylidene fluoride-hexafluoropropylene) (P (VDF-HFP) ) has a content of the hexafluoropropylene copolymerized unit within the above mentioned range, , an affinity of the spinning polymer may be improved with the increase of the content of the hexafluoropropylene copolymerized unit, which is further benefit to improve the adhesiveness of the obtained fiber layer.

It should be understood that, both the second and the first solvents may be any solvent that may dissolve the spinning polymer. Under the condition that, the spinning polymer is at least one selected from a group consisting of polyvinylidene fluoride, and poly (vinylidene fluoride-hexafluoropropylene) , the first solvent may be at least one selected from a group consisting of N, N-dimethylformamide, and N, N-dimethylacetamide. The second solvent may be at least one selected from a group consisting of N-methylpyrrolidone, dimethylsulfoxide, tetramethylurea, and triethyl phosphate.

In order to improve the construction safety, in some embodiments, the first solvent has a flash point ranging from about 58 ℃ to about 70 ℃, and the second solvent has a flash point ranging from about 65 ℃ to about 117 ℃. The first and second solvents fulfilling the above mentioned conditions show a high construction safety property, and low requirements for construction environment, which may reduce construction costs to a certain extent.

According to the present disclosure, the spinning solution is used for preparing the fiber layer via electrostatic spinning in subsequential steps. Thereby, the spinning polymer in the spinning solution may have a concentration used in electrostatic spinning. In some embodiments, the spinning polymer of the spinning solution has a concentration ranging from about 5 wt％to about 30 wt％. In some embodiments, the spinning polymer of the spinning solution has a concentration ranging from about 10 wt％to about 25 wt％. When a relative molecular weight of a polymer and the other conditions are constant, the concentration of the spinning solution is a crucial factor which may affect entanglement of molecular chains in the spinning solution.

According to different concentrations and molecular chain morphology, a polymer solution usually present in three types, i.e. a dilute solution, a sub-concentrated solution and a concentrated solution. Molecular chains are separated from each other and uniformly disposed in the dilute solution, and overlapped, interspersed and tangled with each other with the increase of the concentration of the polymer solution. A critical concentration of the dilute solution differing from the sub-concentrated solution is defined as a contacting concentration under which the molecular chains start to contact and then overlap with each other. A critical concentration of the sub-concentrated solution differing from the concentrated solution is defined as a tangling concentration under which the molecular chains start to intersperse and tangle with each other. According to the present disclosure, a filament forming may be effectively ensured when the spinning solution has above concentration, and which may be even more better with the increase of the concentration of the spinning solution in the above mentioned concentration range and thus increase of the entanglement of the molecular chains.

According to the present disclosure, a next step is electrostatic spinning on the separator with the spinning solution. The method of electrostatic spinning is known to those skilled in the related art. For example, the electrostatic spinning may be performed on the separator under the adjusted conditions as follow: receiving distance, temperature, humidity, moving velocity of a needle, voltage, flow velocity, rotating speed of a metal drum, and etc. After the step of electrostatic spinning, the separator may be removed and then be optionally hot-rolled under a condition including a temperature of about 25 ℃ to about 60 ℃ and a pressure of about 0.5 MPa to about 5 MPa, finally may be blast-dried for 24 h under a temperature of about 50 ℃.

Through above steps of the method of the present disclosure, the polymer composite membrane according to the present disclosure may be obtained. In the method according to the present disclosure, via using the mixed solvent, the fiber layer obtained by electrostatic spinning shows not only a high porosity, but also a good adhesiveness, especially a good adhesion between filaments inside the fiber layer, which may be benefit to effectively bond the electrodes with the separator to form an integrated body.

Usually, the electrostatic spinning includes two manners: needle spinning and needle-free spinning. In the present disclosure, both manners are applicable.

According to the present disclosure, while the flow velocity varies within the above mentioned range, the fiber layer with a suitable fiber diameter may be ensured to be obtained, and the needle may be effectively avoid to be clogged, thus a spinning process is ensured to be smoothly performed. Especially in the premise of the use of the mixed solvent according to the present disclosure, in combination with the flow velocity to be controled within the above mentioned range, a fiber layer with an excellent porosity and adhesiveness may be obtained. With the spinning temperature and humidity both varying within the above mentioned range, in conjunction with the use of the fore-mentioned mixed solvent, a drying process after a smooth filamentation from the fiber filaments obtained by spinning may be ensured, which may prevent the fiber filaments from being conglutinated to avoid a reduction of the porosity, and further to avoid a reduction of the adhesiveness of the fiber layer. With the voltage varying within the above mentioned range, the spinning solution may be effectively activated to form a jet, so as to form an effective stretching in an electric field and to obtain the fiber filaments with a suitable diameter, which may ensure the forms of the fiber filaments, and may be also benefit to improve the porosity and the adhesiveness of the fiber layer.

In some embodiments, the electrostatic spinning is performed under a condition further including: a receiving distance ranging from about 6 cm to about 15 cm, a moving velocity of a needle ranging from about 6 mm/sec to about 7 mm/sec, a flow velocity ranging from about 0.3 mL/h to about 5 mL/h, a spinning temperature ranging from about 25 ℃ to about 70 ℃, an inner diameter of the needle of about 0.46 mm, a spinning humidity ranging from about 10％to about 60％, and a voltage ranging from about 5 kV to about 20 kV. In some embodiments, the condition for the needle spinning further includes: a voltage ranging from about 7 kV to about 15 kV, and a rotating speed of a metal drum ranging from about 100 rpm to about 300 rpm.

Through a large number of experiments, it was found by the inventor of the present disclosure that, in the premise of the use of the mixed solvent described above, in conjunction with the electrostatic spinning process under the above mentioned conditions, a volatilization speed of the mixed solvent may be well matched with a formation speed of the fiber filaments, thus a fiber layer with a good appearance, a high adhesiveness, and a better adhesiveness between the filaments therein may be obtained, and the porosity of the fiber layer is more than 95％.

According to embodiments of the present disclosure, the fiber diameter and the thickness of the fiber layer obtained by the method of the present disclosure may be varied in a large range, which may be adjusted according to specific process conditions. In some embodiments, the fiber layer has a fiber diameter ranging from about 100 nm to about 1500 nm, and a thickness ranging from about 0.5 μm to about 5 μm.

In some embodiments, the fiber layer obtained from the above mentioned method has an areal density ranging from about 0.2 g/m² to about 3 g/m².

According to the present disclosure, electrostatic spinning may be performed on just one surface of the separator, or may be performed on both surfaces of the separator. In some embodiments, both surfaces of the separator are provided with the fiber layer via electrostatic spinning. In the embodiment, electrostatic spinning is performed on one surface of the separator followed by optionally hot-rolling and drying, then the same operations are performed on the other surface of the separator.

According to the present disclosure, the method further includes:

coating the slurry on a surface of the separator to obtain an inorganic particle layer； and

electrostatic spinning on a surface of the inorganic particle layer with the spinning solution.

According to the present disclosure, the inorganic particle is at least one selected from a group consisting of Al₂O₃ (including α-, β-, γ-type) , SiO₂, BaSO₄, TiO₂, CuO, MgO, LiAlO₂, ZrO₂, carbon nanotube (CNT) , BN, SiC, Si₃N₄, WC, BC, AlN, Fe₂O₃, BaTiO₃, MoS₂, α–V₂O₅, PbTiO₃, TiB₂, CaSiO₃, molecular sieve, clay, and kaolin. In some embodiments, the inorganic particle is Al₂O₃, SiO₂, or BaSO₄. In some embodiments, the inorganic particle has the diameter ranging from about 50 nm to about 3 μm.

The binder is used for adhering the inorganic particle in the formed inorganic particle layer, and providing an adhesion to the surface of the separator. In some embodiments, the binder is at least one selected from a group consisting of polyvinylidene fluoride (PVDF) , poly (vinylidene fluoride-hexafluoropropylene) (P (VDF-HFP) ) , polymethyl methacrylate (PMMA) , polyacrylonitrile (PAN) , polyimide (PI) , polyvinyl pyrrolidone (PVP) , polyethylene oxide (PEO) , polyvinyl alcohol (PVA) , sodium carboxymethyl cellulose (Na-CMC) , and styrene-butadiene rubber (SBR) .

The third solvent is used for providing a certain fluidity to the slurry, which is facilitate to construction, and the third solvent may be removed during the subsequent process to form the inorganic particle layer including only the binder and the inorganic particle. In some embodiments, the third solvent is at least one selected from a group consisting of N-methylpyrrolidone (NMP) , N, N-dimethylformamide (DMF) , N, N-dimethylacetamide (DMAc) , toluene, methylene chloride, chloroform, deionized water, and ethanol.

According to the present disclosure, while the separator is provided with the inorganic particle layer at first, then the fiber layer may be firmly adhered to the inorganic particle layer, which may improve the peeling strength of the polymer composite membrane on one hand, on the other hand, the inorganic particle between the separator and the fiber layer may provide an excellent anti-heat-shrink performance to the polymer composite membrane. In addition, according to the present disclosure, in the premise of the use of mixed solvent described above to perform the electrostatic spinning, a formation speed of the fiber filaments and a drying speed during the electrostatic spinning may also be adjusted to match with each other, which may further improve the adhesion between the fiber layer and the inorganic particle layer.

In the present disclosure, the thickness of the inorganic particle layer may be varied in a large range. In some embodiments, the inorganic particle layer has a thickness ranging from about 0.5 μm to about 3 μm.

As mentioned above, both surfaces of the separator may be provided with the fiber layer. Similarly, both surfaces of the separator may be provided with the inorganic particle layer at first, then the surface of each inorganic particle layer may be provided with the fiber layer respectively.

According to the present disclosure, the polymer composite membrane obtained from the method according to the present disclosure is also provided. The technical features and correspond technical effects of the polymer composite membrane are as mentioned above, which will not be described herein.

According to the present disclosure, a gel electrolyte is also provided, the gel electrolyte includes a polymer composite membrane according to the present disclosure or obtainable by the method according to the present disclosure, and an electrolyte adsorbed to the polymer composite membrane.

Similar to the methods known in prior arts, the gel electrolyte may be formed by swelling after absorbing the electrolyte with the polymer membrane according to the present disclosure.

In specific, the polymer composite membrane may be cut to be of a suitable size, then vacuum dried for 12 h under a temperature of about 50 ℃, following quickly transferred to a glove box with a content of oxygen less than 3 ppm and a content of water less than 3 ppm. Then the polymer composite membrane may be immersed into the electrolyte for 24 h.

In some embodiments, the electrolyte is known to those skilled in the related art, and includes a lithium salt and a fourth solvent. The lithium salt may be any suitable ionizable lithium salt, for example, selected from the group consisting of lithium hexafluorophosphate (LiPF₆) , lithium perchlorate (LiClO₄) , and lthium tetrafluoroborate (LiBF₄) . The fourth solvent is selected from the group consisting of ethylene carbonate (EC) , propylene carbonate (PC) , dimethyl carbonate (DMC) , methyl ethyl carbonate (EMC) , diethyl carbonate (DEC) , vinylene carbonate (VC) , or combinations thereof. In some embodiments, the lithium salt in the electrolyte has a concentration ranging from about 0.8 mol/L to about 1.5 mol/L.

A lithium ion battery is also provided, and includes an anode, a cathode, and a gel electrolyte according to the present disclosure, and the gel electrolyte is disposed between the anode and the cathode.

A method of preparing the lithium ion battery mentioned above is similar to any conventional method of preparing a polymer lithium ion battery in the related art. In some embodiments, the method of preparing the lithium ion battery includes steps of: separating a positive electrode sheet from a negative electrode sheet via the polymer composite membrane in a drying room, wounding the whole assembly to form a core, and injecting the electrolyte into the core, sealing to obtain the lithium ion battery.

The positive electrode and the negative electrode are known to those skilled in the related art, which will not be described herein.

Hereinafter, the present disclosure will be described in detail with reference to the following embodiments.

EMBODIMENT 1

The present embodiment provides a polymer composite membrane and a preparation method thereof, a gel electrolyte and a lithium ion battery.

(1) Preparation of a multiple-layer membrane

SiO₂ particles with an average diameter of 1 μm, a binder PVDF and solvent NMP were mixed according to a mass ratio of 19: 1: 200 to obtain a slurry. Both surfaces of a PE separator having a thickness of 11 μm were coated with the slurry. After stoving, a three-layer membrane A1 with a thickness of 15 μm was obtained, in which the thickness of the PE separator was 11 μm, and each surface of the PE separator was coated with a SiO₂ particle layer having a thickness of 2 μm.

(2) Preparation of a spinning solution

Solvent NMP and solvent DMF were evenly mixed according to a mass ratio of 7: 3 to obtain a mixed solvent, and then PVDF was added and sufficiently dissolved in the mixed solvent through magnetic stirring in a water bath at 50 ℃ to obtain a PVDF spinning solution B1 with a PVDF concentration of 15 wt％.

(3) Preparation of a polymer composite membrane

A metal drum used as a receiving device was wrapped with the three-layer membrane A1 obtained from step (1) , then electrostatic spinning was performed on a surface of three-layer membrane A1 with the PVDF spinning solution B1 in a manner of needle spinning. The condition for the needle spinning was adjusted as follow: a receiving distance of 12 cm, a temperature of 25 ℃, a humidity of 50％, an inner diameter of the needle of 0.46 mm, a moving velocity of the needle of 6.6 mm/sec, a voltage of 10 kV, a flow velocity of 0.3 mL/h, and a rotating speed of the metal drum of 200 rpm. After the process of electrostatic spinning, the three-layer membrane A1 was removed and blast-dried for 24 h under a temperature of 50 ℃. Then the electrostatic spinning was performed on the other surface of the three-layer membrane A1 under the same condition. After which, the three-layer membrane A1 was removed and blast-dried for 24 h under a temperature of 50 ℃, so as to obtain a polymer composite membrane C1, of which each surface was formed with a fiber layer having a thickness of 1.2 μm respectively. The fiber diameter of the fiber layer of the polymer composite membrane C1 in a SEM image was measured and recorded by a transmission electron microscope (TEM) Macrography Application, thus the average fiber diameter of the fiber layer was calculated to be 176 nm. The areal density of the fiber layer was 0.22 g/m² calculated by a gravimetric method.

The structure of the polymer composite membrane C1 was shown in Fig. 1. As can be seen, the polymer composite membrane C1 included sequentially laminated the fiber layer, the inorganic particle layer, the separator, the inorganic particle layer and the fiber layer.

(4) Preparation of a lithium ion battery

The polymer composite membrane C1 was cut to be of a suitable size and vacuum dried for 12 h under a temperature of 50 ℃. Then a LiCoO₂ positive electrode sheet and a graphite negative electrode sheet dried and separated from each other with the cut polymer composite membrane C1 were wound to obtain a core in a drying room. After casing, injecting, hot-pressing for 4 h under a temperature of 85 ℃ and a pressure of 1 MPa, vacuum sealing, and etc for conventional production process of a soft pack battery, a SL281643-typed soft pack lithium ion battery was obtained, recorded as E1.

EMBODIMENT 2

(1) Preparation of a multiple-layer membrane

Al₂O₃ particles with an average diameter of 615 nm, a binder PEO and solvent deionized water were mixed according to a mass ratio of 9: 1: 67 to obtain a slurry. Both surfaces of a PE separator having a thickness of 11 μm were coated with the slurry. After stoving, a three-layer membrane A2 with a thickness of 15 μm was obtained, in which the thickness of the PE separator was 11 μm, and each surface of the PE separator was coated with an Al₂O₃ particle layer having a thickness of 2 μm.

(2) Preparation of a spinning solution

Solvent NMP and solvent DMF were evenly mixed according to a mass ratio of 5: 5 to obtain a mixed solvent, and then P (VDF-HFP) with HFP copolymerized unit of 5 wt％was added and sufficiently dissolved in the mixed solvent through magnetic stirring in a water bath at 50 ℃ to obtain a P (VDF-HFP) spinning solution B2 with a P (VDF-HFP) concentration of 20 wt％.

(3) Preparation of a polymer composite membrane

A metal drum used as a receiving device was wrapped with the three-layer membrane A2 obtained from step (1) , then electrostatic spinning was performed on a surface of three-layer membrane A2 with the P (VDF-HFP) spinning solution B2 in a manner of needle spinning. The condition for the needle spinning was adjusted as follow: a receiving distance of 12 cm, a temperature of 25 ℃, a humidity of 50％, an inner diameter of the needle of 0.46 mm, a moving velocity of the needle of 6.6 mm/sec, a voltage of 10 kV, a flow velocity of 1 mL/h, and a rotating speed of the metal drum of 200 rpm. After the process of electrostatic spinning, the three-layer membrane A2 was removed and blast-dried for 24 h under a temperature of 50 ℃. Then the electrostatic spinning was performed on the other surface of the three-layer membrane A2 under the same condition. After which, the three-layer membrane A2 was removed and blast-dried for 24 h under a temperature of 50 ℃ so as to obtain a polymer composite membrane C2, of which each surface was formed with a fiber layer having a thickness of 1.3 μm respectively. Fig. 2 showed a surface microstructure of the polymer composite membrane C2. The fiber diameter of the fiber layer of the polymer composite membrane C2 in a SEM image was measured and recorded by a transmission electron microscope (TEM) Macrography Application, thus the average fiber diameter of the fiber layer was calculated to be 723 nm. The areal density of the fiber layer is 1.24 g/m² calculated by a gravimetric method.

(4) Preparation of a lithium ion battery

The polymer composite membrane C2 was cut to be of a suitable size and vacuum dried for 12 h under a temperature of 50 ℃. Then a LiCoO₂ positive electrode sheet and a graphite negative electrode sheet dried and separated from each other with the cut polymer composite membrane C2were wound to obtain a core in a drying room. After casing, injecting, hot-pressing for 4 h under a temperature of 85 ℃ and a pressure of 1 MPa, vacuum sealing, and etc for conventional production process of a soft pack battery, a SL281643-typed soft pack lithium ion battery was obtained, recorded as E2.

EMBODIMENT 3

The present embodiment provides a polymer composite membrane and a preparation method of thereof, a gel electrolyte and a lithium ion battery.

(1) Preparation of a multiple-layer membrane

BaSO₄ particles with an average diameter of 500 nm, a binder PVDF and solvent NMP were mixed according to a mass ratio of 24: 1: 500 to obtain a slurry. Both surfaces of a PE separator having a thickness of 11 μm were coated with the slurry. After stroving, a three-layer membrane A3 with a thickness of 15 μm was obtained, in which the thickness of the PE separator was 11 μm, and each surface of the PE separator was coated with a BaSO₄ particle layer having a thickness of 2 μm.

(2) Preparation of a spinning solution

Solvent NMP and solvent DMF were evenly mixed according to a mass ratio of 5: 5 to obtain a mixed solvent, and then P (VDF-HFP) with HFP copolymerized unit of 10 wt％was added and sufficiently dissolved in the mixed solvent through magnetic stirring in a water bath at 50 ℃ to obtain a P (VDF-HFP) spinning solution B3 with a P (VDF-HFP) concentration of 15 wt％.

(3) Preparation of a polymer composite membrane

A metal drum used as a receiving device was wrapped with the three-layer membrane A3 obtained from step (1) , then electrostatic spinning was performed on a surface of three-layer membrane A3 with the P (VDF-HFP) spinning solution B3 in a manner of needle-free spinning. The condition for the needle-free spinning was adjusted as follow: an electrode distance of 18.2 cm, a temperature of 25 ℃, a humidity of 35％, a moving velocity of a liquid pool of 540 mm/sec, a moving velocity of a substrate of 150 mm/min, an anode voltage of 40 kV, a cathode voltage of -20 kV, and a voltage difference of 60 kV. After the process of electrostatic spinning, the three-layer membrane A3 was rolled up and blast-dried for 50 min under a temperature of 50 ℃. Then the electrostatic spinning was performed on the other surface of the three-layer membrane under the same condition. After which, the three-layer membrane A3 was blast-dried for 24 h under a temperature of 50 ℃ so as to obtain a polymer composite membrane C3, of which each surface was formed with a fiber layer having a thickness of 2 μm respectively. Fig. 3 showed a surface microstructure of the polymer composite membrane C3. The fiber diameter of the fiber layer of the polymer composite membrane C3 in a SEM image was measured and recorded by a transmission electron microscope (TEM) Macrography Application, thus the average fiber diameter of the fiber layer was calculated to be 510 nm. The areal density of the fiber layer was 1.62 g/m² calculated by a gravimetric method.

(4) Preparation of a lithium ion battery

The polymer composite membrane C3 was cut to be of a suitable size and vacuum dried for 12 h under a temperature of 50 ℃. Then a LiCoO₂ positive electrode sheet and a graphite negative electrode sheet dried and separated from each other via the cut polymer composite membrane C3were wound to obtain a core in a drying room. After casing, injecting, hot-pressing for 4 h under a temperature of 85 ℃ and a pressure of 1 MPa, vacuum sealing, and etc for conventional production process of a soft pack battery, a SL281643-typed soft pack lithium ion battery was obtained, recorded as E3.

EMBODIMENT 4

(1) Preparation of a multiple-layer membrane

BaSO₄ particles with an average diameter of 500 nm, a binder PEO and solvent deionized water were mixed according to a mass ratio of 19: 1: 200 to obtain a slurry. Both surfaces of a PE separator having a thickness of 11 μm were coated with the slurry. After stroving, a three-layer membrane A4 with a thickness of 15 μm was obtained, in which the thickness of the PE separator was 11 μm, and each surface of the PE separator was coated with a BaSO₄ particle layer having a thickness of 2 μm.

(2) Preparation of a spinning solution

Solvent NMP and solvent DMF were evenly mixed according to a mass ratio of 3: 7 to obtain a mixed solvent, and then P (VDF-HFP) with HFP copolymerized unit of 15 wt％was added and sufficiently dissolved in the mixed solvent through magnetic stirring in a water bath at 50 ℃ to obtain a P (VDF-HFP) spinning solution B4 with a P (VDF-HFP) concentration of 15 wt％.

(3) Preparation of a polymer composite membrane

A metal drum used as a receiving device was wrapped with the three-layer membrane A4 obtained from step (1) , then electrostatic spinning was performed on a surface of three-layer membrane A4 with the P (VDF-HFP) spinning solution B4 in a manner of needle spinning. The condition for the needle spinning was adjusted as follow: a receiving distance of 12 cm, a temperature of 40 ℃, a humidity of 30％, an inner diameter of the needle of 0.46 mm, a moving velocity of the needle of 6.6 mm/sec, a voltage of 10 kV, a flow velocity of 0.3 mL/h, and a rotating speed of the metal drum of 200 rpm. After the process of electrostatic spinning, the three-layer membrane A4 was removed and blast-dried for 24 h under a temperature of 50 ℃. Then the electrostatic spinning was performed on the other surface of the three-layer membrane A4 under the same condition. After which, the three-layer membrane A4 was removed and blast-dried for 24 h under a temperature of 50 ℃ so as to obtain a polymer composite membrane C4, of which each surface was formed with a fiber layer having a thickness of 2 μm respectively. The average fiber diameter of the fiber layer of the polymer composite membrane C4 was 543 nm. The areal density of the fiber layer was 0.37 g/m² calculated by a gravimetric method.

(4) Preparation of a lithium ion battery

The polymer composite membrane C4 was cut to be of a suitable size and vacuum dried for 12 h under a temperature of 50 ℃. Then a LiCoO₂ positive electrode sheet and a graphite negative electrode sheet dried and separated from each other via the cut polymer composite membrane C4were wound to obtain a core in a drying room. After casing, injecting, hot-pressing for 4 h under a temperature of 85 ℃ and a pressure of 1 MPa, vacuum sealing, and etc for conventional production process of a soft pack battery, a SL281643-typed soft pack lithium ion battery was obtained, recorded as E4.

EMBODIMENT 5

(1) Preparation of a multiple-layer membrane

Al₂O₃ particles with an average diameter of 615 nm, a binder PVDF and solvent NMP were mixed according to a mass ratio of 9: 1: 67 to obtain a slurry. Both surfaces of a PE separator having a thickness of 11 μm were coated with the slurry. After stroving, a three-layer membrane A5 with a thickness of 15 μm was obtained, in which the thickness of the PE separator was 11 μm, and each surface of the PE separator was coated with an Al₂O₃ particle layer having a thickness of 2 μm.

(2) Preparation of a spinning solution

Solvent NMP and solvent DMF were evenly mixed according to a mass ratio of 7: 3 to obtain a mixed solvent, and then P (VDF-HFP) with HFP copolymerized unit of 5 wt％was added and sufficiently dissolved in the mixed solvent through magnetic stirring in a water bath at 50 ℃ to obtain a P (VDF-HFP) spinning solution B5 with a P (VDF-HFP) concentration of 25 wt％.

(3) Preparation of a polymer composite membrane

A metal drum used as a receiving device was wrapped with the three-layer membrane A5 obtained from step (1) , then electrostatic spinning was performed on a surface of three-layer membrane A5 with the P (VDF-HFP) spinning solution B5 in a manner of needle spinning. The condition for the needle spinning was adjusted as follow: a receiving distance of 12 cm, a temperature of 40 ℃, a humidity of 50％, an inner diameter of the needle of 0.46 mm, a moving velocity of the needle of 6.6 mm/sec, a voltage of 10 kV, a flow velocity of 1 mL/h, and a rotating speed of the metal drum of 200 rpm. After the process of electrostatic spinning, the three-layer membrane A5 was removed and blast-dried for 24 h under a temperature of 50 ℃. Then the electrostatic spinning was performed on the other surface of the three-layer membrane A5 under the same condition. After which, the three-layer membrane A5 was removed and blast-dried for 24 h under a temperature of 50 ℃ so as to obtain a polymer composite membrane C5, of which each surface was formed with a fiber layer having a thickness of 2 μm respectively. The average fiber diameter of the fiber layer was 639 nm. The areal density of the fiber layer was 1.97 g/m² calculated by a gravimetric method.

(4) Preparation of a lithium ion battery

The polymer composite membrane C5 was cut to be of a suitable size and vacuum dried for 12 h under a temperature of 50 ℃. Then a LiCoO₂ positive electrode sheet and a graphite negative electrode sheet dried and separated from each other via the cut polymer composite membrane C5 were wound to obtain a core in a drying room. After casing, injecting, hot-pressing for 4 h under a temperature of 85 ℃ and a pressure of 1 MPa, vacuum sealing, and etc for conventional production process of a soft pack battery, a SL281643-typed soft pack lithium ion battery was obtained, recorded as E5.

EMBODIMENT6

(1) Preparation of a multiple-layer membrane

Al₂O₃ particles with an average diameter of 615 nm, a binder PEO and solvent deionized water were mixed according to a mass ratio of 24: 1: 500 to obtain a slurry. Both surfaces of a PE separator having a thickness of 11 μm were coated with the slurry. After stroving, a three-layer membrane A6 with a thickness of 15 μm was obtained, in which the thickness of the PE separator was 11 μm, and each surface of the PE separator was coated with an Al₂O₃ particle layer having a thickness of 2 μm.

(2) Preparation of a spinning solution

Solvent NMP and solvent DMF were evenly mixed according to a mass ratio of 5: 5 to obtain a mixed solvent, and then P (VDF-HFP) with HFP copolymerized unit of 5 wt％was added and sufficiently dissolved in the mixed solvent through magnetic stirring in a water bath at 50 ℃ to obtain a P (VDF-HFP) spinning solution B6 with a P (VDF-HFP) concentration of 25 wt％.

(3) Preparation of a polymer composite membrane

A metal drum used as a receiving device was wrapped with the three-layer membrane A6 obtained from step (1) , then electrostatic spinning was performed on a surface of three-layer membrane A6 with the P (VDF-HFP) spinning solution B6 in a manner of needle spinning. The condition for the needle spinning was adjusted as follow: a receiving distance of 12 cm, a temperature of 70 ℃, a humidity of 40％, an inner diameter of the needle of 0.46 mm, a moving velocity of the needle of 6.6 mm/sec, a voltage of 10 kV, a flow velocity of 0.6 mL/h, and a rotating speed of the metal drum of 200 rpm. After the process of electrostatic spinning, the three-layer membrane A6 was removed and blast-dried for 24 h under a temperature of 50 ℃. Then the electrostatic spinning was performed on the other surface of the three-layer membrane A6 under the same condition. After which, the three-layer membrane A6 was removed and blast-dried for 24 h under a temperature of 50 ℃ so as to obtain a polymer composite membrane C6, of which each surface was formed with a fiber layer having a thickness of 2.5 μm respectively. Fig. 4 showed a surface microstructure of the polymer composite membrane C6. The average fiber diameter of the fiber layer was 1042 nm. The areal density of the fiber layer was 1.41 g/m² calculated by a gravimetric method.

Another PE separator was provided. Then the electrostatic spinning was performed on both surfaces of the PE separator under the same condition to obtain a polymer composite membrane with a fiber layer the same with that of C6. Then the fiber layer was peeled off to be tested and recorded as C6-2.

(4) Preparation of a lithium ion battery

The polymer composite membrane C6 was cut to be of a suitable size and vacuum dried for 12 h under a temperature of 50 ℃. Then a LiCoO₂ positive electrode sheet and a graphite negative electrode sheet dried and separated from each other via the cut polymer composite membrane C6 were wound to obtain a core in a drying room. After casing, injecting, hot-pressing for 4 h under a temperature of 85 ℃ and a pressure of 1 MPa, vacuum sealing, and etc for conventional production process of a soft pack battery, a SL281643-typed soft pack lithium ion battery was obtained, recorded as E6.

EMBODIMENT 7

(1) Preparation of a multiple-layer membrane

SiO₂ particles with an average diameter of 1 μm, a binder PVDF and solvent NMP were mixed according to a mass ratio of 19: 1: 200 to obtain a slurry. Both surfaces of a PE separator having a thickness of 11 μm were coated with the slurry. After stroving, a three-layer membrane A7 with a thickness of 15 μm was obtained, in which the thickness of the PE separator was 11 μm, and each surface of the PE separator was coated with a SiO₂ particle layer having a thickness of 2 μm.

(2) Preparation of a spinning solution

Solvent NMP and solvent DMF were evenly mixed according to a mass ratio of 3: 7 to obtain a mixed solvent, and then P (VDF-HFP) with HFP copolymerized unit of 10 wt％was added and sufficiently dissolved in the mixed solvent through magnetic stirring in a water bath at 50 ℃ to obtain a P (VDF-HFP) spinning solution B7 with a P (VDF-HFP) concentration of 20 wt％.

(3) Preparation of a polymer composite membrane

A metal drum used as a receiving device was wrapped with the three-layer membrane A7 obtained from step (1) , then electrostatic spinning was performed on a surface of three-layer membrane A7 with the P (VDF-HFP) spinning solution B7 in a manner of needle spinning. The condition for the needle spinning was adjusted as follow: a receiving distance of 12 cm, a temperature of 70 ℃, a humidity of 30％, an inner diameter of the needle of 0.46 mm, a moving velocity of the needle of 6.6 mm/sec, a voltage of 10 kV, a flow velocity of 0.6 mL/h, and a rotating speed of the metal drum of 200 rpm. After the process of electrostatic spinning, the three-layer membrane A7 was removed, hot-rolled under a temperature of 50 ℃ and a pressure of 1 MPa, and then blast-dried for 24 h under a temperature of 50 ℃. Then the electrostatic spinning was performed on the other surface of the three-layer membrane A7 under the same condition. After which, the three-layer membrane A7 was removed and blast-dried for 24 h under a temperature of 50 ℃ so as to obtain a polymer composite membrane C7, of which each surface was formed with a fiber layer having a thickness of 2 μm respectively. Fig. 5 showed a surface microstructure of the polymer composite membrane C7. The average fiber diameter of the fiber layer was 1059 nm. The areal density of the fiber layer was 1.84 g/m² calculated by a gravimetric method.

(4) Preparation of a lithium ion battery

The polymer composite membrane C7 was cut to be of a suitable size and vacuum dried for 12 h under a temperature of 50 ℃. Then a LiCoO₂ positive electrode sheet and a graphite negative electrode sheet dried and separated from each other via the cut polymer composite membrane C2 were wound to obtain a core in a drying room. After casing, injecting, hot-pressing for 4 h under a temperature of 85 ℃ and a pressure of 1 MPa, vacuum sealing, and etc for conventional production process of a soft pack battery, a SL281643-typed soft pack lithium ion battery was obtained, recorded as E7.

EMBODIMENT 8

The preparation of a polymer composite membrane was substantially similar to EMBODIMENT 2, with an exception that: the PE separator was not coated with an inorganic particle layer, and an electrostatic spinning was directly performed on both surfaces of the PE separator.

A lithium ion battery recorded as E8 was obtained.

COMPARATIVE EMBODIMENT 1

The present comparative embodiment provides a polymer composite membrane and a preparation method thereof, a gel electrolyte and a lithium ion battery for comparatively illustrate the present disclosure.

(1) Preparation of a multiple-layer membrane

A three-layer membrane AR1 was prepared according to the step (1) of EMBODIMENT 1.

(2) Preparation of a spinning solution

A P (VDF-HFP) spinning solution BR1 with a P (VDF-HFP) concentration of 15 wt％was prepared substantially similar to the step (2) of EMBODIMENT 1, with an exception that: using a single solvent DMF instead of the mixed solvent of EMBODIMENT 1.

(3) Preparation of a polymer composite membrane

A polymer composite membrane CR1 was prepared substantially similar to the step (3) of EMBODIMENT 1 via the three-layer membrane AR1 and the P (VDF-HFP) spinning solution BR1. The average fiber diameter of the fiber layer of the polymer composite membrane CR1 was 162 nm, the thickness of the fiber layer was 2.5 μm. Fig. 6 showed a surface microstructure of the polymer composite membrane CR1.

(4) Preparation of a lithium ion battery

A SL281643-typed soft pack lithium ion battery recorded as ER1 was prepared substantially similar to the step (4) of EMBODIMENT 1 via the polymer composite membrane CR1.

COMPARATIVE EMBODIMENT 2

(1) Preparation of a multiple-layer membrane

A three-layer membrane AR2 was prepared according to the step (1) of EMBODIMENT 6.

(2) Preparation of a spinning solution

A P (VDF-HFP) spinning solution BR2 with a P (VDF-HFP) concentration of 25 wt％was prepared substantially similar to the step (2) of EMBODIMENT 6, with an exception that: using a single solvent NMP instead of the mixed solvent of EMBODIMENT 6.

(3) Preparation of a polymer composite membrane

A polymer composite membrane CR2 was prepared substantially similar to the step (3) of EMBODIMENT 6 via the three-layer membrane AR2 and the P (VDF-HFP) spinning solution BR2. Most area of the polymer composite membrane CR2 was black, and the area distributed with fiber was very small, which can be seen in Fig. 7. The thickness of the fiber layer of the polymer composite membrane CR2 was 1.5 μm.

(4) Preparation of a lithium ion battery

A SL281643-typed soft pack lithium ion battery recorded as ER2 was prepared substantially similar to the step (4) of EMBODIMENT 6 via the polymer composite membrane CR2.

COMPARATIVE EMBODIMENT 3

P(VDF-HFP) with a content of the HFP copolymerized unit in the P (VDF-HFP) of 5 wt％, DMC and acetone were mixed according to a mass ratio of 5: 2: 93, and then sufficiently dissolved through magnetic stirring under a room temperature to obtain a P (VDF-HFP) solution with a P (VDF-HFP) concentration of 5 wt％.

Then the P (VDF-HFP) solution was coated on a surface of a PE separator having a thickness of 12 μm. After blast-drying under 50 ℃, the other surface of the PE separator was also coated with the P(VDF-HFP) solution. After blast-drying under 50 ℃, a porous membrane recorded as CR3 was obtained, in which a thickness of the coating layer was 1.5 μm, an areal density of the coating layer was calculated to be 1.85 g/m². Fig. 8 showed a surface microstructure of the porous membrane CR3.

A polytetrafluoroethylene (PTFE) membrane was provided. Then both surfaces of the PTFE membrane were coated with the P (VDF-HFP) solution via a coating method to obtain a porous membrane with a coating layer the same with that of CR3. Then the coating layer was peeled off to be tested and recorded as CR3-2.

A SL281643-typed soft pack lithium ion battery recorded as ER3 was prepared substantially similar to the step (4) of EMBODIMENT 2 via the porous membrane CR3.

COMPARATIVE EMBODIMENT 4

A polymer composite membrane was prepared substantially similar to the steps of COMPARATIVE EMBODIMENT 1, with an exception that: the PE separator was not coated with an inorganic particle layer, and electrostatic spinning was directly performed on both surfaces of the PE separator.

A lithium ion battery recorded as ER4 was obtained.

Performance Testing

1. SEM (scanning electron microscopy)

The micro morphologies of the polymer composite membranes obtained from

EMBODIMENTS

2, 3, 6, 7, and COMPARATIVE EMBODIMENTS 1-2 and the porous membrane obtained from COMPARATIVE EMBODIMENT 3 were observed with a scanning electron microscope (SEM, JEOL, JSM-7600FE) . As shown in Figs 2-8.

As can be seen in Figs. 2-8, in the SEM images of the polymer composite membranes obtained from

EMBODIMENTS

2, 3, 6, and 7, the inorganic particle layer located inside of the polymer composite membrane was visible, as the fiber layer was very thin and had a high porosity.

The fiber diameters of the fiber layers of the polymer composite membranes obtained from

EMBODIMENTS

2, 3, and 6 were very uniform, while the fiber filaments in the fiber layer of the polymer composite membrane in EMBODIMENT 7 was very flat because of a process of hot-rolling to the multiple-layer membrane. Thus, the fiber layer may be denser by a process of hot-rolling.

The polymer composite membrane obtained from EMBODIMENT 3 was prepared by electrostatic spinning in a manner of needle-free spinning, and the fiber filaments in the fiber layer was of a reticular structure, which was benefit for improving a mechanical strength of the fiber layer.

The polymer composite membrane obtained from COMPARATIVE EMBODIMENT 1 via a single solvent DMF having a low boiling point showed a good fiber morphology, but the filaments were too fluffy, thus the inorganic particle layer located inside the polymer composite membrane could not be observed.

The polymer composite membrane obtained from COMPARATIVE EMBODIMENT 2 via a single solvent NMP having a high boiling point showed that: most of the area of the polymer composite membrane CR2 was black, and the area distributed with fiber was very small, the inorganic particle layer located inside the polymer composite membrane could not be observed because a formation speed of the fiber filaments was much faster than a volatilization speed of the solvent, thus part of the fiber filaments could be leveled.

The porous membrane obtained from COMPARATIVE EMBODIMENT 3 via a coating method was porous. As can be seen from the morphology, the porosity of the fiber layer in the polymer composite membrane of the present disclosure was much bigger than that of the coating layer.

It could be known from the above embodiments that, the fiber layer in the polymer composite membranes obtained from

EMBODIMENTS

2, 3, 6, and 7 were firmly adhered to the inorganic particle layer, and won’t be peeled off； and the filaments inside the fiber layer also showed a good adhesiveness. The fiber layer in the polymer composite membrane obtained from COMPARATIVE EMBODIMENT 1 showed a poor adhesion to the inorganic particle layer because of an application of the single solvent DMF having a low boiling point, and the formation speed of the fiber filaments being much slower than the volatilization speed of the solvent DMF. The fiber layer in the polymer composite membrane obtained from COMPARATIVE EMBODIMENT 2 showed a good adhesion to the inorganic particle layer, because the formation speed of the fiber filaments was much slower than the volatilization speed of the solvent NMP, thus part of the fiber filaments could be leveled.

(2) Porosity

The porosities of the fiber layers of the polymer composite membranes obtained from EMBODIMENTS 1-8 and

COMPARATIVE EMBODIMENTS

1, 2 and 4, and the coating layer of the porous membrane obtained from COMPARATIVE EMBODIMENTS 3 were calculated by a formula below:

Porosity (％) ＝ (1-ρ_M/ρ_P) ×100％＝ [1-ρ_f/ (ρ_p·d) ] ×100％,

where ρ_M is the apparentdensity of the fiber layer of the polymer composite membrane, ρ_f is the areal density of the fiber layer, d is the thickness of the fiber layer, and ρ_P is the density of the polymer itself in the fiber layer.

The results were recorded in Table 1.

Table 1

As shown in the Table 1, the porosity of the fiber layer of the polymer composite membrane obtained from the method according to the present disclosure was much higher than that of the coating layer obtained from the method of precipitation phase separation via evaporation of solvent. In addition, the method in COMPARATIVE EMBODIMENT 2 used pure NMP as the solvent, and part of the fiber layer was leveled, thus the porosity thereof was very low (53 ％) .

(3) Liquid Absorbency

The fiber layer C6-2 obtained from EMBODIMENT 6 and the coating layer CR3-2 obtained from COMPARATIVE EMBODIMENT 3 were cut into a disc with a diameter of 17 mm respectively, vacuum dried under the room temperature and weighed. The weighed fiber layer and coating layer were immersed into the lithium salt solutions (i.e. electrolytes) for 24 hours respectively. Then the fiber layer and the coating layer were taken out and the electrolytes on the surface thereof were removed respectively, the fiber layer and the coating layer were weighted to calculate the liquid absorbency thereof respectively. All the operations were performed in a glove box filled with argon gas.

The liquid absorbency (i.e. lithium salt absorbency) was calculated by a formula:

Liquid Absorbency (％) ＝ (W2-W1) /W1 × 100％

where W1 is the original weight of the dry layer, and W2 is the weight of the dry layer after absorbing the lithium salt (g) .

The results were recorded in Table 2.

Table 2

As shown in the Table 2, the lithium salt absorbency of the fiber layer of the polymer composite membrane obtained from the method according to the present disclosure was much higher than that of the coating layer obtained from the method of precipitation phase separation via evaporation of solvent.

(4) Heat-shrink Performance

The three-layer membrane A2, polymer composite membrane C2 obtained from EMBODIMENT 2 and polymer composite membrane C8 obtained from EMBODIMENT 8 were cut to be of a certain size and baked in an oven for 1h under 120 ℃ . The size of the baked membranes was measured. And, a heat-shrinkage refers to a ratio of the size difference of the membrane to the size of membrane before baking. The results were recorded in Table 3.

Table 3

As shown in Fig. 3, the heat-shrinkage of the polymer composite membrane obtained from spinning on the three-layer membrane was lower than that of the three-layer membrane, which indicated that the anti-heat-shrink performance of the polymer composite membrane was increased, and the safety performance of the lithium ion battery may be improved.

Moreover, the longitudinal heat-shrinkage of the polymer composite membrane C2 was 1.13％, which was much lower than that of the polymer composite membrane C8 (4.67％) , which indicated that the anti-heat-shrink performance of the polymer composite membrane obtained from spinning on the three-layer membrane was higher than that of the polymer composite membrane obtained from spinning on the PE separator, and the safety performance of the lithium ion battery may be improved.

(5) Peeling Strength

The polymer composite membranes C1-C8 and CR1-CR2, CR4 obtained from EMBODIMENTS 1-8 and

COMPARATIVE EMBODIMENTS

1, 2 and 4 were cut to be of a size larger than 150mm × 50mm. One surface of each of the cut membranes was adhered with a scotch tape having a size of 120mm × 40mm. After three times rolling under a pressure of 1 MPa, the rolled membranes were fixed onto a mold by a double-sided adhesive on the other suface without the scotch tape. Then the peeling strength of the membranes was tested by an electronic universal testing machine and recorded in Table 4.

Table 4

As shown in the Table 4, the peeling strength of the fiber layer on the inorganic particle layer was much higher that of the fiber layer on the PE separator. In other words, the fiber layer showed a high adhesiveness to the inorganic particle layer than to the PE separator.

(6) Electrical Conductivity

The three-layer membrane A2 and polymer composite membrane C2 obtained from EMBODIMENT 2, fiber layer C6-2 obtained from EMBODIMENT 6, and porous membrane CR3 and coating layer CR3-2 obtained from COMPARATIVE EMBODIMENT 3 were placed between two stainless steel (SS) electrodes respectively to absorb enough amount of electrolytes, then sealed in 2016 type Li/LiCoO₂ polymer lithium ion batteries respectively and tested by a AC impedance test, in which the intersection of a line with the real axis was the resistance of the gel electrolyte. The ionic conductivity σ was calculated by the following formula:

σ＝ L/A·R,

where L is the thickness of the gel electrolyte, A is the surface area of the electrode that contacts with the membrane, and R is the resistance of the gel electrolyte. The results were shown in Table 5.

Table 5

As shown in the Table 5, the ionic conductivity of the polymer composite membrane obtained from the method according to the present disclosure was higher than that of the three-layer membrane, and also higher than that of the membrane with the coating layer, which was benefit for the quick migration of the lithium ions. In addition, the ionic conductivity of the fiber layer was also much higher than that of the coating layer, because of the high porosity of the fiber layer.

(7) Rate Discharge Ability

The lithium ion batteries E2 obtained from EMBODIMENT 2 and ER3 obtained from COMPARATIVE EMBODIMENT 3 were fully charged to 4.35V under 0.5C current via a constant current and voltage in the room temperature with an off current of 0.02C, then discharged to 3.0V under 0.5C current, 1C current, 5C current, 8C current, and 10C current, respectively. The discharging capacities were recorded. in addition, the ratios of the discharging capacities under 0.5C current, 1C current, 5C current, 8C current, and 10C current to the discharging capacity under 0.5C current were calculate and recorded. The results were shown in Table 6.

Table 6

As shown in the Table 6, the high rate discharge ability (≥8C) of the lithium ion battery including the polymer composite membrane obtained from the method according to the present disclosure was much higher than that of the lithium ion battery including the polymer composite membrane having the coating layer.

(8) DC Resistance

The lithium ion batteries E2 and E6 obtained from

EMBODIMENTS

2, 6 and ER3 obtained from COMPARATIVE EMBODIMENT 3 were charged to 4.35V under a current of 180 mA (1C) via a constant current and voltage with an off current of 3.6 mA (0.02C) . Then the DC resistance of the battery was tested and calculated according to following steps:

1. pausing for 1 s,

2. discharging under a constant current of 180 mA (1C) for 1 s, with a lower limit voltage of 2.5 V, testing the voltage per 10 ms, and recording the last voltage as V1； and

3. discharging under a constant current of 900 mA (5C) for 1 s, with a lower limit voltage of 2.5 V, testing the voltage per 10 ms, and recording the last voltage as V2.

The

above steps

2 and 3 were cycled for 3 times, the voltages on the third cycle were recorded.

The DC resistance (RDC) was calculated by a formula:

RDC ＝ (V1-V2) / (0.9-0.18) × 1000 , where the unit for RDC is mΩ.

The results were shown in Table 7.

Table 7

	RDC (mΩ)
Battery E2	267
Battery E6	278
Battery ER3	513

As shown in Table 7, the RDC of the lithium ion battery including the polymer composite membrane obtained from the method according to the present disclosure was far less than that of the lithium ion battery including the porous membrane, which corresponded to the rate discharge ability of the lithium ion battery. That’s because the polymer composite membrane according to the present disclosure shows a high porosity, a good adhesiveness to the electrodes, a high electrical conductivity, and a small polarization during the process of charge and discharge when used in the lithium-ion battery.

As can be seen from the above test that, under the same testing condition, the SL281643-typed soft pack lithium ion battery including the polymer composite membrane according to the present disclosure showed a lower DC resistance (<300mΩ) , and a good high-rate discharge ability (ratio of 10C to 0.2C more than 80％) . The fiber layer of the polymer composite membrane showed a high porosity (70-98％) and a high liquid absorbency (200-1300％) , the polymer composite membrane showed a good anti-heat-shrink performance, a high ionic conductivity (>7×10^-4S/cm) , in which the ionic conductivity of the single fiber layer was also very high (>5.5×10^-3S/cm) , and the adhesiveness between the fiber layer and the inorganic particle layer was also very strong, with a peeling strength ranging from 0.03 kN/m to 0.5 kN/m.

Reference throughout this specification to “an embodiment, ” “some embodiments, ” “one embodiment” , “another example, ” “an example, ” “a specific example, ” or “some examples, ” means that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the present disclosure. Thus, the appearances of the phrases such as “in some embodiments, ” “in one embodiment” , “in an embodiment” , “in another example, ” “in an example, ” “in a specific example, ” or “in some examples, ” in various places throughout this specification are not necessarily referring to the same embodiment or example of the present disclosure. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments or examples.

Although explanatory embodiments have been shown and described, it would be appreciated by those skilled in the art that changes, alternatives, and modifications all falling into the scope of the claims and their equivalents may be made in the embodiments without departing from spirit and principles of the disclosure.

Claims

A method of preparing a polymer composite membrane, comprising:

providing a separator,

providing a spinning solution comprising a mixed solvent and a spinning polymer dissolved in the mixed solvent, the mixed solvent comprising a first solvent having a boiling point ranging from about 150 ℃ to about 170 ℃, a second solvent having a boiling point ranging from about 175 ℃ to about 220 ℃, and a mass ratio of the first solvent and the second solvent ranging from about 7: 3 to about 3: 7； and

electrostatic spinning on the separator with the spinning solution to obtain the polymer composite membrane having the separator and a fiber layer coated on the separator.
The method of claim 1, wherein the spinning polymer is at least one selected from a group consisting of polyvinylidene fluoride, and poly (vinylidene fluoride-hexafluoropropylene) ；

the first solvent is at least one selected from a group consisting of N, N-dimethylformamide, and N, N-dimethylacetamide； and

the second solvent is at least one selected from a group consisting of N-methylpyrrolidone, dimethylsulfoxide, tetramethylurea, and triethyl phosphate.
The method of claim 1 or 2, wherein the spinning polymer is poly (vinylidene fluoride-hexafluoropropylene) with a hexafluoropropylene copolymerized unit ranging from about 5 wt％ to about 15 wt％.
The method of any one of claims 1 to 3, wherein the spinning polymer in the spinning solution has a concentration ranging from about 5 wt％ to about 30 wt％.
The method of any one of claims 1 to 4, wherein the first solvent has a flash point ranging from about 58 ℃ to about 70 ℃, and the second solvent has a flash point ranging from about 65 ℃ to about 117 ℃.
The method of any one of claims 1 to 5, wherein the separator is a polyolefin separator.
The method of any one of claims 1 to 6, wherein the electrostatic spinning is performed in a manner of needle spinning under a condition comprising: a flow velocity ranging from about 0.3 mL/h to about 5 mL/h, a spinning temperature ranging from about 25 ℃ to about 70 ℃, a spinning humidity ranging from about 10％ to about 60％, and a voltage ranging from about 5 kV to about 20 kV.
The method of claim 7, wherein the electrostatic spinning is performed under the condition further comprising: a receiving distance ranging from about 6 cm to about 15 cm, a moving velocity of the needle ranging from about 6 mm/sec to about 7 mm/sec, an inner diameter of the needle of about 0.46 mm, the voltage ranging from about 7 kV to about 15 kV, and a rotating speed of a metal drum ranging from about 100 rpm to about 300 rpm.
The method of any one of claims 1 to 6, wherein the electrostatic spinning is performed in a manner of needle-free spinning under a condition comprising: a spinning temperature ranging from about 25 ℃ to about 70 ℃, a spinning humidity ranging from about 10％ to about 60％, a moving velocity of a liquid pool ranging from 0 to about 2000 mm/sec, a moving velocity of a substrate ranging from 0 to about 20000 mm/min, an anode voltage ranging from about 0 to about 150 kV, a cathode voltage ranging from about -50 kV to about 0 kV, and a voltage difference ranging from about 20 kV to about 100 kV.
The method of any one of claims 1 to 9, wherein both surfaces of the separator are provided with the fiber layer with the electrostatic spinning.
The method of any one of claims 1 to 10, wherein the fiber layer has a porosity ranging from about 70％ to about 98％, and a peel strength ranging from about 0.03 kN/m to about 0.50 kN/m.
The method of any one of claims 1 to 11, wherein the fiber layer has a liquid absorbency ranging from about 200％ to about 1300％.
The method of any one of claims 1 to 12, wherein the fiber layer has an areal density ranging from about 0.2 g/m² to about 3 g/m².
The method of any one of claims 1 to 13, wherein the fiber layer has a fiber diameter ranging from about 100 nm to about 1500 nm, and a thickness ranging from about 0.5 μm to about 5 μm.
The method of any one of claims 1 to 14, further comprising:

providing a slurry comprising an inorganic particle, a third solvent and a binder；

coating the slurry on a surface of the separator to obtain an inorganic particle layer on the surface of the separator； and

electrostatic spinning on the separator coated with the inorganic particle layer with the spinning solution.
The method of claim 15, wherein a mass ratio of the inorganic particle, the third solvent and the binder is about 9-24: 67-500: 1.
The method of claim 15 or 16, wherein the inorganic particle is at least one selected from a group consisting of Al₂O₃, SiO₂, BaSO₄, TiO₂, CuO, MgO, LiAlO₂, ZrO₂, carbon nanotube, BN, SiC, Si₃N₄, WC, BC, AlN, Fe₂O₃, BaTiO₃, MoS₂, α–V₂O₅, PbTiO₃, TiB₂, CaSiO₃, molecular sieve, clay, and kaolin；

the binder is at least one selected from a group consisting of polyvinylidene fluoride, poly (vinylidene fluoride-hexafluoropropylene) , polymethyl methacrylate, polyacrylonitrile, polyimide, polyvinyl pyrrolidone, polyethylene oxide, polyvinyl alcohol, sodium carboxymethyl cellulose, and styrene-butadiene rubber； and

the third solvent is at least one selected from a group consisting of N-methylpyrrolidone, N, N-dimethylformamide, N, N-dimethylacetamide, toluene, methylene chloride, chloroform, deionized water, and ethanol.
The method of any one of claims 15 to 17, wherein the inorganic particle has a diameter ranging from about 50 nm to about 3 μm.
The method of any one of claims 15 to 18, wherein the inorganic particle layer has a thickness ranging from about 0.5 μm to about 3 μm.
The method of any one of claims 15 to 19, wherein both surfaces of the separator are provided with the inorganic particle layer.
The method of claim 20, wherein the electrostatic spinning is performed on both surfaces of the separator coated with the inorganic particle layer respectively.
A polymer composite membrane, obtained by a method of any one of claims 1 to 21, comprising:

a separator, and

a fiber layer on the separator, having a peel strength more than 0.03 kN/m.
The polymer composite membrane of claim 22, wherein the fiber layer has a porosity more than 70％.
The polymer composite membrane of claim 22 or 23, wherein the fiber layer has a porosity ranging from about 70％ to about 98％, and a peel strength ranging from about 0.03 kN/m to about 0.50 kN/m.
The polymer composite membrane of any one of claims 22 to 24, wherein the fiber layer has a liquid absorbency ranging about 200％ to about 1300％.
The polymer composite membrane of any one of claims 22 to 25, wherein the fiber layer is made of polyvinylidene fluoride, poly (vinylidene fluoride-hexafluoropropylene) , or a combination thereof.
The polymer composite membrane of any one of claims 22 to 26, wherein the fiber layer is made of poly (vinylidene fluoride-hexafluoropropylene) with a hexafluoropropylene copolymerized unit ranging from about 5 wt％ to about 15 wt％.
The polymer composite membrane of any one of claims 22 to 27, wherein the fiber layer has an areal density ranging from about 0.2 g/m² to about 3 g/m².
The polymer composite membrane of any one of claims 22 to 28, wherein the fiber layer has a fiber diameter ranging from about 100 nm to about 1500 nm, and a thickness ranging from about 0.5 μm to about 5 μm.
The polymer composite membrane of any one of claims 22 to 29, wherein both surfaces of the separator are provided with the fiber layer.
The polymer composite membrane of any one of claims 22 to 30, wherein the separator is a polyolefin separator.
The polymer composite membrane of any one of claims 22 to 31, further comprising an inorganic particle layer between the separator and the fiber layer；

wherein the inorganic particle layer comprises:

an inorganic particle selected from a group consisting of Al₂O₃, SiO₂, BaSO₄, TiO₂, CuO, MgO, LiAlO₂, ZrO₂, carbon nanotube, BN, SiC, Si₃N₄, WC, BC, AlN, Fe₂O₃, BaTiO₃, MoS₂, α–V₂O₅, PbTiO₃, TiB₂, CaSiO₃, molecular sieve, clay, kaolin, and combinations thereof； and

a binder selected from a group consisting of polyvinylidene fluoride, poly (vinylidene fluoride-hexafluoropropylene) , polymethyl methacrylate, polyacrylonitrile, polyimide, polyvinyl pyrrolidone, polyethylene oxide, polyvinyl alcohol, sodium carboxymethyl cellulose, styrene-butadiene rubber, and combinations thereof.
The polymer composite membrane of claim 32, wherein the inorganic particle has a diameter ranging from about 50 nm to about 3 μm.
The polymer composite membrane of claim 32 or 33, wherein a mass ratio of the inorganic particle and the binder is about 9-24: 1.
The polymer composite membrane of any one of claims 32 to 34, wherein the inorganic particle layer has a thickness ranging from about 0.5 μm to about 3 μm.
The polymer composite membrane of any one of claims 32 to 35, wherein both surfaces of the separator are provided with the inorganic particle layer.
A gel electrolyte, comprising：

a polymer composite membrane according to any one of claims 22 to 36, or obtainable by a method according to any one of claims 1 to 21； and

an electrolyte adsorbed to the polymer composite membrane.
The gel electrolyte of claim 37, wherein the electrolyte comprises:

a lithium salt being any one selected from a group consisting of lithium hexafluorophosphate, lithium perchlorate, and lithium tetrafluoroborate, having a concentration ranging from about 0.8 mol/L to about 1.5 mol/L； and

a fourth solvent selected from a group consisting of ethylene carbonate, propylene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, vinylene carbonate, and combinations thereof.
A lithium ion battery, comprising:

an anode,

a cathode, and

a gel electrolyte according to claim 37 or 38, disposed between the anode and the cathode.