CN101997102B - Lithium ion battery diaphragm and manufacturing method thereof - Google Patents
Lithium ion battery diaphragm and manufacturing method thereof Download PDFInfo
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Abstract
The invention discloses a lithium ion battery diaphragm and a manufacturing method thereof. The diaphragm comprises composite microporous membranes in layers B, A and B, wherein the layer A is polyethylene, or copolymer of polyethylene and 0-50% of other olefins, or copolymer of polyethylene and 0-50% of other polyolefin, and the layer B is polyvinylidene fluoride or copolymer of hexafluoropropylene-vinylidene fluoride; the diaphragm is extruded and bidirectionally stretched by the three layers B, A and B, wherein the layer A and the layer B are made through a thermally induced phase separation method; and the stretching strength of the diaphragm is 120-170 MPa. The lithium ion battery diaphragm provided by the invention has the advantages of low pore closing temperature, high diaphragm breaking temperature and high mechanical strength, and can effectively improve the safety performance of the lithium ion battery using the diaphragm.
Description
[technical field]
The present invention relates to a kind of lithium ion battery, particularly relate to a kind of lithium ion battery separator and manufacture method thereof.
[background technology]
Lithium ion battery separator is a kind of little porous material, and material itself is insulator, rises to separate positive/negative plate in battery, prevents the effect of internal short-circuit, and micropore sees through electrolyte simultaneously, guarantees that ionic conduction forms current circuit.So, lithium ion battery separator is specific micropore except needs have, it is had outside good ion permeability, what pay special attention to is its safety in utilization in battery, prevents that namely positive/negative plate from directly contacting the formation internal short-circuit, requires barrier film that higher mechanical strength is arranged, good chemical stability, shrinkage is little, and the closed pore effect is arranged under high temperature, molten broken temperature high.
Existing lithium ion battery separator mainly contains the polyethylene monofilm, polypropylene monofilm, or polypropylene, polyethylene/polyacrylic sandwich diaphragm.Poly fusing point is between 125 ℃-140 ℃, and polyacrylic fusing point is between 160 ℃-170 ℃.Because the polypropylene material fusing point is higher, so contain the barrier film of polypropylene material, better high temperature safety is arranged, show that shrinkage is relatively less, broken film temperature is high.But barrier film also must have the closed pore effect to ensure battery security, so list just has the too high defective of closed pore temperature take dystectic polypropylene as material, polypropylene, polyethylene/polyacrylic sandwich diaphragm is just better in the high temperature safe sex expression.But existing polypropylene, polyethylene/polyacrylic sandwich diaphragm is to cause the mode that forms micropore between the crystalline region amorphous area to make barrier film by the hard elastics presoma simple tension to the highly crystalline orientation, mechanical strength is lower, in use procedure, the battery winding short circuit ratio is high, affects production efficiency.
Recently there are many researchs to concentrate on the polyethylene monofilm the dystectic resin of coating and improve its high temperature safety.Propose the porous layer of coating one deck poly on barrier film as patent CN200580051172, thereby form integrated composite membrane, because its coating composition fusing point is very high, the broken film temperature of barrier film is reached more than 250 ℃.Patent CN200480034190 proposes the surface-coated fluororesin and makes through hole, also can improve the resistance to elevated temperatures of barrier film.But coating process is larger to coating layer thickness evenness control difficulty on the one hand, and the coating processing speed is slower, is generally several m/mins, affects production efficiency, is difficult for heavy industrialization.Although the coating high-melting-point substances can improve the broken film temperature of barrier film on the other hand, can cause the Partial Blocking in hole due to coating fluid, barrier film internal resistance meeting is increased.For this reason, usually use porosity large, the fast barrier film of air permeability is as matrix, even there is like this part micropore blocked, still there is the abundant micropore can conducting ion, and can not make the internal resistance increase too large, but use porosity excessive, the too fast barrier film matrix of air permeability can make the mechanical strength variation of barrier film.
[summary of the invention]
The technical problem that the present invention solves is the problem that in prior art, lithium ion battery separator closed pore broken film temperature is not suitable for and mechanical strength is low, and the spy provides a kind of lithium ion battery new types of diaphragm.
The present invention also provides the preparation method of lithium ion battery new types of diaphragm.
The invention provides a kind of lithium ion battery separator, comprise the B/A/B three-layer composite microporous membrane, wherein, described A layer is the copolymer of other alkene of polyethylene or ethene and 0-50% or other polyolefinic mixtures of polyethylene and 0-50%; Described B layer is the copolymer of Kynoar or hexafluoropropylene-vinylidene; Described barrier film forms through the three-layer co-extruded pressure of B/A/B, biaxial tension, and wherein, A layer, B layer are made by thermally induced phase separation; The hot strength of described barrier film is at 120-170Mpa.
The present invention also provides the preparation method of this lithium ion battery separator, comprises the following steps:
Step a, polymer and diluent blending extrusion in extruder that the A layer is used, the polymer that the B layer is used, diluent and inorganic filler blending extrusion in extruder, then pass through in die head compoundly with three layers of B/A/B, after co-extrusion, chilling obtains composite sheet; Wherein, A layer polymer used is the copolymer of other alkene of polyethylene or ethene and 0-50% or other polyolefinic mixtures of polyethylene and 0-50%, and B layer polymer used is the copolymer of Kynoar or hexafluoropropylene-vinylidene;
Step b, will composite sheet become film through after biaxial tension;
Step c, the film that step b is obtained pass through the eluting solvent wash-out, and dry;
Steps d, the film thermal typing that step c is obtained.
Lithium ion battery separator provided by the invention has adopted the B/A/B three-layer composite microporous membrane, wherein the A layer adopts and contains the copolymer of polyethylene or ethene or contain polyethylene mixture, the low characteristics of poly closed pore temperature have been utilized, be compounded with poly microporous barrier, can make diaphragm closed pore temperature in the scope of 125 ℃-135 ℃; And the B layer adopts the copolymer of Kynoar or hexafluoropropylene-vinylidene, on the one hand that to utilize it be the higher crystalline polymer of a kind of fusing point, after stretched orientation, melt strength is improved, the barrier film hot strength make the broken film temperature of barrier film can reach 170 ℃, more than can reach 120MPa simultaneously.That to utilize it be a kind of polarity macromolecule on the other hand, better to the adhesion of battery positive/negative plate with respect to non-polar high polymer barrier films such as polyethylene polypropylene, barrier film and pole piece are fitted tightr, prevented from that material and Li dendrite pierce through barrier film in the generation of the gap of barrier film and negative plate, effectively reduced self-discharge of battery.
[description of drawings]
Fig. 1 is lithium ion battery separator internal resistance and the closed pore broken film temperature tester structural representation that uses in the present invention.
Fig. 2 is lithium ion battery separator internal resistance and the closed pore broken film temperature tester modular construction schematic diagram that uses in the present invention.
Fig. 3 is lithium ion battery separator internal resistance and the closed pore broken film temperature tester modular construction schematic diagram that uses in the present invention
[embodiment]
Describe lithium ion battery separator provided by the invention and preparation method thereof in detail below in conjunction with specific embodiment.
The invention provides a kind of lithium ion battery separator, comprise the B/A/B three-layer composite microporous membrane, wherein, described A layer is the copolymer of other alkene of polyethylene or ethene and 0-50% or other polyolefinic mixtures of polyethylene and 0-50%; Described B layer is the copolymer of Kynoar or hexafluoropropylene-vinylidene; Described barrier film forms through the three-layer co-extruded pressure of B/A/B, biaxial tension, and wherein, A layer, B layer are made by thermally induced phase separation; The hot strength of described barrier film is 120-170Mpa.The hot strength of barrier film is used the standard method of test test of GB/T1040.3-2006.
The polymer that described A layer is selected from contains a large amount of polyethylene compositions, is to utilize its fusing point lower, and lower closed pore temperature can be provided.One or more of copolymer that it can select low density polyethylene (LDPE), medium density polyethylene, high density polyethylene (HDPE), ultra-high molecular weight polyethylene or contain a large amount of polyethylene part.For the consideration to barrier film intensity and processability, preferred high density polyethylene (HDPE), its fusing point is 132 ℃～136 ℃, molecular weight is 1 * 10
5～1 * 10
6Further preferred, molecular weight is 1 * 10
5-1 * 10
6High density polyethylene (HDPE) and molecular weight be 4 * 10
3-1 * 10
4The mixture of low density polyethylene (LDPE), the fusing point of described low density polyethylene (LDPE) is that the mass percent that 125 ℃～128 ℃, content account for mixture is 5%～20%, with further reduction closed pore temperature.
The polymer poly vinylidene that described B layer is selected from or the copolymer of hexafluoropropylene-vinylidene not only can improve the broken film temperature of barrier film, and can reduce self-discharge of battery with polyethylene, polyacrylic barrier film relatively merely.For to the considering of processability, mechanical strength and broken film temperature, the molecular weight of the copolymer of described Kynoar, hexafluoropropylene-vinylidene is preferably 2.0 * 10
5-5 * 10
5, fusing point is 145 ℃-170 ℃.
The Thickness Ratio of described B layer and described A layer is 1: 2-1: 4.The gross thickness of described B/A/B three-layer composite microporous membrane is 20-50um, and the thickness of described B layer is 4-8um, and too thick cost is too high, and too thin being difficult to processes evenly.
Other alkene in the copolymer of other alkene of described ethene and 0-50% are one or more in propylene, 1-butylene, 4-methyl-1-pentene, 1-hexene.Other polyolefin in the polyolefinic mixture of other of described polyethylene and 0-50% are one or more that contain in the polymer of ethene, propylene, 1-butylene, 4-methyl-1-pentene, 1-hexene.
Described lithium ion battery separator also comprises the application of described barrier film in lithium ion battery.
The present invention also provides the preparation method of described lithium ion battery separator, comprises the following steps:
Step a, polymer and diluent blending extrusion in extruder that the A layer is used, the polymer that the B layer is used, diluent and inorganic filler blending extrusion in extruder, then pass through in die head compoundly with three layers of B/A/B, after co-extrusion, chilling obtains composite sheet; Wherein, A layer polymer used is the copolymer of other alkene of polyethylene or ethene and 0-50% or other polyolefinic mixtures of polyethylene and 0-50%, and B layer polymer used is the copolymer of Kynoar or hexafluoropropylene-vinylidene;
Step b, will composite sheet become film through after biaxial tension;
Step c, the film that step b is obtained pass through the eluting solvent wash-out, and dry;
Steps d, the film thermal typing that step c is obtained.
The preparation method of above-mentioned barrier film, in step a, the polyethylene of A layer or contain the relatively easy fusion plastification of poly resin, therefore can adopt directly from double screw extruder charge door pan feeding, the mode of side direction feeding diluent, also can adopt first with polyethylene or contain poly resin and diluent fully mix molten swollen after, then the mode that adds from spout.The polyethylene of A layer or contain poly resin and can select the double screw extruder of draw ratio more than 40, melting mixing between 190 ℃～250 ℃.The Kynoar of B layer or the copolymer of hexafluoropropylene-vinylidene, due to certain polarity being arranged, the resin melting viscosity is larger, with respect to the polyethylene of A layer or contain poly resin and will more be difficult to mixing, therefore need to select the stronger double screw extruder of mixing ability, can select the double screw extruder of draw ratio more than 60, melting mixing between 200 ℃～250 ℃.Simultaneously at the rapid stirring that must heat up before adding extruder, can add extruder after fully swelling is even dissolved together with diluent and inorganic filler in blender, can be in homogenizer 90 ℃ stirred 2～10 hours.
In above-mentioned steps a, diluent used is that the difficult solvent flashing of higher boiling point of good intermiscibility is arranged with A layer and B layer under high temperature, plays two kinds of effects of plasticising and pore-forming.Therefore, described diluent is selected from a kind of in nonane, decane, atoleine, dioctyl phthalate (DOP).
In step a, in polymer and diluent that the A layer is used, the percentage of diluent accounts for 50%-80wt%, and the barrier film porosity is too low very little for content, and processing and forming is difficult too much for content.In the polymer that the B layer is used and diluent and inorganic filler, the percentage of diluent accounts for 30%～60wt%, and the percentage of inorganic filler accounts for 5%～20%.Mainly between 0.1～0.6um, porosity is 30%～50% to the pore-size distribution of A layer.The inorganic filler that the B layer is used, effect have been mainly porosity and the apertures that increase to stretch forms, and it can be the oxide etc. of the elements such as siliceous, calcium, aluminium, germanium, zirconium, and particle diameter is that 10～100nm is the best; Specifically can be selected from and be nano-calcium carbonate or nano silicon.
Mainly between 0.5～1.5um, porosity is 50%～80% to the pore-size distribution of A layer.Aperture and the porosity of B layer are larger than A layer, can reduce like this membrane surface ionic conduction resistance, can also guarantee that the diluent of A layer in step c can be by wash-out is complete fully.
Chilling speed in step a is preferably in per minute more than 50 ℃, can make like this Microphase Separation finer and closely woven, helps to form many and fine and closely woven micropore.
In step b, biaxial tension can be step drawing, can be also synchro-draw.Step drawing is compared with synchro-draw can realize relatively large stretching ratio, makes barrier film that larger mechanical strength be arranged; Synchro-draw stretches with respect to distributing, and the more approaching circle of the micropore of formation is conducive to the conduction of ion, so both respectively have characteristics.During step drawing, the stretching ratio of longitudinal stretching is 3～6 times, preheat temperature is 115～125 ℃, and draft temperature is 110～120 ℃, and setting temperature is 115～125 ℃, preheat temperature will make the abundant preheating of diaphragm higher than draft temperature, draft temperature can not be too low, otherwise the fluororesin necking phenomenon can be very serious, and setting temperature is as far as possible high, and indulge and draw the distance between horizontal drawing as far as possible short, keep vertical pulling-on piece temperature to prevent cooling as far as possible; The preheat temperature of cross directional stretch is 120～130 ℃, and draft temperature is 115～125 ℃, and setting temperature is 120～130 ℃, and stretching ratio is 5～8 times.During synchro-draw, preheat temperature is 120～130 ℃, and draft temperature is 115～125 ℃, and setting temperature is 120～130 ℃, and each direction of stretching ratio is 2～4 times, and multiplying power is easily disconnected film during greater than 4 times.
In step c, described eluting solvent is the good solvent of diluent described in step a, and the poor solvent of polymer is selected from a kind of in alkane, halogenated hydrocarbons, ketone.
In steps d, described thermal finalization is in order to reduce the film contraction distortion, is to be 120～130 ℃ in preheat temperature, and draft temperature is 115～125 ℃, setting temperature is under 120～130 ℃ of conditions, film to be carried out the little multiplying power cross directional stretch of 1～2 times, and heat-setting time is preferably in 0.5～5min.In type-approval process, in order further to reduce to shrink, can also do 5%～10% retraction in the typing section.
The following example is to further explanation of the present invention and explanation, and the present invention is not constituted any limitation.
Embodiment 1
High density polyethylene (HDPE) (weight average molecular weight 2.5 * 10 with 27 (% by weight)
5, 133 ℃ of fusing points) and the low density polyethylene (LDPE) (weight average molecular weight 5.5 * 10 of 3 (% by weight)
3, 125 ℃ of fusing points) add double screw extruder A (diameter 75mm, L/D=60, strong mixed milling type), with measuring pump side to the atoleine that pumps into 70 (% by weight) (kinematic viscosity 90cst/40 ℃), melting mixing is modulated into solution A under the condition of 200 ℃, 200 rev/mins again.
PVDF (weight average molecular weight 3.5 * 10 with 40 (% by weight)
5, 163 ℃ of fusing points), the DOP (kinematic viscosity 70cst/40 ℃) of 50 (% by weight), after adding stirred tank to mix swelling 8h with the nano-calcium carbonate calcium powder (particle diameter 50nm) of 10 (% by weight), add double screw extruder B (diameter 75mm, L/D=68, strong mixed milling type), melting mixing is modulated into solution B under the condition of 220 ℃, 200 rev/mins.
Two kinds of solution B/A/B in T-shaped die head is three-layer co-extruded, and the Thickness Ratio of three layers is 1: 3: 1, in surface temperature is to draw slab on the slab roller of 15 ℃, obtains the sheet that thickness is 0.8mm.
Sheet is carried out the substep biaxial tension, stretching ratio for vertical * horizontal=4 * 5 times, wherein vertically draw 123 ℃ of preheat temperatures, 118 ℃ of draft temperatures, 123 ℃ of setting temperatures; Horizontal 127 ℃ of the preheat temperatures of drawing, 122 ℃ of draft temperatures, 125 ℃ of setting temperatures.The film that obtains after stretching is clean, dry with butanone.
Then with film 125 ℃ of preheat temperatures, 122 ℃ of draft temperatures, 125 ℃ of thermal finalization 1min of setting temperature stretch 1.6 times, the retraction 6%.Obtain the three-layer composite microporous membrane A1 of thick 25um.
PVDF in embodiment 1 is become weight average molecular weight 2.5 * 10
5, 158 ℃ of fusing points, other are identical with embodiment 1, obtain three-layer composite microporous membrane A2.
PVDF in embodiment 1 is become the copolymer of PVDF-HFP, weight average molecular weight 2.0 * 10
5, 155 ℃ of fusing points, other are identical with embodiment 1, obtain three-layer composite microporous membrane A3.
Embodiment 4
The resin of B layer in embodiment 1 is not added nano-calcium carbonate, use the PVDF (weight average molecular weight 3.5 * 10 with 40 (% by weight)
5, 163 ℃ of fusing points), the DOP (kinematic viscosity 70cst/40 ℃) of 60 (% by weight), other are identical with embodiment 1, obtain three-layer composite microporous membrane A4.
Embodiment 5
Low density polyethylene (LDPE) with the high density polyethylene (HDPE) and 3 (% by weight) of 27 (% by weight) in the raw material of solution A in embodiment 1, become the high density polyethylene (HDPE) of 30 (% by weight), do not add low density polyethylene (LDPE), other are identical with embodiment 1, obtain three-layer composite microporous membrane A5.
Embodiment 6
B/A/B threeply degree ratio in embodiment 1 is become 1: 2: 1, and other are identical with embodiment 1, obtain three-layer composite microporous membrane A6.
Slab thickness in embodiment 1 is become 0.5mm, and the substep biaxial tension changes synchronous bidirectional into and stretches, stretching ratio for vertical * horizontal=3.5 * 3.5 times, wherein preheat temperature is 127 ℃, 122 ℃ of draft temperatures, 125 ℃ of setting temperatures, other are identical with embodiment 1, obtain three-layer composite microporous membrane A7.
Comparative Examples 1
High density polyethylene (HDPE) (weight average molecular weight 2.5 * 10 with 30 (% by weight)
5, 133 ℃ of fusing points) add double screw extruder (diameter 75mm, L/D=60, strong mixed milling type), with measuring pump side to the atoleine that pumps into 70 (% by weight) (kinematic viscosity 90cst/40 ℃), melting mixing is modulated into solution under the condition of 200 ℃, 200 rev/mins again.
By T-shaped die head, be cooling slab on the slab roller of 15 ℃ with solution in surface temperature, obtain the sheet of thickness 0.8mm.
Sheet is carried out the substep biaxial tension, stretching ratio for vertical * horizontal=4 * 5 times, wherein vertically draw 123 ℃ of preheat temperatures, 118 ℃ of draft temperatures, 123 ℃ of setting temperatures; Horizontal 127 ℃ of the preheat temperatures of drawing, 122 ℃ of draft temperatures, 125 ℃ of setting temperatures.The film that obtains after stretching is clean, dry with heptane.
Then with film 125 ℃ of preheat temperatures, 122 ℃ of draft temperatures, 125 ℃ of thermal finalization 1min of setting temperature stretch 1.6 times, the retraction 5%.Obtain the individual layer PE film D1 of thick 25um.
Comparative Examples 2
High density polyethylene (HDPE) (weight average molecular weight 2.5 * 10 with 30 (% by weight)
5, 133 ℃ of fusing points) add double screw extruder (diameter 75mm, L/D=60, strong mixed milling type), with measuring pump side to the atoleine that pumps into 70 (% by weight) (kinematic viscosity 90cst/40 ℃), melting mixing is modulated into solution under the condition of 200 ℃, 200 rev/mins again.
By T-shaped die head, be cooling slab on the slab roller of 15 ℃ with solution in surface temperature, obtain the sheet of thickness 0.5mm.
Sheet is carried out the substep biaxial tension, stretching ratio for vertical * horizontal=4 * 5 times, wherein vertically draw 123 ℃ of preheat temperatures, 118 ℃ of draft temperatures, 123 ℃ of setting temperatures; Horizontal 127 ℃ of the preheat temperatures of drawing, 122 ℃ of draft temperatures, 125 ℃ of setting temperatures.The film that obtains after stretching is clean, dry with heptane.
Then with film 125 ℃ of preheat temperatures, 122 ℃ of draft temperatures, 125 ℃ of thermal finalization 1min of setting temperature stretch 1.6 times, the retraction 5%.Obtain the individual layer PE film of thick 16um.
PVDF (weight average molecular weight 3.5 * 10 with 5 (% by weight)
5, 163 ℃ of fusing points) become solution with the acetone of 95 (% by weight) in stirrer for mixing, then add and account for total solution weight than 5 ‰ toluene, be mixed into the coating fluid of transparent homogeneous.Again with the individual layer PE film immersion that obtains in coating fluid, then the PE film is taken out and makes the acetone volatilization, then make the toluene volatilization 90 ℃ of bakings, obtain both sides and all be coated with PVDF, gross thickness is the film D2 of 25um.
Comparative Examples 3
The atoleine that the formula rate that the atoleine of the high density polyethylene (HDPE) and 70 (% by weight) of 30 (% by weight) in comparative example 2 is modulated into solution changes the high density polyethylene (HDPE) and 80 (% by weight) of 20 (% by weight) into is modulated into solution.Other are identical with comparative example 2, obtain barrier film D3.
Method of testing
1, air permeability: use the standard of GB5402-2003, with the test of GURLEY Permeability gauge.
2, porosity: use standard and the method test of ASTM D2873.
3, pore-size distribution: use the mercury injection method test.
4, diluent residual quantity: take the barrier film of 10g, weight is accurate to 1mg, and note is m1.With scissors, barrier film is shredded into tiny fragment, then clean 5min with analytically pure hexane in ultrasonic cleaner, again take barrier film weight after solvent fully volatilizees, weight is accurate to 1mg, and note is m2.Diluent residual quantity computing formula is (m1-m2)/m1 * 100%.
5, shrinkage: use standard and the method test of GB 12027-2004.
6, hot strength: use standard and the method for testing test of GB/T1040.3-2006.
7, closed pore broken film temperature: Fig. 1 shows the testing apparatus figure of closed pore broken film temperature.With two thickness nickel foil (A, B) that is 10um, with a slice nickel foil A, be labeled as the 9 square parts that stay each 10mm of length and width on slide 2, cover fixing (seeing Fig. 2) simultaneously with insulating tape 8 (dash area in figure).On the ceramic wafer 4 that is connected with thermocouple 3, another nickel foil of load B, be labeled as 10 (seeing Fig. 3), use the lithium fluoroborate (solvent is isobutyl carbonate propylene glycol ester/ethylene carbonate ester/gamma-butyrolacton=1/1/2) of 1mol/l as electrolyte, with placed on it with the above-mentioned electrolyte dipping test barrier film sample 1 of 2 hours, the slide that nickel foil A is arranged is sticked in load thereon, and then load silicon rubber 5.It is arranged on electric hot plate 7, applies with hydraulic press 6 under the pressure state of 1.5MPa, with the speed of 15 ℃/min, temperature is warmed up to 200 ℃ from 25 ℃.Exchanging 1V, under the condition of 1kHz, measuring resistance changes.During with 25 ℃ the resistance of barrier film is as barrier film resistance, and the temperature when reaching 1000 Ω with resistance is as closed pore temperature, reaches temperature when resistance falls after rise to 1000 Ω again after closed pore state as broken film temperature.Electrode interface is labeled as 11 in the drawings.
The test result that obtains according to above-mentioned method of testing is recorded in table 1.
Table 1 test result
Annotate: in embodiment 1-7, therefore pore-size distribution has difference to be divided into two parts due to A, B two-layer aperture, what the aperture was little is the A layer, and what the aperture was large is the B layer.Wherein embodiment 4 is due to plus nano calcium carbonate not, and the aperture of B layer is obviously little than other embodiment.In Comparative Examples 1, only has the PE layer; In Comparative Examples 2-3, the small-bore is the PE layer, and the large aperture is the PVDF layer of coating.
Can find out from the test result of top table 1, the broken film temperature of the barrier film of embodiment 1-7 preparation is all greater than 169 ℃, the broken film temperature of the barrier film of making apparently higher than Comparative Examples 1-3; And the MD hot strength of the barrier film of embodiment 1-7 and TD hot strength obviously are better than the barrier film of Comparative Examples especially; And the porosity of the barrier film of embodiment 1-7 preparation is higher.Therefore, lithium ion battery separator closed pore temperature provided by the invention is low, and broken film temperature is high, and mechanical strength is high, can effectively improve the security performance of the lithium ion battery that uses barrier film of the present invention.
Self discharge relatively
Making with embodiment 1 the three layers of barrier film of B/A/B that obtain is made into battery and is denoted as battery A, be made into battery with three layers of barrier film of PP/PE/PP of 25um and be denoted as battery B, battery A, B all are charged to the state of 3.85V, after placing a period of time, contrast its self discharge by comparing its voltage drop.Battery production method is as follows:
With cobalt acid lithium powder, acetylene black conductor powder, polyvinylidene fluoride and 1-METHYLPYRROLIDONE (NMP) according to 100: 3: 2: 40 weight ratio is evenly mixed, be modulated into anode sizing agent, this slurry is coated on the plus plate current-collecting body aluminium foil, then dry, compressing tablet, be cut into the anode pole piece of 485 * 44mm, this positive plate contains 6.9g cobalt acid lithium powder.
With graphite, electric conduction of carbon fiber agent, polytetrafluoroethylene and deionized water according to 100: 1.5: 3: 135 weight ratio is mixed, be modulated into cathode size, this slurry is coated on the negative current collector Copper Foil, then dry, compressing tablet, be cut into the cathode pole piece of 477 * 44mm, contain 3.2g graphite on this negative plate.
Take LiPF6 as lithium salts, be in the solvent of 1: 1, to make its concentration reach 1mol/L with its volume ratio that is dissolved into EC and EMC, obtain nonaqueous electrolytic solution.
With the overlapping rectangular cell battery core that is wound into of the above-mentioned positive plate of making, negative plate and barrier film, be inserted in 5 * 34 * 50mm cubic battery, inject electrolyte and sealing, then ageing in a conventional manner, sealing obtains LP053450 type square lithium ion secondary battery.The design capacity of this battery is 1C=900mAh.
Table 2 self discharge compare test result
| Initial voltage (V) | After 15 days (V) | After 30 days (V) | After 45 days (V) | After 60 days (V) | |
| Battery A | 3.850 | 3.844 | 3.838 | 3.834 | 3.832 |
| Battery B | 3.850 | 3.847 | 3.846 | 3.845 | 3.845 |
Can find out from the test result of top table 2, the self discharge of the lithium ion battery of use barrier film of the present invention is significantly little than the self discharge of the lithium ion battery of using three layers of barrier film of PP/PE/PP, and battery performance is better.
Claims (10)
1. a lithium ion battery separator, comprise the B/A/B three-layer composite microporous membrane, and wherein, described A layer is that molecular weight is 1 * 10
5-1 * 10
6High density polyethylene (HDPE) and molecular weight be 4 * 10
3-1 * 10
4The mixture of low density polyethylene (LDPE), the fusing point of described low density polyethylene (LDPE) is that the mass percent that 125 ℃～128 ℃, content account for mixture is 5%-20%; Described B layer is the copolymer of Kynoar or hexafluoropropylene-vinylidene; Described barrier film forms through the three-layer co-extruded pressure of B/A/B, biaxial tension, and wherein, A layer, B layer are made by thermally induced phase separation; The hot strength 120-170Mpa of described barrier film;
The Thickness Ratio of described B layer and described A layer is 1:2-1:4;
The gross thickness of described B/A/B three-layer composite microporous membrane is 20-50um, and the thickness of described B layer is 4-8um.
2. lithium ion battery separator according to claim 1, wherein, the molecular weight of the copolymer of described Kynoar and hexafluoropropylene-vinylidene is 2.0 * 10
5-5 * 10
5, fusing point is 145 ℃-170 ℃.
3. the application of lithium ion battery separator as claimed in claim 1 or 2 in lithium ion battery.
4. the preparation method of a lithium ion battery separator comprises the following steps:
Step a, polymer and diluent blending extrusion in extruder that the A layer is used, the polymer that the B layer is used, diluent and inorganic filler blending extrusion in extruder, then pass through in die head compoundly with three layers of B/A/B, after co-extrusion, chilling obtains composite sheet; Wherein, described A layer is that molecular weight is 1 * 10
5-1 * 10
6High density polyethylene (HDPE) and molecular weight be 4 * 10
3-1 * 10
4The mixture of low density polyethylene (LDPE), the fusing point of described low density polyethylene (LDPE) is that the mass percent that 125 ℃～128 ℃, content account for mixture is 5%-20%, B layer polymer used is the copolymer of Kynoar or hexafluoropropylene-vinylidene;
Step b, will composite sheet become film through after biaxial tension;
Step c, the film that step b is obtained pass through the eluting solvent wash-out, and dry;
Steps d, the film thermal typing that step c is obtained.
5. the preparation method of lithium ion battery separator according to claim 4, wherein, in step a, described inorganic filler is nano-calcium carbonate or nano silicon.
6. the preparation method of lithium ion battery separator according to claim 4, wherein, in step a, described diluent is selected from a kind of in nonane, decane, atoleine, dioctyl phthalate.
7. the preparation method of lithium ion battery separator according to claim 4, wherein, in step a, in polymer and diluent that the A layer is used, the percentage of diluent accounts for 50wt%-80wt%; In the polymer that the B layer is used and diluent and inorganic filler, the percentage of diluent accounts for 30wt%～60wt%, and the percentage of inorganic filler accounts for 5wt%～20wt%.
8. the preparation method of lithium ion battery separator according to claim 4, wherein, in step b, biaxial tension is synchro-draw, and draft temperature is 115-125 ℃, and each direction of stretching ratio is 2～4 times.
9. the preparation method of lithium ion battery separator according to claim 4, wherein, in step b, biaxial tension is step drawing, and the stretching ratio of longitudinal stretching is 3～6 times, and draft temperature is 110～120 ℃; The stretching ratio of cross directional stretch is 5～8 times, and draft temperature is 115～125 ℃.
10. the preparation method of lithium ion battery separator according to claim 4, wherein, in steps d, described thermal finalization is under 120-130 ℃ of condition, film to be carried out 1-2 stretching doubly, the time is 0.5-5min.
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