Preparation method and application of aqueous mixed binder

Abstract

The invention discloses a preparation method and application of a water-based mixed binder, and relates to the technical field of water-based binders. The water-based mixed binder is obtained by mixing and compounding a water-based polyurethane emulsion and polyvinylidene fluoride. Wherein in the preparation process of the waterborne polyurethane emulsion, amino-terminated polyacrylic acid-catechol is added to serve as a chain extender for use, the amino-terminated polyacrylic acid-catechol is prepared by initiating polymerization of 4-allyl catechol, alkenyl siloxane and an acrylic monomer, and after polyacrylic acid-catechol is prepared, the amino-terminated polyacrylic acid-catechol is added to serve as a chain extender for use; and carrying out an ultraviolet irradiation click reaction on the obtained product and mercaptoacetic acid ethanolamine ester. The waterborne polyurethane emulsion prepared by the invention can greatly improve the electrochemical performance of the lithium battery, and especially has a great improvement effect on the peel strength, the cycle discharge specific capacity and the coulombic efficiency of the lithium battery.

Description

Technical Field

[0001] The present invention relates to the technical field of aqueous binders, and particularly to a preparation method and application of an aqueous mixed binder. Background Art

[0002] Environmentally friendly lithium-ion batteries are widely used in portable devices and are considered the next-generation power source for electric vehicles, hybrid electric vehicles, and smart grids. The main materials of lithium-ion battery electrodes are active material powders, conductive agents, and polymer binders. Among them, the performance of the binder has a very great influence on the stability and cycle rate of the battery.

[0003] Common binders include polyvinylidene fluoride (PVDF) binder, polyacrylic acid (PAA) binder, chitosan (CTS) binder, carboxymethyl cellulose (CMC-Li, CMC-Na, etc.) binder, styrene-butadiene rubber (SBR) binder, polytetrafluoroethylene (PTFE) binder, and their derivative binders. Among them, although PVDF is widely used in lithium-ion batteries, at conventional operating temperatures, the high crystallinity of PVDF will cause molecules with electrolytes to be difficult to circulate, thereby increasing the charge and discharge load of the battery; in addition, if the drying rate of the battery is not appropriate, the shrinkage rate of PVDF does not match the shrinkage rate of the current collector, which will cause the coating to separate from the current collector. Over time, due to the stress inside the electrode, the electrode layer may partially or completely peel off from the current collector, resulting in poor load characteristics and capacity attenuation; at the same time, it has the disadvantages of high price, difficult to recycle, and requires the use of volatile organic solvents for processing. Although the other binders are not inferior to conventional PVDF in terms of battery performance, there are still certain deficiencies in some performances, such as: CMC-Na and PAA have problems such as insufficient bonding strength and high brittleness; the cycle stability of CTS fails to meet the standard.

[0004] In summary, to solve the above problems, the present invention provides an aqueous mixed binder, which can effectively improve the electrochemical performance of lithium-ion batteries, especially in terms of peel strength, cyclic discharge specific capacity, Coulomb efficiency, etc. Summary of the Invention

[0005] The purpose of the present invention is to provide a preparation method and application of an aqueous mixed binder to solve the problems raised in the above background art.

[0006] To solve the above technical problems, the present invention provides the following technical solutions:

[0007] A preparation method of an aqueous mixed binder, comprising the following steps:

[0008] Step 1: Add polyether diol and polyester diol into a reaction vessel. After vacuum dehydration at 100 °C for 0.5 - 2 h, add diisocyanate thereto, then cool down to 88 - 95 °C and react for 1 - 5 h; cool down to 42 - 48 °C, add a diol chain extender and amino-terminated polyacrylate-catechol thereto, then heat up to 72 - 78 °C and react for 1 - 5 h; cool down to 42 - 48 °C, add a catalyst thereto, then heat up to 68 - 75 °C and react for 3 - 6 h; cool down to 42 - 48 °C, discharge the material, and add a certain amount of deionized water under high-speed stirring for emulsification. After emulsification, add a post-chain extender and react for 1 - 3 h to prepare an aqueous polyurethane emulsion;

[0009] Step 2: Mix and compound the aqueous polyurethane emulsion and polyvinylidene fluoride to obtain an aqueous mixed binder.

[0010] Further, the polyether diol includes, but is not limited to, one or more combinations of those with models N210, N-220, N-310, N-320, and N-330.

[0011] Further, the polyester diol is a non-ionic polyester diol with the model YmerN-120.

[0012] Further, the diisocyanate includes, but is not limited to, one or more combinations of isophorone diisocyanate, hexamethylene diisocyanate, 2,4-toluene diisocyanate, diphenylmethane diisocyanate, and 4,4'-dicyclohexylmethane diisocyanate.

[0013] Further, the diol chain extender includes, but is not limited to, one or two combinations of 2,2-dimethylolpropionic acid and 2,2-dimethylolbutyric acid.

[0014] Further, the catalyst includes, but is not limited to, one or more combinations of stannous octoate, stannous isooctoate, dibutyltin dilaurate, and dibutyltin laurate.

[0015] Further, the post-chain extender includes, but is not limited to, one or two combinations of ethylenediamine and isophorone diamine.

[0016] Further, the raw materials required for preparing the aqueous polyurethane emulsion include the following components in parts by weight: 25 - 30 parts of polyether diol, 25 - 30 parts of polyester diol, 18 - 24 parts of diisocyanate, 2 - 4 parts of diol chain extender, 1 - 2 parts of amino-terminated polyacrylate-catechol, 0.15 - 0.2 part of catalyst, 0.5 - 2 parts of post-chain extender, and 60 - 80 parts of deionized water.

[0017] Further, the preparation method of the amino-terminated polyacrylate-catechol is as follows:

[0018] (1) Add 4-allylcatechol, alkenylsiloxane, and acrylic monomer to toluene with a weight five times that of the sum of the three, stir and mix to obtain a monomer solution; add a part of the monomer solution to a reaction vessel, add azobisisobutyronitrile, and react at 70-80 °C for 2-6 h under nitrogen protection. Then, add the remaining reaction solution to the reaction system in 1-3 portions, with each reaction lasting 1-4 h. After vacuum distillation, polyacrylic acid-catechol is obtained.

[0019] (2) Add polyacrylic acid-catechol, mercaptoacetic acid ethanolamine ester, and photoinitiator to toluene with a weight five times that of the sum of the three, stir and mix, irradiate with ultraviolet light, and stir and react for 1-3 h. After vacuum distillation, amino-terminated polyacrylic acid-catechol is obtained.

[0020] Further, the alkenylsiloxane includes, but is not limited to, one or a combination of more of vinyl-terminated polydimethylsiloxane, phenylvinyl silicone oil, and polydimethylmethylvinylsiloxane.

[0021] Further, the acrylic monomer includes, but is not limited to, one or a combination of more of methacrylic acid, acrylic acid, acrylamide, and hydroxyethyl acrylate.

[0022] Further, the mass ratio of 4-allylcatechol, alkenylsiloxane, acrylic monomer, azobisisobutyronitrile, and toluene is (0.1-0.3):(0.8-1):2:(0.01-0.02).

[0023] Further, the mass ratio of polyacrylic acid-catechol, mercaptoacetic acid ethanolamine ester, and photoinitiator is 1:(0.2-0.4):(0.001-0.005).

[0024] Further, the mass ratio of the aqueous polyurethane emulsion and polyvinylidene fluoride is (0.5-1.5):1.

[0025] By initiating the polymerization of 4-allylcatechol, alkenylsiloxane, and acrylic monomer, polyacrylic acid-catechol is prepared; then, through a click reaction, mercaptoacetic acid ethanolamine ester is used to cap it to obtain amino-terminated polyacrylic acid-catechol, which is used as a chain extender to participate in the preparation of aqueous polyurethane, increasing the molecular chain length and crosslinking density of the aqueous polyurethane. Moreover, since amino-terminated polyacrylic acid-catechol contains polyacrylic acid chain ends, siloxane chain ends, and catechol groups, the polyacrylic acid chain ends are rich in functional groups and can form good interactions with the active substances in the battery, enhancing the adhesion while protecting the active substances; the siloxane chain ends and catechol groups can further assist to significantly improve the adhesion performance of the binder.

[0026] Further, for the application of the aqueous mixed binder, lithium iron phosphate particles, conductive carbon black, and the aqueous mixed binder are stirred and mixed, and sodium carboxymethylcellulose is added to prepare a slurry, which is then coated onto a current collector for the preparation of a battery;

[0027] The specific method for preparing the slurry is as follows: (1) Sodium carboxymethylcellulose is added to deionized water and stirred and mixed to prepare a 2-3 wt% sodium carboxymethylcellulose solution; polyvinylidene fluoride is added to N-methylpyrrolidone and stirred and mixed to prepare a 4-6 wt% polyvinylidene fluoride solution; (2) Lithium iron phosphate particles, conductive carbon black, aqueous polyurethane emulsion, and polyvinylidene fluoride solution are added to the sodium carboxymethylcellulose solution and stirred and mixed to prepare a slurry.

[0028] Further, the mass ratio of the lithium iron phosphate particles, conductive carbon black, aqueous polyurethane emulsion, and polyvinylidene fluoride solution is (6-8):(0.5-1.5):(0.5-1.5):1.

[0029] Furthermore, the mass ratio of the lithium iron phosphate particles, conductive carbon black, aqueous polyurethane emulsion, and polyvinylidene fluoride solution is 6:1.5:1.5:1 or 7:1:1:1 or 8:0.5:0.5:1.

[0030] Compared with the prior art, the beneficial effects achieved by the present invention are as follows:

[0031] (1) In the present invention, amino-terminated polyacrylic acid-catechol is prepared and participates in the preparation of aqueous polyurethane, increasing the internal molecular chain length and crosslinking density of the aqueous polyurethane, and introducing polypropylene chain ends, siloxane chain ends, and catechol groups, thereby comprehensively obtaining an aqueous polyurethane emulsion that can promote adhesion;

[0032] (2) The aqueous polyurethane emulsion prepared in the present invention is added to polyvinylidene fluoride to obtain an aqueous mixed binder; in subsequent applications, sodium carboxymethylcellulose is additionally added, which has good dispersibility in an aqueous medium and can make the aqueous polyurethane emulsion and polyvinylidene fluoride more uniformly dispersed, thereby better improving the electrochemical performance of lithium batteries, especially in terms of peel strength, cyclic discharge specific capacity, and Coulomb efficiency. Specific Embodiments

[0033] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0034] It should be noted that the following parts are by weight parts, and there are no special restrictions on the purchasing manufacturers of the raw materials involved in the present invention. Exemplarily, they include:

[0035] In the following examples, isophorone diisocyanate, polypropylene glycol (N-220, Mn = 2000), trimethylolpropane polyethylene glycol monomethyl ether (Ymer N-120, Mn = 1000), and 2,2-dimethylolpropionic acid are all of industrial purity; dibutyltin dilaurate, stannous octoate, ethylenediamine, and polyvinylidene fluoride are all of analytical purity; sodium carboxymethyl cellulose is of battery grade; all are purchased from Macklin Biochemical Technology Co., Ltd.;

[0036] Lithium iron phosphate, of battery grade, is purchased from Zhangzhou Wanbao Energy Technology Co., Ltd.;

[0037] Acetylene black, of battery grade, is purchased from Tianjin Youmeng Chemical Technology Co., Ltd.;

[0038] The separator, model Celgard 2400, is purchased from Celgard LLC;

[0039] Lithium foil, of battery grade, is purchased from Tianjin Zhongneng Lithium Industry Co., Ltd.;

[0040] 4-Allylpyrocatechol and vinyl-terminated polydimethylsiloxane, CAS No.: 68083-19-2, are both of industrial purity and are both purchased from Zhengzhou Alpha Chemical Co., Ltd.;

[0041] Ethanolamine thioglycolate, of industrial purity, CAS No.: 126-97-6, is purchased from Hubei Jusheng Technology Co., Ltd.;

[0042] The photoinitiator, of analytical purity, model: IHT-PI 432, is purchased from Shanghai Jinjinle Industrial Co., Ltd.

[0043] The rest are all commercially available; in addition, each of the following parts is 100 g.

[0044] Example 1: A preparation method and application of an aqueous mixed binder:

[0045] Step 1:

[0046] 1. Preparation of amino-terminated polyacrylic acid-catechol: (1) Mix 0.2 parts of 4-allyl catechol, 0.9 parts of vinyl-terminated polydimethylsiloxane, 2 parts of acrylic acid monomers (a mixture of methacrylic acid, acrylic acid, acrylamide, and hydroxyethyl acrylate in a mass ratio of 4:4:1:1), and 15.5 parts of toluene evenly to obtain a monomer solution; Add one-third of the monomer solution into the reaction vessel, add 0.015 parts of azobisisobutyronitrile, and react at 75 °C for 4 h under nitrogen protection. Then, add the remaining reaction solution to the reaction system in 2 portions, each reaction for 3 h, and obtain polyacrylic acid-catechol through vacuum distillation; (2) Add 2 parts of polyacrylic acid-catechol, 0.6 parts of mercaptoacetic acid ethanolamine ester, 0.004 parts of photoinitiator, and 13 parts of toluene into the reaction vessel, stir and mix, irradiate with ultraviolet light, stir and react for 1 h, and obtain amino-terminated polyacrylic acid-catechol through vacuum distillation;

[0047] 2. Preparation of aqueous polyurethane emulsion: Add 28 parts of N-220 and 28 parts of Ymer N-120 into a three-necked flask equipped with a reflux condenser and a thermometer, vacuum dehydrate at 100 °C for 1 h, then add 22 parts of isophorone diisocyanate, and cool down to 90 °C to react for 3 h; Cool down to 45 °C, add 3 parts of 2,2-dimethylolpropionic acid and 1.5 parts of amino-terminated polyacrylic acid-catechol, then heat up to 75 °C to react for 2 h; Cool down to 45 °C, add 0.18 parts of catalyst (a mixture of dibutyltin dilaurate and stannous octoate in a mass ratio of 1:1), then heat up to 70 °C to react for 4 h; Cool down to 45 °C, discharge the material, and add 70 parts of deionized water under high-speed stirring for emulsification. After emulsification, add 1.2 parts of ethylenediamine and react for 2 h to prepare an aqueous polyurethane emulsion;

[0048] Step two: (1) Add sodium carboxymethylcellulose to deionized water, stir and mix to prepare a sodium carboxymethylcellulose solution with a concentration of 2.5 wt%; Add polyvinylidene fluoride to N-methylpyrrolidone, stir and mix to prepare a 5 wt% polyvinylidene fluoride solution; (2) Add lithium iron phosphate particles, conductive carbon black, aqueous polyurethane emulsion, and polyvinylidene fluoride solution to the sodium carboxymethylcellulose solution, stir and mix to prepare a slurry for standby;

[0049] Among them, the mass ratio of the lithium iron phosphate particles, conductive carbon black, aqueous polyurethane emulsion, and polyvinylidene fluoride is 6:1.5:1.5:1; The addition amount of sodium carboxymethylcellulose is 0.3% of the total mass of the lithium iron phosphate particles, conductive carbon black, aqueous polyurethane emulsion, and polyvinylidene fluoride.

[0050] The following is based on Example 1, and Examples 2-3 and Comparative Example 1 are set as follows:

[0051] Example 2: Example 2 is based on Example 1 and adjusted as follows: the mass ratio of the lithium iron phosphate particles, conductive carbon black, aqueous polyurethane emulsion, and polyvinylidene fluoride is 7:1:1:1.

[0052] Example 3: Example 3 is based on Example 1 and adjusted as follows: the mass ratio of the lithium iron phosphate particles, conductive carbon black, aqueous polyurethane emulsion, and polyvinylidene fluoride is 8:0.5:0.5:1.

[0053] Comparative Example 1: Comparative Example 1 is based on Example 1 and adjusted as follows: polyvinylidene fluoride solution is used as the binder; the mass ratio of the lithium iron phosphate particles, conductive carbon black, and polyvinylidene fluoride is 8:1:1.

[0054] Performance Test: (1) Coat the slurries obtained from the above examples and comparative examples onto aluminum foil. After coating, place it in a vacuum oven at 100 °C for 6 h, and then place it in a glove box filled with high-purity argon (more than 99.5%) for 24 h. After drying, ensure that the average loading of the active material is 0.8 mg / cm 2 , and finally cut it into a positive electrode disc with a diameter of 6 mm; (2) In a glove box filled with high-purity argon (more than 99.5%), use a lithium sheet as the negative electrode, and a separator with the model of Celgard 2400 as the separator, assemble the electrode sheets into a CR2025 type button battery, and inject a commercial 4.35 V electrolyte to obtain a button battery; (3) After the button battery stands for 12 h, conduct peel strength test, cycle performance test, and mechanical property test of the adhesive film respectively.

[0055] The test methods are as follows:

[0056] ① Peel Strength Test: Cut the dried electrode sheet into a strip with a size of 20 mm × 100 mm, paste 3M tape on the surface of the electrode sheet, and then use a multi-functional tensile testing machine to conduct a 180° peel test on the electrode sheet at a tensile speed of 100 mm / min to compare the bonding performance of different compounded binders;

[0057] ② Cycle Performance Test: Use a Neware battery tester to test the cycle performance at a current density of 1C at 25 °C;

[0058] ③ Mechanical Property Test of the Adhesive Film: Cut the adhesive film into a dumbbell shape with a size of 4 mm × 20 mm, use an intelligent tensile machine to test it at a tensile speed of 200 mm / min, repeat the test 3 times, and take the average value.

[0059] The specific test data are shown in Table 1 below:

[0060] Table 1

[0061]

[0062] Conclusion: By comparing the data of the examples and comparative examples in Table 1 above, it can be found that the examples with the compounded aqueous polyurethane emulsion are superior to Comparative Example 1 in terms of peel strength, tensile strength, and cycling performance. The maximum peel strength of the electrode in the examples can reach 0.51 N / cm. After 500 cycles, the Coulombic efficiency has good performance, reaching a maximum of 97.5%. The reason is that the addition of the aqueous polyurethane emulsion can enhance the hydrogen bond force between the active material and the binder, making it less likely to fall off, so the rate of capacity decay slows down. By comparing the data of Examples 1 to 3, it can be found that the slurry obtained by mixing lithium iron phosphate particles, conductive carbon black, aqueous polyurethane emulsion, and polyvinylidene fluoride solution in a mass ratio of 6:1.5:1.5:1 in Example 3, and the assembled battery has significantly better electrochemical performance than Examples 1 and 2.

[0063] In summary, by compounding the aqueous polyurethane emulsion in polyvinylidene fluoride, the present invention can effectively improve the electrochemical performance of the battery, and has broad prospects.

[0064] Finally, it should be noted that the above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A preparation method of an aqueous mixed binder, characterized in that: It includes the following steps: Step 1: Add polyether diol and polyester diol into a reaction vessel. After vacuum dehydration at 100 °C for 0.5 - 2 h, add diisocyanate thereto, then cool down to 88 - 95 °C and react for 1 - 5 h; cool down to 42 - 48 °C, add a diol chain extender and amino-terminated polyacrylic acid-catechol thereto, then heat up to 72 - 78 °C and react for 1 - 5 h; cool down to 42 - 48 °C, add a catalyst thereto, then heat up to 68 - 75 °C and react for 3 - 6 h; cool down to 42 - 48 °C, discharge the material, and add a certain amount of deionized water under high-speed stirring for emulsification. After emulsification, add a post-chain extender and react for 1 - 3 h to prepare an aqueous polyurethane emulsion; Step 2: Mix and compound the aqueous polyurethane emulsion and polyvinylidene fluoride to obtain an aqueous mixed binder.

2. The preparation method of an aqueous hybrid binder according to claim 1, characterized in that: The polyether diol includes one or a combination of more than one of the types N210, N-220, N-310, N-320, N-330; the polyester diol is a non-ionic polyester diol with the type YmerN-120; the diisocyanate includes one or a combination of more than one of isophorone diisocyanate, hexamethylene diisocyanate, 2,4-toluene diisocyanate, diphenylmethane diisocyanate, 4,4'-dicyclohexylmethane diisocyanate; the diol chain extender includes one or a combination of two of 2,2-dimethylolpropionic acid and 2,2-dimethylolbutyric acid; the catalyst includes one or a combination of more than one of stannous octoate, stannous isooctoate, dibutyltin dilaurate, dibutyltin laurate; the post-chain extender includes one or a combination of two of ethylenediamine and isophorone diamine.

3. The preparation method of an aqueous hybrid binder according to claim 1, characterized in that: The raw materials required for preparing the aqueous polyurethane emulsion include the following components in parts by weight: 25 - 30 parts of polyether diol, 25 - 30 parts of polyester diol, 18 - 24 parts of diisocyanate, 2 - 4 parts of diol chain extender, 1 - 2 parts of amino-terminated polyacrylic acid-catechol, 0.15 - 0.2 part of catalyst, 0.5 - 2 parts of post-chain extender, and 60 - 80 parts of deionized water.

4. The preparation method of an aqueous hybrid binder according to claim 3, characterized in that: The preparation method of the amino-terminated polyacrylic acid-catechol is as follows: (1) Mix 4-allyl catechol, alkenyl siloxane, acrylic monomer, and toluene evenly to obtain a monomer solution; add part of the monomer solution into a reaction vessel, add azobisisobutyronitrile, and under nitrogen protection, react at 70 - 80 °C for 2 - 6 h, then add the remaining reaction solution to the reaction system in 1 - 3 portions, react for 1 - 4 h each time, and obtain polyacrylic acid-catechol through vacuum distillation; (2) Add polyacrylic acid-catechol, mercaptoacetic acid ethanolamine ester, photoinitiator, and toluene into a reaction vessel, stir and mix, irradiate with ultraviolet light, stir and react for 1 - 3 h, and obtain amino-terminated polyacrylic acid-catechol through vacuum distillation.

5. The preparation method of an aqueous mixed binder according to claim 4, characterized in that: The alkenyl siloxane includes one or a combination of more than one of vinyl-terminated polydimethylsiloxane, phenyl vinyl silicone oil, and polydimethylmethylvinylsiloxane; the acrylic monomer includes one or a combination of more than one of methacrylic acid, acrylic acid, acrylamide, and hydroxyethyl acrylate.

6. The preparation method of an aqueous mixed binder according to claim 4, characterized in that: The mass ratio of the 4-allylcatechol, vinylsiloxane, acrylic monomer, and azobisisobutyronitrile is (0.1 - 0.3):(0.8 - 1):2:(0.01 - 0.02); the mass ratio of the polyacrylic acid-catechol, mercaptoacetic acid ethanolamine ester, and photoinitiator is 1:(0.2 - 0.4):(0.001 - 0.005).

7. The preparation method of an aqueous hybrid binder according to claim 1, characterized in that: The mass ratio of the aqueous polyurethane emulsion and polyvinylidene fluoride is (0.5 - 1.5):

1.

8. Use of an aqueous hybrid binder according to any one of claims 1 to 7, characterized in that: Stir and mix lithium iron phosphate particles, conductive carbon black, and an aqueous mixed binder, and add sodium carboxymethylcellulose to prepare a slurry for the preparation of a battery; Among them, the preparation method of the slurry is specifically as follows: (1) Add sodium carboxymethylcellulose to deionized water, stir and mix to prepare a 2 - 3 wt% sodium carboxymethylcellulose solution; add polyvinylidene fluoride to N-methylpyrrolidone, stir and mix to prepare a 4 - 6 wt% polyvinylidene fluoride solution; (2) Add the lithium iron phosphate particles, conductive carbon black, the aqueous polyurethane emulsion described in claim 1, and the polyvinylidene fluoride solution to the sodium carboxymethylcellulose solution, stir and mix to prepare a slurry.

9. Use of an aqueous hybrid binder according to claim 8, characterized in that: The mass ratio of the lithium iron phosphate particles, conductive carbon black, aqueous polyurethane emulsion, and polyvinylidene fluoride is (6 - 8):(0.5 - 1.5):(0.5 - 1.5):

1.

10. The application of an aqueous hybrid binder according to claim 9, characterized in that: The mass ratio of the lithium iron phosphate particles, conductive carbon black, aqueous polyurethane emulsion, and polyvinylidene fluoride is 6:1.5:1.5:1 or 7:1:1:1 or 8:0.5:0.5:1.

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