CN110676515A - Preparation method of low-temperature high-energy-density lithium ion battery - Google Patents

Preparation method of low-temperature high-energy-density lithium ion battery Download PDF

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CN110676515A
CN110676515A CN201910796330.9A CN201910796330A CN110676515A CN 110676515 A CN110676515 A CN 110676515A CN 201910796330 A CN201910796330 A CN 201910796330A CN 110676515 A CN110676515 A CN 110676515A
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additive
electrolyte
temperature
negative electrode
positive electrode
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CN110676515B (en
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耿铁成
胡琪卉
张琦
朱其峰
文亚洲
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Baoli New Energy Technology Co ltd
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Dragon Technology (ningxia) Co Ltd
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    • H—ELECTRICITY
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    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
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    • H01M4/00—Electrodes
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    • H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
    • H01M4/50—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of manganese
    • H01M4/505—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of manganese of mixed oxides or hydroxides containing manganese for inserting or intercalating light metals, e.g. LiMn2O4 or LiMn2OxFy
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    • H01M4/52—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of nickel, cobalt or iron
    • H01M4/525—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of nickel, cobalt or iron of mixed oxides or hydroxides containing iron, cobalt or nickel for inserting or intercalating light metals, e.g. LiNiO2, LiCoO2 or LiCoOxFy
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Abstract

The invention claims a preparation method of a low-temperature energy type lithium ion power battery, which comprises the following steps: the preparation method of the low-temperature rechargeable lithium ion power battery cell comprises the steps of pole piece manufacturing, secondary rolling, liquid injection, standing, secondary liquid injection, formation, standing and capacity grading. The process disclosed by the invention is simple to operate, the material with excellent low-temperature dynamics is selected, and the processes of secondary rolling and secondary liquid injection are adopted, so that the high energy density is ensured, meanwhile, the sufficient wettability of the electrolyte and the anode and cathode active materials is improved, and the electrochemical reaction is facilitated. And the electrolyte of a proper low-temperature additive is selected, so that the uniformity, compactness and stability of the generated SEI film are ensured, and the influence on the charge and discharge of the battery cell caused by overlarge impedance at low temperature is prevented.

Description

Preparation method of low-temperature high-energy-density lithium ion battery
Technical Field
The invention relates to the technical field of batteries, in particular to the technical field of lithium ion batteries, and particularly relates to a preparation method of a low-temperature high-energy-density power battery cell.
Background
With the rapid development of the industry in China, the continuous development pace is restricted by environmental pollution and resource shortage. In 2012, new energy automobiles are listed as one of seven strategic industries in China, the new energy automobiles enter a high-speed development stage from now on, power batteries are the main energy systems of modern electric automobiles and are one of three major core components, and the key technology of the new energy automobiles needs to be broken through urgently and is the biggest obstacle hindering the development of the electric automobiles.
The traditional power battery is used at low temperature, and the charging time is prolonged when the traditional power battery is reflected to a user side. Since lithium tends to precipitate from the battery when charging is performed at a low temperature, the battery is generally heated slowly before charging, and then charged after the battery temperature rises to a normal temperature. The heating device not only increases the cost of the battery PACK, but also loses the energy density of the entire PACK system. Therefore, the energy density of the battery pack is improved, and not only a power single battery cell with high energy density needs to be developed, but also the single battery cell needs to have a low-temperature charging function.
Disclosure of Invention
In order to overcome the defects in the prior art, the invention provides a preparation method of a low-temperature high-energy-density lithium ion battery.
The preparation method of the low-temperature high-energy density lithium ion battery provided by the invention comprises the following steps:
s1, preparing the positive plate: mixing and dispersing a nickel cobalt lithium manganate positive electrode material and a positive electrode conductive agent, continuously dropwise adding an oil system NMP solution, mixing and stirring uniformly to reach the viscosity of 4000-10000mPa.s, preparing positive electrode slurry, coating the positive electrode slurry on an aluminum foil, drying, rolling and die-cutting to obtain a positive electrode sheet;
s2, preparing the negative plate: mixing and dispersing the amorphous carbon-coated graphite and a negative electrode conductive agent, adding a water-based binder, and uniformly mixing and stirring to reach the viscosity of 2000-8000 mPa.s to obtain negative electrode slurry; coating the negative electrode slurry on the carbon-coated copper foil, drying, and then performing secondary rolling and die cutting to obtain a negative electrode sheet;
and S3, packaging of the battery: placing the positive plate obtained in the step S1 and the negative plate obtained in the step S2 with diaphragms for lamination, then welding tabs, and packaging by a polymer aluminum plastic film;
s4, liquid injection, formation and volume grading: injecting liquid for the first time, injecting first electrolyte, standing for the first time, and pressurizing to form the electrolyte; and then standing for the second time, injecting a second electrolyte, standing for the third time, and finally grading and aging at normal temperature to prepare the lithium ion battery.
In the preparation method of the lithium ion battery, the nickel cobalt lithium manganate positive electrode material (NCM) is selected from one of NCM523, NCM622 and NCM811, and the addition amount of the NCM is 95-98% of that of the positive electrode material;
the N-methylpyrrolidone NMP solution contains 5% -12% of PVDF polyvinylidene fluoride, and the addition amount of NMP is 1.0-3% of the positive electrode material;
the positive electrode conductive agent is selected from at least one of conductive carbon black, carbon nano tubes and graphene, and the addition amount of the positive electrode conductive agent is 0.5-3% of that of the positive electrode material.
In the preparation method of the lithium ion battery, the graphite coated by the amorphous carbon is soft carbon or hard carbon-coated artificial graphite, and the addition amount of the graphite is 94-97.5 percent of the negative electrode material;
the negative electrode conductive agent is selected from at least one of water-based carbon nanotube slurry, conductive carbon black and carbon fiber, and the addition amount of the negative electrode conductive agent is 0.5-2.5% of the negative electrode material;
the water-based binder is at least one of water-based SBR/PVA/PAA, and CMC is added, and accounts for 20-50% of the binder; the addition amount of the aqueous binder is 1.5-4% of the negative electrode material.
The percentages of the above addition amounts are weight fractions.
In the preparation method of the lithium ion battery, in the step S2, the secondary rolling process of the copper foil coated after drying comprises the step of compacting the copper foil for the first time to the density of 1.55-1.65g/cm3Laying aside for 1-24 hr, and compacting for the second time to obtain the product with density of 1.60-1.75g/cm3。
In the preparation method of the lithium ion battery, in step S4, the first electrolyte is injected after packaging, namely the first electrolyte is injected, the edge is pre-sealed, after standing for 1-24 hours at 50-60 ℃, the second electrolyte is sealed for one time, and the vacuum degree required by the second sealing process is less than or equal to-70 KPa; pressurizing at 50-60 deg.C to form a mixture with 0.1-50% SOC, 0.3-80% SOC, and clamp pressure of 4.76-7.93Kgf/cm2The formation temperature is 35-60 ℃; standing for the second time after formation, wherein the temperature is room temperature for 1-24 hours; the injection amount is 10 percent of the injection amount of the primary injection during the secondary injection, the standing is carried out for three times for 1 to 8 hours after the injection, and the standing temperature is 35 to 60 ℃; the secondary sealing parameter requires that the packaging strength is more than or equal to 50N/15mm, and the packaging vacuum degree is less than or equal to-30 KPa.
In the preparation method of the lithium ion battery, the primary injection is a first electrolyte, and the first electrolyte is prepared by mixing electrolyte lithium salt, an organic solvent and an additive; wherein the addition amount of the lithium salt is 1-1.3M, the additive accounts for 2-7% of the total weight, and the balance is an organic solvent.
The organic solvent is a 1:1:1 mixture of EC, DEC and DMC, the electrolyte lithium salt is lithium hexafluorophosphate with the concentration of 1M,
the additive comprises a negative electrode film forming additive, a PS additive and a high-temperature additive, wherein the negative electrode film forming additive is VC, and the addition amount of the VC accounts for 1-3% of the total weight; the PS additive accounts for 1-3% of the total weight, and the high-temperature additive LiBOB accounts for 0.5-2% of the total weight.
In the preparation method of the lithium ion battery, the secondary injection is a second electrolyte, and the second electrolyte is prepared by mixing electrolyte lithium salt, an organic solvent and an additive;
wherein the addition amount of the lithium salt is 1-1.3M, the additive accounts for 4-12% of the total weight, and the balance is organic solvent;
the organic solvent is a 1:1:1 mixture of EC, DEC and DMC, the electrolyte lithium salt is lithium hexafluorophosphate, and the concentration is 1M;
the additives comprise a first, a second and a third additive; the first additive is EP, the adding amount is 3% -10% of the total weight fraction, the second additive is one selected from LITFSI, LIFSI and LIFTFSI, the adding amount is 00.5% -2% of the total weight fraction, and the third additive is LiPO2F2The addition amount is 00.5-2% of the total weight fraction.
The weight energy density of the lithium ion battery reaches 250-280 Wh/kg.
The preparation method of the lithium ion battery comprises the following steps: the positive active substance adopts nickel cobalt lithium manganate, mixed positive conductive agent and NMP as solvent; the negative electrode adopts graphite coated by amorphous carbon, is mixed with a negative electrode conductive agent/binder, adopts a water system slurry mixing process, and is secondarily rolled; during liquid injection, a secondary liquid injection process is adopted, two different electrolytes are respectively injected before and after formation, and a high-temperature standing process is adopted to ensure enough liquid retention; and (5) preparing the lithium ion battery through capacity grading and aging.
Due to the adoption of the technology, the invention has the beneficial effects that:
1. by adjusting the components of the electrolyte, selecting a solvent with low melting point and low viscosity and adding a functional low-temperature additive, the high ionic mobility at low temperature is ensured. And aiming at different requirements of different voltages on the electrolyte and the additive characteristics of the electrolyte in formation and subsequent circulation of the battery, the secondary injection adopts electrolytes with different proportions, so that the stability of SEI film formation is ensured.
2. By adopting secondary injection and a special standing process, the method ensures that the electrolyte can completely infiltrate the amorphous carbon-coated graphite, effectively improves the safety and prolongs the cycle life.
3. Through the reasonable collocation of the conductive agents with different dimensions and the carbon-coated copper foil with lower interface resistance, the migration channel of lithium ions is ensured.
4. The graphite coated by the amorphous carbon has excellent low-temperature dynamic performance, can not cause metal lithium deposition when charged at low temperature, can support the use of the battery cell at low temperature, and ensures the safety and the cycle life.
5. The nickel cobalt lithium manganate is adopted as the positive active substance, so that the gram capacity is high, and the high energy density of the battery cell is ensured.
Drawings
Fig. 1 is a comparative low temperature charge curve of a lithium ion battery of example 1 of the present invention at-20 ℃ and a lithium ion battery of a comparative example of the prior art.
Fig. 2 is a plot of cycle life versus cycle life for the lithium ion battery of example 1 of the present invention at-20 c versus a comparative example of the prior art.
Detailed Description
In order to make the aforementioned objects, features and advantages of the present invention more comprehensible, specific embodiments thereof will be described in detail with reference to the following examples. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used in the description of the invention herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.
Comparative example lithium ion power cell prepared according to prior art
Firstly, preparing a positive plate: adding a positive electrode active material NCM622 and a positive electrode conductive agent SP into a stirrer, mixing and dispersing, adding a proper amount of NMP solution containing a binder PVDF, mixing at a high speed, continuously adding the NMP solution containing the binder PVDF until the viscosity reaches 5000mPa.s, and preparing into slurry.
Wherein the weight ratio of the positive active substance to the positive conductive agent to the PVDF is 94-96:2.5-4:1.5-2, and the positive conductive agent consists of conductive carbon black and carbon nano tubes in the weight ratio of 1:1.
And coating the anode slurry on an aluminum foil, drying, rolling and die cutting to obtain the anode plate.
Secondly, preparing a negative plate: adding a negative electrode active material (artificial graphite) and a negative electrode conductive agent (conductive carbon black) into a stirrer, mixing and dispersing, adding a proper amount of CMC-containing aqueous solution, mixing at a high speed until the viscosity is 7000mPa.s, adding SBR, and stirring at a low speed to obtain negative electrode slurry.
The weight ratio of the negative electrode active material, the negative electrode conductive agent and the negative electrode binder is 96:2:2, and the negative electrode binder consists of CMC and SBR according to the weight ratio of 1:1 (the proportion of the negative electrode).
And coating the cathode slurry on a common copper foil, drying, and then rolling and die cutting once to obtain the cathode sheet.
The copper foil thickness in the carbon-coated copper foil is 8 μm.
Thirdly, packaging the battery: and (5) laminating the pole pieces of S1 and S2 with a diaphragm, welding the pole lugs, and packaging by using a polymer aluminum plastic film.
Fourthly, the liquid injection process is to inject liquid once after packaging, pre-seal edges, lay aside for 24 hours at normal temperature,
the vacuum degree of the secondary sealing process is required to be less than or equal to-70 KPa; pressurizing at 60 ℃ to form, wherein the formation process is that 0.1C is charged to 50% SOC, 0.3C is charged to 80% SOC, the clamp pressure is 4.76-7.93Kgf/cm2, and the formation temperature is 35-60 ℃; after the formation, the mixture was allowed to stand twice at room temperature for 24 hours. (the manufacturers can adjust the parameters according to different machines, the formation current and the capacity can be slightly different, and the electrolyte is generally purchased from electrolyte manufacturers).
Example 1
The preparation method of the low-temperature high-energy-density lithium ion power battery comprises the following steps:
firstly, preparing a positive plate: adding the positive electrode active material NCM622 and the positive electrode conductive agent into a stirrer, mixing and dispersing, continuously adding an NMP solution containing 10% of PVDF, mixing at high speed until the viscosity is 5000mPa.s, and preparing into slurry. The weight ratio of the positive electrode active material to the positive electrode conductive agent to the PVDF is 97.5:1:1.5, and the positive electrode conductive agent is composed of conductive carbon black and carbon nano tubes in the weight ratio of 1:1.
And coating the anode slurry on an aluminum foil, drying, rolling and die cutting to obtain the anode plate.
Secondly, preparing a negative plate: adding a negative electrode active material (artificial graphite) and a negative electrode conductive agent (conductive carbon black) into a stirrer, mixing and dispersing, adding a proper amount of CMC-containing aqueous solution, mixing at a high speed until the viscosity is 7000mPa.s, adding SBR, and stirring at a low speed to obtain negative electrode slurry.
The weight ratio of the negative electrode active material, the negative electrode conductive agent and the negative electrode binder is 96:2:2, and the negative electrode binder consists of CMC and SBR according to the weight ratio of 1:1 (the proportion of the negative electrode).
And coating the negative electrode slurry on the carbon-coated copper foil, drying, and then performing secondary rolling and die cutting to obtain the negative electrode sheet.
The thickness of the copper foil in the carbon-coated copper foil is 6 μm, and the thickness of the carbon-coated layer is 0.5 μm.
Thirdly, packaging the battery: and (5) laminating the pole pieces of S1 and S2 with a diaphragm, welding the pole lugs, and packaging by using a polymer aluminum plastic film.
Fourthly, the liquid injection process is to inject liquid for one time after packaging, pre-seal edges, and place for 24 hours at 60 ℃ for two times, wherein the vacuum degree required by the two sealing processes is less than or equal to-70 KPa; pressurizing at 60 ℃ to form, wherein the formation process is that 0.1C is charged to 50% SOC, 0.3C is charged to 80% SOC, the clamp pressure is 4.76-7.93Kgf/cm2, and the formation temperature is 35-60 ℃; after the formation, the mixture was allowed to stand twice at room temperature for 24 hours.
Wherein the electrolyte is prepared by uniformly mixing electrolyte lithium salt, an organic solvent and an additive during primary liquid injection; the organic solvent is a mixture of EC, DEC and DMC in a ratio of 1:1:1, the lithium salt is lithium hexafluorophosphate with the concentration of 1M, the additive comprises a negative electrode film forming additive, a PS additive and a high-temperature additive, the negative electrode film forming additive is VC with the addition of 1%; the addition amount of the PS additive is 1 percent, and the addition amount of the high-temperature additive LiBOB is 0.5 percent.
The injection amount is 10% of the injection amount of the primary injection during the secondary injection, and the secondary injection is kept for 8 hours for three times, wherein the keeping temperature is 35-60 ℃; the secondary sealing parameter requires that the packaging strength is more than or equal to 50N/15mm, and the packaging vacuum degree is less than or equal to-30 KPa. Grading and aging according to a conventional process.
The secondary injection is a second electrolyte which is prepared by uniformly mixing electrolyte lithium salt, an organic solvent and an additive; the organic solvent is a 1:1:1 mixture of EC, DEC and DMC, and the lithium salt is lithium hexafluorophosphate with the concentration of 1M; the additive is EP, the addition amount is 3 percent by weight, LITFSI, the addition amount is 2 percent by weight, LiPO2F2The addition amount is 1% by weight.
The low-temperature charging curve at-20 ℃ of the lithium ion battery of embodiment 1 of the present invention and the low-temperature charging curve of the lithium ion battery of the prior art are shown in fig. 1. Fig. 2 is a low-temperature cycle life curve of the lithium ion battery of example 1 at-20 ℃ and a low-temperature cycle life curve of the lithium ion battery of the prior art. The comparison curve shows that the charging capacity is improved at the temperature of minus 20 ℃ and the capacity fading is reduced during the circulation according to the method of the process, so that more times of circulation can be realized.
Example 2
Different from the first embodiment, the weight ratio of the positive electrode active material, the positive electrode conductive agent and the PVDF is 96:2:2, and the positive electrode conductive agent is composed of conductive carbon black and carbon nanotubes with the weight ratio of 1:1.
Example 3
Different from the first embodiment, the additive of the first electrolyte is VC, and the addition amount is 3%; the PS addition is 2%, and the high-temperature additive LiBOB is 2%.
Example 4
Different from the first embodiment, the negative electrode sheet is prepared by the following steps: adding a negative electrode active material (artificial graphite) and a negative electrode conductive agent (carbon fiber) into a stirrer, mixing and dispersing, adding a proper amount of aqueous solution containing CMC, mixing at a high speed until the viscosity is regulated, adding PAA, and stirring at a low speed to obtain negative electrode slurry.
The weight ratio of the negative electrode active material, the negative electrode conductive agent and the negative electrode binder is 96:1:3, and the negative electrode binder consists of CMC and PAA according to the weight ratio of 1:2 (negative electrode ratio).
Example 5
The difference from the first example is that the positive electrode active material was NCM 811.
The foregoing shows and describes the general principles, essential features, and advantages of the invention. It will be understood by those skilled in the art that the present invention is not limited by the foregoing examples, which are provided to illustrate the principles of the invention, and that various changes and modifications may be made without departing from the spirit and scope of the invention, which is also intended to be covered by the appended claims. The scope of the invention is defined by the appended claims and equivalents thereof.

Claims (10)

1. The preparation method of the low-temperature high-energy density lithium ion battery comprises the following steps:
s1, preparing the positive plate: mixing and dispersing a nickel cobalt lithium manganate positive electrode material and a positive electrode conductive agent, continuously dropwise adding an oil system NMP solution, mixing and stirring uniformly to reach the viscosity of 4000-10000mPa.s, preparing positive electrode slurry, coating the positive electrode slurry on an aluminum foil, drying, rolling and die-cutting to obtain a positive electrode sheet;
s2, preparing the negative plate: mixing and dispersing the amorphous carbon-coated graphite and a negative electrode conductive agent, adding a water-based binder, and uniformly mixing and stirring to reach the viscosity of 2000-8000 mPa.s to obtain negative electrode slurry; coating the negative electrode slurry on the carbon-coated copper foil, drying, and then performing secondary rolling and die cutting to obtain a negative electrode sheet;
and S3, packaging of the battery: placing the positive plate obtained in the step S1 and the negative plate obtained in the step S2 with diaphragms for lamination, then welding tabs, and packaging by a polymer aluminum plastic film;
s4, liquid injection, formation and volume grading: injecting liquid for the first time, injecting first electrolyte, standing for the first time, and pressurizing to form the electrolyte; and then standing for the second time, injecting a second electrolyte, standing for the third time, and finally grading and aging at normal temperature to prepare the lithium ion battery.
2. The method according to claim 1, wherein in S1, the NMP solution contains 5-12% PVDF polyvinylidene fluoride; the addition amount of NMP is 1.0-3% of the positive electrode material.
3. The preparation method of claim 1, wherein in S1, the addition amount of the lithium nickel cobalt manganese oxide positive electrode material is 95% -98% of the positive electrode material.
4. The method according to claim 1, wherein the positive electrode conductive agent is at least one selected from the group consisting of conductive carbon black, carbon nanotubes and graphene, and the amount of the positive electrode conductive agent added is 0.5 to 3% of the positive electrode material in S1.
5. The method according to claim 1, wherein the conductive agent of the negative electrode in S2 is at least one selected from the group consisting of aqueous carbon nanotube slurry, conductive carbon black, and carbon fiber, and the amount of the conductive agent added is 0.5% to 2.5% of the negative electrode material.
6. The method according to claim 1, wherein in S2, the aqueous binder is at least one selected from the group consisting of aqueous SBR/PVA/PAA, and CMC is added, wherein CMC accounts for 20-50% of the negative binder; the addition amount of the aqueous binder is 1.5-4% of the negative electrode material.
7. The preparation method according to claim 1, wherein in S2, the secondary rolling process of the negative electrode sheet comprises a first compaction density of 1.55-1.65g/cm3Laying aside for 1-24 hr, and compacting for the second time to obtain the product with density of 1.60-1.75g/cm3。
8. The preparation method according to claim 1, wherein in S4, the liquid injection process is one-time liquid injection after packaging, that is, injecting the first electrolyte, pre-sealing edges, standing at high temperature for 1-24 hours, and then performing one-time secondary sealing, wherein the secondary sealing process requires a vacuum degree of less than or equal to-70 KPa; high-temperature pressurizing to form a mixture, wherein the formation process comprises charging 0.1C to 50% SOC, charging 0.3C to 80% SOC, and clamping pressure is 4.76-7.93Kgf/cm2The formation temperature is 35-60 ℃; standing for the second time after formationRoom temperature is 1-24 hours; the injection amount in the secondary injection is 10 percent of that in the primary injection, standing is carried out for 1-8 hours for three times after the injection, and the standing temperature is 35-60 ℃; the secondary sealing parameter requires that the packaging strength is more than or equal to 50N/15mm, and the packaging vacuum degree is less than or equal to-30 KPa.
9. The preparation method according to claim 8, wherein the primary electrolyte is a first electrolyte prepared by mixing an electrolyte lithium salt, an organic solvent and an additive; wherein the addition amount of the lithium salt is 1-1.3M, the additive accounts for 2-7% of the total weight, and the balance is organic solvent;
the organic solvent is a mixture of EC, DEC and DMC in a ratio of 1:1:1, the electrolyte lithium salt is lithium hexafluorophosphate with the concentration of 1M,
the additive comprises a negative electrode film forming additive, a PS additive and a high-temperature additive, wherein the negative electrode film forming additive is VC, and the addition amount of the VC accounts for 1-3% of the total weight; the PS additive accounts for 1-3% of the total weight, and the high-temperature additive LiBOB accounts for 0.5-2% of the total weight.
10. The preparation method according to claim 8, wherein the secondary electrolyte is a second electrolyte prepared by mixing an electrolyte lithium salt, an organic solvent and an additive; wherein the addition amount of the lithium salt is 1-1.3M, the additive accounts for 4-12% of the total weight, and the balance is organic solvent;
the organic solvent is a mixture of EC, DEC and DMC in a ratio of 1:1:1, the electrolyte lithium salt is lithium hexafluorophosphate with the concentration of 1M;
the additives comprise a first, a second and a third additive; the first additive is EP, the adding amount is 3-10% of the total weight fraction, the second additive is one of LITFSI, LIFSI and LIFTFSI, the adding amount is 0.5-2% of the total weight fraction, and the third additive is LiPO2F2The addition amount is 0.5 to 2 percent of the total weight fraction.
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