Binder and lithium ion battery using the same

Abstract

The present application discloses a binder for a lithium ion battery, which comprises a polymer obtained through emulsion polymerization of a monomer in the presence of a reactive emulsifying agent. The binder is used in fabrication of a lithium ion electrode plate, whereby a thin film formed on the surface of an electrode membrane and fine channels formed in the electrode membrane with the use of a conventional emulsifying agent during the electrode membrane-forming process are eliminated, and the lithium ion conductivity of the electrode membrane is improved. Meanwhile, with the use of the reactive emulsifying agent, the bonding effect of the binder and the stability of the electrode membrane are improved, thereby greatly improving the charging rate and cycle life of the lithium ion battery.

Claims

1. A binder for a lithium ion battery, comprising a polymer, wherein the polymer is obtained through emulsion polymerization of a monomer in the presence of a reactive emulsifying agent; the monomer comprises at least one compound having a chemical structure of Formula I: ##STR00011## wherein R.sup.11 is hydrogen or R.sup.11 is selected from an alkyl group having 1 to 20 carbon atoms; R.sup.12 is hydrogen or R.sup.12 is selected from an alkyl group having 1 to 20 carbon atoms; R.sup.13 is hydrogen or R.sup.13 is selected from an alkyl group having 1 to 20 carbon atoms; and R.sup.14 is hydrogen or R.sup.14 is selected from an alkyl group having 1 to 20 carbon atoms; the reactive emulsifying agent is selected from a compound with a chemical structure comprising a moiety of Formula IV: ##STR00012## wherein R.sup.41 is hydrogen or R.sup.41 is selected from an alkyl group having 1 to 20 carbon atoms; and R.sup.42 is hydrogen or R.sup.42 is selected from an alkyl group having 1 to 20 carbon atoms.

2. The binder for a lithium ion battery according to claim 1, wherein R.sup.41 and R.sup.42 are all hydrogen.

3. The binder for a lithium ion battery according to claim 1, wherein the reactive emulsifying agent is selected from at least one of a maleate and a derivative thereof, wherein the maleate and the derivative thereof are selected from at least one of a compound with a chemical structure of Formula IX: ##STR00013## where R.sup.91 is hydrogen or R.sup.91 is selected from an alkyl group having 1 to 20 carbon atoms; R.sup.92 is hydrogen or R.sup.92 is selected from an alkyl group having 1 to 10 carbon atoms; R.sup.93 is hydrogen or R.sup.93 is selected from an alkyl group having 1 to 10 carbon atoms; and M.sup.+ is selected from at least one of Li.sup.+, Na.sup.+, K.sup.+, Rb.sup.+, Cs.sup.+, and NH.sub.4.sup.+.

4. The binder for a lithium ion battery according to claim 1, wherein the weight ratio of the reactive emulsifying agent to the monomer is 1-3:100.

5. The binder for a lithium ion battery according to claim 1, wherein the polymer has a glass transition temperature from 0° C. to 50° C.

6. The binder for a lithium ion battery according to claim 1, wherein the polymer has a weight average molecular weight from 300 kDa to 1,500 kDa.

7. The binder for a lithium ion battery according to claim 1, wherein the binder has a solid content from 25 wt % to 50 wt %.

8. An electrode plate, comprising at least one of the binders according to claim 1.

9. A lithium ion battery, comprising at least one of the binders according to claim 1.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) Hereinafter, this application is described in detail below with reference to examples, but the application is not limited thereto.

(2) In the examples, the reactive emulsifying agent sodium dodecyl maleate is synthesized following the process described in the literature [Journal of Hunan University (Natural Science), 2008, 35(10): 55-59], the thickener, sodium carboxymethyl cellulose (CMC), is one having a weight average molecular weight of 200-300 kDa; the conductive carbon black Super-P is one having a specific surface area of 60-70 m.sup.2/g and a density of 160±10 Kg/m.sup.3. The binder, polyvinylidene fluoride (PVDF), is one having a weight average molecular weight of 600-700 kDa; and the binder, styrene-butadiene rubber (SBR), is one having a weight average molecular weight of 70-80 kDa.

(3) In the comparative examples, octyl phenol polyoxyethylene ether has a molecular weight of 646.34, a pH (1% aqueous solution) of 8.0-9.0, and a clouding point (1% aqueous solution) of greater than 0° C.

Example 1. Preparation of Binder Samples B1-B17

(4) Preparation of Binder Sample B1:

(5) 3 parts by weight of sodium dodecyl maleate was used as an emulsifying agent. A mixture of 48.5 parts by weight of methyl methacrylate, 48.5 parts by weight of n-butyl acrylate and 3 parts by weight of acrylic acid was used as monomers. Ammonium persulfate was used as a polymerization initiator, and an aqueous initiator solution was obtained by dissolving 2 parts by weight of ammonium persulfate in 50 parts by weight of deionized water.

(6) 3 parts by weight of sodium dodecyl maleate and 150 parts by weight of deionized water were added to a four-neck reactor, and heated to 60° C. with stirring, to obtain an aqueous emulsifying agent solution after sodium dodecyl maleate was completely dissolved. ⅓ of the monomers was added to the four-neck reactor, mixed with and emulsified by the aqueous emulsifying agent solution by stirring, and then heated to 75° C. ⅓ of the aqueous initiator solution was added to the four-neck reactor, to initiate the polymerization. The remaining ⅔ of the monomers and the remaining ⅔ of the aqueous initiator solution were evenly added dropwise after the system became blue and no obvious reflux was produced. The addition of the monomers was completed in 2 hrs, followed by the aqueous initiator solution at a later time. After addition, the system was maintained at 75° C. for 2 hrs, and then cooled to obtain a binder designated as sample B1.

(7) Preparation of Binder Samples B2-B17:

(8) The specific steps were as described in the preparation of B1, except that the species and amounts of the emulsifying agent, the monomers, and the initiator, and the polymerization temperature and time were changed. The resulting samples were designated as B2-B17 respectively.

(9) The binder numbers and respective species and amounts of the emulsifying agent, the monomers, and the initiator, and the polymerization temperature and time are shown in Table 1.

(10) TABLE-US-00001 TABLE 1 Aqueous emulsifying agent solution Aqueous Species and initiator solution amount (parts by Amount Species and Species and Amount weight) of of water amount (parts by amount (parts of water Polymerization Binder emulsifying (parts by weight) of by weight) of (parts by Temperature No. agent weight) monomers initiator weight) Time B1 Sodium dodecyl 150 Methyl Ammonium 50 75° C. maleate 3 methacrylate 48.5 persulfate 2 2 h n-butyl acrylate 48.5 Acrylic acid 3 B2 Sodium dodecyl 150 Methyl Ammonium 50 75° C. maleate 2 methacrylate 48.5 persulfate 2 2 h n-butyl acrylate 48.5 Acrylic acid 3 B3 Sodium dodecyl 150 Methyl Ammonium 50 75° C. maleate 1 methacrylate 48.5 persulfate 2 2 h n-butyl acrylate 48.5 Acrylic acid 3 B4 Sodium dodecyl 150 Methyl Ammonium 50 80° C. maleate 3 methacrylate 48.5 persulfate 2 h n-butyl acrylate 0.3 48.5 Acrylic acid 3 B5 Sodium dodecyl 150 Methyl Ammonium 50 80° C. maleate 3 methacrylate 48.5 persulfate 1 2 h n-butyl acrylate 48.5 Acrylic acid 3 B6 Sodium dodecyl 150 Methyl Ammonium 50 80° C. maleate 3 methacrylate 48.5 persulfate 3 2 h n-butyl acrylate 48.5 Acrylic acid 3 B7 Sodium dodecyl 150 Methyl Ammonium 150 75° C. maleate 3 methacrylate 48.5 persulfate 2 2 h n-butyl acrylate 48.5 Acrylic acid 3 B8 Sodium dodecyl 70 Methyl Ammonium 30 75° C. maleate 3 methacrylate 48.5 persulfate 2 2 h n-butyl acrylate 48.5 Acrylic acid 3 B9 Sodium dodecyl 150 Styrene 19.6 Ammonium 50 70° C. maleate 3 Methyl acrylate persulfate 2 3 h 27 Ethyl acrylate 50.4 Acrylic acid 3 B10 Sodium dodecyl 150 Styrene 28.9 Ammonium 50 65° C. maleate 3 Methyl acrylate persulfate 2 4 h 27 Ethyl acrylate 41.1 Acrylic acid 3 B11 Sodium dodecyl 150 Styrene 37.6 Ammonium 50 50° C. maleate 3 Methyl acrylate persulfate 2 5 h 27 Ethyl acrylate 32.4 Acrylic acid 3 B12 Sodium dodecyl 150 Styrene 45.8 Ammonium 50 75° C. maleate 3 Methyl acrylate persulfate 2 2 h 27 Ethyl acrylate 24.2 Acrylic acid 3 B13 Sodium allyl 150 Methyl Ammonium 50 75° C. hydroxyalkyl methacrylate 48.5 persulfate 2 2 h sulfonate 3 n-butyl acrylate 48.5 Acrylic acid 3 B14 Ammonium 150 Methyl Ammonium 50 75° C. allyloxy methacrylate 48.5 persulfate 2 2 h nonylphenol n-butyl acrylate polyoxyethylene 48.5 ether sulfate 3 Acrylic acid 3 B15 Sodium allyl 150 Methyl Ammonium 50 75° C. ether sulfonate 3 methacrylate 48.5 persulfate 2 2 h n-butyl acrylate 48.5 Acrylic acid 3 B16 Sodium 150 Methyl Ammonium 50 75° C. acrylamido methacrylate 48.5 persulfate 2 2 h sulfonate 3 n-butyl acrylate 48.5 Acrylic acid 3 B17 Sodium alkyl 150 Methyl Ammonium 50 75° C. allylsuccinate methacrylate 48.5 persulfate 2 2 h sulfonate 3 n-butyl acrylate 48.5 Acrylic acid 3

Comparative Example 1

(11) Preparation of Binder DB1

(12) The specific steps were the same as described in the preparation of B1, except that the emulsifying agent was 2.3 parts by weight of sodium dodecyl sulfate and 0.8 part by weight of octyl phenol polyoxyethylene ether. The resulting binder was designated as DB1.

Example 2. Determination of Molecular Weight and Glass Transition Temperature of Polymer in the Binder

(13) The weight average molecular weight of the polymer in the binders B1-B17 obtained in Example 1 and the binder DB1 obtained in Comparative Example 1 was determined/calculated by gel permeation chromatography. See GB/T 21863-2008 for specific steps/process.

(14) The glass transition temperature of the polymer in the binders B1-B17 obtained in Example 1 and the binder DB1 obtained in Comparative Example 1 was determined/calculated by differential scanning calorimetry. See GB/T 19466—Plastic: Differential Scanning Calorimetry (DSC) for specific steps/process.

(15) The results are shown in Table 2.

(16) TABLE-US-00002 TABLE 2 Weight average Binder molecular weight Glass transition No. (×10 kDa) temperature (° C.) B1  78 5 B2  80 5 B3  81 5 B4  93 8 B5  87 6 B6  63 4 B7  77 5 B8  79 5 B9  81 5 B10 80 15 B11 77 25 B12 79 35 B13 81 5 B14 83 5 B15 78 5 B16 82 5 B17 83 5 DB1 79 4

Example 3. Fabrication of Electrode Plate and Lithium Ion Battery

(17) Fabrication of a Positive Electrode Plate:

(18) Lithium cobaltate (molecular formula LiCoO.sub.2) as the positive electrode active material, the conductive carbon black Super-P as the conductive agent, and the binder were uniformly dispersed in the solvent N-methyl pyrrolidone (NMP), to prepare a positive electrode slurry. The positive electrode slurry had a solid content of 75 wt %, and the solid ingredients consisted of 97 wt % of lithium cobaltate, 1.6 wt % of the binder polymer and 1.4 wt % of the conductive carbon black Super-P. The positive electrode slurry was evenly coated in an amount of 0.018 g/cm.sup.2 onto an aluminium foil with a thickness of 12 μm and used as a positive electrode current collector. The aluminium foil was then oven dried at 85° C., cold pressed, trimmed, cut into pieces, slit into stripes, dried at 85° C. for 4 hrs under vacuum, and then subjected to tab welding, to obtain a positive electrode plate.

(19) Fabrication of a Negative Electrode Plate

(20) Artificial graphite as the negative electrode active material, the conductive carbon black Super-P as the conductive agent, and the binder were uniformly mixed in deionized water, to prepare a negative electrode slurry. The negative electrode slurry had a solid content of 50 wt %, and the solid ingredients consisted of 96.5 wt % of artificial graphite, 1.0 wt % of the conductive carbon black Super-P, and 2.5 wt % of the binder polymer. The negative electrode slurry was evenly coated in an amount of 0.0089 g/cm.sup.2 onto a copper foil with a thickness of 8 μm and used as a negative electrode current collector. The aluminium foil was then oven dried at 85° C., cold pressed, trimmed, cut into pieces, slit into stripes, dried at 110° C. for 4 hrs under vacuum, and then subjected to tab welding, to obtain a negative electrode plate.

(21) Preparation of Electrolyte Solution

(22) In a dry chamber, ethylene acetate (EC), propylene carbonate (PC) and diethyl carbonate (DEC) were uniformly mixed at a weight ratio of EC:PC:DEC=30:30:40, to obtain a non-aqueous organic solvent. LiPF.sub.6 was added to the non-aqueous organic solvent, to obtain a 1 mol/L solution of LiPF.sub.6, which was precisely the electrolyte solution.

(23) Fabrication of Lithium Ion Battery:

(24) A polyethylene film with a thickness of 12 μm was used as a separator.

(25) The steps were specifically as follows.

(26) The positive electrode plate, the separator and the negative electrode plate were superimposed in sequence such that the separator is switched between the positive electrode plate and the negative electrode plate for separation. The system was then rolled into a square bare battery core having a thickness of 8 mm, a width of 60 mm, and a length of 130 mm. The bare battery core was packaged in an aluminium foil bag, oven dried at 75° C. under vacuum for 10 hrs. A non-aqueous electrolyte solution L1.sup.# was injected, vacuum encapsulated, and stood for 24 hrs. The battery was charged to 4.35 V at a constant current of 0.1 C (160 mA), then charged at a constant voltage of 4.35 V until the current dropped to 0.05 C (80 mA), and then discharged to 3.0 V at a constant current of 0.1 C (160 mA). The charge and discharge process was repeated 2 times. Finally, the battery was charged to 3.85 V at a constant current of 0.1 C (160 mA), to finish the fabrication of the lithium ion secondary battery.

(27) The battery numbers and the binder used in respective electrode plates are shown in Table 3.

(28) TABLE-US-00003 TABLE 3 Battery Binder in positive Binder in negative No. electrode plate electrode plate C1  B1  B1  C2  B2  B2  C3  B3  B3  C4  B4  B4  C5  B5  B5  C6  B6  B6  C7  B7  B7  C8  B8  B8  C9  B9  B9  C10 B10 B10 C11 B11 B11 C12 B12 B12 C13 B13 B13 C14 B14 B14 C15 B15 B15 C16 B16 B16 C17 B17 B17 C18 B1  Styrene-butadiene rubber (SBR, present in the binder in an amount of 50% by weight) binder C19 polyvinylidene fluoride (PVDF, present in B1  the binder in an amount of 10% by weight) binder DC1 DB1 DB1

Example 4. Test of Charging Characteristics of Lithium Ion Secondary Batteries

(29) The charging characteristics of lithium ion secondary batteries C1-C19 and DC1 fabricated in Example 3 were tested respectively.

(30) The process was specifically as follows.

(31) The lithium ion secondary batteries were charged to 4.35 V at a constant current of 6 C at 25° C., and then charged at a constant voltage of 4.35 V till the current was 0.05 C. The sampling time interval was 5 s. The time to 50%, 80%, and 100% SOC were derived from raw data.

(32) TABLE-US-00004 TABLE 4 Battery Time to 50% SOC at Time to 80% SOC Time to 100% SOC at No. 6 C (min) at 6 C (min) 6 C (min) C1  7 18.5 49.8 C2  6.8 18 49 C3  7 18 49 C4  5 15 45 C5  6 16 47 C6  8 20 56 C7  6.7 18.3 49.4 C8  7 18.1 49.6 C9  7.1 18 49 C10 7.2 18 49.1 C11 6.9 18.1 48.9 C12 7.2 18 48.8 C13 7 18.3 49 C14 7.1 18 49.2 C15 6.9 17.8 48.8 C16 7 17.9 48.9 C17 6.7 17.7 49 C18 9 23 58.5 C19 8.5 21 57 DC1 10 25 61

Example 5. Test of Cycle Performance of Lithium Ion Secondary Batteries

(33) The cycle performance of lithium ion secondary batteries C1-C19 and DC1 fabricated in Example 3 were tested respectively.

(34) The process was specifically as follows. The lithium ion secondary batteries were charged to 4.35 V at a constant current of 6 C at 25° C., then charged at a constant voltage of 4.35 V till the current was 0.05 C, and then discharged to 3.0 V at a constant current of 1 C. This was one round of charge-discharge cycle. The discharge capacity is that of the first cycle. After being stood for half an hour, the charge-discharge cycling test of the lithium ion secondary batteries was conducted following the process above.

(35) The discharge capacity at the cycles 100, 200, 300, 400 and 500 was recorded and the capacity retention rate was calculated.
Capacity retention rate of lithium ion secondary battery after N rounds of cycles (%)=[Discharge capacity of the Nth cycle/Discharge capacity of the 1.sup.st cycle]×100%.

(36) The lithium ion batteries after 500 rounds of cycles were disassembled, the thickness of the anode plates as measured to calculate the increase rate in the thickness.
Increase rate in thickness of anode plate (%)=(Thickness after 500 rounds of cycles−initial thickness)/initial thickness×100%

(37) The test results are shown in Table 5.

(38) TABLE-US-00005 TABLE 5 Capacity retention rate Increase rate in Battery after N rounds of cycles (%) thickness after 500 No. 100 200 300 400 500 rounds of cycles (%) C1 96.2 93.9 93 92.1 90.8 15.4 C2 96 94.1 92.9 92 91 15.5 C3 96 94 93 92 91 15.3 C4 96 93.8 93.1 92.2 91 15.7 C5 96 94 93 92 91 15.4 C6 96 94 93 92 91 15.4 C7 95.8 94 93 92 91 15.5 C8 96.3 94 93 92 91 15.4 C9 96 94 93 92 91 15.5 C10 95.5 93.6 92.7 91.6 90.8 15.9 C11 94.9 93.7 91.9 91 89.4 16.5 C12 94 93 91 90 89 16.8 C13 95.7 93.9 92.7 91.7 90.8 15.3 C14 96 94 93 92 91 15.5 C15 96 93.6 92.5 91.9 90.7 15.4 C16 96 94 93 92 91 15.5 C17 96 94 93 92 91 15.5 C18 93.8 92.7 90.9 89.8 88.7 16.8 C19 95.1 93.2 91.6 91.3 89.8 15.5 DC1 91.6 90.8 89.3 88.5 86.5 17.6

(39) It can be seen from the data in Table 4 that compared with DC1, the time required by C1-C19 to charge to the same SOC is shorter. The performance can be improved by at least 8% in terms of the time required to charge to 80% SOC. The reactive emulsifying agent is superior to the conventional emulsifying agent in the dynamics, whereby the charging rate is increased. As shown through comparison of C4-C6, with the increase of the molecular weight of the polymer, the charging rate to attain 80% SOC is elevated by 25%, because a longer molecule chain is more beneficial to the conduction of lithium ions. The discharging rate has no obvious change with several varying reactive emulsifying agents. Compared with C1, the charging rate of C18 and C19 is lower because the ion conductivity of styrene-butadiene rubber and polyvinylidene fluoride is inferior to that of an acrylate binder.

(40) It can be seen from the data in Table 5 that C1-C19 are better than DC1 in terms of the cycle performance and the increase rate in thickness of the electrode plate. After 500 rounds of cycle, the capacity retention rates of C1-C19 are all above 88.7%, which is higher than that (86.5%) of the control group by 2.2%. After 500 rounds of cycle, the increase rate in thickness of the electrode plate of C1-C19 are all below 16.8%, which is lower than that (17.6%) of the control group by 0.8%. As shown through comparison of C9-C12, with the increase of the glass transition temperature, the increase rate in thickness of the anode plate trends to rise gradually. This is because after separation, the particles cannot be pulled back by the binder due to the weakening of the rebound resilience, the lowering of the tenacity, and the increasing of the plasticity of the binder with the increase of the glass transition temperature, thereby causing the thickness of the electrode plate to increase.

(41) In summary, the descriptions above are merely a few examples of this application, which are not intended to limit this application in any way. Although this application is disclosed as above in connection with preferred examples, this application is not limited thereto. Some variations and modifications made by any person of skill in the art based on the technical contents disclosed above without departing from the scope of the technical solution of this application provide equivalent embodiments of this application, which also fall within the scope of the technical solution.