POSITIVE ELECTRODE SHEET CONTAINING HIGH-SAFETY HEAT-SENSITIVE COATING AND LITHIUM-ION BATTERY

Abstract

A positive electrode sheet containing a high-safety heat-sensitive coating and a lithium-ion battery containing the positive electrode sheet are provided. The heat-sensitive coating includes a first conductive agent, a first binder, heat-sensitive polymer microspheres and an auxiliary agent, and has a conductive property at normal temperature and also has advantages of increasing a contact area between an active material and a current collector, improving the conductivity, and effectively reducing the polarization of battery and the like; when the use temperature of the positive electrode sheet reaches 120° C. and above, the heat-sensitive polymer microspheres will melt to form a plurality of continuous electron blocking layers, current blocking occurs in the coating, an internal blocking forms inside the battery, thereby preventing the occurrence of further thermal runaway of lithium-ion battery.

Claims

1. A positive electrode sheet, comprising a positive current collector, a heat-sensitive coating and a positive active material layer, wherein the heat-sensitive coating and the positive active material layer are sequentially arranged on a surface of the positive current collector; the heat-sensitive coating comprises heat-sensitive polymer microspheres, a first conductive agent, a first binder and an auxiliary agent; and the positive active material layer comprises a positive active material, a second conductive agent and a second binder.

2. The positive electrode sheet according to claim 1, further comprising a composite fusion layer, wherein the heat-sensitive coating, the composite fusion layer and the positive active material layer are sequentially arranged on a surface of the positive current collector; and the composite fusion layer comprises the positive active material, the heat-sensitive polymer microspheres, the first conductive agent, the first binder, the second conductive agent, the second binder and the auxiliary agent.

3. The positive electrode sheet according to claim 1, wherein the heat-sensitive coating comprises the following components by weight percentage: 1.1-95 wt % of the heat-sensitive polymer microspheres, 2.9-48.9 wt % of the first conductive agent, 2-40 wt % of the first binder and 0.1-10 wt % of the auxiliary agent.

4. The positive electrode sheet according to claim 2, wherein the heat-sensitive coating comprises the following components by weight percentage: 1.1-95 wt % of the heat-sensitive polymer microspheres, 2.9-48.9 wt % of the first conductive agent, 2-40 wt % of the first binder and 0.1-10 wt % of the auxiliary agent.

5. The positive electrode sheet according to claim 1, wherein the positive active material layer comprises the following components by weight percentage: 80-99 wt % of the positive active material, 0.5-10 wt % of the second conductive agent and 0.5-10 wt % of the second binder.

6. The positive electrode sheet according to claim 2, wherein the positive active material layer comprises the following components by weight percentage: 80-99 wt % of the positive active material, 0.5-10 wt % of the second conductive agent and 0.5-10 wt % of the second binder.

7. The positive electrode sheet according to claim 2, wherein a thickness of the current collector is 0.1 μm-20 μm; and/or a thickness of the heat-sensitive coating is 0.1 μm-5 μm; and/or a thickness of the composite fusion layer is 0.001-0.5 μm; and/or a thickness of the positive active material layer is 5-100 μm.

8. The positive electrode sheet according to claim 4, wherein a thickness of the current collector is 0.1 μm-20 μm; and/or a thickness of the heat-sensitive coating is 0.1 μm-5 μm; and/or a thickness of the composite fusion layer is 0.001-0.5 μm; and/or a thickness of the positive active material layer is 5-100 μm.

9. The positive electrode sheet according to claim 1, wherein a particle size of the heat-sensitive polymer microsphere is 100 nm-3.0 μm.

10. The positive electrode sheet according to of claim 2, wherein a particle size of the heat-sensitive polymer microsphere is 100 nm-3.0 μm.

11. The positive electrode sheet according to claim 1, wherein a heat-sensitive temperature of the heat-sensitive polymer microsphere is 115° C.-160° C.

12. The positive electrode sheet according to claim 2, wherein a heat-sensitive temperature of the heat-sensitive polymer microsphere is 115° C.-160° C.

13. The positive electrode sheet according to claim 1, wherein the heat-sensitive polymer microsphere is selected from at least one of polyethylene, polypropylene, polyamide, polyester amide, polystyrene, polyvinyl chloride, polyester, polyurethane, olefin copolymer or copolymer modified by monomer thereof.

14. The positive electrode sheet according to claim 2, wherein the heat-sensitive polymer microsphere is selected from at least one of polyethylene, polypropylene, polyamide, polyester amide, polystyrene, polyvinyl chloride, polyester, polyurethane, olefin copolymer or copolymer modified by monomer thereof.

15. The positive electrode sheet according to claim 1, wherein a separator resistance of the positive electrode sheet is less than 10Ω.

16. The positive electrode sheet according to claim 2, wherein a separator resistance of the positive electrode sheet is less than 10Ω.

17. The positive electrode sheet according to claim 1, wherein in the heat-sensitive coating, the heat-sensitive polymer microspheres account for 1.1-95 vol % of a total volume of the heat-sensitive coating.

18. The positive electrode sheet according to claim 2, wherein in the heat-sensitive coating, the heat-sensitive polymer microspheres account for 1.1-95 vol % of a total volume of the heat-sensitive coating.

19. A secondary battery, comprising the positive electrode sheet according to claim 1.

20. The secondary battery according to claim 19, wherein the positive electrode sheet further comprises a composite fusion layer, wherein the heat-sensitive coating, the composite fusion layer and the positive active material layer are sequentially arranged on a surface of the positive current collector; and the composite fusion layer comprises the positive active material, the heat-sensitive polymer microspheres, the first conductive agent, the first binder, the second conductive agent, the second binder and the auxiliary agent.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0060] FIG. 1 is a structural schematic diagram of a positive electrode sheet of the present disclosure.

[0061] FIG. 2 is a schematic diagram of a resistance test of a positive electrode sheet of the present disclosure.

[0062] FIG. 3 is a graph showing resistance test results of positive electrode sheets of Example 10, Comparative Example 1, and Comparative Example 2.

[0063] FIG. 4 is a graph showing results of ARC test of batteries prepared in Example 10 and Comparative Examples 1-2.

[0064] FIG. 5 is an SEM image of a cross-section of a coating area in a positive electrode sheet prepared in Example 1.

[0065] FIG. 6 is an SEM image of a cross-section of a coating area in a positive electrode sheet prepared in Example 8.

[0066] FIG. 7 is an SEM image of a cross-section of a coating area in a positive electrode sheet prepared in Example 10.

DESCRIPTION OF EMBODIMENTS

[0067] The present disclosure will be described in further detail below in connection with specific examples. It should be understood that the following examples are merely intended to exemplarily illustrate and explain the disclosure, and should not be construed as limiting the protection scope of the present disclosure. All techniques implemented on the basis of the above content of the present disclosure are encompassed within the scope of the present disclosure intended to be protected.

[0068] The experimental methods used in the following examples are conventional methods unless otherwise specified; the reagents, materials and the like used in the following examples, unless otherwise specified, may be obtained from the commercial route. All heat-sensitive polymer microspheres used in the following examples are purchased from the commercial route.

[0069] In the description of the present disclosure, it should be noted that terms “first”, “second” and the like are only for the purpose of description, instead of indicating or implying the relative importance.

Example 1

[0070] S1: preparing heat-sensitive coating slurry: 1000 g of hydrofluoroether, 2.9 g of CNT (Carbon Nano Tube), 2 g of polyvinylidene fluoride and 0.1 g of triethyl phosphate were uniformly mixed and filtered through a 100 mesh screen to give a filtrate; 95 g of heat-sensitive polymer microspheres of polyethylene with a particle size of 1.5 μm were added to the filtrate, uniformly mixed and filtered through the 100 mesh screen again to obtain the heat-sensitive coating slurry.

[0071] S2: preparing a positive active material layer slurry: 500 g of NMP (N-Methylpyrrolidone), 96 g of lithium cobaltate, 2 g of conductive carbon black as a conductive agent, and 2 g of polyvinylidene fluoride as a binder were uniformly mixed to obtain the positive active material layer slurry.

[0072] S3: preparing a positive electrode sheet: the heat-sensitive coating slurry in S1 was coated on a surface of an aluminum foil current collector, and dried at 110° C. for 12 h to obtain the current collector having the heat-sensitive coating on its surface; the positive active material layer slurry in S2 was coated on the surface of the current collector having the heat-sensitive coating on its surface, dried, and subjected to tableting and cutting treatments to obtain the positive electrode sheet, where the heat-sensitive coating and the positive active material layer were permeated into each other to form a composite fusion layer during drying and tableting, and the thickness of the composite fusion layer was observable by SEM and EDS (Energy Dispersive Spectrometer) analysis.

[0073] S4: preparing a negative electrode sheet: 400 g of deionized water, 97 g of graphite, 0.5 g of conductive carbon black, 1 g of CMC, 1.5 g of styrene-butadiene rubber were uniformly mixed, coated on the negative current collector, and dried, where the drying process belonged to a conventional process in the art.

[0074] S5: preparing a lithium-ion battery: the obtained positive electrode sheet, the obtained negative electrode sheet and a separator were subjected to laminating, winding, etc. to give a lithium-ion battery cell, followed by drying, electrolyte injection, formation, and encapsulation to obtain the lithium-ion battery with high safety.

Example 2

[0075] S1: preparing a heat-sensitive coating slurry: 200 g of water, 10 g of acetylene black, 20 g of CNT, 30 g of styrene-butadiene rubber, 10 g of carboxymethyl cellulose and 10 g of polyethylene glycol were uniformly mixed and filtered through a 100 mesh screen to give a filtrate; 16.7 g of heat-sensitive polymer microspheres of ethylene-propylene copolymer with a particle size of 1.5 μm and 3.3 g of heat-sensitive polymer microspheres of ethylene-propylene copolymer with a particle size of 1.2 μm were added to the filtrate, uniformly mixed and then filtered through a 100 mesh screen again to obtain the heat-sensitive coating slurry.

[0076] S2: preparing a positive active material layer slurry: 500 g of NMP, 96 g of lithium cobaltate, 2 g of conductive carbon black as the conductive agent, 2 g of polyvinylidene fluoride as the binder were uniformly mixed to obtain the positive active material layer slurry.

[0077] S3: preparing a positive electrode sheet: the heat-sensitive coating slurry in S1 was coated on a surface of an aluminum foil current collector, and dried at 80° C. for 72 h to obtain the current collector having the heat-sensitive coating on its surface; the positive active material layer slurry in S2 was coated on the surface of the current collector having the heat-sensitive coating on its surface, dried, and subjected to tableting and cutting treatments to obtain the positive electrode sheet.

[0078] S4: preparing a negative electrode sheet: 350 g of deionized water, 90 g of graphite, 7 g of silicon monoxide (SiO.sub.x, 0<x<2), 0.5 g of conductive carbon black, 1 g of CMC, 1.5 g of styrene-butadiene rubber were uniformly mixed, coated on a negative current collector, and dried, where the drying process belonged to a conventional process in the art.

[0079] S5: preparing a lithium-ion battery: the obtained positive electrode sheet, the obtained negative electrode sheet and a separator were subjected to laminating, winding, etc. to give a lithium-ion battery cell, followed by drying, electrolyte injection, formation, and encapsulation to obtain the lithium-ion battery with high safety.

Examples 3-12 and Comparative Examples 1-2

[0080] The preparing process refers to the preparation of the oil-based heat-sensitive coating in Example 1 or refers to the preparation of the water-based heat-sensitive coating in Example 2, and the distinguishing features are shown in Table 1-Table 3.

TABLE-US-00001 TABLE 1 Addition amounts and drying conditions of heat-sensitive coating components in Examples 1-12 and Comparative Examples 1-2 Heat- sensitive Drying Drying polymer Conductive Auxiliary temperature time No. Solvent/g microsphere/g agent/g Binder/g agent/g (h) (h) Example 1  1000 95   2.9  2 0.1 110 12 Example 2   200 20   30   40 10    80 72 Example 3   500 1.1 48.9  40 10    85 18 Example 4   400 76   10   10 4   100 20 Example 5   800 65   22   10 3    90 24 Example 6   300 52   26   15 7    95 60 Example 7   500 46   35   15 4   100 70 Example 8   700 35   40   20 5   105 36 Example 9   600 38   34   20 8    90 50 Example 10  600 70   20    8 2   100 48 Example 11  600 70   20    8 2   100 48 Example 12  600 70   20    8 2   100 48 Comparative — — — — — — — Example 1  Comparative  600 — 20    8 2   100 48 Example 2 

TABLE-US-00002 TABLE 2 Composition of heat-sensitive coating slurries in Examples 1-12 and Comparative Examples 1-2 Heat- Heat sensitive sensitive polymer temperature Conductive Auxiliary No. Solvent microsphere (° C.) agent Binder agent Example 1  Hydrofluoroether Polyethylene 130 CNT Polyvinylidene Triethyl fluoride phosphate Example 2  Water Ethylene- 135 Acetylene Styrene- Polyethylene propylene black: butadiene glycol copolymer carbon rubber: with the nano-tube carboxymethyl particle (a mass cellulose size of ratio of (a mass ratio 1.5 μm and E2) of 3:1) ethylene- propylene copolymer with the particle size of 1.2 μm at a mass ratio of 6:1 Example 3  Acetone Polypropylene 140 Conductive Polyvinylidene Sodium carbon fluoride- dodecyl black: hexafluoropropylene benzene graphene sulfonate (a mass ratio of 1:2) Example 4  Water Polyethylene 115 Graphene: Styrene- Polyethylene carbon butadiene glycol nano-tube rubber: (a mass carboxymethyl ratio of cellulose 1:4) (a mass ratio of 1:1) Example 5  Acetone Polypropylene 160 Acetylene Polyvinylidene Sodium black fluoride dodecyl benzene sulfonate Example 6  Tetrahydrofuran Ethylene- 130 Conductive Polyvinylidene Sodium vinyl carbon fluoride hexadecyl acetate black sulfonate Copolymer Example 7  Water Polyurethane 130 Carbon Acrylate Polyoxyethylene nano tube Example 8  Water Propylene- 115 Carbon Ethylene- Stearic acid ethylene- nano tube: vinyl acetate acrylate Conductive copolymer copolymer carbon black (a mass ratio of 1:2) Example 9  Pyridine Propylene- 135 Conductive Polytetrafluoroethylene Branched ethylene- carbon chain acrylate black: alcohol copolymer graphene (a mass ratio of 1:2) Example 10 Water Polyethylene 120 Conductive Styrene- Polyethylene carbon butadiene glycol black rubber: carboxymethyl cellulose (a mass ratio of 1:1) Example 11 Water Polyethylene 120 Conductive Styrene- Polyethylene carbon butadiene glycol black rubber: carboxymethyl cellulose (a mass ratio of 1:1) Example 12 Water Polyethylene 120 Conductive Styrene- Polyethylene carbon butadiene glycol black rubber: carboxymethyl cellulose (a mass ratio of 1:1) Comparative — — — — — — Example 1  Comparative — — — Carbon Styrene- Polyethylene Example 2  nano-tube: butadiene glycol conductive rubber: carbon carboxymethyl black (1:1) cellulose (1:1)

TABLE-US-00003 TABLE 3 Structures of positive electrode sheets in Examples 1-12 and Comparative Examples 1-2 Thickness Thickness Thickness of heat- Thickness of positive of current sensitive of fusion electrode No. collector/(μm) coating/(μm) layer/(μm) layer/(μm) Example 1 12 2.5 0.5 50 Example 2 13 2 0.08 100 Example 3 10 0.2 0.01 15 Example 4 20 0.5 0.5 45 Example 5 2 1 0.4 80 Example 6 5 2 0.1 60 Example 7 10 4 0.005 45 Example 8 11 0.5 0.5 50 Example 9 15 5 0.001 70 Example 10 10 3 0.01 50 Example 11 10 3 0.01 60 Example 12 10 1 0.5 50 Comparative 10 — — 50 Example 1 Comparative 10 3 0.001 50 Example 2

[0081] Experimental Data

[0082] A resistance test of an electrode sheet: ACCFILM film resistance test instrument is used, where the instrument uses a voltage-controllable dual-probe resistance to directly test the overall resistance of the electrode sheet, and the output measurement value is the resistance of electrode sheet.

[0083] The test process includes: allowing the probe to have a proper surface smoothness, pressurizing the probes with 10N, and performing a test, where the test instrument was placed in an oven with an initial temperature of 20° C., the temperature of oven is raised to 145° C. at a heating rate of 2° C./min, the data is recorded in real time, and the result is as shown in FIG. 3.

[0084] A method for testing the internal resistance of battery by alternating current impedance includes: performing an alternating current impedance test on the lithium-ion battery with PGSTAT302N chemical workstation of Metrohm in the range of 100 KHz-0.1 mHz at 25° C.

[0085] A test method of the cycle performance of battery includes: performing a charging and discharging test on the lithium-ion battery on cabinet of LAND battery charging and discharging test instrument under test conditions of 25° C., 50.0% humidity and charging and discharging at 1 C/1 C.

[0086] A thermal test method of battery includes: detecting battery states at different temperatures by an adiabatic rate calorimeter PhiTEC I (ARC) of HEL, England.

[0087] A test method of a cross-section of the electrode sheet of battery includes: coating a heat-sensitive coating slurry on an aluminum foil current collector, then coating a positive active material layer slurry, drying, subjecting a sample to cross-cutting, vacuuming, spraying with gold, preparing sample, and observing the cross-section of the coating area by Hitachi thermal field emission scanning electron microscope SU5000.

[0088] 1. Electrode sheet resistance test result: FIG. 3 is a graph of a change of the resistance values of positive electrode sheets in Example 10, Comparative Example 1 and Comparative Example 2 as a function of the temperature. In FIG. 3, from comparison of the resistance test results of the positive electrode sheets of Example 10, Comparative Example 1, and Comparative Example 2, it is found that the resistances of the positive electrode sheets of Comparative Example 1 and Comparative Example 2 slightly decrease as the temperature increases; and the resistance of the positive electrode sheet of Example 10 changes little along with the temperature in the range of 20° C.-115° C. At 115° C.-125° C., the heat-sensitive polymer microspheres in the positive electrode sheet of Example 10 will melt to form a plurality of continuous electron blocking layers, current blocking occurs in the heat-sensitive coating and an internal blocking is formed inside the battery, thereby blocking the flow of ions and electrons, forming a heat-sensitive effect, and further improving the safety performance of battery.

[0089] 2. The batteries prepared in Example 10 and Comparative Examples 1-2 are tested with an adiabatic acceleration calorimeter PhiTEC I (ARC) of HEL, England. The internal temperature of the instrument is increased at a rate of 0.14° C./min, and the temperature of battery is tested, where the test results are as shown in FIG. 4. By comparing the ARC test results of the batteries prepared in Example 10 and Comparative Examples 1-2, it is found that a thermal runaway occurs at about 180° C. in the batteries of Comparative Examples 1-2, and the batteries burn violently; and the thermal runaway occurs at about 200° C. in the battery of Example 10.

[0090] Main reasons: during the temperature of battery rising from 100° C. to 180° C., there exists SEI (solid electrolyte interface) film cracks and intense reaction of the positive electrode with the electrolyte, especially in a range of 160° C.-180° C., the phenomena such as severe thermal runaway and fire may occur. With respect to the battery of the present disclosure, during the battery temperature rising from 100° C. to 180° C., when reaching the heat-sensitive temperature, a blocking layer is formed inside the battery, which can effectively reduce the effects of the cracking of the SEI film and the thermal reaction between the positive electrode and the electrolyte on the battery, prolong the safety time of battery, and increase the thermal runaway temperature of battery.

[0091] 3. The batteries prepared in Examples 1-12 and Comparative Examples 1-2 are subjected to an EIS (Electrochemical Impedance Spectroscopy) test and a battery cycle performance test, and the test results are as follows:

TABLE-US-00004 Cycle Internal numbers at Serial resistance of 80% capacity number battery (mΩ) retention Example 1 46.35 1190 Example 2 70.49 830 Example 3 10.25 5630 Example 4 34.21 1620 Example 5 53.76 940 Example 6 46.25 1010 Example 7 35.34 1535 Example 8 44.91 1300 Example 9 49.57 1030 Example 10 45.72 1200 Example 11 47.82 960 Example 12 44.14 1170 Comparative 43.93 1000 Example 1 Comparative 46.14 1150 Example 2

[0092] By comparing EIS test results of the batteries prepared in Examples 1-12 and Comparative Examples 1-2, it is found that:

[0093] 1) In the batteries prepared in Examples 1-12, a thickness of the positive layer, that is, the positive active material layer, in the positive electrode sheet increases, and an internal resistance of the battery increased accordingly. The main reason is that increased thickness of the electrode sheet results in the increase of the transmission paths of lithium ions and electrons, and the increase of the impedance of the battery, thereby causing the decrease of the effective cycle of battery. However, the internal resistance and cycling performance of the batteries prepared in Examples 1-12 meet the requirements of conventional projects.

[0094] 2) For the batteries prepared in Examples 10-12, the difference of batteries in Examples 10-11 lies in that the thicknesses of the positive layers in the positive electrode sheets are different, where, compared with Example 10, the thickness of the positive electrode layer in the positive electrode sheet in Example 11 increases by 10 μm, the transmission paths of lithium ions and electrons in the electrode sheet increase, the impedance of the battery increases, and the cyclic performance of the battery slightly decreases, but meets the use requirements of the conventional projects; and the difference of batteries in Example 10 and Example 12 lies in that the thicknesses of the heat-sensitive coatings are different, where, compared with Example 10, the thickness of the heat-sensitive coating in Example 12 decreases by 2 μm, which has little effect on the performance of battery and belongs to the normal range.

[0095] 3) Experimental Results of Example 10, Comparative Example 1 and Comparative Example 2:

[0096] Internal resistance of battery: Comparative Example 1<Example 10<Comparative Example 2, the main reason for this lies in that there is no bottom coating in the positive electrode sheet in Comparative Example 1, resulting in a relatively low internal resistance of battery. Overall, there is little difference between the internal resistances of their batteries.

[0097] Cycle number of battery: Example 10≈Comparative Example 2>Comparative Example 1, the main reason is that there is no bottom coating in the positive electrode sheet Comparative Example 1, and although the internal resistance of battery in the early stage is slightly small, the battery cycle performance will be affected by the battery polarization, the increase of dynamic internal resistance, the unevenness of positive electrode and so son along with the cycling of battery.

[0098] By comparing the cycle performance test results of batteries prepared in Examples 1-12 and Comparative Examples 1-2, it is found that the coating in the positive electrode sheet of the present disclosure has the following advantages: suppressing the polarization of battery and reducing the increase of dynamic internal resistance in the cycle process; the structure of the battery in the present disclosure can improve the consistency of positive electrode and increase the cycle life of battery; the present disclosure adopts a multi-layer structure of positive electrode sheet, thereby avoiding direct contact between the positive electrode layer and the current collector; and the heat-sensitive coating of the present disclosure can enhance the adhesion force between the positive active material and the current collector, and reduce the manufacturing cost of the electrode sheet.

[0099] 4. The cross sections of the heat-sensitive coating areas in the positive electrode sheets prepared in Example 1, Example 8 and Example 10 are observed by Hitachi new thermal field emission scanning electron microscope SU5000. The test results are as shown in FIG. 5, FIG. 6 and FIG. 7.

[0100] By comparing the cross-sectional SEM test results of the heat-sensitive coating areas of Example 1, Example 8 and Example 10, it is found that the heat-sensitive microspheres in the heat-sensitive coating of Example 1 account for about 95 vol % of the total volume of the heat-sensitive coating; the heat-sensitive microspheres in the heat-sensitive coating of Example 8 account for about 35 vol % of the total volume of the heat-sensitive coating; and the heat-sensitive microspheres in the heat-sensitive coating of Example 10 accounted for about 70 vol % of the total volume of the heat-sensitive coating.

[0101] The experimental results show that the lithium-ion battery assembled with the positive electrode sheet in the present disclosure has better safety compared with the conventional lithium-ion battery.

[0102] As above, the implementations of the present disclosure have been described. However, the present disclosure is not limited to the above implementations. Any modification, equivalent replacement and improvement made within the spirit and the principle of the present disclosure shall be included within the protection scope of the present disclosure.