Negative plate, preparation method thereof and lithium battery

Abstract

The invention relates to the technical field of battery manufacturing, and discloses a negative plate, a preparation method thereof and a lithium battery. The preparation method of the negative electrode plate comprises the following steps: spraying a cation solution on a primary electrode plate product of which the surface is coated with negative electrode active slurry before the primary electrode plate product is fed into a drying device; feeding the primary pole piece product with the cationic solution sprayed on the surface into a drying device for drying; the negative active slurry comprises soluble alginate and an adhesive; cations in the cation solution are metal ions which can be subjected to ionic crosslinking with carboxyl on a guluronic acid unit in the alginate to form a three-dimensional network structure; and the adhesive is selected from at least one of styrene-butadiene copolymer and styrene-acrylate. According to the preparation method provided by the invention, the distribution uniformity of the adhesive in the negative electrode active layer can be improved, and the adhesion capability of the negative electrode active layer to the current collector is improved, so that the rate and cycle performance of the battery are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of battery material manufacturing, and more particularly to a negative electrode sheet, a preparation method thereof, and a lithium battery. Background Art

[0002] The negative electrode is an important part of a lithium battery. A reasonable negative electrode slurry formulation and its preparation method are important conditions for the performance of a lithium battery to be exerted. Currently, in the manufacturing process of lithium batteries, a certain amount of binder such as SBR emulsion or styrene-acrylate emulsion is usually added to the negative electrode slurry to provide the adhesion between negative electrode active materials and between the negative electrode active material and the current collector. However, during the coating and drying process, the surface solvent evaporates rapidly to form a solid layer, and the surface tension drives the binder to migrate to the surface.

[0003] The floating of the binder results in the attenuation of the peel strength and flexibility of the electrode sheet, which may cause the coating to peel off from the current collector; at the same time, the internal resistance of the battery increases, reducing the rate performance and cycle life of the battery.

[0004] In view of this, the present invention is specifically proposed. Summary of the Invention

[0005] The purpose of the present invention is to provide a negative electrode sheet, a preparation method thereof, and a lithium battery, which improve the problems that during the preparation of the existing negative electrode sheet, when the negative electrode slurry is dried, the floating of the binder leads to uneven distribution of the binder in the prepared negative electrode sheet and inability to provide good adhesion, resulting in poor peel strength of the electrode sheet prepared with the negative electrode slurry and fast attenuation rate of the cycle capacity.

[0006] The present invention is implemented as follows:

[0007] In a first aspect, the present invention provides a method for preparing a negative electrode sheet, including:

[0008] Spraying a cationic solution on the initial product of the electrode sheet with a negative electrode active slurry coated on its surface before feeding it into a drying device;

[0009] Feeding the initial product of the electrode sheet with the cationic solution sprayed on its surface into the drying device for drying;

[0010] The negative electrode active slurry includes soluble alginate and a binder;

[0011] The cation in the cationic solution is a metal ion that can undergo ionic crosslinking with the carboxyl group on the guluronic acid unit in the alginate to form a three-dimensional network structure;

[0012] By weight, the binder is 1 to 25 parts, the alginate is 1 to 4 parts, and the cation is 0.01 to 0.05 parts;

[0013] The binder is selected from at least one of styrene-butadiene copolymer and styrene-acrylate.

[0014] In an alternative embodiment, the alginate is at least one of sodium alginate and potassium alginate.

[0015] In an alternative embodiment, the valence state of the metal ion ≥ 2 + .

[0016] In an alternative embodiment, the metal ion is selected from at least one of Mg 2+ , Zn 2+ , Ba 2+ , Cu 2+ , Fe 3+ , Al 3+ and Ca 2+ ;

[0017] Preferably, the metal ion is Ca 2+ .

[0018] In an alternative embodiment, the solute in the cation solution is selected from at least one of CaCl2, Ca(NO3)2, Ca(C2H3O2)2, CaBr2 and CaI2.

[0019] In an alternative embodiment, by weight, the negative electrode active paste further includes 68 - 96 parts of negative electrode active material, 0 - 5 parts of conductive agent, and 0.1 - 2 parts of dispersant.

[0020] In a second aspect, the preparation method provided by the present invention includes at least one of the following features (1) - (3), and the negative electrode active material is selected from at least one of natural graphite and artificial graphite;

[0021] Optionally, the conductive agent is at least one of carbon black, graphene, and carbon nanotubes;

[0022] Optionally, the dispersant is polyvinylpyrrolidone.

[0023] In an alternative embodiment, the preparation method of the negative electrode active paste includes:

[0024] Mixing the dispersant, the conductive agent with the first portion of deionized water uniformly to obtain a first paste, and the fineness of the first paste is 30 - 150 μm;

[0025] Mixing the first paste with the alginate uniformly to obtain a second paste;

[0026] Mixing a part of the negative electrode active material, the second portion of deionized water with the second paste uniformly and stirring until the fineness is 30 - 100 μm to obtain a third paste;

[0027] Mixing the remaining part of the negative electrode active material with the third paste uniformly and stirring until the fineness is 5 - 30 μ μm to obtain a fourth paste;

[0028] Mix the binder with the fourth slurry evenly to make the viscosity of the mixed system 4000 - 8000 cps, and evacuate for 25 - 35 min to obtain the negative electrode active slurry.

[0029] Thirdly, the present invention provides a negative electrode sheet prepared by the preparation method according to any one of the foregoing embodiments.

[0030] Fourthly, the present invention provides a lithium battery including the negative electrode sheet according to the foregoing embodiment.

[0031] The present invention has the following beneficial effects:

[0032] In the preparation method of the negative electrode sheet provided by the present invention, alginate is used to replace conventional sodium carboxymethyl cellulose as a thickening agent, and a cationic solution is sprayed before the negative electrode active slurry is fed into the oven. The carboxyl groups on the guluronic acid units of the alginate in the negative electrode slurry can undergo ionic crosslinking with the cations to form a three-dimensional network structure, thereby turning the negative electrode slurry into a gel state; and the negative electrode slurry is an aqueous system, and the binders (styrene-butadiene copolymer and styrene-acrylate) exist in the form of microemulsion droplets in the slurry. During the drying process of the electrode sheet, the binders will be restricted by the three-dimensional network structure, and their floating will be inhibited, so that the binders can be more evenly distributed in the active layer of the electrode sheet. Therefore, the preparation method provided by the present invention can improve the uniformity of the distribution of the binders in the negative electrode active layer, improve the adhesion ability of the negative electrode active layer to the current collector, and thus improve the rate and cycle performance of the battery. Detailed Embodiments

[0033] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Those not specified in the embodiments are carried out according to conventional conditions or conditions recommended by the manufacturer. Reagents or instruments not specified by the manufacturer are all conventional products that can be obtained through commercial purchase.

[0034] The features and properties of the present invention will be further described in detail below in conjunction with embodiments.

[0035] A preparation method of a negative electrode sheet provided by an embodiment of the present invention includes:

[0036] Spray a cationic solution on the initial product of the electrode sheet with the negative electrode active slurry coated on its surface before it is fed into the drying device;

[0037] Feed the initial product of the electrode sheet with the cationic solution sprayed on its surface into the drying device for drying;

[0038] The negative electrode active slurry includes soluble alginate and a binder;

[0039] The cation in the cation solution is a metal ion that can undergo ionic crosslinking with the carboxyl groups on the guluronic acid units in alginate to form a three-dimensional network structure;

[0040] By weight, the binder is 1 to 25 parts, the alginate is 1 to 4 parts, and the cation is 0.01 to 0.05 parts;

[0041] The binder is selected from at least one of styrene-butadiene copolymer (SBR) and styrene acrylate.

[0042] In the preparation process of the negative electrode sheet provided by the present invention, alginate is used to replace conventional sodium carboxymethyl cellulose as a thickener, and a cation solution is sprayed before the negative electrode active slurry is sent into the oven. The carboxyl groups on the guluronic acid units of the alginate in the negative electrode slurry can undergo ionic crosslinking with the cations to form a three-dimensional network structure, thereby turning the negative electrode slurry into a gel state; and the negative electrode slurry is an aqueous system, and the binder (styrene-butadiene copolymer and styrene acrylate) exists in the form of microemulsion droplets in the slurry. During the drying process of the electrode sheet, the binder will be restricted by the three-dimensional network structure, and its floating will be inhibited, so that the binder can be more evenly distributed in the active layer of the electrode sheet. Therefore, the preparation process provided by the present invention can improve the uniformity of the distribution of the binder in the negative electrode active layer, improve the adhesion ability of the negative electrode active layer to the current collector, and thus improve the rate and cycle performance of the battery.

[0043] Optionally, the alginate is at least one of sodium alginate and potassium alginate.

[0044] Optionally, the valence state of the metal ion ≥ 2 + . Alginate has a high content of guluronic acid. When its solution encounters divalent or higher-valent cations (especially calcium ions), these cations can undergo ionic crosslinking with the carboxyl groups on the guluronic acid units to form a three-dimensional network structure, thereby causing the solution to turn into a gel state.

[0045] It should be noted that precisely because the slurry will turn into a gel state after spraying calcium ions, metal ions that can undergo ionic crosslinking with the carboxyl groups on the guluronic acid units cannot be added to the slurry, which will cause the slurry to present a gel state and make it impossible to coat the slurry well on the current collector.

[0046] Optionally, the metal ion is selected from at least one of Mg 2+ , Zn 2+ , Ba 2+ , Cu 2+ , Fe 3+ , Al 3+ and Ca 2+ ; preferably, the metal ion is Ca 2+ .

[0047] Optionally, the solute in the cation solution is selected from at least one of CaCl2, Ca(NO3)2, Ca(C2H3O2)2, CaBr2, and CaI2.

[0048] Optionally, by weight, excluding water, the total amount of other components in the negative electrode active paste is 100 parts. The negative electrode active paste further includes 68 - 96 parts of negative electrode active material, 0 - 5 parts of conductive agent, and 0.1 - 2 parts of dispersant.

[0049] Optionally, the negative electrode active material is selected from at least one of natural graphite and artificial graphite.

[0050] Optionally, the conductive agent is at least one of carbon black, graphene, and carbon nanotubes.

[0051] Optionally, the dispersant is polyvinylpyrrolidone.

[0052] Furthermore, the method for preparing the negative electrode active paste includes:

[0053] S1. Prepare the first paste

[0054] Add the first portion of deionized water into the stirring kettle of a planetary slurry mixer, then add the dispersant and the conductive agent and mix and stir. Measure the fineness every 15 minutes until the fineness of the slurry reaches 30 - 150 μm to obtain the first paste. The amount of the first portion of deionized water is 3 - 20% (such as 3%, 5%, 10%, or 20%) of the total mass of the negative electrode material (all substances participating in forming the slurry except water).

[0055] S2. Prepare the second paste

[0056] Mix the first paste and alginate uniformly to obtain the second paste.

[0057] S3. Prepare the third paste

[0058] Mix about 70% of the negative electrode active material, the second portion of deionized water, and the second paste and stir. Measure the fineness every 15 minutes until the fineness of the slurry reaches 30 - 100 μm to obtain the third paste. The mass of the second portion of deionized water is 35 - 55% (such as 35%, 40%, 50%, or 55%) of the total mass of the negative electrode material.

[0059] S4. Prepare the fourth paste

[0060] Mix the remaining negative electrode active material and the third paste and stir. Measure the fineness every 15 minutes until the fineness of the slurry reaches 5 - 30 μm to obtain the fourth paste.

[0061] S5. Prepare the negative electrode active paste

[0062] Mixing the adhesive and the fourth slurry evenly;

[0063] The viscosity of the mixed system is tested. If the viscosity is between 4000 and 8000 cps, it meets the requirements and the subsequent operation is carried out. If the viscosity is too high and not within the above range, the viscosity of the mixed system is adjusted to 4000 to 8000 cps by adding deionized water.

[0064] After the viscosity meets the requirement, vacuum is applied for 25 to 35 minutes (eg, 25 minutes, 30 minutes or 35 minutes) to obtain the negative electrode active slurry.

[0065] The preparation method of the negative electrode sheet is specifically as follows:

[0066] A negative electrode active slurry is coated on the surface of the current collector by a coating machine, and the electrode sheet coated with the negative electrode active slurry is transported to an oven. In the process of transporting the electrode sheet to the oven, before entering the oven, a cationic solution is sprayed on the electrode sheet, and the cations in the cationic solution enter the negative electrode active slurry and react with the binder to form a three-dimensional network structure;

[0067] The electrode sheets sprayed with cationic solution are sent into the oven, the solvent evaporates, and then rolled to obtain the negative electrode sheets.

[0068] The negative electrode sheet provided in the embodiment of the present invention is prepared by the preparation method provided in the embodiment of the present invention.

[0069] The battery provided by the embodiment of the present invention includes the negative electrode sheet provided by the embodiment of the present invention.

[0070] Example 1

[0071] By weight, 1 part of a dispersant (polyvinyl pyrrolidone), 2 parts of a conductive agent (carbon black), and 11.5 parts of deionized water are mixed and stirred uniformly to obtain a first slurry with a fineness of 30 to 150 μm;

[0072] Add 2 parts of sodium alginate to the first slurry and stir thoroughly until the sodium alginate is dissolved to obtain a second slurry;

[0073] Add 56 parts of negative electrode active material (natural graphite) and 40 parts of deionized water to the second slurry, stir and mix until the fineness is 30-100 μm to obtain a third slurry;

[0074] Add 24 parts of negative electrode active material (natural graphite) to the third slurry and stir until the fineness is 5 to 30 μm to obtain a fourth slurry;

[0075] 15 parts of binder (SBR) were added to the fourth slurry and stirred for 20 minutes. The viscosity was then measured. The viscosity was about 6000 cps, which met the requirement. The negative electrode active slurry was then vacuumed for 30 minutes to obtain.

[0076] The negative electrode active slurry is coated on the current collector to obtain a preliminary pole piece, and the preliminary pole piece is sent into an oven. Before entering the oven, a calcium chloride solution is evenly sprayed on the surface of the preliminary pole piece, and the weight portion of calcium ions in the calcium chloride solution is 0.03 parts.

[0077] It is sent into an oven and dried at 95°C, and then rolled to obtain a negative electrode sheet.

[0078] Example 2

[0079] By weight, 2 parts of a dispersant (polyvinyl pyrrolidone), 5 parts of a conductive agent (carbon black), and 20 parts of deionized water are mixed and stirred uniformly to obtain a first slurry with a fineness of 30 to 150 μm;

[0080] Add 4 parts of sodium alginate to the first slurry, and stir thoroughly until the sodium alginate is dissolved to obtain a second slurry;

[0081] Add 50 parts of negative electrode active material (natural graphite) and 55 parts of deionized water to the second slurry, stir and mix until the fineness is 30-100 μm to obtain a third slurry;

[0082] Add 18 parts of negative electrode active material (natural graphite) to the third slurry and stir until the fineness is 5 to 30 μm to obtain a fourth slurry;

[0083] 25 parts of binder (SBR) were added to the fourth slurry and stirred for 20 minutes. The viscosity was then measured. The viscosity was about 4000 cps, which met the requirement. The negative electrode active slurry was then vacuumed for 30 minutes to obtain.

[0084] The negative electrode active slurry is coated on the current collector to obtain a preliminary pole piece, and the preliminary pole piece is sent into an oven. Before entering the oven, a calcium chloride solution is evenly sprayed on the surface of the preliminary pole piece, and the weight portion of calcium ions in the calcium chloride solution is 0.05 parts.

[0085] It is sent into an oven and dried at 100°C, and then rolled to obtain a negative electrode sheet.

[0086] Example 3

[0087] By weight, 0.1 part of a dispersant (polyvinyl pyrrolidone) and 3 parts of deionized water are mixed and stirred uniformly to obtain a first slurry with a fineness of 30 to 150 μm;

[0088] Add 1 part of sodium alginate to the first slurry, and stir thoroughly until the sodium alginate is dissolved to obtain a second slurry;

[0089] Add 67 parts of negative electrode active material (natural graphite) and 35 parts of deionized water to the second slurry, stir and mix until the fineness is 30-100 μm to obtain a third slurry;

[0090] Add 29 parts of negative electrode active material (natural graphite) to the third slurry and stir until the fineness reaches 5 - 30 μm to obtain the fourth slurry;

[0091] Add 1 part of binder (styrene - acrylate) to the fourth slurry and stir for 20 min, then measure the viscosity. The viscosity is about 8000 cps, which meets the requirements. After that, evacuate for 30 min to obtain the negative electrode active slurry.

[0092] Coat the negative electrode active slurry onto the current collector to obtain the initial product of the electrode sheet. Send the initial product of the electrode sheet into the oven. Before entering the oven, evenly spray calcium nitrate solution on the surface of the initial product of the electrode sheet. The weight fraction of calcium ions in the calcium nitrate solution is 0.01 part.

[0093] Send it into the oven and dry at 90 °C, then roll - press to obtain the negative electrode sheet.

[0094] Example 4

[0095] This example is basically the same as Example 2, except that: replace the calcium chloride solution with an MgCl2 solution with 0.03 parts by mass of magnesium ions.

[0096] Example 5

[0097] This example is basically the same as Example 2, except that: replace the calcium chloride solution with an FeCl3 solution with 0.05 parts by mass of iron ions.

[0098] Comparative Example

[0099] This comparative example is basically the same as Example 2, except that: do not spray calcium chloride solution before sending the initial product of the electrode sheet into the oven.

[0100] Experimental Example

[0101] Cut the negative electrode sheets prepared in each example and comparative example and assemble them into 5 Ah batteries. The composition of the active layer of the positive electrode sheet is lithium iron phosphate with a mass ratio of LFP∶SP∶CNTs∶PVDF = 96.5∶1∶0.5∶2. The current collectors are 13 μm aluminum foil and 6 μm copper foil. The separator is a 20 μm PP membrane. The electrolyte is EC∶DMC∶EMC = 1∶1∶1 (mass ratio), 1 mol / L LiPF.

[0102] Measure the peel strength of the active layer on the negative electrode sheets prepared in each example and comparative example. The test method:

[0103] Take the rolled negative electrode sheet and cut the electrode sheet with a customized die. The length is 200 mm * width is 20 mm; Take a flat thin steel plate with a length of about 300 mm and a width of about 60 mm; Stick a double-sided tape (length 250 mm * width 20 mm) in the center of the steel plate; Uncover the double-sided tape and attach the electrode sheet to the tape; Insert the steel plate with the electrode sheet pasted and fixed into the lower clamp of the tensile machine and fix it vertically; Insert the electrode sheet without tape into the upper clamp of the tensile machine and fix it so that the electrode sheet attached to the tape and the electrode sheet fixed by the upper clamp of the tensile machine form 180°; After fixing the test sample, calibrate and zero, set the test width, the stripping length of the electrode sheet is 100 mm, the stripping speed is 50 mm / min, and then start the test to obtain the average value of the stripping strength. Record the average value of the stripping strength in Table 1.

[0104] Measure the electrochemical performance of the batteries prepared in each example and comparative example, and record the test results in Table 1.

[0105] Table 1 Performance of the negative electrode sheets of each example and comparative example

[0106]

[0107] It can be seen from Table 1 that the negative electrode sheets provided by each example of the present invention all have better electrochemical performance after being assembled into batteries.

[0108] Comparing Examples 2, 4 and 5 with the comparative example, it can be seen that the performance of Examples 2, 4 and 5 is better, which indicates that spraying the cationic solution before the electrode sheet is sent into the oven can improve the performance of the negative electrode sheet;

[0109] Comparing Example 2 with Examples 4 and 5, the effect of Example 2 is slightly better, indicating that calcium ions can better improve the electrochemical performance of the negative electrode sheet compared with other cations acting in the negative electrode slurry.

[0110] In summary, for the preparation method of the negative electrode sheet provided by the present invention, alginate is used to replace the conventional sodium carboxymethylcellulose as a thickening agent, and a cationic solution is sprayed before the negative electrode active slurry is sent into the oven. The carboxyl group on the guluronic acid unit of alginate in the negative electrode slurry can undergo ionic cross-linking with the cation to form a three-dimensional network structure, thereby turning the negative electrode slurry into a gel state; and the negative electrode slurry is an aqueous system, and the binders (styrene-butadiene copolymer and styrene-acrylate) exist in the form of microemulsion droplets in the slurry. During the drying process of the electrode sheet, the binders will be restricted by the three-dimensional network structure, and their floating will be inhibited, so that the binders can be more evenly distributed in the active layer of the electrode sheet. Therefore, the preparation process provided by the present invention can improve the uniformity of the distribution of the binders in the negative electrode active layer, improve the bonding ability of the negative electrode active layer to the current collector, and thus improve the rate and cycle performance of the battery.

[0111] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, 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 method for preparing a negative electrode sheet, characterized in that: include: Spraying the cationic solution on the primary electrode sheet with the surface coated with the negative electrode active slurry before being sent to the drying device; The electrode pieces with the cationic solution sprayed on the surface are sent to the drying device for drying; The negative electrode active slurry includes soluble alginate and a binder; The cations in the cation solution are metal ions that can undergo ionic crosslinking with the carboxyl groups on the guluronic acid units in the alginate to form a three-dimensional network structure; In terms of weight, the adhesive is 1 to 25 parts, the alginate is 1 to 4 parts, and the cation is 0.01 to 0.05 parts; The adhesive is selected from at least one of styrene-butadiene copolymer and styrene-acrylate.

2. The preparation method according to claim 1, characterized in that: The alginate is at least one of sodium alginate and potassium alginate.

3. The preparation method according to claim 1, characterized in that: The valence of the metal ion is ≥2 + .

4. The preparation method according to claim 1, characterized in that: The metal ions are selected from Mg²⁺, Zn²⁺, Ba²⁺, Cu²⁺, Fe³⁺, Al³⁺ and Ca 2+ At least one of the following; Preferably, the metal ion is Ca 2+ .

5. The preparation method according to claim 1, characterized in that: The solute in the cationic solution is selected from at least one of CaCl2, Ca(NO3)2, Ca(C2H3O2)2, CaBr2 and CaI2.

6. The preparation method according to claim 1, characterized in that: In terms of weight, the negative electrode active slurry further includes 68-96 parts of negative electrode active material, 0-5 parts of conductive agent and 0.1-2 parts of dispersant.

7. The preparation method according to claim 6, characterized in that: The invention comprises at least one of the following features (1) to (3), wherein the negative electrode active material is selected from at least one of natural graphite and artificial graphite; Optionally, the conductive agent is at least one of carbon black, graphene and carbon nanotubes; Optionally, the dispersant is polyvinyl pyrrolidone.

8. The preparation method according to claim 6, characterized in that: The method for preparing the negative electrode active slurry comprises: The dispersant, the conductive agent and the first portion of deionized water are uniformly mixed to obtain a first slurry, wherein the fineness of the first slurry is 30-150 μm; The first slurry and the alginate are uniformly mixed to obtain a second slurry; Evenly mixing a portion of the negative electrode active material, a second portion of deionized water and the second slurry and stirring until the fineness is 30-100 μm to obtain a third slurry; The remaining negative electrode active material is mixed evenly with the third slurry and stirred until the fineness is 5-30 μm to obtain a fourth slurry; The binder and the fourth slurry are uniformly mixed to make the viscosity of the mixed system 4000-8000 cps, and vacuum is applied for 25-35 minutes to obtain the negative electrode active slurry.

9. A negative electrode sheet, characterized in that: The method is prepared according to any one of claims 1 to 8.

10. A lithium battery, characterized in that: Comprising the negative electrode sheet as claimed in claim 9.

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