Negative electrode slurry and preparation method thereof, negative electrode plate, sodium secondary battery and electric device

Abstract

The invention provides negative electrode slurry and a preparation method thereof, a negative electrode plate, a sodium secondary battery and an electric device. The negative electrode slurry comprises a negative electrode active material and a modifier coated on the surface of the negative electrode active material, the negative electrode active material comprises one or more of hard carbon and hard carbon coated graphite, and the modifier comprises carboxymethyl cellulose salt with the weight-average molecular weight of 20000-150000. The negative electrode slurry has low gel and foaming phenomena, and can improve the processing performance and storage performance of the sodium secondary battery.

Description

Technical Field

[0001] The present application relates to the technical field of sodium batteries, and in particular to a negative electrode slurry and a preparation method thereof, a negative electrode sheet, a sodium secondary battery and an electrical device. Background Art

[0002] In recent years, secondary batteries have been widely used in energy storage power systems such as hydropower, thermal, wind and solar power stations, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace and other fields.

[0003] Compared with lithium secondary batteries, sodium secondary batteries have greater competitive advantages due to their abundant sodium resources and wide distribution. However, due to the limitations of the sodium secondary battery preparation process, such as the gelation and foaming phenomenon in the preparation of the negative electrode slurry, sodium secondary batteries cannot be mass-produced to meet market demand. Summary of the invention

[0004] The present application is made in view of the above-mentioned problems, and its purpose is to provide a negative electrode slurry, aiming to improve the gel and foaming phenomena of the negative electrode slurry, so as to enhance the processing performance and storage performance of the sodium secondary battery.

[0005] In a first aspect of the present application, a negative electrode slurry for a sodium secondary battery is provided, the negative electrode slurry comprising a negative electrode active material and a modifier coated on the surface of the negative electrode active material, the negative electrode active material comprising one or more of hard carbon and hard carbon-coated graphite, the modifier comprising a carboxymethyl cellulose salt having a weight average molecular weight of 20,000 to 150,000.

[0006] Compared with sodium carboxymethyl cellulose with a weight average molecular weight greater than 300,000, the modifier containing carboxymethyl cellulose salt with a weight average molecular weight of 20,000 to 150,000 can make the modifier have higher stretchability and lower steric hindrance in the negative electrode slurry due to the reduction of molecular weight, which is conducive to the coating between the modifier and the hard carbon. In other words, the modifier containing carboxymethyl cellulose salt with a weight average molecular weight of 20,000 to 150,000 can rely on the SP of the main chain and the hard carbon surface. 2The hybridized CC bond forms a strong intermolecular effect, so that the modifier is coated on the hard carbon surface. The coating structure formed by the modifier and the hard carbon is convenient for adjusting the pore distribution of the hard carbon, reducing the probability of water molecules in the negative electrode slurry entering the hard carbon pores, and thus reducing the degree of foaming of the negative electrode slurry. More importantly, the side chains of the modifier containing carboxymethyl cellulose salt with a weight average molecular weight of 20,000 to 150,000 can be combined with the oxygen-containing functional groups on the hard carbon surface, which can reduce the degree of gelation of the negative electrode slurry caused by the side chains of carboxymethyl cellulose sodium with a weight average molecular weight greater than 300,000 and the oxygen-containing functional groups on the hard carbon surface forming a cross-linked structure, and the side chains of the carboxymethyl cellulose salt coated on the hard carbon surface can also form a space charge layer, which is conducive to increasing the repulsion between hard carbon particles, thereby reducing the degree of gelation of the negative electrode slurry caused by the agglomeration of hard carbon particles, improving the processability of the negative electrode slurry, and improving the yield, which is conducive to the mass production of sodium secondary batteries. At the same time, it is also conducive to improving the storage performance and cycle performance of sodium secondary batteries.

[0007] In any embodiment, the modifier includes a carboxymethyl cellulose salt having a weight average molecular weight of 40,000 to 100,000.

[0008] Further controlling the weight average molecular weight of the carboxymethyl cellulose salt in the modifier to 40,000 to 100,000 is beneficial to further improve the processability of the negative electrode slurry, as well as the cycle performance and storage performance of the sodium secondary battery.

[0009] In any embodiment, the carboxymethyl cellulose salt includes one or more of lithium carboxymethyl cellulose, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, rubidium carboxymethyl cellulose, and cesium carboxymethyl cellulose.

[0010] In any embodiment, the mass ratio of the modifier to the negative electrode active material is 0.001 to 0.011, and can be 0.002 to 0.0065.

[0011] Controlling the mass ratio of the modifier and the negative electrode active material within an appropriate range is beneficial to providing enough modifier to coat the hard carbon surface to form a coating structure, improving the gelation and foaming phenomena of the negative electrode slurry, and reducing the impact of excessively thick coating on the hard carbon surface due to excessive proportion of the modifier on battery performance.

[0012] In any embodiment, based on the total mass of solids in the negative electrode slurry, the mass content of the modifier is 0.1% to 1%, and can be optionally 0.2% to 0.6%.

[0013] In any embodiment, based on the total mass of solids in the negative electrode slurry, the mass content of the negative electrode active material is 92% to 97%, and optionally 92% to 95%.

[0014] Controlling the mass content of the modifier within an appropriate range can not only provide enough modifier to coat the hard carbon surface to form a coating structure, but also reduce the impact of excessively thick coating on the hard carbon surface due to excessive mass content of the modifier on battery performance.

[0015] The mass content of the negative electrode active material is controlled within a suitable range to provide sufficient sodium insertion sites so that the sodium secondary battery has a high capacity.

[0016] In any embodiment, the solid content of the negative electrode slurry is 45% to 55%, and can be optionally 48% to 53%.

[0017] The negative electrode slurry is controlled within a suitable range so that the negative electrode slurry has a certain fluidity for subsequent processing.

[0018] In any embodiment, the negative electrode slurry further includes at least one of a conductive agent, a dispersant, a binder, and a plasticizer.

[0019] In any embodiment, the conductive agent includes one or more of Super P, Ketjen black, acetylene black, carbon nanotubes, and graphene, and optionally includes Super P; and / or

[0020] The dispersant comprises sodium carboxymethyl cellulose having a weight average molecular weight of 400,000 to 1,000,000; and / or

[0021] The binder includes one or more of styrene-butadiene rubber, polyacrylic acid, polyacrylate, polyamide, poly(acrylonitrile-acrylate), poly(styrene-acrylate), and optionally includes styrene-butadiene rubber; and / or

[0022] The plasticizer includes 1,3-butanediol.

[0023] Sodium carboxymethyl cellulose with a weight average molecular weight of 400,000 to 1,000,000 as a dispersant is beneficial to improving the dispersion uniformity of the negative electrode slurry and improving the processability of the negative electrode slurry. The above-mentioned conductive agent and binder can make the negative electrode sheet prepared from the negative electrode slurry have excellent conductivity and adhesion. The above-mentioned plasticizer can make the negative electrode sheet prepared from the negative electrode slurry have excellent toughness and reduce the cracking of the negative electrode sheet.

[0024] In any embodiment, the bubble volume per unit mass of hard carbon in the negative electrode slurry does not exceed 2.5 mL / g.

[0025] The negative electrode slurry has a lower degree of foaming, which can improve the processability of the negative electrode sheet and is beneficial to the improvement of the storage performance and cycle performance of the sodium secondary battery.

[0026] In any embodiment, the viscosity of the negative electrode slurry after being placed at 25° C. for 48 hours changes to 3000 mPa·s to 12000 mPa·s.

[0027] The viscosity of the negative electrode slurry after being placed at 25°C for 48 hours changes from 3000mPa·s to 12000mPa·s, and its gelation phenomenon is significantly improved, which is beneficial to the subsequent processing of the negative electrode slurry, as well as the improvement of the storage performance and cycle performance of the sodium secondary battery.

[0028] The second aspect of the present application provides a method for preparing a negative electrode slurry, comprising: coating a modifier on the surface of a negative electrode active material to obtain a viscous mixture; dispersing the viscous mixture in a solvent to prepare a negative electrode slurry; wherein the negative electrode active material comprises hard carbon and / or hard carbon-coated graphite, the modifier comprises a carboxymethyl cellulose salt having a weight average molecular weight of 20,000 to 150,000, and the carboxymethyl cellulose salt comprises one or more of lithium carboxymethyl cellulose, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, rubidium carboxymethyl cellulose, and cesium carboxymethyl cellulose.

[0029] Coating the modifier on the surface of the negative electrode active material can effectively reduce the gel degree of the negative electrode slurry. Specifically, since the modifier containing the carboxymethyl cellulose salt with a weight average molecular weight of 20,000 to 150,000 has higher stretchability in the negative electrode slurry, it is conducive to the coating between the modifier and the hard carbon. Therefore, the coating structure formed by the modifier and the hard carbon is convenient for adjusting the pore distribution of the hard carbon, reducing the probability of water molecules in the negative electrode slurry entering the hard carbon pores, thereby reducing the degree of foaming of the negative electrode slurry. Furthermore, the side chains of the modifier containing carboxymethyl cellulose salt with a weight average molecular weight of 20,000 to 150,000 can combine with the oxygen-containing functional groups on the hard carbon surface, which can reduce the degree of gelation of the negative electrode slurry caused by the side chains of the dispersant combining with the oxygen-containing functional groups on the hard carbon surface to form a cross-linked structure, and the side chains of the carboxymethyl cellulose salt coated on the hard carbon surface can also form a space charge layer, which is beneficial to increase the repulsion between hard carbon particles, thereby reducing the degree of gelation of the negative electrode slurry caused by the agglomeration of hard carbon particles, improving the processability of the negative electrode slurry, and increasing the yield, which is beneficial to the mass production of sodium secondary batteries. At the same time, it is also beneficial to improve the storage performance and cycle performance of sodium secondary batteries.

[0030] In any embodiment, coating the surface of the negative electrode active material with the modifier specifically comprises: kneading the raw material containing the negative electrode active material and the modifier in a solvent to obtain the adhesive mixture. The negative electrode active material is first kneaded with the modifier in a solvent so that the modifier can be effectively coated on the surface of the negative electrode active material to increase the coating rate.

[0031] In any embodiment, the kneading time is 30 min to 120 min, and can be optionally 40 min to 80 min.

[0032] Controlling the kneading time within an appropriate range is beneficial to the formation of a coating structure between the modifier and the hard carbon, improving the foaming and gelling phenomena of the negative electrode slurry, enhancing the processability of the negative electrode slurry, and improving the storage performance and cycle performance of the sodium secondary battery.

[0033] In any embodiment, the solid content of the adhesive mixture is 60% to 68%, and optionally 60% to 65%.

[0034] In any embodiment, the preparation method specifically comprises:

[0035] Stirring and mixing the conductive agent, the negative electrode active material and the modifier to obtain a dry mixed mixture;

[0036] kneading the dry blended mixture in a solvent to obtain the adhesive mixture;

[0037] The adhesive mixture, the dispersant and the solvent are stirred and mixed to obtain a first adhesive solution;

[0038] The first glue solution and the plasticizer are stirred and mixed, and a binder is added and stirred evenly to obtain the negative electrode slurry.

[0039] The preparation method can significantly improve the foaming and gelling phenomena of the negative electrode slurry, enhance the processability of the negative electrode slurry, and enhance the storage performance and cycle performance of the sodium secondary battery.

[0040] The third aspect of the present application provides a negative electrode plate, which includes a negative electrode active material and a modifier coated on the surface of the negative electrode active material, wherein the negative electrode active material includes hard carbon and / or hard carbon-coated graphite, and the modifier includes a carboxymethyl cellulose salt with a weight average molecular weight of 20,000 to 150,000, and the carboxymethyl cellulose salt includes one or more of lithium carboxymethyl cellulose, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, rubidium carboxymethyl cellulose, and cesium carboxymethyl cellulose.

[0041] The negative electrode sheet has good quality and stability, which is beneficial to improving the processing performance, storage performance and cycle stability of sodium secondary batteries.

[0042] The fourth aspect of the present application provides a sodium secondary battery, comprising the negative electrode plate of the third aspect of the present application, or a negative electrode plate prepared by the negative electrode slurry of the first aspect of the present application or the negative electrode slurry prepared by the preparation method of the second aspect.

[0043] The fifth aspect of the present application provides an electrical device, comprising the sodium secondary battery of the fourth aspect of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 It is a schematic diagram of the gelation mechanism of the negative electrode slurry;

[0045] Figure 2 This is an optical microscope photo of the negative electrode slurry after coating;

[0046] Figure 3 is a schematic diagram of a coating structure formed by a modifier and hard carbon in one embodiment of the present application;

[0047] Figure 4 is a schematic diagram of bonding between a modifier and hard carbon in one embodiment of the present application;

[0048] Figure 5 Schematic diagram of the division of gel state test results of negative electrode slurry after storage at 25° C. for 48 hours in one embodiment of the present application;

[0049] Figure 6 is a schematic diagram of a sodium secondary battery according to an embodiment of the present application;

[0050] Figure 7 yes Figure 6 An exploded view of a sodium secondary battery according to an embodiment of the present application is shown;

[0051] Figure 8 is a schematic diagram of a battery module according to an embodiment of the present application;

[0052] Fig. 9 is a schematic diagram of a battery pack according to an embodiment of the present application;

[0053] Fig.10 yes Fig. 9 An exploded view of a battery pack according to an embodiment of the present application is shown;

[0054] Fig.11 Schematic diagram of an electrical device using a sodium secondary battery according to an embodiment of the present application as a power source.

[0055] Description of reference numerals:

[0056] 1 battery pack; 2 upper box; 3 lower box; 4 battery module; 5 sodium secondary battery; 51 shell; 52 electrode assembly; 53 top cover assembly. DETAILED DESCRIPTION

[0057] Below, the negative electrode slurry and its preparation method, negative electrode plate, sodium secondary battery and electric device of the present application are described in detail with appropriate reference to the drawings. However, there are cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following descriptions are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0058] "Scope" disclosed in the present application is limited in the form of lower limit and upper limit, and a given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of a special range. The scope limited in this way can be including end values ​​or not including end values, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a scope. For example, if the scope of 60-120 and 80-110 is listed for a specific parameter, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range values ​​1 and 2 are listed, and if the maximum range values ​​3, 4 and 5 are listed, the following scope can be all expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In the present application, unless otherwise specified, the numerical range "ab" represents the abbreviation of any real number combination between a and b, wherein a and b are real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" are listed in this document, and "0-5" is just an abbreviation of these numerical combinations. In addition, when a parameter is expressed as an integer ≥ 2, it is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0059] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0060] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0061] If there is no special explanation, all steps of the present application can be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), which means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0062] If there is no special explanation, the "include" and "comprising" mentioned in this application are open-ended or closed-ended. For example, the "include" and "comprising" may mean that other components not listed may also be included or only the listed components may be included or only the listed components may be included.

[0063] Unlike lithium secondary batteries, sodium secondary batteries often use hard carbon as their negative electrode active material. Since the hard carbon precursor is rich in a large number of heteroatoms such as hydrogen, oxygen, and nitrogen, the prepared hard carbon surface is rich in a large number of oxygen-containing functional groups (oxygen content> 10%), and the side chains of sodium carboxymethyl cellulose (weight average molecular weight greater than 300,000), a commonly used dispersant in the negative electrode slurry, are easy to combine with the oxygen-containing functional groups on the hard carbon surface to form a cross-linked structure, causing the gel of the negative electrode slurry (such as Figure 1 As shown), the hard carbon particles are small, with irregular surfaces, large roughness, large specific surface area, and high surface energy, which leads to the agglomeration of hard carbon particles, further aggravating the gelation of negative electrode slurry. In addition, there are pores in the hard carbon particles, and the solvent water in the negative electrode slurry will slowly enter the pores of the hard carbon and discharge the gas in the pores of the hard carbon, causing the negative electrode slurry to foam, further deteriorating the gelation. The capacity of hard carbon mainly includes two stages. The first stage is at 1.5V~0.1V (vs Na / Na + ) capacity comes from Na + Adsorption process at the surface defects of hard carbon, the second stage at 0.1V (vs Na / Na + )The following capacity contributions come from Na + The filling process in the micropores of hard carbon. In order to increase the capacity of the negative electrode, the porosity in the hard carbon is often increased in the prior art. While the high porosity increases the capacity of the hard carbon, it also brings more serious process problems, making the negative electrode slurry containing hard carbon face more serious foaming and gel problems. The gelation and foaming of the negative electrode slurry are not conducive to the subsequent coating of the negative electrode slurry, and have a great impact on the mass production and performance of sodium secondary batteries. Therefore, it is necessary to provide a negative electrode slurry with significantly improved gelation and foaming to meet market demand.

[0064] [Anode slurry]

[0065] Based on this, the present application proposes a negative electrode slurry for a sodium secondary battery, which includes a negative electrode active material and a modifier coated on the surface of the negative electrode active material, the negative electrode active material includes one or more of hard carbon and hard carbon-coated graphite, and the modifier includes a carboxymethyl cellulose salt with a weight average molecular weight of 20,000 to 150,000.

[0066] In some embodiments, the weight average molecular weight of the carboxymethyl cellulose salt can be 20,000, 30,000, 40,000, 50,000, 60,000, 70,000, 80,000, 90,000, 100,000, 110,000, 120,000, 130,000, 140,000, 150,000 or a value in a range consisting of any two of the foregoing.

[0067] Usually, due to the properties of hard carbon itself and the reaction between it and conventional sodium carboxymethyl cellulose (weight average molecular weight greater than 300,000), the negative electrode slurry using hard carbon as the negative electrode active material is prone to foaming and gelling. The occurrence of foaming and gelling will affect the subsequent coating, such as the occurrence of missing coating of the negative electrode slurry (such as Figure 2 As shown in the figure, the black spots in the optical microscope photo of the negative electrode sheet represent the problems of missing coating of the negative electrode slurry) and high weight loss rate of the negative electrode sheet. Missing coating of the negative electrode slurry or high weight loss rate of the negative electrode sheet will affect the uniformity of distribution of the negative electrode active material, causing the CB value of the negative electrode sheet to fluctuate and there is a risk of serious sodium precipitation, which will also affect the cycle performance and storage performance of the sodium secondary battery.

[0068] It can be understood that, compared with sodium carboxymethyl cellulose having a weight average molecular weight greater than 300,000, the weight average molecular weight of the modifier containing carboxymethyl cellulose salt having a weight average molecular weight of 20,000 to 150,000 is reduced, so that the modifier has higher stretchability and lower steric hindrance in the negative electrode slurry, which is beneficial to the coating between the modifier and the hard carbon, such as Figure 3 and Figure 4 As shown ( Figure 3 The gas inside the pores of medium-hard carbon is nitrogen because the hard carbon is prepared in a nitrogen atmosphere and the nitrogen generated during the preparation process is adsorbed inside the pores). The modifier containing carboxymethyl cellulose salt with a weight average molecular weight of 20,000 to 150,000 can rely on the SP of the main chain and the hard carbon surface. 2The hybridized CC bond forms a strong intermolecular effect, so that the modifier is coated on the hard carbon surface. The coating structure formed by the modifier and the hard carbon is convenient for adjusting the pore distribution of the hard carbon, reducing the probability of water molecules in the negative electrode slurry entering the hard carbon pores, and thus reducing the degree of foaming of the negative electrode slurry. More importantly, the side chains of the modifier containing carboxymethyl cellulose salt with a weight average molecular weight of 20,000 to 150,000 can be combined with the oxygen-containing functional groups on the hard carbon surface, which can reduce the degree of gelation of the negative electrode slurry caused by the side chains of carboxymethyl cellulose sodium with a weight average molecular weight greater than 300,000 and the oxygen-containing functional groups on the hard carbon surface forming a cross-linked structure, and the side chains of the carboxymethyl cellulose salt coated on the hard carbon surface can also form a space charge layer, which is conducive to increasing the repulsion between hard carbon particles, thereby reducing the degree of gelation of the negative electrode slurry caused by the agglomeration of hard carbon particles, improving the processability of the negative electrode slurry, and improving the yield, which is conducive to the mass production of sodium secondary batteries. At the same time, it is also conducive to improving the storage performance and cycle performance of sodium secondary batteries.

[0069] In this article, "the weight loss rate of the uncold-pressed negative electrode sheet" can characterize the content of water entering the hard carbon pores. The mass lost by the uncold-pressed negative electrode sheet when baked at 140°C is the mass of water. The lower the weight loss rate of the uncold-pressed negative electrode sheet, the fewer water molecules enter the hard carbon pores, which is more conducive to improving the foaming phenomenon of the negative electrode slurry.

[0070] In this article, "rheology" can characterize whether the negative electrode slurry is shear thickening or shear thinning under the action of shear force. Shear thinning is beneficial to the coating of the negative electrode slurry, while shear thickening is not beneficial to the coating of the negative electrode slurry.

[0071] In some embodiments, the modifier includes a carboxymethyl cellulose salt having a weight average molecular weight of 40,000 to 100,000.

[0072] Further controlling the weight average molecular weight of the carboxymethyl cellulose salt in the modifier to 40,000 to 100,000 is beneficial to further improve the processability of the negative electrode slurry, as well as the cycle performance and storage performance of the sodium secondary battery.

[0073] In some embodiments, the carboxymethyl cellulose salt includes one or more of lithium carboxymethyl cellulose, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, rubidium carboxymethyl cellulose, and cesium carboxymethyl cellulose.

[0074] In some embodiments, the carboxymethyl cellulose salt is sodium carboxymethyl cellulose.

[0075] In some embodiments, the carboxymethylcellulose salt is lithium carboxymethylcellulose.

[0076] In some embodiments, the mass ratio of the modifier to the negative electrode active material is 0.001 to 0.011, and can be 0.002 to 0.0065.

[0077] In some embodiments, the mass ratio of the modifier to the negative electrode active material may be 0.001, 0.002, 0.003, 0.004, 0.005, 0.006, 0.0065, 0.007, 0.008, 0.009, 0.01, 0.011 or a value in a range consisting of any two of the foregoing points.

[0078] Controlling the mass ratio of the modifier and the negative electrode active material within an appropriate range is beneficial to providing enough modifier to coat the hard carbon surface to form a coating structure, improving the gelation and foaming phenomena of the negative electrode slurry, and reducing the risk of excessively thick coating on the hard carbon surface due to excessive proportion of the modifier, which aggravates the risk of sodium precipitation and the impact on battery performance.

[0079] In some embodiments, based on the total mass of solids in the negative electrode slurry, the mass content of the modifier is 0.1% to 1%, and optionally 0.2% to 0.6%.

[0080] In some embodiments, based on the total mass of solids in the negative electrode slurry, the mass content of the modifier can be selected to be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1% or a value in the range consisting of any two of the above points.

[0081] Controlling the mass content of the modifier within an appropriate range can take into account the processing performance, storage performance and cycle performance of the sodium secondary battery. If the content of the modifier is too low, it cannot effectively form a coating structure with the hard carbon, slowing down the gelation and foaming of the negative electrode slurry; if the content of the modifier is too high, it is easy to cause the hard carbon surface coating to be too thick, aggravating the risk of sodium precipitation in the sodium secondary battery, and also not conducive to battery performance.

[0082] In some embodiments, based on the total mass of solids in the negative electrode slurry, the mass content of the negative electrode active material is 92% to 97%, and optionally 92% to 95%.

[0083] In some embodiments, based on the total mass of solids in the negative electrode slurry, the mass content of the negative electrode active material is 92%, 92.5%, 93%, 93.5%, 94%, 94.5%, 95%, 95.5%, 96%, 96.5%, 97% or a value in the range consisting of any two of the above points.

[0084] The mass content of the negative electrode active material is controlled within a suitable range to provide sufficient sodium insertion sites so that the sodium secondary battery has a high capacity.

[0085] In some embodiments, the solid content of the negative electrode slurry is 45% to 55%, and optionally 48% to 53%.

[0086] In some embodiments, the solid content of the negative electrode slurry may be selected to be 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55% or a value in a range consisting of any two of the above points.

[0087] The negative electrode slurry is controlled within a suitable range so that the negative electrode slurry has a certain fluidity for subsequent processing.

[0088] In some embodiments, the negative electrode slurry further includes at least one of a conductive agent, a dispersant, a binder, and a plasticizer.

[0089] In some embodiments, the conductive agent includes one or more of Super P, Ketjen black, acetylene black, carbon nanotubes, and graphene, and may optionally include Super P.

[0090] In some embodiments, the dispersant includes sodium carboxymethyl cellulose having a weight average molecular weight of 400,000 to 1,000,000.

[0091] In some embodiments, the binder includes one or more of styrene-butadiene rubber, polyacrylic acid, polyacrylate, polyamide, poly(acrylonitrile-acrylate), poly(styrene-acrylate), and optionally includes styrene-butadiene rubber.

[0092] In some embodiments, the plasticizer includes 1,3,-butanediol.

[0093] Sodium carboxymethyl cellulose with a weight average molecular weight of 400,000 to 1,000,000 as a dispersant is beneficial to improving the dispersion uniformity of the negative electrode slurry and improving the processability of the negative electrode slurry. The above-mentioned conductive agent and binder can make the negative electrode sheet prepared from the negative electrode slurry have excellent conductivity and adhesion. The above-mentioned plasticizer can make the negative electrode sheet prepared from the negative electrode slurry have excellent toughness and reduce the cracking of the negative electrode sheet.

[0094] In some embodiments, the bubble volume per unit mass of hard carbon in the negative electrode slurry is no more than 2.5 mL / g.

[0095] In this article, the bubble volume per unit mass of hard carbon in the negative electrode slurry can be tested by any known method. As an example, the negative electrode slurry (the mass of hard carbon in the negative electrode slurry is m, in g) is sealed in a sealed bag; the gas change is tested by the drainage method. The initial volume is V0, in mL, and the test volume after 48h is V1, in mL. Then the bubble volume per unit mass of hard carbon in the negative electrode slurry = (V1-V0) / m, in mL / g.

[0096] In some embodiments, the bubble volume per unit mass of hard carbon in the negative electrode slurry is 0 mL / g, 0.2 mL / g, 0.5 mL / g, 0.8 mL / g, 1 mL / g, 1.2 mL / g, 1.5 mL / g, 1.8 mL / g, 2 mL / g, 2.2 mL / g, 2.5 mL / g or a value in the range formed by any two of the above points.

[0097] The negative electrode slurry has a lower degree of foaming, which can improve the processability of the negative electrode sheet and is beneficial to the improvement of the storage performance and cycle performance of the sodium secondary battery.

[0098] In some embodiments, the viscosity of the negative electrode slurry after being placed at 25° C. for 48 hours changes from 3000 mPa·s to 12000 mPa·s.

[0099] In this article, the viscosity change of the negative electrode slurry after being placed at 25°C for 48 hours can be tested by any known method. As an example, a rotary viscosity meter is used for measurement: first, the initial viscosity of the negative electrode slurry is measured using a rotary viscosity meter, a suitable rotor is selected, the viscometer rotor is fixed, and the negative electrode slurry is placed under the viscometer rotor. The negative electrode slurry just submerges the scale line of the rotor. The instrument model: Shanghai Fangrui NDJ-5S, rotor: 63# (2000-10000mPa·s), 64# (10000-50000mPa·s), speed: 12 rpm, test temperature: 25°C, test time is 5 minutes, and the data is read after the display is stable. After standing at 25°C for 48 hours, the viscosity is tested using the same test method. The viscosity change of the negative electrode slurry after being placed at 25°C for 48 hours is the difference between the viscosity of the negative electrode slurry after standing for 48 hours and the initial viscosity of the negative electrode slurry.

[0100] In some embodiments, the viscosity of the negative electrode slurry after being placed at 25° C. for 48 hours changes to 3000 mPa·s, 4000 mPa·s, 5000 mPa·s, 6000 mPa·s, 7000 mPa·s, 8000 mPa·s, 9000 mPa·s, 10000 mPa·s, 11000 mPa·s, 12000 mPa·s or a value in the range formed by any two of the above points.

[0101] The viscosity of the negative electrode slurry after being placed at 25°C for 48 hours changes from 3000mPa·s to 12000mPa·s, and its gelation phenomenon is significantly improved, which is beneficial to the subsequent processing of the negative electrode slurry, as well as the improvement of the storage performance and cycle performance of the sodium secondary battery.

[0102] The second aspect of the present application provides a method for preparing a negative electrode slurry, comprising: coating a modifier on the surface of a negative electrode active material to obtain a viscous mixture; dispersing the viscous mixture in a solvent to prepare a negative electrode slurry; wherein the negative electrode active material comprises hard carbon and / or hard carbon-coated graphite, the modifier comprises a carboxymethyl cellulose salt having a weight average molecular weight of 20,000 to 150,000, and the carboxymethyl cellulose salt comprises one or more of lithium carboxymethyl cellulose, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, rubidium carboxymethyl cellulose, and cesium carboxymethyl cellulose.

[0103] As used herein, a "sticky mixture" refers to a dough-like mixture.

[0104] Coating the modifier on the surface of the negative electrode active material can effectively reduce the gel degree of the negative electrode slurry. Specifically, since the modifier containing the carboxymethyl cellulose salt with a weight average molecular weight of 20,000 to 150,000 has higher stretchability in the negative electrode slurry, it is conducive to the coating between the modifier and the hard carbon. Therefore, the coating structure formed by the modifier and the hard carbon is convenient for adjusting the pore distribution of the hard carbon, reducing the probability of water molecules in the negative electrode slurry entering the hard carbon pores, thereby reducing the degree of foaming of the negative electrode slurry. Furthermore, the side chains of the modifier containing carboxymethyl cellulose salt with a weight average molecular weight of 20,000 to 150,000 can combine with the oxygen-containing functional groups on the hard carbon surface, which can reduce the degree of gelation of the negative electrode slurry caused by the side chains of the dispersant combining with the oxygen-containing functional groups on the hard carbon surface to form a cross-linked structure, and the side chains of the carboxymethyl cellulose salt coated on the hard carbon surface can also form a space charge layer, which is beneficial to increase the repulsion between hard carbon particles, thereby reducing the degree of gelation of the negative electrode slurry caused by the agglomeration of hard carbon particles, improving the processability of the negative electrode slurry, and increasing the yield, which is beneficial to the mass production of sodium secondary batteries. At the same time, it is also beneficial to improve the storage performance and cycle performance of sodium secondary batteries.

[0105] In some embodiments, coating the modifier on the surface of the negative electrode active material specifically includes: kneading a raw material containing the negative electrode active material and the modifier in a solvent to obtain the adhesive mixture.

[0106] In this article, "kneading" refers to the operation of adding a small amount of liquid (or binder) to solid powder so that the liquid uniformly wets the inside and surface of the powder particles to prepare a uniform plastic material.

[0107] The negative electrode active material is first kneaded with the modifier in a solvent, so that the modifier can be effectively coated on the surface of the negative electrode active material to improve the coating rate.

[0108] In some embodiments, the kneading time is 30 min to 120 min, and optionally 40 min to 80 min.

[0109] In some embodiments, the kneading time may be 30 min, 40 min, 50 min, 60 min, 70 min, 80 min, 90 min, 100 min, 110 min, 120 min or a value in a range consisting of any two of the above points.

[0110] Controlling the kneading time within an appropriate range is beneficial to the formation of a coating structure between the modifier and the hard carbon, improving the foaming and gelling phenomena of the negative electrode slurry, enhancing the processability of the negative electrode slurry, and improving the storage performance and cycle performance of the sodium secondary battery.

[0111] In some embodiments, the solid content of the adhesive mixture is 60% to 68%, and optionally 60% to 65%.

[0112] In some embodiments, the solid content of the adhesive mixture can be selected to be 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68% or a value in a range consisting of any two of the above points.

[0113] In some embodiments, the preparation method specifically includes: stirring and mixing the conductive agent, the negative electrode active material and the modifier at a stirring speed of 300 rpm to 800 rpm and a stirring time of 10 min to 60 min to obtain a dry mixed mixture; kneading the dry mixed mixture in a solvent to obtain a viscous mixture; stirring and mixing the viscous mixture, the dispersant and the solvent at a stirring speed of 1200 rpm to 1800 rpm and a stirring time of 30 min to 60 min to obtain a first glue solution; stirring and mixing the first glue solution and the plasticizer, and then adding a binder, continuing to stir evenly at a stirring speed of 500 rpm to 800 rpm and a stirring time of 10 min to 30 min to obtain the negative electrode slurry.

[0114] The preparation method can significantly improve the foaming and gelling phenomena of the negative electrode slurry, enhance the processability of the negative electrode slurry, and enhance the storage performance and cycle performance of the sodium secondary battery.

[0115] [Negative electrode]

[0116] The negative electrode plate includes a negative electrode current collector and a negative electrode film layer located on at least one side of the negative electrode current collector.

[0117] As an example, the negative electrode current collector has two surfaces opposite to each other in its thickness direction, and the negative electrode film layer is disposed on any one or both of the two opposite surfaces of the negative electrode current collector.

[0118] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, as the metal foil, a copper foil may be used. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0119] In some embodiments, the negative electrode film layer is prepared from the negative electrode slurry in some embodiments or the negative electrode slurry prepared by the preparation method in some embodiments.

[0120] The negative electrode sheet can be prepared in the following manner: the negative electrode slurry is extrusion coated or transfer coated on the negative electrode current collector, and placed in an oven at a temperature of 110°C to 130°C with a coating speed of 25 m / min, and the negative electrode sheet is obtained by cold pressing and slitting.

[0121] In some embodiments, the negative electrode film layer includes a negative electrode active material and a modifier coated on the surface of the negative electrode active material, the negative electrode active material includes hard carbon and / or hard carbon-coated graphite, the modifier includes a carboxymethyl cellulose salt with a weight average molecular weight of 20,000 to 150,000, and the carboxymethyl cellulose salt includes one or more of carboxymethyl cellulose lithium, carboxymethyl cellulose sodium, carboxymethyl cellulose potassium, carboxymethyl cellulose rubidium, and carboxymethyl cellulose cesium.

[0122] [Positive electrode]

[0123] The positive electrode sheet includes a positive electrode current collector and a positive electrode material layer located on at least one side of the positive electrode current collector.

[0124] As an example, the positive electrode current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode material layer is disposed on any one or both of the two opposite surfaces of the positive electrode current collector.

[0125] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0126] In some embodiments, the positive electrode material layer includes a positive electrode active material, and the positive electrode active material can be a positive electrode active material for batteries well-known in the art. As an example, the positive electrode active material can include at least one of the following materials: layered transition metal oxides, polyanion compounds, or Prussian blue compounds. However, the present application is not limited to these materials, and other conventional materials that can be used as the positive electrode active material of the battery can also be used. These positive electrode active materials can be used alone or in combination of two or more. Among them, the Prussian blue compound includes Na x P[R(CN)6] δ ·zH2O, where P and R are each independently selected from at least one of transition metal elements, 0 < x ≤ 2, 0 < δ ≤ 1, and 0 ≤ z ≤ 10; the polyanion compound includes Na b Me c (PO4) d O2X, where A includes one or more of H, Li, Na, K, and NH4, Me includes one or more of Ti, Cr, Mn, Fe, Co, Ni, V, Cu, and Zn, X includes one or more of F, Cl, and Br, 0 < b ≤ 4, 0 < c ≤ 2, 1 ≤ d ≤ 3; the layered transition metal oxide includes Na a M b Fe c O2, M includes transition metal ions, 0.67 < a < 1.1, 0.5 < b < 1, 0 < c < 0.5.

[0127] In some embodiments, the positive electrode material layer may further optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0128] In some embodiments, the positive electrode material layer may further optionally include a conductive agent. As an example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0129] In some embodiments, the positive electrode plate can be prepared by the following method: dispersing the above components for preparing the positive electrode plate, such as the positive electrode active material, the conductive agent, the binder, and any other components, in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; extruding and coating or transfer coating the above positive electrode slurry on a positive electrode current collector aluminum foil with a thickness of 13 μm; in an oven, within the range of 110°C to 130°C, the coating speed is 30 m / min, and then through cold pressing and slitting, the positive electrode plate is obtained.

[0130] [Electrolytes]

[0131] The electrolyte plays the role of conducting ions between the positive electrode and the negative electrode. The present application has no specific restrictions on the type of electrolyte, which can be selected according to needs. For example, the electrolyte can be liquid, gel or all-solid.

[0132] In some embodiments, the electrolyte is an electrolyte solution, which includes an electrolyte salt and a solvent.

[0133] In some embodiments, the electrolyte includes an electrolyte salt selected from at least one of NaPF6, NaBF4, NaN(SO2F)2(NaFSI), NaClO4, NaAsF6, NaB(C2O4)2(NaBOB), NaBF2(C2O4)(NaDFOB), NaN(SO2RF)2, and NaN(SO2F)(SO2RF), wherein RF is represented by C b F 2b+1 , b is an integer between 1 and 10, and can be optionally an integer between 1 and 3.

[0134] In some embodiments, the electrolyte salt is selected from one or more of NaPF6, NaN(SO2F)2, NaN(CF3SO2)2, NaB(C2O4)2, and NaBF2(C2O4). In some embodiments, the electrolyte salt is selected from one or more of NaPF6, NaN(SO2RF)2, and NaBF2(C2O4). In some embodiments, RF is -CF3, -C2F5, or -CF2CF2CF3.

[0135] In some embodiments, the electrolyte includes a solvent, and the solvent includes at least one of a chain carbonate, a chain carboxylic acid ester, a cyclic carbonic acid, an ether solvent, a sulfone solvent, and a nitrile solvent. In some embodiments, the chain carbonate includes at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC), methyl isopropyl carbonate (MIPC), methyl butyl carbonate, ethyl propyl carbonate, dipropyl carbonate, and dibutyl carbonate. In some embodiments, the chain carbonate includes at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and methyl propyl carbonate (MPC). In some embodiments, the chain carboxylic acid ester includes at least one of methyl formate (MF), ethyl formate (EF), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), ethyl butyrate (EB), methyl acetate (MA), ethyl acetate (EA), and propyl acetate (PA). In some embodiments, the linear carboxylic acid ester includes at least one of methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl acetate (MA), ethyl acetate (EA), and propyl acetate (PA). In some embodiments, the ether solvent includes at least one of dioxolane (DOL), tetrahydrofuran (THF), 2-methyltetrahydrofuran (2Me-THF), tetrahydropyran (THP), 1,2-dimethoxyethane (DME), diethylene glycol dimethyl ether (DG), 1,2-diethoxyethane, and 1,2-dibutoxyethane.

[0136] In some embodiments, the electrolyte may further include additives, such as negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, additives that improve battery high or low temperature performance, etc.

[0137] [Isolation film]

[0138] In some embodiments, the sodium secondary battery further includes a separator, which can be any known porous structure separator with good chemical stability and mechanical stability.

[0139] In some embodiments, the material of the isolation membrane can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride. The isolation membrane can be a single-layer film or a multi-layer composite film. When the isolation membrane is a multi-layer composite film, the materials of each layer can be the same or different.

[0140] [Sodium secondary battery]

[0141] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator may be formed into an electrode assembly by a winding process or a lamination process.

[0142] In some embodiments, the sodium secondary battery may include an outer package that can be used to encapsulate the electrode assembly and the electrolyte.

[0143] In some embodiments, the outer packaging of the sodium secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the sodium secondary battery can also be a soft package, such as a bag-type soft package. The material of the soft package can be plastic, and as the plastic, polypropylene, polybutylene terephthalate, and polybutylene succinate can be listed.

[0144] The shape of the sodium secondary battery in this application can be cylindrical, square or any other shape. For example, Figure 6 A sodium secondary battery 5 having a square structure is used as an example.

[0145] In some embodiments, reference Figure 7 , the outer packaging may include a shell 51 and a cover plate 53. Among them, the shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate are enclosed to form a accommodating cavity. The shell 51 has an opening connected to the accommodating cavity, and the cover plate 53 can be covered on the opening to close the accommodating cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can form an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the accommodating cavity. The electrolyte is infiltrated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the sodium secondary battery 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[0146] In some embodiments, the sodium secondary batteries can be assembled into a battery module. The number of sodium secondary batteries contained in the battery module can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0147] Figure 8 4 is an example of a battery module. Figure 8 In the battery module 4, the plurality of sodium secondary batteries 5 may be arranged in sequence along the length direction of the battery module 4. Of course, they may also be arranged in any other manner. Further, the plurality of sodium secondary batteries 5 may be fixed by fasteners.

[0148] Optionally, the battery module 4 may further include a housing having a housing space, and the plurality of sodium secondary batteries 5 are housed in the housing space.

[0149] In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art according to the application and capacity of the battery pack.

[0150] Fig. 9and Fig.10 1 is a battery pack 1 as an example. Fig. 9 and Fig.10 The battery pack 1 may include a battery box and a plurality of battery modules 4 disposed in the battery box. The battery box includes an upper box body 2 and a lower box body 3. The upper box body 2 can cover the lower box body 3 and form a closed space for accommodating the battery modules 4. The plurality of battery modules 4 can be arranged in the battery box in any manner.

[0151] In addition, the present application also provides an electrical device, which includes at least one of the sodium secondary battery, battery module, or battery pack provided in the present application. The sodium secondary battery, battery module, or battery pack can be used as a power source for the electrical device, and can also be used as an energy storage unit for the electrical device. The electrical device may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but are not limited thereto.

[0152] As the electrical device, a sodium secondary battery, a battery module or a battery pack may be selected according to its usage requirements.

[0153] Fig.11 The power consumption device is taken as an example. The power consumption device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. In order to meet the power consumption device's requirements for high power and high energy density of sodium secondary batteries, a battery pack or a battery module can be used.

[0154] Another example of a device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is usually required to be thin and light, and a sodium secondary battery may be used as a power source.

[0155] Example

[0156] Hereinafter, the embodiments of the present application will be described. The embodiments described below are exemplary and are only used to explain the present application, and should not be construed as limiting the present application. If no specific techniques or conditions are indicated in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. If the manufacturer is not indicated in the reagents or instruments used, they are all conventional products that can be obtained commercially.

[0157] 1. Preparation method

[0158] Example 1

[0159] 1) Negative electrode slurry

[0160] The mass ratio of the negative electrode active material hard carbon, conductive agent Super P, sodium carboxymethyl cellulose with a weight average molecular weight of 80,000 (Chongqing Lihong), sodium carboxymethyl cellulose with a weight average molecular weight of 400,000 (Nippon Paper), plasticizer 1,3-butanediol and binder styrene-butadiene rubber (SBR) in the preparation process is 95:0.5:0.4:1.0:0.1:3.

[0161] The negative electrode active material hard carbon, the conductive agent Super P and sodium carboxymethyl cellulose with a weight average molecular weight of 80,000 were mixed at a stirring speed of 600 rpm for 40 minutes to obtain a dry mixed mixture;

[0162] The dry mixed mixture was mixed with solvent water and kneaded for 60 minutes to obtain an adhesive mixture with a solid content of 65%;

[0163] The adhesive mixture and sodium carboxymethyl cellulose with a weight average molecular weight of 400,000 are mixed, and solvent water is added and stirred evenly at a stirring speed of 1600 rpm for 45 minutes to obtain a first adhesive solution;

[0164] The first glue solution was mixed with the plasticizer 1,3-butanediol, and stirred evenly, and then the binder styrene-butadiene rubber (SBR) was added, and the stirring was continued at a speed of 700 rpm and a stirring time of 25 min to obtain a negative electrode slurry with a solid content of 50%;

[0165] The temperature of the above preparation process was controlled at 25±3°C.

[0166] The bubble volume per unit mass of hard carbon in the negative electrode slurry is 0 mL / g, and the viscosity change of the negative electrode slurry after being placed at 25° C. for 48 hours is 3240 mPa·s.

[0167] 2) Negative electrode

[0168] The negative electrode slurry was extrusion coated on a negative electrode current collector copper foil with a thickness of 8 μm, and placed in an oven at a temperature of 120° C. and a coating speed of 25 m / min. The negative electrode sheet was obtained through cold pressing and slitting.

[0169] 3) Positive electrode

[0170] The positive electrode active material Na 7 / 9 Cu 2 / 9 Fe 1 / 9 Mn 2 / 3O2, conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) are fully stirred and mixed in an N-methylpyrrolidone solvent system in a weight ratio of 90:5:5 to obtain a positive electrode slurry; the positive electrode slurry is extrusion-coated on a positive electrode current collector aluminum foil with a thickness of 13 μm; in an oven, the temperature is 120°C and the coating speed is 30 m / min, and then the positive electrode sheets are obtained by cold pressing and slitting.

[0171] 4) Electrolyte

[0172] In an argon atmosphere glove box (H2O content <10ppm, O2 content <1ppm), propylene carbonate (PC) and ethyl methyl carbonate (EMC) were mixed in a mass ratio of 30 / 70, and 1M NaPF6 sodium salt was dissolved and stirred evenly to prepare an electrolyte.

[0173] 5) Isolation film

[0174] A 12 μm polyethylene (PE) porous polymer film was used as the isolation membrane.

[0175] 6) Preparation of batteries

[0176] The positive electrode sheet, the separator, and the negative electrode sheet are stacked in order, so that the separator is located between the positive and negative electrode sheets to isolate the positive and negative electrode sheets, and the bare battery cell is wound, and the pole ears are welded. The bare battery cell is placed in an outer package, and the prepared electrolyte is injected into the dried battery cell. After packaging, standing, formation, shaping, capacity testing and other processes, the sodium secondary battery product of Example 1 is obtained.

[0177] The sodium secondary batteries of Examples 2 to 17 are prepared in a similar manner to the sodium secondary battery of Example 1, but the preparation parameters in the negative electrode slurry preparation method are adjusted. The different preparation parameters are detailed in Table 1.

[0178] The preparation method of the sodium secondary battery of Example 18 is similar to that of the sodium secondary battery of Example 1, but the preparation parameters in the negative electrode slurry preparation method are adjusted, as follows:

[0179] The mass ratio of the negative electrode active material hard carbon, conductive agent Super P, sodium carboxymethyl cellulose with a weight average molecular weight of 80,000 (Chongqing Lihong), sodium carboxymethyl cellulose with a weight average molecular weight of 400,000 (Nippon Paper), plasticizer 1,3-butanediol and binder styrene-butadiene rubber (SBR) in the preparation process is 95:0.5:0.4:1.0:0.1:3.

[0180] The negative electrode active material hard carbon, the conductive agent Super P and the sodium carboxymethyl cellulose with a weight average molecular weight of 80,000 were mixed, and solvent water was added and kneaded for 60 minutes to obtain a sticky mixture with a solid content of 65%;

[0181] The adhesive mixture, sodium carboxymethyl cellulose with a weight average molecular weight of 400,000, plasticizer 1,3-butanediol and binder styrene-butadiene rubber (SBR) were mixed, and solvent water was added and stirred evenly at a stirring speed of 1700 rpm for 50 minutes to obtain a negative electrode slurry with a solid content of 50%;

[0182] The temperature of the above preparation process was controlled at 25±3°C.

[0183] The bubble volume per unit mass of hard carbon in the negative electrode slurry is 0.7 mL / g, and the viscosity change of the negative electrode slurry after being placed at 25° C. for 48 hours is 9030 mPa·s.

[0184] The sodium secondary battery of Example 19 is similar to the sodium secondary battery preparation method of Example 1, but the preparation steps in the negative electrode slurry preparation method are adjusted and prepared in a one-step method, as follows:

[0185] The mass ratio of the negative electrode active material hard carbon in the preparation process: conductive agent Super P, sodium carboxymethyl cellulose with a weight average molecular weight of 80,000 (Chongqing Lihong), sodium carboxymethyl cellulose with a weight average molecular weight of 400,000 (Nippon Paper), plasticizer 1,3-butanediol and binder styrene-butadiene rubber (SBR) is 95:0.5:0.4:1.0:0.1:3.

[0186] The negative electrode active material hard carbon, the conductive agent Super P, the sodium carboxymethyl cellulose with a weight average molecular weight of 80,000, the sodium carboxymethyl cellulose with a weight average molecular weight of 400,000, the binder styrene-butadiene rubber (SBR), the plasticizer 1,3-butanediol and the solvent water were mixed at a stirring speed of 1200 rpm for 2 hours to obtain a negative electrode slurry with a solid content of 50%;

[0187] The temperature of the above preparation process was controlled at 25±3°C.

[0188] The bubble volume per unit mass of hard carbon in the negative electrode slurry is 2.2 mL / g, and the viscosity change of the negative electrode slurry after being placed at 25° C. for 48 hours is 12000 mPa·s.

[0189] The sodium secondary batteries of Comparative Examples 1 to 3 are prepared in a similar manner to the sodium secondary battery of Example 1, but the weight average molecular weight of the modifier is adjusted, as shown in Table 1.

[0190] 2. Performance Test

[0191] 1. Negative electrode slurry

[0192] 1) Gel state test of negative electrode slurry after standing for 48 hours

[0193] After the negative electrode slurry was allowed to stand for 48 hours at 25° C., a steel ruler was used to pick up the negative electrode slurry in the beaker, and the gel state of the negative electrode slurry was determined based on the flow state of the negative electrode slurry.

[0194] The gel-free state is that the negative electrode slurry flows naturally and continuously, and the negative electrode slurry flows on the surface of the steel ruler without agglomeration. Figure 5 a) as shown;

[0195] The slight gel state is that the negative electrode slurry flows naturally and continuously, but the fluid is thin, and the negative electrode slurry is basically spread flat on the surface of the steel ruler, with slight small pieces, such as Figure 5 b) as shown;

[0196] The moderate gel state is that the negative electrode slurry drips naturally and intermittently; it does not flow continuously, and the negative electrode slurry cannot be spread evenly on the surface of the steel ruler, and there are obvious block agglomerations, such as Figure 5 c) as shown;

[0197] Severe gel state means that the negative electrode slurry cannot flow down in a stream, but falls off in lumps or remains directly on the steel ruler and cannot flow down. Figure 5 d).

[0198] 2) Filtration performance test

[0199] Take a 500ml beaker and place it at the lower end of the 200-mesh filter stand. Take 500ml of the negative electrode slurry and place it in the filter to filter. Record the time when the volume of the negative electrode slurry in the beaker reaches 300ml. Record the time.

[0200] 3) Test of bubble volume per unit mass of hard carbon in negative electrode slurry

[0201] The negative electrode slurry (the mass of hard carbon in the negative electrode slurry is m, in g) is sealed in a sealed bag; the gas change is tested by the water displacement method. The initial volume is V0, in mL, and the test volume after 48 hours is V1, in mL. Then the bubble volume per unit mass of hard carbon in the negative electrode slurry = (V1-V0) / m, in mL / g.

[0202] 4) Rheological test

[0203] The test instrument is Anton Paar rheometer. During the test, the negative electrode slurry is placed between the upper and lower plates, and the temperature of the negative electrode slurry is controlled by a heating device; the motor is controlled to rotate at a certain speed (i.e., shear rate), and the torque required to maintain this speed is measured (i.e., the resistance of the negative electrode slurry, which can be converted into shear stress); then the viscosity of the sample can be obtained by calculation. The instrument can be controlled to test at a continuously changing speed, so that the viscosity of the negative electrode slurry at different shear rates can be obtained. As the shear rate increases, the viscosity of the negative electrode slurry decreases, and the rheology is judged to be shear thinning, that is, there is no shear thickening phenomenon, and it is judged to be "none". As the shear rate increases, the viscosity of the negative electrode slurry increases, that is, there is a shear thickening phenomenon, and the rheology is judged to be "yes".

[0204] 2. Negative electrode

[0205] 1) Weight loss test of the negative electrode sheet without cold pressing

[0206] After coating, the negative electrode sheet that has not been cold pressed is punched into small discs of fixed size, and its weight is m1; it is baked at 140°C for 6 minutes, and its weight is then measured m2. The weight loss rate of the negative electrode sheet that has not been cold pressed = (m1-m2) / m1.

[0207] 3. Battery

[0208] 1) Cycle performance

[0209] At 25°C, the prepared battery was charged to 3.95V at a constant current of 1C, then charged at a constant voltage of 3.95V until the current dropped to 0.05C. After standing for 10 minutes, it was discharged to 1.5V at a constant current of 1C. This was one charge / discharge cycle of the battery. The capacity of the first discharge was 100%, and the charge and discharge cycles were repeated. When the discharge capacity decayed to 80%, the test was stopped and the number of cycles was recorded. The number of cycles when the capacity retention rate reached 80% was used as an indicator for evaluating the battery cycle performance.

[0210] 2) Storage performance

[0211] At 25°C, the prepared battery was charged to 3.95V at a constant current of 1C, then charged at a constant voltage of 3.95V until the current dropped to 0.05C, and after standing for 10 minutes, discharged to 1.5V at a constant current of 1C. This is a charge / discharge cycle of the battery, with the capacity of the first discharge being 100%. The battery was then placed in a 60°C oven, and after being stored for a period of time, the battery was taken out and the above charging steps were repeated again, and the discharge capacity was recorded until the discharge capacity decayed to 80%, the test was stopped, and the storage days were recorded. The storage days when the capacity retention rate reached 80% were used as an indicator to evaluate the battery storage performance.

[0212] III. Analysis of test results of various embodiments and comparative examples

[0213] The batteries of the embodiments and comparative examples were prepared according to the above method, and various performance parameters were measured. The results are shown in Tables 1 and 2 below.

[0214] Table 1

[0215]

[0216] Table 2

[0217]

[0218] The negative electrode slurries in Examples 1 to 19 all include a negative electrode active material and a modifier coated on the surface of the negative electrode active material, the negative electrode active material includes hard carbon, and the modifier includes sodium carboxymethyl cellulose with a weight average molecular weight of 20,000 to 150,000.

[0219] From the comparison of Examples 1, 4 to 7, 12 to 15 with Comparative Example 3, it can be seen that the introduction of the sodium carboxymethyl cellulose modifier in the present application is beneficial to slowing down the gelation of the negative electrode slurry after standing for 48 hours at 25°C, reducing the volume of bubbles in the negative electrode slurry and the weight loss rate of the uncold-pressed negative electrode sheet, improving the filtration performance of the negative electrode slurry, and improving the cycle performance and storage performance of the sodium secondary battery.

[0220] From the comparison of Examples 1, 4 to 7, 12 to 15 with Comparative Examples 1 to 2, it can be seen that controlling the weight average molecular weight of sodium carboxymethyl cellulose in the modifier to be 20,000 to 150,000 is beneficial to slowing down the gelation of the negative electrode slurry after standing for 48 hours at 25°C, reducing the volume of bubbles in the negative electrode slurry and the weight loss rate of the uncold-pressed negative electrode sheet, and improving the cycle performance and storage performance of the sodium secondary battery.

[0221] As can be seen from Examples 1 to 3, 8 to 11, the mass ratio of the modifier to the negative electrode active material is controlled to be 0.001 to 0.011, so as to slow down the gelation of the negative electrode slurry after standing for 48 hours at 25°C, reduce the volume of bubbles in the negative electrode slurry and the weight loss rate of the uncold pressed negative electrode sheet, so that the sodium secondary battery has excellent cycle performance and storage performance. As can be seen from the comparison between Examples 1 to 3, 9, 11 and Examples 8 and 10, further controlling the mass ratio of the modifier to the negative electrode active material to be 0.002 to 0.0065 is conducive to further improving the cycle performance and storage performance of the sodium secondary battery.

[0222] As can be seen from Examples 1 to 3, based on the total mass of the solids in the negative electrode slurry, the mass content of the negative electrode active material is controlled to be 92% to 97% to slow down the gelation of the negative electrode slurry after standing for 48 hours at 25°C, reduce the volume of bubbles in the negative electrode slurry and the weight loss rate of the uncold pressed negative electrode sheet, so that the sodium secondary battery has excellent cycle performance and storage performance. As can be seen from the comparison between Examples 1 to 2 and Example 3, based on the total mass of the solids in the negative electrode slurry, further controlling the mass content of the negative electrode active material to be 92% to 95% is conducive to further slowing down the gelation of the negative electrode slurry after standing for 48 hours at 25°C, reducing the weight loss rate of the uncold pressed negative electrode sheet, and improving the cycle performance and storage performance of the sodium secondary battery.

[0223] From the comparison between Examples 1, 5 to 6 and Examples 4 and 7, it can be seen that controlling the weight average molecular weight of sodium carboxymethyl cellulose in the modifier to 40,000 to 100,000 is beneficial to further reduce the volume of bubbles in the negative electrode slurry and improve the cycle performance and storage performance of the sodium secondary battery.

[0224] As can be seen from Examples 1, 8 to 11, based on the total mass of the solids in the negative electrode slurry, the mass content of the modifier is controlled to be 0.1% to 1%, so as to slow down the gelation of the negative electrode slurry after standing for 48 hours at 25°C, reduce the volume of bubbles in the negative electrode slurry and the weight loss rate of the uncold pressed negative electrode sheet, so that the sodium secondary battery has excellent cycle performance and storage performance. As can be seen from the comparison between Examples 1, 9, and 11 and Examples 8 and 10, based on the total mass of the solids in the negative electrode slurry, further controlling the mass content of the modifier to be 0.2% to 0.6% is conducive to further improving the cycle performance and storage performance of the sodium secondary battery.

[0225] As can be seen from Examples 1, 12 to 15, the kneading time in the adhesive mixture is controlled to be 30 min to 120 min to slow down the gelation of the negative electrode slurry after standing for 48 hours at 25°C, reduce the volume of bubbles in the negative electrode slurry and the weight loss rate of the uncold pressed negative electrode sheet, so that the sodium secondary battery has excellent cycle performance and storage performance. As can be seen from the comparison between Examples 1, 13 to 14 and Examples 12 and 15, the kneading time in the adhesive mixture is controlled to be 40 min to 80 min, which is conducive to further improving the cycle performance and storage performance of the sodium secondary battery.

[0226] As can be seen from Examples 1, 16 to 17, the solid content of the adhesive mixture is controlled to be 60% to 68% to slow down the gelation of the negative electrode slurry after standing at 25°C for 48 hours, reduce the volume of bubbles in the negative electrode slurry and the weight loss rate of the uncold pressed negative electrode sheet, so that the sodium secondary battery has excellent cycle performance and storage performance. As can be seen from the comparison between Examples 1, 16 and Example 17, further controlling the solid content of the adhesive mixture to be 60% to 65% is conducive to further slowing down the gelation of the negative electrode slurry after standing at 25°C for 48 hours, reducing the weight loss rate of the uncold pressed negative electrode sheet, and improving the cycle performance and storage performance of the sodium secondary battery.

[0227] It can be seen from Examples 1, 18, and 19 that the one-step preparation method, the two-step preparation method, or the four-step preparation method for the negative electrode slurry can slow down the gelation of the negative electrode slurry after standing for 48 hours at 25°C, reduce the volume of bubbles in the negative electrode slurry and the weight loss rate of the uncold pressed negative electrode sheet, so that the sodium secondary battery has excellent cycle performance and storage performance. From the comparison of Examples 1 and 18 with Example 19, it can be seen that the negative electrode slurry, compared with the one-step method for preparing the negative electrode slurry, the two-step preparation method or the four-step preparation method is more conducive to reducing the volume of bubbles in the negative electrode slurry and the weight loss rate of the uncold pressed negative electrode sheet, so that the sodium secondary battery has excellent cycle performance and storage performance.

[0228] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and the embodiments having the same structure as the technical idea and exerting the same effect within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the main purpose of the present application, various modifications that can be thought of by those skilled in the art to the embodiments and other methods of combining some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A negative electrode slurry for a sodium secondary battery, characterized in that: The invention comprises a negative electrode active material and a modifier coated on the surface of the negative electrode active material, wherein the negative electrode active material comprises one or more of hard carbon and hard carbon-coated graphite, and the modifier comprises a carboxymethyl cellulose salt with a weight average molecular weight of 20,000 to 150,000.

2. The negative electrode slurry according to claim 1, characterized in that: The modifier includes a carboxymethyl cellulose salt having a weight average molecular weight of 40,000 to 100,000.

3. The negative electrode slurry according to claim 1, characterized in that: The carboxymethyl cellulose salt includes one or more of lithium carboxymethyl cellulose, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, rubidium carboxymethyl cellulose, and cesium carboxymethyl cellulose.

4. The negative electrode slurry according to any one of claims 1 to 3, characterized in that: The mass ratio of the modifier to the negative electrode active material is 0.001 to 0.

011.

5. The negative electrode slurry according to any one of claims 1 to 3, characterized in that: The mass ratio of the modifier to the negative electrode active material is 0.002 to 0.0065.

6. The negative electrode slurry according to any one of claims 1 to 3, characterized in that: Based on the total mass of solids in the negative electrode slurry, the mass content of the modifier is 0.1% to 1%.

7. The negative electrode slurry according to any one of claims 1 to 3, characterized in that: Based on the total mass of the solid in the negative electrode slurry, the mass content of the modifier is 0.2% to 0.6%.

8. The negative electrode slurry according to any one of claims 1 to 3, characterized in that: Based on the total mass of solids in the negative electrode slurry, the mass content of the negative electrode active material is 92% to 97%.

9. The negative electrode slurry according to any one of claims 1 to 3, characterized in that: Based on the total mass of solids in the negative electrode slurry, the mass content of the negative electrode active material is 92% to 95%.

10. The negative electrode slurry according to any one of claims 1 to 3, characterized in that: The solid content of the negative electrode slurry is 45% to 55%.

11. The negative electrode slurry according to any one of claims 1 to 3, characterized in that: The solid content of the negative electrode slurry is 48% to 53%.

12. The negative electrode slurry according to any one of claims 1 to 3, characterized in that: The negative electrode slurry further includes at least one of a conductive agent, a dispersant, a binder and a plasticizer.

13. The negative electrode slurry according to any one of claims 1 to 3, characterized in that: The bubble volume per unit mass of hard carbon in the negative electrode slurry does not exceed 2.5 mL / g.

14. The negative electrode slurry according to any one of claims 1 to 3, characterized in that: The viscosity of the negative electrode slurry after being placed at 25° C. for 48 hours changes to 3000 mPa·s to 12000 mPa·s.

15. The negative electrode slurry according to claim 12, characterized in that: The conductive agent includes one or more of Super P, Ketjen black, acetylene black, carbon nanotubes, and graphene.

16. The negative electrode slurry according to claim 12, characterized in that: The conductive agent includes Super P.

17. The negative electrode slurry according to claim 12, characterized in that: The dispersant includes sodium carboxymethyl cellulose with a weight average molecular weight of 400,000 to 1,000,000.

18. The negative electrode slurry according to claim 12, characterized in that: The binder includes one or more of styrene-butadiene rubber, polyacrylic acid, polyacrylate, polyamide, poly(acrylonitrile-acrylate), and poly(styrene-acrylate).

19. The negative electrode slurry according to claim 12, characterized in that: The binder includes styrene-butadiene rubber.

20. The negative electrode slurry according to claim 12, characterized in that: The plasticizer includes 1,3-butanediol.

21. A method for preparing a negative electrode slurry, characterized in that: The preparation method comprises: coating the modifier on the surface of the negative electrode active material to obtain an adhesive mixture; Dispersing the adhesive mixture in a solvent to prepare the negative electrode slurry; The negative electrode active material includes one or more of hard carbon and hard carbon-coated graphite, and the modifier includes a carboxymethyl cellulose salt with a weight average molecular weight of 20,000 to 150,000.

22. The preparation method according to claim 21, characterized in that: The carboxymethyl cellulose salt includes one or more of lithium carboxymethyl cellulose, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, rubidium carboxymethyl cellulose, and cesium carboxymethyl cellulose.

23. The preparation method according to claim 21, characterized in that: The method of coating the modifier on the surface of the negative electrode active material specifically includes: The raw material containing the negative electrode active material is kneaded with the modifier in a solvent to obtain the adhesive mixture.

24. The preparation method according to claim 23, characterized in that: The kneading time is 30 min to 120 min.

25. The preparation method according to claim 23, characterized in that: The kneading time is 40 min to 80 min.

26. The preparation method according to any one of claims 21 to 23, characterized in that: The solid content of the adhesive mixture is 60% to 68%.

27. The preparation method according to any one of claims 21 to 23, characterized in that: The solid content of the adhesive mixture is 60% to 65%.

28. The preparation method according to any one of claims 21 to 23, characterized in that: The preparation method specifically comprises: Stirring and mixing the conductive agent, the negative electrode active material and the modifier to obtain a dry mixed mixture; kneading the dry blended mixture in a solvent to obtain the adhesive mixture; The adhesive mixture, the dispersant and the solvent are stirred and mixed to obtain a first adhesive solution; The first glue solution and the plasticizer are stirred and mixed, and a binder is added and stirred evenly to obtain the negative electrode slurry.

29. A negative electrode plate, characterized in that: The negative electrode plate includes a negative electrode active material and a modifier coated on the surface of the negative electrode active material, the negative electrode active material includes one or more of hard carbon and hard carbon-coated graphite, and the modifier includes a carboxymethyl cellulose salt with a weight average molecular weight of 20,000 to 150,000.

30. The negative electrode plate according to claim 29, characterized in that: The carboxymethyl cellulose salt includes one or more of lithium carboxymethyl cellulose, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, rubidium carboxymethyl cellulose, and cesium carboxymethyl cellulose.

31. A sodium secondary battery, characterized in that: A negative electrode sheet comprising the negative electrode sheet of claim 29 or 30, or a negative electrode sheet prepared by the negative electrode slurry of any one of claims 1 to 20 or the negative electrode slurry prepared by the preparation method of any one of claims 21 to 28.

32. An electrical device, characterized in that: Includes the sodium secondary battery as claimed in claim 31.

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