Negative electrode slurry and preparation method therefor, negative electrode sheet, sodium secondary battery, and electric device

Abstract

A negative electrode slurry and a preparation method therefor, a negative electrode sheet, a sodium secondary battery, and an electric device. The negative electrode slurry comprises a negative electrode active material and a modifier that coats the surface of the negative electrode active material. The negative electrode active material comprises one or more of hard carbon and graphite coated with hard carbon. The modifier comprises carboxymethyl cellulose salt having a weight-average molecular weight of 20,000-150,000. The negative electrode slurry exhibits a low occurrence degree of gelation and foaming, and can improve the processing performance and the storage performance of the sodium secondary battery.

Description

Negative electrode slurry and preparation method thereof, negative electrode sheet, sodium secondary battery and electrical device

[0001] Cross-references

[0002] This application claims priority to Chinese Patent Application No. 202311524591.8 filed on November 15, 2023, entitled “Negative electrode slurry and preparation method thereof, negative electrode sheet, sodium secondary battery and electrical device”, which is incorporated herein by reference in its entirety. Technical Field

[0003] 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 plate, a sodium secondary battery and an electrical device. Background Art

[0004] In recent years, secondary batteries have been widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power stations, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace and other fields.

[0005] Compared to lithium-ion batteries, sodium-ion batteries offer a significant competitive advantage due to their abundant and widespread sodium resources. However, limitations in their manufacturing process, such as gelation and foaming during the negative electrode slurry preparation process, have prevented mass production of sodium-ion batteries to meet market demand.

[0006] Summary of the Invention

[0007] 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 sodium secondary batteries.

[0008] 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, and the modifier comprising a carboxymethyl cellulose salt having a weight average molecular weight of 20,000 to 150,000.

[0009] 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 extensibility 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 2The hybridized C-C bonds form strong intermolecular interactions, allowing the modifier to coat the hard carbon surface. The coating structure formed by the modifier and the hard carbon facilitates the adjustment of the hard carbon's pore distribution, reducing the probability of water molecules in the negative electrode slurry entering the hard carbon pores, thereby reducing the degree of negative electrode slurry foaming. 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 bind to the oxygen-containing functional groups on the hard carbon surface, reducing the degree of gelation in the negative electrode slurry caused by the cross-linking structure formed by the side chains of carboxymethyl cellulose sodium with a weight-average molecular weight greater than 300,000. The side chains of the carboxymethyl cellulose salt coated on the hard carbon surface also form a space charge layer, which helps to increase the repulsion between hard carbon particles, thereby reducing the degree of gelation in the negative electrode slurry caused by hard carbon particle agglomeration. This improves the processability of the negative electrode slurry and increases the yield, which is beneficial for the mass production of sodium secondary batteries and also helps to enhance the storage performance and cycle performance of sodium secondary batteries.

[0010] In any embodiment, the modifier includes a carboxymethyl cellulose salt having a weight average molecular weight of 40,000 to 100,000.

[0011] 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 improving the processability of the negative electrode slurry, as well as improving the cycle performance and storage performance of the sodium secondary battery.

[0012] In any embodiment, the carboxymethylcellulose salt includes one or more of lithium carboxymethylcellulose, sodium carboxymethylcellulose, potassium carboxymethylcellulose, rubidium carboxymethylcellulose, and cesium carboxymethylcellulose.

[0013] In any embodiment, the mass ratio of the modifier to the negative electrode active material is 0.001 to 0.011, and can be optionally 0.002 to 0.0065.

[0014] 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 the excessively thick hard carbon surface coating layer on battery performance due to an excessively high proportion of the modifier.

[0015] 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 optionally 0.2% to 0.6%.

[0016] 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%, optionally 92% to 95%.

[0017] 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.

[0018] Controlling the mass content of the negative electrode active material within an appropriate range provides sufficient sodium insertion sites to enable the sodium secondary battery to have a high capacity.

[0019] In any embodiment, the solid content of the negative electrode slurry is 45% to 55%, and optionally 48% to 53%.

[0020] The negative electrode slurry is controlled within a suitable range so that the negative electrode slurry has a certain fluidity for subsequent processing.

[0021] In any embodiment, the negative electrode slurry further includes at least one of a conductive agent, a dispersant, a binder, and a plasticizer.

[0022] 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

[0023] The dispersant includes sodium carboxymethyl cellulose having a weight average molecular weight of 400,000 to 1,000,000; and / or

[0024] The binder comprises one or more of styrene-butadiene rubber, polyacrylic acid, polyacrylate, polyamide, poly(acrylonitrile-acrylate), poly(styrene-acrylate), and optionally styrene-butadiene rubber; and / or

[0025] Plasticizers include 1,3-butanediol.

[0026] 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.

[0027] In any embodiment, the bubble volume per unit mass of hard carbon in the negative electrode slurry does not exceed 2.5 mL / g.

[0028] The negative electrode slurry has a low 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.

[0029] In any embodiment, the viscosity of the negative electrode slurry after being left at 25° C. for 48 hours changes from 3000 mPa·s to 12000 mPa·s.

[0030] 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 and the improvement of the storage performance and cycle performance of the sodium secondary battery.

[0031] 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 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 carboxymethyl cellulose lithium, carboxymethyl cellulose sodium, carboxymethyl cellulose potassium, carboxymethyl cellulose rubidium, and carboxymethyl cellulose cesium.

[0032] Coating the surface of the negative electrode active material with a modifier can effectively reduce the degree of gelation in the negative electrode slurry. Specifically, because the modifier containing a carboxymethyl cellulose salt with a weight-average molecular weight of 20,000 to 150,000 has higher extensibility in the negative electrode slurry, it facilitates the coating between the modifier and the hard carbon. The coating structure formed by the modifier and the hard carbon facilitates the adjustment of 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 in 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 and thereby reduce the degree of gelation of the negative electrode slurry caused by the agglomeration of hard carbon particles, improve the processability of the negative electrode slurry, increase the yield, and be 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.

[0033] In any embodiment, coating the surface of the negative electrode active material with the modifier specifically includes kneading a raw material containing the negative electrode active material and the modifier in a solvent to form a viscous mixture. Kneading the negative electrode active material and the modifier in the solvent first allows the modifier to be effectively coated on the surface of the negative electrode active material, thereby increasing the coating rate.

[0034] In any embodiment, the kneading time is 30 min to 120 min, and optionally 40 min to 80 min.

[0035] 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.

[0036] In any embodiment, the solid content of the adhesive mixture is 60% to 68%, and optionally 60% to 65%.

[0037] In any embodiment, the preparation method specifically comprises:

[0038] stirring and mixing the conductive agent, the negative electrode active material and the modifier to obtain a dry mix;

[0039] kneading the dry blend in a solvent to obtain a viscous mixture;

[0040] Stirring and mixing the adhesive mixture, the dispersant, and the solvent to obtain a first adhesive solution;

[0041] The first glue solution and the plasticizer are stirred and mixed, and the binder is added and stirred evenly to obtain the negative electrode slurry.

[0042] The above preparation method can significantly improve the foaming and gelling phenomena of the negative electrode slurry, enhance the processability of the negative electrode slurry, and improve the storage performance and cycle performance of the sodium secondary battery.

[0043] 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, 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 carboxymethyl cellulose lithium, carboxymethyl cellulose sodium, carboxymethyl cellulose potassium, carboxymethyl cellulose rubidium, and carboxymethyl cellulose cesium.

[0044] 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.

[0045] The fourth aspect of the present application provides a sodium secondary battery, comprising the negative electrode sheet of the third aspect of the present application, or a negative electrode sheet 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.

[0046] 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

[0047] FIG1 is a schematic diagram of the gelation mechanism of the negative electrode slurry;

[0048] FIG2 is an optical microscope photograph of the negative electrode after coating with slurry;

[0049] FIG3 is a schematic diagram of a coating structure formed by a modifier and hard carbon in one embodiment of the present application;

[0050] FIG4 is a schematic diagram of the bonding between the modifier and the hard carbon in one embodiment of the present application;

[0051] FIG5 is a schematic diagram showing the classification of gel state test results of the negative electrode slurry after storage at 25° C. for 48 hours in one embodiment of the present application;

[0052] FIG6 is a schematic diagram of a sodium secondary battery according to an embodiment of the present application;

[0053] FIG7 is an exploded view of the sodium secondary battery according to one embodiment of the present application shown in FIG6 ;

[0054] FIG8 is a schematic diagram of a battery module according to an embodiment of the present application;

[0055] FIG9 is a schematic diagram of a battery pack according to an embodiment of the present application;

[0056] FIG10 is an exploded view of the battery pack according to one embodiment of the present application shown in FIG9 ;

[0057] FIG. 11 is a schematic diagram of an electric device using a sodium secondary battery as a power source according to an embodiment of the present application.

[0058] Explanation of reference numerals: 1 battery pack; 2 upper case; 3 lower case; 4 battery module; 5 sodium secondary battery; 51 housing; 52 electrode assembly; 53 top cover assembly. DETAILED DESCRIPTION

[0059] Below, the embodiments of the negative electrode slurry and its preparation method, negative electrode sheet, sodium secondary battery and electric device of the present application are described in detail with appropriate reference to the drawings. However, there may be 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 structures 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 description 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.

[0060] " range " disclosed in the present application is limited in the form of lower limit and upper limit, and 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 special range. The scope limited in this way can be to include end value or not include end value, and can be arbitrarily combined, that is, any lower limit can form a range with any upper limit combination. For example, if the scope of 60-120 and 80-110 is listed for specific parameters, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range value 1 and 2 are listed, and if the maximum range value 3,4 and 5 are listed, then the following range can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this 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 all real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this 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.

[0061] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0062] 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.

[0063] Unless otherwise specified, all steps of the present application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating 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), indicating 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.

[0064] Unless otherwise specified, the terms "include" and "comprising" used in this application may be open-ended or closed-ended. For example, "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0065] 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 heteroatoms such as hydrogen, oxygen, and nitrogen, the surface of the prepared hard carbon is rich in oxygen-containing functional groups (oxygen content > 10%), and the side chains of sodium carboxymethyl cellulose (weight average molecular weight greater than 300,000), a dispersant commonly used in the negative electrode slurry, are easily combined 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 (as shown in Figure 1). In addition, the hard carbon particles are small, the surface is irregular, the roughness is large, the specific surface area is large, and the surface energy is high, which causes the hard carbon particles to tend to agglomerate, further aggravating the gel of the 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, discharge the gas in the hard carbon pores, cause the negative electrode slurry to foam, and further deteriorate the gel. The capacity of hard carbon mainly includes two stages. The first stage is between 1.5V and 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.

[0066] [Anode slurry]

[0067] 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.

[0068] 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 values.

[0069] Generally, 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 (as shown in Figure 2, the black spots in the optical microscope photo of the negative electrode sheet represent the missing coating of the negative electrode slurry) and the high weight loss rate of the negative electrode sheet. The missing coating of the negative electrode slurry or the high weight loss rate of the negative electrode sheet will affect the uniformity of the distribution of the negative electrode active material, resulting in fluctuations in the CB value of the negative electrode sheet and the risk of serious sodium precipitation, and will also affect the cycle performance and storage performance of the sodium secondary battery.

[0070] It can be understood that 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 has a lower weight average molecular weight, 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, as shown in Figures 3 and 4 (the gas inside the pores in the hard carbon in Figure 3 is nitrogen because the hard carbon is prepared under a nitrogen atmosphere and the nitrogen generated during the preparation process, and the nitrogen 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 between the main chain and the hard carbon surface. 2 The hybridized C-C bonds form strong intermolecular interactions, allowing the modifier to coat the hard carbon surface. The coating structure formed by the modifier and the hard carbon facilitates the adjustment of the hard carbon's pore distribution, reducing the probability of water molecules in the negative electrode slurry entering the hard carbon pores, thereby reducing the degree of negative electrode slurry foaming. 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 bind to the oxygen-containing functional groups on the hard carbon surface, reducing the degree of gelation in the negative electrode slurry caused by the cross-linking structure formed by the side chains of carboxymethyl cellulose sodium with a weight-average molecular weight greater than 300,000. The side chains of the carboxymethyl cellulose salt coated on the hard carbon surface also form a space charge layer, which helps to increase the repulsion between hard carbon particles, thereby reducing the degree of gelation in the negative electrode slurry caused by hard carbon particle agglomeration. This improves the processability of the negative electrode slurry and increases the yield, which is beneficial for the mass production of sodium secondary batteries and also helps to enhance the storage performance and cycle performance of sodium secondary batteries.

[0071] 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.

[0072] In this article, "rheological properties" 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 conducive to the coating of the negative electrode slurry.

[0073] In some embodiments, the modifier includes a carboxymethyl cellulose salt having a weight average molecular weight of 40,000 to 100,000.

[0074] 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 improving the processability of the negative electrode slurry, as well as improving the cycle performance and storage performance of the sodium secondary battery.

[0075] 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.

[0076] In some embodiments, the carboxymethylcellulose salt is sodium carboxymethylcellulose.

[0077] In some embodiments, the carboxymethylcellulose salt is lithium carboxymethylcellulose.

[0078] In some embodiments, the mass ratio of the modifier to the negative electrode active material is 0.001 to 0.011. In some embodiments, the mass ratio of the modifier to the negative electrode active material is 0.002 to 0.0065.

[0079] 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 values.

[0080] 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 modifier, which aggravates the risk of sodium precipitation and the impact on battery performance.

[0081] In some embodiments, the mass content of the modifier is 0.1% to 1% based on the total mass of the solids in the negative electrode slurry. In some embodiments, the mass content of the modifier is 0.2% to 0.6%.

[0082] In some embodiments, based on the total mass of the solids in the negative electrode slurry, the mass content of the modifier can be selected as 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.

[0083] Controlling the modifier content within an appropriate range can balance the processing, storage, and cycling performance of sodium secondary batteries. If the modifier content is too low, it cannot effectively form a coating structure with the hard carbon, mitigating gelation and foaming in the negative electrode slurry. If the modifier content is too high, the hard carbon surface coating can become too thick, exacerbating the risk of sodium precipitation in sodium secondary batteries and negatively impacting battery performance.

[0084] In some embodiments, the negative electrode active material content is 92% to 97% by mass based on the total mass of solids in the negative electrode slurry. In some embodiments, the negative electrode active material content is 92% to 95% by mass.

[0085] In some embodiments, 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, based on the total mass of the solids in the negative electrode slurry.

[0086] Controlling the mass content of the negative electrode active material within an appropriate range provides sufficient sodium insertion sites to enable the sodium secondary battery to have a high capacity.

[0087] In some embodiments, the solid content of the negative electrode slurry is 45% to 55%. In some embodiments, the solid content of the negative electrode slurry is 48% to 53%.

[0088] In some embodiments, the solid content of the negative electrode slurry may 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.

[0089] The negative electrode slurry is controlled within a suitable range so that the negative electrode slurry has a certain fluidity for subsequent processing.

[0090] In some embodiments, the negative electrode slurry further includes at least one of a conductive agent, a dispersant, a binder, and a plasticizer.

[0091] In some embodiments, the conductive agent includes one or more of Super P, Ketjen black, acetylene black, carbon nanotubes, and graphene. In some embodiments, the conductive agent includes Super P.

[0092] In some embodiments, the dispersant includes sodium carboxymethyl cellulose having a weight average molecular weight of 400,000 to 1,000,000.

[0093] In some embodiments, the binder comprises one or more of styrene-butadiene rubber, polyacrylic acid, polyacrylate, polyamide, poly(acrylonitrile-acrylate), poly(styrene-acrylate). In some embodiments, the binder comprises styrene-butadiene rubber.

[0094] In some embodiments, the plasticizer includes 1,3,-butanediol.

[0095] 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.

[0096] In some embodiments, the bubble volume per unit mass of hard carbon in the negative electrode slurry does not exceed 2.5 mL / g.

[0097] In this article, the bubble volume per unit mass of hard carbon in the anode slurry can be measured using any known method. As an example, the anode slurry (where the mass of hard carbon in the anode slurry is m, expressed in g) is sealed in a sealed bag and the gas evolution is measured using the water displacement method. The initial volume is V0, expressed in mL, and the measured volume after 48 hours is V1, expressed in mL. The bubble volume per unit mass of hard carbon in the anode slurry is then calculated as (V1 - V0) / m, expressed in mL / g.

[0098] 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 consisting of any two of the above points.

[0099] The negative electrode slurry has a low 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.

[0100] 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.

[0101] In this document, the viscosity change of the negative electrode slurry after 48 hours at 25°C can be measured using any known method. For example, a rotary viscometer is used: First, the initial viscosity of the negative electrode slurry is measured using a rotary viscometer. A suitable viscometer rotor is selected and secured. The negative electrode slurry is placed below the viscometer rotor, just covering the scale line. Instrument model: Shanghai Fangrui NDJ-5S, rotors: 63# (2000-10000 mPa·s), 64# (10000-50000 mPa·s), speed: 12 rpm, test temperature: 25°C, test time: 5 minutes, and read the data after the display stabilizes. After 48 hours at 25°C, the viscosity is measured using the same test method. The viscosity change of the negative electrode slurry after 48 hours at 25°C is the difference between the viscosity of the negative electrode slurry after 48 hours and the initial viscosity of the negative electrode slurry.

[0102] 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 a range consisting of any two of the above points.

[0103] 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 and the improvement of the storage performance and cycle performance of the sodium secondary battery.

[0104] 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 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.

[0105] As used herein, a "gooey mixture" refers to a dough-like mixture.

[0106] Coating the surface of the negative electrode active material with a modifier can effectively reduce the degree of gelation in the negative electrode slurry. Specifically, because the modifier containing a carboxymethyl cellulose salt with a weight-average molecular weight of 20,000 to 150,000 has higher extensibility in the negative electrode slurry, it facilitates the coating between the modifier and the hard carbon. The coating structure formed by the modifier and the hard carbon facilitates the adjustment of 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 in 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 and thereby reduce the degree of gelation of the negative electrode slurry caused by the agglomeration of hard carbon particles, improve the processability of the negative electrode slurry, increase the yield, and be 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.

[0107] In some embodiments, coating the surface of the negative electrode active material with the modifier specifically includes: kneading a raw material containing the negative electrode active material and the modifier in a solvent to obtain the adhesive mixture.

[0108] In this context, "kneading" refers to the operation of adding a small amount of liquid (or binder) to solid powder so that the liquid uniformly wets the interior and surface of the powder particles to prepare a uniform plastic material.

[0109] 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, thereby improving the coating rate.

[0110] In some embodiments, the kneading time is 30 min to 120 min. In some embodiments, the kneading time is 40 min to 80 min.

[0111] 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 within a range consisting of any two of the above points.

[0112] 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.

[0113] In some embodiments, the adhesive mixture has a solid content of 60% to 68%. In some embodiments, the adhesive mixture has a solid content of 60% to 65%.

[0114] 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.

[0115] In some embodiments, the preparation method specifically includes: stirring and mixing a conductive agent, a negative electrode active material and a 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 sticky mixture; stirring and mixing the sticky mixture, a dispersant and a 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; stirring and mixing the first glue and a plasticizer, and then adding a binder and 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.

[0116] The above preparation method can significantly improve the foaming and gelling phenomena of the negative electrode slurry, enhance the processability of the negative electrode slurry, and improve the storage performance and cycle performance of the sodium secondary battery.

[0117] [Negative electrode]

[0118] 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.

[0119] 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 either or both of the two opposite surfaces of the negative electrode current collector.

[0120] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer base layer and a metal layer formed on at least one surface of the polymer base material. 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 base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0121] 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.

[0122] The negative electrode sheet can be prepared by the following method: extruding and coating or transfer coating the above-mentioned negative electrode paste on the negative electrode current collector, and placing it in an oven within the range of 110 °C to 130 °C, with a coating speed of 25 m / min, and obtaining the negative electrode sheet through cold pressing and slitting.

[0123] 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 graphite coated with hard carbon. The modifier includes a carboxymethyl cellulose salt with a weight average molecular weight of 20,000 to 150,000. 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.

[0124] [Positive electrode sheet]

[0125] 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.

[0126] As an example, the positive electrode current collector has two surfaces opposite to each other in its own thickness direction, and the positive electrode material layer is provided on either or both of the two opposite surfaces of the positive electrode current collector.

[0127] In some embodiments, the positive electrode current collector can be a metal foil or a composite current collector. For example, as the metal foil, aluminum foil can be used. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (such as 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.).

[0128] 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 a battery 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 oxide, polyanion compound, or Prussian blue compound. However, the present application is not limited to these materials, and other conventional materials that can be used as battery positive electrode active materials 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 bMe c (PO4) d O2X, wherein 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; layered transition metal oxide includes Na a M b Fe c O2, M including transition metal ions, 0.67 <a<1.1,0.5<b<1,0<c<0.5。

[0129] 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), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin.

[0130] In some embodiments, the positive electrode material layer may further include a conductive agent. For 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.

[0131] In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is extrusion coated or transfer coated on a positive electrode current collector aluminum foil with a thickness of 13 μm; in an oven at a temperature of 110°C to 130°C, the coating speed is 30 m / min, and then the positive electrode sheet is obtained by cold pressing and slitting.

[0132] [Electrolytes]

[0133] The electrolyte conducts ions between the positive and negative electrodes. This application does not specify the type of electrolyte, and the electrolyte can be selected based on the needs. For example, the electrolyte can be liquid, gel, or solid.

[0134] In some embodiments, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent.

[0135] 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.

[0136] 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.

[0137] In some embodiments, the electrolyte includes a solvent, and the solvent includes at least one of a chain carbonate, a chain carboxylate, 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 carboxylate 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.

[0138] In some embodiments, the electrolyte may further include additives. For example, the additives may include 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, and additives that improve battery high or low temperature performance.

[0139] [Isolation film]

[0140] In some embodiments, the sodium secondary battery further includes a separator, which can be any known porous separator with good chemical and mechanical stability.

[0141] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film. When the separator is a multi-layer composite film, the materials of each layer can be the same or different.

[0142] [Sodium secondary battery]

[0143] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

[0144] In some embodiments, the sodium secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0145] 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, or a steel shell. Alternatively, the outer packaging of the sodium secondary battery can be a soft shell, such as a pouch-type soft shell. The soft shell can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0146] The shape of the sodium secondary battery in this application can be cylindrical, square or any other shape. For example, FIG6 shows a sodium secondary battery 5 with a square structure as an example.

[0147] In some embodiments, referring to FIG7 , the outer packaging may include a shell 51 and a cover plate 53. 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 enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the separator can be formed into an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving 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.

[0148] In some embodiments, 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. The specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0149] Figure 8 illustrates an exemplary battery module 4. Referring to Figure 8 , within the battery module 4, multiple sodium secondary batteries 5 may be arranged sequentially along the length of the battery module 4. Of course, any other arrangement is also possible. Furthermore, the multiple sodium secondary batteries 5 may be secured together using fasteners.

[0150] Optionally, the battery module 4 may further include a housing having an accommodation space, and the plurality of sodium secondary batteries 5 are accommodated in the accommodation space.

[0151] 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 based on the application and capacity of the battery pack.

[0152] Figures 9 and 10 illustrate an example battery pack 1. Referring to Figures 9 and 10 , the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box comprises an upper case 2 and a lower case 3. The upper case 2 can be placed over the lower case 3 to form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0153] 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 is not limited thereto.

[0154] As the electrical device, a sodium secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0155] Figure 11 shows an example of an electric device. This device can be a pure electric vehicle, hybrid electric vehicle, or plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of sodium secondary batteries, a battery pack or battery module can be used.

[0156] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is generally required to be lightweight and thin, and may use a sodium secondary battery as a power source.

[0157] Example

[0158] Below, the embodiment of the present application is described. The embodiment described below is exemplary and is only used to explain the present application, and is not to be construed as limiting the present application. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used that do not specify the manufacturer are conventional products that can be obtained commercially.

[0159] 1. Preparation method

[0160] Example 1

[0161] 1) Negative electrode slurry

[0162] 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.

[0163] 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 mix;

[0164] The dry mix was mixed with solvent water and kneaded for 60 min to obtain an adhesive mixture with a solid content of 65%;

[0165] The adhesive mixture and sodium carboxymethyl cellulose with a weight average molecular weight of 400,000 were mixed, and solvent water was added and stirred evenly at a stirring speed of 1600 rpm for 45 minutes to obtain a first adhesive solution;

[0166] 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 stirred continuously at a stirring speed of 700 rpm for 25 minutes to obtain a negative electrode slurry with a solid content of 50%;

[0167] The temperature of the above preparation process was controlled at 25±3°C.

[0168] 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.

[0169] 2) Negative electrode

[0170] The above 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.

[0171] 3) Positive electrode

[0172] The positive electrode active material Na 7 / 9 Cu 2 / 9 Fe 1 / 9 Mn 2 / 3 O2, 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 above positive electrode slurry is extrusion-coated on a positive electrode current collector aluminum foil with a thickness of 13μm; in an oven, the temperature is 120°C and the coating speed is 30m / min, and then the positive electrode sheets are obtained by cold pressing and slitting.

[0173] 4) Electrolyte

[0174] 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.

[0175] 5) Isolation film

[0176] A 12 μm polyethylene (PE) porous polymer film was used as the separator.

[0177] 6) Preparation of batteries

[0178] The positive electrode sheet, the separator, and the negative electrode sheet are stacked in order, so that the separator is placed between the positive and negative electrode sheets to isolate the positive and negative electrode sheets. The bare battery cell is wound, the tabs are welded, and the bare battery cell is placed in an outer package. The above-prepared electrolyte is injected into the dried battery cell, and then the sodium secondary battery product of Example 1 is obtained after packaging, standing, formation, shaping, and capacity testing.

[0179] The sodium secondary batteries of Examples 2 to 17 were prepared in a similar manner to that of Example 1, except that the preparation parameters in the negative electrode slurry preparation method were adjusted. The different preparation parameters are detailed in Table 1.

[0180] 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:

[0181] 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.

[0182] Mix 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, add solvent water, and knead for 60 minutes to obtain a sticky mixture with a solid content of 65%;

[0183] 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%;

[0184] The temperature of the above preparation process was controlled at 25±3°C.

[0185] 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.

[0186] The sodium secondary battery of Example 19 is prepared in a similar manner to that of Example 1, but the preparation steps in the negative electrode slurry preparation method are adjusted to adopt a one-step preparation method, as follows:

[0187] 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.

[0188] The negative electrode active material hard carbon, conductive agent Super P, sodium carboxymethyl cellulose with a weight average molecular weight of 80,000, sodium carboxymethyl cellulose with a weight average molecular weight of 400,000, binder styrene-butadiene rubber (SBR), plasticizer 1,3-butanediol and 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%;

[0189] The temperature of the above preparation process was controlled at 25±3°C.

[0190] 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 12,000 mPa·s.

[0191] The sodium secondary batteries of Comparative Examples 1 to 3 were prepared in a similar manner to the sodium secondary battery of Example 1, except that the weight average molecular weight of the modifier was adjusted. See Table 1 for details.

[0192] 2. Performance Testing

[0193] 1. Anode slurry

[0194] 1) Gel state test of negative electrode slurry after 48 hours of standing

[0195] 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.

[0196] In the non-gel state, the negative electrode slurry flows naturally and continuously, and the negative electrode slurry flows flat on the surface of the steel ruler without agglomeration, as shown in FIG5a ;

[0197] In the slight gel state, the negative electrode slurry flows naturally and continuously, but the fluid is thin. The negative electrode slurry is basically spread evenly on the surface of the steel ruler, with slight small pieces, as shown in Figure 5b);

[0198] 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, with obvious block agglomerations, as shown in Figure 5c);

[0199] The severe gel state is that the negative electrode slurry cannot flow down in a stream, but falls into lumps or remains directly on the steel ruler and cannot flow down, as shown in Figure 5d).

[0200] 2) Filtration performance test

[0201] Take a 500ml beaker and place it at the lower end of a 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.

[0202] 3) Test of bubble volume per unit mass of hard carbon in negative electrode slurry

[0203] Seal the anode slurry (the mass of hard carbon in the anode slurry is m, in g) in a sealed bag and measure the gas evolution using the displacement method. The initial volume is V0, in mL, and the measured volume after 48 hours is V1, in mL. The bubble volume per unit mass of hard carbon in the anode slurry is calculated as (V1 - V0) / m, in mL / g.

[0204] 4) Rheological test

[0205] The test instrument is an 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 perform the 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 rheological property is judged to be shear thinning, that is, there is no shear thickening phenomenon, and it is judged as "none". As the shear rate increases, the viscosity of the negative electrode slurry increases, that is, there is a shear thickening phenomenon, and the rheological property is judged to be "yes".

[0206] 2. Negative electrode

[0207] 1) Weight loss rate test of the negative electrode sheet without cold pressing

[0208] After coating and before cold pressing, punch the negative electrode sheet into small discs of a fixed size and weigh them as m1. Bake them at 140°C for 6 minutes and weigh them as m2. The weight loss rate of the negative electrode sheet before cold pressing = (m1-m2) / m1.

[0209] 3. Battery

[0210] 1) Cycle 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. 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 cycle was 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 to evaluate the battery cycle performance.

[0212] 2) Storage performance

[0213] At 25°C, the prepared battery was charged at a constant current of 1C to 3.95V, then charged at a constant voltage of 3.95V until the current dropped to 0.05C. After standing for 10 minutes, it was discharged at a constant current of 1C to 1.5V. This was one charge / discharge cycle of the battery, with the capacity at the first discharge being 100%. The battery was then placed in a 60°C oven for a period of time, then taken out and the above charging steps were repeated. 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 until the capacity retention rate reached 80% were used as an indicator to evaluate the battery storage performance.

[0214] 3. Analysis of test results of various embodiments and comparative examples

[0215] Batteries of various examples and comparative examples were prepared according to the above methods, and various performance parameters were measured. The results are shown in Tables 1 and 2 below.

[0216] Table 1

[0217] Table 2

[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, and 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, and 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 1-3 and 8-11, controlling the mass ratio of modifier to negative electrode active material to 0.001-0.011 can 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, and thus achieve excellent cycling and storage performance of the sodium secondary battery. A comparison of Examples 1-3, 9, and 11 with Examples 8 and 10 shows that further controlling the mass ratio of modifier to negative electrode active material to 0.002-0.0065 can further improve the cycling and storage performance of the sodium secondary battery.

[0222] As can be seen from Examples 1-3, 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 92% to 97%, thereby slowing down the gelation of the negative electrode slurry after standing at 25°C for 48 hours, 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. As can be seen from the comparison of Examples 1-2 with Example 3, further controlling the mass content of the negative electrode active material to 92% to 95%, based on the total mass of the solids in the negative electrode slurry, is beneficial for 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.

[0223] From the comparison of Examples 1, 5 to 6 with 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, the modifier content is controlled to 0.1% to 1% by weight, based on the total weight of the solids in the negative electrode slurry, 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, and thus enable the sodium secondary battery to have excellent cycle performance and storage performance. As can be seen from the comparison of Examples 1, 9, and 11 with Examples 8 and 10, further controlling the modifier content to 0.2% to 0.6% by weight, based on the total weight of the solids in the negative electrode slurry, is beneficial for further improving the cycle performance and storage performance of the sodium secondary battery.

[0225] As can be seen from Examples 1, 12, and 15, controlling the kneading time in the adhesive mixture to 30 to 120 minutes 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 reduce the weight loss rate of the uncold-pressed negative electrode sheet, thereby enabling the sodium secondary battery to have excellent cycling and storage performance. A comparison of Examples 1, 13, and 14 with Examples 12 and 15 shows that controlling the kneading time in the adhesive mixture to 40 to 80 minutes is beneficial for further improving the cycling and storage performance of the sodium secondary battery.

[0226] As can be seen from Examples 1, 16, and 17, controlling the solid content of the adhesive mixture to 60% to 68% can 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 reduce the weight loss rate of the uncold-pressed negative electrode sheet, thereby enabling the sodium secondary battery to have excellent cycling and storage performance. As can be seen from a comparison of Examples 1 and 16 with Example 17, further controlling the solid content of the adhesive mixture to 60% to 65% is beneficial in 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 cycling and storage performance of the sodium secondary battery.

[0227] As can be seen from Examples 1, 18, and 19, the one-step, two-step, or four-step preparation methods for the negative electrode slurry can all 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 reduce the weight loss rate of the uncold-pressed negative electrode sheet, thereby enabling the sodium secondary battery to have excellent cycle performance and storage performance. A comparison of Examples 1 and 18 with Example 19 shows that, compared to preparing the negative electrode slurry using the one-step method, the two-step or four-step preparation methods are 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, thereby enabling the sodium secondary battery to have 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 merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine 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 or 2, 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 4, 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 5, 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 6, 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 7, 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 8, 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 9, 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 10, 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 11, 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 12, 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 13, 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 any one of claims 12 to 14, 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 any one of claims 12 to 15, characterized in that: The conductive agent includes Super P.

17. The negative electrode slurry according to any one of claims 12 to 16, 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 any one of claims 12 to 17, 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 any one of claims 12 to 18, characterized in that: The binder includes styrene-butadiene rubber.

20. The negative electrode slurry according to any one of claims 12 to 19, 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 or 22, characterized in that: The step 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 any one of claims 21 to 23, characterized in that The kneading time is 30 min to 120 min.

25. The preparation method according to any one of claims 21 to 24, characterized in that: The kneading time is 40 min to 80 min.

26. The preparation method according to any one of claims 21 to 25, 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 26, 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 27, 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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