Sodium ion battery, battery pack and energy storage system

Abstract

The invention provides a sodium ion battery, a battery pack and an energy storage system. The sodium ion battery comprises a positive pole piece and a negative pole piece, wherein the negative pole piece comprises a negative pole current collector, an inner material layer arranged on the surface of at least one side of the negative pole current collector and an outer material layer arranged on the surface of the inner material layer; the inner material layer and the outer material layer both contain a negative electrode active material, the mass fraction of an additive in the inner material layer is greater than 0%, the mass fraction of an additive in the outer material layer is greater than or equal to 0%, and the content of the additive in the inner material layer is greater than that of the additive in the outer material layer. And the ionic conductivity of the additive is greater than that of the negative electrode active material, so that the sodium precipitation probability of the sodium ion battery is reduced, and the cycle performance of the sodium ion battery is improved.

Description

Technical Field

[0001] The present application relates to the field of energy storage, and specifically designs a sodium ion battery, a battery pack, and an energy storage system. Background Art

[0002] As a new type of secondary battery with abundant resources and low cost, the rapid development of sodium-ion batteries is of great significance to new energy fields such as large-scale energy storage and low-speed electric vehicles. Like lithium-ion batteries, sodium-ion batteries also face the problem of sodium precipitation when used at low temperatures and high-rate charging scenarios. The main reason for sodium precipitation is the difficulty of sodium ion diffusion in the solid phase, which leads to a higher concentration of sodium ions on the outer surface of the negative electrode, causing polarization. This reduces the negative electrode potential below the sodium precipitation reaction potential, causing sodium precipitation, and thus affecting the cycle performance of the sodium-ion battery. Summary of the Invention

[0003] The present application provides a sodium ion battery, a battery pack, and an energy storage system to reduce the probability of sodium precipitation in the sodium ion battery and improve the cycle performance of the sodium ion battery.

[0004] In a first aspect, the present application provides a sodium ion battery, which includes a positive electrode sheet and a negative electrode sheet, the negative electrode sheet including a negative electrode current collector and an inner material layer provided on at least one side surface of the negative electrode current collector and an outer material layer provided on the surface of the inner material layer; the inner material layer and the outer material layer both contain negative electrode active material, the mass fraction of the additive in the inner material layer is greater than 0%, the mass fraction of the additive in the outer material layer is ≥0%, the content of the additive in the inner material layer is greater than the content of the additive in the outer material layer, and the ionic conductivity of the additive is greater than the ionic conductivity of the negative electrode active material.

[0005] In the sodium-ion battery of the present application, two layers of negative electrode material are provided on the surface of the negative electrode current collector, wherein the content of the additive in the inner material layer is higher than the content of the additive in the outer material layer. During the operation of the sodium-ion battery, the sodium ions transmitted to the negative electrode plate can be induced to be transmitted to the inner material layer under the action of the additive. Therefore, the sodium-ion battery of the present application can increase the transmission rate of sodium ions in the negative electrode material layer and the uniformity of deposition by adding additives to the inner material, reduce polarization, and alleviate the sodium precipitation problem caused by the enrichment and deposition of sodium ions on the surface of the outer material layer, thereby improving the cycle performance of sodium ions and improving the low-temperature, high-rate charge and discharge performance of the sodium-ion battery.

[0006] Wherein, the negative electrode active material includes but is not limited to at least one of hard carbon, soft carbon, artificial graphite, or natural graphite. Therefore, when the negative electrode active material is hard carbon, the ionic conductivity of the additive is greater than the ionic conductivity of the hard carbon. When the negative electrode active material is soft carbon, the ionic conductivity of the additive is greater than the ionic conductivity of the soft carbon. When the negative electrode active material is artificial graphite, the ionic conductivity of the additive is greater than the ionic conductivity of the artificial graphite. When the negative electrode active material is natural graphite, the ionic conductivity of the additive is greater than the ionic conductivity of the natural graphite.

[0007] The ionic conductivity (σ) of the additive may be greater than 10 -4 S cm -1 As an example, the additives in the sodium ion battery of the present application include but are not limited to at least one of Al2O3, Na3PS4, NaSbS4, Na3PSe4, and sodium superion conductors. Among them, the sodium superion conductor can be, for example, Na3Zr2Si2PO 12 , Na4Fe3(PO4)3P2O7, or at least one of Na3V2(PO4)3.

[0008] Since soft carbon, hard carbon, artificial graphite, or natural graphite have a longer low potential platform when used as negative electrode active materials, they are more likely to reach the sodium precipitation potential under polarization. Therefore, by adding additives, the potential of the negative electrode sheets of soft carbon, hard carbon, etc. can be prevented from decreasing, so that they are above the sodium precipitation potential, thereby avoiding sodium precipitation.

[0009] The inner material layer must contain additives, while the outer material layer may or may not contain additives. When additives are added to the outer material layer, the content of the additives in the inner material layer must be greater than that in the outer material layer.

[0010] In an optional implementation, the mass fraction of the additive in the outer layer material is ≤2%. For example, the content of the additive in the outer layer material is 0. Furthermore, in addition to the negative electrode active material and the additive, the inner layer material layer may also include substances such as a conductive agent and a binder. In addition to the negative electrode active material, the outer layer material layer may also include substances such as a conductive agent and a binder. In an optional implementation, the difference between the content of the additive in the inner layer material layer and the content of the additive in the outer layer material layer may be greater than or equal to 3%.

[0011] In an optional implementation, the ratio of the thickness of the inner material layer to the thickness of the outer material layer is 0.1-10. The thickness of the inner material layer can be higher,

[0012] In the sodium ion battery of the present application, the additive may have higher electronic conductivity in addition to higher ionic conductivity. In an optional implementation, the additive includes a core and a coating layer, the coating layer is carbon, and the core is a compound capable of transporting sodium ions. Exemplarily, the core includes at least one of Al2O3, Na3PS4, NaSbS4, Na3PSe4, and a sodium superion conductor. The carbon in the coating layer can achieve electron conduction and improve the electronic conductivity of the additive. The compound in the core can achieve ion conduction and improve the sodium ion conductivity of the additive. Therefore, selecting an additive with a carbon coating layer can make the negative electrode plate have higher ionic conductivity and higher electronic conductivity, so as to utilize the high-rate charge and discharge performance of the negative electrode plate.

[0013] In an optional implementation, the weight of the additive is 0.1%-15% of the total weight of the negative electrode active material. Optionally, the mass fraction of the additive in the inner layer material may be 1%, 3%, 5%, 6%, 8%, 10%, 12%, 14% or 15%, etc. The mass fraction of the additive in the outer layer material layer is 0-12%. Optionally, the mass fraction of the additive in the outer layer material may be 0%, 1%, 3%, 5%, 6%, 8%, 10% or 12%, etc. Since the additive does not provide additional capacity, excessive additive content will affect the capacity of the negative electrode sheet. Too little additive addition will not significantly improve the effect of sodium precipitation in the battery. Therefore, in the implementation of the present application, by optimizing the mass proportion of the additive in the negative electrode active material, it can play a role in balancing the uniform transmission of sodium ions in the inner and outer layers without affecting the capacity of the sodium ion battery or significantly reducing the capacity of the sodium ion battery.

[0014] In the negative electrode sheet, since the porosity of carbon materials such as soft carbon, hard carbon, and graphite used as negative electrode active materials is high, the high porosity will affect the transmission of ions and electrons, resulting in local precipitation of sodium ions. Additives can fill the pores in the negative electrode active material to form a bridge for ion transmission, so that the sodium ions are more evenly distributed and deposited in the negative electrode active material. In an optional implementation, the ratio of the median particle size D50 of the additive to the median particle size D50 of the negative electrode active material is 0.1-1, such as 0.2-0.8, and another example is 0.4-0.6. Using additives of the above particle size for pore filling can, on the one hand, improve the ion and electron transmission performance, reduce the resistance of the negative electrode sheet, and thereby reduce the capacity loss of the sodium ion battery during the charge and discharge process, and improve the low-temperature charge and discharge rate performance of the sodium ion battery. On the other hand, selecting the right additive can reduce the negative electrode porosity. Since the capacity retention rate of the battery cell is higher at a lower filling coefficient, the reduction of the negative electrode porosity can help reduce the filling coefficient of the electrolyte of the sodium ion battery and improve the capacity retention rate of the sodium ion battery.

[0015] In a second aspect, the present application provides a battery pack comprising a plurality of batteries, wherein the plurality of batteries are connected in series or in parallel; the battery is the sodium ion battery of the first aspect of the present application.

[0016] In a third aspect, the present application provides an energy storage system, which includes a power converter and at least one battery pack of the present application; the power converter is used to convert the voltage output by the battery pack into power and output it to a power grid or a load, and / or, convert the voltage output by an external power supply into power and output it to the battery pack.

[0017] The technical effects that can be achieved in the second and third aspects mentioned above can be described with reference to the corresponding effects in the first aspect mentioned above, and will not be repeated here.

[0018] Among them, the data in the above-mentioned possible implementation methods of the present application, such as the ratio of the median particle size of the additive to the median particle size of the negative electrode active material, the proportion of the additive, etc., when measured, the values within the scope of engineering measurement error should be understood to be within the range specified in the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 A schematic structural diagram of a sodium ion battery provided in an embodiment of the present application;

[0020] Figure 2 Schematic diagram of the structure of a negative electrode sheet according to an embodiment;

[0021] Figure 3 Schematic diagram of the structure of a negative electrode sheet according to an embodiment;

[0022] Figure 4 A schematic structural diagram of additive particles according to an embodiment;

[0023] Figure 5 A schematic diagram of the connection structure of an energy storage system.

[0024] Reference numerals:

[0025] 11-positive electrode sheet; 12-negative electrode sheet; 121-negative electrode current collector; 122-inner material layer; 123-outer material layer;

[0026] 124-negative electrode active material; 125-additive; 126-conductive agent; 127-core; 128-coating layer; 13-separator; 14-electrolyte;

[0027] 100-battery pack; 200-power converter; 300-grid; 400-load; 500-photovoltaic panel. DETAILED DESCRIPTION

[0028] In order to make the purpose, technical solutions and advantages of this application clearer, this application will be further described in detail below with reference to the accompanying drawings.

[0029] The terms used in the following embodiments are only for the purpose of describing specific embodiments and are not intended to limit the present application. As used in the specification and appended claims of this application, the singular expressions "a", "an", "above", "the", and "this" are intended to also include expressions such as "one or more", unless the context clearly indicates otherwise.

[0030] References to "one embodiment" or "some embodiments" in this specification mean that a particular feature, structure, or characteristic described in conjunction with that embodiment is included in one or more embodiments of the present application. Thus, phrases such as "in one embodiment," "in some embodiments," "in other embodiments," and "in yet other embodiments" appearing in various places in this specification do not necessarily refer to the same embodiment, but rather mean "one or more but not all embodiments," unless otherwise specifically emphasized. The terms "including," "comprising," "having," and variations thereof mean "including but not limited to," unless otherwise specifically emphasized.

[0031] Figure 1 This is a schematic diagram of the structure of a sodium ion battery. Figure 1 As shown, the sodium ion battery includes a positive electrode sheet, a separator, a negative electrode sheet, and an electrolyte. The separator is placed between the positive electrode sheet and the negative electrode sheet, and the electrolyte infiltrates the positive electrode sheet, the negative electrode sheet, and the separator. Figure 1 As shown, during charging, sodium ions escape from the positive electrode sheet, pass through the electrolyte and through the separator 13, and then embed into the negative electrode sheet. Electrons flow from the external circuit from the positive electrode sheet 11 to the negative electrode sheet 12, and combine with the sodium ions embedded in the negative electrode sheet to form sodium metal. During discharge, the sodium metal decomposes into sodium ions and electrons. Sodium ions escape from the negative electrode material, pass through the electrolyte and through the separator 13, and then embed into the positive electrode sheet. Electrons flow from the external circuit from the negative electrode sheet 12 to the positive electrode sheet 11, powering external terminal devices. The electrolyte is the carrier for sodium ions to be transmitted between the positive and negative electrodes. The separator 13 is a non-conductive film layer that can pass sodium ions but prevents electrons from passing through, thereby separating the positive and negative electrodes to prevent short circuits.

[0032] Among them, reference Figure 1 The positive electrode sheet 11 typically includes a positive electrode current collector and a positive electrode material layer. The positive electrode current collector can be a metal foil such as copper foil or aluminum foil. The positive electrode material layer includes a positive electrode active material capable of sodium insertion and removal. The positive electrode active material can be selected from three major categories of materials: transition metal layered oxide systems, polyanionic compounds, and Prussian blue systems.

[0033] The primary function of the separator is not only to prevent direct contact between the positive and negative electrodes, which could cause a short circuit, but also to provide a porous channel for the transport of sodium ions. The separator can be a polyolefin separator or a glass fiber separator. Polyolefin separators can be, for example, polyethylene or polypropylene.

[0034] The electrolyte is a transport carrier for ions in sodium ion batteries and is typically composed of a sodium salt, a solvent, and an additive. The sodium salt may be, for example, one or more selected from sodium hexafluorophosphate, sodium tetrafluoroborate, sodium hexafluoroarsenate, sodium perchlorate, sodium bis(fluorosulfonyl)imide, sodium bis(trifluoromethanesulfonyl)imide, sodium bis(oxalatoborate), and sodium difluorooxalatoborate. The solvent may be an aqueous solvent or a non-aqueous solvent, and may be selected based on the type of sodium salt and additive. The additive may include, for example, one or more of vinylidene ester, 1,3-propane sultone, fluoroethylene carbonate, fluoromethylethylene carbonate, dimethyl sulfate, and methylvinyl sulfate.

[0035] Continue to refer to Figure 1 During charging, the limited sodium conductivity of the negative electrode active material causes uneven sodium ion transport and distribution within the negative electrode material layer, making sodium ions more likely to accumulate on the surface of the negative electrode. This accumulation of sodium ions lowers the potential of the entire negative electrode. If the potential of the negative electrode is too low, sodium ions can precipitate on the surface of the negative electrode, forming irreversible sodium metal dendrites.

[0036] This application addresses the problem of sodium precipitation on the surface of sodium-ion batteries due to insufficient sodium conductivity of the negative electrode active material. The sodium-ion battery of the present application embodiment improves the uniformity of sodium ion transport in the negative electrode material layer by adding an additive, thereby preventing sodium ion deposition on the surface of the negative electrode plate and reducing the probability of sodium precipitation on the surface of the negative electrode plate.

[0037] Figure 2 FIG. 1 is a schematic diagram of the structure of the negative electrode sheet according to an embodiment of the present invention. Figure 2 As shown, the negative electrode sheet includes a negative electrode current collector 121, with a negative electrode material layer provided on at least one surface of the negative electrode current collector 121. The negative electrode material layer can be provided on one surface of the negative electrode current collector 121, or on both surfaces of the negative electrode current collector 121. The negative electrode current collector 121 can be a metal foil, such as copper foil or aluminum foil. The negative electrode material layer includes an inner material layer 122 and an outer material layer 123, which are stacked. The inner material layer 122 is provided between the negative electrode current collector 121 and the outer material layer 123.

[0038] The inner material layer 122 contains a negative electrode active material 124 and an additive 125. The additive 125 is a substance having an ion conductivity greater than that of the negative electrode active material 124. The mass fraction of the additive 125 in the inner material layer is greater than 0.

[0039] Exemplarily, the negative electrode active material may be at least one of hard carbon, soft carbon, artificial graphite, or natural graphite. The additive may be at least one of Al2O3, Na3PS4, NaSbS4, Na3PSe4, and a sodium superion conductor. Among them, a sodium superion conductor (NASICON) refers to a substance in a solid state, whose crystal structure is a three-dimensional network structure, which allows sodium ions to pass through it, exhibiting an abnormally high ionic conductivity. Common sodium superion conductors are sodium-based polyanion compounds, which are compounds with a three-dimensional network structure formed by strong covalent bonds between polyanion polyhedra and transition metal ion (sodium ion) polyhedra. Exemplarily, a sodium superion conductor may be, for example, Na3Zr2Si2PO 12 , Na4Fe3(PO4)3P2O7, or at least one of Na3V2(PO4)3.

[0040] Continue to refer to Figure 2 The outer material layer 123 contains the negative electrode active material 124. However, the outer material layer 123 may contain the additive 125 or may not contain the additive 125. That is, the mass fraction of the additive 125 in the outer material layer 123 is greater than or equal to 0. Figure 2 In the structure shown, the outer material layer 123 does not contain the additive 125 .

[0041] Figure 3 FIG. 1 is a schematic structural diagram of a negative electrode sheet according to another embodiment. Figure 3 In the negative electrode sheet shown, the outer material layer 123 may contain a small amount of additive 125 .

[0042] The ionic conductivity of the additive may be greater than 10 -4 S cm -1 . It can be understood that when the negative electrode active material is hard carbon, a substance with an ion conductivity greater than that of hard carbon needs to be selected as an additive. When the negative electrode active material is soft carbon, a substance with an ion conductivity greater than that of soft carbon needs to be selected as an additive. When the negative electrode active material is artificial graphite, a substance with an ion conductivity greater than that of artificial graphite needs to be selected as an additive. When the negative electrode active material is natural graphite, a substance with an ion conductivity greater than that of natural graphite needs to be selected as an additive.

[0043] In the sodium-ion battery of the embodiment of the present application, the negative electrode material layer adopts a layered design. More additives are added to the inner material layer near the negative electrode current collector, making it easier for sodium ions to diffuse into the inner layer without forming sodium metal precipitation on the surface of the negative electrode material layer, thereby improving the low-temperature and high-rate charging performance of the sodium-ion battery. In one embodiment, the ratio of the thickness of the inner material layer to the thickness of the outer material layer is 0.1-10. Exemplarily, the ratio of the thickness of the inner material layer to the outer material layer can be, for example, 0.1, 0.3, 0.5, 0.6, 0.8, 1.0, 1.2, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 7.0, 7.5, 8.0, 8.5, 9.0, 10, or any value between any two of the above values. If the thickness of the inner material layer is too thick, the content of the additive is relatively high, which will affect the overall capacity of the sodium-ion battery. If the thickness of the outer material layer is too thick, it is difficult for sodium ions to penetrate the outer material layer and reach the inner material layer. Therefore, when the thickness ratio of the inner material layer to the outer material layer is within the above range, it can ensure that the sodium ion battery has less capacity loss and can also achieve uniform transmission of sodium ions.

[0044] The weight proportion of the additive in the total negative electrode active material is 0.1%-15%, such as 1%-12%, such as 3%-12%, and such as 5%-10%. For example, the weight proportion of the additive in the negative electrode active material can be 0.1%, 0.5%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, 4.2%, 4.5%, 4.8%, 5%, 5.2%, 5.5%, 5.8%, 6%, 7.2%, 7.5%, 7.8%, 8%, 8.2%, 8.5%, 8.8%, 9%, 9.2%, 9.5%, 9.8%, 10%, 11%, 12%, 13%, 14%, or 15%, or a value between any two of the above values. Too high an additive content can cause significant capacity loss in sodium-ion batteries. Too low an additive content can lead to inefficient sodium ion transport. Therefore, a 0.1%-15% additive content can improve uniform sodium ion transport without sacrificing capacity.

[0045] In order to better improve the sodium ion battery, in one embodiment, the difference w1-w2 between the mass fraction w1 of the additive in the inner material layer and the mass fraction w2 of the additive in the outer material layer is 3%-10%. For example, the difference between the two can be 3%, 4%, 5%, 6%, 7%, 8%, 9%, or 10% or a value between any two of the above values.

[0046] The mass fraction of the additive in the inner material layer may be 1%-15%, such as 2%-13%, such as 3%-12%, and such as 5%-11%. For example, the mass fraction of the additive in the inner material layer is typically, but not limited to, 1%, 1.2%, 1.5%, 1.8%, 2%, 3%, 4%, 5%, 6%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, or 15%, or a value between any two of the above values.

[0047] The mass fraction of the additive in the outer layer material is ≤2%, such as ≤1.5%, such as ≤1%, and further such as 0. For example, the mass fraction of the additive in the outer layer material layer is typically, but not limited to, 0%, 0.1%, 0.2%, 0.3%, 0.5%, 0.8%, 1.0%, 1.2%, 1.4%, 1.6%, 1.8%, or 2%, or a value between any two of the above values. To improve the stability of the SEI film between the outer layer material layer and the electrolyte, in one embodiment, the content of the additive in the outer layer material layer is 0.

[0048] In addition to having high ionic conductivity, the additive may also have high electronic conductivity. To improve the electronic conductivity of the additive, the surface layer of the additive may be an electronic conductive layer.

[0049] Figure 4 FIG. 1 is a schematic diagram of the structure of an additive according to an embodiment of the present invention. Figure 4 As shown, the additive includes a core 127 and a coating layer 128, and the coating layer 128 is carbon or other materials with high electronic conductivity. The core 127 is a compound capable of transporting sodium ions. Exemplarily, the core 127 includes at least one of Al2O3, Na3PS4, NaSbS4, Na3PSe4, and a sodium superion conductor. The carbon in the coating layer 128 can achieve electron conduction and improve the electronic conductivity of the additive. The compound in the core 127 can achieve ion conduction and improve the sodium ion conductivity of the additive. Therefore, selecting an additive with a carbon coating layer can make the negative electrode plate have higher ion conductivity and higher electronic conductivity, so as to achieve the high-rate charge and discharge performance of the negative electrode plate.

[0050] In the negative electrode sheet, since the porosity of carbon materials such as soft carbon, hard carbon, and graphite used as negative electrode active materials is high, the high porosity will affect the transmission of ions and electrons, resulting in local precipitation of sodium ions. Additives can fill the pores in the negative electrode active material to form a bridge for ion transmission, so that sodium ions are more evenly distributed and deposited in the negative electrode active material. In an optional implementation, the ratio of the median particle size D50 of the additive to the median particle size D50 of the negative electrode active material is 0.01-1, such as 0.2-1, such as 0.2-0.8, such as 0.3-0.8, such as 0.4-0.8, and such as 0.4-0.6. The median particle size D50 of the additive can be, for example, 2μm-8μm.

[0051] By optimizing the particle size of the additive, it can be filled into the gaps between the particles of the negative electrode active material, which helps to strengthen the connectivity of the particles of the negative electrode active material and form a better electronic and ionic conductive network. In addition, filling the additive can also reduce the porosity of the negative electrode material layer, reducing the amount of electrolyte added and the cost of battery cell raw materials. Reducing the negative electrode porosity can help reduce the electrolyte injection coefficient of sodium ion batteries and improve the capacity retention rate of sodium ion batteries.

[0052] It is understandable that the negative electrode sheet of the embodiment of the present application, such as Figure 2 and Figure 3 As shown, in addition to the negative electrode active material, the conductive agent 126 may also include a binder and a conductive agent. The binder may be selected from one or more of styrene-butadiene rubber and polyacrylonitrile. The conductive agent 126 may be selected from one or more of acetylene black, conductive carbon black, carbon fiber, carbon nanotubes, and graphene.

[0053] The sodium ion battery of the embodiment of the present application, the preparation method thereof may include the following steps:

[0054] 1) applying a slurry containing a negative electrode active material, a conductive agent, a binder and additives to the surface of the negative electrode current collector and drying the slurry to form an inner material layer;

[0055] 2) Coating a slurry containing a negative electrode active material, a conductive agent, a binder and optional additives on the surface of the inner material layer and drying the slurry to obtain a negative electrode sheet.

[0056] 3) Prepare a positive electrode sheet, and assemble a battery using the positive and negative electrode sheets. After adding an electrolyte, the battery undergoes processes such as standing, formation, aging, and capacity separation to obtain a sodium ion battery.

[0057] The sodium ion battery of the embodiment of the present application has application scenarios including but not limited to household energy storage, industrial and commercial energy storage, site energy backup power, data center backup power, smart photovoltaic backup power scenarios, electric vehicles, electric two-wheelers or electric three-wheelers, etc.

[0058] The sodium-ion batteries of the embodiments of the present application can be assembled into a battery module or battery pack. In addition to the sodium-ion batteries of the embodiments of the present application, the battery module or battery pack may also include a BMS management system.

[0059] The preparation method and performance of the sodium ion battery of the present application will be further described in detail below with reference to the examples.

[0060] Example 1

[0061] This embodiment is a sodium ion battery, and the preparation method of the sodium ion battery comprises the following steps:

[0062] S1. Preparation of additives

[0063] Conductive carbon black and β-Al2O3, where the average particle size of β-Al2O3 is 2 μm, are mixed in a mass percentage ratio of 1:2 and transferred to a high-speed ball mill. The grinding balls are steel balls with a diameter of 2 mm and ball milling is performed at a speed of 600 rpm for 6 hours to obtain carbon-layer-coated β-Al2O3 particles.

[0064] S2. Preparation of negative electrode sheet

[0065] 1) Preparing an inner layer slurry: hard carbon or soft carbon powder (median particle size 8 μm), conductive carbon black, styrene-butadiene rubber (SBR), sodium carboxymethyl cellulose (CMC), and the carbon-coated β-Al2O3 particles prepared in step S1 are mixed in a mass ratio of 92:1.5:3:1.5:2 and then stirred and dispersed in water to form a uniform slurry.

[0066] 2) Preparation of outer layer slurry: hard carbon or soft carbon powder (median particle size 8 μm), conductive carbon black, styrene-butadiene rubber (SBR), and sodium carboxymethyl cellulose (CMC) are mixed in a mass ratio of 94:1.5:3:1.5 and then stirred and dispersed in water to form a uniform slurry.

[0067] 3) The inner layer slurry is applied to the surface of the aluminum foil serving as the negative electrode current collector. After drying, an inner layer of material is formed on the surface of the negative electrode current collector. The outer layer of slurry is then applied to the inner layer of material and dried. The electrode sheet after the two coatings are then rolled and slit to obtain the negative electrode sheet. The thickness of the inner layer of material is 90 μm, and the thickness of the outer layer of material is 30 μm.

[0068] S3. Preparation of positive electrode sheet

[0069] NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3O2 (or NaCuxFeyMnzO2, Prussian blue, NaFePO4, etc.) is mixed with conductive carbon black and adhesive polyvinylidene fluoride (PVDF) in a mass ratio of 92:3:5 and dispersed in the organic solvent N-methylpyrrolidone (NMP) to obtain a uniform and stable positive electrode slurry, which is then transferred to the positive electrode collector for coating, drying, rolling, slitting and other steps to obtain the positive electrode sheet.

[0070] S4. Preparation of sodium ion batteries

[0071] A battery was assembled using positive and negative electrode sheets, and an electrolyte (1M NaPF6 in DMC:EC:EMC=1:1:1Vol%) was added. After standing, forming, aging, and capacity separation, a sodium ion battery was obtained.

[0072] Example 2

[0073] This embodiment is a sodium-ion battery, which differs from Example 1 in that the inner layer slurry ratio is different. In this embodiment, the inner layer slurry ratio is: hard carbon, conductive carbon black, styrene-butadiene rubber, sodium carboxymethyl cellulose, and carbon-coated β-Al2O3 particles in a mass ratio of 89:1.5:3:1.5:5.

[0074] Example 3

[0075] This embodiment is a sodium-ion battery, which differs from Example 1 in that the inner layer slurry ratio is different. In this embodiment, the inner layer slurry ratio is: hard carbon, conductive carbon black, styrene-butadiene rubber, sodium carboxymethyl cellulose, and carbon-coated β-Al2O3 particles in a mass ratio of 84:1.5:3:1.5:10.

[0076] Example 4

[0077] This embodiment is a sodium ion battery, which differs from the first embodiment in that the conductive carbon black used in the preparation of the additive is replaced with carbon nanotubes. The rest of the preparation process is the same as that of the first embodiment.

[0078] Example 5

[0079] This embodiment is a sodium ion battery, which differs from the first embodiment in that the β-Al2O3 used in the preparation of the additive is replaced with NaSbS4. The rest of the preparation process is the same as that of the first embodiment.

[0080] Example 6

[0081] This embodiment is a sodium ion battery, which differs from the embodiment 1 in that the preparation process of the additive is different. The specific preparation process of the additive is as follows: a carbon source (phenolic resin, epoxy resin, glucose, etc.) is mixed with 3 μm Na3Zr2Si2PO 12The mixture was added to water at a mass ratio of 1:5 and stirred, thoroughly mixed, and then vacuum dried. The dried powder was calcined at 700°C for 2 hours in an argon atmosphere and then cooled to room temperature. The calcined material was crushed to obtain a carbon-coated solid electrolyte material.

[0082] The rest of the preparation process is the same as that in Example 1.

[0083] Example 7

[0084] This embodiment is a sodium ion battery, which differs from embodiment 1 in that the preparation process of the negative electrode plate is different. In the negative electrode plate of this embodiment, the thickness of the inner material layer is 80 μm, and the thickness of the outer material layer is 50 μm.

[0085] Comparative Example 1

[0086] This comparative example is a sodium ion battery, and the preparation method of the sodium ion battery comprises the following steps:

[0087] S1. Preparation of additives

[0088] Conductive carbon black and β-Al2O3, where the average particle size of β-Al2O3 is 2 μm, are mixed in a mass percentage ratio of 1:2 and transferred to a high-speed ball mill. The grinding balls are steel balls with a diameter of 2 mm and ball milling is performed at a speed of 600 rpm for 6 hours to obtain carbon-layer-coated β-Al2O3 particles.

[0089] S2. Preparation of negative electrode sheet

[0090] 1) Preparation of negative electrode slurry: hard carbon or soft carbon powder (median particle size 8 μm), conductive carbon black, styrene-butadiene rubber, and sodium carboxymethyl cellulose are mixed in a mass ratio of 94:1.5:3:1.5 and then stirred and dispersed in water to form a uniform slurry.

[0091] 2) The negative electrode slurry is coated on the surface of the aluminum foil serving as the negative electrode current collector, and after drying, the negative electrode sheet is obtained by rolling and slitting. The thickness of the negative electrode material layer is 120 μm.

[0092] S3. Preparation of positive electrode sheet

[0093] NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2 (or NaCuxFeyMnzO2, Prussian blue, NaFePO4, etc.) is mixed with conductive carbon black and adhesive polyvinylidene fluoride (PVDF) in a mass ratio of 92:3:5 and dispersed in the organic solvent N-methylpyrrolidone (NMP) to obtain a uniform and stable positive electrode slurry, which is then transferred to the positive electrode collector for coating, drying, rolling, slitting and other steps to obtain the positive electrode sheet.

[0094] S4. Preparation of sodium ion batteries

[0095] A battery was assembled using positive and negative electrode sheets, and an electrolyte (1M NaPF6 in DMC:EC:EMC=1:1:1Vol%) was added. After standing, forming, aging, and capacity separation, a sodium ion battery was obtained.

[0096] Comparative Example 2

[0097] This comparative example is a sodium ion battery, and the preparation method of the sodium ion battery comprises the following steps:

[0098] S1. Preparation of additives

[0099] Conductive carbon black and β-Al2O3, where the average particle size of β-Al2O3 is 2 μm, are mixed in a mass percentage ratio of 1:2 and transferred to a high-speed ball mill. The grinding balls are steel balls with a diameter of 2 mm and ball milling is performed at a speed of 600 rpm for 6 hours to obtain carbon-layer-coated β-Al2O3 particles.

[0100] S2. Preparation of negative electrode sheet

[0101] 1) preparing a negative electrode slurry: mixing hard carbon or soft carbon powder (median particle size 8 μm), conductive carbon black, styrene-butadiene rubber, sodium carboxymethyl cellulose (CMC), and the carbon-coated β-Al2O3 particles prepared in step S1 in a mass ratio of 91:1.5:3:1.5:3, and then stirring and dispersing them in water to form a uniform slurry.

[0102] 2) The inner layer slurry is coated on the surface of the aluminum foil used as the negative electrode current collector, and after drying, the negative electrode sheet is obtained by rolling and slitting. The thickness of the negative electrode material layer is 120 μm.

[0103] S3. Preparation of positive electrode sheet

[0104] NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2 (or NaCuxFeyMnzO2, Prussian blue, NaFePO4, etc.) is mixed with conductive carbon black and adhesive polyvinylidene fluoride (PVDF) in a mass ratio of 92:3:5 and dispersed in the organic solvent N-methylpyrrolidone (NMP) to obtain a uniform and stable positive electrode slurry, which is then transferred to the positive electrode collector for coating, drying, rolling, slitting and other steps to obtain the positive electrode sheet.

[0105] S4. Preparation of sodium ion batteries

[0106] A battery was assembled using positive and negative electrode sheets, and an electrolyte (1M NaPF6 in DMC:EC:EMC=1:1:1Vol%) was added. After standing, forming, aging, and capacity separation, a sodium ion battery was obtained.

[0107] The batteries of each embodiment and comparative example were tested for rated capacity, injection system, negative electrode resistivity, capacity retention after 500 cycles at 25°C @ 1C, DCR (mΩ) at 25°C -100% SOC, and sodium deposition after charge and discharge cycles. The specific test results are listed in Tables 1 and 2.

[0108] The testing process of the above parameters is as follows:

[0109] 1) Rated capacity

[0110] Place the battery in a 25°C environment, charge it to 3.95V at a constant current and constant voltage of 0.2C, then charge it at a constant voltage until the current drops to 0.05C, let it stand for 30 minutes, then discharge it to 2V at 0.2C, let it stand for 30 minutes, repeat this cycle twice, and take the second discharge capacity as the rated capacity.

[0111] 2) Injection coefficient

[0112] Filling coefficient = filling volume / battery cell design capacity. This solution uses a filling coefficient of 3.5g / Ah.

[0113] 3) Resistivity of the negative electrode

[0114] Use a film resistance tester to take a section of electrode (width 5-9cm, length 20-30cm); move the electrode position so that the test probe falls on six positions on the electrode, obtain 6 electrode resistivity values, and take the average as the resistivity of the electrode.

[0115] 4) Capacity retention after 500 cycles at 25°C @ 1C:

[0116] Place the sodium ion battery in an oven at a constant temperature of 25±3℃, charge it to 3.95V at a constant current and constant voltage of 1C, then charge it at a constant voltage until the current drops to 0.05C, let it stand for 30 minutes, and then discharge it to 2.0V at a constant current of 1C. Repeat this cycle 500 times, record the discharge capacity of the 1st and 500th cycles, and calculate the capacity retention rate of the cycle according to the following formula:

[0117] Capacity retention (%)=discharge capacity at the 500th cycle / discharge capacity at the 1st cycle×100%.

[0118] 5) DC internal resistance DCR

[0119] The battery was placed in a 25°C environment, charged at a constant current and constant voltage of 0.2C to 3.95V, then charged at a constant voltage until the current dropped to 0.05C, and allowed to stand for 30 minutes; the cell was discharged at a constant current of 0.2C for 30 seconds to obtain the discharge end voltage V1, then discharged at a constant current of 1C for 5 seconds to obtain the discharge end voltage V2. There was no standing time between the two constant current discharge rates, and DCR = (V1-V2) / 0.8C.

[0120] 6) Sodium precipitation test method

[0121] The battery was tested using the rated capacity method to obtain the pre-test rated capacity. The battery was then placed in an incubator at -10°C / 15°C and allowed to rest for 4 hours. The battery was then charged at a constant current and voltage of 0.1C / 0.15C / 0.2C / 0.5C / 1C at the corresponding temperature to a value of 3.95V / 0.05C, and allowed to rest for 30 minutes. The cell was then discharged at a constant current of 0.2C to 2.0V and allowed to rest for 30 minutes. This cycle was repeated 10 times. The battery was returned to room temperature and allowed to rest for 4 hours. The rated capacity test method was then used to obtain the post-test rated capacity. The battery was then disassembled to determine if sodium precipitation had occurred at the negative electrode.

[0122] Sodium precipitation loss = (rated capacity before test - rated capacity after test) / rated capacity before test × 100%.

[0123] Table 1

[0124]

[0125] Table 2

[0126]

[0127] It can be seen from the test data in Table 1 that in the sodium ion batteries of Examples 1-8, the resistivity of the negative electrode sheet is significantly lower than that of Comparative Example 1, and the capacity retention rate is higher. Compared with Comparative Example 2, the rated capacity of the sodium ion battery of the embodiment of the present application is higher, and the capacity retention rate is also higher. Combined with the test data in Table 2, it can be seen that the amount of sodium precipitation of the sodium ion battery of the embodiment of the present application at low temperature and room temperature is significantly lower than the amount of sodium precipitation of Comparative Example 1 and Comparative Example 2. After the charge and discharge cycle, the sodium precipitation amount of the sodium ion battery of the embodiment of the present application is generally less than 2%. However, after the same charge and discharge cycle, the sodium ion battery of Comparative Example 1 has already had sodium precipitation problems at low temperature, and the sodium precipitation amount at 15°C has reached 8%. Compared with the sodium ion battery of the embodiment of the present application, the sodium precipitation performance has been significantly deteriorated. This shows that by adding an appropriate amount of additives to the inner material layer, the sodium precipitation performance of the sodium ion battery can be significantly improved.

[0128] The sodium ion battery of Comparative Example 2 does not adopt a layered design, and is filled with additive particles. The overall electrode conductivity is more significantly improved, but the surface heterogeneous particles are not conducive to the integrity of the SEI film, and the interface stability during the cycle is reduced, which is not conducive to long-cycle performance. In addition, the electrode resistivity / DCR of this comparative example is low, which makes the rate of sodium precipitation higher, but once sodium precipitation occurs, the amount of sodium loss will be more significant. Therefore, the sodium ion battery of Comparative Example 2 has an increase in sodium precipitation relative to Comparative Example 1, but also an increase relative to Example 1 of the present application. It shows that when the additives in the negative electrode material layer are evenly distributed, the effect on the uneven distribution of sodium ion deposition in the inner and outer layers is small, and the elimination force is insufficient. Therefore, it will also cause the problem of excessive sodium ion deposition in the outer layer, affecting the amount of sodium precipitation.

[0129] The relevant test data of Examples 1 to 3 show that when the content of the additive in the inner material layer is different, it will have a certain impact on the capacity and capacity retention rate of the sodium ion battery.

[0130] The relevant test data of Example 1 and Example 4 show that when the outer material layer also contains additives, the capacity retention rate of the sodium ion battery will also be increased, but it will have a certain impact on the capacity of the sodium ion battery.

[0131] The relevant test data of Example 1 and Examples 5-7 show that, among the additives, the selection of the conductive material of the coating layer has basically no effect on the cycle performance of the sodium ion battery. The selection of the ion conductor material of the inner core has basically no effect on the cycle performance of the sodium ion battery. Combined with Table 2, in Example 5, when the carbon material of the coating layer is changed, the sodium precipitation performance is improved. The sodium ion batteries of Examples 6-7 have a slightly increased amount of sodium precipitation relative to Example 1. This shows that different materials of additives have a certain impact on the sodium precipitation problem of sodium ion batteries.

[0132] The relevant test data of Example 1 and Example 8 show that when the thickness of the inner material layer and the outer material layer is different, there will be a certain impact on the rated capacity of the sodium ion battery. As the thickness of the inner material layer decreases, the capacity retention rate of the sodium ion battery tends to decrease. Combined with Table 2, the ratio of the thickness of the inner material layer to the thickness of the outer material layer needs to be controlled within an appropriate range. Otherwise, the inner material layer is too thin, and the effect on the transmission of sodium ions in the overall negative electrode material layer is too small, and it does not play a decisive role. If the outer material layer is too thick, it will have an adverse effect on the diffusion of sodium ions, hinder the transmission of sodium ions, and also cannot solve the sodium precipitation problem of the sodium ion battery.

[0133] Based on the same technical concept, an embodiment of the present application provides a battery pack comprising multiple batteries according to the embodiments of the present application. The multiple batteries can be connected in series or in parallel via external connectors. Since the battery pack includes all the technical features of the battery, it also includes all the beneficial effects of the battery, which will not be elaborated here.

[0134] Based on the same technical concept, an embodiment of the present application provides an energy storage system. Figure 5 This is a schematic diagram of the connection structure of an energy storage system. Figure 5 As shown, the energy storage system includes the battery pack 100 and the power converter 200 of the present application. The power converter 200 is used to convert the voltage output by the battery pack 100 into power and output it to the power grid 300 or the external load 400, and / or the power converter 200 is used to convert the voltage output by the external power supply into power and output it to the battery pack 100. The battery pack 100 can be connected to the photovoltaic module 500, and the photovoltaic module 500 is used to charge the battery pack 100. Since the energy storage system includes the battery pack 100, the battery pack 100 includes all the technical features of the battery. Therefore, the energy storage system also includes all the beneficial effects of the battery, which will not be repeated here.

[0135] The above are only specific embodiments of the present application, but the scope of protection of this application is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in this application should be included in the scope of protection of this application. Therefore, the scope of protection of this application should be based on the scope of protection of the claims.

Claims

1. A sodium ion battery, characterized in that It includes a positive electrode sheet and a negative electrode sheet, the negative electrode sheet includes a negative electrode collector and an inner material layer provided on the surface of at least one side of the negative electrode collector and an outer material layer provided on the surface of the inner material layer; the inner material layer and the outer material layer both contain negative electrode active materials, the mass fraction of the additive in the inner material layer is greater than 0, the mass fraction of the additive in the outer material layer is ≥0%, the content of the additive in the inner material layer is greater than the content of the additive in the outer material layer, and the ionic conductivity of the additive is greater than the ionic conductivity of the negative electrode active material.

2. The sodium ion battery according to claim 1, characterized in that The difference between the mass fraction of the additive in the inner material layer and the mass fraction of the additive in the outer material layer is ≥3%.

3. The sodium ion battery according to claim 1 or 2, characterized in that The mass fraction of the additive in the outer layer material is ≤2%.

4. The sodium ion battery according to any one of claims 1 to 3, characterized in that The additive includes at least one of Al2O3, Na3PS4, NaSbS4, Na3PSe4, and a sodium superion conductor.

5. The sodium ion battery according to any one of claims 1 to 4, characterized in that The additive includes a core and a coating layer, the coating layer is carbon, and the core includes at least one of Al2O3, Na3PS4, NaSbS4, Na3PSe4, and a sodium superion conductor.

6. The sodium ion battery according to any one of claims 1 to 5, characterized in that The ratio of the median particle size D50 of the additive to the median particle size D50 of the negative electrode active material is 0.01-1.

7. The sodium ion battery according to any one of claims 1 to 6, characterized in that The weight of the additive is 1-15% of the total weight of the negative electrode active material.

8. The sodium ion battery according to any one of claims 1 to 7, characterized in that The ratio of the thickness of the inner material layer to the thickness of the outer material layer is 0.1-10.

9. The sodium ion battery according to any one of claims 1 to 8, characterized in that The negative electrode active material includes at least one of hard carbon, soft carbon, artificial graphite, or natural graphite.

10. A battery pack, characterized in that: The battery pack includes multiple batteries, and the multiple batteries are connected in series or in parallel; the battery is a sodium ion battery as described in any one of claims 1 to 9.

11. An energy storage system, characterized in that: The energy storage system includes a power converter and at least one battery pack as described in claim 10; the power converter is used to convert the voltage output by the battery pack into power and output it to the power grid or load, and / or, convert the voltage output by an external power supply into power and output it to the battery pack.

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