Negative plate and sodium ion battery

Abstract

The invention provides a negative plate and a sodium ion battery. The negative plate comprises a negative current collector and a negative active layer positioned on at least one surface of the negative current collector; the negative active layer comprises hard carbon, a granular fluorine-containing binder and a first conductive agent, and the first conductive agent comprises a multi-walled carbon nanotube; the Dv50 of the hard carbon and the average particle size D of the granular fluorine-containing binder meet the condition that DV50 / D is more than or equal to 30 and less than or equal to 60; the length-diameter ratio d of the multi-walled carbon nanotube is less than or equal to 200. When the negative plate is applied to the battery, a stable and uniform SEI film can be formed, the interface impedance of the negative plate is reduced, and the probability of side reaction between the negative plate and electrolyte is reduced, so that the low-temperature discharge performance and the high-temperature cycle performance of the battery are improved.

Description

Technical Field

[0001] This invention relates to the field of electrochemical technology, specifically to a negative electrode and a sodium-ion battery. Background Technology

[0002] With the rapid development of the new energy vehicle industry, the demand for lithium-ion batteries is also increasing. However, global lithium resources are limited and unevenly distributed. Therefore, the development of new, high-performance, and low-cost alternative energy storage devices has attracted much attention. Sodium-ion batteries have a similar working principle to lithium-ion batteries, and sodium resources are far more abundant in the Earth's crust than lithium resources. Therefore, sodium-ion batteries are considered a potential alternative to lithium-ion batteries.

[0003] The negative electrode active material is one of the key factors affecting sodium-ion batteries. Currently, hard carbon is the commonly used negative electrode active material for sodium-ion batteries. However, existing hard carbon has poor conductivity, resulting in an uneven SEI film on the surface of the negative electrode active layer. This leads to a high interfacial impedance of the negative electrode sheet, and the SEI film is prone to partial rupture during cycling, resulting in an increase in electrolyte side reactions at the interface and severe gas generation in the battery. Summary of the Invention

[0004] In view of this, the present invention provides a negative electrode sheet that, when applied to a battery, can form a stable and uniform SEI film, reduce the interfacial impedance of the negative electrode sheet, and reduce the probability of side reactions between the negative electrode sheet and the electrolyte, thereby improving the low-temperature discharge performance and high-temperature cycle performance of the battery.

[0005] The present invention provides a sodium-ion battery including the above-mentioned negative electrode, which has excellent high and low temperature performance (low temperature discharge performance and high temperature cycle performance).

[0006] In detail, in a first aspect, the present invention provides a negative electrode sheet, wherein the negative electrode sheet includes a negative electrode current collector and a negative electrode active layer located on at least one surface of the negative electrode current collector;

[0007] The negative electrode active layer includes hard carbon, particulate fluorinated binder and a first conductive agent, wherein the first conductive agent includes multi-walled carbon nanotubes.

[0008] The Dv50 of the hard carbon and the average particle size D of the granular fluorinated binder satisfy: 30 ≤ D V 50 / D≤60;

[0009] The aspect ratio d of the multi-walled carbon nanotubes is ≤200.

[0010] The negative electrode as described above, wherein the negative electrode satisfies at least one of the following conditions:

[0011] a、40≤D V50 / D≤50;

[0012] b, d ≤ 100;

[0013] c. The granular fluorinated binder is at least one of polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl fluoride, polyhexafluoropropylene, fluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer, and tetrafluoroethylene-hexafluoropropylene copolymer.

[0014] In the negative electrode sheet as described above, in the negative electrode active layer, at least a portion of the particulate aqueous binder is located on at least a portion of the surface of the hard carbon;

[0015] The coverage S of the particulate fluorinated binder on the surface of a single hard carbon cell satisfies: 80 ≤ S ≤ 400;

[0016] Preferably, 150 ≤ S ≤ 250.

[0017] The negative electrode sheet as described above, wherein the average particle size D of the particulate fluorinated binder satisfies: 50nm≤D≤350nm;

[0018] Preferably, 100nm≤D≤250nm.

[0019] The negative electrode sheet as described above, wherein the negative electrode active layer further includes a second conductive agent, the second conductive agent being selected from at least one of acetylene black and furnace black;

[0020] Preferably, the average particle size D1 of the second conductive agent satisfies: 30nm≤D1≤200nm.

[0021] The negative electrode as described above, wherein 3μm≤D V 50≤12μm.

[0022] The negative electrode as described above, wherein the hard carbon D V 50 with the D of the hard carbon V 90 satisfies: 1≤D V 90 / D V 50≤6; and / or,

[0023] The BET of the hard carbon is 0.3m. 2 / g~8m 2 / g.

[0024] The negative electrode sheet described above has a compaction density PD of 0.75 g / cm³. 3 ≤PD≤1.45g / cm 3 ; and / or,

[0025] The porosity of the negative electrode active layer is 20% to 70%.

[0026] The present invention provides a sodium-ion battery, wherein the negative electrode sheet is as described above.

[0027] The sodium-ion battery described above further includes an electrolyte comprising propylene sulfite and vinylene carbonate.

[0028] In the sodium-ion battery described above, the mass percentage of propylene sulfite in the electrolyte is η1, where 1.5% ≤ η1 ≤ 5%; and / or,

[0029] In the electrolyte, the mass percentage of vinylene carbonate is η2, where 0.1% ≤ η2 ≤ 1%.

[0030] In the sodium-ion battery described above, 50nm≤D≤200nm, 3%≤η1≤5%, 0.5%≤η2≤1%; or,

[0031] 200nm≤D≤350nm, 1.5%≤η1≤3%, 0.1%≤η2≤0.5%.

[0032] In the sodium-ion battery described above, the electrolyte further includes ethyl methyl carbonate and propylene carbonate.

[0033] In the sodium-ion battery described above, the electrolyte contains ethyl methyl carbonate at a mass percentage of η3 and propylene carbonate at a mass percentage of η4.

[0034] 0.46≤D / 100(η3+η4)≤5;

[0035] Preferably, η3 < η4, 30% ≤ η3 ≤ 50%, and 40% ≤ η4 ≤ 60%.

[0036] The sodium-ion battery described above further includes a positive electrode sheet, which comprises a positive current collector and a positive active layer located on at least one surface of the positive current collector, the positive active layer comprising a positive active material.

[0037] The positive electrode active material includes at least one of layered oxides, polyanionic compounds, Prussian blue compounds, and sodium titanium oxides.

[0038] In the negative electrode sheet provided by this invention, the Dv50 of the hard carbon and the average particle size D of the particulate fluorinated binder in the negative electrode active layer satisfy: 30 ≤ D VThe aspect ratio (d) of the multi-walled carbon nanotubes is 50 / D≤60, and the aspect ratio (d) of the multi-walled carbon nanotubes is ≤200. In this negative electrode active layer, the multi-walled carbon nanotubes, granular fluorinated binder, and hard carbon are more uniformly dispersed. The granular fluorinated binder contains abundant CF bonds, which exhibit superior swelling properties with the electrolyte and better affinity for it. When applied to batteries, this allows for a more uniform distribution of the electrolyte within the negative electrode, particularly on the surface of the hard carbon, forming a more uniform SEI film. This avoids the phenomenon of impeded metal ion transport and increased local potential caused by an uneven SEI film, thereby preventing the deposition of metal ions on the negative electrode surface and improving the battery's high and low temperature performance.

[0039] The sodium-ion battery provided by the present invention includes the above-mentioned negative electrode sheet, and the sodium-ion battery has excellent high and low temperature performance. Attached Figure Description

[0040] Figure 1 This is a surface SEM image of the negative electrode sheet in Embodiment 1 of the present invention at a certain magnification.

[0041] Figure 2 This is a surface SEM image of the negative electrode sheet in Embodiment 1 of the present invention at another magnification. Detailed Implementation

[0042] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0043] In sodium-ion batteries, the negative electrode active material is usually hard carbon. However, hard carbon has poor conductivity, which makes the SEI film formed on the surface of the negative electrode uneven. This results in a large interfacial impedance of the negative electrode, which is detrimental to the low-temperature performance of the battery. Furthermore, the uneven SEI film is prone to rupture during battery cycling, leading to an increase in side reactions between the electrolyte and the negative electrode, which is detrimental to the high-temperature performance of the battery.

[0044] In view of this, the present invention provides a negative electrode sheet. By regulating the composition and structure of the negative electrode active layer in the negative electrode sheet, the sizes of hard carbon, particulate fluorinated binder and multi-walled carbon nanotubes in the negative electrode active layer meet a specific relationship. This allows the negative electrode sheet to form a stable SEI film when applied to a battery, reducing the interfacial impedance of the negative electrode sheet and the probability of side reactions between the negative electrode sheet and the electrolyte, thereby improving the high and low temperature performance of the battery.

[0045] A first aspect of the present invention provides a negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector and a negative electrode active layer located on at least one surface of the negative electrode current collector;

[0046] The negative electrode active layer includes hard carbon, particulate fluorinated binder and a first conductive agent, the first conductive agent including multi-walled carbon nanotubes;

[0047] The average particle size D of the hard carbon Dv50 and the granular fluorinated binder satisfies: 30 ≤ D V 50 / D≤60;

[0048] The aspect ratio d of multi-walled carbon nanotubes is ≤200.

[0049] In this invention, multi-walled carbon nanotubes in the negative electrode active layer can increase the ion transport channels in the negative electrode sheet, thereby improving the ion transport performance of the negative electrode sheet. When the aspect ratio of the multi-walled carbon nanotubes is ≤200, the ion transport performance of the negative electrode sheet can be improved while ensuring the mechanical properties of the negative electrode active layer. Furthermore, when the Dv50 of the hard carbon and the average particle size D of the particulate fluorinated binder satisfy: 30 ≤ D... V A DV / D ratio of 50 / D ≤ 60 allows for more uniform dispersion of hard carbon, multi-walled carbon nanotubes, and granular fluorinated binders in the negative electrode active layer. The granular fluorinated binder contains abundant CF bonds, which exhibit superior swelling properties with the electrolyte and better affinity for it. When applied to batteries, this allows for more uniform electrolyte distribution within the negative electrode, particularly on the surface of the hard carbon, forming a more uniform SEI film. This prevents the obstruction of metal ion (e.g., sodium ion) transport due to an uneven SEI film, which increases local potential and thus avoids the deposition of metal ions (e.g., sodium ions) on the negative electrode surface, improving the battery's high and low temperature performance. When DV50 / D ≤ 30, the average particle size of the granular fluorinated binder is too large relative to the active particles, potentially leading to excessive electrolyte concentration in localized areas on the active particle surface. This results in an excessively thick SEI film in certain regions, obstructing metal ion transport, increasing local overpotential, and subsequently causing uneven metal deposition, metal precipitation, and reduced high-temperature performance. When 60≤DV50 / D, the average particle size of the granular fluorinated binder is too small relative to the active particles, resulting in a small contact area between the binder and the active material. Consequently, the exposed area of ​​the negative electrode active material is large, which has a relatively small effect on promoting SEI film formation and cannot improve the uniformity of SEI film formation.

[0050] In this invention, after discharging a battery (e.g., a sodium-ion battery) to 0% SOC, the negative electrode is disassembled and removed. It is then soaked in dimethyl carbonate (DMC) solvent at 60°C for 5 hours, followed by rinsing with DMC to remove salts (e.g., sodium salts) adhering to the negative electrode. After calcination in a tube furnace at 500°C for 3 hours under a nitrogen atmosphere, the residual material of the negative electrode active layer is gently scraped off from the surface of the negative electrode current collector to obtain a composite material. The composite material is measured using a Malvern particle size analyzer. The testing steps are as follows: The composite material is dispersed in deionized water containing a dispersant (e.g., nonylphenol polyoxyethylene ether, content 0.02–0.03 wt%) to form a mixture. The mixture is ultrasonicated for 2 minutes and then placed in a Malvern particle size analyzer for testing to obtain the D... V 50.

[0051] In some implementations, SEM can be used to observe the surface of the electrode to obtain a surface SEM image of the electrode. Different regions can be selected in the surface SEM image of the electrode, and the average particle size of the fluorinated PVDF can be measured separately. The average value can then be calculated. For example, 10 regions can be measured and the average value can then be calculated.

[0052] In some implementations, the surface of the electrode can be observed using SEM to obtain a surface SEM image of the electrode. Different regions can be selected in the surface SEM image of the electrode, and the length and diameter of the multi-walled carbon nanotubes can be measured to obtain the aspect ratio. Then, the average value can be calculated. For example, 10 regions can be measured and the average value can be calculated.

[0053] The particulate fluorinated binder of the present invention can be any particulate fluorinated binder commonly used in the art. Exemplarily, the particulate fluorinated binder can be at least one selected from polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl fluoride, polyhexafluoropropylene, fluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer, and tetrafluoroethylene-hexafluoropropylene copolymer. In particular, the particulate PVDF contains abundant CF bonds, which exhibit superior swelling properties with the electrolyte and better affinity for the electrolyte, thereby further improving the ion-conducting capacity of the negative electrode.

[0054] The inventors discovered in their research that when 40 ≤ D V When 50 / D≤50, multi-walled carbon nanotubes, particulate fluorinated binders, and hard carbon can be dispersed more uniformly, thereby forming a more uniform and dense SEI film, reducing the interfacial impedance of the negative electrode and the probability of side reactions between the negative electrode and the electrolyte, thus improving the high and low temperature performance of the battery.

[0055] Furthermore, when d≤100, the ion transport performance of the negative electrode can be further improved while ensuring the mechanical properties of the negative electrode.

[0056] In some embodiments of the present invention, in the negative electrode active layer, at least a portion of the particulate fluorinated binder is located on at least a portion of the surface of the hard carbon;

[0057] The coverage S of the particulate fluorinated binder on a single hard carbon surface satisfies: 80≤S≤400.

[0058] In this invention, the particulate fluorinated binder can be located on the entire surface of the hard carbon or on a portion of its surface. The coverage number S of the particulate fluorinated binder on the surface of a single hard carbon refers to the number of particulate fluorinated binders in contact with the surface of the hard carbon. In some embodiments, the surface morphology of the negative electrode can be observed by scanning electron microscopy, and the number of particulate fluorinated binder particles on one side of a single hard carbon particle can be counted. This number is then multiplied by 6 (the six sides of a graphite particle) to obtain the S1 of that hard carbon particle. The number of PVDF particles on the surface of a total of 10 hard carbon particles is counted and denoted as S1, S2, S3...S10, respectively. Finally, S = (S1 + S2... + S10) / 10 is obtained.

[0059] When S meets the above range, the fluorinated binder can be more uniformly dispersed on the surface of hard carbon, improving the mechanical properties of the negative electrode active layer, and can make the electrolyte more uniformly wet the negative electrode sheet, forming a stable SEI film, thereby improving the high and low temperature performance of the battery.

[0060] Preferably, 150 ≤ S ≤ 250.

[0061] Furthermore, when the average particle size D of the particulate fluorinated binder satisfies 50nm ≤ D ≤ 350nm, the particulate aqueous binder is less prone to agglomeration, and the contact area between the particulate aqueous binder and the hard carbon is suitable, resulting in an appropriate exposed hard carbon area. This allows for the formation of a uniform, stable, and appropriately thick SEI film, thereby improving the battery's low-temperature discharge performance and high-temperature cycling performance. Preferably, 150nm ≤ D ≤ 250nm.

[0062] In some embodiments of the present invention, the negative electrode active layer further includes a second conductive agent. When the second conductive agent is selected from at least one of acetylene black and furnace black, the first conductive agent and the second conductive agent in the negative electrode active layer can form a rich conductive network with the particulate fluorinated binder, further improving the conductivity of the negative electrode sheet. In addition, the network formed by the first conductive agent, the second conductive agent and the particulate fluorinated binder can provide attachment sites for the negative electrode SEI film, promote the formation of a stable and uniform SEI film, and thus improve the high and low temperature performance of the battery.

[0063] Furthermore, when the average particle size D1 of the second conductive agent satisfies 30nm ≤ D1 ≤ 200nm, the conductivity of the negative electrode can be further improved, and a more uniform and stable SEI film can be formed, thereby improving the high and low temperature performance of the battery. In some embodiments, SEM testing can be performed on the surface of the electrode to obtain a surface SEM image of the electrode. Ten different second conductive agents can be selected from the SEM image, and the average particle size of each second conductive agent can be obtained. Finally, the average value is calculated to obtain D1.

[0064] The inventors discovered in their research that when 3μm≤D V When 50 ≤ 12 μm, the ion transport performance of the negative electrode can be further improved while ensuring the stability of the negative electrode, thereby forming a more uniform SEI film and improving the high and low temperature performance of the battery.

[0065] In some embodiments of the present invention, the D of hard carbon V 50 and hard carbon D V 90 satisfies: 1≤D V 90 / D V 50≤6 indicates that the hard carbon particles in the negative electrode active layer are uniform in size, which helps to form a more uniform SEI film.

[0066] Furthermore, the BET of hard carbon is 0.3m. 2 / g~8m 2 At a ratio of / g, the electrochemical performance of the battery can be improved while ensuring the mechanical properties of the negative electrode.

[0067] In this invention, after discharging a battery (e.g., a sodium-ion battery) to 0% SOC, the negative electrode is disassembled and removed. It is then soaked in dimethyl carbonate (DMC) solvent at 60°C for 5 hours, followed by rinsing with DMC to remove adhering salts (e.g., sodium salts). After calcination in a tube furnace at 500°C for 3 hours under a nitrogen atmosphere, the residual material in the negative electrode active layer is gently scraped off from the surface of the negative electrode current collector to obtain a composite material. The material is then measured using a Malvern particle size analyzer. The testing steps are as follows: The composite material is dispersed in deionized water containing a dispersant (e.g., nonylphenol polyoxyethylene ether, content 0.02–0.03 wt%) to form a mixture. The mixture is ultrasonicated for 2 minutes and then placed in a Malvern particle size analyzer for testing, yielding a DV90. The specific surface area of ​​the composite material is obtained by testing with a nitrogen adsorption surface area analyzer.

[0068] The inventors also discovered that when the compaction density PD of the negative electrode sheet satisfies: 0.75 g / cm³ 3 ≤PD≤1.45g / cm 3 At the same time, the energy density of the battery can be further improved while ensuring its high and low temperature performance.

[0069] When the porosity of the negative electrode active layer is 20% to 70%, the electrolyte wettability of the negative electrode can be improved while ensuring the mechanical properties of the negative electrode, thereby forming a more uniform and stable SEI film and improving the high and low temperature performance of the battery.

[0070] The present invention also provides a method for preparing the above-mentioned negative electrode sheet, specifically comprising: mixing hard carbon, conductive agent, particulate fluorinated binder, solvent and dispersant evenly to obtain a negative electrode active slurry;

[0071] A negative electrode active slurry is coated onto at least one surface of a negative electrode current collector, and after drying, a negative electrode sheet including a negative electrode active layer is obtained.

[0072] The negative electrode current collector can be any one of single-sided aluminum foil, double-sided aluminum foil, porous aluminum foil, or carbon-coated aluminum foil.

[0073] The conductive agent includes a first conductive agent and a second conductive agent;

[0074] The dispersant is sodium carboxymethyl cellulose. Sodium carboxymethyl cellulose can improve the dispersion uniformity of the negative electrode active slurry, thereby obtaining a negative electrode sheet with uniform hard carbon dispersion.

[0075] The present invention can further improve the electrochemical performance of the battery by selecting the content of each component in the negative electrode active layer. For example, in some embodiments, the mass ratio of the first conductive agent, the second conductive agent, and the particulate fluorinated binder in the negative electrode active layer is (0.2–8):(0.1–1.5):(0.5–5). Preferably, the mass ratio of the first conductive agent, the second conductive agent, and the particulate fluorinated binder in the negative electrode active layer is (1–5):(0.2–1.2):(1–2.5).

[0076] In some embodiments, the mass ratio of hard carbon, first conductive agent, second conductive agent, sodium carboxymethyl cellulose, and particulate PVDF in the negative electrode active layer is (88-95):(1-5):(0.2-1.2):(0.6-1.5):(1-2.5).

[0077] In some embodiments, the negative electrode active layer further includes a second binder, which may be polyacrylic acid, acrylic acid-acrylonitrile copolymer, polybutyl acrylate, polyethyl acrylate, polybutyl methacrylate, polymethyl methacrylate, methyl methacrylate-styrene copolymer, acrylate-acrylonitrile copolymer, or butadiene-isobutyl acrylate copolymer. When a second binder is included, the bonding strength and cohesion between the active particles can be further improved, preventing the active particles from loosening and falling off, thereby further enhancing the safety performance of the battery.

[0078] In the negative electrode active layer, the mass ratio of hard carbon, first conductive agent, second conductive agent, sodium carboxymethyl cellulose, particulate PVDF, and second binder is (88-95):(1-5):(0.2-1.2):(0.6-1.5):(1-2.5):(0-1.2). For example, in the negative electrode active layer, the mass ratio of hard carbon, furnace black, multi-walled carbon nanotubes, sodium carboxymethyl cellulose, particulate PVDF, and polyacrylic acid-acrylonitrile copolymer is 92.2:4:0.8:0.6:1.5:0.9.

[0079] A second aspect of the present invention provides a sodium-ion battery, comprising the negative electrode sheet of the first aspect.

[0080] It is understood that the sodium-ion battery of the present invention also includes a positive electrode, a separator, an electrolyte, and an outer packaging. In the present invention, a sodium-ion battery can be prepared by a method including the following steps: stacking a positive electrode, a separator, and a negative electrode to form an electrode assembly, or stacking a positive electrode, a separator, and a negative electrode and then winding them to form an electrode assembly; then placing the electrode assembly in an outer packaging; injecting an electrolyte into the outer packaging; and finally sealing and forming the battery to form a sodium-ion battery.

[0081] The sodium-ion battery of the present invention, having a negative electrode sheet in the first aspect, exhibits excellent low-temperature discharge performance and high-temperature cycle performance.

[0082] In some embodiments of the present invention, the positive electrode sheet includes a positive current collector and a positive active layer located on at least one surface of the positive current collector, the positive active layer comprising a positive active material.

[0083] The positive electrode active material includes at least one of layered oxides, polyanionic compounds, Prussian blue compounds, and sodium titanium oxides.

[0084] It is understandable that the positive electrode active layer can be located on one surface of the positive electrode current collector to form a positive electrode sheet, or the positive electrode active layer can be located on both surfaces of the positive electrode current collector to form a positive electrode sheet.

[0085] The aforementioned positive electrode can be better matched with the negative electrode in the first aspect, enabling the sodium-ion battery to have both excellent low-temperature discharge performance and high-temperature cycle performance.

[0086] In this invention, the layered oxide may include NaCoO2, NaNiO2, NaFeO2, and layered oxides containing doped elements (e.g., Na...). 0.90 Fe 0.25 Ni 0.25 Mn 0.45 Zn 0.05At least one of O2); polyanionic compounds may include sodium iron phosphate (NaFePO4) and / or sodium vanadium phosphate (Na3V2(PO4)3); Prussian blue compounds may include Na3Fe(CN)6; sodium titanium oxides may include Na2Ti3O7.

[0087] It is understood that the positive electrode active layer also includes a conductive agent and a binder. The conductive agent can be at least one of conductive carbon black, conductive graphite, multi-walled carbon nanotubes, single-walled carbon nanotubes, conductive carbon fiber, and graphene; the binder can be at least one of polyvinylidene fluoride (PVDF) and polyimide.

[0088] The positive current collector can be either aluminum foil or carbon-coated aluminum foil.

[0089] In some embodiments, the positive electrode sheet is prepared by a method including the following steps: mixing a positive electrode active material, a conductive agent and a binder to form a positive electrode active slurry;

[0090] The positive electrode active slurry is coated onto the positive electrode current collector, and after drying, a positive electrode sheet including the positive electrode active layer is obtained.

[0091] In some embodiments of the present invention, the electrolyte includes propylene sulfite (PS) and vinylene carbonate (VC). PS and VC can further promote the formation of the SEI film and can prevent excessive solvent in the electrolyte from embedding into the gaps of the negative electrode active layer, thus ensuring the high-temperature cycle performance of the sodium-ion battery.

[0092] The use of these two additives can help the SEI film form and prevent excessive PC from being embedded in the interlayer pores of activated carbon, thus ensuring the high-temperature performance of sodium-ion batteries.

[0093] The present invention can also select the mass percentage of PS and VC in the electrolyte to better match the electrolyte with the negative electrode, thereby further improving the electrochemical performance of sodium-ion batteries.

[0094] In some embodiments of the present invention, the mass percentage of propylene sulfite in the electrolyte is η1, where 1.5% ≤ η1 ≤ 5%; and / or,

[0095] When the mass percentage of vinylene carbonate in the electrolyte is η2, and 0.1%≤η2≤1%, it can work synergistically with the negative electrode to promote the formation of the SEI film and improve the high-temperature cycle performance of sodium-ion batteries while ensuring the ion transport capacity of the electrolyte.

[0096] In particular, when 50nm≤D≤200nm, 3%≤η1≤5%, and 0.5%≤η2≤1%, or when 200nm≤D≤350nm, 1.5%≤η1≤3%, and 0.1%≤η2≤0.5%, the electrolyte can be better matched with the fluorinated binder to promote the formation of a stable and uniform SEI film and improve the high-temperature cycle performance of sodium-ion batteries. Specifically, when 50nm≤D≤200nm, the particle size of the particulate fluorinated binder is relatively small. Under the same S condition, the contact area between the particulate fluorinated binder and hard carbon is small, resulting in a larger exposed area of ​​hard carbon. Appropriately increasing the content of PS and VC can form a sufficient and dense SEI film, reducing the risk of high-temperature gas generation in sodium-ion batteries and thus improving the high-temperature cycle performance of sodium-ion batteries. Conversely, when 200nm≤D≤350nm, the particle size of the particulate fluorinated binder is relatively large, resulting in a larger contact area with hard carbon, which can promote the formation of the SEI film. Adding a small amount of PS and VC can obtain a thin and dense SEI film, preventing the SEI film from being too thick and thus improving the high-temperature cycle performance of sodium-ion batteries.

[0097] In some embodiments of the present invention, when the electrolyte further includes ethyl methyl carbonate (EMC) and propylene carbonate (PC), the electrolyte can be better matched with the particulate fluorinated binder in the negative electrode active layer, thereby improving the affinity of the negative electrode active layer for the electrolyte and thus improving the low-temperature discharge performance of the sodium-ion battery.

[0098] Furthermore, when the mass percentage of ethyl methyl carbonate in the electrolyte is η3 and the mass percentage of propylene carbonate is η4, 0.46≤D / 100(η3+η4)≤5. When D, η3, and η4 satisfy the above relationship, EMC and PC can be matched with the particulate fluorinated binder. On the one hand, this promotes the uniformity of SEI film formation, improves the electrolyte wetting uniformity of the negative electrode, reduces uneven sodium deposition caused by uneven sodium ion transport, reduces the formation of sodium dendrites, and thus reduces the risk of short circuits and thermal runaway. On the other hand, it optimizes the thickness and thermal stability of the SEI film. Consequently, the sodium-ion battery exhibits superior low-temperature discharge performance and high-temperature cycling performance.

[0099] Furthermore, when η3 < η4, 30% ≤ η3% ≤ 50%, and 40% ≤ η4% ≤ 60%, the polarity of the electrolyte can be better improved. According to the principle of like dissolves like, electrolytes with higher polarity have a stronger affinity with particulate fluorinated binders, which can improve the electrolyte wetting performance of the negative electrode and thus improve the low-temperature discharge performance of sodium-ion batteries. In particular, the polar CF bonds in particulate PVDF have a stronger affinity with highly polar electrolytes, which can further improve the low-temperature discharge performance of sodium-ion batteries.

[0100] The inventors also discovered that when 50nm ≤ D ≤ 350nm, and η3 < η4, 30% ≤ η3 ≤ 50%, and 40% ≤ η4 ≤ 60%, sodium-ion batteries can achieve superior low-temperature discharge performance and high-temperature cycle performance. This is because, at this concentration, the particulate aqueous binder is less prone to agglomeration, and the contact area between the particulate aqueous binder and hard carbon is suitable, resulting in an appropriate exposed hard carbon area. This allows for the formation of a uniform, stable, and appropriately thick SEI film. Furthermore, the electrolyte containing the aforementioned concentrations exhibits superior wettability of the negative electrode, promoting uniform ion transport in the negative electrode and improving the low-temperature discharge performance of the sodium-ion battery. Additionally, at high temperatures, the probability of side reactions between the negative electrode and the electrolyte decreases, improving the high-temperature cycle performance of the sodium-ion battery.

[0101] In a specific embodiment, the electrolyte contains PC, DEC, and EMC as solvents, and the mass ratio of PC, DEC, and EMC is 50:10:40. The electrolyte also includes, by mass percentage: VC 0.3%, PS 2.5%, DTD 2%, NaPO2F2 0.5%, NaFSI 7%, and NaPF6 6%.

[0102] The present invention will be further described below with reference to specific embodiments:

[0103] Example 1

[0104] The battery in this embodiment is prepared by a method including the following steps:

[0105] 1) Preparation of positive electrode sheet

[0106] Na, a layered metal oxide cathode material 0.90 Fe 0.25 Ni 0.25 Mn 0.45 Zn 0.05 O2, polyvinylidene fluoride (PVDF) binder, conductive carbon black, and multi-walled carbon nanotubes are mixed together, and N-methylpyrrolidone (NMP) is added. The mixture is stirred under the action of a dual planetary stirrer until the mixture becomes homogeneous and fluid, thus obtaining the positive electrode active slurry.

[0107] The positive electrode active slurry is uniformly coated on both surfaces of an aluminum foil with a thickness of 12μm. After drying at 100℃ for 36 hours and vacuum treatment, an electrode sheet is obtained. The electrode sheet is then rolled and cut to obtain a positive electrode sheet including the positive electrode active layer.

[0108] The solid content of the positive electrode active slurry is 60%; in the positive electrode active layer, the mass ratio of positive electrode material, PVDF, conductive carbon black and multi-walled carbon nanotubes is 94:2:3:1.

[0109] 2) Preparation of negative electrode sheet

[0110] Hard carbon, conductive carbon black, multi-walled carbon nanotubes, granular PVDF and sodium carboxymethyl cellulose (mass ratio 93:3:1:1.5:1.5) were mixed evenly, deionized water was added, and the mixture was stirred by a double planetary stirrer to obtain a negative electrode active slurry (the solid content of the negative electrode active slurry was 45%).

[0111] The negative electrode active slurry is coated on both surfaces of the copper foil, dried, and then rolled and die-cut to obtain a negative electrode sheet including the negative electrode active layer.

[0112] 3) Preparation of electrolyte

[0113] In a reaction vessel filled with argon gas and with qualified water and oxygen content, propylene carbonate (PC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), vinylene carbonate (VC), propylene sulfite (PS), and ethylene sulfate (DTD) are mixed evenly. Then, sodium difluorophosphate (NaPO2F2), sodium difluorosulfonamide (NaFSI), and sodium hexafluorophosphate (NaPF6) are added and stirred evenly to prepare the electrolyte.

[0114] The electrolyte contains the solvents PC, DEC, and EMC, with a mass ratio of 50:10:40. The electrolyte also includes, by mass percentage: VC 0.3%, PS 2.5%, DTD 2%, NaPO2F2 0.5%, NaFSI 7%, and NaPF6 6%.

[0115] 4) Preparation of the diaphragm

[0116] Alumina, polyvinylidene fluoride-hexafluoropropylene, and acetone are mixed evenly to obtain a coating slurry. The coating slurry is coated on both surfaces of a polyethylene diaphragm substrate and dried to obtain a diaphragm including an alumina layer.

[0117] In the alumina layer, the mass ratio of alumina to polyvinylidene fluoride-hexafluoropropylene is 8:2;

[0118] 5) Preparation of secondary batteries

[0119] The positive electrode, negative electrode, and separator obtained above are stacked in the order of positive electrode, separator, and negative electrode, and then wound to obtain an electrode assembly.

[0120] The electrode assembly is placed in the outer packaging aluminum foil, and the electrolyte prepared in step 3) is injected into the outer packaging. After vacuum sealing, standing, formation, shaping and sorting, a secondary battery is obtained.

[0121] Other battery parameters are shown in Table 1.

[0122] Examples 2-29, Comparative Examples 1-3

[0123] The preparation methods of the batteries in Examples 2-29 and Comparative Examples 1-3 are basically the same as those in Example 1, with the differences shown in Table 1.

[0124] Performance testing

[0125] The following performance tests were performed on the batteries in the examples and comparative examples, and the results are shown in Table 1.

[0126] 1. SEM testing

[0127] The surface of the negative electrode in Example 1 was subjected to SEM testing to obtain a surface SEM image of the negative electrode. Figure 1 This is a surface SEM image of the negative electrode sheet in Embodiment 1 of the present invention at a certain magnification. Figure 2 This is a surface SEM image of the negative electrode sheet in Embodiment 1 of the present invention at another magnification. Figure 1 and Figure 2 In the diagram, the green arrow marks hard carbon with a Dv50 of 8 μm, the red circle marks particulate PVDF, and the blue rhombus marks multi-walled carbon nanotubes as the first conductive agent. The Dv50 of hard carbon and the average particle size D of the particulate fluorinated binder satisfy the following conditions: DV50 / D is 44.4; the aspect ratio d of the multi-walled carbon nanotubes is 150.

[0128] 2. Electrochemical performance testing

[0129] The charge / discharge capacity is denoted as C1 when the battery is fully charged to the upper limit voltage at room temperature (25℃) and then discharged to the lower limit voltage at 0.2C. The discharge capacity is denoted as C2 when the battery is fully charged to the upper limit voltage at room temperature (25℃) and then discharged to the lower limit voltage at 0.2C at a low temperature (-30℃). The ratio of C2 to C1 is denoted as the low temperature discharge retention rate.

[0130] The residual capacity and recovery capacity of the battery were measured after 60 days at 60℃ and 100% SOC, and the percentage of residual capacity and recovery capacity were calculated.

[0131] Table 1

[0132]

[0133]

[0134]

[0135] As can be seen from Table 1, when the negative electrode sheet of the present invention is applied to a battery, it can improve the low-temperature discharge performance and high-temperature cycle performance of the battery.

[0136] Furthermore, as can be seen from Examples 1, 2, 7, and 21-23, by selecting the coverage number of the particulate fluorinated binder on a single hard carbon surface, the low-temperature discharge performance and high-temperature cycling performance of the battery can be optimized.

[0137] As can be seen from Examples 1, 3, 10, 17 and 20, by selecting the Dv50 of hard carbon and the average particle size D of particulate fluorinated binder, the hard carbon and particulate fluorinated binder in the negative electrode active layer can be dispersed more uniformly, thereby improving the low-temperature discharge performance and high-temperature cycle performance of the battery.

[0138] As can be seen from Examples 17 and 25, and Examples 24 and 26, by selecting the content of EMC and PC in the electrolyte, EMC and PC can be matched with the particulate fluorinated binder, thereby improving the low-temperature discharge performance and high-temperature cycle performance of the battery.

[0139] As can be seen from Examples 1, 5 and 8, by selecting the aspect ratio of multi-walled carbon nanotubes, the low-temperature discharge performance and high-temperature cycling performance of the battery can be improved.

[0140] As can be seen from Examples 1, 6, 15, and 16, by selecting the content of η1 and η2 in the electrolyte, the low-temperature discharge performance and high-temperature cycle performance of the battery can be improved. Furthermore, as can be seen from Examples 1 and 6, matching D with η1 and η2 can further improve the low-temperature discharge performance and high-temperature cycle performance of the battery.

[0141] As can be seen from Examples 1 and 9, by selecting the average particle size of the second conductive agent, the low-temperature discharge performance and high-temperature cycling performance of the battery can be improved.

[0142] As can be seen from Examples 1, 7, and 27-29, through the D... V 90 / D V Selecting 50 can improve the battery's low-temperature discharge performance and high-temperature cycle performance.

[0143] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A negative electrode sheet, characterized in that, The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer located on at least one surface of the negative electrode current collector; The negative electrode active layer includes hard carbon, particulate fluorinated binder and a first conductive agent, wherein the first conductive agent includes multi-walled carbon nanotubes. The Dv50 of the hard carbon and the average particle size D of the granular fluorinated binder satisfy: 30 ≤ D V 50 / D≤60; The aspect ratio d of the multi-walled carbon nanotubes is ≤200.

2. The negative electrode sheet according to claim 1, characterized in that, The negative electrode sheet must satisfy at least one of the following conditions: a、40≤D V 50 / D≤50; b, d ≤ 100; c. The granular fluorinated binder includes at least one of polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl fluoride, polyhexafluoropropylene, fluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer, and tetrafluoroethylene-hexafluoropropylene copolymer; d、3μm≤D V 50≤12μm。 3. The negative electrode sheet according to claim 1 or 2, characterized in that, In the negative electrode active layer, at least a portion of the particulate fluorinated binder is located on at least a portion of the surface of the hard carbon; The coverage S of the particulate fluorinated binder on the surface of a single hard carbon cell satisfies: 80 ≤ S ≤ 400; Preferably, 150 ≤ S ≤ 250; and / or, The average particle size D of the granular fluorinated binder satisfies: 50nm≤D≤350nm; Preferably, 100nm≤D≤250nm.

4. The negative electrode sheet according to any one of claims 1-3, characterized in that, The negative electrode active layer further includes a second conductive agent, which is selected from at least one of acetylene black and furnace black. Preferably, the average particle size D1 of the second conductive agent satisfies: 30nm≤D1≤200nm.

5. The negative electrode sheet according to any one of claims 1-4, characterized in that, The D of the hard carbon V 50 with the D of the hard carbon V 90 satisfies: 1≤D V 90 / D V 50≤6; and / or, The BET of the hard carbon is 0.3m. 2 / g~8m 2 / g; and / or, The compaction density PD of the negative electrode sheet satisfies: 0.75 g / cm³. 3 ≤PD≤1.45g / cm 3 ; and / or, The porosity of the negative electrode active layer is 20% to 70%.

6. A sodium-ion battery, characterized in that, Includes the negative electrode sheet as described in any one of claims 1-5.

7. The sodium-ion battery according to claim 6, characterized in that, It also includes an electrolyte, which comprises propylene sulfite and vinylene carbonate.

8. The sodium-ion battery according to claim 7, characterized in that, In the electrolyte, the mass percentage of propylene sulfite is η1, where 1.5% ≤ η1 ≤ 5%; and / or, In the electrolyte, the mass percentage of vinylene carbonate is η2, where 0.1% ≤ η2 ≤ 1%. Preferably, 50nm ≤ D ≤ 200nm, 3% ≤ η1 ≤ 5%, 0.5% ≤ η2 ≤ 1%; or, 200nm≤D≤350nm, 1.5%≤η1≤3%, 0.1%≤η2≤0.5%.

9. The sodium-ion battery according to any one of claims 7-8, characterized in that, The electrolyte also includes ethyl methyl carbonate and propylene carbonate; Preferably, in the electrolyte, the mass percentage of methyl ethyl carbonate is η3 and the mass percentage of propylene carbonate is η4; 0.46≤D / 100(η3+η4)≤5; Preferably, η3 < η4, 30% ≤ η3 ≤ 50%, and 40% ≤ η4 ≤ 60%.

10. The sodium-ion battery according to claim 9, characterized in that, The battery further includes a positive electrode sheet, which includes a positive current collector and a positive active layer located on at least one surface of the positive current collector, the positive active layer comprising a positive active material. The positive electrode active material includes at least one of layered oxides, polyanionic compounds, Prussian blue compounds, and sodium titanium oxides.

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