Positive plate and sodium ion battery

Abstract

The embodiment of the invention relates to the technical field of batteries, and provides a positive plate and a sodium ion battery, the positive plate comprises a positive current collector and a positive active coating located on at least one surface of the positive current collector; the positive electrode active coating comprises a first active sub-coating and a second active sub-coating, the second active sub-coating is coated on at least one surface of the positive electrode current collector, and the first active sub-coating is coated on the surface of the second active sub-coating; the first active sub-coating comprises a Prussian blue analogue positive electrode material, and the second active sub-coating comprises a layered metal oxide positive electrode material; and the surface resistance of the positive plate is 40 to 1000 m omega. And the needling safety of the sodium ion battery can be effectively improved.

Description

Technical Field

[0001] This application relates to the technical field of batteries, and particularly to a positive electrode sheet and a sodium-ion battery. Background Art

[0002] Due to its advantages such as low cost and abundant raw materials, sodium-ion batteries have become important battery products in the new energy field.

[0003] The positive electrode materials of the positive electrode sheet of sodium-ion batteries mainly include layered metal oxides, polyanionic positive electrode materials, Prussian blue analogs, etc. Among them, layered metal oxides have the highest tap density and are the positive electrode materials with the fastest industrialization speed. However, layered metal oxides have a higher cost compared to other positive electrode materials, and the nail penetration safety of sodium-ion batteries constructed based on positive electrode sheets including layered metal oxides is poor. Therefore, for sodium-ion battery systems based on layered metal oxides, it is urgent to develop work such as cost reduction and safety improvement, which will help to be more quickly launched into the market. Summary of the Invention

[0004] Embodiments of this application provide a positive electrode sheet and a sodium-ion battery, which can effectively improve the nail penetration safety of sodium-ion batteries and reduce the cost of sodium-ion batteries.

[0005] In a first aspect, embodiments of this application provide a positive electrode sheet, including:

[0006] A positive electrode current collector and a positive electrode active coating located on at least one surface of the positive electrode current collector;

[0007] The positive electrode active coating includes a first active sub-coating and a second active sub-coating. The second active sub-coating is coated on at least one surface of the positive electrode current collector, and the first active sub-coating is coated on the surface of the second active sub-coating;

[0008] The first active sub-coating includes a Prussian blue analog positive electrode material, and the second active sub-coating includes a layered metal oxide positive electrode material;

[0009] The surface resistance of the positive electrode sheet is 40 mΩ to 1000 mΩ.

[0010] Optionally, the surface resistance of the positive electrode sheet is 100 mΩ to 600 mΩ.

[0011] Optionally, the general formula of the Prussian blue analog positive electrode material is as follows:

[0012] Na x M y Co z [Fe(CN)6] p ·vH2O;

[0013] Among them, 1 < x < 2, 0.8 < y < 1, 0 < z < 0.2, 0.7 < p < 1, 0.1 < v < 0.8, and M is selected from one or more of iron, manganese, nickel, and copper;

[0014] and / or,

[0015] The chemical general formula of the layered metal oxide cathode material is Na b Ni a1 Fe a2 Mn a3 N c O2;

[0016] Among them, 0.67 ≤ b ≤ 0.95, a1 + a2 + a3 + c = 1, 0 ≤ a2 < 0.3, 0.3 < a3 < 0.6, 0 < c < 0.15, and N is selected from one or more of copper, zinc, titanium, magnesium, aluminum, and zirconium.

[0017] Optionally, the water content of the Prussian blue analogue cathode material at 200 °C is 0.5% - 5%.

[0018] Optionally, the D50 particle size of the Prussian blue analogue cathode material is 0.7 μm - 3 μm;

[0019] and / or,

[0020] The D10 particle size of the Prussian blue analogue cathode material is 0.2 μm - 1 μm;

[0021] and / or,

[0022] The D90 particle size of the Prussian blue analogue cathode material is 2 μm - 6 μm;

[0023] and / or,

[0024] The D50 particle size of the layered metal oxide cathode material is 3 μm - 8 μm;

[0025] and / or,

[0026] The D10 particle size of the layered metal oxide cathode material is 2 μm - 4 μm;

[0027] and / or,

[0028] The D90 particle size of the layered metal oxide cathode material is 15 μm - 25 μm.

[0029] Optionally, the D50 particle size A of the Prussian blue analogue cathode material and the D50 particle size B of the layered metal oxide cathode material satisfy the following formula:

[0030] 0.1 ≤ A / B ≤ 0.75;

[0031] and / or

[0032] The D10 particle size C of the layered metal oxide cathode material and the D90 particle size D of the layered metal oxide cathode material satisfy the following formula:

[0033] 0.1 ≤ C / D ≤ 0.2;

[0034] and / or

[0035] The D10 particle size E of the Prussian blue analogue cathode material and the D90 particle size F of the Prussian blue analogue cathode material satisfy the following formula:

[0036] 0.03 ≤ E / F ≤ 0.3.

[0037] Optionally, the mass ratio of the Prussian blue analogue cathode material to the layered metal oxide cathode material is (10 - 90):(90 - 10).

[0038] Optionally, the thickness of the positive electrode sheet is 90 μm to 200 μm;

[0039] and / or

[0040] The thickness ratio of the first active sub - coating to the second active sub - coating is (1:9):(9:1);

[0041] and / or

[0042] The tap density of the first active sub - coating is 1.4 g / cm 3 to 1.8 g / cm 3 ;

[0043] and / or

[0044] The tap density of the second active sub - coating is 3.0 g / cm 3 to 3.3 g / cm 3 .

[0045] In a second aspect, an embodiment of the present application provides a sodium - ion battery, including the positive electrode sheet according to any one of the first aspect.

[0046] Optionally, the application voltage window of the sodium - ion battery is (3.9 V - 4.1 V) - (1.5 V - 2.2 V).

[0047] The positive electrode sheet and sodium-ion battery provided by the embodiments of the present application can effectively improve the safety of the sodium-ion battery and reduce costs by coating a Prussian blue analogue positive electrode material and a layered metal oxide positive electrode material in the active layer of the positive electrode sheet, through the synergistic effect of the Prussian blue analogue positive electrode material and the layered metal oxide positive electrode material. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 It is a schematic diagram of a positive electrode sheet provided by the embodiments of the present application;

[0049] Figure 2 It is a SEM image of the cross-section of the positive electrode sheet of Embodiment 1 of the present application. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0050] To make the objectives, technical solutions, and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present application. Apparently, the described embodiments are some, but not all, of the embodiments of the present application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present application without creative efforts shall fall within the protection scope of the present application.

[0051] In the embodiments of the present application, terms such as "first" and "second" are used to distinguish identical or similar items with basically the same functions and effects, and do not limit their sequence. Those skilled in the art can understand that terms such as "first" and "second" do not limit the quantity and execution order, and "first" and "second" do not necessarily mean different.

[0052] It should be noted that in the embodiments of the present application, words such as "exemplary" or "for example" are used to represent examples, illustrations, or explanations. Any embodiment or design solution described as "exemplary" or "for example" in the present application should not be construed as being more preferred or having more advantages than other embodiments or design solutions. Exactly, using words such as "exemplary" or "for example" aims to present relevant concepts in a specific manner.

[0053] As mentioned above, currently in sodium-ion batteries, layered metal oxides are mostly used as the positive electrode material of the positive electrode sheet. However, layered metal oxides are prone to oxygen release reactions in the high state of charge (SOC) state, causing the temperature of the battery to rise and leading to thermal runaway. The puncture safety of the battery is poor. Moreover, the material cost of layered metal oxides is high, resulting in a high battery cost. To address the above problems, the embodiments of the present application provide a positive electrode sheet and a battery, which can effectively improve the safety of the sodium-ion battery and reduce costs by jointly using a layered metal oxide positive electrode material and a Prussian blue analogue positive electrode material in the positive electrode sheet.

[0054] In a first aspect, the present application provides a positive electrode sheet, which includes a positive electrode current collector and a positive electrode active coating (which can also be referred to as a positive electrode active material layer) coated on at least one surface of the positive electrode current collector.

[0055] The positive electrode current collector includes two relatively arranged surfaces for coating the positive electrode active coating. In the present application, the positive electrode active coating can be coated only on one surface of the positive electrode current collector, or on both surfaces of the positive electrode current collector simultaneously.

[0056] The present application does not limit the specific material of the positive electrode current collector. For example, it can be aluminum foil or the like.

[0057] As Figure 1 shown, the positive electrode active coating 1 includes a first active sub - coating 11 and a second active sub - coating 12. The second active sub - coating is located between the positive electrode current collector 2 and the first active sub - coating 11, and the first active sub - coating 11 is located above the second active sub - coating 12. It should be understood that Figure 1 is a schematic diagram of the positive electrode active coating being coated only on one surface of the positive electrode current collector.

[0058] The first active sub - coating 11 includes a Prussian blue analogue positive electrode material, and the second active sub - coating 12 includes a layered metal oxide positive electrode material.

[0059] The surface resistance of the positive electrode sheet is 40 mΩ to 1000 mΩ. For example, 40 mΩ, 100 mΩ, 200 mΩ, 300 mΩ, 400 mΩ, 500 mΩ, 600 mΩ, 700 mΩ, 800 mΩ, 900 mΩ, 1000 mΩ, or the range composed of any two of the above values.

[0060] In the present application, coating the first active sub - coating including a Prussian blue analogue on top of the second active sub - coating including a layered metal oxide can reduce the direct contact between the layered metal oxide and the electrolyte and the separator, delay the accumulation and release of heat inside the battery, thereby improving the safety performance of the entire battery cell. Also, the moisture in the Prussian blue analogue structure can improve the safety performance. When the battery cell undergoes thermal runaway and the temperature exceeds 200 °C, the water molecules in the structure will be thermally desorbed, and this process is an endothermic reaction, which can reduce the temperature of the battery cell thermal runaway, thereby effectively improving the safety of the battery. At the same time, the cost of the Prussian blue analogue is relatively low. Introducing the Prussian blue analogue into the positive electrode material can effectively reduce the cost of the positive electrode sheet.

[0061] Moreover, in an electrochemical system, the more electrons are enriched closer to the current collector, and the more ions are enriched farther away from the current collector. Therefore, to enhance the electrochemical reaction, the electronic conductivity should be enhanced near the current collector, while the ionic conductivity should be enhanced on the side far from the current collector. The electronic conductivity of the Prussian blue analogue is less than that of the layered metal oxide, but the ionic conductivity is greater than that of the layered metal oxide. In this application, by disposing the second active sub-coating including the layered metal oxide on the side close to the positive current collector, more conductive paths can be formed, reducing the electron transfer resistance. By disposing the first active sub-coating including the Prussian blue analogue on the side far from the positive current collector, ion conduction can be promoted, thereby improving the rate performance and cycle stability of the battery.

[0062] Therefore, in this application, by adjusting the ratio between the first active sub-coating and the second active sub-coating to adjust the surface resistance of the positive electrode sheet within the above range, a good ion and electron conduction network can be formed, enabling the battery to simultaneously have the characteristics of high rate performance, high cycle stability, high safety and low cost.

[0063] In one embodiment, the surface resistance of the positive electrode sheet is preferably: 100 mΩ to 600 mΩ. For example, the surface resistance of the positive electrode sheet can be 100 mΩ, 150 mΩ, 200 mΩ, 250 mΩ, 300 mΩ, 350 mΩ, 400 mΩ, 450 mΩ, 500 mΩ, 550 mΩ, 600 mΩ, or any range composed of any two of the above values. When the surface resistance of the positive electrode sheet is within the above range, the battery has more excellent discharge rate performance and can meet the application requirements of high-rate discharge.

[0064] In one embodiment, the chemical general formula of the Prussian blue analogue cathode material is as follows:

[0065] Na x M y Co z [Fe(CN)6] p ·vH2O, Formula 1;

[0066] In Formula 1, 1 < x < 2, 0.8 < y < 1, 0 < z < 0.2, 0.7 < p < 1.

[0067] When the Prussian blue analogue structural formula is controlled within the above range, it has a high capacity and voltage; in this structural formula, the redox potential of Co 2+ / Co 3+ is 3.85 V / 3.75 V, which can match the voltage window of the layered metal oxide; if the Co 2+ content is too high, it will lead to an increase in material cost and a decrease in structural stability; when Co 2+ is not contained in the structureWhen the voltage window of the Prussian blue analogue is narrower than that of the layered metal oxide, it is difficult to match the voltage windows of the two. When the positive electrode plate is charged and discharged in the voltage window of the Prussian blue analogue, it is difficult to utilize the capacity of the layered metal oxide in the high potential range (>3.8V). When the positive electrode plate is charged and discharged in the voltage window of the layered metal oxide, gas generation in the battery cell will occur during the cycling process and voltage drops are likely to occur during long-term cycling.

[0068] In Formula 1, M is a transition metal ion selected from at least one of Fe, Mn, Ni, and Cu. Doping with the transition metal M can form a high entropy effect, slow down the volume strain during charge and discharge, and significantly improve its cycle stability.

[0069] In Formula 1, 0.1 < v < 0.8. When the water content in the Prussian blue analogue structural formula is controlled within the above range, the Prussian blue analogue has good structural stability and a long cycle life. When the water content is too high, the specific capacity of the Prussian blue analogue positive electrode decreases and gas generation in the battery cell is likely to occur. When the water content is too low, the structural stability of the Prussian blue analogue positive electrode material is poor, and problems such as dissolution of transition metals result in poor cycle stability.

[0070] In one embodiment, the water content of the Prussian blue analogue positive electrode material at 200°C is 0.5% to 5%. For example, the water content at 200°C can be 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, or any range composed of any two of the above values. When the water is controlled within the above range, the safety performance of the battery cell can also be improved. If the battery cell undergoes thermal runaway and the temperature exceeds 200°C, the water molecules in the structure will be thermally removed, and this process is an endothermic reaction, thus reducing the temperature of the battery cell thermal runaway.

[0071] In one embodiment, the chemical general formula of the layered metal oxide positive electrode material is as follows:

[0072] Na b Ni a1 Fe a2 Mn a3 N c O2, Formula 2;

[0073] In Formula 2, 0.67 ≤ b ≤ 0.95, a1 + a2 + a3 + c = 1, 0 ≤ a2 < 0.3, 0.3 < a3 < 0.6, 0 < c < 0.15. When the structural formula of the layered metal oxide positive electrode material is controlled within the above range, the layered metal oxide can be prevented from undergoing a phase change at high voltages (e.g., 4.0V), so that the layered metal oxide can be charged and discharged in a relatively high voltage window, having a high specific capacity and structural stability.

[0074] In Formula 2, N is selected from one or more of copper, zinc, titanium, magnesium, aluminum, and zirconium. Doping transition metals in the layered metal oxide cathode material can slow down the volume strain and reduce the dissolution of transition metals, thereby enhancing the cycle stability of the battery and reducing the increase in DCR during the cycling process.

[0075] In one embodiment, both the layered metal oxide cathode material and the Prussian blue analogue cathode material are single-crystal materials or materials with a single-crystal core. Compared with polycrystalline materials, single-crystal materials have more excellent high-temperature storage performance and high-temperature cycling performance. Secondly, when polycrystalline materials are arranged at the bottom layer, during the compaction process, polycrystalline grains may break, resulting in an increase in specific surface area, making the battery cell prone to gas generation during the cycling process and deteriorating the performance of the battery cell.

[0076] In one embodiment, the D50 particle size of the Prussian blue analogue cathode material is 0.7 μm - 3 μm. For example, the D50 particle size of the Prussian blue analogue cathode material is 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, or the range composed of any two of the above values. Within this range, the Prussian blue analogue cathode particles have good rate performance and a relatively high tap density.

[0077] In one embodiment, the D10 particle size of the Prussian blue analogue cathode material is 0.2 μm - 1 μm. For example, the D10 particle size of the Prussian blue analogue cathode material is 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1.0 μm, or the range composed of any two of the above values.

[0078] In one embodiment, the D90 particle size of the Prussian blue analogue cathode material is 2 μm - 6 μm. For example, the D90 particle size of the Prussian blue analogue cathode material is 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, 5.5 μm, 6 μm, or the range composed of any two of the above values.

[0079] In one embodiment, the D50 particle size of the layered metal oxide cathode material is 3 μm - 8 μm. For example, the D50 particle size of the layered metal oxide cathode material is 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, or a range composed of any two of the above values. Within this range, the layered metal oxide cathode material has good storage performance and rate performance. When the D50 particle size of the layered metal oxide cathode material is too small, the specific surface area is large, and the residual alkali content at the surface and interface of the cathode material is higher. The residual alkali is prone to side reactions with the electrolyte, and gas generation in the battery cell is likely to occur during storage. When the D50 of the layered metal oxide cathode material is too large, the transport path of sodium ions in the material is longer, and the rate performance of the material is worse.

[0080] In one embodiment, the D50 particle size of the layered metal oxide cathode material is preferably 3 μm - 6 μm, and the layered metal oxide cathode material has both high-temperature storage performance and rate performance.

[0081] In one embodiment, the D10 particle size of the layered metal oxide cathode material is 2 μm - 4 μm. For example, the D10 particle size of the layered metal oxide cathode material is 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, or a range composed of any two of the above values. Controlling the D10 particle size of the layered metal oxide cathode material within the above range can reduce the side reaction between the electrolyte and the cathode material during storage, thereby better improving the chemical stability of the battery cell and further extending the storage life of the battery.

[0082] In one embodiment, the D90 particle size of the layered metal oxide cathode material is 15 μm - 25 μm. For example, the D90 particle size of the layered metal oxide cathode material is 15 μm, 17 μm, 19 μm, 20 μm, 21 μm, 23 μm, 25 μm, or a range composed of any two of the above values. Controlling the D90 particle size of the layered metal oxide cathode material within the above range can reduce the diffusion path of sodium ions and has good rate performance.

[0083] In one embodiment, the D50 particle size A of the Prussian blue analogue cathode material and the D50 particle size B of the layered metal oxide cathode material satisfy the following formula:

[0084] 0.1 ≤ A / B ≤ 0.75;

[0085] When the D50 particle size A of the Prussian blue analogue cathode material and the D50 particle size of the layered metal oxide cathode material satisfy the above relationship, the particle size matching effect between the Prussian blue analogue cathode material and the layered metal oxide cathode material can be improved, the porosity of the upper layer of the electrode sheet is greater than that of the lower layer, the electrolyte infiltration ability of the composite cathode electrode sheet can be enhanced, and thus the fast charging performance can be improved.

[0086] In one embodiment, the D10 particle size C and the D90 particle size D of the layered metal oxide cathode material satisfy the following formula:

[0087] 0.1 ≤ C / D ≤ 0.2;

[0088] Since controlling the D10 particle size of the layered metal oxide cathode material can improve the high-temperature storage life of the battery, and controlling the D90 particle size of the layered metal oxide cathode material can improve the rate performance of the battery, therefore, controlling C / D within the above range can balance the high-temperature storage life and rate performance of the battery to be in a good state.

[0089] In one embodiment, the D10 particle size E and the D90 particle size F of the Prussian blue analogue cathode material satisfy the following formula:

[0090] 0.03 ≤ E / F ≤ 0.3;

[0091] Controlling the D10 particle size of the Prussian blue analogue can improve the high-temperature storage life of the battery, and controlling the D90 particle size of the Prussian blue analogue can improve the rate performance and cycle stability of the battery. Therefore, controlling E / F within the above range can balance the high-temperature storage life and rate cycle performance of the battery to be in a good state.

[0092] In one embodiment, in the positive active coating, the mass ratio of the Prussian blue analogue cathode material to the layered metal oxide cathode material is (10 - 90):(90 - 10). For example, the mass ratio of the Prussian blue analogue cathode material to the layered metal oxide cathode material can be 10:90, 20:80, 30:70, 40:60, 50:50, 60:40, 70:30, 80:20, 90:10. The ratio of (50 - 90):(50 - 10) is preferred. When the content of the layered metal oxide cathode material is too high, the watt-hour cost and safety performance of the battery cell will be affected; when the content of the Prussian blue analogue cathode material is too high, the volume energy density of the battery cell may be affected. Therefore, when the above two cathode materials are within the above range, the composite cathode sheet takes into account the volume energy density, cost and safety, and has excellent comprehensive performance.

[0093] In one embodiment, the tap density of the first active sub - coating is 1.4 g / cm 3 ~1.8 g / cm 3 。For example, the tap density of the first active sub - coating can be 1.4 g / cm 3 、1.5 g / cm 3 、1.6 g / cm 3 、1.7 g / cm 3 、1.8 g / cm 3 , or the range composed of any two of the above values.

[0094] In one embodiment, the tap density of the second active sub - coating is 3.0 g / cm 3 ~3.3 g / cm 3 。For example, the tap density of the second active sub - coating can be 3.0 g / cm 3 、3.05 g / cm 3 、3.1 g / cm 3 、3.15 g / cm 3 、3.2 g / cm 3 、3.25 g / cm 3 、3.3 g / cm 3 , or the range composed of any two of the above values.

[0095] When the tap density of the first active sub - coating and the tap density of the second active sub - coating are adjusted to the above ranges, the cell cost and the volumetric energy density can be taken into account. That is, the tap density of the layered metal oxide close to the current collector layer is high, and the material cost is relatively high, while the tap density of the Prussian blue analogue cathode material far from the current collector layer is low, but the specific capacity is higher and the material cost is low. Considering the characteristics of the two materials, this composite cathode sheet reduces the watt - hour cost of the cell compared with using only the layered metal oxide cathode material, and increases the volumetric energy density compared with using only the Prussian blue analogue cathode material.

[0096] In one embodiment, the thickness of the cathode sheet is 90 μm to 200 μm. For example, the thickness of the cathode sheet can be 90 μm, 100 μm, 110 μm, 120 μm, 130 μm, 140 μm, 150 μm, 160 μm, 170 μm, 180 μm, 190 μm, 200 μm or the range composed of any two of the above values. When the thickness of the cathode sheet is within the above range, the cathode sheet has good processability and at the same time takes into account the rate performance.

[0097] In one embodiment, in the positive electrode active coating, the thickness ratio of the first sub - coating to the second active sub - coating is (1:9):(9:1); within this range, the positive electrode sheet can take into account factors such as cost and safety.

[0098] In one embodiment, the first sub - coating further includes: a first binder and a first conductive agent, and the second sub - coating further includes: a second binder and a second conductive agent.

[0099] In the first sub - coating, the mass ratio of the Prussian blue analogue positive electrode material to the first binder and the first conductive agent is (90 - 96):(2 - 5):(2 - 5). In the second sub - coating, the mass ratio of the layered metal oxide positive electrode material to the second binder and the second conductive agent is (92 - 96):(2 - 5):(2 - 5). When the composite positive electrode sheet composed of the two active substances is within the above range, the composite positive electrode sheet has a lower surface resistance, thus having better rate performance; in addition, under the above formula, the composite positive electrode sheet also has a higher stripping rate, which can improve the manufacturing ability of the subsequent battery cells.

[0100] In a second aspect, the present application also provides a sodium - ion battery, including the positive electrode sheet described in any of the above embodiments. Therefore, the sodium - ion battery of the present application has good cycle performance, high safety and low cost.

[0101] In one embodiment, the application voltage window of the sodium - ion battery is within (3.9V - 4.1V) - (1.5V - 2.2V); when the sodium - ion battery assembled from this composite positive electrode sheet is used within this voltage range, it has a higher energy density; when the upper - limit voltage is too low, the capacity of the layered metal oxide is difficult to exert; when the upper - limit voltage is too high, the structural stability of the layered metal oxide is poor, resulting in poor cycle stability of the battery cell; for the lower - limit voltage, according to specific downstream applications, it can be adjusted within 1.5V - 2.2V, so as to take into account both energy density and usage requirements.

[0102] It should be understood that the sodium - ion battery provided by the embodiments of the present application further includes a negative electrode sheet, an electrolyte and a separator, and the negative electrode sheet, the electrolyte and the separator can all be commercially available products well - known to those skilled in the art or products prepared by conventional preparation methods.

[0103] The present invention will be further described below through specific examples and comparative examples.

[0104] Example 1

[0105] 1) Preparation of the positive electrode sheet

[0106] Prepare a slurry with Prussian blue analogue as the positive electrode active substance, and use the active substance (Na 1.89 Mn0.45 Fe 0.5 Co 0.05 [Fe(CN)6] 0.92 (·0.25H2O), conductive agent carbon black (Super-P), carbon nanotubes (CNT), and binder polyvinylidene fluoride (PVDF) are uniformly dispersed in the solvent N-methylpyrrolidone (NMP) according to the mass ratio of 94:2:1:3 to prepare the positive electrode slurry PB-1.

[0107] Prepare a slurry with layered metal oxide as the positive electrode active material. The active material (Na 0.713 Ni 0.283 Mn 0.572 Ti 0.0798 Mg 0.0332 Zn 0.032 O2), conductive agent carbon black (Super-P), and binder polyvinylidene fluoride (PVDF) are uniformly dispersed in the solvent N-methylpyrrolidone (NMP) according to the mass ratio of 94:3:3 to prepare the positive electrode slurry NFM-1.

[0108] The slurry PB-1 is uniformly coated on both sides of the aluminum foil current collector, and the coating is dried. The drying temperature is 100 °C and the drying time is 5 min. Then, the slurry NFM-1 is coated on the surface of the dried lower layer coating and dried at 100 °C for 5 min. The dried electrode sheet is rolled to 110 μm to obtain a composite electrode sheet. In this example, the mass ratio of the Prussian blue analogue in PB-1 to the layered metal oxide in NFM-1 is 60:40, the compaction density PD1 of the first active sub-coating is 1.6 g / cm 3 , and the compaction density PD2 of the second active sub-coating is 3.15 g / cm 3 . The D50 particle size (D50-1) of the Prussian blue analogue positive electrode material is 1.5 μm, the D10 particle size (D10-1) of the Prussian blue analogue positive electrode material is 0.6 μm, the D90 particle size (D90-1) of the Prussian blue analogue positive electrode material is 4 μm, the D50 particle size (D50-2) of the layered metal oxide positive electrode material is 5 μm, the D10 particle size (D10-2) of the layered metal oxide positive electrode material is 3 μm, and the D90 particle size (D90-3) of the layered metal oxide positive electrode material is 20 μm.

[0109] 2) Preparation of negative electrode sheet

[0110] Mix 92.5 wt% of the negative electrode hard carbon active material, 4.8 wt% of the conductive agent (4% conductive carbon black and 0.8% carbon nanotubes), 2.7 wt% of the binder (sodium polyacrylate PAA-Na, styrene-butadiene copolymer, sodium carboxymethyl cellulose CMC-Na = 44.4%:29.6%:25.9%), and N-methylpyrrolidone to obtain the negative electrode active layer slurry. Coat this slurry on both functional surfaces of the negative electrode current collector, a 12-μm corona-treated aluminum foil, with a coating areal density of 5 mg / cm 2 , and after drying at 100 °C, roll press and cut to obtain a negative electrode sheet with a compaction density of 0.95 g / cm 3 ;

[0111] 3) Preparation of the electrolyte

[0112] The above electrolyte may include an organic solvent and a sodium salt electrolyte. As an example, the organic solvent may be one or more of ethylene carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, propylene sulfite, methyl acetate, ethyl propionate, fluoroethylene carbonate, diethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, methyl tert-butyl ether; the sodium salt electrolyte may be one or more of sodium hexafluorophosphate, sodium bis(fluorosulfonyl)imide, sodium bis(trifluoromethanesulfonyl)imide, sodium trifluoromethanesulfonate, sodium tetrafluoroborate, sodium difluorophosphate, sodium perchlorate, sodium chloride. In this example, the electrolyte composition used is 1 mol / L NaPF6, 0.2 mol / L NaFSI, and the solvent is PC:DEC:EC with a mass ratio of 1:1.63:0.28. Based on the total mass of the electrolyte, the additives are 1% wt of vinylene carbonate (VC) and 2% wt of 1,3-propane sultone (PS).

[0113] 4) Preparation of the separator

[0114] The separator can be various materials in the art suitable for separators in electrochemical energy storage devices. For example, it can be at least one of polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester, and natural fibers.

[0115] 5) Preparation of the sodium-ion battery

[0116] Form an electrode assembly by laminating the positive electrode sheet, the negative electrode sheet, and the separator, then encapsulate it in an aluminum-plastic film and dry it, inject the electrolyte, and after formation, obtain the sodium-ion battery.

[0117] Examples 2 - 5 are basically the same as Example 1 in the preparation process. The difference lies in that in the preparation of the positive electrode sheet, the mass ratio of the Prussian blue analogue in PB-1 to the layered metal oxide in NFM-1 was adjusted. The specific adjustments are shown in Table 2.

[0118] Examples 6 - 9 are basically the same as Example 1 in the preparation process. The difference lies in that in the preparation of the positive electrode sheet, the compaction density of the first active sub-coating and the compaction density of the second active sub-coating were adjusted. The specific adjustments are shown in Table 2.

[0119] Examples 10 - 15 are basically the same as Example 1 in the preparation process. The difference lies in that in the preparation of the positive electrode sheet, the particle sizes of the Prussian blue analogue positive electrode material and the layered metal oxide positive electrode material were adjusted. The specific adjustments are shown in Table 2.

[0120] Examples 16 - 27 are basically the same as Example 1 in the preparation process. The difference lies in that in the preparation of the positive electrode sheet, different Prussian blue analogue positive electrode materials and / or layered metal oxide positive electrode materials were selected. The specific adjustments are shown in Table 3.

[0121] Comparative Example 1

[0122] Comparative Example 1 is basically the same as Example 1 in the preparation process. The difference lies in that in the preparation of the positive electrode sheet, the mass ratio of the Prussian blue analogue in PB-1 to the layered metal oxide in NFM-1 was adjusted to 40:60.

[0123] Comparative Example 2

[0124] Comparative Example 2 is basically the same as Example 1 in the preparation process. The difference lies in that in the preparation of the positive electrode sheet, the mass ratio of the Prussian blue analogue in PB-1 to the layered metal oxide in NFM-1 was adjusted to 95:5.

[0125] Comparative Example 3

[0126] Comparative Example 3 is basically the same as Example 1 in the preparation process. The difference lies in that in the preparation of the positive electrode sheet, the mass ratio of the Prussian blue analogue in PB-1 to the layered metal oxide in NFM-1 was adjusted to 5:95.

[0127] Comparative Example 4

[0128] Comparative Example 4 is basically the same as Example 1 in the preparation process. The difference lies in that in the preparation of the positive electrode sheet, the slurry PB-1 was not coated.

[0129] Comparative Example 5

[0130] Comparative Example 5 is basically the same as Example 1 in the preparation process. The difference lies in that in the preparation of the positive electrode sheet, the slurry NFM-1 was not coated.

[0131] Experimental Example

[0132] 1. Moisture Content Determination

[0133] The slurries PB-1 prepared in the above-mentioned examples and comparative examples were subjected to thermogravimetric analysis using a thermogravimetric analyzer, and the change in the mass of the slurry PB-1 over time was recorded to obtain a thermogravimetric (TG) curve, thereby calculating the moisture content of the slurry PB-1.

[0134] 2. Sheet Resistance Measurement

[0135] The positive electrode sheets prepared in the above-mentioned examples and comparative examples were cut into cuboids of the same size, and the current and voltage difference of each positive electrode sheet were measured using the four-probe method. The sheet resistance of the positive electrode sheet was calculated based on the measured current and voltage difference.

[0136] 3. Room Temperature Cycle Stability:

[0137] At 25 °C, the secondary batteries prepared in the above-mentioned examples and comparative examples were charged at a constant current of 1C to the upper limit voltage of 3.9V, then charged at a constant voltage until the current was 0.05C. After standing for 5 minutes, the secondary batteries were discharged at a constant current of 1C to the lower limit voltage of 1.5V, and the discharge capacity at this time was recorded as the discharge capacity of the first cycle. The secondary batteries were subjected to cyclic charge and discharge tests according to the above method, and the discharge capacity after each cycle was recorded. The capacity retention rate (%) of the secondary battery after 300 cycles at 25 °C = discharge capacity after 300 cycles / discharge capacity of the first cycle × 100%.

[0138] 4. Rate Performance Test:

[0139] The secondary batteries prepared in the above-mentioned examples and comparative examples were left standing in an environment of 25 °C for 30 minutes, then charged at a constant current of 0.5C to the upper limit voltage of 3.9V, and charged at a constant voltage until 0.05C. After standing for 30 minutes, they were then discharged at a constant current of 0.5C to the lower limit voltage of 1.5V, and the discharge capacity at this time was recorded as the actual capacity C1 of the battery cell; after standing for 30 minutes, they were charged at a constant current of 0.5C to the upper limit voltage of 3.9V again, and charged at a constant voltage until 0.05C. After standing for 30 minutes, they were then discharged at a constant current of 10C to the lower limit voltage of 1.5V, and the discharge capacity at this time was recorded as C2. The rate performance is the discharge capacity at 10C divided by the discharge capacity at 0.5C, i.e., C2 / C1 * 100%.

[0140] 5. Penetration Safety Test:

[0141] Place the secondary batteries prepared in the above examples and comparative examples in an incubator at 25°C and let them stand for 30 minutes to allow the batteries to reach a constant temperature. Charge the batteries at a constant current of 1C to the upper limit voltage of 3.9V, and then charge them at a constant voltage until the current is cut off at 0.05C. Transfer the fully charged sodium-ion batteries to a nail penetration tester, keep the test environment temperature at 25°C ± 2°C, use a steel nail with a diameter of 5mm to uniformly penetrate the center of the sodium-ion battery at a speed of 25mm / s, and keep it for 1 hour. Record the temperature rise on the surface of the core that has been punctured by the nail.

[0142] 6. High-temperature storage performance test:

[0143] Place the secondary batteries prepared in the above examples and comparative examples in an environment of about 25°C and charge them at a constant current of 0.5C to the upper limit voltage of 3.9V, and then charge them at a constant voltage until the current is lower than 0.05C to make the batteries in a fully charged state. Measure the thickness of the fully charged battery before storage. Place the fully charged batteries in an oven at about 85°C for storage for about 24h, then measure their thickness after storage and calculate their expansion rate.

[0144] 7. Full specific capacity performance test:

[0145] Place the secondary batteries prepared in the above examples and comparative examples in an environment of about 25°C and charge them at a constant current of 0.5C to the upper limit voltage of 3.9V, and then charge them at a constant voltage until the current is lower than 0.05C to make the batteries in a fully charged state. Discharge at 0.5C to 1.5V, record the discharge time, and calculate the specific capacity performance.

[0146] Table 1

[0147]

[0148]

[0149] Referring to Table 1, compared with Comparative Example 4, in Examples 1 - 5, and Comparative Examples 1 - 3, 5, by adding a Prussian blue analog cathode material coating to the cathode sheet, the temperature rise during nail penetration of the sodium-ion batteries prepared from this cathode sheet is significantly reduced. Moreover, the higher the mass ratio of the Prussian blue analog cathode material to the layered metal oxide cathode material, the more obvious the effect of reducing the temperature rise during nail penetration of the sodium-ion battery.

[0150] Furthermore, relative to Comparative Examples 1, 3, and 4 (low gram capacity), Comparative Examples 3 and 4 (increased temperature upon puncture), and Comparative Examples 2 and 5 (poor rate performance and low volume energy density (too high content of Prussian blue analogue positive electrode material)), Examples 1 to 5, by controlling the mass ratio of the Prussian blue analogue positive electrode material to the layered metal oxide positive electrode material to be (50-90): (50-10), and controlling the surface resistance of the positive electrode sheet to be between 40 mΩ and 1000 mΩ, the sodium ion battery prepared from the positive electrode sheet simultaneously has the characteristics of high rate performance, high cycle stability, high safety and low cost.

[0151] Table 2

[0152]

[0153] Table 2 - continued

[0154]

[0155] Referring to Table 2, relative to Examples 8 and 9, Examples 1, 6 and 7, the compaction density of the first active sub-coating and the compaction density of the second active sub-coating are controlled within the corresponding range, and the sodium ion battery prepared by the positive electrode sheet has high rate performance, high cycle stability and high safety at the same time. When the compaction density of the first active sub-coating and the compaction density of the second active sub-coating are less than the range, the room temperature cycle stability of the sodium ion battery prepared by the positive electrode sheet is slightly reduced, and when the compaction density of the first active sub-coating and the compaction density of the second active sub-coating are greater than the range, the rate performance of the sodium ion battery prepared by the positive electrode sheet is slightly reduced.

[0156] Furthermore, relative to Examples 12-15, Examples 1, 9 and 10, when the different particle sizes of the Prussian blue analogue positive electrode material and the layered metal oxide positive electrode material are controlled within the corresponding particle size range, and when the ratio of the D50 particle size of the Prussian blue analogue positive electrode material to the layered metal oxide positive electrode material is controlled within the corresponding ratio range, the sodium ion battery prepared from the positive electrode sheet simultaneously has higher rate performance, higher cycle stability and high safety.

[0157] When the ratio of the D50 particle size of the Prussian blue analog positive electrode material to the layered metal oxide positive electrode material is too small, the rate performance of the sodium ion battery prepared from the positive electrode sheet is slightly reduced, and the high-temperature storage expansion rate is slightly increased. When the ratio of the D50 particle size of the Prussian blue analog positive electrode material to the layered metal oxide positive electrode material is too large, the rate performance of the sodium ion battery prepared from the positive electrode sheet is slightly reduced.

[0158] When the particle size of the Prussian blue analogue positive electrode material and the layered metal oxide positive electrode material is too small, the rate performance of the sodium ion battery prepared from the positive electrode sheet decreases slightly, and the high-temperature storage expansion rate increases slightly. When the particle size of the Prussian blue analogue positive electrode material and the layered metal oxide positive electrode material is too large, the rate performance of the sodium ion battery prepared from the positive electrode sheet decreases slightly.

[0159] Table 3

[0160]

[0161] Table 3 - continued

[0162]

[0163]

[0164] Referring to Table 3, relative to Examples 1 and 16, when only one type of M is selected in the Prussian blue positive electrode material in Example 17, the room temperature cycle stability and rate performance of the sodium ion battery prepared by the positive electrode sheet are significantly reduced. When the chemical formula of the Prussian blue positive electrode material in Examples 18 and 19 is not within the general formula provided in the present application, the room temperature cycle stability and rate performance of the sodium ion battery prepared by the positive electrode sheet are significantly reduced, and the high temperature storage expansion rate is slightly increased.

[0165] Further, relative to Examples 1 and 20, when the sodium ion content of the layered metal oxide positive electrode material in Examples 21 and 22 is high (out of range), the positive electrode full electric capacity of the sodium ion battery prepared by the positive electrode sheet is high, but the room temperature cycle stability and rate performance are significantly reduced, and the needle puncture temperature rise is significantly increased. In particular, in Example 21, when the layered metal oxide positive electrode material does not contain element N, the high temperature storage expansion rate of the sodium ion battery is significantly increased. In Example 23, when the layered metal oxide positive electrode material is significantly different from the general formula provided in the present application, the full electric capacity of the sodium ion battery is significantly reduced.

[0166] Furthermore, in Examples 24-27, when the water content of the Prussian blue positive electrode material is too high, the positive electrode full electric capacity and rate performance of the sodium ion battery prepared from the positive electrode sheet are significantly reduced compared to Example 1. However, the higher the water content of the Prussian blue positive electrode material, the smaller the needle puncture temperature rise of the sodium ion battery.

[0167] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements 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 positive electrode sheet, characterized in that, Comprising: a positive current collector and a positive active coating located on at least one surface of the positive current collector; the positive active coating includes a first active sub-coating and a second active sub-coating, the second active sub-coating is coated on at least one surface of the positive current collector, and the first active sub-coating is coated on the surface of the second active sub-coating; the first active sub-coating includes a Prussian blue analogue cathode material, and the second active sub-coating includes a layered metal oxide cathode material; the sheet resistance of the positive electrode sheet is 40 mΩ to 1000 mΩ.

2. The positive electrode sheet according to claim 1, wherein the sheet resistance of the positive electrode sheet is 100 mΩ to 600 mΩ.

3. The positive electrode sheet according to claim 1 or 2, characterized in that, The general formula of the Prussian blue analogue cathode material is as follows: Na x M y Co z [Fe(CN)6] p ·vH2O; wherein, 1 < x < 2, 0.8 < y < 1, 0 < z < 0.2, 0.7 < p < 1, 0.1 < v < 0.8, and M is selected from one or more of iron, manganese, nickel, and copper; and / or The chemical general formula of the layered metal oxide cathode material is Na b Ni a1 Fe a2 Mn a3 N c O2; wherein, 0.67 ≤ b ≤ 0.95, a1 + a2 + a3 + c = 1, 0 ≤ a2 < 0.3, 0.3 < a3 < 0.6, 0 < c < 0.15, and N is selected from one or more of copper, zinc, titanium, magnesium, aluminum, and zirconium.

4. The positive electrode sheet according to claim 3, wherein The water content of the Prussian blue analogue cathode material at 200 °C is 0.5% to 5%.

5. The positive electrode sheet according to claim 1, characterized in that, The D50 particle size of the Prussian blue analogue cathode material is 0.7 μm - 3 μm; and / or the D10 particle size of the Prussian blue analogue cathode material is 0.2 μm - 1 μm; and / or the D90 particle size of the Prussian blue analogue cathode material is 2 μm - 6 μm; and / or the D50 particle size of the layered metal oxide cathode material is 3 μm - 8 μm; and / or the D10 particle size of the layered metal oxide cathode material is 2 μm - 4 μm; and / or the D90 particle size of the layered metal oxide cathode material is 15 μm - 25 μm.

6. The positive electrode sheet according to claim 1, characterized in that, The D50 particle size A of the Prussian blue analogue cathode material and the D50 particle size B of the layered metal oxide cathode material satisfy the following formula: 0.1 ≤ A / B ≤ 0.75; and / or the D10 particle size C of the layered metal oxide cathode material and the D90 particle size D of the layered metal oxide cathode material satisfy the following formula: 0.1 ≤ C / D ≤ 0.2; and / or the D10 particle size E of the Prussian blue analogue cathode material and the D90 particle size F of the Prussian blue analogue cathode material satisfy the following formula: 0.03 ≤ E / F ≤ 0.

3.

7. The positive electrode sheet according to claim 1, characterized in that, The mass ratio of the Prussian blue analogue cathode material to the layered metal oxide cathode material is (10 - 90):(90 - 10).

8. The positive electrode sheet according to any one of claims 1, characterized in that, The thickness of the positive electrode sheet is 90 μm to 200 μm; and / or the thickness ratio of the first active sub-coating to the second active sub-coating is (1:9):(9:1); and / or The compacted density of the first active sub-coating is 1.4 g / cm 3 ~1.8 g / cm 3 ; and / or The compacted density of the second active sub-coating is 3.0 g / cm 3 ~3.3 g / cm 3 .

9. A sodium-ion battery, characterized in that, Comprising the positive electrode sheet according to any one of claims 1 - 8.

10. The sodium ion battery according to claim 9, characterized in that, The application voltage window of the sodium ion battery is (3.9V - 4.1V) - (1.5V - 2.2V).

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