Positive pole piece of sodium ion battery and sodium ion battery

Abstract

According to the sodium-ion battery positive pole piece and the sodium-ion battery, the bottom layer of the sodium-ion battery positive pole piece is coated with a polycrystalline sodium-ion battery positive pole active material with primary particles, the sodium-ion battery positive pole piece has the characteristics of good rate capability and high capacity exertion, and can provide relatively high capacity for the positive pole piece; the lithium ion battery is a single-crystal layered oxide sodium electric material, has the characteristics of primary granularity, less side reaction with an electrolyte, good gas production effect and the like, can reduce the side reaction between inner polycrystals and the electrolyte, and has the characteristics of high charge-discharge voltage, good material stability and weak oxidizability to the electrolyte as the outer layer is coated with a NaxNyOz material; and the oxidation of the electrolyte by the layered sodium electric material can be effectively protected, and the gas production in the circulation process is reduced.

Description

Technical Field

[0001] The present invention relates to the technical fields of sodium-ion secondary batteries and materials, and particularly relates to a composite positive electrode sheet for a sodium-ion secondary battery and a sodium-ion battery. Background Art

[0002] Compared with lithium-ion secondary batteries, sodium-ion secondary batteries have significant advantages such as low cost and stable raw materials, and have basically the same production process as lithium-ion batteries, showing great application prospects in the fields of two-wheel vehicles, household energy storage, and large-scale energy storage. Currently, the widely studied layered transition metal oxides have problems such as poor crystal stability, severe side reactions with electrolytes, and poor cycle performance and cycle gas generation performance during charge and discharge. To improve the cycle gas generation characteristics, the positive electrode materials usually adopt surface coating or doping methods to inhibit the crystal volume strain of active particles and reduce the side reactions caused by the direct contact between the active substance and the electrolyte.

[0003] To improve the cycle characteristics of layered sodium batteries, invention patents CN 114843524 A and CN 115207352A respectively disclose that by coating electrochemically inert metal carbonates (Na2CO3) or phosphates (NaTi2(PO4)3) or other substances on the surface of active materials to reduce the surface activity of the materials, the side reactions caused by the direct contact between the active substance and the electrolyte can be reduced, effectively improving the crystal structure stability of the positive electrode material during long-term cycling and the cycle service life of the electrode sheet. However, the coating of electrochemically inert substances will reduce the surface conductivity and increase the battery impedance. Patent CN 115172671 A reports a preparation process of a composite electrode sheet that enhances the cycle stability of layered oxide materials and Prussian blue-based composite materials through iron-based polyanion compounds. CN111029553A adds polyanion compounds to oxides containing transition metal elements, enabling the polyanion compounds to be evenly filled between the particles of the oxides containing transition metal elements. Combining with the stable sodium storage structure of the polyanion compounds, it can counteract the volume change of sodium ions during insertion and extraction, thereby buffering the volume expansion of the positive electrode material during charge and discharge, reducing the contact area between the positive electrode material and the electrolyte, and improving the cycle stability of the positive electrode material. However, the introduction of polyanions will significantly reduce the energy density of layered oxygen transition metal oxides.

[0004] To improve cycling and cycle gas generation, the present invention comprehensively improves the gas generation problem of sodium batteries through a special design of multi-layer coating of the positive electrode sheet. Summary of the Invention

[0005] In order to overcome the deficiencies of the prior art, the purpose of the present invention is to provide a positive electrode sheet for a sodium-ion battery and a sodium-ion battery.

[0006] The solution adopted by the present invention is as follows:

[0007] In a first aspect, the present invention provides a positive electrode sheet for a sodium-ion battery, comprising a current collector, on which a first coating layer is provided, the first coating layer containing a positive electrode active material, the positive electrode active material being a polycrystalline composite layered oxide positive electrode material, a second coating layer is provided on the first coating layer, the second coating layer containing a positive electrode active material, the positive electrode active material being a single crystal composite layered oxide positive electrode material, a third coating layer is provided on the second coating layer, and the third coating layer contains the positive electrode active material Na x N y O z , the valence of the N element is M, and X + My - 2Z = 0 is satisfied, and the preferred elements for N are Zr and Cr.

[0008] In the present invention, the polycrystalline composite layered oxide positive electrode material in the first coating layer contains a sodiated insertion compound Na 1+a Ni x Mn y M b O2, where M is a doping element, -0.1 ≤ a ≤ 0.1, 0.05 ≤ x ≤ 0.95, 0.05 ≤ y ≤ 0.95, b ≥ 0 and x + y + b = 1; the doping element M is selected from at least one of Fe, Cu, Co, Mg, Al, Ti, Cr, Li, Ca, Zn, Sr, Y, Zr, La, F, Si, P, and B.

[0009] In the present invention, the single crystal composite layered oxide positive electrode material in the second coating layer contains a sodiated insertion compound Na 1+a Ni x Mn y M b O2, where M is a doping element, -0.1 ≤ a ≤ 0.1, 0.05 ≤ x ≤ 0.95, 0.05 ≤ y ≤ 0.95, b ≥ 0 and x + y + b = 1; the doping elements include at least one of Fe, Cu, Co, Mg, Al, Ti, Cr, Li, Ca, Zn, Sr, Y, Zr, La, F, Si, P, and B.

[0010] In the present invention, the first and second coatings containing the sodiated insertion compounds can be obtained by commercial channels or synthesized by experimental methods. The synthesis method is as follows:

[0011] (1) Prepare an aqueous solution containing nickel salt, iron salt, and M salt, pump the mixed salt solution of the above solution into a reactor, and at the same time add an NaOH solution to the reactor for coprecipitation; the precipitate is washed with deionized water and dried to obtain a transition metal compound precursor Ni x Mn y M b(OH)2;

[0012] (2) The dried precursor powder is fully mixed with Na2CO3;

[0013] (3) The mixture in step (2) is calcined in air at 800 - 900 °C and naturally cooled to room temperature to obtain a sodium - containing intercalation compound of the first coating layer.

[0014] (4) The mixture in step (2) is calcined in air at 900 - 1100 °C and naturally cooled to room temperature to obtain a sodium - containing intercalation compound of the second coating layer.

[0015] In the present invention, the concentration of the mixed salt solution in step (1) is 0.5 - 2.5 M, and the relative contents of nickel salt, iron salt, and M salt in the mixed salt solution satisfy the proportional relationship of the corresponding elements in the sodium - containing intercalation compound Na 1+a Ni x Mn y M b O2.

[0016] In the present invention, the concentration of the NaOH solution in step (1) is 0.5 - 2.5 M.

[0017] In the present invention, the coprecipitation temperature in step (1) is 35 - 60 °C, and the reaction time is 12 - 64 h.

[0018] In the present invention, the nickel salt, iron salt, and M salt in step (1) can be nitrate, sulfate, or chloride.

[0019] In the present invention, the drying temperature in step (1) is 80 - 100 °C, and the drying time is 5 - 12 h.

[0020] In the present invention, the molar ratio of the precursor powder to Na2CO3 in step (2) is 1:2 - 2:1.

[0021] In the present invention, the heating rate of the calcination in step (3) is 2 - 5 °C / min, and the calcination time is 8 - 15 h.

[0022] In the present invention, the heating rate of the calcination in step (3) is 2 - 5 °C / min, and the calcination time is 8 - 20 h.

[0023] In the present invention, for the third coating layer Na x N y O zIt can be synthesized by the solid-phase sintering method. During the synthesis, the sodium source and the N source are mixed evenly according to the stoichiometric ratio. Among them, the sodium source can be at least one of sodium nitrate, sodium carbonate, sodium acetate, sodium hydroxide, and sodium citrate. The N source is one of chromium nitrate and zirconium nitrate. The mixed materials are placed in an inert atmosphere furnace and sintered at 800-1000 °C for 6-10 h, then cooled and taken out, and obtained after pulverization. Preferably, the inert atmosphere can be nitrogen or argon.

[0024] In the present invention, the current collector is one of an aluminum foil current collector, an aluminum-based composite current collector, a perforated aluminum foil current collector, and a perforated aluminum-based composite current collector;

[0025] In the present invention, the thickness of the current collector is 8-16 μm; the surface energy value of the current collector is ≥30 dyn / cm;

[0026] In the present invention, the first coating layer further comprises a positive electrode conductive agent and a positive electrode binder; the positive electrode conductive agent is usually one or a combination of conductive carbon black, conductive graphite, Ketjen black, acetylene black, carbon nanotubes, graphene, etc. The positive electrode binder is usually polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE). The mass ratio of the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder in the first coating layer is (90-98):(1-5):(1-5);

[0027] In the present invention, the positive electrode active material in the first coating layer comprises primary particles, and the primary particles form secondary particles. The D50 particle size of the primary particles is 0.1 μm - 2.5 μm, and the D50 average particle size of the secondary particles is 3 - 20 μm.

[0028] In the present invention, the specific surface area of the positive electrode active material in the first coating layer is 0.1 m 2 / g ≤ BET ≤ 5 m 2 / g. Exemplarily, BET can be 0.1, 0.5, 1, 2, 3, 4, 5 or a range composed of any two of the above values.

[0029] In the present invention, the second coating layer further comprises a positive electrode conductive agent and a positive electrode binder; the positive electrode conductive agent is selected from one or a combination of conductive carbon black, conductive graphite, Ketjen black, acetylene black, carbon nanotubes, graphene, etc. The positive electrode binder is selected from polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE). The mass ratio of the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder in the second coating layer is (90-98):(1-5):(1-5);

[0030] In the present invention, the positive electrode active material in the second coating layer only comprises primary particles, and the D50 particle size of the primary particles is 3 μm - 15 μm.

[0031] In the present invention, the specific surface area of the positive electrode active material in the second coating layer is 0.1 m 2 / g ≤ BET ≤ 2 m 2 / g. Exemplarily, BET can be 0.1, 0.2, 0.5, 1.5, 2 or a range composed of any two of the above values.

[0032] The third coating layer further comprises a positive electrode conductive agent and a positive electrode binder; the positive electrode conductive agent is selected from one or a combination of conductive carbon black, conductive graphite, Ketjen black, acetylene black, carbon nanotubes, graphene, etc. The positive electrode binder is selected from polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE). The mass ratio of the positive electrode active material, the positive electrode conductive agent and the positive electrode binder in the third coating layer is (90 - 98):(1 - 5):(1 - 5);

[0033] The D50 particle size of the positive electrode active material in the third coating layer is 1 μm - 15 μm, such as 1 μm, 3 μm, 5 μm, 7 μm, 10 μm, 12 μm, 15 μm.

[0034] The specific surface area of the positive electrode material in the third coating layer is 0.1 m 2 / g ≤ BET ≤ 5 m 2 / g. Exemplarily, BET can be 0.1, 0.5, 1, 2, 3, 4, 5, or a range composed of any two of the above values.

[0035] The areal density of the first coating layer is 0 - 20 mg / cm 2 , the areal density of the second coating layer is 0 - 20 mg / cm 2 , and the areal density of the third coating layer is 1 - 5 mg / cm 2 .

[0036] Those skilled in the art understand that the above-mentioned positive electrode plate is single-sided coated, and in the manufacture of the battery, a method of double-sided coating is used.

[0037] On the other hand, the present invention also provides a sodium ion battery, and the sodium ion battery includes the above-mentioned positive electrode plate.

[0038] The preparation method of the sodium ion battery includes the following steps:

[0039] (1) Preparation of coating layer (one): Prepare a positive electrode binder, a positive electrode conductive agent and a positive electrode active material according to a mass ratio; sequentially add the positive electrode binder, the positive electrode conductive agent and the positive electrode active material to a positive electrode solvent, and homogenize to obtain a positive electrode slurry one;

[0040] (2) Preparation of coating layer (II): preparing a positive electrode binder, a positive electrode conductor and a positive electrode active material according to a mass ratio; sequentially adding the positive electrode binder, the positive electrode conductor and the positive electrode active material to a positive electrode solvent, and homogenizing to obtain a positive electrode slurry II;

[0041] (3) Preparation of coating layer (III): preparing a positive electrode binder, a positive electrode conductor and a positive electrode active material according to a mass ratio; sequentially adding the positive electrode binder, the positive electrode conductor and the positive electrode active material to a positive electrode solvent, and homogenizing to obtain a positive electrode slurry III;

[0042] (4) The positive electrode slurry 1 is uniformly coated on the positive electrode current collector, and after drying, a coating layer (1) is formed.

[0043] (5) The positive electrode slurry 2 is uniformly coated on the coating layer (1), and after drying, a coating layer (2) is formed.

[0044] (6) The positive electrode slurry (3) is uniformly coated on the coating layer (2), and after drying, a coating layer (3) is formed.

[0045] (7) Rolling and cutting the positive electrode sheets to obtain positive electrode sheets;

[0046] (8) preparing a negative electrode active material anti-settling agent, a negative electrode binder, a negative electrode conductive agent and a negative electrode active material, stirring them evenly to form a negative electrode slurry, and coating the negative electrode slurry evenly on a negative electrode current collector to obtain a negative electrode sheet;

[0047] (9) The positive electrode obtained in step (1-7), the negative electrode obtained in step (8), the electrolyte, the separator and the outer packaging structure are assembled, injected, formed and divided in sequence to obtain a sodium ion battery.

[0048] In one embodiment, in the step (1-3), the cathode solvent is nitrogen methyl pyrrolidone (NMP) or dimethylformamide (DMF) as the cathode dispersion solvent. After the obtained cathode slurry is coated on the cathode current collector, the cathode solvent therein will dry and volatilize and will not exist in the battery. It is understood by those skilled in the art that the step (1-3) can be carried out in a blender, and the solid content of the obtained slurry is generally 50-80%.

[0049] In the present invention, the negative electrode sheet in step (8) comprises a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector. The present application has no particular limitation on the negative electrode current collector, as long as the purpose of the present application can be achieved, for example, it may comprise copper foil, aluminum foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, foamed nickel, foamed copper or a composite current collector.

[0050] In the present invention, in the step (8), the negative electrode active material includes at least one of metal oxides, metal sulfides, metal phosphides, Sb-based negative electrode materials, hard carbon, soft carbon, and metallic sodium, etc. The anti-settling agent is usually sodium carboxymethyl cellulose (CMC). The negative electrode conductive agent is selected from one or a combination of conductive carbon black, conductive graphite, Ketjen black, acetylene black, carbon nanotubes, graphene, etc. The binder is selected from styrene-butadiene latex, acrylic acid-based.

[0051] The present invention has no special requirements for the negative electrode active material, anti-settling agent, negative electrode conductive agent, and negative electrode binder in the above step (8), and they are common materials currently in use.

[0052] The mass ratio of the negative electrode active material, anti-settling agent, negative electrode conductive agent, and negative electrode binder is (90 - 97.5):(0.5 - 1.5):(1 - 5):(1 - 3.5). After being stirred evenly, a negative electrode slurry is formed.

[0053] Those skilled in the art understand that the negative electrode slurry is uniformly coated on the negative electrode current collector, and after drying, it is subjected to double-roll pressing and slitting to obtain a negative electrode sheet.

[0054] Those skilled in the art understand that the assembly, liquid injection, formation, and grading in the step (9) are all conventional means for manufacturing batteries in the art.

[0055] In a specific embodiment, in the step (9), the positive electrode sheet, separator, and negative electrode sheet are stacked and wound together to form a bare battery cell. The bare battery cell is, from the inside to the outside, the positive electrode sheet, separator, and negative electrode sheet in sequence; the bare battery cell is put into a shell and welded, and then placed in a vacuum oven at 70 - 100 °C (such as 75 °C, 80 °C, 85 °C, 90 °C, and 95 °C) and baked for 24 - 48 h (such as 24 h, 36 h, and 44 h) until the water content of the positive electrode sheet < 200 ppm (such as 100 ppm and 150 ppm) and the water content of the negative electrode sheet < 300 ppm (such as 150 ppm and 200 ppm), completing the assembly process to obtain an assembly; then the assembly is injected with electrolyte and left for 24 - 48 h (such as 30 h, 38 h, and 44 h) to complete the liquid injection process; then it is charged with a small current (such as 0.05C, 0.1C, and 0.2C) to 50 - 80% (such as 60% and 70%) of the battery cell capacity to complete the formation process; then grading is carried out.

[0056] Those skilled in the art understand that when the positive electrode sheet, separator, and negative electrode sheet are stacked and wound together to form a bare battery cell, it is necessary to ensure that the negative electrode sheet completely wraps the positive electrode sheet. Preferably, the widths of the positive electrode sheet, the separator, and the negative electrode sheet increase in sequence.

[0057] The beneficial effects of the present invention are as follows:

[0058] The present invention coats a polycrystalline sodium-based cathode material with primary particles on the bottom layer, which has good rate performance and high capacity utilization, and can provide a high capacity for the cathode electrode. The second coating layer of the cathode electrode is a single-crystalline layered oxidation material, which has primary particle size, few side reactions with the electrolyte, and good gas generation effect, etc., and can reduce the side reactions between the inner-layer polycrystalline material and the electrolyte. The outer layer is coated with Na x N y O z material, which has a high charge-discharge voltage, good material stability, and weak oxidizing property to the electrolyte, and can effectively protect the layered sodium-based material from being oxidized by the electrolyte and reduce gas generation during the cycling process. Description of the Drawings

[0059] Figure 1 Schematic diagram of the polycrystalline layer oxygen material in Example 1;

[0060] Figure 2 Schematic diagram of the single-crystalline layer oxygen material in Example 2; Detailed Embodiments

[0061] The devices and raw materials used in the following examples and comparative examples are as follows:

[0062] Cathode solvent: N-methylpyrrolidone, abbreviated as NMP, electronic grade, manufactured by Shandong Qingyun Changxin Chemical Technology Co., Ltd.;

[0063] Cathode binder: polyvinylidene fluoride, model PVDF5130, manufactured by SOLVAF;

[0064] Conductive carbon black, model Superp, purity > 99wt%, manufactured by Imerys Graphite & Carbon;

[0065] Aluminum foil: model 1235-H18, thickness 14μm, aluminum purity > 99wt%, manufactured by Nanan Aluminum Co., Ltd.;

[0066] Anode binder: styrene-butadiene latex, model BM451B, manufactured by ZEON;

[0067] Anode active material: hard carbon, model Type2, manufactured by Kuraray

[0068] Anti-settling agent: sodium carboxymethyl cellulose, model CMC2200, manufactured by Daicel Corporation.

[0069] Nickel sulfate hexahydrate: chemically pure, manufactured by Shanghai Reagent Co., Ltd., Sinopharm Group;

[0070] Manganese sulfate monohydrate: chemically pure, manufactured by Shanghai Reagent Co., Ltd., Sinopharm Group;

[0071] Ferric sulfate heptahydrate: chemically pure, Sinopharm Shanghai Reagent Co., Ltd.

[0072] Sodium hydroxide: AR, 96%, Aladdin reagent;

[0073] Sodium carbonate: purity>99.5%, Aladdin reagent;

[0074] Copper nitrate trihydrate: AR, Aladdin reagent;

[0075] Sodium nitrate: AR,>99%, Aladdin reagent;

[0076] Chromium nitrate nonahydrate: AR,>99%, Aladdin reagent;

[0077] Example 1 (S1)

[0078] (1)Ni 1 / 3 Fe 1 / 3 Mn 1 / 3 For the synthesis of (OH)2, a mixed salt solution containing the required aqueous solution stoichiometric amounts of nickel sulfate hexahydrate, manganese sulfate monohydrate, and iron sulfate heptahydrate with a total concentration of 2M was pumped into a continuous stirred tank reactor, and a 2M NaOH solution was added to the reactor at the same time. The solution temperature was maintained at 50°C during the entire coprecipitation process, and the entire reaction time was 48h. The precipitate was washed with deionized water and dried in air at 90°C for 8h.

[0079] (2) The dried precursor powder and Na2CO3 were mixed in a molar ratio of 2:1 using a rotary mixer for 18 hours.

[0080] (3) The mixture of step (2) was calcined at 880°C in air at a heating rate of 3°C / min for 12 h, and naturally cooled to room temperature to obtain a first coating layer containing a sodium-containing intercalation compound NaNi 1 / 3 Mn 1 / 3 Fe 1 / 3 The particles of the positive electrode active material are secondary spherical particles, the primary particle size is 0.1-2.5 μm, and the particle size D50 of the secondary particles is 10 μm, and the specific surface area is 3.1 g / m 2 .

[0081] (4) Using dry process, the secondary spherical sodium positive electrode material NaNi 1 / 3 Mn 1 / 3 Fe 1 / 3 O2 (5000.0 g), positive electrode conductive agent SP (157.8 g), positive electrode binder PVDF 5130 (105.3 g) were slowly stirred in a stirrer for 30 min, and then 2850 g of nitrogen methyl pyrrolidone (NMP for short, positive electrode solvent) was added and stirred for 3 h to make it evenly dispersed, to obtain positive electrode slurry 1;

[0082] (5) Ni 0.3 Fe 0.3 Mn 0.3 Cu 0.1 (5) Synthesis of Ni(OH)₂: A mixed salt solution with a total concentration of 2 M, containing stoichiometric amounts of nickel sulfate hexahydrate, manganese sulfate monohydrate, iron sulfate heptahydrate, and copper nitrate trihydrate in an aqueous solution, was pumped into a continuously stirred tank reactor. At the same time, a 2 M NaOH solution was added to the reactor. The solution temperature was maintained at 50 °C throughout the coprecipitation process, and the total reaction time was 36 h. The precipitate was washed with deionized water and dried in air at 90 °C for 8 h.

[0083] (6) The dried precursor powder was mixed with Na₂CO₃ in a molar ratio of 2:1 using a rotary mixer for 18 h to mix thoroughly.

[0084] (7) The mixture from step (6) was calcined in air at 960 °C at a heating rate of 3 °C / min for 15 h and then naturally cooled to room temperature to obtain a sodium-containing intercalation compound NaNi 0.3 Mn 0.3 Fe 0.3 Cu 0.1 O₂. Among them, the primary particles of the positive electrode active material are spherical, and the particle size D50 of the particles is 8 μm, and the specific surface area is 4.0 g / m 2 .

[0085] (8) Using a dry process, the single-crystal sodium battery positive electrode material NaNi 0.3 Mn 0.3 Fe 0.3 Cu 0.1 O₂

[0086] (5000.0 g), the positive electrode conductive agent SP (157.8 g), and the positive electrode binder PVDF 5130 (105.3 g) were slowly stirred in a blender for 30 min, and then 2850 g of N-methylpyrrolidone (abbreviated as NMP, the positive electrode solvent) was added and stirred for 3 h to disperse evenly to obtain the positive electrode slurry two;

[0087] (9) The third coating layer NaGrO₂ can be synthesized by a solid-phase sintering method. In the synthesis, sodium nitrate and chromium nitrate nonahydrate were mixed evenly according to the stoichiometric ratio of 1:1. The mixed material was placed in an inert atmosphere argon furnace and sintered at 900 °C for 6 - 10 h, cooled and taken out, and then obtained after pulverization. The particle size D50 of the particles is 5 μm, and the specific surface area is 4.5 g / m 2 .

[0088] (10) Using a dry process, the sodium-ion battery cathode material NaGrO2 (5000.0 g), the cathode conductive agent SP (157.8 g), and the cathode binder PVDF 5130 (105.3 g) are slowly stirred in a blender for 30 min, and then 2850 g of N-methylpyrrolidone (abbreviated as NMP, the cathode solvent) is added, and stirred for 3 h to disperse evenly, obtaining the cathode slurry three;

[0089] (11) The cathode slurry one is evenly coated on the 14-μm aluminum foil, and the coating areal density is 8 mg / cm2. After drying, a coating layer one is formed. The slurry two is evenly coated on the coating layer one, and the coating areal density is 5 mg / cm2. After drying, a coating layer two is formed. The slurry three is evenly coated on the coating layer two, and after drying, a coating layer three is formed, and the coating areal density is 3 mg / cm2. After three-layer coating, the final cathode electrode sheet is formed. Then, through calendaring and slitting, the cathode electrode sheet is obtained.

[0090] (12) Using a dry process, the hard carbon anode material (5000.0 g), the anode conductive agent SP (106.4 g), and the anti-settling agent sodium carboxymethyl cellulose CMC (53.2 g) are slowly stirred for 30 min, and then 2000 g of deionized water is added, and stirring continues for 3 h. Finally, the anode binder SBR (398.9 g) is added, and stirred in a blender for 45 min to obtain the anode slurry; the anode slurry is evenly coated on a 14-μm aluminum foil current collector, and after drying, through calendaring and slitting, the anode electrode sheet is obtained;

[0091] (13) The cathode electrode sheet obtained in steps (1-4), the anode electrode sheet obtained in step (5), the electrolyte composed of ethylene carbonate EC (mass fraction 13%), ethyl methyl carbonate EMC (mass fraction 5%), propylene carbonate PC (mass fraction 80%), fluoroethylene carbonate FEC (mass fraction 2%), and sodium hexafluorophosphate (mass fraction 12.5%), the PE separator with an alumina ceramic layer of 2-μm thickness coated on both sides (the thickness of the PE separator itself is 14 μm), and the outer packaging structure are assembled, injected with electrolyte, formed, and separated and capacitanced in sequence according to the conventional method to obtain the sodium-ion battery.

[0092] Examples 2-5 (S2-5) and Comparative Examples 1-5 (D1-5)

[0093] Prepare the sodium-ion battery according to the method of Example 1. Among them,

[0094] 1. In Example 2, compared with Example 1, only the following differences exist:

[0095] In steps (1-4), the cathode material is different, which is NaNi 0.3 Mn 0.4 Fe 0.3O2, the primary particle size is 0.1 - 2.5 μm, the particle size D50 is 9.5 μm, and the specific surface area is 3.3 g / m 2 .

[0096] 2. In Example 3, compared with Example 1, only the following is different:

[0097] In steps (5 - 7), the cathode material is different, which is NaNi 0.3 Mn 0.28 Fe 0.37 Cu 0.05 O2, the particle size D50 is 5 μm, and the specific surface area is 4.1 g / m 2 .

[0098] 3. In Example 4, compared with Example 1, only the following is different:

[0099] The areal density of coating layer 1 is increased from 8 mg / cm 2 to 10 mg / cm 2 , and the areal density of coating layer 3 is increased from 3 mg / cm 2 to 5 mg / cm 2

[0100] 4. In Example 5, compared with Example 1, only the following is different:

[0101] The areal density of coating layer 2 is decreased from 5 mg / cm 2 to 3 mg / cm 2 , and the areal density of coating layer 3 is increased from 3 mg / cm 2 to 5 mg / cm 2 .

[0102] 5. In Comparative Example 1, compared with Example 1, only the following is different:

[0103] The coating of coating layer 2 is cancelled

[0104] 6. In Comparative Example 2, compared with Example 1, only the following is different:

[0105] The coating of coating layer 3 is cancelled

[0106] 7. Comparative Example 3

[0107] Compared with Example 1, the coating order of the first coating layer and the second coating layer is interchanged.

[0108] 8. Comparative Example 4

[0109] Compared with Example 1, the coating order of the first coating layer and the third coating layer is interchanged.

[0110] 9. Comparative Example 5

[0111] Compared with Example 1, the coating orders of the second coating layer and the third coating layer are interchanged.

[0112] Performance test:

[0113] Test method:

[0114] Cyclic test: The prepared soft-pack battery is charged at a constant current of 0.5C to 4.0V, charged at a constant voltage of 4.0V to 0.05C, then discharged at 1C to 1.5V, and then cycled until the discharge capacity reaches 80% of the initial capacity and then stopped.

[0115] Gas generation test for 500 cycles: The initial volume of the battery cell is V1, and it is cycled 500 times according to the method of the above cyclic test, and the test volume is V2. The initial capacity of the battery cell is C. The gas generation amount after 500 cycles is calculated as = (V2 - V1) / C.

[0116] Performance of the lithium-ion batteries prepared in Table 1 S1-6 and D1-2

[0117]

[0118]

[0119] From the comparison between Examples 1-5 and Comparative Examples 1-5, it can be seen that the sodium-ion battery of the present invention has significantly improved cyclic performance compared with Comparative Examples 1-5. There are obvious improvements in gas generation during cycling.

Claims

1. A positive electrode sheet of a sodium-ion battery, comprising a current collector, a first coating layer is provided on the current collector, the first coating layer contains a positive electrode active material, the positive electrode active material is a polycrystalline composite layered oxide positive electrode material, a second coating layer is provided on the first coating layer, the second coating layer contains a positive electrode active material, the positive electrode active material is a single crystal composite layered oxide positive electrode material, a third coating layer is provided on the second coating layer, and the third coating layer contains the positive electrode active material Na x N y O z , the valence of the N element is M, and it satisfies X + My - 2Z = 0. The preferred elements of N are Zr and Cr.

2. The positive electrode sheet according to claim 1, wherein The polycrystalline composite layered oxide cathode material in the first coating layer and the single-crystalline composite layered oxide cathode material in the second coating layer contain a sodiated intercalation compound Na 1+a Ni x Mn y M b O2, where M is a doping element, -0.1 ≤ a ≤ 0.1, 0.05 ≤ x ≤ 0.95, 0.05 ≤ y ≤ 0.95, b ≥ 0 and x + y + b = 1; the doping element M is selected from at least one of Fe, Cu, Co, Mg, Al, Ti, Cr, Li, Ca, Zn, Sr, Y, Zr, La, F, Si, P, and B.

3. The positive electrode sheet according to claim 1 or 2, characterized in that The third coating layer Na x N y O z Synthesized by the solid-phase sintering method. In the synthesis, a sodium source and an N source are mixed evenly according to the stoichiometric ratio. Preferably, the sodium source is at least one of sodium nitrate, sodium carbonate, sodium acetate, sodium hydroxide, and sodium citrate; preferably, the N source is one of chromium nitrate and zirconium nitrate; the mixed materials are placed in an inert atmosphere furnace and sintered at 800-1000°C for 6-10 hours, cooled and taken out, and obtained after being pulverized.

4. The positive electrode sheet according to any one of claims 1 to 3, characterized in that, The current collector is one of an aluminum foil current collector, an aluminum-based composite current collector, a perforated aluminum foil current collector, and a perforated aluminum-based composite current collector; and / or, the thickness of the current collector is 8-16 μm; the surface energy value of the current collector is ≥30 dyn / cm.

5. The positive electrode sheet according to any one of claims 1-4, characterized in that, The first coating layer, the second coating layer, and the third coating layer further contain a positive electrode conductive agent and a positive electrode binder; the positive electrode conductive agent is usually one or a combination of conductive carbon black, conductive graphite, Ketjen black, acetylene black, carbon nanotubes, graphene, etc. The positive electrode binder is usually polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE). The mass ratio of the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder in the first coating layer, the second coating layer, and the third coating layer is (90-98):(1-5):(1-5).

6. The positive electrode sheet according to any one of claims 1-5, characterized in that, The positive electrode active material in the first coating layer contains primary particles, and the primary particles form secondary particles. The D50 particle size of the primary particles is 0.1 μm-2.5 μm, and the D50 average particle size of the secondary particles is 3-20 μm; and / or, the specific surface area of the positive electrode active material in the first coating layer is 0.1 m 2 / g ≤ BET ≤ 5 m 2 / g.

7. The positive electrode sheet according to any one of claims 1-6, characterized in that, The positive electrode active material in the second coating layer only contains primary particles, and the D50 particle size of the primary particles is 3 μm-15 μm; and / or, the specific surface area of the positive electrode active material in the second coating layer is 0.1 m 2 / g ≤ BET ≤ 2m 2 / g.

8. The positive electrode sheet according to any one of claims 1-7, characterized in that, The D50 particle size of the third coated cathode active material is 1 μm - 15 μm; and / or, the specific surface area of the third coated cathode material is 0.1 m 2 / g ≤ BET ≤ 5 m 2 / g.

9. The positive electrode sheet according to any one of claims 1-8, characterized in that, The areal density of the first coating layer is 0 - 20 mg / cm 2 , and the areal density of the second coating layer is 0 - 20 mg / cm 2 , and the areal density of the third coating layer is 1 - 5 mg / cm 2 .

10. A sodium ion battery, the sodium ion battery includes a positive electrode sheet according to any one of claims 1-9.

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