A positive electrode additive and a positive electrode active material, and a preparation method and application thereof

Abstract

The application belongs to the field of sodium ion batteries, and relates to a positive electrode additive and a positive electrode active material as well as a preparation method and application thereof. The preparation method of the positive electrode additive comprises the following steps: S1, mixing a multi-chain carbon source, a crosslinking agent and a sodium source agent to obtain a mixture; S2, performing crosslinking calcination, pre-carbonization calcination and in-situ growth calcination on the mixture under the protection of an inert gas at specific temperatures in sequence, crushing and screening the obtained calcination product to obtain the positive electrode additive with a particle size Dv50 of 2 microns or less. The positive electrode additive provided by the application can effectively improve the conductivity and cycle stability of a polyanion type positive electrode material.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion batteries, specifically relating to a positive electrode additive and a positive electrode active material, their preparation methods, and applications. Background Technology

[0002] Sodium-ion batteries are reversible secondary batteries, operating on a similar principle to lithium-ion batteries, primarily relying on the insertion and extraction of sodium ions between the positive and negative electrodes. Sodium-ion batteries can achieve high-rate charging, and their energy density and cycle life are superior to lead-acid batteries. The theoretical cost of sodium-ion batteries is lower than that of lithium-ion batteries, and with the ramp-up of material production capacity and the gradual maturation of the industry chain, the cost of sodium-ion batteries will decline rapidly. It is projected that by 2025, my country's sodium-ion battery market will reach 60 GWh. The main cathode materials for sodium-ion batteries include transition metal oxides, polyanionic types, Prussian blue, and their analogues. Among these, the polyanionic cathode material Na... x M y [(XO m ) n- [ ] is a multifaceted framework structure composed of sodium, transition metals, and anions, where M is a metal element with variable valence, and X is at least one of P, S, V, Si, As, B, Mo, W, and Ge. Although this polyanionic cathode material has good structural and thermal stability, and a high voltage plateau in sodium-ion cells, resulting in longer cycle life and higher safety, it suffers from poor conductivity, which severely affects the energy density and power density of sodium-ion batteries. Furthermore, imparting high conductivity to anionic cathode materials requires complex material synthesis processes, which hinders their industrial application.

[0003] To address the poor conductivity of polyanionic cathode materials, carbon coating or elemental doping and substitution are generally used to modify them. Carbon coating has two approaches: (1) In-situ carbon coating: Inorganic or organic carbon sources are mixed with precursor materials to form Na x M y [(XO m ) n- The precursor of [ ], during high-temperature sintering of the precursor, the carbon source decomposes, dehydrates, carbonizes, and uniformly coats Na. x M y [(XO m ) n- On the surface, the escaped gas plays a certain role in creating pores, increasing the specific surface area of ​​the material to some extent. Furthermore, the in-situ generated carbon not only inhibits particle growth in the synthesized material but also suppresses particle agglomeration. The excellent conductivity of carbon itself and the small particle size of the synthesized material both enhance the performance of Na+. x M y [(XOm ) n- (2) Non-in-situ coating: the carbon source and Na x M y [(XO m ) n- Na was obtained by high-temperature calcination of the precursor together with the precursor. x M y [(XO m ) n- The composite material of ] / C, which can form high-purity Na before coating. x M y [(XO m ) n- The selection of carbon sources is also very rich. Although this method is simple to operate, it cannot uniformly coat carbon onto Na. x M y [(XO m ) n- ]Surface, Na x M y [(XO m ) n- The uniformity of material properties is difficult to guarantee. There are two main methods for element doping and substitution: (1) Using non-metallic elements for ectopic substitution, specifically the breaking of anion-oxygen bonds, and introducing non-metallic elements such as P, S, B and halogen elements at the breaking point. The substitution of non-metallic elements improves the charge exchange path, so that the polyanionic material itself can also conduct electrons, thereby improving the conductivity; (2) Using metal ions (such as Mg, Nb, Al, Ti, etc.) for modification. These metal ions occupy the position of transition metal M in polyanionic cathode materials, causing M to exhibit multiple valence states. This can create more metal polyhedral bonding points, and the electronic conductivity can be improved by 3 to 8 orders of magnitude.

[0004] For example, CN115939345A discloses a method for preparing a nitrogen-doped carbon-coated modified polyanionic cathode material. This method involves sequentially adding an alkali metal source, an iron source, a phosphate source, a carbon source, a nitrogen source, and a dopant element source to a solvent, followed by low-temperature spray drying granulation, calcination, and sieving to obtain the nitrogen-doped carbon-coated polyanionic cathode material. This method, through nitrogen doping and carbon coating, utilizes the synergistic effect of CN to increase the electronic conductivity of the cathode material and improve its cycle stability. However, it requires spray drying at relatively low temperatures and necessitates more equipment to achieve the required heat transfer and drying conditions. For example, CN114639829A discloses a metal-doped polyanionic cathode material and its preparation method. This method involves adding a mixed solution of deionized water and ethylene glycol to a raw material solid-phase mixture, reacting the mixture in a reactor at a high temperature to obtain an Fe-doped precursor, thoroughly grinding each part of the precursor, and then calcining it to obtain the Fe-doped polyanionic cathode material. Fe doping shortens the VO bond length and stabilizes the O atom, thereby improving stability and electrochemical kinetics. 3+ The addition of Na as a substitute + The diffusion coefficient and electronic conductivity are high, but the hydrothermal method cannot guarantee the stability and uniformity of Fe-doped precursor nuclei growth, and the low yield of the hydrothermal method makes it unsuitable for large-scale commercial application. Summary of the Invention

[0005] One of the objectives of this invention is to overcome the shortcomings of poor conductivity and cycle stability of existing polyanionic cathode materials, and to provide a new cathode additive that can effectively improve the conductivity and cycle stability of polyanionic cathode materials.

[0006] After in-depth and extensive research, the inventors of this invention discovered that using starch with a linear-to-branched mass ratio of (20%–30%):(70%–80%) as a carbon source, and simultaneously performing cross-linking calcination, pre-carbonization calcination, and in-situ growth calcination at temperatures of 80–130°C, 200–280°C, and 600–700°C, the resulting cathode additive can effectively improve the conductivity and cycle stability of polyanionic cathode materials. The reasons for this are speculated to be: selecting linear and amylopectin in specific proportions as carbon sources can improve the degree of branching, enrich the conductive network structure, and form a porous framework structure; the ultra-low cross-linking calcination temperature (80–130°C) can ensure that the porous framework structure of starch is not destroyed while maintaining cross-linking, thus stabilizing the porous framework structure; and the moderate pre-carbonization calcination temperature (200–280°C) can allow small molecules that affect conductivity, such as H2O, in the porous framework structure to fully volatilize without affecting the framework structure. A relatively high in-situ growth and calcination temperature (600–700℃) ensures that the starch is fully carbonized. The resulting cathode additive not only possesses a strong and uniform interpenetrating conductive network structure with high conductivity, but its strong conductivity and small particle size also allow it to form a micro-nano hierarchical structure with the polyanionic cathode material, filling the crystal defects and gaps. This creates an organic conductive network, reducing the resistivity of the powder and electrode layers. The macroscopic reduction in resistivity manifests as a shortening of the sodium ion (Na₂O₃) ion exchange rate. + The cathode additive, by increasing the distance of solid-phase diffusion within the cathode particles, enhances the electron diffusion channels within the cathode active material and improves the electron transport rate between particles. Furthermore, the hierarchical structure formed by the cathode additive and cathode material particle sizes increases the contact area with the electrolyte, shortens the ion transport path, and lowers the diffusion barriers and corresponding diffusion distances of sodium ions along different paths. This improves the overall reaction kinetics of the material and significantly enhances the cycle performance of sodium-ion secondary batteries. Based on these principles, this invention was completed.

[0007] Specifically, the method for preparing the positive electrode additive provided by the present invention includes the following steps:

[0008] S1. Mix a multi-chain carbon source, a crosslinking agent and a sodium source, wherein the multi-chain carbon source contains amylose and amylopectin in a mass ratio of (20% to 30%):(70% to 80%) to obtain a mixture;

[0009] S2. The mixture is subjected to cross-linking calcination, pre-carbonization calcination and in-situ growth calcination in sequence under inert gas protection. The temperature of cross-linking calcination C1 is 80-130℃, the temperature of pre-carbonization calcination is 200-280℃, and the temperature of in-situ growth calcination is 600-700℃. The resulting calcination product is crushed and sieved to obtain a positive electrode additive with a particle size Dv50 of less than 2μm.

[0010] The second objective of this invention is to provide a positive electrode additive prepared by the above method.

[0011] A third objective of this invention is to provide a positive electrode active material, wherein the positive electrode active material contains a polyanionic positive electrode material and the aforementioned positive electrode additive.

[0012] The fourth objective of this invention is to provide the application of the above-mentioned positive electrode active material in sodium-ion batteries.

[0013] The cathode additive provided by this invention can significantly improve the conductivity and cycle stability of polyanionic cathode materials. In practical application, no additional modification of the polyanionic cathode material is required; the additive can be directly added to the polyanionic cathode material. The cathode active material provided by this invention has a powder resistivity of 70–80 Ω*cm at 10 MPa, and the electrode resistivity prepared from it at 10 MPa is 30–40 Ω*cm. Furthermore, the preparation process of the cathode additive provided by this invention is simple and efficient. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of the hierarchical structure between the cathode additive and the polyanionic cathode material. Detailed Implementation

[0015] In the preparation process of the positive electrode additive (Na2O / C) provided by this invention, step S1 involves mixing a multi-chain carbon source, a crosslinking agent, and a sodium source to obtain a mixture. The multi-chain carbon source contains amylose and amylopectin, preferably composed of both. This imparts a rich conductive network structure to the positive electrode additive, laying a good foundation for improving the conductivity of the polyanionic positive electrode material. The mass ratio of amylose to amylopectin is (20%–30%):(70%–80%), for example, 20%:80%, 22%:78%, 24%:76%, 26%:74%, 28%:72%, 30%:70%, or any value between them. The multi-chain carbon source can be derived from corn, soybeans, etc.

[0016] In this invention, in step S1, the preferred mass ratio of the multi-chain carbon source, crosslinking agent, and sodium source agent is 1:(0.15-0.3):(0.1-0.3). At this ratio, the components can achieve a perfect synergistic effect, resulting in a cathode additive that not only has a richer conductive network structure but also disperses more uniformly in the polyanionic cathode material, thus enabling more effective conductivity. The crosslinking agent can be any existing substance that can crosslink the multi-chain carbon source, including but not limited to at least one of phosphorus oxychloride, sodium hexametaphosphate, and citric acid. The sodium source agent can be any existing substance that can provide a sodium source, including but not limited to at least one of disodium hydrogen phosphate, sodium acetate, sodium hydrosulfide, and sodium oxalate. Based on 1 part by weight of the multi-chain carbon source, the amount of crosslinking agent is preferably 0.15 to 0.3 parts by weight, such as 0.15, 0.18, 0.2, 0.22, 0.24, 0.26, 0.28, 0.3 parts by weight or any value between them; the amount of sodium source is preferably 0.1 to 0.3 parts by weight, such as 0.1, 0.12, 0.14, 0.16, 0.18, 0.2, 0.22, 0.24, 0.26, 0.28, 0.3 parts by weight or any value between them.

[0017] In this invention, in step S1, the mixing of the multi-chain carbon source, crosslinking agent, and sodium source can be carried out in various existing mixing equipment, such as a V-type mixer. The mixing conditions typically include a rotational speed of 20–25 rpm, such as 20 rpm, 22 rpm, 24 rpm, 25 rpm, or any value between them; and a mixing time of 40–60 min, such as 40 min, 42 min, 45 min, 48 min, 50 min, 52 min, 55 min, 58 min, 60 min, or any value between them.

[0018] In the preparation process of the positive electrode additive provided by the present invention, in step S2, the mixture is subjected to cross-linking calcination, pre-carbonization calcination and in-situ growth calcination in sequence under inert gas protection. The cross-linking calcination temperature C1 is 80-130℃, the pre-carbonization calcination temperature is 200-280℃, and the in-situ growth calcination temperature is 600-700℃. The obtained calcination product is crushed and sieved to obtain a positive electrode additive with a particle size Dv50 of less than 2μm.

[0019] In this invention, in step S2, the crosslinking calcination temperature C1 must be controlled between 80 and 130°C, such as 80°C, 85°C, 90°C, 95°C, 100°C, 105°C, 110°C, 115°C, 120°C, 125°C, 130°C, or any value between them. Furthermore, the heating rate V1 of the crosslinking calcination is preferably 1 to 3°C / min, such as 1, 1.2, 1.5, 1.8, 2, 2.2, 2.5, 2.8, 3°C / min, or any value between them. The holding time T1 of the crosslinking calcination is preferably 2 to 3 hours, such as 2 hours, 2.2 hours, 2.4 hours, 2.6 hours, 2.8 hours, 3 hours, or any value between them.

[0020] In this invention, in step S2, the pre-carbonization roasting temperature C2 must be controlled between 200 and 280°C, such as 200°C, 205°C, 210°C, 215°C, 220°C, 225°C, 230°C, 235°C, 240°C, 245°C, 250°C, 255°C, 260°C, 265°C, 270°C, 275°C, 280°C, or any value between them. Furthermore, the pre-carbonization roasting heating rate V2 is preferably 1 to 3°C / min, such as 1, 1.2, 1.5, 1.8, 2, 2.2, 2.5, 2.8, 3°C / min, or any value between them. The pre-carbonization roasting holding time T2 is preferably 3 to 4 hours, such as 3 hours, 3.2 hours, 3.4 hours, 3.6 hours, 3.8 hours, 4 hours, or any value between them.

[0021] In this invention, in step S2, the in-situ growth and calcination temperature C3 must be controlled between 600 and 700°C, such as 600°C, 605°C, 610°C, 615°C, 620°C, 625°C, 630°C, 635°C, 640°C, 645°C, 650°C, 655°C, 660°C, 665°C, 670°C, 675°C, 680°C, 685°C, 690°C, 695°C, 700°C, or any value between them. Furthermore, the heating rate V3 of the in-situ growth and calcination is preferably 1 to 3°C / min, such as 1, 1.2, 1.5, 1.8, 2, 2.2, 2.5, 2.8, 3°C / min, or any value between them. The holding time T3 for in-situ growth calcination is preferably 3 to 4 hours, such as 3 hours, 3.2 hours, 3.4 hours, 3.6 hours, 3.8 hours, 4 hours or any value between them.

[0022] In this invention, in step S2, the calcined product is crushed and sieved. The crushing and sieving conditions are such that the particle size Dv50 of the positive electrode additive is less than 2 μm, preferably 0.8 μm to 1.2 μm, such as 0.8 μm, 0.9 μm, 1 μm, 1.1 μm, 1.2 μm, or any value between them. The particle size Dv50 of the polyanionic positive electrode material is generally 5 to 8 μm, such as 5 μm, 5.5 μm, 6 μm, 6.5 μm, 7 μm, 7.5 μm, 8 μm, or any value between them. Figure 1 As shown, controlling the particle size Dv50 of the positive electrode additive to below 2μm, preferably between 0.8μm and 1.2μm, is more conducive to forming a micro-nano hierarchical structure between the positive electrode additive and the polyanionic positive electrode material, filling the crystal defects and gaps of the polyanionic positive electrode material, thereby forming a conductive network and reducing the resistivity of the powder and electrode layers.

[0023] In this invention, in step S2, the inert gas can be nitrogen and / or a group 0 element gas. The group 0 element gas can be, for example, at least one of helium, neon, argon, etc.

[0024] The positive electrode active material provided by this invention contains a polyanionic positive electrode material and the aforementioned positive electrode additive. The preferred mass ratio of the polyanionic positive electrode material to the positive electrode additive is 1:(3%–5%), such as 1:3%, 1:3.2%, 1:3.4%, 1:3.6%, 1:3.8%, 1:4%, 1:4.2%, 1:4.4%, 1:4.6%, 1:4.8%, 1:5%, or any value between them.

[0025] In this invention, the polyanionic cathode material refers to a compound with a three-dimensional network structure formed by strong covalent bonds connecting polyanionic polyhedra and transition metal ion polyhedra, and its general formula can be, for example, Na. x M y [(XO m ) n- In this context, M is a metallic element with a variable valence state, such as at least one of Ti, V, Cr, Mn, Fe, Co, Ni, Ca, Mg, Al, Nb, etc.; X is at least one of P, S, V, Si, As, B, Mo, W, Ge, etc.; x is 1 to 4, specifically 1, 2, 3, or 4; y is 1 to 3, specifically 1, 2, or 3; m is 3 to 4, specifically 3 or 4; and n is 0 to 3, specifically 0, 1, 2, or 3. Specifically, examples of polyanionic cathode materials include, but are not limited to, at least one of NaFePO4, Na2FeP2O7, Na3V2(PO4)3, and Na2MnSiO4.

[0026] The present invention will be described in detail below through embodiments.

[0027] In the following examples, comparative examples, and test cases, amylose YD1 was purchased from Hubei Watson Chemical Technology Co., Ltd., grade 9005-82-7; amylopectin YD2 was purchased from Henan Zunting Biotechnology Co., Ltd., grade 9037-22-3; positive electrode conductive paste CNT was purchased from Jiangsu Tiannai Technology Co., Ltd., grade LB120-50; conductive agent SP was purchased from Henan Yinghui Chemical Co., Ltd., grade 1333-86-4; positive electrode binder PVDF was purchased from Shenzhen Liyou New Energy Technology Co., Ltd., grade TOB-PVDF-6020; negative electrode active material hard carbon was purchased from Shenzhen Zhongyue Chemical Co., Ltd., grade BSHC-260; negative electrode binder CMC was purchased from Shanghai Wanzhao Fine Chemical Co., Ltd., grade WSG-B20H; and styrene-butadiene rubber was purchased from Dongguan Donglin Polymer Materials Co., Ltd., grade SN-307R.

[0028] Example 1

[0029] S1. Place 5 kg of multi-chain carbon source (a mixture of amylose YD1 and amylopectin YD2 in a mass ratio of 20%:80%), 0.75 kg of crosslinking agent (phosphorus oxychloride), and 0.5 kg of sodium source agent (disodium hydrogen phosphate) into a V-type mixer and mix them at a speed of 20 rpm for 40 min to obtain mixture A.

[0030] S2. Mixture A is placed in a box furnace with nitrogen as the protective gas (gas flow rate controlled at 2 L / min) and subjected to cross-linking calcination, pre-carbonization calcination, and in-situ growth calcination sequentially. The cross-linking calcination temperature C1 is 80℃, the heating rate V1 is 2℃ / min, and the holding time T1 is 2h; the pre-carbonization calcination temperature C2 is 220℃, the heating rate V2 is 1℃ / min, and the holding time T2 is 3h; the in-situ growth calcination temperature C3 is 600℃, the heating rate V3 is 1℃ / min, and the holding time T3 is 3h. The calcined product is then subjected to air jet milling and classification, with the particle size Dv50 controlled to be 0.8μm, to obtain the positive electrode additive.

[0031] S3. The positive electrode additive and the polyanionic positive electrode material NaFePO4 (Dv50 is 5.6μm) are mixed evenly in a batch mixer at a mass ratio of 3%:1 to obtain the positive electrode active material NCN.

[0032] The conditions for each step are shown in Table 1.

[0033] Example 2

[0034] S1. Place 5 kg of multi-chain carbon source (a mixture of amylose YD1 and amylopectin YD2 in a mass ratio of 25%:75%), 1 kg of crosslinking agent (sodium hexametaphosphate), and 0.75 kg of sodium source agent (sodium acetate) in a V-type mixer and mix them at a speed of 23 rpm for 50 min to obtain mixture A.

[0035] S2. Mixture A was placed in a box furnace with nitrogen as the protective gas (flow rate controlled at 2 L / min) and subjected to cross-linking calcination, pre-carbonization calcination, and in-situ growth calcination sequentially. The cross-linking calcination temperature C1 was 130℃, the heating rate V1 was 1℃ / min, and the holding time T1 was 3 h; the pre-carbonization calcination temperature C2 was 260℃, the heating rate V2 was 1℃ / min, and the holding time T2 was 3 h; the in-situ growth calcination temperature C3 was 650℃, the heating rate V3 was 1℃ / min, and the holding time T3 was 4 h. The calcined product was then subjected to air jet milling and fractionation, with the particle size Dv50 controlled to 1 μm to obtain the positive electrode additive.

[0036] S3. The positive electrode additive and the polyanionic positive electrode material NaFePO4 (Dv50 is 5.6μm) are mixed evenly in a batch mixer at a mass ratio of 4%:1 to obtain the positive electrode active material NCN.

[0037] The conditions for each step are shown in Table 1.

[0038] Example 3

[0039] S1. Place 5 kg of multi-chain carbon source (a mixture of amylose YD1 and amylopectin YD2 in a mass ratio of 30%:70%), 1.5 kg of crosslinking agent (citric acid), and 1 kg of sodium source agent (sodium hydrosulfide) into a V-type mixer and mix them. Set the speed to 25 rpm and the mixing time to 60 min to obtain mixture A.

[0040] S2. Mixture A was placed in a box furnace with nitrogen as the protective gas (flow rate controlled at 2 L / min) and subjected to cross-linking calcination, pre-carbonization calcination, and in-situ growth calcination sequentially. The cross-linking calcination temperature C1 was 120℃, the heating rate V1 was 2℃ / min, and the holding time T1 was 2 h; the pre-carbonization calcination temperature C2 was 280℃, the heating rate V2 was 2℃ / min, and the holding time T2 was 4 h; the in-situ growth calcination temperature C3 was 700℃, the heating rate V3 was 2℃ / min, and the holding time T3 was 3 h. The calcined product was then subjected to air jet milling and fractionation, with the particle size Dv50 controlled to 1.2 μm to obtain the positive electrode additive.

[0041] S3. The positive electrode additive and the polyanionic positive electrode material NaFePO4 (Dv50 is 5.6μm) are mixed evenly in a batch mixer at a mass ratio of 5%:1 to obtain the positive electrode active material NCN.

[0042] The conditions for each step are shown in Table 1.

[0043] Example 4

[0044] The positive electrode additive and positive electrode active material were prepared according to the method of Example 1, except that the polyanionic positive electrode material NaFePO4 was replaced by the same weight of the polyanionic positive electrode material Na2FeP2O7 (Dv50 of 7.2 μm). The remaining steps and conditions were the same as in Example 1, and the positive electrode additive and positive electrode active material NCN were obtained. The conditions for each step are shown in Table 1.

[0045] Example 5

[0046] The positive electrode additive and positive electrode active material were prepared according to the method of Example 1, except that the polyanionic positive electrode material NaFePO4 was replaced by the same weight of the polyanionic positive electrode material Na3V2(PO4)3 (Dv50 of 6.3 μm). The remaining steps and conditions were the same as in Example 1, and the positive electrode additive and positive electrode active material NCN were obtained. The conditions for each step are shown in Table 1.

[0047] Blank example

[0048] Compared to Example 1, the positive electrode additive Na2O / C was not synthesized; only the polyanionic positive electrode material NaFePO4 (Dv50 is 5.6 μm) was synthesized.

[0049] Comparative Example 1

[0050] The positive electrode additive and positive electrode active material were prepared according to the method of Example 1, except that no crosslinking agent was used in the preparation of the positive electrode additive. The remaining steps and conditions were the same as in Example 1, and the positive electrode additive and positive electrode active material NCN were obtained. The conditions for each step are shown in Table 1.

[0051] Comparative Example 2

[0052] The positive electrode additive and positive electrode active material were prepared according to the method of Example 1, except that no sodium source agent was used in the preparation of the positive electrode additive. The remaining steps and conditions were the same as in Example 1, and the positive electrode additive and positive electrode active material NCN were obtained. The conditions for each step are shown in Table 1.

[0053] Comparative Example 3

[0054] The cathode additive and cathode active material were prepared according to the method of Example 1, except that in the preparation of the cathode additive, the mass ratio of amylose to amylopectin in the multi-chain carbon source was 10%:90%. The remaining steps and conditions were the same as in Example 1, and the cathode additive and cathode active material NCN were obtained. The conditions for each step are shown in Table 1.

[0055] Comparative Example 4

[0056] The cathode additive and cathode active material were prepared according to the method of Example 1, except that the crosslinking calcination temperature was controlled at 180°C during the preparation of the cathode additive. The remaining steps and conditions were the same as in Example 1, resulting in the cathode additive and cathode active material NCN. The conditions for each step are shown in Table 1.

[0057] Comparative Example 5

[0058] The cathode additive and cathode active material were prepared according to the method of Example 1, except that the pre-carbonization calcination temperature was controlled at 180°C during the preparation of the cathode additive. The remaining steps and conditions were the same as in Example 1, resulting in the cathode additive and cathode active material NCN. The conditions for each step are shown in Table 1.

[0059] Comparative Example 6

[0060] The cathode additive and cathode active material were prepared according to the method of Example 1, except that the pre-carbonization calcination temperature was controlled at 400°C during the preparation of the cathode additive. The remaining steps and conditions were the same as in Example 1, resulting in the cathode additive and cathode active material NCN. The conditions for each step are shown in Table 1.

[0061] Comparative Example 7

[0062] The cathode additive and cathode active material were prepared according to the method of Example 1. The difference was that in the preparation process of the cathode additive, the air jet milling and classification particle size Dv50 were controlled at 5 μm. The remaining steps and conditions were the same as in Example 1, and the cathode additive and cathode active material NCN were obtained. The conditions for each step are shown in Table 1.

[0063] Test case

[0064] The positive electrode active material NCN, positive electrode conductive paste CNT (graphene@carbon nanotube composite conductive paste), positive electrode conductive agent SP (Super-P), and positive electrode binder PVDF (polyvinylidene fluoride) obtained in the above embodiments and comparative examples were mixed at a mass ratio of 97:0.5:1:1.5. N-methylpyrrolidone (NMP) solvent was added and the mixture was stirred evenly in a planetary vacuum mixer to obtain the positive electrode paste. The positive electrode paste was uniformly coated onto the surface of carbon-coated aluminum foil and dried in a high-temperature oven to obtain a positive electrode sheet. Subsequently, the positive electrode sheet was rolled, slit, and die-cut to obtain a rectangular positive electrode sheet with a thickness of 172±0.5 μm.

[0065] Hard carbon (anode active material), SP (Super-P) (anode conductive agent), CMC (sodium carboxymethyl cellulose) (anode binder), and SBR (styrene-butadiene rubber) were mixed in a mass ratio of 95:1:1.5:2.5. Deionized water was added as solvent, and the mixture was stirred evenly in a planetary vacuum mixer to obtain anode slurry. The anode slurry was uniformly coated onto the surface of copper foil and dried in a high-temperature oven to obtain anode sheets. The anode sheets were then rolled, slit, and die-cut to obtain rectangular anode sheets with a thickness of 172±0.5 μm.

[0066] An electrolyte was prepared by mixing dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethylene carbonate (EC) in a volume ratio of 1:1:1 as solvents, 1 mol / L NaPF6 as solute, and 1% vinylene carbonate (VC) as an additive, using magnetic stirring.

[0067] (1) Powder resistivity: The NCN powder of the positive electrode active material obtained in the above examples and comparative examples was pressed into a thin disc with a diameter of 101±0.5mm and a thickness of 41±0.5mm. Then, the powder resistivity was tested using a four-probe method. The test temperature was 20℃~30℃, the temperature fluctuation during the test was ±1℃, the pressure was 10MPa, and the holding time was 10s. The results are shown in Table 2.

[0068] (2) Electrode resistivity: The rectangular positive and negative electrodes prepared by the above method were cut into rectangles of 5cm × 10cm. Then, the resistivity of the electrodes was tested using an electrode resistivity meter at a pressure of 10MPa and a holding time of 10s. The results are shown in Table 2.

[0069] (3) Capacity and capacity retention rate: Using an alumina-coated PE film as a separator, the above-prepared positive electrode, separator and negative electrode are stacked in sequence to obtain a bare cell. Then, the bare cell is welded with positive aluminum tabs and negative copper tabs, and put into an aluminum-plastic shell. After drying, the moisture content of the bare cell reaches the standard, and then the electrolyte is injected. Then, the cell is sealed on the top side, placed at high temperature, formed, finally sealed and shaped to obtain a sodium-ion secondary battery.

[0070] At 25°C, the sodium-ion secondary batteries prepared above were subjected to the following tests using a nebula testing system: capacity performance was tested at a rate of 0.1C; rate cycle performance was tested at rates of 0.3C and 1C, respectively, and the capacity during charge and discharge processes was recorded, up to 500 cycles, and the capacity cycle retention rate of the sodium-ion batteries was determined. The results are shown in Table 2.

[0071]

[0072]

[0073] As shown in Table 2, Examples 1-5 show significant improvements in both powder resistivity and electrode resistivity compared to the blank example and Comparative Examples 1-7. Examples 1-5 exhibit approximately 8.9%-26.4% higher specific capacity at 0.1C, approximately 11.4%-29.3% higher capacity retention at 0.3C, and approximately 8.1%-20.8% higher capacity retention at 1C compared to the blank example and Comparative Examples 1-7. Therefore, the cathode additive obtained using the method provided by this invention can significantly improve the conductivity and cycle stability of polyanionic cathode materials.

[0074] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention without departing from the principles and spirit of the present invention.

Claims

1. A method for preparing a positive electrode additive, characterized in that, The method includes the following steps: S1. Mix a multi-chain carbon source, a crosslinking agent and a sodium source, wherein the multi-chain carbon source contains amylose and amylopectin in a mass ratio of (20%~30%):(70%~80%) to obtain a mixture; S2. The mixture is subjected to cross-linking calcination, pre-carbonization calcination and in-situ growth calcination in sequence under inert gas protection. The temperature of cross-linking calcination C1 is 80~130℃, the temperature of pre-carbonization calcination C2 is 200~280℃, and the temperature of in-situ growth calcination C3 is 600~700℃. The calcined product is crushed and sieved to obtain a positive electrode additive with a particle size Dv50 of less than 2μm.

2. The method for preparing the positive electrode additive according to claim 1, characterized in that, In step S1, the mass ratio of the multi-chain carbon source, crosslinking agent, and sodium source agent is 1:(0.15~0.3):(0.1~0.3).

3. The method for preparing the positive electrode additive according to claim 1, characterized in that, In step S1, the crosslinking agent is selected from at least one of phosphorus oxychloride, sodium hexametaphosphate, and citric acid.

4. The method for preparing the positive electrode additive according to claim 1, characterized in that, In step S1, the sodium source agent is selected from at least one of disodium hydrogen phosphate, sodium acetate, sodium hydrosulfide, and sodium oxalate.

5. The method for preparing the positive electrode additive according to claim 1, characterized in that, In step S2, the heating rate V1 of the crosslinking calcination is 1~3℃ / min, and the holding time T1 is 2~3h.

6. The method for preparing the positive electrode additive according to claim 1, characterized in that, In step S2, the heating rate V2 of the pre-carbonization roasting is 1~3℃ / min, and the holding time T2 is 3~4h.

7. The method for preparing the positive electrode additive according to claim 1, characterized in that, In step S2, the heating rate V3 of the in-situ growth calcination is 1~3℃ / min, and the holding time T3 is 3~4h.

8. The method for preparing the positive electrode additive according to claim 1, characterized in that, In step S2, the crushing and sieving conditions result in a particle size Dv50 of 0.8 μm to 1.2 μm for the positive electrode additive.

9. The positive electrode additive prepared by the method according to any one of claims 1 to 8.

10. A positive electrode active material, characterized in that, The positive electrode active material contains a polyanionic positive electrode material and the positive electrode additive as described in claim 9.

11. The positive electrode active material according to claim 10, characterized in that, The mass ratio of the polyanionic cathode material to the cathode additive is 1:(3%~5%).

12. The positive electrode active material according to claim 10, characterized in that, The general formula of the polyanionic cathode material is Na. x M y [(XO m ) n- M is a metallic element with a variable valence state, X is at least one of P, S, V, Si, As, B, Mo, W and Ge, x is 1~4, y is 1~3, m is 3~4 and n is 0~3.

13. The application of the positive electrode active material according to any one of claims 10 to 12 in a sodium-ion battery.

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