Long-circulation high-voltage sodium ion positive electrode material and preparation method thereof

Abstract

The invention discloses a long-circulation high-voltage sodium ion positive electrode material and a preparation method, the positive electrode material comprises an inner core and a coating layer arranged on the surface of the inner core, the inner core comprises an NMPP precursor, the coating layer is a polyacrylonitrile carbon-based coating layer, and the mass ratio of the NMPP precursor to the polyacrylonitrile carbon-based coating layer is 1: (0.003-0.05). The NMPP / C material has the characteristic of long circulation, and the N-doped graphitized carbon layer formed by carbonization of polyacrylonitrile realizes multi-dimensional synergistic reinforcement of an NMPP precursor by improving electronic conductivity, providing a rapid Na < + > transmission channel, buffering volume change, blocking electrolyte erosion and forming chemical bonding, so that the performance of the NMPP / C material is improved, and the service life of the NMPP / C material is prolonged. And the carbon-coated NMPP / C has better electrochemical performance in a sodium ion battery with high power and long service life.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of sodium ion batteries, and particularly relates to a long-cycle high-voltage sodium ion positive electrode material and a preparation method thereof. BACKGROUND

[0002] As a sodium storage material, the polyanion compound has the advantages of simple synthesis method, high structural stability, and easy scale production, and is the most potential positive electrode material for developing low-cost sodium ion batteries.

[0003] At present, the polyanion positive electrode material still faces many challenges in practical application. The theoretical specific capacity of sodium iron pyrophosphate phosphate is 129 mAh / g, the discharge average voltage is 3.0 V, and the low energy density is an important reason for limiting its large-scale application. The discharge average voltage of sodium manganese pyrophosphate phosphate (NMPP) material can reach 3.7 V due to the presence of manganese element, which is much higher than 3.0 V of sodium iron pyrophosphate phosphate. In recent years, it has attracted the attention of scientific researchers. However, due to the presence of P-O bond, the intrinsic electronic conductivity of the polyanion material is poor. The surface of the material is usually coated with amorphous carbon. The ordinary carbon coating has the disadvantages of poor uniformity and weak interface bonding force. At the same time, the manganese element has a large volume change in the charging and discharging process due to the Jahn-Teller distortion, which easily causes the peeling of the surface carbon layer, leads to the dissolution of manganese, and finally leads to poor cycle performance, especially not conducive to the improvement of long cycle performance, which limits its large-scale application. Similarly, sodium iron pyrophosphate phosphate has the problems of poor conductivity and large volume change in the charging and discharging process. The interface bonding force between the ordinary carbon coating and sodium iron pyrophosphate phosphate is weak, which easily causes peeling and leads to poor cycle performance. SUMMARY

[0004] In view of the problems of poor conductivity and large volume change in the charging and discharging process of the polyanion positive electrode material sodium iron pyrophosphate phosphate and sodium manganese pyrophosphate phosphate in the prior art, the application provides a long-cycle high-voltage sodium ion positive electrode material and a preparation method thereof to improve the electronic conductivity, interface stability and cycle performance of the material.

[0005] In order to achieve the above-mentioned purpose, in a first aspect, the application provides a long-cycle high-voltage sodium ion positive electrode material. The positive electrode material comprises a core and a coating layer arranged on the surface of the core. The core comprises a NMPP precursor, and the coating layer is a polyacrylonitrile carbon-based coating layer. The mass ratio of the NMPP precursor to the polyacrylonitrile carbon-based coating layer is 1: (0.003-0.05). The polyacrylonitrile coating NMPP precursor is obtained by adding the NMPP precursor, acrylonitrile and methyl methacrylate into dimethyl ether solvent, heating and stirring, and then filtering and drying. The polyacrylonitrile carbon-based coating layer coated with the NMPP precursor is obtained by carbonizing the NMPP coated with polyacrylonitrile.

[0006] Preferably, the mass ratio of the acrylonitrile to the methyl methacrylate is 1:(0.05-0.12).

[0007] In a second aspect, the present application provides a preparation method of a long-circulation high-voltage sodium-ion positive electrode material, comprising the following steps: S1, dissolving a sodium source, a manganese source and a phosphorus source in deionized water, performing sand milling to obtain slurry A, and performing spray drying on the slurry A to obtain a precursor B; S2, sintering the precursor B prepared in step S1 under a sintering atmosphere to obtain an NMPP precursor; S3, adding the NMPP precursor prepared in step S2, acrylonitrile and methyl methacrylate into a dimethyl ether solvent, heating and stirring, and then performing filtration and drying to obtain an NMPP coated with polyacrylonitrile; S4, carbonizing the NMPP coated with polyacrylonitrile prepared in step S3 to obtain the long-circulation high-voltage sodium-ion positive electrode material; the carbonization temperature is 280-350°C, and the carbonization time is 2-10h.

[0008] Preferably, in step S1, the sodium source is at least one of sodium carbonate, sodium bicarbonate, disodium hydrogen phosphate, sodium dihydrogen phosphate and sodium pyrophosphate; the manganese source is at least one of manganese carbonate, manganese oxalate, manganese nitrate and manganese oxide; the phosphorus source is at least one of ammonium dihydrogen phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate and phosphoric acid.

[0009] Preferably, in step S1, the particle size D50 of the slurry A is ≤0.4μm.

[0010] Preferably, in step S1, the particle size D50 of the precursor B is 5-25μm.

[0011] Preferably, in step S2, the sintering atmosphere is nitrogen or argon; the sintering temperature is 450-650°C, and the sintering time is 6-18h.

[0012] Preferably, in step S3, the mass ratio of the NMPP precursor to the acrylonitrile is 1:(0.05-0.3).

[0013] Preferably, in step S3, the mass ratio of the acrylonitrile to the methyl methacrylate is 1:(0.05-0.12).

[0014] In a third aspect, the present application provides a sodium ion battery, comprising a positive electrode sheet, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer arranged on the positive electrode current collector, and the positive electrode active material layer comprises the long-cycle high-voltage sodium ion positive electrode material or the long-cycle high-voltage sodium ion positive electrode material prepared by the preparation method.

[0015] The present application has the following beneficial effects: The NMPP / C material in the present application has the characteristics of long cycle, and the N-doped graphitized carbon layer formed by carbonization of polyacrylonitrile has the advantages of improving electronic conductivity, providing fast Na + The transmission channel, buffer volume change, electrolyte erosion and chemical bonding are realized, and the NMPP precursor is synergistically strengthened in multiple dimensions, so that the carbon-coated NMPP / C has better electrochemical performance in the sodium ion battery with high power and long service life. The liquid phase coating method of the present application prepares NMPP / PAN, and more C-N bonds are formed in the carbon coating layer after carbonization. The thickness of the layer is controllable, and the internal nitrogen (such as pyrrole nitrogen) can form a coordination effect with the surface manganese element, thereby enhancing the binding energy of the coating layer and the core, making the coating layer more stable, avoiding the peeling of the carbon coating layer and the dissolution of manganese due to the Jahn-Teller effect during the cycle of NMPP, and improving the cycle performance. BRIEF DESCRIPTION OF DRAWINGS

[0016] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the embodiments. It should be understood that the following drawings only show some embodiments of the present application, and therefore should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can also be obtained without creative labor.

[0017] Figure 1 Preparation method flow chart of the long-cycle high-voltage sodium ion positive electrode material prepared in Example 1 of the present application; Figure 2 N1S spectrum of the long-cycle high-voltage sodium ion positive electrode material prepared in Example 1 of the present application; Figure 3 XPS Mn2P spectrum of the long-cycle high-voltage sodium ion positive electrode material prepared in Example 1 and Comparative Example 1 of the present application; Figure 4 Cycle curve diagram of the battery prepared in Example 1 and Comparative Example 1 of the present application after 500 cycles; Figure 5 Microstructure scanning electron microscope (SEM) diagram of the long-cycle high-voltage sodium ion positive electrode material prepared in Example 1 of the present application. DETAILED DESCRIPTION

[0018] In the present application, the orientation words such as "upper", "lower", "left", "right" are generally understood in connection with the orientation shown in the drawings and the actual application, unless otherwise specified.

[0019] In addition, the terms "first", "second", "third", etc. are used only for descriptive purposes and should not be construed as implying or suggesting relative importance or an indicated number of the specified technical features. Thus, the features defined with "first", "second", etc. can explicitly or implicitly include one or more of the features. In the description of the present application, the meaning of "a plurality of" is two or more, unless otherwise explicitly specified and limited.

[0020] In the present application, unless otherwise explicitly specified and limited, the "on" or "under" of the first feature to the second feature can be that the first and second features are in direct contact, or the first and second features are indirectly in contact through an intermediate medium. Moreover, the "over", "above" and "on" of the first feature to the second feature can be that the first feature is directly above or obliquely above the second feature, or only indicates that the horizontal height of the first feature is higher than that of the second feature. The "under", "below" and "under" of the first feature to the second feature can be that the first feature is directly below or obliquely below the second feature, or only indicates that the horizontal height of the first feature is less than that of the second feature.

[0021] The endpoints of the ranges and any values disclosed herein are not limited to the precise values recited as the exact dimensions are not critical to the present application. Any numerical value, however, can be expressed as a range by either adding or subtracting a small percentage (e.g., 1-10%) from the stated value in the disclosure to account for variations, measurement errors, and the like. For numerical ranges expressed as "from X to Y", it is intended that values including the lower and upper limits of the range are expressly stated. For values which are less than one, one unit in the disclosed range is considered to be 0.0001, 0.001, 0.01 or 0.1 as appropriate. For values which are greater than one, one unit is considered to be 1, 10, 100 or 1000 as appropriate. These amounts are only examples and are not intended to exclude other defined values. The terms "optionally", "optional", and "may" mean that the subsequently described event or circumstance can or can not occur, or that the subsequently described event or circumstance is not required, but can occur.

[0022] The applicant of the present application found that, due to the larger ionic radius of sodium ions, the sodium ion battery has better rate performance compared with the lithium ion battery. In addition, the positive and negative electrode current collectors of the sodium ion battery can use aluminum foil which has relatively low cost, and the similar process technology as the lithium ion battery makes the sodium ion battery compatible with the existing lithium ion battery production equipment in the process of industrialization development, reduces the technical bottleneck, and can further reduce the production cost. The sodium ion battery as a low-cost and high-safety product has very broad application prospects. The polyanion compound as a sodium storage material has the advantages of simple synthesis method, high structural stability and easy large-scale production, and is the most potential positive electrode material for developing low-cost sodium ion batteries. The research on the sodium storage performance of the polyanion compound has very important practical significance for promoting the industrialization of sodium ion batteries.

[0023] Currently, the polyanion positive electrode material still faces many challenges in practical application. The theoretical specific capacity of sodium iron pyrophosphate is 129 mAh / g, and the discharge average voltage is 3.0 V. The low energy density is an important reason limiting its large-scale application. Sodium manganese pyrophosphate material has a discharge average voltage of 3.7 V due to the presence of manganese element, which is much higher than 3.0 V of sodium iron pyrophosphate. In recent years, it has attracted the attention of scientific researchers. The sodium ion battery sodium iron phosphate (NaFePO4) positive electrode material has a low electronic conductivity of only 10 -8 ~10 -9 Scm -1 , which leads to significant electrode polarization and limited rate performance. Although the sodium ion battery sodium iron pyrophosphate (NFPP) carbon-coated positive electrode material has advantages in structural stability and theoretical capacity, it still faces many defects in practical application: the original NFPP has extremely low electronic conductivity, which leads to rapid capacity decay during high-rate discharge; Na + The diffusion resistance in the NFPP structure is large, and the ionic conductivity is insufficient, which makes the charge-discharge platform under high current density uneven, limiting the rapid charge-discharge performance. In the actual preparation process, the carbon layer often appears incomplete coverage, uneven thickness, which leads to local electronic path disconnection, and further causes capacity instability and cycle life decline. NFPP is sensitive to moisture and oxygen in the air, and is easy to form an oxidation layer or absorb moisture on the surface, causing electrochemical performance decay, especially in low temperature environment. It needs multiple steps of ball milling, pre-sintering, carbon source mixing and high temperature calcination, which has high energy consumption, narrow process window and high production cost.

[0024] In order to solve the above problems, the embodiment of the application provides a long-cycle high-voltage sodium ion positive electrode material. The positive electrode material comprises a core and a coating layer arranged on the surface of the core. The core comprises an NMPP precursor, and the coating layer is a polyacrylonitrile carbon-based coating layer. The mass ratio of the NMPP precursor to the polyacrylonitrile carbon-based coating layer is 1:(0.003-0.05), which can be one of 1:0.003, 1:0.01, 1:0.02, 1:0.03, 1:0.04 and 1:0.05 or a range value of any two. The NMPP precursor, acrylonitrile and methyl methacrylate are added into dimethyl ether solvent, heated and stirred, and then filtered and dried to realize polyacrylonitrile coating of the NMPP precursor. The NMPP coated with polyacrylonitrile is carbonized to obtain a polyacrylonitrile carbon-based coating layer for coating the NMPP precursor.

[0025] The NMPP / C material in the application has the characteristics of long cycle, and the capacity retention rate is as high as 84.86% after 500 cycles. The N-doped graphitized carbon layer formed by carbonization of polyacrylonitrile improves the electronic conductivity and provides fast Na+ The multi-dimensional synergistic enhancement of the NMPP precursor, achieved through the construction of transport channels, buffering of volume changes, protection against electrolyte erosion, and formation of chemical bonds, results in superior electrochemical performance of carbon-coated NMPP / C in high-power, long-life sodium-ion batteries. This invention utilizes a liquid-phase coating method to prepare NMPP / PAN, which, after carbonization, yields a nitrogen-doped carbon coating layer with controllable thickness. Simultaneously, the internal pyrrole nitrogen forms a coordination effect with the surface manganese element, enhancing the binding energy between the coating layer and the core. This makes the coating layer more stable and prevents carbon coating layer peeling and manganese dissolution caused by volume changes due to the Jan Taylor effect during NMPP cycling, thus improving cycle performance.

[0026] In this embodiment, NMPP suffers from poor conductivity and excessive volume deformation during charge and discharge. Ordinary carbon coating exhibits weak interfacial bonding with NMPP, leading to easy peeling and poor cycle performance. Therefore, flexible carbon coating is required. During high-temperature carbonization, polyacrylonitrile forms a nitrogen-doped carbon layer, which reduces charge transfer impedance and significantly improves the overall conductivity of the electrode. The uniform carbon coating provides a continuous electron-ion conductive network, reducing ion diffusion resistance and maintaining high capacity even at high discharge rates. The carbon layer isolates the active material from direct contact with the electrolyte, suppressing side reactions of electrolyte decomposition, preventing particle agglomeration and volume expansion, and improving structural integrity and cycle life.

[0027] Optionally, in one embodiment, the polyacrylonitrile carbon-based coating layer uses acrylonitrile as the main monomer, and copolymerizes it with methyl methacrylate to obtain a copolymerized polyacrylonitrile resin. This resin is dissolved in dimethyl ether solvent, achieving a more effective and tighter coating of the NMPP precursor. The coating layer is then carbonized. During high-temperature carbonization, polyacrylonitrile transforms into a dense graphitized carbon layer, forming an outer shell. Polyacrylonitrile contains a large number of nitrile groups (-CN), leaving nitrogen atoms during carbonization to form an N-doped carbon structure, improving conductivity and active site density. The dense outer shell formed by polyacrylonitrile coating enables uniform heat transfer during pyrolysis, reducing volume shrinkage and crack formation within the precursor, ensuring the integrity of the final carbon material layer. The interaction between polyacrylonitrile and the NMPP precursor forms a network structure before carbonization, resulting in a more uniform pore size distribution and a higher specific surface area after carbonization. Traditionally, the interaction between pure carbon coatings and precursors is primarily based on van der Waals forces, which are relatively weak. However, in carbon layers coated with polyacrylonitrile (PAT), the pyrrole nitrogen can form weak coordination interactions with Mn, which are far stronger than van der Waals forces, resulting in a more stable coating. PAT-coated carbon products often exhibit better flexibility and compressive strength, making them suitable for electrode materials. Furthermore, the residual π bonds in the cyclized PAT chains enhance electronic conductivity channels, which, combined with the conductive framework of NMPP itself, results in an overall conductivity significantly higher than that of uncoated carbon materials.

[0028] Optionally, in an embodiment, the mass ratio of the acrylonitrile to the methyl methacrylate can be one of 1:0.05, 1:0.06, 1:0.07, 1:0.08:1:0.09, 1:0.1, 1:0.11, 1:0.12 or a range value of any two thereof. A suitable ratio of acrylonitrile to methyl methacrylate can provide a required polymerization initiator for polymerization. When added too little, the chain initiation rate is lower than the chain termination rate, the AN-MA copolymerization conversion rate is <75%, a large amount of unreacted monomer remains in the solution, resulting in only a fragmented PAN coating layer on the surface of the NMPP, and there can be exposed areas. When added too much, chain growth occurs, the coating layer strength decreases significantly, and the coating layer is prone to peeling during the cleaning or transfer process before the secondary sintering, and the coating layer thickness is uneven (deviation >15%), some areas are too thick, and the carbon layer cracks after secondary sintering; in addition, impurities generated by decomposition of the excess initiator will react with the NMPP surface during secondary sintering, forming an impurity phase and reducing the ion diffusion coefficient of the NMPP.

[0029] The embodiment of the present application also provides a preparation method of a long-circulation high-voltage sodium ion positive electrode material, comprising steps 201-203. Step 201, dissolving a sodium source, a manganese source and a phosphorus source in deionized water, the sodium source:manganese source:phosphorus source=(3.98-4.15):(2.6-3):4, obtaining slurry A after sand milling, and performing spray drying on the slurry A to obtain a precursor B; In an embodiment of the present application, the precursor prepared by spray drying has significant advantages in morphology, composition uniformity, particle size controllability, subsequent sintering performance and process scalability. The spherical precursor provides an ideal substrate for uniform deposition of subsequent carbon coating or other functional layers, can realize uniform and dense coating, and is beneficial to solve the problem of uneven traditional primary carbon coating.

[0030] In an embodiment of the present application, the sodium source:manganese source:phosphorus source=(3.98-4.15):(2.6-3):4 can be one of 3.98:2.6:4, 4:2.7:4, 4.02:2.8:4, 4.04:2.9:4, 4.06:2.9:4, 4.08:2.9:4, 4.1:2.8:4, 4.12:2.9:4, 4.15:3:4 or a range value of any two thereof.

[0031] Optionally, in an embodiment, the particle size D50 of the slurry A is ≤0.4 μm, and the particle size D50 of the slurry A can be one of 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm or a range value of any two thereof. High-speed ball milling or high-speed centrifugal grinding is used to uniformly disperse metal ions and avoid agglomeration during subsequent atomization.

[0032] Optionally, in an embodiment, the particle size D50 of the precursor B is 5-25 μm, the particle size D50 of the precursor B can be one of 5 μm, 10 μm, 15 μm, 20 μm, 25 μm or a range value of any two of them, and the uniform spherical particles are beneficial to the densification during subsequent high-temperature sintering, and improve the crystal integrity of the material. In the subsequent sintering process, it can be quickly densified to form a fine and uniform grain structure, significantly improve the ion / electron conduction path, thereby improving the rate performance and cycle life of the battery. By spray drying the slurry, the particle size range value of the precursor B is controlled by adjusting the inlet air temperature, outlet air temperature, feeding speed and atomizer speed.

[0033] Optionally, in an embodiment, the sodium source is at least one of sodium carbonate, sodium bicarbonate, disodium hydrogen phosphate, sodium dihydrogen phosphate, and sodium pyrophosphate. Among them, sodium carbonate can directly provide Na + in the high-temperature solid-phase reaction, and release CO2 during the heating process, causing slight gas expansion, which helps to form a porous structure and improve the ion diffusion rate of the final material; when sodium bicarbonate is used as the sodium source, it can supply Na + at a lower temperature, reducing the impact of high-temperature sintering on the oxidation state of manganese; disodium hydrogen phosphate has the functions of supplying Na+ and PO4 3- , and is weakly alkaline after being dissolved in water, which can provide a buffering effect during synthesis to keep the system pH stable and prevent manganese ions from being excessively oxidized or generating undesirable secondary phases at high temperatures. Sodium dihydrogen phosphate provides Na + and phosphate, but is weakly acidic in water and can be used to adjust the acid-base balance of the system; sodium pyrophosphate directly provides Na + and pyrophosphate, eliminating the need for additional phosphate neutralization or dehydration steps, and the synthesis route is more concise. Pyrophosphate has strong complexing ability for metal ions (such as Mn 2+ ), which can keep the solution stable below 70°C and prevent metal ions from precipitating, thereby improving the uniformity of the precursor. The sodium pyrophosphate obtained by high-temperature dehydration polymerization is itself a precursor of the target product, and after use, it can directly participate in the formation of the target sodium manganese pyrophosphate lattice, reducing the loss of mutual transformation between raw materials. During high-temperature treatment, these sodium sources are easy to disperse uniformly, inhibiting the generation of NaFePO4 and other impurities, thereby improving the electronic conductivity and Na + storage performance of the material.

[0034] Optionally, in an embodiment, the manganese source is at least one of manganese carbonate, manganese oxalate, manganese nitrate, and manganese oxide. After adding the phosphorus source, Mn 2+ and PO4 3- are coordinated and precipitated to form a manganese phosphate precursor with Mn-P-O bonds. Further, it provides a skeleton for high-temperature solid-phase reaction, and Mn 2+The addition of the phosphorus source can adjust the lattice parameters, crystal size and morphology of the precursor, so that the particles are finer and more uniformly dispersed, thereby facilitating subsequent sintering or carbon-nitrogen coating. The manganese source decomposes at high temperatures (e.g., MnCO3→ MnO + CO2), generating micropores or vacancies, and the microporous structure helps Li + Fast diffusion within the material improves the rate performance.

[0035] Optionally, in an embodiment, the phosphorus source is at least one of ammonium dihydrogen phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, and phosphoric acid. The phosphorus source collectively provides sufficient PO4 3- / HPO4 2- / H2PO4 - At high temperatures, P-O-P bonds are formed through dehydration condensation to generate a pyrophosphate skeleton (P2O7 4- ), which is a key structural unit of the manganese-sodium positive electrode material. The phosphoric acid itself and its neutralization products (Na2HPO4, NaH2PO4) collectively constitute a buffer system to prevent the system from being too acidic or too basic, ensuring uniform dispersion of the metal manganese / iron precursor and smooth progress of the subsequent solid-phase reaction.

[0036] Step 202, sintering the precursor B prepared in step 201 under a sintering atmosphere to obtain an NMPP precursor; In an embodiment of the present application, the crystal growth is fully promoted during the sintering process, and the cracks and grain boundary defects within the crystal are significantly reduced, forming a dense crystal framework. By doping at the precursor stage, the element distribution is more uniform after sintering, avoiding the common surface pore and element segregation phenomena during the later doping.

[0037] Optionally, in an embodiment, the sintering atmosphere is nitrogen or argon; the inert gas can prevent oxygen or water vapor from reacting with the precursor, avoiding the formation of oxides or other impurity phases, thereby improving the phase purity of the final NMPP precursor. During high-temperature sintering, a nitrogen environment can inhibit the volatilization or decomposition of nitrogen, ensuring that the N-content is not reduced and ensuring the stoichiometric ratio of the NMPP precursor. Effective prevention of oxidation and loss of nitrogen, maintaining stoichiometry, obtaining high phase purity, dense and uniform NMPP precursor; at the same time, taking into account the cost and safety, reducing the introduction of impurities, ultimately improving the mechanical and electrochemical properties of the material. The above sintering temperature is 450-650°C, which can be one of 450°C, 500°C, 550°C, 600°C, 650°C or any range value of two of them, and the sintering time is 6-18h, which can be one of 6h, 8h, 10h, 12h, 14h, 16h, 18h or any range value of two of them. The range of sintering temperature and sintering time can not only ensure the completion of phase transition, but also avoid excessive grain growth or high energy consumption caused by high temperature, taking into account the material performance and production cost.

[0038] Step 203, adding the NMPP precursor prepared in step 202 and acrylonitrile, methyl methacrylate into dimethyl ether solvent, heating and stirring, and after filtering and drying, polyacrylonitrile-coated NMPP is obtained; In an embodiment of the present application, the heating and stirring condition is stirring at 65°C for 12h. The high molecular chain of polyacrylonitrile provides good flexibility and adhesion, so that the precursor is not easy to break during compaction and heat treatment, and the particle morphology and pore structure are maintained. The polyacrylonitrile coating can improve the decomposition temperature of the material and reduce the thermal weight loss at high temperature.

[0039] Optionally, in an embodiment, the mass ratio of the above-mentioned NMPP precursor to acrylonitrile is 1:(0.05-0.3), which can be one of 1:0.05, 1:0.08, 1:0.11, 1:0.14, 1:0.17, 1:0.2, 1:0.23, 1:0.26, 1:0.3 or a range value of any two thereof. This ratio can regulate the carbon coating amount. Too little carbon coating may result in a too thin coating layer, element leaching, and substrate exposure. Too thick carbon coating may result in a too thick carbon layer, affecting sodium ion migration, and high cost. The mass ratio of the above-mentioned acrylonitrile to the above-mentioned methyl methacrylate can be one of 1:0.05, 1:0.06, 1:0.07, 1:0.08, 1:0.09, 1:0.1, 1:0.11, 1:0.12 or a range value of any two thereof. A suitable ratio of acrylonitrile to methyl methacrylate can provide the required polymerization initiator for polymerization. Too little addition will result in a chain initiation rate lower than the chain termination rate, AN-MA copolymerization conversion <75%, and a large amount of unreacted monomer remaining in the solution, resulting in only a fragmented PAN coating layer on the surface of NMPP, with exposed areas. Excessive addition will result in chain growth, a significant decrease in coating layer strength, and easy demolding during cleaning or transfer before secondary sintering, as well as uneven coating layer thickness (deviation >15%), with some areas being too thick, and carbon layer cracking after secondary sintering. In addition, impurities generated by the decomposition of excessive initiator will react with the surface of NMPP during secondary sintering, generating impurity phases and reducing the ion diffusion coefficient of NMPP.

[0040] Step 204, carbonizing the polyacrylonitrile-coated NMPP prepared in step 203 to obtain a long-cycle high-voltage sodium ion positive electrode material.

[0041] In one embodiment of the present invention, the carbonized polyacrylonitrile coating layer transforms into a nitrogen-rich carbon layer, which can build conductive bridges between NMPP particles, significantly improving the overall electronic conductivity. This dual-carbon structure can effectively suppress the aggregation of NMPP particles, maintaining the small size and uniform dispersion of the active material. The nitrogen atoms (such as pyridine nitrogen and pyrrole nitrogen) remaining in the polyacrylonitrile carbonization product can provide additional electrochemical active sites, enhancing the kinetics of redox reactions, thereby improving the specific capacity and rate performance of the battery. The carbonized polyacrylonitrile coating layer has high mechanical strength and thermal stability, which can buffer volume changes during charge and discharge, reducing structural damage to the electrode material. Coating polyacrylonitrile onto the surface of NMPP and subjecting it to high-temperature carbonization can utilize the high carbon yield and nitrogen doping advantages of polyacrylonitrile, and improve the electrochemical performance and structural stability of NMPP-based materials by forming a continuous conductive network and porous structure.

[0042] Optionally, in one embodiment, the carbonization temperature is 280–350°C, which can be a range of 280°C, 300°C, 320°C, 340°C, and 350°C, or any two of these values. The carbonization time is 2–10 hours, which can be a range of 2 hours, 4 hours, 6 hours, 8 hours, and 10 hours, or any two of these values. By precisely controlling the secondary carbonization temperature, the PAN coating layer is appropriately carbonized, forming more CN bonds. If the temperature is too high, a large amount of nitrogen is lost, CN bonds decrease, and the coating layer becomes closer to ordinary carbon coating. If the carbonization temperature is too low, CN bonds are not formed, and the degree of carbonization is low, resulting in low conductivity.

[0043] In one embodiment of the present invention, when the carbonization temperature is too low, PAN carbonization is incomplete, leaving a large number of uncarbonized polymer fragments. The coating layer has a low degree of graphitization and a loose structure. Although the nitrogen retention rate is high, it exists mostly as unstable pyrrole N and amino N, which are difficult to form coordination bonds (MNC) with metals. When the temperature is too high and over-carbonization occurs, the coating layer becomes more brittle, and a large amount of nitrogen is lost. N escapes in the form of NH3, HCN, etc., and the metal coordination effect almost disappears. At an appropriate temperature, PAN only completes cyclization. There are a large number of defects and flexible segments inside the carbon layer, which can deform like an elastic membrane with the volume expansion / contraction of NMPP, avoiding direct exposure of the matrix to the electrolyte and improving cycle stability.

[0044] In one embodiment of the present invention, the secondary sintering carbonization temperature is much lower than that of most primary sintering temperatures, which will not trigger matrix phase reconstruction or metal particle agglomeration, and can completely preserve the nanoscale active structure of NMPP after primary sintering. The surface of the NFPP matrix after primary sintering usually has small micropores (originating from gas escape during sintering). During secondary sintering, some low molecular weight segments of PAN will melt and penetrate into these micropores, forming a carbon-matrix interlocking structure after cooling, which reduces the porosity of the matrix and enhances the bonding force between the coating layer and the matrix.

[0045] Compared with the direct addition of PAN, the liquid phase coating method of this invention allows AN molecules to be uniformly adsorbed on the substrate surface (including particle gaps) and polymerized to form a uniform and continuous PAN film. In contrast, direct solid phase PAN powder coating is prone to uneven coating, large thickness differences, or even bare areas when mixed with the NMPP matrix, which can easily lead to element dissolution, increased impedance, and deterioration of electrical properties.

[0046] This invention prepares NMPP / PAN by liquid-phase coating method. After carbonization, a nitrogen-doped carbon coating layer is obtained. The thickness of this layer is controllable. At the same time, the internal pyrrole nitrogen can form a coordination effect with the surface manganese element, which enhances the bonding ability between the coating layer and the core, making the coating layer more stable. This avoids the carbon coating layer peeling off and manganese dissolution caused by volume changes due to the Jan Taylor effect during NMPP cycling, thus improving cycling performance.

[0047] The present invention provides a sodium-ion battery, including a positive electrode sheet, wherein the positive electrode sheet includes a positive current collector and a positive active material layer disposed on the positive current collector, wherein the positive active material layer includes the long-cycle high-voltage sodium-ion positive electrode material described above or the long-cycle high-voltage sodium-ion positive electrode material prepared by the above preparation method.

[0048] The long-cycle carbon-coated sodium manganese pyrophosphate cathode material of the present invention improves electronic conductivity and accelerates Na+ oxidation. + It diffuses and inhibits metal dissolution and structural degradation, achieving high capacity and excellent long cycle retention, while also possessing the advantages of high rate capability, low temperature adaptability and low cost.

[0049] The sodium-ion battery provided in this embodiment of the invention also includes a negative electrode, a separator, and an electrolyte.

[0050] The negative electrode sheet includes a negative current collector and a negative active material layer disposed on the negative current collector. The negative active material layer may include at least one of artificial graphite, natural graphite, hard carbon, soft carbon, and carbon black.

[0051] In this embodiment, the electrolyte acts as a conductor of ions between the positive and negative electrodes. The electrolyte can be liquid, gel-like, or entirely solid. In some embodiments, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent, wherein the electrolyte salt is a lithium salt. No specific limitations are imposed here.

[0052] The present invention will be described in detail below through embodiments.

[0053] Example 1 A long-cycle, high-voltage sodium-ion cathode material, the preparation steps of which include: S1: Weigh 4 mol sodium dihydrogen phosphate and 3 mol manganese carbonate and dissolve them in 5 L of deionized water. After sand milling, slurry A is obtained with a particle size D50 of 0.3 μm. The slurry is then spray-dried to obtain the precursor.

[0054] S2: The precursor was sintered at 550℃ for 10h under a nitrogen atmosphere to obtain sodium manganese pyrophosphate (NMPP) precursor; the slurry was spray dried with an inlet air temperature of 270℃, an outlet air temperature of 95℃, a feed rate of 55ml / min, and an atomizer speed of 200HZ to obtain sodium manganese pyrophosphate (NMPP) precursor with a D50 of 19μm.

[0055] S3: Add 500g of NMPP precursor, 50g of acrylonitrile (AN), and 5.56g of methyl methacrylate (MA) to 1L of dimethyl ether (DME), stir at 60℃ for 12h, filter and dry to obtain NMPP coated with polyacrylonitrile.

[0056] S4: Carbonizing NMPP / PAN at 300℃ for 4 hours yields a long-cycle, high-voltage sodium-ion cathode material.

[0057] Example 2 The only difference from Example 1 is that in the preparation of precursor B in step (1), the sodium source is adjusted to 2 mol sodium pyrophosphate, the manganese source is 1.45 mol manganese oxalate, and the phosphorus source is 2 mol ammonium dihydrogen phosphate. The remaining steps are the same as in Example 1.

[0058] Example 3 The only difference from Example 1 is that, in step (1) during the preparation of precursor B, the particle size D50 of slurry A is adjusted to 0.25 μm. This can be achieved by adjusting the sand milling time, etc.

[0059] Example 4 The only difference from Example 1 is that in the preparation of precursor B in step (1), the slurry is spray-dried with an inlet air temperature of 250°C, an outlet air temperature of 90°C, a feed rate of 50 ml / min, an atomizer speed of 250 Hz, and the particle size D50 of precursor B is adjusted to 15 μm.

[0060] Example 5 The only difference from Example 1 is that, in step (2) of the NMPP precursor preparation process, the sintering temperature is adjusted to 580°C and the sintering time is 12h.

[0061] Example 6 The only difference from Example 1 is that, in step (2) of the NMPP precursor preparation process, the sintering atmosphere is adjusted to argon.

[0062] Example 7 The only difference from Example 1 is that in step (3) during the preparation of NMPP coated with polyacrylonitrile, the mass ratio of NMPP precursor to acrylonitrile is adjusted to 1:0.2.

[0063] Example 8 The only difference from Example 1 is that in step (4) of the preparation of long-cycle high-voltage sodium ion cathode material, the carbonization temperature is adjusted to 300°C and the carbonization time is 3h.

[0064] Example 9 The only difference from Example 1 is that in step (3) during the preparation of NMPP coated with polyacrylonitrile, the mass ratio of acrylonitrile to methyl methacrylate is adjusted to 1:0.08.

[0065] Example 10 The only difference from Example 1 is that, in step (2) of the NMPP precursor preparation process, the sintering temperature is adjusted to 600°C and the sintering time is 18h.

[0066] Example 11 The only difference from Example 1 is that, in step (2) of the NMPP precursor preparation process, the sintering temperature is adjusted to 650°C and the sintering time is 10h.

[0067] Comparative Example 1 A carbon-coated sodium manganese pyrophosphate, differing from Example 1 in that the PAN coating is omitted and a conventional carbon source is used for coating, and the preparation steps include: S1: Weigh 4 mol sodium dihydrogen phosphate, 3 mol manganese carbonate and 30 g glucose and dissolve them in 5 L of deionized water. After sand milling, slurry A is obtained with a particle size D50 of 0.3 μm. The slurry is then spray-dried to obtain the precursor.

[0068] S2: The precursor was sintered at 550°C for 10 hours under a nitrogen atmosphere to obtain a long-cycle, high-voltage sodium-ion cathode material.

[0069] Comparative Example 2 A carbon-coated sodium manganese pyrophosphate, differing from Example 1 in that the NMPP:AN ratio is 1:0.4, and the specific preparation steps include: S1: Weigh 4 mol sodium dihydrogen phosphate and 3 mol manganese carbonate and dissolve them in 5 L of deionized water. After sand milling, slurry A is obtained with a particle size D50 of 0.3 μm. The slurry is then spray-dried to obtain the precursor.

[0070] S2: The precursor was sintered at 550°C for 10 h under a nitrogen atmosphere to obtain sodium manganese pyrophosphate (NMPP) precursor.

[0071] S3: Add 500g of NMPP precursor, 200g of acrylonitrile (AN), and 21g of methyl methacrylate (MA) to 1L of dimethyl ether (DME), stir at 65°C for 12 hours, and obtain NMPP / PAN after filtration and drying.

[0072] S4: Carbonizing NMPP / PAN at 270℃ for 4 hours yields a long-cycle, high-voltage sodium-ion cathode material.

[0073] Comparative Example 3 A carbon-coated sodium manganese pyrophosphate, differing from Example 1 in that the NMPP / PAN carbonization temperature is 400°C and the ratio of NMPP precursor to acrylonitrile is different, specifically including the following steps: S1: Weigh 4 mol sodium dihydrogen phosphate and 3 mol manganese carbonate and dissolve them in 5 L of deionized water. After sand milling, slurry A is obtained with a particle size D50 of 0.3 μm. The slurry is then spray-dried to obtain the precursor.

[0074] S2: The precursor was sintered at 550°C for 10 h under a nitrogen atmosphere to obtain sodium manganese pyrophosphate (NMPP) precursor.

[0075] S3: Add 500g of NMPP precursor, 25g of acrylonitrile (AN), and 5.56g of methyl methacrylate (MA) to 1L of dimethyl ether (DME), stir at 65°C for 12 hours, and obtain NMPP / PAN after filtration and drying.

[0076] S4: Carbonizing NMPP / PAN at 400℃ for 4 hours yields a long-cycle, high-voltage sodium-ion cathode material.

[0077] Figure 1 This is a flowchart illustrating the preparation method of the long-cycle high-voltage sodium-ion cathode material obtained in Example 1. The NMPP / C prepared by the coating method exhibits excellent cycle stability.

[0078] Figure 2The XPS N1S diagram of Example 1 shows C≡N bonds, indicating that the coating method introduces N and forms C≡N.

[0079] Figure 3 The XPS Mn2P spectra of Example 1 and Comparative Example 1 are shown. Compared with Comparative Example 1, the Mn 2p3 / 2 binding energy of Example 1 shifts to a higher binding energy by 0.5 eV. This is due to the formation of N-Mn coordination and the increase in electron cloud density of Mn.

[0080] Figure 4 The battery cycling curves show that after 500 cycles, Example 1 retains a capacity of 84.86%, while Comparative Example 1 retains only 51.10%, indicating that this method can significantly improve cycle stability.

[0081] Figure 5 The image shows an SEM image of Example 1. As can be seen from the image, the carbon coating layer is compact and has low porosity, indicating that the coating layer forms a good interfacial contact with the matrix, which improves cycle stability.

[0082] (1) Preparation of the positive electrode sheet of sodium-ion battery The positive electrode materials prepared in each embodiment and comparative example were mixed with conductive carbon (Super P) and binder polyvinylidene fluoride (PVDF) at a mass ratio of 8:1:1. After grinding, an appropriate amount of 1-methyl-2-pyrrolidone (NMP) solvent was added, and the mixture was placed in a homogenizer and vibrated for 10 minutes to uniformly coat the slurry onto carbon-coated aluminum foil. After drying, the aluminum foil coated with black slurry was punched into a circular electrode sheet with a diameter of 14 mm using a slicing machine. Then, a certain pressure was applied to the electrode sheet using a pressing machine to press it into a sheet. Finally, the electrode sheet was placed in a vacuum oven and vacuum dried at 120°C for 12 hours to obtain the positive electrode sheet for sodium-ion batteries.

[0083] (2) Preparation of sodium-ion batteries The prepared sodium-ion battery positive electrode sheet was used as the working electrode, and metallic sodium was used as the counter electrode. A 1 mol / L NaClO4 / PC:EMC:FEC (49:49:2) organic electrolyte was used to assemble a coin cell in a glove box filled with argon atmosphere.

[0084] (3) Electrical performance testing The electrochemical performance of the prepared sodium-ion battery was tested using conventional methods in the art, with a test voltage range of 2.0–4.0 V. The test results are shown below.

[0085] Table 1 Performance Test Results

[0086] As shown in Table 1, the NMPP / C material of the present invention has the characteristic of long cycle life, with a capacity retention rate of up to 84.86% after 500 cycles.

[0087] Comparing Example 1 and Example 2, it can be seen that the cathode material prepared by using different sodium, manganese and phosphorus sources can still maintain a high capacity and cycle retention rate.

[0088] As can be seen from the comparison between Example 1 and Example 3, reducing the particle size of the slurry during the sand milling stage can appropriately improve the cycle stability.

[0089] As can be seen from the comparison between Example 1 and Example 4, the cathode material prepared by using different precursor particle sizes can still maintain a high capacity and cycle retention rate.

[0090] As can be seen from the comparison between Example 1 and Example 5, the cathode materials prepared by using different sintering temperatures and sintering times can still maintain high capacity and cycle retention.

[0091] As can be seen from the comparison between Example 1 and Example 6, the cathode material prepared by argon sintering atmosphere can still maintain a high capacity and cycle retention rate.

[0092] As can be seen from the comparison between Example 1 and Example 7, the cathode materials prepared by using different proportions of sodium body to acrylonitrile mass ratio can still maintain high capacity and cycle retention.

[0093] Comparing Example 1 and Example 8, it can be seen that the cathode materials prepared by using different carbonization times of the coating layer can still maintain high capacity and cycle retention.

[0094] As can be seen from the comparison between Example 1 and Example 9, the cathode materials prepared by using different ratios of acrylonitrile and methyl methacrylate can still maintain high capacity and cycle retention.

[0095] Comparing Example 1 and Example 10, it can be seen that cathode materials prepared by using different sintering temperatures and times for the precursor can still maintain high capacity and cycle retention.

[0096] As can be seen from the comparison between Example 1 and Comparative Example 1, the polyacrylonitrile coating of the present invention can significantly improve the cycle stability of NMPP.

[0097] As can be seen from the comparison between Example 1 and Comparative Example 2, an appropriate amount of acrylonitrile can improve cycle stability, while the effect is poor when the coating amount is low.

[0098] As can be seen from the comparison between Example 1 and Comparative Example 3, a higher coating carbonization temperature will reduce cycle stability.

[0099] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A long-cycle, high-voltage sodium-ion cathode material, characterized in that, The cathode material includes a core and a coating layer disposed on the surface of the core. The core includes an NMPP precursor, and the coating layer is a polyacrylonitrile carbon-based coating layer. The mass ratio of the NMPP precursor to the polyacrylonitrile carbon-based coating layer is 1:(0.003~0.05). The NMPP precursor is coated with polyacrylonitrile by adding the NMPP precursor, acrylonitrile, and methyl methacrylate to a dimethyl ether solvent, heating and stirring, and then filtering and drying. The polyacrylonitrile-coated NMPP is then carbonized to obtain the polyacrylonitrile carbon-based coating layer coated with the NMPP precursor.

2. The long-cycle high-voltage sodium-ion cathode material according to claim 1, characterized in that, The mass ratio of acrylonitrile to methyl methacrylate is 1:(0.05-0.12).

3. The method for preparing the long-cycle high-voltage sodium-ion cathode material according to any one of claims 1-2, characterized in that, Includes the following steps: S1. Dissolve sodium source, manganese source and phosphorus source in deionized water, and then mill them to obtain slurry A. Spray dry slurry A to obtain precursor B. S2. The precursor B obtained in step S1 is sintered in a sintering atmosphere to obtain the NMPP precursor. S3. The NMPP precursor obtained in step S2 is added to acrylonitrile and methyl methacrylate in dimethyl ether solvent, heated and stirred, filtered and dried to obtain NMPP coated with polyacrylonitrile. S4: Carbonize the NMPP coated with polyacrylonitrile obtained in step S3 to obtain a long-cycle high-voltage sodium-ion cathode material; the carbonization temperature is 280-350℃ and the carbonization time is 2-10h.

4. The method for preparing the long-cycle high-voltage sodium-ion cathode material according to claim 3, characterized in that, In step S1, the sodium source is at least one of sodium carbonate, sodium bicarbonate, disodium hydrogen phosphate, sodium dihydrogen phosphate, and sodium pyrophosphate. The manganese source is at least one of manganese carbonate, manganese oxalate, manganese nitrate, and manganese oxide. The phosphorus source is at least one of ammonium dihydrogen phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, and phosphoric acid.

5. The method for preparing the long-cycle high-voltage sodium-ion cathode material according to claim 3, characterized in that, In step S1, the particle size D50 of the slurry A is ≤0.4μm.

6. The method for preparing the long-cycle high-voltage sodium-ion cathode material according to claim 3, characterized in that, In step S2, the particle size D50 of the precursor B is 5μm~25μm.

7. The method for preparing the long-cycle high-voltage sodium-ion cathode material according to claim 3, characterized in that, In step S2, the sintering atmosphere is nitrogen or argon; the sintering temperature is 450–650°C, and the sintering time is 6–18 h.

8. The method for preparing the long-cycle high-voltage sodium-ion cathode material according to claim 3, characterized in that, In step S3, the mass ratio of the NMPP precursor to acrylonitrile is 1:(0.05-0.3).

9. The method for preparing the long-cycle high-voltage sodium-ion cathode material according to claim 3, characterized in that, In step S3, the mass ratio of acrylonitrile to methyl methacrylate is 1:(0.05-0.12).

10. A sodium-ion battery, characterized in that, The material includes a positive electrode sheet, which includes a positive current collector and a positive active material layer disposed on the positive current collector. The positive active material layer includes the long-cycle high-voltage sodium-ion positive electrode material as described in any one of claims 1 to 2 or the long-cycle high-voltage sodium-ion positive electrode material prepared by the preparation method described in any one of claims 3 to 9.

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