Oxide-coated ion-doped sodium ferric sulfate positive electrode material as well as preparation method and application thereof

Abstract

The invention discloses an oxide-coated ion-doped sodium ferric sulfate positive electrode material as well as a preparation method and application thereof. The positive electrode material is formed by compounding a sodium ferric sulfate bulk phase material doped with various valence metal cations and anions and coated with carbon similar to a positive temperature coefficient thermosensitive metal oxide. Wherein the metal cations with various valence states are doped on Na or Na / Fe ion sites of the sodium ferric sulfate, the doped anions occupy part of three-dimensional S-O tetrahedron positions, the inorganic carbon in the carbon compound is uniformly distributed in the sodium ferric sulfate bulk phase material, and the organic carbon is uniformly distributed outside the sodium ferric sulfate bulk phase material. The positive electrode plate assembled battery prepared from the positive electrode material has the advantages of high reversible charge-discharge capacity, excellent cycle performance and rate capability and high electron and ion migration rate in a low-temperature environment. The method is low in raw material cost, simple in synthesis method, short in production period and suitable for large-scale continuous production.

Description

Technical Field

[0001] This invention relates to a sodium ferric sulfate cathode material, its preparation method, and its application, specifically to an oxide-coated ion-doped sodium ferric sulfate cathode material, its preparation method, and its application. Background Technology

[0002] Sodium-ion batteries (NIBs) have become an ideal alternative to lithium-ion batteries in the field of electrochemical storage due to their abundant raw materials and cost-effectiveness. Because of their low energy density and high safety, NIBs are more suitable for low-speed electric vehicles and large-scale energy storage. The cathode material, as the heart of the sodium-ion battery, plays a crucial role. Polyanionic materials, as a type of polyaluminum-containing material... + The sodium-ion battery cathode material with the storage site has the chemical formula NaA'A''M. y (X a O b ) z D w In this system, A' and A'' are different sodium ion storage sites, M = one or more of Ti, V, Cr, Ni, Mn, Fe, Co, Ca, Mg, Al, etc., and X is S, P, Si, B, Mo, etc. With its stable three-dimensional structure and ultra-low raw material cost, it has attracted the attention of many researchers.

[0003] Sodium-ion batteries work on a similar principle to lithium-ion batteries, utilizing Na+. + The conversion of electrical energy into chemical energy is achieved through the extraction and insertion between the positive and negative electrode materials. However, compared to lithium-ion batteries, due to the... + The radius (1.02 Å) is greater than that of Li. + With a radius of 0.76 Å, the diffusion barrier of Na compounds is always higher than that of Li compounds. At low temperatures, Na... + While sodium iron sulfate (FeSU) batteries have a higher migration energy barrier and a lower ion migration rate, their low cost and high performance compared to lead-acid batteries have earned the trust of researchers and are poised to become a hot material for future development, driven by the rapid development of energy storage fields such as electric vehicles. Therefore, they must be able to quickly adapt to outdoor application scenarios with varying temperatures and large temperature ranges. Thus, to address the challenges of the energy storage market in harsh environments and to enhance the electrochemical kinetics performance of NIBs in low-temperature (LT) environments, it is urgent to develop sodium iron sulfate (FeSU) as a polyanionic cathode material for NIBs, thereby promoting the commercial application of sodium battery products in low-temperature environments.

[0004] Currently, the improvement of battery low-temperature performance mainly focuses on the development of electrolyte materials. From the perspective of modifying cathode materials, the general approach is to: (1) increase the ion diffusion rate and electron transfer of the material to counteract the potential polarization under low-temperature conditions; (2) reduce the polarization of the material in Na+. +(3) The cathode material should have strong environmental adaptability and low raw material cost, and be commercially applicable.

[0005] CN119181789A discloses a tellurium and niobium-doped cathode material and its preparation method, which modifies the material by doping tellurium and niobium in the bulk phase and coating it with carbon nitride on the surface. However, at low temperatures, the thermodynamic conditions required for electron or ion migration are more stringent. Although Te / Nb co-doping stabilizes the crystal structure of the material, even with surface carbon source modification, it is still difficult to effectively improve the electrochemical performance of the material.

[0006] CN118315560A discloses a composite cathode material, its preparation method, and its application. This material improves its electronic conductivity through surface modification using metal oxides, nitrogen-doped carbon materials, and carbon nanotube coatings. However, since the material's kinetic properties are affected not only by electron transfer but also by ion migration within the bulk phase, even with improved electron migration rates and reduced ohmic polarization at low temperatures, the concentration polarization and electrochemical polarization during the electrochemical reaction are exacerbated due to the difficulty in effectively improving the ion migration rate within the bulk phase. This makes it difficult to effectively improve the material's low-temperature electrochemical performance.

[0007] CN114229818A discloses a method for preparing in-situ graphene-doped low-temperature lithium iron phosphate cathode material. The method modifies the surface of lithium iron phosphate by coating it with a carbon layer, forming a porous coating layer with carbon vacancies to improve the interfacial-surface dynamics of the material. However, at low temperatures, the thermodynamic activation energy of ions / electrons increases, and the ion migration rate within the bulk phase of the material slows down, becoming the rate-controlling step in the electrochemical reaction. Since the method only improves the electron migration rate and interfacial ion dynamics through surface-interfacial carbon source modification, it does not address the rate-controlling step in the electrochemical reaction process, thus making it difficult to effectively improve the low-temperature electrochemical performance of the material.

[0008] CN119230826A discloses a dual-doped sodium iron sulfate cathode material, its preparation method, and its applications. Although the co-doping with transition metals Co and Ni slightly alters the electron cloud density distribution around TM-O-Fe, weakening the Fe-O binding energy, enhancing electron delocalization, and improving electronic conductivity, the similar radii and electronegativity of Co and Ni with Fe make it difficult to change the ion diffusion channels in the bulk phase of the material. Simultaneously, the activation energy barrier increases at low temperatures, making it difficult to improve the ion diffusion rate, leading to intensified concentration polarization during the electrochemical reaction and hindering effective improvement of the material's electrochemical performance.

[0009] CN108172794A discloses a composite cathode material, its preparation method, and its applications. This material's electronic dynamics performance is improved through metal oxide coating and carbon layer modification. However, at low temperatures, the resistivity of the metal oxide, conductive carbon layer, and bulk material increases significantly with decreasing temperature, making it difficult to effectively utilize the material's low-temperature electrochemical performance.

[0010] In summary, there is an urgent need to synergistically improve the bulk ion diffusion rate and electronic conductivity of materials, and reduce Na+. + The process of insertion and extraction of internal structural stress stabilizes the material structure. The resulting positive electrode sheet assembled battery exhibits high reversible charge-discharge capacity, excellent cycle performance, high rate performance, and fast electron and ion migration rate in low-temperature environments. This oxide-coated ion-doped sodium iron sulfate positive electrode material has applications and uses. It also has a low raw material cost, simple synthesis method, short production cycle, and is suitable for large-scale continuous production. Summary of the Invention

[0011] The technical problem to be solved by the present invention is to overcome the above-mentioned defects of the prior art and provide an oxide-coated ion-doped sodium iron sulfate positive electrode material and its application, which has high reversible charge-discharge capacity, excellent cycle performance and high rate performance, and fast electron and ion migration rate in the battery assembly under low temperature environment.

[0012] The further technical problem to be solved by the present invention is to overcome the above-mentioned defects of the prior art and provide a method for preparing oxide-coated ion-doped sodium iron sulfate cathode material with low raw material cost, simple synthesis method, short production cycle and suitable for large-scale continuous production.

[0013] The technical solution adopted by this invention to solve its technical problem is as follows: An oxide-coated ion-doped sodium ferric sulfate cathode material, wherein the cathode material is composed of a sodium ferric sulfate bulk material coated with a carbon composite containing multiple valence metal cations and anions, using a positive temperature coefficient (PTC) thermistor metal oxide. The multiple valence metal cations are doped at the Na or Na / Fe ion sites of the sodium ferric sulfate, and the anions occupy some of the three-dimensional SO tetrahedral positions. Furthermore, the inorganic carbon in the carbon composite is uniformly distributed within the sodium ferric sulfate bulk material, while the organic carbon is uniformly distributed outside the sodium ferric sulfate bulk material. Based on the original structure, the bulk phase of the cathode material of this invention is further modified by doping metal cations at the Na or Na / Fe ion sites of the sodium ferric sulfate, utilizing a dual-site, dual-ion, dual-modification method to alter the Na and Fe metal-oxygen bond interaction, thereby broadening the Na... + Migration channels are introduced to improve the structural stability of the bulk phase of sodium ferric sulfate materials; specific anions are introduced to occupy some three-dimensional SO tetrahedral positions to alter the partial Fe2O content in the bulk phase of sodium ferric sulfate. 10The dimer-SO4 tetrahedral arrangement, combined with metal cation doping stabilizing the material structure through charge compensation mechanism, and the two-dimensional triangular structure of anion doping increasing the Na content in the material, all contribute to the material's Na+ content. + The migration pathway stabilized the crystal structure of the material during cycling and accelerated the migration of bulk Na. + Ion transport rate. A positive temperature coefficient (PTC) quasi-thermosensitive metal oxide is coated onto sodium ferric sulfate material, synergistically constructing a highly efficient conductive network at low temperatures with inorganic / organic conductive carbon uniformly distributed inside and outside the bulk sodium ferric sulfate material. This invention utilizes surface-bulk phase electronic / ionic dual modification methods—macroscopic surface morphology modification and microscopic elemental lattice site control—to improve the electron transfer rate and reduce charge transfer impedance, even when the conductivity of high-conductivity materials with negative temperature coefficients decreases at low temperatures. Simultaneously, the PTC quasi-thermosensitive metal oxide coating enhances the electron transfer rate and reduces charge transfer impedance. Furthermore, anion and cation doping bulk phase modification constructs a stable crystal framework structure, lowering the sodium ion migration barrier and widening ion migration channels. Thus, through bulk structure construction, channel design, and increased surface conductivity, the electronic conductivity and kinetic properties of the material at low temperatures are synergistically improved, effectively enhancing the material's electrochemical performance.

[0014] Preferably, the oxide-coated ion-doped sodium iron sulfate cathode material has a hollow microsphere structure with an average particle size of 5–35 μm (more preferably 10–25 μm). During spray drying, the solute-solvent ratio in a given volume of droplet is theoretically fixed. However, excessively large particle sizes lead to increased porosity, which is detrimental to improving material compaction and tap density. Conversely, while excessively small spherical particles increase the specific surface area of ​​the material, they also increase side reactions between the electrolyte and the electrode material, resulting in a significant reduction in the material's initial coulombic efficiency.

[0015] Preferably, the mass fraction of the positive temperature coefficient (PTC) thermistor metal oxide in the cathode material is 0.1–8.5 wt% (more preferably 2.2–8.2 wt%). In addition to exhibiting its high electronic conductivity at low temperatures and improving the conductivity of the composite material, the thermistor metal oxide coating can also reduce side reactions between the bulk material and the electrolyte during charge-discharge processes at low temperatures, mitigating volume changes in the CEI film.

[0016] Preferably, the positive temperature coefficient (PTC) thermistor oxide includes one or more of cerium oxide, zinc oxide, tin oxide, or indium oxide. The resistivity of the PTC thermistor oxide decreases or slightly increases with decreasing temperature, indicating that temperature does not inhibit the conductivity of the PTC thermistor oxide. More preferably, the coating oxide is one or more of cerium oxide, zinc oxide, or tin oxide. The PTC thermistor oxide is at the nanoscale.

[0017] Preferably, the multiple valence metal cations include two or more valence metal cations among monovalent, divalent, or higher valence metal cations.

[0018] Preferably, the monovalent metal cation includes K ions, etc.

[0019] Preferably, the divalent or higher metal cations include one or more of Ca, Al, or Ti ions.

[0020] Unlike existing methods that use single metal ions to dope iron sites, the cathode material of this invention uses metal doping elements with ionic radii greater than Na. + Large or high-valence inert metal ions, from stabilizing the crystal lattice phase and broadening Na + From the perspective of migration pathways, low-valence, large-radius ions (>1.02 Å) occupy Na sites, broadening the Na₂O₃ space. + Ion migration occurs through migration channels in different directions without participating in redox reactions. Meanwhile, high-valence inert metal ions enter the lattice, occupying Na or Na / Fe sites. They do not participate in the redox reactions during charge / discharge processes, but alter the Fe-O bond length through a charge compensation mechanism, affecting the electron cloud distribution and thus supporting the Na... + The bulk structure stabilizes during charging and discharging, reducing structural collapse. Therefore, the metal cations doped in the cathode material of this invention act as lattice support elements, do not participate in chemical reactions, and remain stable in Na+ phases. + During the insertion / extraction process, internal stress in the material can be relieved and ion channels can be widened.

[0021] Preferably, the anion includes borate ions, etc. (BO3 anion) 3- The arrangement of two-dimensional planar triangular structures instead of the three-dimensional tetrahedral structure of SO leads to local lattice distortion in the material, improves the migration channels of sodium ions, and promotes ion transport.

[0022] Preferably, the chemical formula of the sodium ferric sulfate bulk material doped with multiple valence state metal cations and anions is Na. 2.6-x-ny M x N y Fe 1.7 (SO4) 3-z (BO3) 2z / 3 Where 0 < x ≤ 0.15, 0 < y ≤ 0.08, 0 < z ≤ 0.25 (more preferably 0.03 ≤ x ≤ 0.05, 0.01 ≤ y ≤ 0.04, 0.01 ≤ z ≤ 0.05), M represents a monovalent metal cation, N represents a divalent or higher metal cation, n represents the valence state of N, n = 2, 3, 4, and satisfies the law of conservation of charge.

[0023] Preferably, the doping amounts of the M and N metal cations relative to the Na ions are 0.01–7.0 at% (more preferably 0.5–5.0 at%, even more preferably 1.0–2.0 at%) and 0.01–3.7 at% (more preferably 0.1–3.0 at%, even more preferably 0.3–2.3 at%), respectively. Excessive or insufficient doping of the low-valence M and high-valence N metal ions will lead to a decrease in overall electrochemical performance. If the metal ion doping amount is too high, it occupies too many Na / Fe sites, resulting in insufficient Na participation in redox reactions. + The reduced quantity of Na alters the electron cloud density state of M / NO, resulting in a more stable bulk structure, increased electron localization, and improved Na... + Ion migration barriers are detrimental to the electrochemical performance of materials at low temperatures. If the amount of metal ion doping is too small, it will lead to insufficient widening of ion migration channels at specific sodium sites, small degree of lattice distortion, and unstable bulk structure, resulting in the inability to fully realize the electrochemical performance.

[0024] Preferably, the doping amount of borate ions relative to sulfate ions is 0.01–6.0 at% (more preferably 0.1–3.0 at%, and even more preferably 0.3–1.3 at%). Compared to pyrophosphate and phosphate, borate ions can form a BO planar triangular structure, resulting in two-dimensional lattice distortion. If BO3… 3- Excessive doping can lead to severe lattice distortion in the material, causing structural instability and collapse during charge-discharge cycles; if BO3... 3- If the doping amount is too low, the lattice distortion of the bulk material will be weak, and the ion migration channels will be reduced.

[0025] Preferably, the carbon composite in the cathode material has a mass fraction of 0.3–4.0 wt% (more preferably 0.3–4.0 wt%, and even more preferably 2.0–3.2 wt%). Excessive carbon content leads to increased raw material costs, a decreased proportion of active material in the electrode material, and a decrease in the material's specific energy; insufficient carbon content makes it difficult to improve the material's electronic conductivity and construct an effective conductive network.

[0026] The technical solution adopted by the present invention to further solve its technical problem is as follows: a method for preparing oxide-coated ion-doped sodium iron sulfate cathode material, comprising the following steps: (1) Sodium source, ferrous source, sulfate source, multivalent metal cation dopant, borate anion dopant and organic acid antioxidant are added to water, stirred and dissolved, and then inorganic conductive carbon source slurry is added. After stirring and mixing, spray drying is carried out to obtain anion and cation doped sodium ferric sulfate precursor material. (2) The anion and cation doped sodium ferric sulfate precursor material obtained in step (1) is mixed with a positive temperature coefficient thermosensitive metal oxide coating agent and added to a low boiling point organic solvent. The solvent is evaporated and coated, and then vacuum dried to obtain metal oxide coated anion and cation doped sodium ferric sulfate precursor material. (3) The metal oxide obtained in step (2) is coated with the anion and cation doped sodium iron sulfate precursor material and sintered under a protective atmosphere to obtain oxide-coated ion doped sodium iron sulfate cathode material.

[0027] The inventive idea of ​​the method of the present invention is as follows: In step (1), an organic acid antioxidant is first added to provide a suitable pH environment to ensure the stable existence of ferrous ions. During the sintering process, the organic acid antioxidant is carbonized as an outer organic carbon conductive coating layer. After adding an inorganic conductive carbon source, the mixture is stirred evenly and spray drying is used to make the material precipitate in a homogeneous phase and avoid the generation of associated crystal water. In step (2), the hollow microsphere particle precursor obtained in the previous step is mixed with a thermosensitive nano metal oxide and a solvent evaporation reaction is carried out in a low-boiling-point organic solvent to obtain a metal oxide coated precursor material. In step (3), the obtained precursor material is sintered at low temperature to remove the crystal water in the material, increase the crystallinity of the material, and promote the diffusion of doped ions to form the obtained phase.

[0028] Preferably, in step (1), the molar ratio of sodium, ferrous, sulfate, metal cation and borate in the sodium source, ferrous source, sulfate source, metal cation dopant and borate anion dopant matches the molar ratio of the corresponding substances in the chemical formula.

[0029] Preferably, in step (1), the amount of the organic acid antioxidant is such that its carbonization mass accounts for 0.1–1.5 wt% (more preferably 0.6–1.3 wt%, and even more preferably 0.8–1.2 wt%) of the oxide-coated ion-doped sodium iron sulfate cathode material. The organic acid antioxidant can control the pH value of the solution environment to be maintained between 2 and 8, ensuring that Fe is maintained during stirring. 2+ It exists stably, preventing oxidation or hydrolysis. Simultaneously, during sintering, the organic acid antioxidant carbonizes and decomposes, forming an outer conductive organic carbon coating. If the amount of organic acid antioxidant is too small, it is difficult to ensure the Fe... 2+ The organic carbon antioxidant remains stable throughout the reaction process. However, excessive use of organic carbon antioxidants can lead to a low proportion of active substances in the material, resulting in lower conductivity and increased electrochemical polarization during charge and discharge. Different organic acid antioxidants yield different carbonization qualities at different sintering temperatures. Carbonization quality = mass of organic acid antioxidant * residual percentage after carbonization at the corresponding temperature, which is obtained from thermogravimetric analysis.

[0030] Preferably, in step (1), the amount of water used is such that the concentration of dissolved ferrous ions is 1.0–2.0 mol / L (more preferably 1.2–1.8 mol / L). During spray drying, the volume of atomized droplets is fixed under fixed process parameters. If the concentration of the prepared material is too low, the proportion of water molecules in the solvent will increase, and the hollow volume will increase greatly when forming microspheres, which will seriously affect the cycle stability of the material during charge and discharge. If the concentration of the prepared material is too high, the formed microspheres will have a rough and thick wall layer.

[0031] Preferably, in step (1), the stirring and dissolving temperature is 25-30°C, the stirring speed is 300-500 rpm, and the time is 15-25 min.

[0032] Preferably, in step (1), the solid content of the inorganic conductive carbon source slurry is 3-10% (more preferably 4-8%). If the solid content is too high, the consistency and stability of the slurry will be poor, and it will easily agglomerate into a gel, which is not conducive to long-term use, and the cost of raw materials will increase. If the solid content is too low, the solid content of the entire system will be reduced, affecting the particle size and conductivity of the material, and reducing the electrochemical performance of the material.

[0033] Preferably, in step (1), the amount of inorganic conductive carbon source slurry used is such that the mass fraction of inorganic conductive carbon source in the theoretical mass of oxide-coated ion-doped sodium iron sulfate cathode material is 0.1–2.5 wt% (more preferably 0.2–2.2 wt%, and even more preferably 1.0–2.0 wt%). Sodium iron sulfate material has low intrinsic electronic conductivity, and the addition of inorganic carbon source can improve the electronic conductivity of the material. If the amount of inorganic conductive carbon source is too high, it will reduce the proportion of active material in the material system and increase the material production cost. If the amount of inorganic conductive carbon source is too low, the material conductivity will be low, and the electrochemical performance will be difficult to effectively utilize.

[0034] Preferably, in step (1), the stirring temperature is 25-30°C, the rotation speed is 700-900 rpm, and the time is 15-25 min.

[0035] Preferably, in step (1), the feed solution is continuously stirred at 700-900 rpm while the spray drying feed is being fed.

[0036] Preferably, in step (1), the process parameters for spray drying are: induced draft fan frequency of 20–40 Hz (more preferably 30–40 Hz), inlet air temperature of 200–300 °C (more preferably 220–240 °C), and outlet air temperature of 100–140 °C (more preferably 110–130 °C). During spray drying, the atomized droplets instantly enter a higher temperature environment from room temperature, causing the water solvent to evaporate instantly and allowing the solute to precipitate more homogeneously. Matching the inlet air temperature with the induced draft fan frequency can control the size of the atomized droplets; if the inlet air temperature is too high, the sphericity of the material will be severely damaged; the outlet air temperature should be ≥ the solvent evaporation temperature to reduce the generation of water of crystallization.

[0037] Preferably, in step (1), the sodium source includes one or more of sodium sulfate, sodium nitrate, sodium acetate or sodium formate, and their hydrates.

[0038] Preferably, in step (1), the ferrous source includes one or more of ferrous sulfate and / or ferrous nitrate, and their hydrates. When the ferrous source is ferrous sulfate, it can be partially used as a sulfuric acid source.

[0039] Preferably, in step (1), the sulfuric acid source includes sulfuric acid, and one or more of ammonium sulfate or sodium sulfate and their hydrates. More preferably, the sulfuric acid is dilute sulfuric acid with a concentration of 1.5–2.5 mol / L. When the sulfuric acid source is sodium sulfate, it can be used as a partial sodium source.

[0040] Preferably, in step (1), the multivalent metal cation dopant includes one or more of the following: sulfate, nitrate, acetate, chloride, or soluble organometallic salts of multivalent metal dopant elements, and their hydrates or solutions. More preferably, the metal cation dopant includes one or more of the following: sulfate, nitrate, acetate, chloride, or soluble organometallic salts of K, Ca, Al, or Ti, and their hydrates or solutions. When the metal cation dopant is a sulfate, it can partially serve as a sulfuric acid source.

[0041] Preferably, in step (1), the borate anion dopant includes boric acid, and one or more of borate anion dopant and its hydrate.

[0042] Preferably, in step (1), the organic acid antioxidant includes one or more of ascorbic acid, citric acid or tannic acid.

[0043] Preferably, in step (1), the inorganic conductive carbon source includes one or more of carbon quantum dots, carbon nanotubes, Ketjen black, SuperP, acetylene black, or graphene.

[0044] Preferably, in step (2), the amount of the positive temperature coefficient (PTC) thermistor-sensitive metal oxide coating agent is such that its mass fraction in the theoretical mass of the oxide-coated ion-doped sodium ferric sulfate cathode material is 0.1–8.5 wt% (more preferably 2.2–8.2 wt%). If too much PTC thermistor-sensitive metal oxide is used, although it reduces surface side reactions, it increases raw material costs and hinders the Na at the material interface. + Migration can actually lead to a decrease in electrochemical performance; if the amount of positive temperature coefficient thermistor oxide is too small, the electronic conductivity and electrochemical performance of the material will be difficult to improve effectively in low-temperature environments.

[0045] Preferably, in step (2), the solid-liquid ratio of the positive temperature coefficient thermosensitive metal oxide coating agent to the low-boiling-point organic solvent is 1:400-750 (more preferably 1:500-600). If the amount of solid is too large, the liquid mixing reaction will be insufficient, which will make it difficult for the metal oxide to fully coat the material surface; if the amount of liquid is too large, the liquid evaporation time will be too long, and Fe element will dissolve from the sodium ferric sulfate precursor.

[0046] Preferably, in step (2), the solvent evaporation coating refers to stirring at 60–90°C (more preferably 70–80°C) for 1–5 hours (more preferably 3–4 hours) until the solvent is evaporated to dryness. The method of this invention uses solvent evaporation for coating. Controlling the evaporation temperature at the boiling point of the organic solvent during the liquid flow evaporation reaction is more conducive to the rapid formation of the composite material; if the evaporation temperature is too low, the reaction time will be too long, resulting in the dissolution of Fe elements; if the evaporation temperature is too high, the raw materials will not be sufficiently mixed.

[0047] Preferably, in step (2), the vacuum drying temperature is 70-90°C, the vacuum degree is -0.05--0.10MPa (more preferably -0.06--0.10MPa), and the time is 3-6h (more preferably 4-5h).

[0048] Preferably, in step (2), the low-boiling-point organic solvent includes one or more of ethanol, propanol, tetrahydrofuran, or ethylenediamine. More preferably, the low-boiling-point organic solvent is ethanol.

[0049] Preferably, in step (3), before sintering, the system is ventilated with a protective atmosphere at room temperature for 1.5 to 2.5 hours to ensure that the sintering system is filled with a protective atmosphere and to remove other gases.

[0050] Preferably, in step (3), the sintering refers to: heating from room temperature to 350-400℃ (more preferably 370-380℃) at a rate of 1-5℃ / min, and sintering for 6-12 hours (more preferably 10-12 hours). The method of this invention enables the sodium ferric sulfate precursor to undergo a dehydration reaction of its water of crystallization through one-step sintering, while simultaneously forming a well-structured oxide coating of the sodium ferric sulfate material. If the sintering temperature is too high or the time is too long, it will lead to the decomposition of sulfate ions in the material, resulting in a distorted material structure and a decrease in specific capacity and cycle performance. If the sintering temperature is too low or the time is too short, the crystallinity of the material will be poor.

[0051] Preferably, in step (3), the protective atmosphere includes one or more of nitrogen, argon, and hydrogen-argon or a mixture of hydrogen and nitrogen. The nitrogen, argon, or atmosphere used to prepare the mixture in this invention are all high-purity atmospheres with a purity ≥ 99.999%.

[0052] The technical solution adopted by the present invention to further solve its technical problem is as follows: the application of oxide-coated ion-doped sodium iron sulfate cathode material, and the cathode electrode sheet made of the oxide-coated ion-doped sodium iron sulfate cathode material is used to assemble sodium-ion batteries.

[0053] The beneficial effects of this invention are as follows: (1) The cathode material of the present invention has a specific oxide coating, which greatly improves the electronic conductivity of the electrode material at low temperature and stabilizes Na + Dynamic properties of materials during insertion / extraction processes; high-valence Ca 2+ Al 3+ Ti 4+ Metal ion doping into the Na / Fe sites in the crystal lattice alleviates the volume expansion and contraction caused by stress changes during charge-discharge cycles, maintaining good cycle stability; low-valence metal K + In-situ doping occupies Na1 sites, broadening Na + Migration channels along the a-axis accelerate ion migration rates; the planar triangular BO occupies part of the three-dimensional SO tetrahedral position, altering the partial Fe2O content of sodium ferric sulfate. 10 The arrangement of the dimer and SO tetrahedra causes lattice distortion and increases the amount of Na. + Migration channels; the initial powder conductivity of the cathode material of this invention is as high as 4.38 S / m at low temperatures, and Na... + The migration rate is 0.05–4.5 × 10⁻⁶. -9 This indicates good electronic and ionic conductivity; (2) The battery assembled from the positive electrode sheet made of the positive electrode material of the present invention has a reversible discharge specific capacity of up to 87.51 mAh / g and an initial coulombic efficiency of up to 90.86% at 0.1C rate. At 1C rate, the first-cycle discharge specific capacity is up to 83.52 mAh / g and the capacity retention rate after 100 cycles is up to 93.86%. The discharge specific capacity at 10C and 20C is up to 77.41 mAh / g and 73.57 mAh / g, respectively. At low temperature conditions at 10C rate, the capacity retention rate after 250 cycles can still reach 80%. This shows that the present invention achieves atomic lattice site regulation through surface thermosensitive material coating design and micro-angle anion and cation co-doping, comprehensively and synergistically improving the low temperature electrochemical performance of battery materials, and successfully preparing sodium iron sulfate material for wide temperature range sodium-ion batteries. The energy storage and power batteries produced by the positive electrode material of the present invention are suitable for low temperature harsh environments and can replace lead-acid batteries for low-speed electric vehicles in low temperature environments. They can also be used as storage devices for energy storage systems or devices in harsh environments. (3) The method of the present invention has low raw material cost, simple synthesis method, short production cycle, and is suitable for large-scale continuous production. Attached Figure Description

[0054] Figure 1 This is cerium oxide coated with Na in Example 1 of the present invention. 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.97 (BO3) 0.046 / 3 SEM images of the cathode material (where the scale bars for (a) to (d) are 20 μm, 5 μm, 2 μm, and 1 μm, respectively). Figure 2 yes Figure 1 (b) EDS plot; Figure 3 This is cerium oxide coated with Na in Example 1 of the present invention. 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.97 (BO3) 0.046 / 3 XRD patterns of the cathode material and the microsphere-structured carbon composite sodium iron sulfate cathode material obtained in Comparative Example 1; Figure 4 This is cerium oxide coated with Na in Example 1 of the present invention. 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.97 (BO3) 0.046 / 3 FTIR images of the cathode material and the microsphere-structured carbon composite sodium iron sulfate cathode material obtained in Comparative Example 1; Figure 5This is cerium oxide coated with Na in Example 1 of the present invention. 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.97 (BO3) 0.046 / 3 XPS images of the cathode material; Figure 6 This is cerium oxide coated with Na in Example 1 of the present invention. 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.97 (BO3) 0.046 / 3 Positive electrode material, Example 4: Tin oxide coated Na 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 Powder resistivity diagrams of the positive electrode material and the microsphere-structured carbon composite sodium iron sulfate positive electrode material obtained in Comparative Example 1 in a low-temperature (-20℃) environment; Figure 7 This is cerium oxide coated with Na in Example 1 of the present invention. 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.97 (BO3) 0.046 / 3 GITT plots of the cathode material and the microsphere-structured carbon composite sodium iron sulfate cathode material obtained in Comparative Example 1 at low temperature (-20℃); Figure 8 This is cerium oxide coated with Na in Example 1 of the present invention. 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.97 (BO3) 0.046 / 3 The battery assembled from the positive electrode sheet made of the positive electrode material and the microsphere structured carbon composite sodium iron sulfate positive electrode material obtained in Comparative Example 1, under the first charge and discharge curves at -20℃, voltage range of 2.0 to 4.5V, and current rate of 0.1 C (11mA / g); Figure 9 This is cerium oxide coated with Na in Example 1 of the present invention. 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.97 (BO3) 0.046 / 3The battery assembled from the positive electrode sheet made of the positive electrode material and the microsphere structured carbon composite sodium iron sulfate positive electrode material obtained in Comparative Example 1, under the current rate cycling diagram of the voltage range of 2.0 to 4.5V and multiple current density rates (1C=110mA / g) at -20℃. Figure 10 This is cerium oxide coated with Na in Example 1 of the present invention. 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.97 (BO3) 0.046 / 3 The battery assembled from the positive electrode sheet made of the positive electrode material and the microsphere structured carbon composite sodium iron sulfate positive electrode material obtained in Comparative Example 1, under the conditions of -20℃, the cycling curves of the battery under the voltage range of 2.0 to 4.5V and the current rate of 1C (1C=110mA / g). Figure 11 This is cerium oxide coated with Na in Example 1 of the present invention. 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.97 (BO3) 0.046 / 3 Cycling curves of a battery assembled with a positive electrode sheet made of positive electrode material at -20℃, in a voltage range of 2.0 to 4.5V and a current rate of 10 C (1C=110mA / g); Figure 12 This is a SEM image of the microsphere-structured carbon composite sodium ferric sulfate cathode material obtained in Comparative Example 1 of this invention; Figure 13 This is the XPS image of the microsphere-structured carbon composite sodium ferric sulfate cathode material obtained in Comparative Example 1 of this invention; Figure 14 This is a SEM image of the oxide-coated carbon composite sodium iron sulfate cathode material obtained in Comparative Example 2 of this invention; Figure 15 This is the EDS diagram of the oxide-coated carbon composite sodium iron sulfate cathode material obtained in Comparative Example 2 of this invention. Detailed Implementation

[0055] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0056] The aqueous carbon nanotube conductive paste used in the embodiments and comparative examples of this invention has a solid content of 5% and was purchased from Xiamen Kaina Graphene Technology Co., Ltd.; the temperature-sensitive nano-metal oxide used in the embodiments and comparative examples of this invention was purchased from Zhejiang Zhitai Nano-Micro New Materials Co., Ltd.; the nitrogen gas used in the embodiments and comparative examples of this invention is a high-purity atmosphere with a purity ≥99.999%; in the embodiments and comparative examples of this invention, thermogravimetric analysis showed that the residual percentage of ascorbic acid after carbonization at 380℃ was 40.11%; the raw materials or chemical reagents used in the embodiments and comparative examples of this invention, unless otherwise specified, were obtained through conventional commercial channels. The electrochemical performance tests and powder resistance tests of the embodiments and comparative examples of this invention were all conducted under low-temperature conditions, namely -20℃.

[0057] Cerium oxide coated Na 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 Example 1 of positive electrode material The cerium oxide coated Na 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 The cathode material is a carbon composite material with thermistor-sensitive cerium oxide nano-coated K. + Ti 4+ BO3 3- It is made of doped sodium ferric sulfate bulk material; wherein, K + Ti is doped at the Na sites of sodium ferric sulfate. 4+ Doping at the Na / Fe sites of sodium ferric sulfate, BO3 3- Planar triangular BO doping occupies some SO tetrahedral positions, and the inorganic carbon in the carbon composite is uniformly distributed within the sodium ferric sulfate bulk phase material, while the organic carbon is uniformly distributed outside the sodium ferric sulfate bulk phase material; the cerium oxide coating Na 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 The cathode material has a hollow microsphere structure with an average particle size of 20 μm; the mass fraction of the thermosensitive nano-cerium oxide in the cathode material is 3.98 wt%; in the cathode material, K + Ti 4+ Relative to Na + The doping amounts were 1.63 at%, 1.06 at%, and BO3, respectively. 3- Compared to SO4 2-The doping amount is 0.52 at; the mass fraction of the carbon composite in the cathode material is 3.0 wt%.

[0058] like Figure 1 , 2 As shown, in an embodiment of the present invention, cerium oxide is coated with Na 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 The cathode material has a hollow microsphere structure with an average particle size of 20 μm. The surface of the microsphere particles is covered with metal oxides, carbon nanotubes and organic carbon. In the material cross-section, EDS was used to find that Na, Fe, K, Ti and B elements are uniformly distributed in the material, proving that K, Ti and B elements have been successfully doped into the material lattice.

[0059] like Figure 3 As shown, in an embodiment of the present invention, cerium oxide is coated with Na 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 The XRD diffraction peaks of the cathode material all correspond to the standard card PDF#97-025-2403. Among them, the main crystal plane reflection peaks at 15.52°, 22.74°, 28.71° and 32.12° correspond to (200), (130), (-112) and (240) respectively, proving that sodium iron sulfate material was successfully synthesized. At the same time, there are also crystal plane reflection peaks corresponding to the standard card PDF#01-073-7747 for nano-metal cerium oxide, indicating that there is a small amount of metal cerium oxide in the composite material.

[0060] like Figure 4 As shown, in an embodiment of the present invention, cerium oxide is coated with Na 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 The cathode material at a wavenumber of 591 cm⁻¹ -1 988cm -1 and 1045cm -1 Strong Fe-O vibrational peaks, SO symmetric stretching vibrational peaks, and SO4 asymmetric stretching vibrational peaks were observed, indicating the successful preparation of sodium ferric sulfate material; the wavenumbers were approximately 420–510 cm⁻¹. -1 Between these peaks, weaker and additional Me-O vibrational peaks of K and Ti-O were observed, indicating the presence of trace amounts of K and Ti doping; at a wavenumber of 650 cm⁻¹ -1At 1150 cm⁻¹, a peak corresponding to the BOB bending vibration was found. -1 At this location, a stretching vibration peak corresponding to the BO bond was found, indicating that BO3 3- Successfully incorporated into the material.

[0061] like Figure 5 As shown, in an embodiment of the present invention, cerium oxide is coated with Na 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 The XPS full spectrum of the cathode material contains Na1s, Fe2p, S2p, Ti2p, K2s, O1s, C1s, and B1s elemental orbitals. Among these, the high-valence metallic element Ti, with its strong electronegativity, is particularly prominent. 4+ Doping into the crystal lattice causes a shift in the distribution of the Fe electron cloud in the Ti-O-Fe structure, exacerbating the localization of d-orbital electrons and making Fe... 2+ 2p 3 / 2 and 2p 1 / 2 The binding energy increases, corresponding to 711.5 eV and 724.8 eV for Fe2p; the peak at a binding energy of 191.9 eV corresponds to B1s.

[0062] The cerium oxide-coated Na in this embodiment of the invention was tested using inductively coupled plasma atomic emission spectrometry (ICP). 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 The Ce element in the cathode material, when converted to CeO2, has a mass fraction of 3.98 wt%; through sulfur and carbon analysis, the mass fraction of carbon composites in the cathode material is 3.0 wt%.

[0063] like Figure 6 As shown, cerium oxide coated with Na in this embodiment of the invention... 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 The cathode material was subjected to powder resistance testing at low temperature, and its initial powder conductivity was 2.62 S / m, indicating that the electronic conductivity of the material was improved after the surface was synergistically modified by metal oxides and carbon layers.

[0064] like Figure 7 As shown, the cerium oxide-coated Na in this embodiment of the invention was tested using galvanostatic intermittent titration (GITT). 2.456 K 0.04Ti 0.026 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 During the charging and discharging process, Na... + The migration rate is mainly in the range of 0.09 to 4.5 × 10⁻⁶. -9 .

[0065] Cerium oxide coated Na 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 Example 1 of the preparation method of positive electrode material (1) Add 6.977g (0.04912mol) of anhydrous sodium sulfate, 18.90g (0.068mol) of ferrous sulfate heptahydrate, 1mL of dilute sulfuric acid (concentration 2mol / L, 0.002mol), 0.157g (0.0016mol) of potassium acetate, 0.354g (0.00104mol) of tetrabutyl titanate, 0.0379g (0.000613mol) of boric acid and 0.51g of ascorbic acid (after carbonization, equivalent to 1.079wt% of the theoretical mass of the cathode material). Add 50 mL of deionized water and stir at 25 °C and 400 rpm for 20 min to dissolve. Then add 7.3 g of aqueous carbon nanotube conductive slurry (carbon nanotubes are equivalent to 1.924 wt% of the theoretical mass of the cathode material). Stir and mix at 25 °C and 800 rpm for 20 min. The resulting feed solution is then continuously stirred at 800 rpm and spray-dried at a fan frequency of 30 Hz, an inlet air temperature of 220 °C, and an outlet air temperature of 120 °C to obtain Na. 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 xH2O (x=2,4) precursor materials; (2) The precursor material obtained in step (1) is mixed with 0.758g of thermosensitive nano-metal cerium oxide (equivalent to 4wt% of the theoretical mass of the cathode material) and added to 400mL of ethanol. The mixture is stirred in a water bath at 80℃ for 3h until it evaporates to dryness. The solvent is evaporated and coated. The solid after evaporation is placed in a vacuum drying oven at 80℃ and a vacuum degree of -0.08MPa and vacuum dried for 4h to obtain cerium oxide coated anion and cation doped sodium iron sulfate precursor material. (3) The cerium oxide-coated anion and cation doped sodium ferric sulfate precursor material obtained in step (2) is placed in a tube furnace. First, it is purged with high-purity nitrogen atmosphere at room temperature for 2 hours to ensure that the tube furnace is filled with high-purity nitrogen atmosphere and other gases are removed. Then, under high-purity nitrogen atmosphere, the temperature is increased from room temperature to 380℃ at a rate of 5℃ / min and sintered for 10 hours to obtain cerium oxide-coated Na 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 Positive electrode material.

[0066] Cerium oxide coated Na 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 Application Example 1 of Cathode Material The cerium oxide coated Na as described in Example 1 of this invention 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 Positive electrode sheets made of positive electrode material are used to assemble sodium-ion batteries.

[0067] Battery assembly: Under conditions where the dew point is below -30°C, weigh 0.0800g of the cerium oxide-coated Na described in Example 1 of this invention. 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 The positive electrode material was mixed with 0.0100g of conductive carbon black as a conductive agent and 0.0100g of PVDF (polyvinylidene fluoride) as a binder. After being mixed evenly, it was coated on aluminum foil to form a positive electrode sheet. In a vacuum glove box, a sodium metal sheet was used as the negative electrode, the battery separator was a Whatman GF / D glass fiber separator, and the electrolyte was 1mol / L NaClO4 (EC:DMC=1:1 (vol%) + 5%FEC) to assemble a CR2032 coin cell.

[0068] Battery performance testing: Electrochemical performance and cycle performance tests were conducted on the assembled batteries in the range of 2 to 4.5V and at an environment of -20℃, including the first charge-discharge specific capacity and first coulombic efficiency at 0.1C, the discharge specific capacity and capacity retention after 100 cycles at a current rate of 1C, and the discharge specific capacity at different high rates.

[0069] like Figure 8 As shown, the cerium oxide coated Na in Example 1 of this invention 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 A sodium-ion battery assembled with a positive electrode sheet made of the positive electrode material exhibits an initial discharge specific capacity of 87.51 mAh / g and an initial coulombic efficiency as high as 90.86% at a low temperature of 0.1C. This indicates that at a low temperature of -20℃, the thermosensitive nano-metal oxide significantly reduces the polarization of the material during charge and discharge, thus improving conductivity. Co-doping of cations and anions reduces the Na+ polarization. + The migration barrier facilitates the release of charging and discharging capacity.

[0070] like Figure 9 As shown, the cerium oxide coated Na in Example 1 of this invention 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 Sodium-ion batteries assembled with positive electrode sheets made of positive electrode material showed discharge specific capacities of 76.57 mAh / g and 72.83 mAh / g at low temperatures of 10C and 20C, respectively, indicating that K + Doping broadens the range of Na + Transport channels enable materials to have higher Na content. + The extraction and insertion rates result in excellent rate performance; while Ti 4+ Doping stabilizes the material structure, and the material maintains a similar discharge specific capacity under repeated rate tests, indicating that the material has excellent structural reversibility.

[0071] like Figure 10 As shown, the cerium oxide coated Na in Example 1 of this invention 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 A sodium-ion battery assembled with a positive electrode sheet made of positive electrode material exhibits a discharge specific capacity of 82.16 mAh / g at a 1C rate (starting from the 4th cycle) under low-temperature conditions. After 100 cycles, the discharge specific capacity is 75.75 mAh / g, with a capacity retention of 92.20%, indicating that K... + Ti 4+ Doping can effectively expand ion channels, stabilize crystal structure, and help improve the cycling performance of materials.

[0072] like Figure 11As shown, the cerium oxide coated Na in Example 1 of this invention 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 Sodium-ion batteries assembled with positive electrode sheets made of positive electrode material can still retain 80% of their capacity after 250 cycles at a 10C rate under low-temperature conditions.

[0073] Cerium oxide coated Na 2.43 K 0.04 Al 0.026 Ti 0.013 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 Example 2 of positive electrode material The cerium oxide coated Na 2.43 K 0.04 Al 0.026 Ti 0.013 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 The cathode material is a carbon composite material with thermistor-sensitive cerium oxide nano-coated K. + Al 3+ Ti 4+ BO3 3- It is made of doped sodium ferric sulfate bulk material; wherein, K + Al is doped at the Na sites of sodium ferric sulfate. 3+ Ti 4+ Doping at the Na / Fe sites of sodium ferric sulfate, BO3 3- Planar triangular BO doping occupies some SO tetrahedral positions, and the inorganic carbon in the carbon composite is uniformly distributed within the sodium ferric sulfate bulk phase material, while the organic carbon is uniformly distributed outside the sodium ferric sulfate bulk phase material; the cerium oxide coating Na 2.43 K 0.04 Al 0.026 Ti 0.013 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 The cathode material has a hollow microsphere structure with an average particle size of 20 μm; the mass fraction of the thermosensitive nano-cerium oxide in the cathode material is 4.02 wt%; in the cathode material, K + Al 3+ Ti 4+ Relative to Na + The doping concentrations were 1.65 at%, 1.07 at%, and 0.53 at%, respectively. 3- Compared to SO42- The doping amount is 0.52 at; the mass fraction of the carbon composite in the cathode material is 3.01 wt%.

[0074] Upon testing, the cerium oxide coated Na in this embodiment of the invention... 2.43 K 0.04 Al 0.026 Ti 0.013 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 The cathode material has a hollow microsphere structure with an average particle size of 20 μm. The surface of the microsphere particles is covered with metal oxides, carbon nanotubes and organic carbon. In the material cross-section, EDS was used to find that Na, Fe, K, Al, Ti and B elements are uniformly distributed in the material, proving that K, Al, Ti and B elements have been successfully doped into the material lattice.

[0075] Upon testing, the cerium oxide coated Na in this embodiment of the invention... 2.43 K 0.04 Al 0.026 Ti 0.013 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 The diffraction peaks of the positive electrode material all correspond to the standard card PDF#97-025-2403, proving that sodium iron sulfate material was successfully synthesized. At the same time, some crystal reflection peaks also correspond to the standard card PDF#01-075-2325 for aluminum sulfate, indicating that the material contains Al doping elements. The crystal reflection peaks corresponding to the standard card PDF#01-073-7747 for nano-sized cerium oxide indicate that there is a small amount of cerium oxide in the composite material.

[0076] Upon testing, the cerium oxide coated Na in this embodiment of the invention... 2.43 K 0.04 Al 0.026 Ti 0.013 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 The cathode material at a wavenumber of 591 cm⁻¹ -1 988cm -1 and 1045cm -1 Strong Fe-O vibrational peaks, SO symmetric stretching vibrational peaks, and SO4 asymmetric stretching vibrational peaks were observed, indicating the successful preparation of sodium ferric sulfate material; the wavenumbers were approximately 420–510 cm⁻¹. -1 Between these peaks, weaker and additional Me-O vibrational peaks of Al-O, KO, and Ti-O were observed, indicating the presence of trace amounts of K, Al, and Ti elemental doping; at a wavenumber of 648 cm⁻¹ -1At 1146 cm⁻¹, a peak corresponding to the BOB bending vibration was found. -1 At this location, a stretching vibration peak corresponding to the BO bond was found, indicating that BO3 3- Successfully incorporated into the material.

[0077] Testing revealed that the cerium oxide-coated Na in this embodiment of the invention... 2.43 K 0.04 Al 0.026 Ti 0.013 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 The XPS full spectrum of the cathode material contains Na1s, Fe2p, S2p, Ti2p, K2s, Al3p, O1s, C1s, and B1s elemental orbitals. Among these, the high-valence metallic element Ti, with its strong electronegativity, is particularly prominent. 4+ Doping into the crystal lattice causes a shift in the distribution of the Fe electron cloud in the Ti-O-Fe structure, exacerbating the localization of d-orbital electrons and making Fe... 2+ 2p 3 / 2 and 2p 1 / 2 The binding energy increases, corresponding to 711.6 eV and 724.9 eV for Fe2p; the peak at a binding energy of 191.6 eV corresponds to B1s.

[0078] ICP testing of cerium oxide-coated Na in embodiments of the present invention 2.43 K 0.04 Al 0.026 Ti 0.013 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 The Ce element in the cathode material, when converted to CeO2, has a mass fraction of 4.02 wt%; through sulfur and carbon analysis, the mass fraction of carbon composites in the cathode material is 3.01 wt%.

[0079] Upon testing, the cerium oxide coated Na in the embodiments of the present invention was found to be... 2.43 K 0.04 Al 0.026 Ti 0.013 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 The cathode material was subjected to powder resistance testing at low temperature, and its initial powder conductivity was 2.25 S / m, indicating that the electronic conductivity of the material was improved after the surface was synergistically modified by metal oxides and carbon layers.

[0080] Testing revealed that the cerium oxide-coated Na in this embodiment of the invention passed the GITT test. 2.43 K 0.04 Al0.026 Ti 0.013 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 During the charging and discharging process, Na... + The migration rate is 0.05–4.3 × 10⁻⁶. -9 .

[0081] Cerium oxide coated Na 2.43 K 0.04 Al 0.026 Ti 0.013 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 Example 2 of the preparation method of positive electrode material The difference between this embodiment and Method Embodiment 1 is only that: in step (1), 0.347g (0.00052mol) of aluminum sulfate octadeca was added, 6.977g (0.04912mol) of anhydrous sodium sulfate was replaced with 6.904g (0.0486mol) of anhydrous sodium sulfate, 1mL of dilute sulfuric acid (concentration 2mol / L, 0.002mol) was replaced with 0.5mL of dilute sulfuric acid (concentration 2mol / L, 0.001mol), and 0.3612g (0.001mol) of tetrabutyl titanate was replaced with 0.177g (0.00052mol) of tetrabutyl titanate. After carbonization of ascorbic acid, it is equivalent to 1.08wt% of the theoretical mass of the cathode material, and the carbon nanotubes are equivalent to 1.927wt% of the theoretical mass of the cathode material. Finally, the precursor Na was obtained. 2.43 K 0.04 Al 0.026 Ti 0.013 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 ·xH2O (x=2,4) precursor material; in step (3), cerium oxide coated Na is finally obtained. 2.43 K 0.04 Al 0.026 Ti 0.013 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 Positive electrode material. Same method as Example 1.

[0082] Cerium oxide coated Na 2.43 K 0.04 Al 0.026 Ti 0.013 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 Application Example 2 of Cathode Material The cerium oxide coated Na described in Example 2 of this invention 2.43 K 0.04 Al 0.026 Ti 0.013 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 Positive electrode sheets made of positive electrode material are used to assemble sodium-ion batteries.

[0083] Battery assembly: Same as in Application Example 1.

[0084] Battery performance test: Same as in application example 1, the test results are shown in Table 1.

[0085] Cerium oxide coated Na 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.955 (BO3) 0.03 Example 3 of positive electrode material The cerium oxide coated Na 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.955 (BO3) 0.03 The cathode material is a carbon composite material with thermistor-sensitive cerium oxide nano-coated K. + Ti 4+ BO3 3- It is made of doped sodium ferric sulfate bulk material; wherein, K + Ti is doped at the Na sites of sodium ferric sulfate. 4+ Doping at the Na / Fe sites of sodium ferric sulfate, BO3 3- Planar triangular BO doping occupies some SO tetrahedral positions, and the inorganic carbon in the carbon composite is uniformly distributed within the sodium ferric sulfate bulk phase material, while the organic carbon is uniformly distributed outside the sodium ferric sulfate bulk phase material; the cerium oxide coating Na 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.955 (BO3) 0.03 The cathode material has a hollow microsphere structure with an average particle size of 18 μm; the mass fraction of the thermosensitive nano-cerium oxide in the cathode material is 4.06 wt%; in the cathode material, K + Ti 4+ Relative to Na + The doping amounts were 1.63 at%, 1.06 at%, and BO3, respectively. 3- Compared to SO4 2-The doping amount is 1.02 at; the mass fraction of the carbon composite in the cathode material is 3.0 wt%.

[0086] Upon testing, the cerium oxide coated Na in this embodiment of the invention... 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.955 (BO3) 0.03 The cathode material has a hollow microsphere structure with an average particle size of 18 μm. The surface of the microsphere particles is covered with metal oxides, carbon nanotubes and organic carbon. In the material cross-section, EDS was used to find that Na, Fe, K, Ti and B elements are uniformly distributed in the material, which proves that K, Ti and B elements have been successfully doped into the material lattice.

[0087] Upon testing, the cerium oxide coated Na in this embodiment of the invention... 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.955 (BO3) 0.03 The diffraction peaks of the positive electrode material all correspond to the standard card PDF#97-025-2403, proving that sodium iron sulfate material was successfully synthesized; at the same time, there are also crystal reflection peaks corresponding to the standard card PDF#01-073-7747 of nano-metal cerium oxide, indicating that there is a small amount of metal cerium oxide in the composite material.

[0088] Upon testing, the cerium oxide coated Na in this embodiment of the invention... 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.955 (BO3) 0.03 The cathode material at a wavenumber of 591 cm⁻¹ -1 988cm -1 and 1045cm -1 Strong Fe-O vibrational peaks, SO symmetric stretching vibrational peaks, and SO4 asymmetric stretching vibrational peaks were observed, indicating the successful preparation of sodium ferric sulfate material; at wavenumbers of approximately 405–508 cm⁻¹... -1 Between these peaks, weaker and additional Me-O vibrational peaks of K and Ti-O were observed, indicating the presence of trace amounts of K and Ti element doping; at a wavenumber of 655 cm⁻¹ -1 At 1152 cm⁻¹, a peak corresponding to the BOB bending vibration was found. -1 At this location, a stretching vibration peak corresponding to the BO bond was found, indicating that BO3 3- Successfully incorporated into the material.

[0089] Upon testing, the cerium oxide coated Na in this embodiment of the invention...2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.955 (BO3) 0.03 The XPS full spectrum of the cathode material contains Na1s, Fe2p, S2p, Ti2p, K2s, O1s, C1s, and B1s elemental orbitals. Among these, the high-valence metallic element Ti, with its strong electronegativity, is particularly prominent. 4+ Doping into the crystal lattice shifts the distribution of Fe electron clouds in the Ti-O-Fe structure, intensifying the localization of d-orbital electrons and leading to Fe... 2+ 2p 3 / 2 and 2p 1 / 2 The binding energy increases, corresponding to 711.5 eV and 724.8 eV for Fe2p; the peak at a binding energy of 191.6 eV corresponds to B1s.

[0090] ICP testing of cerium oxide-coated Na in embodiments of the present invention 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.955 (BO3) 0.03 The Ce element in the cathode material, when converted to CeO2, has a mass fraction of 4.06 wt%; through sulfur and carbon analysis, the mass fraction of carbon composites in the cathode material is 3.0 wt%.

[0091] Upon testing, the cerium oxide coated Na in the embodiments of the present invention was found to be... 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.955 (BO3) 0.03 The cathode material was subjected to powder resistance testing at low temperature, and its initial powder conductivity was 2.31 S / m, indicating that the electronic conductivity of the material was improved after the surface was synergistically modified by metal oxides and carbon layers.

[0092] Testing revealed that the cerium oxide-coated Na in this embodiment of the invention passed the GITT test. 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.955 (BO3) 0.03 During the charging and discharging process, Na... + The migration rate is 0.08–4.4 × 10⁻⁶. -9 .

[0093] Cerium oxide coated Na 2.456 K 0.04 Ti0.026 Fe 1.7 (SO4) 2.955 (BO3) 0.03 Example 3 of the preparation method of positive electrode material The difference between this embodiment and Method Embodiment 1 is only that in step (1), 1 mL of dilute sulfuric acid (concentration 2 mol / L, 0.002 mol) is replaced with 0.55 mL of dilute sulfuric acid (concentration 2 mol / L, 0.0011 mol), and 0.0379 g (0.000613 mol) of boric acid is replaced with 0.0742 g (0.0012 mol) of boric acid. After carbonization of ascorbic acid, it is equivalent to 1.081 wt% of the theoretical mass of the cathode material, and carbon nanotubes are equivalent to 1.930 wt% of the theoretical mass of the cathode material. Finally, the precursor Na is obtained. 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.955 (BO3) 0.03 ·xH2O (x=2,4); In step (3), cerium oxide coated with Na is finally obtained. 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.955 (BO3) 0.03 Positive electrode material. Same method as Example 1.

[0094] Cerium oxide coated Na 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.955 (BO3) 0.03 Application Example 3 of Cathode Material The cerium oxide coated Na described in Example 3 of this invention 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.955 (BO3) 0.03 Positive electrode sheets made of positive electrode material are used to assemble sodium-ion batteries.

[0095] Battery assembly: Same as in Application Example 1.

[0096] Battery performance test: Same as in application example 1, the test results are shown in Table 1.

[0097] Tin oxide coated Na 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.977 (BO3)0.046 / 3 Example 4 of positive electrode material The tin oxide coated Na 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 The cathode material is a carbon composite material with thermistor-sensitive nano-tin oxide coating. + Ti 4+ BO3 3- It is made of doped sodium ferric sulfate bulk material; wherein, K + Ti is doped at the Na sites of sodium ferric sulfate. 4+ Doping at the Na / Fe sites of sodium ferric sulfate, BO3 3- Planar triangular BO doping occupies some SO tetrahedral positions, and the inorganic carbon in the carbon composite is uniformly distributed within the sodium ferric sulfate bulk phase material, while the organic carbon is uniformly distributed outside the sodium ferric sulfate bulk phase material; the tin oxide coating Na 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 The cathode material has a hollow microsphere structure with an average particle size of 19 μm; the mass fraction of the thermosensitive nano-tin oxide in the cathode material is 8.1 wt%; in the cathode material, K + Ti 4+ Relative to Na + The doping amounts were 1.63 at%, 1.06 at%, and BO3, respectively. 3- Compared to SO4 2- The doping amount is 0.52 at; the mass fraction of the carbon composite in the cathode material is 2.9 wt%.

[0098] Upon testing, the tin oxide coated Na in this embodiment of the invention... 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 The cathode material has a hollow microsphere structure with an average particle size of 19 μm. The surface of the microsphere particles is covered with metal oxides, carbon nanotubes and organic carbon. In the material cross-section, EDS was used to find that Na, Fe, K, Ti and B elements are uniformly distributed in the material, proving that K, Ti and B elements have been successfully doped into the material lattice.

[0099] Upon testing, the tin oxide coated Na in this embodiment of the invention... 2.456 K 0.04 Ti 0.026 Fe 1.7(SO4) 2.977 (BO3) 0.046 / 3 The diffraction peaks of the positive electrode material all correspond to the standard card PDF#97-025-2403, proving that sodium ferric sulfate material was successfully synthesized; at the same time, there are also crystal reflection peaks corresponding to the standard card PDF#00-001-0625 of nano-metallic tin oxide, indicating that there is a small amount of metallic tin oxide in the composite material.

[0100] Upon testing, the tin oxide coated Na in this embodiment of the invention... 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 The cathode material at a wavenumber of 591 cm⁻¹ -1 988cm -1 and 1045cm -1 Strong Fe-O vibrational peaks, SO symmetric stretching vibrational peaks, and SO4 asymmetric stretching vibrational peaks were observed, indicating the successful preparation of sodium ferric sulfate material; the wavenumbers were approximately 412–525 cm⁻¹. -1 Between these peaks, weaker and additional Me-O vibrational peaks of K and Ti-O were observed, indicating the presence of trace amounts of K and Ti element doping; at a wavenumber of 648 cm⁻¹ -1 At 1158 cm⁻¹, a peak corresponding to the BOB bending vibration was found. -1 At this location, a stretching vibration peak corresponding to the BO bond was found, indicating that BO3 3- Successfully incorporated into the material.

[0101] Upon testing, the tin oxide coated Na in this embodiment of the invention... 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 The XPS full spectrum of the cathode material contains Na1s, Fe2p, S2p, Ti2p, K2s, O1s, C1s, and B1s elemental orbitals. Among these, the high-valence metallic element Ti, with its strong electronegativity, is particularly prominent. 4+ Doping into the crystal lattice shifts the distribution of Fe electron clouds in the Ti-O-Fe structure, intensifying the localization of d-orbital electrons and leading to Fe... 2+ 2p 3 / 2 and 2p 1 / 2 The binding energy increases, corresponding to 711.5 eV and 724.8 eV for Fe2p; the peak at a binding energy of 191.7 eV corresponds to B1s.

[0102] ICP testing of tin oxide coated Na in this embodiment of the invention 2.456 K0.04 Ti 0.026 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 The Sn element in the cathode material, when converted to SnO2, has a mass fraction of 8.1 wt%; through sulfur and carbon analysis, the mass fraction of carbon composites in the cathode material is 2.9 wt%.

[0103] like Figure 6 As shown, the tin oxide coated with Na in this embodiment of the invention... 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 The cathode material was subjected to powder resistance testing at low temperature, and its initial powder conductivity was 4.38 S / m, indicating that the electronic conductivity of the material was improved after the surface was synergistically modified by metal oxides and carbon layers.

[0104] Testing revealed that the tin oxide-coated Na in this embodiment of the invention passed the GITT test. 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 During the charging and discharging process, Na... + The migration rate is 0.1–4.4 × 10⁻⁶. -9 .

[0105] Tin oxide coated Na 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 Example 4 of the preparation method of positive electrode material The difference between this embodiment and Method Embodiment 1 is only that: in step (1), 0.51g of ascorbic acid is replaced with 0.523g of ascorbic acid (equivalent to 1.058wt% of the theoretical mass of the cathode material after carbonization); in step (2), 0.758g of nano-cerium oxide is replaced with 1.606g of nano-tin oxide (equivalent to 8.1wt% of the theoretical mass of the cathode material), and carbon nanotubes are equivalent to 1.842wt% of the theoretical mass of the cathode material, finally obtaining tin oxide-coated anion and cation doped sodium iron sulfate precursor material; in step (3), the final product is tin oxide-coated Na 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3Positive electrode material. Same method as Example 1.

[0106] Tin oxide coated Na 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 Application Example 4 of Cathode Material The tin oxide coated with Na described in Example 4 of this invention 2.456 K 0.04 Ti 0.026 Fe 1.7 (SO4) 2.977 (BO3) 0.046 / 3 Positive electrode sheets made of positive electrode material are used to assemble sodium-ion batteries.

[0107] Battery assembly: Same as in Application Example 1.

[0108] Battery performance test: Same as in application example 1, the test results are shown in Table 1.

[0109] Comparative Example 1 (1) 7.387 g (0.052 mol) of anhydrous sodium sulfate, 18.91 g (0.068 mol) of ferrous sulfate heptahydrate and 0.46 g of ascorbic acid were added to 50 mL of deionized water. After stirring and dissolving for 20 min at 25 °C and 400 rpm, 7.3 g of aqueous carbon nanotube conductive slurry was added. After stirring and mixing for 20 min at 25 °C and 800 rpm, the resulting feed solution was continuously stirred at 800 rpm. Spray drying was carried out at a blower frequency of 30 Hz, an inlet air temperature of 220 °C and an outlet air temperature of 120 °C to obtain the carbon composite sodium ferric sulfate precursor material Na. 2.6 Fe 1.7 (SO4)3·nH2O / Vc / C (n=2,4); (2) The carbon composite sodium ferric sulfate precursor material obtained in step (1) is placed in a tube furnace. First, it is purged with high-purity nitrogen atmosphere at room temperature for 2 hours to ensure that the tube furnace is filled with high-purity nitrogen atmosphere and other gases are removed. Then, under high-purity nitrogen atmosphere, the temperature is increased from room temperature to 380℃ at a rate of 5℃ / min and sintered for 10 hours to obtain hollow microsphere structure carbon composite sodium ferric sulfate cathode material Na. 2.6 Fe 1.7 (SO4)3.

[0110] like Figure 12 As shown, the average particle size of the hollow microsphere structured carbon composite sodium iron sulfate cathode material obtained in the comparative example of this invention is 15 μm, and its morphology has an obvious hollow microsphere structure, which to some extent proves that the precursor production and preparation used spray drying technology.

[0111] Upon examination, EDS analysis of the material profile revealed that the Na, Fe, S, O, and C elements were uniformly distributed in the hollow microsphere structure carbon composite sodium ferric sulfate cathode material obtained in the comparative example of this invention.

[0112] like Figure 3 As shown, the XRD crystal plane diffraction peaks of the hollow microsphere structure carbon composite sodium iron sulfate cathode material obtained in the comparative example of this invention correspond to the standard card PDF#97-025-2403. Among them, the reflection peaks of multiple crystal planes at 15.52°, 22.74°, 28.71° and 32.12° correspond to (200), (130), (-112) and (240) respectively, proving that sodium iron sulfate material was successfully synthesized and has good crystallinity.

[0113] like Figure 4 As shown, the hollow microsphere structured carbon composite sodium ferric sulfate cathode material obtained in the comparative example of this invention is only used at a wavenumber of 591 cm⁻¹. -1 988cm -1 and 1045cm -1 The presence of strong Fe-O vibration peaks, SO symmetric stretching vibration peaks, and SO4 asymmetric stretching vibration peaks indicates that a pure-phase sodium ferric sulfate material was successfully prepared without the presence of other elemental doping.

[0114] like Figure 13 As shown, the XPS full spectrum of the hollow microsphere carbon composite sodium iron sulfate cathode material obtained in the comparative example of this invention contains Na1s, Fe2p, S2p, O1s, and C1s elemental orbitals, but no high-valence metal ions or B1s orbitals; peak fitting revealed that Fe... 2+ 2p 3 / 2 and 2p 1 / 2 The binding energies are 711.2 eV and 724.6 eV, respectively.

[0115] Through sulfur and carbon analysis, the mass fraction of carbon composite in the hollow microsphere structure carbon composite sodium iron sulfate cathode material obtained in the comparative example of this invention is 3.011 wt%.

[0116] like Figure 6 As shown, the hollow microsphere structure carbon composite sodium iron sulfate cathode material obtained by the present invention was subjected to powder resistance testing at low temperature. Its initial powder conductivity was only 0.262 S / m, indicating that the electronic conductivity of the material is very low when the material surface is not modified by metal oxides or carbon layers.

[0117] like Figure 7 As shown, the hollow microsphere structured carbon composite sodium ferric sulfate cathode material obtained in this invention exhibits Na… during charge and discharge processes. +Its migration rate is only 0.01 to 2.5 × 10⁻⁶. -11 The migration rate is far slower than that of particles in the oxide-coated ion-doped sodium iron sulfate cathode material of Example 1 of this invention.

[0118] The positive electrode sheet made from the hollow microsphere structure carbon composite sodium iron sulfate positive electrode material obtained in the comparative example of this invention is used to assemble sodium-ion batteries.

[0119] Battery assembly: Same as in Application Example 1.

[0120] Battery performance test: Same as application example 1.

[0121] like Figure 8 As shown, the sodium-ion battery assembled from the positive electrode sheet made of the hollow microsphere structure carbon composite sodium iron sulfate positive electrode material obtained in the comparative example of the present invention has a first discharge specific capacity of only 70.45 mAh / g and a first charge-discharge coulombic efficiency of 89.07% under low temperature conditions of 0.1C.

[0122] like Figure 9 As shown, the sodium-ion battery assembled from the positive electrode sheet made of the hollow microsphere structure carbon composite sodium iron sulfate positive electrode material obtained in the comparative example of the present invention has a discharge specific capacity of only 63.95 mAh / g and 62.21 mAh / g at low temperature conditions of 10C and 20C, respectively.

[0123] like Figure 10 As shown, the sodium-ion battery assembled from the positive electrode sheet made of the hollow microsphere structure carbon composite sodium iron sulfate positive electrode material obtained in the comparative example of the present invention has a discharge specific capacity of only 68.82 mAh / g at a 1C rate under low temperature conditions (starting from the 4th cycle) and a discharge specific capacity of only 65.05 mAh / g after 100 cycles, with a capacity retention rate of 94.52%.

[0124] Comparative Example 2 (1) Replace 0.46g ascorbic acid with 0.52g ascorbic acid (equivalent to 1.094wt% of the theoretical mass of the cathode material after carbonization), and carbon nanotubes are equivalent to 1.915wt% of the theoretical mass of the cathode material; the rest is the same as step (1) of Comparative Example 1. (2) The carbon composite sodium iron sulfate precursor material obtained in step (1) is mixed with 0.763g of thermosensitive nano-metal cerium oxide (equivalent to 4wt% of the theoretical mass of the cathode material) and added to 400mL of ethanol. The mixture is stirred in a water bath at 80℃ for 3h until it evaporates to dryness. The solvent is evaporated and coated. The solid after evaporation is placed in a vacuum drying oven at 80℃ and a vacuum degree of -0.08MPa and vacuum dried for 4h to obtain cerium oxide coated carbon composite sodium iron sulfate precursor material. (3) Following step (2) of Comparative Example 1, a hollow microsphere structure cerium oxide-coated carbon composite sodium ferric sulfate cathode material Na was obtained.2.6 Fe 1.7 (SO4)3.

[0125] like Figure 14 , 15 As shown, the hollow microsphere structure cerium oxide-coated carbon composite sodium iron sulfate cathode material obtained in the comparative example of the present invention has an average particle size of 25 μm and its morphology has an obvious hollow microsphere structure. In the material cross-section image, EDS was used to find that Na, Fe, C and Ce elements are uniformly distributed in the material, and no other elements were detected.

[0126] Testing revealed that the crystal plane diffraction peaks of the hollow microsphere structure cerium oxide-coated carbon composite sodium ferric sulfate cathode material obtained in the comparative example of this invention all corresponded to the standard card PDF#97-025-2403, proving the successful synthesis of sodium ferric sulfate material; at the same time, the cerium oxide PDF#01-073-7747 crystal plane reflection peak was also present.

[0127] The Ce element in the hollow microsphere structure cerium oxide-coated carbon composite sodium iron sulfate cathode material obtained in the comparative example of this invention was tested by ICP, and the mass fraction of CeO2 was 4.03 wt%. Through sulfur and carbon analysis, the mass fraction of carbon composite in the cathode material was 3.0 wt%.

[0128] Testing revealed that the hollow microsphere structure cerium oxide-coated carbon composite sodium iron sulfate cathode material obtained in the comparative example of this invention exhibited an initial powder conductivity of 2.05 S / m under low-temperature conditions. Although the electronic conductivity of the material was improved after surface modification with metal oxides, it was far less than the improvement effect on electronic conductivity achieved by the oxide-coated ion-doped sodium iron sulfate cathode material of Example 1 of this invention through the synergistic modification of metal oxides and carbon layers.

[0129] Testing revealed that the hollow microsphere structure cerium oxide-coated carbon composite sodium ferric sulfate cathode material obtained in this invention exhibited Na... + The migration rate is only 0.05–3.8 × 10⁻⁶. -11 The migration rate is far slower than that of particles in the oxide-coated ion-doped sodium iron sulfate cathode material of Example 1 of this invention.

[0130] The positive electrode sheet made from the hollow microsphere structure cerium oxide-coated carbon composite sodium iron sulfate positive electrode material obtained in the comparative example of this invention was used to assemble a sodium-ion battery.

[0131] Battery assembly: Same as in Application Example 1.

[0132] Battery performance test: Same as in application example 1, the test results are shown in Table 1.

[0133] Comparative Example 3 The only difference between this comparative example and Comparative Example 1 is that in step (1), 1.312 g (0.008 mol) of sodium phosphate, 18.91 g (0.068 mol) of ferrous sulfate heptahydrate, 5.68 g (0.04 mol) of anhydrous sodium sulfate and 0.427 g of ascorbic acid (equivalent to 0.959 wt% of the theoretical mass of the cathode material after carbonization) were added to 50 mL of deionized water. After stirring and dissolving for 20 min at 25 °C and 400 rpm, 7.3 g of aqueous carbon nanotube conductive slurry (carbon nanotubes equivalent to 2.043 wt% of the theoretical mass of the cathode material) was added and spray-dried to finally obtain the composite sodium ferric sulfate cathode material precursor Na. 2.6 Fe 1.7 (SO4) 2.7 (PO4) 0.2 ·xH2O / Vc / C(x=2,4); (2) Finally, hollow microsphere structure composite anion in-situ doped sodium ferric sulfate cathode material Na was obtained 2.6 Fe 1.7 (SO4) 2.7 (PO4) 0.2 / C. Comparative Example 1.

[0134] Testing revealed that the hollow microsphere composite anion-doped sodium iron sulfate cathode material obtained in the comparative example of this invention has an average particle size of 22 μm and exhibits a distinct hollow microsphere structure. In the material cross-section image, EDS analysis revealed that Na, Fe, S, and P elements are uniformly distributed in the material, and no other elements were detected.

[0135] Testing revealed that the crystal plane diffraction peaks of the hollow microsphere composite anion-doped sodium ferric sulfate cathode material obtained in the comparative example of this invention all corresponded to the standard card PDF#97-025-2403, proving that sodium ferric sulfate material was successfully synthesized, and no other crystal plane reflection peaks were found in the XRD diffraction pattern.

[0136] Testing revealed that the hollow microsphere composite anion-doped sodium ferric sulfate cathode material obtained in the comparative example of this invention exhibited a chromium content of 984.3 cm⁻¹. -1 There exists a PO4 at that location. 3- Infrared vibrational peaks caused by group vibrations.

[0137] Through sulfur and carbon analysis, the mass fraction of carbon composite in the hollow microsphere structure composite anion in situ doped sodium iron sulfate cathode material obtained in the comparative example of this invention is 3.0 wt%.

[0138] Testing revealed that the hollow microsphere composite anion-doped sodium ferric sulfate cathode material obtained in the comparative example of this invention exhibited an initial powder conductivity of 0.534 S / m under low-temperature conditions, indicating that the electronic conductivity of the material is very low when the material surface is not modified with metal oxides or carbon layers.

[0139] Testing revealed that the hollow microsphere composite anion-doped sodium ferric sulfate cathode material obtained in this comparative invention exhibited Na... + Its migration rate is only 0.02–2.6 × 10⁻⁶. -11 The migration rate of PO4 is far slower than that of the particles in the oxide-coated ion-doped sodium ferric sulfate cathode material of Example 1 of this invention, indicating that PO4... 3- While doping can reduce stress changes within a material and stabilize the crystal structure to some extent, it cannot increase ion migration channels and thus limits internal ion migration.

[0140] The positive electrode sheet made from the hollow microsphere structure composite anion-doped sodium ferric sulfate positive electrode material obtained in the comparative example of this invention was used to assemble a sodium-ion battery.

[0141] Battery assembly: Same as in Application Example 1.

[0142] Battery performance test: Same as in application example 1, the test results are shown in Table 1.

[0143] Comparative Example 4 The only difference between this comparative example and Comparative Example 1 is that in step (1), 7.387 g (0.052 mol) of anhydrous sodium sulfate, 16.69 g (0.06 mol) of ferrous sulfate heptahydrate, 2.248 g (0.008 mol) of nickel sulfate heptahydrate and 0.427 g of ascorbic acid (equivalent to 1.008 wt% of the theoretical mass of the cathode material after carbonization) were added to 50 mL of deionized water. After stirring and dissolving for 20 min at 25 °C and 400 rpm, 7.3 g of aqueous carbon nanotube conductive slurry (carbon nanotubes equivalent to 1.995 wt% of the theoretical mass of the cathode material) was added and spray-dried to finally obtain the hollow microsphere inorganic carbon composite sodium ferric sulfate cathode material precursor Na. 2.6 Ni 0.2 Fe 1.5 (SO4)3·xH2O / Vc / C (x=2,4); In step (2), the hollow microsphere structure composite cation in-situ co-doped sodium ferric sulfate cathode material Na is finally obtained. 2.6 Ni 0.2 Fe 1.5 (SO4)3 / C. Same as Comparative Example 1.

[0144] Upon testing, the average particle size of the microsphere-structured composite cation in-situ co-doped sodium iron sulfate cathode material obtained in the comparative example of this invention is 21 μm, and its morphology exhibits a distinct hollow microsphere structure. In the material cross-section image, EDS analysis revealed that Na, Fe, S, and Ni elements are uniformly distributed in the material, and no other elements were detected.

[0145] Testing revealed that the crystal plane diffraction peaks of the hollow microsphere composite cation in-situ co-doped sodium iron sulfate cathode material obtained in the comparative example of this invention all corresponded to the standard card PDF#97-025-2403, proving the successful synthesis of sodium iron sulfate material. Simultaneously, some crystal plane reflection peaks also corresponded to the standard card PDF#01-072-1195 for nickel sulfate, indicating that the material contains Ni doping elements.

[0146] Through sulfur and carbon analysis, the mass fraction of carbon composite in the hollow microsphere structure composite cation in situ co-doped sodium iron sulfate cathode material obtained in the comparative example of this invention is 3.0 wt%.

[0147] Testing revealed that the hollow microsphere composite cation in-situ co-doped sodium iron sulfate cathode material obtained in the comparative example of this invention exhibited an initial powder conductivity of 0.572 S / m at a low temperature of -20°C. This indicates that the electronic conductivity of the material is very low when the material surface is not modified with metal oxides or carbon layers.

[0148] Testing revealed that the hollow microsphere composite cation-co-doped sodium ferric sulfate cathode material obtained in this comparative invention exhibited Na... + Its migration rate is only 0.04–2.55 × 10⁻⁶. -11 The migration rate of particles in the oxide-coated ion-doped sodium iron sulfate cathode material of Example 1 of this invention is far slower than that in the present invention, indicating that Ni... 2+ Although doping alters the electron cloud density between Me and O, increases electron delocalization, and improves the intrinsic conductivity of the material, it limits ion migration because it cannot change the internal migration channels of the material.

[0149] The positive electrode sheet made from the hollow microsphere structure composite cation in-situ co-doped sodium iron sulfate positive electrode material obtained in the comparative example of this invention was used to assemble a sodium-ion battery.

[0150] Battery assembly: Same as in Application Example 1.

[0151] Battery performance test: Same as in application example 1, the test results are shown in Table 1.

[0152] Battery assembly: Same as in Application Example 1.

[0153] Table 1. Comparison of electrical and cycle performance test results of batteries assembled from positive electrode sheets made of positive electrode materials from Examples 1-4 and Comparative Examples 1-4 at a low temperature of -20℃.

[0154] Note: In the table, the conductivity was measured using a four-probe powder resistance meter at a low temperature (-20℃).

[0155] As shown in Table 1, the cathode materials in Examples 1-4 of this invention have a specific temperature-sensitive metal oxide coating. Under low-temperature conditions, according to the Arrhenius equation, the electron conduction of the electrode material requires more reaction activation energy. E The synergistic coating of metal oxides and organic carbon sources, along with the compositing of inorganic carbon sources, provides a better conductive network for electrochemical reactions. Through the synergistic effect of in-situ co-doping of low-valent cations and high-valent metal ions, the directional occupation of Na and Na / Fe sites in the material lattice is regulated, broadening the sodium ion migration channels and inhibiting Na+ ion migration. + During extraction and insertion, the material undergoes irreversible phase transitions and stabilizes its crystal structure; the BO planar triangular structure occupies some SO tetrahedral positions, altering a portion of the Fe2O in the bulk phase. 10 The dimer arrangement creates lattice distortion, increasing the amount of Na in the bulk structure. + Migration channels; Testing revealed that batteries assembled from the positive electrode sheets made from the obtained material exhibited a reversible discharge specific capacity of up to 87.51 mAh / g and an initial coulombic efficiency of up to 90.86% at a low-temperature environment with a 1C rate. At a low-temperature environment with a 10C rate, the capacity retention rate after 100 cycles reached 93.86%, and at a low-temperature environment with a 10C rate, the discharge specific capacity reached 77.41 mAh / g. Furthermore, after 250 cycles at a low-temperature environment with a 10C rate, the discharge specific capacity retention rate still reached 80%, while at a high 20C rate, the discharge specific capacity reached 73.57 mAh / g. In summary, this invention, through macroscopic morphological structure modification design combined with microscopic angle cation and anion co-doping to achieve atomic lattice site control, comprehensively improves the cycling performance and rate performance of the material at low temperatures, successfully producing sodium iron sulfate material for sodium-ion batteries with excellent low-temperature performance.

[0156] As shown in Table 1, in Comparative Example 1, no metal cations and BO3 were added. 3- The sodium ferric sulfate material, obtained by doping without coating with a positive temperature-sensitive oxide, has a hollow microsphere structure. Firstly, in Comparative Example 1, low-valence metal ions K were not introduced. + Occupy Na sites and broaden Na + Migration channels that are extracted and inserted during charging and discharging; secondly, in Comparative Example 1, no high-valence metal ions such as Al were introduced. 3+ Ti 4+It occupies some of the Na1 sites shared by Na / Fe, stabilizing Na + The structural stability during the process makes it difficult for the material to maintain high reversibility and small volume changes during charge-discharge cycles. Finally, only inorganic conductive carbon and ascorbic acid-carbonized organic carbon are added as conductive agents to reduce the polarization degree of the material during charge-discharge. Due to the lack of the above conditions, especially at low temperatures, the thermodynamic and kinetic activity of the electrode material will decrease significantly compared to that at room temperature. In summary, Comparative Example 1 is inferior to Example 1 of the present invention in terms of specific capacity, rate capability, and cycle life at low temperatures.

[0157] As shown in Table 1, in Comparative Example 2, compared to Example 1 which simply used a temperature-dependent thermistor oxide cerium oxide for surface coating, the conductivity of the composite material was significantly improved by four-probe powder resistance testing at a low temperature of -20°C. This resulted in a significant increase in the charge-discharge specific capacity of the material at low temperatures compared to Comparative Example 1. However, since the material was not ion-doped, the decrease in temperature would worsen the diffusion rate of intrinsic ions in the bulk phase, increase the migration barrier, and thus limit the electrochemical performance.

[0158] As shown in Table 1, in Comparative Example 3, a trace amount of PO4 was used. 3- Doping to replace BO3 3- Theoretically, PO4 3- With SO4 2- In sodium ferric sulfate, the structure is three-dimensional tetrahedral, and the PO bonds have relatively long bond lengths, which increases the distance between Fe-O dimers, reduces repulsion, and stabilizes the material structure. However, after borate ions enter the crystal lattice, they are arranged in a two-dimensional planar triangular pattern, causing lattice distortion in local areas and changes in local charge density, increasing and opening up Na+. + The migration channels allow the material to fully realize its rate performance at low temperatures, while phosphate only stabilizes the crystal structure by reducing the coulombic efficiency between ferric iron atoms, without effectively improving ion channels. Therefore, doping with borate yields better high-rate performance than doping with phosphate.

[0159] As shown in Table 1, in Comparative Example 4, when the transition metal Ni is doped into the Fe site, due to SO4... 2- The strong inductive effect caused the redox potential of Ni to exceed the test voltage window of 4.5V, while Fe... 3+ / Fe 2+ The reduced number of redox pairs results in a significant decrease in discharge specific capacity at low temperatures compared to the material in Example 1 of this invention.

Claims

1. An oxide-coated ion-doped sodium ferric sulfate cathode material, characterized in that: The cathode material is a bulk iron sulfate material composed of carbon composites doped with various valence metal cations and anions, coated with a positive temperature coefficient thermistor metal oxide. The various valence metal cations are doped at the Na or Na / Fe ion sites of the iron sulfate, and the anions occupy part of the three-dimensional SO tetrahedral positions. The inorganic carbon in the carbon composite is uniformly distributed within the bulk iron sulfate material, and the organic carbon is uniformly distributed outside the bulk iron sulfate material.

2. The oxide-coated ion-doped sodium ferric sulfate cathode material according to claim 1, characterized in that: The oxide-coated ion-doped sodium ferric sulfate cathode material has a hollow microsphere structure with an average particle size of 5–35 μm; the mass fraction of the positive temperature coefficient (PTC) thermistor oxide in the cathode material is 0.1–8.5 wt%; the PTC thermistor oxide includes one or more of cerium oxide, zinc oxide, tin oxide, or indium oxide; the multiple valence metal cations include two or more of monovalent or divalent and higher valence metal cations; the monovalent metal cation includes K ions; the divalent and higher valence metal cations include one or more of Ca, Al, or Ti ions; the anion includes borate ions; the chemical formula of the sodium ferric sulfate bulk material doped with multiple valence metal cations and anions is Na. 2.6-x-ny M x N y Fe 1.7 (SO4) 3-z (BO3) 2z / 3 Wherein, 0 < x ≤ 0.15, 0 < y ≤ 0.08, 0 < z ≤ 0.25, M represents a monovalent metal cation, N represents a divalent or higher metal cation, n represents the valence state of N, n = 2, 3, 4, and satisfies the law of charge conservation; the doping amounts of the M and N metal cations relative to Na ions are 0.01–7.0 at% and 0.01–3.7 at%, respectively; the doping amount of the borate ion relative to the sulfate ion is 0.01–6.0 at%; and the mass fraction of the carbon composite in the cathode material is 0.3–4.0 wt%.

3. A method for preparing the oxide-coated ion-doped sodium iron sulfate cathode material as described in claim 1 or 2, characterized in that, Includes the following steps: (1) Sodium source, ferrous source, sulfate source, multivalent metal cation dopant, borate anion dopant and organic acid antioxidant are added to water, stirred and dissolved, and then inorganic conductive carbon source slurry is added. After stirring and mixing, spray drying is carried out to obtain anion and cation doped sodium ferric sulfate precursor material. (2) The anion and cation doped sodium ferric sulfate precursor material obtained in step (1) is mixed with a positive temperature coefficient thermosensitive metal oxide coating agent and added to a low boiling point organic solvent. The solvent is evaporated and coated, and then vacuum dried to obtain metal oxide coated anion and cation doped sodium ferric sulfate precursor material. (3) The metal oxide obtained in step (2) is coated with the anion and cation doped sodium iron sulfate precursor material and sintered under a protective atmosphere to obtain oxide-coated ion doped sodium iron sulfate cathode material.

4. The method for preparing the oxide-coated ion-doped sodium ferric sulfate cathode material according to claim 3, characterized in that: In step (1), the molar ratios of sodium, ferrous, sulfate, metal cation, and borate elements in the sodium source, ferrous source, sulfate source, metal cation dopant, and borate anion dopant match the molar ratios of the corresponding substances in the chemical formula; the amount of organic acid antioxidant is such that its carbonization mass accounts for 0.1–1.5 wt% of the mass fraction of the oxide-coated ion-doped sodium ferric sulfate cathode material; the amount of water is such that the concentration of dissolved ferrous ions is 1.0–2.0 mol / L; the stirring and dissolving temperature is 25–30°C, the stirring speed is 300–500 rpm, and the time is 15–25 min.

5. The method for preparing the oxide-coated ion-doped sodium ferric sulfate cathode material according to claim 3 or 4, characterized in that: In step (1), the solid content of the inorganic conductive carbon source slurry is 3-10%; the amount of the inorganic conductive carbon source slurry is such that the mass fraction of the inorganic conductive carbon source in the theoretical mass of the oxide-coated ion-doped sodium iron sulfate cathode material is 0.1-2.5 wt%; the stirring temperature is 25-30℃, the rotation speed is 700-900 rpm, and the time is 15-25 min; while the spray drying feed is being prepared, the feed solution is continuously stirred at 700-900 rpm; the process parameters of the spray drying are: the frequency of the induced draft fan is 20-40 Hz, the inlet air temperature is 200-300℃, and the outlet air temperature is 100-140℃.

6. The method for preparing the oxide-coated ion-doped sodium ferric sulfate cathode material according to any one of claims 3 to 5, characterized in that: In step (1), the sodium source includes sodium sulfate, sodium nitrate, sodium acetate, or sodium formate, and one or more of their hydrates; the ferrous source includes ferrous sulfate and / or ferrous nitrate, and one or more of their hydrates; the sulfuric acid source includes sulfuric acid, and one or more of ammonium sulfate or sodium sulfate and their hydrates; the multivalent metal cation dopant includes sulfate, nitrate, acetate, chloride, or soluble organometallic salt of multivalent metal dopant elements, and one or more of their hydrates or solutions; the borate anion dopant includes boric acid, and one or more of borate anion dopant and their hydrates; the organic acid antioxidant includes one or more of ascorbic acid, citric acid, or tannic acid; and the inorganic conductive carbon source includes one or more of carbon quantum dots, carbon nanotubes, Ketjen black, Super P, acetylene black, or graphene.

7. The method for preparing the oxide-coated ion-doped sodium ferric sulfate cathode material according to any one of claims 3 to 6, characterized in that: In step (2), the amount of the positive temperature coefficient (PTC) thermistor oxide coating agent is such that its mass fraction in the theoretical mass of the oxide-coated ion-doped sodium iron sulfate cathode material is 0.1–8.5 wt%; the solid-liquid ratio of the PTC thermistor oxide coating agent to the low-boiling-point organic solvent is 1:400–750; the solvent evaporation coating refers to stirring at 60–90°C for 1–5 h until it is evaporated to dryness; the vacuum drying temperature is 70–90°C, the vacuum degree is -0.05–-0.10 MPa, and the time is 3–6 h; the low-boiling-point organic solvent includes one or more of ethanol, propanol, tetrahydrofuran, or ethylenediamine.

8. The method for preparing the oxide-coated ion-doped sodium ferric sulfate cathode material according to any one of claims 3 to 7, characterized in that: In step (3), before sintering, the system is ventilated with a protective atmosphere at room temperature for 1.5 to 2.5 hours to ensure that the sintering system is filled with a protective atmosphere and to remove other gases. The sintering refers to heating from room temperature to 350 to 400°C at a rate of 1 to 5°C / min and sintering for 6 to 12 hours. The protective atmosphere includes nitrogen, argon, and one or more of hydrogen-argon or hydrogen-nitrogen mixtures.

9. An application of the oxide-coated ion-doped sodium ferric sulfate cathode material as described in claim 1 or 2, characterized in that: The positive electrode sheet made of the oxide-coated ion-doped sodium iron sulfate positive electrode material as described in claim 1 or 2 is used to assemble a sodium-ion battery.

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