Sodium-ion positive electrode material, preparation method, positive electrode and battery

Abstract

The application discloses a sodium ion positive electrode material, a preparation method, a positive electrode and a battery, and the general formula of the positive electrode material is Na x MeO2, wherein Me is one or more of Fe, Ni, Mn, Co, Cu, Ti, Mg and Al, and x is 1.02-1.20; the positive electrode material is prepared by using a metal etching method and by using metal powder and a sodium ion saturated solution. The preparation method can control the sodium embedding amount by adjusting a sodium ion saturation coefficient. The obtained sodium ion positive electrode material precursor product is a single-layer primary particle loose stack, and the crystal phase growth direction of the primary particle can be easily controlled by adjusting control parameters. The structure of the obtained precursor product has many cavities, and a conductive material can be embedded into the cavity structure of the precursor product after sintering, so that the conductivity of the obtained positive electrode material is greatly increased.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery materials technology, specifically to a sodium-ion cathode material, its preparation method, the cathode, and the battery. Background Technology

[0002] In the existing technology, the synthesis methods of sodium-ion battery cathode materials include solid-state method, co-precipitation method, and hydrothermal method.

[0003] CN114388757A discloses a method for preparing Prussian white material, which involves reacting an aqueous solution of inorganic sodium salt, a Na4Fe(CN)6 solution, and an aqueous solution of inorganic metal salt under an anaerobic or oxygen-free atmosphere at 0.2-10 MPa and 50-100℃.

[0004] CN114644361A discloses a method for preparing a multilayer sodium-ion battery cathode material. A mixed salt solution of nickel-rich manganese, nickel, and cobalt is fed into a reaction vessel in parallel with a precipitant solution and a complexing agent solution for co-precipitation reaction. When the particle size D50 reaches 9-13 μm, the reaction is stopped. The obtained precursor is then mixed with sodium salt and sintered to obtain the desired product.

[0005] CN113745507A discloses a method for preparing sodium vanadium oxychloride phosphate cathode material. A vanadium source, a phosphorus source, and a reducing agent are added to water and placed in a reaction vessel. The mixture is heated and stirred at 70℃-80℃. Then, a sodium source, a fluorine source, and a chlorine source are added to the reaction vessel. The reaction is carried out for 30-60 minutes to obtain a homogeneous and transparent solution. The pH is adjusted to 6-10. The obtained product is centrifuged, washed with water, dried, and annealed at 200-600℃ for 1-6 hours to obtain the sodium vanadium oxychloride phosphate cathode material.

[0006] However, existing preparation methods have problems such as high free sodium ion content, low purity, large deviation of sodium ion content in the cathode material lattice, and difficulty in treating the mother liquor. Summary of the Invention

[0007] To address the problems existing in the prior art, this invention provides a sodium-ion cathode material, a preparation method, a cathode, and a battery. The sodium-ion cathode material is prepared by a metal etching method, which is simple, controllable, and has low preparation cost. The resulting sodium-ion cathode material has a high sodium ion content in its crystal lattice, a low free sodium ion content, and high purity. When the cathode material is applied to a sodium-ion battery, the battery exhibits good electrochemical performance.

[0008] To achieve the above objectives, the present invention adopts the following technical solution.

[0009] A sodium-ion cathode material with the general formula Na xMeO2, wherein Me is one or more of the metals Fe, Ni, Mn, Co, Cu, Ti, Mg, and Al, and x is 1.02 to 1.20 (e.g., 1.05, 1.10, 1.15, 1.18).

[0010] Preferably, in the above-mentioned sodium-ion cathode material, the primary particles of the sodium-ion cathode material have a lamellar layer structure, and the secondary particles of the sodium-ion cathode material are spherical structures composed of primary particles, with an interlayer spacing (c) of [missing information]. (For example, The sheet-like layered structure here is formed by the stacking of atoms in the cathode material.

[0011] The interlayer spacing c of the positive electrode material in a conventional sodium-ion battery is approximately Therefore, the interlayer spacing of the primary particles of the sodium-ion cathode material according to the present invention is larger than that of conventional sodium-ion batteries.

[0012] The present invention also provides a method for preparing the above-mentioned sodium ion cathode material, which adopts the following technical solution.

[0013] A method for preparing a sodium-ion cathode material, employing a metal etching method, involves reacting metal micropowder with a saturated sodium-ion solution, and includes the following steps:

[0014] (1) Add water and sodium-containing electrolyte to the reaction vessel and stir until homogeneous to obtain bottom solution A, i.e., sodium ion saturated solution;

[0015] (2) Add the etchant and complexing agent to the base liquid A of step (1) in a certain molar ratio in multiple portions, stir evenly, and obtain solution B;

[0016] (3) The metal micro powder is added to the solution B of step (2) in two steps to carry out the reaction. After the reaction is completed, the reaction solution is concentrated, aged, centrifuged and dried to obtain sodium ion cathode material precursor C.

[0017] (4) The sodium ion cathode material precursor C, sodium salt and additives are mixed and sintered to obtain sodium-rich cathode material.

[0018] In the above preparation method, as a preferred embodiment, in step (1), the sodium-containing electrolyte includes sodium salt, sodium oxide and hydroxide, for example, the electrolyte includes one or more of sodium sulfate, sodium nitrate, sodium carbonate, sodium phosphate, sodium oxide, sodium hydroxide and sodium acetate; and / or, the water is anhydrous water.

[0019] Preferably, the concentration of the base solution A is 2-10 mol / L (e.g., 2.5 mol / L, 3 mol / L, 4 mol / L, 5 mol / L, 6 mol / L, 7 mol / L, 8 mol / L, 9 mol / L).

[0020] In the above preparation method, as a preferred embodiment, in step (2), the etchant includes one or more of acetic acid, nitric acid, sulfuric acid, hydrochloric acid, trifluoromethanesulfonic acid, and trichloroacetic acid; the complexing agent includes one or more of NH3·H2O, Na2CO3, NaHCO3, and CH3COONa.

[0021] In the above preparation method, as a preferred embodiment, in step (2), the molar ratio between the etchant and the complexing agent is 1:0.5-1 (e.g., 1:0.6, 1:0.7, 1:0.8, 1:0.9).

[0022] In the above preparation method, as a preferred embodiment, in step (2), the pH of solution B is 2-6 (e.g., 3, 4, 5).

[0023] In this invention, the etchant is weakly acidic, and the complexing agent is weakly alkaline. Initially, the system needs to be adjusted to acidity to ensure the etching process proceeds normally, but the etching process should not be too rapid. Therefore, this invention uses a certain amount of complexing agent to control the etching reaction rate, ensuring the controllability of the etching process and the reaction cycle, thereby obtaining a favorable primary particle structure for the cathode material. When there is too much etchant, the system becomes too acidic, with a low pH; when there is too much complexing agent, the system becomes too alkaline, with a high pH, ​​reducing the degree of reaction. Both of these situations are detrimental to obtaining a favorable primary particle structure for the cathode material during the synthesis process.

[0024] In this invention, in step (2), the initial pH value of solution B is controlled by controlling the ratio of etchant and complexing agent added. pH essentially reflects the reaction environment in the early stage of the reaction.

[0025] In the above preparation method, as a preferred embodiment, in step (3), the metal powder includes one or more of the following metal powders: iron powder, nickel powder, manganese powder, cobalt powder, copper powder, titanium powder, magnesium powder, aluminum powder, etc.; preferably, the particle size of the metal powder is <20μm (e.g., 5μm, 10μm, 15μm, 18μm).

[0026] In the above preparation method, as a preferred embodiment, in step (3), during a reaction time of 0-2 hours, the metal powder, etchant, and complexing agent are added hourly at a molar ratio of (4-10):1:(0.5-1) to maintain the system pH between 5-7 (e.g., 5.5, 6, 6.5) during the reaction; when the reaction time is >2 hours, the metal powder, etchant, and complexing agent are added hourly at a molar ratio of (1-4):1:(0-0.5) to maintain the system pH between 6-9 (e.g., 6.5, 7, 7.5, 8, 8.5) during the reaction. The amount of metal powder added depends on the reaction time, and the amount added per hour is based on a fixed molar ratio.

[0027] In step (3), adding the metal powder to solution B in step (2) in two steps means adding the metal powder in two stages as described above. The first stage, within a reaction time of 0-2 hours, involves adding the metal powder, etchant, and complexing agent in a specific ratio to ensure the pH of the reaction system is 5-7. The second stage, when the reaction time is greater than 2 hours, involves adding the metal powder, etchant, and complexing agent in another ratio to ensure the pH of the reaction system is 6-9. Preferably, the amount of metal powder added in the first stage is greater than that in the second stage, and the pH of the reaction system in the first stage is lower than that in the second stage.

[0028] In the above preparation method, as a preferred embodiment, in step (3), the reaction temperature is 45-65℃ and the total reaction time is 47-120h.

[0029] In this invention, during the metal etching reaction, in the first two hours, due to excess acid in the system, a large amount of metal powder is added to promote the formation of numerous microcrystals. After reaching a certain saturation concentration, these microcrystals begin to agglomerate and grow. During this period, the pH of the reaction system is controlled between 5 and 7 to control the rate of microcrystal formation, thus facilitating the adjustment of the particle morphology of the target cathode material. After 2 hours of feeding, the grains agglomerate and grow. At this point, the pH of the system is controlled between 6 and 9 to slow down the rate of microcrystal formation, thereby controlling the reaction rate and ensuring the physicochemical properties and appearance of the synthesized particles. Furthermore, during the grain agglomeration and growth reaction, the amount of metal powder added needs to be reduced because the redox reaction between the metal powder and the etchant increases the system pH. Additionally, controlling the system pH between 6 and 9 during the grain agglomeration and growth period is beneficial for the formation of particles with a better lamellar structure.

[0030] Furthermore, this invention employs a stepwise addition of metal powder due to the kinetic differences between the pre-reaction transition period and the stable reaction period. In the pre-reaction transition period, the etching reaction is more intense, with a large number of metal ions reacting to form microcrystals, resulting in weaker kinetics for microcrystal aggregation and growth. In the stable period, the kinetics for microcrystal aggregation and growth are stronger, with the generated microcrystals primarily attached to crystal nuclei for growth. Therefore, adding an excessive amount of metal powder in the early stage of the reaction ensures the formation of sufficient crystal nuclei, while adding an appropriate amount in the later stage ensures the particle growth rate and morphology.

[0031] In addition, the present invention uses a two-step method to add metal micro powder. On the one hand, during the first precipitation reaction, some sodium ions are inserted into the crystal lattice. On the other hand, a solid intermediate is generated. The intermediate has a distinct plate-like structure, which facilitates further sodium insertion. If the metal powder is directly sintered, it is more difficult to control the uniformity and the amount of sodium ion insertion.

[0032] In the above preparation method, as a preferred embodiment, in step (3), an inert gas is introduced during the reaction process to keep the dissolved oxygen content of the slurry <5% (e.g., 1%, 2%, 3%, 4%).

[0033] Preferably, after the reaction is completed, the reaction solution is concentrated to a solid content of 60-80 wt% (e.g., 62%, 65%, 70%, 75%, 78%) and a sodium ion saturation of 1.12-2 (e.g., 1.15, 1.2, 1.3, 1.5, 1.7, 1.8, 1.9), and then aged for 24-72 h (e.g., 25 h, 30, 40, 50, 60, 70), with the pH of the system maintained at 7-10 (e.g., 7.5, 8, 8.5, 9, 9.5).

[0034] In the above preparation method, as a preferred embodiment, in step (4), the sodium salt includes one or more of the following: sodium sulfate, sodium bisulfate, sodium nitrate, sodium carbonate, sodium bicarbonate, sodium phosphate, sodium oxide, sodium hydroxide, and sodium acetate.

[0035] In the above preparation method, as a preferred embodiment, in step (4), the precursor C, sodium salt and additives are mixed and sintered at 400-1000℃ (e.g. 450℃, 500℃, 600℃, 700℃, 800℃, 900℃) for 16-24h (e.g. 18h, 19h, 20h, 21h, 22h, 23h).

[0036] In the above preparation method, as a preferred embodiment, in step (4), the additive includes one or more of the following: carbon powder, iron powder, boric acid, cobalt boric acid, zirconium oxide, aluminum oxide, magnesium oxide, rubidium oxide, cobalt hydroxide, and lithium phosphate. Preferably, based on the weight of the precursor C, the doping amount of the additive is 300-8000 ppm (e.g., 500 ppm, 1000 ppm, 2000 ppm, 3000 ppm, 4000 ppm, 5000 ppm, 6000 ppm, 7000 ppm, 7500 ppm). The doping amount of the additive is calculated based on the weight of the precursor. For example, a doping amount of 500 ppm means that the doping amount of the additive is 0.05% of the weight of the precursor.

[0037] In the above preparation method, as a preferred embodiment, in step (4), the molar ratio between the sodium ion cathode material precursor C and the sodium element (or sodium ion) in the sodium salt is 1:10-1.16 (e.g., 1:1.11, 1:1.12, 1:1.13, 1:1.14, 1:1.15).

[0038] The present invention also provides a positive electrode sheet, comprising the above-described sodium-ion battery positive electrode material or the sodium-ion battery positive electrode material prepared by the above method.

[0039] The present invention also provides a battery comprising the above-described positive electrode.

[0040] In the preparation method of this invention, the essence of metal ion generation is electrochemical principle, namely electrochemical etching. Initially, due to the acidic environment, the oxide on the metal surface reacts with the acidic substance, and then the metal reacts with the acidic substance. Due to the early reaction, a rough interface is formed on the metal surface, and the subsequent reaction rate will be accelerated. In particular, when there are conductive impurities inside the metal, a micro galvanic cell will be formed to accelerate the etching reaction, thereby generating metal ions.

[0041] 4Me+HNO3+5H2O→4Me(OH)2↓+NH3↑

[0042] 4Me+HClO4+4H2O→4Me(OH)2↓+HCl↑

[0043] Other strong oxidizing concentrated acids react in basically the same way (Me is a metal element). Strong oxidizing acids also exhibit oxidizing properties at high concentrations. The metal, oxidizing acid and water react, the metal loses electrons and its valence state increases, and it combines with hydroxide ions to form a precursor. The acid gains electrons and is reduced, and its valence state decreases.

[0044] This invention uses a metal powder and saturated electrolyte system to synthesize sodium ion cathode material, which can effectively reduce the amount of mother liquor, increase the amount of sodium intercalation in the crystal lattice, and form a sodium-rich cathode material. This material has a more obvious plate-like primary particle structure with a larger interlayer spacing, which is conducive to the intercalation and deintercalation of sodium ions. At the same time, due to the high sodium ion content, it can effectively improve the stability of the SEI film and improve the electrochemical performance.

[0045] In this invention, the above-mentioned technical features can be freely combined to form new technical solutions, provided that they do not conflict with each other.

[0046] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0047] 1. Compared with conventional precipitation reactions that produce water during the precipitation process, the synthesis technology of this invention has a high ionic potential in the saturated electrolyte solution after the metal ions are generated, which can undergo redox reactions with water and strong acids to form metal precipitates. The reaction process consumes water; this water-consuming reaction process is beneficial to the stability of sodium ion cathode materials.

[0048] 2. In the preparation method of the present invention, the sodium content is controllable. By adjusting the sodium ion saturation coefficient, the sodium embedding amount can be controlled. Under the prior art, the sodium embedding amount is limited, and the amount of sodium ions lost with the mother liquor is relatively large.

[0049] 3. The sodium ion cathode material precursor product obtained by adopting the technical solution of the present invention is a single layer of loosely stacked primary particles, which is different from the conventional spherical structure precursor. The crystal growth direction of the primary particles can be easily controlled by adjusting the control parameters. When the sodium ion cathode material is prepared by using the same sintering process, the sodium ion cathode material prepared by adopting the technical solution of the present invention has better capacity and cycle retention rate.

[0050] 4. Compared with the prior art, which only coats the surface of the precursor product with carbon to improve the conductivity of the obtained cathode material, the technical solution of the present invention has more cavities in the structure of the precursor product. If the cathode material of the present invention is modified, for example, by mixing it with conductive materials such as conductive graphite or graphene ground into fine powder and then sintering, the conductive material can be embedded in the cavity structure of the precursor product, which greatly increases the conductivity of the obtained cathode material. Attached Figure Description

[0051] Figure 1 The images shown are SEM images of the precursor materials prepared in Examples 1-7 and Comparative Examples 1-3 of this invention.

[0052] Figure 2 The images show SEM images of the sodium ion cathode materials obtained in Examples 1-7 and Comparative Examples 1-3 of this invention.

[0053] Figure 3 The charge-discharge curves of sodium-ion batteries using the sodium-ion cathode materials obtained in Examples 1-7 and Comparative Examples 1-3 of this invention are shown at 0.1C.

[0054] Figure 4 Electrochemical cycling curves at 0.1C for sodium-ion batteries using the sodium-ion cathode materials obtained in Examples 1-7 and Comparative Examples 1-3 of this invention.

[0055] Wherein, a-Example 1, b-Example 2, c-Example 3, d-Example 4, e-Example 5, f-Example 6, g-Example 7, i-Comparative Example 1, j-Comparative Example 2, k-Comparative Example 3. Detailed Implementation

[0056] The present invention will now be described in detail with reference to embodiments. Various examples are provided by way of explanation and not by way of limitation. Indeed, those skilled in the art will recognize that modifications and variations can be made to the invention without departing from its scope or spirit. For example, a feature shown or described as part of one embodiment may be used in another embodiment to produce yet another embodiment. Therefore, it is desirable that the present invention encompass such modifications and variations falling within the scope of the appended claims and their equivalents.

[0057] Unless otherwise specified, all reagents and materials mentioned in this invention are commercially available.

[0058] A method for preparing a sodium-ion cathode material, employing a metal etching method and utilizing a system of metal micropowder and a saturated sodium-ion solution, includes the following steps:

[0059] (1) Add water and electrolyte to the reaction vessel and stir evenly to obtain base solution A, i.e., sodium ion saturated solution, the concentration of the base solution A is 2-10 mol / L (e.g. 2.5 mol / L, 3 mol / L, 4 mol / L, 5 mol / L, 6 mol / L, 7 mol / L, 8 mol / L, 9 mol / L);

[0060] (2) The etchant and complexing agent are added to the base solution A of step (1) in multiple portions according to a certain molar ratio and stirred evenly to obtain solution B. At this time, the pH of solution B is 2-6 (e.g., 3, 4, 5); wherein the molar ratio between the etchant and the complexing agent is 1:0.5-1 (e.g., 1:0.6, 1:0.7, 1:0.8, 1:0.9);

[0061] (3) The metal powder is added to solution B in step (2) in two steps for reaction. After the reaction is completed, the reaction solution is concentrated, aged, centrifuged, and dried to obtain precursor C. During the reaction time of 0-2 hours, the metal powder, etchant, and complexing agent are added hourly at a molar ratio of (4-10):1:(0.5-1) to maintain the pH of the system between 5 and 7 during the reaction. During the reaction time > 2 hours, the molar ratio of metal powder, etchant, and complexing agent is 1-4:1:0-0.5 per hour (e.g., 0.1:1:0.1, 0.5:1:0.1, 1:1:0.1, 2:1:0.1, 3:1:0.1, 4:1:0.1, 1:0.5). Metal powder, etchant, and complexing agent are added in ratios of 1:0.5, 2:1:0.5, 3:1:0.5, 4:1:0.5, 1:1:0.25, 2:1:0.25, 3:1:0.25, and 4:1:0.25 to maintain the pH of the system between 6 and 9 during the reaction. After the reaction, the reaction solution is concentrated to a solid content of 60-80 wt% (e.g., 62%, 65%, 70%, 75%, 78%) and a sodium ion saturation of 1.12-2 (e.g., 1.15, 1.2, 1.3, 1.5, 1.7, 1.8, 1.9), and then aged for 24-72 hours (e.g., 25 hours, 30 hours, 40 hours, 50 hours, 60 hours, 70 hours), maintaining the pH of the system at 7-10 during the aging process.

[0062] (4) The precursor C, sodium salt and additives are mixed and sintered at 400-1000℃ for 16-24h to obtain sodium-rich cathode material; wherein the molar ratio between the precursor C and sodium element in the sodium salt is 1:10-1.16 (e.g., 1:1.11, 1:1.12, 1:1.13, 1:1.14, 1:1.15), and the doping amount of the additive is 300-8000ppm based on the weight of the precursor C.

[0063] In this invention, metal powder is added in two steps in step (3) to obtain precursor C, which facilitates the subsequent embedding of sodium ions into the crystal lattice. On the one hand, considering the limited effect of sodium embedding in a single step, a portion of the sodium ions are embedded into the crystal lattice after the first precipitation reaction; on the other hand, a solid intermediate is generated, which has a distinct plate-like structure, facilitating further sodium embedding. Compared with directly sintering metal powder for sodium embedding, the uniformity and sodium ion embedding amount of the cathode material prepared by the method of this invention are easier to control.

[0064] The addition of metal powder in step (3) of this invention is based on the kinetic differences between the pre-reaction transition period and the reaction stabilization period during the preparation of precursor C. The etching reaction is more intense in the pre-reaction transition period, and a large number of metal ions react to generate microcrystals. The kinetics of microcrystal agglomeration and growth are weak. In the stabilization period, the kinetics of microcrystal agglomeration and growth are stronger, and the generated microcrystals basically attach to crystal nuclei and grow. Therefore, an excessive amount of metal powder is added first to ensure the generation of sufficient crystal nuclei, and then an appropriate amount of metal powder is added in the later stage to ensure the particle growth rate and morphology.

[0065] In step (3), after the metal powder is added, within a reaction time of 0-2 hours, due to the excess acid in the system, it reacts with the large amount of added metal powder to form a large number of tiny crystals. After reaching a certain saturation concentration, the tiny crystals begin to agglomerate and grow. Therefore, during this period, the reaction rate can be effectively controlled by controlling the pH value, adjusting the particle morphology, and achieving excellent physicochemical properties of the material.

[0066] After a reaction time exceeding 2 hours, the microcrystals agglomerate and grow. To slow down the crystallization rate, control the reaction rate, and ensure the physicochemical and appearance properties of the synthesized particles, the amount of metal powder added needs to be reduced. Furthermore, the redox reaction between the metal powder and the etchant increases the system pH, thus requiring pH control. In this invention, after adding the metal powder and reacting for more than 2 hours, the pH is controlled between 6 and 9 to ensure the resulting particles have a good lamellar structure. At higher pH levels, particle sphericity increases, and the lamellar structure is less pronounced; conversely, at lower pH levels, particle stability is poor, they are easily crushed, and processing performance is poor.

[0067] The present invention will be described in detail below with reference to the embodiments. Table 1 lists the relevant experimental conditions of Examples 1-7 and Comparative Examples 1-3 of the present invention. The comparative examples were prepared by conventional liquid-phase precipitation method, and the examples were prepared according to the technology of the present invention. The Na supersaturation of the base liquid and the concentration process mainly affected the Na content of the finished product, and the pH of the process mainly affected the morphology of the material; for specific implementation cases, see Examples 1-7 and Comparative Examples 1-3.

[0068] Table 1. Relevant experimental conditions for Examples 1-7 and Comparative Examples 1-3

[0069]

[0070] Example 1

[0071] A method for preparing a sodium-ion cathode material, employing a metal etching method and utilizing a system of metal micropowder and a saturated sodium-ion solution, includes the following steps:

[0072] (1) Preparation of base solution: Heat 2 kg of anhydrous salt water to 60°C, add anhydrous sodium sulfate, stir at 450 rpm for 2 h, and prepare a 2 mol / L sodium sulfate saturated solution.

[0073] (2) Etching and precipitation: Add 0.2L of 25% ammonia (complexing agent) and 0.091L of 65% concentrated nitric acid (etching agent) to the base solution, stir evenly, and the resulting solution has a pH of 4. Turn on the hot water bath, stabilize the temperature to 60℃, and stir at 450rpm for 2h; then carry out the etching and precipitation reaction at 60℃; during the first 0-2h of the etching and precipitation reaction, add metal powder every hour at a molar ratio of 6:1:0.8 between metal powder, etching agent, and complexing agent, and the system pH is 5 during the reaction; when the reaction is >2h, add metal powder every hour at a molar ratio of 4:1:0.3 between metal powder, etching agent, and complexing agent. Metal powder was added to a reaction vessel. During the reaction, the pH of the system was 6. After 64 hours of reaction, the particle size D50 of the obtained particles was 5±1μm. Feeding was stopped, and the mixture was concentrated to a solid content of 65wt%. The sodium ion supersaturation was adjusted to 1.2, and the mixture was aged for 24 hours. During the aging process, the pH of the system was maintained at 7-10. The precursor was obtained by centrifugation and drying. The metal powder was a mixed metal powder with a Ni:Fe:Mn molar ratio of 1:1:1. The equal amounts of metal powder added here are for ease of preparation and to balance the synergistic effect between the metals.

[0074] (3) Sodium-mixed sintering: The precursor, sodium carbonate, and additives were mixed evenly according to a molar ratio of 1:1.13 between sodium ions in the precursor and sodium carbonate, and sintered at 850℃ for 18h to obtain a sodium-rich cathode material; the additive was alumina, and the amount added was 0.3% of the precursor weight. The weight of the precursor was determined according to the dry weight of the precursor output, and the total Na content in the precursor was calculated by subtracting the amount of Na lost from the mother liquor from the amount of Na added during the sintering process.

[0075] The sodium-rich cathode material obtained in this embodiment has a porosity of 97.3%. Here, porosity refers to open-pore porosity, which is the porosity between the particle pores and the external environment, where ion transfer resistance is minimal. Open-pore porosity = V1 / V2, where V1 is the pore volume of the particle, V2 is the volume of the sphere, which can be calculated from specific surface area data; V2 can be calculated from true density data, which characterizes the packed volume of a solid with no pores.

[0076] Example 2

[0077] This embodiment provides a method for preparing a sodium-ion cathode material, which differs from Example 1 in that:

[0078] A. The solution prepared in step (1) is a saturated sodium carbonate solution;

[0079] B. The concentration of the base solution in step (1) is about 3 mol / L. The base solution is a saturated solution at 60℃.

[0080] The porosity of the sodium-rich cathode material obtained in this embodiment is 97.8%, and the testing method is the same as in Example 1.

[0081] Example 3

[0082] This embodiment provides a method for preparing a sodium-ion cathode material, which differs from Example 1 in that:

[0083] (1) The solution prepared in step (1) is a saturated sodium nitrate solution;

[0084] (2) The concentration of the base solution in step (1) is about 6 mol / L. The base solution is a saturated solution at 60°C.

[0085] Example 4

[0086] This embodiment provides a method for preparing a sodium-ion cathode material, which differs from Example 1 in that:

[0087] In step (2), the sodium ion supersaturation coefficient (i.e., supersaturation) during the aging process is adjusted to 1.6.

[0088] Example 5

[0089] This embodiment provides a method for preparing a sodium-ion cathode material, which differs from Example 1 in that:

[0090] In step (2), the sodium ion supersaturation coefficient during the aging process is adjusted to 2.

[0091] Example 6

[0092] This embodiment provides a method for preparing sodium ion cathode material, which differs from Example 5 in that: in step (2), the pH of the system is adjusted to 7 within 0-2 hours of the etching reaction.

[0093] Example 7

[0094] This embodiment provides a method for preparing sodium ion cathode material. The difference from Example 6 is that in step (2), the etching precipitation reaction is >2h, and the pH of the system is adjusted to 9 during the reaction.

[0095] Comparative Example 1

[0096] This comparative example provides a method for preparing a sodium-ion cathode material using a co-precipitation method. The composition of this sodium-ion cathode material is the same as that of the sodium-ion cathode materials in Examples 1 and 2, all containing Na, Ni, Fe, and Mn, with Al as the dopant element. The preparation method includes the following steps:

[0097] (1) Preparation of base liquid: Add 80L of salt water to the reaction vessel and heat to 60℃;

[0098] (2) Coprecipitation reaction: The ternary feed solution, complexing agent and precipitant are added to the reactor under stirring through their respective inlet pipes in parallel. The reaction temperature is 60℃, the pH value of the system during the reaction is 11.6, the concentration of complexing agent is 6.0g / L, the reaction time is 26h, and the reaction is stopped. The obtained precursor slurry is filtered and washed. The solid product after washing is placed in a 130℃ oven to dry for 24h. It is then sieved through a 350 mesh sieve to remove iron and obtain the precursor material. The ternary feed solution is a mixture of nickel, iron, and manganese, with a molar ratio of Ni, Fe, and Mn of 1:1:1. The total concentration of nickel, iron, and manganese in the ternary feed solution is 220±10 g / L, and the feed rate is 5 L / min. The complexing agent is ammonia monohydrate with a concentration of 200±10 g / L and a feed rate of 0.5 L / min. The precipitant is sodium hydroxide with a concentration of 420±20 g / L and a feed rate of 2.4 L / min.

[0099] (3) Sodium-mixed sintering: Weigh the precursor and Na2CO3, put them in a ball mill and mix them evenly. The molar ratio of the precursor to sodium ions in the sodium salt is 1:1.13. Then, control the oxygen flow rate to 2.5L / min, sinter at 850℃ for 16h, and finally cool to room temperature, grind, and sieve (325 mesh, take the sieve material) to obtain sodium-ion cathode material.

[0100] Comparative Example 2

[0101] This comparative example provides a method for preparing a sodium-ion cathode material, which differs from Comparative Example 1 in that:

[0102] In the co-precipitation reaction step, a saturated sodium sulfate solution at 60°C, along with the ternary feed solution, precipitant, and complexing agent, are simultaneously (or added in parallel) to the reactor to carry out the precipitation reaction. Here, the flow rate of the sodium sulfate solution is 5 ml / min.

[0103] In this comparative example, sodium sulfate was added during the precipitation process, thus the effect of sodium ions on the precipitation reaction exists.

[0104] Comparative Example 3

[0105] This comparative example provides a method for preparing a sodium ion cathode material. The difference between this method and Comparative Example 1 is that in the bottom solution preparation step, the bottom solution is a saturated sodium sulfate solution with a concentration of approximately 2 mol / L.

[0106] Na ion loss characterization

[0107] Through the precipitation process, the mother liquor Na + The content of Na and the free Na in the sintered finished product +The content of sodium ions was used to characterize the loss of sodium ions. Table 2 lists the free sodium ion content distribution and the interlayer spacing of the obtained sodium ion cathode materials in Examples 1-7 and Comparative Examples 1-3 of this invention. The interlayer spacing of the cathode materials was measured by XRD using conventional testing methods. The main parameters for XRD testing were: sample amount 3g, scan range: 10-70°, scan rate 0.5° / min.

[0108] Table 2. Distribution of free sodium ion content and interlayer spacing of sodium ion cathode materials obtained in Examples 1-7 and Comparative Examples 1-3.

[0109]

[0110]

[0111] As shown in Table 2, Comparative Example 1 uses a co-precipitation method to synthesize the precursor, and then introduces sodium ions into the sodium-ion cathode material through sintering. Due to the presence of free Na+ in the sintered product... + The high content means that the actual amount of Na entering the sodium ion cathode material lattice is... + less.

[0112] Comparative Example 2, based on Comparative Example 1, added a saturated sodium sulfate solution during the precipitation process to increase the influence of sodium ions on the dynamic precipitation process. However, the sodium sulfate flow rate was very low at this time, and it quickly flowed away with the mother liquor, resulting in a short-lived effect. This is evident from the free Na in the sintered product in Table 2. + The content shows that, compared to Comparative Example 1, most of the sodium sulfate was carried away with the mother liquor, with only a small portion entering the crystal lattice.

[0113] Comparative Example 3, based on Comparative Example 1, added a saturated sodium salt solution as the base liquid. Due to the higher concentration of the sodium salt solution and the longer reaction time, it increased the Na content in the sodium ion cathode material lattice compared to Comparative Example 1. + The content of.

[0114] Therefore, the three technical solutions in Comparative Examples 1-3, under the premise of conventional processes, gradually verified that sodium can be introduced during the co-precipitation process, and that the amount of sodium introduced increases with the increase of reaction time and concentration, thus reducing loss.

[0115] Furthermore, as shown in Table 2, when the precursor is synthesized using the co-precipitation method, the sodium ion content in the mother liquor (Na) is significantly lower. + The loss of sodium ions is relatively large, especially in the co-current process (Comparative Example 2), where the loss of sodium ions is the greatest. In the precursor synthesis process of this invention, due to the use of metal etching and a saturated sodium ion bottom solution system, as well as the addition of an aging process, the loss of sodium ions is significant. + It is almost never lost with the mother liquor.

[0116] Structural characterization of precursor materials

[0117] The structures of the precursor materials in Examples 1-7 and Comparative Examples 1-3 were characterized using SEM. Figure 1 SEM images of the precursor materials in Examples 1-7 and Comparative Examples 1-3 are shown respectively.

[0118] Depend on Figure 1 It can be seen that the precursor products prepared by the technical solutions of Examples 1-7 are loosely stacked monolayer primary particles with many cavities in the structure; the precursor products prepared by the technical solutions of Comparative Examples 1-3 have a spherical structure.

[0119] In addition, by Figure 1 It can be seen that the crystal growth direction of primary particles can be controlled relatively easily by adjusting the process parameters.

[0120] Material structural characteristics

[0121] The structures of the sodium-ion cathode materials in Examples 1-7 and Comparative Examples 1-3 were characterized by SEM. Figure 2 SEM images of the precursor materials in Examples 1-7 and Comparative Examples 1-3 are shown respectively.

[0122] As can be seen from the figure, the structure of the sodium-ion cathode material after sintering is not significantly different in the electron microscope image.

[0123] Assembly and performance testing of sodium-ion batteries

[0124] Battery assembly

[0125] The positive electrode materials prepared in Examples 1-7 and Comparative Examples 1-3 were mixed with superconducting carbon black SuperP, carbon nanotubes and polyvinylidene fluoride in a mass ratio of 96.3:1.0:0.5:2.2, respectively. A positive electrode slurry was prepared using N-methylpyrrolidone as a solvent, coated onto a 14 μm thick aluminum foil, dried at 120°C, and then compacted under a pressure of 10.0 MPa. The mixture was then cut into positive electrode sheets according to specific specifications. The negative electrode material, conductive agent superconducting carbon black SuperP, sodium carboxymethyl cellulose and styrene-butadiene rubber were mixed in a mass ratio of 95.9:1.0:1.3:1.8, coated onto a 6 μm thick copper foil, dried at 85°C, and then compacted under a pressure of 2.5 MPa. The mixture was then cut into negative electrode sheets. The electrodes are stacked in the following order: negative electrode, separator, positive electrode, separator, and negative electrode again. A total of 11 positive electrode sheets and 12 negative electrode sheets are stacked. After stacking, tabs are welded on, and the cells are sealed with an aluminum-plastic film. Electrolyte is then injected, a small amount of vitamin C additive is added, and the cells are sealed to assemble a sodium-ion soft-pack battery with a nominal capacity of 2.85 Ah. The electrode die-cutting specification is 60*91 mm, and the thickness is calculated based on the number of stacked layers; the 11 positive electrode sheets are approximately 5 mm thick.

[0126] Cyclic performance

[0127] The battery was tested at 25°C on an electrochemical workstation battery testing system. The tested current density was 170 mAh / g, and the charge / discharge voltage window was 2.5-4.3V.

[0128] Figure 3-4 The charge-discharge curves and electrochemical cycle curves at 0.1C of sodium-ion batteries composed of the cathode materials obtained in Examples 1-7 and Comparative Examples 1-3 are shown respectively.

[0129] Depend on Figure 3 It can be seen that the sodium-ion batteries assembled using the sodium-ion cathode materials prepared in Examples 1-7 have relatively similar specific capacities. The specific capacity of the sodium-ion batteries assembled using the sodium-ion cathode materials prepared in Comparative Examples 1-3 is lower than that of Examples 1-7 because the actual sodium ion content in the crystal lattice of the sodium-ion cathode material is lower, and the denser structure is not conducive to capacity utilization, thus reducing the capacity performance of the sodium-ion battery.

[0130] Furthermore, during the preparation of the positive electrode, excessive free sodium can cause the slurry to easily absorb water and agglomerate during homogenization. High water content can lead to a voltage plateau caused by water decomposition during charging, resulting in a slow rise in the charging plateau and consequently, an excessively high charging capacity, i.e., overcharging. Additionally, due to abnormalities such as water absorption and agglomeration during homogenization, unstable areal density can occur during the coating and rolling process. Excessive areal density differences can easily lead to incomplete lithium intercalation or even lithium plating during a single charge-discharge cycle, resulting in a decrease in discharge capacity. Simultaneously, unstable areal density can also increase impedance, further reducing battery capacity.

[0131] Depend on Figure 4 It can be seen that the sodium-ion batteries assembled using the sodium-ion cathode materials prepared in Examples 1-7 all have a capacity retention rate of >92% after 100 cycles. The sodium-ion batteries assembled using the sodium-ion cathode materials of Comparative Examples 1-3 have a lower capacity retention rate than those of Examples 1-7, possibly because the external structure is more fragile and collapses more quickly after cycling.

[0132] Compared with Comparative Examples 1-3, sodium-ion batteries prepared using the same sintering process and the sodium-ion cathode material prepared using the technical solution of the present invention (Examples 1-7) have better capacity and cycle retention.

[0133] Therefore, since the sodium-rich precursor material obtained according to the technical solution of the present invention has a sheet-like feature, compared with conventional cathode materials, the sodium-rich cathode material obtained after sintering can retain more ion diffusion channels, thereby improving the performance of sodium-ion cathode materials and improving the capacity and cycle performance of sodium-ion batteries prepared using it.

Claims

1. A method for preparing a sodium-ion cathode material, characterized in that, The preparation method adopts a metal etching method and is prepared by metal powder and a sodium ion saturated solution, and comprises the following steps: (1) adding water and a sodium-containing electrolyte into a reaction container, stirring to obtain a bottom solution A, i.e., a sodium ion saturated solution; (2) adding an etchant and a complexing agent into the bottom solution A of step (1) in multiple times according to a certain molar ratio, and stirring to obtain a solution B; (3) adding metal powder into the solution B of step (2) in two steps to react, and after the reaction, concentrating, aging, centrifuging and drying the reaction solution to obtain a sodium ion positive electrode material precursor C; (4) mixing the sodium ion positive electrode material precursor C, a sodium salt and an additive, and sintering to obtain a sodium ion positive electrode material. The sodium ion positive electrode material has a general formula of Na x MeO2, wherein Me is one or more of Fe, Ni, Mn, Co, Cu, Ti, Mg, Al metals, and x is 1.02-1.

20.

2. The method for preparing the sodium-ion cathode material according to claim 1, characterized in that, The primary particles of the sodium-ion positive electrode material have a sheet-like layer structure, the secondary particles of the sodium-ion positive electrode material are a spherical structure composed of primary particles, and the interlayer spacing c is 3. The preparation method of the sodium ion positive electrode material according to claim 1, characterized in that, in step (3), the metal powder, the etchant and the complexing agent are added according to a molar ratio of (4-10):1:(0.5-1) per hour within 0-2 hours of reaction time, so that the pH of the system during the reaction is between 5 and 7; and when the reaction time is >2h, the metal powder, the etchant and the complexing agent are added according to a molar ratio of (1-4):1:(0-0.5) per hour, so that the pH of the system during the reaction is between 6 and 9.

4. The method for preparing the sodium-ion cathode material according to claim 1, characterized in that, in step (1), the sodium-containing electrolyte comprises a sodium salt, a sodium oxide and a sodium hydroxide; and / or, in step (2), the etchant comprises one or more of acetic acid, nitric acid, sulfuric acid, hydrochloric acid, triflic acid, trichloroacetic acid; and the complexing agent comprises one or more of NH3·H2O, Na2CO3, NaHCO3 and CH3COONa; and / or, in step (3), the metal powder comprises one or more of iron powder, nickel powder, manganese powder, cobalt powder, copper powder, titanium powder, magnesium powder and aluminum powder; and / or, in step (4), the sodium salt comprises one or more of sodium sulfate, sodium bisulfate, sodium nitrate, sodium carbonate, sodium bicarbonate, sodium phosphate, sodium oxide, sodium hydroxide and sodium acetate; and / or, the additive comprises one or more of carbon powder, iron powder, boric acid, cobalt boric acid, zirconium oxide, aluminum oxide, magnesium oxide, rubidium oxide, cobalt hydroxide and lithium phosphate. in step (1), the electrolyte comprises one or more of sodium sulfate, sodium nitrate, sodium carbonate, sodium phosphate, sodium oxide, sodium hydroxide and sodium acetate; and / or, the water is salt-free water.

5. The method for preparing the sodium-ion cathode material according to claim 1, characterized in that, in step (1), the concentration of the bottom solution A is 2-10 mol / L; and / or, 6. The method for preparing the sodium-ion cathode material according to claim 1, characterized in that, in step (2), the molar ratio between the etchant and the complexing agent is 1:0.5-1; and / or, the pH of the solution B is 2-6; and / or, in step (3), the particle size of the metal powder is <20 μm; and / or, in step (4), the doping amount of the additive based on the weight of the precursor C is 300-8000 ppm; and / or, the molar ratio between the sodium element in the precursor C and the sodium salt is 1:10-1.

16. ​ ​ 7. The method for preparing the sodium-ion cathode material according to claim 1, characterized in that, In step (3), the reaction temperature is 45–65°C, and the total reaction time is 47–120 h; and / or, an inert gas is introduced during the reaction to maintain the dissolved oxygen content of the slurry at <5%; and / or, after the reaction, the reaction solution is concentrated to a solid content of 60–80 wt% and a sodium ion supersaturation coefficient of 1.12–2, and then aged for 24–72 h, maintaining the pH of the system at 7–10 during the aging process; and / or, In step (4), the precursor C, sodium salt and additives are mixed and sintered at 400-1000℃ for 16-24h.

8. A positive electrode sheet characterized by comprising: The positive electrode includes a sodium ion positive electrode material prepared by the preparation method according to any one of claims 1-7.

9. A battery, characterized by Including the positive electrode sheet according to claim 8.

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