A sodium-ion battery single-crystal cathode material, a preparation method and application thereof

Abstract

This invention discloses a monocrystalline cathode material for sodium-ion batteries, its preparation method, and its applications. The preparation method of this monocrystalline cathode material includes the following steps: mixing and sintering waste materials and / or precursor materials with molten salt to obtain a molten salt sintered product; washing the molten salt sintered product with a salt solution, followed by filtration and drying to obtain the monocrystalline cathode material for sodium-ion batteries. This invention is based on molten salt technology, introducing salt solution washing during the washing stage. Based on solvation structure regulation, it inhibits the damage of water molecules to the oxide cathode material, thereby achieving the cleaning of excess molten salt while maintaining the structural integrity of the newly prepared material. The method of this invention has the characteristics of low cost, simple process, and wide applicability. It can realize the unified recycling of waste materials in different states or the direct sodium intercalation of precursor materials, providing technical support for the closed-loop development of the sodium-ion battery industry.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of sodium ion battery cathode materials, and particularly relates to a sodium ion battery single-crystal cathode material and a preparation method and application thereof. BACKGROUND

[0002] With the acceleration of the new energy process, new renewable energy occupies an increasingly important position in daily life, and the application of new energy largely depends on the development of energy storage technology. At present, the commercial mature lithium ion battery is limited by the import dependence of noble metal raw materials such as lithium, nickel and cobalt and the resource abundance, and it is difficult to more guarantee the overall large-scale application demand of power batteries and energy storage fields. The resource-abundant (ranked sixth in crustal element content) and low-cost sodium ion battery is considered to be the key to the current technical breakthrough, and its industrialization development has gradually shown practical value and been recognized by the market. Inspired by the development history of lithium ion batteries, the battery system for large-scale application must face the problem of closed-loop development. Sodium ion batteries, due to their low raw material cost, face the challenge of poor economic benefits in the recycling of retired batteries, and it is urgent to develop applicable low-cost and large-scale recycling technologies to meet the needs of sustainable development. At the same time, under the existing market distribution, how to help sodium ion batteries replace lithium ion batteries or lead-acid batteries in some application scenarios also greatly tests the preparation cost and comprehensive performance of sodium ion batteries.

[0003] The molten salt method is an efficient and simple material recycling and preparation technology. On the one hand, the molten salt, due to the self-saturation sodium supplement feature, can realize unified large-scale regeneration treatment without screening the specific sodium content of waste materials after supplementing sodium from different waste materials, which is beneficial to the direct, rapid and low-cost regeneration of waste materials. On the other hand, based on the higher ion concentration and solid-liquid reaction characteristics, single-crystal materials can be easily prepared. A small amount of interface-exposed cathode material will produce less gas in actual application, reducing the risk of battery bulging and improving the safety of the battery. However, the molten salt usually needs a further cleaning process to remove the excess molten salt after sintering, which is a great challenge to the newly generated sodium ion battery cathode material. Sodium-based oxide cathode materials are sensitive to water, and the structure of the cathode material is easily destroyed in the process of water cleaning molten salt, which cannot effectively match the application of the molten salt technology. In addition, although the existing technology proposes a method for preparing large single-crystal sodium ion battery cathode materials by molten salt, the reversible specific capacity of the obtained material is generally lower than 80 mAh / g, which is also affected by the cleaning process to some extent.

[0004] Therefore, it is very challenging to realize the application of the molten salt method in sodium ion batteries through technical innovation, and even to optimize the material interface on the original basis and thus improve the cycle life. SUMMARY

[0005] The purpose of this invention is to overcome the above-mentioned technical deficiencies and propose a single-crystal cathode material for sodium-ion batteries, its preparation method and application, thereby solving the technical problem that the process of washing away excess molten salt during the preparation of sodium-ion battery cathode materials using the molten salt method in the prior art leads to the re-destruction of the cathode material structure and reduces the cycle life of sodium-ion batteries.

[0006] In a first aspect, the present invention provides a method for preparing a single-crystal cathode material for sodium-ion batteries, comprising the following steps:

[0007] Waste materials and / or precursor materials are mixed with molten salt and sintered to obtain molten salt sintered products;

[0008] The molten salt sintering product was washed with a salt solution, then filtered and dried to obtain a sodium-ion battery single-crystal cathode material.

[0009] In a second aspect, the present invention provides a sodium-ion battery single-crystal cathode material, which is obtained by the preparation method of the sodium-ion battery single-crystal cathode material provided in the first aspect of the present invention.

[0010] Thirdly, the present invention provides the application of the above-mentioned sodium-ion battery single-crystal cathode material in sodium-ion batteries.

[0011] Compared with the prior art, the beneficial effects of the present invention include:

[0012] This invention, based on molten salt technology, introduces a salt solution for cleaning during the washing stage. By modulating the solvation structure, it suppresses the damage of water molecules to the oxide cathode material, thus achieving the cleaning of excess molten salt while maintaining the structural integrity of the newly prepared material. The method of this invention features low cost, simple process, and wide applicability. It can achieve unified recycling of waste materials in different states or direct sodium intercalation of precursor materials, providing technical support for the closed-loop development of the sodium-ion battery industry. Furthermore, the sodium-ion battery cathode material obtained by this invention exhibits a single-crystal growth trend with simultaneous interface optimization, enhanced cycle stability, and improved product competitiveness. Attached Figure Description

[0013] Figure 1 The diagram shows (a) the material phase structure and (b) the change in sodium content during the molten salt repair process in Example 1 of this invention.

[0014] Figure 2 Scanning electron microscope images of (a) waste single crystal material and (b) the repair material prepared in Example 1 of the present invention;

[0015] Figure 3 This is a transmission electron microscope image of the repair material prepared in Example 1 of the present invention;

[0016] Figure 4 (a) First-cycle charge-discharge curve and (b) Cyclic performance diagram of the repair material and the waste single crystal material used in Example 1 of this invention;

[0017] Figure 5 This is a scanning electron microscope image of the repair material prepared in Example 2 of the present invention;

[0018] Figure 6 (a) First-cycle charge-discharge curves and (b) Cyclic performance diagrams of the repair materials prepared in Examples 1-2 of this invention;

[0019] Figure 7 This is a scanning electron microscope image of the repair material prepared in Example 5 of the present invention;

[0020] Figure 8 The first-week charge-discharge curves of the repair materials prepared in Examples 5, 6, 8 and 10 of this invention are shown.

[0021] Figure 9 This is a transmission electron microscope image of the repair material prepared in Example 14 of the present invention;

[0022] Figure 10 This is a scanning electron microscope image of the repair material prepared in Comparative Example 1 of the present invention;

[0023] Figure 11 The first-week charge-discharge curves of the repair material prepared in Comparative Example 1 of this invention and the waste single crystal material used are shown. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0025] Current sodium-ion battery recycling technologies face challenges in terms of economics and scalability. Even with reports of using molten salt methods for sodium-ion battery preparation and recycling, the use of ethanol and water as solvents during the cleaning process inevitably leads to the formation of hydrated phases, resulting in poor overall material performance. The problem of material damage during the cleaning process remains largely unresolved. Sodium-ion battery cathode materials mainly include layered transition metal oxides, polyanionic compounds, and Prussian blue analogs. Among these, layered transition metal oxides are mainly divided into P2-type and O3-type structures. O3-type materials often have higher specific capacity, but they are more sensitive to water, and the molten salt method is less suitable for O3-type materials.

[0026] This invention is based on molten salt technology, introducing a salt solution for cleaning during the cleaning stage. By modulating the solvation structure, it suppresses the damage of water molecules to the oxide cathode material, thus maintaining the structural integrity of the newly prepared material while cleaning excess molten salt. This method is applicable to the preparation of O3-type materials. Further selective annealing after cleaning, depending on the specific circumstances, helps to reconstruct the shallow surface structure of the cathode material. The reconstructed inert interface contributes to extending the material's cycle life.

[0027] Based on this, the present invention is proposed.

[0028] In a first aspect, the present invention provides a method for preparing a single-crystal cathode material for sodium-ion batteries, comprising the following steps:

[0029] S1. The waste material (full name: waste sodium-ion battery cathode material) and / or precursor material (full name: sodium-ion battery cathode material precursor) and molten salt are mixed and sintered to obtain molten salt sintering product;

[0030] S2. The molten salt sintering product is washed with a salt solution, then filtered and dried to obtain a sodium-ion battery single-crystal cathode material.

[0031] In this embodiment, in step S1, the chemical formula of the waste material is Na. x Ni y Mn z Me w O2 and Me are selected from at least one of Fe, Mg, Al, Cu, Co, Sn, Ti, and Zn, wherein 0≤x≤1, 0≤y≤0.5, 0.3≤z≤1.0, 0≤w≤0.4, and y+z+w=1. This invention does not limit the state of the waste material; the waste material can be in any state (single crystal, polycrystalline, charged state, discharged state, etc.), and those skilled in the art can choose according to the actual situation.

[0032] In this embodiment, in step S1, the precursor material is a precursor for preparing the sodium-ion battery cathode material. This invention does not limit the type of precursor material; it can be a carbonate precursor, a hydroxide precursor, or a solid-phase mixed metal salt material, metal oxide material, etc. Those skilled in the art can select the appropriate material based on the specific circumstances, ultimately ensuring that the chemical formula of the sodium-ion battery cathode material (the chemical formula of the sodium-ion battery cathode material is Na) is met. m Ni n Mn o Me p O2 and Me are selected from at least one of Fe, Mg, Al, Cu, Co, Sn, Ti and Zn, wherein 0.44≤m≤1, 0≤n≤0.5, 0.3≤o≤1.0, 0≤p≤0.4, and n+o+p=1.

[0033] In this embodiment, in step S1, the molten salt includes an active component and optionally an inactive component; wherein the active component includes at least one of Na₂CO₃, NaHCO₃, NaOH, NaNO₃, and Na₂O; and the inactive component includes at least one of halogen salts (Cl, I, Br, F, etc.) of Li, Na, K, and Ca, and sulfates. In the molten salt of the present invention, the active component can act as a sodium intercalation component to achieve sodium intercalation, while the inactive component can participate in regulating the melting temperature of the active component, reducing energy consumption, and providing a molten salt environment.

[0034] Preferably, the ratio of the molar amount of sodium in the active component to the theoretical molar amount of the waste material and / or precursor material final product is greater than 1, and further is 1 to 5, including but not limited to 1, 2, 3, 4, 5, etc.

[0035] Preferably, the molten salt comprises an active component, and the active component comprises at least two of Na2CO3, NaHCO3, NaOH, NaNO3, and Na2O.

[0036] Preferably, the molten salt comprises: an active component and an inactive component; wherein the active component comprises at least one of Na2CO3, NaHCO3, NaOH, NaNO3, and Na2O; and the inactive component comprises at least one of halogen salts (Cl, I, Br, F, etc.) of Li, Na, K, and Ca, and sulfates.

[0037] This invention does not limit the proportion of each raw material in the molten salt component. Those skilled in the art can select the proportion based on the phase diagram, and can preferentially select the proportion corresponding to the lower melting point.

[0038] In this embodiment, step S1 includes: mixing and sintering waste materials and / or precursor materials with molten salt and dopant to obtain a molten salt sintered product. This invention, by introducing doping elements during the molten salt sodium replenishment stage, can simultaneously achieve structural strengthening of the sodium-ion battery cathode material, while inhibiting the etching of the material surface by the molten salt and maintaining the integrity of the material structure.

[0039] This invention does not limit the type of dopant element used in the dopant, and those skilled in the art can select it according to the actual situation. In some specific embodiments of this invention, the dopant element is at least one selected from W, Al, Mg, B, Zn, Zr, and Ca.

[0040] Furthermore, the doping element accounts for 0% to 5% of the theoretical molar amount of the waste material and / or precursor material final product, including but not limited to 0%, 0.01%, 0.05%, 0.1%, 0.2%, 0.5%, 1%, 2%, 5%, etc.

[0041] In this embodiment, step S1, the mixing and sintering process includes: first heating to a first temperature and holding at that temperature for a first time, then heating to a second temperature and holding at that temperature for a second time; wherein, the first temperature is the melting temperature of the molten salt, which depends on the phase diagram parameters of the molten salt system, and is generally 200~800℃, and the first time is 2~8h; the second temperature is higher than the first temperature, and the second temperature is generally 600~1200℃, further 750~900℃, and the second time is 1~20h, further 5~18h. This invention promotes the melting and diffusion of the molten salt by holding at the first temperature for a first time, ensuring sufficient contact of the raw materials; if the first temperature is too low or the first time is too short, insufficient contact of the raw materials will result; if the first temperature is too high or the first time is too long, premature sodium insertion will occur, leading to uneven reaction. Holding at the second sintering temperature for a second time is beneficial for high-temperature sodium insertion. If the second temperature is too low or the second time is too short, insufficient sodium insertion will occur; if the second temperature is too high or the second time is too long, sodium volatilization or increased polarization will occur.

[0042] Further, the temperature is increased to the first temperature at a rate of 1~5℃ / min; the temperature is increased to the second temperature at a rate of 1~5℃ / min; after sintering, the temperature is naturally cooled to room temperature.

[0043] In this embodiment, in step S2, the salt in the salt solution is at least one of the following: halogen salts of Li, Na, K, and Ca (F, Cl, I, Br, etc.), sulfates, acetates, and nitrates. The solvent is a mixture of alcohol and water, and the volume percentage of water is 10% to 50%, including but not limited to 10%, 20%, 30%, 40%, and 50%. If the volume percentage of water is too high, the cathode material will be easily damaged; if the volume percentage of water is too low, the salt solution cannot be prepared, and the residue of excessive molten salt cannot be cleaned.

[0044] This invention does not limit the type of alcohol in the salt solution. Those skilled in the art can select it according to the actual situation, such as methanol, ethanol, propanol, glycerol, polyethylene glycol, etc.

[0045] In this embodiment, in step S2, the salt concentration in the salt solution is 10% to 90% of the saturation concentration (i.e., the saturation concentration of salt in the corresponding mixed solvent system), including but not limited to 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, and 90%. By using salt solutions of the above concentrations, this invention can dissolve excess molten salt, allowing the material properties to be fully utilized, reducing polarization, and preventing material damage. If the salt solution concentration is too low, it will also lead to greater damage to the material.

[0046] In this embodiment, the ratio of molten salt sintering product to salt solution is 1g:(50~300)mL.

[0047] In this embodiment, in step S2, the cleaning temperature is room temperature, and the cleaning time is 30~600s (including but not limited to 30s, 60s, 120s, 180s, 240s, 300s, 360s, 420s, 480s, 540s, 600s, etc.), and further 60~300s.

[0048] In this embodiment, during step S2, the filtration process involves rinsing with alcohol and water. Each rinse is completed within 40 seconds until no filtrate flows out, and the rinsing is repeated 2-6 times. This invention utilizes a salt solution to dissolve residual salt, followed by rinsing with alcohol and water to ensure rapid contact between the positive electrode material and the alcohol and water, further removing residual salt.

[0049] This invention does not limit the type of alcohol, and those skilled in the art can select it according to the actual situation. For example, it can be methanol, ethanol, propanol, glycerol, polyethylene glycol, etc.

[0050] In the rinsing process of alcohol and water, the present invention does not limit the way alcohol and water are added. Those skilled in the art can choose according to the actual situation. For example, alcohol and water can be mixed for rinsing, or alcohol and water can be rinsed alternately.

[0051] Specifically, during the filtration process, the method of rinsing with alcohol and water includes: after the salt solution is filtered, maintain a negative pressure state, add alcohol and water to rinse, and drain within 40 seconds during each rinse until no filtrate flows out, and the number of rinses is 2 to 6.

[0052] Specifically, during the filtration process, the liquid-to-solid ratio of alcohol and / or water to the filter cake is 30-80 mL:1g.

[0053] In this embodiment, in step S2, the salt in the salt solution is at least one of the following: halogen salts of Na, K, and Ca (F, Cl, I, Br, etc.), sulfates, acetates, and nitrates. After drying, the process further includes annealing the dried product. The inventors discovered during experiments that when using lithium salt cleaning, no annealing is required to directly obtain the required sodium-ion battery single-crystal cathode material; however, when using non-lithium salt cleaning, annealing is necessary to remove the side effects caused by water on the cathode material surface, thereby obtaining the required sodium-ion battery single-crystal cathode material.

[0054] Furthermore, the annealing process includes holding at 300~1000℃ (more specifically 600~900℃) for 0~15h (more specifically 1~5h). If the annealing temperature is too low or the holding time is too short, the mitigation of the above-mentioned side effects will not be significant; if the annealing temperature is too high or the holding time is too long, the long processing time may lead to excessive phase transformation on the surface.

[0055] Specifically, the temperature is increased to 300-1000°C at a rate of 1-5°C / min, and further to 600-900°C.

[0056] In a second aspect, the present invention provides a sodium-ion battery single-crystal cathode material, which is obtained by the preparation method of the sodium-ion battery single-crystal cathode material provided in the first aspect of the present invention.

[0057] Thirdly, the present invention provides the application of the above-mentioned sodium-ion battery single-crystal cathode material in sodium-ion batteries.

[0058] Example 1

[0059] This embodiment provides a method for preparing sodium-ion battery single-crystal cathode materials by directly regenerating waste sodium-ion battery cathode materials, including the following steps:

[0060] (1) Weigh 3 mmol of waste single crystal material (Na) x Ni 1 / 3 Mn 1 / 3 Fe 1 / 3 O2 (charged state, molar amount of waste material calculated based on electrode loading) and 5.26 mmol NaOH and 0.34 mmol Na2CO3 were thoroughly mixed and then transferred to a crucible. The mixture was first heated to 400℃ in a muffle furnace at a rate of 2℃ / min and held for 4 hours, then heated to 850℃ at a rate of 5℃ / min and held for 12 hours. After natural cooling to room temperature, the molten salt sintered product (i.e., a mixture of transition metal oxide cathode material and excess molten salt) was obtained.

[0061] (2) Prepare 100 mL of 0.2 mol / L NaCl solution using a mixed solvent of water and ethanol (V 水 V 乙醇 =1:4). The molten salt sintering product was poured into NaCl solution and washed at room temperature for 2 min, then filtered. During the filtration process, it was rinsed three times alternately with 20 mL of ethanol and 20 mL of water. During each rinse, it was dried within 40 s until no filtrate flowed out. After the filtration process, the filter cake was dried in a vacuum oven at 80 °C for 30 min. The resulting powder was heated to 750 °C in a muffle furnace at a heating rate of 5 °C / min and held for 1 h to obtain the repair material (i.e., a sodium-ion battery single-crystal cathode material with structure retention and interface enhancement).

[0062] The failure of waste materials mainly stems from structural damage and sodium ion loss, with no significant change in their transition metal composition. After low-temperature sintering in a muffle furnace to remove PVDF and conductive carbon components from the electrode material, the remaining material exhibits a transition metal oxide phase, similar to the raw materials of solid-state methods or precursors in conventional synthesis processes. The product obtained after molten salt treatment reverts to a typical layered oxide structure, with some impurity peaks in its XRD pattern corresponding to the absorption of CO2 from the air by excess molten salt residue, transforming it into Na2CO3 (Na2CO3 component). Figure 1 (a) in the text. After washing and separation with salt solution, Figure 1 The ICP results in (b) show that only a small number of sodium ions were released during the cleaning process. XRD patterns further confirm that the crystal phase of the final repaired material corresponds to a typical R-3m space group structure, with no hydrated phase impurity peaks appearing, indicating that the molten salt method can be applied to the recycling and preparation of oxide cathode materials for sodium-ion batteries via a salt solution cleaning strategy. The morphology of the waste material and the recycled transition metal oxide material is shown in the figures below. Figure 2 As shown, the waste material developed numerous cracks due to long-cycle desodiuming and structural phase transformation, but after regeneration, its morphology recovered to dense micron-sized particles. High-resolution TEM analysis revealed (…). Figure 3 While salt solution cleaning can maintain the stability of the material's crystal structure, under strong H / Na ion exchange, some sodium ions will inevitably be released from the surface of the repaired material. Annealing treatment forms an inert rock salt phase structure within a 5nm thickness range on the surface, thereby suppressing the side reactions between the electrolyte and electrode materials during long-term cycling and improving cycle life.

[0063] This embodiment also provides a method for preparing a sodium-ion battery using the above-mentioned sodium-ion battery single-crystal cathode material, including the following steps:

[0064] The recycled sodium-ion battery monocrystalline cathode material was mixed with conductive carbon black and polyvinylidene fluoride (PVDF, binder) at a mass ratio of 8:1:1, and N-methylpyrrolidone (NMP) was added. The mixture was thoroughly ground and mixed to form a uniform slurry, which was then coated onto an aluminum foil current collector as a test electrode. The electrolyte used was 1M NaClO4 / PC:EMC:FEC (V:V:V=50:45:5). A coin cell was fabricated using a metallic sodium sheet as the counter electrode. The battery performance test results are shown in Table 2 and... Figure 4 as well as Figure 6 .

[0065] Test results: The sodium-ion battery single-crystal cathode material directly regenerated using the method in Example 1 recovered its first-cycle discharge specific capacity (0.1C = 13 mA / g) from 54.7 mAh / g of the waste material to 116.5 mAh / g within a voltage range of 2.0~4.1V, exhibiting a typical charge-discharge curve for O3-type oxide cathode materials. Furthermore, the capacity retention rate after 200 cycles at 1C was 82.5%. Figure 4 ).

[0066] Example 2

[0067] Compared with Example 1, the only difference is that sodium tungstate is introduced simultaneously as a W source during the mixing process of molten salt and waste materials, and the doping amount of W is 0.5% of the molar amount of waste materials. The other processes are completely the same.

[0068] Figure 5 The morphology of the W-doped material shows that, compared to the undoped material with more rough small particles, the surface of the W-doped material tends to be smoother. This is because W, due to its larger ionic radius, easily segregates on the material surface during sintering, thus enhancing the surface structure and suppressing the etching effect of molten salt on the material. In terms of electrochemical performance, the reversible capacity of the W-doped material is further improved, and its cycle stability is also enhanced. Figure 6 ).

[0069] Example 3

[0070] Compared with Example 2, the only difference is that the doping amount of W is 0.2% of the molar amount of waste material.

[0071] Example 4

[0072] Compared with Example 2, the only difference is that the doping amount of W is 2% of the molar amount of waste material.

[0073] Example 5

[0074] The only difference from Example 1 is that the waste material is waste polycrystalline material Na. x Ni 1 / 3 Mn 1 / 3 Fe 1 / 3 O2 (discharge state).

[0075] like Figure 7 As shown, after molten salt treatment, polycrystalline materials also exhibit a trend towards monocrystalline growth.

[0076] Example 6

[0077] Compared with Example 5, the only difference is that aluminum nitrate is introduced simultaneously as an Al source during the mixing process of molten salt and waste materials, and the amount of Al doping is 2% of the molar amount of waste materials. The other processes are completely the same.

[0078] Example 7

[0079] Compared with Example 5, the only difference is that sodium borate is introduced simultaneously as a source of B during the mixing process of molten salt and waste materials, and the amount of B doping is 2% of the molar amount of waste materials. The other processes are completely the same.

[0080] Example 8

[0081] Compared with Example 5, the only difference is that zirconium acetate is introduced simultaneously as a Zr source during the mixing process of molten salt and waste material, and the Zr doping amount is 2% of the molar amount of waste material. The other processes are completely the same.

[0082] Example 9

[0083] Compared with Example 5, the only difference is that magnesium acetate is introduced simultaneously as a Mg source during the mixing process of molten salt and waste materials, and the amount of Mg doping is 2% of the molar amount of waste materials; the other processes are completely the same.

[0084] Example 10

[0085] Compared with Example 5, the only difference is that calcium carbonate is introduced simultaneously as a Ca source during the mixing process of molten salt and waste materials, and the amount of Ca doping is 2% of the molar amount of waste materials. The other processes are completely the same.

[0086] The charge-discharge curves of the repair materials prepared in Examples 5, 6, 8, and 10 during the first week are shown below. Figure 8 As shown, these are all typical O3-type charge-discharge curves. The reversible capacity in the first week at 0.1C is restored to about 120 mAh / g. Further, combined with Table 2, it can be seen that the introduction of doping elements such as Al, Zr, and Ca at a doping level of 2% not only optimizes the reversibility of the material, but also significantly improves the long-term cycling stability.

[0087] Example 11

[0088] Compared to Example 1, the only difference is that the molten salt used is a binary molten salt system composed of 1.71 mmol of active Na2CO3 and 2.57 mmol of inactive NaCl. The temperature was first increased to 700°C in a muffle furnace at a rate of 5°C / min and held for 4 hours, then increased to 850°C at a rate of 3°C / min and held for 12 hours; all other processes were identical.

[0089] Example 12

[0090] Compared with Example 1, the only difference is that the heat preservation time at 850°C is 5 hours.

[0091] Example 13

[0092] Compared with Example 1, the only difference is that the heat preservation time at 850°C is 18 hours.

[0093] Example 14

[0094] Compared to Example 1, the only difference is that the molten salt is replaced with 3 mmol of sodium carbonate; the rest of the process is exactly the same.

[0095] When sodium carbonate is chosen as the sodium source, the material exhibits excessive sodium intercalation in the non-molten salt system, leading to poor material performance. Since the state of waste materials is unknown under actual operating conditions, the molten salt method can restore material performance even with excessive sodium replenishment, facilitating the batch regeneration of waste materials and demonstrating unique advantages in battery lifecycle closure. Under the same treatment conditions, the sodium content of the material in Example 14 remained close to 1 after salt solution washing and annealing, indicating that the actual sodium intercalation exceeded this value (Table 1). TEM also revealed structural decomposition on the outermost surface of the material due to excessive sodium intercalation. Figure 9 (This is accompanied by near-surface structural distortion, resulting in relatively poor material cycle reversibility, but it is still significantly better than waste materials and Comparative Example 1.)

[0096] Example 15

[0097] Compared to Example 1, the only difference is that the waste material is replaced with NaNi 1 / 3 Mn 1 / 3 Fe 1 / 3 The O2 stoichiometric ratio is 1 mmol each of nickel oxide, manganese oxide, and iron oxide (based on the metal element content).

[0098] Comparative Example 1

[0099] Compared with Example 1, the only difference is that in the cleaning process of the molten salt sintering product, a mixed solvent of water and ethanol with a volume ratio of 1:4 is used instead of NaCl solution for cleaning; the other processes are completely the same.

[0100] During the cleaning process of molten salt sintering products, without the use of a salt solution, the transition metal oxide cathode material readily undergoes a strong H / Na ion exchange reaction with water, leading to the re-desodiuming of dense micron-sized particles. Its morphology is as follows: Figure 10 As shown, the water-washed material exhibits significant interlayer cracking, indicating that the repaired material has been re-damaged. Its typical charge-discharge curve is shown below. Figure 11 As shown, the discharge specific capacity at 0.1C is even lower than that of waste materials, corresponding to the charge-discharge curve characteristics after sodium loss upon contact with water.

[0101] Comparative Example 2

[0102] Compared with Example 1, the only difference is that the molten salt sintering product was not annealed after washing and separation; the other processes are completely the same.

[0103] Table 1. Main elemental contents of the repair materials prepared in Example 14 of the present invention

[0104]

[0105] Table 2. Partial electrochemical performance results of the single-crystal cathode material prepared in this invention and the waste materials used.

[0106]

[0107] (In Table 2, "-" indicates that it has not been tested yet)

[0108] As can be seen from Table 2, the electrochemical performance (specific capacity and cycle performance) of the sodium-ion battery single-crystal cathode material prepared in the embodiments of the present invention is significantly improved compared with the waste material. Furthermore, the method of the present invention can realize the unified regeneration of waste materials in different states or the direct sodium intercalation of precursor materials.

[0109] As can be seen from Examples 1 to 10, the introduction of doping elements is beneficial to improving the cycle stability of cathode materials.

[0110] As can be seen from Example 11, the method of the present invention can also be applied to other molten salt systems.

[0111] As can be seen from Examples 1 and 12-13, during the molten salt sintering process, both excessively short and excessively long sintering times are not conducive to further improving the electrochemical performance of single-crystal cathode materials.

[0112] As demonstrated in Examples 1 and 14, self-saturation sodium replenishment is a characteristic of molten salt technology, which is beneficial for material repair under actual working conditions where material states are inconsistent. In contrast, in non-molten salt systems (i.e., single-component systems), excessive sodium replenishment can easily damage the material structure due to excessive sodium intercalation, leading to deteriorated performance. Therefore, developing molten salt preparation technology is of positive significance for the mass regeneration of waste sodium-ion battery materials.

[0113] As can be seen from Example 1 and Comparative Example 1, cleaning with a mixed solvent of water and ethanol cannot effectively solve the problem of the regenerated material's sensitivity to water. Without effectively inhibiting H / Na ion exchange, the repair material is easily damaged again during the cleaning of residual molten salt, leading to repair failure.

[0114] As can be seen from Example 1 and Comparative Example 2, although the salt solution in Example 1 inhibited H / Na ion exchange, a small amount of H proton intercalation still affected the material. Without annealing treatment, the cathode material had almost no performance.

[0115] Compared with the prior art, the beneficial effects of the present invention include:

[0116] (1) Based on the process design of molten salt technology, the present invention uses salt solution to suppress the activity of water during the cleaning stage, thereby stabilizing the structure of sodium-ion battery cathode material during the residual molten salt cleaning process.

[0117] (2) The present invention uses the post-processing process of molten salt sodium intercalation stage and cleaning stage to enhance the material interface, which further improves the material cycle stability; among them, the annealing process after cleaning helps to reconstruct the shallow surface structure of the cathode material, and the reconstructed inert interface is conducive to extending the material cycle life.

[0118] (3) In this invention, doping elements are introduced simultaneously during the sodium intercalation stage of molten salt. On the one hand, the doping elements will form a doped layer on the surface, which is beneficial to suppressing the corrosive effect of molten salt on the material; on the other hand, the strengthening of the surface structure by the doping elements also further suppresses the damage of water to the material structure during the cleaning process, which is beneficial to achieving material stability.

[0119] The specific embodiments of the present invention described above do not constitute a limitation on the scope of protection of the present invention. Any other corresponding changes and modifications made in accordance with the technical concept of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a sodium-ion battery single-crystal cathode material, characterized in that, The method comprises the following steps: mixing and sintering waste materials and / or precursor materials and molten salt to obtain a molten salt sintered product; cleaning the molten salt sintered product with a salt solution, and then performing suction filtration and drying to obtain a sodium ion battery single-crystal cathode material; wherein the molten salt comprises an active component, and the active component comprises at least two of Na2CO3, NaHCO3, NaOH, NaNO3 and Na2O; or the molten salt comprises an active component and a non-active component; wherein the active component comprises at least one of Na2CO3, NaHCO3, NaOH, NaNO3 and Na2O; and the non-active component comprises at least one of halogen salts, sulfate salts of Li, Na, K and Ca; in the salt solution, the salt is at least one of halogen salts, sulfate salts, acetate salts and nitrate salts of Li, Na, K and Ca, and the solvent used is a mixed solvent of alcohol and water, and the volume percentage of water is 10% to 50%; in the salt solution, the concentration of the salt is 10% to 90% of the saturation concentration; in the salt solution, the salt is at least one of halogen salts, sulfate salts, acetate salts and nitrate salts of Li, and after drying, the dried product does not need to be annealed; or in the salt solution, the salt is at least one of halogen salts, sulfate salts, acetate salts and nitrate salts of Na, K and Ca, and after drying, the dried product is further annealed.

2. The method for preparing the sodium-ion battery single-crystal cathode material according to claim 1, characterized in that, The ratio of the molar amount of sodium in the active component to the theoretical molar amount of the final product of the waste materials and / or the precursor materials is greater than 1. 3.The method of claim 1, wherein the sodium-ion battery single-crystal cathode material is prepared by the following steps: preparing a sodium-ion battery single-crystal cathode material precursor; and annealing the sodium-ion battery single-crystal cathode material precursor. The step of mixing and sintering the waste materials and / or the precursor materials and the molten salt comprises mixing and sintering the waste materials and / or the precursor materials, the molten salt and a dopant; wherein the dopant corresponds to at least one of W, Al, Mg, B, Zn, Zr and Ca; and / or the dopant corresponds to 0% to 5% of the theoretical molar amount of the final product of the waste materials and / or the precursor materials. 4.The method of claim 1, wherein the sodium-ion battery single-crystal cathode material is prepared by the following steps: preparing a precursor solution; preparing a sodium-ion battery single-crystal cathode material by a solvothermal method; and annealing the sodium-ion battery single-crystal cathode material. The process of the mixing and sintering comprises first heating to a first temperature for a first time, and then heating to a second temperature for a second time; wherein the first temperature is 200 to 800℃, and the first time is 2 to 8h; the second temperature is greater than the first temperature, the second temperature is 600 to 1200℃, and the second time is 1 to 20h; the temperature is raised to the first temperature at a rate of 1 to 5℃ / min, and the temperature is raised to the second temperature at a rate of 1 to 5℃ / min; after the sintering is completed, the temperature is naturally cooled to room temperature.

5. The method for preparing the sodium-ion battery single-crystal cathode material according to claim 1, characterized in that, The ratio of the amount of the molten salt sintered product to the amount of the salt solution is 1g: (50 to 300)mL; the cleaning temperature is room temperature, and the cleaning time is 30 to 600s. 6.The method of claim 1, wherein the sodium-ion battery single-crystal cathode material is prepared by the following steps: preparing a precursor solution; preparing a sodium-ion battery single-crystal cathode material by a solvothermal method; and annealing the sodium-ion battery single-crystal cathode material. During the suction filtration, alcohol and water are used for washing, each washing process is performed within 40s to suction dry until no filtrate flows out, and the washing frequency is 2 to 6 times. 7.The method of claim 1, wherein the sodium-ion battery single-crystal cathode material is prepared by the following steps: preparing a precursor solution; preparing a sodium-ion battery single-crystal cathode material by a solvothermal method; and annealing the sodium-ion battery single-crystal cathode material. The annealing process comprises heating at 300 to 1000℃ for 0 to 15h, and the heating rate is 1 to 5℃ / min.

8. A sodium-ion battery single-crystalline cathode material, characterized in that, The sodium ion battery single crystal cathode material is obtained by the preparation method of the sodium ion battery single crystal cathode material in any one of claims 1-7.

9. Application of the sodium ion battery single crystal cathode material in claim 8 in a sodium ion battery.

Images