Single-crystal positive electrode material for sodium-ion battery and preparation method and application of single-crystal positive electrode material

Abstract

The invention discloses a sodium ion battery single crystal positive electrode material and a preparation method and application thereof. The preparation method of the single-crystal positive electrode material of the sodium ion battery comprises the following steps: mixing and sintering a waste material and / or a precursor material and molten salt to obtain a molten salt sintering product; and cleaning the molten salt sintering product with a salt solution, and then carrying out suction filtration and drying to obtain the sodium-ion battery single-crystal positive electrode material. On the basis of the molten salt technology, the salt solution is introduced for cleaning in the cleaning stage, and damage of water molecules to the oxide positive electrode material is regulated and controlled based on the solvation structure, so that the structural integrity of the newly prepared material is kept while excessive molten salt cleaning is realized. The method has the characteristics of low cost, simple process, wide application range and the like, unified regeneration of waste materials in different states or direct sodium embedding of precursor materials can be realized, and technical support is provided for 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, it is easy to quickly prepare single-crystal materials. 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 to 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: Waste materials and / or precursor materials are mixed with molten salt and sintered to obtain molten salt sintered products; 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.

[0007] 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.

[0008] Thirdly, the present invention provides the application of the above-mentioned sodium-ion battery single-crystal cathode material in sodium-ion batteries.

[0009] Compared with the prior art, the beneficial effects of the present invention include: 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

[0010] 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. 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; Figure 3 This is a transmission electron microscope image of the repair material prepared in Example 1 of the present invention; 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; Figure 5 Scanning electron microscope image of the repair material prepared for Example 2 of the present application; Figure 6 (a) first week charge-discharge curves and (b) cycle performance chart of the repair material prepared for Examples 1-2 of the present application; Figure 7 Scanning electron microscope image of the repair material prepared for Example 5 of the present application; Figure 8 First week charge-discharge curve chart of the repair material prepared for Examples 5, 6, 8 and 10 of the present application; Figure 9 Transmission electron microscope image of the repair material prepared for Example 14 of the present application; Figure 10 Scanning electron microscope image of the repair material prepared for Comparative Example 1 of the present application; Figure 11 First week charge-discharge curve chart of the repair material prepared for Comparative Example 1 of the present application and the used waste single crystal material. DETAILED DESCRIPTION

[0011] In order to make the purpose, technical scheme and advantages of the present application more clear and explicit, the present application will be further described in detail below in combination with the drawings and examples. It should be understood that the specific examples described herein are only used to explain the present application and do not limit the present application.

[0012] The economic and scale problems of the existing sodium ion battery recycling technology are challenged. Even if there are reports that use molten salt method for sodium ion battery preparation and recycling, but the use of ethanol and water as solvent in the cleaning process will inevitably lead to the presence of hydrated phase, and the overall performance of the material is poor. The problem of damage to the material caused by the cleaning process has not been well solved. The positive electrode materials of sodium ion battery mainly include: layered transition metal oxides, polyanion compounds and prussian blue analogues. Among them, the layered transition metal oxides are mainly divided into P2 type and O3 type structures, and the O3 type material often has a higher specific capacity, but it is more sensitive to water, and the adaptability of the molten salt method to the O3 type material is poorer.

[0013] Based on the molten salt technology, the present application introduces salt solution cleaning in the cleaning stage, and based on the solvation structure regulation to inhibit the damage of water molecules to the oxide positive electrode material, so as to realize the cleaning of excess molten salt while maintaining the structural integrity of the newly prepared material, which can be applied to the preparation of O3 type material. Further annealing process is selectively used after cleaning according to the actual situation, which is helpful to the reconstruction of the surface shallow structure of the positive electrode material. The inert interface of the reconstruction is beneficial to prolong the cycle life of the material.

[0014] Based on this, the present application is proposed.

[0015] In a first aspect, the present application provides a preparation method of a sodium-ion battery single-crystal cathode material, comprising the following steps: S1, mixing and sintering waste materials (collectively referred to as waste sodium-ion battery cathode materials) and / or precursor materials (collectively referred to as sodium-ion battery cathode material precursors) and a molten salt to obtain a molten salt sintering product; S2, washing the molten salt sintering product with a salt solution, followed by suction filtration and drying to obtain a sodium-ion battery single-crystal cathode material.

[0016] In this embodiment, in step S1, the chemical formula of the waste material is Na x Ni y Mn z Me w O2, Me is 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. The present application does not limit the state of the waste material, which can be in any state (single crystal, polycrystal, charged state, discharged state, etc.), and a person skilled in the art can select according to the actual situation.

[0017] In this embodiment, in step S1, the precursor material is a precursor for preparing a sodium-ion battery cathode material. The present application does not limit the type of precursor material, which can be a carbonate precursor, a hydroxide precursor, or a solid-phase mixed metal salt material, a metal oxide material, etc., and a person skilled in the art can select according to the actual situation, as long as it can ultimately meet the chemical formula of the sodium-ion battery cathode material (the chemical formula of the sodium-ion battery cathode material is Na m Ni n Mn o Me p O2, Me is 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.

[0018] In this embodiment, in step S1, the molten salt comprises: an active component and optionally a non-active component; wherein the active component comprises at least one of Na2CO3, NaHCO3, NaOH, NaNO3, Na2O; and the non-active component comprises at least one of halogen salts (Cl, I, Br, F, etc.), sulfate salts of Li, Na, K, Ca. In the molten salt of the present application, the active component can act as a sodium-embedding component to achieve sodium embedding, while the non-active component can participate in adjusting the melting temperature of the active component, reducing energy consumption, and providing a molten salt environment.

[0019] Preferably, the ratio of the molar amount of sodium element in the active component to the final theoretical molar amount of the waste material and / or precursor material is greater than 1, further 1-5, including but not limited to 1, 2, 3, 4, 5, etc.

[0020] Preferably, the molten salt comprises: an active component, and the active component comprises at least two of Na2CO3, NaHCO3, NaOH, NaNO3, Na2O.

[0021] Preferably, 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, Na2O; the non-active component comprises at least one of halogen salts (Cl, I, Br, F, etc.), sulfate salts of Li, Na, K, Ca.

[0022] The present application does not limit the proportion of each raw material in the molten salt component, and those skilled in the art can select according to the phase diagram, and the corresponding proportion when the melting point is lower can be selected preferentially.

[0023] In the embodiment, step S1 comprises: mixing and sintering the waste material and / or precursor material and the molten salt and the dopant to obtain a molten salt sintering product. By introducing the doping element in the sodium supplementing stage of the molten salt, the present application can simultaneously realize the structure strengthening of the sodium ion battery positive electrode material, and inhibit the etching of the molten salt to the material surface, and maintain the integrity of the material structure.

[0024] The present application does not limit the type of doping element used by the dopant, and those skilled in the art can select according to the actual situation. In some specific embodiments of the present application, the doping element is at least one of W, Al, Mg, B, Zn, Zr, Ca.

[0025] Further, the doping element accounts for 0%-5% of the final theoretical molar amount of the waste material and / or precursor material, including but not limited to 0%, 0.01%, 0.05%, 0.1%, 0.2%, 0.5%, 1%, 2%, 5%, etc.

[0026] In the embodiment, in step S1, the process of mixing and sintering includes: first heating to a first temperature and keeping for a first time, and then heating to a second temperature and keeping 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 greater 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. The present application promotes the melting and diffusion of the molten salt by keeping at the first temperature for the first time, so that the raw materials are fully contacted; if the first temperature is too low or the first time is too short, the raw materials will not be fully contacted, and if the first temperature is too high or the first time is too long, sodium will be embedded in advance, and the reaction will be uneven. By keeping at the second sintering temperature for the second time, it is beneficial to high-temperature sodium embedding. If the second temperature is too low or the second time is too short, the sodium embedding will be insufficient, and if the second temperature is too high or the second time is too long, the sodium will be volatilized or the polarization will be increased.

[0027] Further, the temperature is raised to the first temperature at a rate of 1-5℃ / min; the temperature is raised to the second temperature at a rate of 1-5℃ / min; after the sintering is completed, the temperature is naturally cooled to room temperature.

[0028] In the embodiment, in step S2, in the salt solution, the salt is at least one of halide salt (F, Cl, I, Br, etc.), sulfate, acetate, nitrate of Li, Na, K, Ca, the solvent is a mixed solvent of alcohol and water, and the volume fraction of water is 10%-50%, including but not limited to 10%, 20%, 30%, 40%, 50%, etc. If the volume fraction of water is too high, the positive electrode material will be easily damaged; if the volume fraction of water is too low, the salt solution cannot be prepared, and the excess molten salt residue cannot be cleaned.

[0029] The present application does not limit the type of alcohol in the salt solution, and those skilled in the art can select according to the actual situation, for example, it can be methanol, ethanol, propanol, glycerol, polyethylene glycol, etc.

[0030] In the embodiment, in step S2, in the salt solution, the concentration of the salt is 10%-90% of the saturation concentration (i.e., the saturation concentration of the salt under the mixed solvent system), including but not limited to 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, etc. By using the above-mentioned concentration of the salt solution, the present application can dissolve the excess molten salt, so that the material performance can be played, the polarization can be reduced, and the material can not be damaged. If the concentration of the salt solution is too low, the damage to the material will be deeper.

[0031] In the embodiment, the amount ratio of the molten salt sintering product to the salt solution is 1g:(50-300)mL.

[0032] In the embodiment, in step S2, the temperature of the cleaning is room temperature, and the time of the cleaning is 30-600 s (including but not limited to 30 s, 60 s, 120 s, 180 s, 240 s, 300 s, 360 s, 420 s, 480 s, 540 s, 600 s, etc.), further 60-300 s.

[0033] In the embodiment, in step S2, during the suction filtration, the filter cake is washed with alcohol and water, each time the washing is performed within 40 s until no filtrate flows out, and the washing is performed 2-6 times.

[0034] The kind of alcohol is not limited in the present application, and can be selected by those skilled in the art according to actual conditions, for example, can be methanol, ethanol, propanol, glycerol, polyethylene glycol, etc.

[0035] The adding mode of the alcohol and water during the washing is not limited in the present application, and can be selected by those skilled in the art according to actual conditions, for example, the alcohol and water can be mixed and washed, or the alcohol and water can be washed alternately.

[0036] Specifically, during the suction filtration, the filter cake is washed with alcohol and water in the following manner: after the suction filtration of the salt solution is completed, the negative pressure state is maintained, alcohol and water are added for washing, each time the washing is performed within 40 s until no filtrate flows out, and the washing is performed 2-6 times.

[0037] Specifically, during the suction filtration, the liquid-solid ratio of the alcohol and / or water to the filter cake is 30-80 mL:1 g.

[0038] In the embodiment, in step S2, in the salt solution, the salt is at least one of halogen salts (F, Cl, I, Br, etc.), sulfate salts, acetate salts, nitrate salts of Na, K, Ca, and after drying, the dried product is further subjected to annealing treatment. The inventors have found in the test that when lithium salt is used for cleaning, no annealing treatment is needed, and the required sodium ion battery single-crystal cathode material can be directly obtained; when non-lithium salt is used for cleaning, the side effects caused by the influence of water on the surface of the cathode material need to be removed by annealing, so that the required sodium ion battery single-crystal cathode material can be obtained.

[0039] Further, the annealing process includes: heat preservation at 300-1000 ℃ (further 600-900 ℃) for 0-15 h (further 1-5 h). If the annealing temperature is too low or the heat preservation time is too short, the alleviation of the above-mentioned side effects is not obvious; if the annealing temperature is too high or the heat preservation time is too long, the long processing time can cause excessive phase change on the surface.

[0040] Specifically, the temperature is increased to 300-1000°C at a rate of 1-5°C / min, and further to 600-900°C.

[0041] In a second aspect, the present application 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 application.

[0042] In a third aspect, the present application provides the use of the above-mentioned sodium-ion battery single-crystal cathode material in a sodium-ion battery.

[0043] Example 1 The present embodiment provides a method for directly recycling waste sodium-ion battery cathode materials to prepare a sodium-ion battery single-crystal cathode material, which comprises the following steps: (1) Take 3 mmol of waste single-crystal material (Na x Ni 1 / 3 Mn 1 / 3 Fe 1 / 3 O2, the molar amount of the waste material is calculated according to the electrode loading), 5.26 mmol of NaOH, and 0.34 mmol of Na2CO3. After mixing, the mixture is transferred into a crucible. In a muffle furnace, the temperature is first increased to 400°C at a rate of 2°C / min and maintained for 4 h. Then, the temperature is increased to 850°C at a rate of 5°C / min and maintained for 12 h. After natural cooling to room temperature, a molten salt sintered product (i.e., a mixture of transition metal oxide cathode material and excess molten salt) is obtained.

[0044] (2) Prepare 100 mL of a 0.2 mol / L NaCl solution, and the solvent is a mixed solvent of water and ethanol (V 水 :V 乙醇 =1:4). The molten salt sintered product is poured into the NaCl solution and washed at room temperature for 2 min, followed by suction filtration. During the suction filtration process, 20 mL of ethanol and 20 mL of water are used for alternating washing for 3 times, respectively. In each washing process, the suction is performed within 40 s until no filtrate flows out. After the suction filtration process, the filter cake is dried in a vacuum oven at 80°C for 30 min. The obtained powder is heated to 750°C at a rate of 5°C / min in a muffle furnace and maintained for 1 h to obtain a repaired material (i.e., a sodium-ion battery single-crystal cathode material with maintained structure and enhanced interface).

[0045] The failure of the waste material is mainly due to structural damage and sodium ion loss, and the transition metal component has no obvious change. After low-temperature sintering in a muffle furnace to remove PVDF and conductive carbon components in the electrode material, the remaining substance exhibits a transition metal oxide phase, similar to the raw materials of the solid phase method or the precursors in the conventional synthesis process. The product obtained by the molten salt technology restores the typical layered oxide structure phase, and some impurity peaks in the XRD spectrum correspond to the residual molten salt after absorption of CO2 in the air to form Na2CO3 component (a) in Figure 1 (b) in the ICP results of the salt solution cleaning and separation, Figure 1 (b) in the ICP results of the salt solution cleaning and separation, Figure 2 The XRD spectrum further confirms that the crystal phase of the final recovered material corresponds to the typical R-3m space group structure, and there is no hydrated phase impurity peak, indicating that the molten salt method can be applied in the recovery and preparation of sodium ion battery oxide cathode materials through the salt solution cleaning strategy. The morphology of the waste material and the regenerated transition metal oxide material is shown in Figure 3 Although the salt solution cleaning can maintain the stability of the crystal structure of the material, under the strong H / Na ion exchange effect, part of the sodium ions will inevitably be removed from the surface of the recovered material, and an inert rock salt phase structure will be formed on the surface within a thickness of 5 nm after annealing treatment, thereby inhibiting the side reaction between the electrolyte and the electrode material during the long cycle process, and being beneficial to the improvement of the cycle life.

[0046] The above-mentioned method for preparing a sodium ion battery using the sodium ion battery single crystal cathode material prepared in the above embodiment also provides a method for preparing a sodium ion battery, which comprises the following steps: The regenerated sodium ion battery single crystal cathode material is mixed with conductive carbon black and polyvinylidene fluoride (PVDF, binder) at a mass ratio of 8:1:1, and N-methyl pyrrolidone (NMP) is added, and the mixture is ground and mixed to form a uniform slurry, which is coated on an aluminum foil current collector as a test electrode. The electrolyte used is 1M NaClO4 / PC:EMC:FEC (V:V:V=50:45:5), and a metal sodium sheet is used as a counter electrode to form a button cell. The battery performance test results are shown in Table 2 and Figure 4 and Figure 6 .

[0047] Test results: the sodium ion battery single crystal cathode material prepared by directly regenerating by the method of Example 1 has a first cycle discharge specific capacity (0.1C= 13 mA / g) of 54.7mAh / g in the voltage range of 2.0~4.1V, which recovers to 116.5mAh / g, showing a typical O3-type oxide cathode material charge-discharge curve, and the capacity retention rate is 82.5% after 200 cycles at 1C.Figure 4 ).

[0048] Example 2 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.

[0049] 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 ).

[0050] Example 3 The only difference from Example 2 is that the doping amount of W is 0.2% of the molar amount of the waste material.

[0051] Example 4 Compared with Example 2, the only difference is that the doping amount of W is 2% of the molar amount of waste material.

[0052] Example 5 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).

[0053] like Figure 7 As shown, after molten salt treatment, polycrystalline materials also exhibit a trend towards monocrystalline growth.

[0054] Example 6 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.

[0055] Example 7 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 material, and the amount of B doping is 2% of the molar amount of waste material. The other processes are completely the same.

[0056] Example 8 Compared with Example 5, the only difference is that zirconium acetate is introduced as the Zr source during the mixing of the molten salt and the waste materials, and the doping amount of Zr is 2% of the molar amount of the waste materials, and the other processes are completely consistent.

[0057] Example 9 Compared with Example 5, the only difference is that magnesium acetate is introduced as the Mg source during the mixing of the molten salt and the waste materials, and the doping amount of Mg is 2% of the molar amount of the waste materials, and the other processes are completely consistent.

[0058] Example 10 Compared with Example 5, the only difference is that calcium carbonate is introduced as the Ca source during the mixing of the molten salt and the waste materials, and the doping amount of Ca is 2% of the molar amount of the waste materials, and the other processes are completely consistent.

[0059] The first week charge-discharge curves of the repaired materials prepared in Examples 5, 6, 8 and 10 are shown in FIG. 1, which are typical O3-type charge-discharge curves, and the first week reversible capacity is repaired to about 120 mAh / g at 0.1C; further combined with Table 2, it can be seen that the introduction of doping elements such as Al, Zr and Ca at a doping amount of 2% not only optimizes the reversibility of the material, but also significantly improves the long cycle stability. Figure 8

[0060] Example 11 Compared with Example 1, the only difference is that the molten salt is a binary molten salt system composed of 1.71 mmol of active Na2CO3 and 2.57 mmol of inactive NaCl. First, it is heated to 700°C at a rate of 5°C / min in a muffle furnace and kept for 4h, and then heated to 850°C at a rate of 3°C / min and kept for 12h, and the other processes are completely the same.

[0061] Example 12 Compared with Example 1, the only difference is that the 850°C holding time is 5h.

[0062] Example 13 Compared with Example 1, the only difference is that the 850°C holding time is 18h.

[0063] Example 14 Compared with Example 1, the only difference is that the molten salt is replaced by 3 mmol of sodium carbonate, and the other processes are completely consistent.

[0064] ​When sodium source is single sodium carbonate, the material in non-molten salt system shows excessive sodium intercalation behavior, resulting in poor material performance. Since the state of waste materials is unknown in actual working conditions, molten salt method can realize material performance repair under the condition of excessive sodium supplement, which is helpful for batch regeneration treatment of waste materials, and has unique advantages in battery life cycle closed loop. Under the same treatment conditions, the sodium content of the material in Example 14 is still close to 1 after salt solution cleaning and annealing treatment, indicating that the actual sodium intercalation amount exceeds this value (Table 1). TEM also found that the outermost structure of the material was decomposed due to excessive sodium intercalation Figure 9 ), accompanied by near-surface structural distortion, resulting in relatively poor material cycle reversibility, but still significantly better than waste materials and Comparative Example 1.

[0065] Example 15 Compared with Example 1, the only difference is that the waste material is replaced with 1 mmol (calculated by metal element content) of nickel oxide, manganese oxide and iron oxide respectively, which meet the stoichiometric ratio of NaNi 1 / 3 Mn 1 / 3 Fe 1 / 3 O2.

[0066] Comparative Example 1 Compared with Example 1, the only difference is that during the cleaning process of the molten salt sintered product, a mixed solvent of water and ethanol with a volume ratio of 1:4 is used instead of NaCl solution cleaning, and other processes are completely consistent.

[0067] During the cleaning process of the molten salt sintered product, without using salt solution, the transition metal oxide positive electrode material is prone to strong H / Na ion exchange reaction with water, resulting in re-dedoping of the dense micron particles. The morphology of the material is shown in Figure 10 , the interlayer of the water-washed material is obviously cracked, indicating that the repaired material is damaged again. The typical charge-discharge curve is shown in Figure 11 , the 0.1C discharge specific capacity is even lower than that of the waste material, corresponding to the charge-discharge curve characteristics after sodium loss in water.

[0068] Comparative Example 2 Compared with Example 1, the only difference is that after the molten salt sintered product is cleaned and separated, no annealing treatment is performed, and other processes are completely consistent.

[0069] Table 1 Main element content of the repaired material prepared in Example 14 of the present application

[0070] Table 2 Partial electrochemical performance results of the single crystal positive electrode material prepared by the present application and the waste material used

[0071] (“-” in Table 2 means not tested) As can be seen from Table 2, the sodium ion battery single crystal cathode material prepared by the embodiment of the application has significantly improved electrochemical performance (specific capacity and cycle performance) compared with the waste material, and the method of the application can realize unified regeneration of different state waste materials or direct sodium insertion of precursor materials.

[0072] As can be seen from Examples 1-10, the introduction of doping elements is beneficial to improving the cycle stability of the cathode material.

[0073] As can be seen from Example 11, the method of the application is also applicable to other molten salt systems.

[0074] As can be seen from Examples 1 and 12-13, during the molten salt sintering process, too short or too long sintering time is not conducive to further improving the electrochemical performance of the single crystal cathode material.

[0075] As can be seen from Examples 1 and 14, self-saturation sodium supplement is a feature of the molten salt technology, which is beneficial to material repair under actual working conditions where the material states are inconsistent. If the sodium is supplemented excessively in a non-molten salt system (i.e. a single component), the material structure is easily damaged due to excessive sodium insertion, resulting in poor performance. Therefore, the development of molten salt preparation technology has a positive significance for batch regeneration of waste sodium ion battery materials.

[0076] As can be seen from Examples 1 and Comparative Example 1, using a mixed solvent of water and ethanol for cleaning cannot effectively solve the problem of water sensitivity of the regenerated material, and in the absence of effective inhibition of H / Na ion exchange, the repaired material is easily damaged again during the cleaning of residual molten salt, resulting in repair failure.

[0077] As can be seen from Examples 1 and Comparative Example 2, although the salt solution in Example 1 inhibits H / Na ion exchange, there are still a small amount of H protons intercalated into the material, and without annealing treatment, the cathode material has almost no performance.

[0078] Compared with the prior art, the application has the following advantages: (1) The process design based on the molten salt technology of the application uses a salt solution to inhibit the activity of water in the cleaning stage, thereby realizing the stability of the structure of the sodium ion battery cathode material during the cleaning of residual molten salt.

[0079] (2) The post-treatment process of the molten salt sodium insertion stage and the cleaning stage of the application realizes the enhancement of the material interface, which further improves the cycle stability of the material; the annealing process after cleaning is helpful for the reconstruction of the shallow structure of the surface of the cathode material, and the reconstructed inert interface is beneficial to prolonging the cycle life of the material.

[0080] (3) The present application synchronously introduces a doping element in the sodium intercalation stage of the molten salt. On the one hand, the doping element forms a doping layer on the surface, which is beneficial to inhibit the corrosion of the molten salt to the material; on the other hand, the strengthening of the doping element to the surface structure further inhibits the damage of water to the material structure in the cleaning process, which is beneficial to realize the stability of the material.

[0081] The specific embodiments of the application described above do not constitute a limitation on the protection scope of the application. Any various other corresponding changes and modifications made according to the technical concept of the application should be included in the protection scope of the claims of the application.

Claims

1. A method for preparing a single-crystal cathode material for sodium-ion batteries, characterized in that, Includes the following steps: Waste materials and / or precursor materials are mixed with molten salt and sintered to obtain molten salt sintered products; The molten salt sintering product was washed with a salt solution, then filtered and dried to obtain a single-crystal cathode material for sodium-ion batteries; wherein... The salt in the salt solution is at least one of the following: a halogen salt, sulfate, acetate, or nitrate of Li, Na, K, or Ca. The solvent used is a mixture of alcohol and water, with water accounting for 10% to 50% by volume. The salt concentration in the salt solution is 10% to 90% of the saturation concentration.

2. The method for preparing the sodium-ion battery single-crystal cathode material according to claim 1, characterized in that, The molten salt comprises: an active component and optionally an inactive component; wherein... The active component includes at least one of Na2CO3, NaHCO3, NaOH, NaNO3, and Na2O; The inactive component includes at least one of the halide salts and sulfates of Li, Na, K, and Ca.

3. The method for preparing the sodium-ion battery single-crystal cathode material according to claim 2, characterized in that, 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 an inactive component; wherein the active component comprises at least one of Na₂CO₃, NaHCO₃, NaOH, NaNO₃, and Na₂O; the inactive component comprises at least one of a halide salt or sulfate of Li, Na, K, and Ca; and / or, 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.

4. The method for preparing the sodium-ion battery single-crystal cathode material according to claim 1, characterized in that, The step of mixing and sintering waste materials and / or precursor materials with molten salt includes: mixing and sintering waste materials and / or precursor materials with molten salt and dopant; wherein... The dopant element corresponding to the dopant is at least one selected from W, Al, Mg, B, Zn, Zr, and Ca; and / or, The dopant element corresponding to the dopant accounts for 0% to 5% of the theoretical molar amount of the waste material and / or precursor material final product.

5. The method for preparing the sodium-ion battery single-crystal cathode material according to claim 1, characterized in that, 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 200~800℃, and the first time is 2~8h; The second temperature is higher than the first temperature, the second temperature is 600~1200℃, and the second time is 1~20h; 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 allowed to cool naturally to room temperature.

6. The method for preparing the sodium-ion battery single-crystal cathode material according to claim 1, characterized in that, The ratio of the molten salt sintering product to the salt solution is 1g:(50~300)mL; The cleaning temperature is room temperature, and the cleaning time is 30~600s.

7. The method for preparing the sodium-ion battery single-crystal cathode material according to claim 1, characterized in that, During the filtration process, alcohol and water are used for rinsing. Each rinsing is performed within 40 seconds until no filtrate flows out. The number of rinsing times is 2 to 6.

8. The method for preparing the sodium-ion battery single-crystal cathode material according to claim 1, characterized in that, The salt in the salt solution is at least one selected from the group consisting of halogen salts, sulfates, acetates, and nitrates of Na, K, and Ca. The drying process further includes annealing the dried product. The annealing process includes: holding at 300~1000℃ for 0~15h, with a heating rate of 1~5℃ / min.

9. A single-crystal cathode material for sodium-ion batteries, 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 according to any one of claims 1 to 8.

10. The application of the sodium-ion battery single-crystal cathode material as described in claim 9 in sodium-ion batteries.

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