Preparation method of reduced manganese titanium-based phosphate sodium ion battery positive electrode material

Abstract

The present invention discloses a method for preparing a reduced manganese-titanium-based phosphate sodium-ion battery cathode material, comprising the following steps: S1, preparing a first precursor solution; S2, preparing a second precursor solution; S3, preparing a third precursor solution; S4, preparing an ionic precursor solution; S5, spray drying and granulation; and S6, high-temperature sintering, to ultimately obtain the reduced manganese-titanium-based phosphate material. The method for preparing the reduced manganese-titanium-based phosphate sodium-ion battery cathode material of the present invention has the characteristics of strong process operability, excellent electrochemical performance, and high product purity.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium ion batteries, and in particular to a method for preparing a reduced manganese-titanium-based phosphate sodium ion battery cathode material. Background Art

[0002] The performance of cathode materials plays a crucial role in sodium-ion battery systems. A suitable cathode material must possess abundant resources, low cost, excellent rate capability, high structural stability, and high thermodynamic stability. However, the performance of currently available sodium-ion battery cathode materials varies widely, with few meeting all of these requirements simultaneously.

[0003] NASICON-type manganese-titanium-phosphate (MTP) materials, as polyanionic materials for sodium-ion batteries, are characterized by high structural stability, abundant resources, non-toxicity, a high average voltage platform, high capacity utilization, and high energy density in sodium-ion battery systems. These materials are highly valuable cathode materials for low-end power applications. However, the valence of titanium in Mn-titanium-phosphate is tetravalent, resulting in a sodium-poor state (Na₃MnTi(PO₄)₃) and a low charge capacity of ≤120 mAh / g. Furthermore, tetravalent titanium has a strong dissociation ability with water, easily forming titanium dioxide, resulting in uneven distribution of manganese and titanium elements. This affects the material's ultimate phase purity, crystallinity, ionic conductivity, and capacity and rate performance.

[0004] Currently, the synthesis of manganese-titanium-based phosphate materials primarily relies on the sol-gel method; this involves uniformly mixing a sodium source, a transition metal M source, a phosphorus source, and a carbon source, then heating to evaporate the water and form a gel. The gel is then evaporated, ground, and sintered to obtain the final product. However, this process hydrolyzes titanium during the gelation process, forming a titanium dioxide precipitate. This results in an uneven distribution of manganese and titanium elements in the final product. This problem impairs the material's ion diffusion kinetics, poor rate performance, and a significant decrease in capacity in the high-voltage region. Furthermore, the material prepared by this method is sodium-poor (Na3MnTi(PO4)3), resulting in low charge capacity and a low energy density for sodium-ion full-battery systems. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for preparing a reduced manganese titanium-based phosphate sodium ion battery positive electrode material, which has the characteristics of strong process operability, excellent electrochemical performance and high product purity.

[0006] The present invention can be achieved through the following technical solutions:

[0007] The present invention discloses a method for preparing a reduced manganese-titanium-based phosphate sodium ion battery cathode material, comprising the following steps:

[0008] S1. Preparation of a first precursor solution: Under atmosphere protection conditions, a water-soluble titanium source and a reducing agent are mixed and dissolved in water to form a first precursor solution;

[0009] S2. Preparation of a second precursor solution: mixing a water-soluble manganese source, a water-soluble phosphorus source, and an acidic complexing agent, and dissolving them in water to form a second precursor solution;

[0010] S3. Preparation of a third precursor solution: Under temperature control and atmosphere protection conditions, slowly dropwise add the second precursor solution into the first precursor solution and mix well to obtain a third precursor solution;

[0011] S4. Preparation of an ionic precursor solution: adding a weakly acidic sodium salt and a carbon source to the third precursor solution and dissolving them to form a uniform ionic precursor solution;

[0012] S5. Spray drying and granulation: Under atmosphere protection conditions, the ionic precursor solution is spray dried to obtain a precursor powder;

[0013] S6. High temperature sintering: calcining the precursor powder at high temperature in a protective atmosphere and then cooling it naturally to obtain the reduced manganese titanium based phosphate material.

[0014] Furthermore, the chemical formula of the reduced manganese titanium-based phosphate sodium ion battery positive electrode material is Na4MnTi(PO4)3; wherein the valence state of the Mn element is +2, and the valence state of the Ti element is +3; the Na element occupies the alkali metal site in the structure, and its occupancy rate is 100%; the unit cell parameters are 8.85Å≤a≤9.0Å, 21.9Å≤c≤22.5Å, 1469.2Å 3 ≤V≤1475.3Å 3 .

[0015] The cathode material structure of the present invention effectively avoids the following shortcomings of the prior art structure: For example, in the manganese titanium-based phosphate material (Na3MnTi(PO4)3), Ti is tetravalent. Under the action of charge conservation, the structure exists in the form of sodium-poor state and can only accommodate 3 Na + , resulting in a low charge capacity; for example, tetravalent titanium ions can easily deprive water molecules of oxygen and then hydrolyze to form insoluble precipitates. In material synthesis, the formation of titanium-based precipitates will cause uneven distribution of manganese and titanium elements, leading to phase separation of the material during sintering; for example, in the structure of manganese-titanium-based phosphates, Mn and Ti ions occupy the same transition metal sites and have comparable binding energies. During the synthesis process, the uneven mixing of the two elements can easily lead to disordered distribution of elements at the transition metal sites after structure formation, which in turn causes changes in sodium ion occupancy and prevents the material from effectively performing its properties.

[0016] Furthermore, in step S1, the water-soluble titanium source includes one or more of bis(trifluoromethanesulfonic acid) titanocene, titanium isopropoxide, titanium tetrachloride, titanyl sulfate, tetrabutyl titanate, dihydroxybis(ammonium lactate) titanium, and titanium sulfate; the reducing agent includes one or more of sodium sulfite, sodium bisulfite, sodium borohydride, sodium thiosulfate, water and hydrazine, ascorbic acid, lithium aluminum hydride, potassium borohydride, and stannous chloride.

[0017] Furthermore, in step S1, the molar ratio of the water-soluble titanium source to the reducing agent is 1:0.5 to 2. The amount of reducing agent added must ensure that the tetravalent titanium ions are completely reduced to trivalent to avoid co-precipitation of residual tetravalent titanium ions with the subsequent phosphorus source.

[0018] Furthermore, in step S1, the protective atmosphere is one or more of nitrogen, argon, nitrogen-hydrogen, and argon-hydrogen, in order to prevent the reduced trivalent titanium ions from being oxidized to tetravalent titanium ions upon contact with oxygen, thereby affecting the valence of titanium in the final material.

[0019] Furthermore, in step S2, the water-soluble manganese source includes one or more of manganese chloride, manganese acetate, manganese nitrate, and manganese sulfate; the water-soluble phosphorus source includes one or more of ammonium dihydrogen phosphate, phosphoric acid, sodium monohydrogen phosphate, sodium dihydrogen phosphate, and sodium phosphate; the acidic complexing agent includes one or more of citric acid, ascorbic acid, polyacrylic acid, oxalic acid, malic acid, and tartaric acid; and the molar ratio of the water-soluble manganese source to the acidic complexing agent is 1:0.1-5. The acidic complexing agent dissolves in water and releases protons H + and free functional groups, protons H + It can destroy the dissociation equilibrium of manganese hydrogen phosphate precipitation, and the free functional groups can complex divalent manganese ions. The interaction between the two inhibits the formation of manganese hydrogen phosphate precipitation.

[0020] Furthermore, in step S4, the weak acid sodium salt includes one or more of sodium bicarbonate, sodium formate, sodium acetate, sodium sulfate, sodium nitrate, and disodium edetate; the carbon source includes one or more of glucose, maltose, glucose, water-soluble starch, sucrose, citric acid, and lactose; the amount of carbon source added is 1 to 5 times the molar amount of the manganese source. When the amount of carbon source added is too little, trivalent titanium is easily oxidized during the sintering process. When the amount of carbon source added is too much, a large amount of residual carbon will affect the specific capacity of the material.

[0021] Furthermore, in step S3, the temperature range is 0~50°C. When the temperature is too high, the solubility equilibrium between divalent manganese ions and phosphate ions is easily broken, thereby generating manganese hydrogen phosphate precipitate; in step S5, the spray drying outlet temperature is ≥100°C, the moisture content of the precursor powder is ≤2%, and the process is carried out in a protective atmosphere to prevent trivalent titanium from coming into contact with oxygen and moisture and being oxidized to tetravalent titanium.

[0022] Furthermore, in step S6, the protective atmosphere includes H2 and / or CO, and the atmosphere has strong reducing properties, which can inhibit the oxidation of trivalent titanium ions during the sintering process; the sintering conditions are: sintering temperature is 600~750°C, holding time is 8~15H, and the O2 content of the sintering atmosphere is ≤10ppm, so as to ensure sufficient crystal growth of reduced manganese titanium phosphate.

[0023] The present invention provides a method for preparing a reduced manganese-titanium-based phosphate sodium ion battery cathode material, which has the following beneficial effects:

[0024] First, the process is highly operable. The reducing agent can reduce the tetravalent titanium ions in the solution to trivalent ones. The dissociation effect of trivalent titanium ions with water is weak, which can effectively avoid the formation of titanium-based precipitates, thereby ensuring the uniform mixing between manganese and titanium ions, which plays a key role in the nucleation, crystal growth and electrochemical performance of the material in the subsequent sintering process.

[0025] Second, the electrochemical performance is excellent. Trivalent titanium ions are not easily oxidized to tetravalent titanium under nitrogen protection conditions. The excess sodium element, reducing atmosphere and the reducing nature of the pyrolytic carbon source during the sintering process can effectively avoid the oxidation of trivalent titanium, and finally form a sodium-rich phase Na4MnTi(PO4)3 material. The increase in the sodium content in the structure can effectively improve the charging capacity of the material.

[0026] Third, the product boasts high purity. The sodium, transition metal, phosphorus, and carbon sources are all water-soluble compounds, and the solution pH is controlled using a weakly acidic complexing agent and a weakly acidic sodium salt. This effectively prevents chemical reactions between the substances and completely suppresses precipitation reactions during the entire material precursor synthesis process, creating favorable conditions for uniform mixing between the elements and enabling the controllable preparation of high-purity, high-performance materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Attachment Figure 1 1 and 2 are discharge curves of Example 1 and Comparative Example 1. DETAILED DESCRIPTION

[0028] In order to enable those skilled in the art to better understand the technical solution of the present invention, the product of the present invention is further described in detail below with reference to the embodiments and drawings.

[0029] The present invention discloses a method for preparing a reduced manganese-titanium-based phosphate sodium ion battery cathode material, comprising the following steps:

[0030] S1. Preparation of a first precursor solution: Under atmosphere protection conditions, a water-soluble titanium source and a reducing agent are mixed and dissolved in water to form a first precursor solution;

[0031] S2. Preparation of a second precursor solution: mixing a water-soluble manganese source, a water-soluble phosphorus source, and an acidic complexing agent, and dissolving them in water to form a second precursor solution;

[0032] S3. Preparation of a third precursor solution: Under temperature control and atmosphere protection conditions, slowly dropwise add the second precursor solution into the first precursor solution and mix well to obtain a third precursor solution;

[0033] S4. Preparation of an ionic precursor solution: adding a weakly acidic sodium salt and a carbon source to the third precursor solution and dissolving them to form a uniform ionic precursor solution;

[0034] S5. Spray drying and granulation: Under atmosphere protection conditions, the ionic precursor solution is spray dried to obtain a precursor powder;

[0035] S6. High temperature sintering: calcining the precursor powder at high temperature in a protective atmosphere and then cooling it naturally to obtain the reduced manganese titanium based phosphate material.

[0036] Furthermore, the chemical formula of the reduced manganese titanium-based phosphate sodium ion battery positive electrode material is Na4MnTi(PO4)3; wherein the valence state of the Mn element is +2, and the valence state of the Ti element is +3; the Na element occupies the alkali metal site in the structure, and its occupancy rate is 100%; the unit cell parameters are 8.85Å≤a≤9.0Å, 21.9Å≤c≤22.5Å, 1469.2Å 3 ≤V≤1475.3Å 3 .

[0037] Furthermore, in step S1, the water-soluble titanium source includes one or more of bis(trifluoromethanesulfonic acid) titanocene, titanium isopropoxide, titanium tetrachloride, titanyl sulfate, tetrabutyl titanate, dihydroxybis(ammonium lactate) titanium, and titanium sulfate; the reducing agent includes one or more of sodium sulfite, sodium bisulfite, sodium borohydride, sodium thiosulfate, water and hydrazine, ascorbic acid, lithium aluminum hydride, potassium borohydride, and stannous chloride.

[0038] Furthermore, in step S1, the molar ratio of the water-soluble titanium source to the reducing agent is 1:0.5-2.

[0039] Furthermore, in step S1 , the protective atmosphere is one or more of nitrogen, argon, nitrogen-hydrogen, and argon-hydrogen.

[0040] Furthermore, in step S2, the water-soluble manganese source includes one or more of manganese chloride, manganese acetate, manganese nitrate, and manganese sulfate; the water-soluble phosphorus source includes one or more of ammonium dihydrogen phosphate, phosphoric acid, sodium monohydrogen phosphate, sodium dihydrogen phosphate, and sodium phosphate; the acidic complexing agent includes one or more of citric acid, ascorbic acid, polyacrylic acid, oxalic acid, malic acid, and tartaric acid; and the molar ratio of the water-soluble manganese source to the acidic complexing agent is 1:0.1-5.

[0041] Furthermore, in step S3, the temperature range is 0-50°C.

[0042] Furthermore, in step S4, the weak acid sodium salt includes one or more of sodium bicarbonate, sodium formate, sodium acetate, sodium sulfate, sodium nitrate, and disodium edetate; the carbon source includes one or more of glucose, maltose, glucose, water-soluble starch, sucrose, citric acid, and lactose; and the amount of carbon source added is 1 to 5 times the molar amount of the manganese source.

[0043] Furthermore, in step S5, the spray drying outlet air temperature is ≥100°C, and the moisture content of the precursor powder is ≤2%.

[0044] Furthermore, in step S6, the protective atmosphere includes H2 and / or CO; the sintering conditions are: sintering temperature is 600~750°C, holding time is 8~15H, and O2 content in the sintering atmosphere is ≤10ppm.

[0045] Manganese-titanium-based phosphate materials contain tetravalent titanium, and their application is hindered by the problem that tetravalent titanium is easily hydrolyzed during the synthesis process. The present invention provides a reduced manganese-titanium-based phosphate sodium-ion battery cathode material and its preparation method, which improves the material's charge capacity and demonstrates the feasibility of mass production. The preparation process involves mixing a water-soluble titanium source and a strong reducing agent under an inert atmosphere and dissolving them in water to form a dark brown transparent solution (Solution A). Simultaneously, a water-soluble manganese source, a water-soluble phosphorus source, and an acidic complexing agent are mixed and dissolved in water to form a slightly reddish transparent solution (Solution B). Then, while controlling the temperature within a certain range, Solution B is slowly added dropwise to Solution A under an inert atmosphere and mixed until uniform (Solution C). A weakly acidic sodium salt and a carbon source are then added to Solution C and dissolved to form a uniform ionic precursor solution. Finally, the precursor solution is spray-dried (under an inert atmosphere) to obtain a precursor powder, which is then calcined at high temperature in a reducing atmosphere and allowed to cool naturally to obtain the reduced manganese-titanium-based phosphate material. This process relies on a strong reducing agent to reduce tetravalent titanium ions to trivalent, and nitrogen atmosphere protection ensures that the titanium ions remain trivalent throughout the process, ultimately producing a reduced, high-capacity, sodium-rich material (Na4MnTi(PO4)3). Furthermore, the weak dissociation of trivalent titanium ions with water effectively prevents the formation of titanium dioxide precipitation, thereby improving the mixing uniformity of manganese and titanium ions, ultimately achieving the preparation of large-grained, highly crystalline reduced manganese-titanium-based materials.

[0046] Application Example 1 Synthesis of Reduced Na4MnTi(PO4)3 and Its Electrochemical Performance

[0047] Step 1: Under nitrogen protection, water-soluble dihydroxybis(ammonium lactate) titanium and sodium sulfite strong reducing agent are dissolved in water at a molar ratio of 1:0.2 to form a dark brown transparent solution (Solution A). Due to the protective effect of nitrogen, the reduced trivalent titanium is stably present in the solution;

[0048] Step 2: Dissolve water-soluble manganese acetate, water-soluble phosphoric acid, and citric acid in water at a molar ratio of 1:3 to form a reddish transparent solution (Solution B). The acidic complexing agent citric acid acidifies the solution and chelates manganese ions, effectively preventing the formation of manganese hydrogen phosphate precipitates.

[0049] Step 3: Control the temperature at 20°C and, under nitrogen protection, slowly add Solution B dropwise into Solution A and mix thoroughly (Solution C). This step controls the temperature and eliminates the interference of dissolved oxygen, effectively avoiding the formation of manganese and titanium-based precipitates.

[0050] Step 4: Slowly add sodium acetate and glucose to solution C to maintain a balanced stoichiometric ratio and dissolve to form a uniform ionic precursor solution;

[0051] Step 5: The precursor solution is spray-dried under nitrogen protection conditions, with the inlet air temperature controlled at 300°C and the outlet air temperature controlled at 100°C to obtain a dry precursor powder. At the same time, the nitrogen protection can prevent the oxidation of trivalent titanium ions;

[0052] Step 6: Calcine the precursor powder in a nitrogen atmosphere at 700°C for 10 hours. After natural cooling, the reduced Na4MnTi(PO4)3 material is obtained. The material must be stored in a dry or inert atmosphere.

[0053] Under dry conditions, reduced Na4MnTi(PO4)3, AB, and PVDF were mixed in a mass ratio of 9.2:0.4:0.4 to prepare a slurry. A 150um four-sided film preparation device was then used to coat the black slurry onto aluminum foil, and the film was dried in a vacuum drying oven at 100°C for 2 hours. A sheet puncher was used to punch the electrode film into a disc with a radius of 0.6mm. Sodium metal was used as the counter electrode, 1mol / L NaClO4EC+DEC (1:1vol%)+5%FEC was used as the electrolyte, and the separator was a PP / PE / PP three-layer separator. CR2016 button cells were assembled in a glove box.

[0054] The button cell was subjected to constant current charge and discharge tests and related performance tests. The results are shown in Table 1. The current density was 0.1C (1C=170mAh / g). Figure 1 The results show that in the voltage range of 1.5-4.3V, the first-week charging capacity of the electrode is as high as 160mAh / g, reaching 94.1% of its theoretical capacity, which is much higher than 82mAh / g in Comparative Example 1. The main reason is that the sodium ions in the reduced Na4MnTi(PO4)3 material structure are fully occupied, and the charging process is accompanied by the Mn 2+ / Mn 3+ / Mn 4+ and Ti 3+ / Ti 4+ In addition, during the charging process of the electrode, there are three obvious oxidation platforms at 2.1V, 3.6V and 4.04V, with an average voltage of 3.4V, and the discharge process is exactly the same, indicating that Mn 2+ and Ti 4+ The oxidation reaction exhibits regular and orderly changes, indirectly demonstrating the orderly dispersion of elements within the material structure. This is related to the reduced dissociation of reduced trivalent titanium ions, which in turn improves the uniformity of the dispersion of manganese and titanium ions. Furthermore, rate performance testing of the electrode showed a capacity retention rate of 91% at a 5C rate, demonstrating that the ordered structure promotes the intercalation and deintercalation of sodium ions.

[0055] Application Example 2 Synthesis of Reduced Na4MnTi(PO4)3 and Its Electrochemical Performance

[0056] Step 1: Under nitrogen protection, water-soluble titanyl sulfate and sodium thiosulfate, a strong reducing agent, are dissolved in water at a molar ratio of 1:0.5 to form a dark brown transparent solution (Solution A). Due to the protective effect of nitrogen, the reduced trivalent titanium is stabilized in the solution;

[0057] Step 2: Dissolve water-soluble manganese sulfate, water-soluble sodium dihydrogen phosphate, and oxalic acid in water at a molar ratio of 1:3 to form a reddish transparent solution (Solution B). Due to the acidification of the solution by the acidic complexing agent oxalic acid and its chelation of manganese ions, the formation of manganese hydrogen phosphate precipitates is effectively avoided.

[0058] Step 3: Control the temperature at 25°C and, under nitrogen protection, slowly add Solution B dropwise into Solution A and mix thoroughly (Solution C). This step controls the temperature and eliminates the interference of dissolved oxygen, effectively avoiding the formation of manganese and titanium-based precipitates.

[0059] Step 4: Slowly add sodium formate and sucrose to solution C to maintain a balanced stoichiometric ratio and dissolve to form a uniform ionic precursor solution;

[0060] Step 5: The precursor solution is spray-dried under nitrogen protection conditions, with the inlet air temperature controlled at 300°C and the outlet air temperature controlled at 100°C to obtain a dry precursor powder. At the same time, the nitrogen protection can prevent the oxidation of trivalent titanium ions;

[0061] Step 6: Calcine the precursor powder in a nitrogen atmosphere at a temperature of 750°C for 10 hours. After natural cooling, the reduced Na4MnTi(PO4)3 material is obtained. The material must be stored in a dry or inert atmosphere.

[0062] Under dry conditions, reduced Na4MnTi(PO4)3, AB, and PVDF were mixed in a mass ratio of 9.2:0.4:0.4 to prepare a slurry. A 150um four-sided film preparation device was then used to coat the black slurry onto aluminum foil, and the film was dried in a vacuum drying oven at 100°C for 2 hours. A sheet puncher was used to punch the electrode film into a disc with a radius of 0.6mm. Sodium metal was used as the counter electrode, 1mol / L NaClO4EC+DEC (1:1vol%)+5%FEC was used as the electrolyte, and the separator was a PP / PE / PP three-layer separator. CR2016 button cells were assembled in a glove box.

[0063] The button cell was subjected to constant current charge and discharge tests and related performance tests, and the results are shown in Table 1. The current density was 0.1C (1C=170mAh / g). The electrode had a first-week charging capacity of up to 164mAh / g in the voltage range of 1.5-4.3V, reaching 96.5% of its theoretical capacity, corresponding to the presence of a 3-electron oxidation and desodium reaction in the structure, indicating that titanium in the structure exists in a reduced trivalent form. In addition, the average potential of the electrode was 3.39V, and the polarization was small, indicating that the sodium ion diffusion energy barrier in the structure was low, indirectly proving the orderliness of the distribution of manganese and titanium ions in the structure. Therefore, at a rate of 5C, the electrode capacity retention rate was as high as 92.4%, showing excellent rate performance.

[0064] Comparative Example 1 Synthesis and Electrochemical Performance of Na3MnTi(PO4)3

[0065] Step 1: Dissolve water-soluble dihydroxybis(ammonium lactate) titanium in water. After standing naturally, a white flocculent precipitate is produced in the solution, which is related to the hydrolysis of tetravalent titanium ions to form titanium-based precipitates (Solution A).

[0066] Step 2: Dissolve water-soluble manganese acetate, water-soluble phosphoric acid, and citric acid in water at a molar ratio of 1:3 to form a reddish transparent solution (Solution B). The acidic complexing agent acidifies the solution and chelates manganese ions, effectively preventing the formation of manganese hydrogen phosphate precipitates.

[0067] Step 3: At 20°C, slowly add solution B into solution A. Due to the presence of titanium-based precipitates, an emulsion (solution C) is formed after mixing.

[0068] Step 4: Slowly add sodium acetate and glucose to solution C to maintain the stoichiometric balance and dissolve to form a turbid precursor slurry;

[0069] Step 5: spray drying the precursor slurry with the air inlet temperature controlled at 300°C and the air outlet temperature controlled at 100°C to obtain a dry precursor powder;

[0070] Step 6: calcine the precursor powder in a nitrogen atmosphere at 700°C for 10 hours, and then cool it down naturally to obtain Na3MnTi(PO4)3 material.

[0071] Na3MnTi(PO4)3, AB, and PVDF were mixed in a mass ratio of 9.2:0.4:0.4 to prepare a slurry. The black slurry was then coated onto aluminum foil using a 150 μm four-sided film preparation device. The film was then dried in a vacuum drying oven at 100°C for 2 hours. A sheet puncher was used to punch the electrode film into discs with a radius of 0.6 mm. Sodium metal was used as the counter electrode, 1 mol / L NaClO4EC+DEC (1:1 vol%)+5%FEC was used as the electrolyte, and a PP / PE / PP three-layer separator was used as the separator. CR2016 button cells were assembled in a glove box.

[0072] The button cell was subjected to constant current charge and discharge tests and related performance tests. The results are shown in Table 1. The current density was 0.1C (1C=170mAh / g). Figure 1 The results show that within the voltage range of 1.5-4.3V, the first cycle charge capacity of the electrode is only 82mAh / g, which is much lower than that of Application Example 1. This indicates that the number of sodium ions released from the material structure is small, which indirectly proves that the material has a small number of oxidized charges, which is related to the tetravalent titanium ions in the structure. In addition, the electrode only has a large polarization platform at 4.15V, and Mn 2+ / Mn 3+ The oxidation platform at 3.6V for the electrode pair almost completely disappears, while the discharge process corresponds to a large voltage hysteresis in the curve, indicating that the resistance of the sodium ion deintercalation process in the structure is too large. This is mainly due to the presence of titanium-based precipitates during the material synthesis process, which leads to the uneven distribution of manganese and titanium ions in the structure, resulting in sodium ion transition polarization. At the same time, the rate performance test results of the electrode show that at a rate of 5C, its capacity retention rate is only 75.6%. This is due to the uneven distribution of ions in the material and the uneven polarization during the sodium deintercalation process, resulting in excessive polarization of the curve.

[0073] Comparative Example 2 Synthesis and Electrochemical Performance of Na3MnTi(PO4)3

[0074] Step 1: Under nitrogen protection, water-soluble titanyl sulfate and sodium thiosulfate, a strong reducing agent, are dissolved in water at a molar ratio of 1:0.5 to form a dark brown transparent solution (Solution A). Due to the protective effect of nitrogen, the reduced trivalent titanium is stabilized in the solution;

[0075] Step 2: Dissolve water-soluble manganese sulfate and water-soluble sodium dihydrogen phosphate in water at a molar ratio of 1:3 to form a reddish transparent solution. Since no acidic compound is added, a white manganese-based precipitate (Solution B) is produced in the solution.

[0076] Step 3: Control the temperature at 25°C and, under nitrogen, slowly add Solution B dropwise into Solution A to form a uniform suspension (Solution C).

[0077] Step 4: Slowly add sodium formate and sucrose to Solution C to maintain a balanced stoichiometric ratio and form a uniform emulsion after dissolution;

[0078] Step 5: The emulsion is spray-dried under nitrogen protection conditions with the inlet air temperature controlled at 300°C and the outlet air temperature controlled at 100°C to obtain a dry precursor powder. The nitrogen protection can also prevent the oxidation of trivalent titanium ions.

[0079] Step 6: Calcine the precursor powder in a nitrogen atmosphere at a temperature of 750°C for 10 hours. After natural cooling, the reduced Na4MnTi(PO4)3 material is obtained. The material must be stored in a dry or inert atmosphere.

[0080] Under dry conditions, reduced Na4MnTi(PO4)3, AB, and PVDF were mixed in a mass ratio of 9.2:0.4:0.4 to prepare a slurry. A 150um four-sided film preparation device was then used to coat the black slurry onto aluminum foil, and the film was dried in a vacuum drying oven at 100°C for 2 hours. A sheet puncher was used to punch the electrode film into a disc with a radius of 0.6mm. Sodium metal was used as the counter electrode, 1mol / L NaClO4EC+DEC (1:1vol%)+5%FEC was used as the electrolyte, and the separator was a PP / PE / PP three-layer separator. CR2016 button cells were assembled in a glove box.

[0081] The above-mentioned button cell was subjected to constant current charge and discharge tests and related performance tests. The results are shown in Table 1, and the current density is 0.1C (1C=170mAh / g). The electrode has a first-week charging capacity of only 130mAh / g in the voltage range of 1.5-4.3V, which is much lower than the 164mAh / g in Application Example 2. At the same time, the average potential of the electrode is 2.86V, which is lower than the 3.39V in Application Example 2. This is related to the presence of manganese-based precipitates in the material synthesis, indicating that the presence of manganese-based precipitates affects the mixing uniformity of manganese and titanium ions. Although titanium exists in a trivalent form in the material, the disorder of the transition metal ions in the structure increases the sodium ion diffusion energy barrier, resulting in severe polarization and reduced capacity utilization. Therefore, at a rate of 5C, the electrode capacity retention rate is only 86.1%, and the rate performance is poor.

[0082] Table 1 Performance test results

[0083]

[0084] The above embodiments are merely specific embodiments of the present invention, and their descriptions are relatively specific and detailed. However, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art would be able to make various modifications and improvements without departing from the scope of the present invention, and such obvious alternatives are all within the scope of protection of the present invention.

Claims

1. A method for preparing a reduced manganese titanium phosphate sodium ion battery cathode material, characterized in that The following steps are involved: S1. Preparation of a first precursor solution: Under atmosphere protection conditions, a water-soluble titanium source and a reducing agent are mixed and dissolved in water to form a first precursor solution; S2. Preparation of a second precursor solution: mixing a water-soluble manganese source, a water-soluble phosphorus source, and an acidic complexing agent, and dissolving them in water to form a second precursor solution; S3. Preparation of a third precursor solution: Under temperature control and atmosphere protection conditions, slowly dropwise add the second precursor solution into the first precursor solution and mix well to obtain a third precursor solution; S4. Preparation of an ionic precursor solution: adding a weakly acidic sodium salt and a carbon source to the third precursor solution and dissolving them to form a uniform ionic precursor solution; S5. Spray drying and granulation: Under atmosphere protection conditions, the ionic precursor solution is spray dried to obtain a precursor powder; S6. High temperature sintering: calcining the precursor powder at high temperature in a protective atmosphere and then cooling it naturally to obtain the reduced manganese titanium based phosphate material.

2. The method for preparing a reduced manganese titanium-based phosphate sodium ion battery positive electrode material according to claim 1, characterized in that: The chemical formula of the reduced manganese titanium-based phosphate sodium ion battery positive electrode material is Na4MnTi(PO4)3; the valence state of the Mn element is +2, and the valence state of the Ti element is +3; the Na element occupies the alkali metal site in the structure, and its occupancy rate is 100%; the unit cell parameters are 8.85Å≤a≤9.0Å, 21.9Å≤c≤22.5Å, 1469.2Å 3 ≤V≤1475.3Å 3 .

3. The method for preparing a reduced manganese titanium phosphate sodium ion battery positive electrode material according to claim 1, characterized in that: In step S1, the water-soluble titanium source includes one or more of bis(trifluoromethanesulfonic acid) titanocene, titanium isopropoxide, titanium tetrachloride, titanyl sulfate, tetrabutyl titanate, dihydroxybis(ammonium lactate) titanium, and titanium sulfate; the reducing agent includes one or more of sodium sulfite, sodium bisulfite, sodium borohydride, sodium thiosulfate, water and hydrazine, ascorbic acid, lithium aluminum hydride, potassium borohydride, and stannous chloride.

4. The method for preparing a reduced manganese titanium phosphate sodium ion battery positive electrode material according to claim 3, characterized in that: In step S1, the molar ratio of the water-soluble titanium source to the reducing agent is 1:0.5-2.

5. The method for preparing a reduced manganese titanium phosphate sodium ion battery positive electrode material according to claim 1, characterized in that: In step S1 , the protective atmosphere is one or more of nitrogen, argon, nitrogen-hydrogen, and argon-hydrogen.

6. The method for preparing a reduced manganese titanium phosphate sodium ion battery positive electrode material according to claim 1, characterized in that: In step S2, the water-soluble manganese source includes one or more of manganese chloride, manganese acetate, manganese nitrate, and manganese sulfate; the water-soluble phosphorus source includes one or more of ammonium dihydrogen phosphate, phosphoric acid, sodium monohydrogen phosphate, sodium dihydrogen phosphate, and sodium phosphate; the acidic complexing agent includes one or more of citric acid, ascorbic acid, polyacrylic acid, oxalic acid, malic acid, and tartaric acid; and the molar ratio of the water-soluble manganese source to the acidic complexing agent is 1:0.1-5.

7. The method for preparing a reduced manganese titanium phosphate sodium ion battery positive electrode material according to claim 1, characterized in that: In step S3, the temperature range is 0-50°C.

8. The method for preparing a reduced manganese titanium-based phosphate sodium ion battery cathode material according to claim 1, characterized in that: In step S4, the weak acid sodium salt is sodium bicarbonate; the carbon source includes one or more of glucose, maltose, glucose, water-soluble starch, sucrose, citric acid, and lactose; and the amount of the carbon source added is 1 to 5 times the molar amount of the manganese source.

9. The method for preparing a reduced manganese titanium phosphate sodium ion battery positive electrode material according to claim 1, characterized in that: In step S5, the spray drying outlet air temperature is ≥100°C, and the moisture content of the precursor powder is ≤2%.

10. The method for preparing a reduced manganese titanium-based phosphate sodium ion battery cathode material according to claim 1, characterized in that: In step S6, the protective atmosphere includes H2 and / or CO; the sintering conditions are: sintering temperature is 600-750°C, holding time is 8-15 hours, and O2 content in the sintering atmosphere is ≤10ppm.

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