Titanium manganese sodium phosphate positive electrode material with double-phase core-shell structure and preparation method of titanium manganese sodium phosphate positive electrode material

Abstract

The invention discloses a titanium-manganese-sodium phosphate positive electrode material with a double-phase core-shell structure and a preparation method of the titanium-manganese-sodium phosphate positive electrode material, and respective performances of elements are fully exerted by accurately regulating and controlling the ratio of Mn to Ti in an inner core and a surface layer. The preparation method comprises the following steps: 1) performing spray granulation according to a high-manganese and low-titanium ratio to prepare a precursor 1 as an inner core; 2) uniformly compounding low-manganese high-titanium powder with the precursor 1 according to a ratio, and obtaining a precursor 2 by adopting a sol-gel method; and 3) in a protective atmosphere containing reducing gas, tabletting and calcining to prepare the sodium manganese titanium phosphate positive electrode material Na < 3 + 2x > Mn < 1 + x > Ti < 1-x > (PO4) 3 (at) Na < 3-2y > Mn < 1-y > Ti < 1 + y > (PO4) 3 / C with a core-shell structure, 0 < = x < = 0.5, and 0 < = y < = 1, the high-manganese design of the inner core can provide high capacity, the high-titanium layer on the surface can effectively stabilize the structure of the material and reduce the problem that manganese is dissolved out in a negative electrode in the cyclic process of titanium manganese sodium phosphate, and the Ti-C reinforcement phase can effectively reduce the resistance of an interface and improve the electronic conductivity of the material.

Description

Technical Field

[0001] This invention belongs to the field of cathode material technology, and relates to a dual-phase core-shell structure sodium titanium manganese phosphate cathode material and its preparation method. Background Technology

[0002] In the field of battery technology, especially in the research of lithium-ion battery anode materials, improving the electronic conductivity of materials is key to enhancing battery performance. Traditional anode materials, such as graphite, while possessing high specific capacity, have low electronic conductivity, limiting the battery's charge / discharge rate and cycle stability. Furthermore, some manganese-containing materials, although capable of improving electronic conductivity, suffer from manganese leaching, leading to a decline in battery performance. Therefore, developing novel anode materials with high electronic conductivity is an important direction for battery technology development.

[0003] In existing technologies, some researchers have attempted to prepare core-shell structured materials using high-temperature solid-state methods, hoping to improve electronic conductivity and reduce manganese leaching through core-shell structure design. However, this method is complex in ball milling processes and often requires prolonged high-temperature treatment, which not only increases production costs but may also damage the material structure, affecting its electrochemical performance.

[0004] The electronic insulation of phosphate groups leads to low charge transport efficiency, necessitating coating with conductive polymers (such as polypyrrole) or carbon-based materials (graphene, carbon nanotubes), and enhancing interfacial electron transfer through thio / cyano-functionalized binders (such as polyarylene ether nitrile derivatives). However, coating with conductive polymers is prone to uneven coating and easily introduces impurity phases (such as ferric chloride catalysts) during synthesis, adversely affecting the phase purity of the cathode material; furthermore, the complex process hinders industrial production.

[0005] In addition, some researchers have tried to optimize electronic conductivity by adjusting the ratio of manganese and titanium in the material, but these methods are often difficult to control the ratio of elements precisely, and in practical applications, the effect of improving electronic conductivity is limited due to the inhomogeneity in the material preparation process.

[0006] To address the aforementioned issues, this technical solution proposes a novel, simple, and mass-producible method for preparing core-shell structured high-manganese, low-titanium core spheres and low-manganese, high-titanium shell materials to improve electronic conductivity. This method synthesizes precursor materials using both spray granulation and sol-gel methods. Utilizing the wettability of solids and liquids, a uniform phase is formed on the surface of the spherical precursor. Low-temperature (e.g., 350℃) pre-calcination introduces oxygen atoms, which promote local diffusion rather than complete oxidation to form oxygen bridges, thus optimizing interfacial bonding and allowing for tighter contact between the two phases, achieving interface modification. Simultaneously, it improves the compatibility of the material interface with the electrolyte. Precise control of the Mn and Ti ratio enables phase modulation in the core and shell, achieving different functionalizations. The high titanium content in the shell layer, under reducing atmosphere conditions, removes pre-oxidized oxygen atoms, replacing them with carbon, easily forming a Ti-C reinforcing phase, effectively improving the electronic conductivity of the material interface. The unique dual-phase core-shell structure effectively mitigates material volume expansion, reduces manganese dissolution, and improves battery performance and stability. Summary of the Invention

[0007] This invention constructs a unique dual-phase core-shell structure. The high-manganese core design provides high capacity, while the high-titanium surface layer not only effectively stabilizes the material structure and aims to reduce the problem of manganese dissolution at the negative electrode during cycling of sodium titanium manganese phosphate, but also the Ti-C in it can effectively reduce the interfacial resistance.

[0008] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0009] A method for preparing a dual-phase core-shell structured sodium manganese titanium phosphate cathode material includes the following steps:

[0010] a. A high-manganese, low-titanium precursor 1 was prepared by spray granulation and used as the core component;

[0011] b. Weigh the raw materials according to the low manganese and high titanium ratio, and mix them with precursor 1, organic sol and gelling agent to obtain precursor 2 by sol-gel method.

[0012] c. The obtained precursor 2 is calcined at high temperature under a protective atmosphere containing reducing gas to obtain a dual-phase core-shell structured sodium manganese titanium phosphate cathode material with the chemical formula Na. 3+2x Mn 1+x Ti 1-x (PO4)3@Na 3-2y Mn 1-y Ti 1+y (PO4)3 / C, 0≤x≤0.5, 0≤y≤1.

[0013] In the above technical solution, further, in step a, the sodium source, manganese source, titanium source, phosphorus source and carbon source are mixed in a high manganese and low titanium ratio. If the manganese source and the titanium source are soluble, spray granulation is used directly. If the manganese source and the titanium source are insoluble, the precursor 1 needs to be prepared by sand milling-spray granulation, and the sand milling slurry is controlled to be 100-200 nm.

[0014] Furthermore, in the spray granulation process, the inlet temperature is controlled at 200-250℃, and the spray granulation pressure is 0.5-0.8 MPa.

[0015] Furthermore, the sodium source is at least one of sodium carbonate, sodium bicarbonate, sodium acetate, sodium oxalate, sodium nitrate, sodium citrate, and sodium hydroxide;

[0016] The phosphorus source is at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, disodium hydrogen phosphate, and sodium dihydrogen phosphate.

[0017] The titanium source is at least one of tetrabutyl titanate, titanium ammonium lactate chelate, titanium citrate chelate, titanium acetate, titanium oxalate, and titanium dioxide.

[0018] The manganese source is at least one of manganese acetate, manganese oxalate, manganese nitrate, manganese monoxide, manganese dioxide, manganese trioxide, and manganese tetroxide.

[0019] The carbon source is at least one of glucose, starch, citric acid, ascorbic acid, polyvinyl alcohol, polyethylene oxide, polydopamine, polyacrylonitrile, polyvinylpyrrolidone, carbon nanotubes, carbon fibers, and graphene.

[0020] Furthermore, the high-manganese, low-titanium ratio is as follows: Na:Mn:Ti:P = (3-4):(1-1.5):(0-0.5):3, wherein the mass content of carbon used is 2.4-4% (i.e., the mass percentage of carbon in the core component).

[0021] Furthermore, the raw materials in b include a soluble titanium source, which is at least one of tetrabutyl titanate and titanium ammonium lactate chelate, and a soluble manganese source, which is at least one of manganese acetate, manganese oxalate, and manganese nitrate; the organic sol can usually be citric acid sol, and the gelling agent can be ethylene glycol, etc.

[0022] Furthermore, the low-manganese, high-titanium ratio is: Na:Mn:Ti:P = (1-3):(0-1):(1-2):3 by molar ratio, wherein the mass content of carbon used is 0.6-1% (i.e., the mass percentage of carbon in the shell component), and the mass ratio of the raw material of the low-manganese, high-titanium ratio to the precursor 1 is 1:180-220. By controlling the amount of this raw material used in the sol-gel process, the thickness of the shell, i.e., the low-manganese, high-titanium layer, can be controlled to be 30-150 nm; more preferably, the thickness is 50-80 nm. The thickness of this shell layer should not be too thick, as excessive thickness will lead to a decrease in the capacity of the cathode material. The high titanium content in the outer shell (Mn:Ti = 0-1:1-2) results in electrochemical activity far lower than that of the high-manganese core (Mn:Ti = 1+x:1-x). An excessively thick outer shell dilutes the proportion of active material, leading to a decrease in overall capacity (e.g., 0.1C capacity drops from 122 mAh / g to <100 mAh / g). Ion diffusion is hindered: the diffusion path of sodium ions through the thick shell is prolonged, increasing internal resistance and exacerbating charge / discharge plateau polarization (voltage hysteresis >0.3V). Interfacial stress cracking occurs because the difference in thermal expansion coefficients between the core and shell may cause the thick shell to crack during sintering or cycling, compromising the integrity of the core-shell structure. It should also not be too thin, as excessive thinness leads to insufficient structural stability. The high titanium content (Na...) in the outer shell... 3-2 yMn1-yTi1+y(PO4)3) inhibits manganese dissolution and alleviates Jan Taylor distortion. If the thickness is insufficient, the core (high-manganese phase) cannot be completely covered, and manganese ions will still migrate to the electrolyte during cycling, leading to capacity decay and irreversible phase transition; the Ti-C network is discontinuous, and it is difficult for thin layers to form a continuous Ti-C conductive network (carbon source mass ratio 0.6–1%), resulting in increased interfacial resistance and decreased rate performance. Experiments show that when the thickness is <30 nm, the 10C capacity retention rate may drop below 40%.

[0023] Furthermore, the sol-gel method specifically includes: adding precursor 1 to a stirred reactor according to a low manganese and high titanium ratio, followed by adding the low manganese and high titanium formulation, organic sol and gelling agent in sequence, stirring and reacting at 50-90℃ for 5-8h to form a uniform sol-gel system, aging for 12-24h to enhance the gel network structure, and then drying at 120℃ for 12h to obtain a fluffy dry gel. After grinding the dry gel, it is pre-calcined in air at 300-400℃ for 2-4h to remove organic matter and introduce an appropriate amount of oxygen atoms, preferably pre-calcined at 350℃.

[0024] Furthermore, the protective atmosphere in c is nitrogen, argon, or vacuum, and the reducing gas is hydrogen, which accounts for 1-3% of the volume in the protective atmosphere, with an optimal proportion of 2%, and the preferred atmosphere is a combination of nitrogen and 2% hydrogen; the high-temperature calcination is calcination at 600-900℃ for 8-12 hours, and the preferred heating rate is 3℃ / min.

[0025] After obtaining the cathode material, it is crushed by airflow to a particle size of 2.0-5.0μm, and then sieved, iron removed and vacuum packaged.

[0026] Compared with the prior art, the beneficial effects of this technical solution are at least as follows:

[0027] 1. The precursor material was synthesized by two preparation methods: spray granulation and sol-gel. The wettability of solid and liquid was used to form a uniform phase on the surface of the spherical precursor. Oxygen atoms were introduced by low-temperature (350℃) pre-calcination. The oxygen atoms promoted the local diffusion of oxygen rather than complete oxidation to form oxygen bridge bonds, thereby optimizing the interfacial bonding and making the two-phase interface contact tighter, thus achieving the purpose of interface modification.

[0028] 2. Precise control of the Mn and Ti ratio enables phase modulation in the core and shell, achieving different functionalizations. The core is high in manganese and low in titanium, which effectively improves the material's capacity; the shell is low in manganese and high in titanium. This structure effectively reduces manganese dissolution and mitigates the effects of the Jan Taylor effect.

[0029] 3. The high titanium content in the outer shell, under reducing atmosphere conditions, allows pre-oxidized oxygen atoms to be removed and replaced with carbon, easily forming a Ti-C reinforcing phase, effectively improving the electronic conductivity of the material interface. Simultaneously, it effectively stabilizes the material structure, and by introducing a suitable soluble carbon source to form Ti-C with the high titanium layer on the surface, it effectively reduces the interfacial resistance.

[0030] 4. This technical solution can introduce soluble carbon into the core and shell layers respectively, which can effectively improve the electronic conductivity of the material and thus improve battery performance.

[0031] 5. By using both spray granulation and sol-gel preparation methods, this technical solution simplifies the complex ball milling process of high-temperature solid-phase material preparation, improves production efficiency, and is simple enough to be suitable for large-scale production.

[0032] 6. The core-shell structure design not only improves the electronic conductivity of the material, but also enhances the stability of the material in high-temperature environments, thus extending the battery's lifespan. Attached Figure Description

[0033] Figure 1 for Na 3.4 Mn 1.2 Ti 0.8 (PO4)3@Na2Mn 0.5 Ti 1.5 SEM image of (PO4)3 / C precursor.

[0034] Figure 2 for Na 3.4 Mn 1.2 Ti 0.8(PO4)3@Na2Mn 0.5 Ti 1.5 SEM image of (PO4)3 / C cathode material.

[0035] Figure 3 for Na 3+2x Mn 1+x Ti 1-x (PO4)3@Na 3-2y Mn 1-y Ti 1+y Schematic diagram of (PO4)3 / C cathode material. Detailed Implementation

[0036] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0037] This invention proposes a method for preparing a dual-phase core-shell structured sodium manganese titanium phosphate cathode material. By precisely controlling the Mn:Ti ratio in the core and surface layers, the respective properties of each element are fully utilized. This method is implemented through the following steps:

[0038] Example 1

[0039] The first step was to prepare a high-manganese, low-titanium core precursor 1: The ingredients were prepared according to a synthesis amount of 200g, with a molar ratio of Na:Mn:Ti:P:C = 3.4:1.2:0.8:3:0.33. Sodium acetate, manganese acetate, titanium acetate, ammonium dihydrogen phosphate, and glucose were weighed and dissolved in 450mL of deionized water. The mixture was stirred vigorously for 1 hour and then transferred to spray granulation. The inlet temperature was controlled at 220℃, and the spray granulation pressure was 0.6MPa to prepare precursor 1 with a suitable particle size.

[0040] The second step involves preparing the core-shell precursor 2: The ingredients were prepared according to a molar ratio of Na:Mn:Ti:P:C = 2:0.5:1.5:3:0.15. Sodium acetate, manganese acetate, titanium acetate, ammonium dihydrogen phosphate, and glucose were weighed and dissolved in 200 mL of deionized water. The mixture was stirred vigorously for 1 hour. Then, precursor 1 was added at a mass ratio of 1:200 (ingredients to precursor 1), along with 1% polyethylene glycol dispersant. The mixture was stirred for another 2 hours. An appropriate amount of citric acid was added to the solution, and the mixture was stirred and evaporated to dryness at 80°C to obtain a gel. The gel was aged for 12 hours to enhance its network structure. The gel was then dried at 120°C for 12 hours to obtain a fluffy dry gel. After grinding the dry gel, it was pre-calcined at 350°C in air for 3 hours to introduce oxygen atoms at the interface. The SEM images of the obtained product are shown below. Figure 1 As shown.

[0041] The third step involves high-temperature sintering to prepare the core-shell structure material: the obtained precursor 2 is transferred to a box furnace under a protective atmosphere (N2 / 2% H2) for calcination. The heating rate is 3℃ / min, the calcination temperature is 650℃, and the holding time is 10h. The resulting cathode material has the chemical formula Na. 3.4 Mn 1.2 Ti 0.8 (PO4)3@Na2Mn 0.5 Ti 1.5 (PO4)3 / C (its SEM image is as follows) Figure 2 (As shown).

[0042] Example 2

[0043] The first step was to prepare a high-manganese, low-titanium core precursor 1: The ingredients were prepared according to a synthesis dosage of 300g, with a molar ratio of Na:Mn:Ti:P:C = 3.4:1.2:0.8:3:0.33. Sodium carbonate, manganese trioxide, titanium dioxide, ammonium dihydrogen phosphate, and glucose were weighed out, 1% PEG was added, and the mixture was dispersed in 1.2L of deionized water. The mixture was coarsely ground for 30 minutes, then transferred to an ultra-nano mill and ground until the slurry reached a particle size of 150nm. Afterward, it was spray-granulated, with the inlet temperature controlled at 220℃ and the spray-granulation pressure at 0.6MPa, to prepare precursor 1 with a suitable particle size.

[0044] The second step, preparing the kernel-shell precursor 2, is the same as in Example 1;

[0045] The third step involves high-temperature sintering to prepare the core-shell structured material. The process is consistent with that in Example 1, and the resulting cathode material has the chemical formula Na. 3.4 Mn 1.2 Ti 0.8 (PO4)3@Na2Mn 0.5 Ti 1.5 (PO4)3 / C.

[0046] Example 3

[0047] Step 1: Preparation of high-manganese, low-titanium core precursor 1: The raw materials were prepared according to a synthesis amount of 200g, with a molar ratio of Na:Mn:Ti:P:C = 3.3:1.15:0.85:3:0.31. Sodium oxalate, manganese acetate, tetrabutyl titanate, ammonium dihydrogen phosphate, and glucose were weighed out. Tetrabutyl titanate was dissolved in an appropriate amount of anhydrous ethanol, and the remaining metal salts were dissolved in 450mL of deionized water. The mixture was slowly mixed and vigorously stirred for 1h. The mixture was then transferred to spray granulation, with the inlet temperature controlled at 220℃ and the spray granulation pressure at 0.6MPa, to prepare precursor 1 with a suitable particle size.

[0048] The second step, preparing the kernel-shell precursor 2, is the same as in Example 1;

[0049] The third step involves high-temperature sintering to prepare the core-shell structured material. The process is consistent with that in Example 1, and the resulting cathode material has the chemical formula Na. 3.3 Mn 1.15 Ti 0.85 (PO4)3@Na2Mn 0.5 Ti 1.5 (PO4)3 / C.

[0050] Example 4

[0051] Step 1: Preparation of high-manganese, low-titanium core precursor 1: Prepare materials according to a synthesis amount of 200g, with a molar ratio of Na:Mn:Ti:P:C = 4:1.5:0.5:3:0.41. Weigh out sodium oxalate, manganese sulfate, titanium acetate, ammonium dihydrogen phosphate, and glucose, dissolve them in 450mL of deionized water, stir vigorously for 1h, transfer to spray granulation, control the inlet temperature at 220℃, and the spray granulation pressure at 0.6MPa to prepare precursor 1 with suitable particle size.

[0052] The second step, preparing the kernel-shell precursor 2, is the same as in Example 1;

[0053] The third step involves high-temperature sintering to prepare the core-shell structure material, following the same process as in Example 1. The resulting cathode material has the chemical formula Na₄Mn₄. 1.5 Ti 0.5 (PO4)3@Na2Mn 0.5 Ti 1.5 (PO4)3 / C.

[0054] Example 5

[0055] The first step of preparing the high-manganese, low-titanium core precursor 1 is the same as in Example 1;

[0056] The second step involves preparing the core-shell precursor 2: The ingredients are prepared according to a molar ratio of Na:Mn:Ti:P:C = 3:1:1:3:0.27. Sodium acetate, manganese acetate, titanium acetate, ammonium dihydrogen phosphate, and glucose are weighed and dissolved in 200 mL of deionized water. The mixture is stirred vigorously for 1 hour. Then, precursor 1 is added at a mass ratio of 1:200, along with 1% polyethylene glycol dispersant. The mixture is stirred for another 2 hours. An appropriate amount of citric acid is added to the solution, and the mixture is stirred and evaporated to dryness at 80°C to obtain a gel. The gel is aged for 12 hours to enhance its network structure. The gel is then dried at 120°C for 12 hours to obtain a fluffy dry gel. After grinding the dry gel, it is pre-calcined at 350°C for 3 hours to remove organic matter.

[0057] The third step involves high-temperature sintering to prepare the core-shell structured material. The process is consistent with that in Example 1, and the resulting cathode material has the chemical formula Na. 3.4 Mn 1.2 Ti 0.8(PO4)3@Na3MnTi(PO4)3 / C.

[0058] Example 6

[0059] The first step of preparing the high-manganese, low-titanium core precursor 1 is the same as in Example 1;

[0060] The second step involves preparing the core-shell precursor 2: The ingredients are prepared according to a molar ratio of Na:Ti:P:C = 1:2:3:0.15. Sodium acetate, manganese acetate, titanium acetate, ammonium dihydrogen phosphate, and glucose are weighed and dissolved in 200 mL of deionized water. The mixture is stirred vigorously for 1 hour. Then, precursor 1 is added at a mass ratio of 1:200, along with 1% polyethylene glycol dispersant. The mixture is stirred for another 2 hours. An appropriate amount of citric acid is added to the solution, and the mixture is stirred and evaporated to dryness at 80°C to obtain a gel. The gel is aged for 12 hours to enhance its network structure. The gel is then dried at 120°C for 12 hours to obtain a fluffy dry gel. After grinding the dry gel, it is pre-calcined at 350°C for 3 hours to remove organic matter.

[0061] The third step involves high-temperature sintering to prepare the core-shell structured material. The process is consistent with that in Example 1, and the resulting cathode material has the chemical formula Na. 3.4 Mn 1.2 Ti 0.8 (PO4)3@NaTi2(PO4)3 / C.

[0062] Example 7

[0063] The first step of preparing the high-manganese, low-titanium core precursor 1 is the same as in Example 1;

[0064] The second step involves preparing the core-shell precursor 2: The ingredients are prepared according to a molar ratio of Na:Mn:Ti:P:C = 2:0.5:1.5:3:0.3. Sodium acetate, manganese acetate, titanium acetate, ammonium dihydrogen phosphate, and glucose are weighed and dissolved in 200 mL of deionized water. The mixture is stirred vigorously for 1 hour. Then, precursor 1 is added at a mass ratio of 1:200, along with 1% polyethylene glycol dispersant. The mixture is stirred for another 2 hours. An appropriate amount of citric acid is added to the solution, and the mixture is stirred and evaporated to dryness at 80°C to obtain a gel. The gel is aged for 12 hours to enhance its network structure. The gel is then dried at 120°C for 12 hours to obtain a fluffy dry gel. After grinding the dry gel, it is pre-calcined at 350°C for 3 hours to introduce oxygen atoms.

[0065] The third step involves high-temperature sintering to prepare the core-shell structured material. The process is consistent with that in Example 1, and the resulting cathode material has the chemical formula Na. 3.4 Mn 1.2 Ti 0.8 (PO4)3@Na2Mn 0.5 Ti 1.5 (PO4)3 / C.

[0066] Comparative Example 1

[0067] The first step was to prepare a high-manganese, low-titanium core precursor 1: The ingredients were prepared according to a synthesis amount of 200g, with a molar ratio of Na:Mn:Ti:P:C = 3.4:1.2:0.8:3:0.33. Sodium oxalate, manganese acetate, titanium acetate, ammonium dihydrogen phosphate, and glucose were weighed out and dissolved in 450mL of deionized water. The mixture was stirred vigorously for 1 hour and then transferred to spray granulation. The inlet temperature was controlled at 220℃, and the spray granulation pressure was 0.6MPa to prepare precursor 1 with a suitable particle size.

[0068] The second step, high-temperature sintering, follows the same process as in Example 1, yielding a cathode material with the chemical formula Na. 3.4 Mn 1.2 Ti 0.8 (PO4)3 / C.

[0069] Comparative Example 2

[0070] Step 1: Preparation of high-manganese, low-titanium core precursor 1: Prepare the raw materials according to the amount of 200g to be synthesized, with a molar ratio of Na:Mn:Ti:P:C = 4:1.5:0.5:3:0.41. Weigh out sodium oxalate, manganese acetate, titanium acetate, ammonium dihydrogen phosphate and glucose respectively, dissolve them in 450mL of deionized water, stir vigorously for 1h, transfer to spray granulation, control the inlet temperature at 220℃, and the spray granulation pressure at 0.6MPa to prepare precursor 1 with suitable particle size;

[0071] The second step, high-temperature sintering, follows the same process as in Example 1, yielding a cathode material with the chemical formula Na₄Mn. 1.5 Ti 0.5 (PO4)3 / C.

[0072] Comparative Example 3

[0073] The first step of preparing the high-manganese, low-titanium core precursor 1 is the same as in Example 1;

[0074] The second step, preparing the core-shell precursor 2, is the same as in Example 1, except that the pre-oxidation process at 350°C in air for 3 hours is omitted in this step.

[0075] The third step involves high-temperature sintering to prepare the core-shell structured material. The process is consistent with that in Example 1, and the resulting cathode material has the chemical formula Na. 3.4 Mn 1.2 Ti 0.8 (PO4)3@Na2Mn 0.5 Ti 1.5 (PO4)3 / C.

[0076] Comparative Example 4

[0077] The first step of preparing the high-manganese, low-titanium core precursor 1 is the same as in Example 1;

[0078] The second step, preparing the kernel-shell precursor 2, is the same as in Example 1;

[0079] The third step involves high-temperature sintering to prepare the core-shell structure material: the obtained precursor 2 is transferred to a box furnace under a protective atmosphere (N2) without reducing gas for calcination. The heating rate is 3℃ / min, the calcination temperature is 650℃, and the holding time is 10h. The resulting cathode material has the chemical formula Na. 3.4 Mn 1.2 Ti 0.8 (PO4)3@Na2Mn 0.5 Ti 1.5 (PO4)3 / C.

[0080] Na prepared by this invention 3.4 Mn 1.2 Ti 0.8 (PO4)3@Na2Mn 0.5 Ti 1.5 (PO4)3 / C (650℃) microspheres were used as the positive electrode active material for sodium-ion batteries, and the preparation method is as follows: Na 3.4 Mn 1.2 Ti 0.8 (PO4)3@Na2Mn 0.5 Ti 1.5 Using (PO4)3 / C (650℃) microspheres as the active material, acetylene black as the conductive agent, and PVDF as the binder, the mass ratio of active material, acetylene black, and PTFE was 70:20:10. After thorough mixing, a small amount of NMP was added, and the mixture was ultrasonically homogenized. This homogenized mixture was then coated onto aluminum foil to serve as the electrode sheet for the sodium-ion battery. The coated positive electrode sheet was dried in an oven at 80℃ for 24 hours before use. A sodium-ion button battery was assembled using a 1M NaClO4 in EC+PC (1:1)+5% FEC solution as the electrolyte, a sodium sheet as the negative electrode, and Celgard as the separator, along with a 2032 positive and negative electrode battery case. The upper and lower voltage limits were set to 2.5-4.3V for charging and discharging.

[0081] Table 1 shows the electrochemical performance data of the bilayer sodium titanyl manganese phosphate cathode material in different embodiments.

[0082]

[0083] Examples 1 and 2 differ in their core preparation processes. Example 1 uses direct dissolution and spray granulation of soluble raw materials, while Example 2 uses insoluble solid-phase sand milling and spray granulation to synthesize the core material. Both examples exhibit superior electrochemical performance, but due to the difficulty in nano-sizing of sand-milled particles, the electrochemical performance is lower than that of Example 1. Comparing Examples 3, 1, and 4, the specific capacity of the material increases significantly with increasing manganese content in the core. However, excessive Mn content leads to the formation of impurities such as manganese phosphate, thereby reducing material performance. Comparing Examples 1, 5, 6, and Comparative Example 2, it was found that increasing titanium content in the shell enhances the rate performance and long-cycle capacity retention. This can be explained by the open three-dimensional framework and rapid ion diffusion rate of the shell, which facilitates the insertion and extraction of sodium ions. Reducing direct contact between the material and the electrolyte minimizes side reactions, inhibits irreversible phase transitions, and ultimately improves the material's rate performance and cycle stability. Compared to Example 1 and Comparative Example 3, the absence of a low-temperature (350°C) pre-oxidation process during the sol-gel phase resulted in no optimization of the two-phase interface, insufficient sites for carbon introduction, and hindered the formation of the reinforcing phase, leading to a decrease in both rate performance and capacity. Compared to Example 1 and Comparative Example 4, the lack of a reducing atmosphere during the third calcination step increased the material's interfacial resistance, decreased electronic conductivity, and significantly reduced its initial coulombic efficiency and capacity. A comparison of Examples 1 and 7 revealed that appropriately increasing the carbon content in the shell under a reducing atmosphere allows for the formation of a Ti-C structure, effectively reducing interfacial resistance and further enhancing the material's long-cycle performance.

[0084] The embodiments described above are merely some preferred embodiments of the present invention, and are not intended to limit the invention. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the invention. Therefore, all technical solutions obtained by equivalent substitution or equivalent transformation fall within the protection scope of the present invention.

Claims

1. A method for preparing a dual-phase core-shell structured sodium manganese titanium phosphate cathode material, characterized in that, Including the following: a. A high-manganese, low-titanium precursor 1 was prepared by spray granulation and used as the core component; b. Weigh the raw materials according to the low manganese and high titanium ratio, and mix them with precursor 1, organic sol and gelling agent to obtain precursor 2 by sol-gel method. c. The obtained precursor 2 is calcined at high temperature under a protective atmosphere containing reducing gas to obtain a dual-phase core-shell structured sodium manganese titanium phosphate cathode material with the chemical formula Na. 3+2x Mn 1+x Ti 1-x (PO4)3@Na 3-2y Mn 1-y Ti 1+y (PO4)3 / C, 0≤x≤0.5, 0≤y≤1.

2. The preparation method according to claim 1, characterized in that, In step a, sodium, manganese, titanium, phosphorus and carbon sources are mixed in a high manganese and low titanium ratio. If the manganese and titanium sources are soluble, spray granulation is used directly. If the manganese and titanium sources are insoluble, sand milling-spray granulation is used to prepare precursor 1, and the sand milling slurry is controlled to be 100-200 nm.

3. The preparation method according to claim 2, characterized in that, The spray granulation process involves controlling the inlet temperature at 200-250℃ and the spray granulation pressure at 0.5-0.8 MPa.

4. The preparation method according to claim 2, characterized in that, The sodium source is at least one of sodium carbonate, sodium bicarbonate, sodium acetate, sodium oxalate, sodium nitrate, sodium citrate, and sodium hydroxide. The phosphorus source is at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, disodium hydrogen phosphate, and sodium dihydrogen phosphate. The titanium source is at least one of tetrabutyl titanate, titanium ammonium lactate chelate, titanium citrate chelate, titanium acetate, titanium oxalate, and titanium dioxide. The manganese source is at least one of manganese acetate, manganese oxalate, manganese nitrate, manganese monoxide, manganese dioxide, manganese trioxide, and manganese tetroxide. The carbon source is at least one of glucose, starch, citric acid, ascorbic acid, polyvinyl alcohol, polyethylene oxide, polydopamine, polyacrylonitrile, polyvinylpyrrolidone, carbon nanotubes, carbon fibers, and graphene.

5. The preparation method according to claim 1, characterized in that, The high-manganese, low-titanium ratio is as follows: Na:Mn:Ti:P = (3-4):(1-1.5):(0-0.5):3, wherein the mass content of carbon used is 2.4-4%.

6. The preparation method according to claim 1, characterized in that, The raw materials in b include a soluble titanium source, which is at least one of tetrabutyl titanate and titanium ammonium lactate chelate, and a soluble manganese source, which is at least one of manganese acetate, manganese oxalate and manganese nitrate.

7. The preparation method according to claim 1, characterized in that, The low-manganese, high-titanium ratio is: Na:Mn:Ti:P = (1-3):(0-1):(1-2):3 by molar ratio, wherein the mass content of carbon used is 0.6-1%, and the mass ratio of the raw material of the low-manganese, high-titanium ratio to precursor 1 is 1:(180-220).

8. The preparation method according to claim 1, characterized in that, The sol-gel method specifically includes: stirring and reacting at 50-90℃ for 5-8 hours to form a uniform sol-gel system, aging for 12-24 hours, and then drying at 120℃ for 12 hours to obtain a fluffy dry gel. After grinding, it is pre-calcined in air at 300-400℃ for 2-4 hours to remove organic matter and introduce an appropriate amount of oxygen atoms.

9. The preparation method according to claim 1, characterized in that, The protective atmosphere in c is nitrogen, argon, or vacuum, and the reducing gas is hydrogen, which accounts for 1-3% of the volume in the protective atmosphere, with an optimal proportion of 2%; the high-temperature calcination is calcination at 600-900℃ for 8-12 hours.

10. A dual-phase core-shell structured sodium manganese titanium phosphate cathode material, characterized in that, It is prepared by the method described in any one of claims 1-9.

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