Preparation method of low-porosity polyanion secondary battery positive electrode material

Abstract

The present invention discloses a method for preparing a low-porosity polyanion secondary battery positive electrode material, comprising the following steps: S1, preparing an emulsion; S2, spray-drying a precursor dry powder; S3, preparing a saturated solution; S4, densifying the spherical precursor dry powder: slowly spraying the saturated solution obtained in step S3 onto the spherical precursor dry powder obtained in step S2 under heating and stirring conditions, allowing the saturated solution to penetrate into the pores and cavities within the spherical precursor dry powder to fill the pores and form a dense spherical precursor dry powder; S5, high-temperature sintering: calcining the dense spherical precursor dry powder at high temperature, allowing it to cool naturally and crystallize, ultimately forming a low-porosity polyanion secondary battery positive electrode material. The method for preparing a low-porosity polyanion secondary battery positive electrode material of the present invention has the characteristics of high phase purity, high compaction density, excellent electrochemical performance, and low interfacial solubility.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and in particular to a method for preparing a low-porosity polyanion secondary battery positive electrode material. Background Art

[0002] There are many types of polyanionic materials for sodium-ion batteries, including sulfates, phosphates, pyrophosphates, pyrophosphate-phosphates, and fluorophosphates. Currently, the most popular polyanionic materials on the market include sodium vanadium phosphate, sodium ferric pyrophosphate, and sodium ferric sulfate. The synthesis processes for these materials are relatively similar, and spray drying technology is often used to prepare the precursors.

[0003] Spray drying is the most commonly used technique for separating solids and liquids in slurries. Drying temperatures typically exceed 200°C. After atomization through a high-pressure nozzle or high-speed centrifuge, the slurry forms fine droplets. Surface tension often causes these droplets to assume a spherical shape. When exposed to temperatures above 200°C, surface moisture evaporates preferentially, while the water within the droplets, carrying solutes, migrates to the surface, forming hollow, spherical particles. Furthermore, during this migration, the solutes in the slurry reach saturation and precipitate, stacking up against each other and creating numerous irregular gaps. These hollow structures and irregular gaps in the material absorb significant amounts of solvent and binder during the slurrying process for battery cell electrodes, resulting in excessive slurry viscosity, hindering coating processing and preventing the completion of battery cell fabrication. Furthermore, these hollow structures and irregular gaps reduce the material's bulk, tap, and compacted densities, making them incapable of meeting industrial requirements. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for preparing a low-porosity polyanion secondary battery positive electrode material, which has the characteristics of high phase purity, high compaction density, excellent electrochemical performance and low interface solubility.

[0005] The present invention can be achieved through the following technical solutions:

[0006] The present invention discloses a method for preparing a low-porosity polyanion secondary battery positive electrode material, comprising the following steps:

[0007] S1. Emulsion preparation: adding water to a first alkali metal source, a water-insoluble transition metal source, and a first anion source, and grinding them until the particle size reaches nanometer level to form a uniform emulsion;

[0008] S2. Spray drying of the precursor dry powder: spray drying the emulsion to achieve solid-liquid separation to obtain uniformly dried spherical precursor dry powder;

[0009] S3. Preparation of a saturated solution: dissolving the second alkali metal source, the water-soluble transition metal source, the second anion source, and the water-soluble carbon source in water and heating to form a saturated solution;

[0010] S4. Densification treatment of the spherical precursor dry powder: slowly spraying and adding the saturated solution obtained in step S3 to the spherical precursor dry powder obtained in step S2 under heating and stirring conditions, so that the saturated solution penetrates into the pores and cavities inside the spherical precursor dry powder to fill the pores and form a dense spherical precursor dry powder;

[0011] S5. High-temperature sintering: The dense spherical precursor dry powder is calcined at high temperature, and naturally cooled and crystallized to finally form a low-porosity polyanion secondary battery positive electrode material.

[0012] To address the porosity issue, the present invention utilizes a pore-filling technique in various steps to fill the pores and voids within spray-dried spherical particles. This increases the material's bulk density, tap density, and compacted density, improving its processing characteristics. The preparation process involves dissolving or grinding a mixture of an alkali metal source, a transition metal source, an anion source, and a carbon source, followed by spray drying to obtain a precursor dry powder. The dry powder is then sprayed with a solution containing the alkali metal, transition metal, and anion under dynamic heating conditions, allowing it to slowly and evenly penetrate the pores within the particles, thereby achieving the filling effect. Finally, high-temperature sintering is performed to produce solid particles.

[0013] Furthermore, in step S1, the mixed grinding method is sand milling or ball milling, and the mixed grinding method is used to achieve particle pulverization, crushing or atomic splitting in the form of shear force, gravity, and friction. The purpose is to nano-size the non-water-soluble transition metal source, so that it is close to an ionic form and merges with the alkali metal elements and anions, thereby ensuring the uniform dispersion of the elements in the slurry.

[0014] Furthermore, in step S1, the maximum particle size of the solid particles contained in the nano-emulsion is less than 200 nm. Below this particle size, the interfacial energy of the solid particles is large. Under high temperature conditions, the instability of the interfacial elements will promote mutual fusion between the particles, increase the degree of inter-ion mixing, and promote material nucleation.

[0015] Furthermore, in step S2, the spray drying conditions are that the inlet air temperature is ≥260°C and the water content of the spherical precursor dry powder is less than 3%. When the inlet air temperature is higher than 260°C, the water in the droplets can evaporate quickly, and at the same time, a uniform sphere is formed under the action of the surface tension of the droplets. When the water content in the dry powder is lower than 3%, the surfaces of the spherical precursor dry powder are not easy to stick to each other, and the fluidity is good, which is convenient for later pore filling.

[0016] Furthermore, in step S3, the heating temperature is greater than 80°C and the solid content of the saturated solution is greater than 35%. When the heating temperature is higher than 80°C, the kinetic energy of the ions in the solution increases, the solubility increases, the liquid fluidity increases, and the wettability increases, which is conducive to penetration into the solid gaps, and the higher solid content can effectively improve the filling efficiency.

[0017] Furthermore, in step S4, the heating condition is a heating temperature greater than 80°C. The purpose is that when the saturated solution is sprayed onto the spherical precursor dry powder, the droplets will preferentially penetrate into the interior along the gaps on the surface of the particles under the action of interfacial tension, and then the water absorbs heat and evaporates, and the solid remains in the pores. The spraying and evaporation are repeated until all the gaps inside the spherical precursor dry powder are filled to form a dense spherical precursor dry powder.

[0018] Furthermore, in step S4, the saturated solution is sprayed in a pressure-type or centrifugal-type manner. The above method disperses the liquid into um-level small droplets in the form of high pressure and high linear speed, which facilitates its uniform adhesion and penetration on the surface of the spherical precursor dry powder. At the same time, the smaller droplets avoid the adhesion phenomenon caused by the large area of ​​contact between the spherical precursor dry powders, thereby effectively improving the uniformity of filling.

[0019] Furthermore, in step S5, the sintering temperature is greater than 400° C. and the holding time is greater than 3 hours to ensure sufficient crystal growth of the material.

[0020] Preferably, the first alkali metal source and the second alkali metal source are lithium sources or sodium sources, the lithium source is one or more of metallic lithium, lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, lithium hydride, and lithium sulfate; the sodium source is one or more of metallic sodium, sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium formate, sodium acetate, and sodium sulfate;

[0021] Preferably, the water-insoluble transition metal source is an iron source, a cobalt source, a nickel source and a manganese source, the iron source is one or more of elemental iron, iron oxide, ferrous oxalate, ferric phosphate and ferric hydroxide, the cobalt source is one or more of elemental cobalt, cobalt hydroxide, cobalt oxide, cobalt oxalate and cobalt carbonate, the nickel source is one or more of elemental nickel, nickel oxide and nickel hydroxide, and the manganese source is one or more of manganese, manganese oxide, manganese carbonate and manganese oxalate;

[0022] Preferably, the first anion source and the second anion source are phosphorus sources, fluorine sources or sulfur sources, the phosphorus source is one or more of phosphoric acid, sodium monohydrogen phosphate, sodium dihydrogen phosphate, sodium phosphate, ammonium dihydrogen phosphate, pyrophosphoric acid and its salt derivatives, metaphosphoric acid and its salt derivatives, polyphosphoric acid and its salt derivatives, the fluorine source is one or more of hydrogen fluoride, sodium fluoride, ammonium fluoride, and lithium fluoride, and the sulfur source is one or more of sulfuric acid, ammonium sulfate, sodium sulfate / lithium / potassium, and ferrous sulfate.

[0023] Preferably, the water-soluble transition metal source is an iron source, a manganese source, a cobalt source or a nickel source, the iron source is one or more of ferric nitrate, ferric sulfate / ferrous sulfate, ferric acetate, and ferric formate, the manganese source is one or more of manganese nitrate, manganese sulfate, manganese acetate, and manganese chloride, the cobalt source is one or more of cobalt sulfate, cobalt nitrate, and cobalt chloride, and the nickel source is one or more of nickel sulfate, nickel nitrate, and nickel chloride.

[0024] Preferably, the water-soluble carbon source is one or more of sucrose, glucose, citric acid, cyclodextrin, lactose, maltose, polyvinyl alcohol, and polyethylene glycol.

[0025] Furthermore, the positive electrode material of the polyanion secondary battery is a lithium battery material such as lithium iron phosphate, lithium iron manganese phosphate, fluorolithium iron phosphate, fluorolithium iron manganese phosphate, lithium iron sulfate, etc.; sodium iron phosphate, sodium iron pyrophosphate, sodium iron pyrophosphate, sodium vanadium phosphate, sodium vanadium fluorophosphate, sodium iron fluorophosphate, sodium iron sulfate or sodium iron fluorosulfate.

[0026] The present invention provides a method for preparing a low-porosity polyanion secondary battery positive electrode material, which has the following beneficial effects:

[0027] First, the phase purity is high. The present invention fully grinds the solid slurry before spray drying, and the solid particles reach the nanometer level. At the same time, no carbon source that hinders the uniform contact between elements is added in the process. During the spray drying process, the elements can be fully and tightly combined and arranged, which is conducive to the nucleation and growth of crystals during the sintering process, forming larger pure phase single crystal particles.

[0028] Second, the compaction density is high. The present invention fills the spherical precursor dry powder formed after spraying. By selecting water-soluble alkali metals, transition metals and anion sources, a saturated solution is configured. The principle of slow spray penetration is used to slowly deposit the above ions inside the spherical precursor dry powder, ultimately achieving the filling purpose and forming solid spherical particles. The loose density, tap density and compaction density will be greatly improved.

[0029] Third, the electrochemical performance is excellent. The present invention introduces a carbon source in the process of filling the spherical precursor dry powder formed after spraying. The carbon source penetrates and solidifies in the pores inside the particles along with the saturated solution, leaving a continuous carbon source connection network. After high-temperature carbonization, these carbon sources will form a three-dimensional structured connected fast electron conductivity network, greatly improving the electron transmission capacity.

[0030] Fourth, the solubility of the material interface is low, and there is some dissolution of elements at the interface of the polyanion material. The best way to solve this problem is to reduce the specific surface area of ​​the material. The present invention reduces the contact area between the material and the electrolyte to a certain extent by filling the pores, avoids the large-scale dissolution of the material itself, and improves the cyclic stability of the material. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Attachment Figure 1 SEM of low-porosity Na3V2(PO4)3 / C material in Application Example 1;

[0032] Attachment Figure 2 This is the SEM of Na3V2(PO4)3 / C material of Comparative Example 1. DETAILED DESCRIPTION

[0033] 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 embodiments.

[0034] The present invention discloses a method for preparing a low-porosity polyanion secondary battery positive electrode material, comprising the following steps:

[0035] S1. Emulsion preparation: adding water to a first alkali metal source, a water-insoluble transition metal source, and a first anion source, and grinding them until the particle size reaches nanometer level to form a uniform emulsion;

[0036] S2. Spray drying of the precursor dry powder: spray drying the emulsion to achieve solid-liquid separation to obtain uniformly dried spherical precursor dry powder;

[0037] S3. Preparation of a saturated solution: dissolving the second alkali metal source, the water-soluble transition metal source, the second anion source, and the water-soluble carbon source in water and heating to form a saturated solution;

[0038] S4. Densification treatment of the spherical precursor dry powder: slowly spraying and adding the saturated solution obtained in step S3 to the spherical precursor dry powder obtained in step S2 under heating and stirring conditions, so that the saturated solution penetrates into the pores and cavities inside the spherical precursor dry powder to fill the pores and form a dense spherical precursor dry powder;

[0039] S5. High-temperature sintering: The dense spherical precursor dry powder is calcined at high temperature, and naturally cooled and crystallized to finally form a low-porosity polyanion secondary battery positive electrode material.

[0040] Furthermore, in step S1, the mixed grinding method is sand milling or ball milling.

[0041] Furthermore, in step S1, the maximum particle size of the solid particles contained in the nano-emulsion is less than 200 nm.

[0042] Furthermore, in step S2, the spray drying conditions are as follows: the air inlet temperature is ≥260°C, and the water content of the spherical precursor dry powder is less than 3%.

[0043] Furthermore, in step S3, the heating temperature is greater than 80°C, and the solid content of the saturated solution is greater than 35%.

[0044] Furthermore, in step S4, the heating condition is that the heating temperature is greater than 80°C.

[0045] Furthermore, in step S4, the saturated solution is sprayed in a pressure-type or centrifugal-type manner.

[0046] Furthermore, in step S5, the sintering temperature is greater than 400°C and the holding time is greater than 3 hours.

[0047] Preferably, the first alkali metal source and the second alkali metal source are lithium sources or sodium sources, the lithium source is one or more of metallic lithium, lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, lithium hydride, and lithium sulfate; the sodium source is one or more of metallic sodium, sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium formate, sodium acetate, and sodium sulfate;

[0048] Preferably, the water-insoluble transition metal source is an iron source, a cobalt source, a nickel source and a manganese source, the iron source is one or more of elemental iron, iron oxide, ferrous oxalate, ferric phosphate and ferric hydroxide, the cobalt source is one or more of elemental cobalt, cobalt hydroxide, cobalt oxide, cobalt oxalate and cobalt carbonate, the nickel source is one or more of elemental nickel, nickel oxide and nickel hydroxide, and the manganese source is one or more of manganese, manganese oxide, manganese carbonate and manganese oxalate;

[0049] Preferably, the first anion source and the second anion source are phosphorus sources, fluorine sources or sulfur sources, the phosphorus source is one or more of phosphoric acid, sodium monohydrogen phosphate, sodium dihydrogen phosphate, sodium phosphate, ammonium dihydrogen phosphate, pyrophosphoric acid and its salt derivatives, metaphosphoric acid and its salt derivatives, polyphosphoric acid and its salt derivatives, the fluorine source is one or more of hydrogen fluoride, sodium fluoride, ammonium fluoride, and lithium fluoride, and the sulfur source is one or more of sulfuric acid, ammonium sulfate, sodium sulfate / lithium / potassium, and ferrous sulfate.

[0050] Preferably, the water-soluble transition metal source is an iron source, a manganese source, a cobalt source or a nickel source, the iron source is one or more of ferric nitrate, ferric sulfate / ferrous sulfate, ferric acetate, and ferric formate, the manganese source is one or more of manganese nitrate, manganese sulfate, manganese acetate, and manganese chloride, the cobalt source is one or more of cobalt sulfate, cobalt nitrate, and cobalt chloride, and the nickel source is one or more of nickel sulfate, nickel nitrate, and nickel chloride.

[0051] Preferably, the water-soluble carbon source is one or more of sucrose, glucose, citric acid, cyclodextrin, lactose, maltose, polyvinyl alcohol, and polyethylene glycol.

[0052] Furthermore, the positive electrode material of the polyanion secondary battery is a lithium battery material such as lithium iron phosphate, lithium iron manganese phosphate, fluorolithium iron phosphate, fluorolithium iron manganese phosphate, lithium iron sulfate, etc.; sodium iron phosphate, sodium iron pyrophosphate, sodium iron pyrophosphate, sodium vanadium phosphate, sodium vanadium fluorophosphate, sodium iron fluorophosphate, sodium iron sulfate or sodium iron fluorosulfate.

[0053] Application Example 1 Synthesis of Low-Porosity Na3V2(PO4)3 / C and Its Electrochemical Performance

[0054] The preparation steps of low-porosity Na3V2(PO4)3 / C in this embodiment are as follows:

[0055] Step 1: Sodium dihydrogen phosphate and vanadium pentoxide are ground with water in a molar ratio of 3:2 until the solid particle size of vanadium pentoxide in the slurry is Dmax≤100, and the grinding is stopped to form a uniform yellow-brown emulsion; Step 2: The yellow-brown emulsion is spray-dried, and the inlet air temperature is set to 280°C to remove the solvent to achieve solid-liquid separation. At this time, the moisture content of the dried spherical precursor powder is less than 1%; Step 3: Sodium dihydrogen phosphate, ammonium metavanadate, and glucose are added in a molar ratio of 3:2:5 at 9 Under heating conditions of 0°C, water is slowly added to form a saturated solution; Step 4: The spherical precursor dry powder is placed in a blender while controlling the temperature to 90°C, and the saturated solution is sprayed into um-level small droplets using a pressure spray gun, and evaporated while spraying until the dry powder begins to agglomerate. At this time, the liquid is saturated to form a dense spherical precursor dry powder; Step 5: The dense spherical precursor dry powder is kept at 750°C for 10 hours, and naturally cooled and crystallized to obtain a low-porosity Na3V2(PO4)3 / C material.

[0056] Low-porosity Na₃V₂(PO₄)₃ / C material, SurP, and PVDF5130 were mixed and homogenized in a mass ratio of 9.5:0.2:0.3. The solids content was controlled at 55%. The slurry exhibited a viscosity below 1000 Pa.s, indicating good fluidity. This indicates that the binder and solvent did not readily penetrate the material during the slurrying process, remaining as a fluid around the particles. This slurry exhibited good fluidity, which is related to the material's low porosity and facilitates coating processing. The black slurry was then coated onto aluminum foil using a 150 μm four-sided coating machine. The film was dried in a vacuum oven at 100°C for 2 hours. The electrode film was punched out into discs with a radius of 0.6 mm using a sheet puncher. CR2016 button cell batteries were assembled in a glove box using sodium metal as the counter electrode, 1 mol / L NaClO₄EC+DEC (1:1 vol%)+5%FEC as the electrolyte, and a PP / PE / PP three-layer separator.

[0057] Figure 1This is an SEM image of the low-porosity Na3V2(PO4)3 / C material. The particles are slightly adhered to each other. This is due to the slightly excessive amount of saturated solution sprayed during the pore filling process. It is not difficult to see that the surface of each particle is smooth and dense, with no obvious pores remaining. From the partially broken particles, it can also be seen that the particles after pore filling are no longer hollow, and the degree of solidification is increased. In contrast, the comparative example 1 ( Figure 2 The material is a hollow spherical particle, and the spherical shell is formed by the accumulation of irregular primary particles with high porosity. The physical and chemical properties test in Table 1 shows that the compacted density of the material is as high as 2.35g / cm 3 , the specific surface area is only 4.5m 2 / g, compared with 1.95g / cm in Comparative Example 1 3 The compacted density and 13.8m 2 / g, the improvement is obvious, indicating that after the pore-filling treatment, the density of the particles increases, the specific surface area decreases, and the compaction density increases.

[0058] Table 1 shows that the reversible discharge capacity of the low-porosity Na3V2(PO4)3 / C electrode at a rate of 0.1C (1C=120mAh / g) is 118mAh / g, which is higher than the 109mAh / g in Comparative Example 1. This difference is related to the uniformity of the precursor ion dispersion and the integrity of the crystal growth during sintering. By adding the carbon source after spraying, we have reduced the effect of the carbon source on the ion uniformity during grinding and spraying to a certain extent, thereby regulating the growth state of the crystal during sintering and improving its capacity. In addition, Table 1 shows that the reversible capacity of the electrode is 105mAh / g at a rate of 10C, and the retention rate relative to 0.1C is 90.6%, showing excellent rate performance, which is related to the construction of the carbon conductive network, effectively improving the conductivity of the electrons, and finally the electrode has a capacity retention rate of up to 98.6% after 1000 cycles, showing excellent cycle stability. Compared with Comparative Example 1 (94.4%), the higher capacity retention rate of this electrode is related to its low porosity. The lower porosity reduces the contact area between the material and the electrolyte to a certain extent, reduces the occurrence of interfacial side reactions, and thus increases its structural stability during long cycles.

[0059] Application Example 2 Synthesis and Electrochemical Performance of Low-Porosity Na4Fe3(PO4)2P2O7 / C

[0060] The preparation steps of low-porosity Na4Fe3(PO4)2P2O7 / C in this embodiment are as follows:

[0061] Step 1: Sodium dihydrogen phosphate and iron oxide are ground with water in a molar ratio of 4:1.5 until the solid particle size of iron oxide in the slurry is Dmax≤50, and then the grinding is stopped to form a uniform reddish-brown emulsion; Step 2: The reddish-brown emulsion is spray-dried, and the inlet air temperature is set to 300°C to remove the solvent to achieve solid-liquid separation. At this time, the moisture content of the dried spherical precursor powder is less than 0.5%; Step 3: Sodium dihydrogen phosphate, ferric nitrate nonahydrate, and glucose are added in a molar ratio of 4:3:7 at 90 °C heating conditions, slowly add water to form a saturated solution; Step 4: Place the spherical precursor dry powder in a blender while controlling the temperature to 90°C, and use a pressure spray gun to spray the saturated solution into um-level small droplets, evaporating while spraying until the dry powder begins to agglomerate. At this time, the liquid is saturated to form a dense spherical precursor dry powder; Step 5: Keep the dense spherical precursor dry powder at 650°C for 12 hours, and naturally cool it down for crystallization to obtain low-porosity Na4Fe3(PO4)2P2O7 / C material.

[0062] Low-porosity Na₄Fe₃(PO₄)₂P₂Oₐ / C material, SurP, and PVDF5130 were mixed in a mass ratio of 9.5:0.2:0.3 to prepare a slurry with a solids content of 55%. The slurry exhibited a viscosity below 1000 Pa.s, demonstrating good fluidity. This is attributed to the separation of the solid and liquid phases during the slurrying process, indicating that the liquid is less likely to adsorb within the solid particles, indirectly reflecting the material's low porosity. Consequently, the slurry exhibited good processability. The black slurry was then coated onto aluminum foil using a 150 μm four-sided film preparation device. The film was dried in a vacuum oven at 100°C for 2 hours. The electrode film was punched into discs with a radius of 0.6 mm using a sheet puncher. CR2016 button cell batteries were assembled in a glove box using sodium metal as the counter electrode, 1 mol / L NaClO₄EC+DEC (1:1 vol%)+5%FEC as the electrolyte, and a PP / PE / PP three-layer separator.

[0063] The physical and chemical properties test in Table 1 shows that the compacted density of the material is as high as 2.30g / cm 3 , the specific surface area is only 5.6m 2 / g, compared with 1.85g / cm in Comparative Example 2 3 The compacted density and 16.7m 2 / g, the improvement is significant, indicating that after the pore-filling treatment, the effective volume density of the material increases, the specific surface area decreases, and the compaction density increases.

[0064] Table 1 shows that the reversible discharge capacity of the low-porosity Na4Fe3(PO4)2P2O7 / C electrode at a rate of 0.1C (1C=129mAh / g) is 112mAh / g, which is higher than the 98mAh / g in Comparative Example 1. The difference in performance is related to the degree of crystallinity of the material, and the degree of crystallinity is related to the uniformity of ion mixing during the precursor preparation process, indicating that the form of post-addition of the carbon source can improve the degree of dispersion between ions, thereby improving its degree of crystallinity and promoting the release of capacity. In addition, Table 1 shows that the reversible capacity of the electrode is 103mAh / g at a rate of 10C, and the retention rate relative to 0.1C is 91.96%, showing excellent rate performance, which is related to the construction of the carbon conductive network, effectively improving the electrical conductivity of the electrons, and finally the electrode has a capacity retention rate of up to 99.5% after 1000 cycles, with almost no capacity decay, which is related to the lower porosity of the material, which reduces the interface side reaction between the material and the electrolyte to a certain extent, thereby increasing its cycle stability.

[0065] Comparative Example 1 Synthesis and Electrochemical Performance of Na3V2(PO4)3 / C

[0066] The preparation steps of Na3V2(PO4)3 / C in this embodiment are as follows:

[0067] Step 1: Sodium dihydrogen phosphate, vanadium pentoxide, and glucose are added with water in a molar ratio of 3:2:3 and ground until the solid particle size Dmax of vanadium pentoxide in the slurry is ≤100, then the grinding is stopped to form a uniform yellowish-brown emulsion; Step 2: The yellowish-brown emulsion is spray-dried, the inlet air temperature is set to 280°C, the solvent is removed, and solid-liquid separation is achieved. At this time, the moisture content of the dried spherical precursor dry powder is less than 1%; Step 4: The spherical precursor dry powder is kept at 750°C for 10 hours, and naturally cooled and crystallized to obtain Na3V2(PO4)3 / C material.

[0068] After the Na3V2(PO4)3 / C material, SurP and PVDF5130 were mixed and slurried in a mass ratio of 9.5:0.2:0.3, the solid content was controlled at 55%. The viscosity of the slurry was higher than 5000Pa.S during discharge, and the fluidity was poor, which affected the uniformity and consistency of the subsequent coating. Compared with the phenomenon in Application Example 1, it is shown that the material will adsorb binders and solvents during the slurrying process, resulting in a decrease in the amount of fluid liquid around the material particles, so that the slurry shows a certain viscosity, which is directly related to the high porosity of the material itself, which is not conducive to the subsequent coating process of the material. Afterwards, the black slurry was coated on aluminum foil using a 150um four-sided preparation device, a relatively smooth and flat membrane was selected, and the membrane was dried in a vacuum drying oven at 100°C for 2 hours. The electrode film was punched into a disc with a radius of 0.6 mm using a punching machine. Metallic sodium was used as the counter electrode, 1 mol / L NaClO4EC+DEC (1:1 vol%)+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.

[0069] Figure 2 This is the SEM image of the Na3V2(PO4)3 / C material. The particles have high sphericity and are regular hollow spherical particles. The shell is composed of nanoparticles stacked together, and there are a lot of pores between the nanoparticles. The physical and chemical properties test in Table 1 shows that the compacted density of the material is only 1.95g / cm 3 , with a specific surface area of ​​up to 13.8m 2 / g, compared with Application Example 1, the material physical properties caused by the larger porosity are seriously deteriorated, affecting the normal processing and utilization of the material.

[0070] Table 1 shows that the reversible discharge capacity of Na3V2(PO4)3 / C electrode at 0.1C (1C=120mAh / g) rate is 109mAh / g, which is much lower than the reversible capacity of the material in Application Example 1. It is explained that the introduction of a carbon source in the initial stage of the material will affect ion distribution, causing crystal growth to be hindered during sintering, and it is impossible to form a high-crystallinity large-particle single crystal, and the capacity is limited. At the same time, a large number of pores remain inside the material after spraying, and the presence of the pores has a shielding effect on crystal growth, which also affects the integrity of the crystal and the performance of the capacity. In addition, Table 1 shows that the reversible capacity of the electrode at 10C rate is 73mAh / g, and the retention rate relative to 0.1C is 66.9%, showing poor rate performance, which is related to the grain growth size being too small, integrity being too low, and the ion migration channel being poor, thereby causing the rate performance of the material to decline. Ultimately, the electrode is cycled through 1000 weeks, and the capacity retention rate is only 94.4%, and the cycle stability is poor. Compared with Application Example 1 (98.6%), the higher porosity of the electrode increases the contact area with the electrolyte, the solubility of transition metals and anions at the material interface increases, the interfacial side reactions intensify, and the cycle stability decreases.

[0071] Comparative Example 2 Synthesis and Electrochemical Performance of Na4Fe3(PO4)2P2O7 / C

[0072] The preparation steps of Na4Fe3(PO4)2P2O7 / C in this embodiment are as follows:

[0073] Step 1: Sodium dihydrogen phosphate, iron oxide, and glucose are added with water in a molar ratio of 4:1.5:3.5 and ground until the solid particle size Dmax of iron oxide in the slurry is ≤50, then the grinding is stopped to form a uniform reddish-brown emulsion; Step 2: The reddish-brown emulsion is spray-dried, the inlet air temperature is set to 300°C, the solvent is removed, and solid-liquid separation is achieved. At this time, the moisture content of the dried spherical precursor dry powder is less than 0.5%; Step 3: The dense spherical precursor dry powder is kept at 650°C for 12 hours, and naturally cooled and crystallized to obtain Na4Fe3(PO4)2P2O7 / C material.

[0074] After homogenizing Na4Fe3(PO4)2P2O7 / C, SurP, and PVDF5130 in a mass ratio of 9.5:0.2:0.3, with a solids content of 55%, the slurry exhibited a viscosity exceeding 7000 Pa·s, indicating poor fluidity. Some portions of the slurry exhibited a jelly-like consistency, making it difficult to coat. This suggests that during the slurrying process, most of the liquid was absorbed into the solid material particles through the pores, resulting in a viscous slurry. The black slurry was then coated onto aluminum foil using a 150 μm four-sided film preparation device. Smooth and uniform film sheets were selected and dried in a vacuum oven at 100°C for 2 hours. The electrode films were punched out into 0.6 mm radius discs using a sheet puncher. CR2016 button cell batteries were assembled in a glove box using sodium metal as the counter electrode, 1 mol / L NaClO4EC+DEC (1:1 vol%)+5%FEC as the electrolyte, and a PP / PE / PP three-layer separator.

[0075] The physical and chemical properties test in Table 1 shows that the compacted density of the material is only 1.85g / cm 3 , with a specific surface area of ​​up to 16.7m 2 / g, compared with Application Example 2, the results are significantly degraded, which is related to the hollow structure and large pore distribution of the material. The electrochemical performance test results in Table 1 show that the reversible discharge capacity of the Na4Fe3(PO4)2P2O7 / C electrode at a rate of 0.1C (1C=129mAh / g) is 98mAh / g, which is lower than the performance in Application Example 2. This shows that the large pore distribution and carbon content in the preparation process of the material precursor hinder the epitaxial growth of the crystal, and the large amount of nano-grain interface capacity loss generated is serious, which ultimately leads to a low effective capacity of the material. In addition, Table 1 shows that the reversible capacity of the electrode is 63 mAh / g at a rate of 10 C, and the retention rate relative to 0.1 C is 64.3%, which is poor rate performance. This is related to the low integrity of a large number of nano-grains and the obstruction of interfacial ion transport. Finally, after 1000 cycles, the capacity retention rate of the electrode is only 95.3%, which is more serious than that of Application Example 2. This is related to the higher porosity of the material, which will aggravate the dissolution of the material in the electrolyte and the generation of side reactions to a certain extent, thereby aggravating the deterioration of its cycle performance.

[0076] Table 1 Performance test results of different embodiments

[0077]

[0078] 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 low-porosity polyanion secondary battery positive electrode material, characterized in that The following steps are involved: S1. Emulsion preparation: adding water to a first alkali metal source, a water-insoluble transition metal source, and a first anion source, and grinding them until the particle size reaches nanometer level to form a uniform emulsion; S2. Spray drying of the precursor dry powder: spray drying the emulsion to achieve solid-liquid separation to obtain uniformly dried spherical precursor dry powder; S3. Preparation of a saturated solution: dissolving the second alkali metal source, the water-soluble transition metal source, the second anion source, and the water-soluble carbon source in water and heating to form a saturated solution; S4. Densification treatment of the spherical precursor dry powder: slowly spraying and adding the saturated solution obtained in step S3 to the spherical precursor dry powder obtained in step S2 under heating and stirring conditions, so that the saturated solution penetrates into the pores and cavities inside the spherical precursor dry powder to fill the pores and form a dense spherical precursor dry powder; S5. High-temperature sintering: The dense spherical precursor dry powder is calcined at high temperature, and naturally cooled and crystallized to finally form a low-porosity polyanion secondary battery positive electrode material.

2. The method for preparing a low-porosity polyanion secondary battery positive electrode material according to claim 1, wherein: In step S1 , the mixed grinding method is sand milling or ball milling.

3. The method for preparing a low-porosity polyanion secondary battery positive electrode material according to claim 1, wherein: In step S1 , the maximum particle size of the solid particles contained in the nano-emulsion is less than 200 nm.

4. The method for preparing a low-porosity polyanion secondary battery positive electrode material according to claim 1, wherein: In step S2, the spray drying conditions are as follows: the air inlet temperature is ≥260°C, and the water content of the spherical precursor dry powder is less than 3%.

5. The method for preparing a low-porosity polyanion secondary battery positive electrode material according to claim 1, wherein: In step S3, the heating temperature is greater than 80° C., and the solid content of the saturated solution is greater than 35%.

6. The method for preparing a low-porosity polyanion secondary battery positive electrode material according to claim 1, wherein: In step S4, the heating condition is that the heating temperature is greater than 80°C.

7. The method for preparing a low-porosity polyanion secondary battery positive electrode material according to claim 1, wherein: In step S4, the saturated solution is sprayed in a pressure-type or centrifugal-type manner.

8. The method for preparing a low-porosity polyanion secondary battery positive electrode material according to claim 1, wherein: In step S5, the sintering temperature is greater than 400° C. and the holding time is greater than 3 h.

9. The method for preparing a low-porosity polyanion secondary battery positive electrode material according to claim 1, characterized in that: The first alkali metal source and the second alkali metal source are lithium sources or sodium sources, the lithium source is one or more of metallic lithium, lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, lithium hydride, and lithium sulfate; the sodium source is one or more of metallic sodium, sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium formate, sodium acetate, and sodium sulfate; The water-insoluble transition metal source is an iron source, a cobalt source, a nickel source, and a manganese source. The iron source is one or more of elemental iron, iron oxide, ferrous oxalate, ferric phosphate, and ferric hydroxide. The cobalt source is one or more of elemental cobalt, cobalt hydroxide, cobalt oxide, cobalt oxalate, and cobalt carbonate. The nickel source is one or more of elemental nickel, nickel oxide, and nickel hydroxide. The manganese source is one or more of manganese, manganese oxide, manganese carbonate, and manganese oxalate. The first anion source and the second anion source are a phosphorus source, a fluorine source or a sulfur source, the phosphorus source is one or more of phosphoric acid, sodium monohydrogen phosphate, sodium dihydrogen phosphate, sodium phosphate, ammonium dihydrogen phosphate, pyrophosphoric acid and its salt derivatives, metaphosphoric acid and its salt derivatives, polyphosphoric acid and its salt derivatives, the fluorine source is one or more of hydrogen fluoride, sodium fluoride, ammonium fluoride, and lithium fluoride, and the sulfur source is one or more of sulfuric acid, ammonium sulfate, sodium sulfate / lithium / potassium sulfate, and ferrous sulfate; The water-soluble transition metal source is an iron source, a manganese source, a cobalt source or a nickel source, the iron source is one or more of ferric nitrate, ferric sulfate / ferrous sulfate, ferric acetate, and ferric formate, the manganese source is one or more of manganese nitrate, manganese sulfate, manganese acetate, and manganese chloride, the cobalt source is one or more of cobalt sulfate, cobalt nitrate, and cobalt chloride, and the nickel source is one or more of nickel sulfate, nickel nitrate, and nickel chloride; The water-soluble carbon source is one or more of sucrose, glucose, citric acid, cyclodextrin, lactose, maltose, polyvinyl alcohol, and polyethylene glycol.

10. The method for preparing a low-porosity polyanion secondary battery positive electrode material according to claim 1, characterized in that: The positive electrode material of the polyanion secondary battery is lithium iron phosphate, lithium iron manganese phosphate, fluorolithium iron phosphate, fluorolithium iron manganese phosphate, lithium iron sulfate and other lithium battery materials; sodium iron phosphate, sodium iron pyrophosphate, sodium iron pyrophosphate, sodium vanadium phosphate, sodium vanadium fluorophosphate, sodium iron fluorophosphate, sodium iron sulfate or sodium iron fluorosulfate.

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