Core-shell composite structure secondary battery positive electrode material and preparation method thereof

Abstract

The present invention discloses a core-shell composite secondary battery positive electrode material and a preparation method thereof. The core-shell composite secondary battery positive electrode material is LiMPO4@Na4M3(PO4)2P2O7, comprising a LiMPO4 core and a Na4M3(PO4)2P2O7 shell, where M is one or more transition metal elements. The preparation method comprises the following steps: S1, preparation of an emulsion; S2, separation of raw material powders; S3, pre-sintering of the core structure; S4, mixing of a second precursor powder; and S5, high-temperature calcination of the core-shell structure. The preparation method of the core-shell composite secondary battery positive electrode material of the present invention has the characteristics of strong structural controllability, high specific capacity utilization, high compaction density, and good cycle stability.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and in particular to a core-shell composite structure secondary battery positive electrode material and a preparation method thereof. Background Art

[0002] Polyanion sodium-ion batteries (NABs) have gained market favor due to their high structural stability, low cost, and excellent cycle stability. Currently, the most widely studied polyanion material system is composite sodium iron phosphate (Na4Fe3(PO4)2P2O7). This system consists of alkali metal groups, transition metal groups, and two anion groups arranged in a co-located point / surface / line pattern to form a three-dimensional framework. This structure is difficult to prepare in pure form, resulting in low actual capacity and significant challenges for industrialization.

[0003] Based on comprehensive considerations of cost, process feasibility and product stability, the production of composite sodium iron phosphate is often synthesized by the solid-phase method, which uses a water-soluble sodium source, a water-insoluble transition metal source (MPO4) and a water-soluble phosphorus source as raw materials, and the material is prepared through processes such as grinding, mixing, spray drying and sintering.

[0004] However, the water-insoluble MPO4 is difficult to dissolve during the grinding process, and its particles are difficult to grind into an amorphous state, resulting in uneven distribution of sodium, metal M and phosphorus elements, which easily induces the production of inactive NaMPO4 impurities and affects the electrochemical performance of the composite sodium iron phosphate.

[0005] Specifically, using MPO4 crystals as raw materials to prepare Na4M3(PO4)2P2O7 is a common method in the current industry. However, after high-temperature sintering, MPO4 crystals have high crystallinity and large particle size. Traditional grinding equipment can only destroy the surface structure during the grinding process, making it amorphous, while the area within 30nm of its core is still highly crystalline, which can easily lead to uneven ion distribution. During the sintering process, sodium ions are embedded in the MPO4 lattice to form an inactive NaMPO4 phase, resulting in a reduction in the material capacity. Summary of the Invention

[0006] The purpose of the present invention is to provide a method for preparing a core-shell composite structure secondary battery positive electrode material, which has the characteristics of strong structural controllability, high gram capacity utilization, high compaction density and good cycle stability.

[0007] The present invention can be achieved through the following technical solutions:

[0008] The present invention discloses a method for preparing a core-shell composite structure secondary battery positive electrode material. The core-shell composite structure secondary battery positive electrode material is LiMPO4@Na4M3(PO4)2P2O7, comprising a LiMPO4 core and a Na4M3(PO4)2P2O7 shell, where M is one or more of the transition metal elements manganese, iron, cobalt, and nickel. The preparation method comprises the following steps:

[0009] S1. Preparation of emulsion: mixing lithium source and MPO4 crystals with dispersant and water in a stoichiometric ratio, and wet grinding to form a uniform emulsion;

[0010] S2. Separation of raw material powder: drying the emulsion to achieve solid-liquid separation and obtain dry raw material powder;

[0011] S3. Pre-sintering of the core structure: Pre-sintering the raw material powder at low temperature under a protective atmosphere to complete the construction of the LiMPO4 core structure, and then cooling naturally to obtain the first precursor powder;

[0012] S4. Mixing the second precursor powder: mixing the first precursor powder, the sodium source, the pyrophosphate, and the carbon source, and dry grinding to form a uniform second precursor powder;

[0013] S5. High-temperature calcination of core-shell structure: Under a protective atmosphere, the second precursor powder is calcined at a high temperature to complete the construction of the Na4M3(PO4)2P2O7 shell structure. After natural cooling, the LiMPO4@Na4M3(PO4)2P2O7 core-shell composite structure secondary battery positive electrode material is obtained.

[0014] The present invention constructs a LiMPO4@Na4M3(PO4)2P2O7 core-shell composite structure, consisting of a LiMPO4 core and a Na4M3(PO4)2P2O7 shell. This composite structure combines high capacity with high compaction. The inventive technique involves mixing a small amount of lithium source and MPO4 crystals with a dispersant and water in a stoichiometric ratio, grinding them uniformly, and then spray-drying them. Because lithium ions have a smaller radius, they are more compatible with the MPO4 lattice. During the low-temperature pre-sintering process, lithium ions can slowly embed into the non-amorphized lattice within the MPO4 particles, forming the LiMPO4 core material. Then, sodium source and pyrophosphate are further added, mixed and ground evenly, and then sintered at high temperature. The amorphous part of the MPO4 particles crystallizes and grows with elements such as sodium and pyrophosphate to form a Na4M3(PO4)2P2O7 shell. Thanks to the higher capacity and compaction density of the active LiMPO4 core, the LiMPO4@Na4M3(PO4)2P2O7 core-shell composite structure exhibits better performance than the single Na4M3(PO4)2P2O7 material.

[0015] Furthermore, in step S1, the molar ratio of lithium element in the lithium source and transition metal M in the MPO4 crystal is 1:15-20; when the molar ratio is too low, an inactive NaMPO4 phase will be derived from the LiMPO4 core, and when the molar ratio is too high, a Li4M3(PO4)2P2O7 phase will be accompanied in the Na4M3(PO4)2P2O7 shell, and its structural stability is poor.

[0016] Furthermore, in step S3, the low-temperature pre-sintering conditions are: sintering temperature 300-500°C, holding time 0.5-2H; when the sintering temperature is too low, lithium ions cannot effectively diffuse into the MPO4 lattice to generate the LiMPO4 core, and too high a temperature will reduce production efficiency and increase energy consumption.

[0017] Furthermore, in step S5, the high-temperature calcination conditions are: calcination temperature 500-650° C., and holding time 2-15 hours, to ensure sufficient crystal growth of the material.

[0018] Furthermore, in step S1, the MPO4 crystal contains one or more transition metal compounds selected from the group consisting of MnPO4, FePO4, CoPO4, and NiPO4.

[0019] Furthermore, in step S4, the pyrophosphate is one or more of pyrophosphoric acid, sodium pyrophosphate, and ammonium pyrophosphate.

[0020] Furthermore, in steps S1 and S4, the grinding is one or more of planetary ball milling, high-energy ball milling, and sand milling; such grinding methods use mechanical force or shear force to achieve the purpose of uniform dispersion between materials.

[0021] Furthermore, in step S2, the drying method is one or more of evaporative crystallization, spray drying, freeze drying, vacuum drying, rotary evaporation drying, and flash drying, and the purpose of solid-liquid separation is achieved by the above drying methods.

[0022] Furthermore, in step S3, the protective atmosphere is hydrogen and / or carbon monoxide, and such protective atmosphere is an atmosphere capable of reducing transition metals at a certain temperature.

[0023] Furthermore, in step S5, the protective atmosphere is one or more of nitrogen, argon, and helium, and such protective atmosphere is a non-oxidizing atmosphere.

[0024] Furthermore, in step S1, the lithium source is one or more of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, lithium nitrate, lithium sulfate, lithium chloride, lithium fluoride, lithium oxalate, and lithium citrate.

[0025] Furthermore, in step S4, the sodium source is one or more of sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium formate, sodium acetate, sodium sulfate, sodium nitrate, sodium chloride, sodium citrate, and sodium gluconate.

[0026] Furthermore, in step S1, the dispersant is one or more of polyethylene glycol series, polyvinyl alcohol series, polyvinyl pyrrolidone series, polyacrylic acid series, and carboxymethyl cellulose series.

[0027] Furthermore, in step S4, the carbon source is one or more of citric acid, starch, maltose, sucrose, lactose, glucose, phenolic resin, carbon fiber, carbon nanotubes, graphene, and graphite.

[0028] Another aspect of the present invention is to protect a secondary battery positive electrode material, which is prepared using the above preparation method.

[0029] The present invention provides a core-shell composite structure secondary battery positive electrode material and a preparation method thereof, which has the following beneficial effects:

[0030] First, the structure is highly controllable. During the grinding process of MPO4 crystals, the surface layer will partially dissolve and become amorphous under the action of shear force, while the core will still be highly crystalline. Since the radius of lithium ions (68pm) is smaller than that of sodium ions (102pm), their migration rate and diffusion potential in MPO4 crystals are faster than those of sodium ions. During the first sintering and low-temperature pre-sintering process, they will preferentially embed into the MPO4 crystal to form the LiMPO4 core. After that, sodium source and pyrophosphate are added for mixing. During the second sintering process, the dissolved and amorphous MPO4 part combines with the sodium source and pyrophosphate to form the Na4M3(PO4)2P2O7 outer core, and finally forms the LiMPO4@Na4M3(PO4)2P2O7 core-shell structure.

[0031] Second, the capacity utilization rate is high. The Na4M3(PO4)2P2O7 material prepared using MPO4 crystals as a precursor contains an inactive NaMPO4 phase, resulting in a low capacity utilization of the NaMPO4@Na4M3(PO4)2P2O7 core-shell structure. Inspired by the high theoretical specific capacity of LiMPO4 (170mAh / g), we introduced lithium ions into the MPO4 crystals to construct the LiMPO4@Na4M3(PO4)2P2O7 core-shell structure. This structure significantly improves the capacity compared to the NaMPO4@Na4M3(PO4)2P2O7 core-shell structure.

[0032] Third, the compaction density is high. The theoretical true density of LiMPO4 material is 3.6g / cm 3 , the corresponding actual achievable compaction density is 2.6g / cm3 , while the theoretical true density of Na4M3(PO4)2P2O7 material is only 3.1g / cm 3 The actual achievable compaction density is only 1.9 g / cm 3 Therefore, the construction of the LiMPO4@Na4M3(PO4)2P2O7 core-shell structure will significantly improve the compaction density of the Na4M3(PO4)2P2O7 material.

[0033] Fourth, good cycle stability: The stability of the electrode material and the electrode-electrolyte interface film is the most important parameter affecting the battery cycle stability. The sodium-ion battery prepared with Na4M3(PO4)2P2O7 as the positive electrode has a negative electrode interface SEI film composed mainly of sodium carbonate, sodium fluoride and other components that are easily soluble in the electrolyte, resulting in the continuous consumption of sodium ions in the positive electrode and a continuous decrease in cycle capacity. However, the sodium-ion battery prepared with LiMPO4@Na4M3(PO4)2P2O7 as the positive electrode has a negative electrode interface SEI film composed of lithium carbonate and lithium fluoride components that are not easily soluble, which can greatly improve the battery's cycle stability. DETAILED DESCRIPTION

[0034] 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.

[0035] The present invention discloses a method for preparing a core-shell composite structure secondary battery positive electrode material. The core-shell composite structure secondary battery positive electrode material is LiMPO4@Na4M3(PO4)2P2O7, comprising a LiMPO4 core and a Na4M3(PO4)2P2O7 shell, where M is one or more of the transition metal elements manganese, iron, cobalt, and nickel. The preparation method comprises the following steps:

[0036] S1. Preparation of emulsion: mixing lithium source and MPO4 crystals with dispersant and water in a stoichiometric ratio, and wet grinding to form a uniform emulsion;

[0037] S2. Separation of raw material powder: drying the emulsion to achieve solid-liquid separation and obtain dry raw material powder;

[0038] S3. Pre-sintering of the core structure: Pre-sintering the raw material powder at low temperature under a protective atmosphere to complete the construction of the LiMPO4 core structure, and then cooling naturally to obtain the first precursor powder;

[0039] S4. Mixing the second precursor powder: mixing the first precursor powder, the sodium source, the pyrophosphate, and the carbon source, and dry grinding to form a uniform second precursor powder;

[0040] S5. High-temperature calcination of core-shell structure: Under a protective atmosphere, the second precursor powder is calcined at a high temperature to complete the construction of the Na4M3(PO4)2P2O7 shell structure. After natural cooling, the LiMPO4@Na4M3(PO4)2P2O7 core-shell composite structure secondary battery positive electrode material is obtained.

[0041] Furthermore, in step S1, the molar ratio of the lithium element in the lithium source to the transition metal M in the MPO4 crystal is 1:15-20.

[0042] Furthermore, in step S3, the low-temperature pre-sintering conditions are: sintering temperature 300-500° C., and holding time 0.5-2 hours.

[0043] Furthermore, in step S5, the high-temperature calcination conditions are: calcination temperature 500-650° C., and holding time 2-15 hours.

[0044] Furthermore, in step S1, the MPO4 crystal contains one or more transition metal compounds selected from the group consisting of MnPO4, FePO4, CoPO4, and NiPO4.

[0045] Furthermore, in step S4, the pyrophosphate is one or more of pyrophosphoric acid, sodium pyrophosphate, and ammonium pyrophosphate.

[0046] Furthermore, in steps S1 and S4, the grinding is one or more of planetary ball milling, high energy ball milling, and sand milling.

[0047] Furthermore, in step S2, the drying method is one or more of evaporative crystallization, spray drying, freeze drying, vacuum drying, rotary evaporation drying, and flash drying, and the purpose of solid-liquid separation is achieved by the above drying methods.

[0048] Furthermore, in step S3, the protective atmosphere is hydrogen and / or carbon monoxide.

[0049] Furthermore, in step S5, the protective atmosphere is one or more of nitrogen, argon, and helium.

[0050] Furthermore, in step S1, the lithium source is one or more of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, lithium nitrate, lithium sulfate, lithium chloride, lithium fluoride, lithium oxalate, and lithium citrate.

[0051] Furthermore, in step S4, the sodium source is one or more of sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium formate, sodium acetate, sodium sulfate, sodium nitrate, sodium chloride, sodium citrate, and sodium gluconate.

[0052] Furthermore, in step S1, the dispersant is one or more of polyethylene glycol series, polyvinyl alcohol series, polyvinyl pyrrolidone series, polyacrylic acid series, and carboxymethyl cellulose series.

[0053] Furthermore, in step S4, the carbon source is one or more of citric acid, starch, maltose, sucrose, lactose, glucose, phenolic resin, carbon fiber, carbon nanotubes, graphene, and graphite.

[0054] Another aspect of the present invention is to protect a secondary battery positive electrode material, which is prepared using the above preparation method.

[0055] Example 1 Synthesis and electrochemical performance of LiFePO4@Na4Fe3(PO4)2P2O7 core-shell structure

[0056] This embodiment relates to a core-shell composite structure secondary battery positive electrode material, the core-shell composite structure secondary battery positive electrode material is LiMPO4@Na4M3(PO4)2P2O7, including a LiMPO4 core and a Na4M3(PO4)2P2O7 shell, and the preparation method is as follows:

[0057] S1. Preparation of emulsion: lithium hydroxide and FePO4 crystals were mixed and ground with PEG8000 (added in an amount of 2% of the weight of the FePO4 crystals) in a molar ratio of 1:15, with the solid content controlled at 40%, and ground until the FePO4 particle size D90 ≤ 100 nm to form a uniform emulsion;

[0058] S2. Separation of raw material powder: spray drying the emulsion at an air inlet temperature of 220°C and an air outlet temperature of 80°C to achieve solid-liquid separation and obtain dry raw material powder;

[0059] S3. Pre-sintering of the core structure: In a nitrogen-hydrogen reducing atmosphere, the raw material powder is kept at 400°C for 1 hour to allow lithium ions to slowly diffuse into the non-amorphized lattice of the FePO4 particles, completing the construction of the LiFePO4 core structure. The powder is then cooled naturally to obtain a black first precursor powder.

[0060] S4. Mixing the second precursor powder: Mix the black first precursor powder, sodium hydroxide, and pyrophosphoric acid in a weight ratio of 64.5:22.8:12.7 with glucose (added in an amount of 1% of the total solid content), and grind them in a ball mill with a ball-to-material ratio of 20:1 for 10 hours to form a uniform gray second precursor powder;

[0061] S5. High-temperature calcination of core-shell structure: Under a nitrogen inert atmosphere, the second precursor powder was calcined at 550°C for 12 hours to realize the construction of the Na4Fe3(PO4)2P2O7 shell structure. After natural cooling, the LiFePO4@Na4Fe3(PO4)2P2O7 core-shell composite structure material was obtained.

[0062] The LiFePO4@Na4Fe3(PO4)2P2O7 core-shell composite material, SurP, and PVDF5130 were mixed in a mass ratio of 9.5:0.2:0.3, and the solid content was controlled at 58%. The viscosity of the slurry was 2000~3000mPa·s, and the fluidity was good. Table 1 shows that the specific surface area of ​​the core-shell composite material is only 6.5m 3 / g, the powder compaction density is as high as 2.35g / cm 3 , compared with the material in Comparative Example 1, the specific surface area is lower and the powder compaction density is higher, indicating that the core-shell composite structure material has a higher degree of particle density and a lower porosity, which is related to the higher degree of solidification of the LiFePO4 core growth, which can reduce the adsorption of the slurry during the homogenization process to a certain extent and improve its processing performance. Afterwards, a 150um four-sided preparation device was used to coat the black slurry on 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, with metallic sodium as the counter electrode, 1mol / L NaClO4EC+DEC (1:1vol%) + 5%FEC as the electrolyte, and a PP / PE / PP three-layer diaphragm as the diaphragm. CR2016 button batteries were assembled in a glove box.

[0063] The electrochemical performance test results in Table 1 show that the discharge capacity of the material is 131.5 mAh / g at a rate of 0.1C (1C = 129 mAh / g), and the first-week coulombic efficiency is 95.2%, which is much higher than the discharge gram capacity of 103.2 mAh / g and the first-week coulombic efficiency of 86.3% in Comparative Example 1. The difference in electrochemical performance is related to the presence of the LiFePO4 core structure. The material has a theoretical gram capacity of 170 mAh / g and a first-week coulombic efficiency of 98.0%. Therefore, the capacity and first-week coulombic efficiency of the LiFePO4@Na4Fe3 (PO4) 2P2O7 core-shell composite structure material will be greatly improved compared to the single Na4Fe3 (PO4) 2P2O7 material. Finally, the electrode has a capacity retention rate of up to 98.6% after 1000 cycles at 1C, showing excellent cycle stability. Compared with Comparative Example 1 (94.1%), the higher capacity retention rate of this battery is related to the electrochemical activity of the LiFePO4@Na4Fe3(PO4)2P2O7 core-shell composite structure material. The volume changes during the deintercalation and intercalation of sodium / lithium ions are similar, and the lattice is not prone to cracks. However, the single Na4Fe3(PO4)2P2O7 material often contains inactive NaFePO4 phase. As sodium ions are deintercalated, cracks are easily generated between it and the Na4Fe3(PO4)2P2O7 crystals, causing side reactions and affecting the cyclic stability of the material.

[0064] Example 2 Synthesis and electrochemical performance of LiMnPO4@Na4Mn3(PO4)2P2O7 core-shell structure

[0065] This embodiment relates to a core-shell composite structure secondary battery positive electrode material, the core-shell composite structure secondary battery positive electrode material is LiMPO4@Na4M3(PO4)2P2O7, including a LiMPO4 core and a Na4M3(PO4)2P2O7 shell, and the preparation method is as follows:

[0066] S1. Preparation of emulsion: lithium carbonate and MnPO4 crystals were mixed and ground with PEG4000 (added in an amount of 1.5% of the weight of FePO4 crystals) in a molar ratio of 0.5:15, with the solid content controlled at 35%, and ground until the FePO4 particle size D90 ≤ 80 nm to form a uniform emulsion;

[0067] S2. Separation of raw material powder: spray drying the emulsion at an air inlet temperature of 250°C and an air outlet temperature of 90°C to achieve solid-liquid separation and obtain dry raw material powder;

[0068] S3. Pre-sintering of the core structure: In an argon-hydrogen reducing atmosphere, the raw material powder is kept at 350°C for 2 hours to allow lithium ions to slowly diffuse into the non-amorphized lattice of the MnPO4 particles, completing the construction of the LiMnPO4 core structure. The powder is then cooled naturally to obtain a black first precursor powder.

[0069] S4. Mixing the second precursor powder: Mix the black first precursor powder, sodium hydroxide, and pyrophosphoric acid in a weight ratio of 68.2:18.5:13.3 with sucrose (added in an amount of 0.5% of the total solid content), and grind them in a ball mill with a ball-to-material ratio of 20:1 for 20 hours to form a uniform gray second precursor powder;

[0070] S5. High-temperature calcination of core-shell structure: Under a nitrogen inert atmosphere, the second precursor powder is calcined at 650°C for 20 hours to realize the construction of the Na4Mn3(PO4)2P2O7 shell structure. After natural cooling, the LiMnPO4@Na4Mn3(PO4)2P2O7 core-shell composite structure material is obtained.

[0071] The LiMnPO4@Na4Mn3(PO4)2P2O7 core-shell composite material, SurP, and PVDF5130 were mixed in a mass ratio of 9.5:0.2:0.3, and the solid content was controlled at 58%. The viscosity of the slurry was 1500~2500mPa·s, and the fluidity was good. Table 1 shows that the specific surface area of ​​the core-shell composite material is only 5.9m 3 / g, the powder compaction density is as high as 2.28g / cm 3 , it shows a lower specific surface area and higher compaction than the single Na4Mn3(PO4)2P2O7 material, which indirectly reflects the density and solidity of the core-shell composite structure, which is beneficial to reduce the adsorption of the glue during processing, thereby improving the processing performance of the material at the battery end. Afterwards, the black slurry was coated on aluminum foil using a 150um four-sided preparation device, and the film was dried in a vacuum drying oven at 100°C for 2 hours. The electrode film was punched out into a disc with a radius of 0.6mm using a sheet puncher. Sodium metal was used as the counter electrode, 1mol / L NaClO4EC+DEC (1:1vol%) + 5%FEC was used as the electrolyte, and the diaphragm was a PP / PE / PP three-layer diaphragm. CR2016 button batteries were assembled in a glove box.

[0072] The electrochemical performance test results in Table 1 show that the material has a discharge capacity of 126.4 mAh / g and a first-week coulombic efficiency of 93.5% at a rate of 0.1C (1C = 129 mAh / g), which is much higher than the electrochemical performance of the Na4Mn3(PO4)2P2O7 material in Comparative Example 2. From a structural perspective, the theoretical capacity of the LiMnPO4 core is as high as 172 mAh / g, and the olivine-type structure is stable during the lithium ion deintercalation process. There is no capacity irreversibility caused by structural changes, and the first-week coulombic efficiency is as high as 99%. Therefore, the LiMnPO4@Na4Mn3(PO4)2P2O7 core-shell composite structure material has a higher theoretical capacity and first-week coulombic efficiency than the single Na4Mn3(PO4)2P2O7 material. Finally, the electrode has a capacity retention rate of up to 97.2% after 1000 cycles at 1C, showing excellent cycle stability. Compared with Comparative Example 2 (89.6%), the higher capacity retention rate of this battery is related to the structural stability of the core-shell composite material during the sodium / lithium deintercalation process. The volume expansion rates of the two phases of materials are similar, the grain boundaries are not easily split, and there are relatively few side reactions, which is conducive to maintaining the integrity of the structure during the cycle, thereby improving its cycle performance.

[0073] Comparative Example 1 Synthesis and electrochemical properties of Na4Fe3(PO4)2P2O7 material

[0074] This embodiment relates to Na4Fe3(PO4)2P2O7 material, and its preparation method is as follows:

[0075] S1. Preparation of emulsion: FePO4 crystals, sodium hydroxide, and pyrophosphoric acid were mixed and ground in a molar ratio of 3:4:0.5 with PEG8000 (added in an amount of 2% of the weight of the FePO4 crystals) and glucose (added in an amount of 1% of the total solid content). The solid content was controlled at 40%, and the mixture was ground until the FePO4 particle size D90 was ≤100 nm to form a uniform emulsion.

[0076] S2. High-temperature calcination of Na4Fe3(PO4)2P2O7 material: spray dry the above emulsion with an air inlet temperature of 220°C and an air outlet temperature of 80°C to achieve solid-liquid separation and obtain a dry precursor powder; Step 3: Under a nitrogen inert atmosphere, calcine the precursor powder at 550°C for 12 hours, and obtain Na4Fe3(PO4)2P2O7 material after natural cooling.

[0077] The Na4Fe3(PO4)2P2O7 material, SurP, and PVDF5130 were mixed in a mass ratio of 9.5:0.2:0.3 to form a slurry. The solid content was controlled at 58%, and the discharge viscosity was 5000~7000mPa·s. The slurry had poor fluidity and serious problems such as sticking to the roller and demoulding occurred during the coating process. Table 1 shows that the material has a specific surface area of ​​8.9m 3 / g, the powder compaction density is 1.89g / cm 3 , which is much lower than the core-shell composite material in Example 1, indicating that the Na4Fe3(PO4)2P2O7 material prepared with FePO4 as a precursor has low crystallinity, high particle porosity, large specific surface area, reduced material compaction density, and stronger adsorption capacity for the glue during the pulping process, resulting in a higher discharge viscosity under the same conditions and difficulty in coating processing. Afterwards, a 150um four-sided preparation device was used to coat the black slurry on 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, with metallic sodium as the counter electrode, 1mol / L NaClO4EC+DEC (1:1vol%) + 5%FEC as the electrolyte, and a PP / PE / PP three-layer diaphragm as the diaphragm. CR2016 button batteries were assembled in a glove box.

[0078] The electrochemical performance test results in Table 1 show that the discharge capacity of the material is only 103.2 mAh / g at a rate of 0.1C (1C=129 mAh / g), and the first-week coulombic efficiency is 86.3%, which is lower than that of the core-shell composite structure material in Example 1. The reason is that the Na4Fe3(PO4)2P2O7 material prepared with FePO4 as a precursor often contains an inactive NaFePO4 phase, the presence of which may cause partial capacity loss of the material. In addition, the pyrophosphate group is distorted during the sodium deintercalation process of the Na4Fe3(PO4)2P2O7 material, which makes it impossible for some sodium ions to be reintercalated into the structure, thereby causing a decrease in the first-week coulombic efficiency of the material. Finally, after 1000 cycles at 1C, the capacity retention rate of the electrode is only 94.1%, and the cycle stability is poor. This is related to the presence of inactive NaFePO4 phase in the material. The Na4Fe3(PO4)2P2O7 material has volume expansion / contraction during the process of sodium deintercalation, while the NaFePO4 phase does not have the behavior of sodium deintercalation. This causes peeling at the grain boundary between the two phases, which is prone to side reactions with the electrolyte, resulting in a decrease in the material's cycle stability.

[0079] Comparative Example 2 Synthesis and electrochemical properties of Na4Mn3(PO4)2P2O7 materials

[0080] This embodiment relates to Na4Mn3(PO4)2P2O7 material. Its preparation method is:

[0081] S1. Emulsion preparation: MnPO4 crystals, sodium hydroxide, and pyrophosphoric acid were mixed and ground in a molar ratio of 3:4:0.5 with PEG4000 (added in an amount of 1.5% of the weight of the FePO4 crystals) and sucrose (added in an amount of 0.5% of the total solid content). The solid content was controlled at 35%, and the mixture was ground until the FePO4 particle size D90 was ≤80nm to form a uniform emulsion.

[0082] S2. Separation of precursor powder: spray drying the emulsion at an air inlet temperature of 250°C and an air outlet temperature of 90°C to achieve solid-liquid separation and obtain dry precursor powder;

[0083] S3. High-temperature calcination of Na4Mn3(PO4)2P2O7 materials: Under a nitrogen inert atmosphere, the precursor powder was calcined at 650°C for 20 hours, and the Na4Mn3(PO4)2P2O7 material was obtained after natural cooling.

[0084] The Na4Mn3(PO4)2P2O7 material, SurP, and PVDF5130 were mixed and homogenized in a mass ratio of 9.5:0.2:0.3. The solid content was controlled at 58%. The viscosity of the slurry was 8000~10000mPa·s, which had poor fluidity. The leveling property during the coating process was poor, the surface density was uneven, and there was a problem of powder falling off after rolling. Table 1 shows that the material has a surface area of ​​about 8.2m 3 / g, the powder compaction density is only 1.74g / cm 3 , which is worse than the LiMnPO4@Na4Mn3(PO4)2P2O7 core-shell composite structure material. This shows that the single Na4Mn3(PO4)2P2O7 material has high porosity and low crystallinity during the sintering process, resulting in an increase in specific surface area. During the processing, the glue is adsorbed in the pores of the material, increasing the viscosity and affecting the subsequent coating. The black slurry was then coated on aluminum foil using a 150um four-sided preparation device, and the film was dried in a vacuum drying oven at 100°C for 2 hours. The electrode film was punched out into discs with a radius of 0.6mm using a sheet puncher. Metal sodium 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 batteries were assembled in a glove box.

[0085] The electrochemical performance test results in Table 1 show that the material has a discharge capacity of 196.8 mAh / g at a rate of 0.1C (1C = 129 mAh / g), and a first-cycle coulombic efficiency of only 85.1%, which is far lower than the electrochemical performance of the core-shell composite structure material in Example 2. When the Na4Mn3(PO4)2P2O7 material is prepared using MnPO4, an inactive NaMnPO4 phase is often produced due to uneven mixing between ions, resulting in a low actual capacity. At the same time, due to the easy distortion of the pyrophosphate in the Na4Mn3(PO4)2P2O7 material, some sodium ions cannot be re-inserted into the Na4Mn3(PO4)2P2O7 structure after being released, resulting in a low first-cycle coulombic efficiency. Ultimately, after 1000 cycles at 1C, the capacity retention rate of the electrode was only 89.6%, indicating poor cycling performance. Compared with Example 2 (97.2%), the reduction in cycle stability is related to the side reactions at the grain boundaries. When sodium ions are repeatedly embedded and deintercalated in the structure, the interface contact portion between the inactive NaMnPO4 phase and the active Na4Mn3(PO4)2P2O7 phase will split due to volume change. The electrolyte penetrates into this site and decomposes, causing further deterioration of the battery and ultimately a decrease in cycle stability.

[0086] Table 1 Performance test results

[0087]

[0088] Example 3

[0089] This embodiment relates to a method for preparing a positive electrode material for a secondary battery with a core-shell composite structure. The positive electrode material for the core-shell composite structure secondary battery is LiMPO4@Na4M3(PO4)2P2O7, which includes a LiMPO4 core and a Na4M3(PO4)2P2O7 shell. The preparation method includes the following steps:

[0090] S1. Preparation of emulsion: mixing lithium source and MPO4 crystals with dispersant and water in a stoichiometric ratio, and wet grinding to form a uniform emulsion;

[0091] S2. Separation of raw material powder: drying the emulsion to achieve solid-liquid separation and obtain dry raw material powder;

[0092] S3. Pre-sintering of the core structure: Pre-sintering the raw material powder at low temperature under a protective atmosphere to complete the construction of the LiMPO4 core structure, and then cooling naturally to obtain the first precursor powder;

[0093] S4. Mixing the second precursor powder: mixing the first precursor powder, the sodium source, the pyrophosphate, and the carbon source, and dry grinding to form a uniform second precursor powder;

[0094] S5. High-temperature calcination of core-shell structure: Under a protective atmosphere, the second precursor powder is calcined at a high temperature to complete the construction of the Na4M3(PO4)2P2O7 shell structure. After natural cooling, the LiMPO4@Na4M3(PO4)2P2O7 core-shell composite structure secondary battery positive electrode material is obtained.

[0095] In step S1 of this embodiment, M is a transition metal element, manganese, and the molar ratio of the lithium element in the lithium source to the transition metal M in the MPO4 crystal is 1: 20. The MPO4 crystal is a transition metal compound, MnPO4.

[0096] In step S2 of the embodiment, the drying method is evaporative crystallization, and the purpose of solid-liquid separation is achieved by the above drying method.

[0097] In step S3 of this embodiment, the low-temperature pre-sintering conditions are: sintering temperature 500° C., holding time 1 hour, and protective atmosphere of hydrogen.

[0098] In step S5 of this embodiment, the high-temperature calcination conditions are: calcination temperature 650° C., holding time 8 hours, and protective atmosphere nitrogen.

[0099] In step S4 of this embodiment, the pyrophosphate is pyrophosphoric acid or sodium pyrophosphate; and the grinding is performed by planetary ball milling.

[0100] In this embodiment, the lithium source is lithium carbonate; the sodium source is sodium carbonate or sodium bicarbonate; the dispersant is a polyethylene glycol series; and the carbon source is citric acid or starch.

[0101] Example 4

[0102] This embodiment relates to a method for preparing a positive electrode material for a secondary battery with a core-shell composite structure. The positive electrode material for the core-shell composite structure secondary battery is LiMPO4@Na4M3(PO4)2P2O7, which includes a LiMPO4 core and a Na4M3(PO4)2P2O7 shell. The preparation method includes the following steps:

[0103] S1. Preparation of emulsion: mixing lithium source and MPO4 crystals with dispersant and water in a stoichiometric ratio, and wet grinding to form a uniform emulsion;

[0104] S2. Separation of raw material powder: drying the emulsion to achieve solid-liquid separation and obtain dry raw material powder;

[0105] S3. Pre-sintering of the core structure: Pre-sintering the raw material powder at low temperature under a protective atmosphere to complete the construction of the LiMPO4 core structure, and then cooling naturally to obtain the first precursor powder;

[0106] S4. Mixing the second precursor powder: mixing the first precursor powder, the sodium source, the pyrophosphate, and the carbon source, and dry grinding to form a uniform second precursor powder;

[0107] S5. High-temperature calcination of core-shell structure: Under a protective atmosphere, the second precursor powder is calcined at a high temperature to complete the construction of the Na4M3(PO4)2P2O7 shell structure. After natural cooling, the LiMPO4@Na4M3(PO4)2P2O7 core-shell composite structure secondary battery positive electrode material is obtained.

[0108] In step S1 of this embodiment, M is the transition metal element iron, and the molar ratio of the lithium element in the lithium source to the transition metal M in the MPO4 crystal is 1:18. The MPO4 crystal is a transition metal compound FePO4.

[0109] In step S2 of the embodiment, the drying method is spray drying or freeze drying, and the purpose of solid-liquid separation is achieved by the above drying methods.

[0110] In step S3 of this embodiment, the low-temperature pre-sintering conditions are: sintering temperature 400° C., holding time 0.5 h, and protective atmosphere carbon monoxide.

[0111] In step S5 of this embodiment, the high-temperature calcination conditions are: calcination temperature 600° C., holding time 2 hours, and protective atmosphere argon.

[0112] In step S4 of this embodiment, the pyrophosphate is pyrophosphoric acid or ammonium pyrophosphate; and the grinding is planetary high-energy ball milling.

[0113] In this embodiment, the lithium source is lithium dihydrogen phosphate and lithium nitrate; the sodium source is sodium sulfate, sodium nitrate, sodium chloride, sodium citrate, and sodium gluconate; the dispersant is a polyvinyl pyrrolidone series, a polyacrylic acid series, and a carboxymethyl cellulose series; and the carbon source is lactose, glucose, phenolic resin, carbon fiber, carbon nanotubes, graphene, and graphite.

[0114] Example 5

[0115] This embodiment relates to a method for preparing a positive electrode material for a secondary battery with a core-shell composite structure. The positive electrode material for the core-shell composite structure secondary battery is LiMPO4@Na4M3(PO4)2P2O7, which includes a LiMPO4 core and a Na4M3(PO4)2P2O7 shell. The preparation method includes the following steps:

[0116] S1. Preparation of emulsion: mixing lithium source and MPO4 crystals with dispersant and water in a stoichiometric ratio, and wet grinding to form a uniform emulsion;

[0117] S2. Separation of raw material powder: drying the emulsion to achieve solid-liquid separation and obtain dry raw material powder;

[0118] S3. Pre-sintering of the core structure: Pre-sintering the raw material powder at low temperature under a protective atmosphere to complete the construction of the LiMPO4 core structure, and then cooling naturally to obtain the first precursor powder;

[0119] S4. Mixing the second precursor powder: mixing the first precursor powder, the sodium source, the pyrophosphate, and the carbon source, and dry grinding to form a uniform second precursor powder;

[0120] S5. High-temperature calcination of core-shell structure: Under a protective atmosphere, the second precursor powder is calcined at a high temperature to complete the construction of the Na4M3(PO4)2P2O7 shell structure. After natural cooling, the LiMPO4@Na4M3(PO4)2P2O7 core-shell composite structure secondary battery positive electrode material is obtained.

[0121] In step S1 of this embodiment, M is the transition metal element cobalt, and the molar ratio of the lithium element in the lithium source to the transition metal M in the MPO4 crystal is 1:15. The MPO4 crystal is a transition metal compound CoPO4.

[0122] In step S2 of the embodiment, the drying method is freeze drying, vacuum drying, and rotary evaporation drying, and the purpose of solid-liquid separation is achieved by the above drying methods.

[0123] In step S3 of this embodiment, the low-temperature pre-sintering conditions are: sintering temperature 300°C, holding time 2 hours. The protective atmosphere is hydrogen and carbon monoxide.

[0124] In step S5 of this embodiment, the high-temperature calcination conditions are: calcination temperature 500° C., holding time 15 hours, and protective atmosphere nitrogen, argon, or helium.

[0125] In step S4 of this embodiment, the pyrophosphate is ammonium pyrophosphate; and the grinding is planetary sand grinding.

[0126] In this embodiment, the lithium source is lithium fluoride, lithium oxalate, and lithium citrate; the sodium source is sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium citrate, and sodium gluconate; the dispersant is a polyethylene glycol series and a polyvinyl alcohol series; and the carbon source is lactose, glucose, phenolic resin, and carbon fiber.

[0127] Example 6

[0128] This embodiment relates to a method for preparing a positive electrode material for a secondary battery with a core-shell composite structure. The positive electrode material for the core-shell composite structure secondary battery is LiMPO4@Na4M3(PO4)2P2O7, which includes a LiMPO4 core and a Na4M3(PO4)2P2O7 shell. The preparation method includes the following steps:

[0129] S1. Preparation of emulsion: mixing lithium source and MPO4 crystals with dispersant and water in a stoichiometric ratio, and wet grinding to form a uniform emulsion;

[0130] S2. Separation of raw material powder: drying the emulsion to achieve solid-liquid separation and obtain dry raw material powder;

[0131] S3. Pre-sintering of the core structure: Pre-sintering the raw material powder at low temperature under a protective atmosphere to complete the construction of the LiMPO4 core structure, and then cooling naturally to obtain the first precursor powder;

[0132] S4. Mixing the second precursor powder: mixing the first precursor powder, the sodium source, the pyrophosphate, and the carbon source, and dry grinding to form a uniform second precursor powder;

[0133] S5. High-temperature calcination of core-shell structure: Under a protective atmosphere, the second precursor powder is calcined at a high temperature to complete the construction of the Na4M3(PO4)2P2O7 shell structure. After natural cooling, the LiMPO4@Na4M3(PO4)2P2O7 core-shell composite structure secondary battery positive electrode material is obtained.

[0134] In step S1 of this embodiment, M is a transition metal element such as iron or nickel, and the molar ratio of the lithium element in the lithium source to the transition metal M in the MPO4 crystal is 1:16. The MPO4 crystal contains transition metal compounds such as FePO4 and NiPO4.

[0135] In step S2 of the embodiment, the drying method is evaporative crystallization or spray drying, and the purpose of solid-liquid separation is achieved by the above drying methods.

[0136] In step S3 of this embodiment, the low-temperature pre-sintering conditions are: sintering temperature 4300°C, holding time 1.5 hours. The protective atmosphere is hydrogen and carbon monoxide.

[0137] In step S5 of this embodiment, the high-temperature calcination conditions are: calcination temperature 550° C., holding time 6 hours, and protective atmosphere of nitrogen or helium.

[0138] In step S4 of this embodiment, the pyrophosphate is pyrophosphoric acid or sodium pyrophosphate; and the grinding is planetary ball milling or high-energy ball milling.

[0139] In this embodiment, the lithium source is lithium citrate; the sodium source is sodium nitrate, sodium chloride, sodium citrate, sodium gluconate; the dispersant is polyethylene glycol series, polyvinyl alcohol series; the carbon source is citric acid, starch, maltose, sucrose, lactose, glucose.

[0140] 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 core-shell composite structure secondary battery positive electrode material, wherein the core-shell composite structure secondary battery positive electrode material is LiMPO4@Na4M3(PO4)2P2O7, comprising a LiMPO4 core and a Na4M3(PO4)2P2O7 shell, wherein M is one or more transition metal elements, characterized in that The preparation method comprises the following steps: S1. Preparation of emulsion: mixing lithium source and MPO4 crystals with dispersant and water in a stoichiometric ratio, and wet grinding to form a uniform emulsion; S2. Separation of raw material powder: drying the emulsion to achieve solid-liquid separation and obtain dry raw material powder; S3. Pre-sintering of the core structure: Pre-sintering the raw material powder at low temperature under a protective atmosphere to complete the construction of the LiMPO4 core structure, and then cooling naturally to obtain the first precursor powder; S4. Mixing the second precursor powder: mixing the first precursor powder, the sodium source, the pyrophosphate, and the carbon source, and dry grinding to form a uniform second precursor powder; S5. High-temperature calcination of the core-shell structure: calcining the second precursor powder at high temperature under a protective atmosphere to complete the construction of the Na4M3(PO4)2P2O7 shell structure, and obtaining the LiMPO4@Na4M3(PO4)2P2O7 core-shell composite structure secondary battery positive electrode material after natural cooling; In step S1, the molar ratio of lithium element in the lithium source to transition metal M in the MPO4 crystal is 1:15-20; the MPO4 crystal contains one or more transition metal compounds selected from MnPO4, FePO4, CoPO4, and NiPO4; In step S3, the low-temperature pre-sintering conditions are: sintering temperature 300-500°C, and holding time 0.5-2 hours.

2. The method for preparing a core-shell composite structure secondary battery positive electrode material according to claim 1, characterized in that: In step S5, the high-temperature calcination conditions are: calcination temperature 500-650° C., and holding time 2-15 hours.

3. The method for preparing a core-shell composite structure secondary battery positive electrode material according to claim 1, characterized in that: In step S4 , the pyrophosphate is one or more of pyrophosphoric acid, sodium pyrophosphate, and ammonium pyrophosphate.

4. The method for preparing a core-shell composite structure secondary battery positive electrode material according to claim 1, characterized in that: In steps S1 and S4, the grinding is one or more of planetary ball milling, high energy ball milling, and sand milling; In step S2, the drying method is one or more of evaporative crystallization, spray drying, freeze drying, vacuum drying, rotary evaporation drying, and flash drying, and the purpose of solid-liquid separation is achieved by the above drying methods.

5. The method for preparing a core-shell composite structure secondary battery positive electrode material according to claim 1, characterized in that: In step S3, the protective atmosphere is hydrogen and / or carbon monoxide; In step S5 , the protective atmosphere is one or more of nitrogen, argon, and helium.

6. The method for preparing a core-shell composite structure secondary battery positive electrode material according to claim 1, characterized in that: In step S1, the lithium source is one or more of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, lithium nitrate, lithium sulfate, lithium chloride, lithium fluoride, lithium oxalate, and lithium citrate; In step S4, the sodium source is one or more of sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium formate, sodium acetate, sodium sulfate, sodium nitrate, sodium chloride, sodium citrate, and sodium gluconate.

7. The method for preparing a core-shell composite structure secondary battery positive electrode material according to claim 1, characterized in that: In step S1, the dispersant is one or more of the polyethylene glycol series, polyvinyl alcohol series, polyvinyl pyrrolidone series, polyacrylic acid series, and carboxymethyl cellulose series; in step S4, the carbon source is one or more of citric acid, starch, maltose, sucrose, lactose, glucose, phenolic resin, carbon fiber, carbon nanotubes, graphene, and graphite.

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