A doped olivine sodium iron phosphate positive electrode material, a preparation method and application thereof

Abstract

The application provides a doped olivine sodium iron phosphate positive electrode material and a preparation method and application thereof. x A y FePO4; 0.6<=x<=0.95, 0.05<=y<=0.4; A is a doping ion, and the doping ion includes any one or a combination of at least two of Li, Be, Mg or Al ions. The preparation method comprises the following steps: mixing a metal source, olivine iron phosphate and water, drying to obtain olivine iron phosphate containing the metal source; pre-burning the olivine iron phosphate containing the metal source to obtain olivine iron phosphate penetrated with metal source ions; mixing a sodium source and the olivine iron phosphate penetrated with metal source ions, and re-burning to obtain doped olivine sodium iron phosphate. The method makes the doped metal ions more uniform, does not damage the structure of the olivine iron phosphate, makes the structure more stable, and effectively improves the cycle performance of the olivine sodium iron phosphate.

Description

Technical Field

[0001] This invention relates to the field of sodium iron phosphate battery technology, and in particular to a doped olivine sodium iron phosphate cathode material, its preparation method, and its application. Background Technology

[0002] With the increasing global demand for clean energy, the need for efficient, safe, and environmentally friendly batteries in fields such as electric vehicles and energy storage systems is also constantly growing. Against this backdrop, sodium-ion batteries have become a research hotspot due to their advantages such as high energy density, long cycle life, low cost, and environmental friendliness. Olivine-type sodium iron phosphate is a common cathode material for sodium iron phosphate batteries, possessing a stable crystal structure and good electrochemical performance. However, during charge and discharge, the large radius of sodium ions causes significant changes in the unit cell volume, leading to the formation of new lattice defects and deteriorating the material's cycle performance. Therefore, the cycle performance of olivine-type sodium iron phosphate needs further improvement, which limits its application in large-scale energy storage systems.

[0003] To improve the cycling performance of olivine-type sodium iron phosphate (SOF), researchers typically employ doping methods, incorporating other metallic elements into the SOF. This method alters the crystal and electronic structures of SOF, thereby enhancing its cycling performance. For example, a common approach involves obtaining olivine-type iron phosphate through chemical ion delithiation, then pre-calcining a homogeneous mixture of magnesium acetate and olivine-type iron phosphate, followed by a secondary calcination using sodium acetate. This method effectively improves the cycling performance of SOF.

[0004] Although existing modification methods involve doping with metal elements, which can improve cycle performance to some extent, they still have the following drawbacks: First, the resulting materials have poor thermodynamic stability, making it impossible to dope into the crystal lattice at high temperatures and easily causing damage to the olivine structure. Second, it may alter other properties of olivine-type sodium iron phosphate, such as energy density and power density, which is detrimental to battery applications. Finally, the stability and reproducibility of the resulting materials need to be improved; otherwise, the long-term performance and safety of the battery will be affected.

[0005] In view of this, the present invention is hereby proposed. Summary of the Invention

[0006] One objective of this invention is to provide a doped sodium iron phosphate cathode material with the molecular formula Na. x A yFePO4, wherein 0.6≤x≤0.95, 0.05≤y≤0.4; A is a dopant ion, which includes any one of Li, Be, Mg or Al ions. This invention, by doping sodium olivine iron phosphate with specific metal source ions, better stabilizes the structure of sodium olivine iron phosphate and effectively improves its cycling performance.

[0007] A second objective of this invention is to provide a method for preparing a doped sodium iron phosphate cathode material. This method further improves the cycle performance of sodium iron phosphate without altering other properties (such as energy density and power density); simultaneously, it simplifies the doping process of the metal source, reduces production costs, and meets the needs of large-scale production; furthermore, it improves the stability and repeatability of the doping method, thereby ensuring the lifespan and safety performance of sodium batteries.

[0008] A third objective of this invention is to provide an application of the aforementioned doped sodium iron phosphate cathode material in the preparation of olivine-type sodium iron phosphate batteries.

[0009] To achieve the above-mentioned objectives of this invention, the following technical solution is adopted: the molecular formula of the doped olivine iron sodium phosphate cathode material is Na x A y FePO4.

[0010] Where 0.6≤x≤0.95, x can be, for example, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, etc.

[0011] Where 0.05≤y≤0.4; y can be, for example, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, etc.

[0012] Wherein, A is a dopant ion, and the dopant ion includes any one or a combination of at least two of Li, Be, Mg or Al ions.

[0013] In this invention, the raw materials for preparing the doped sodium iron phosphate cathode material include: olivine iron phosphate and a metal source.

[0014] Preferably, the molar ratio of the metal source to olivine iron phosphate is (0.08–0.3):1, for example, it can be 0.08:1, 0.09:1, 0.1:1, 0.11:1, 0.12:1, 0.13:1, 0.14:1, 0.15:1, 0.16:1, 0.17:1, 0.18:1, 0.19:1, 0.2:1, 0.21:1, 0.22:1, 0.23:1, 0.24:1, 0.25:1, 0.26:1, 0.27:1, 0.28:1, 0.29:1, 0.3:1, etc.

[0015] Preferably, the olivine iron phosphate is nano-sized olivine iron phosphate.

[0016] Preferably, the primary particle size of the nano-sized olivine iron phosphate is 50-300 nm, for example, it can be 50 nm, 60 nm, 80 nm, 100 nm, 120 nm, 140 nm, 160 nm, 180 nm, 200 nm, 220 nm, 240 nm, 260 nm, 280 nm, 300 nm, etc.

[0017] Preferably, the metal source includes any one or a combination of at least two of the following: organic salts of metals, inorganic salts of metals, or oxides of metals.

[0018] Preferably, the metal source is selected from any one or a combination of at least two of lithium source, beryllium source, magnesium source or aluminum source, and is preferably a magnesium source.

[0019] Preferably, the organic salt includes any one or a combination of at least two of formate, acetate, oxalate, lactate, citrate or gluconate.

[0020] Preferably, the inorganic salt includes any one or a combination of at least two of carbonates, silicates, chlorides, borates, or sulfates.

[0021] More specifically, the magnesium source includes an organic salt of magnesium, preferably any one or a combination of at least two of magnesium formate, magnesium acetate, magnesium oxalate, magnesium lactate, magnesium citrate, or magnesium gluconate.

[0022] More specifically, the magnesium source includes inorganic salts of magnesium, preferably any one or a combination of at least two of magnesium carbonate, magnesium silicate, magnesium chloride, magnesium borate, or magnesium sulfate.

[0023] More specifically, the beryllium source includes organic salts of beryllium, preferably any one or a combination of at least two of beryllium formate, beryllium acetate, beryllium oxalate, beryllium lactate, beryllium citrate, or beryllium gluconate.

[0024] More specifically, the beryllium source includes inorganic salts of beryllium, preferably any one or a combination of at least two of beryllium carbonate, beryllium silicate, beryllium chloride, beryllium borate, or beryllium sulfate.

[0025] More specifically, the lithium source includes an organic salt of lithium, preferably any one or a combination of at least two of lithium formate, lithium acetate, lithium oxalate, lithium lactate, lithium citrate or lithium gluconate.

[0026] More specifically, the lithium source includes inorganic salts of lithium, preferably any one or a combination of at least two of lithium carbonate, lithium silicate, lithium chloride, lithium borate or lithium sulfate.

[0027] More specifically, the aluminum source includes organic salts of aluminum, preferably any one or a combination of at least two of aluminum formate, aluminum acetate, aluminum oxalate, aluminum lactate, aluminum citrate, or aluminum gluconate.

[0028] More specifically, the aluminum source includes inorganic salts of aluminum, preferably any one or a combination of at least two of aluminum carbonate, aluminum silicate, aluminum chloride, aluminum borate, or aluminum sulfate.

[0029] Secondly, the present invention provides a method for preparing a sodium iron phosphate cathode material doped with olivine, the method comprising the following steps:

[0030] The metal source, olivine ferric phosphate, and water are stirred and mixed to dissolve and uniformly disperse the metal source throughout the system. The mixture is then dried to obtain olivine ferric phosphate with a uniformly dispersed metal source. The metal source is any one or a combination of at least two of lithium, beryllium, magnesium, or aluminum sources.

[0031] The metal-containing olivine iron phosphate is pre-calcined to obtain olivine iron phosphate permeated with metal source ions.

[0032] Sodium source and olivine iron phosphate permeated with metal source ions are mixed and recalcined to obtain sodium olivine iron phosphate doped with metal ions.

[0033] In this invention, firstly, by mixing a metal source, olivine ferric phosphate, and water, followed by drying, the metal ions provided by the metal source are more evenly distributed in the olivine ferric phosphate. Secondly, the olivine ferric phosphate containing the metal source is pre-fired to ensure that the metal ions can effectively penetrate into the olivine ferric phosphate. Finally, the sodium source and the olivine ferric phosphate permeated with metal source ions are mixed and refired a second time, resulting in more uniform doped metal ions. Furthermore, ion permeation doping at low temperatures does not damage the structure of the olivine ferric phosphate. This two-step ion permeation process further stabilizes the structure.

[0034] In this invention, the metal source is any one or a combination of at least two of lithium, beryllium, magnesium, or aluminum sources, which can effectively improve the cycle performance of sodium iron phosphate olivine without significantly affecting its energy density and power density, thus benefiting battery applications.

[0035] Preferably, the nano-sized olivine ferric phosphate needs to be pretreated before being mixed with the metal source: the nano-sized olivine ferric phosphate is dried in a vacuum environment.

[0036] In this invention, nano-sized olivine iron phosphate is selected as the base material and pretreated (heated and dried in a vacuum environment) to ensure its purity and activity, thus preparing it for the subsequent doping process and making it more conducive to the subsequent penetration of metal ions into olivine iron phosphate.

[0037] Preferably, the drying temperature is 70-90°C, for example, 70°C, 72°C, 74°C, 76°C, 78°C, 80°C, 82°C, 84°C, 86°C, 88°C, 90°C, etc.

[0038] Preferably, the drying time is 6 to 18 hours, for example, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, 13 hours, 14 hours, 15 hours, 16 hours, 17 hours, 18 hours, etc. Preferably, the mass ratio of olivine iron phosphate to water is 1:(0.9 to 1.5), for example, 1:0.9, 1:1, 1:1.1, 1:1.2, 1:1.3, 1:1.4, 1:1.5, etc.

[0039] Preferably, the mixing method includes mechanical stirring and / or ball milling.

[0040] Preferably, the mechanical stirring speed is 400-600 rpm, for example, 400 rpm, 420 rpm, 440 rpm, 460 rpm, 480 rpm, 500 rpm, 520 rpm, 540 rpm, 560 rpm, 580 rpm, 600 rpm, etc.

[0041] Preferably, the mechanical stirring time is 1 to 6 hours, for example, it can be 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, 5.5 hours, 6 hours, etc.

[0042] Preferably, the ball milling media comprises zirconia balls and / or silicon carbide balls.

[0043] Preferably, the ball-to-material ratio of the ball mill is (1-5):1, for example, it can be 1:1, 2:1, 3:1, 4:1, 5:1, etc.

[0044] Preferably, the rotational speed of the ball mill is 600 to 1000 rpm, for example, 600 rpm, 700 rpm, 800 rpm, 900 rpm, 1000 rpm, etc.

[0045] Preferably, the ball milling time is 1 to 3 hours, for example, it can be 1 hour, 1.2 hours, 1.4 hours, 1.6 hours, 1.8 hours, 2 hours, 2.2 hours, 2.4 hours, 2.6 hours, 2.8 hours, 3 hours, etc.

[0046] Preferably, the drying process includes spray drying and / or heat drying.

[0047] Preferably, the air inlet temperature of the spray dryer is 150-300℃, for example, it can be 150℃, 160℃, 180℃, 200℃, 220℃, 240℃, 260℃, 280℃, 300℃, etc.

[0048] Preferably, the outlet temperature of the spray dryer is 80-120°C, for example, it can be 80°C, 85°C, 90°C, 95°C, 100°C, 105°C, 110°C, 115°C, 120°C, etc.

[0049] Preferably, the heating and drying temperature is 70-90℃, for example, it can be 70℃, 72℃, 75℃, 78℃, 80℃, 82℃, 85℃, 88℃, 90℃, etc.

[0050] Preferably, the heating and evaporation are carried out under stirring.

[0051] Preferably, the stirring speed for heating and drying is 400-600 rpm, for example, 400 rpm, 420 rpm, 440 rpm, 460 rpm, 480 rpm, 500 rpm, 520 rpm, 540 rpm, 560 rpm, 580 rpm, 600 rpm, etc.

[0052] Preferably, the heating and drying time is 4 to 6 hours, for example, 4 hours, 4.2 hours, 4.4 hours, 4.6 hours, 4.8 hours, 5 hours, 5.2 hours, 5.4 hours, 5.6 hours, 5.8 hours, 6 hours, etc.

[0053] In this invention, based on the method of mixing evenly by mechanical stirring or ball milling, followed by spray drying or heating to dry, the mixing and drying process parameters can be further optimized so that metal ions can be better and more evenly distributed in olivine iron phosphate.

[0054] Preferably, the preheating temperature is 280-400℃, for example, it can be 280℃, 285℃, 290℃, 295℃, 300℃, 305℃, 310℃, 315℃, 320℃, 330℃, 340℃, 350℃, 360℃, 370℃, 380℃, 390℃, 400℃, etc.

[0055] Preferably, the pre-burning time is 8 to 24 hours, for example, it can be 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, 13 hours, 14 hours, 15 hours, 16 hours, 17 hours, 18 hours, 19 hours, 20 hours, 21 hours, 22 hours, 23 hours, 24 hours, etc.

[0056] It should be noted that in this invention, the mixed materials are pre-calcined. The pre-calcination temperature and time can be adjusted according to the actual situation to ensure that the corresponding metal ions can penetrate into the olivine iron phosphate more effectively.

[0057] Preferably, the sodium source includes organic sodium salts and / or inorganic sodium salts.

[0058] Preferably, the organic sodium salt includes any one or a combination of at least two of sodium formate, sodium acetate, sodium oxalate, sodium lactate, sodium polyacrylate, pentasodium diethylenetriaminepentamethylphosphonate, sodium carboxymethyl cellulose, sodium fluoride, sodium citrate, or sodium gluconate.

[0059] Preferably, the inorganic sodium salt includes any one or a combination of at least two of sodium carbonate, sodium bicarbonate, sodium silicate, sodium chloride, sodium borate, or sodium sulfate.

[0060] Preferably, the molar ratio of the sodium source and the olivine iron phosphate permeated with metal source ions is (0.6-0.95):1, for example, it can be 0.6:1, 0.65:1, 0.7:1, 0.75:1, 0.8:1, 0.85:1, 0.9:1, 0.95:1, etc.

[0061] Preferably, the reheating temperature is 280-400℃, for example, it can be 280℃, 285℃, 290℃, 295℃, 300℃, 305℃, 310℃, 315℃, 320℃, 330℃, 340℃, 350℃, 360℃, 370℃, 380℃, 390℃, 400℃, etc.

[0062] Preferably, the reheating time is 10 to 24 hours, for example, it can be 10 hours, 10.5 hours, 11 hours, 11.5 hours, 12 hours, 12.5 hours, 13 hours, 13.5 hours, 14 hours, 16 hours, 17 hours, 18 hours, 19 hours, 20 hours, 21 hours, 22 hours, 23 hours, 24 hours, etc.

[0063] In this invention, the re-firing process further includes furnace cooling, pulverization, and sieving in sequence.

[0064] Preferably, the screening method is vibrating screening or direct screen screening.

[0065] Preferably, the sieving uses a sieve with a mesh size of 50 to 300 (e.g., 50 mesh, 60 mesh, 80 mesh, 100 mesh, 120 mesh, 140 mesh, 160 mesh, 180 mesh, 200 mesh, 220 mesh, 250 mesh, 280 mesh, 300 mesh, etc.).

[0066] Thirdly, the present invention provides an application of the doped sodium iron phosphate cathode material described above in the preparation of cathode materials for sodium-ion batteries.

[0067] Compared with the prior art, the present invention has the following beneficial effects:

[0068] (1) The method of the present invention mixes the doped metal source with olivine iron phosphate evenly and then pre-fires it. Then, it mixes the pre-fired material with sodium source and fires it again. This makes the doped metal ions more uniform. Moreover, the structure of olivine iron phosphate is not destroyed by ion permeation doping at low temperature. The two-step ion permeation process makes the structure more stable.

[0069] (2) The method described in this invention can effectively improve the cycle performance of sodium iron phosphate olivine by doping with lithium ions, beryllium ions, magnesium ions and aluminum ions, without significantly affecting its energy density and power density, which is beneficial to the application of batteries.

[0070] (3) The method described in this invention can improve stability and repeatability, the process steps are simple, the conditions are easy to control, and the stability and repeatability are high, which is conducive to ensuring the service life and safety performance of the battery.

[0071] (4) The raw materials and methods used in the method described in this invention are all environmentally friendly and pollution-free, which meets the current social demand for green energy. Attached Figure Description

[0072] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0073] Figure 1 SEM image of sodium olivine phosphate doped with metal ions provided in Example 1.

[0074] Figure 2EDS diagram of sodium iron phosphate ions doped with metal ions provided in Example 1.

[0075] Figure 3 EDS diagram of olivine iron phosphate sodium magnesium ions doped with metal ions provided in Example 1.

[0076] Figure 4 The XRD pattern of sodium iron phosphate doped with metal ions provided in Example 1. Detailed Implementation

[0077] Unless otherwise defined herein, the scientific and technical terms used in conjunction with this invention shall have the meanings commonly understood by one of ordinary skill in the art. The meaning and scope of terms shall be clear; however, in any case of potential ambiguity, the definitions provided herein shall prevail over any dictionary or foreign definitions. In this application, unless otherwise stated, the use of "or" means "and / or". Furthermore, the use of the term "comprising" and other forms is non-limiting.

[0078] It should be noted that specific details are set forth in the following description to provide a full understanding of the invention. However, the invention can be practiced in many ways other than those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0079] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0080] The present invention will be further illustrated by the following examples. Unless otherwise specified, the materials in the examples are prepared according to existing methods or purchased directly from the market.

[0081] Example 1

[0082] This embodiment provides a method for preparing a sodium iron phosphate cathode material doped with olivine, the method comprising the following steps:

[0083] S1. Preprocessing:

[0084] Nanoscale olivine iron phosphate (average primary particle size of 200 nm) after lithium iron phosphate delithiation was selected as the base material and placed in a vacuum drying oven and dried at 80°C for 12 hours.

[0085] S2, a mixture of magnesium source and olivine iron phosphate:

[0086] Magnesium acetate and ferric olivine phosphate were mixed evenly at a molar ratio of 0.1:1, and then dissolved in deionized water (the amount of deionized water added was 1.2 times the mass of ferric olivine phosphate). The dissolution was carried out by stirring at room temperature. Then the temperature was raised to 80°C, and the mixture was heated and stirred at a speed of 500 rpm for 5 hours. After the solution was completely evaporated, magnesium-containing ferric olivine phosphate was obtained.

[0087] S3, Preheating:

[0088] Magnesium-containing olivine ferric phosphate was pre-calcined in a muffle furnace at a temperature of 300°C for 10 hours to obtain magnesium-permeated olivine ferric phosphate.

[0089] S4, Reheating:

[0090] The pre-calcined material was cooled to room temperature, and then mixed with sodium acetate at a molar ratio of 0.8:1 according to the mixing steps in S2 above. The resulting mixture was then recalcined at a temperature of 300°C for 12 hours to obtain the crude product.

[0091] S5. Post-processing:

[0092] The crude product obtained from recalcination was cooled to room temperature; then the agglomerated particles were dispersed by grinding; finally, it was sieved using a 200-mesh sieve to obtain the magnesium-doped sodium olivine phosphate product.

[0093] Figure 1 SEM image of sodium iron phosphate doped with metal elements, as provided in Example 1. Figure 1 As shown, the final magnesium-doped sodium olivine phosphate product has a primary particle size that is basically consistent with that of nano-sized sodium olivine phosphate (average primary particle size of 200 nm).

[0094] Figure 2 EDS diagram of sodium iron phosphate ions doped with metal elements provided in Example 1. Figure 3 EDS diagram of sodium magnesium phosphate ions doped with metal elements, as shown in Example 1. Figure 2 and Figure 3 As shown, Na ions and Mg ions are evenly distributed throughout the entire system of sodium iron phosphate olivine.

[0095] Figure 4 The XRD pattern of sodium iron phosphate doped with metal elements, as provided in Example 1. Figure 4 As shown, the characteristic peaks corresponding to the XRD mainly contain two crystal phases, namely Fe corresponding to the PDF#97-015-6834 card. 0.732 Mg2.268 The presence of (PO4)2 and the olivine-type NaFePO4 corresponding to PDF#97-016-9118 indicates that magnesium ions successfully entered the interior of the olivine iron phosphate crystal without damaging its structure, thus preserving the material's activity and providing sufficient assurance for subsequent successful sodium intercalation. On the other hand, the relative intensity of the characteristic peaks qualitatively indicates that olivine-type NaFePO4 is still the main component, indicating that Mg ion doping was successful, and magnesium-doped olivine-type NaFePO4 cathode material was prepared.

[0096] Example 2

[0097] This embodiment provides a method for preparing a sodium iron phosphate cathode material doped with olivine, the method comprising the following steps:

[0098] S1. Preprocessing:

[0099] Nanoscale olivine iron phosphate (average primary particle size of 200 nm) after lithium iron phosphate delithiation was selected as the base material and placed in a vacuum drying oven and dried at 80°C for 12 hours.

[0100] S2, a mixture of lithium source and olivine iron phosphate:

[0101] Lithium lactate and ferric olivine phosphate were mixed evenly at a molar ratio of 0.1:1, and then dissolved in deionized water (the amount of deionized water added was 1.2 times the mass of ferric olivine phosphate). The dissolution was carried out by stirring at room temperature. Then the temperature was raised to 80°C, and the mixture was heated and stirred at a speed of 500 rpm for 5 hours. After the solution was completely evaporated, lithium-containing ferric olivine phosphate was obtained.

[0102] S3, Preheating:

[0103] Lithium-containing olivine iron phosphate was pre-calcined in a muffle furnace at a temperature of 300°C for 10 hours to obtain lithium-permeable olivine iron phosphate.

[0104] S4, Reheating:

[0105] The pre-calcined material was cooled to room temperature, and then mixed with sodium lactate at a molar ratio of 0.9:1 according to the mixing steps in S2 above. The resulting mixture was then recalcined at a temperature of 300°C for 12 hours to obtain the crude product.

[0106] S5. Post-processing:

[0107] The crude product obtained from the re-firing was cooled to room temperature; then the agglomerated particles were dispersed by grinding; finally, it was sieved using a 200-mesh sieve to obtain the lithium-doped sodium iron phosphate olivine.

[0108] Example 3

[0109] This embodiment provides a method for preparing a sodium iron phosphate cathode material doped with olivine, the method comprising the following steps:

[0110] S1. Preprocessing:

[0111] Nanoscale olivine iron phosphate (average primary particle size of 200 nm) after lithium iron phosphate delithiation was selected as the base material and placed in a vacuum drying oven and dried at 80°C for 12 hours.

[0112] S2, a mixture of aluminum source and olivine iron phosphate:

[0113] Aluminum oxalate and ferric olivine phosphate were mixed evenly at a molar ratio of 0.1:1, and then dissolved in deionized water (the amount of deionized water added was 1.2 times the mass of ferric olivine phosphate). The dissolution was carried out by stirring at room temperature. Then the temperature was raised to 80°C, and the mixture was heated and stirred at a speed of 500 rpm for 5 hours. After the solution was completely evaporated, aluminum-containing ferric olivine phosphate was obtained.

[0114] S3, Preheating:

[0115] Olivine iron phosphate containing aluminum source was pre-calcined in a muffle furnace at a temperature of 300℃ for 10 hours to obtain olivine iron phosphate permeated with aluminum ions.

[0116] S4, Reheating:

[0117] The pre-calcined material was cooled to room temperature, and then mixed with sodium oxalate at a molar ratio of 0.7:1 according to the mixing steps in S2 above. The resulting mixture was then recalcined at a temperature of 300°C for 12 hours to obtain the crude product.

[0118] S5. Post-processing:

[0119] The crude product obtained from re-firing is cooled to room temperature; then the agglomerated particles are dispersed by grinding; finally, it is sieved through a 200-mesh sieve to obtain the aluminum-doped sodium olivine iron phosphate product.

[0120] Example 4

[0121] This embodiment provides a method for preparing a cathode material doped with sodium iron phosphate and olivine phosphate. The only difference from Example 1 is that in step S1, nano-sized sodium iron phosphate is replaced with an equimolar amount of micron-sized sodium iron phosphate (average particle size of 2 μm). The other steps are completely consistent with Example 1.

[0122] Example 5

[0123] This embodiment provides a method for preparing a sodium iron phosphate cathode material doped with olivine phosphate. The only difference from Example 1 is that the mixing steps of S2 and the magnesium source olivine phosphate are different, as follows: magnesium acetate and olivine phosphate are mixed evenly at a molar ratio of 0.1:1, and then dissolved in deionized water (the amount of deionized water added is 1.2 times the mass of olivine phosphate); then the solution is completely evaporated by spray drying. The inlet temperature of the spray dryer is 200°C and the outlet temperature is 100°C to obtain magnesium source olivine phosphate; the other steps are completely the same as in Example 1.

[0124] Example 6

[0125] This embodiment provides a method for preparing a sodium iron phosphate cathode material doped with olivine. The only difference from Embodiment 1 is that the pre-calcination temperature of S3 is 250°C and the pre-calcination time is 24h; the other steps are completely consistent with Embodiment 1.

[0126] Example 7

[0127] This embodiment provides a method for preparing a sodium iron phosphate cathode material doped with olivine. The only difference from Example 1 is that the pre-calcination temperature of S3 is 400°C and the pre-calcination time is 6 hours; the other steps are completely the same as in Example 1.

[0128] Example 8

[0129] This embodiment provides a method for preparing a sodium iron phosphate cathode material doped with olivine. The only difference from Example 1 is that the re-firing temperature of S4 is 250°C and the pre-firing time is 24h; the other steps are completely the same as in Example 1.

[0130] Example 9

[0131] This embodiment provides a method for preparing a sodium iron phosphate cathode material doped with olivine. The only difference from Example 1 is that the re-firing temperature of S4 is 400°C and the pre-firing time is 6 hours; the other steps are completely the same as in Example 1.

[0132] Example 10

[0133] This embodiment provides a method for preparing a sodium iron phosphate cathode material doped with olivine. The only difference from Example 1 is that magnesium acetate in S2 is replaced with an equimolar amount of beryllium acetate; the other steps are completely the same as in Example 1.

[0134] Example 11

[0135] This comparative example provides a method for preparing a sodium iron phosphate cathode material doped with olivine. The only difference from Example 1 is that the olivine iron phosphate does not undergo the S1 pretreatment; the other steps are completely consistent with Example 1.

[0136] Example 12

[0137] This comparative example provides a method for preparing a sodium iron phosphate cathode material doped with olivine. The only difference from Example 1 is that the S3 pre-calcination step is omitted. Instead, sodium acetate and magnesium-containing olivine iron phosphate are directly mixed at a molar ratio of 0.8:1 according to the mixing step S2 described above. The resulting mixture is then recalcined at a temperature of 300°C for 22 hours to obtain a crude product. All other steps are completely consistent with those in Example 1.

[0138] Comparative Example 1

[0139] This comparative example provides a method for preparing a sodium iron phosphate cathode material doped with olivine. The only difference from Example 1 is that magnesium acetate is replaced with an equimolar amount of manganese acetate, while the other steps are completely consistent with Example 1.

[0140] Comparative Example 2

[0141] This comparative example provides a method for preparing a doped sodium iron phosphate cathode material. The only difference from Example 1 is that no doping is performed. Sodium acetate and sodium iron phosphate are directly mixed in a molar ratio of 1:1 according to the mixing steps in S2 above. The resulting mixture is then calcined at a temperature of 300°C for 22 hours to obtain a crude product. The other steps are completely consistent with those in Example 1.

[0142] Performance testing

[0143] Test samples: sodium iron phosphate doped with metal elements provided in Examples 1-12, and sodium iron phosphate doped with metal elements provided in Comparative Examples 1-2;

[0144] Test method:

[0145] (1) Battery assembly: The above test samples were weighed with acetylene black (conductive agent) and PVDF (binder) in a mass ratio of 8:1:1, ground in a mortar for a period of time, mixed evenly, and then N-methylpyrrolidone (NMP) was added. Grinding continued until a uniform black viscous slurry was obtained. The prepared slurry was placed on aluminum foil and coated into a film of uniform thickness with a scraper. A sodium metal sheet was used as the counter electrode, a glass fiber membrane was used as the separator, and 1 mol / L NaClO4 / PC (propylene carbonate) was used as the electrolyte. The CR2032 coin cell was assembled in an argon atmosphere glove box.

[0146] (2) Test the cycle performance, energy density and specific capacity of the corresponding batteries respectively.

[0147] The specific test results are shown in Table 1:

[0148] Table 1

[0149]

[0150] As shown in Table 1, the prepared sodium iron olivine phosphate was tested for performance, including cycle performance, energy density, and specific capacity. It was found that the sodium iron olivine phosphate improved by the method described in this invention achieved a capacity retention rate of over 88.5% to 98.2% after 200 cycles, with an initial discharge specific energy of 326.9 to 357.3 mWh / g and an initial discharge specific capacity of 122.8 to 134.2 mAh / g.

[0151] As fully illustrated in Comparative Example 2, the method of the present invention can effectively improve the cycle performance of sodium iron phosphate by doping olivine iron phosphate with metal ions such as lithium ions, beryllium ions, magnesium ions, and aluminum ions. This helps to promote the application of olivine-type sodium iron phosphate batteries in large-scale energy storage systems and meet the market demand for efficient, safe, and environmentally friendly batteries. It can also reduce the production cost of batteries and improve their performance and safety, thus having high commercial value and social benefits.

[0152] Furthermore, Examples 1 and 4 illustrate that when using olivine-type ferric phosphate as a matrix, a larger primary particle size of the matrix hinders the uniform diffusion of ions into the material under the same preparation conditions, resulting in poor capacity and cycle performance. Examples 1 and 5 demonstrate that spray drying can achieve certain effects, but it affects the uniformity of the components, causing ferric phosphate to be in a sodium-deficient state, thus resulting in abnormal first-efficiency exceeding 100%. Examples 1 and 6-9 show that magnesium ion diffusion requires a certain amount of thermal power, and lowering the pre-sintering temperature is not conducive to the uniform diffusion of magnesium ions throughout the system, thereby affecting subsequent sodium intercalation and significantly impacting electrochemical performance. However, when the sintering temperature reaches above 400°C, some olivine-type ferric phosphate begins to deactivate, undergoing structural transformation and deteriorating the electrochemical performance of the material.

[0153] Examples 1 and 11 demonstrate that pretreatment effectively removes internal moisture from iron phosphate, ensuring correct proportions in post-processing and reducing operational errors during material preparation. Examples 1 and 12 demonstrate that double sintering not only prevents the batch-wise, effective, and uniform embedding of dopant ions but also provides a certain pinning effect, improving the stability of doped ions and thus effectively enhancing cycle performance in electrochemical applications.

[0154] Example 1 and Comparative Example 1 illustrate that magnesium ions and other mentioned ions can enter the interior of the olivine iron phosphate lattice, while manganese ions and other ions cannot enter the interior of the lattice due to issues such as ionic radius. Therefore, they not only do not significantly improve the electrochemical performance of the material, but also deteriorate the material.

[0155] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A sodium iron phosphate cathode material doped with olivine, characterized in that, The molecular formula of the doped olivine sodium iron phosphate positive electrode material is Na x A y FePO4; Where 0.6≤x≤0.95, 0.05≤y≤0.4; Wherein, A is a dopant ion, and the dopant ion includes any one or a combination of at least two of Li, Be, Mg or Al ions; The raw materials for preparing the doped sodium iron phosphate cathode material include: olivine iron phosphate and a metal source; The preparation method of the doped sodium iron phosphate cathode material includes the following steps: The metal source, olivine ferric phosphate, and water are stirred and mixed, and then dried to obtain olivine ferric phosphate with a uniformly dispersed metal source; wherein the metal source is any one or a combination of at least two of lithium source, beryllium source, magnesium source, or aluminum source. The metal-containing olivine iron phosphate is pre-calcined to obtain olivine iron phosphate permeated with metal source ions. Sodium source and olivine iron phosphate permeated with metal source ions are mixed and recalcined to obtain sodium olivine iron phosphate doped with metal ions. The olivine iron phosphate is nano-sized olivine iron phosphate. The primary particle size of the nano-sized olivine iron phosphate is 50~300 nm. The drying process includes heating and evaporation; The pre-firing temperature is 280~300℃; The reheating temperature is 280~400℃.

2. The doped sodium iron phosphate cathode material according to claim 1, characterized in that, The molar ratio of the metal source to olivine iron phosphate is (0.08~0.3):

1.

3. The doped sodium iron phosphate cathode material according to claim 1, characterized in that, The metal source includes any one or a combination of at least two of the following: organic salts of metals, inorganic salts of metals, or oxides of metals.

4. The doped sodium iron phosphate cathode material according to claim 1, characterized in that, The metal source is selected from any one or a combination of at least two of the following: lithium source, beryllium source, magnesium source, or aluminum source.

5. The doped olivine-based sodium iron phosphate cathode material according to claim 1, characterized in that, The metal source is a magnesium source.

6. The sodium iron phosphate cathode material doped with olivine according to claim 3, characterized in that, The organic salt includes any one or a combination of at least two of formate, acetate, oxalate, lactate, citrate or gluconate.

7. The sodium iron phosphate cathode material doped with olivine according to claim 3, characterized in that, The inorganic salt includes any one or a combination of at least two of the following: carbonates, silicates, chlorides, borates, or sulfates.

8. The method for preparing the doped sodium iron phosphate cathode material according to any one of claims 1-7, characterized in that, The preparation method includes the following steps: The metal source, olivine ferric phosphate, and water are stirred and mixed, and then dried to obtain olivine ferric phosphate with a uniformly dispersed metal source; wherein the metal source is any one or a combination of at least two of lithium source, beryllium source, magnesium source, or aluminum source. The metal-containing olivine iron phosphate is pre-calcined to obtain olivine iron phosphate permeated with metal source ions. Sodium source and olivine iron phosphate permeated with metal source ions are mixed and recalcined to obtain sodium olivine iron phosphate doped with metal ions. The olivine iron phosphate is nano-sized olivine iron phosphate. The primary particle size of the nano-sized olivine iron phosphate is 50~300 nm. The drying process includes heating and evaporation; The pre-firing temperature is 280~300℃; The reheating temperature is 280~400℃.

9. The method for preparing the doped sodium iron phosphate cathode material according to claim 8, characterized in that, The mass ratio of olivine iron phosphate to water is 1:(0.9~1.5).

10. The method for preparing the doped sodium iron phosphate cathode material according to claim 8, characterized in that, The mixing methods include mechanical stirring and / or ball milling.

11. The method for preparing the doped olivine iron sodium phosphate cathode material according to claim 10, characterized in that, The mechanical stirring speed is 400~600 rpm, and the time is 1~6 h.

12. The method for preparing the doped sodium iron phosphate cathode material according to claim 10, characterized in that, The ball milling media include zirconia balls and / or silicon carbide balls, with a ball-to-material ratio of (1~5):1, a rotation speed of 600~1000 rpm, and a time of 1~3 h.

13. The method for preparing the doped olivine iron sodium phosphate cathode material according to claim 8, characterized in that, The heating and evaporation are carried out under stirring, with the stirring temperature at 70~90℃, the stirring speed at 400~600 rpm, and the time at 6~18 h.

14. The method for preparing the doped olivine iron sodium phosphate cathode material according to claim 8, characterized in that, The pre-firing time is 8~24 h.

15. The method for preparing the doped sodium iron phosphate cathode material according to claim 8, characterized in that, The sodium source includes organic sodium salts and / or inorganic sodium salts.

16. The method for preparing the doped olivine iron sodium phosphate cathode material according to claim 15, characterized in that, The organic sodium salt includes any one or a combination of at least two of sodium formate, sodium acetate, sodium oxalate, sodium lactate, sodium polyacrylate, pentasodium diethylenetriaminepentamethylphosphonate, sodium carboxymethyl cellulose, sodium citrate, or sodium gluconate.

17. The method for preparing the doped sodium iron phosphate cathode material according to claim 15, characterized in that, The inorganic sodium salt includes any one or a combination of at least two of sodium carbonate, sodium bicarbonate, sodium silicate, sodium chloride, sodium borate, or sodium sulfate.

18. The method for preparing the doped sodium iron phosphate cathode material according to claim 8, characterized in that, The molar ratio of sodium source and olivine iron phosphate permeated with metal source ions is (0.6~0.95):

1.

19. The method for preparing the doped sodium iron phosphate cathode material according to claim 8, characterized in that, The reheating time is 10-24 hours.

20. The method for preparing the doped sodium iron phosphate cathode material according to claim 8, characterized in that, The process after reheating also includes furnace cooling, crushing, and sieving.

21. The method for preparing the doped sodium iron phosphate cathode material according to claim 20, characterized in that, The screening method is either vibrating screening or direct screen screening.

22. The method for preparing the doped sodium iron phosphate cathode material according to claim 20, characterized in that, The sieving process uses a 50-300 mesh screen.

23. The application of the doped sodium iron phosphate cathode material according to any one of claims 1-7 in the preparation of cathode materials for sodium-ion batteries.

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