A mixed polyanion sodium-ion battery cathode material and a preparation method thereof

Abstract

The application relates to a mixed polyanion sodium ion battery positive electrode material and a preparation method thereof, and the preparation method is specifically as follows: iron source, phosphorus source, sodium source and carbon source are sequentially added into water, stirring and dissolving are carried out, a complexing agent is added for complexing to obtain a clear solution; the clear solution is subjected to spray drying treatment to obtain a precursor; the precursor is calcined under an inert gas atmosphere to obtain the target product Na x+0.12 Fe x‑0.56 (P2O7)2(PO4) x‑3 , wherein x is 4, 5, 7 or 9. Compared with the prior art, the application has the characteristics of low cost, simple synthesis, excellent electrochemical performance, good capacity, charge-discharge performance and cycle stability and the like.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery technology, and relates to a hybrid polyanion sodium-ion battery cathode material and its preparation method. Background Technology

[0002] Since the 20th century, the energy crisis has been a major global issue. The overuse of fossil fuels has led to severe resource shortages and environmental pollution. Finding sustainable renewable energy sources (such as wind, solar, and tidal energy) and improving energy efficiency have become crucial tasks for researchers. However, the intermittent and random nature of these renewable energy sources remains a major bottleneck restricting large-scale energy storage. Since the successful commercialization of lithium-ion batteries in 1991, their outstanding energy and power densities have enabled their widespread application in fields ranging from electric vehicles to high-end portable electronic devices. However, with increasing demand for lithium-ion batteries, the uneven distribution of lithium resources and high mining costs have significantly limited their application in large-scale energy storage systems, making the search for alternative battery materials urgent. Sodium, with its abundant resources and even geographical distribution, and its less likely status as a strategic resource compared to lithium, especially given its similar synthesis process and intercalation / deintercalation mechanism to lithium-based materials, has attracted widespread attention.

[0003] The relatively low specific capacity and energy density of cathode materials have always been a key obstacle to their widespread application. Developing low-cost, high-capacity, and long-life sodium-ion battery cathode materials has been crucial for the commercial success of large-scale energy storage systems. Currently, the most researched sodium-ion battery cathode materials mainly include transition metal oxides, polyanionic materials, and Prussian blue materials. Among them, polyanionic materials are composed of polyanionic groups and transition metal elements, and most have an open three-dimensional framework to accommodate larger sodium ions, effectively reducing the volume change caused by sodium ion insertion and extraction, thus enhancing the structural stability of the material. Secondly, polyanions have a tunable inductive effect on the material, easily yielding high-voltage materials, resulting in higher redox potentials. This is beneficial for achieving higher energy density, higher structural stability, better cycle performance, and tunable redox potentials, giving polyanionic materials a significant advantage in the field of large-scale energy storage systems. Iron-based sodium phosphate battery cathode materials have been widely reported by researchers due to their high cost-effectiveness, environmental friendliness, and high cycle stability. For example, FePO4, NaFePO4, Na2FePO4F, Na3Fe2(PO4)3, and Na2FeP2O7. However, these materials generally suffer from low specific capacity and poor cycle performance, failing to adequately meet energy storage requirements. Ha et al. (K.Ha, S.Woo, D.Mok, et al. Na...) 4-α M2+α / 2 (P2O7)2(2 / 3≤α≤7 / 8,M=Fe,Fe 0.5 Mn 0.5 (Mn): A promising sodium ion cathode for Na-ion batteries, Adv. Energy Mater., 2013, 3, 770-776. https: / / doi.org / 10.1002 / aenm.201200825) This study first demonstrated that Na... 3.12 Fe 2.44 The electrochemical properties of (P₂O₇)₂ are considered to increase with the number of reversible sodium ions. 3.12 Fe 2.44 (P₂O₇)₂ yielded a specific stoichiometric ratio of Na₂FeP₂O₇ (97 mAh·g) -1 Higher capacity (117.6 mAh·g) -1 However, the presence of large pyrophosphate groups is not conducive to further improving the theoretical specific capacity of the material, and the voltage plateau also needs to be improved.

[0004] "Hybrid polyanion" cathode materials have rich structural diversity and redox potential. The combination of various polyanions and different degrees of induction effect are conducive to the regulation of redox potential, which often makes them exhibit high operating voltage, good energy density and long cycle life. Kim et al. (H. Kim, I. Park, S. Lee, et al. Understanding the electrochemical mechanism of the new iron-based mixed-phosphate Na4Fe3(PO4)2(P2O7) in a Na rechargeable battery. Chem. Mater. 2013, 25, 3614-3622. https: / / doi.org / 10.1021 / cm4013816) synthesized for the first time an iron-based mixed-phosphate cathode material, Na4Fe3(PO4)2(P2O7), using hydroxyphosphate (NaFePO4) and macropyrophosphate (Na2FeP2O7) as ligands. The open framework of the polyanionic compound and the P2O7 dimer can rotate and twist to adapt to structural changes, exhibiting a high reversible capacity (113 mA·h·g). -1 ) and a suitable average operating voltage (3.1V (vs Na / Na) +Cao et al. (Y.Cao, C.Yang, Y.Liu, et al. A new polyanion Na3Fe2(PO4)P2O7 cathode with high electrochemical performance for sodium-ion batteries, ACS Energy Lett., 2020, 5, 3788-3796. https: / / doi.org / 10.1021 / acsenergylett.0c01902) reported another novel polyanion cathode material, Na3Fe2(PO4)(P2O7), exhibiting high electrochemical performance of 119 mA·h·g. -1 Both materials exhibit high theoretical capacity and small volume change. Although both materials demonstrate excellent electrochemical performance, their theoretical capacities are relatively low. Summary of the Invention

[0005] The purpose of this invention is to overcome the defects of the prior art by providing a hybrid polyanionic sodium-ion battery cathode material and its preparation method. This invention is low-cost, easy to synthesize, has excellent electrochemical performance, and exhibits good capacity, charge-discharge performance, and cycle stability.

[0006] The objective of this invention can be achieved through the following technical solutions:

[0007] One of the technical solutions of this invention is to provide a hybrid polyanionic sodium-ion battery cathode material, the general chemical formula of which is Na. x+0.12 Fe x-0.56 (P₂O₇)₂(PO₄) x-3 , where x is 4, 5, 7 or 9.

[0008] One of the technical solutions of the present invention is to provide a method for preparing a hybrid polyanionic sodium-ion battery cathode material, the method comprising the following steps:

[0009] (1) Add iron source, phosphorus source, sodium source and carbon source in sequence to dissolve in water, stir to dissolve, and add complexing agent to complex to obtain a clear solution;

[0010] (2) The clarified solution was spray-dried to obtain a solid powder precursor;

[0011] (3) The precursor is calcined in an inert gas atmosphere to obtain the target product.

[0012] Further, in step (1), the iron source includes ferric nitrate, ferric citrate or ferric acetate, the phosphorus source includes pyrophosphate and phosphate, the pyrophosphate includes disodium dihydrogen pyrophosphate, the phosphate includes ammonium dihydrogen phosphate, and the sodium source includes sodium acetate, sodium oxalate, sodium carbonate or sodium citrate.

[0013] Furthermore, the carbon source in step (1) includes citric acid, glucose, sucrose or oleic acid, and the carbon content accounts for 2-10% of the mass of the precursor in step (2).

[0014] Furthermore, the complexing agent in step (1) includes oxalic acid, citric acid, malic acid, or maleic acid.

[0015] Further, in step (1), the molar ratio of iron source, pyrophosphate, phosphate, sodium source and complexing agent is 3.44:2:1:4.12:10.32, the solid-liquid mass ratio is (0.047-0.141):1, and x is 4.

[0016] Further, in step (1), the molar ratio of iron source, pyrophosphate, phosphate, sodium source and complexing agent is 4.44:2:2:5.12:13.32, the solid-liquid mass ratio is (0.049-0.147):1, and x is 5.

[0017] Further, in step (1), the molar ratio of iron source, pyrophosphate, phosphate, sodium source and complexing agent is 6.44:2:4:7.12:19.32, the solid-liquid mass ratio is (0.052-0.156):1, and x is 7.

[0018] Further, in step (1), the molar ratio of iron source, pyrophosphate, phosphate, sodium source and complexing agent is 8.44:2:6:9.12:25.32, the solid-liquid mass ratio is (0.053-0.159):1, and x is 9.

[0019] Furthermore, the calcination temperature in step (3) is 400-600℃ and the time is 1-10h.

[0020] The basic idea of ​​this invention is as follows: Unlike the research of Kim et al. (who synthesized iron-based mixed phosphate cathode material Na4Fe3(PO4)2(P2O7) using hydroxyphosphate NaFePO4 and macropyrophosphate Na2FeP2O7 as ligands), we have developed an iron-based pyrophosphate Na4Fe3(PO4)2(P2O7) with a higher theoretical capacity. 3.12 Fe 2.44 Different proportions of phosphate PO4 are introduced into (P2O7)2. 3- Non-stoichiometric mixed phosphates Na with different ratios of phosphate and pyrophosphate were obtained. x+0.12 Fe x-0.56 (P₂O₇)₂(PO₄) x-3(x = 4, 5, 7, 9) Cathode materials. Simultaneously, the introduction of more reversible sodium ions and the reduction of the Na / Fe ratio were achieved, effectively increasing the theoretical capacity of the material. It was proven that as the x value increases, the Na / Fe ratio continuously decreases, and the number of Fe ions participating in the redox reaction increases, meaning an increase in theoretical capacity. When x = 4, 5, 7, 9, its theoretical capacities are 126.3, 131.5, 137.8, and 141.3 mAh·g, respectively. -1 Furthermore, especially compared to the stoichiometric ratio of Na4Fe3(PO4)2(P2O7), it contains a higher amount of pyrophosphate P2O7. 4- Proportion of Na 5.12 Fe 4.44 The cycling stability of (P2O7)2(PO4)2 was also improved accordingly. Compared with stoichiometric iron-based mixed phosphates, the prepared material showed significant improvements in capacity, charge-discharge performance, and long-term cycling stability, making it a promising electrode material.

[0021] Compared with the prior art, the present invention has the following advantages:

[0022] (1) This invention introduces iron salts, pyrophosphates, phosphates and sodium salts in different proportions and obtains a uniform precursor by spray drying. In the subsequent heat treatment crystallization process, the iron source, phosphorus source and sodium source can react and crystallize quickly, which shortens the diffusion process during crystallization and improves the crystallization efficiency.

[0023] (2) The spray drying method used in this invention has many advantages in the production of battery electrode materials, such as uniform mixing of reactants, easy formation of nano-secondary particles, simple operation, and abundant output.

[0024] (3) The present invention selects iron-based materials, which are relatively low-cost and abundant in Earth's resources, and have certain cost advantages, which are conducive to industrial production.

[0025] (4) Compared with stoichiometric iron-based mixed phosphates, such as Na4Fe3(PO4)2(P2O7) and Na3Fe2(PO4)(P2O7), the present invention achieves higher capacity and energy density of iron-based mixed phosphate cathode materials. Attached Figure Description

[0026] Figure 1 Na in the embodiments of the present invention x+0.12 Fe x-0.56 (P₂O₇)₂(PO₄) x-3 The X-ray diffraction (XRD) pattern;

[0027] Figure 2 Na in Embodiment 1 of the present invention 4.12Fe 3.44 Scanning electron microscope (SEM) image of (P2O7)2(PO4);

[0028] Figure 3 Na in Embodiment 2 of the present invention 5.12 Fe 4.44 SEM image of (P2O7)2(PO4)2;

[0029] Figure 4 Na in Embodiment 3 of the present invention 7.12 Fe 6.44 SEM image of (P2O7)2(PO4)4;

[0030] Figure 5 Na in Embodiment 4 of the present invention 9.12 Fe 8.44 SEM image of (P2O7)2(PO4)6;

[0031] Figure 6 Na in the embodiments of the present invention x+0.12 Fe x-0.56 (P₂O₇)₂(PO₄) x-3 Charge and discharge curves;

[0032] Figure 7 Na in the embodiments of the present invention x+0.12 Fe x-0.56 (P₂O₇)₂(PO₄) x-3 Charging / discharging specific capacity at different rates;

[0033] Figure 8 Na in the embodiments of the present invention x+0.12 Fe x-0.56 (P₂O₇)₂(PO₄) x-3 The cyclic performance diagram. Detailed Implementation

[0034] The present invention will now be described in detail with reference to specific embodiments. These embodiments are based on the technical solution of the present invention and provide detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0035] Unless otherwise specified, the equipment used in the following embodiments is conventional equipment in the art; unless otherwise specified, the reagents used are commercially available products or prepared by conventional methods in the art. In the following embodiments, unless otherwise described in detail, conventional experimental methods in the art can be used.

[0036] In this embodiment, X-ray diffraction was performed using a D8 ADVANCE (Brute, Germany) microscope. The analytical conditions were Cu Kα radiation, tube voltage of 40 kV, scanning speed of 0.01° / s, and scanning range of 5°–55°. The scanning electron microscope was a JSM-7800F (Hitachi, Japan). Electrochemical experiments were conducted using a Wuhan Landian system (CT2001A).

[0037] Example 1:

[0038] A hybrid polyanionic sodium-ion battery cathode material and its preparation method include the following steps:

[0039] First, 9.45 g of ferric nitrate nonahydrate solid was dissolved in 500 mL of deionized water. Then, 3.02 g of disodium dihydrogen pyrophosphate, 0.78 g of ammonium dihydrogen phosphate, 0.067 g of sodium acetate, 8.7 g of oxalic acid, and 1.5 g of sucrose were added sequentially. The clarified solution was then spray-dried (220 °C, 400 r / min) to obtain a solid powder precursor. Finally, the precursor was heat-treated in a tube furnace under a nitrogen atmosphere (airflow rate 20 mL / min). -1 ), calcined at 500℃ for 10 hours, to obtain Na 4.12 Fe 3.44 (P2O7)2(PO4)(x=4) material.

[0040] Example 2:

[0041] A hybrid polyanionic sodium-ion battery cathode material and its preparation method are basically the same as in Example 1, except that 9.87g of ferric nitrate nonahydrate solid is dissolved in 500mL of deionized water, and then 2.44g of disodium dihydrogen pyrophosphate, 1.27g of ammonium dihydrogen phosphate, 0.51g of sodium acetate, 9.1g of oxalic acid, and 1.5g of sucrose are added sequentially to obtain Na... 5.12 Fe 4.44 (P2O7)2(PO4)2(x=5) material.

[0042] Example 3:

[0043] A hybrid polyanionic sodium-ion battery cathode material and its preparation method are basically the same as in Example 1, except that 10.41g of ferric nitrate nonahydrate solid is dissolved in 500mL of deionized water, and then 1.78g of disodium dihydrogen pyrophosphate, 1.84g of ammonium dihydrogen phosphate, 1.02g of sodium acetate, 9.74g of oxalic acid and 1.5g of sucrose are added sequentially to obtain Na 7.12 Fe 6.44 (P2O7)2(PO4)4(x=7) material.

[0044] Example 4:

[0045] A hybrid polyanionic sodium-ion battery cathode material and its preparation method are basically the same as in Example 1, except that 10.57g of ferric nitrate nonahydrate solid is dissolved in 500mL of deionized water, and then 1.38g of disodium dihydrogen pyrophosphate, 2.14g of ammonium dihydrogen phosphate, 1.30g of sodium acetate, 9.83g of oxalic acid, and 1.5g of sucrose are added sequentially to obtain Na... 9.12 Fe 8.44 (P2O7)2(PO4)6(x=9) material.

[0046] like Figure 1 As shown, all four materials exhibit high crystallinity: Na 5.12 Fe 4.44 (P₂O₇)₂(PO₄)₂ does not contain the NaFePO₄ phase, making it a pure phase material; Na 4.12 Fe 3.44 (P2O7)2(PO4) and Na 7.12 Fe 6.44 (P₂O₇)₂(PO₄)₄ contains a small amount of NaFePO₄ phase, but still maintains a high purity; Na 9.12 Fe 8.44 (P2O7)2(PO4)6 contains a significant amount of NaFePO4 phase.

[0047] like Figure 2 As shown, nanoscale particles agglomerate and grow, and are uniformly distributed. Spray drying technology effectively reduces particle size and shortens diffusion time during crystallization by atomizing the precursor solution and converting it into solid powder, thus greatly improving the crystallization rate and promoting the formation of nanoparticles.

[0048] like Figure 3 As shown, all four materials exhibit high specific capacity at a rate of 0.1C, especially Na. 5.12 Fe 4.44 (P₂O₇)₂(PO₄)₂ has a capacity of 125.2 mAh·g -1 The reversible capacity is close to the theoretical capacity, representing a breakthrough in both capacity and performance compared to the reversible capacity of Kim et al. without subsequent doping and coating modifications; Na 4.12 Fe 3.44 (P₂O₇)₂(PO₄), Na 7.12 Fe 6.44 (P2O7)2(PO4)4 and Na 9.12 Fe 8.44 (P₂O₇)₂(PO₄)₆ also have 120.7 mAh·g -1 122.2mAh·g -1 and 116.8mAh·g -1The reversible capacity. The four materials differ primarily in their voltage plateau around 2.5V: Na... 5.12 Fe 4.44 (P2O7)2(PO4)2 and Na 9.12 Fe 8.44 (P₂O₇)₂(PO₄)₆ has no voltage plateau, while Na 4.12 Fe 3.44 (P₂O₇)₂(PO₄) has a very obvious voltage plateau; Na 7.12 Fe 6.44 The voltage plateau of (P2O7)2(PO4)4 is relatively weak.

[0049] like Figure 4 As shown, all four materials exhibit good rate performance, Na 5.12 Fe 4.44 (P₂O₇)₂(PO₄)₂ is even more outstanding, with charge / discharge specific capacities of 121.0 mAh·g⁻¹ at rates of 0.3C, 0.5C, 1C, 2C, 5C, 10C, 20C, 30C, 40C, 50C, and 60C, respectively. -1 120.5mAh·g -1 118.4mAh·g -1 112.5mAh·g -1 107.2mAh·g -1 103.1mAh·g -1 101.3mAh·g -1 100.0mAh·g -1 97.8 mAh·g -1 95.3mAh·g -1 and 93.3mAh·g -1 .

[0050] like Figure 5 As shown, after 3000 cycles at a discharge rate of 20C, Na... 5.12 Fe 4.44 (P₂O₇)₂(PO₄)₂, Na 7.12 Fe 6.44 (P₂O₇)₂(PO₄)₄, Na 4.12 Fe 3.44 (P2O7)2(PO4) and Na 9.12 Fe 8.44 The capacity retention rates of (P₂O₇)₂(PO₄)₆ were 87.0%, 85.1%, 84.0%, and 79.7%, respectively, indicating that Na x+0.12 Fe x-0.56 (P₂O₇)₂(PO₄) x-3The cathode materials (x = 4, 5, 7, 9) all exhibit excellent long-cycle stability, among which Na... 5.12 Fe 4.44 (P2O7)2(PO4)2 is superior.

[0051] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A hybrid polyanionic sodium-ion battery cathode material, characterized in that, The general chemical formula of the cathode material is Na. x+0.12 Fe x-0.56 (P₂O₇)₂(PO₄) x-3 , where x is 4, 5, 7 or 9; The method for preparing the hybrid polyanionic sodium-ion battery cathode material includes the following steps: (1) Add iron source, phosphorus source, sodium source and carbon source in sequence to dissolve in water, stir to dissolve, and add complexing agent to complex to obtain a clear solution; (2) The precursor was obtained by spray drying the clarified solution; (3) The precursor is calcined in an inert gas atmosphere to obtain the target product; In step (1), the iron source includes ferric nitrate, ferric citrate or ferric acetate, the phosphorus source includes pyrophosphate and phosphate, the pyrophosphate includes disodium dihydrogen pyrophosphate, the phosphate includes ammonium dihydrogen phosphate, and the sodium source includes sodium acetate, sodium oxalate, sodium carbonate or sodium citrate. The complexing agent in step (1) includes oxalic acid, citric acid, malic acid, or maleic acid; Contains a high amount of pyrophosphate P2O7 4- The capacity, charge-discharge performance, and long-cycle stability of cathode materials with a lower proportion and less NaFePO4 phase were correspondingly improved.

2. The hybrid polyanionic sodium-ion battery cathode material according to claim 1, characterized in that, The carbon source in step (1) includes citric acid, glucose, sucrose or oleic acid, and the carbon content accounts for 2-10% of the mass of the precursor in step (2).

3. The hybrid polyanionic sodium-ion battery cathode material according to claim 1, characterized in that, In step (1), the molar ratio of iron source, pyrophosphate, phosphate, sodium source and complexing agent is 3.44:2:1:4.12:10.32, the solid-liquid mass ratio is (0.047-0.141):1, and x is 4.

4. The hybrid polyanionic sodium-ion battery cathode material according to claim 1, characterized in that, In step (1), the molar ratio of iron source, pyrophosphate, phosphate, sodium source and complexing agent is 4.44:2:2:5.12:13.32, the solid-liquid mass ratio is (0.049-0.147):1, and x is 5.

5. The hybrid polyanionic sodium-ion battery cathode material according to claim 1, characterized in that, In step (1), the molar ratio of iron source, pyrophosphate, phosphate, sodium source and complexing agent is 6.44:2:4:7.12:19.32, the solid-liquid mass ratio is (0.052-0.156):1, and x is 7.

6. The hybrid polyanionic sodium-ion battery cathode material according to claim 1, characterized in that, In step (1), the molar ratio of iron source, pyrophosphate, phosphate, sodium source and complexing agent is 8.44:2:6:9.12:25.32, the solid-liquid mass ratio is (0.053-0.159):1, and x is 9.

7. The hybrid polyanionic sodium-ion battery cathode material according to claim 1, characterized in that, In step (3), the calcination temperature is 400-600 ℃ and the time is 1-10 h.

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