High-manganese phosphate positive electrode material and application thereof in sodium ion battery

Abstract

The application relates to a phospho-pyrophosphate composite polyanion positive electrode material for a sodium ion battery, a preparation method and application thereof in a sodium ion battery; the phospho-pyrophosphate composite polyanion positive electrode has a structure shown in formula I: Na4Fe 3‑x‑1.5y‑ z Mn x V y A z (PO4)2P2O7@C formula I the phospho-pyrophosphate composite polyanion compound: when having the structure shown in formula I: 1.5 < x < 2.8; 2) A is selected from 1 kind and 2 kinds of Mg 2+ 、Al 3+ 、Cr 3+ 、Ga 3+ 、Co 2+ 、Ni 2+ 、Cu 2+ 、Zn 2+ 、Ge 2+ 、Zr 4+ 、Nb 3+ / 4+ 、Mo 3+ 、In 3+ 、Sn 4+ 、Sb 3+ 、Te 4+ 、Ce 4+ 、Pr 4+ 、Nd 3+ 、Pm 3+ 、Ta 3+ / 4+ 、W 4+ 、Bi 5+ 、Po 6+ 、Cm 4+ 、Bk 4+ The application improves the ionic conductivity and electronic conductivity of the material through cell structure regulation and carbon coating strategies, and the prepared phospho-pyrophosphate composite polyanion compound electrode material has high specific capacity, good rate performance and cycle stability as a sodium ion battery positive electrode material.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of sodium ion batteries, and relates to a sodium ion battery electrode material, in particular to a phosphoric pyrophosphate composite polyanion compound electrode material, a preparation method and application of the electrode material in a sodium ion battery electrode material. BACKGROUND

[0002] Developing clean and renewable energy has become an important strategy for the development of countries around the world. In order to realize the continuous and stable integration of renewable energy into the power grid system, it is necessary to develop supporting energy storage technology. Lithium ion batteries are widely used in large-scale energy storage fields due to high energy density and long cycle life. However, in recent years, the limitation of lithium source has led to high prices of lithium ion battery cathode materials. Sodium is more abundant and more evenly distributed in the earth's crust, so it has a much lower price. Sodium ions and lithium ions have similar physical and chemical properties and storage mechanisms. Therefore, sodium ion batteries have great application potential in user-side energy storage and large-scale energy storage fields. The cathode material is one of the key components that determine the performance of sodium ion batteries, so designing a cathode material with high specific capacity and high stability is the key to realizing the practicality of sodium ion batteries in large-scale energy storage.

[0003] Polyanion sodium ion battery cathode materials such as phosphoric pyrophosphate iron (manganese vanadium) sodium have become the preferred cathode material for sodium ion batteries due to their advantages of structural stability, high safety, high redox potential, etc. In this structure, the oxidation and reduction potentials of manganese ions and vanadium ions are high, which can improve the overall charge and discharge voltage of the material, thereby improving the energy density of the sodium ion battery full cell. However, manganese ions and vanadium ions undergo a Jahn-Teller distortion process during charging and discharging, which causes a series of adverse changes in the crystal structure at the level of sodium ion diffusion channels, transition metal dissolution, and alkali metal ion and transition metal ion mixing, thereby causing problems such as decreased rate performance and poor cycle stability. SUMMARY

[0004] In view of the defects of existing sodium ion battery cathode materials, the application provides a novel phosphoric pyrophosphate composite polyanion compound. The co-doping of manganese ions, vanadium ions and other transition metal ions precisely regulates the cathode cell structure, improves the redox potential of the material, and improves the ionic and electronic conductivities. By using different types of carbon sources and reducing agents to construct a good carbon coating layer interface, the dissolution behavior of manganese ions during the cycle process is inhibited, and the sodium ion transmission rate is improved. Therefore, the rate performance and cycle stability of the material are improved as a whole.

[0005] ADVANTAGEOUS EFFECTS

[0006] In order to solve the problems of low redox potential and low specific capacity of the sodium iron pyrophosphate material itself, the application provides a novel structure of phosphoric acid pyrophosphate composite polyanion compound, the redox potential of the structure is improved by accurately controlling the composite ratio of iron, manganese and vanadium, so that the specific capacity of the material in the charging and discharging process is improved.

[0007] Meanwhile, the ion conductivity and electronic conductivity of the material are improved by doping part of metal ions at the transition metal sites (iron, manganese and vanadium) in the crystal cell structure, the crystal cell structure is stabilized, the band gap of the material is reduced, the reversible electrochemical deintercalation / intercalation process of inert sodium ions in the structure is realized, and the multi-electron transfer process of more than three electrons is realized, so that the specific capacity of the material is improved.

[0008] Meanwhile, different carbon sources and reducing agents are selected to realize uniform carbon coating of the material, a macromolecular carbon is formed by the esterification process of citric acid and ethylene glycol to construct a continuous carbon network, the interface conductivity of the material and the reversibility of the sodium ion deintercalation / intercalation process are improved, the volume expansion problem caused by the sodium ion deintercalation / intercalation is relieved to a certain extent, the lattice stress of the material is buffered to prevent the destruction of the electrode material, and the dissolution of transition metal ions in the cycle process is inhibited, so that the cycle stability of the material is further improved.

[0009] The application improves the ion conductivity and electronic conductivity of the material by the crystal cell structure regulation and carbon coating strategy, and the prepared phosphoric acid pyrophosphate composite polyanion compound electrode material has high specific capacity, good rate performance and cycle stability as a positive electrode material of a sodium ion battery. BRIEF DESCRIPTION OF DRAWINGS

[0010] Figure 1 The XRD graph is for sample 1#;

[0011] Figure 2 The SEM characterization graph is for sample 1#;

[0012] Figure 3 The charge-discharge cycle test graph of the coin-type half battery is for sample 1#. DETAILED DESCRIPTION

[0013] Example 1-Na4Fe 0.65 Mn 1.75 V 0.3 Cr 0.05 Al 0.05 (PO4)2P2O7@C

[0014] 1) 500mL of deionized water was added to a beaker, heated to 60℃ in a water bath, and 0.3mol of citric acid C6H 10O8, 0.3 mol ethylene glycol C2H6O2, 0.065 mol iron nitrate nonahydrate FeN3O9·9H2O, 0.175 mol manganese nitrate tetrahydrate MnN2O6·4H2O, 0.03 mol ammonium metavanadate NH4VO3, 0.005 mol chromium nitrate nonahydrate CrN3O9·9H2O, 0.005 mol aluminum nitrate nonahydrate AlN3O9·9H2O, stirring until fully dissolved;

[0015] 2) 0.4 mol of sodium dihydrogen phosphate dihydrate NaH2PO4·2H2O (sodium source / phosphorus source) was added to the above aqueous solution, and stirring was continued until the raw materials were completely dissolved, to obtain a precursor solution;

[0016] 3) The precursor solution was pumped into a spray drying device protected by a nitrogen atmosphere for spray drying (the inlet temperature was 160°C, the outlet temperature was 95°C, the feed flow rate was 60 ml / min, and the compressed nitrogen pressure was 0.8 KPa), to obtain a precursor powder;

[0017] 4) The precursor powder was then transferred into a high-temperature tube furnace protected by an argon atmosphere for heat treatment, the first-stage heat treatment temperature was 400°C, the heat treatment time was 5 h, the second-stage heat treatment temperature was 650°C, and the heat treatment time was 10 h, to obtain the final product after heat treatment, denoted as sample 1#

[0018] 5) The sample 1# was tested as follows:

[0019] a. Refined-XRD and ICP (mainly detecting elements: Na / Fe / Mn / V / Cr / Al / P) tests, the Refined-XRD showed that the product after heat treatment was consistent with the crystal structure of Na4Fe3(PO4)2P2O7, belonging to the R3c space group, and Cr and Al were distributed in the transition metal (Fe / Mn) site, and the ICP test showed that the element ratio between Na:Fe:Mn:V:Cr:Al:P elements was 4:0.65:1.75:0.3:0.05:0.05:4, i.e. Na4Fe 0.65 Mn 1.75 V 0.3 Cr 0.05 Al 0.05 (PO4)2P2O7@C material, consistent with the Refined-XRD structure;

[0020] b. XPS test was performed on the iron, manganese, vanadium, chromium and aluminum elements in sample 1#, and the peak separation results showed that the iron and manganese in the above electrode were Fe 2+ , Mn 2+ , V 3+ , Cr 3+ , and Al 3+ ;

[0021] c. The band gap width of the prepared sample 1# was tested by UV-visible absorption spectroscopy, and the test result was 3.25 eV;

[0022] d. XRD, SEM characterization, carbon content (TG) test and button half-cell charge and discharge cycle test were performed on the prepared sample 1#. Figure 1 ) It can be seen that the synthesized material is pure phase (purity up to 99.95%), and from SEM ( Figure 2 ) It can be seen that the particle size is uniform and small, with a particle size of 3μm-5μm. From the button half-cell 1C charge and discharge cycle test ( Figure 3 ) It can be seen that the positive electrode material has good cycle stability.

[0023] 6) Sample 1# was used as the active material and mixed evenly with the conductive agent acetylene black and the binder polyvinylidene fluoride in a mass ratio of 8:1:1. An appropriate amount of solvent N-methylpyrrolidone was added and mixed evenly to form a paste, which was applied to the aluminum current collector. After drying, it was cut into discs with a diameter of 14 mm. The active material surface density was 1.1 to 3.2 mg / cm 2 (Here is 2.1 mg / cm 2 ), using this as the positive electrode of the battery, metallic sodium as the negative electrode, 1M NaClO4 / EC / DEC / 5%FEC (EC / DEC V / V=1:1) as the electrolyte, and a glass fiber membrane as the separator. The assembled battery was subjected to charge and discharge tests. The charge and discharge cycles were performed 10 times at 0.2C / 0.2C in the voltage range of 1.5V-4.5V. The discharge capacity and discharge voltage of the stable cycle were recorded in Table 1. The battery was then charged to 4.5V and then cut off. The positive electrode sheet was disassembled from the battery in a glove box, washed with DMC for 3-5 times, and then evaporated to dryness in the glove box.

[0024] a. Take part of the electrode for ICP test (main detection elements: Na:Fe:Mn:V:Cr:Al:P). The ICP test results show that the element ratio between Na:Fe:Mn:V:Cr:Al:P is 0.4:0.65:1.75:0.3:0.05:0.05:4, that is, when charged to 4.5V, there are 3.6 reversible sodium ions in the structure for deintercalation;

[0025] b. At the same time, XPS test was performed on the iron, manganese, vanadium, chromium and aluminum elements in the above-mentioned electrode. The peak separation results showed that the iron and manganese in the above-mentioned electrode were Fe 3+ 、Mn 3+ 、V 5+ Cr 3+ 、Al 3+ , namely Fe 2+ / 3+ and Mn 2+ / 3+The single electron transfer redox reaction occurred between V 3+ / 5+ The two electron transfer reaction occurred between Cr 3+ , Al 3+ No redox reaction occurred in the above process, i.e. the number of electron transfer in the above process > 1e - The number of redox deintercalation of sodium ions > 3;

[0026] c. The volume change of the unit cell during the charging and discharging process was about 2% analyzed by Re-fined-XRD technology; the inert sodium at the Na2 site participated in the redox intercalation and deintercalation during the charging and discharging process, i.e. the activation of the inert sodium ions in the unit cell structure was realized;

[0027] 7) At the same time, the capacity retention rate of the battery after 200 times of 1C / 1C cycles was 90%, recorded in Table 1, and the separator was taken out after the battery was disassembled in the glove box, washed 3-5 times with DMC, and evaporated to dryness in the glove box. The percentage of the mass of dissolved manganese on the separator to the mass of manganese in the positive electrode structure was 3*10 -10 , recorded in Table 1

[0028] Example 2-Na4Fe 0.695 Mn 1.75 V 0.3 Cr 0.05 Al 0.02 (PO4)2P2O7@C

[0029] The other examples and comparative examples have the same process and conditions as Example 1, except that:

[0030] 1) Add 500 mL of deionized water to a beaker, heat to 60°C in a water bath, and then add 0.3 mol of citric acid C6H 10 O8, 0.3 mol of ethylene glycol C2H6O2, 0.0695 mol of iron nitrate nonahydrate FeN3O9·9H2O, 0.175 mol of manganese nitrate tetrahydrate MnN2O6·4H2O, 0.03 mol of ammonium metavanadate NH4VO3, 0.005 mol of chromium nitrate nonahydrate CrN3O9·9H2O, and 0.002 mol of aluminum nitrate nonahydrate AlN3O9·9H2O, and stir until fully dissolved;

[0031] 4) The final product is obtained after heat treatment, denoted as sample 2#;

[0032] 5) The sample 2# is tested as follows:

[0033] a, ICP test the element ratio between Na:Fe:Mn:V:Cr:Al:P elements is 4:0.695:1.75:0.3:0.05:0.02:4, that is, Na4Fe 0.695 Mn 1.75 V 0.3 Cr 0.05 Al 0.02 (PO4)2P2O7@C material, consistent with the Refined-XRD structure;

[0034] c, the band gap width of the prepared sample 2# is tested by ultraviolet-visible absorption spectrum, and the test result is 3.35eV;

[0035] d, the prepared sample 2# is characterized by XRD, SEM, carbon content (TG) test and half cell charging and discharging cycle test, it can be seen from the XRD that the synthesized material is a pure phase (the purity is as high as 99.65%), it can be seen from the SEM that the particle size is uniform and small, the particle size is 4μm-6μm, and it can be seen from the 1C charging and discharging cycle test of the half cell that the positive electrode material has good cycle stability.

[0036] 6) Take sample 2# as active material, and perform charging and discharging cycle under 0.2C / 0.2C condition in 1.5V-4.5V voltage interval for 10 times, record the data of stable cycle discharge specific capacity in table 1, after charging to 4.5V, cut off, take out the positive electrode sheet from the battery in the glove box, wash 3-5 times with DMC, and evaporate in the glove box;

[0037] a, take part of the electrode sheet for ICP (mainly detect elements: Na:Fe:Mn:V:Cr:Al:P) test, the ICP test result shows that the element ratio between Na:Fe:Mn:V:Cr:Al:P elements is 0.55:0.695:1.75:0.3:0.05:0.02:4, that is, 3.45 reversible sodium ions are deintercalated in the structure after charging to 4.5V;

[0038] b, the volume change of the unit cell in the charging and discharging process is about 3% by Re-fined-XRD technology; 7) At the same time, the capacity retention rate of the battery after 200 times of 1C / 1C cycle is 88%, which is recorded in table 1, and the separator is taken out after the battery is disassembled in the glove box, washed 3-5 times with DMC, and evaporated in the glove box, the percentage of the mass of dissolved manganese on the separator to the mass of manganese in the positive electrode structure is 4*10 -10 , recorded in table 1

[0039] Example 3-Na4Fe 0.5 Mn 1.75 V0.3 Cr 0.05 Al 0.15 (PO4)2P2O7@C

[0040] The processes and conditions of other embodiments and comparative examples are the same as those of Example 1, except that:

[0041] 1) Add 500 mL of deionized water to a beaker, heat it in a water bath to 60°C, and then add 0.3 mol of citric acid monohydrate (C6H 10 O8, 0.3 mol ethylene glycol C2H6O2, 0.05 mol iron nitrate nonahydrate FeN3O9·9H2O, 0.175 mol manganese nitrate tetrahydrate MnN2O6·4H2O, 0.03 mol ammonium metavanadate NH4VO3, 0.005 mol chromium nitrate nonahydrate CrN3O9·9H2O, 0.015 mol aluminum nitrate nonahydrate AlN3O9·9H2O, stir until fully dissolved;

[0042] 4) After heat treatment, the final product is obtained, which is recorded as sample 3#;

[0043] 5) Test sample 3# as follows:

[0044] a. The element ratio of Na:Fe:Mn:V:Cr:Al:P in ICP test is 4:0.5:1.75:0.3:0.05:0.15:4, which is Na4Fe 0.5 Mn 1.75 V 0.3 Cr 0.05 Al 0.15 (PO4)2P2O7@C material, which is consistent with the Refined-XRD structure;

[0045] c. The band gap width of the prepared sample 3# was tested by UV-visible absorption spectroscopy, and the test result was 3.5eV;

[0046] d. The prepared sample 3# was subjected to XRD, SEM characterization, carbon content (TG) test and button half-cell charge-discharge cycle test. XRD showed that the synthesized material was pure phase (purity up to 99.4%). SEM showed that the particle size was uniform and small, with a particle size of 3μm-7μm. The button half-cell 1C charge-discharge cycle test showed that the positive electrode material had good cycle stability.

[0047] 6) Sample 3# as active material, 10 times of charge-discharge cycle in 0.2C / 0.2C condition in 1.5V-4.5V voltage interval, record the data of stable cycle discharge specific capacity in Table 1, after charging to 4.5V, cut off, disassemble the positive electrode sheet from the battery in the glove box, clean 3-5 times using DMC, and evaporate dry in the glove box;

[0048] a, take part of the electrode sheet for ICP test (mainly test elements: Na:Fe:Mn:V:Cr:Al:P), ICP test results show that the element ratio between Na:Fe:Mn:V:Cr:Al:P elements is 0.85:0.5:1.75:0.3:0.05:0.15:4, that is, 3.15 reversible sodium ions are deintercalated in the structure when charged to 4.5V;

[0049] b, the volume change of the unit cell in the charge-discharge process is about 1.6% by Re-fined-XRD technology; 7) At the same time, the capacity retention rate of the battery after 200 times of 1C / 1C cycle is 94%, recorded in Table 1, and the separator is disassembled from the battery in the glove box, cleaned 3-5 times using DMC, and evaporated dry in the glove box, the percentage of the mass of dissolved manganese on the separator to the mass of manganese in the positive electrode structure is 2.5*10 -10 , recorded in Table 1

[0050] Example 4-Na4Fe 0.68 Mn 1.75 V 0.3 Cr 0.02 Al 0.05 (PO4)2P2O7@C

[0051] Other examples and comparative examples have the same process and conditions as Example 1, except that:

[0052] 1) Add 500 mL of deionized water to a beaker, heat to 60°C in a water bath, and then add 0.3 mol of citric acid C6H 10 O8, 0.3 mol of ethylene glycol C2H6O2, 0.068 mol of iron nitrate nonahydrate FeN3O9·9H2O, 0.175 mol of manganese nitrate tetrahydrate MnN2O6·4H2O, 0.03 mol of ammonium metavanadate NH4VO3, 0.002 mol of chromium nitrate nonahydrate CrN3O9·9H2O, and 0.005 mol of aluminum nitrate nonahydrate AlN3O9·9H2O, and stir until fully dissolved;

[0053] 4) The final product is obtained after heat treatment, denoted as sample 4#;

[0054] 5) Test sample 4# as follows:

[0055] a、ICP test the element ratio between Na:Fe:Mn:V:Cr:Al:P elements is 4:0.68:1.75:0.3:0.02:0.05:4, that is, Na4Fe 0.68 Mn 1.75 V 0.3 Cr 0.02 Al 0.05 (PO4)2P2O7@C material, consistent with the Refined-XRD structure;

[0056] c、The prepared sample 4# was tested for band gap width by ultraviolet-visible absorption spectrum, and the test result was 3.4eV;

[0057] d、The prepared sample 4# was characterized by XRD, SEM, carbon content (TG) test and 1C charge-discharge cycle test of the half battery, from which it can be seen that the synthesized material is a pure phase (the purity is as high as 99.2%), from the SEM it can be seen that the particle size is uniform and the particle size is small, the particle size is 3μm-6μm, and from the 1C charge-discharge cycle test of the half battery it can be seen that the positive electrode material has good cycle stability

[0058] 6) Take sample 4# as active material, and perform charge-discharge cycle under 0.2C / 0.2C condition in 1.5V-4.5V voltage range for 10 times, record the data of stable cycle discharge specific capacity in Table 1, after charging to 4.5V, cut off, take out the positive electrode sheet from the battery in the glove box, wash 3-5 times with DMC, and evaporate in the glove box;

[0059] a、Take part of the electrode sheet for ICP test (mainly detect elements: Na:Fe:Mn:V:Cr:Al:P), the ICP test result shows that the element ratio between Na:Fe:Mn:V:Cr:Al:P elements is 0.8:0.5:1.75:0.3:0.05:0.15:4, that is, 3.2 reversible sodium ions are deintercalated in the structure after charging to 4.5V;

[0060] b、The volume change of the unit cell in the charge-discharge process is about 2.3% by Re-fined-XRD technology; 7) At the same time, the capacity retention rate of the battery after 200 times of 1C / 1C cycle is 90%, recorded in Table 1, and the separator is taken out after the battery is disassembled in the glove box, washed 3-5 times with DMC, and evaporated in the glove box, the percentage of the mass of dissolved manganese on the separator to the mass of manganese in the positive electrode structure is 2.99*10 -10 , recorded in Table 1

[0061] Example 5-Na4Fe0.5 Mn 1.75 V 0.3 Cr 0.15 Al 0.05 (PO4)2P2O7@C

[0062] Other embodiments and the process and conditions of the comparative example are the same as example 1, except that:

[0063] 1) 500 mL of deionized water was added to a beaker, heated to 60°C in a water bath, and 0.3 mol of citric acid C6H 10 O8, 0.3 mol of ethylene glycol C2H6O2, 0.05 mol of iron nitrate nonahydrate FeN3O9·9H2O, 0.175 mol of manganese nitrate tetrahydrate MnN2O6·4H2O, 0.03 mol of ammonium metavanadate NH4VO3, 0.015 mol of chromium nitrate nonahydrate CrN3O9·9H2O, and 0.005 mol of aluminum nitrate nonahydrate AlN3O9·9H2O were added in sequence, and stirred until fully dissolved;

[0064] 4) After heat treatment, the final product was obtained, denoted as sample 5#;

[0065] 5) The sample 5# was tested as follows:

[0066] a. ICP test of the element ratio between Na:Fe:Mn:V:Cr:Al:P elements was 4:0.5:1.75:0.3:0.15:0.05:4, i.e. Na4Fe 0.5 Mn 1.75 V 0.3 Cr 0.15 Al 0.05 (PO4)2P2O7@C material, consistent with the Refined-XRD structure;

[0067] c. The prepared sample 5# was tested for band gap width by ultraviolet-visible absorption spectrum, and the test result was 3.2 eV;

[0068] d. The prepared sample 5# was characterized by XRD, SEM, carbon content (TG) test, and coin-type half-cell charge-discharge cycle test. From the XRD, it can be seen that the synthesized material is a pure phase (with a purity as high as 99.2%), from the SEM, it can be seen that the particle size is uniform and small, with a particle size of 2 μm-4 μm, and from the coin-type half-cell 1C charge-discharge cycle test, it can be seen that the positive electrode material has good cycle stability.

[0069] 6) Sample 5# is used as active material, and is subjected to charge-discharge cycling under 0.2C / 0.2C conditions in the voltage range of 1.5V-4.5V for 10 times, and the data of the stable cycle discharge specific capacity is recorded in Table 1, and after charging to 4.5V, the cut-off is performed, and the positive electrode sheet is disassembled from the battery in the glove box, and is washed 3-5 times using DMC, and is evaporated to dryness in the glove box;

[0070] a, part of the electrode sheet is subjected to ICP (mainly detecting elements: Na:Fe:Mn:V:Cr:Al:P) test, and the ICP test result shows that the element ratio among Na:Fe:Mn:V:Cr:Al:P elements is 0.86:0.5:1.75:0.3:0.15:0.05:4, that is, 3.14 reversible sodium ions are deintercalated in the structure after charging to 4.5V;

[0071] b, the volume change of the unit cell in the charge-discharge process is about 2.2% by Re-fined-XRD technology; 7) at the same time, the capacity retention rate of the battery after 200 times of 1C / 1C cycling is 89%, which is recorded in Table 1, and the separator is disassembled from the battery in the glove box, and is washed 3-5 times using DMC, and is evaporated to dryness in the glove box, and the percentage of the mass of the dissolved manganese on the separator to the mass of the manganese in the positive electrode structure is 2.89*10 -10 , which is recorded in Table 1

[0072] Example 6 Na4Fe 0.95 Mn 1.75 V 0.1 Cr 0.05 Al 0.05 (PO4)2P2O7@C

[0073] The other examples and comparative examples are the same as Example 1, and the difference is that:

[0074] 1) 500mL of deionized water is added to a beaker, and the water is heated to 60°C in a water bath, and 0.3mol of citric acid C6H 10 O8, 0.3mol of ethylene glycol C2H6O2, 0.095mol of iron nitrate nonahydrate FeN3O9·9H2O, 0.175mol of manganese nitrate tetrahydrate MnN2O6·4H2O, 0.01mol of ammonium metavanadate NH4VO3, 0.005mol of chromium nitrate nonahydrate CrN3O9·9H2O, and 0.005mol of aluminum nitrate nonahydrate AlN3O9·9H2O are sequentially added and stirred until fully dissolved;

[0075] 4) The final product is obtained after heat treatment, and is recorded as sample 6#;

[0076] 5) The sample 6# is subjected to the following tests:

[0077] a、ICP test the element ratio between Na:Fe:Mn:V:Cr:Al:P elements is 4:0.95:1.75:0.1:0.05:0.05:4, that is, Na4Fe 0.95 Mn 1.75 V 0.1 Cr 0.05 Al 0.05 (PO4)2P2O7@C material, consistent with the Refined-XRD structure;

[0078] c、The prepared sample 6# was tested for band gap width by ultraviolet-visible absorption spectrum, and the test result was 3.41eV;

[0079] d、The prepared sample 6# was characterized by XRD, SEM, carbon content (TG) test and charge-discharge cycle test of button half cell, from which it can be seen that the synthesized material is a pure phase (the purity is as high as 99.5%), from the SEM it can be seen that the particle size is uniform and the particle size is small, the particle size is 1μm-5μm, from the 1C charge-discharge cycle test of the button half cell it can be seen that the positive electrode material has good cycle stability.

[0080] 6) Take sample 6# as active material, and perform charge-discharge cycle under 0.2C / 0.2C condition in 1.5V-4.5V voltage range for 10 times, record the data of stable cycle discharge specific capacity in Table 1, after charging to 4.5V, cut off, take out the positive electrode sheet from the battery in the glove box, wash 3-5 times with DMC, and evaporate in the glove box;

[0081] a、Take part of the electrode sheet for ICP test (mainly detect elements: Na:Fe:Mn:V:Cr:Al:P), the ICP test result shows that the element ratio between Na:Fe:Mn:V:Cr:Al:P elements is 0.88:0.5:1.75:0.1:0.05:0.05:4, that is, 3.12 reversible sodium ions are deintercalated in the structure after charging to 4.5V;

[0082] b、The volume change of the unit cell in the charge-discharge process is about 3% by Re-fined-XRD technology; 7) At the same time, the capacity retention rate of the battery after 200 times of 1C / 1C cycle is 91%, recorded in Table 1, and the separator is taken out after the battery is disassembled in the glove box, washed 3-5 times with DMC, and evaporated in the glove box, the percentage of the mass of dissolved manganese on the separator to the mass of manganese in the positive electrode structure is 3.3*10 -10 , recorded in Table 1

[0083] Example 7 Na4Fe0.5 Mn 1.75 V 0.4 Cr 0.05 Al 0.05 (PO4)2P2O7@C

[0084] Other embodiments and the process and conditions of the comparative example are the same as example 1, except that:

[0085] 1) 500 mL of deionized water was added to a beaker, heated to 60°C in a water bath, and 0.3 mol of citric acid C6H 10 O8, 0.3 mol of ethylene glycol C2H6O2, 0.05 mol of iron nitrate nonahydrate FeN3O9·9H2O, 0.175 mol of manganese nitrate tetrahydrate MnN2O6·4H2O, 0.04 mol of ammonium metavanadate NH4VO3, 0.005 mol of chromium nitrate nonahydrate CrN3O9·9H2O, and 0.005 mol of aluminum nitrate nonahydrate AlN3O9·9H2O were added in sequence and stirred until fully dissolved;

[0086] 4) After heat treatment, the final product was obtained, denoted as sample 7#;

[0087] 5) The sample 7# was tested as follows:

[0088] a. ICP test of the element ratio between Na:Fe:Mn:V:Cr:Al:P elements was 4:0.5:1.75:0.4:0.05:0.05:4, i.e. Na4Fe 0.5 Mn 1.75 V 0.4 Cr 0.05 Al 0.05 (PO4)2P2O7@C material, consistent with the Refined-XRD structure;

[0089] c. The band gap width of the prepared sample 7# was tested by UV-Vis absorption spectrum, and the test result was 3.21 eV;

[0090] d. The prepared sample 7# was tested by XRD, SEM characterization, carbon content (TG) test, and coin-type half-cell charge-discharge cycle test. From the XRD, it can be seen that the synthesized material is a pure phase (with a purity as high as 99.7%), from the SEM, it can be seen that the particle size is uniform and small, with a particle size of 3 μm-6 μm, and from the coin-type half-cell 1C charge-discharge cycle test, it can be seen that the positive electrode material has good cycle stability.

[0091] 6) Sample 7# as active material, 10 times of charge-discharge cycle in 0.2C / 0.2C condition in 1.5V-4.5V voltage interval, record the data of stable cycle discharge specific capacity in Table 1, after charging to 4.5V, cut off, disassemble the positive electrode sheet from the battery in the glove box, clean 3-5 times using DMC, and evaporate dry in the glove box;

[0092] a, take part of the electrode sheet for ICP test (mainly test elements: Na:Fe:Mn:V:Cr:Al:P), ICP test results show that the element ratio between Na:Fe:Mn:V:Cr:Al:P elements is 0.67:0.5:1.75:0.4:0.05:0.05:4, that is, 3.33 reversible sodium ions are deintercalated in the structure when charged to 4.5V;

[0093] b, the volume change of the unit cell in the charge-discharge process is about 1.7% by Re-fined-XRD technology; 7) At the same time, the capacity retention rate of the battery after 200 times of 1C / 1C cycle is 94%, which is recorded in Table 1, and the separator is disassembled in the glove box, cleaned 3-5 times using DMC, and evaporated dry in the glove box, the percentage of the mass of dissolved manganese on the separator to the mass of manganese in the positive electrode structure is 2.9*10 -10 , recorded in Table 1

[0094] Example 8 Na4Fe 0.85 Mn 1.55 V 0.3 Cr 0.05 Al 0.05 (PO4)2P2O7@C

[0095] Other examples and comparative examples have the same process and conditions as Example 1, except that:

[0096] 1) Add 500 mL of deionized water to a beaker, heat to 60°C in a water bath, and then add 0.3 mol of citric acid C6H 10 O8, 0.3 mol of ethylene glycol C2H6O2, 0.085 mol of iron nitrate nonahydrate FeN3O9·9H2O, 0.155 mol of manganese nitrate tetrahydrate MnN2O6·4H2O, 0.03 mol of ammonium metavanadate NH4VO3, 0.005 mol of chromium nitrate nonahydrate CrN3O9·9H2O, and 0.005 mol of aluminum nitrate nonahydrate AlN3O9·9H2O, and stir until fully dissolved;

[0097] 4) The final product is obtained after heat treatment, denoted as sample 8#;

[0098] 5) Test sample 8# as follows:

[0099] a、ICP test the element ratio between Na:Fe:Mn:V:Cr:Al:P elements is 4:0.85:1.55:0.3:0.05:0.05:4, that is, Na4Fe 0.85 Mn 1.55 V 0.3 Cr 0.05 Al 0.05 (PO4)2P2O7@C material, consistent with the Refined-XRD structure;

[0100] c、The prepared sample 8# is tested for band gap width by ultraviolet-visible absorption spectrum, and the test result is 3.26eV;

[0101] d、The prepared sample 8# is characterized by XRD, SEM, carbon content (TG) test and charge-discharge cycle test of button half cell, from which it can be seen that the synthesized material is a pure phase (the purity is as high as 99.5%), from the SEM it can be seen that the particle size is uniform and the particle size is small, the particle size is 4μm-8μm, and from the 1C charge-discharge cycle test of the button half cell it can be seen that the positive electrode material has good cycle stability.

[0102] 6) Take sample 8# as active material, and perform charge-discharge cycle under 0.2C / 0.2C condition in 1.5V-4.5V voltage range for 10 times, record the data of stable cycle discharge specific capacity in Table 1, after charging to 4.5V, cut off, take out the positive electrode sheet from the battery in the glove box, wash 3-5 times with DMC, and evaporate in the glove box;

[0103] a、Take part of the electrode sheet for ICP test (mainly detect elements: Na:Fe:Mn:V:Cr:Al:P), the ICP test result shows that the element ratio between Na:Fe:Mn:V:Cr:Al:P elements is 0.78:0.85:1.55:0.3:0.05:0.05:4, that is, 3.22 reversible sodium ions are deintercalated in the structure after charging to 4.5V;

[0104] b、The volume change of the unit cell in the charge-discharge process is about 1.65% by Re-fined-XRD technology;

[0105] 7) At the same time, the capacity retention rate of the battery after 200 times of 1C / 1C cycle is 95%, which is recorded in Table 1, and the separator is taken out after the battery is disassembled in the glove box, washed 3-5 times with DMC, and evaporated in the glove box, the percentage of the mass of dissolved manganese on the separator to the mass of manganese in the positive electrode structure is 2.1*10 -10 , recorded in Table 1

[0106] Example 9 Na4Fe 0.3 Mn 2.1 V 0.3 Cr 0.05 Al 0.05 (PO4)2P2O7@C

[0107] Other examples and the process and conditions of the comparative example are the same as Example 1, except that:

[0108] 1) 500 mL of deionized water was added to a beaker, heated to 60°C in a water bath, and 0.3 mol of citric acid C6H 10 O8, 0.3 mol of ethylene glycol C2H6O2, 0.03 mol of iron nitrate FeN3O9·9H2O, 0.21 mol of manganese nitrate MnN2O6·4H2O, 0.03 mol of ammonium metavanadate NH4VO3, 0.005 mol of chromium nitrate CrN3O9·9H2O, and 0.005 mol of aluminum nitrate AlN3O9·9H2O were added in sequence, and stirred until fully dissolved;

[0109] 4) After heat treatment, the final product was obtained, denoted as sample 9#;

[0110] 5) The sample 9# was tested as follows:

[0111] a. ICP test of the element ratio between Na:Fe:Mn:V:Cr:Al:P elements was 4:0.3:2.1:0.3:0.05:0.05:4, i.e. Na4Fe 0.3 Mn 2.1 V 0.3 Cr 0.05 Al 0.05 (PO4)2P2O7@C material, consistent with the Refined-XRD structure;

[0112] c. The prepared sample 9# was tested for band gap width by UV-Vis absorption spectrum, and the test result was 3.4 eV;

[0113] d. The prepared sample 9# was characterized by XRD, SEM, carbon content (TG) test, and coin-type half-cell charge-discharge cycle test. From the XRD, it can be seen that the synthesized material is a pure phase (with a purity as high as 99.8%), from the SEM, it can be seen that the particle size is uniform and small, with a particle size of 3 μm-8 μm, and from the coin-type half-cell 1C charge-discharge cycle test, it can be seen that the positive electrode material has good cycle stability.

[0114] 6) Sample 9# as active material, 10 times of charge-discharge cycle in 0.2C / 0.2C condition in 1.5V-4.5V voltage interval, the data of stable cycle discharge specific capacity were recorded in Table 1, after charging to 4.5V, cut-off, the positive electrode sheet was disassembled from the battery in the glove box, washed 3-5 times using DMC, and evaporated to dryness in the glove box;

[0115] a, part of the electrode sheet was taken for ICP test (mainly detecting elements: Na:Fe:Mn:V:Cr:Al:P), the ICP test results showed that the element ratio between Na:Fe:Mn:V:Cr:Al:P elements was 0.75:0.3:2.1:0.3:0.05:0.05:4, i.e. 3.25 reversible sodium ions were deintercalated in the structure when charged to 4.5V;

[0116] b, the volume change of the unit cell during charge-discharge process was about 3% by Re-fined-XRD technology; 7) At the same time, the capacity retention rate of the battery after 200 times of 1C / 1C cycle was 90%, which was recorded in Table 1, and the separator was disassembled from the battery in the glove box, washed 3-5 times using DMC, and evaporated to dryness in the glove box, the percentage of the mass of dissolved manganese on the separator to the mass of manganese in the positive electrode structure was 4.2*10 -10 , recorded in Table 1

[0117] Comparative example

[0118] Comparative example 1-Na4Fe 0.725 Mn 1.75 V 0.3 Cr 0.05 Al0(PO4)2P2O7@C

[0119] 1) 500mL of deionized water was added to a beaker, heated to 60°C in a water bath, and 0.3mol of citric acid C6H 10 O8, 0.3mol of ethylene glycol C2H6O2, 0.0725mol of iron nitrate nonahydrate FeN3O9·9H2O, 0.175mol of manganese nitrate tetrahydrate MnN2O6·4H2O, 0.03mol of ammonium metavanadate NH4VO3, and 0.005mol of chromium nitrate nonahydrate CrN3O9·9H2O were added in turn and stirred until completely dissolved;

[0120] 2) 0.4mol of sodium dihydrogen phosphate dihydrate NaH2PO4·2H2O (sodium source / phosphorus source) was added to the above aqueous solution, and the stirring was continued until the raw materials were completely dissolved to obtain a precursor solution;

[0121] 3) Pumping the precursor solution into a spray drying apparatus protected by a nitrogen atmosphere for spray drying (inlet temperature of 160°C, outlet temperature of 95°C, feed flow rate of 60 ml / min, compressed nitrogen pressure of 0.8 kPa) to obtain a precursor powder;

[0122] 4) The precursor powder was then transferred to a high-temperature tube furnace protected by an argon atmosphere for heat treatment. The first stage heat treatment temperature was 400°C and the heat treatment time was 5 hours. The second stage heat treatment temperature was 650°C and the heat treatment time was 10 hours. The final product was obtained after heat treatment and was recorded as sample A#

[0123] 5) Test sample A# as follows:

[0124] a. Refined-XRD and ICP (main detection elements: Na / Fe / Mn / V / Cr / P) tests. Refined-XRD shows that the product after heat treatment is consistent with the crystal structure of Na4Fe3(PO4)2P2O7 and belongs to the R3c space group. ICP test shows that the element ratio between Na:Fe:Mn:V:Cr:P is 4:0.725:1.75:0.3:0.05:4. Cr is distributed in the transition metal (Fe / Mn) position, that is, Na4Fe 0.7 Mn 1.75 V 0.3 Cr 0.05 Al 0.05 (PO4)2P2O7@C material, which is consistent with the Refined-XRD structure;

[0125] b. XPS test of iron, manganese, vanadium and chromium in sample A# shows that the peak separation results show that the iron and manganese in the above-mentioned electrode are Fe 2+ 、Mn 2+ 、V 3+ Cr 3+ ;

[0126] c. The band gap width of the prepared sample A# was tested by UV-visible absorption spectroscopy, and the test result was 3.6 eV;

[0127] d. The prepared sample A# was subjected to XRD, SEM characterization, carbon content (TG) test and button half-cell charge-discharge cycle test. XRD showed that the synthesized material was pure phase (purity up to 98%), SEM showed that the particle size was uniform and small, with a particle size of 3μm-5μm, and the button half-cell 1C charge-discharge cycle test showed that the positive electrode material had good cycle stability.

[0128] 6) Sample A# was used as the active material and mixed evenly with the conductive agent acetylene black and the binder polyvinylidene fluoride in a mass ratio of 8:1:1. An appropriate amount of solvent N-methylpyrrolidone was added and mixed evenly to form a paste, which was applied to the aluminum current collector. After drying, it was cut into discs with a diameter of 14 mm. The active material surface density was 1.1 to 3.2 mg / cm 2 (Here is 2.1 mg / cm 2 ), using this as the positive electrode of the battery, metallic sodium as the negative electrode, 1M NaClO4 / EC / DEC / 5%FEC (EC / DEC V / V=1:1) as the electrolyte, and a glass fiber membrane as the separator. The assembled battery was subjected to charge and discharge tests. The charge and discharge cycles were performed 10 times at 0.2C / 0.2C in the voltage range of 1.5V-4.5V. The discharge capacity data of the stable cycle are recorded in Table 1. The battery was then charged to 4.5V and then cut off. The positive electrode sheet was disassembled from the battery in a glove box, washed with DMC for 3-5 times, and then evaporated to dryness in the glove box.

[0129] a. Take part of the electrode for ICP test (main detection elements: Na:Fe:Mn:V:Cr:P). The ICP test results show that the element ratio between Na:Fe:Mn:V:Cr:P is 1.1:0.725:1.75:0.3:0.05:4, that is, when charged to 4.5V, there are 2.9 reversible sodium ions in the structure for deintercalation;

[0130] b. At the same time, XPS test was performed on the iron, manganese, vanadium and chromium elements in the above-mentioned electrode. The peak separation results showed that the iron and manganese in the above-mentioned electrode were Fe 3+ 、Mn 3+ 、V 5+ Cr 3+ , namely Fe 2+ / 3+ and Mn 2+ / 3+ A single electron transfer redox reaction occurs between the 3 + / 5+ Two electron transfer reactions occurred, and Cr 3+ No redox reaction occurs in the above process, that is, the number of electron transfers in the above process is > 1e - / Valence-changing transition metal ions, the number of sodium ions undergoing redox deintercalation is >3;

[0131] c. Re-fined-XRD analysis revealed that the volume change of the unit cell during charge and discharge was approximately 5%. During charge and discharge, the inert sodium at the Na2 site participated in the redox insertion and extraction, activating the inert sodium ions in the unit cell structure.

[0132] 7) After 200 cycles of 1C / 1C, the capacity retention of the battery is 82%, recorded in Table 2, and after disassembling the battery in the glove box, the separator is taken out, washed 3-5 times with DMC, and evaporated to dryness in the glove box, the percentage of the mass of manganese dissolved from the separator to the mass of manganese in the positive electrode structure is 7.5*10 -10 , recorded in Table 2

[0133] Comparative Example 2 - Na4Fe 0.275 Mn 1.75 V 0.3 Cr 0.05 Al 0.3 (PO4)2P2O7@C

[0134] Other comparative examples and comparative example processes and conditions are the same as Comparative Example 1, except that:

[0135] 1) 500 mL of deionized water is added to a beaker, heated to 60°C in a water bath, and 0.3 mol of citric acid C6H 10 O8, 0.3 mol of ethylene glycol C2H6O2, 0.0275 mol of iron nitrate nonahydrate FeN3O9·9H2O, 0.175 mol of manganese nitrate tetrahydrate MnN2O6·4H2O, 0.03 mol of ammonium metavanadate NH4VO3, 0.005 mol of chromium nitrate nonahydrate CrN3O9·9H2O, and 0.03 mol of aluminum nitrate nonahydrate AlN3O9·9H2O are added in sequence, and stirred until fully dissolved;

[0136] 4) Sample B#

[0137] 5) Sample B# is tested as follows:

[0138] a, Refined-XRD and ICP (mainly test elements: Na / Fe / Mn / V / Cr / Al / P) tests, Refined-XRD shows that the product after heat treatment is consistent with the crystal structure of Na4Fe3(PO4)2P2O7, belonging to the R3c space group, and the element ratio of ICP test Na:Fe:Mn:V:Cr:Al:P elements is 4:0.275:1.75:0.3:0.05:0.3:4, Cr and Al are distributed in the transition metal (Fe / Mn) site, i.e. Na4Fe 0.275 Mn 1.75 V 0.3 Cr 0.05 Al 0.3 (PO4)2P2O7@C material, consistent with the Refined-XRD structure;

[0139] b、XPS test of iron, manganese, vanadium, chromium, aluminum elements in sample B#, peak separation results Iron and manganese in the above electrode plate are Fe 2+ , Mn 2+ , V 3+ , Cr 3+ , Al 3+ ;

[0140] c, the band gap width of the prepared sample B# is tested by ultraviolet-visible absorption spectrum, and the test result is 3.7eV;

[0141] d, XRD, SEM characterization, carbon content (TG) test and button half cell charge-discharge cycle test are carried out on the prepared sample B#, it can be seen from XRD that the synthesized material is pure phase (the purity is as high as 97%), it can be seen from SEM that the particle size is uniform and the particle size is small, the particle size is 4μm-10μm, and it can be seen from the 1C charge-discharge cycle test of button half cell that the positive electrode material has good cycle stability.

[0142] 6) sample B# is used as active material, and charge-discharge cycle is carried out under 0.2C / 0.2C condition in 1.5V-4.5V voltage range for 10 times, the data of stable cycle discharge specific capacity are recorded in table 2, after charging to 4.5V, the cut-off is carried out, the positive electrode plate is disassembled from the battery in the glove box, and is washed 3-5 times by using DMC, and is evaporated dry in the glove box;

[0143] a, part of the electrode plate is taken for ICP (mainly detects elements: Na:Fe:Mn:V:Cr:Al:P) test, and the ICP test result shows that the element ratio of Na:Fe:Mn:V:Cr:Al:P elements is 1.25:0.275:1.75:0.3:0.05:0.3:4, that is, 2.75 reversible sodium ions are deintercalated in the structure after charging to 4.5V;

[0144] b, XPS test of iron, manganese, vanadium, chromium, aluminum elements in the above electrode plate, peak separation results Iron and manganese in the above electrode plate are Fe 3+ , Mn 3+ , V 5+ , Cr 3+ , Al 3+ , that is, the single electron transfer redox reaction occurs between Fe 2+ / 3+ and Mn 2+ / 3+ , while V 3+ / 5+ occurs two electron transfer reaction, and Cr 3+ , Al 3+ does not occur redox reaction in the above process, that is, the number of electron transfer in the above process is >1e -redox deintercalation of sodium ions > 3; c, the volume change of the unit cell during charging and discharging process was about 3% analyzed by Re-fined-XRD technology; 7) at the same time, the capacity retention rate of the battery after 200 times of 1C / 1C cycle was 86%, recorded in Table 1, and after disassembling the battery in the glove box, the separator was taken out, washed 3-5 times with DMC, and evaporated in the glove box, the percentage of the mass of dissolved manganese on the separator to the mass of manganese in the positive electrode structure was 4*10 -10 , recorded in Table 2

[0145] Comparative Example 3-Na4Fe 0.725 Mn 1.75 V 0.3 Cr0Al 0.05 (PO4)2P2O7@C

[0146] The other comparative examples and comparative examples process and conditions are the same as comparative example 1, and the difference is that:

[0147] 1) 500 mL of deionized water was added to a beaker, heated to 60°C in a water bath, and 0.3 mol of citric acid C6H 10 O8, 0.3 mol of ethylene glycol C2H6O2, 0.0725 mol of iron nitrate nonahydrate FeN3O9·9H2O, 0.175 mol of manganese nitrate tetrahydrate MnN2O6·4H2O, 0.03 mol of ammonium metavanadate NH4VO3, and 0.005 mol of aluminum nitrate nonahydrate AlN3O9·9H2O were added in turn, and stirred until fully dissolved;

[0148] 4) recorded as sample C#

[0149] 5) sample C# was tested as follows:

[0150] a, Refined-XRD and ICP (mainly test elements: Na / Fe / Mn / V / Al / P) test, Refined-XRD shows that the product after heat treatment is consistent with the crystal structure of Na4Fe3(PO4)2P2O7, belonging to R3c space group, Al is distributed in the transition metal (Fe / Mn) site, ICP test element ratio of Na:Fe:Mn:V:Al:P is 4:0.725:1.75:0.3:0.05:4, that is, Na4Fe 0.725 Mn 1.75 V 0.3 Al 0.05 (PO4)2P2O7@C material, consistent with the Refined-XRD structure; b, XPS test of iron, manganese, vanadium and aluminum elements in sample C#, the peak separation results show that the iron and manganese in the above electrode sheet are Fe2+ Mn 2+ V 3+ Al 3+ ;

[0151] c, the prepared sample C# was tested for band gap width by ultraviolet-visible absorption spectrum, and the test result was 3.4eV;

[0152] d, the prepared sample C# was tested by XRD, SEM characterization, carbon content (TG) test and button half-cell charge-discharge cycle test, it can be seen from the XRD that the synthesized material is a pure phase (the purity is as high as 98%), it can be seen from the SEM that the particle size is uniform and small, the particle size is 4μm-8μm, and it can be seen from the button half-cell 1C charge-discharge cycle test that the positive electrode material has good cycle stability.

[0153] 6) The sample C# was used as an active material, and was subjected to charge-discharge cycle under the condition of 0.2C / 0.2C in the voltage range of 1.5V-4.5V for 10 times, the data of the discharge specific capacity of the stable cycle were recorded in Table 2, and then charged to 4.5V and cut off, the positive electrode sheet was disassembled from the battery in the glove box, washed 3-5 times with DMC, and then evaporated to dryness in the glove box;

[0154] a, part of the electrode sheet was subjected to ICP (mainly detecting elements: Na:Fe:Mn:V:Al:P) test, and the ICP test result showed that the element ratio among Na:Fe:Mn:V:Al:P elements was 1.09:0.725:1.75:0.3:0.05:4, that is, 2.91 reversible sodium ions were deintercalated in the structure when charged to 4.5V;

[0155] b, at the same time, the iron, manganese, vanadium, chromium and aluminum elements in the above electrode sheet were subjected to XPS test, and the peak separation result showed that the iron and manganese in the above electrode sheet were Fe 3+ Mn 3+ V 5+ Al 3+ , that is, the single electron transfer redox reaction occurred between Fe 2+ / 3+ and Mn 2+ / 3+ , while V 3+ / 5+ occurred two electron transfer reaction, and Al 3+ did not occur redox reaction in the above process, that is, the number of electron transfer in the above process >1e - / variable valence transition metal ion, and the number of sodium ions subjected to redox deintercalation >3;

[0156] c. The volume change of the unit cell during the charge and discharge process was analyzed by Re-fined-XRD technology, which was about 4%; 7) At the same time, the capacity retention rate of the battery after 200 cycles of 1C / 1C was 85%, which is recorded in Table 1. The battery was disassembled in a glove box and the diaphragm was removed. After washing with DMC for 3-5 times, it was evaporated in the glove box. The mass of manganese dissolved on the diaphragm was detected by ICP technology. The percentage of the mass of manganese in the positive electrode structure was 3.5*10 -10 , recorded in Table 2

[0157] Comparative Example 4-Na4Fe 0.275 Mn 1.75 V 0.3 Cr 0.3 Al 0.05 (PO4)2P2O7@C

[0158] The processes and conditions of other embodiments and comparative examples are the same as those of Comparative Example 1, except that:

[0159] 1) Add 500 mL of deionized water to a beaker, heat it in a water bath to 60°C, and then add 0.3 mol of citric acid monohydrate (C6H 10 O8, 0.3 mol ethylene glycol C2H6O2, 0.0275 mol iron nitrate nonahydrate FeN3O9·9H2O, 0.175 mol manganese nitrate tetrahydrate MnN2O6·4H2O, 0.03 mol ammonium metavanadate NH4VO3, 0.03 mol chromium nitrate nonahydrate CrN3O9·9H2O, 0.005 mol aluminum nitrate nonahydrate AlN3O9·9H2O, stir until fully dissolved;

[0160] 4) Recorded as sample D#

[0161] 5) Perform the following tests on sample D#:

[0162] a. Refined-XRD and ICP (main detection elements: Na / Fe / Mn / V / Cr / Al / P) tests. Refined-XRD shows that the product after heat treatment is consistent with the crystal structure of Na4Fe3(PO4)2P2O7, belonging to the R3c space group. ICP test shows that the element ratio between Na:Fe:Mn:V:Al:P is 4:0.275:1.75:0.3:0.3:0.05:4. Cr and Al are distributed in the transition metal (Fe / Mn) position, that is, Na4Fe 0.275 Mn 1.75 V 0.3 Cr 0.3 Al 0.05 (PO4)2P2O7@C material, which is consistent with the Refined-XRD structure;

[0163] b. XPS test of iron, manganese, vanadium, chromium, aluminum elements in sample D#, peak separation results Iron and manganese in the above electrode plate are Fe 2+ , Mn 2+ , V 3+ , Cr 3+ , Al 3+ ;

[0164] c. The band gap width of the prepared sample D# is tested by UV-visible absorption spectrum, and the test result is 3.6eV;

[0165] d. XRD, SEM characterization, carbon content (TG) test and charge-discharge cycle test of the prepared sample D# are carried out, it can be seen from XRD that the synthesized material is a pure phase (the purity is as high as 97%), it can be seen from SEM that the particle size is uniform and small, and the particle size is 4μm-8μm, and it can be seen from the 1C charge-discharge cycle test of the half battery that the positive electrode material has good cycle stability.

[0166] 6) Sample D# is used as active material, and charge-discharge cycle is carried out under 0.2C / 0.2C condition in 1.5V-4.5V voltage range for 10 times, the data of discharge specific capacity of stable cycle are recorded in Table 1, and then charged to 4.5V and cut off, the positive electrode plate is disassembled from the battery in the glove box, washed 3-5 times with DMC, and then evaporated in the glove box;

[0167] a. Part of the electrode plate is taken for ICP (mainly detecting elements: Na:Fe:Mn:V:Cr:Al:P) test, and the ICP test result shows that the element ratio of Na:Fe:Mn:V:Cr:Al:P elements is 1.3:0.275:1.75:0.3:0.3:0.05:4, that is, 2.7 reversible sodium ions are deintercalated in the structure when charged to 4.5V;

[0168] b. XPS test of iron, manganese, vanadium, chromium, aluminum elements in the above electrode plate, peak separation results Iron and manganese in the above electrode plate are Fe 3+ , Mn 3+ , V 5+ , Al 3+ , that is, single electron transfer redox reaction occurs between Fe 2+ / 3+ and Mn 2+ / 3+ , while V 3+ / 5+ occurs two electron transfer reaction, and Al 3+ does not occur redox reaction in the above process, that is, the number of electron transfer in the above process >1e - / variable valence transition metal ion, and the number of sodium ions occurring redox deintercalation >3;

[0169] c. The volume change of the unit cell during the charge and discharge process was about 4.2% by the Re-fined-XRD technique; 7) At the same time, the capacity retention rate of the battery after 200 times of 1C / 1C cycling was 84%, recorded in Table 2, and the separator was taken out after the battery was disassembled in the glove box, washed 3-5 times with DMC, and evaporated in the glove box. The percentage of the mass of dissolved manganese on the separator to the mass of manganese in the positive electrode structure was 3.4*10 -10 , recorded in Table 2.

[0170] Comparative Example 5-Na4Fe 1.1 Mn 1.75 V0Cr 0.05 Al 0.05 (PO4)2P2O7@C

[0171] The other comparative examples and comparative processes and conditions are the same as those of Comparative Example 1, except that:

[0172] 1) 500 mL of deionized water was added to a beaker, heated to 60°C in a water bath, and 0.3 mol of citric acid C6H 10 O8, 0.3 mol of ethylene glycol C2H6O2, 0.11 mol of iron nitrate nonahydrate FeN3O9·9H2O, 0.175 mol of manganese nitrate tetrahydrate MnN2O6·4H2O, 0.005 mol of chromium nitrate nonahydrate CrN3O9·9H2O, and 0.005 mol of aluminum nitrate nonahydrate AlN3O9·9H2O were added in sequence and stirred until fully dissolved;

[0173] 4) Sample E#

[0174] 5) The sample E# was tested as follows:

[0175] a. Refined-XRD and ICP (mainly test elements: Na / Fe / Mn / Cr / Al / P) tests, Refined-XRD showed that the product after heat treatment was consistent with the crystal structure of Na4Fe3(PO4)2P2O7, belonging to the R3c space group, and the element ratio of Na:Fe:Mn:V:Al:P elements was 4:1.1:1.75:0.05:0.05:4 by ICP test, and Cr and Al were distributed in the transition metal (Fe / Mn) site, i.e. Na4Fe 1.1 Mn 1.75 Cr 0.05 Al 0.05 (PO4)2P2O7@C material, consistent with the Refined-XRD structure; b. XPS test was performed on the iron, manganese, chromium, and aluminum elements in sample E#, and the peak separation results showed that the iron and manganese in the above electrode were Fe 2+ , Mn2+ Cr 3+ Al 3 + ;

[0176] c, the prepared sample E# was tested for band gap width by ultraviolet-visible absorption spectrum, and the test result was 3.7eV;

[0177] d, the prepared sample E# was characterized by XRD, SEM, carbon content (TG) test and half-cell charging and discharging cycle test. It can be seen from the XRD that the synthesized material is a pure phase (the purity is as high as 97.5%), and it can be seen from the SEM that the particle size is uniform and small, and the particle size is 4μm-8μm. It can be seen from the 1C charging and discharging cycle test of the half-cell that the positive electrode material has good cycle stability.

[0178] 6) The sample E# was used as an active material, and charging and discharging cycles were carried out under the condition of 0.2C / 0.2C in the voltage range of 1.5V-4.5V for 10 times. The data of the discharge specific capacity of the stable cycle were recorded in Table 2. After charging to 4.5V, the cutoff was made, and the positive electrode sheet was disassembled from the battery in the glove box, washed 3-5 times with DMC, and then evaporated to dryness in the glove box;

[0179] a, part of the electrode sheet was subjected to ICP (mainly detecting elements: Na:Fe:Mn:Cr:Al:P) test. The ICP test result showed that the element ratio between Na:Fe:Mn:Cr:Al:P elements was 1.1:1.1:1.75:0.05:0.05:4, that is, 2.9 reversible sodium ions were deintercalated in the structure when charged to 4.5V;

[0180] b, at the same time, XPS test was carried out on the iron, manganese, chromium and aluminum elements in the above electrode sheet, and the peak separation result showed that the iron and manganese in the above electrode sheet were Fe 3+ , Mn 3+ , Cr 3+ , Al 3+ , that is, a single electron transfer redox reaction occurred between Fe 2+ / 3+ and Mn 2+ / 3+ , while Al 3+ , Cr 3+ did not undergo redox reaction in the above process, that is, the number of electron transfer in the above process was about 1e - / variable valence transition metal ion, and the number of sodium ions undergoing redox deintercalation was about 3;

[0181] c. The volume change of the unit cell during the charge and discharge process was about 6% by Re-fined-XRD technology; 7) At the same time, the capacity retention rate of the battery after 200 times of 1C / 1C cycle was 80%, recorded in Table 1, and the separator was taken out after the battery was disassembled in the glove box, washed 3-5 times with DMC, and evaporated in the glove box. The percentage of the mass of dissolved manganese on the separator to the mass of manganese in the positive electrode structure was 6*10 -10 , recorded in Table 2.

[0182] Comparative Example 6-Na4Fe 0.35 Mn 1.75 V 0.5 Cr 0.05 Al 0.05 (PO4)2P2O7@C

[0183] Other comparative examples and comparative processes and conditions are the same as those of Comparative Example 1, except that:

[0184] 1) 500 mL of deionized water was added to a beaker, heated to 60°C in a water bath, and 0.3 mol of citric acid C6H 10 O8, 0.3 mol of ethylene glycol C2H6O2, 0.035 mol of iron nitrate nonahydrate FeN3O9·9H2O, 0.175 mol of manganese nitrate tetrahydrate MnN2O6·4H2O, 0.05 mol of ammonium metavanadate NH4VO3, 0.005 mol of chromium nitrate nonahydrate CrN3O9·9H2O, and 0.005 mol of aluminum nitrate nonahydrate AlN3O9·9H2O were added in sequence and stirred until fully dissolved;

[0185] 4) Sample F#

[0186] 5) Sample F# was tested as follows:

[0187] a. Refined-XRD and ICP (mainly test elements: Na / Fe / Mn / Cr / V / Al / P) tests, Refined-XRD showed that the product after heat treatment was consistent with the crystal structure of Na4Fe3(PO4)2P2O7, belonging to the R3c space group, and the element ratio of ICP test Na:Fe:Mn:V:Al:P elements was 4:0.35:1.75:0.5:0.05:0.05:4, Cr and Al were distributed in the transition metal (Fe / Mn) site, i.e. Na4Fe 0.35 Mn 1.75 V 0.5 Cr 0.05 Al 0.05 (PO4)2P2O7@C material, consistent with the Refined-XRD structure;

[0188] b. XPS test of iron, manganese, vanadium, chromium, aluminum elements in sample F#, peak separation results Iron and manganese in the above electrode plate are Fe 2+ , Mn 2+ , V 3+ , Cr 3+ , Al 3+ ;

[0189] c. The band gap width of the prepared sample F# is tested by UV-visible absorption spectrum, and the test result is 3.6eV;

[0190] d. XRD, SEM characterization, carbon content (TG) test and charge-discharge cycle test of the prepared sample F# are carried out, it can be seen from XRD that the synthesized material is a pure phase (the purity is as high as 98%), it can be seen from SEM that the particle size is uniform and small, and the particle size is 5-8μm, and it can be seen from the 1C charge-discharge cycle test of the half battery that the positive electrode material has good cycle stability.

[0191] 6) Take sample F# as active material, carry out charge-discharge cycle under 0.2C / 0.2C condition in 1.5V-4.5V voltage range for 10 times, record the data of discharge specific capacity of stable cycle in table 2, after charging to 4.5V, cut off, take out the positive electrode plate from the battery in the glove box, wash 3-5 times with DMC, and evaporate in the glove box;

[0192] a. Take part of the electrode plate for ICP (mainly detect elements: Na:Fe:Mn:V:Cr:Al:P) test, ICP test result shows that the element ratio of Na:Fe:Mn:V:Cr:Al:P elements is 0.69:0.35:1.75:0.5:0.05:0.05:4, that is, 2.8 reversible sodium ions are deintercalated in the structure after charging to 4.5V;

[0193] b. At the same time, XPS test of iron, manganese, vanadium, chromium, aluminum elements in the above electrode plate, peak separation results Iron and manganese in the above electrode plate are Fe 3+ , Mn 3+ , V 5+ , Cr 3+ , Al 3+ , that is, single electron transfer redox reaction occurs between Fe 2+ / 3+ and Mn 2+ / 3+ , while Al 3+ , Cr 3+ do not occur redox reaction in the above process, that is, the number of electron transfer in the above process >1e - / variable valence transition metal ion, and the number of sodium ions occurring redox deintercalation >3;

[0194] c. The volume change of the unit cell during the charge and discharge process was about 5% by Re-fined-XRD technology; 7) At the same time, the capacity retention rate of the battery after 200 times of 1C / 1C cycle was 88%, recorded in Table 1, and the separator was taken out after the battery was disassembled in the glove box, washed 3-5 times with DMC, and evaporated in the glove box. The percentage of the mass of dissolved manganese on the separator to the mass of manganese in the positive electrode structure was 4.8*10 -10 , recorded in Table 2.

[0195] Comparative Example 7-Na4Fe1Mn 1.4 V 0.3 Cr 0.05 Al 0.05 (PO4)2P2O7@C

[0196] Other comparative examples and comparative processes and conditions are the same as Comparative Example 1, and the difference is that:

[0197] 1) 500 mL of deionized water was added to a beaker, heated to 60°C in a water bath, and 0.3 mol of citric acid C6H 10 O8, 0.3 mol of ethylene glycol C2H6O2, 0.1 mol of iron nitrate nonahydrate FeN3O9·9H2O, 0.14 mol of manganese nitrate tetrahydrate MnN2O6·4H2O, 0.03 mol of ammonium metavanadate NH4VO3, 0.005 mol of chromium nitrate nonahydrate CrN3O9·9H2O, and 0.005 mol of aluminum nitrate nonahydrate AlN3O9·9H2O were added in sequence and stirred until fully dissolved;

[0198] 4) Sample G#

[0199] 5) Sample G# was tested as follows:

[0200] a. Refined-XRD and ICP (mainly test elements: Na / Fe / Mn / Cr / V / Al / P) tests, Refined-XRD showed that the product after heat treatment was consistent with the crystal structure of Na4Fe3(PO4)2P2O7, belonging to the R3c space group, and the element ratio of ICP test Na:Fe:Mn:V:Cr:Al:P elements was 4:1:1.4:0.3:0.05:0.05:4, Cr and Al were distributed in the transition metal (Fe / Mn) site, i.e. Na4Fe1Mn 1.4 V 0.3 Cr 0.05 Al 0.05 (PO4)2P2O7@C material, consistent with the Refined-XRD structure;

[0201] b. XPS test of iron, manganese, vanadium, chromium and aluminum in sample G#. Peak separation results show that the iron and manganese in the above-mentioned electrode are Fe 2+ 、Mn 2+ 、V 3+ Cr 3+ 、Al 3+ ;

[0202] c. The band gap width of the prepared sample F# was tested by UV-visible absorption spectroscopy, and the test result was 3.4 eV;

[0203] d. The prepared sample G# was subjected to XRD, SEM characterization, carbon content (TG) test and button half-cell charge and discharge cycle test. XRD shows that the synthesized material is pure phase (purity up to 97%), SEM shows that the particle size is uniform and the particle size is small, with a particle size of 3μm-8μm. The button half-cell 1C charge and discharge cycle test shows that the positive electrode material has good cycle stability.

[0204] 6) Using sample G# as the active material, charge and discharge cycles were performed 10 times at 0.2C / 0.2C in the voltage range of 1.5V-4.5V. The discharge capacity data of the stable cycle are recorded in Table 2. The battery was then post-charged to 4.5V and then cut off. The positive electrode was removed from the battery in a glove box, washed with DMC 3-5 times, and then evaporated to dryness in the glove box.

[0205] a. Take part of the electrode for ICP test (main detection elements: Na:Fe:Mn:V:Cr:Al:P). The ICP test results show that the element ratio between Na:Fe:Mn:V:Cr:Al:P is 0.97:1:1.4:0.3:0.05:0.05:4, that is, when charged to 4.5V, there are 3.03 reversible sodium ions in the structure for deintercalation;

[0206] b. At the same time, XPS test was performed on the iron, manganese, vanadium, chromium and aluminum elements in the above-mentioned electrode. The peak separation results showed that the iron and manganese in the above-mentioned electrode were Fe 3+ 、Mn 3+ 、V 5+ Cr 3+ 、Al 3+ , namely Fe 2+ / 3+ and Mn 2+ / 3+ A single electron transfer redox reaction occurs between the two, and Al 3+ , Cr 3+ No redox reaction occurs in the above process, that is, the number of electron transfers in the above process is > 1e - / Valence-changing transition metal ions, the number of sodium ions undergoing redox deintercalation is >3;

[0207] c. The volume change of the unit cell during the charge and discharge process was analyzed by Re-fined-XRD technology, which was about 5%; 7) At the same time, the capacity retention rate of the battery after 200 cycles of 1C / 1C was 88%, which is recorded in Table 1. The battery was disassembled in a glove box and the diaphragm was removed. After washing with DMC for 3-5 times, it was evaporated in the glove box. The mass of manganese dissolved on the diaphragm was detected by ICP technology. The percentage of the mass of manganese dissolved in the positive electrode structure was 5*10 -10 , recorded in Table 2.

[0208] Comparative Example 8-Na4Fe 1.4 Mn1V 0.3 Cr 0.05 Al 0.05 (PO4)2P2O7@C

[0209] The processes and conditions of other comparative examples and comparative examples are the same as those of Comparative Example 1, except that:

[0210] 1) Add 500 mL of deionized water to a beaker, heat it in a water bath to 60°C, and then add 0.3 mol of citric acid monohydrate (C6H 10 O8, 0.3 mol ethylene glycol C2H6O2, 0.14 mol iron nitrate nonahydrate FeN3O9·9H2O, 0.1 mol manganese nitrate tetrahydrate MnN2O6·4H2O, 0.03 mol ammonium metavanadate NH4VO3, 0.005 mol chromium nitrate nonahydrate CrN3O9·9H2O, 0.005 mol aluminum nitrate nonahydrate AlN3O9·9H2O, stir until fully dissolved;

[0211] 4) Recorded as sample H#

[0212] 5) Test sample H# as follows:

[0213] a. Refined-XRD and ICP (main detection elements: Na / Fe / Mn / Cr / V / Al / P) tests. Refined-XRD shows that the product after heat treatment is consistent with the crystal structure of Na4Fe3(PO4)2P2O7 and belongs to the R3c space group. ICP test shows that the element ratio between Na:Fe:Mn:V:Cr:Al:P is 4:1.4:1:0.3:0.05:0.05:4. Cr and Al are distributed in the transition metal (Fe / Mn) position, that is, Na4Fe 1.4 Mn1V 0.3 Cr 0.05 Al 0.05 (PO4)2P2O7@C material, which is consistent with the Refined-XRD structure;

[0214] b、XPS test of iron, manganese, vanadium, chromium, aluminum elements in sample H#, peak separation results Iron and manganese in the above electrode plate are Fe 2+ , Mn 2+ , V 3+ , Cr 3+ , Al 3+ ;

[0215] c、Band gap width test of prepared sample H# by UV-visible absorption spectrum, the test result is 3.6eV;

[0216] d、XRD, SEM characterization, carbon content (TG) test and coin type half cell charge-discharge cycle test of prepared sample H#, from XRD it can be seen that the synthesized material is a pure phase (purity up to 97.2%), from SEM it can be seen that the particle size is uniform and the particle size is small, the particle size is 2μm-7μm, from the 1C charge-discharge cycle test of the coin type half cell it can be seen that the positive electrode material has good cycle stability.

[0217] 6) Take sample H# as active material, carry out charge-discharge cycle under 0.2C / 0.2C condition in 1.5V-4.5V voltage range for 10 times, record the data of stable cycle discharge specific capacity in table 2, after charging to 4.5V, cut off, take out the positive electrode plate from the battery in the glove box, wash 3-5 times with DMC, and evaporate in the glove box;

[0218] a、Take part of the electrode plate for ICP (mainly detect elements: Na:Fe:Mn:V:Cr:Al:P) test, ICP test result shows that the element ratio between Na:Fe:Mn:V:Cr:Al:P elements is 0.99:1.4:1:0.3:0.05:0.05:4, that is, 3.01 reversible sodium ions are deintercalated in the structure after charging to 4.5V;

[0219] b、XPS test of iron, manganese, vanadium, chromium, aluminum elements in the above electrode plate, peak separation results Iron and manganese in the above electrode plate are Fe 3+ , Mn 3+ , V 5+ , Cr 3+ , Al 3+ , that is, single electron transfer redox reaction occurs between Fe 2+ / 3+ and Mn 2+ / 3+ , while Al 3+ , Cr 3+ do not undergo redox reaction in the above process, that is, the number of electron transfer in the above process >1e - / variable valence transition metal ion, the number of sodium ions undergoing redox deintercalation >3;

[0220] c. The volume change of the unit cell during the charge and discharge process was about 4% by Re-fined-XRD technology; 7) At the same time, the capacity retention rate of the battery after 200 times of 1C / 1C cycles was 86%, recorded in Table 1, and the separator was taken out after the battery was disassembled in the glove box, washed 3-5 times with DMC, and evaporated in the glove box. The percentage of the mass of dissolved manganese on the separator to the mass of manganese in the positive electrode structure was 4*10 -10 , recorded in Table 2.

[0221] Comparative Example 9-Na4Fe 0.65 Mn 1.75 V 0.3 Cr 0.05 Al 0.05 (PO4)2P2O7@C

[0222] 1) Add 500 mL of deionized water to a beaker, heat to 60°C in a water bath, and then add 0.3 mol of citric acid C6H 10 O8, 0.3 mol of ethylene glycol C2H6O2, 0.065 mol of iron nitrate nonahydrate FeN3O9·9H2O, 0.175 mol of manganese nitrate tetrahydrate MnN2O6·4H2O, 0.03 mol of ammonium metavanadate NH4VO3, 0.005 mol of chromium nitrate nonahydrate CrN3O9·9H2O, and 0.005 mol of aluminum nitrate nonahydrate AlN3O9·9H2O, and stir until fully dissolved;

[0223] 4) Sample K#

[0224] 5) The sample K# was tested as follows:

[0225] a. Refined-XRD and ICP (mainly test elements: Na / Fe / Mn / V / Cr / Al / P) tests, Refined-XRD shows that the product after heat treatment is consistent with the crystal structure of Na4Fe3(PO4)2P2O7, belonging to the R3c space group, and the ICP test shows that the element ratio between Na:Fe:Mn:V:Cr:Al:P elements is 4:0.65:1.75:0.3:0.05:0.05:4. Cr and Al are distributed in the transition metal (Fe / Mn) site, i.e. Na4Fe 0.65 Mn 1.75 V 0.3 Cr 0.05 Al 0.05 (PO4)2P2O7@C material, consistent with the Refined-XRD structure;

[0226] b, XPS test of iron, manganese, vanadium, chromium, aluminum elements in sample K#, peak separation results Iron and manganese in the above electrode plate are Fe 2+ , Mn 2+ , V 3+ , Cr 3+ , Al 3+ ;

[0227] c, the band gap width of the prepared sample K# is tested by UV-visible absorption spectrum, and the test result is 3.5eV;

[0228] d, XRD, SEM characterization, carbon content (TG) test and button half cell charge-discharge cycle test are carried out on the prepared sample K#, it can be seen from XRD that the synthesized material is pure phase (the purity is as high as 96.8%), it can be seen from SEM that the particle size is uniform and the particle size is small, the particle size is 8μm-10μm, and it can be seen from the 1C charge-discharge cycle test of button half cell that the positive electrode material has good cycle stability.

[0229] 2) sample K# is used as active material, and charge-discharge cycle is carried out under 0.2C / 0.2C condition in 1.5V-4.5V voltage range for 10 times, the data of stable cycle discharge specific capacity are recorded in table 2, after charging to 4.5V, the cut-off, the positive electrode plate is disassembled from the battery in the glove box, washed 3-5 times with DMC, and then evaporated in the glove box;

[0230] a, part of the electrode plate is taken for ICP (mainly detects elements: Na:Fe:Mn:V:Cr:P) test, and the ICP test result shows that the element ratio of Na:Fe:Mn:V:Cr:P elements is 1:0.65:1.75:0.3:0.05:0.05:4, that is, after charging to 4.5V, three reversible sodium ions in the structure are deintercalated;

[0231] b, XPS test of iron, manganese, vanadium, chromium, aluminum elements in the above electrode plate, peak separation results Iron and manganese in the above electrode plate are Fe 3+ , Mn 3+ , V 4+ , Cr 3+ , Al 3+ , that is, the single electron transfer redox reaction occurs between Fe 2+ / 3+ and Mn 2+ / 3+ , V 3+ / 4+ occurs single electron transfer reaction, and Cr 3+ , Al 3+ does not occur redox reaction in the above process, that is, the number of electron transfer in the above process is about 1e -about 3; c, the volume change of the unit cell during the charge and discharge process was about 8% by Re-fined-XRD technology; 7) At the same time, the capacity retention rate of the battery after 200 times of 1C / 1C cycles was 80%, recorded in Table 1, and after the battery was disassembled in the glove box, the separator was taken out, washed 3-5 times with DMC, and evaporated in the glove box, the percentage of the mass of dissolved manganese on the separator to the mass of manganese in the positive electrode structure was 7*10 -10 , recorded in Table 2

[0232] Comparative Example 10-Na4Fe 0.65 Mn 1.75 V 0.3 Cr 0.05 Al 0.05 (PO4)2P2O7@C

[0233] 1) Add 500 mL of deionized water to a beaker, heat to 60°C in a water bath, and then add 0.3 mol of citric acid C6H 10 O8, 0.3 mol of ethylene glycol C2H6O2, 0.065 mol of iron nitrate FeN3O9·9H2O, 0.175 mol of manganese nitrate MnN2O6·4H2O, 0.03 mol of ammonium metavanadate NH4VO3, 0.005 mol of chromium nitrate CrN3O9·9H2O, and 0.005 mol of aluminum nitrate AlN3O9·9H2O, and stir until fully dissolved;

[0234] 4) Denoted as sample L#

[0235] 5) The sample L# was tested as follows:

[0236] a, Refined-XRD and ICP (mainly test elements: Na / Fe / Mn / V / Cr / Al / P) tests, Refined-XRD shows that the product after heat treatment is consistent with the crystal structure of Na4Fe3(PO4)2P2O7, belonging to the R3c space group, and ICP tests the element ratio between Na:Fe:Mn:V:Cr:Al:P elements as 4:0.65:1.75:0.3:0.05:0.05:4, Cr and Al are distributed in the transition metal (Fe / Mn) site, i.e. Na4Fe 0.65 Mn 1.75 V 0.3 Cr 0.05 Al 0.05 (PO4)2P2O7@C material, consistent with the Refined-XRD structure;

[0237] b. XPS test of iron, manganese, vanadium, chromium, aluminum elements in sample L#, peak separation results Iron and manganese in the above electrode plate are Fe 2+ , Mn 2+ , V 3+ , Cr 3+ , Al 3+ , respectively.

[0238] c. The band gap width of the prepared sample K# is tested by UV-visible absorption spectrum, and the test result is 3.63 eV.

[0239] d. XRD, SEM characterization, carbon content (TG) test and coin-type half-cell charge-discharge cycle test are performed on the prepared sample L#. It can be seen from the XRD that the synthesized material is a pure phase (the purity is as high as 97%), and it can be seen from the SEM that the particle size is uniform and small, and the particle size is 8 μm-15 μm. It can be seen from the 1C charge-discharge cycle test of the coin-type half-cell that the positive electrode material has good cycle stability.

[0240] 2) Sample L# is used as active material, and charge-discharge cycle is carried out under 0.2C / 0.2C condition in 1.5V-4.5V voltage range for 10 times. The data of stable cycle discharge specific capacity are recorded in Table 2. After charging to 4.5V, the cutoff is carried out. The positive electrode plate is disassembled from the battery in the glove box, washed 3-5 times with DMC, and then evaporated in the glove box;

[0241] a. Part of the electrode plate is taken for ICP (mainly detecting elements: Na:Fe:Mn:V:Cr:P) test. The ICP test result shows that the element ratio among Na:Fe:Mn:V:Cr:P elements is 1.02:0.65:1.75:0.3:0.05:0.05:4, that is, 2.98 reversible sodium ions are deintercalated after charging to 4.5V.

[0242] b. XPS test of iron, manganese, vanadium, chromium, aluminum elements in the above electrode plate, peak separation results Iron and manganese in the above electrode plate are Fe 3+ , Mn 3+ , V 4+ , Cr 3+ , Al 3+ , respectively. 2+ / 3+ , Mn 2+ / 3+ , V 3+ / 4+ , Cr 3+ , Al 3+ , that is, the single electron transfer redox reaction occurs between Fe 2+ / 3+ and Mn 2+ / 3+ , V 3+ / 4+ occurs single electron transfer reaction, and Cr 3+ , Al 3+ do not occur redox reaction in the above process, that is, the number of electron transfer in the above process is about 1e -about 3; c, the volume change of the unit cell during the charging and discharging process is about 10% analyzed by the Re-fined-XRD technology; 7) at the same time, the capacity retention rate of the battery after 200 times of 1C / 1C cycles is 79%, recorded in Table 1, and after the battery is disassembled in the glove box, the separator is taken out, washed 3-5 times using DMC, and evaporated in the glove box, the percentage of the mass of the dissolved manganese on the separator to the mass of the manganese in the positive electrode structure is 8*10 -10 , recorded in Table 2

[0243] Example (Table 1)

[0244]

[0245] Comparative Example (Table 2)

[0246]

[0247] Conclusion

[0248] A novel structure of phosphoric acid pyrophosphate composite polyanion compound and a preparation method thereof are provided in the present application. By accurately controlling the ratio of iron, manganese, vanadium and transition metal ions, the redox potential of the material is improved, the ionic conductivity and electronic conductivity of the material are improved, and the unit cell structure is stabilized. Therefore, the actual gram capacity, rate performance and cycle stability of the material in the application process as a sodium ion battery positive electrode material are further improved. The prepared Na4Fe 0.65 Mn 1.75 V 0.3 Cr 0.05 Al 0.05 (PO4)2P2O7@C has a discharge gram capacity of 156 mAh / g at 0.2C and a median voltage of 3.7V. The discharge gram capacity is 122 mAh / g at 1C and 109 mAh / g at 5C. The capacity retention rate is as high as 90% after 200 cycles of charging and discharging at 1C.

[0249] The doping of transition metal sites (iron, manganese, vanadium) in the unit cell structure has a great influence on improving the ionic conductivity and electronic conductivity of the material, stabilizing the unit cell structure, and reducing the band gap of the material. By appropriate doping, such as Cr 3+ (containing unpaired electrons), Al 3+(Al-O covalent bond stable cell structure) can help the reversible electrochemical de-intercalation of inert sodium ions in the structure, realize multi-electron transfer of more than three electrons, and improve the specific capacity and cycle stability of the material. However, excessive transition metal site doping will reduce the redox active site in the electrochemical process, resulting in a decrease in specific capacity. Through the examples and comparative examples in the present application, it is found that when Na4Fe 3-x-1.5y-z Mn x V y A z In the structure of (PO4)2P2O7@C, when x is 1.75, y is 0.3, and z satisfies 0.25≥z≥0.1, the prepared material has excellent rate performance and capacity retention rate, the discharge specific capacity at 0.2C can reach 137-156 mAh / g, the median voltage is 3.52V-3.7V, the discharge specific capacity at 1C can reach 109-122 mAh / g, the discharge specific capacity at 5C can reach 100-109 mAh / g, and the capacity retention rate is as high as 89%-94% after 200 cycles of charge and discharge at 1C. (For example: Examples 1-4, Comparative Examples 1-4)

[0250] When the proportion of the doping elements is fixed, the proportion of iron, manganese and vanadium three active transition metal ions has a more significant influence on the electrochemical behavior of the material, thereby affecting the rate performance and capacity retention rate of the material under the cycle condition. When Na4Fe 3-x-1.5y-z Mn x V y A z In the structure of (PO4)2P2O7@C, when x is 1.75, z is 0.2, and y satisfies 0.5≥y≥0.1, the prepared material has excellent rate performance and capacity retention rate, the discharge specific capacity at 0.2C can reach 135-150 mAh / g, the median voltage is 3.43V-3.52V, the discharge specific capacity at 1C can reach 108-119 mAh / g, the discharge specific capacity at 5C can reach 100-111 mAh / g, and the capacity retention rate is as high as 89%-94% after 200 cycles of charge and discharge at 1C. (For example: Examples 6-8, Comparative Examples 5-6) When Na4Fe 3-x-1.5y-z Mn x V y A zWhen Na4Fe (PO4) 2P2O7@C structure, y is 0.3, z is 0.2, and the value of x is in the range of 2.25≥x≥1.5, the prepared material has excellent rate performance and capacity retention rate, the discharge gram capacity at 0.2C can reach 140-142mAh / g, the median voltage is 3.40V-3.65V, the discharge gram capacity at 1C can reach 112-117mAh / g, the discharge gram capacity at 5C can reach 105-108mAh / g, and the capacity retention rate is as high as 90%-95% after 200 cycles of charge-discharge at 1C.

[0251] When Na4Fe (PO4) 2P2O7@C structure, y is 0.3, z is 0.2, and the value of x is in the range of 2.25≥x≥1.5, the prepared material has excellent rate performance and capacity retention rate, the discharge gram capacity at 0.2C can reach 140-142mAh / g, the median voltage is 3.40V-3.65V, the discharge gram capacity at 1C can reach 112-117mAh / g, the discharge gram capacity at 5C can reach 105-108mAh / g, and the capacity retention rate is as high as 90%-95% after 200 cycles of charge-discharge at 1C. 3-x-1.5y-z Mn x V y A z When Na4Fe (PO4) 2P2O7@C structure, y is 0.3, z is 0.2, and the value of x is in the range of 2.25≥x≥1.5, the prepared material has excellent rate performance and capacity retention rate, the discharge gram capacity at 0.2C can reach 140-142mAh / g, the median voltage is 3.40V-3.65V, the discharge gram capacity at 1C can reach 112-117mAh / g, the discharge gram capacity at 5C can reach 105-108mAh / g, and the capacity retention rate is as high as 90%-95% after 200 cycles of charge-discharge at 1C.

[0252] When Na4Fe (PO4) 2P2O7@C structure, y is 0.3, z is 0.2, and the value of x is in the range of 2.25≥x≥1.5, the prepared material has excellent rate performance and capacity retention rate, the discharge gram capacity at 0.2C can reach 140-142mAh / g, the median voltage is 3.40V-3.65V, the discharge gram capacity at 1C can reach 112-117mAh / g, the discharge gram capacity at 5C can reach 105-108mAh / g, and the capacity retention rate is as high as 90%-95% after 200 cycles of charge-discharge at 1C. 3-x-1.5y-z Mn x V y A z When Na4Fe (PO4) 2P2O7@C structure, y is 0.3, z is 0.2, and the value of x is in the range of 2.25≥x≥1.5, the prepared material has excellent rate performance and capacity retention rate, the discharge gram capacity at 0.2C can reach 140-142mAh / g, the median voltage is 3.40V-3.65V, the discharge gram capacity at 1C can reach 112-117mAh / g, the discharge gram capacity at 5C can reach 105-108mAh / g, and the capacity retention rate is as high as 90%-95% after 200 cycles of charge-discharge at 1C.

Claims

1. A phosphate-pyrophosphate composite polyanionic compound, characterized in that: The phosphate pyrophosphate composite polyanionic compound has a structure shown in Formula I: Na4Fe 3-x-1.5y-z Mn x V y A z (PO4)2P2O7@C Formula I 1) The phosphate-pyrophosphate composite polyanionic compound: when having the structure shown in Formula I: 1.5<x<2.8; 0＜y＜0.5； 0＜z＜0.3； 2) A is selected from Al 3+ Cr 3+ One or two of 3) x, y, and z should ensure that the phosphate-pyrophosphate complex polyanionic compound designed in Formula I is electrically neutral.

2. The phosphate pyrophosphate composite polyanionic compound according to claim 1, characterized in that The mass content of C is 1wt%-12wt%; x, y, and z should ensure that the phosphate-pyrophosphate complex polyanionic compound designed in Formula I is electrically neutral, that is, satisfy the following relationship: When A is selected from Al 3+ Cr 3+ , it is necessary to satisfy 4+2*(3-x-1.5yz)+x+y+)+3z=10.

3. A method for preparing the phosphate pyrophosphate composite polyanionic compound according to claim 1 or 2, characterized in that: The specific steps include: Step 1) using one or more of a solid phase calcination method, a sol-gel method, and a solvent thermal method to mix a sodium source, an iron source, a manganese source, a vanadium source, an aluminum source, a phosphorus source, a carbon source, and a reducing agent to obtain a solid substance; The mixing method is one or more of ball milling, jar milling, sand milling, water dissolution, mechanical stirring, and melting; Step 2) heat-treating the solid material obtained in step 1); the solid material must be processed into a powder state by crushing, ball milling, or pot milling before heat treatment; The heat treatment is carried out under a specific flow atmosphere, and the specific atmosphere is selected from an inert atmosphere, which is one or more of argon, nitrogen, and helium, or an inert atmosphere gas containing a hydrogen volume concentration greater than 0 and less than 20%.

4. The method for preparing the compound according to claim 3, wherein The solid phase calcination method described in step 1) includes the following process: The sodium source, iron source, manganese source, vanadium source, A source, phosphorus source, carbon source and reducing agent are mixed uniformly to obtain a solid precursor; When insoluble raw materials are present in the sodium source, iron source, manganese source, vanadium source, A source, phosphorus source, carbon source, and reducing agent, the mixing method is one or more of ball milling, jar milling, or sand milling, and a solvent is further added during mixing to obtain a precursor slurry, wherein the solvent is water, ethanol, or a mixed solvent of water and ethanol, the mass of water in the mixed solvent accounts for 30-80wt% of the total mass of the mixed solvent, and the mass of ethanol accounts for 20-70wt% of the total mass of the mixed solvent. When the particle size range of the precursor slurry is 0.5μm-1.0μm, the precursor slurry is pumped into a spray dryer atomizer through a peristaltic pump to collect powder; after drying the precursor slurry, a solid precursor with a particle size of 20μm-30μm is obtained; When the sodium source, iron source, manganese source, vanadium source, A source, phosphorus source, carbon source, and reducing agent do not contain insoluble raw materials, when the mixing method is dissolution, a solvent is further added to the mixture to obtain a precursor solution, wherein the solvent is water. The precursor solution is pumped into the atomizer of the spray dryer through a peristaltic pump to collect powder; after drying the precursor slurry, a solid precursor with a particle size of 20 μm-30 μm is obtained; When the sodium source, iron source, manganese source, vanadium source, A source, phosphorus source, carbon source, and reducing agent are mixed in a melting manner, the raw materials are placed in a graphite crucible, and the temperature is rapidly raised to a temperature at which the raw materials are fluid, and the temperature is maintained for 0.5-1 hour, and then rapidly cooled to room temperature within 2-5 minutes to obtain a molten raw material; the molten raw material is ball-milled to obtain a solid precursor, wherein the particle size of the solid precursor is 25 μm-35 μm; the ball milling speed is 300 r / min-800 r / min; and the ball milling time is 2 h-8 h; The spray drying method controls the feed flow rate to 20 mL / min to 60 mL / min, the inlet air temperature to 120° C. to 195° C., the exhaust air temperature to 80° C. to 110° C., and the compressed air pressure to 0.03 KPa to 0.9 KPa. The precursor slurry is pumped into the atomizer of the spray dryer through a peristaltic pump to collect the powder. After drying, the precursor slurry obtains a precursor powder with a particle size of 20 μm to 30 μm.

5. The preparation method according to claim 3 or 4, characterized in that: The phosphate pyrophosphate composite polyanionic compound has a structure shown in Formula I, wherein the molar ratio of the sodium source, the iron source, the manganese source, the vanadium source, the A source, and the phosphorus source is 4:(3-x-1.5yz):x:y:z:4, and the molar ratio of the sodium source, the iron source, the manganese source, the vanadium source, the A source, and the phosphorus source is based on the molar amount of the sodium element, the iron element, the manganese element, the vanadium element, the A element, and the phosphorus element, respectively; The amount of the reducing agent added should ensure that the molar ratio of the reducing agent, iron source, manganese source, vanadium source, and A source is 3-4:3-x-1.5yz:x:y:z; and the iron, manganese, and vanadium in the phosphate pyrophosphate composite polyanionic compound should be Fe 2+ 、Mn 2+ 、V 3 + .

6. The preparation method according to claim 3 or 4, characterized in that The sodium source is selected from one or more of sodium hydroxide, sodium ethoxide, sodium ascorbate, sodium carbonate, trisodium phosphate, sodium phenolate, sodium carboxymethyl starch, sodium formate, sodium nitrite, sodium bicarbonate, sodium acetate, sodium citrate, sodium oxalate, disodium hydrogen phosphate, sodium dihydrogen phosphate, sodium pyrophosphate, and sodium hydrogen pyrophosphate; The iron source is selected from one or more of iron powder, ferroferric oxide, ferrous oxide, ferrous carbide, ferrous oxide, ferric oxalate, ferrous oxalate (dihydrate), ferrous lactate, ferric phosphate, ferric phosphide, ferric pyrophosphate, ferrous citrate, ferric nitrate, ferrous nitrate, ferric hydroxide, EDTA iron-sodium salt, ferric sulfate, ferrous sulfate, ferric chloride, ferrous chloride, sodium ferrocyanide, ferric acetate, ferrous ammonium sulfate, ferric citrate, ammonium ferric citrate, and sodium ferric citrate succinate; The manganese source is selected from one or more of manganese powder, manganese monoxide, manganese dioxide, manganese trioxide, manganese tetraoxide, manganese hydroxide, manganese carbonate, manganese sulfate, manganese chloride, manganese nitrate, manganese phosphate, manganous phosphate, manganese phosphide, manganese dihydrogen phosphate, manganese nitrate tetrahydrate, manganese oxalate, and manganese acetate; The vanadium source is selected from one or more of vanadium powder, vanadium carbide, sodium metavanadate, ammonium metavanadate, sodium orthovanadate, vanadyl oxalate, vanadium oxide, vanadium nitride, vanadium pentoxide, vanadium tetroxide, vanadyl sulfate, vanadyl acetylacetonate, and vanadium acetylacetonate; The source A is selected from one or more of oxides, hydroxides, carbonates, sulfates, chlorides, sulfates, phosphates, and acetylacetonates corresponding to the metal ions; The phosphorus source is selected from one or more of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, metaphosphoric acid, hypophosphorous acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, sodium hexametaphosphate, pyrophosphoric acid, sodium pyrophosphate, sodium hydrogen pyrophosphate, iron phosphate, iron phosphide, and manganese phosphate; The carbon source is selected from one or more of ethylene glycol, starch, carboxymethyl starch, sodium carboxymethyl starch, sodium alginate, citric acid, oxalic acid, ammonium citrate, ascorbic acid, formaldehyde, acetaldehyde, lactic acid, malic acid, glucose, sucrose, maltose, maltodextrin, dopamine, carbon nanotubes, redox graphene, phenolic resin, polyvinyl pyrrolidone, tannic acid, polyethylene glycol, and aniline; The reducing agent is selected from one or more of ethylene glycol, starch, carboxymethyl starch, sodium carboxymethyl starch, sodium alginate, citric acid, oxalic acid, ammonium citrate, ascorbic acid, formaldehyde, acetaldehyde, lactic acid, malic acid, glucose, sucrose, maltose, maltodextrin, dopamine, carbon nanotubes, phenolic resin, polyvinyl pyrrolidone, tannic acid, polyethylene glycol, and aniline; The addition of the carbon source and the reducing agent should ensure that the mass content of carbon in the phosphate-pyrophosphate composite polyanionic compound is 1 wt % to 12 wt %.

7. The preparation method according to claim 3 or 4, characterized in that The ball milling requires the addition of a ball milling medium. The ball milling speed in step 1) of claim 3 is 200-400 rpm / min, and the ball milling time is 2-4 h. The ball milling medium is one or more of zirconia beads, agate beads, and zirconium silicate beads, with a ball-to-material ratio of 1.5-7; the diameter of the ball milling medium is 2-10 mm; The sanding process requires the addition of a sanding medium, the sanding speed is 1500-3000 rpm / min, and the sanding time is 1-8 hours; The sand grinding medium is one or more of natural sand beads, zirconia beads, and agate beads, with a ball-to-material ratio of 1-5; and the diameter of the sand grinding medium is 0.1-1 mm.

8. Use of the phosphate-pyrophosphate composite polyanionic compound according to any one of claims 1 to 2 in a sodium ion battery, characterized in that: Phosphate-pyrophosphate complex polyanionic compounds are used as active substances in electrode materials for sodium ion batteries.

9. The use according to claim 8, characterized in that Phosphate-pyrophosphate complex polyanionic compound as positive electrode active material for sodium ion battery.

10. The use according to claim 9, characterized in that In the positive electrode material, the content of the phosphate pyrophosphate composite polyanion compound electrode material is 60-98 wt %; The positive electrode material further contains a conductive agent and a binder, wherein the mass ratio of the phosphate pyrophosphate composite polyanion compound, the conductive agent and the binder is (60-98) wt%: (1-39) wt%: (1-39) wt%; The conductive agent is one or more of carbon black, conductive graphite, carbon fiber, carbon nanotube, and graphene; The binder is one or more of polyvinylidene fluoride: PVDF5130, HSV900, and kynar761A.

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