Iron-manganese-based Prussian blue positive electrode material and preparation method and application thereof

Abstract

The invention discloses an iron-manganese-based Prussian blue positive electrode material and a preparation method thereof. The molecular formula of the positive electrode material is Na < 2-x > Fe Mn M < c > [Fe (CN) 6], wherein M is a divalent ion of any one or more than two elements of Ni, Mg, Cu, Zn, Co and Ca, x is more than or equal to 0 and less than or equal to 2, a + b is more than or equal to 0.9 and less than or equal to 0.98, c is more than or equal to 0.02 and less than or equal to 0.1, and a + b + c is equal to 1. The positive electrode material disclosed by the invention is stable in structure, and has high specific discharge capacity and wide voltage range (the upper voltage limit is greater than or equal to 3.65 V) when being used as the positive electrode material of the magnesium ion battery; meanwhile, the positive electrode material can also be used for a sodium ion battery and also has excellent electrochemical performance.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and in particular to an iron-manganese-based Prussian blue positive electrode material and a preparation method thereof. Background Art

[0002] Lithium-ion batteries have been widely used due to the popularization of various electronic devices and new energy vehicles, and have become one of the most competitive secondary batteries in the market. However, lithium resources are limited and unevenly distributed, resulting in high prices. In addition, lithium-ion batteries have poor safety and are subject to the risk of thermal runaway. It is necessary to develop secondary battery technologies with richer resources and greater safety. Sodium-ion batteries have become a research hotspot in recent years due to their lower cost and relatively better safety, and have begun to gradually advance commercialization. However, since both lithium and sodium negative electrodes will produce dendrites, which can pierce the diaphragm and cause safety problems, it is not suitable for lithium-ion batteries and sodium-ion batteries to use metallic lithium and sodium as negative electrodes. Instead, other negative electrode materials can be used, which limits the storage capacity of the battery.

[0003] Magnesium ranks eighth in the Earth's crust and is abundant in reserves. It is crucial for reducing battery costs and helps reduce reliance on lithium raw materials in battery manufacturing. Recycling is also simpler, reducing pollution. Furthermore, magnesium negative electrodes do not form dendrites, allowing the use of metallic magnesium as the negative electrode. Direct use of metallic magnesium can increase the battery's storage capacity and improve battery performance without creating safety issues. Therefore, magnesium batteries, using magnesium metal as the negative electrode material, allow for higher energy density. Furthermore, magnesium-ion batteries offer higher energy storage efficiency, are cheaper, and are safer than lithium-ion batteries. The widespread availability of magnesium-ion batteries plays an important role in promoting the development of electric vehicles and distributed energy storage technologies.

[0004] However, the current positive electrode materials for magnesium-ion batteries generally have problems such as low discharge specific capacity, narrow voltage range and poor cycle stability; in particular, the voltage upper limit of the positive electrode materials currently studied in existing technologies is generally around 2 to 3V, which is a large gap from the voltage range of lithium-ion batteries and sodium-ion batteries, which further increases the difficulty of promoting the application of magnesium-ion batteries. Summary of the Invention

[0005] In view of the above-mentioned deficiencies in the prior art, the object of the present invention is to provide an iron-manganese-based Prussian blue positive electrode material and a preparation method thereof, so as to solve the problems of low discharge specific capacity, narrow voltage range and poor cycle stability of magnesium ion battery positive electrode materials in the prior art.

[0006] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0007] An iron-manganese-based Prussian blue positive electrode material, the molecular formula of the positive electrode material is Na 2-xFe a Mn b M c [Fe(CN)6]; wherein M is a divalent ion of any one or more elements selected from Ni, Mg, Cu, Zn, Co, and Ca, 0≤x≤2, 0.9≤a+b≤0.98, 0.02≤c≤0.1, and a+b+c=1.

[0008] Preferably, 0≤x<1; and the positive electrode material is applied to magnesium ion batteries or sodium ion batteries.

[0009] The present invention provides a method for preparing an iron-manganese-based Prussian blue positive electrode material. The specific steps for preparing the positive electrode material are as follows:

[0010] Step 1: Ignoring vacancies and lattice water content, according to the molecular formula Na 2-x Fe a Mn b M c [Fe(CN)6], calculate Fe 2+ 、Mn 2+ and M 2+ The amount of the compound corresponding to the element; wherein the molar ratio of Fe, Mn and M is a:b:c, and 0.9≤a+b≤0.98, 0.02≤c≤0.1, a+b+c=1;

[0011] Step 2: mixing sodium ferrocyanide or its hydrate with ascorbic acid to prepare solution A; wherein the molar ratio of sodium ferrocyanide or its hydrate to ascorbic acid in solution A is 1:(0.1-0.5);

[0012] Step 3: Fe 2+ 、Mn 2+ and M 2+ The corresponding compound of the element is mixed with ascorbic acid to prepare solution B; in solution B, Fe 2+ 、Mn 2+ and M 2+ The molar ratio of the total molar amount of the compound corresponding to the element to ascorbic acid is 1:(0.1-0.5), and a+b+c=1;

[0013] Step 4: Sodium citrate is prepared alone to form solution C; alternatively, sodium citrate is mixed with ascorbic acid or oxalic acid to form solution C; in solution C, the concentration of sodium citrate is 0.6 mol / L to 1.8 mol / L, and the molar ratio of ascorbic acid or oxalic acid to sodium citrate is (0.05 to 0.25):1; then, under an inert atmosphere, solution A and solution B are respectively introduced into solution C via peristaltic pumps for 5.5 to 7.5 hours;

[0014] Step 5: The mixed material in step 4 is allowed to stand for 8 to 16 hours, and then washed and dried to obtain the positive electrode material.

[0015] Preferably, M is a divalent ion of any one or two or more elements selected from the group consisting of Ni, Mg, Cu, Zn, Co and Ca.

[0016] Preferably, Fe 2+ 、Mn 2+ and M 2+ The corresponding compounds are chlorides or sulfates.

[0017] Preferably, in step 4, when solution C is a mixture of sodium citrate and ascorbic acid or oxalic acid, the molar ratio of sodium citrate to ascorbic acid or oxalic acid is 1:(0.05-0.25).

[0018] Preferably, in step 4, the inert atmosphere is a nitrogen atmosphere or an argon atmosphere.

[0019] Preferably, in step 5, after washing with water and alcohol for multiple times, the mixture is dried at 120° C. to 240° C. for 10 h to 30 h.

[0020] Preferably, the cleaning method is one of centrifugal washing, suction filtration washing or filter press washing.

[0021] Preferably, the drying method includes vacuum drying, nitrogen atmosphere drying or argon atmosphere drying.

[0022] Preferably, the drying process is staged drying, first drying at 120°C to 160°C for 4h to 12h, and then drying at 170°C to 240°C for 6h to 18h.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] 1. The positive electrode material of the present invention has a stable structure. When used as a positive electrode material for magnesium ion batteries, it not only has a high discharge specific capacity but also has a wide voltage range (voltage upper limit ≥ 3.65V). At the same time, the positive electrode material can also be used in sodium ion batteries and also has excellent electrochemical properties.

[0025] 2. The present invention adopts a co-precipitation method to prepare the positive electrode material. The metal elements used are from a wide range of sources, and the amount of other metal elements with higher costs is small, which makes the overall production cost of the positive electrode material low. At the same time, by separating the main reaction materials and adding them separately, the probability of ion collision is reduced, the reaction rate is reduced, and the crystallinity and structural stability of the material are improved. The whole process is simple and controllable, and has prospects for industrial application. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1This is the first charge and discharge curve of the magnesium ion battery at 1.0 to 3.65 V when the Prussian blue material prepared in Example 1 is at a current density of 0.1 C (1 C = 130 mA / g).

[0027] Figure 2 These are the charge and discharge curves of the magnesium ion battery at the 1st and 100th times when the material prepared in Example 1 is at a current density of 1C.

[0028] Figure 3 This is the first charge and discharge curve of the magnesium ion battery at 1.5 to 3.8 V when the current density is 1C for the material prepared in Example 1.

[0029] Figure 4 This is the first charge and discharge curve of the magnesium ion battery at 1.0-3.65V when the current density is 0.1C for the material prepared in Comparative Example 1.

[0030] Figure 5 This is a comparison chart of the cycle performance of magnesium ion batteries prepared in Example 1 and Example 2 and Comparative Example 1 at a current density of 1C and a voltage range of 1.0 to 3.65V.

[0031] Figure 6 is a SEM image of the material prepared in Example 1.

[0032] Figure 7 is the XRD pattern of the material prepared in Example 1.

[0033] Figure 8 This is the first charge and discharge curve of the sodium ion battery when the material prepared in Example 1 is at a current density of 0.1C (1C=150mA / g).

[0034] Figure 9 This is a cycle performance diagram of a sodium ion battery of the material prepared in Example 1 when the current density is 1C and the voltage range is 2.0 to 4.0V. DETAILED DESCRIPTION

[0035] The present invention will be described clearly and completely with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments derived by persons of ordinary skill in the art based on the present invention are within the scope of protection of the present invention.

[0036] Unless otherwise indicated in specific cases, the numerical ranges listed herein include the upper and lower limits, and all integers and fractions within the range, and are not limited to the specific values listed when defining the range.

[0037] 1. An iron-manganese-based Prussian blue cathode material

[0038] The molecular formula of the positive electrode material of the present invention is Na 2-x Fe a Mn b M c [Fe(CN)6]; wherein M is a divalent ion of any one or more elements selected from Ni, Mg, Cu, Zn, Co, and Ca, 0≤x≤2, 0.9≤a+b≤0.98, 0.02≤c≤0.1, and a+b+c=1.

[0039] In some embodiments of the present invention, 0≤x<1, due to the Na + It is usually excessive, so X is generally 0, that is, the Na content is usually 2. The washing process will cause a certain loss of Na due to different washing degrees. Therefore, the value of 2-x in the positive electrode material of the present invention is between 1 and 2.

[0040] The cathode material of the present invention has advantages in both structure and performance. The channels occupied by sodium ions can synergistically promote the diffusion of magnesium ions. The introduction of Mn can provide additional charge compensation as a valence-varying element, contribute to high specific capacity, and can improve the voltage platform (≥3.65V). However, the introduction of Mn is prone to distortion. By adjusting the ratio of Fe to Mn, the lattice can be stabilized and the voltage range can be widened. M is a doping / substituting element. A small amount of M introduced into the molecular formula can reduce vacancy defects and water content in the material by occupying the lattice and interstitial sites of the Prussian blue framework. Excessive vacancies and water content will reduce the structural stability of the material.

[0041] 2. A method for preparing an iron-manganese-based Prussian blue positive electrode material

[0042] Step 1: Ignoring vacancies and lattice water content, according to the molecular formula Na 2-x Fe a Mn b M c [Fe(CN)6], calculate Fe 2+ 、Mn 2+ and M 2+ The amount of the compound corresponding to the element; wherein the molar ratio of Fe, Mn and M is a:b:c, and 0.9≤a+b≤0.98, 0.02≤c≤0.1, a+b+c=1;

[0043] Step 2: mixing sodium ferrocyanide or its hydrate with ascorbic acid to prepare solution A; wherein the molar ratio of sodium ferrocyanide or its hydrate to ascorbic acid in solution A is 1:(0.1-0.5);

[0044] Step 3: Fe 2+ 、Mn 2+ and M 2+The corresponding compound of the element is mixed with ascorbic acid to prepare solution B; in solution B, Fe 2+ 、Mn 2+ and M 2+ The molar ratio of the total molar amount of the compound corresponding to the element to ascorbic acid is 1:(0.1-0.5), and a+b+c=1;

[0045] Step 4: Sodium citrate is prepared alone to form solution C; alternatively, sodium citrate is mixed with ascorbic acid or oxalic acid to form solution C; in solution C, the concentration of sodium citrate is 0.6 mol / L to 1.8 mol / L, and the molar ratio of ascorbic acid or oxalic acid to sodium citrate is (0.05 to 0.25):1; then, under an inert atmosphere, solution A and solution B are respectively introduced into solution C via peristaltic pumps for 5.5 to 7.5 hours;

[0046] Step 5: The mixed material in step 4 is allowed to stand for 8 to 16 hours, and then washed and dried to obtain the positive electrode material.

[0047] After conducting in-depth research on the preparation method of Prussian blue material, the present invention found that the reaction rate of Prussian blue material is extremely fast during synthesis. This is mainly caused by the violent collision between ions after mixing all the raw materials, which produces a large number of vacancies and crystal water in the obtained material, affecting the growth effect of the material at the microscopic level, and ultimately causing the material crystallinity to deteriorate and the structural stability to deteriorate. To this end, the present invention adopts a coprecipitation method to prepare the positive electrode material, wherein each raw material is made into a corresponding solution and added step by step. This method can avoid the problem of excessive reaction rate when each solution is added to each other in the prior art, reduce the collision probability of ions, control the coprecipitation order and speed of each element, make the distribution of each element more uniform, and reduce the grain boundary defects caused by local concentration gradients; at the same time, this slow growth can promote the orderly coordination of [Fe (CN) 6] octahedron with each metal ion, reduce the crystal water content, reduce vacancy formation, and make the obtained Prussian blue material have a rigid framework. The material finally prepared has excellent stability.

[0048] In some embodiments of the present invention, M is a divalent ion of any one or more of Ni, Mg, Cu, Zn, Co, and Ca. The M element is a doping element and can be any one of Ni, Mg, Cu, Zn, Co, and Ca, or a combination of any two or more of these elements. For example, M can be Mg, or a combination of Mg and Ni.

[0049] In some embodiments of the present invention, in step 3, Fe 2+ 、Mn 2+ and M 2+ The corresponding compounds are chlorides or sulfates. 2+The corresponding compound can be FeCl2 or FeSO4, Mn 2+ The corresponding compound can be MnCl2 or MnSO4, M 2+ The corresponding compound can be any one or more of NiCl2, MgCl2, CuCl2, ZnCl2, CoCl2, and CaCl2; or M 2+ The corresponding compound can be any one or more of NiSO4, MgSO4, CuSO4, ZnSO4, CoSO4, and CaSO4; it can also be a mixture of chloride and sulfate.

[0050] In some embodiments of the present invention, in step 4, when solution C is a mixture of sodium citrate and ascorbic acid or oxalic acid, the molar ratio of sodium citrate to ascorbic acid or oxalic acid is 1:(0.05-0.25). That is, the molar ratio of sodium citrate to ascorbic acid is 1:(0.05-0.25); or, the molar ratio of sodium citrate to oxalic acid is 1:(0.05-0.25).

[0051] In some embodiments of the present invention, in step 4, the inert atmosphere is a nitrogen atmosphere or an argon atmosphere.

[0052] In some embodiments of the present invention, in step 5, after washing with water and alcohol for multiple times, the material is dried at 120°C to 240°C for 10 hours to 30 hours. The washing method is one of centrifugal washing, suction filtration washing or filter press washing. The drying method includes one of vacuum drying, nitrogen atmosphere drying or argon atmosphere drying. The drying process is staged drying, first drying at 120°C to 160°C for 4 hours to 12 hours, and then drying at 170°C to 240°C for 6 hours to 18 hours. Compared with non-staged drying, staged drying can make the drying more complete and can protect the structure of the material from being easily damaged by high temperature.

[0053] 2. Examples and Comparative Examples

[0054] Example 1

[0055] Step 1: According to the molecular formula Na 2-x Fe 0.4 Mn 0.57 Mg 0.03 The molar ratio of each non-sodium metal element in [Fe(CN)6] is first calculated to determine the mass of the corresponding compound required to synthesize 0.1 mol of the material.

[0056] Step 2: Sodium ferrocyanide decahydrate and ascorbic acid were added to a beaker at a molar ratio of 1:0.15, and deionized water was added to dissolve the mixture, which was recorded as solution A.

[0057] Step 3: Calculate the total molar amount of ferrous sulfate, manganese sulfate, and magnesium sulfate to 1, then add them to a beaker with ascorbic acid at a molar ratio of 1:0.15, and add deionized water to dissolve them. This is recorded as Solution B.

[0058] Step 4: Sodium citrate and ascorbic acid were added to a beaker at a molar ratio of 1:0.1, and deionized water was added to dissolve them into a 1.2 mol / L solution, which was recorded as liquid C. Liquid A and liquid B were then added dropwise to liquid C simultaneously through a peristaltic pump. The addition took 6 hours, and a nitrogen atmosphere was continuously introduced until the addition was complete.

[0059] Step 5: After the addition is completed, the material is allowed to stand for 12 hours, then centrifuged and washed three times and vacuum dried at 120°C for 8 hours, and then vacuum dried at 180°C for 12 hours to obtain the final required Na2Fe 0.4 Mn 0.57 Mg 0.03 [Fe(CN)6] Prussian blue material.

[0060] Example 2

[0061] Step 1: According to the molecular formula Na 2-x Fe 0.5 Mn 0.45 Mg 0.03 Ni 0.02 The molar ratio of each non-sodium metal element in [Fe(CN)6] is first calculated to determine the mass of the corresponding compound required to synthesize 0.1 mol of the material.

[0062] Step 2: Sodium ferrocyanide decahydrate and ascorbic acid were added to a beaker at a molar ratio of 1:0.2, and deionized water was added to dissolve the mixture, which was recorded as solution A.

[0063] Step 3: The total molar amount of ferrous sulfate, manganese sulfate, magnesium sulfate, and nickel sulfate is calculated as 1, and then added to a beaker with ascorbic acid at a molar ratio of 1:0.2, and deionized water is added to dissolve, which is recorded as solution B.

[0064] Step 4: Sodium citrate and ascorbic acid were added to a beaker at a molar ratio of 1:0.05, and deionized water was added to dissolve them into a 1.2 mol / L solution, which was recorded as liquid C. Liquid A and liquid B were then added dropwise to liquid C simultaneously through a peristaltic pump. The addition took 6 hours, and a nitrogen atmosphere was continuously introduced until the addition was complete.

[0065] Step 5: After the addition is completed, the material is allowed to stand for 12 hours, then the material is centrifuged and washed three times and vacuum dried at 130°C for 8 hours, and then vacuum dried at 200°C for 12 hours to obtain the final required Na2Fe 0.5 Mn 0.45 Mg 0.03 Ni 0.02[Fe(CN)6] Prussian blue material.

[0066] Example 3

[0067] Step 1: According to the molecular formula Na 2-x Fe 0.46 Mn 0.46 Cu 0.05 Ni 0.03 The molar ratio of each non-sodium metal element in [Fe(CN)6] is first calculated to determine the mass of the corresponding compound required to synthesize 0.1 mol of the material.

[0068] Step 2: Sodium ferrocyanide and ascorbic acid were then added to a beaker at a molar ratio of 1:0.25, and deionized water was added to dissolve them, which was recorded as solution A.

[0069] Step 3: The total molar amount of ferrous sulfate, manganese sulfate, copper sulfate, and nickel sulfate is calculated as 1, and then added to a beaker with ascorbic acid at a molar ratio of 1:0.25, and deionized water is added to dissolve, which is recorded as solution B.

[0070] Step 4: Sodium citrate and ascorbic acid were added to a beaker at a molar ratio of 1:0.1, and deionized water was added to dissolve them into a 1.5 mol / L solution, which was recorded as liquid C. Liquid A and liquid B were then added dropwise to liquid C simultaneously through a peristaltic pump. The addition took 7 hours, and a nitrogen atmosphere was continuously introduced until the addition was complete.

[0071] Step 5: After the addition is completed, the material is allowed to stand for 15 hours, then centrifuged and washed three times and vacuum dried at 140°C for 10 hours and then at 190°C for 12 hours to obtain the final desired Na2Fe 0.46 Mn 0.46 Cu 0.05 Ni 0.03 [Fe(CN)6] Prussian blue material.

[0072] Comparative Example 1

[0073] The method was adjusted based on Example 1, except that the M element was not added and the other steps were exactly the same as those in Example 1 to prepare Na2Fe 0.4 Mn 0.6 [Fe(CN)6].

[0074] 3. Product Characterization and Performance Testing

[0075] Taking Example 1 as an example, the Prussian blue materials prepared in Example 1 and Comparative Example 1 were assembled into button batteries for charge and discharge tests, and were subjected to scanning electron microscopy and XRD test analysis.

[0076] like Figure 1As shown, the material prepared in Example 1 is assembled into a magnesium ion battery for the first charge and discharge curve test. It can be seen that in the wide voltage range of 1.0 to 3.65 V, the material in Example 1 has a discharge specific capacity of more than 115 mAh / g, and the energy density is 249 Wh / Kg as the positive electrode material.

[0077] like Figure 2 As shown in FIG, the material prepared in Example 1 has a discharge capacity of about 95 mAh / g at a current density of 1C, and the capacity retention rate after 100 cycles is greater than 91%, indicating that the material prepared in Example 1 has good structural stability. Figure 3 As shown, the material prepared in Example 1 has a discharge specific capacity greater than 100 mAh / g in a current density of 1 C and a voltage range of 1.5 to 3.8 V.

[0078] Figure 4 The first charge and discharge curve of the magnesium ion battery prepared by the material of Comparative Example 1 is shown in the figure. Although Comparative Example 1 has a high discharge capacity of more than 132mAh / g and a high energy density of 290Wh / Kg based on the positive electrode material at a current density of 0.1C, Figure 5 It can be seen that the capacity retention rate of Comparative Example 1 after 100 cycles at a current density of 1C is only 49.46%, while the material retention rate of Example 1 prepared by divalent ion doping reaches 91.23%. After further adjusting the element ratio and process of the material according to Example 2, the capacity retention rate of Example 2 after 100 cycles at a current density of 1C reaches 98.11%, indicating that the material has excellent cycle stability when used as a positive electrode material in magnesium ion batteries.

[0079] Figure 6 and Figure 7 They are the morphology and XRD test spectrum of the material prepared in Example 1. It can be seen that the material has a good bulk single crystal morphology and high crystallinity.

[0080] In addition, the performance test of the sodium ion battery was also carried out on the Prussian blue material prepared in Example 1. Figure 8 and Figure 9 As shown, in sodium ion batteries, the material has a discharge specific capacity greater than 151 mAh / g and a capacity retention rate of 95.35% after 100 cycles at a current density of 1C, which shows that the positive electrode material of the present invention can also be well applied to sodium ion batteries.

[0081] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the technical solutions. Those skilled in the art should understand that modifications or equivalent replacements of the technical solutions of the present invention that do not depart from the purpose and scope of the technical solutions of the present invention should be included in the scope of the claims of the present invention.

Claims

1. An iron-manganese-based Prussian blue positive electrode material, characterized in that The molecular formula of the positive electrode material is Na 2-x Fe a Mn b M c [Fe(CN)6]; wherein M is a divalent ion of any one or more elements selected from Ni, Mg, Cu, Zn, Co, and Ca, 0≤x≤2, 0.9≤a+b≤0.98, 0.02≤c≤0.1, and a+b+c=1.

2. The positive electrode material according to claim 1, characterized in that 0≤x<1; and the positive electrode material is applied to magnesium ion batteries or sodium ion batteries.

3. A method for preparing an iron-manganese-based Prussian blue positive electrode material, characterized in that: The positive electrode material according to any of claims 1 to 2 is prepared by the following specific steps: Step 1: Ignoring vacancies and lattice water content, according to the molecular formula Na 2-x Fe a Mn b M c [Fe(CN)6], calculate Fe 2+ 、Mn 2+ and M 2+ The amount of the compound corresponding to the element; wherein the molar ratio of Fe, Mn and M is a:b:c, and 0.9≤a+b≤0.98, 0.02≤c≤0.1, a+b+c=1; Step 2: mixing sodium ferrocyanide or its hydrate with ascorbic acid to prepare solution A; wherein the molar ratio of sodium ferrocyanide or its hydrate to ascorbic acid in solution A is 1:(0.1-0.5); Step 3: Fe 2+ 、Mn 2+ and M 2+ The corresponding compound of the element is mixed with ascorbic acid to prepare solution B; in solution B, Fe 2 + 、Mn 2+ and M 2+ The molar ratio of the total molar amount of the compound corresponding to the element to ascorbic acid is 1:(0.1-0.5), and a+b+c=1; Step 4: Sodium citrate is prepared alone to form solution C; alternatively, sodium citrate is mixed with ascorbic acid or oxalic acid to form solution C; in solution C, the concentration of sodium citrate is 0.6 mol / L to 1.8 mol / L, and the molar ratio of ascorbic acid or oxalic acid to sodium citrate is (0.05 to 0.25):1; then, under an inert atmosphere, solution A and solution B are respectively introduced into solution C via peristaltic pumps for 5.5 to 7.5 hours; Step 5: The mixed material in step 4 is allowed to stand for 8 to 16 hours, and then washed and dried to obtain the positive electrode material.

4. The preparation method according to claim 3, characterized in that M is a divalent ion of any one or two or more elements selected from the group consisting of Ni, Mg, Cu, Zn, Co and Ca.

5. The preparation method according to claim 3, characterized in that: In step 3, Fe 2+ 、Mn 2+ and M 2+ The corresponding compounds are chlorides or sulfates.

6. The preparation method according to claim 3, characterized in that: In step 4, the inert atmosphere is a nitrogen atmosphere or an argon atmosphere.

7. The preparation method according to claim 3, characterized in that: In step 5, after washing with water and alcohol for multiple times, the product is dried at 120° C. to 240° C. for 10 h to 30 h.

8. The preparation method according to claim 7, characterized in that: The cleaning method is one of centrifugal washing, suction filtration washing or filter press washing.

9. The preparation method according to claim 7, characterized in that: The drying method includes vacuum drying, nitrogen atmosphere drying or argon atmosphere drying.

10. The preparation method according to claim 7, characterized in that: The drying process is staged drying, first drying at 120° C. to 160° C. for 4 h to 12 h, and then drying at 170° C. to 240° C. for 6 h to 18 h.

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