Vanadium-doped manganese iron lithium phosphate cathode material, preparation method and application thereof

Abstract

The application discloses a vanadium-doped manganese iron lithium phosphate positive electrode material and a preparation method and application thereof, and belongs to the technical field of lithium batteries. In the preparation of the positive electrode material, lithium carbonate and a carbon source are ball-milled and mixed, lithium source and the carbon source are pre-dispersed to avoid agglomeration and promote uniform coating of the carbon layer, manganese iron phosphate, vanadyl oxalate and ammonium dihydrogen phosphate are subsequently added and ball-milled and mixed, pre-calcination is performed to form a precursor, vanadium elements are doped at manganese iron positions, a carbon source and urea are added and high-temperature calcination is performed, ammonia gas generated by decomposition of the urea is reacted with vanadium elements, volatilization of the vanadium elements at high temperatures is effectively inhibited, a nitrogen-doped carbon coating layer and a vanadium nitride conductive phase are generated, and the conductivity and structural stability of the material are further enhanced. By introducing vanadium elements, the application overcomes the shortcomings of existing manganese iron lithium phosphate materials in terms of electrochemical performance, improves the cycle stability and rate performance of the material, and exhibits more excellent performance in the charging and discharging process.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium batteries, and particularly relates to a vanadium-doped lithium manganese iron phosphate positive electrode material and a preparation method and application thereof. BACKGROUND

[0002] The lithium manganese iron phosphate positive electrode material (LiMn x Fe 1-x PO4, abbreviated as LMFP) is a new type of phosphate lithium ion battery positive electrode material. It is a solid solution material formed by doping a certain proportion of manganese (Mn) elements on the basis of lithium iron phosphate (LiFePO4, abbreviated as LFP). There are some problems to be overcome at present: in terms of capacity and cycle life, the capacity attenuation is fast during the charging and discharging process, and the cycle life is short; the battery rate performance is poor, and the performance decreases obviously during high-rate charging and discharging. The electronic conductivity is low, and the charge transfer impedance is large, resulting in large energy loss of the battery during the charging and discharging process. It is found that the use of vanadium doping can significantly improve the capacity and cycle life of the battery, promote the lithium ion intercalation and deintercalation process by forming vanadium oxide, thereby improving the energy storage capacity and service life of the battery. At the same time, vanadium doping can also improve the rate performance of the battery and increase the transmission speed of lithium ions, so that the battery can still maintain good performance during high-rate charging and discharging.

[0003] However, vanadium doping in the application of lithium manganese iron phosphate can also cause some new problems, such as structural stability problems, volatilization of vanadium elements during high-temperature sintering, destruction of the crystal structure by excessive doping, and influence on the cycle life. On the other hand, there are also problems of vanadium occupying lithium sites, hindering lithium ion diffusion, reducing rate performance, and causing kinetic performance decline. Moreover, there are differences in crystal structure between lithium vanadate and lithium manganese iron phosphate formed by adding vanadium elements, and it is difficult to form a stable interface by direct mechanical mixing (ball milling) in the conventional process, which can easily cause particle cracking due to volume expansion difference during the charging and discharging process, and aggravate capacity attenuation. SUMMARY

[0004] The purpose of the present application is to provide a vanadium-doped lithium manganese iron phosphate positive electrode material and a preparation method and application thereof, which significantly improve the cycle stability of the positive electrode material.

[0005] The purpose of the present application can be achieved by the following technical solutions:

[0006] The present application provides a preparation method of a vanadium-doped lithium manganese iron phosphate positive electrode material, comprising the following steps:

[0007] S1, mixing lithium carbonate and a carbon source by ball milling to obtain a mixed lithium source;

[0008] S2, adding manganese iron phosphate, vanadyl oxalate and ammonium dihydrogen phosphate to the mixed lithium source, ball milling and mixing, and drying to obtain a mixture;

[0009] S3, pre-calcining the dried mixture to obtain a mixed precursor;

[0010] S4, ball-milling the mixed precursor, a carbon source and urea, and high-temperature calcining to obtain a lithium manganese iron phosphate and lithium vanadium phosphate composite positive electrode material.

[0011] In the above preparation method, the lithium carbonate and the carbon source are ball-milled and mixed to pre-disperse the lithium source and the carbon source to avoid agglomeration and promote uniform coating of the carbon layer and improve the conductivity. The manganese iron phosphate, vanadyl oxalate and ammonium dihydrogen phosphate are subsequently ball-milled and mixed, which is different from the traditional vanadium doping process. In the present application, the lithium source, the phosphorus source, the manganese source, the iron source and the vanadium source are mixed and then reacted as raw materials. In the present application, the lithium manganese iron phosphate is prepared by using manganese iron phosphate and lithium carbonate as raw materials, and the lithium vanadium phosphate is prepared by reacting vanadyl oxalate, ammonium dihydrogen phosphate and lithium carbonate. Two cross-reaction systems are formed to avoid occupation of lithium sites by vanadium. In the present application, the doping site of vanadium is the manganese iron site, which can improve the electronic conductivity of the material, avoid distortion of the structure and improve the cycle life.

[0012] Further, in S1, the carbon source is at least one of glucose, sucrose and citric acid. Small-molecule organic matter is used as the carbon source. Due to its pyrolysis characteristics, it can generate a porous or loose carbon layer, which is helpful for the diffusion of lithium ions and the transmission of electrons.

[0013] Further, in S1, the ball-milling medium in the ball-milling process is anhydrous ethanol, and the ball-milling time is 30-50 min.

[0014] Further, the mass of the carbon source in S1 is 3%-5% of the total raw material mass for preparation of the vanadium-doped lithium manganese iron phosphate positive electrode material. The lithium carbonate and the carbon source are ball-milled first to ensure uniform adhesion of the carbon source to the surface of the lithium source, and a continuous conductive network is formed by subsequent high-temperature carbonization.

[0015] Further, the lithium source, the manganese iron phosphate, the vanadyl oxalate and the ammonium dihydrogen phosphate are added in a molar ratio of Li:Fe:Mn:V:P=(3-8):1:1:(0.5-4):(3-8) according to the elements contained.

[0016] Further, in S2, the ball-milling time is 1-2 h.

[0017] Further, in S2, the drying is carried out in an oven at 80-120℃ for 2-5 h.

[0018] Further, the pre-calcining process is set as follows:

[0019] The temperature is raised to 650-700℃ at a temperature raising rate of 3-5℃ / min, and calcined for 3-5h under the protection of a gas with a volume ratio of argon to hydrogen being 95:5. During the pre-calcination, the lithium carbonate and other raw materials are partially decomposed and form a precursor with incomplete crystallization. The pre-calcination can make the particle size distribution of the precursor more uniform, and reduce the impurity content.

[0020] Compared with a pure inert atmosphere (such as Ar), a small amount of H2 in the mixed gas of Ar / H2 (95 / 5) can enhance the reducing property, prevent the oxidation of Mn / V elements (such as Mn 2+ → Mn 3+ , and ensure the structural stability of the material.

[0021] Further, in S4, the carbon source is the same as that in S1.

[0022] Further, the mass ratio of the mixed precursor, the carbon source and urea is 1:(0.01-0.02):(0.01-0.02).

[0023] The lithium manganese iron phosphate has an olivine structure, as does the lithium iron phosphate. This structure has high stability, and the cycle life of the lithium manganese iron phosphate is generally lower than that of the lithium iron phosphate. The lithium vanadium phosphate is doped at the lithium site, and the lithium vanadium phosphate has a monoclinic structure. The three-dimensional lithium ion deintercalation channel makes the structure more stable, and the lithium vanadium phosphate has excellent safety performance due to its stable chemical properties. The lithium vanadium phosphate with a monoclinic crystal structure is composed of a three-dimensional framework of VO6 octahedra and PO4 tetrahedra sharing oxygen atom vertices. Each VO6 octahedron is connected to six PO4 tetrahedra through vertices, and each PO4 tetrahedron is connected to four VO6 octahedra. Through this connection mode, a three-dimensional network unit structure is formed, and Li + The Li is located in the position forming 12 tetrahedral voids in the unit cell. Since the PO4 tetrahedra separate the VO6 octahedra, the VO6 octahedra cannot be directly connected to each other, resulting in poor electronic conductivity.

[0024] Urea can produce ammonia gas in the process of high-temperature decomposition, and the ammonia gas can react with the residual vanadium element after pre-calcination to form vanadium nitride, which has high conductivity and can be used as a "conductive bridge" to connect phosphate particles and establish an electron migration channel. The nitrogen-doped carbon layer has a stronger interface with the positive electrode material, and as a coating layer, it can reduce the direct contact between the positive electrode and the electrolyte, inhibit the electrolyte side reaction, reduce the interface impedance, and avoid the dissolution of manganese element at high potential. If urea is added in S1, the ammonia gas produced will react with vanadium to form more vanadium nitride, which will occupy a large amount of vanadium element and inhibit the formation of lithium vanadium phosphate; in contrast, the present application adds urea in S4, at this time the vanadium element has formed the precursor of lithium vanadium phosphate, and in the process of high-temperature calcination, lithium vanadium phosphate further reacts and crystallizes completely, and the structure is stable, at this time part of the vanadium element material that has not formed crystals can react with ammonia gas to generate vanadium nitride, which can not only inhibit the volatilization of vanadium element in the high-temperature calcination environment, but also can be distributed in the grain boundary in the form of fine particle vanadium nitride to inhibit the expansion and cracking of the crystal grain, build a continuous composite positive electrode material conductive network, form a stable interface, inhibit the capacity decay, and the nitride phase can inhibit the loss of lattice oxygen and relieve the structure collapse under high pressure.

[0025] Further, the process of high-temperature calcination is set as follows:

[0026] The temperature is raised to 750-800℃ at a temperature rising rate of 3-5℃ / min, and calcination is carried out under the protection of argon gas:hydrogen gas=95:5 by volume ratio for 5-7h. The complete crystallization temperature of lithium manganese iron phosphate is generally above 700℃, and the complete crystallization temperature of lithium vanadium phosphate is generally above 750℃. Through high-temperature calcination, impurity phases in the positive electrode material can be reduced, and lithium manganese iron phosphate and lithium vanadium phosphate can be completely crystallized to establish a complete lithium ion diffusion channel.

[0027] The two-stage calcination through pre-calcination and high-temperature calcination in the above preparation can avoid particle agglomeration or composition segregation caused by direct high temperature. Both ball milling and high-temperature solid phase method are mature processes, which are easy to scale up and suitable for large-scale preparation of high-performance positive electrode materials.

[0028] The present application also provides a vanadium-doped lithium manganese iron phosphate positive electrode material prepared by the above preparation method.

[0029] The present application also provides the application of a vanadium-doped lithium manganese iron phosphate positive electrode material prepared by the above preparation method, which is applied in the fields of lithium ion batteries, solid-state batteries, etc.

[0030] The present application has the following advantages:

[0031] (1) The vanadium-doped lithium manganese iron phosphate positive electrode material prepared in the present invention has good electrochemical properties. Compared with the lithium manganese iron phosphate positive electrode material not doped with vanadium, the present invention overcomes the shortcomings of the existing lithium manganese iron phosphate material in electrochemical properties by introducing vanadium, improves the cycle stability and rate performance of the material, and shows better performance during the charge and discharge process.

[0032] (2) After pre-calcination to form a precursor, a carbon source and urea are added and mixed for high-temperature calcination. The sintering temperature is controlled at 600-800°C using a low-temperature solid-phase method. The ammonia generated by the decomposition of urea reacts with the vanadium element, effectively suppressing the volatilization of the vanadium element at high temperatures and ensuring precise control of the doping concentration.

[0033] (3) By introducing carbon source and urea for composite doping, a nitrogen-doped carbon coating layer and a vanadium nitride conductive phase were generated, further enhancing the conductivity and structural stability of the material.

[0034] (4) The open three-dimensional framework monoclinic structure of lithium vanadium phosphate and the high ionic conductivity of vanadium element jointly promote the rapid diffusion of lithium ions, thereby enhancing the rate performance of the material.

[0035] (5) The raw materials of the positive electrode material are preferably selected. Manganese iron phosphate and lithium carbonate are used as raw materials to prepare lithium manganese iron phosphate. Vanadium oxalate, ammonium dihydrogen phosphate and lithium carbonate are reacted to prepare lithium vanadium phosphate, forming two cross-reaction systems to avoid vanadium occupying the lithium site. In the present invention, the doping site of the vanadium element is the manganese iron site, which can improve the electronic conductivity of the material, avoid structural distortion, and improve the cycle life. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The present invention will be further described below with reference to the accompanying drawings.

[0037] Figure 1 1C charge-discharge curves of the positive electrode materials prepared in Example 1, Example 2 and Comparative Example 1 of the present invention;

[0038] Figure 2 1 is the XRD diagram of the positive electrode materials prepared in Example 1 and Example 2 of the present invention. DETAILED DESCRIPTION

[0039] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0040] Example 1

[0041] Take lithium carbonate and 5% of the total raw material mass of glucose, place it in a zirconium oxide ball mill jar, add anhydrous ethanol, ball mill mix for 30 min, get mixed lithium source. Add manganese iron phosphate, vanadyl oxalate and ammonium dihydrogen phosphate to the mixed lithium source according to the element molar ratio Li:Fe:Mn:V:P=8:1:1:4:8, ball mill mix for 60 min, take out and dry in a 100℃ oven for 2h, get the mixed material. The mixed material is heated to 680℃ at a rate of 3℃ / min, pre-calcined for 5h under the protection of gas (volume ratio of argon:hydrogen=95:5), get the mixed precursor. Add 1% of the mass of glucose and 1% of the mass of urea to the mixed precursor, ball mill mix for 30 min, take out and dry in a 100℃ oven, heat to 750℃ at a rate of 3℃ / min, calcine for 6h under the protection of gas (volume ratio of argon:hydrogen=95:5), get the carbon-coated manganese iron lithium phosphate and vanadium lithium phosphate composite positive electrode material.

[0042] Example 2

[0043] The difference from Example 1 is only that the element molar ratio Li:Fe:Mn:V:P is adjusted to 9:3:3:2:9.

[0044] Take lithium carbonate and 5% of the total raw material mass of glucose, place it in a zirconium oxide ball mill jar, add anhydrous ethanol, ball mill mix for 30 min, get mixed lithium source. Add manganese iron phosphate, vanadyl oxalate and ammonium dihydrogen phosphate to the mixed lithium source according to the element molar ratio Li:Fe:Mn:V:P=9:3:3:2:9, ball mill mix for 60 min, take out and dry in a 100℃ oven for 2h, get the mixed material. The mixed material is heated to 680℃ at a rate of 3℃ / min, pre-calcined for 5h under the protection of gas (volume ratio of argon:hydrogen=95:5), get the mixed precursor. Add 1% of the mass of glucose and 1% of the mass of urea to the mixed precursor, ball mill mix for 30 min, take out and dry in a 100℃ oven, heat to 750℃ at a rate of 3℃ / min, calcine for 6h under the protection of gas (volume ratio of argon:hydrogen=95:5), get the carbon-coated manganese iron lithium phosphate and vanadium lithium phosphate composite positive electrode material.

[0045] Example 3

[0046] The difference from Example 1 is only that the element molar ratio Li:Fe:Mn:V:P is adjusted to 5:1:1:2:5.

[0047] The lithium carbonate and 5% of the total raw material mass of glucose were weighed, placed in a zirconium oxide ball mill jar, anhydrous ethanol was added, and ball milling was performed for 30 min to obtain a mixed lithium source. The mixed lithium source was further ball-milled for 60 min according to the molar ratio of the elements Li:Fe:Mn:V:P = 5:1:1:2:5 to add manganese iron phosphate, vanadyl oxalate, and ammonium dihydrogen phosphate, and then taken out and dried in a 100°C oven for 2 h to obtain a mixture. The mixture was heated to 680°C at a rate of 3°C / min, pre-calcined for 5 h under the protection of a gas (volume ratio of argon:hydrogen = 95:5) to obtain a mixed precursor. 1% of glucose and 1% of urea were added to the mixed precursor, ball-milled for 30 min, taken out and dried in a 100°C oven, heated to 750°C at a rate of 3°C / min, and calcined for 6 h under the protection of a gas (volume ratio of argon:hydrogen = 95:5) to obtain a carbon-coated manganese iron lithium phosphate and vanadium lithium phosphate composite positive electrode material.

[0048] Example 4

[0049] The difference from Example 1 is only that the molar ratio of the elements Li:Fe:Mn:V:P is adjusted to 8:1:1:3.8:8.

[0050] The lithium carbonate and 5% of the total raw material mass of glucose were weighed, placed in a zirconium oxide ball mill jar, anhydrous ethanol was added, and ball milling was performed for 30 min to obtain a mixed lithium source. The mixed lithium source was further ball-milled for 60 min according to the molar ratio of the elements Li:Fe:Mn:V:P = 5:1:1:2:5 to add manganese iron phosphate, vanadyl oxalate, and ammonium dihydrogen phosphate, and then taken out and dried in a 100°C oven for 2 h to obtain a mixture. The mixture was heated to 680°C at a rate of 3°C / min, pre-calcined for 5 h under the protection of a gas (volume ratio of argon:hydrogen = 95:5) to obtain a mixed precursor. 1% of glucose and 1% of urea were added to the mixed precursor, ball-milled for 30 min, taken out and dried in a 100°C oven, heated to 750°C at a rate of 3°C / min, and calcined for 6 h under the protection of a gas (volume ratio of argon:hydrogen = 95:5) to obtain a carbon-coated manganese iron lithium phosphate and vanadium lithium phosphate composite positive electrode material.

[0051] Example 5

[0052] The difference from Example 1 is only that the molar ratio of the elements Li:Fe:Mn:V:P is adjusted to 8:1:1:4.2:8.

[0053] The lithium carbonate and 5% of the total raw material mass of glucose were weighed, placed in a zirconium oxide ball mill jar, anhydrous ethanol was added, and ball milling was performed for 30 min to obtain a mixed lithium source. The mixed lithium source was further ball-milled for 60 min according to the molar ratio of the elements Li:Fe:Mn:V:P = 8:1:1:4.2:8 to add manganese iron phosphate, vanadyl oxalate, and ammonium dihydrogen phosphate, and then taken out and dried in a 100°C oven for 2 h to obtain a mixture. The mixture was heated to 680°C at a rate of 3°C / min, pre-calcined for 5 h under the protection of a gas (volume ratio of argon:hydrogen = 95:5) to obtain a mixed precursor. 1% of glucose and 1% of urea were added to the mixed precursor, ball-milled for 30 min, taken out and dried in a 100°C oven, heated to 750°C at a rate of 3°C / min, and calcined for 6 h under the protection of a gas (volume ratio of argon:hydrogen = 95:5) to obtain a carbon-coated manganese iron lithium phosphate and vanadium lithium phosphate composite positive electrode material.

[0054] Example 6

[0055] The difference from Example 1 is that the amount of urea added to the mixed precursor is increased to 1.5%.

[0056] The lithium carbonate and 5% of the total raw material mass of glucose were weighed, placed in a zirconium oxide ball mill jar, anhydrous ethanol was added, and ball milling was performed for 30 min to obtain a mixed lithium source. The mixed lithium source was further ball-milled for 60 min according to the molar ratio of the elements Li:Fe:Mn:V:P = 8:1:1:4:8 to add manganese iron phosphate, vanadyl oxalate, and ammonium dihydrogen phosphate, and then taken out and dried in a 100°C oven for 2 h to obtain a mixture. The mixture was heated to 680°C at a rate of 3°C / min, pre-calcined for 5 h under the protection of a gas (volume ratio of argon:hydrogen = 95:5) to obtain a mixed precursor. 1% of glucose and 1.5% of urea were added to the mixed precursor, ball-milled for 30 min, taken out and dried in a 100°C oven, heated to 750°C at a rate of 3°C / min, and calcined for 6 h under the protection of a gas (volume ratio of argon:hydrogen = 95:5) to obtain a carbon-coated manganese iron lithium phosphate and vanadium lithium phosphate composite positive electrode material.

[0057] Example 7

[0058] The difference from Example 1 is that the amount of urea added to the mixed precursor is increased to 2%.

[0059] The lithium carbonate and 5% of the total raw material mass of glucose were weighed, placed in a zirconium oxide ball mill jar, and anhydrous ethanol was added. The mixture was ball milled for 30 min to obtain a mixed lithium source. The mixed lithium source was ball milled for 60 min with the addition of manganese iron phosphate, vanadyl oxalate, and ammonium dihydrogen phosphate according to the molar ratio of the elements Li:Fe:Mn:V:P=8:1:1:4:8. The mixture was dried in a 100°C oven for 2 h to obtain a mixture. The mixture was heated to 680°C at a rate of 3°C / min, and pre-calcined for 5 h under the protection of a gas (volume ratio of argon:hydrogen=95:5) to obtain a mixed precursor. The mixed precursor was ball milled for 30 min with the addition of 1% of the mass of glucose and 2% of urea. The mixture was dried in a 100°C oven and heated to 750°C at a rate of 3°C / min. The mixture was calcined for 6 h under the protection of a gas (volume ratio of argon:hydrogen=95:5) to obtain the positive electrode material.

[0060] Comparative Example 1

[0061] The difference from Example 1 is that vanadyl oxalate is not added when preparing the positive electrode material.

[0062] The lithium carbonate and 5% of the total raw material mass of glucose were weighed, placed in a zirconium oxide ball mill jar, and anhydrous ethanol was added. The mixture was ball milled for 30 min to obtain a mixed lithium source. The mixed lithium source was ball milled for 60 min with the addition of manganese iron phosphate, vanadyl oxalate, and ammonium dihydrogen phosphate according to the molar ratio of the elements Li:Fe:Mn:V:P=8:1:1:4:8. The mixture was dried in a 100°C oven for 2 h to obtain a mixture. The mixture was heated to 680°C at a rate of 3°C / min, and pre-calcined for 5 h under the protection of a gas (volume ratio of argon:hydrogen=95:5) to obtain a mixed precursor. The mixed precursor was ball milled for 30 min with the addition of 1% of the mass of glucose and 2% of urea. The mixture was dried in a 100°C oven and heated to 750°C at a rate of 3°C / min. The mixture was calcined for 6 h under the protection of a gas (volume ratio of argon:hydrogen=95:5) to obtain the positive electrode material.

[0063] Comparative Example 2

[0064] The difference from Example 1 is that urea is not added when preparing the positive electrode material, and an equal mass of glucose is used to replace the urea.

[0065] The lithium carbonate and 5% of the total raw material mass of glucose were weighed, placed in a zirconium oxide ball mill jar, and added with anhydrous ethanol. The mixture was ball milled for 30 min to obtain a mixed lithium source. The mixed lithium source was added with manganese iron phosphate, vanadyl oxalate, and ammonium dihydrogen phosphate according to the molar ratio of the elements Li:Fe:Mn:V:P = 8:1:1:4:8, and ball milled for 60 min. The mixture was dried in a 100°C oven for 2 h to obtain a mixed material. The mixed material was heated to 680°C at a rate of 3°C / min, and pre-calcined for 5 h under the protection of a gas (volume ratio of argon:hydrogen = 95:5) to obtain a mixed precursor. 2% of the mass of glucose was added to the mixed precursor, which was ball milled for 30 min and dried in a 100°C oven. The mixture was heated to 750°C at a rate of 3°C / min and calcined for 6 h under the protection of a gas (volume ratio of argon:hydrogen = 95:5) to obtain the positive electrode material.

[0066] Comparative Example 3

[0067] The difference from Example 1 is that the positive electrode material is not pre-calcined, but directly calcined in one step.

[0068] The lithium carbonate and 5% of the total raw material mass of glucose were weighed, placed in a zirconium oxide ball mill jar, and added with anhydrous ethanol. The mixture was ball milled for 30 min to obtain a mixed lithium source. The mixed lithium source was added with manganese iron phosphate, vanadyl oxalate, and ammonium dihydrogen phosphate according to the molar ratio of the elements Li:Fe:Mn:V:P = 8:1:1:4:8, and ball milled for 60 min. The mixture was dried in a 100°C oven for 2 h to obtain a mixed material. The mixed material was added with 1% of the mass of glucose and 1% of the mass of urea, which was ball milled for 30 min and dried in a 100°C oven. The mixture was heated to 750°C at a rate of 3°C / min and calcined for 11 h under the protection of a gas (volume ratio of argon:hydrogen = 95:5) to obtain the positive electrode material.

[0069] The positive electrode materials prepared in Examples 1-5 and Comparative Examples 1-3 were assembled into button cells. The positive electrode material was mixed with a conductive agent (conductive carbon black) and a binder (polyvinylidene fluoride) at a mass ratio of 8:2:1, and N-methyl-2-pyrrolidone was added to make a slurry. The slurry was ground and coated on an aluminum foil, and the positive electrode sheet was obtained after drying. Lithium sheets were used as negative electrode materials, polypropylene microporous membranes were used as separators, and 1 mol / L LiPF6 was used as an electrolyte to prepare button cells.

[0070] The assembled button cells were tested for electrochemical performance. The specific capacity of the button cells was tested at 1C, 5C, and 10C different rates under constant temperature conditions at 25°C. The sample batteries were tested for charge and discharge cycles at 1C rate, and the cycle test number was 500 times. The results are shown in Table 1. The charge and discharge test curves of Examples 1, 2, and Comparative Example 1 at 25°C are shown in Figure 1The cathode materials of Example 1 and Example 2 were subjected to XRD testing, and the XRD diffraction patterns were as shown. Figure 2 shown.

[0071]

[0072] As can be seen from Table 1, the discharge capacity of Example 1 at 1C reaches 145.2mAh / g, the discharge capacity of Example 2 at 1C reaches 146.1mAh / g, and the capacity of Comparative Example 1 is 142.6mAh / g. The XRD pattern of Example 1 shows a clear diffraction peak of lithium vanadium phosphate, which greatly improves the rate performance of the material. 0.5 Fe 0.5 The molar ratio of lithium manganese iron phosphate to lithium vanadium phosphate (Li3V2(PO4)3) in Example 2 is 6:1. At this time, the positive electrode material has a high capacity, but its cycle stability is poor. Example 1 has the best overall performance. Example 4 reduces the proportion of vanadium, and Example 5 increases the proportion of vanadium. At this time, too high a vanadium content will lead to component segregation, affecting electrochemical performance. Too low a vanadium content will lead to an excess of lithium, forming a Li3N impurity phase, reducing material purity and specific capacity. At the urea content ratio in Example 6, the positive electrode material has high capacity characteristics and improved cycle performance.

[0073] It should be noted that, in this document, relational terms such as first and second, etc., are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprises," "comprising," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that includes a list of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, article, or apparatus.

[0074] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a vanadium-doped lithium manganese iron phosphate positive electrode material, characterized in that: The following steps are involved: S1, mixing lithium carbonate and a carbon source by ball milling to obtain a mixed lithium source; S2, adding ferromanganese phosphate, vanadyl oxalate and ammonium dihydrogen phosphate to the mixed lithium source, ball milling and mixing, and drying to obtain a mixed material; S3, pre-calcining the dried mixture to obtain a mixed precursor; S4, ball-milling the mixed precursor, carbon source and urea, and calcining at high temperature to obtain a carbon-coated lithium manganese iron phosphate and lithium vanadium phosphate composite positive electrode material, and introducing a carbon source and urea for composite doping to generate a nitrogen-doped carbon coating layer and a vanadium nitride conductive phase; The carbon source in S1 is at least one of glucose, sucrose and citric acid; The carbon source in S4 is the same as the carbon source in S1; The process of pre-calcination is as follows: The temperature was raised to 650-700°C at a heating rate of 3-5°C / min, and calcined for 3-5 hours under a gas protection atmosphere with a volume ratio of argon to hydrogen of 95:

5. The process of high temperature calcination is as follows: The temperature was raised to 750-800° C. at a heating rate of 3-5° C. / min, and calcined for 5-7 h under a gas protection of argon:hydrogen with a volume ratio of 95:

5.

2. The method for preparing a vanadium-doped lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: The mass of the carbon source in S1 is 3%-5% of the total mass of raw materials for preparing vanadium-doped lithium manganese iron phosphate positive electrode material.

3. The method for preparing a vanadium-doped lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: The mixed lithium source, ferromanganese phosphate, vanadyl oxalate and ammonium dihydrogen phosphate are added according to the molar ratio of the elements contained: Li:Fe:Mn:V:P=(3-8):1:1:(0.5-4):(3-8).

4. The method for preparing a vanadium-doped lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: In S2, the drying is carried out in an oven at 80-120°C for 2-5 hours.

5. The method for preparing a vanadium-doped lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: The mass ratio of the mixed precursor, the carbon source and the urea is 1:(0.01-0.02):(0.01-0.02).

6. A vanadium-doped lithium manganese iron phosphate positive electrode material, characterized in that: The vanadium-doped lithium manganese iron phosphate positive electrode material is prepared by the preparation method according to any one of claims 1 to 5.

7. An application of a vanadium-doped lithium manganese iron phosphate positive electrode material, characterized in that: The vanadium-doped lithium manganese iron phosphate positive electrode material prepared by the preparation method according to any one of claims 1 to 5 is applied to lithium-ion batteries or solid-state batteries.

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