Vanadium-doped lithium manganese iron phosphate positive electrode material and preparation method and application thereof

Abstract

The invention discloses a vanadium-doped lithium manganese iron 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, the lithium source and the carbon source are pre-dispersed to avoid agglomeration, and uniform coating of a carbon layer is promoted; then ferromanganese phosphate, vanadyl oxalate and ammonium dihydrogen phosphate are added for ball-milling mixing and pre-calcination to form a precursor, doping of the vanadium element at the ferromanganese position is achieved, a carbon source and urea are added for mixing and high-temperature calcination, ammonia gas generated by urea decomposition reacts with the vanadium element, and the volatilization problem of the vanadium element at the high temperature is effectively inhibited; and a nitrogen-doped carbon coating layer and a vanadium nitride conductive phase are generated, so that the conductivity and the structural stability of the material are further enhanced. The vanadium element is introduced, so that the defect of an existing lithium manganese iron phosphate material in the aspect of electrochemical performance is overcome, the cycling stability and the rate capability of the material are improved, and the material shows more excellent performance in the charging and discharging process.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium batteries, and particularly relates to a vanadium-doped lithium iron manganese phosphate cathode material, a preparation method thereof, and an application thereof. Background Art

[0002] Lithium iron manganese phosphate cathode material (LiMn x Fe 1-x PO4, abbreviated as LMFP) is a new type of phosphate-based lithium-ion battery cathode material. It is a solid solution material formed by doping a certain proportion of manganese (Mn) element on the basis of lithium iron phosphate (LiFePO4, abbreviated as LFP). At present, there are some problems that need to be overcome urgently: in terms of capacity and cycle life, the capacity decays rapidly during the charge and discharge process, and the cycle life is short; the battery rate performance is not good, and the performance drops significantly during high-rate charge and discharge. The electronic conductivity is relatively low, and the charge transfer impedance is relatively large, resulting in large energy loss during the charge and discharge process of the battery. It is found that vanadium doping can significantly improve the capacity and cycle life of the battery. By forming vanadium oxides, it promotes the insertion and extraction process of lithium ions, 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, increase the transmission speed of lithium ions, and enable the battery to still maintain good performance during high-rate charge and discharge.

[0003] However, vanadium doping in the application of lithium iron manganese phosphate is also prone to cause some new problems, such as structural stability problems. During the high-temperature sintering process, vanadium elements volatilize, and excessive doping destroys the crystal structure, affecting the cycle life. On the other hand, there is also a problem that vanadium occupies the lithium site, hindering the diffusion of lithium ions, reducing the rate performance, and causing a decline in kinetic performance. And there are differences in crystal structures between lithium vanadium phosphate and lithium iron manganese phosphate formed by adding vanadium elements. In the conventional process, direct mechanical mixing (ball milling) is difficult to form a stable interface, and during the charge and discharge process, particle cracking is easily caused due to volume expansion differences, exacerbating capacity decay. Summary of the Invention

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

[0005] The purpose of the present invention can be achieved by the following technical solutions: The present invention provides a preparation method of a vanadium-doped lithium iron manganese phosphate cathode material, including the following steps: S1. Ball-mill and mix lithium carbonate and a carbon source to obtain a mixed lithium source; S2. Add lithium iron manganese phosphate, vanadyl oxalate, and ammonium dihydrogen phosphate to the mixed lithium source, ball-mill and mix, and dry to obtain a mixed material; S3. Pre-calcine the dried mixed material to obtain a mixed precursor; S4. Mix the mixed precursor, carbon source, and urea by ball milling, and then perform high-temperature calcination to obtain a composite cathode material of lithium iron manganese phosphate and lithium vanadium phosphate.

[0006] In the above preparation method, first mix lithium carbonate and the carbon source by ball milling to pre-disperse the lithium source and the carbon source to avoid agglomeration, promote the uniform coating of the carbon layer, and improve the conductivity. Subsequently, add lithium iron manganese phosphate, vanadyl oxalate, and ammonium dihydrogen phosphate and mix them by ball milling. Different from the traditional vanadium doping process, in which the lithium source, phosphorus source, manganese source, iron source, and vanadium source are used as raw materials and mixed and reacted respectively, in the present invention, lithium iron manganese phosphate is prepared using lithium iron manganese phosphate and lithium carbonate as raw materials, and lithium vanadium phosphate is prepared by the reaction of vanadyl oxalate, ammonium dihydrogen phosphate, and lithium carbonate, forming two cross-reaction systems to avoid vanadium occupying lithium sites. 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.

[0007] Furthermore, in S1, the carbon source is at least one of glucose, sucrose, and citric acid. Using small molecule organic compounds as the carbon source, due to their pyrolysis characteristics, a porous or loose carbon layer can be formed, which helps the diffusion of lithium ions and electron transport.

[0008] Furthermore, in S1, the ball milling medium in the ball milling mixing process is anhydrous ethanol, and the ball milling mixing time is 30 - 50 min.

[0009] Furthermore, the mass of the carbon source in S1 is 3% - 5% of the total mass of the raw materials for preparing the vanadium-doped lithium iron manganese phosphate cathode material. First, ball mill lithium carbonate and the carbon source to ensure that the carbon source is evenly attached to the surface of the lithium source, and then form a continuous conductive network through high-temperature carbonization.

[0010] Furthermore, the mixed lithium source, lithium iron manganese phosphate, vanadyl oxalate, and ammonium dihydrogen phosphate are added according to the molar ratio of the elements Li:Fe:Mn:V:P = (3 - 8):1:1:(0.5 - 4):(3 - 8).

[0011] Furthermore, in S2, the ball milling mixing time is 1 - 2 h.

[0012] Furthermore, in S2, the drying is carried out in an oven at 80 - 120 °C for 2 - 5 h.

[0013] Furthermore, the process settings of the pre-calcination are as follows: Heat up at a heating rate of 3 - 5 °C / min to 650 - 700 °C, and calcine for 3 - 5 h under the gas protection of an argon:hydrogen volume ratio of 95:5. During the pre-calcination process, lithium carbonate and other raw materials are partially decomposed to 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 at the same time.

[0014] Using a mixed gas of Ar / H2 (95 / 5), compared with a pure inert atmosphere (such as Ar), a small amount of H2 can enhance the reducibility and prevent the oxidation of Mn / V elements (such as Mn 2+ →Mn 3+ ), ensuring the structural stability of the material.

[0015] Furthermore, in S4, the carbon source is the same as the carbon source in S1.

[0016] Furthermore, the mass ratio of the mixed precursor, carbon source and urea is 1:(0.01 - 0.02):(0.01 - 0.02).

[0017] Lithium iron manganese phosphate, like lithium iron phosphate, has an olivine structure. This structure has high stability. The cycle life of lithium iron manganese phosphate is usually lower than that of lithium iron phosphate, and the battery capacity decays rapidly due to the dissolution problem of manganese elements. Lithium vanadium phosphate is doped at the lithium site. Lithium vanadium phosphate has a monoclinic crystal structure. Its three-dimensional lithium ion insertion / extraction channels make its structure more stable. At the same time, due to its relatively stable chemical properties, it shows excellent performance in terms of safety. Monoclinic crystal structured lithium vanadium phosphate is composed of a three-dimensional framework in which VO6 octahedrons and PO4 share the oxygen atom vertices. Each VO6 octahedron is connected to 6 PO4 tetrahedrons through vertices, and each PO4 tetrahedron is connected to 4 VO6 octahedrons. Through this connection method, a three-dimensional network unit structure is formed. Li + is located at the position forming 12 tetrahedral voids in the unit cell. Since the PO4 tetrahedrons separate the VO6 octahedrons, the VO6 octahedrons cannot be directly connected to each other, resulting in relatively poor electronic conductivity.

[0018] Ammonia gas will be generated during the high-temperature decomposition of urea. The ammonia gas reacts with the residual vanadium elements after pre-calcination to form vanadium nitride. Vanadium nitride has high conductivity and can be used as a "conductive bridge" to connect phosphate particles and establish an electron migration channel. Moreover, the nitrogen-doped carbon layer has a stronger interface binding with the cathode material. The nitrogen-doped carbon layer, as a coating layer, can reduce the direct contact between the cathode and the electrolyte, inhibit the side reactions of the electrolyte, reduce the interfacial impedance, and avoid the dissolution of manganese elements at high potentials. If urea is added in S1, the ammonia gas generated at this time will react with vanadium elements to form more vanadium nitride, occupying a large amount of vanadium elements, which will inhibit the formation of lithium vanadium phosphate. In contrast, in this application, urea is added in S4. At this time, the vanadium elements have formed the precursor of lithium vanadium phosphate. During the high-temperature calcination process, lithium vanadium phosphate further reacts and crystallizes completely, and the structure is stable. At this time, some uncrystallized vanadium element substances can react with ammonia gas to form vanadium nitride, which can not only inhibit the volatilization of vanadium elements in the high-temperature calcination environment, but also be distributed in the grain boundaries in the form of fine particles of vanadium nitride to inhibit the expansion and cracking of grains, construct a continuous composite cathode material conductive network, form a stable interface, inhibit capacity decay, and the nitride phase can inhibit the loss of lattice oxygen and relieve the structural collapse under high voltage.

[0019] Furthermore, the process of the high-temperature calcination is set as follows: Heat up to 750 - 800 °C at a heating rate of 3 - 5 °C / min, and calcine for 5 - 7 h under the gas protection with a volume ratio of argon:hydrogen = 95:5. The complete crystallization temperature of lithium iron manganese phosphate is generally above 700 °C, and the complete crystallization temperature of lithium vanadium phosphate is generally above 750 °C. Through high-temperature calcination, the impurity phases in the cathode material can be reduced, the crystallization of lithium iron manganese phosphate and lithium vanadium phosphate can be completed, and a complete lithium-ion diffusion channel can be established.

[0020] Through the two-stage calcination of pre-calcination and high-temperature calcination in the above preparation, the particle agglomeration or composition segregation caused by direct high temperature can be avoided. Both ball milling and high-temperature solid-phase method are mature processes, which are easy to scale up production and are suitable for large-scale preparation of high-performance cathode materials.

[0021] The present invention also provides a vanadium-doped lithium iron manganese phosphate cathode material, which is prepared by the preparation method described above.

[0022] The present invention also provides an application of a vanadium-doped lithium iron manganese phosphate cathode material prepared by the preparation method described above, which is applied to fields such as lithium-ion batteries and solid-state batteries.

[0023] Advantages of the present invention: (1) The vanadium-doped lithium iron manganese phosphate cathode material prepared in the present invention has good electrochemical performance. Compared with the lithium iron manganese phosphate cathode material without vanadium doping, by introducing vanadium element in the present invention, the deficiencies of the existing lithium iron manganese phosphate material in terms of electrochemical performance are overcome, the cycle stability and rate performance of the material are improved, and more excellent performance is shown during charge and discharge.

[0024] (2) After the precursor is formed by pre-calcination, a carbon source and urea are added and calcined at high temperature. The sintering temperature is controlled at 600 - 800 °C by the low-temperature solid-phase method, and the ammonia gas generated by the decomposition of urea reacts with the vanadium element, effectively inhibiting the volatilization problem of the vanadium element at high temperature and ensuring the precise control of the doping concentration.

[0025] (3) By introducing a carbon source and urea for composite doping, a nitrogen-doped carbon coating layer and a vanadium nitride conductive phase are generated, further enhancing the conductivity and structural stability of the material.

[0026] (4) The open three-dimensional framework monoclinic structure of lithium vanadium phosphate and the high ionic conductivity of the vanadium element jointly promote the rapid diffusion of lithium ions, thereby enhancing the rate performance of the material.

[0027] (5) Preferably, the raw materials of the positive electrode material are used to prepare lithium iron manganese phosphate with manganese iron phosphate and lithium carbonate, and vanadium lithium phosphate is prepared by reacting vanadium oxalate, ammonium dihydrogen phosphate and lithium carbonate to form two cross-reaction systems to avoid vanadium occupying lithium sites. In the present invention, the doping site of vanadium element is the manganese iron site, which can improve the electronic conductivity of the material, avoid the distortion of the structure and improve the cycle life. Description of the Drawings

[0028] The present invention will be further described below with reference to the drawings.

[0029] Figure 1 It is the 1C charge and discharge curve graph of the positive electrode materials prepared in Example 1, Example 2 and Comparative Example 1 of the present invention; Figure 2 It is the XRD pattern of the positive electrode materials prepared in Example 1 and Example 2 of the present invention. Specific Embodiments

[0030] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0031] Example 1 Weigh lithium carbonate and glucose accounting for 5% of the total raw material mass, place them in a zirconia ball milling tank, add absolute ethanol, and ball mill and mix for 30 minutes to obtain a mixed lithium source. Then, add lithium iron manganese phosphate, vanadium oxalate and ammonium dihydrogen phosphate to the mixed lithium source according to the molar ratio of the contained elements Li:Fe:Mn:V:P = 8:1:1:4:8, and ball mill and mix for 60 minutes. Take it out and dry it in an oven at 100°C for 2 hours to obtain a mixed material. The mixed material is heated to 680°C at a rate of 3°C / min, and pre-calcined for 5 hours under the protection of gas (volume ratio argon:hydrogen = 95:5) to obtain a mixed precursor. Add 1% glucose and 1% urea by mass to the mixed precursor, ball mill and mix for 30 minutes, take it out and dry it in an oven at 100°C, and heat it to 750°C at a rate of 3°C / min, and calcine for 6 hours under the protection of gas (volume ratio argon:hydrogen = 95:5) to obtain a carbon-coated lithium iron manganese phosphate and vanadium lithium phosphate composite positive electrode material.

[0032] Example 2 The difference from Example 1 is only that the molar ratio of elements Li:Fe:Mn:V:P is adjusted to 9:3:3:2:9.

[0033] Weigh lithium carbonate and glucose accounting for 5% of the total raw material mass, place them in a zirconia ball milling tank, add absolute ethanol, and ball mill and mix for 30 min to obtain a mixed lithium source. Then, add lithium iron phosphate, vanadyl oxalate, and ammonium dihydrogen phosphate to the mixed lithium source according to the molar ratio of elements Li:Fe:Mn:V:P = 9:3:3:2:9, ball mill and mix for 60 min, take it out and dry it in an oven at 100 °C for 2 h to obtain a mixed material. The mixed material is heated to 680 °C at a rate of 3 °C / min and pre-calcined for 5 h under the protection of gas (volume ratio of argon:hydrogen = 95:5) to obtain a mixed precursor. Add 1% glucose and 1% urea by mass to the mixed precursor, ball mill and mix for 30 min, take it out and dry it in an oven at 100 °C, heat it to 750 °C at a rate of 3 °C / min, and calcine it for 6 h under the protection of gas (volume ratio of argon:hydrogen = 95:5) to obtain a carbon-coated lithium iron manganese phosphate and lithium vanadium phosphate composite cathode material.

[0034] Example 3 The difference from Example 1 is only that the molar ratio of elements Li:Fe:Mn:V:P is adjusted to 5:1:1:2:5.

[0035] Weigh lithium carbonate and glucose accounting for 5% of the total raw material mass, place them in a zirconia ball milling tank, add absolute ethanol, and ball mill and mix for 30 min to obtain a mixed lithium source. Then, add lithium iron phosphate, vanadyl oxalate, and ammonium dihydrogen phosphate to the mixed lithium source according to the molar ratio of elements Li:Fe:Mn:V:P = 5:1:1:2:5, ball mill and mix for 60 min, take it out and dry it in an oven at 100 °C for 2 h to obtain a mixed material. The mixed material is heated to 680 °C at a rate of 3 °C / min and pre-calcined for 5 h under the protection of gas (volume ratio of argon:hydrogen = 95:5) to obtain a mixed precursor. Add 1% glucose and 1% urea by mass to the mixed precursor, ball mill and mix for 30 min, take it out and dry it in an oven at 100 °C, heat it to 750 °C at a rate of 3 °C / min, and calcine it for 6 h under the protection of gas (volume ratio of argon:hydrogen = 95:5) to obtain a carbon-coated lithium iron manganese phosphate and lithium vanadium phosphate composite cathode material.

[0036] Example 4 The difference from Example 1 is only that the molar ratio of elements Li:Fe:Mn:V:P is adjusted to 8:1:1:3.8:8.

[0037] Weigh lithium carbonate and glucose which is 5% of the total raw material mass, place them in a zirconia ball milling jar, add absolute ethanol, and ball mill and mix for 30 min to obtain a mixed lithium source. Then, add lithium iron phosphate, vanadyl oxalate, and ammonium dihydrogen phosphate to the mixed lithium source according to the molar ratio of the contained elements Li:Fe:Mn:V:P = 8:1:1:3.5:8, ball mill and mix for 60 min, take it out and dry it in an oven at 100 °C for 2 h to obtain a mixed material. The mixed material is 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. Add 1% of glucose and 1% of urea by mass to the mixed precursor, ball mill and mix for 30 min, take it out and dry it in an oven at 100 °C, heat it to 750 °C at a rate of 3 °C / min, and calcine it for 6 h under the protection of a gas (volume ratio of argon:hydrogen = 95:5) to obtain a carbon-coated lithium iron manganese phosphate and lithium vanadium phosphate composite cathode material.

[0038] Example 5 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.

[0039] Weigh lithium carbonate and glucose which is 5% of the total raw material mass, place them in a zirconia ball milling jar, add absolute ethanol, and ball mill and mix for 30 min to obtain a mixed lithium source. Then, add lithium iron phosphate, vanadyl oxalate, and ammonium dihydrogen phosphate to the mixed lithium source according to the molar ratio of the contained elements Li:Fe:Mn:V:P = 8:1:1:4.2:8, ball mill and mix for 60 min, take it out and dry it in an oven at 100 °C for 2 h to obtain a mixed material. The mixed material is 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. Add 1% of glucose and 1% of urea by mass to the mixed precursor, ball mill and mix for 30 min, take it out and dry it in an oven at 100 °C, heat it to 750 °C at a rate of 3 °C / min, and calcine it for 6 h under the protection of a gas (volume ratio of argon:hydrogen = 95:5) to obtain a carbon-coated lithium iron manganese phosphate and lithium vanadium phosphate composite cathode material.

[0040] Example 6 The difference from Example 1 is only that 1% of urea by mass of the mixed precursor is increased to 1.5%.

[0041] Weigh lithium carbonate and glucose which is 5% of the total raw material mass, place them in a zirconia ball milling jar, add absolute ethanol, and ball mill and mix for 30 min to obtain a mixed lithium source. Then, add lithium iron phosphate, vanadyl oxalate, and ammonium dihydrogen phosphate to the mixed lithium source according to the molar ratio of the contained elements Li:Fe:Mn:V:P = 8:1:1:4:8, ball mill and mix for 60 min, take it out and dry it in an oven at 100 °C for 2 h to obtain a mixed material. The mixed material is 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. Add 1% of glucose and 1.5% of urea based on the mass of the mixed precursor, ball mill and mix for 30 min, take it out and dry it in an oven at 100 °C, heat it to 750 °C at a rate of 3 °C / min, and calcine for 6 h under the protection of a gas (volume ratio of argon:hydrogen = 95:5) to obtain a carbon-coated lithium iron phosphate and lithium vanadium phosphate composite cathode material.

[0042] Example 7 The difference from Example 1 is only that the urea which is 1% of the mass of the mixed precursor is increased to 2%.

[0043] Weigh lithium carbonate and glucose which is 5% of the total raw material mass, place them in a zirconia ball milling jar, add absolute ethanol, and ball mill and mix for 30 min to obtain a mixed lithium source. Then, add lithium iron phosphate, vanadyl oxalate, and ammonium dihydrogen phosphate to the mixed lithium source according to the molar ratio of the contained elements Li:Fe:Mn:V:P = 8:1:1:4:8, ball mill and mix for 60 min, take it out and dry it in an oven at 100 °C for 2 h to obtain a mixed material. The mixed material is 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. Add 1% of glucose and 2% of urea based on the mass of the mixed precursor, ball mill and mix for 30 min, take it out and dry it in an oven at 100 °C, heat it to 750 °C at a rate of 3 °C / min, and calcine for 6 h under the protection of a gas (volume ratio of argon:hydrogen = 95:5) to obtain a cathode material.

[0044] Comparative Example 1 The difference from Example 1 is that vanadyl oxalate is not added when preparing the cathode material.

[0045] Weigh lithium carbonate and glucose accounting for 5% of the total raw material mass, place them in a zirconia ball milling jar, add absolute ethanol, and ball mill and mix for 30 min to obtain a mixed lithium source. Then, add lithium iron phosphate, vanadyl oxalate, and ammonium dihydrogen phosphate to the mixed lithium source according to the molar ratio of the contained elements Li:Fe:Mn:P = 2:1:1:2, ball mill and mix for 60 min, take it out and dry it in an oven at 100 °C for 2 h to obtain a mixed material. The mixed material is 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. Add 1% glucose and 1% urea of its mass to the mixed precursor, ball mill and mix for 30 min, take it out and dry it in an oven at 100 °C, heat it to 750 °C at a rate of 3 °C / min, and calcine for 6 h under the protection of a gas (volume ratio of argon:hydrogen = 95:5) to obtain the cathode material.

[0046] Comparative Example 2 The difference from Example 1 is that urea is not added when preparing the cathode material, and glucose of equal mass is used to replace urea.

[0047] Weigh lithium carbonate and glucose accounting for 5% of the total raw material mass, place them in a zirconia ball milling jar, add absolute ethanol, and ball mill and mix for 30 min to obtain a mixed lithium source. Then, add lithium iron phosphate, vanadyl oxalate, and ammonium dihydrogen phosphate to the mixed lithium source according to the molar ratio of the contained elements Li:Fe:Mn:V:P = 8:1:1:4:8, ball mill and mix for 60 min, take it out and dry it in an oven at 100 °C for 2 h to obtain a mixed material. The mixed material is 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. Add 2% glucose of its mass to the mixed precursor, ball mill and mix for 30 min, take it out and dry it in an oven at 100 °C, heat it to 750 °C at a rate of 3 °C / min, and calcine for 6 h under the protection of a gas (volume ratio of argon:hydrogen = 95:5) to obtain the cathode material.

[0048] Comparative Example 3 The difference from Example 1 is that when preparing the cathode material, pre-calcination is not carried out, and it is directly calcined in one step.

[0049] Weigh lithium carbonate and glucose accounting for 5% of the total raw material mass, place them in a zirconia ball milling tank, add absolute ethanol, and ball mill and mix for 30 min to obtain a mixed lithium source. Then, add lithium iron phosphate, vanadyl oxalate, and ammonium dihydrogen phosphate to the mixed lithium source according to the molar ratio of elements Li:Fe:Mn:V:P = 8:1:1:4:8, ball mill and mix for 60 min, take it out and dry it in an oven at 100 °C for 2 h to obtain a mixed material. Add 1% glucose and 1% urea by mass of the material before mixing, ball mill and mix for 30 min, take it out and dry it in an oven at 100 °C, heat it up to 750 °C at a rate of 3 °C / min, and calcine it for 11 h under the protection of gas (volume ratio of argon:hydrogen = 95:5) to obtain the cathode material.

[0050] Assemble the cathode materials prepared in Examples 1 - 5 and Comparative Examples 1 - 3 into coin cells. The cathode material, conductive agent (conductive carbon black), and binder (polyvinylidene fluoride) are mixed at a mass ratio of 8:2:1, and N-methyl-2-pyrrolidone is added to make a slurry, which is ground and coated on an aluminum foil. After drying, a cathode sheet is obtained. Using a lithium sheet as the anode material, a polypropylene microporous membrane as the separator, and 1 mol / L LiPF6 as the electrolyte, coin cells are prepared.

[0051] Perform electrochemical performance tests on the assembled coin cells. Under the constant temperature condition of 25 °C, test the specific capacity of the coin cells at different rates of 1C, 5C, and 10C respectively, and under the condition of 1C rate, conduct charge-discharge cycle tests on the sample cells. The number of cycle tests is 500 weeks respectively. The results are shown in Table 1. The curves of Examples 1, 2, and Comparative Example 1 during charge-discharge tests at 25 °C are as Figure 1 shown. Take the cathode materials of Example 1 and Example 2 for XRD tests. The XRD diffraction patterns are as Figure 2 shown.

[0052]

[0053] It can be seen from Table 1 that the discharge specific capacity of Example 1 at 1C reaches 145.2 mAh / g, the discharge specific capacity of Example 2 at 1C reaches 146.1 mAh / g, the specific capacity of Comparative Example 1 is 142.6 mAh / g, and obvious diffraction peaks of lithium vanadium phosphate appear in the XRD pattern of Example 1, greatly improving the rate performance of the material. In Examples 1 - 3, the molar ratio of elements in the preparation of the cathode material was adjusted. Based on lithium manganese 0.5 Fe 0.5The elemental ratios in lithium iron phosphate manganese (LiFePO4Mn) and lithium vanadium phosphate (Li3V2(PO4)3). In Example 2, the molar ratio of lithium iron phosphate manganese to lithium vanadium phosphate is 6:1. At this time, the capacity of the cathode material is relatively high, but its cycle stability is poor. The comprehensive performance of Example 1 is the best. In Example 4, the proportion of vanadium element is reduced, and in Example 5, the proportion of vanadium element is increased. At this time, too much vanadium element will lead to composition segregation, affecting the electrochemical performance, and too little vanadium element will lead to an excess of lithium element, generating a Li3N impurity phase, reducing the material purity and the specific capacity. At the urea content ratio of Example 6, the cathode material has high-capacity characteristics and improved cycle performance.

[0054] It should be noted that in this text, relational terms such as first and second are only used 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 term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements not only includes those elements, but also includes other elements not expressly listed, or also includes elements inherent to such process, method, article or device.

[0055] Although the embodiments of the present invention have been shown and described, those of ordinary skill in the art can understand that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A preparation method of a vanadium-doped lithium iron manganese phosphate cathode material, characterized in that, It includes the following steps: S1. Ball-mill and mix lithium carbonate with a carbon source to obtain a mixed lithium source; S2. Add lithium iron phosphate, vanadyl oxalate and ammonium dihydrogen phosphate to the mixed lithium source, ball-mill and mix them, and dry to obtain a mixed material; S3. Pre-calcine the dried mixed material to obtain a mixed precursor; S4. Ball-mill and mix the mixed precursor, a carbon source and urea, and carry out high-temperature calcination to obtain a composite cathode material of lithium iron phosphate and lithium vanadium phosphate.

2. The preparation method of a vanadium-doped lithium iron manganese phosphate cathode material according to claim 1, wherein, In S1, the carbon source is at least one of glucose, sucrose and citric acid.

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

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

5. The preparation method of a vanadium-doped lithium iron manganese phosphate cathode material according to claim 1, wherein In S2, the drying is carried out in an oven at 80-120°C for 2-5 h.

6. The preparation method of a vanadium-doped lithium iron manganese phosphate cathode material according to claim 1, wherein, The process settings for the pre-calcination are as follows: Heat up at a heating rate of 3-5°C / min to 650-700°C, and calcine for 3-5 h under the gas protection of argon:hydrogen with a volume ratio of 95:

5.

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

8. The preparation method of a vanadium-doped lithium iron manganese phosphate cathode material according to claim 1, characterized in that, The process settings for the high-temperature calcination are as follows: Heat up at a heating rate of 3-5°C / min to 750-800°C, and calcine for 5-7 h under the gas protection of argon:hydrogen with a volume ratio of 95:

5.

9. A vanadium-doped lithium iron manganese phosphate cathode material, characterized in that, The vanadium-doped lithium iron phosphate cathode material is prepared by the preparation method described in any one of claims 1-8.

10. Application of a vanadium-doped lithium iron manganese phosphate cathode material, characterized in that, The vanadium-doped lithium iron phosphate cathode material prepared by the preparation method described in any one of claims 1-8 is applied to fields such as lithium-ion batteries and solid-state batteries.

Images