Anion-cation co-doped modified lithium manganese iron phosphate positive electrode material as well as preparation method and application thereof

Abstract

The invention discloses an anion and cation co-doped modified lithium manganese iron phosphate positive electrode material as well as a preparation method and application thereof. The chemical general formula of the positive electrode material is Li (Mn0. 6Fe0. 4) 1-x (MxNy) PO4, M is a modified cation, and N is a modified anion; the modified cation elements are at least two of V, Ti, Co, Ni, Cu and Zn; the modified anion element is at least one of halogens; the ratio of the stoichiometric number x to the stoichiometric number y of the modified cations to the modified anions is 1: (1-2). The preparation process of the material is as follows: ferromanganese oxalate is obtained by a coprecipitation method, and then co-doping of anion and cation elements is realized by a two-stage roasting process, so that the crystallinity of the material is effectively ensured while a small amount of oxalate in the material is removed. When the material is used for preparing a positive electrode of a lithium ion battery, the cycle stability and the rate capability of the battery can be greatly improved, the retention rate of 300 cycles at 1C is 94.7%, and the maximum cycle life can reach more than 2000 cycles.

Description

[0001] Technical solution

[0002] The present invention relates to a modified lithium iron manganese phosphate cathode material, specifically to a cathode material of lithium iron manganese phosphate modified by co-doping of anions and cations, a preparation method thereof and an application thereof, belonging to the technical field of battery materials. Background technology

[0003] With the continuous consumption of global fossil fuels, the demand for new energy is increasing day by day. Compared with traditional zinc-manganese dry batteries and lead-acid batteries, lithium-ion batteries have gradually become a research hotspot due to their high specific energy, low carbon environmental protection and rich raw materials. Among them, the cathode material is the main factor determining the performance of lithium-ion batteries. The spinel-type cathode material (lithium manganate) has the advantages of low production cost and good safety performance. However, due to the Jahn-Teller effect of Mn 3+ , its cycling performance and electrochemical stability are relatively low; the olivine-type cathode material (lithium iron phosphate) does not have the Jahn-Teller effect during charge and discharge, but has a low energy density and is prone to loss of active lithium. Therefore, lithium iron manganese phosphate with a higher voltage platform has become a new research hotspot. It has a higher voltage platform than lithium iron phosphate, but due to the introduction of Mn 2+ , the dissolution of manganese occurs the Jahn-Teller effect, and the geometric structure of the material is distorted during charge and discharge, resulting in a slight decrease in cycle stability and cycle life. Therefore, it is of great significance to prepare a lithium iron manganese phosphate cathode material with good electrochemical performance and to carry out modification research on this basis to improve its capacity retention rate and ensure its cycle life during charge and discharge for the development of the lithium-ion battery industry.

[0004] Some progress has been made in the domestic and foreign research on lithium iron manganese phosphate cathode materials, but there are still some challenges and problems. In addition to the simple solid-phase reaction, a series of new strategies have been explored for the current synthesis methods of lithium iron phosphate, such as sol-gel method, carbothermal reduction, microwave synthesis, co-precipitation and solvothermal synthesis, etc. Peng et al. prepared LiMn 0.8 Fe 0.2 PO 4 / C composite materials by spray drying and high-temperature sintering. The uniformly conformal conductive carbon network between nanocrystals and particles provides good electrochemical kinetics. The indentation reversible discharge capacity at 1 C is 148 mAh / g respectively. The retention rate of this composite material after 500 cycles at 1 C reaches 95%, showing good cycling performance. Yangon et al. used PVP as an anti-agglomerant and successfully prepared LiMn 0.8 Fe 0.2 PO 4, it has a discharge capacity as high as 160.6 mAh / g at 0.05 C and exhibits good high-rate performance; it maintains a capacity of 113 mAh / g at 10 C, and after 50 discharge cycles at 0.5 C, the capacity decreases from 146.5 to 143 mAh / g, with a retention rate of 97.6%. In addition to the research on synthesis schemes, how to optimize its internal structure, increase the ion diffusion coefficient, and improve the surface conductivity has also become a research hotspot. Among them, the method of element doping is the most important. Under the doping effect of elements, the resistance of charge transfer and polarization phenomenon can be effectively reduced, and lattice defects can be increased, thereby enhancing the electronic conductivity of LiFePO 4 and improving the diffusion rate of lithium ions. YU Songmin et al. synthesized lithium iron manganese phosphate cathode material by solvothermal method. During the synthesis process, LiF was used as the F source to modify the material by O-site doping. After modification, the discharge specific capacity, rate performance, and cycling performance of the lithium iron manganese phosphate cathode material were significantly improved. After F doping, the initial Coulomb efficiency of the material at 0.1 C increased from 90% to 93%, and the discharge specific capacity reached 153 mAh / g. Compared with LMFP / C, the capacity of LMFP / C-1%F at 5 C high rate increased from 88 mAh / g to 106 mAh / g.

[0005] However, the above modification processes all have the following problems. On the one hand, due to the Jahn-Teller effect of Mn 3+ , lattice distortion occurs in the material, resulting in the dissolution of Mn 2+ during cycling and the destruction of the material structure; on the other hand, although carbon coating and doping can improve conductivity, the intrinsic electronic conductivity of LMFP (<10 -9 S / cm) is still much lower than that of materials such as lithium cobalt oxide (LCO). Its capacity decays significantly at high current densities. For example, the material of the YU team can reach an initial capacity of 150 mAh / g at low current density, but the capacity decreases significantly when the rate is increased to 5 C (such as only 88 mAh / g for undoped LMFP / C). This patent uses a modification method of co-doping with multi-valent high-valent cations (V 3+ , Ti 4+ ) combined with surface modification of anions (F - ) to prepare a vanadium, titanium, and fluorine co-doped lithium iron manganese phosphate cathode material. The capacity of the material can reach more than 160 mAh / g at low current density, and the capacity basically does not decay after 200 cycles at 1 C, with a retention rate of 99%. At the same time, due to the synergistic effect of multiple elements, the material also has a discharge specific capacity of 130 mAh / g at a high current density of 5 C, showing excellent electrochemical performance. Summary of the Invention

[0006] Aiming at the problems of the existing technology, the first object of the present invention is to provide a cathode material of lithium iron manganese phosphate modified by co-doping of anions and cations. The cathode material is modified by the synergistic doping of multiple anions and cations, which can significantly improve the capacity and rate performance of the material on the basis of ensuring the cycle stability of the material.

[0007] The second object of the present invention is to provide a preparation method of a cathode material of lithium iron manganese phosphate modified by co-doping of anions and cations. The method uses the co-precipitation method to obtain manganese iron oxalate, and then through a two-stage calcination process, the co-doping of anion and cation elements is realized. While removing a small amount of oxalate in the material, the crystallinity of the material is effectively guaranteed.

[0008] The third object of the present invention is to provide an application of a cathode material of lithium iron manganese phosphate modified by co-doping of anions and cations, which is used to prepare the cathode of a lithium-ion battery. Based on the special doping structure of the above material, when it is used to prepare the cathode of a lithium-ion battery, compared with the existing technology, it has more excellent technical effects, can significantly enhance the binding force between the active substance and the current collector, and greatly improve the cycle stability and rate performance of the battery. After testing, the lithium-ion battery prepared with this material can reach a first-cycle capacity of 168.2 mAh / g at 0.1 C, a first-cycle discharge specific capacity of 147.8 mAh / g at 1 C, and the capacity decays to 140 mAh / g after 300 cycles, with a retention rate of 94.7%. It can also maintain a capacity close to 130 mAh / g at 5 C.

[0009] To achieve the above technical object, the present invention provides a cathode material of lithium iron manganese phosphate modified by co-doping of anions and cations, including: the chemical general formula of the cathode material is Li(Mn 0.6 Fe 0.4 ) 1-x (M x N y )PO 4 , where M is a modified cation and N is a modified anion; the modified cation element is at least two of V, Ti, Co, Ni, Cu, and Zn; the modified anion element is at least one of halogens; the ratio of the stoichiometric numbers x and y of the modified cation to the modified anion is 1:1 to 2.

[0010] The cathode material provided by the present invention is doped and modified with multiple anions and cations. Among them, the doping of multiple cations, such as V 3+ doping can introduce additional free electrons, Ti 4+Doping can form a conductive network to improve the conductivity of the material, and can replace the Mn / Fe sites of the lithium iron manganese phosphate cathode material, reducing the Li-Fe mixing during charge and discharge, effectively reducing the concentration of anti-site defects in the material, and improving the conductivity and structural stability of the material. However, with the introduction of cations, VO x , Li x TiO y and other impurity phases may be introduced, resulting in a certain reduction in the cycle stability of the material. Therefore, halogen anions such as F - are selected as the third doping element for surface carbon doping and coating modification. By forming C-F bonds, the lithium iron manganese phosphate cathode material is surface-modified to improve the interface stability of the material, inhibit the dissolution of Mn 2+ and Fe 2+ , reduce the reduction of the cycle performance of the material caused by lattice distortion caused by co-doping, and at the same time form more active sites, reduce the diffusion barrier of Li + , and greatly improve the lithium ion diffusion coefficient (D Li + ). Finally, under the synergistic modification of anions and cations, the dual improvement of the discharge specific capacity and cycle stability of the lithium iron manganese phosphate cathode material is achieved.

[0011] As a preferred solution, the modified cationic elements are V and Ti.

[0012] As a preferred solution, the modified anionic elements are F and / or Cl.

[0013] As a preferred solution, the stoichiometric number x of the modified cation ranges from 0.2 to 0.5.

[0014] As a preferred solution, the ratio of x to y is 1:1.3 to 1.6.

[0015] The present invention also provides a preparation method of a cathode material for lithium iron manganese phosphate modified by co-doping of anions and cations. A soluble reducing agent, an iron source and a manganese source are uniformly dropped into an oxalate precipitating agent for coprecipitation reaction to obtain manganese iron oxalate; the modified raw materials including a lithium source, a phosphorus source, a modified cation source and a modified anion source and manganese iron oxalate are mechanically activated together and then calcined to obtain the product.

[0016] The present invention uses the oxalate coprecipitation method to prepare the lithium iron manganese phosphate cathode material. Ammonium oxalate is selected as the precipitating agent and uniformly dropped into the prepared manganese iron solution. After the manganese iron oxalate precursor is ball-milled and mixed with the lithium source, the phosphorus source and the carbon source, a two-stage calcination temperature system is used to remove a small amount of oxalate radicals in the material and ensure the crystallinity of the material, obtaining the lithium iron manganese phosphate cathode material.

[0017] As a preferred embodiment, the soluble reducing agent is at least one of sugars, vitamins, sodium borohydride, and potassium borohydride.

[0018] As a preferred embodiment, the iron source is at least one of organic ferrous salts and inorganic ferrous salts.

[0019] As a preferred embodiment, the manganese source is manganese phosphate and / or manganese sulfate.

[0020] As a preferred embodiment, the modified raw material further contains a carbon supplementing agent, and the addition amount of the carbon supplementing agent is 2-10 wt% of the sum of the masses of the modified raw material and manganese iron oxalate in terms of the mass of the C element. Further preferably, the carbon supplementing agent is at least one of ascorbic acid, glucose, and sucrose.

[0021] As a preferred embodiment, when the modified cationic element is V and Ti, the modified cation source is ammonium metavanadate and titanium dioxide.

[0022] As a preferred embodiment, when the modified anionic element is F, the modified anion source is lithium fluoride.

[0023] As a preferred embodiment, the conditions for the coprecipitation reaction are: the temperature is 20-100 °C, and the time is 2-24 h. Further preferably, the conditions for the coprecipitation reaction are: the temperature is 40-80 °C, and the time is 6-12 h.

[0024] As a preferred embodiment, the mechanical activation method is wet ball milling, and the conditions are: using zirconia as the grinding balls, the ball milling medium is water and / or ethanol, the ball-to-material ratio is 5:1-10:1, the rotation speed is 200-400 rpm / min, and the time is 6-12 h.

[0025] As a preferred embodiment, the calcination treatment method is two-stage roasting, and the process is: heating from room temperature to 400-500 °C at a rate of 3-10 °C / min, holding for 2-6 h for the first-stage roasting, and then heating to 600-800 °C at a rate of 3-10 °C / min and holding for 4-10 h for the second-stage roasting. After the roasting is completed, it is cooled to room temperature with the furnace.

[0026] The present invention also provides an application of the cathode material of lithium iron manganese phosphate modified by co-doping of cations and anions for preparing the cathode of a lithium ion battery.

[0027] The present invention prepares the cathode material of lithium iron manganese phosphate modified by synergistic doping of cations and anions, and uses the doping of V 3+ to weaken the interaction existing between the Li-O chemical bonds, introduce additional free electrons, and combine with the doping of Ti 4+ to reduce the Li Fe -Fe LiThe concentration of inversion defects can, to a certain extent, improve the reversible capacity of the material. On this basis, further doping modification of F - is carried out to give play to the synergistic effect of multi-doping elements of V 3+ , Ti 4+ and F - . The surface carbon-coated layer of the lithium iron manganese phosphate cathode material is modified to form stable C-F bonds, inhibit the dissolution of Mn 2+ and Fe 2+ , reduce the lattice distortion caused by V 3+ , Ti 4+ , effectively improve the cycle stability of the material, at the same time reduce the interfacial impedance between the electrode and the electrolyte, increase the carrier concentration of the carbon layer, effectively improve the lithium ion diffusion coefficient, and finally while ensuring the cycle stability of the material, gradually increase the discharge specific capacity, better meeting the various requirements for lithium iron manganese phosphate in the current market.

[0028] Compared with the prior art, the beneficial technical effects of the technical solution provided by the present invention are as follows:

[0029] 1) The cathode material provided by the present invention is modified by co-doping of anionic and cationic multi-ions, and on the basis of ensuring the cycle stability of the material, significantly improves the capacity and rate performance of the material.

[0030] 2) The preparation method provided by the present invention uses the co-precipitation method to obtain manganese iron oxalate, and then through a two-stage calcination process, realizes the co-doping of anionic and cationic elements, while removing a small amount of oxalate radicals in the material, effectively ensuring the crystallinity of the material.

[0031] 3) In the technical solution provided by the present invention, based on the special doping structure of the above material, when it is used to prepare the cathode of a lithium ion battery, compared with the prior art, it has more excellent technical effects, can significantly enhance the binding force between the active material and the current collector, greatly improve the cycle stability and rate performance of the battery. After testing, the lithium ion battery prepared with this material can reach a first-cycle capacity of 168.2 mAh / g at 0.1 C, a first-cycle discharge specific capacity of 147.8 mAh / g at 1 C, the capacity basically does not decay after 200 cycles, and the retention rate can reach 99%. After continuing to cycle 300 times, the capacity decays to 140 mAh / g, and the retention rate is 95%. It can also maintain a capacity close to 130 mAh / g at 5 C. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 For the lithium iron manganese phosphate (LiMn 0.6 Fe 0.4 PO 4 ) and vanadium-doped lithium iron manganese phosphate materials (Li(Mn 0.6 Fe0.4 ) 1-x V x PO 4 XRD distribution pattern of

[0033] Figure 2 SEM images of the intermediate obtained in Example 4 and the final product obtained in Example 6 of the present invention;

[0034] Among them, Figure 2 (a) is the SEM image of the manganese iron oxalate intermediate prepared in Example 4 at a magnification of 500×, Figure 2 (b) is the SEM image of the manganese iron oxalate intermediate prepared in Example 4 at a magnification of 2000×., Figure 2 (c) is the SEM image of the lithium iron manganese phosphate cathode material prepared in Example 4 at a magnification of 500×, Figure 2 (d) is the SEM image of the lithium iron manganese phosphate cathode material prepared in Example 4 at a magnification of 2000×, Figure 2 (e) is the SEM image of the modified lithium iron manganese phosphate cathode material prepared in Example 4 at a magnification of 500×.

[0035] Figure 3 Infrared spectra of lithium iron manganese phosphate obtained in Example 4 and Comparative Examples 1 to 3 of the present invention;

[0036] Among them, Figure 3 (a) is the infrared spectrum of lithium iron manganese phosphate obtained in Example 4 and Comparative Examples 1 to 3 in the range of 400 - 1400 cm -1 , Figure 3 (b) is the infrared spectrum of lithium iron manganese phosphate obtained in Example 4 and Comparative Examples 1 to 3 in the range of 800 - 1200 cm -1 .

[0037] Figure 4 Electrochemical cycling performance curves of vanadium, titanium and fluorine co-doped lithium iron manganese phosphate prepared in Examples 1 to 6 of the present invention;

[0038] Figure 5 Electrochemical cycling performance curves of lithium iron manganese phosphate prepared in Example 4 and Comparative Examples 1 to 3 of the present invention;

[0039] Figure 6 Rate performance curves of vanadium, titanium and fluorine co-doped lithium iron manganese phosphate prepared in Examples 1 to 6;

[0040] Figure 7 Rate performance curves of lithium iron manganese phosphate prepared in Example 4 and Comparative Examples 1 to 3 of the present invention;

[0041] Figure 8 GITT test curves and lithium ion diffusion coefficient diagrams of lithium iron manganese phosphate prepared in Comparative Examples 1 to 2 and Examples 3 to 5 of the present invention;

[0042] Among them, Figure 8 (a) is the GITT test curve of lithium iron manganese phosphate prepared in Comparative Example 1, Figure 8 (b) is the GITT test curve of lithium iron manganese phosphate prepared in Comparative Example 2, Figure 8 (c) is the GITT test curve of vanadium-titanium-fluorine co-doped lithium iron manganese phosphate prepared in Example 3, Figure 8 (d) is the GITT test curve of vanadium-titanium-fluorine co-doped lithium iron manganese phosphate prepared in Example 4, Figure 8 (e) is the GITT test curve of vanadium-titanium-fluorine co-doped lithium iron manganese phosphate prepared in Example 3. Detailed implementation mode

[0043] Next, the technical solutions of the present invention will be clearly and completely described in conjunction with the accompanying drawings of the specification. Obviously, the described implementation modes are only part of the implementation modes of the present invention, rather than all implementation modes. Based on the implementation modes in the present invention, all other implementation modes obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0044] Moreover, the technical solutions between various implementation modes of the present invention can be combined with each other, but it must be based on the ability of those of ordinary skill in the art to implement. When the combination of technical solutions results in contradictions or cannot be implemented, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0045] For those not specified in the examples, they are carried out according to conventional conditions or the conditions recommended by the manufacturer. For reagents or instruments not specified in the manufacturer, they are all conventional products that can be obtained through market purchase.

[0046] Unless otherwise defined, all technical and scientific terms used in the present invention are the same as those understood by those of ordinary skill in the art of the present technology and the description of the present invention. Any methods, devices, and materials similar or equivalent to the existing technology described in the embodiments of the present invention can also be used to implement the present invention.

[0047] Example 1

[0048] This example provides a cathode material of lithium iron manganese phosphate modified by co-doping of anions and cations, which is doped and modified with V, Ti, and F, and its chemical formula is Li(Mn 0.6 Fe 0.4 ) 1-x-y V x Ti y PO 4 F z , where x = 0.35%, y = 0.5%, z = 0.25%, and the specific preparation process is as follows:

[0049] 1) Dissolve 0.20 g of ascorbic acid, 2.03 g of manganese sulfate, and 2.23 g of ferrous sulfate in 30 mL of pure water medium to obtain solution A;

[0050] 2) Prepare 3.69 g of ammonium oxalate into a 270 mL solution and transfer it to a three-necked flask;

[0051] 3) Slowly drip solution A into the three-necked flask containing dissolved ammonium oxalate through a constant flow pump, while introducing a protective atmosphere of Ar 2 Prevent the oxidation of low-valent metal ions. The whole process is heated in an oil bath at 40 °C for more than 6 h. After centrifugation and drying, manganese iron oxalate Mn 0.6 Fe 0.4 C 2 O 4 ;

[0052] 4) Weigh ammonium metavanadate, titanium dioxide, lithium fluoride corresponding to the doping amount according to the stoichiometric coefficients of the chemical formula, mix them with 0.74 g of lithium carbonate, 2.30 g of ammonium dihydrogen phosphate, 0.26 g of glucose, manganese iron oxalate, and small balls of different particle sizes, put them into a vacuum ball mill tank, add 10 mL of ethanol as the ball milling medium, use a planetary ball mill, ball mill for 8 h, and the ball milling speed is 450 r / min. After the ball milling is completed, obtain the lithium iron manganese phosphate precursor through drying and sieving in sequence;

[0053] 5) Carry out two-stage calcination on the obtained precursor in a protective atmosphere of Ar 2 The process is as follows: heat from room temperature to 425 °C at a rate of 5 °C / min, hold for 4 h for the first-stage calcination, then heat to 650 °C at a rate of 5 °C / min, hold for 6 h for the second-stage calcination. After the calcination is completed, let it react with the furnace to room temperature to obtain the modified lithium iron manganese phosphate cathode material.

[0054] To verify the electrochemical performance of the above cathode material, the present invention also dissolves it with a binder and a conductive agent in N-methylpyrrolidone to prepare a slurry, coats it on an aluminum foil, and after vacuum drying, assembles it as a battery cathode to form a lithium-ion battery, and tests its electrochemical performance. The test results show that the lithium-ion battery has an initial capacity of 143.4 mAh / g at 0.1 C, and the initial discharge specific capacity at 1 C reaches 125.4 mAh / g. After 200 cycles, the capacity decays to 118.3 mAh / g, and the retention rate is 94%.

[0055] Example 2

[0056] This example is exactly the same as Example 1, except that: the addition amount of lithium fluoride is different, z = 0.5%.

[0057] To verify the electrochemical performance of the above-mentioned cathode material, the present invention also dissolves it together with a binder and a conductive agent in N-methylpyrrolidone to prepare a slurry, which is coated on an aluminum foil. After vacuum drying, it is used as the cathode of a battery to assemble a lithium-ion battery, and its electrochemical performance is tested. The test results show that the first-cycle capacity of this lithium-ion battery is 139.4 mAh / g at 0.1 C, the first-cycle discharge specific capacity reaches 125.6 mAh / g at 1 C, and the capacity decays to 116.8 mAh / g after 300 cycles, with a retention rate of 93%.

[0058] Example 3

[0059] This example is exactly the same as Example 1, except that the addition amount of lithium fluoride is different, and z = 0.75%.

[0060] To verify the electrochemical performance of the above-mentioned cathode material, the present invention also dissolves it together with a binder and a conductive agent in N-methylpyrrolidone to prepare a slurry, which is coated on an aluminum foil. After vacuum drying, it is used as the cathode of a battery to assemble a lithium-ion battery, and its electrochemical performance is tested. The test results show that the first-cycle capacity of this lithium-ion battery is 162.2 mAh / g at 0.1 C, the first-cycle discharge specific capacity reaches 141.7 mAh / g at 1 C, and the capacity decays to 134.6 mAh / g after 200 cycles, with a retention rate of 95%.

[0061] Example 4

[0062] This example is exactly the same as Example 1, except that the addition amount of lithium fluoride is different, and z = 0.875%.

[0063] To verify the electrochemical performance of the above-mentioned cathode material, the present invention also dissolves it together with a binder and a conductive agent in N-methylpyrrolidone to prepare a slurry, which is coated on an aluminum foil. After vacuum drying, it is used as the cathode of a battery to assemble a lithium-ion battery, and its electrochemical performance is tested. The test results show that the first-cycle capacity of this lithium-ion battery is 168.2 mAh / g at 0.1 C, the first-cycle discharge specific capacity reaches 147.8 mAh / g at 1 C, and the capacity remains at 146 mAh / g after 200 cycles, with a retention rate of 99%.

[0064] Example 5

[0065] This example is exactly the same as Example 1, except that the addition amount of lithium fluoride is different, and z = 1%.

[0066] To verify the electrochemical performance of the above-mentioned cathode material, the present invention also dissolves it with a binder and a conductive agent in N-methylpyrrolidone to prepare a slurry, coats the slurry on an aluminum foil, and after vacuum drying, uses it as the cathode of a battery to assemble a lithium-ion battery, and tests its electrochemical performance. The test results show that the lithium-ion battery has an initial capacity of 157.7 mAh / g at 0.1 C, an initial discharge specific capacity of 140.5 mAh / g at 1 C, and after 200 cycles, the capacity decays to 131.8 mAh / g, with a retention rate of 94%.

[0067] Example 6

[0068] This example is exactly the same as Example 1, and the difference lies only in that: the addition amount of lithium fluoride is different, z = 1.15%.

[0069] To verify the electrochemical performance of the above-mentioned cathode material, the present invention also dissolves it with a binder and a conductive agent in N-methylpyrrolidone to prepare a slurry, coats the slurry on an aluminum foil, and after vacuum drying, uses it as the cathode of a battery to assemble a lithium-ion battery, and tests its electrochemical performance. The test results show that the lithium-ion battery has an initial capacity of 157.7 mAh / g at 0.1 C, an initial discharge specific capacity of 140.5 mAh / g at 1 C, and after 200 cycles, the capacity decays to 128.7 mAh / g, with a retention rate of 92%.

[0070] Comparative Example 1

[0071] This comparative example is exactly the same as Example 1, and the difference lies in that: no doping and modification elements are used, and its chemical formula is Li(Mn 0.6 Fe 0.4 )PO 4 .

[0072] To verify the electrochemical performance of the above-mentioned cathode material, the present invention also dissolves it with a binder and a conductive agent in N-methylpyrrolidone to prepare a slurry, coats the slurry on an aluminum foil, and after vacuum drying, uses it as the cathode of a battery to assemble a lithium-ion battery, and tests its electrochemical performance. The test results show that the lithium-ion battery has an initial capacity of 158.9 mAh / g at 0.1 C, an initial capacity of 127.7 mAh / g at 1 C, and after 200 cycles, the capacity decays to 116.2 mAh / g, with a retention rate of 91%.

[0073] Comparative Example 2

[0074] This comparative example is exactly the same as Example 1, and the difference lies in that: only V modification is used, and its chemical formula is Li(Mn 0.6 Fe 0.4 ) 1-x V x PO 4 (x = 0.35%).

[0075] To verify the electrochemical performance of the above-mentioned cathode material, the present invention also dissolves it together with a binder and a conductive agent in N-methylpyrrolidone to prepare a slurry, which is coated on an aluminum foil. After vacuum drying, it is used as the cathode of the battery to assemble a lithium-ion battery, and its electrochemical performance is tested. The test results show that the first-cycle capacity of this lithium-ion battery is 165.7 mAh / g at 0.1 C, the first-cycle discharge specific capacity reaches 138.9 mAh / g at 1 C, and the capacity decays to 127.4 mAh / g after 200 cycles, with a retention rate of 91%.

[0076] Comparative Example 3

[0077] This comparative example is exactly the same as Example 1, except that: This comparative example is exactly the same as Example 1, except that: only V and Ti are used for modification, and its chemical formula is Li(Mn 0.6 Fe 0.4 ) 1-x-y V x Ti y PO 4 (x = 0.35%, y = 0.5%).

[0078] To verify the electrochemical performance of the above-mentioned cathode material, the present invention also dissolves it together with a binder and a conductive agent in N-methylpyrrolidone to prepare a slurry, which is coated on an aluminum foil. After vacuum drying, it is used as the cathode of the battery to assemble a lithium-ion battery, and its electrochemical performance is tested. The test results show that the first-cycle capacity of this lithium-ion battery is 163.1 mAh / g at 0.1 C, the first-cycle discharge specific capacity reaches 143.3 mAh / g at 1 C, and the capacity decays to 130.2 mAh / g after 200 cycles, with a retention rate of 90%.

[0079] To better illustrate the beneficial technical effects of the technical solutions provided by the present invention, the present invention also conducts a series of tests and characterizations on the above-mentioned examples and comparative examples. The specific results are as follows:

[0080] Figure 1 For the lithium iron phosphate (LiMn 0.6 Fe 0.4 PO 4 ) and vanadium-doped lithium iron phosphate material (Li(Mn 0.6 Fe 0.4 ) 1-x V x PO 4 ) prepared in Comparative Examples 1 and 2 of the present invention, the XRD distribution diagrams are shown. It can be seen from the images that both can correspond one by one to the standard cards, proving that the lattice of lithium iron phosphate is less affected by trace ion doping. V 3+Compared with the unmodified material, some main peaks of the modified lithium iron manganese phosphate shift towards smaller angles, indicating that vanadium has successfully entered the lattice of the lithium iron manganese phosphate cathode material, increasing the lattice defects inside the material.

[0081] Figure 2 (a) and Figure 2 (b) are the morphology diagrams of ammonium manganese iron oxalate synthesized by the oxalate coprecipitation method in Example 4 at different magnification factors. It can be seen from the figure that it is cubic block-shaped with uniform size and regular morphology, and the particle size is about 10 μm. After magnification, it can be seen that the block-shaped morphology is formed by stacking square flake-like particles; Figure 2 (c) and Figure 2 (d) are the morphologies of the lithium iron manganese phosphate material obtained in Example 4 at different magnification factors. It is a spherical morphology with uneven particle size formed by the aggregation of irregular small particles, and the particle size is about 50 - 100 μm. This is mainly because a high-temperature ball milling reaction is required during the preparation process, resulting in the cubic block morphology formed by the stacking of precursors being ground and damaged, and then re-aggregating to form irregular spherical particles; Figure 2 (e) is the modified lithium iron manganese phosphate cathode material obtained in Example 4. After being modified by the synergistic doping of vanadium, titanium and fluorine, the lithium iron manganese phosphate is transformed into more regular spherical particles with a particle size of about 50 μm and a more uniform morphology.

[0082] Figure 3 (a) is the infrared spectrum (FT-IR) of the vanadium, titanium and fluorine co-doped lithium iron manganese phosphate prepared in Example 4 of the present invention and the lithium iron manganese phosphate prepared in Comparative Examples 1 - 3 in the range of 400 - 1400 cm -1 . Since the PO 4 tetrahedron shares points or edges with the FeO 6 and LiO 6 octahedron, the vibration of the P-O bond is related to the surrounding lithium ions and iron ions to a certain extent. Therefore, the vibration of the absorption band near 1000 cm -1 can be used to characterize the anti-site defect concentration of olivine structure materials. Compared with Comparative Example 1, the two groups of samples in Comparative Example 2 and Comparative Example 3 show that the stretching vibrations of the P-O, Fe-O, and Mn-O bonds in the ranges of 900 - 1200 cm -1 , 620 - 650 cm -1 , and 550 - 580 cm -1 shift towards smaller wavenumbers, the binding of Fe 2+ increases, the anti-site defect concentration of Li Fe -Fe Li decreases, and the discharge specific capacity of the corresponding materials shows a slight increase. Figure 3 (b) is the vanadium, titanium and fluorine co-doped lithium iron manganese phosphate prepared in Example 4 and the lithium iron manganese phosphate prepared in Comparative Examples 1 - 3 in the range of 700 - 1200 cm-1 The infrared spectra (FT-IR) showed that compared with Comparative Examples 1 to 3, there was a stretching vibration of C-F at 1068 cm -1 in Example 4, indicating that the addition of F - effectively modified the surface carbon layer.

[0083] Figure 4 The electrochemical cycling performance curves of the lithium iron manganese phosphate (Li(Mn 0.6 Fe 0.4 )) 1-x-y V x Ti y PO 4 F z co-doped and modified in Examples 1 to 6 at 1 C are shown. It can be seen from the images that the electrochemical performance of the material shows an inverted saddle shape of first increasing and then decreasing with the doping amount of F - . With the increase of the doping ratio of F - , both the discharge specific capacity and the cycling stability of the obtained cathode material are improved. When the doping ratio of F - Z = 0.875%, the electrochemical performance of the material is optimal. Further increasing the doping ratio of F - instead leads to a decline in the electrochemical performance of the material, indicating that the doping amount of the modifying element must be within the range required by the present invention, and neither too high nor too low can achieve the corresponding technical effects.

[0084] Figure 5 The electrochemical cycling performance curves of the materials obtained in Example 4 and Comparative Examples 1 to 3 at 1 C are shown. It can be seen from the figure that the electrochemical performance of the material obtained in Comparative Example 1 is the worst. In Comparative Example 2, V element was doped compared with Comparative Example 1, and its discharge specific capacity increased slightly, but compared with Comparative Example 3 and Example 4, its discharge specific capacity was still relatively low, and the reversible discharge specific capacity at 1 C was only 130 mA / g. Therefore, on this basis, further co-doping modification of V 3+ , Ti 4+ was carried out. The Li(Mn 0.6 Fe 0.4 ) 1-x-y V x Ti y PO 4 prepared in Comparative Example 3 (x = 0.35%, y = 0.5%) had an initial capacity of 143.3 mAh / g at 1 C, and the capacity decayed to 130.2 mAh / g after 200 cycles, with a retention rate of 90%. Although the discharge specific capacity increased to a certain extent, due to the possible presence of TiO 4+ Li 2、 TiO x TiO yImpurity phases, and due to the influence of high-valent cation doping on the material structure, resulting in a slight decrease in its cycling stability; to solve this problem, this patent selects F - as the third element to co-modify the surface carbon layer, and inhibits Mn 2+ and Fe 2+ dissolution through the formation of C-F bonds, alleviates the influence of lattice distortion caused by high-valent cation doping such as V 3+ and Ti 4+ on the cycling stability of the material. The Li(Mn 0.6 Fe 0.4 ) 1-x-y V x Ti y PO 4 F z prepared in Example 4 (x = 0.35%, y = 0.5%, z = 0.875%) has an initial capacity of 147.8 mAh / g at 1C, a capacity of 146 mAh / g after 200 cycles, a retention rate of 99%, and continues to cycle to 300 cycles and then the capacity decays to 141.4 mAh / g, with a retention rate above 95%. Finally, under the co-modification of V 3+ and Ti 4+ and F - , a lithium iron phosphate cathode material with simultaneously improved capacity and cycling performance is obtained.

[0085] Figure 6 are the rate performance curves of the materials obtained in Examples 1 to 6. The discharge specific capacity of Example 4 is close to 162.5 mAh / g at 0.1C, which is consistent with the cycling test results, and can also maintain a capacity close to 130 mAh / g at 5C. When the doping amount decreases or increases, the discharge specific capacity of the material at different rates also decreases to a certain extent, but at high current densities of 1C and 5C, the capacity remains above 120 mAh / g and 100 mAh / g, showing that the materials provided by the present invention not only have excellent cycling stability but also excellent rate performance.

[0086] Figure 7 are the rate performance curves of the materials obtained in Example 4 and Comparative Examples 1 to 3. The lithium iron phosphate material prepared in Comparative Example 1 has a discharge specific capacity of only 120 mAh / g at 1C, and the capacity decays to 100 mAh / g at a high rate of 5C. As the number of doping element types increases, the rate performance of the battery gradually improves. After co-doping modification with V 3+ and Ti 4+ and F - , the lithium iron phosphate cathode material can also maintain a discharge specific capacity of 130 mAh / g at a high current density of 5C, which is consistent with the results of the electrochemical cycling performance test, and the rate performance has been greatly improved.

[0087] Figure 8 For Comparative Examples 1-2, the GITT test curves and lithium ion diffusion coefficients of Examples 3-5. Figure 8 (a) is the GITT test curve of lithium iron manganese phosphate prepared in Comparative Example 1. During the discharge process, the lithium ion diffusion coefficient (D Li + ) is 3.05×10 -15 ; Figure 8 (b) is the GITT test curve of V 3+ -doped lithium iron manganese phosphate prepared in Comparative Example 2. During the discharge process, the lithium ion diffusion coefficient (D Li + ) is 2.74×10 -15 Compared with the lithium iron manganese phosphate cathode material prepared in Comparative Example 1, the lithium ion diffusion coefficient has a small increase, which is mainly due to the substitution of Mn / Fe sites by high-valent cations such as V 3+ , Ti 4+ etc., resulting in cation vacancies due to charge compensation, providing more paths for lithium ion migration, and the D Li + increases, and the corresponding discharge specific capacity also increases to a certain extent; Figure 8 (c) is the GITT test curve of vanadium-titanium-fluorine co-doped lithium iron manganese phosphate prepared in Example 3, where the F - doping amount is 0.75%, and the corresponding lithium ion diffusion coefficient (D Li + ) is 6.35×10 -14 ; Figure 8 (d) is the GITT test curve of vanadium-titanium-fluorine co-doped lithium iron manganese phosphate prepared in Example 4, where the F - doping amount is 0.875%, and the corresponding lithium ion diffusion coefficient (D Li + ) is 5.3×10 -14 , compared with Comparative Examples 1-2, the lithium ion diffusion coefficient of the vanadium-titanium-fluorine co-doped modified lithium iron manganese phosphate cathode material obtained in Examples 3-4 has been greatly improved (about one order of magnitude). Combining FT-IR analysis, it is mainly due to the formation of C-F bonds in the carbon coating layer on the material surface, which improves the interfacial interaction between the electrode and the electrolyte, reduces the interfacial impedance, provides some migration paths for lithium ions in the material, and reduces the energy barrier during migration, ultimately promoting the rapid migration of lithium ions and realizing a large increase in the lithium ion diffusion coefficient; Figure 8 (e) is the GITT test curve of vanadium-titanium-fluorine co-doped lithium iron manganese phosphate prepared in Example 5, where the F - doping amount is 1%, and the corresponding lithium ion diffusion coefficient (D Li + ) is 8.35×10-15 , compared with Examples 3-4, the reduction amplitude is significantly lower, which is consistent with the relevant test results of electrochemical performance, indicating that the lithium iron manganese phosphate cathode material Li(Mn 0.6 Fe 0.4 ) 1-x-y V x Ti y PO 4 F z (x = 0.35%, y = 0.5%, z = 0.875%) has excellent electrochemical performance.

Claims

1. An anion-cation co-doped modified lithium manganese iron phosphate positive electrode material, characterized in that: include: The chemical formula of the positive electrode material is Li(Mn 0.6 Fe 0.4 ) 1-x (M x N y )PO4, wherein M is a modified cation and N is a modified anion; the modified cation element is at least two of V, Ti, Co, Ni, Cu and Zn; the modified anion element is at least one of the halogens; and the ratio of the stoichiometric numbers x to y of the modified cation and the modified anion is 1:1~2.

2. The anion-cation co-doped modified lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: The modified cation elements are V and Ti; the modified anion elements are F and / or Cl.

3. The anion-cation co-doped modified lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: The stoichiometric number x of the modified cation ranges from 0.2 to 0.5; the ratio of x to y is 1:1.3 to 1.

6.

4. The method for preparing a cation-anion co-doped modified lithium manganese iron phosphate positive electrode material according to any one of claims 1 to 3, characterized in that: A soluble reducing agent, an iron source and a manganese source are uniformly dropped into an oxalate precipitant for co-precipitation reaction to obtain ferromanganese oxalate; modified raw materials including a lithium source, a phosphorus source, a modified cation source and a modified anion source and ferromanganese oxalate are mechanically activated together and then calcined to obtain the product.

5. The method for preparing an anion-cation co-doped modified lithium manganese iron phosphate positive electrode material according to claim 4, characterized in that: The soluble reducing agent is at least one of sugars, vitamins, sodium borohydride and potassium borohydride; the iron source is at least one of organic ferrous salt and inorganic ferrous salt; the manganese source is manganese phosphate and / or manganese sulfate.

6. The method for preparing an anion-cation co-doped modified lithium manganese iron phosphate positive electrode material according to claim 4, characterized in that: The modified raw material also contains a carbon supplement, and the amount of the carbon supplement, calculated by the mass of the C element, is 2-10wt% of the sum of the mass of the modified raw material and ferromanganese oxalate.

7. The method for preparing an anion-cation co-doped modified lithium manganese iron phosphate positive electrode material according to claim 4, characterized in that: When the modified cation element is V and Ti, the modified cation source is ammonium metavanadate and titanium dioxide; when the modified anion element is F, the modified anion source is lithium fluoride.

8. The method for preparing an anion-cation co-doped modified lithium manganese iron phosphate positive electrode material according to claim 4, characterized in that: The conditions of the coprecipitation reaction are: temperature of 20-100°C and time of 2-24h; the conditions of wet ball milling by mechanical activation are: using zirconium oxide as grinding balls, ball milling medium of water and / or ethanol, ball-to-material ratio of 5:1-10:1, rotation speed of 200-400rpm / min, and ball milling time of 6-12h.

9. The method for preparing an anion-cation co-doped modified lithium manganese iron phosphate positive electrode material according to claim 4, characterized in that: The calcination treatment method is a two-stage roasting, and the process is: heating from room temperature to 400-500°C at 3-10°C / min, keeping warm for 2-6 hours for a first-stage roasting, and then heating to 600-800°C at 3-10°C / min, keeping warm for 4-10 hours for a second-stage roasting, and cooling to room temperature with the furnace after the roasting is completed.

10. The use of an anion-cation co-doped modified lithium manganese iron phosphate positive electrode material according to any one of claims 1 to 3, characterized in that: Used to prepare the positive electrode of lithium-ion batteries.

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