A nitrogen-sulfur co-doped lithium iron manganese phosphate positive electrode material and a preparation process thereof

Abstract

The present application relates to the field of lithium manganese iron phosphate positive electrode material, and particularly to a nitrogen and sulfur co-doped lithium manganese iron phosphate positive electrode material and a preparation process thereof, which is used to solve the problems of manganese ion elution and extremely low electronic conductivity; the nitrogen and sulfur co-doped lithium manganese iron phosphate positive electrode material is obtained by directly adding the nitrogen and sulfur doped lithium manganese iron phosphate powder into a graphene-nano zirconium dioxide composite dispersion liquid, then in-situ adding aniline and initiating polymerization, so that polyaniline is directly generated on the surface of lithium manganese iron phosphate particles and between graphene layers; the polyaniline in the preparation process can improve the conductivity; the zirconium dioxide can inhibit manganese elution; the graphene provides a two-dimensional support structure to enhance mechanical stability; N and S doping synergistically improves the conductivity and ion diffusion rate of the lithium manganese iron phosphate positive electrode material; and the prepared nitrogen and sulfur co-doped lithium manganese iron phosphate positive electrode material has a wide application prospect in new energy automobile power batteries and portable electronic device power supplies.

Description

Technical Field

[0001] This invention relates to the field of lithium manganese iron phosphate cathode materials, specifically to a nitrogen-sulfur co-doped lithium manganese iron phosphate cathode material and its preparation process. Background Technology

[0002] Lithium manganese iron phosphate (LFP) has become an important research direction for lithium-ion battery cathode materials due to its high theoretical specific capacity, moderate operating voltage, low cost, and good safety. However, LFP has two major drawbacks: firstly, Mn... 3+ / Mn 2+ The significant Jahn-Teller effect of redox couples leads to lattice distortion during charge and discharge, severely impacting the material's cycle stability and causing rapid capacity decay after multiple charge-discharge cycles. Secondly, lithium manganese iron phosphate (LFP) exhibits extremely low intrinsic conductivity and a low lithium-ion diffusion coefficient, resulting in severe polarization during high-rate charge-discharge cycles and a significant decrease in energy conversion efficiency, failing to meet the demands of fast-charging applications. These two major drawbacks limit the large-scale commercial application of LFP in lithium-ion batteries, necessitating breakthroughs in technology through material modification and other methods.

[0003] Therefore, developing a modification technology for lithium manganese iron phosphate that can synergistically improve conductivity, suppress manganese leaching, and enhance cycle stability has become an urgent problem to be solved in this field. Summary of the Invention

[0004] In order to overcome the above-mentioned technical problems, the purpose of this invention is to provide a nitrogen-sulfur co-doped manganese iron phosphate cathode material and its preparation process.

[0005] The objective of this invention can be achieved through the following technical solutions:

[0006] In a first aspect, this application provides a nitrogen-sulfur co-doped lithium manganese iron phosphate cathode material, comprising the following raw materials in parts by weight:

[0007] 9.4-12.5 parts of nitrogen / sulfur-doped lithium manganese iron phosphate powder, 50-70 mL of graphene nanosheet-nanozirconium dioxide composite dispersion, 10-14 parts of hydrochloric acid, 0.5-0.7 parts of aniline, and 10-14 parts of ammonium persulfate solution;

[0008] The ammonium persulfate solution is a mixture of ammonium persulfate and deionized water in a ratio of 1.23 g: 10 mL; the concentration of the hydrochloric acid is 1 mol / L.

[0009] The graphene nanosheet-zirconia nanocomposite dispersion is prepared by the following steps:

[0010] Step a1: Add graphene nanosheets, concentrated sulfuric acid, and concentrated nitric acid to a beaker and ultrasonically disperse them for 30-35 minutes at a power of 380-400W. Then transfer the mixture to a constant temperature water bath at 45-50℃ and magnetically stir for 2-3 hours. After that, pour the mixture into ice water and then vacuum filter it. Wash the filter cake with distilled water until the pH of the filtrate is 6-7 to remove residual acid. Then disperse the filtrate in deionized water and ultrasonically disperse it for 20-25 minutes to obtain a graphene nanosheet dispersion.

[0011] Step a2: Add nano-zirconia to the graphene nanosheet dispersion and ultrasonically disperse for 60-65 minutes at a power of 450-500W. Use intermittent ultrasonication, working for 30 seconds and pausing for 10 seconds each time. Then transfer to a constant temperature stirrer and magnetically stir for 30-35 minutes at room temperature to obtain a graphene nanosheet-nano-zirconia composite dispersion.

[0012] In a preferred embodiment of the present invention, the ratio of graphene nanosheets, concentrated sulfuric acid, concentrated nitric acid and deionized water used in step a1 is 0.1-0.3g: 30-90mL: 10-30mL: 50-150mL.

[0013] In a preferred embodiment of the present invention, the graphene nanosheets in step a1 have a thickness of 6-8 nm and a length of 25 μm; the concentrated sulfuric acid has a mass fraction of 98%; and the concentrated nitric acid has a mass fraction of 68%.

[0014] In a preferred embodiment of the present invention, the ratio of the nano-zirconia and graphene nanosheet dispersion in step a2 is 0.2-0.4g: 50-100mL.

[0015] In a preferred embodiment of the present invention, the nano-zirconia particles in step a2 have a particle size of 20-50 nm.

[0016] The nitrogen / sulfur-doped lithium manganese iron phosphate powder is prepared by the following steps:

[0017] Lithium carbonate, manganese acetate, ferrous oxalate, ammonium dihydrogen phosphate, urea, thiourea, and anhydrous ethanol were added to an agate ball mill jar at a ball-to-material ratio of 8:1 and ball-milled at 280-300 r / min for 4-5 hours. After ball milling, the slurry was transferred to an evaporating dish and vacuum dried at 55-60℃ for 8-9 hours. The slurry was then ground through a 100-mesh sieve and placed in a tube furnace. Argon gas was introduced at a flow rate of 50 mL / min for 30 minutes. The temperature was first increased to 480-500℃ at a rate of 5℃ / min and held for 2-3 hours. Then, the temperature was increased to 700-720℃ at a rate of 3℃ / min and held for 8-9 hours. After sintering, the slurry was kept in an argon atmosphere and allowed to cool naturally to room temperature. Finally, it was ground in an agate mortar and passed through a 200-mesh sieve to obtain nitrogen / sulfur-doped manganese iron phosphate powder.

[0018] In a preferred embodiment of the present invention, the ratio of lithium carbonate, manganese acetate, ferrous oxalate, ammonium dihydrogen phosphate, urea, thiourea, and anhydrous ethanol is 0.372-0.380g: 1.231-1.251g: 0.090-0.095g: 0.115-0.120g: 0.048-0.050g: 0.060-0.062g: 10-20mL.

[0019] Secondly, this application provides a process for preparing a nitrogen-sulfur co-doped lithium manganese iron phosphate cathode material, comprising the following steps:

[0020] Nitrogen / sulfur-doped lithium manganese iron phosphate powder was added to a graphene nanosheet-zirconia composite dispersion and ultrasonically dispersed for 30-35 minutes at a power of 380-400W, using intermittent ultrasonication with a 2-minute pause every 10 minutes. The dispersion was then transferred to a three-necked flask equipped with a stirrer, thermometer, gas delivery tube, and constant-pressure dropping funnel. Hydrochloric acid was added, and nitrogen gas was introduced for protection. The mixture was stirred at 25-30℃ and 300-400 rpm for 10-12 minutes. Aniline was then added dropwise while stirring, controlling the dropping rate. The rate of addition is 1-2 drops / s. After the addition is complete, the mixture is stirred for 25-30 minutes. Then, ammonium persulfate solution is added dropwise at a rate of 1-2 drops / s. After the addition is complete, the mixture is stirred for 6-7 hours. After the reaction is complete, the reaction product is centrifuged. The precipitate is washed 4-6 times with anhydrous ethanol and deionized water by alternating centrifugation at a speed of 8000 r / min for 10 minutes each time. Then, it is placed in a vacuum drying oven and dried at a temperature of 45-50℃ for 12-13 hours. Finally, it is ground through a 200-mesh sieve to obtain nitrogen-sulfur co-doped manganese iron phosphate cathode material.

[0021] The beneficial effects of this invention are:

[0022] This invention discloses a nitrogen-sulfur co-doped lithium manganese iron phosphate cathode material and its preparation process. The process involves directly adding nitrogen / sulfur-doped lithium manganese iron phosphate powder to a graphene-nanozirconium dioxide composite dispersion, followed by in-situ dropwise addition of aniline to initiate polymerization. This allows polyaniline to be directly generated on the surface of the lithium manganese iron phosphate particles and between the graphene sheets, forming a hierarchical structure with lithium manganese iron phosphate as the core, zirconium dioxide as the adsorption layer, and polyaniline as the coating layer. Graphene is connected through the conjugated structure of polyaniline to form a continuous conductive network, ultimately yielding the nitrogen-sulfur co-doped lithium manganese iron phosphate cathode material. In this preparation process, polyaniline can fill voids through conjugated π bonds, synergistically constructing a continuous conductive network with graphene to improve conductivity. Zirconium dioxide can inhibit manganese leaching; graphene provides a two-dimensional support structure to enhance mechanical stability; and N and S doping synergistically improves the conductivity and ion diffusion rate of the lithium manganese iron phosphate cathode material. The prepared nitrogen-sulfur co-doped lithium manganese iron phosphate cathode material has broad application prospects in new energy vehicle power batteries and portable electronic device power supplies, and can promote the sustainable development of related industries.

[0023] N and S atoms are primarily doped by replacing O atoms in the olivine lattice of lithium manganese iron phosphate, forming doped energy levels. Simultaneously, a small number of N and S atoms that do not fully enter the lattice can form a doped amorphous layer on the particle surface. The lone pair electrons of N and the 3p orbitals of S can provide additional charge carriers. The radii of N and S atoms are slightly larger than those of O, and doping slightly increases the olivine lattice parameters of lithium manganese iron phosphate, reducing the Li-type doping efficiency. + The diffusion activation energy and the diffusion activation energy work together to enhance conductivity and ion diffusion rate.

[0024] The conjugated π-bond structure of polyaniline can fill the gaps between nitrogen / sulfur-doped lithium manganese iron phosphate and graphene, forming a conductive pathway of "nitrogen / sulfur-doped lithium manganese iron phosphate-polyaniline-graphene," solving the problem of conductive dead zones caused by graphene agglomeration and improving the overall conductivity uniformity of the material. The flexible polyaniline molecular chain can undergo elastic deformation during charge / discharge with lattice expansion / contraction, absorbing stress. Simultaneously, the amino groups of polyaniline can form hydrogen bonds with the hydroxyl groups on the surface of nitrogen / sulfur-doped lithium manganese iron phosphate, enhancing the adhesion between the coating layer and the substrate and preventing coating layer detachment during cycling. Furthermore, the protonated amino groups of polyaniline can react with dissolved Mn... 2+ Electrostatic adsorption occurs, and Mn 2+ It is fixed within the coating layer, reducing its migration into the electrolyte and assisting zirconium dioxide in improving its anti-dissolution effect.

[0025] Zirconia particles are ultrasonically adsorbed onto the surface of lithium manganese iron phosphate, forming a uniform nanoscale physical barrier layer that effectively prevents Mn from entering the lithium iron phosphate plant. 2+ Through penetration, oxygen vacancies in zirconium dioxide can interact with Mn. 2+ Forming stable Zr-O-Mn bonds, Mn 2+Anchored at the coating interface, reducing manganese leaching; the high hardness and chemical inertness of zirconium dioxide can protect the nitrogen / sulfur-doped manganese iron phosphate matrix from electrolyte corrosion, preventing the collapse of the olivine structure due to HF etching; zirconium dioxide nanoparticles can fill the pores of polyaniline, reducing electrolyte penetration and extending the material's cycle life.

[0026] Graphene's high thermal conductivity can quickly disperse the Joule heat generated during charging and discharging, thereby reducing the battery's operating temperature and improving safety. Graphene's sheet structure can encapsulate nitrogen / sulfur-doped manganese iron phosphate particles, reducing particle agglomeration. Graphene's mechanical strength can enhance the tensile strength of the coating layer, preventing cracking of the coating layer due to volume changes during cycling. Detailed Implementation

[0027] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0028] Example 1:

[0029] This embodiment describes a preparation process for a nitrogen-sulfur co-doped lithium manganese iron phosphate cathode material, including the following steps:

[0030] Step s1: Add 0.1g of graphene nanosheets (6nm thick and 25μm long), 30mL of 98% concentrated sulfuric acid and 10mL of 68% concentrated nitric acid to a beaker. Sonicate the mixture for 30min at 380W. Then transfer it to a 45℃ constant temperature water bath and stir magnetically for 2h. Pour the mixture into ice water and then vacuum filter it. Wash the filter cake with distilled water until the pH of the filtrate is 6 to remove residual acid. Then disperse the filtrate in 50mL of deionized water and sonicate for 20-25min to obtain a graphene nanosheet dispersion.

[0031] Step s2: Add 0.2g of nano-zirconia (the particle size of nano-zirconia is 20nm) to 50mL of graphene nanosheet dispersion, and ultrasonically disperse for 60min at a power of 450W. Use intermittent ultrasonication, working for 30s and pausing for 10s each time. Then transfer to a constant temperature stirrer and magnetically stir for 30-35min at room temperature to obtain graphene nanosheet-nano-zirconia composite dispersion.

[0032] Step s3: Add 0.372g lithium carbonate, 1.231g manganese acetate, 0.090g ferrous oxalate, 0.115g ammonium dihydrogen phosphate, 0.048g urea, 0.060g thiourea, and 10mL anhydrous ethanol to an agate ball mill jar. The ball-to-material ratio is 8:1. The mixture is ball-milled at 280r / min for 4h. After ball milling, the slurry is transferred to an evaporating dish and vacuum-dried at 55℃ for 8h. The mixture is then ground through a 100-mesh sieve and placed in a tube furnace. Argon gas is introduced at a flow rate of 50mL / min for 30min. The temperature is first increased to 480℃ at a rate of 5℃ / min and held for 2h. Then, the temperature is increased to 700℃ at a rate of 3℃ / min and held for 8h. After sintering, the mixture is kept in an argon atmosphere and allowed to cool naturally to room temperature. The mixture is then ground in an agate mortar and passed through a 200-mesh sieve to obtain nitrogen / sulfur-doped manganese iron phosphate powder.

[0033] Step s4: Add 9.4 parts of nitrogen / sulfur-doped lithium manganese iron phosphate powder to 50 parts of graphene nanosheet-nanozirconia composite dispersion. Disperse the mixture ultrasonically for 50 min at 380 W, using intermittent sonication, working for 10 min and pausing for 2 min. Then transfer the mixture to a three-necked flask equipped with a stirrer, thermometer, gas delivery tube, and constant-pressure dropping funnel. Add 10 parts of 1 mol / L hydrochloric acid, purge with nitrogen for protection, and stir for 10 min at 25 °C and a stirring rate of 300 r / min. Then, while stirring, add 0.5 parts of aniline dropwise, controlling the dropping rate to 1. The reaction was carried out at a rate of 1 drop / s. After the addition was complete, the mixture was stirred for 25 min. Then, 10 parts of ammonium persulfate solution (ammonium persulfate solution is a mixture of ammonium persulfate and deionized water in a ratio of 1.23 g: 10 mL) were added drop by drop at a rate of 1 drop / s. After the addition was complete, the mixture was stirred for 6 h. After the reaction was completed, the reaction product was centrifuged. The precipitate was washed 4 times by alternating centrifugation with anhydrous ethanol and deionized water at a speed of 8000 r / min for 10 min each time. Then, it was placed in a vacuum drying oven and dried at 45℃ for 12 h. The product was then ground through a 200-mesh sieve to obtain nitrogen-sulfur co-doped manganese iron phosphate cathode material.

[0034] Example 2:

[0035] This embodiment describes a preparation process for a nitrogen-sulfur co-doped lithium manganese iron phosphate cathode material, including the following steps:

[0036] Step s1: Add 0.2g of graphene nanosheets (7nm thick and 25μm long), 60mL of 98% concentrated sulfuric acid and 20mL of 68% concentrated nitric acid to a beaker. Sonicate the mixture for 33min at 390W. Then transfer it to a 47℃ constant temperature water bath and stir magnetically for 2.5h. Pour the mixture into ice water and then vacuum filter it. Wash the filter cake with distilled water until the pH of the filtrate is 6 to remove residual acid. Then disperse the filtrate in 100mL of deionized water and sonicate for 23min to obtain a graphene nanosheet dispersion.

[0037] Step s2: Add 0.3g of nano-zirconia (the particle size of nano-zirconia is 35nm) to 75mL of graphene nanosheet dispersion, and ultrasonically disperse for 63min at a power of 470W. Use intermittent ultrasonication, working for 30s and pausing for 10s each time. Then transfer to a constant temperature stirrer and magnetically stir for 33min at room temperature to obtain graphene nanosheet-nano-zirconia composite dispersion.

[0038] Step s3: Add 0.376g lithium carbonate, 1.241g manganese acetate, 0.093g ferrous oxalate, 0.117g ammonium dihydrogen phosphate, 0.049g urea, 0.061g thiourea, and 15mL anhydrous ethanol to an agate ball mill jar. At a ball-to-material ratio of 8:1, ball mill at 290 r / min for 4.5 h. After ball milling, transfer the slurry to an evaporating dish and vacuum dry at 57℃ for 8.5 h. Grind... The sample was passed through a 100-mesh sieve and then placed in a tube furnace. Argon gas was introduced at a flow rate of 50 mL / min for 30 min. The temperature was first increased to 490℃ at a heating rate of 5℃ / min and held for 2.5 h. Then the temperature was increased to 710℃ at a heating rate of 3℃ / min and held for 8.5 h. After sintering, the sample was kept in an argon atmosphere and allowed to cool naturally to room temperature. Then it was ground with an agate mortar and passed through a 200-mesh sieve to obtain nitrogen / sulfur-doped manganese iron phosphate powder.

[0039] Step s4: Add 10.5 parts of nitrogen / sulfur-doped lithium manganese iron phosphate powder to 60 parts of graphene nanosheet-nanozirconia composite dispersion. Disperse the mixture ultrasonically for 55 min at a power of 390 W, using intermittent sonication, with each 10 min interval followed by a 2 min pause. Then transfer the mixture to a three-necked flask equipped with a stirrer, thermometer, gas delivery tube, and constant-pressure dropping funnel. Add 12 parts of 1 mol / L hydrochloric acid, purge with nitrogen for protection, and stir for 11 min at 27 °C and a stirring rate of 350 r / min. Then, while stirring, add 0.6 parts of aniline dropwise, controlling the dropping rate to 1. The reaction was carried out at a rate of 1 drop / s. After the addition was complete, the mixture was stirred for 27 min. Then, 12 parts of ammonium persulfate solution (ammonium persulfate solution is a mixture of ammonium persulfate and deionized water in a ratio of 1.23 g: 10 mL) were added dropwise. The dropping rate was controlled at 1 drop / s. After the addition was complete, the mixture was stirred for 6.5 h. After the reaction was completed, the reaction product was centrifuged. The precipitate was washed 5 times with anhydrous ethanol and deionized water by alternating centrifugation at a speed of 8000 r / min for 10 min each time. Then, it was placed in a vacuum drying oven and dried at 47℃ for 12.5 h. The product was then ground through a 200-mesh sieve to obtain nitrogen-sulfur co-doped manganese iron phosphate cathode material.

[0040] Example 3:

[0041] This embodiment describes a preparation process for a nitrogen-sulfur co-doped lithium manganese iron phosphate cathode material, including the following steps:

[0042] Step s1: Add 0.3g of graphene nanosheets (8nm thick and 25μm long), 90mL of 98% concentrated sulfuric acid and 30mL of 68% concentrated nitric acid to a beaker. Sonicate the mixture for 35min at 400W. Then transfer it to a 50℃ constant temperature water bath and stir magnetically for 3h. Pour the mixture into ice water and then vacuum filter it. Wash the filter cake with distilled water until the pH of the filtrate is 7 to remove residual acid. Then disperse the filtrate in 150mL of deionized water and sonicate for 25min to obtain a graphene nanosheet dispersion.

[0043] Step s2: Add 0.4g of nano-zirconia (the particle size of nano-zirconia is 50nm) to 100mL of graphene nanosheet dispersion, and ultrasonically disperse for 65min at a power of 500W. Use intermittent ultrasonication, working for 30s and pausing for 10s each time. Then transfer to a constant temperature stirrer and magnetically stir for 35min at room temperature to obtain graphene nanosheet-nano-zirconia composite dispersion.

[0044] Step s3: Add 0.380g lithium carbonate, 1.251g manganese acetate, 0.095g ferrous oxalate, 0.120g ammonium dihydrogen phosphate, 0.050g urea, 0.062g thiourea, and 20mL anhydrous ethanol to an agate ball mill jar. The ball-to-material ratio is 8:1. The mixture is ball-milled at 300r / min for 5h. After ball milling, the slurry is transferred to an evaporating dish and vacuum-dried at 60℃ for 9h. The mixture is then ground through a 100-mesh sieve and placed in a tube furnace. Argon gas is introduced at a flow rate of 50mL / min for 30min. The temperature is first increased to 500℃ at a rate of 5℃ / min and held for 3h. Then, the temperature is increased to 720℃ at a rate of 3℃ / min and held for 9h. After sintering, the mixture is kept in an argon atmosphere and allowed to cool naturally to room temperature. The mixture is then ground in an agate mortar and passed through a 200-mesh sieve to obtain nitrogen / sulfur-doped manganese iron phosphate powder.

[0045] Step s4: Add 12.5 parts of nitrogen / sulfur-doped lithium manganese iron phosphate powder to 70 parts of graphene nanosheet-nanozirconium dioxide composite dispersion, and ultrasonically disperse for 60 min at a power of 400W, using intermittent ultrasonication, working for 10 min and pausing for 2 min each time. Then transfer to a three-necked flask equipped with a stirrer, thermometer, gas delivery tube and constant pressure dropping funnel, add 14 parts of 1mol / L hydrochloric acid, purge with nitrogen protection, and stir for 12 min at a temperature of 30℃ and a stirring rate of 400 r / min. Then add 0.7 parts of aniline dropwise while stirring, controlling the dropping rate to 2. The reaction was carried out at a rate of 2 drops / s. After the addition was complete, the mixture was stirred for 30 min. Then, 14 parts of ammonium persulfate solution (ammonium persulfate solution is a mixture of ammonium persulfate and deionized water in a ratio of 1.23 g: 10 mL) were added dropwise. The dropping rate was controlled at 2 drops / s. After the addition was complete, the mixture was stirred for 7 h. After the reaction was completed, the reaction product was centrifuged. The precipitate was washed 6 times with anhydrous ethanol and deionized water by alternating centrifugation at a speed of 8000 r / min for 10 min each time. Then, it was placed in a vacuum drying oven and dried at 50℃ for 13 h. The product was then ground through a 200-mesh sieve to obtain nitrogen-sulfur co-doped manganese iron phosphate cathode material.

[0046] Comparative Example 1:

[0047] This comparative example illustrates a preparation process for a coated lithium manganese iron phosphate cathode material, including the following steps:

[0048] Step s1: Add 0.1g of graphene nanosheets (6nm thick and 25μm long), 30mL of 98% concentrated sulfuric acid and 10mL of 68% concentrated nitric acid to a beaker. Sonicate the mixture for 30min at 380W. Then transfer it to a 45℃ constant temperature water bath and stir magnetically for 2h. Pour the mixture into ice water and then vacuum filter it. Wash the filter cake with distilled water until the pH of the filtrate is 6 to remove residual acid. Then disperse the filtrate in 50mL of deionized water and sonicate for 20-25min to obtain a graphene nanosheet dispersion.

[0049] Step s2: Add 0.2g of nano-zirconia (the particle size of nano-zirconia is 20nm) to 50mL of graphene nanosheet dispersion, and ultrasonically disperse for 60min at a power of 450W. Use intermittent ultrasonication, working for 30s and pausing for 10s each time. Then transfer to a constant temperature stirrer and magnetically stir for 30-35min at room temperature to obtain graphene nanosheet-nano-zirconia composite dispersion.

[0050] Step s3: Add 0.372g lithium carbonate, 1.231g manganese acetate, 0.090g ferrous oxalate, 0.115g phosphoric acid, and 10mL anhydrous ethanol to an agate ball mill jar. The ball-to-material ratio is 8:1. The mixture is ball-milled at 280r / min for 4 hours. After ball milling, the slurry is transferred to an evaporating dish and vacuum-dried at 55℃ for 8 hours. The slurry is then ground through a 100-mesh sieve and placed in a tube furnace. Argon gas is introduced at a flow rate of 50mL / min for 30 minutes. The temperature is first increased to 480℃ at a rate of 5℃ / min and held for 2 hours. Then, the temperature is increased to 700℃ at a rate of 3℃ / min and held for 8 hours. After sintering, the mixture is kept in an argon atmosphere and allowed to cool naturally to room temperature. Finally, it is ground in an agate mortar and passed through a 200-mesh sieve to obtain lithium manganese iron phosphate powder.

[0051] Step s4: Add 9.4 parts of lithium manganese iron phosphate powder to 50 parts of graphene nanosheet-nanozirconium dioxide composite dispersion, and ultrasonically disperse for 50 min at a power of 380W, using intermittent ultrasonication, working for 10 min and pausing for 2 min. Then transfer to a three-necked flask equipped with a stirrer, thermometer, gas delivery tube and constant pressure dropping funnel, add 10 parts of 1mol / L hydrochloric acid, purge with nitrogen protection, and stir for 10 min at a temperature of 25℃ and a stirring rate of 300 r / min. Then add 0.5 parts of aniline dropwise while stirring, controlling the dropping rate to 1. The reaction was carried out at a rate of 1 drop / s. After the addition was complete, the mixture was stirred for 25 min. Then, 10 parts of ammonium persulfate solution (ammonium persulfate solution is a mixture of ammonium persulfate and deionized water in a ratio of 1.23 g: 10 mL) were added drop by drop at a rate of 1 drop / s. After the addition was complete, the mixture was stirred for 6 h. After the reaction was completed, the reaction product was centrifuged. The precipitate was washed 4 times by alternating centrifugation with anhydrous ethanol and deionized water at a speed of 8000 r / min for 10 min each time. Then, it was placed in a vacuum drying oven and dried at 45℃ for 12 h. The product was then ground through a 200-mesh sieve to obtain lithium manganese iron phosphate cathode material.

[0052] Comparative Example 2:

[0053] This comparative example illustrates a preparation process for a nitrogen-sulfur co-doped lithium manganese iron phosphate cathode material, including the following steps:

[0054] Step s1: 0.372g lithium carbonate, 1.231g manganese acetate, 0.090g ferrous oxalate, 0.115g ammonium dihydrogen phosphate, 0.048g urea, 0.060g thiourea, and 10mL anhydrous ethanol were added to an agate ball mill jar at a ball-to-material ratio of 8:1 and ball-milled at 280r / min for 4h. After ball milling, the slurry was transferred to an evaporating dish and vacuum dried at 55℃ for 8h. The slurry was then ground through a 100-mesh sieve and placed in a tube furnace. Argon gas was introduced at a flow rate of 50mL / min for 30min. The temperature was first increased to 480℃ at a rate of 5℃ / min and held for 2h. Then, the temperature was increased to 700℃ at a rate of 3℃ / min and held for 8h. After sintering, the argon atmosphere was maintained, and the mixture was allowed to cool naturally to room temperature. The mixture was then ground in an agate mortar and passed through a 200-mesh sieve to obtain nitrogen / sulfur-doped manganese iron phosphate material.

[0055] Comparative Example 3:

[0056] This comparative example illustrates a preparation process for a lithium manganese iron phosphate cathode material, including the following steps:

[0057] Step s1: Add 0.372g lithium carbonate, 1.231g manganese acetate, 0.090g ferrous oxalate, 0.115g phosphoric acid, and 10mL anhydrous ethanol to an agate ball mill jar at a ball-to-material ratio of 8:1 and ball mill at 280r / min for 4h. After ball milling, transfer the slurry to an evaporating dish and vacuum dry at 55℃ for 8h. Grind the slurry through a 100-mesh sieve and then place it in a tube furnace. Purge the argon gas at a flow rate of 50mL / min for 30min. First, heat the slurry to 480℃ at a heating rate of 5℃ / min and hold for 2h. Then, heat the slurry to 700℃ at a heating rate of 3℃ / min and hold for 8h. After sintering, maintain the argon atmosphere and allow it to cool naturally to room temperature. Then grind the slurry in an agate mortar and pass it through a 200-mesh sieve to obtain lithium manganese iron phosphate material.

[0058] Preparation of the positive electrode:

[0059] 5g of polyvinylidene fluoride (CAS No.: 24937-79-9), 10g of conductive carbon black, 85g of cathode material (including nitrogen-sulfur co-doped lithium manganese iron phosphate cathode material in Examples 1-3, lithium manganese iron phosphate cathode material in Comparative Example 1, nitrogen / sulfur doped lithium manganese iron phosphate material in Comparative Example 2, and lithium manganese iron phosphate material in Comparative Example 3) and 150mL of N-methylpyrrolidone were placed in a beaker and mechanically stirred at 500r / min for 4h to form a slurry. The slurry was coated onto aluminum foil with a single-sided thickness of 80μm using a doctor blade coater. The foil was pre-dried in a 60℃ forced-air drying oven for 30min and then transferred to a 120℃ vacuum drying oven for 12h. The dried electrode was compacted using a roller press with a pressure controlled at 110MPa and cut into electrode sheets with a diameter of 14mm to obtain the cathode.

[0060] Battery manufacturing:

[0061] Using lithium foil as the negative electrode and Celgard 2400 polypropylene microporous membrane as the separator, the electrolyte was 1 mol / L LiPF6 (EC:DEC=1:1, v / v). The positive electrode, negative electrode, separator and electrolyte were assembled in an argon glove box with water and oxygen content of less than 1 ppm. After assembly, the battery was left to stand for 24 h to obtain the battery (CR2032 button cell).

[0062] Performance testing:

[0063] The nitrogen-sulfur co-doped manganese iron phosphate cathode materials of Examples 1-3 and Comparative Examples 1-3 were tested for discharge specific capacity at 0.1C and 5C rates under charge / discharge cutoff voltages of 3.0-4.3V; capacity retention after 100 cycles at 5C charge / discharge rate was tested; and conductivity was tested.

[0064] The test results are shown in Table 1:

[0065] Table 1: Test Results Summary Table

[0066]

[0067] Referring to Table 1, based on the comparison between Examples 1-3 and Comparative Examples 1-3, it can be seen that the nitrogen-sulfur co-doped manganese iron phosphate cathode material of the present invention has excellent conductivity, rate performance and cycle stability.

[0068] Based on the comparison between Example 1 and Comparative Example 1, it can be seen that Example 1 uses a composite material (graphene-nanozirconium dioxide) to coat nitrogen-sulfur co-doped lithium manganese iron phosphate; while Comparative Example 1 uses the same composite material to coat undoped lithium manganese iron phosphate. Nitrogen-sulfur co-doping can regulate the crystal structure of lithium manganese iron phosphate, which can optimize the electronic orbital distribution of the material, improve the electron / ion conduction ability of the bulk, enhance the stability of the crystal structure, and reduce the structural collapse caused by lithium deintercalation during charging and discharging. Therefore, the performance of Example 1 is significantly better than that of Comparative Example 1.

[0069] Based on the comparison between Example 1 and Comparative Example 2, it can be seen that Example 1 is a graphene-nanozirconium dioxide composite dispersion coated with nitrogen-sulfur co-doped manganese iron phosphate; Comparative Example 2 is a nitrogen-sulfur co-doped manganese iron phosphate without the coating of this composite material; the graphene-nanozirconium dioxide composite layer has a synergistic effect, graphene can construct a continuous electron transport network and improve conductivity, and nanozirconium dioxide can inhibit the agglomeration of material particles, while enhancing the interfacial stability between the electrode and the electrolyte and reducing side reactions; therefore, the performance of Example 1 is better than that of Comparative Example 2.

[0070] Based on the comparison between Example 1 and Comparative Example 3, it can be seen that Example 1 adopts a dual modification scheme of nitrogen-sulfur co-doping and graphene-nanozirconia composite material coating; Comparative Example 3 is the original lithium manganese iron phosphate without any doping or coating modification; nitrogen-sulfur doping and composite material coating have a synergistic effect, nitrogen-sulfur doping optimizes the electrochemical activity and structural stability of the material body, and composite material coating improves conductivity and interface compatibility. The combination of the two improves the performance in all aspects; therefore, the performance of Example 1 is far superior to that of Comparative Example 3.

[0071] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0072] The above description is merely an example and illustration of the present invention. Those skilled in the art can make various modifications or additions to the specific embodiments described or use similar methods to replace them, as long as they do not deviate from the invention or exceed the scope defined in this application, they should all fall within the protection scope of the present invention.

Claims

1. A nitrogen-sulfur co-doped lithium iron manganese phosphate cathode material, characterized in that, Raw materials comprising the following components by weight: 9.4-12.5 parts of nitrogen / sulfur-doped lithium manganese iron phosphate powder, 50-70 mL of graphene nanosheet-nanozirconium dioxide composite dispersion, 10-14 parts of hydrochloric acid, 0.5-0.7 parts of aniline, and 10-14 parts of ammonium persulfate solution; The ammonium persulfate solution is a mixture of ammonium persulfate and deionized water in a ratio of 1.23 g: 10 mL; the concentration of the hydrochloric acid is 1 mol / L. The graphene nanosheet-zirconia nanocomposite dispersion is prepared by the following steps: Step a1: Graphene nanosheets, concentrated sulfuric acid and concentrated nitric acid are ultrasonically dispersed, then magnetically stirred, then vacuum filtered, washed, then dispersed in deionized water, and ultrasonicated to obtain a graphene nanosheet dispersion. Step a2: Add nano-zirconia to the graphene nanosheet dispersion, disperse by ultrasonication, then transfer to a constant temperature stirrer and stir magnetically to obtain a graphene nanosheet-nano-zirconia composite dispersion. 2.The nitrogen and sulfur co-doped lithium iron manganese phosphate positive electrode material of claim 1, characterized in that, The ratio of graphene nanosheets, concentrated sulfuric acid, concentrated nitric acid and deionized water used in step a1 is 0.1-0.3g: 30-90mL: 10-30mL: 50-150mL.

3. The nitrogen-sulfur co-doped lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The graphene nanosheets in step a1 have a thickness of 6-8 nm and a length of 25 μm. 4.The nitrogen and sulfur co-doped lithium iron manganese phosphate positive electrode material of claim 1, wherein, The concentrated sulfuric acid in step a1 has a mass fraction of 98%. 5.The nitrogen and sulfur co-doped lithium iron manganese phosphate cathode material of claim 1, wherein, The concentrated nitric acid in step a1 has a mass fraction of 68%.

6. The nitrogen-sulfur co-doped manganese iron phosphate cathode material according to claim 1, characterized in that, The ratio of the nano-zirconia and graphene nanosheet dispersion in step a2 is 0.2-0.4g: 50-100mL. 7.The nitrogen and sulfur co-doped lithium iron manganese phosphate cathode material of claim 1, wherein, The nano-zirconia particles in step a2 have a particle size of 20-50 nm. 8.The nitrogen and sulfur co-doped lithium iron manganese phosphate cathode material of claim 1, wherein, The nitrogen / sulfur-doped lithium manganese iron phosphate powder is prepared by the following steps: Lithium carbonate, manganese acetate, ferrous oxalate, ammonium dihydrogen phosphate, urea, thiourea, and anhydrous ethanol were ball-milled. The slurry was then transferred to an evaporating dish, dried, ground, and sieved. The powder was then sintered, naturally cooled, ground, and sieved again to obtain nitrogen / sulfur-doped manganese iron phosphate powder. 9.The nitrogen and sulfur co-doped lithium iron manganese phosphate cathode material of claim 8, wherein, The ratio of lithium carbonate, manganese acetate, ferrous oxalate, ammonium dihydrogen phosphate, urea, thiourea, and anhydrous ethanol is 0.372-0.380g: 1.231-1.251g: 0.090-0.095g: 0.115-0.120g: 0.048-0.050g: 0.060-0.062g: 10-20mL.

10. A process for preparing a nitrogen-sulfur co-doped lithium iron manganese phosphate cathode material, characterized in that, The preparation of the nitrogen-sulfur co-doped manganese iron phosphate cathode material as described in any one of claims 1-9 includes the following steps: Nitrogen / sulfur-doped lithium manganese iron phosphate powder was added to a graphene nanosheet-zirconia composite dispersion and ultrasonically dispersed. The mixture was then transferred to a three-necked flask, hydrochloric acid was added and stirred, aniline was added and reacted, followed by ammonium persulfate solution. The mixture was centrifuged, washed, dried, ground and sieved to obtain nitrogen-sulfur co-doped lithium manganese iron phosphate cathode material.

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