Magnesium-doped and carbon nanotube-modified lithium manganese iron phosphate material and preparation method thereof

Abstract

The application relates to the technical field of lithium batteries, in particular to a magnesium-doped and carbon nanotube-doubly-modified lithium manganese iron phosphate material and a preparation method. 2+ The material body phase structure stability is optimized by doping, the lattice stress is relieved, and the lithium ion diffusion rate is improved, a three-dimensional conductive network formed by CNTs connects the internal particles of the agglomerated particles and the external contact, and the two can synergistically reduce polarization and improve the rate performance; the material has less capacity attenuation and higher coulomb efficiency, and also has good lithium ion transmission effect, and the capacity is greatly improved.

Description

Technical Field

[0001] This invention relates to the field of lithium battery technology, and more specifically, to magnesium-doped and carbon nanotube-modified lithium manganese iron phosphate materials and their preparation methods. Background Technology

[0002] Lithium iron manganese phosphate (LFMP) has become a highly promising cathode material in the field of lithium-ion batteries due to its high voltage platform (~4.1V), excellent thermal stability and low cost, but its practical application is limited by the energy density bottleneck.

[0003] On the one hand, the dual voltage plateau (Fe / Mn redox reaction) leads to high-voltage decomposition and capacity decay of the electrolyte, while structural degradation caused by manganese dissolution further weakens cycle stability; on the other hand, the material's intrinsic conductivity is poor, and the low compaction density (1.8-2.3 g / cm³) of the nanoparticles... 3 It significantly reduces the volumetric energy density.

[0004] In the current research, lithium manganese iron phosphate (LFMP) is modified by introducing heterogeneous ions for lattice doping. Furthermore, to address the aggregation problem during synthesis, a three-dimensional interconnected conductive network is constructed using in-situ carbon-based materials (such as carbon nanotubes and graphene). The network's pores provide lithium-ion diffusion channels within the aggregates, thereby mitigating the negative impact of aggregation on electrode kinetics. However, while the above modification strategy improves the cycle life and discharge specific capacity of LFMP, its energy density remains limited by the intrinsic voltage plateau (~4.1V) and compaction density (<2.5 g / cm³). 3 The inherent bottleneck of ). Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide magnesium-doped and carbon nanotube-modified lithium manganese iron phosphate materials and their preparation methods.

[0006] The technical solution of the present invention to solve the above-mentioned technical problems is as follows:

[0007] This invention provides a method for preparing a magnesium-doped and carbon nanotube-modified lithium manganese iron phosphate material. A mixed solution is prepared using magnesium, phosphorus, manganese, iron, and lithium source components. The mixed solution is then mixed with carbon nanotubes, and the lithium manganese iron phosphate material is generated in situ through a hydrothermal reaction.

[0008] Based on the above technical solution, the present invention can be further improved as follows.

[0009] Furthermore, the molar percentage of the magnesium source component is 0.1-3 mol% of the phosphorus source component.

[0010] Furthermore, the magnesium source component includes at least one of magnesium sulfate, magnesium carbonate, and magnesium acetate.

[0011] Furthermore, based on the theoretically generated lithium manganese iron phosphate material as 100%, the mass percentage of the carbon nanotubes is 0.05wt% to 2wt%.

[0012] Furthermore, the lithium source component includes at least one of lithium carbonate, lithium acetate, lithium hydroxide, and lithium dihydrogen phosphate; the phosphorus source component includes at least one of phosphoric acid, ammonium dihydrogen phosphate, and lithium dihydrogen phosphate; the manganese source component includes at least one of manganese oxalate, manganese sulfate, and manganese acetate; and the iron source component includes at least one of ferrous oxalate, ferric acetate, and ferrous sulfate; wherein the molar ratio of lithium to phosphorus is 1:1 to 3:1, and the molar ratio of manganese source to iron source is 5:5 to 9:1.

[0013] Furthermore, the following steps are included:

[0014] S1. Add the phosphorus source solution dropwise to the lithium source solution to obtain solution A;

[0015] S2. Mix the iron source solution, magnesium source solution and manganese source solution to obtain solution B;

[0016] S3. Add the solution B dropwise to the solution A to obtain a first mixed solution, and add the carbon nanotubes to the first mixed solution to obtain a second mixed solution;

[0017] S4. The second mixed solution is heated and subjected to the hydrothermal reaction to obtain the reactants;

[0018] S5. The precipitate in the reactants is washed, dried and ground to obtain the lithium manganese iron phosphate powder;

[0019] S6. Sinter the lithium manganese iron phosphate powder to obtain the material.

[0020] Further, in step S4, the hydrothermal reaction involves introducing the second mixed solution into a polytetrafluoroethylene liner, heating and reacting it in a sealed stainless steel environment to obtain the reactants; the heating reaction temperature is 160–200°C, and the time is 8–16 hours.

[0021] Furthermore, in step S6, the lithium manganese iron phosphate powder is mixed with a carbon source and then sintered. The sintering conditions are annealing at 500-700°C for 1-6 hours under an inert atmosphere.

[0022] The present invention also provides a magnesium-doped and carbon nanotube-modified lithium manganese iron phosphate material, which is prepared by the method described above.

[0023] The present invention also provides a lithium battery, wherein the positive electrode material of the lithium battery is lithium manganese iron phosphate as described above.

[0024] The beneficial effects of this invention are as follows:

[0025] (1) The preparation method of the magnesium-doped and carbon nanotube-modified lithium manganese iron phosphate material of the present invention, wherein Mg 2+ Doping optimizes the bulk structure stability of the material, alleviates lattice stress, and improves the lithium-ion diffusion rate.

[0026] (2) The method for preparing magnesium-doped and carbon nanotube-modified lithium manganese iron phosphate material of the present invention utilizes a three-dimensional conductive network constructed by CNTs to connect the internal particles of the aggregated particles with the external environment.

[0027] (3) The method for preparing magnesium-doped and carbon nanotube-modified lithium manganese iron phosphate material of the present invention utilizes the high aspect ratio and mechanical strength of CNTs to construct a fast conductive framework that penetrates the electrode, in conjunction with Mg 2+ The lattice stabilization effect reduces polarization and improves rate performance;

[0028] (4) The method for preparing magnesium-doped and carbon nanotube-modified lithium manganese iron phosphate material of the present invention adopts a dual-modification strategy, which effectively improves the energy density and cycle life of the battery. When applied to lithium batteries, it will have excellent electrochemical performance.

[0029] (5) The preparation method of magnesium-doped and carbon nanotube-modified lithium manganese iron phosphate material of the present invention is simple and easy to control, and the whole process is green and environmentally friendly.

[0030] (6) The magnesium-doped and carbon nanotube-modified lithium manganese iron phosphate material of the present invention has less capacity decay and higher coulombic efficiency, as well as good lithium-ion transport effect, and the capacity is greatly improved.

[0031] (7) The lithium battery of the present invention has effectively improved energy density and cycle life. Attached Figure Description

[0032] Figure 1 The XRD pattern of the magnesium-doped and carbon nanotube-modified lithium manganese iron phosphate composite material of the present invention is shown in the example. Figure 1 In the image, 'a' represents the overall XRD pattern of LFMMP-CNTs. Figure 1 b is Figure 1 A magnified view of part 'a';

[0033] Figure 2 SEM images of the preparation method of the magnesium-doped and carbon nanotube-modified lithium manganese iron phosphate composite material of the present invention are shown in the examples. Figure 2 The scale bar for a in the middle is 200 nm. Figure 2 The scale bar for b in the middle is 1 μm;

[0034] Figure 3This invention provides a method for preparing the magnesium-doped and carbon nanotube-modified lithium manganese iron phosphate composite material, along with electrochemical test results from examples and comparative examples. Figure 3 Figure 'a' shows the rate performance test results for the two samples. Figure 3 Figure b shows the first charge-discharge curves of the two samples at a current density of 0.1C. Figure 3 In the figure, c represents the cycling performance curves of the two samples at a current density of 1C. Detailed Implementation

[0035] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0036] The present invention discloses a method for preparing magnesium-doped and carbon nanotube-modified lithium manganese iron phosphate material, which involves preparing a mixed solution using magnesium, phosphorus, manganese, iron, and lithium source components, mixing the mixed solution with carbon nanotubes, and generating the lithium manganese iron phosphate composite material in situ through a hydrothermal reaction.

[0037] The preparation method of the present invention, doped with Mg 2+ In-situ growth of carbon nanotubes (CNTs) was combined with cathode materials, and Mg... 2+ Doping optimizes the bulk structural stability of the material, alleviates lattice stress, and improves the lithium-ion diffusion rate. The three-dimensional conductive network constructed by CNTs connects the internal particles of the aggregated particles with the external environment. The high aspect ratio and mechanical strength of CNTs are used to construct a fast conductive framework that runs through the electrode, synergistically with Mg. 2+ The lattice stabilizing effect can further reduce polarization and improve rate performance; the preparation method of the present invention, by adopting the above-mentioned dual modification strategy, effectively improves the energy density and cycle life of the battery, and its application in lithium batteries will result in excellent electrochemical performance.

[0038] Specifically, Mg 2+ The effects of doping are as follows:

[0039] (1) Adjusting lattice parameters: Incorporating smaller Mg ions at the Fe / Mn sites can extend the Li-O bond, increase the interstitial positions in octahedral LiO6, and stabilize the olivine structure during lithium insertion / extraction, thereby enabling the electrochemical process of Li... + Fast transmission.

[0040] (2) Suppressing lattice distortion: The strong oxygen affinity of Mg (high Mg-O bond energy) can stabilize PO4. 3- The tetrahedral structure mitigates the Li-phase degradation during charging and discharging. + The lattice stress induced by insertion / extraction reduces volume expansion, thereby enhancing the stability of lithium manganese iron phosphate.

[0041] (3) Enhance electronic conductivity: Mg 2+ Doping can introduce lattice defects (such as cation vacancies or interstitial ions), forming localized electronic states and increasing intrinsic electronic conductivity; Mg 2+ Doping can adjust the redox potential of Fe / Mn and suppress Mn. 3+ disproportionation reaction (Mn) 3+ →Mn 2+ +Mn 4+ This reduces structural distortion caused by the Jahn-Teller effect and maintains the stability of the electron transport path.

[0042] However, simply introducing magnesium does not prevent the aggregation of the generated LFMP nanoparticles. Aggregation leads to insufficient contact between the aggregated particles and the electrolyte, resulting in capacity loss in the LFMP material. While simply adding conductive CNTs after LFMP synthesis can enhance the conductivity between nanoparticles, it does not improve the intrinsic conductivity of LFMP.

[0043] Because, in order to improve conductivity and lithium conductivity, this invention uses Mg 2+ After doping and LFMP synthesis, conductive agents such as CNTs are added for dual modification, which improves both the external conductivity and the intrinsic conductivity of the material. The method of in-situ synthesis of LFMP on CNTs mitigates the effects of LFMP aggregation, while CNTs also act as conductive agents to improve the inter-material conductivity.

[0044] The conventional method of adding conductive agents is to add them after LFMP synthesis. However, this method does not work to prevent LFMP agglomeration because LFMP has already occurred during synthesis. Therefore, this method can only increase the conductivity between particles.

[0045] Therefore, in the preparation method of the present invention, LFMP nanoparticles are directly grown on the surface of CNTs to form an alternative type of covered agglomeration.

[0046] Preferably, the molar percentage of the magnesium source component in this invention is 0.1-3 mol% of the phosphorus source component.

[0047] Preferably, the magnesium source component includes at least one of magnesium sulfate, magnesium carbonate, and magnesium acetate.

[0048] Preferably, based on the mass of the theoretically generated lithium manganese iron phosphate material as 100%, the mass percentage of carbon nanotubes is 0.05wt% to 2wt%.

[0049] Preferably, the lithium source component includes at least one of lithium carbonate, lithium acetate, lithium hydroxide, and lithium dihydrogen phosphate; the phosphorus source component includes at least one of phosphoric acid, ammonium dihydrogen phosphate, and lithium dihydrogen phosphate; the manganese source component includes at least one of manganese oxalate, manganese sulfate, and manganese acetate; and the iron source component includes at least one of ferrous oxalate, ferric acetate, and ferrous sulfate.

[0050] Preferably, the molar ratio of lithium to phosphorus is 1:1 to 3:1, and the molar ratio of manganese source to iron source is 5:5 to 9:1.

[0051] The preparation method of the present invention specifically includes the following steps:

[0052] S1. Add the phosphorus source solution dropwise to the lithium source solution to obtain solution A.

[0053] S2. Mix the iron source solution, magnesium source solution and manganese source solution to obtain solution B.

[0054] Preferably, the lithium source solution is a water-ethylene glycol solution of lithium hydroxide, the phosphorus source solution is an ethylene glycol solution of phosphate, the iron source solution is an ethylene glycol solution of FeSO4·7H2O, and the manganese source solution is a water-ethylene glycol solution of MnSO4·H2O.

[0055] S3. Add solution B dropwise to solution A to obtain the first mixed solution. Add CNTs to the first mixed solution to obtain the second mixed solution.

[0056] Preferably, CNTs can be pretreated. The pretreatment method is as follows: a certain amount of commercial CNTs is weighed, ethylene glycol is added, and the mixture is subjected to vigorous stirring and ultrasonic treatment, which are repeated three times each. The dispersed material is placed in a stainless steel reactor and kept at 180°C for 5 hours. After natural cooling, the resulting solution is filtered and repeatedly washed and dried with anhydrous ethanol and deionized water to obtain the pretreated CNTs.

[0057] S4. The second mixed solution is heated and subjected to a hydrothermal reaction to obtain the reactants.

[0058] Preferably, the hydrothermal reaction involves introducing the second mixed solution into a polytetrafluoroethylene liner, heating and reacting it in a sealed stainless steel environment to obtain the reactants; the heating temperature is 160–200°C, and the time is 8–16 hours.

[0059] S5. Wash, dry and grind the precipitate in the reactants to obtain lithium manganese iron phosphate powder;

[0060] S6. Sinter the lithium manganese iron phosphate powder to obtain the composite material.

[0061] Preferably, lithium manganese iron phosphate powder is mixed with a carbon source and then sintered. The sintering conditions are annealing at 500-700°C for 1-6 hours under an inert atmosphere.

[0062] Preferably, the carbon source is at least one of glucose and sucrose.

[0063] More preferably, the mass of the carbon source is 0 to 30 wt% of the mass of the composite powder.

[0064] The preparation method of the present invention is simple and easy to control, and the whole process is green and environmentally friendly.

[0065] The magnesium-doped and carbon nanotube-modified lithium manganese iron phosphate composite material of the present invention has less capacity decay and higher coulombic efficiency, exhibiting good lithium-ion transport performance and significantly improved capacity.

[0066] The lithium battery of the present invention has a positive electrode material of lithium manganese iron phosphate composite material as described above; the energy density and cycle life of the lithium battery can be effectively improved.

[0067] The following examples illustrate this point.

[0068] Example

[0069] This embodiment uses the method of the present invention to prepare Mg 2+ The specific preparation process of the LFMP composite material with doping and CNT dual modification is as follows:

[0070] (1) Pretreatment of carbon nanotubes:

[0071] Weigh a certain amount of commercial carbon nanotubes into a beaker, add 60 ml of ethylene glycol, and vigorously stir and sonicate the solution three times each. Place the dispersed material into a stainless steel reactor and keep it at 180°C for 5 hours. Allow it to cool naturally, filter the resulting solution, wash it repeatedly with anhydrous ethanol and deionized water, and dry the obtained CNTs.

[0072] (2) Raw material mixing:

[0073] Weigh out 30 mmol of LiOH, 11 mmol of H3PO4, 2 mmol of FeSO4·7H2O, and 8 mmol of MnSO4·H2O; replace an equal amount of ferrous sulfate with magnesium sulfate at a mass fraction of 1% to make the molar ratio of Li, Fe, Mg, Mn, and P conform to LiFe 0.19 Mg 0.01 Mn 0.8 PO4.

[0074] A lithium source solution was obtained by dissolving LiOH in a mixed solution of 2 ml water and 20 ml ethylene glycol; a phosphorus source solution was obtained by dispersing H3PO4 in 20 ml ethylene glycol; an iron source solution was obtained by dissolving FeSO4·7H2O in 10 ml ethylene glycol; and a manganese source solution was obtained by dispersing MnSO4·H2O in a mixed solution of 3 ml water and 10 ml ethylene glycol.

[0075] H3PO4 solution is added dropwise to LiOH solution to form solution A; FeSO4·7H2O and MnSO4·H2O are mixed evenly to form solution B; solution B is added dropwise to solution A and stirred evenly to obtain the first mixed solution.

[0076] Add 0.2 wt% of pretreated carbon nanotubes to the first mixed solution to obtain the second mixed solution.

[0077] (3) Preparation of dual-modified LFMMP-CNTs composite materials by hydrothermal reaction:

[0078] The second mixed solution was poured into a 100ml polytetrafluoroethylene liner, sealed in a stainless steel reactor, and placed in a muffle furnace at 180℃ for 10h with a heating rate of 5℃ / min to obtain the product.

[0079] The precipitate in the product was obtained, and it was washed three times with deionized water and ethanol respectively and centrifuged. Finally, a 15wt% sucrose (0.614g / ml) solution of the positive electrode material was added and mixed evenly.

[0080] The product was dried in an oven at 60°C; the product was then calcined at 650°C for 2 hours under an inert atmosphere, and the double-modified cathode material LFMMP-CNTs was obtained after natural cooling in a tube furnace.

[0081] Comparative Example

[0082] This comparative example modifies the LFMMP cathode material using only the solvothermal method. The chemical formula of this cathode material is LiFe. 0.2 Mn 0.8 The specific preparation steps for PO4 are as follows:

[0083] Weigh out 30 mmol of LiOH, 11 mmol of H3PO4, 2 mmol of FeSO4·7H2O and 8 mmol of MnSO4·H2O; dissolve LiOH in a mixed solution of 2 ml water and 20 ml ethylene glycol; disperse H3PO4 in 20 ml ethylene glycol; dissolve FeSO4·7H2O in 10 ml ethylene glycol; disperse MnSO4·H2O in a mixed solution of 3 ml water and 10 ml ethylene glycol.

[0084] H3PO4 solution is added dropwise to LiOH solution to form solution A; FeSO4·7H2O and MnSO4·H2O are mixed evenly to form solution B; solution B is added dropwise to solution A and stirred evenly to obtain a mixed solution.

[0085] Pour the mixed solution into a 100ml polytetrafluoroethylene-lined container, seal it in a stainless steel reactor, and place it in a muffle furnace at 180℃ for 10 hours, with a heating rate of 5℃ / min.

[0086] The precipitate in the product was obtained, and it was washed three times with deionized water and ethanol respectively and centrifuged. Finally, a 15wt% sucrose (0.614g / ml) solution of the positive electrode material was added and mixed evenly.

[0087] The product was dried in an oven at 60°C; the product was then calcined at 650°C for 2 hours under an inert atmosphere, and the sample LFMMP was obtained after natural cooling in a tube furnace.

[0088] The LFMMP-CNTs of the embodiment were tested, and the test results are as follows:

[0089] XRD analysis was performed on LFMMP-CNTs, and the results are as follows: Figure 1 As shown. According to Figure 1 It can be seen that the diffraction peaks are sharp and without impurity peaks, indicating that the cathode material LFMMP-CNTs has an olivine structure and good crystallinity. The modification did not affect the material synthesis process and no impurities were generated. There is a weak peak with a wide diffraction angle at 26°, which is the diffraction peak of C, indicating that CNTs have been combined with LFMMP cathode material.

[0090] Figure 2 The image shows an SEM image of LFMMP-CNTs. It can be seen that the synthesized LFMMP primary particles are nanoparticles of about 40 nm in size, with irregular shapes and smooth surfaces. These nanoparticles are synthesized and grown on the surface of CNTs in situ. CNTs are aggregates formed by many single carbon nanotubes connected to each other.

[0091] A series of electrochemical tests were performed on the LFMMP-CNTs of the examples and the LFMP of the comparative example to examine the performance of LFMP and LFMMP-CNTs under different electrochemical performance tests. The results are as follows: Figure 3 As shown.

[0092] pass Figure 3 It can be seen that the dual-modified LFMMP-CNTs-modified material exhibits less capacity decay and higher coulombic efficiency, indicating that the Mg doping... 2+The use of carbon nanotubes to modify the material improves conductivity and the contact between the active material and the electrolyte, thereby promoting lithium-ion transport and increasing capacity.

[0093] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a magnesium-doped and carbon nanotube-modified lithium manganese iron phosphate material, characterized in that, A mixed solution was prepared using magnesium, phosphorus, manganese, iron, and lithium source components. The mixed solution was then mixed with carbon nanotubes, and the lithium manganese iron phosphate material was generated in situ through a hydrothermal reaction. The molar ratio of manganese source to iron source is 5:5 to 9:1; S1. Add the phosphorus source solution dropwise to the lithium source solution to obtain solution A; S2. Mix the iron source solution, magnesium source solution and manganese source solution to obtain solution B; S3. Add the solution B dropwise to the solution A to obtain a first mixed solution, and add the carbon nanotubes to the first mixed solution to obtain a second mixed solution; The carbon nanotubes are first pretreated by weighing the carbon nanotubes, adding ethylene glycol, and performing vigorous stirring and ultrasonic treatment, repeating each three times. The dispersed material is then placed in a stainless steel reactor and kept at 180°C for 5 hours. After natural cooling, the resulting solution is filtered and repeatedly washed and dried with anhydrous ethanol and deionized water to obtain the pretreated carbon nanotubes. S4. The second mixed solution is heated and subjected to the hydrothermal reaction to obtain the reactants; The hydrothermal reaction involves introducing the second mixed solution into a polytetrafluoroethylene liner, heating and reacting it in a sealed stainless steel environment to obtain the reactants; the heating temperature is 160~200℃, and the time is 8~16h. S5. The precipitate in the reactants is washed, dried and ground to obtain the lithium manganese iron phosphate powder; S6. Sinter the lithium manganese iron phosphate powder to obtain the lithium manganese iron phosphate material.

2. The preparation method of a magnesium-doped and carbon nanotube-modified lithium manganese iron phosphate material according to claim 1, characterized in that, The molar percentage of the magnesium source component is 0.1-3 mol of the phosphorus source component.

3. The method for preparing a magnesium-doped and carbon nanotube-modified lithium manganese iron phosphate material according to claim 2, characterized in that, The magnesium source component includes at least one of magnesium sulfate, magnesium carbonate, and magnesium acetate.

4. The preparation method of a magnesium-doped and carbon nanotube-modified lithium manganese iron phosphate material according to claim 1, characterized in that, Based on the theoretically generated lithium manganese iron phosphate material as 100%, the mass percentage of the carbon nanotubes is 0.05wt%~2wt%.

5. A method for preparing a magnesium-doped and carbon nanotube-modified lithium manganese iron phosphate material according to any one of claims 1-4, characterized in that, The lithium source component includes at least one of lithium carbonate, lithium acetate, lithium hydroxide, and lithium dihydrogen phosphate; the phosphorus source component includes at least one of phosphoric acid, ammonium dihydrogen phosphate, and lithium dihydrogen phosphate; the manganese source component includes at least one of manganese oxalate, manganese sulfate, and manganese acetate; the iron source component includes at least one of ferrous oxalate, ferric acetate, and ferrous sulfate; wherein the molar ratio of lithium to phosphorus is 1:1 to 3:

1.

6. The method for preparing a magnesium-doped and carbon nanotube-modified lithium manganese iron phosphate material according to claim 1, characterized in that, In step S6, the lithium manganese iron phosphate powder is mixed with a carbon source and then sintered. The sintering conditions are: annealing at 500~700℃ for 1~6 hours under an inert atmosphere.

7. A magnesium-doped and carbon nanotube-modified lithium manganese iron phosphate material, characterized in that, It is prepared by the method described in any one of claims 1-6.

8. A lithium battery, characterized in that, The positive electrode material of the lithium battery is lithium manganese iron phosphate as described in claim 7.

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