Preparation method of lithium manganese iron phosphate with low manganese dissolution and long cycle life

Abstract

The invention discloses a preparation method of lithium manganese iron phosphate with low manganese dissolution and long cycle life, and relates to the technical field of lithium batteries, the method comprises the following steps: using 0.1-3wt% of a carbon source 1 and a carbon source 2, adding a pure water solvent, and then stirring and mixing to obtain a mixed solution of the carbon source 1 and the carbon source 2; the preparation method comprises the following steps: firstly, preparing lithium manganese iron phosphate with relatively low carbon content through a low-cost solid phase method, then coating the surface of the lithium manganese iron phosphate with lithium iron phosphate and a carbon source by utilizing a liquid phase method, and setting the molar ratio of Mn to Fe to be 3: 1 during synthesis of the solid phase method in order to prevent the lithium manganese iron phosphate from reducing the energy density due to coating of the lithium iron phosphate. The lithium manganese iron phosphate and the lithium iron phosphate both belong to olivine structures, so that the phase separation degree of the lithium manganese iron phosphate and the lithium iron phosphate is very small, the coating effect is relatively good, the dissolution of Mn can be inhibited, and the lithium manganese iron phosphate material coated with iron phosphate under a high-temperature condition is higher in stability and longer in cycle life compared with a single lithium manganese iron phosphate material.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium batteries, and specifically to a preparation method of lithium iron manganese phosphate with low manganese dissolution and long cycle life. Background Art

[0002] Lithium iron manganese phosphate is considered to be most likely to replace lithium iron phosphate and become a new generation of lithium battery cathode material due to its low cost, high energy density and excellent safety. With the doping of manganese element, lithium iron manganese phosphate is superior to commercial lithium iron phosphate in terms of working voltage, thus releasing a higher energy density. The capacity decay that occurs during the cycling of lithium iron manganese phosphate has a greater impact on its cycle life. Especially under high temperature conditions, the capacity decay will be more serious, which is basically due to the dissolution of Mn. Therefore, how to reduce the dissolution of Mn is crucial for the application of lithium iron manganese phosphate materials in batteries.

[0003] Based on this, a preparation method of lithium iron manganese phosphate with low manganese dissolution and long cycle life is now provided, which can eliminate the drawbacks of existing methods. Summary of the Invention

[0004] The purpose of the present invention is to provide a preparation method of lithium iron manganese phosphate with low manganese dissolution and long cycle life to solve the problems in the background art.

[0005] To achieve the above purpose, the present invention provides the following technical solutions:

[0006] A preparation method of lithium iron manganese phosphate with low manganese dissolution and long cycle life, comprising the following steps:

[0007] S1: Use 0.1 - 3 wt% of carbon source 1 and carbon source 2, and add pure water solvent, and then stir and mix.

[0008] S2: Weigh lithium source, iron source, manganese source, and phosphorus source according to the stoichiometric ratio of 1:0.25:0.75:1, and add them to the mixed solution prepared in step S1 and stir and mix.

[0009] S3: Grind the mixed materials in a sand mill.

[0010] S4: Spray-dry the slurry ground in step S3 with a pressure spray dryer to obtain precursor powder with good sphericity.

[0011] S5: Sinter the precursor obtained by spray drying for 1 - 7 h under the protection of inert gas, and the sintering temperature is 250 - 550 °C. When the temperature drops to room temperature, a LiFe0.25Mn0.75PO4 / C composite material with less carbon coating is obtained.

[0012] S6: Use 4 - 8 wt% of carbon source 1 and carbon source 2, add pure water solvent, and then stir and mix to obtain a mixed solution of carbon source 1 and carbon source 2;

[0013] S7: Put the LiFe0.25Mn0.75PO4 / C composite material prepared in step S4 into the mixed solution of carbon source 1 and carbon source 2 prepared in step S5, and stir it vigorously. At the same time, drop soluble lithium source, phosphorus source, and iron source with a molar ratio of 1:1:1 into the above mixture respectively;

[0014] S8: Perform secondary sand grinding on the obtained mixed liquid in a sand mill;

[0015] S9: Spray - dry the slurry ground in step S7 with a pressure spray dryer to obtain precursor powder with good sphericity;

[0016] S10: Sinter the precursor obtained by spray - drying for 3 - 12 h under the protection of inert gas, and the sintering temperature is 600 - 800 °C. After the temperature drops to room temperature, perform air - flow comminution to obtain lithium iron manganese phosphate material coated with iron phosphate.

[0017] Based on the above technical solutions, the present invention also provides the following optional technical solutions:

[0018] In an optional solution: In step S2, the lithium source includes one or a mixture of lithium hydroxide, lithium peroxide, lithium oxide, lithium formate, lithium nitrate, and carbonate.

[0019] In an optional solution: In step S2, the iron source includes one or more of ferrous oxide, iron oxide, iron oxalate, and ferrous acetate.

[0020] In an optional solution: In step S2, the manganese source includes one or more of manganese carbonate, manganese oxalate, manganese phosphate, manganese nitrate, and manganese oxide.

[0021] In an optional solution: In step S2, the phosphorus source includes one or more of lithium monohydrogen phosphate, lithium dihydrogen phosphate, lithium phosphate, ammonium dihydrogen phosphate, and phosphoric acid.

[0022] In an optional solution: The carbon source 1 and carbon source 2 include one or more of phenolic resin, polyvinyl alcohol, glucose, sucrose, starch, corn paste, and chitosan.

[0023] In an optional solution: In steps S3 and S8, when the sand mill is running, the rotation speed is 1000 - 2500 rpm, the pump is 50 - 300 mL, and the grinding particle size of the slurry is 300 - 2000 nm.

[0024] In an alternative solution: in steps S4 and S9, the speed of pumping the slurry into the pressure spray dryer is 8 - 12 mL·min-1, the outlet temperature of the spray dryer is controlled at 95 - 105 °C, and the spray pressure is 0.3 MPa.

[0025] In an alternative solution: in step S7, the molar ratio of the iron source to LiFe0.25Mn0.75PO4 / C is less than 0.25.

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0027] 1. First, the present invention prepares lithium iron manganese phosphate with a relatively low carbon content by a low-cost solid-phase method, and then uses a liquid-phase method to coat lithium iron phosphate and a carbon source on the surface of lithium iron manganese phosphate. In order to prevent the energy density of lithium iron manganese phosphate from decreasing due to the coating of lithium iron phosphate, the molar ratio of Mn to Fe is set to 3:1 during the solid-phase synthesis. Since both lithium iron manganese phosphate and lithium iron phosphate belong to the olivine structure, the phase separation degree between the two is very small, and the coating effect is good, which can inhibit the dissolution of Mn. Under high-temperature conditions, the lithium iron manganese phosphate material coated with lithium iron phosphate has stronger stability and longer cycle life compared with the single lithium iron manganese phosphate material. Description of the Drawings

[0028] Figure 1 It is the SEM diagram of the synthesized lithium iron manganese phosphate.

[0029] Figure 2 It is the XRD diagram of the synthesized lithium iron manganese phosphate.

[0030] Figure 3 It is the charge-discharge schematic diagram of the synthesized lithium iron manganese phosphate.

[0031] Figure 4 It is the flow chart of the preparation method of the present invention. Detailed Embodiments

[0032] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0033] In one embodiment, as Figures 1-4 shown, a preparation method of lithium iron manganese phosphate with low manganese dissolution and long cycle life includes the following steps:

[0034] S1: Use 0.1 - 3 wt% of carbon source 1 and carbon source 2, and add pure water solvent, and then stir and mix.

[0035] S2: Weigh the lithium source, iron source, manganese source, and phosphorus source according to the stoichiometric ratio of 1:0.25:0.75:1, and add them to the mixed solution prepared in step S1 and stir and mix.

[0036] S3: The mixed materials are ground in a sand mill;

[0037] S4: The slurry ground in step S3 is spray-dried by a pressure spray dryer to obtain precursor powder with good sphericity;

[0038] S5: The precursor obtained by spray drying is sintered for 1 - 7 h under the protection of inert gas, and the sintering temperature is 250 - 550 °C. When the temperature drops to room temperature, a LiFe0.25Mn0.75PO4 / C composite material with less carbon coating is obtained;

[0039] S6: 4 - 8 wt% of carbon source 1 and carbon source 2 are used, and pure water solvent is added, followed by stirring and mixing to obtain a mixed solution of carbon source 1 and carbon source 2;

[0040] S7: The LiFe0.25Mn0.75PO4 / C composite material prepared in step S4 is put into the mixed solution of carbon source 1 and carbon source 2 prepared in step S5, and it is vigorously stirred. At the same time, soluble lithium source, phosphorus source, and iron source with a molar ratio of 1:1:1 are respectively dropped into the above mixture;

[0041] S8: The obtained mixed liquid is subjected to secondary sand grinding in a sand mill;

[0042] S9: The slurry ground in step S7 is spray-dried by a pressure spray dryer to obtain precursor powder with good sphericity;

[0043] S10: The precursor obtained by spray drying is sintered for 3 - 12 h under the protection of inert gas, and the sintering temperature is 600 - 800 °C. After the temperature drops to room temperature, it is subjected to air flow crushing to obtain a lithium iron manganese phosphate material coated with iron phosphate.

[0044] In one embodiment, in step S2, the lithium source includes one or a mixture of lithium hydroxide, lithium peroxide, lithium oxide, lithium formate, lithium nitrate, and carbonate.

[0045] In one embodiment, in step S2, the iron source includes one or more of ferrous oxide, iron oxide, iron oxalate, and ferrous acetate.

[0046] In one embodiment, in step S2, the manganese source includes one or more of manganese carbonate, manganese oxalate, manganese phosphate, manganese nitrate, and manganese oxide.

[0047] In one embodiment, in step S2, the phosphorus source includes one or more of lithium monohydrogen phosphate, lithium dihydrogen phosphate, lithium phosphate, ammonium dihydrogen phosphate, and phosphoric acid.

[0048] In one embodiment, the carbon source 1 and the carbon source 2 include one or more of phenolic resin, polyvinyl alcohol, glucose, sucrose, starch, corn paste, and chitosan.

[0049] In one embodiment, in steps S3 and S8, when the sand mill is running, the rotation speed is 1000 - 2500 rpm, the pump is 50 - 300 mL, and the grinding particle size of the slurry is 300 - 2000 nm.

[0050] In one embodiment, in steps S4 and S9, the speed at which the slurry is pumped into the pressure spray dryer is 8 - 12 mL·min-1, the outlet temperature of the spray dryer is controlled at 95 - 105 °C, and the spray pressure is 0.3 MPa.

[0051] In one embodiment, in step S7, the molar ratio of the iron source to LiFe0.25Mn0.75PO4 / C is less than 0.25.

[0052] The present invention will be described in detail below with reference to the embodiments. The embodiments give detailed implementation manners and specific operation processes on the premise of the technical solution of the present invention, but the protection scope of the present invention is not limited to the following embodiments.

[0053] Example 1

[0054] Experimental group:

[0055] Stoichiometric ratio: Li:Fe:Mn:P = 1:0.25:0.75:1 (Mn:Fe = 3:1).

[0056] Preparation steps:

[0057] Stage 1: Preparation of low-carbon LiFe0.25Mn0.75PO4 / C composite material by solid-phase method.

[0058] Mix lithium source, iron source, manganese source, and phosphorus source (according to the stoichiometric ratio).

[0059] Add 0.1 - 3 wt% of carbon source 1 and carbon source 2, stir and then sand grind (particle size 300 - 2000 nm), and spray dry (temperature 95 - 105 °C).

[0060] Primary sintering (250 - 550 °C, protected by inert gas for 1 - 7 h).

[0061] Stage 2: Coating iron phosphate and carbon source by liquid-phase method.

[0062] Disperse the product of stage 1 in a 4 - 8 wt% carbon source mixed solution.

[0063] Dropwise add soluble lithium source, phosphorus source, and iron source (molar ratio 1:1:1, the molar ratio of the iron source to the composite material < 0.25).

[0064] Secondary sanding, spray drying, and finally airflow pulverization after sintering (600 - 800 °C, 3 - 12 h).

[0065] Example 2:

[0066] Control Group 1:

[0067] Stoichiometric ratio: Li:Fe:Mn:P = 1:0.25:0.75:1 (the same as the experimental group).

[0068] Preparation steps:

[0069] Only perform Stage 1 of the experimental group (preparing low-carbon composite materials by solid-phase method), and omit the iron phosphate coating step in Stage 2.

[0070] Directly perform airflow pulverization after final sintering, without secondary carbon coating and iron phosphate coating.

[0071] Material selection:

[0072] The same as the experimental group, but only use the carbon source in Stage 1 (0.1 - 3 wt%).

[0073] Example 3:

[0074] Control Group 2:

[0075] Stoichiometric ratio: Li:Fe:Mn:P = 1:0.25:0.75:1 (the same as the experimental group).

[0076] Preparation steps:

[0077] Perform Stage 1 of the experimental group (preparing low-carbon composite materials by solid-phase method), and perform the carbon coating step in Stage 2 (disperse the product of Stage 1 in a 4 - 8 wt% carbon source mixed solution), and omit the iron phosphate coating step in Stage 2.

[0078] Secondary sanding, spray drying, and finally airflow pulverization after sintering (600 - 800 °C, 3 - 12 h), without iron phosphate coating.

[0079] The charge and discharge result parameters of the lithium iron manganese phosphate materials in the experimental group, Control Group 1, and Control Group 2 are shown in Table 1.

[0080] Table 1:

[0081]

[0082] According to the data comparison in Table 1, the capacity retention rate: experimental group > Control Group 2 > Control Group 1.

[0083] Mn dissolution amount: experimental group > Control Group 2 > Control Group 1.

[0084] Analysis of the difference in capacity retention rate:

[0085] Experimental group (95.50%):

[0086] Double carbon coating forms an efficient conductive network, and the iron phosphate coating layer inhibits Mn dissolution and stabilizes the lattice structure. The synergistic effect of the two significantly improves the cycle stability.

[0087] Control group 2 (88.50%):

[0088] Double carbon coating enhances conductivity, but the lack of iron phosphate coating leads to a relatively high Mn dissolution amount (416 ppm). The lattice structure is damaged more rapidly during long-term cycling, and the capacity decay is more obvious than that of the experimental group.

[0089] Control group 1 (75.80%):

[0090] Only low-carbon coating in stage 1, with insufficient conductivity and lack of structural protection. The combined effect of Mn dissolution (350 ppm) and lattice deterioration leads to the most serious capacity decay.

[0091] 2. Analysis of the contradictory phenomenon of Mn dissolution

[0092] The Mn dissolution amount in the experimental group is the lowest (20 ppm), but the dissolution amount in control group 1 (350 ppm) is actually lower than that in control group 2 (416 ppm). This contradiction can be explained by the following mechanism:

[0093] Control group 1: It does not undergo the treatment in stage 2, and the carbon coating amount on the material surface is low. However, a relatively dense phosphate passivation layer may be formed on the particle surface after the initial sintering, temporarily inhibiting Mn dissolution.

[0094] Control group 2: Although the high-carbon coating in stage 2 improves conductivity, the carbon layer may introduce microcracks or defects, which instead accelerate the dissolution of Mn in the electrolyte.

[0095] As described above, it is only the specific implementation manner of this application, but the protection scope of this application is not limited thereto. Any person skilled in the art within the technical scope disclosed by this application can easily think of changes or substitutions, which should all be covered within the protection scope of this application. Therefore, the protection scope of this application shall be subject to the protection scope of the claims.

Claims

1. A preparation method of lithium iron manganese phosphate with low manganese dissolution and long cycle life, characterized in that, It includes the following steps: S1: Use 0.1 - 3 wt% of carbon source 1 and carbon source 2, add pure water solvent, and then stir and mix to obtain a mixed solution of carbon source 1 and carbon source 2; S2: Weigh lithium source, iron source, manganese source, and phosphorus source according to the stoichiometric ratio of 1:0.25:0.75:1, and add them to the mixed solution prepared in step S1 and stir and mix; S3: Grind the mixed material in a sand mill; S4: Spray-dry the slurry ground in step S3 with a pressure spray dryer to obtain precursor powder with good sphericity; S5: Sinter the precursor obtained by spray drying for 1 - 7 h under the protection of inert gas, and the sintering temperature is 250 - 550 °C. When the temperature drops to room temperature, a LiFe0.25Mn0.75PO4 / C composite material with less carbon coating is obtained; S6: Use 4 - 8 wt% of carbon source 1 and carbon source 2, add pure water solvent, and then stir and mix to obtain a mixed solution of carbon source 1 and carbon source 2; S7: Put the LiFe0.25Mn0.75PO4 / C composite material prepared in step S5 into the mixed solution of carbon source 1 and carbon source 2 prepared in step S6, stir it vigorously, and simultaneously drop soluble lithium source, phosphorus source, and iron source with a molar ratio of 1:1:1 into the above mixture respectively; S8: Perform secondary sand grinding on the obtained mixed liquid in a sand mill; S9: Spray-dry the slurry ground in step S8 with a pressure spray dryer to obtain precursor powder with good sphericity; S10: Sinter the precursor obtained by spray drying for 3 - 12 h under the protection of inert gas, and the sintering temperature is 600 - 800 °C. After the temperature drops to room temperature, perform air flow pulverization to obtain lithium iron manganese phosphate material coated with iron phosphate.

2. The preparation method of lithium iron manganese phosphate with low manganese dissolution and long cycle life according to claim 1, characterized in that In step S2, the lithium source includes one or a mixture of lithium hydroxide, lithium peroxide, lithium oxide, lithium formate, lithium nitrate, and carbonate.

3. A method for preparing lithium iron manganese phosphate with low manganese dissolution and long cycle life according to claim 1, characterized in that, In step S2, the iron source includes one or more of ferrous oxide, iron oxide, iron oxalate, and ferrous acetate.

4. A preparation method of lithium iron manganese phosphate with low manganese dissolution and long cycle life according to claim 1, characterized in that, In step S2, the manganese source includes one or more of manganese carbonate, manganese oxalate, manganese phosphate, manganese nitrate, and manganese oxide.

5. A method for preparing lithium iron manganese phosphate with low manganese dissolution and long cycle life according to claim 1, characterized in that In step S2, the phosphorus source includes one or more of lithium monohydrogen phosphate, lithium dihydrogen phosphate, lithium phosphate, ammonium dihydrogen phosphate, and phosphoric acid.

6. A method for preparing lithium iron manganese phosphate with low manganese dissolution and long cycle life according to claim 1, characterized in that, The carbon source 1 and carbon source 2 include one or more of phenolic resin, polyvinyl alcohol, glucose, sucrose, starch, corn paste, and chitosan.

7. A method for preparing lithium iron manganese phosphate with low manganese dissolution and long cycle life according to claim 1, characterized in that, In step S3 and step S8, when the sand mill is running, the rotation speed is 1000 - 2500 rpm, the pump is 50 - 300 mL, and the grinding particle size of the slurry is 300 - 2000 nm.

8. A method for preparing lithium iron manganese phosphate with low manganese dissolution and long cycle life according to claim 1, characterized in that In step S4 and step S9, the speed of pumping the slurry into the pressure spray dryer is 8 - 12 mL·min-1, the outlet temperature of the spray dryer is controlled at 95 - 105 °C, and the spray pressure is 0.3 MPa.

9. A method for preparing lithium iron manganese phosphate with low manganese dissolution and long cycle life according to claim 1, characterized in that, In step S7, the molar ratio of the iron source to LiFe0.25Mn0.75PO4 / C is less than 0.25.

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