Gradient composite coated modified lithium manganese iron phosphate cathode material and its preparation method

Abstract

This invention discloses a gradient composite-coated modified lithium manganese iron phosphate (LMFP) cathode material and its preparation method, belonging to the field of cathode material technology. This invention pretreats the LMFP precursor with a citric acid-ethanol mixed solution to expose the active crystal facets. Subsequently, a three-dimensional gradient coating structure is sequentially constructed on the material surface, consisting of a polydopamine (PDA) chemically bonded layer, an MXene nanosheet intermediate layer, and a polyaniline (PANI) conductive outer layer, thereby increasing the electronic conductivity of the LMFP cathode material to 10. ‑3 The capacity retention rate (1C) is in the range of S / cm, with a capacity retention rate of >90% after 500 cycles. The coating layer is not easy to fall off, and it effectively inhibits the dissolution of manganese ions.

Description

Technical Field

[0001] This invention relates to the field of cathode material technology, specifically to a gradient composite coated modified lithium manganese iron phosphate cathode material and its preparation method. Background Technology

[0002] Lithium manganese iron phosphate (LiFe) x Mn 1-x Lithium manganese iron phosphate (LMFP) is a novel cathode material for lithium-ion batteries. This material combines the advantages of lithium iron phosphate (LFP) and lithium manganese phosphate (LMP), achieving performance optimization through partial replacement of Fe and Mn. Due to its high energy density, high safety, and low cost, it is considered an important development direction in the fields of power batteries and energy storage. While LFP cathode materials possess advantages such as high energy density, high safety, and low cost, they also have some performance bottlenecks. Coating modification is a crucial approach to addressing these bottlenecks, significantly improving its electrochemical performance and stability, and providing technical support for its widespread application in the new energy field.

[0003] Traditional carbon coating modification improves the electronic conductivity of cathode material particles by coating them with a conductive carbon layer, thereby enhancing their rate performance and cycle stability. However, traditional carbon coating technology also has some limitations, the most significant being the weak bonding between the carbon layer and the cathode material matrix, leading to delamination after long cycles.

[0004] Therefore, there is an urgent need to develop a new method for modifying and coating lithium manganese iron phosphate cathode materials to improve their performance and meet application requirements. Summary of the Invention

[0005] The technical problem to be solved by this invention is to overcome the shortcomings of the prior art and provide a gradient composite coated modified lithium manganese iron phosphate cathode material and its preparation method, thereby increasing the electronic conductivity of the lithium manganese iron phosphate cathode material to 10. -3 The capacity retention rate (1C) is in the range of S / cm, with a capacity retention rate of >90% after 500 cycles. The coating layer is not easy to fall off, and it effectively inhibits the dissolution of manganese ions.

[0006] The technical solution of this invention is as follows:

[0007] On one hand, the present invention provides a method for preparing a gradient composite coated modified lithium manganese iron phosphate cathode material, comprising the following steps:

[0008] S1 Preparation of Activated Material: The LMFP precursor was placed in a citric acid-ethanol mixed solution, and after sonication, centrifugation, and drying, the activated material was obtained.

[0009] S2 Polydopamine (PDA) Inner Layer Coating: The activated material is dispersed in Tris-HCl (tris(hydroxymethyl)aminomethane) hydrochloride buffer, dopamine hydrochloride is added and stirred, and a PDA layer is formed by oxidative polymerization. The PDA-coated material is obtained by centrifugation.

[0010] S3 MXene intermediate layer construction: Ti3AlC2 was etched by HF and then peeled off to obtain a single layer of Ti3C2T. x MXene nanosheets were prepared into an MXene dispersion; PDA coating material was placed in a vacuum reactor and impregnated with the MXene dispersion, so that the MXene nanosheets were embedded in the interlayer gaps of the PDA, and then dried;

[0011] S4 Polyaniline (PANI) outer layer vapor deposition: The material obtained in step S3 is transferred to a vapor deposition reactor, aniline monomer vapor is introduced, and ammonium persulfate is used as an initiator to carry out a vapor-phase polymerization reaction at -15 to 5°C for 2-6 hours to form a PANI conductive network layer, thus obtaining PANI@MXene nanosheets@PDA@LMFP cathode material.

[0012] Preferably, in step S1, the volume ratio of citric acid to ethanol in the citric acid-ethanol mixed solution is 1:(3-5).

[0013] Preferably, in step S1, the ultrasonic frequency is 40-80kHz and the ultrasonic time is 20-30min.

[0014] Preferably, in step S1, the (010) crystal plane exposure rate of the activated material is >60%, and the specific surface area is 20-25 m². 2 / g.

[0015] Preferably, in step S2, the pH of the Tris-HCl buffer solution is 8.5-9.

[0016] Preferably, in step S2, the concentration of dopamine hydrochloride is 0.5-1 g / mL, and the concentration of the activating material is 20-30 g / mL.

[0017] Preferably, in step S2, the stirring temperature is 25-35℃, the stirring time is 6-10h, and the thickness of the PDA layer is 2-5nm.

[0018] Preferably, in step S3, the thickness of the MXene nanosheets is 0.8-1.2 nm, the concentration of the MXene dispersion is 1-3 mg / mL, the pressure of the vacuum reactor is ≤100 Pa, the drying temperature is 110-130 °C, and the drying time is 12 h.

[0019] Preferably, in step S4, when aniline monomer vapor is introduced, the carrier gas is a mixture of atomized ammonium persulfate aqueous solution and Ar, the introduction rate is 5-15 mL / min, the concentration of ammonium persulfate aqueous solution is 5 wt.%, the volume ratio of atomized ammonium persulfate aqueous solution to Ar in the mixed gas is 1:(5-10), and the humidity is controlled <5%RH; the thickness of the PANI conductive network layer is 10-15 nm, and the pore size is 2-5 nm; the mass ratio of aniline monomer to the material obtained in step S3 is 1:(10-15), and the molar ratio of ammonium persulfate to aniline monomer is 1:(1-2).

[0020] On the other hand, the present invention provides a gradient composite coated modified lithium manganese iron phosphate cathode material prepared by the above preparation method.

[0021] Compared with the prior art, the present invention has the following advantages:

[0022] This invention pretreats the LMFP precursor with a citric acid-ethanol mixed solution to expose the active crystal faces, and then sequentially constructs a three-dimensional gradient coating structure on the material surface, consisting of a polydopamine (PDA) chemically bonded layer, an MXene nanosheet intermediate layer, and a polyaniline (PANI) conductive outer layer. The MXene layer is embedded into the interlayer gaps of the PDA layer using a vacuum impregnation method to form a π-π stacked interface; the outer PANI layer is coated using a low-temperature vapor deposition process to achieve a nano-network coating. This invention achieves the following through the construction of an organic-inorganic composite gradient interface: 1) strong coordination bonds between the PDA layer and Mn / Fe sites inhibit manganese dissolution; 2) MXene nanosheets construct three-dimensional electron channels; and 3) the PANI outer layer provides elastic buffering and electrolyte isolation. The modification method of this invention increases the electronic conductivity of the lithium manganese iron phosphate cathode material to 10. -3 The capacity retention rate (1C) is in the range of S / cm, with a capacity retention rate of >90% after 500 cycles. The coating layer is not easy to fall off, and it effectively inhibits the dissolution of manganese ions. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the structure of the gradient composite coated modified lithium manganese iron phosphate cathode material of the present invention.

[0024] Figure 2 This is a SEM image of the gradient composite-coated modified lithium manganese iron phosphate cathode material prepared in Example 1 of this invention. Detailed Implementation

[0025] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of this invention will be clearly and completely described below in conjunction with the embodiments of this invention.

[0026] Example 1

[0027] The preparation method of the gradient composite coated modified lithium manganese iron phosphate cathode material in this embodiment includes the following steps:

[0028] S1 Preparation of Activated Material: 1 kg of LMFP precursor powder (D50 = 0.5 μm, specific surface area 13 m²) was used. 2 / g) was placed in 200mL of a citric acid-ethanol mixed solution (citric acid to ethanol volume ratio of 1:3) and sonicated at 40kHz for 30min. After centrifugation and drying, (010) crystal face exposure rate >60% and specific surface area 21.5m² were obtained. 2 / g of activating material;

[0029] S2 PDA inner layer coating: 1 kg of activated material was dispersed in 5 L of Tris-HCl buffer solution with pH=8.5, 25 g of dopamine hydrochloride was added, and the mixture was stirred at 25 °C for 6 h. A PDA layer with a thickness of 2 nm was formed by oxidative polymerization. The PDA-coated material was obtained by centrifugation and denoted as PDA@LMFP.

[0030] S3 MXene intermediate layer construction: A 1nm thick monolayer Ti3C2T was prepared by etching Ti3AlC2 with HF and then peeling it off. x MXene nanosheets were prepared into a 1 mg / mL MXene dispersion (water as solvent); PDA@LMFP was placed in a vacuum reactor at 50 Pa and immersed in the MXene dispersion for 2 h. MXene nanosheets were embedded into the interlayer gaps of PDA through capillary action to form a π-π stacked structure with a spacing ≤ 5 nm. The mixture was then dried at 110 °C for 12 h.

[0031] S4 Polyaniline (PANI) Outer Layer Vapor Phase Deposition: 1 kg of the material obtained in step S3 was transferred to a vapor phase deposition reactor. A mixture of atomized 5 wt.% ammonium persulfate aqueous solution and Ar (humidity controlled <5% RH) at a volume ratio of 1:5 was used as the carrier gas. 67 g of aniline monomer vapor was introduced at a rate of 10 mL / min. 162 g of ammonium persulfate was used as the initiator. The vapor phase polymerization reaction was carried out at -10 °C for 2 h to form a PANI conductive network layer with a thickness of 10 nm and a pore size of 3 nm, resulting in the PANI@MXene nanosheet@PDA@LMFP cathode material, the structure of which is as follows. Figure 1 As shown, the SEM image is as follows Figure 2 As shown in the SEM image, the material surface is uniformly coated.

[0032] Example 2

[0033] The preparation method of the gradient composite coated modified lithium manganese iron phosphate cathode material in this embodiment includes the following steps:

[0034] S1 Preparation of Activated Material: 1 kg of LMFP precursor powder (D50 = 0.5 μm, specific surface area 13 m²) was used. 2 / g) was placed in 200mL of a citric acid-ethanol mixed solution (citric acid to ethanol volume ratio of 1:4) and sonicated at 60kHz for 25min. After centrifugation and drying, (010) crystal face exposure rate >60% and specific surface area 23.6m² were obtained. 2 / g of activating material;

[0035] S2 PDA inner layer coating: 1 kg of activated material was dispersed in 5 L of Tris-HCl buffer solution with pH=8.5, 35 g of dopamine hydrochloride was added, and the mixture was stirred at 30 °C for 8 h. A PDA layer with a thickness of 3 nm was formed by oxidative polymerization. The PDA-coated material was obtained by centrifugation and denoted as PDA@LMFP.

[0036] S3 MXene intermediate layer construction: A 0.8 nm thick monolayer Ti3C2T was prepared by etching Ti3AlC2 with HF and then peeling it off. x MXene nanosheets were prepared into a 2 mg / mL MXene dispersion. PDA@LMFP was placed in a vacuum reactor at 50 Pa and immersed in the MXene dispersion for 2 h. The MXene nanosheets were embedded into the interlayer gaps of PDA through capillary action to form a π-π stacked structure with a spacing ≤ 5 nm. The mixture was then dried at 120 °C for 12 h.

[0037] S4 Polyaniline (PANI) outer layer vapor deposition: 1 kg of the material obtained in step S3 was transferred to a vapor deposition reactor. A mixture of 5 wt.% ammonium persulfate aqueous solution and Ar (humidity controlled <5%RH) with a volume ratio of 1:6 was used as the carrier gas. 83.3 g of aniline monomer vapor was introduced at a rate of 5 mL / min. 201.4 g of ammonium persulfate was used as the initiator. The vapor-phase polymerization reaction was carried out at 0 °C for 4 h to form a PANI conductive network layer with a thickness of 12 nm and a pore size of 3.5 nm, thus obtaining the PANI@MXene nanosheet@PDA@LMFP cathode material.

[0038] Example 3

[0039] The preparation method of the gradient composite coated modified lithium manganese iron phosphate cathode material in this embodiment includes the following steps:

[0040] S1 Preparation of Activated Material: 1 kg of LMFP precursor powder (D50 = 0.5 μm, specific surface area 13 m²) was used. 2 / g) was placed in 200mL of a citric acid-ethanol mixed solution (citric acid to ethanol volume ratio of 1:5) and sonicated at 80kHz for 20min. After centrifugation and drying, (010) crystal face exposure rate >60% and specific surface area 24.3m² were obtained. 2 / g of activating material;

[0041] S2 PDA inner layer coating: 1 kg of activated material was dispersed in 5 L of Tris-HCl buffer solution with pH=9, 50 g of dopamine hydrochloride was added, and the mixture was stirred at 35 °C for 10 h. A PDA layer with a thickness of 5 nm was formed by oxidative polymerization. The PDA coating material was obtained by centrifugation and denoted as PDA@LMFP.

[0042] S3 MXene intermediate layer construction: A 1.2 nm thick Ti3C2 monolayer was prepared by etching Ti3AlC2 with HF and then peeling it off. x MXene nanosheets were prepared into a 3 mg / mL MXene dispersion. PDA@LMFP was placed in a vacuum reactor at 50 Pa and immersed in the MXene dispersion for 2 h. The MXene nanosheets were embedded into the interlayer gaps of PDA through capillary action to form a π-π stacked structure with a spacing ≤ 5 nm. The mixture was then dried at 130 °C for 12 h.

[0043] S4 Polyaniline (PANI) outer layer vapor deposition: 1 kg of the material obtained in step S3 was transferred to a vapor deposition reactor. A mixture of 5 wt.% ammonium persulfate aqueous solution and Ar (humidity controlled <5%RH) with a volume ratio of 1:10 was used as the carrier gas. 100 g of aniline monomer vapor was introduced at a rate of 15 mL / min. 241.7 g of ammonium persulfate was used as the initiator. The vapor-phase polymerization reaction was carried out at 5 °C for 6 h to form a PANI conductive network layer with a thickness of 15 nm and a pore size of 5 nm, thus obtaining PANI@MXene nanosheets@PDA@LMFP cathode material.

[0044] Comparative Example 1

[0045] Comparative Example 1 directly uses LMFP precursor as lithium manganese iron phosphate cathode material.

[0046] Comparative Example 2

[0047] The difference from Example 1 is that step S1 is omitted, and the LMFP precursor powder is directly used to replace the activation material in step S2.

[0048] Comparative Example 3

[0049] The difference from Example 1 is that step S2 is omitted, and the activated material obtained in step S1 is used directly to replace PDA@LMFP in step S3.

[0050] Comparative Example 4

[0051] The difference from Example 1 is that step S3 is omitted, and the PDA@LMFP obtained in step S2 is used directly to replace the material obtained in step S3.

[0052] Comparative Example 5

[0053] The difference from Example 1 is that step S4 is omitted, and the material obtained in step S3 is directly used as the final product, denoted as MXene nanosheet@PDA@LMFP cathode material.

[0054] The positive electrode materials of Examples 1-3 and Comparative Examples 1-5, acetylene black, and polyvinylidene fluoride (PVDF) binder were mixed evenly in a weight ratio of 94:3:3 and dispersed in an N-methylpyrrolidone (NMP) solution to obtain a paste-like mixture. The mixture was coated onto aluminum foil and vacuum dried overnight at 90°C to obtain an electrode sheet. The assembly of the Li / LMFP button cell (2016 model) was carried out in a glove box filled with high-purity Ar: a lithium metal sheet was used as the negative electrode, a polypropylene membrane was used as the separator, 1 mol of LiPF6 was dissolved in ethyl carbonate / dimethyl carbonate (EC / DMC) (1:1, volume ratio) as the electrolyte, and the above electrode sheet was used as the positive electrode material of the battery.

[0055] The batteries assembled in Examples 1-3 and Comparative Examples 1-5 were subjected to performance tests. The charging and discharging performance of the lithium-ion batteries was tested at room temperature on a Blue Electric testing system, with a test voltage range of 2.5-4.5V. The test results are shown in Table 1.

[0056] Table 1. Performance test results of batteries in Examples 1-3 and Comparative Examples 1-5

[0057]

[0058] As can be seen from the test results of Examples 1-3 in Table 1, with the increase of the thickness of the PDA and PANI coating layers, the conductivity and ion diffusion coefficient of the prepared cathode material increase, and the dissolution of Mn decreases, thereby improving the discharge capacity and cycle life of the material. As can be seen from Example 1 and Comparative Example 1, the performance indicators of Example 1 are significantly better than those of Comparative Example 1. This is because the strong coordination bond between the PDA layer and the Mn / Fe sites inhibits manganese dissolution; MXene nanosheets construct three-dimensional electron channels, improving electronic conductivity; and the PANI outer layer provides elastic buffering and electrolyte isolation, improving the ion diffusion coefficient. Under the combined effect of these factors, the energy density and cycle life of the material are ultimately improved. As can be seen from Example 1 and Comparative Example 2, the performance of Comparative Example 2 is slightly lower than that of Example 1, indicating that citric acid pretreatment significantly improves the proportion of active crystal faces and interfacial reaction kinetics of the LMFP precursor through multiple mechanisms such as directional etching, surface activation, and defect control, laying the foundation for subsequent coating modification. As can be seen from Example 1 and Comparative Example 3, the main performance differences between the two are in Mn dissolution and cycling performance. This indicates that the PDA layer coating of LMFP effectively inhibits manganese dissolution through multiple mechanisms such as strong interfacial bonding and transition metal protection, significantly improving the electrochemical performance and cycling stability of LMFP. As can be seen from Example 1 and Comparative Example 4, the electronic conductivity and discharge capacity of Example 1 are significantly higher than those of Comparative Example 4, indicating that the three-dimensional electronic channels constructed by MXene nanosheets can significantly improve electronic conductivity, thereby increasing the discharge capacity of the material. As can be seen from Example 1 and Comparative Example 5, the ion diffusion coefficient and Mn dissolution of Example 1 are significantly higher than those of Comparative Example 5, indicating that the porous structure of PANI may be beneficial for electrolyte wetting, accelerating lithium-ion diffusion kinetics. Simultaneously, the coating layer can physically isolate the electrolyte from the active material, reducing manganese ion dissolution (especially under high voltage or high temperature conditions), thereby alleviating the capacity decay problem.

Claims

1. A method for preparing gradient composite coated modified lithium manganese iron phosphate cathode material, characterized in that, Includes the following steps: S1 Preparation of Activated Material: The LMFP precursor was placed in a citric acid-ethanol mixed solution, and after sonication, centrifugation, and drying, the activated material was obtained; S2 polydopamine inner layer coating: The activated material was dispersed in Tris-HCl buffer, dopamine hydrochloride was added and stirred to form a PDA layer, and the PDA-coated material was obtained by centrifugation; S3 MXene intermediate layer construction: Ti3AlC2 was etched by HF and then peeled off to obtain a single layer of Ti3C2T. x MXene nanosheets were prepared into an MXene dispersion; PDA-coated material was placed in a vacuum reactor and impregnated with the MXene dispersion, followed by drying; S4 Polyaniline outer layer vapor deposition: The material obtained in step S3 is transferred to a vapor deposition reactor, aniline monomer vapor is introduced, and ammonium persulfate is used as an initiator to carry out a vapor phase polymerization reaction at -15 to 5°C for 2-6 hours to form a PANI conductive network layer, thus obtaining PANI@MXene nanosheets@PDA@LMFP cathode material.

2. The preparation method of the gradient composite coated modified lithium manganese iron phosphate cathode material as described in claim 1, characterized in that, In step S1, the volume ratio of citric acid to ethanol in the citric acid-ethanol mixed solution is 1:(3-5).

3. The preparation method of the gradient composite coated modified lithium manganese iron phosphate cathode material as described in claim 1, characterized in that, In step S1, the ultrasonic frequency is 40-80kHz and the ultrasonic time is 20-30min.

4. The preparation method of the gradient composite coated modified lithium manganese iron phosphate cathode material as described in claim 1, characterized in that, In step S1, the exposure rate of the (010) crystal plane of the activated material is >60%, and the specific surface area is 20-25m². 2 / g.

5. The method for preparing the gradient composite coated modified lithium manganese iron phosphate cathode material as described in claim 1, characterized in that, In step S2, the pH of the Tris-HCl buffer is 8.5-9.

6. The method for preparing the gradient composite coated modified lithium manganese iron phosphate cathode material as described in claim 1, characterized in that, In step S2, the concentration of dopamine hydrochloride is 0.5-1 g / mL, and the concentration of the activating material is 20-30 g / mL.

7. The method for preparing the gradient composite coated modified lithium manganese iron phosphate cathode material as described in claim 1, characterized in that, In step S2, the stirring temperature is 25-35℃, the stirring time is 6-10h, and the thickness of the PDA layer is 2-5nm.

8. The method for preparing the gradient composite coated modified lithium manganese iron phosphate cathode material as described in claim 1, characterized in that, In step S3, the thickness of the MXene nanosheets is 0.8-1.2 nm, the concentration of the MXene dispersion is 1-3 mg / mL, the pressure of the vacuum reactor is ≤100 Pa, the drying temperature is 110-130℃, and the drying time is 12 h.

9. The preparation method of the gradient composite coated modified lithium manganese iron phosphate cathode material as described in claim 1, characterized in that, In step S4, when aniline monomer vapor is introduced, the carrier gas is a mixture of atomized ammonium persulfate aqueous solution and Ar, the introduction rate is 5-15 mL / min, the concentration of ammonium persulfate aqueous solution is 5 wt.%, the volume ratio of atomized ammonium persulfate aqueous solution to Ar in the mixed gas is 1:(5-10), and the humidity is controlled <5%RH; the thickness of the PANI conductive network layer is 10-15 nm, and the pore size is 2-5 nm; the mass ratio of aniline monomer to the material obtained in step S3 is 1:(10-15), and the molar ratio of ammonium persulfate to aniline monomer is 1:(1-2).

10. The gradient composite coated modified lithium manganese iron phosphate cathode material prepared by the preparation method according to any one of claims 1-9.

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