Gradient composite coated modified lithium manganese iron phosphate positive electrode material and preparation method thereof

Abstract

The invention discloses a gradient composite coated modified lithium manganese iron phosphate positive electrode material and a preparation method thereof, and belongs to the technical field of positive electrode materials. An LMFP precursor is pretreated through a citric acid-ethanol mixed solution, an active crystal face is exposed, and then a three-dimensional gradient coating structure of a polydopamine (PDA) chemical bonding layer, an MXene nanosheet middle layer and a polyaniline (PANI) conductive outer layer is sequentially constructed on the surface of the material, so that the electronic conductivity of the lithium manganese iron phosphate positive electrode material is increased to 10 <-3 > S / cm magnitude, and the performance of the lithium manganese iron phosphate positive electrode material is improved. The capacity retention ratio (1C) of 500 cycles is larger than 90%, the coating layer is not prone to falling off, and meanwhile dissolution of manganese ions is effectively inhibited.

Description

Technical Field

[0001] The present invention relates to the technical field of cathode materials, and particularly relates to a gradient composite-coated modified lithium iron manganese phosphate cathode material and a preparation method thereof. Background Art

[0002] Lithium iron manganese phosphate (LiFe x Mn 1-x PO 4 , abbreviated as LMFP) is a new type of cathode material for lithium-ion batteries. This material combines the advantages of lithium iron phosphate (LFP) and lithium manganese phosphate (LMP). By partially replacing Fe and Mn, the performance is optimized. Due to its characteristics such as high energy density, high safety, and low cost, it is considered an important development direction in the fields of power batteries and energy storage. Although the lithium iron manganese phosphate cathode material has advantages such as high energy density, high safety, and low cost, it has some performance bottlenecks. Coating modification is an important way to solve the performance bottlenecks of the lithium iron manganese phosphate cathode material, which can significantly improve its electrochemical performance and stability, and provide technical support for its wide application in the new energy field.

[0003] Traditional carbon coating modification improves the electronic conductivity of the material by coating a layer of conductive carbon on the surface of the cathode material particles, thereby improving its rate performance and cycle stability. However, traditional carbon coating technology also has some limitations. The most important problem is the weak bonding force between the carbon layer and the cathode material matrix, resulting in the problem of peeling after long cycles.

[0004] Therefore, it is urgent to develop a new coating modification method for lithium iron manganese phosphate cathode materials to improve the performance of lithium iron manganese phosphate cathode materials and meet the usage requirements. Summary of the Invention

[0005] The technical problem to be solved by the present invention is: overcoming the deficiencies of the prior art, providing a gradient composite-coated modified lithium iron manganese phosphate cathode material and a preparation method thereof, so that the electronic conductivity of the lithium iron manganese phosphate cathode material is increased to 10 -3 S / cm magnitude, the capacity retention rate (1C) after 500 cycles > 90%, the coating layer is not easy to fall off, and at the same time, the dissolution of manganese ions is effectively inhibited.

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

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

[0008] S1 Prepare the activated material: Place the LMFP precursor in a citric acid-ethanol mixed solution, and after ultrasonic treatment, centrifugation, and drying, obtain the activated material;

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

[0010] S3 MXene intermediate layer construction: Ti 3 AlC 2 After being etched by HF and peeled off, monolayer Ti 3 C 2 T x MXene nanosheets are prepared into an MXene dispersion; the PDA-coated material is placed in a vacuum reactor and impregnated with the MXene dispersion to embed the MXene nanosheets into the gaps of the PDA layer, 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 h to form a PANI conductive network layer, and the PANI@MXene nanosheet@PDA@LMFP cathode material is obtained.

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

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

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

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

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

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

[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 ≤ 100 Pa; the drying temperature is 110-130 °C, and the drying time is 12 h.

[0019] Preferably, in step S4, when introducing aniline monomer vapor, the carrier gas is a mixed gas 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 and modified lithium iron manganese phosphate cathode material prepared by the above preparation method.

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

[0022] In the present invention, the LMFP precursor is pretreated with a citric acid-ethanol mixed solution to expose the active crystal plane, and then a three-dimensional gradient coating structure of a polydopamine (PDA) chemical bonding layer, a MXene nanosheet intermediate layer, and a polyaniline (PANI) conductive outer layer is sequentially constructed on the material surface. Among them, the MXene layer is embedded in the gap of the PDA layer by vacuum impregnation to form a π-π stacking interface; the outer layer PANI adopts a low-temperature chemical vapor deposition process to achieve nano-network coating. By constructing an organic-inorganic composite gradient interface, the present invention realizes: 1) strong coordination bonds between the PDA layer and Mn / Fe sites inhibit manganese dissolution; 2) MXene nanosheets construct three-dimensional electron channels; 3) the PANI outer layer provides elastic buffering and electrolyte isolation. The modification method of the present invention increases the electronic conductivity of the lithium iron manganese phosphate cathode material to 10 -3 S / cm order of magnitude, the capacity retention rate (1C) after 500 cycles > 90%, the coating layer is not easily peeled off, and at the same time, manganese ion dissolution is effectively inhibited. Description of the Drawings

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

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

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

[0026] Example 1

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

[0028] S1 Preparation of activated material: Place 1 kg of LMFP precursor powder (D50 = 0.5 μm, specific surface area 13 m 2 / g) in 200 mL of citric acid-ethanol mixed solution (volume ratio of citric acid to ethanol is 1:3), and ultrasonically treat it at 40 kHz for 30 min. After centrifugation and drying, an activated material with a (010) crystal plane exposure rate > 60% and a specific surface area of 21.5 m 2 / g is obtained;

[0029] S2 PDA inner layer coating: Disperse 1 kg of activated material in 5 L of Tris-HCl buffer solution with pH = 8.5, add 25 g of dopamine hydrochloride, stir at 25 °C for 6 h, and form a PDA layer with a thickness of 2 nm through oxidative polymerization. Centrifuge to obtain the PDA-coated material, denoted as PDA@LMFP;

[0030] S3 MXene intermediate layer construction: After etching Ti 3 AlC 2 with HF and peeling, a 1-nm-thick single-layer Ti 3 C 2 T x MXene nanosheets are prepared and formulated into a 1 mg / mL MXene dispersion (solvent is water); Place PDA@LMFP in a vacuum reactor at 50 Pa and impregnate it with the MXene dispersion for 2 h. Through capillary action, the MXene nanosheets are embedded in the gaps of the PDA layer to form a π-π stacking structure with a spacing ≤ 5 nm, and dry it at 110 °C for 12 h;

[0031] S4 Polyaniline (PANI) outer layer vapor deposition: Transfer 1 kg of the material obtained in step S3 to a vapor deposition reactor, use a mixed gas of atomized 5 wt.% ammonium persulfate aqueous solution and Ar with a volume ratio of 1:5 (control humidity < 5% RH) as the carrier gas, introduce aniline monomer vapor at a rate of 10 mL / min, use 162 g of ammonium persulfate as the initiator, and carry out a gas-phase polymerization reaction 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, and obtain the PANI@MXene nanosheet@PDA@LMFP cathode material, whose structure is as Figure 1 shown, and the SEM photo is as Figure 2 shown. It can be seen from the SEM photo that the surface of the material is uniformly coated.

[0032] Example 2

[0033] The preparation method of the gradient composite-coated modified lithium iron manganese phosphate cathode material of 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 m 2 / g) was placed in 200 mL of a citric acid-ethanol mixed solution (volume ratio of citric acid to ethanol is 1:4) and sonicated at 60 kHz for 25 min. After centrifugation and drying, an activated material with a (010) crystal plane exposure rate > 60% and a specific surface area of 23.6 m 2 / g was obtained;

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

[0036] S3 Construction of MXene intermediate layer: Ti 3 AlC 2 After being etched with HF and peeled, single-layer Ti 3 C 2 T x MXene nanosheets with a thickness of 0.8 nm were prepared and formulated into a 2 mg / mL MXene dispersion; PDA@LMFP was placed in a vacuum reactor at 50 Pa and impregnated with the MXene dispersion for 2 h. The MXene nanosheets were embedded into the gaps of the PDA layer through capillary action to form a π-π stacking structure with a spacing ≤ 5 nm, and it was dried at 120 °C for 12 h;

[0037] S4 Gas-phase deposition of the outer layer of polyaniline (PANI): 1 kg of the material obtained in step S3 was transferred to a gas-phase deposition reactor. A mixed gas of atomized 5 wt.% ammonium persulfate aqueous solution and Ar with a volume ratio of 1:6 (controlled humidity < 5% RH) was used as the carrier gas, and 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, and a gas-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, obtaining the PANI@MXene nanosheet@PDA@LMFP cathode material.

[0038] Example 3

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

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

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

[0042] S3 MXene intermediate layer construction: Ti 3 AlC 2 After being etched with HF and peeled off, a 1.2 - nm - thick single - layer Ti 3 C 2 T x MXene nanosheets were prepared, and a 3 mg / mL MXene dispersion was formulated; PDA@LMFP was placed in a vacuum reactor at 50 Pa and impregnated with the MXene dispersion for 2 h. Through capillary action, the MXene nanosheets were embedded into the gaps of the PDA layer to form a π - π stacking structure with a spacing ≤ 5 nm, and 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 mixed gas of atomized 5 wt.% ammonium persulfate aqueous solution and Ar with a volume ratio of 1:10 (controlled humidity < 5% RH) was used as the carrier gas, and 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, and a gas - 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, obtaining the PANI@MXene nanosheets@PDA@LMFP cathode material.

[0044] Comparative Example 1

[0045] In Comparative Example 1, the LMFP precursor was directly used as the lithium iron manganese phosphate cathode material.

[0046] Comparative Example 2

[0047] The difference from Example 1 is that step S1 was not carried out, and the LMFP precursor powder was directly used to replace the activated material in step S2.

[0048] Comparative Example 3

[0049] The difference from Example 1 is that step S2 is not carried out, and the activated material obtained in step S1 is directly used to replace PDA@LMFP in step S3.

[0050] Comparative Example 4

[0051] The difference from Example 1 is that step S3 is not carried out, and the PDA@LMFP obtained in step S2 is directly used to replace the material obtained in step S3.

[0052] Comparative Example 5

[0053] The difference from Example 1 is that step S4 is not carried out, and the material obtained in step S3 is directly used as the final product, denoted as MXene nanosheet@PDA@LMFP cathode material.

[0054] The cathode 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 then dispersed in an N-methylpyrrolidone (NMP) solution to obtain a paste-like mixture; the mixture was coated on an aluminum foil and dried overnight in a vacuum at 90 °C to obtain a pole piece. The assembly of the Li / LMFP button battery (model 2016) was carried out in a glove box filled with high-purity Ar: a lithium metal sheet was used as the negative electrode, a polypropylene film was used as the separator, and 1 mol of LiPF 6 dissolved in ethylene carbonate / dimethyl carbonate (EC / DMC) (1:1, volume ratio) was used as the electrolyte, and the above-mentioned pole piece was used as the cathode 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 tests of the lithium-ion batteries were carried out on a BlueTEC test system at room temperature, and the test voltage range was 2.5-4.5 V. The test results are shown in Table 1:

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

[0057]

[0058] It can be seen from the test results of Examples 1-3 in Table 1 that as the thickness of the PDA and PANI coating layers increases, the conductivity and ion diffusion coefficient of the prepared cathode material increase, and the Mn dissolution decreases, thereby improving the discharge capacity and cycle life of the material. It can be seen from Example 1 and Comparative Example 1 that all 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 Mn dissolution; the MXene nanosheets construct a three-dimensional electron channel to improve the electronic conductivity; while the PANI outer layer provides elastic buffering and electrolyte isolation to improve the ion diffusion coefficient. Under the combined action of the above, the energy density and cycle life of the material are ultimately improved. It can be seen from Example 1 and Comparative Example 2 that the performance of Comparative Example 2 is slightly lower than that of Example 1, indicating that the citric acid pretreatment significantly improves the proportion of active crystal planes and the interfacial reaction kinetics of the LMFP precursor through multiple mechanisms such as directional etching, surface activation, and defect regulation, laying a foundation for subsequent coating modification. It can be seen from Example 1 and Comparative Example 3 that the main performance differences between the two are in the Mn dissolution amount and cycle performance, indicating that the PDA layer coating on LMFP effectively inhibits Mn dissolution through multiple mechanisms such as strong interfacial binding and transition metal protection, and significantly improves the electrochemical performance and cycle stability of LMFP. It can be seen from Example 1 and Comparative Example 4 that the electronic conductivity and discharge capacity of Example 1 are significantly higher than those of Comparative Example 4, indicating that the three-dimensional electron channel constructed by the MXene nanosheets can significantly improve the electronic conductivity, thereby improving the discharge capacity of the material. It can be seen from Example 1 and Comparative Example 5 that 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 to electrolyte infiltration, accelerating the lithium ion diffusion kinetics, and at the same time the coating layer can physically isolate the electrolyte from the active material, reducing the dissolution of manganese ions (especially under high voltage or high temperature conditions), thereby alleviating the problem of capacity decay.

Claims

1. A method for preparing a gradient composite coated modified lithium manganese iron phosphate positive electrode material, characterized in that: The following steps are involved: S1: preparing activated material: placing LMFP precursor in a citric acid-ethanol mixed solution, ultrasonicating, centrifuging, and drying to obtain activated material; S2 Polydopamine inner layer coating: The activated material is dispersed in Tris-HCl buffer, dopamine hydrochloride is added and stirred to form a PDA layer, and the PDA coating material is obtained by centrifugation; S3 MXene intermediate layer construction: Ti3AlC2 was etched by HF and then peeled off to obtain a single layer Ti3C2T x MXene nanosheets are prepared into a MXene dispersion; the PDA-coated material is placed in a vacuum reactor and immersed in the MXene dispersion, and then dried; 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 and obtain a PANI@MXene nanosheet@PDA@LMFP positive electrode material.

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

3. The method for preparing the gradient composite coated modified lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: In step S1, the ultrasonic frequency is 40-80 kHz, and the ultrasonic time is 20-30 min.

4. The method for preparing the gradient composite coated modified lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: In step S1, the (010) crystal plane exposure rate of the activated material is greater than 60%, and the specific surface area is 20-25m 2 / g.

5. The method for preparing the gradient composite coated modified lithium manganese iron phosphate positive electrode material according to 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 positive electrode material according to claim 1, characterized in that: In step S2, the concentration of dopamine hydrochloride is 0.5-1 g / mL, and the concentration of the activation material is 20-30 g / mL.

7. The method for preparing the gradient composite coated modified lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: In step S2, the stirring temperature is 25-35°C, 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 positive electrode material according to claim 1, characterized in that: In step S3, the thickness of the MXene nanosheet 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.

9. The method for preparing the gradient composite coated modified lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: In step S4, when aniline monomer vapor is introduced, the carrier gas is a mixed gas of atomized ammonium persulfate aqueous solution and Ar, the introduction rate is 5-15 mL / min, the concentration of the ammonium persulfate aqueous solution is 5 wt.%, the volume ratio of the atomized ammonium persulfate aqueous solution to Ar in the mixed gas is 1:(5-10), and the humidity is controlled to be less than 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 the aniline monomer to the material obtained in step S3 is 1:(10-15), and the molar ratio of ammonium persulfate to the aniline monomer is 1:(1-2).

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

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