Modified lithium iron phosphate-based positive electrode material as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of secondary battery energy storage, and discloses a modified lithium iron phosphate-based positive electrode material and a preparation method and application thereof.The preparation method comprises the steps that the lithium iron phosphate-based positive electrode material is ultrasonically dispersed in a solution containing alkali metal ions, then dispersion liquid is washed and dried, and the modified lithium iron phosphate-based positive electrode material is obtained. The lithium iron phosphate-based positive electrode material coated with the uniform amorphous composite interface is obtained. According to the method, the problem that the primary carbon coating layer is difficult to completely coat due to scale dependence is effectively solved. The thickness of the coating layer obtained by the method is uniform and adjustable, the actual lithium storage specific capacity of the lithium iron phosphate-based material is not obviously influenced, and when the coating layer is applied to the lithium battery positive electrode material, the out-of-control electrolyte decomposition induced by lattice cracks accompanied by repeated lithium intercalation / deintercalation in the charging and discharging process is obviously inhibited; and the electrochemical properties such as coulombic efficiency, capacity holding rate and cycling stability are effectively improved. Meanwhile, the method is simple, effective, low in cost, easy to amplify, high in universality and suitable for large-scale production of the lithium storage positive electrode material.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary battery energy storage, and in particular to a modified lithium iron phosphate-based positive electrode material and a preparation method and application thereof. Background Art

[0002] In secondary battery applications, the energy storage density of materials, especially the energy storage density of positive electrode materials, directly affects the overall energy storage density of the battery, which requires that the positive electrode materials must have as high a tap density as possible. However, a high tap density means that the positive electrode material must form a unique micron aggregate with a certain distribution to reduce additional unnecessary gaps, which objectively requires that the material particle size distribution is different. In addition, commonly used lithium-ion battery positive electrode materials, such as LiFePO4 and LiCoO2, mostly exhibit poor intrinsic conductivity. In order to meet high-power applications, they need to be kinetically modified, such as constructing nano-particle micron aggregates, shortening the ion diffusion path, and applying a small amount of carbon coating to accelerate the charge conduction between particles. However, the carbon coating process must not only consider the effects of the coating content, annealing process, etc. on the intrinsic properties of the positive electrode material (such as stress, defects, actual capacity, etc.), but also consider the dependence of the coating process on the particle size. Therefore, it is still very challenging to achieve a moderately uniform coating layer for positive electrode materials with a large particle size distribution, especially the practicality of the coating interface layer.

[0003] When the electrode contacts the electrolyte, a so-called solid electrolyte interface will spontaneously form, which has good ion conductivity and electronic insulation. A stable solid electrolyte interface is of great significance to the stability of the electrode and the stable operation of the battery. Especially for positive electrode materials, an unstable interface will cause the electrolyte to decompose repeatedly and form a large amount of by-products, which will not only consume the electrolyte, but also deteriorate the charge conduction process between the electrode and the electrolyte. For positive electrode materials, this interface is also called the cathode electrolyte interface. For lithium-ion battery positive electrode materials, such as commercial lithium iron phosphate-based materials Li x Fe y M z PO4, due to its uneven particle size, has a large number of exposed uncoated areas. After contacting with the electrolyte, a primary cathode electrolyte interface will be formed first. After repeated lithium insertion / extraction processes, a large number of crack areas will be generated inside the grains due to the repeated action of initial stress and subsequent stress. When the cracks extend to the carbon coating area, the composite coating layer of the coated carbon and the primary cathode electrolyte interface shows strong stability; however, when the cracks extend to the uncoated area, it will lead to rupture of the primary cathode interface, and the deep extension of the electrolyte and the subsequent continuous formation of amorphous by-products, resulting in uncontrolled decomposition of the electrolyte. This is the commercial lithium iron phosphate-based material Li x Fe y M zThis is one of the important reasons why the lithium storage properties of PO4 materials are still attenuated.

[0004] Therefore, there is an urgent need to provide a practical, easy and scalable interface preparation method; at the same time, this method does not depend on the size of the parent particles, and the thickness of the coating layer is adjustable, so as not to affect the energy storage capacity of the commercial material itself, and can effectively inhibit the uncontrolled decomposition process of the electrolyte caused by crack expansion, thereby further improving the capacity retention rate, cycle stability, and coulombic efficiency of commercial lithium storage positive electrode materials. Summary of the invention

[0005] In view of the above problems, the present invention provides a modified lithium iron phosphate-based positive electrode material and a preparation method and application thereof, which can prepare a modified lithium iron phosphate-based positive electrode material uniformly dispersed in a commercial lithium iron phosphate-based material Li at room temperature. x Fe y M z The amorphous composite interface of PO4 powder is beneficial to inhibit the uncontrolled electrolyte decomposition caused by cracks formed by repeated insertion and extraction of lithium, further improving its energy storage capacity retention rate, cycle stability, coulombic efficiency, etc.

[0006] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0007] A method for preparing a modified lithium iron phosphate-based positive electrode material, the specific process is to disperse a lithium iron phosphate-based material powder containing a small amount of carbon coating in a solution containing alkali metal ions, disperse, stand, wash, and dry, so as to prepare the surface-modified lithium iron phosphate-based positive electrode material;

[0008] Wherein, the lithium iron phosphate-based material powder containing a small amount of carbon is C / Li with a carbon content of 1-10% wt. x Fe y M z PO4 composite material, in which C is coated on Li in the form of a coating layer x Fe y M z The surface of PO4.

[0009] Preferably, the solute alkali metal cation in the alkali metal ion solution is one or more of sodium, potassium, calcium, magnesium or aluminum, and the anion is one or more of chloride ion, bromide ion, iodide ion, acetate ion or tetrafluoroborate ion; the solvent in the alkali metal ion solution is one or more of deionized water, ethanol, methanol or ethylene glycol.

[0010] Preferably, the dispersion and standing process is one or a combination of stirring dispersion or ultrasonic dispersion or long-term standing, and the time is 5 minutes to 24 hours.

[0011] Preferably, the concentration of the alkali metal solution is 0.1-2.5ML -1.

[0012] Preferably, the ratio of the lithium iron phosphate-based powder to the alkali metal ion solution is 1 g: (3-50) mL.

[0013] Preferably, the washing process is one or a combination of deionized water and ethanol washing, and the washing times are 1-2 times; the drying process is drying in an air atmosphere blast drying oven at 50-100°C for 30min-24h;

[0014] Preferably, the surface modification is obtained by local conversion of lithium iron phosphate-based materials and alkali metal ion solutions into an amorphous composite interface; the amorphous composite interface is continuous and uniform, with adjustable thickness, and contains one or more of potassium, calcium, magnesium, and aluminum; the average thickness of the amorphous composite interface is 5-15nm.

[0015] The present invention also provides a modified lithium iron phosphate-based positive electrode material prepared according to the preparation method.

[0016] Preferably, the particle size of the modified lithium iron phosphate-based positive electrode material is 50-500 nm, the tap density is 2.0-5.0 g / cc, and the specific surface area is 10-40 m 2 / g.

[0017] The present invention also provides an application of the surface modified lithium iron phosphate-based material in lithium secondary batteries such as lithium ion batteries, lithium metal batteries and solid-state batteries.

[0018] Specifically, the above lithium iron phosphate-based material is used in the preparation of lithium secondary battery electrode materials.

[0019] More specifically, the amorphous composite interface obtained by the above surface modification method is effective in inhibiting the degradation of commercial lithium iron phosphate-based materials Li x Fe y M z PO4 is used in uncontrolled electrolyte decomposition caused by cracks formed by repeated insertion and extraction of lithium.

[0020] By adopting the above technical solution, the beneficial effects of the present invention are:

[0021] The present invention obtains a lithium iron phosphate-based material uniformly coated with an amorphous composite interface layer through chemical adsorption of room temperature solution and in-situ local conversion. The coating layer obtained by this method has a uniform and adjustable thickness, does not significantly affect the actual lithium storage capacity of the lithium iron phosphate-based material, and is applied to lithium battery positive electrode materials, which significantly inhibits the uncontrolled decomposition of the electrolyte induced by the lattice cracks accompanying the repeated insertion / extraction of lithium during the charge and discharge process, and effectively improves the electrochemical properties such as coulomb efficiency, capacity retention rate, and cycle stability. In addition, this method does not depend on the size of the parent particles, the interface thickness is adjustable, and it is simple, easy to implement, and low-cost. It is suitable for further modification of large-scale commercial materials, making up for the uneven coating problem caused by the large scale difference of the carbon coating method in the large-scale preparation process, and does not significantly affect the intrinsic energy storage capacity of commercial electrode materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 The polycrystalline powder diffraction patterns of the corresponding samples of Comparative Example 1 and Example 1 of the present invention;

[0023] Figure 2 This is a transmission electron microscope high-angle annular dark field image of the sample corresponding to Example 1 of the present invention;

[0024] Figure 3 TEM element imaging distribution diagram of the sample corresponding to Example 1 of the present invention;

[0025] Figure 4 Transmission electron microscope images of the corresponding samples of Comparative Example 1 and Example 1 of the present invention;

[0026] Figure 5 The corresponding batteries of Comparative Example 2, Example 2-1 and Example 2-2 of the present invention are at 200mA g -1 The charge and discharge capacity diagram of constant current charge and discharge under the current density;

[0027] Figure 6 The corresponding batteries of Comparative Example 2, Example 2-1 and Example 2-2 are at 200mAg -1 Coulomb efficiency diagram of constant current charge and discharge at current density;

[0028] Figure 7 Transmission electron microscope images of the corresponding samples of Comparative Example 3 and Example 3. DETAILED DESCRIPTION

[0029] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0030] The raw materials, reagents, etc. used in the following examples can be obtained through commercial channels.

[0031] Example 1

[0032] A method for preparing a modified lithium iron phosphate-based positive electrode material comprises the following steps:

[0033] Prepare 30 mL of 0.5 M calcium acetate aqueous solution in a 50 mL centrifuge tube, evenly disperse 2 g of commercial lithium iron phosphate-based material powder therein, let stand for 6 hours, filter, and recover the filtrate for repeated use. Wash the filter cake twice with deionized water and once with anhydrous ethanol, and then dry it in a forced air drying oven at 80°C under air atmosphere for 6 hours to obtain a lithium iron phosphate-based material powder evenly coated with an amorphous composite interface, which is a modified lithium iron phosphate-based positive electrode material.

[0034] The sample corresponding to Example 1 is the modified lithium iron phosphate-based positive electrode material coated with the amorphous composite interface.

[0035] Comparative Example 1

[0036] The sample corresponding to Comparative Example 1 is a small amount of carbon-coated commercial lithium iron phosphate-based material powder, which was purchased without any post-processing.

[0037] The corresponding samples of Example 1 and Comparative Example 1 were characterized by a polycrystalline powder diffractometer. Figure 1 The polycrystalline powder diffraction pattern (XRD) of the sample shows that all diffraction peaks of the samples corresponding to Example 1 and Comparative Example 1 are consistent with the orthorhombic structure standard lithium iron phosphate (PDF No. 40-1499), and there is no unidentified diffraction peak. This indicates that the sample corresponding to Example 1 does not introduce any heterogeneous crystalline impurity phase compared to the sample corresponding to Comparative Example 1, which means that the calcium ion solution treatment process does not introduce any heterogeneous crystalline phase into the lithium iron phosphate-based material.

[0038] The energy dispersive spectrometer of the transmission electron microscope was used to analyze the corresponding samples of Example 1. Figure 2 This is a high-angle annular dark field image (HAADF-STEM) of the sample. Figure 2 From the particle size in the graph, it can be seen that the particle size of the interface modified lithium iron phosphate-based material powder after treatment with calcium ion solution is about 50-500nm.

[0039] The energy dispersive spectrometer of the transmission electron microscope was used to analyze the corresponding samples of Example 1. Figure 3 This is the element distribution image of the sample in scanning transmission mode. The results show that a uniform coating layer containing calcium is formed on the surface of the particles.

[0040] The samples of Example 1 and Comparative Example 1 were characterized by high-resolution imaging using a transmission electron microscope. Figure 4This is a high-resolution transmission electron microscopy (HRTEM) image of the sample. Figure 4 It can be seen that the calcium ion solution treatment process forms a continuous amorphous phase with a thickness of about 5nm on the surface of commercial LiFePO4 grains, which is consistent with Figure 1 The XRD results are consistent with those of

[0041] comprehensive Figure 1-4 It can be seen that a uniform modified amorphous calcium-based composite interface can be formed on the surface of commercial lithium iron phosphate-based material powder by treating with room temperature calcium ion aqueous solution.

[0042] Example 2

[0043] The manufacturing method of the battery of Example 2 is the same as that of Comparative Example 2, except that the commercial positive electrode materials used are different. The positive electrode material of Example 2-1 is a commercial lithium iron phosphate-based powder treated with a 0.5M calcium tetrafluoroborate aqueous solution, while the positive electrode material of Example 2-2 is a commercial lithium iron phosphate-based powder treated with a 0.5M calcium chloride aqueous solution.

[0044] Battery assembly preparation: 80wt.% modified lithium iron phosphate powder, 10wt.% acetylene black, and 10wt.% polyvinylidene fluoride are uniformly dispersed in N-methylpyrrolidone to form a slurry. The mixed slurry is evenly scraped on aluminum foil, dried at 80℃ for 12h, and cut into lithium iron phosphate positive electrode sheets with a diameter of 10mm. The active material loading is 1.5-4mg cm -2 The lithium iron phosphate-based positive electrode sheet and the lithium sheet were used as the positive and negative electrodes, respectively, the polypropylene film was used as the separator, and the 1M LiPF6 ethylene carbonate / diethyl carbonate (mass ratio 1:1) solution was used as the electrolyte, and a 2032-type button cell was assembled in a glove box.

[0045] Comparative Example 2

[0046] A method for preparing a battery comprises the following steps:

[0047] 80wt.% commercial lithium iron phosphate powder, 10wt.% acetylene black, and 10wt.% polyvinylidene fluoride were uniformly dispersed in N-methylpyrrolidone to form a slurry. The mixed slurry was evenly scraped onto aluminum foil, dried at 80℃ for 12h, and cut into lithium iron phosphate positive electrode sheets with a diameter of 10mm. The active material loading was 1.5-4mg cm -2 The lithium iron phosphate-based positive electrode sheet and the lithium sheet were used as the positive and negative electrodes, respectively, the polypropylene film was used as the separator, and the 1M LiPF6 ethylene carbonate / diethyl carbonate (mass ratio 1:1) solution was used as the electrolyte, and a 2032-type button cell was assembled in a glove box.

[0048] The applicant tested the product effect and the results are as follows:

[0049] The batteries obtained in Example 2 and Comparative Example 2 were respectively -1 Constant current charge and discharge test was performed under the current and voltage window of 2.0-4.2V. Figure 5 The figure is the charge and discharge capacity diagram of the battery. After 500 cycles, the discharge capacity retention rate of the battery in comparative example 2 is only 51.2%, that is, the discharge capacity is 64 mAh g -1 In sharp contrast, under the same test conditions, the discharge capacity retention rate of the battery in Example 2-1 is 67.2%, that is, the discharge capacity is 82 mAh g -1 Under the same test conditions, the discharge capacity retention rate of the battery in Example 2-2 is 74.8%, that is, the discharge capacity is 92 mAh g -1 The results show that after the commercial lithium iron phosphate-based powder is treated with calcium ion solution, an amorphous calcium-based composite interface is formed on the surface, and its capacity retention rate and cycle stability are significantly improved.

[0050] Figure 6 The figure is the coulombic efficiency of the battery during charge and discharge. During 500 cycles, the coulombic efficiency of the battery in Comparative Example 2 is generally lower than 99% on average, while the coulombic efficiency of the batteries in Examples 2-1 and 2-2 is generally higher than 99%. The results show that after the commercial lithium iron phosphate-based powder is treated with a calcium ion solution and an amorphous calcium-based composite interface is formed on the surface, its coulombic efficiency is significantly improved.

[0051] Example 3

[0052] A transmission electron microscopy sample preparation of a cycled battery electrode material is disclosed. The battery of Example 2-2 is disassembled in a glove box after cycling, the positive electrode is immersed in diethyl carbonate, the electrolyte adsorbed on the surface is washed off, and the battery is dried. The active material on the electrode is directly scraped onto a copper mesh for transmission electron microscopy characterization.

[0053] Comparative Example 3

[0054] The sample preparation process of Comparative Example 3 is the same as that of Example 3, except that the disassembled battery is the battery after the cycle of Comparative Example 2.

[0055] The applicant further verified the product effect, and the results are as follows:

[0056] Example 3 and Comparative Example 3 were characterized by transmission electron microscopy. Figure 7It is a high-resolution transmission electron microscope photo of the sample after cycling. Comparative Example 3, that is, the commercial lithium iron phosphate-based powder after cycling, after repeated insertion and extraction of lithium, the cracks generated form crack extension zones, which extend to the grain boundary lacking carbon coating areas. The penetration of the electrolyte will lead to the formation of a secondary electrolyte interface phase, resulting in a large amount of amorphous phase in the diffusion zone, which comes from the uncontrolled decomposition of the electrolyte; Different from this, Example 3, that is, the commercial lithium iron phosphate-based powder coated with a calcium-based amorphous interface, due to the presence of an amorphous calcium-based composite interface, the crack extension to the interface electrolyte penetration is suppressed, and Fourier transform shows that the same crystal structure is maintained in the crack zone and the boundary zone, and a large amount of amorphous phase is not formed. The results show that the presence of the calcium-based amorphous composite interface effectively inhibits the uncontrolled electrolyte decomposition caused by cracks formed by repeated insertion and extraction of lithium in the commercial lithium iron phosphate-based powder, thereby improving the electrochemical properties of the material such as capacity retention rate, cycle stability, and coulomb efficiency.

[0057] The above description is a detailed description of the preferred feasible embodiments of the present invention, but the embodiments are not intended to limit the scope of the patent application of the present invention. All equivalent changes or modified changes completed under the technical spirit suggested by the present invention should fall within the patent scope covered by the present invention.

Claims

1. A method for preparing a modified lithium iron phosphate-based positive electrode material, characterized in that: The specific process is to disperse a small amount of carbon-coated lithium iron phosphate-based material powder in a solution containing alkali metal ions, disperse, stand, wash, and dry, so as to prepare the surface-modified lithium iron phosphate-based positive electrode material; Wherein, the lithium iron phosphate-based material powder containing a small amount of carbon is C / Li with a carbon content of 1-10% wt. x Fe y M z PO4 composite material, in which C is coated on Li in the form of a coating layer x Fe y M z The surface of PO4.

2. The method for preparing the modified lithium iron phosphate-based positive electrode material according to claim 1, characterized in that: The solute alkali metal cation in the alkali metal ion solution is one or more of sodium, potassium, calcium, magnesium or aluminum, and the anion is one or more of chloride ion, bromide ion, iodide ion, acetate ion or tetrafluoroborate ion; the solvent in the alkali metal ion solution is one or more of deionized water, ethanol, methanol or ethylene glycol.

3. The method for preparing the modified lithium iron phosphate-based positive electrode material according to claim 1, characterized in that: The dispersion and standing process is one or a combination of stirring dispersion or ultrasonic dispersion or long-term standing, and the time is 5 minutes to 24 hours.

4. The method for preparing a modified lithium iron phosphate-based positive electrode material according to claim 1 or 2, characterized in that: The concentration of the alkali metal solution is 0.1-2.5ML -1 .

5. The method for preparing the modified lithium iron phosphate-based positive electrode material according to claim 1, characterized in that: The dosage ratio of the lithium iron phosphate-based powder to the alkali metal ion solution is 1 g: (3-50) mL.

6. The method for preparing the modified lithium iron phosphate-based positive electrode material according to claim 1, characterized in that: The washing process is one of deionized water washing and ethanol washing or a combination of the two, and the washing times are 1-2 times; the drying process is drying in an air atmosphere blast drying oven at 50-100° C. for 30 min-24 h.

7. The method for preparing a modified lithium iron phosphate-based positive electrode material according to claim 1, characterized in that: The surface modification is obtained by chemical adsorption of lithium iron phosphate-based materials and alkali metal ion solution to locally transform them into an amorphous composite interface; the amorphous composite interface is continuous and uniform, with adjustable thickness, and contains one or more of potassium, calcium, magnesium, and aluminum; the average thickness of the amorphous composite interface is 5-15nm.

8. A modified lithium iron phosphate-based positive electrode material prepared according to the preparation method according to any one of claims 1 to 7.

9. The modified lithium iron phosphate-based positive electrode material according to claim 8, characterized in that: The modified lithium iron phosphate-based positive electrode material has a particle size of 50-500 nm, a tap density of 2.0-5.0 g / cc, and a specific surface area of ​​10-40 m 2 / g.

10. Use of the surface modified lithium iron phosphate-based material according to claim 8 or 9 in a lithium secondary battery such as a lithium ion battery, a lithium metal battery, or a solid-state battery.

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